[Cell Cycle 7:6, 691-697; 15 March 2008]; ©2008 Landes Bioscience
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Coordination of cell cycle exit and differentiation of neuronal progenitors Panagiotis K. Politis,1,* Dimitra Thomaidou2 and Rebecca Matsas2,* 1Center
of Basic Research; Biomedical Research Foundation; Academy of Athens; Athens, Greece; 2Laboratory of Cellular and Molecular Neurobiology; Hellenic Pasteur Institute; Athens, Greece
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Early in development, a population of competent ectodermal cells are committed to neural fate to form the neural plate, and later produce the neural tube, of which the anterior part generates the forebrain and the posterior part the spinal cord. The spinal cord develops from a small number of highly plastic neural stem/progenitor cells that proliferate, acquire regional identities and generate a progressively restricted repertoire of cell types, first neurons and later oligodendroglial and astroglial cells. The initial induction of neural fate, during formation of the neural plate, does not affect the proliferative capacity of the cells.13 It is only later that committed precursors are instructed to become post-mitotic, as progenitors exit the cell cycle, cease to proliferate and begin to differentiate into neurons and glial cells. Similarly to other brain regions, as for example in cortical development, initiation of differentiation pathways in the spinal cord, either neurogenic or gliogenic, appears to be connected with cell cycle control systems that instruct whether stem/progenitor cells will maintain their proliferative capacity or will differentiate into the appropriate neural cell types.2 This regulation is essential for generation of the appropriate number of neurons, as well as neuronal subtypes, in the adult spinal cord and the establishment of proper wiring of neuronal circuits. Therefore complex relationships are believed to take place between cell cycle components and factors regulating differentiation of neural stem/progenitor cells. As soon as the early neural tube is formed, it already acquires dorso-ventral organization. The proliferation rate of neural progenitors is higher in the dorsal as compared with the ventral part, whereas the opposite is true for the differentiation rate (Fig. 1).14 In this early neural tube, the region between the roof and floor plate is densely occupied by neural progenitors. The identity of these progenitors is modulated along the dorso-ventral axis by extrinsic signals that activate hierarchies of transcription factors, expressed in a region- and cell-specific manner, to regionalize the early neural tube.15,16 These transcription factors act to subdivide the ventricular zone (VZ) into defined progenitor domains with restricted developmental potential, and subsequently, to establish distinct differentiation programs in the neurons that emerge from each domain. This tight regulation leads to the generation of the different neuronal subtypes in the spinal cord, including ventral motor neurons as well as dorsal and ventral interneurons (Fig. 1). Newly-born neurons migrate laterally out of the ventricular zone into their final positions in the periphery of the spinal cord (mantle zone, MZ), where they become incorporated into the local neuronal circuitry. Upon progression of development,
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During development, co-ordinate regulation of cell cycle exit and differentiation of neuronal precursors is essential for generation of appropriate number of neurons and proper wiring of neuronal circuits. Recent studies have identified some of the molecules implicated in the regulation of these cellular events, but the complex machinery that orchestrates these processes into a coherent developmental program remains unclear. BM88/Cend1 is a neuronal protein associated in vivo with terminal neuron-generating divisions, marking the exit of proliferative cells from the cell cycle. Genetic studies in neural cell lines, neural stem/progenitor cells using the neurosphere system and in the developing chicken neural tube in vivo have shown that BM88/Cend1 is a dual function molecule co-ordinating cell cycle exit and differentiation of neuronal progenitors. These studies have thus shed light on a molecular determinant that participates, along with other known and possibly still unknown regulators, in the complex processes by which a progenitor cell becomes a mature neuron.
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Key words: BM88/Cend1, p53, retinoblastoma protein pRb, cyclin D1, neural stem cells, proneural genes, embryonic development, spinal cord
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Introduction
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The generation of the central nervous system (CNS) is governed by a tightly regulated balance between neural stem/progenitor cell proliferation and differentiation to distinctive neural phenotypes.1-3 During embryonic development multipotential progenitors generate gradually more restricted precursors that will finally produce neuronal or glial progeny.4-6 Current observations have highlighted the existence of mechanisms coupling cell cycle exit and differentiation as well as functional cross-talk between intrinsic factors controlling these two mechanisms. A number of key factors regulating cell cycle progression have been implicated in cell fate determination and differentiation of neuronal precursors, while specification- and/or differentiation-inducing molecules are beginning to emerge as cell cycle regulators.7-12 *Correspondence to: Panagiotis K Politis; Center of Basic Research; Biomedical Research Foundation; Academy of Athens; Soranou Efesiou 4; Athens 11527 Greece; Tel.: 0030.210.6597479; Fax: 0030.210.6597545; Email:
[email protected]/ Rebecca Matsas; Laboratory of Cellular and Molecular Neurobiology; Hellenic Pasteur Institute; 127 Vas. Sofias Avenue; Athens 11521 Greece; Tel.: 0030.210.6478843; Fax: 0030.210.6478833; Email:
[email protected] Submitted: 12/22/07; Accepted: 01/01/08 Previously published online as a Cell Cycle E-publication: http://www.landesbioscience.com/journals/cc/article/5550 www.landesbioscience.com
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Cell cycle exit and differentiation of neuronal progenitors
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VZ precursors give first rise to neurons (early neurogenic phase) and then glia (late gliogenic phase). A large number of basic Helix-Loop-Helix (bHLH) proneural genes and homeodomain transcription factors have been identified that simultaneously promote cell cycle exit and initiate the general neurogenic and cell-fate specific differentiation programs in neural precursors. However, in most cases these genes are expressed transiently in neural progenitors and are downregulated before progenitor cells exit the proliferative zone and begin to differentiate.9,17-19 Therefore their ability to sustain cell cycle withdrawal and potentiate full neuronal differentiation relies on the induction of downstream genes that can further implement neuronal differentiation programs.20 To unveil these complex processes, it is important to understand the molecular mechanisms coupling cell cycle exit and neuronal differentiation. Especially, since it appears that post-mitotic neurons, although highly differentiated, must constantly keep their cell cycle in check throughout life to suppress re-entry into a proliferative state.21,22 Otherwise relaxation of this control mechanism, as in conditions of stress, may lead to cell cycle re-initiation and entrance into a vulnerable state often leading to neuronal Figure 1. Schematic representation of the regionalization patterns of the 6 types of dorsal neurons death. This extra view discusses the function of and the 5 types of ventral neurons in correlation with the proliferation and differentiation gradients BM88/Cend1 (for cell cycle exit and neuronal in the developing neural tube. A large number of molecular markers characterizing different types differentiation 1, NCBI nomenclature at www. of neurons in the spinal cord have been used to identify distinct domains of dorsal and ventral ncbi.nih.gov), a novel neuronal lineage-specific neuronal populations, as well as, the corresponding progenitor domains that generate these cell regulator of the cell cycle, in the synchronization populations. These cells differentiate to give the six types of dorsal interneurons (dI), dI1-dI6. In a similar manner the ventral spinal cord is subdivided in five progenitor domains: vp0, vp1, vp2, of cell cycle exit and differentiation of neuronal pMN and vp3, which generate 5 distinct groups of neuronal populations: v0, v1, v2, motor neuprogenitors in the developing nervous system rons (MN) and v3. The medio-lateral and dorso-ventral gradients of BM88/Cend1 expression are inversely correlated with proliferation gradient, while they are proportionate to the differentiation with particular focus in the spinal cord. BM88/Cend1 was first identified as a protein gradients existing in the spinal cord. widely expressed in terminally differentiated neurons of the adult mammalian central and peripheral nervous The in vivo Expression Pattern of BM88/Cend1 Suggests a systems.23,24 BM88/Cend1 cloned from porcine, mouse, human Role in the Control of Proliferation versus Differentiation of and, more recently, chick brain is an integral membrane protein Neuronal Progenitors composed of two 22–23 kDa polypeptide chains linked together by disulphide bridges.25-27 It is anchored to the membrane of intracelThe first indication that BM88/Cend1 may be implicated in the lular organelles, including the outer membrane of mitochondria, regulation of proliferation versus differentiation decisions in neural the endoplasmic reticulum and other electrolucent vesicles, via a precursors during CNS development came from its in vivo pattern transmembrane domain, in a way that the bulk of the protein faces of expression. Although, initially, BM88/Cend1 was identified as a towards the cytoplasm.24-26 Interestingly, the human BM88/Cend1 protein expressed in post-mitotic neurons,23,39 subsequent studies gene is localized in chromosome 11p15.5, a region associated with revealed that it marks neuronal cells all along the different stages of human diseases.25 The human 11p15.5 region contains an impor- the neuronal lineage both in rodents and in the chick.12,27,40 BM88/ tant tumour-suppression locus implicated in several childhood and Cend1 is expressed at low levels in neuronal progenitors, while its adult cancers,28-30 and it is also associated with the overgrowth expression is distinctly upregulated in young post-mitoticas well as genetic disorder Beckwith-Wiedemann syndrome (BWS).31-37 in mature neurons.24,40 Importantly, BM88/Cend1 is expressed in Characterization of the human BM88/Cend1 promoter revealed neural stem/progenitor cells and radial glia in the embryonic forean 88 bp proximal promoter fragment which is sufficient to confer brain and spinal cord at a time window when these cells are destined neuron-specific transcriptional activity.38 This short but potent to generate neurons while it ceases to be expressed when they give promoter fragment may be useful for the development of novel, rise to glial cells. In neuronal birthday studies in the embryonic neuron-specific gene therapy tools. cortex, it was shown that BM88/Cend1 is mainly associated in vivo 692
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Cell cycle exit and differentiation of neuronal progenitors
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BM88/Cend1 is a Dual Function Molecule Promoting Cell Cycle Exit and Neuronal Differentiation during Embryonic Development
Cend1 misexpression in the chick neural tube induced ectopic generation of cholinergic motor neurons (MNs) only in the pMN domain of the VZ, from which under normal conditions MNs are generated and not in more dorsal or ventral regions, from where interneurons are produced. As endogenous BM88/Cend1 expression is not confined to specific progenitor populations, these results suggest that BM88/Cend1 acts in generic neuron differentiation rather than in the specification of distinct neuronal identities. How does BM88/Cend1 exert its dual function on cell cycle exit and neuronal differentiation? Evidence from in vitro studies42 indicates that BM88/Cend1 is sufficient and necessary for proper regulation of cell cycle exit via (a) activation of the p53-pRb signaling pathway that controls the balance between cell cycle progression and exit and (b) cyclin D1 downregulation and cytoplasmic sequestration which is a key event for progenitor cell survival and differentiation into post-mitotic neurons (Fig. 2A).43 On the other hand, both in vitro and in vivo studies have provided evidence that BM88/Cend1 forms part of the signalling pathway(s) activated by proneural genes to trigger neuronal differentiation.12,38 In the developing chick neural tube, BM88/Cend1 appears to be induced by forced expression of the proneural gene Mash1.12 Moreover, the proximal promoter of the human BM88/Cend1 gene contains an E-box consensus sequence that represents a putative DNA-binding domain for bHLH transcription factors. Indeed the BM88/Cend1 promoter can be directly transactivated by the bHLH proneural gene neurogenin 1.38 Thus it seems that BM88/Cend1 acts downstream of proneural genes, although the operation of positive or negative feedback loops cannot be excluded at present evidence (Fig. 2B).
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with neuron-generating divisions, marking the exit of neural stem/ progenitor cells from the cell cycle.40 The apparently low expression of BM88/Cend1 in neuronal progenitors is readily elevated upon precursor to neuron switch and can be induced by retinoic acid,12,27,41,42 or other differentiation promoting agents, such as the histone deacetylase inhibitor trichostatin A (our unpublished observations). In the embryonic mouse and chick spinal cord a medio-lateral and dorso-ventral gradient of BM88/Cend1 expression is apparent, with lower BM88/Cend1 levels in the neural stem/ progenitor cell population of the VZ and higher in the differentiated cells of the MZ in the medio-lateral axis. In the dorso-vental axis, highest BM88/Cend1 expression levels are evident in ventral and lowest in dorsal areas.27 These expression gradients are, on one hand, inversely correlated with the proliferation gradients and, on the other hand, proportionate to the differentiation gradients existing in the spinal cord, where proliferation is higher in dorsal and medial areas, in contrast to differentiation which persists in ventral and lateral areas (Fig. 1).14 This intimate correlation between BM88/Cend1 expression and the progression of progenitor cells towards neuronal differentiation suggested that BM88/Cend1 may be functionally involved in this process.
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To address whether BM88/Cend1 is implicated in the regulation of neuronal differentiation, we analyzed the impact of gain- and loss-of-function approaches in different experimental systems: first, in vitro in (a) neuroblastoma cell lines, (b) isolated primary neural progenitors and (c) in the neurosphere system; and second, in vivo in the embryonic chick neural tube.12,42 Our overall analysis suggested that BM88/Cend1 exerts dual action on neuron generation: first, it strongly increases the probability that neuronal progenitors exit the cell cycle and start differentiating and, second, it drives postmitotic cells to become terminally differentiated neurons. The result was precocious and overt neuronal differentiation. This effect was produced through a strong antiproliferative action of BM88/Cend1 on neural progenitors, which was particularly prominent not only in vitro, but also in vivo in the chick neural tube suggesting that BM88/Cend1 participates in the switch to a postmitotic state of neural stem/progenitor cells (Fig. 2). The capacity of BM88/Cend1 to drive neural progenitors out of the cell cycle was accompanied by its ability to antagonize the Notch signaling pathway. Forced expression of BM88/Cend1 in the early neural tube led neural progenitors to overcome the anti-differentiating barrier of lateral inhibition by downregulating Notch1 and its downstream effector protein, Hes5, thus giving birth to large numbers of neighbouring post-mitotic neurons in the otherwise Notch1 expressing area of the VZ.12 As a result, misexpression of BM88/Cend1 in VZ precursors was sufficient to prematurely initiate and successfully conclude their differentiation program producing terminally differentiated neurons within the VZ. These ectopic neurons expressed characteristic molecular markers of the neuronal cytoskeleton, such as βIII-tubulin and neurofilament proteins as well as appropriate transcription factors and neurotransmitters. Interestingly, the subtype identity of ectopic neurons was not influenced by BM88/Cend1 but was determined by their position along the dorso-ventral axis. For example, BM88/
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The Role of BM88/Cend1 in Synchronization of Cell Cycle Exit with Neuronal Differentiation
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A key question arising from these observations is whether the inhibition of cell cycle progression exerted by BM88/Cend1, is in itself sufficient to cause premature and ectopic neuronal differentiation in the spinal cord. Previous studies have suggested that components of the cell cycle machinery, such as the cyclin-dependent kinase inhibitors p27Kip1 or its Xenopous homologue p27Xic1 and the mammalian p21Cip1, may influence positively neural specification and differentiation.44-50 However, studies in the developing mouse retina indicated that, even though p27Kip1 negatively regulates progenitor cell proliferation, it has no dramatic effects on cell fate specification and differentiation,51 as observed with the Xenopus cyclin-dependent kinase inhibitors. In addition, it appears that cell cycle inhibitors can influence either neurogenesis or gliogenesis depending on the time that the cell exits the cell cycle and that neurogenesis is induced only when proneural genes, especially the bHLH type, are co-expressed.52 In the chick spinal cord, we observed that forced cell cycle exit of neuronal precursors, by means of overexpression of p27Kip1, failed to reproduce the effect of BM88/Cend1 on neuronal differentiation.12 This implies that precocious and ectopic neurogenesis is not caused only by the action of BM88/Cend1 on cell cycle exit but by a second independent function of this molecule that efficiently couples and synchronizes the two events. Failure of p27Kip1 to mimic the effect of BM88/Cend1 on neuronal differentiation in the chick neural tube does not preclude its involvement in differentiation/migration events within a different cellular context, such as that of the mammalian cerebral cortex where p27kip1 was shown
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to promote neuronal differentiation of cortical progenitors independently of its function in suppressing cell cycle progression.10,53 Indeed, such observations highlight the influence of environmental factors within the broader concept of context-dependent gene function. In line with the emerging idea that cell cycle progression and neuronal differentiation are commonly regulated, a number of recent studies suggest that alike BM88/Cend1, a number of cell cycle regulatory genes have an important role in neuronal differentiation during CNS development. Again, this proposed function for cell cycle genes in neuronal differentiation is not a mere consequence of deregulated cell cycle properties, but represents distinct and novel functions for these genes often mediated by distinct domains. In particular, roles for p57Kip2, Rb/E2Fs and Geminin in neuronal differentiation have been described and separated from their Figure 2. (A) BM88/Cend1 forms part of the p53-cyclin D1-pRb signaling pathway leading to cell cycle arrest function in cell cycle control. For at the G phase of the cell cycle. BM88/Cend1 induces the action of p53 and its downstream effector p21 0 example, p57Kip2 is implicated in while it interferes with cyclin D1 signalling. (B) BM88/Cend1 defines a late molecular switch for neurogenthe differentiation of two distinct esis. Schematic drawing illustrating the proposed role of BM88/Cend1 in the regulation of cell cycle exit and neuronal populations in the CNS, differentiation of neuronal precursors. During early stages of vertebrate development the processes leading from neural stem cells to neural progenitors destined towards a neuronal or glial fate and, later to committed the amacrine neurons of the retina neuronal precursors are regulated by a hierarchy of transcription factors. In this hierarchical process, proneural and the dopaminergic neurons of the genes have a dual capacity to promote progenitor cell specification and induce neurogenesis. BM88/Cend1 midbrain.46,54 In addition, Geminin acts by coordinately regulating cell cycle exit and generic neuronal differentiation most probably downstream negatively regulates neuronal differ- of proneural genes. entiation via interaction with Brg1, Conversely, a number of pro-differentiation genes have also been a member of the SWI/SNF family of chromatin remodeling enzymes. This interaction inhibits the association of key regulators implicated in regulation of both cell cycle exit and induction of of neurogenesis, such as neurogenin and NeuroD, with Brg1 and differentiation and/or specification.1 This group includes Prospero consequently abolishes their ability to activate the transcription of in Drosophila and its mammalian homologue Prox1,62-64 Numb in neuron specific genes. Notably, the interaction domain in Geminin Drosophila and vertebrates,65-68 Phox2b in vertebrates,8,69 PC3/Tis21 for Brg1 is distinct from the domain that mediates its function in cell in mammals,70 other homeodomain genes, including Lims, MNR2, cycle regulation,55-58 again suggesting distinct domains for different Islet115,16,71 and finally bHLH proneural genes.20 Moreover, the dual functions. Similarly, the pRb/E2F pathway has been implicated in effect of Prox1 in the developing chick neural tube, similar to that neuronal differentiation of a specific subset of cholinergic neurons of BM88/Cend1, on progenitor cell proliferation and differentiain the retina, independently from cell cycle control. The separation tion (our unpublished observations), is in agreement with the well of these functions was manifested by the observation that, although documented role of Prox1 in the mammalian retina.62 Since Prospero, the pRb dependent defect in neuronal differentiation is rescued the Drosophila homologue of Prox1, also regulates the transition in the retinae of pRb/E2F3 double deficient mice, the defect in from mitotically active progenitor cells to terminally differentiated ectopic proliferation remains despite rescued differentiation.59 neurons,63 our data point to a well conserved regulatory pathway for Moreover, another two cell cycle signaling components, Cyclin D2 neurogenesis from flies to vertebrates. Although segregation of cell cycle progression events from and p107, are implicated in the regulation of neuronal differentiation in the cerebellum and cortex, respectively.60,61 Collectively, differentiation inducing events is a feasible task when dealing with these observations indicate that many important roles for cell cycle components of the cell cycle machinery, this endeavor becomes more genes exist in regulating neuronal differentiation, further suggesting difficult when trying to separate into distinct mechanisms the action a functional cross-talk between components of the cell cycle regula- of bi-functional proteins whose primary function has originally been tory machinery and differentiation promoting factors during nervous associated with neuronal differentiation. From a conceptual point of view, genetic manipulations that result in generation of post-mitotic system development. 694
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neural progenitors, which remain into an undifferentiated state, is an acceptable event. Instead, it is an oxymoron to envisage the generation of cycling neurons. Therefore, it is more difficult to argue that differentiation-inducing molecules can also be bi-functional by directly interfering with cell cycle progression. In the case of BM88/ Cend1 we have obtained such proof by analyzing and comparing the effect of its overexpression in cell lines of neural origin, which endogenously express the molecule and are inherently capable of neuronal differentiation, with cell lines of non-neural origin that lack both of these characteristics. While overexpression of BM88/Cend1 in neural cell lines resulted in both cell cycle exit and differentiation towards a neuronal phenotype, its overexpression in 3T3 fibroblasts triggered cell cycle exit but, apparently because of absence of the appropriate cellular machinery required for neuronal differentiation, drove the cells towards a pro-apoptotic pathway.42 In this way we managed to separate the effect of BM88/Cend1 on cell cycle progression from its effect on neuronal differentiation and obtain clear evidence for its bi-functionality. Most importantly, we unveiled a novel, potentially exciting anti-tumour action for this molecule that may become of use in cancer therapeutics.
the observation that Cyclin D1 re-expression is observed in neurons from patients with neurodegenerative diseases.74-76,81 In agreement, it has been reported that forced cyclin D1 expression in the nucleus of differentiated neurons results in apoptotic induction.43Collectively these lines of evidence converge on the hypothesis that BM88/Cend1 might be implicated in the mechanisms that suppress the induction of cell cycle in mature post-mitotic neurons.
Failure of Cell Cycle Arrest and Neuronal Degeneration in the Adult Nervous System
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From the initial identification of BM88/Cend1 as a component of mature neurons several years ago,23,24,39 a clearer picture of its biology has emerged as a neuronal-specific regulator of cell cycle exit and differentiation of neuronal precursors in the developing nervous system. We have found that this molecule is expressed in CNS cells all along the neuronal lineage in a characteristic fashion: at low levels in neural stem/progenitor cells and at higher levels in post-mitotic differentiated neurons.12,27,40 Genetic manipulation of its expression in neural cell lines, neural stem/progenitor cells using the neurosphere system and in the developing chicken neural tube in vivo have uncovered its involvement in regulating proper control of cell cycle progression/exit and differentiation of neuronal precursors. Our studies have thus shed light on yet another molecular determinant that participates, along with other known and possibly still unknown regulators, in the complex processes by which a progenitor cell becomes a mature neuron. The high expression levels of BM88/Cend1 which persist in mature neurons suggest additional functions for this molecule in the adult. The challenging hypothesis that BM88/Cend1 may be neuroprotective by acting to maintain throughout life cell cycle arrest in fully differentiated post-mitotic neurons in the adult, remains to be explored.
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The high expression levels of BM88/Cend1 in terminally differentiated neurons suggest an important, yet unidentified, function of this molecule in post-mitotic neurons. Experiments in BM88/Cend1overexpressing neuroblastoma cells indicate a neuroprotective role against diverse neurotoxic stimuli (our unpublished observations). Interestingly, recent data propose that the cell cycle of terminally differentiated neurons must be kept under constant check and that relaxation of this control mechanism leads to re-initiation of the cell cycle and entrance into a vulnerable state, often leading to neuronal death.21,22 De-regulation of cell cycle arrest in neurons may arise in the case of neurodegenerative diseases and can direct neuronal cells into an apoptosis-prone state that eventually leads to death. This concept was proposed to explain the re-expression of various cell cycle proteins, usually found only in actively dividing cells, in neurons from patients with Alzheimer’s disease.72,73 These proteins include cyclins A, B, D, E, as well as cyclin dependent kinases, PCNA and Ki67.72,74-81 There are also reports of cell cycle protein re-expression in other neurodegenerative diseases, such as amyotrophic lateral sclerosis, ataxia telangiectasia, Parkinson’s disease and in brain insults, such as stroke.82-92 In this regard, it is noteworthy that BM88/Cend1 has recently emerged as part of a protein interaction network associated with inherited human ataxias, a group of diseases characterized by degeneration of cerebellar Purkinje cells.93 Therefore BM88/ Cend1 in terminally differentiated adult neurons could function as a life-long suppressor of cell cycle and exert through this mechanism a neuroprotective role. In support of this hypothesis come our data showing that BM88/Cend1 participates in the p53-pRb-cyclin D1 pathway42 that controls progenitor cell survival and differentiation into post-mitotic neurons.43 The p53-pRb pathway and Cyclin D1 are extremely important for neuronal survival, since in mice with homozygous null alleles of pRb, there is a massive apoptotic neuronal cell death.94-96 The neuronal death in pRb knockouts is dependent on both p53 and E2F1 transcription factors.97,98 The importance of downregulating Cyclin D1 in post-mitotic neurons is emphasized by
Concluding Remarks
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Acknowledgements
Our work was supported by a grant from the European Union Research and Technological Development Program to R.M., the Hellenic General Secretariat for Research and Technology Grants YB11 to R.M. and YB26 to R.M. and D.T., and a Greece–Germany Promotion of Exchange and Scientific Collaboration (IKYDA) grant to D.T. P.K.P. was recipient of a postdoctoral scholarship from the Hellenic State Scholarship Foundation (IKY). References 1. Cremisi F, Philpott A, Ohnuma S. Cell cycle and cell fate interactions in neural development. Curr Opin Neurobiol 2003; 13:26-33. 2. Ohnuma S, Harris WA. Neurogenesis and the cell cycle. Neuron 2003; 40:199-208. 3. Ohnuma S, Philpott A, Harris WA. Cell cycle and cell fate in the nervous system. Curr Opin Neurobiol 2001; 11:66-73. 4. Desai AR, McConnell SK. Progressive restriction in fate potential by neural progenitors during cerebral cortical development. Development 2000; 127:2863-72. 5. Mori T, Buffo A, Gotz M. The novel roles of glial cells revisited: the contribution of radial glia and astrocytes to neurogenesis. Curr Top Dev Biol 2005; 69:67-99. 6. Temple S, Qian X. Vertebrate neural progenitor cells: subtypes and regulation. Curr Opin Neurobiol 1996; 6:11-7. 7. Dou CL, Li S, Lai E. Dual role of brain factor-1 in regulating growth and patterning of the cerebral hemispheres. Cereb Cortex 1999; 9:543-50. 8. Dubreuil V, Hirsch MR, Pattyn A, Brunet JF, Goridis C. The Phox2b transcription factor coordinately regulates neuronal cell cycle exit and identity. Development 2000; 127:5191-201. 9. Heins N, Cremisi F, Malatesta P, Gangemi RM, Corte G, Price J, Goudreau G, Gruss P, Gotz M. Emx2 promotes symmetric cell divisions and a multipotential fate in precursors from the cerebral cortex. Mol Cell Neurosci 2001; 18:485-502. 10. Nguyen L, Besson A, Roberts JM, Guillemot F. Coupling cell cycle exit, neuronal differentiation and migration in cortical neurogenesis. Cell Cycle 2006; 5:2314-8.
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