[52] Martinez-Murillo, R., Caro, L., and Nieto-Sampedro, M. Lesion-in- ... [58] Ramon y Cajal, S. Histotogie du Systeme Nerveux de fHomme et des. Vertehres.
Restorative Neurology and Neuroscience 19 (2001) 85-94
Regulation of intrinsic regenerative properties and axonal plasticity in cerebellar Purkinje cells Ferdinando Rossi , Annalisa Buffo and Piergiorgio Strata Ri/ci Lcvi Monlakini Center for Brain Repair. Department ofNeuroseienee. University of Turin. Corso Raffaellu 30. 1-10/25 Turin. Italy Received 20 April 2001; accepted 31 July 2001
Abstract Axon regeneration in the matnmalian brain requires that injured neurons upregulate a specific set of growth-associated genes. To investigate the mechanisms that control the intrinsic growth properties of adult central neurons, we have examined the response to injury and regenerative potential of different cerebellar and precerebellar neuron populations. Axotomised neurons in the inferior olive, deep ceiebellar nuclei and lateral reticular nucleus upregulate growth-associated molecules and regenerate their neurites into growth-permissive transplants. In contrast, Purkinje cells fail to respond to injury and show extremely poor regenerative capabilities. Targeted overexpression of GAP-43 promotes Purkinje axon plasticity, indicating that the weak regenerative potential of these neurons is mainly due to the inability to activate growth-associated genes. Application of neutralising antibodies against the myelin-associated protein Nogo-A induces cell body changes and axonal sprouting in intact Purkinje cells. In addition, immature injured Purkinje cells respond to axotomy and regenerate transected neurites, but they progressively lose this ability during postnatal development in parallel with myelin formation and the establishment of intracortical connections. These results indicate that the intrinsic growth potential of Purkinje cells is constitutively inhibited by environmental signals directed at stabilising the mature connectivity and preventing aberrant neuritic plasticity. Such a strict control eventually leads to restrict the regenerative capabilities of these neurons after injury. Keywords: axon regeneration, growth-associated genes, Purkinje cells, Nogo-A, myelin, axotomy, cerebellum
1. Introduction Although axon regeneration in the mammalian central nervous system (CNS) is primarily hampered by adverse environmental conditions [26,64,70], successful long-distance neurite growth also requires that the injured neurons express a specific set of genes, partially recapitulating the ontogenetic growth program [5,24,25,28,69], The ability to activate regeneration-associated genes is different among distinct neuronal phenotypes in the mature nervous system, and it is strongly influenced by context-dependent conditions. This suggests that the expression of these molecules is controlled * Corresponding author. Tel.: +39 011 6707705; Fax: +39 011 6707708; Email: ferdinando.rossifriunito.it. 0922-6028/01/S8.00 'O 2001, IOS Press
by sophisticated regulatoi^ tnechanisms, whose understanding is a crucial step towards the goal of efficient brain repair. In the peripheral nervous systetn (PNS) neurons react to axotomy and upregulate growth-associated molecules irrespective ofthe distance between injury and the cell body, although the response is faster for proximal than distal lesions [45,49], Similar changes can be induced in uninjured neurons following blockade of axonal transport [77] or target remodelling [75], suggesting that the expression of growth-associated molecules is constitutively regulated by retrograde inhibitory cues. Indeed, the cell body response to axotomy can be attenuated or suppressed by application of NGF [33,53] or other target-derived factors [4], and axotomy-like changes can be elicited in intact PNS neurons following administration of anti-NGF antibodies [33,65], On the other hand, molecules
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inferior olivary neurons lateral reticular neurons deep nuclei neurons atrophy slow cell death cell body reaction c-Jun JunD NOS GAP-43 L1 CHL1
Purkinje cells survival axon reaction (torpedoes, arciform fibres)
axotomy
weak cell body reaction (rare neurons close to the lesion)
\ no regeneration delayed sprouting
regeneration into growth-permissive environment Fig. 1. Difterential response to injury and regenerative potential of adult cerebellar and precerebellar neurons. Following axotomy, neurons of the inferior olive, lateral reticular nucleus and deep cerebellar nuclei show a vigorous cell body reaction, with upregulation of several growth-associated molecules. These axotomised neurons undergo slow atrophy or degeneration, but they are able to regenerate their axons into a growth-pemiissive environment. In contrast, Purkinje cells are characterised by a weak cell body response, strong survival properties, and poor regenerative capabilities.
have been identified, such as leukemia inhibitory factor (LIF) [66], which are released following injury and enhance the reaction of affected nerve cells. Altogether, these observations indicate that the expression of growth-associated genes in PNS neurons is regulated through a balance between targetderived negative signals and lesion-induced positive cues. In the CNS, both the basal expression of growth-associated molecules and the intensity ofthe response to axotomy are extremely variable among different neuron phenotypes [3,37,50], In addition, within a single neuron population the strength and duration of cell body changes, and the ensuing regenerative potential, are strongly dependent on lesion conditions, such as the distance between the cell body and the injury site [18,27,40,72], the presence of uninjured collateral branches [48], and the affected axon branch, as for dorsal root ganglion neurons [ 13,42,59,69], Several types of injured CNS neurons are sensitive to positive signals provided by growthpromoting transplants [7,14,41,60,73] or neurotrophic factors directly applied to the soma [46], On the other hand, evidence has been obtained that target-derived negative cues suppress growth-associated gene programs also in the CNS [34,35]. Nevertheless, the well-estahlished notion that axon injuries close to the cell body induce stronger responses than distant ones suggests that additional cues delivered along the neurite contribute to regulate the intrinsic growth potential of central neurons [67,68], All these data depict a complex scenario in which the expression of growth-associated molecules in CNS neurons results from the interplay between multiple negative and positive environmental factors, which interact with distinct neuron phenotypes, each endowed with its peculiar sensitivity to regulatory cues.
Most ofthe effector molecules responsible for this regulation and their mechanisms of action are still unknown. Furthermore, the functional significance of such a strict control on neuronal growth and plasticity is not fully understood. To address some of these issues, in the last few years we have studied the response to injury and regenerative potential of cerebellar and precerebellar neuron populations. These neurons, and particularly Purkinje cells, show some very peculiar features that lnake them a most suitable model to investigate the mechanisms regulating the intrinsic growth properties of CNS neurons. 2. Different types of cerebellar axons show opposite regenerative capabilities into growth permissive transplants Because of the ordered arrangement of afferent and efferent systems of the cerebellar cortex, which run parallel to each other along the axial white matter of cortical lobules, it is possible to transect simultaneously different axon populations and compare their response to injury and regenerative capabilities in front of the same environmental conditions [61], Axotomised Purkinje cells undergo a characteristic axon reaction, including the appearance of axonal torpedoes along the initial neuritic segment and the hypertrophy of the recurrent collateral branches, while the severed corticofugal stump remains apposed to the lesion site [19,57], Despite these axonal modifications, injured Purkinje cells do not undergo clear degenerative or atrophic changes and survive to axotomy for vety long times [9,19], In addition, starting from several weeks after injury, in parallel with the appear-
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ance of growth-pennissive molecules in the injury scar, numerous thin processes sprout from the transected corticofugal stumps and grow in the white inatter and in the adjacent granular layer [22], Severed olivocerebellar axons also remain apposed to the lesioti site for several weeks after lesion, but they appear progressively thinner and undergo slow degenerative changes [61], The affected cell bodies shrink, the detidrites become atrophic, and about 65 % of inferior olivary neurons die within two months after axotomy made at the inferior cerebellar peduncle [8], Neither axon type is able to grow spontaneously into the adult cerebellar tissue. However, the regenerative potential of the injured neurons can be assessed by placing growth-permissive transplants in the injury track to provide the severed processes with favourable environmental conditions. Transected olivocerebellar axons vigorously regenerate into embryonic cerebellar or neocortical transplants [61 ] as well as into dissociated Schwann cell grafts placed into the lesion site [6,71], Most surprisingly, Purkinje axons are not able to regenerate into the same embryonic neural transplants, and similar results are obtained with grafts of embryonic deep cerebellar nuclei [61], thus ruling out the possibility that adult Purkinje neurites fail to regenerate because ofthe competition with their embryonic counterparts. In addition, Purkinje axons show very poor, if any, growth capabilities also when confronted with Schwann cell grafts [6], in line with other reports showing that they cannot regenerate into a peripheral nerve stump implanted into the cerebellum [11,17,74], Purkinje cells and inferior olivary neurons thus display different morphological reactions to axon injury, and they are endowed with opposite regenerative capabilities (Fig, 1), Axotomised inferior olivary neurons undergo regressive phenomena and, in some instances, cell death, but they can vigorously regenerate their axons when provided with favourable environmental conditions. On the contrary, axotomised Purkitije cells do not show atrophic changes, but they are unable to regenerate their axons into any ofthe tested growth-permissive transplants, 3. The regenerative potential of cerebellar neurons is related to the strength of their cellular response to injury The opposite regenerative potential shown by olivocerebellar and Purkinje axons confronted with the same environmental conditions may be attributed either to a differential ability to upregulate growth-associated molecules or to a different sensitivity to the growth-promoting cues provided by the microenvironment. Indeed, although all the different tissues or cells grafted into the injury site are endowed with growth-pennissive/promoting properties, distinct neuron populations may not share the sarne sensitivity to such extrinsic cues. However, the constant results obtained with different kinds of transplants make this possibility very unlikely. Rather, the comparative analysis ofthe reaction to axotomy reveals that the regenerative potential ofthe different neuron types is related to the strength of their cell body response (Fig. 1),
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Axotomised neurons in the deep nuclei and lateral reticular nucleus, whose neurites are known to regenerate into growth-pennissive transplants [2,11,54,74], show a strong upregulation of transcription factors c-Jun and JunD, growth-associated protein GAP-43, and NOS, revealed by immunocytochemistry or NADPH diaphorase histochemistry [78], The expression of these inarkers builds up within a few days after injury and persists for several weeks in nutnerous neurons scattered throughout the examined nuclei. These results have been recently confirmed and extended in another report showing that injured deep nuclear neurons also upregulate the cell recognition molecules LI and CHLl [11], Upregulation ofthe same molecules as well as ofthe developmentally-regulated calcitonin-gene related peptide also occurs in axotomised inferior olivary neurons [8,62,71], which are also characterised by a strong basal expression of GAP-43 in the intact brain [15,47,78]. Contrary to the strong response observed in the afferent systems to the cerebellar cortex, Purkinje cells show an extremely weak reactivity. Following large surgical transections encompassing several cerebellar lobules, the vast majority of axototnised Purkinje cells does not express any of the examined markers, except for rare neurons in the vicinity of the injury site that transiently upregulate c-Jun, JunD, CAP-23 and NADPH diaphorase, but not LI and CHLl [11] or GAP-43 [74,78], although a mild transient upregulation of GAP-43 mRNA has been recently observed [76]. To date, the otTly marker, which is consistently overexpressed in these injured neurons, is the P75 low-affinity neurotrophin receptor [21,52,74], Several reports indicate that cell body changes of injured neurons can be enhanced and prolonged in the presence of growth-permissive/promoting transplants [7,11,14,41,60,73]. However, no expression of growth-associated molecules can be induced in axotomised Purkinje cells by embryonic neural transplants placed in the injury track [78] or peripheral nerve stumps implatitcd in the cerebellar parenchyma [11], Thus, the regenerative potential ofthe different cerebellar and precerebellar neuron populations is clearly related to the strength of their cell body response to axotomy (Fig. 1). In this context, Purkinje cells are unable to set up a spontaneous reaction to injury and they also appear to be insensitive to environmental cues that may boost their intrinsic growth potential. 4. GAP-43 overexpression affects both neurite growth and survival capabilities of axotomised Purkinje cells If Purkinje cells fail to regenerate their axons because of their weak cell body reaction to axotomy, then their intrinsic growth potential might be increased by artificially inducing the overexpression of growth-associated genes. To pursue this strategy, transgenic mice have been generated, in which the GAP-43 gene is expressed under the control of the Purkinje cell-specific L7 promoter [9], These mice show a strong basal expression of GAP-43 in all Purkinje cells, but there are no anatomical abnormalities in the intact cerebellum. However,
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Fig. 2. Regukuion ol ininnsic Pui kinjc cell growth properlies by Nogo-A mycliii-associaletl piolcin. The Purkinjo axon slem us well as the ascending portion of the recurrent collateral branches are covered by a thick niyelin sheath. Application of neutralising anti-Nogo-A antibodies (IN-1 or 472) to the adult cerebellum induces cell body changes in intact Purkinje cells accompanied by axonal sprouting in the deeper portions of the granular layer (dark grey), which are normally devoid of terminal Purkinje axon branches.
a clear phenotype is revealed following axotomy. Starting from a few days after injury, Purkinje axons sprout numerous new processes, which gradually expand to form dense meshworks in the vicinity of the injury site. Despite these conspicuous growth phenomena the transected Purkinje neurites remain uncapable of long-distance regeneration into embryonic neural or Schwann cell grafts. In addition, contrary to their wild-type counterparts, the injured transgenic Purkinje cells undergo a slow degeneration, which can be prevented by the grafts placed in the injury track. Overexpression of GAP-43 thus reverses the behaviour of injured Purkinje cells by enhancing both their growth potential and their sensitivity to axon injury. A similar link between GAP-43 expression, axon growth and neuronal survival has been also observed in other systems [1,30,36,76]. Together with the well-established bipotential action of c-Jun [37], this indicates that complex molecular crosstalks between intracelkilar pathways determine whether the injured neurons will go into death or regeneration. In this respect, it is worth underlying that the effects of GAP-43 overexpression on Purkinje cell
survival and axon growth are not evident in intact animals, suggesting that some lesion-induced modifications are required to reveal transgene activity. On the other hand, although GAP-43 overexpression does promote Purkinje axon growth, it is not sufficient on its own to confer a full regenerative capability to the injured neuron. Indeed, recent evidence shows that long-distance axon regeneration requires the coordinate expression of several growth-associated genes [5]. 5. Environmental control of growth-associated gene expression and axon plasticity in Purkinje cells The extremely weak Purkinje cell response to axotomy may be due either to an intrinsic inability to activate growthassociated genes or to the action of extrinsic regulatory signals, which suppress their expression. Intrinsic inhibitory mechanisms are likely important, since several growth-associated molecules, including GAP-43 [78], LI and CHLl [11], are not upregulated by adult injured Purkinje cells, and GAP-43 is absent (or expressed at low levels) even during
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F,g. 3. Reciprocal distribution of myelinated axon profiles and Purkinje neurites in lobules 1 and X of the adult rat cerebellum. The figure illustrates the results of quantitative estimauons of the density of myelin basic protein (MBP) immunolabelled axons and calbindin (CaBP) immunopositive Purkinje neurites in the granular layer ot lobules I and X from three adult rats. Left panel shows a sagittal section of the rat cerebellum, in which the different lobules are indicated For each ammaUhree pairs of adjacent sections labelled for either marker have been analysed. On every section, the density of labelled axon profiles has been estimated on 200 X 250 ^tm areas (represented by the boxes in the left panel), to which a grid was superimposed. The mean numbers of labelled axons crossing the grid are reported ,n the histogram. Note the reciprocal distribution of myelinated profiles and Purkinje axon branches in the two lobules suggesting that the number and extension of Purkinje axon branches in the granular layer are inversely related to the local amount of myelin.
normal Purkinje cell developinent [15,32,76]. On the other hand, other molecules related to the cell body response to injury, such as c-Jun, JunD, NOS and CAP-23, can be upregulated by rare Purkinje cells in the vicinity of the injury site [78], indicating that these neurons are able to set up some sort of reaction, at least when specific conditions are met. The close vicinity of reactive Purkinje cells to the lesion site suggests that the response may be elicited by injury/inflammation-related molecules [28]. Although this hypothesis cannot be completely ruled out, the area of the transected lobules in which reactive Purkinje cells are localised is much smaller than the region affected by intense glial modifications [19]. In addition, the same distribution pattern of responsive Purkinje cells is observed following axotomy made in organotypic cerebellar cultures [78], where the contribution of blood-borne inflammatory elements is minimal or absent. Examination of axotomised Purkinje cells in the organotypic cultures shows that cell body changes only occur in those neurons that maintain a very short axon stump, not longer than a few hundred microns [78]. In other words, Purkinje cells represent an extreme case of the "critical distance of axotomy to the cell body" that in other systems usually ranges from several hundred microns to a few millimetres [18,72]. This phenomenon has been attributed to the activity of retrogradely-transported negative signals issued by elements distributed along the axon, which have to be removed in order to elicit cell body changes [67,68]. This
view is supported by the observation that colchicine application induces the expression of injury/growth-associated molecules in intact Purkinje cells [56,78]. However, assuming that Purkinje cells are actually influenced by such retrograde cues, it is not clear why they can only react after extremely proximal injuries. Two different features of the Purkinje axon may account for this peculiar behaviour. First, recurrent collateral branches emanate a few hundred microns from the origin of the stem Purkinje neurite and temiinate on nearby Purkinje cells [55,58]. Because of the very proximal origin, these branches are spared in most axotomised Purkinje cells, which may still draw regulatory cues and/or trophic support from their cortical targets [57]. Second, a thick myelin sheath covers both the whole corticofugal stump and the ascending portion of the recurrent branches [55,58], suggesting that myelin-associated molecules may also contribute to regulate gene expression in Purkinje cells. Among myelin-associated molecules, Nogo-A [12] is one of the most likely candidates. In addition to its well-established growth-cone collapsing activity, this protein is thought to exert a constitutive regulatory function on axon growth and plasticity in the intact CNS, as witnessed by its role in targeting GAP-43 to terminal neuritic domains [43] and in preventing aberrant growth of developing axonal tracts [39,63]. Adult intact Purkinje cells express high levels of the Nogo-A receptor [29], suggesting a continuous interaction with this myelin-associated molecule.
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•l regeneration survival c-Jun expression -2 §
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Fig. 4. Evolution of the response to injury, survival capability and regenerative potential of Purkinje cells axotomised during postnatal development. The graph summarizes data from ref 32, about the percentage of injured Purkinje cells which upregulate c-Jun (circles, left y axis), survive to the lesion (triangles, left y axis), and about the number of transected Purkinje axons growing across 100 |im of interface between host cerebellum and embryonic neural grafts (squares, right y axis). Note the reciprocal evolution of the cell body response (represented by c-Jun expression), resistance to injury and growth potential during the first two weeks of postnatal cerebellar development. P = postnatal day, ad = adult.
Application of the Nogo-A neutralising antibody IN-1 to organotypic slices of postnatal cerebellum induces a strong upregulation of c-Jun in numerous Purkinje cells, whereas only a few reactive neurons are seen in control slices [78], The same results can be obtained in vivo (Fig, 2), A single intraparenchymal injection of the IN-1 Fab fragment in the intact cerebellum of adult rats elicits the activation of c-Jun, JunD and NADPH-diaphorase reactivity in numerous intact Purkinje cells, and similar changes occur in their axotomised counterparts when the antibody is injected in transected lobules. The cell body modifications are accompanied by axon growth phenomena [10], The intact Purkinje axons sprout numerous new processes that invade the deeper portions ot the granular layer. All these effects, which reverse within a few weeks after injection, can be faithfully replicated by means of the antibody 472 [10], raised against the inhibitory dornain of a recombinant Nogo-A protein [12], Functional neutralization of Nogo-A thus induces the upregulation of injury/growth-associated molecules and axon sprouting in intact Purkinje cells (Fig, 2), The time course of these phenomena indicates that this myelin-associated molecule constitutively regulates the intrinsic growth properties of Purkinje cells by a dual action exerted locally on the axon itself and retrogradely on the cell body [10], In the antibodytreated cerebella, the newly formed sprouts invade portions of the granular layer that are usually devoid of terminal Purkinje axon branches. Indeed, in most of the cerebellar lobuli and folia the terminal arbours of recurrent Purkinje axon collaterals - the infraganglionic plexus - are confined within a restricted region in the upper portion of the granular layer. This typical pattern is not present in lobules IX and X, where a dense meshwork of terminal Purkinje axon branches covers the whole depth of the granular layer [78], Interestingly, anti-myelin basic protein (MBP) staining shows an in-
verse distribution pattern of myelinated axon profiles, which are dense in the granular layer of lobules I-VIII and sparse in lobules IX and X (Fig, 3), This observation further supports the conclusion that myelin-associated molecules, and namely Nogo-A, exert a constitutive control on Purkinje axons to prevent aberrant growth and confine terminal branches within precise domains of the cerebellar cortical layers. Application of anti-Nogo-A antibodies to the cerebellum does not induce clear cell body changes in other cerebellar or precerebellar neurons, whose myelinated axons also run through the injected regions of the cerebellar cortex. In line with the results of focal demyelination experiments in the spinal cord [38], this indicates that upregulation of growth associated genes can only be obtained when myelin-derived negative cues have been efficiently neutralised along a substantial extent of the axon length. In addition, however, it cannot be excluded that distinct neuron phenotypes may be differentially sensitive to these regulatory cues. Nevertheless, it is worth mentioning that c-Jun upregulation can be induced in neurons of the cerebral cortex by local application of IN-1 antibodies (our unpublished observation), as well as in ponto-cerebellar and septo-hippocampal neurons after experimental demyelination [51], 6. Evolution of the Purkinje cell response to injury during postnatal cerebellar development The typical features of the response to axotomy of adult Purkinje cells - i,e, a weak cell body reaction, a strong resistance to injury and poor regenerative capabilities - are not shared by their immature counterparts. In vitro studies have shown that late embryonic or early postnatal Purkinje cells are viable in organotypic cultures and vigorously regenerate their axons even into the mature cerebellar environment [20], The same neurons massively die in an apoptotic manner when explanted between postnatal days (P) 1 and 5 [31], In contrast, after P7 numerous Purkinje cells survive, but they completely fail to regenerate their axons [20], Lesion/transplantation experiments in vivo reveal the peculiar evolution of Purkinje cell response to injury and regenerative potential during postnatal cerebellar development [32], Virtually all Purkinje cells axotomised during the first postnatal week (at P3 and P6) show a strong reaction irrespective of their position relative to the injury site. Cell body changes include an intense c-Jun upregulation accompanied by a mild, but consistent, expression of GAP-43, whereas NADPH-diaphorase reactivity is unchanged. The injured neurons massively degenerate within a few days after lesion, but their loss can be partially prevented by embryonic neural grafts placed in the lesion track. The reaction of the juvenile axotomised Purkinje cells also includes compensatory phenomena. Many transected axons develop growth cones and sprout new processes that grow into the immature cerebellar white matter and cortical layers. In addition, numerous Purkinje axons elongate into embryonic neocortical transplants, where they form extensive terminal plexuses.
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Fig. 5. Development ot the extrinsic regulation of Purkinje cell growth potential. During the second postnatal week, major events of Purkinje cell development are the growth and remodelling of recurrent plexuses as well as the myelination of the axon, which progresses in a retrograde manner. The maturation of intracortical connectivity and myelin sheath provide the Purkinje cell with retrograde trophic support and regulatory cues that stabilise the neuron and restrict axonal plas^ticity withm defined terminal domains. The same cues may be eventually responsible to promote survival and restrict regeneration following injury of adult cells.
These features progressively change when the lesion is made during the second postnatal week (at P9 and PI2): the cell body reaction gradually becomes weaker, while the injured neurons acquire resistance to injury and lose their regenerative capabilities both in situ and into the grafts. Thus, cell body changes, resistance to injury and regenerative potential of Purkinje cells evolve in a reciprocal manner during the first two postnatal weeks (Fig. 4), What are the crucial events taking place during this period that are responsible for completely reversing the behaviour and fate of axotomised Purkinje cells? Major moiphogenetic processes occur in the cerebellum during postnatal development. Among others, however, some events might be particularly relevant for the evolution of Purkinje cell survival and growth properties. Although Purkinje axons reach their targets in the deep nuclei before birth [23], during the first two postnatal weeks they are still engaged in intense growth phenomena, involving both the development of the intracortical plexus and the interstitial elongation of the corticofugal neurite, required to match the growth of the whole cerebellar mass. Recurrent collateral branches and terminal plexuses mature during the second postnatal week [16,58] in parallel with the appearance of the morphological reaction of transected Purkinje axons [32]. Most interestingly, the hypertrophy of the recurrent collateral plexus, which has been interpreted as a compensatory reaction
to the loss of trophic support from target neurons in the deep nuclei [57], only occurs in animals lesioned at P9 or later, in coincidence with a conspicuous increase of axotomised Purkinje cell survival [32]. In addition, myelination of Purkinje axons develops during the same period according to a retrograde pattern, starting from the central white matter and progressing in a centrifugal manner towards the cortex [44,79]. Taken together, these observations suggest that maturation of Purkinje axons and of their glial microenvironment may be major events responsible for changing the properties of these neurons after injury (Fig, 5). The development of the intracortical plexuses and the associated connectivity likely provide the injured neurons with trophic support that prevents their degeneration and retrograde negative cues that suppress their growth potential. In addition, the concomitant formation of myelin sheaths contributes to stabilise Purkinje axons, restrict their plasticity to specific cortical domains and inhibit the expression of growth-associated genes. In other words, a set of coordinated developmental processes, whose primary aim is to stabilise the mature Purkinje axon and its connectivity, eventually leads to promote survival and restrict regeneration of these neurons after injury. Indeed, the sprouting phenomena observed in long-term injured Purkinje cells [19,22] or following application of antiNogo-A antibodies [10] suggest that these neurons are en-
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dowed with an intrinsic propensity for axon plasticity that has to be strictly controlled in order to avoid the formation of aberrant disadaptive connections. Specific experiments are now needed to test this hypothesis and to address numerous related issues that still remain unanswered. In any case, the elucidation of the cellular/molecular processes responsible for restricting Purkinje cell intrinsic growth potential may provide important insights on the general mechanisms that regulate growth and repair processes in the mammalian CNS. Acknowledgements Supported by Italian Telethon (grant n, 1130), Fondazione Cavalieri Ottolenghi of Turin, References [I] Aigner, L., Arber, S., Kapfhammer, J.P, Laux, T., Schneider, C , Botteri, F., Brenner, H,-R. and Caroni, P. Overexpression of the neural growth-associated protein GAP-43 induces nerve sprouting in the adult nervous system of transgenic mice. Cell S3 (1995)269-278. [2] Armengol, J.A., Sotelo, C , Angaut, P and Alvarado-Mallart, R.M. Organization of host afferents to cerebellar grafts implanted into kainate lesioned cerebellum in adult rats. Hodological evidence for the specificity of host-graft interactions. Eur. J. Neuro.sci. 1 (1989) 75-93. [3] Barron, K,D. Neuronal responses to axotomy; consequences and possibilities for rescue from permanent atrophy or cell death. In F.J. Seil (Ed.). Neural Regeneration and Transplantation, Alan Liss, New York, 1989, pp. 79-99. [4] Blottner, D. and Herdegen, T. Neuroprotective Fibroblast Growth Factor type-2 down-regulates the c-Jun transcription factor in axotomized sympathetic preganglionic neurons of adult rat. Neuroscience 82(1998)283-292. [5] Bomze, H.M., Bulsara, K.R., Iskandar, B.J., Caroni, P and Skene, J.H.P Spinal axon regeneration evoked by replacing two growth cone proteins in adult neurons. Nat. Neuro.sci. 4 (2001) 38-43. [6] Bravin, M., Savio, T., Strata, P and Rossi, F. Olivocerebellar axon regeneration and target reinnervation following dissociated Schwann cell grafts in surgically injured cerebella of adult rats. Eur J. Neurosci. 9(1997)2634-2649. [7] Broude, E., McAtee, M., Kelley, M.S. and Bregman, B.S. c-Jun expression in adult rat dorsal root ganglion neurons: differential response after central or peripheral axotomy. E.xp. Neurol. 148 (1997) 367-377. [8] Buffo, A., Fronte, M., Oestreicher, A.B. and Rossi, F. Degenerative phenomena and reactive modifications of the adult rat inferior olivary neurons following axotomy and disconnection from their targets. Neuroscience 85 (1998) 587-604. [9] Buffo, A., Holtmaat, A.J.D.G, Savio, T., Verbeek, S,, Oberdick, J., Oestreicher, A.B., Gispen, W.H., Verhaagen, J., Rossi, F. and Strata, P Targeted overexpression of the neurite growth-associated protein B-50/GAP-43 in cerebellar Purkinje cells induces sprouting in response to axotomy, but does not allow axon regeneration into growth permissive transplants. J. Neurosci. 17 (1997) 8778-8791. [10] Buffo, A., Zagrebelsky, M., Huber, A.B., Skerra, A., Schwab, M,E., Strata, P, and Rossi, F. Application of neutralising antibodies against Nl-35/250 myelin-associated neurite growth inhibitory proteins to the adult rat cerebellum induces sprouting of uninjured Purkinje cell axons. J. Neurosci. 20 (2000) 2275-2286. [II] Chaiksuksunt, V., Zhang, Y., Anderson, PN., Campbell, G., Vaudano, b., Schachner M., and Lieberman, A.R. Axonal regeneration from CNS neurons in the cerebellutn and brainstem of adult rats: correlation
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