Alternative promoter usage and alternative splicing ... - CiteSeerX

Page 1 of 38 Articles in PresS. Physiol Genomics (October 2, 2007). doi:10.1152/physiolgenomics.00192.2007

Alternative promoter usage and alternative splicing contribute to mRNA heterogeneity of mouse monocarboxylate transporter 2

Shelley X.L. Zhang,1 Tina R. Searcy,2 Yiman Wu,1 David Gozal,1,2 Yang Wang1,2

1

Kosair Children’s Hospital Research Institute, Department of Pediatrics, 2Department of

Pharmacology and Toxicology, University of Louisville, Louisville, KY 40202

Running head: MCT2 mRNA heterogeneity and alternative promoters

Corresponding Author: Yang Wang Department of Pediatrics University of Louisville 570 S. Preston Street, Ste. 211 Louisville, KY 40202 Tel: 502-852-4071 Fax: 502-852-2215 E-mail: [email protected]

Copyright © 2007 by the American Physiological Society.

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2 ABSTRACT

Expression patterns of monocarboxylate transporter 2 (MCT2) display mRNA diversity in a tissue-specific fashion. We cloned and characterized multiple mct2 5’-cDNA ends from the mouse and determined the structural organization of the mct2 gene. We found that transcription of this gene was initiated from 5 independent genomic regions that spanned greater than 80 kb on chromosome 10, resulting in 5 unique exon 1 variants (exons 1a, 1b, 1c, 1d, and 1e) that were then spliced to the common exon 2. Alternative splicing of four internal exons (exons AS1, AS2, AS3, and exon 3) greatly increased the complexity of mRNA diversity. While exon 1c was relatively commonly used for transcription initiation in various tissues, other exon 1 variants were used in a tissue-specific fashion, especially exons 1b and 1d that were used exclusively for testis-specific expression. Sequence analysis of 5’-flanking regions upstream of exons 1a, 1b, and 1c revealed the presence of numerous potential binding sites for ubiquitous transcription factors in all three regions and for transcription factors implicated in testis-specific or hypoxia-induced gene expression in the 1b region. Transient transfection assays demonstrated that each of the three regions contained a functional promoter and that the in vitro, cell type-specific activities of these promoters were consistent with the tissue-specific expression pattern of the mct2 gene in vivo. These results indicate that tissue-specific expression of the mct2 gene is controlled by multiple alternative promoters and that both alternative promoter usage and alternative splicing contribute to the remarkable mRNA diversity of the gene.

Key words: gene expression, transcriptional regulation, mRNA diversity, lactate metabolism, hypoxia

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3 INTRODUCTION

Lactate movement across the cellular membrane (“intercellular lactate shuttle”) relies on a specific transport mechanism that is afforded by the proton-linked monocarboxylate transporters (MCTs) (22, 27). There is also evidence that these transporters may be responsible for lactate movement across the mitochondrial membrane (“intracellular lactate shuttle”), which facilitates lactate oxidation (7, 12, 33). MCTs catalyze the facilitated diffusion of lactate with a proton, a process that does not require energy input other than that provided by the concentration gradients of lactate and protons. Recent studies have implicated a critical role for MCTs in coordinating lactate metabolism under both physiological and pathological conditions. For example, in skeletal muscle cells the amount of MCT correlates with the capacity of lactate oxidation (35), while in cardiac myocytes MCT is operating close to its maximal capacity, and may become rate-limiting under hypoxic conditions (58). It remains largely unknown, however, how lactate transporters are regulated under conditions such as hypoxia/ischemia, in which enhanced lactate metabolism is desired (23, 33, 48, 52, 62). Interestingly, a recent study reported that at least one of the genes coding for lactate transporters was upregulated by hypoxia at the transcriptional level via a hypoxia-inducible factor 1 (HIF-1)-dependent mechanism (56). The MCT gene family is comprised of multiple members, among which MCT1 (SLC16A1) and MCT2 (SLC16A7) have been relatively well characterized. These two transporters, although being similar in substrate selectivity and transport capability, have significantly different tissue/cell distribution, prompting the hypothesis that members of the MCT gene family perform distinct physiological functions (19, 29, 46). This hypothesis has been supported by recent studies showing functional relevance of neuron-specific expression of MCT2 and astroglial cellspecific expression of MCT1 in lactate traffic between these two cell types in the brain (11, 28, 39, 44). Furthermore, evidence suggests that MCT1 and MCT2 are differentially regulated by

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4 distinct molecular mechanisms. Notably, while the MCT1 gene produces a single mRNA transcript in various tissues under the control of a single promoter (15, 21), the MCT2 gene produces a variety of mRNA transcripts with different molecular weights whose relative abundance is tissue-dependent (25, 29, 31, 40). However, the molecular mechanisms that underlie MCT2 mRNA diversity and the biological relevance of the various mRNA species in determining the overall expression of MCT2 are currently unknown. We hypothesized that the MCT2 gene was a complex locus whose expression was controlled by alternative promoters and alternative splicing patterns. In the present study, we cloned and characterized multiple mct2 5’-cDNA ends from the mouse and determined the structural organization of the mouse mct2 gene. We demonstrate that the mouse mct2 gene is controlled by at least five alternative promoters that initiate transcription of this gene from distinct locations in a large genomic region and give rise to a variety of mRNA transcripts with distinct 5’ ends. Alternative splicing of multiple internal exons further increases the complexity of mRNA diversity. We further demonstrate that alternative promoter usage is responsible for the reported tissue-specific expression pattern of this important gene.

MATERIALS AND METHODS

5’-RACE (Rapid Amplification of cDNA Ends) Tissue samples from the brain, the skeletal muscle, the lung, and the testis, which are known to possess active lactate transporter activity, were harvested from normal male C57BL/6 mice. Total cellular RNA was extracted using the TRI reagent (Molecular Research Center Inc.). An improved 5’-RACE protocol was performed using the RLM-RACE kit (Ambion) to isolate fulllength 5’-cDNA ends. Briefly, Total cellular RNA was treated with calf intestinal alkaline phosphatase to remove the 5’ phosphate from all 5’-truncated mRNA transcripts that lack the

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5 cap structure. The RNA samples were then treated with tobacco acid pyrophosphatase to remove the cap structure from transcripts with a complete 5’ end, leaving an intact 5’ monophosphate. An RNA adapter was ligated to those transcripts with the intact 5’ monophosphate and the RNA samples were reverse transcribed using decamer random primers and Superscript III reverse transcriptase (Invitrogen). The resulting cDNA pool was used for the 1st round of PCR amplification using a sense primer compatible with the 5’ part of the adapter (5’-GCTGATGGCGATGAATGAACACTG-3’) and a gene-specific antisense primer localized in exon 5 of the mouse mct2 gene (GSP1, 5’-CAATGGAGATAAAGGACGCACAGA-3’) (29). The PCR product was used as templates for the 2nd round of PCR using a nested sense primer compatible with the 3’ part of the adapter (5’CGCGGATCCGAACACTGCGTTTGCTGGCTTTGATG-3’) and a nested gene-specific antisense primer localized in exon 4 of the mouse mct2 gene (GSP2, 5’CCAACACACTACTGATGGGACCTC-3’) (29). Products from the 2nd round of PCR were then subcloned into the pCR II vector (TA cloning kit, Invitrogen) and subject to DNA sequence analysis on both strands. The design of the RACE strategy was such that all mRNA diversities upstream of exon 4 where translation of mct2 was initiated would be identified (Fig. 1A/B).

Exon 1-specific RT-PCR and Southern blotting The first strand cDNA was synthesized with total cellular RNA (5 Gg) derived from various mouse organs using random primers and SuperScript III reverse transcriptase. A 532-bp fragment of mouse GAPDH was amplified to verify the quality of all cDNA samples (sense: 5’AGCCTCGTCCCGTAGACAA-3’; antisense: 5’-CCTTCCACAATGCCAAAGTT-3’). The overall expression pattern of mct2 in these organs was assessed using a primer pair subtending the common region of the mct2 cDNA (sense in exon 4: 5’-ACACCACCTCCAGTCAGAT-3’; antisense in exon 6: 5’-AACCTTTTGTGCTGTTGAT-3’) (29). Five exon 1-specific sense primers (1a: 5’-CTCCGAGTCCCCAACAAGTG-3’; 1b: 5’-GGGCTGACTCACTCCGTTGT-3’; 1c: 5’-

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6 GCACCGGGTGTCAGAGTAC-3’; 1d: 5’-TTCTGACAGGCTCTCCATGA-3’; and 1e: 5’ATCGCATTAAACATCTTCATTCA-3’) were designed based upon the results obtained from cDNA cloning and were paired with a common antisense primer (5’GCACCGGGTGTCAGAGTAC-3’) localized in exon 4 for PCR amplification of cDNA samples. PCR amplifications were performed in a total volume of 50 Gl for 30 cycles using Platinum Taq polymerase (Invitrogen). PCR products were size-fractionated by agarose gel electrophoresis and transferred to GeneScreen Plus membranes (DuPont). Southern blots were hybridized with [ -32P]ATPlabeled oligonucleotides. An oligonucleotide localized in exon 6 (5’CCTTCCAGCCATAGTTGTT-3’) was used to hybridize with PCR products amplified from the common region to detect the overall expression pattern of mct2 in various organs. An oligonucleotide localized in exon 4 but internally to the common antisense primer used in the PCR (5’-AGGCTCTGATGGCATTTCTG-3’) was used to hybridize with PCR products amplified with exon 1-specific primers to detect tissue-specific expression patterns of each exon 1 variants. Radioactive signals were recorded and analyzed using a PhosphorImager (STORM 840 and ImageQuant 1.2 software, Molecular Dynamics).

Real-time RT-PCR Relative expression levels of exon 1-specific mct2 transcripts in several important organs were further examined with real-time RT-PCR using the Mx4000 Multiple Quantitative PCR System (Stratagene) and the BrilliantTM quantitative PCR core reagent (Stratagene). cDNA derived from 100 ng total RNA was used as template. New sets of primers were designed for this purpose: exon 1a, sense 5’-CAACAAGTGGCTGATGCG-3’, antisense 5’GGTGGTGGGTAGAATATTTAAGC-3’; exon 1b, sense 5’-CTCCGTTGTGATACTTTAATCC-3’, antisense 5’-GTGGTGGGTAGAATATTTAAGC-3’; exon 1d, sense 5’ACTTTAAGGAGATGCTGGAAG-3’, antisense 5’-TGGGTAGAATATTTAAGCCTAGC-3’; and

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7 exon 1e, sense 5’-TCGCATTAAACATCTTCATTCAC-3’, antisense 5’GATGACACTTTAACAGCTTTGG-3’. Exon 1c was not included in this assessment because it was technically impossible to design a primer pair that would detect all variants of 1c-specific transcripts. A common TaqMan probe (Integrated DNA Technologies) was used for all PCR reactions. This probe was labeled at the 5’ end with a fluorescent reporter (6-carboxy-fluorescin, 6-FAM) and at the 3’ end with a quencher dye (Black Hole QuencherTM, BHQ-1) and incorporated locked nucleic acids (LNAs) to increase the binding specificity: 5’ctgGagGctGctCtacc-3’ (capitalized letters indicate LNAs). The PCR was performed for 40 cycles (30 s at 95°C and 30 s at 60°C) following denaturing for 10 min at 95°C. PCR products were continuously measured by the sequence detector and normalized by the endogenous reference ß-actin (sense 5’-ATCTACGAGGGCTATGCTCTCC-3’, antisense 5’GCTGTGGTGGTGAAGCTGTAG-3’, and TaqMan probe 5’-CCTGCGTCTGGACCTGGCTGGC3’). Three replicates for each RNA sample were performed. Results are expressed as relative levels based on calibers derived from series dilutions.

Western blotting analysis Frozen tissue samples were pulverized and homogenized in sample buffer containing 50 mM Tris-HCl (pH 7.5), 5 mM EDTA, 10 mM EGTA, 10 mM benzamidine, 50 Gg/ml phenylmethylsulfonyl fluoride, 10 Gg/ml aprotinin, 10 Gg/ml leupeptin, 10 Gg/ml pepstatin A, and 10% glycerol (vol/vol) (all chemicals from Sigma-Aldrich). The homogenate was then incubated on a rocking platform in the presence of 20 mM 3-[(3-cholamidopropyl)dimethylammonio]-1propanesulfonate (CHAPS, Sigma-Aldrich) at 4°C for 4 h. Following centrifugation at 14,000 g for 30 min, the supernatant was collected as total cellular protein. Protein concentration was determined using a modified Lowry method (DC Protein Assay Kit, Bio-Rad). Protein samples (80 Gg/lane) were then separated on 10% polyacrylamide/SDS gels and transferred by electroblotting onto nitrocellulose membrane, which was dyed with reversible Ponceau staining

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8 for verification of equal loading and transfer efficiency. After being blocked in 5% nonfat milk (Bio-Rad), membranes were incubated with an anti-MCT2 polyclonal antibody (Chemicon), followed by incubation with a horseradish peroxidase-conjugated secondary antibody (Cell Signaling Technology). Signal detection was facilitated with enhanced chemiluminescence (ECL kit, GE Lifesciences).

Characterization of genomic organization All newly identified cDNA sequences and the known 5’-cDNA sequence of the mouse mct2 were compared with the database of the Mouse Genome Project to confirm their authenticity and to determine their exonic-intronic boundaries, their positions in the mouse genome, and their relative positions to one another. Sequences in the 5’-flanking regions upstream of the exon 1 variants were then extracted from the genomic contig (GenBank accession number: NT_039502). The composition of these regions, including the presence of repetitive elements and potential binding sites for transcription factors, were analyzed using Genetics Computer Group sequence analysis software package and Eukaryotic Transcription Factor Database release 7.4.

Promoter-reporter Assay DNA fragments containing approximately 3 kb of the 5’-flanking region and a part of the exon 1 sequence were generated for exons 1a (-2706/+506, relative to the most 5’ transcription start site, similarly hereinafter), 1b (-3183/+112), and 1c (-3059/+181), respectively, by PCR using Pfx polymerase and mouse genomic DNA as template (1a, sense: 5’GGCTCTCCTGCGTTTGTCTTCT-3’; antisense: 5’-ATCTGCTCTCCCTTCCGTCTCT-3’; 1b, sense: 5’-ATCCAGCCTCCGATTGTTCTAC-3’; antisense: 5’GAGACATGGGACACAGGTTCTTC-3’; and 1c, sense: 5’-CCTTTATGCTGGAGGGAGAATG-3’; antisense: 5’-AGCACCGCCTTTACCTGACTCT-3’). The 1a and 1b fragments were directly

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9 inserted, whereas the 1c fragment was digested with FspI and the 3240-bp fragment was then inserted, into the BglII site, which had been made blunt-ended, in the poly-linker of the promoterless luciferase reporter vector pGL2/Basic (Promega) such that the expression of the firefly luciferase reporter was under the control of the inserted mouse genomic DNA fragments. Transient expression of these promoter-reporter constructs was assessed in LA-4 (mouse lung epithelium, ATCC) and Neuro-2a (mouse neuroblastoma, ATCC) cells. Cells were grown in 6well plates to 40-50% confluence and were transfected with 2 Gg of the DNA constructs and 4 Gl of GenePORTER 2 reagent (Gene Therapy Systems) per well. The pGL2/Control vector (Promega) whose reporter was under the control of the robust heterologous SV40 promoter was used as a positive control and the promoter-less pGL2/Basic vector as a negative control. A renilla luciferase expression vector (phRL-TK, Promega) was co-transfected to control for transfection efficiency across wells. Forty-eight hours after transfection, cells were lysed and assayed for luciferase activity using the Dual-Luciferase® Reporter Assay System (Promega) and a microplate reader (Victor3V, PerkinElmer). A total of six batches of transfection were performed, with each being conducted in duplicated wells.

RESULTS

Cloning of multiple exon 1 variants and their genomic organization A total of 68 5’-RACE clones derived from the 4 organs, including the brain, the skeletal muscle, the lung, and the testis, were analyzed. The 5’ ends of these clones were believed to represent true transcription start sites since the improved 5’-RACE protocol targeted only capped mRNA transcripts. A comparison with the overall structural organization of the mouse mct2 gene demonstrated that all clones contained a common 3’ sequence that was identical to the 5’ end of exon 4, but divergent 5’ sequences that occurred upstream of the first nucleotide of

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10 exon 2 (Fig 1A). Mapping of these 5’-cDNA sequences with the Mouse Genome Database indicated that these divergent sequences were led by four unique exons designated as exons 1a, 1b, 1c, and 1d, respectively, based on the order in which they were identified (Fig. 1). Of note, the 1a sequence was similar to the 5’ end of the published mct2 cDNA sequence (29), but had a longer 5’ end. While exons 1a, 1b, and 1d were spliced directly to the common exon 2, exon 1c was spliced to exon 2 via complex combinations of three alternatively spliced internal exons, termed AS1, AS2, and AS3 based on their 5’ to 3’ order in the mouse genome (Fig. 1, 2A/B). A search of mouse ESTs identified an additional exonic sequence derived from the mct2 gene (Accession number AI315949). This unique sequence was originally cloned from the kidney and comprised of, in the 5’ to 3’ order, a 5’-leader exon, AS2, AS3, exon 2, and exon 4 (Fig. 1, 2A/B). The 5’-leader exon of this EST was deemed to be an exon 1 (1e) rather than an internal exon although it was localized between AS1 and AS2 (Fig. 2A/B) because it was not alternatively spliced in any transcripts initiated from exon 1c and it was not preceded by a splicing acceptor sequence. These novel 5’-exonic sequences, including five leading exons and three internal exons, were distributed over a genomic region greater than 80 kb in mouse chromosome 10 (Fig. 2A). All exon-intron boundaries conformed to the GT/AG rule (36). Interestingly, as the EST (AI315949), the vast majority of the 5’-RACE clones lacked exon 3. Alternative splicing of this internal exon further increased the complexity of splicing patterns utilized by this gene (Fig. 2C). Since stop codons were evident in all three translation reading frames upstream of the initiating AUG in exon 4, all newly identified sequences represented mct2 mRNA diversity within the 5’-untranslated region (5’-UTR) and did not affect the structure of the encoded protein.

Tissue-specific expression of alternative exon 1 variants An exon 1-specific RT-PCR approach was used to determine the expression patterns of each mouse mct2 exon 1 in normal tissues. Consistent with the trend noticed in 5’-RACE, the

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11 alternative exon 1 variants were expressed in a tissue-specific fashion when compared with PCR amplifications subtending the common region (exon 4-6, within the open reading frame) (Fig. 3). Notably, exons 1b and 1d were expressed almost exclusively in the testis, although very low levels of 1b were detected in a few other tissues. In contrast, exons 1a, 1c, and 1e were expressed in multiple tissues. In particular, exon 1c was ubiquitously expressed in all tissues with relatively uniform intensity, whereas exons 1a and 1e were also expressed in multiple tissues but the expression intensity varied greatly among different tissues. For example, 1a was highly expressed in the lung, the liver, the kidney, and the bladder, but was only marginally expressed in brain tissues, and 1e was mainly expressed in the kidney and the heart. In line with the finding made in 5’-RACE, exon 3 was found to be excluded from most mct2 transcripts under normal conditions, especially those initiated from exons 1a, 1b, and 1d (Fig. 3A). The identities of the high and low molecular-weight PCR amplicons were examined by cloning and sequencing and were confirmed to be those with or without exon 3, respectively. Additional confirmation was obtained by using an oligonucleotide probe located in exon 3 to reprobe the Southern blots (Fig. 3B). Meanwhile, mct2 mRNA transcripts initiated from exons 1c and 1e were accentuated by intricate alternative splicing of AS1, AS2, AS3, and exon 3 with a tissue-specific flavor (Fig. 3). For example, more than 10 different PCR products could be recognized from transcripts initiated from 1c, although only 5 such variants were physically cloned in 5’-RACE as shown in Fig. 2B. A similar pattern was observed with 1e-initiated transcripts. Given the extraordinary complexity of these 1c- and 1e-initiated transcripts, no additional attempt was made to clarify the molecular nature of these PCR products. Interestingly, sizes and relative densities of these PCR products differed among different tissues, suggesting the existence of tissue-specific and regulated alternative splicing mechanisms. The relative expression levels of exon 1a-, 1b-, 1d-, and 1e-specific transcripts in several important organs, including the brain, the lung, the liver, the kidney, the heart, and the testis,

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12 were further assessed with real-time RT-PCR for better quantification (Fig. 4). While confirming the general trends observed in experiments using RT-PCR plus Southern blotting regarding the tissue-specific usage of alternative exon 1s and relative expression levels of each exon 1 variant in various tissues, real-time RT-PCR revealed additional findings. For example, it revealed that the expression level of 1a in the kidney was substantially higher than, rather than being similar to, those in the lung and the liver (Fig. 4A). It also detected marginal expression of 1d in both the brain and the lung (Fig. 4C), which was not seen using RT-PCR plus Southern blotting (Fig. 3A). Conversely, marginal expression of 1b in the brain and the kidney and 1e in the brain and the lung as seen using RT-PCR plus Southern blotting (Fig. 3A) was not detected with real-time RT-PCR (Fig. 4C/D). The overall expression patterns of alternative exon 1 variants in various tissues assessed using both methods are summarized in Table 1. Interestingly, mct2 mRNA expression (Fig. 3C) was not always correlated quantitatively to its protein expression (Fig. 5) in all tissues. For instance, although mct2 mRNA was expressed in the lung at a level similar to those in the brain and in the testis, its protein was expressed in the former at a much lower level. On the other hand, while mct2 mRNA was barely detectable in the skeletal muscle, its protein was expressed in this tissue at a level similar to those in the liver and the heart.

Cell type-specific usage of alternative promoters Transient transfection of promoter-reporter constructs containing approximately 3 kb of 1a, 1b, or 1c 5’-flanking genomic regions demonstrated that both the 1a and the 1c regions contained functional promoters (Fig. 6).Furthermore, transcriptional activities of these two promoters in this in vitro assay generally displayed cell type-specific patterns that were consistent with their in vivo tissue-specific usage as reflected by the expression patterns of their respective exon 1 variants (Fig. 3).For example, the 1a promoter was highly active in LA-4 cells (mouse lung epithelium) as it was in the lung, while it was only marginally active in Neuro-2A

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13 cells (mouse neuroblastoma) as it was in the brain (Fig. 6). Interestingly, the 1c promoter was highly active in Neuro-2A cells (Fig. 6B) as it was in the brain, but its activity in LA-4 cells compared with that in Neuro-2A cells (Fig. 6A) seemed to be lower than its activity in the lung compared with that in the brain. This minor discrepancy in 1c promoter activities in vivo vs. in vitro may reflect the possibility that the 3-kb genomic region upstream of exon 1c lacks certain cis-elements that are important for maximum promoter activity in LA-4 cells. Alternatively, it may reflect the difference between the activity of a promoter assessed in a tissue and that assessed in a specific cell type. Although significant 1b promoter activity was not detected in these two cell types as expected based upon the unique testis-specific expression pattern of this promoter in vivo, preliminary studies suggested that a 5’-deletion 1b construct (-1254/+112) was moderately active in LA-4 cells (data not shown). Thus, the approximately 3-kb genomic regions upstream of exons 1a, 1b, and 1c, respectively, represent functional, alternative promoters of the mouse mct2 gene, which are likely involved in the tissue-/cell type-specific expression of the gene. Sequence inspection of the proximal regions (-1200 bp) of the three promoters revealed that only 1b contained a TATA-like element (CATAAAWG) in the core promoter region near the transcription start site (–23) (Fig. 7). However, numerous potential binding sites for ubiquitous transcription factors were evident in all three promoters (Fig. 7). For example, the 1a promoter comprised two CAAT boxes (-180 and -789), a strong Sp1 site (-382), an NF-Y site (-618), and two AP1 sites (-892 and -1003). In the 1b promoter, a CAAT box (-157) and an Sp1 site (-373) were closely localized with the TATA-like element. Interestingly, this region contained 3 copies of the steroidogenic factor 1 (SF-1) biding site (252, -364, and -1132) that was thought to be important to testis-specific gene expression (4, 32). And finally, the 1c promoter possessed two strong Sp1 sites (33 and 151), two insulin response elements (IRE, 35 and 244), one of which overlapped with the proximal Sp1 site, an upstream CAAT box (-832), a GATA site (-271), and three AP-1 sites (188, -332, and –564).

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14

DISCUSSION

The current study shows that the mouse mct2 gene is a highly complex locus whose expression in diverse tissues is likely initiated by multiple alternative promoters. This notion is supported by the following lines of evidence: (1) transcription of the mct2 gene starts from five distinct exon 1 variants that are distributed over a large genomic region; (2) the usage of these exon 1 variants displays a tissue-specific fashion; (3) the 5’-genomic regions flanking these exon 1 variants contain arrays of cis-acting regulatory elements constituting common promoters; (4) three of the 5’-flanking regions have been tested in vitro, each of which has been shown to contain a functional promoter; and (5) the in vitro, cell type-specific activities of the promoters are consistent with the tissue-specific expression pattern of the gene in vivo. Interestingly, the human and the rat homologues appear to utilize the same transcriptional strategy. For example, both genes are transcribed from multiple first exons, as deduced from cDNA sequences deposited in the GenBank database (Accession numbers AF049608 and AF058056 for hMCT2; X97445, U62316, and BC081701 for rMCT2). A recent study further demonstrated the presence of two distinct promoters in the human MCT2 gene (56). The fact that this intricate transcriptional strategy is conserved across several species suggests that it may serve important biological functions. Once regarded as “unconventional”, alternative promoter usage is now recognized as a common mechanism in the transcriptional regulation of mammalian genes (3, 30, 55, 64). A recent analysis of 21,000 full-length mouse cDNA clones has revealed that 9% of the mouse genes contain alternative first exons (61), whereas a subsequent analysis of 67,000 human transcripts has shown that approximately 18% of human genes may use alternative promoter (30). Alternative promoters create genomic diversity and flexibility and, in turn, provide a

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15 framework for a given gene to effectively respond to diverse physiological and pathological signals, including cell type-, developmental stage-, and environmental stress-specific signals. Many important genes whose diverse and complex function under various conditions requires accurate and exquisite transcriptional regulation are equipped with multiple promoters. For instance, the human dystrophin gene uses five independent promoters to direct its transcription in a cell-specific and developmentally controlled fashion (1), while at least ten promoters are responsible for tissue-specific expression of the human neuronal NO synthase in the brain, the skeletal muscle, the testis, and other organs (59, 60). Other examples include the genes for cMyc (6, 17), platelet-derived growth factor ß receptor (57), aromatase cytochrome P450 (24), and glucocorticoid receptor (34). In the case of the mouse mct2 gene, it is apparent that the use of alternative promoters is at least important for tissue-specific expression of the gene, as shown in the current study. While it remains to be determined whether such mechanism is also involved in the expression of this gene in response to development (5, 9, 49), differentiation (31), metabolic stress (45), trauma (47), hormones (9, 20), and hypoxia (63), the interesting composition of the 1b promoter appears to suggest that this particular region might be involved in the induction of mct2 by the latter two types of signals. Indeed, this genomic region not only contains multiple copies of the SF-1 biding site that is thought to be important to testis-specific gene expression (4, 32), but also features clusters of potential binding sites for transcription factors implicated in hypoxia-induced gene expression (Fig. 7), including two HIF-1 sites (53, 54), four STAT sites (26), three NF- B sites (50, 51), and three C/EBP/p300 sites (2, 16). In this regard, a recent study demonstrated that both the mouse and the human MCT4 genes were upregulated by hypoxia through HIF-1 -dependent transcriptional activation (56). It is possible that HIF-1 plays a central role in regulating the expression of lactate transporters and, in turn, lactate metabolism under hypoxic conditions. However, it was noted that the human MCT1

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16 promoter and the two human MCT2 promoters tested in the same study were not responsive to hypoxia (56). The fact that all five mct2 promoters are active in the testis appears to contradict the concept of “alternative promoters”. It is possible that in this case multiple promoters serve as a safeguard mechanism that secures proper expression of a vital gene. Recent studies have implicated MCT2 in spermatogenesis and other important testicular functions (9, 10, 20, 25). A sustained lactate transporter activity might be vital to these functions, such that additional security measures may have developed through evolution to preserve the expression of this transporter. Indeed, two of the five mct2 promoters are shown to be designated specifically for this organ. It would be a fascinating topic as to how these promoters are coordinated to achieve the level of expression of mct2 proper to the vital organ of the testis. Alternatively, the five promoters might direct mct2 expression in different types of cells within the testis. The biological significance of these promoters within this important organ will require additional investigation. In addition to alternative promoter usage, alternative splicing of pre-mRNA transcripts, especially those derived from exons 1c and 1e, is a major contributor to mct2 mRNA diversity (Fig. 3). The alternative splicing patterns of these transcripts are so complex that it is difficult, if not impossible, to determine the molecular identity for each one of them. However, alternative inclusion of exon 1a, 1b, 1d, or 1e in 1c transcripts has been ruled out since Southern blotting analysis of the 1c-specific PCR products using probes located in these exons did not detect any bands (data not shown). It is conceivable that the various 1c transcripts are products of alternative splicing of internal exons AS1, AS2, AS3, and possibly exon 3 and that the 1e transcripts are derived from alternative splicing of exons AS2, AS3, and possibly exon 3. Of note, while exon 3 was predominantly excluded among mct2 transcripts from C57BL/6 mice as shown in the current study, it was included in a cDNA cloned from 129 mice (Accession number AF058054) (29). It is unclear whether the latter reflects a true strain-based preference in the usage of this exon or just a random occurrence in a single cDNA sequence.

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17 Although all mct2 mRNA diversities, resulting from both alternative promoter usage and alternative splicing, are restricted to the 5’-UTR and hence are not expected to affect MCT2 protein structure, their potential significance cannot be ignored. The 5’-UTR of eukaryotic mRNA has long been known to play crucial roles in posttranscriptional regulation of gene expression through the modulation of RNA transport (8), translational efficiency (37, 60), and RNA stability (13). Several recent studies have compared the effect of 5’-UTR variants from a single gene on translation and have revealed significant differences in translational efficiency among naturally occurring mRNA variants from the neuronal NO synthase (38, 60), the endothelial argininosuccinate synthase (41), and the RARbeta2 (42). Considering the substantial differences among the mct2 mRNA variants in their primary composition and possibly in their secondary structure, these transcripts are likely translated at different efficiencies. While it needs to be experimentally confirmed, data presented in the current study showing discrepancies between mRNA and protein levels in some organs are supportive of this notion. Furthermore, recent studies demonstrated that the mct2 indeed was regulated at the translational level (14, 25, 43). It is thus reasonable to assume at this time that the remarkable mRNA diversity of the mouse mct2 gene is more than a mere consequence of the need to initiate transcription from different genomic locations, and in fact may represent an additional regulatory mechanism that insures appropriate expression of this important gene under various conditions in diverse tissues. In summary, we demonstrate that transcription of the mct2 gene is initiated from five distinct variants of exon 1, likely under the control of five alternative promoters, and that a large number of mature mRNA are produced from these pre-mRNA through alternative splicing of multiple internal exons in the 5’-UTR. Both alternative promoter usage and alternative splicing may contribute to the tissue-specific expression pattern of mct2 and may also be involved in development-specific and environmental stress-induced expression of this important gene.

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18 ACKNOWLEDGEMENTS

This study was supported in part by NIH (R01HL074369 and P50HL060296), AHA (0455418B), DARPA (N66001-02-C-8056), and the Center for Genetics and Molecular Medicine, University of Louisville. Nucleotide sequences of the 5 exon 1 variants and 3 alternatively spliced exons of the mouse mct2 gene identified in this study have been deposited in GenBank (Accession numbers: DQ140161-140168).

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19 REFERENCES

1.

Ahn AH, and Kunkel LM. The structural and functional diversity of dystrophin. Nat Genet 3: 283-291, 1993.

2.

Arany Z, Huang LE, Eckner R, Bhattacharya S, Jiang C, Goldberg MA, Bunn HF, and Livingston DM. An essential role for p300/CBP in the cellular response to hypoxia. Proc Natl Acad Sci U S A 93: 12969-12973, 1996.

3.

Ayoubi TA, and Van De Ven WJ. Regulation of gene expression by alternative promoters. Faseb J 10: 453-460., 1996.

4.

Barbara PS, Moniot B, Poulat F, Boizet B, and Berta P. Steroidogenic factor-1 regulates transcription of the human anti- mullerian hormone receptor. J Biol Chem 273: 29654-29660, 1998.

5.

Baud O, Fayol L, Gressens P, Pellerin L, Magistretti P, Evrard P, and Verney C. Perinatal and early postnatal changes in the expression of monocarboxylate transporters MCT1 and MCT2 in the rat forebrain. J Comp Neurol 465: 445-454., 2003.

6.

Bentley DL, and Groudine M. Novel promoter upstream of the human c-myc gene and regulation of c-myc expression in B-cell lymphomas. Mol Cell Biol 6: 3481-3489, 1986.

7.

Benton CR, Campbell SE, Tonouchi M, Hatta H, and Bonen A. Monocarboxylate transporters in subsarcolemmal and intermyofibrillar mitochondria. Biochem Biophys Res Commun 323: 249-253, 2004.

8.

Bi J, Hu X, Loh HH, and Wei LN. Mouse kappa-opioid receptor mRNA differential transport in neurons. Mol Pharmacol 64: 594-599, 2003.

9.

Boussouar F, Mauduit C, Tabone E, Pellerin L, Magistretti PJ, and Benahmed M. Developmental and hormonal regulation of the monocarboxylate transporter 2 (MCT2) expression in the mouse germ cells. Biol Reprod 69: 1069-1078, 2003.

Page 20 of 38

20 10.

Brauchi S, Rauch MC, Alfaro IE, Cea C, Concha, II, Benos DJ, and Reyes JG. Kinetics, molecular basis, and differentiation of L-lactate transport in spermatogenic cells. Am J Physiol Cell Physiol 288: C523-534, 2005.

11.

Broer S, Rahman B, Pellegri G, Pellerin L, Martin JL, Verleysdonk S, Hamprecht B, and Magistretti PJ. Comparison of lactate transport in astroglial cells and monocarboxylate transporter 1 (MCT 1) expressing Xenopus laevis oocytes. Expression of two different monocarboxylate transporters in astroglial cells and neurons. J Biol Chem 272: 30096-30102., 1997.

12.

Brooks GA, Dubouchaud H, Brown M, Sicurello JP, and Butz CE. Role of mitochondrial lactate dehydrogenase and lactate oxidation in the intracellular lactate shuttle. Proc Natl Acad Sci U S A 96: 1129-1134, 1999.

13.

Chen CY, Del Gatto-Konczak F, Wu Z, and Karin M. Stabilization of interleukin-2 mRNA by the c-Jun NH2-terminal kinase pathway. Science 280: 1945-1949, 1998.

14.

Chenal J, and Pellerin L. Noradrenaline enhances the expression of the neuronal monocarboxylate transporter MCT2 by translational activation via stimulation of PI3K/Akt and the mTOR/S6K pathway. J Neurochem 102: 389-397, 2007.

15.

Cuff MA, and Shirazi-Beechey SP. The human monocarboxylate transporter, MCT1: genomic organization and promoter analysis. Biochem Biophys Res Commun 292: 10481056, 2002.

16.

Ebert BL, and Bunn HF. Regulation of transcription by hypoxia requires a multiprotein complex that includes hypoxia-inducible factor 1, an adjacent transcription factor, and p300/CREB binding protein. Mol Cell Biol 18: 4089-4096, 1998.

17.

Eick D, Polack A, Kofler E, Lenoir GM, Rickinson AB, and Bornkamm GW. Expression of P0- and P3-RNA from the normal and translocated c-myc allele in Burkitt's lymphoma cells. Oncogene 5: 1397-1402, 1990.

Page 21 of 38

21 18.

Fishbein WN, Merezhinskaya N, and Foellmer JW. Relative distribution of three major lactate transporters in frozen human tissues and their localization in unfixed skeletal muscle. Muscle Nerve 26: 101-112, 2002.

19.

Garcia CK, Brown MS, Pathak RK, and Goldstein JL. cDNA cloning of MCT2, a second monocarboxylate transporter expressed in different cells than MCT1. J Biol Chem 270: 1843-1849., 1995.

20.

Goddard I, Florin A, Mauduit C, Tabone E, Contard P, Bars R, Chuzel F, and Benahmed M. Alteration of lactate production and transport in the adult rat testis exposed in utero to flutamide. Mol Cell Endocrinol 206: 137-146., 2003.

21.

Hadjiagapiou C, Borthakur A, Dahdal RY, Gill RK, Malakooti J, Ramaswamy K, and Dudeja PK. Role of USF1 and USF2 as potential repressor proteins for human intestinal monocarboxylate transporter 1 promoter. Am J Physiol Gastrointest Liver Physiol 288: G1118-1126, 2005.

22.

Halestrap AP, and Price NT. The proton-linked monocarboxylate transporter (MCT) family: structure, function and regulation. Biochem J 343: 281-299., 1999.

23.

Halestrap AP, Wang X, Poole RC, Jackson VN, and Price NT. Lactate transport in heart in relation to myocardial ischemia. Am J Cardiol 80: 17A-25A., 1997.

24.

Harada N, Utsumi T, and Takagi Y. Tissue-specific expression of the human aromatase cytochrome P-450 gene by alternative use of multiple exons 1 and promoters, and switching of tissue-specific exons 1 in carcinogenesis. Proc Natl Acad Sci U S A 90: 11312-11316, 1993.

25.

Jackson VN, Price NT, Carpenter L, and Halestrap AP. Cloning of the monocarboxylate transporter isoform MCT2 from rat testis provides evidence that expression in tissues is species-specific and may involve post-transcriptional regulation. Biochem J 324: 447-453., 1997.

Page 22 of 38

22 26.

Joung YH, Park JH, Park T, Lee CS, Kim OH, Ye SK, Yang UM, Lee KJ, and Yang YM. Hypoxia activates signal transducers and activators of transcription 5 (STAT5) and increases its binding activity to the GAS element in mammary epithelial cells. Exp Mol Med 35: 350-357., 2003.

27.

Juel C. Lactate-proton cotransport in skeletal muscle. Physiol Rev 77: 321-358., 1997.

28.

Kasischke KA, Vishwasrao HD, Fisher PJ, Zipfel WR, and Webb WW. Neural activity triggers neuronal oxidative metabolism followed by astrocytic glycolysis. Science 305: 99103, 2004.

29.

Koehler-Stec EM, Simpson IA, Vannucci SJ, Landschulz KT, and Landschulz WH. Monocarboxylate transporter expression in mouse brain. Am J Physiol 275: E516-524, 1998.

30.

Landry JR, Mager DL, and Wilhelm BT. Complex controls: the role of alternative promoters in mammalian genomes. Trends Genet 19: 640-648., 2003.

31.

Lin RY, Vera JC, Chaganti RS, and Golde DW. Human monocarboxylate transporter 2 (MCT2) is a high affinity pyruvate transporter. J Biol Chem 273: 28959-28965., 1998.

32.

Lynch JP, Lala DS, Peluso JJ, Luo W, Parker KL, and White BA. Steroidogenic factor 1, an orphan nuclear receptor, regulates the expression of the rat aromatase gene in gonadal tissues. Mol Endocrinol 7: 776-786, 1993.

33.

McClelland GB, and Brooks GA. Changes in MCT 1, MCT 4, and LDH expression are tissue specific in rats after long-term hypobaric hypoxia. J Appl Physiol 92: 1573-1584, 2002.

34.

McCormick JA, Lyons V, Jacobson MD, Noble J, Diorio J, Nyirenda M, Weaver S, Ester W, Yau JL, Meaney MJ, Seckl JR, and Chapman KE. 5'-heterogeneity of glucocorticoid receptor messenger RNA is tissue specific: differential regulation of variant transcripts by early-life events. Mol Endocrinol 14: 506-517, 2000.

Page 23 of 38

23 35.

McCullagh KJ, Poole RC, Halestrap AP, O'Brien M, and Bonen A. Role of the lactate transporter (MCT1) in skeletal muscles. Am J Physiol 271: E143-150., 1996.

36.

Mount SM. A catalogue of splice junction sequences. Nucleic Acids Research 10: 459472, 1982.

37.

Nanbru C, Lafon I, Audigier S, Gensac MC, Vagner S, Huez G, and Prats AC. Alternative translation of the proto-oncogene c-myc by an internal ribosome entry site. J Biol Chem 272: 32061-32066, 1997.

38.

Newton DC, Bevan SC, Choi S, Robb GB, Millar A, Wang Y, and Marsden PA. Translational regulation of human neuronal nitric-oxide synthase by an alternatively spliced 5'-untranslated region leader exon. J Biol Chem 278: 636-644., 2003.

39.

Pellerin L, Pellegri G, Bittar PG, Charnay Y, Bouras C, Martin JL, Stella N, and Magistretti PJ. Evidence supporting the existence of an activity-dependent astrocyteneuron lactate shuttle. Dev Neurosci 20: 291-299., 1998.

40.

Pellerin L, Pellegri G, Martin JL, and Magistretti PJ. Expression of monocarboxylate transporter mRNAs in mouse brain: support for a distinct role of lactate as an energy substrate for the neonatal vs. adult brain. Proc Natl Acad Sci U S A 95: 3990-3995., 1998.

41.

Pendleton LC, Goodwin BL, Flam BR, Solomonson LP, and Eichler DC. Endothelial argininosuccinate synthase mRNA 5'-untranslated region diversity. Infrastructure for tissue-specific expression. J Biol Chem 277: 25363-25369, 2002.

42.

Peng X, Mehta RG, Tonetti DA, and Christov K. Identification of novel RARbeta2 transcript variants with short 5'-UTRs in normal and cancerous breast epithelial cells. Oncogene 24: 1296-1301, 2005.

43.

Pierre K, Debernardi R, Magistretti PJ, and Pellerin L. Noradrenaline enhances monocarboxylate transporter 2 expression in cultured mouse cortical neurons via a translational regulation. J Neurochem 86: 1468-1476, 2003.

Page 24 of 38

24 44.

Pierre K, Magistretti PJ, and Pellerin L. MCT2 is a major neuronal monocarboxylate transporter in the adult mouse brain. J Cereb Blood Flow Metab 22: 586-595., 2002.

45.

Pierre K, Parent A, Jayet PY, Halestrap AP, Scherrer U, and Pellerin L. Enhanced expression of three monocarboxylate transporter isoforms in the brain of obese mice. J Physiol 2007.

46.

Price NT, Jackson VN, and Halestrap AP. Cloning and sequencing of four new mammalian monocarboxylate transporter (MCT) homologues confirms the existence of a transporter family with an ancient past. Biochem J 329: 321-328., 1998.

47.

Prins ML, and Giza CC. Induction of monocarboxylate transporter 2 expression and ketone transport following traumatic brain injury in juvenile and adult rats. Dev Neurosci 28: 447-456, 2006.

48.

Py G, Eydoux N, Lambert K, Chapot R, Koulmann N, Sanchez H, Bahi L, Peinnequin A, Mercier J, and Bigard AX. Role of hypoxia-induced anorexia and right ventricular hypertrophy on lactate transport and MCT expression in rat muscle. Metabolism 54: 634644, 2005.

49.

Rafiki A, Boulland JL, Halestrap AP, Ottersen OP, and Bergersen L. Highly differential expression of the monocarboxylate transporters MCT2 and MCT4 in the developing rat brain. Neuroscience 122: 677-688., 2003.

50.

Sasaki H, Ray PS, Zhu L, Otani H, Asahara T, and Maulik N. Hypoxia/reoxygenation promotes myocardial angiogenesis via an NF kappa B-dependent mechanism in a rat model of chronic myocardial infarction. J Mol Cell Cardiol 33: 283-294., 2001.

51.

Schmedtje JF, Jr., Ji YS, Liu WL, DuBois RN, and Runge MS. Hypoxia induces cyclooxygenase-2 via the NF-kappaB p65 transcription factor in human vascular endothelial cells. J Biol Chem 272: 601-608, 1997.

Page 25 of 38

25 52.

Schurr A, Payne RS, Miller JJ, Tseng MT, and Rigor BM. Blockade of lactate transport exacerbates delayed neuronal damage in a rat model of cerebral ischemia. Brain Res 895: 268-272., 2001.

53.

Semenza GL. HIF-1: mediator of physiological and pathophysiological responses to hypoxia. J Appl Physiol 88: 1474-1480, 2000.

54.

Semenza GL. Hypoxia-inducible factor 1: master regulator of O2 homeostasis. Curr Opin Genet Dev 8: 588-594, 1998.

55.

Trinklein ND, Aldred SJ, Saldanha AJ, and Myers RM. Identification and functional analysis of human transcriptional promoters. Genome Res 13: 308-312, 2003.

56.

Ullah MS, Davies AJ, and Halestrap AP. The plasma membrane lactate transporter MCT4, but not MCT1, is up-regulated by hypoxia through a HIF-1alpha-dependent mechanism. J Biol Chem 281: 9030-9037, 2006.

57.

Vu TH, Martin GR, Lee P, Mark D, Wang A, and Williams LT. Developmentally regulated use of alternative promoters creates a novel platelet-derived growth factor receptor transcript in mouse teratocarcinoma and embryonic stem cells. Mol Cell Biol 9: 4563-4567, 1989.

58.

Wang X, Levi AJ, and Halestrap AP. Substrate and inhibitor specificities of the monocarboxylate transporters of single rat heart cells. Am J Physiol 270: H476-484., 1996.

59.

Wang Y, Newton DC, Miller TL, Teichert AM, Phillips MJ, Davidoff MS, and Marsden PA. An alternative promoter of the human neuronal nitric oxide synthase gene is expressed specifically in Leydig cells. Am J Pathol 160: 369-380, 2002.

60.

Wang Y, Newton DC, Robb GB, Kau CL, Miller TL, Cheung AH, Hall AV, VanDamme S, Wilcox JN, and Marsden PA. RNA diversity has profound effects on the translation of neuronal nitric oxide synthase. Proc Natl Acad Sci U S A 96: 12150-12155, 1999.

61.

Zavolan M, van Nimwegen E, and Gaasterland T. Splice variation in mouse full-length cDNAs identified by mapping to the mouse genome. Genome Res 12: 1377-1385, 2002.

Page 26 of 38

26 62.

Zhang F, Vannucci SJ, Philp NJ, and Simpson IA. Monocarboxylate transporter expression in the spontaneous hypertensive rat: effect of stroke. J Neurosci Res 79: 139145, 2005.

63.

Zhang SX, Wang J, Miller JJ, Mwasaru EF, Gozal D, and Wang Y. Whole-body hypoxic preconditioning (WBHPC) protects mice against acute hypoxia by improving organ function in the lung. FASEB J 17: A1281., 2003 (Abstract).

64.

Zhang T, Haws P, and Wu Q. Multiple variable first exons: a mechanism for cell- and tissue-specific gene regulation. Genome Res 14: 79-89, 2004.

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27 FIGURE LEGENDS

Figure 1. Mouse mct2 5’-mRNA structure. A. Various 5’-leader sequences are spliced to a common exon 2, which is then spliced, with or without the alternatively spliced exon 3, to exon 4 where translation initiates. Shaded area represents coding region. The size of each exon is indicated. *31 represents the size of the noncoding region of exon 4. Bent arrows represent transcription start sites. Locations of the gene-specific antisense primers used in the 5’-RACE are depicted as arrowheads GSP1 and GSP2. B. Nucleotide sequences of the 5 exon 1 variants and 3 alternatively spliced exons of the mouse mct2 gene (GenBank Accession numbers: DQ140161-140168).

Figure 2. Structural organization and alternative splicing patterns of mouse mct2 5’ exons. A. Exon/intron location on genomic DNA. Exonic sequences are depicted as vertical boxes. Five exon 1 variants and alternatively spliced exons span a genomic region greater than 80 kb upstream of exon 2. B. Splicing patterns of mouse mct2 5’ exons. Exons 1a, 1b, and 1d are spliced to the common exon 2; whereas exons 1c and 1e are spliced to exon 2 with various combinations of 3 alternatively spliced exons, AS1, AS2, and AS3. Those shown in the figure were the ones physically cloned using 5’-RACE (1e represents EST AI315949). Bent arrows depict transcription start sites. C. Alternative splicing patterns involving the deletion/inclusion of exon 3 that is positioned between the common exons 2 and 4. Alternative splicing of this exon greatly increases the number of mct2 mRNA species.

Figure 3. Tissue-specific expression of mct2 exon 1 variants and their splicing patterns. A. RTPCR amplification was performed using varied exon 1-specific sense primers and a common antisense primer located in exon 4. An oligonucleotide probe located in exon 4 was used for Southern blot analysis that hybridized to PCR products both with (larger bands) and without

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28 (smaller bands) exon 3. B. The same blots hybridized with an oligonucleotide probe located in exon 3 that detected only PCR products with exon 3. C. PCR amplification of the coding region between exons 4 and 6 was used to assess the overall mct2 expression. Blots shown are representative results from four different experiments.

Figure 4. Real-time RT-PCR assessment of relative expression levels of mct2 exon 1 variants in various tissues. A. Exon 1a is expressed in multiple tissues at different levels. It is highly expressed in the kidney. B. Exon 1b is exclusively expressed in the testis. C. Exon 1d is almost exclusively expressed in the testis with marginal expression in the brain and the lung. D. Exon 1e is selectively expressed in the kidney, the heart, and the testis. Data are mean±SE; n=4 for each group; Levels lower than 1% of the highest level in each PCR reaction were deemed as undetected.

Figure 5. Western blotting analysis of MCT2 protein expression in various tissues. Shown is a representative blot from 4 independent experiments. The size and the general distribution of the indicated band are consistent with those of MCT2 reported in earlier studies using noncommercial MCT2 antibodies (18, 25). The Z42 kD MCT2 protein is expressed in multiple tissues at different levels.

Figure 6. Cell type-specific promoter activities of selected mct2 promoters assessed with promoter-reporter assay. A. Reporter activities in LA-4 cells (mouse lung epithelium) transiently transfected with promoter-reporter constructs. The 1a promoter is the most active in these cells as it is in the lung in vivo. B. Reporter activities in Neuo-2A cells (mouse neuroblastoma) where the 1c promoter is the most active as it is in brain tissues in vivo. The promoterless pGL2/Basic vector was used as a negative control. Data are mean±SE; n=6 for each group; *P