Lack of Purkinje cell loss in adult rat cerebellum following protracted axotomy: degenerative changes and regenerative attempts of the severed axons

Is inferior olive central to the pathophysiology of essential tremor? No

Essential tremor (ET) represents one of the commonest movement disorder worldwide and is the most common tremor disorder. ET manifests with various combinations of motor and nonmotor symptoms. The clinical hallmark is a kinetic tremor of upper limbs. Historically, the pathogenesis of ET has been based on the hypothesis of an overactivity of the inferior olive (inferior olive hypothesis: IOH) where the inferior olive would act as the central pace-maker of ET, resulting in impaired electrophysiological discharges of the olivo-cerebellar tract. The absence of structural alterations in post-mortem studies of the inferior olive is a striking argument against the IOH. Furthermore, neuroimaging studies point towards the implication of the cerebello-thalamo-cerebral pathway rather than the IO, and the harmaline model which has been considered as an animal model of ET presents important weaknesses. By contrast, a series of experiments by Louis et al. have provided convincing evidence of impaired wiring of the Purkinje cell microcircuitry and progressive neurodegeneration of the cerebellar cortex. The Purkinje neuron appears as the primary culprit (Purkinjopathy). The cerebellar cortex hypothesis (CCH) has solid neuropathological signatures, unlike the purely physiological IOH. Rather than a dysregulatory electrophysiological disorder suggested by IOH, ET is a clinical-pathological entity similar to late onset neurodegenerative disorders such as Parkinson's disease or Alzheimer's disease. The CCH emphasizes the need to develop novel therapeutic strategies in order to maintain or promote the cerebellar reserve. The modern reconceptualization of ET in a genuine cerebellar disorder is cleaning the IOH to the light of histopathological studies. ET falls in the large basket of the neurodegenerative diseases and we have entered into a novel formulation of the disease pathogenesis with direct impacts on future therapies.

Gga3 deletion and a GGA3 rare variant associated with late onset Alzheimer's disease trigger BACE1 accumulation in axonal swellings

Axonal dystrophy, indicative of perturbed axonal transport, occurs early during Alzheimer's disease (AD) pathogenesis. Little is known about the mechanisms underlying this initial sign of the pathology. This study proves that Golgi-localized γ-ear-containing ARF binding protein 3 (GGA3) loss of function, due to Gga3 genetic deletion or a GGA3 rare variant that cosegregates with late-onset AD, disrupts the axonal trafficking of the β-site APP-cleaving enzyme 1 (BACE1) resulting in its accumulation in axonal swellings in cultured neurons and in vivo. We show that BACE pharmacological inhibition ameliorates BACE1 axonal trafficking and diminishes axonal dystrophies in Gga3 null neurons in vitro and in vivo. These data indicate that axonal accumulation of BACE1 engendered by GGA3 loss of function results in local toxicity leading to axonopathy. Gga3 deletion exacerbates axonal dystrophies in a mouse model of AD before β-amyloid (Aβ) deposition. Our study strongly supports a role for GGA3 in AD pathogenesis, where GGA3 loss of function triggers BACE1 axonal accumulation independently of extracellular Aβ, and initiates a cascade of events leading to the axonal damage distinctive of the early stage of AD.

Pattern of Neuronal and Axonal Damage, Glial Response, and Synaptic Changes in Rat Cerebellum within the First Week following Traumatic Brain Injury

We examined damage and repair processes in the rat cerebellum within the first week following moderate traumatic brain injury (TBI) induced by lateral fluid percussion injury (LFPI) over the left parietal cortex. Rats were killed 1, 3, or 7 days after the injury or sham procedure. Fluoro-Jade B staining revealed 2 phases of neurodegenerative changes in the cell bodies and fibers: first, more focal, 1 day after the LFPI, and second, widespread, starting on post-injury day 3. Purkinje cell loss was detected in posterior lobule IX 1 day following LFPI. Apoptosis was observed in the cerebellar cortex, on days 1 and 7 following LFPI, and was not caspase- or apoptosis-inducing factor (AIF)-mediated. AIF immunostaining indicated axonal damage in the cerebellar white matter tracts 3- and 7-days post-injury. Significant astrocytosis and microgliosis were noticed on day 7 following LFPI at the sites of neuronal damage and loss. Immunohistochemical labeling with the presynaptic markers synaptophysin and growth-associated protein-43 revealed synaptic perturbations already on day 1 that were more pronounced at later time points following LFPI. These results provide new insights into pathophysiological alterations in the cerebellum and their mechanisms following cerebral TBI.

Inappropriate Intrusion of an Axonal Mitochondrial Anchor into Dendrites Causes Neurodegeneration

Syntaphilin (SNPH) is a major mitochondrial anchoring protein targeted to axons and excluded from dendrites. In this study, we provide in vivo evidence that this spatial specificity is lost in Shiverer (Shi) mice, a model for progressive multiple sclerosis (MS), resulting in inappropriate intrusion of SNPH into dendrites of cerebellar Purkinje cells with neurodegenerative consequences. Thus, reconstituting dendritic SNPH intrusion in SNPH-KO mice by viral transduction greatly sensitizes Purkinje cells to excitotoxicity when the glutamatergic climbing fibers are stimulated. Finally, we demonstrate in vitro that overexpression of SNPH in dendrites compromises neuronal viability by inducing N-methyl-D-aspartate (NMDA) excitotoxicity, reducing mitochondrial calcium uptake, and interfering with quality control of mitochondria by blocking somal mitophagy. Collectively, we propose that inappropriate immobilization of dendritic mitochondria by SNPH intrusion produces excitotoxicity and suggest that interception of dendritic SNPH intrusion is a therapeutic strategy to combat neurodegeneration.

Essential tremor pathology: neurodegeneration and reorganization of neuronal connections

Essential tremor (ET) is the most common tremor disorder globally and is characterized by kinetic tremor of the upper limbs, although other clinical features can also occur. Postmortem studies are a particularly important avenue for advancing our understanding of the pathogenesis of ET; however, until recently, the number of such studies has been limited. Several recent postmortem studies have made important contributions to our understanding of the pathological changes that take place in ET. These studies identified abnormalities in the cerebellum, which primarily affected Purkinje cells (PCs), basket cells and climbing fibres, in individuals with ET. We suggest that some of these pathological changes (for example, focal PC axonal swellings, swellings in and regression of the PC dendritic arbor and PC death) are likely to be primary and degenerative. By contrast, other changes, such as an increase in PC recurrent axonal collateral formation and hypertrophy of GABAergic basket cell axonal processes, could be compensatory responses to restore cerebellar GABAergic tone and cerebellar cortical inhibitory efficacy. Such compensatory responses are likely to be insufficient, enabling the disease to progress. Here, we review the results of recent postmortem studies of ET and attempt to place these findings into an anatomical-physiological disease model.

Disruption of Tmem30a results in cerebellar ataxia and degeneration of Purkinje cells

Phospholipids are asymmetrically distributed across mammalian plasma membrane with phosphatidylserine (PS) and phosphatidylethanolamine concentrated in the cytoplasmic leaflet of the membrane bilayer. This asymmetric distribution is dependent on a group of P4-ATPases named PS flippases. The proper transport and function of PS flippases require a β-subunit transmembrane protein 30 A (TMEM30A). Disruption of PS flippases led to several human diseases. However, the roles of TMEM30A in the central nervous system remain elusive. To investigate the role of Tmem30a in the cerebellum, we developed a Tmem30a Purkinje cell (PC)-specific knockout (KO) mouse model. The Tmem30a KO mice displayed early-onset ataxia and progressive PC death. Deficiency in Tmem30a led to an increased expression of Glial fibrillary acidic protein and astrogliosis in regions with PC loss. Elevated C/EBP homologous protein and BiP expression levels indicated the presence of endoplasmic reticulum stress in the PCs prior to visible cell loss. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) analysis suggested that apoptotic cell death occurred in the cerebellum. Our data demonstrate that loss of Tmem30a in PCs results in protein folding and transport defects, a substantial decrease in dendritic spine density, increased astrogliosis and PC death. Taken together, our data demonstrate an essential role of Tmem30a in the cerebellum PCs.

Elavl3 is essential for the maintenance of Purkinje neuron axons

Neuronal Elav-like (nElavl or neuronal Hu) proteins are RNA-binding proteins that regulate RNA stability and alternative splicing, which are associated with axonal and synaptic structures. nElavl proteins promote the differentiation and maturation of neurons via their regulation of RNA. The functions of nElavl in mature neurons are not fully understood, although Elavl3 is highly expressed in the adult brain. Furthermore, possible associations between nElavl genes and several neurodegenerative diseases have been reported. We investigated the relationship between nElavl functions and neuronal degeneration using Elavl3^−/− mice. Elavl3^−/− mice exhibited slowly progressive motor deficits leading to severe cerebellar ataxia, and axons of Elavl3^−/− Purkinje cells were swollen (spheroid formation), followed by the disruption of synaptic formation of axonal terminals. Deficit in axonal transport and abnormalities in neuronal polarity was observed in Elavl3^−/− Purkinje cells. These results suggest that nElavl proteins are crucial for the maintenance of axonal homeostasis in mature neurons. Moreover, Elavl3^−/− mice are unique animal models that constantly develop slowly progressive axonal degeneration. Therefore, studies of Elavl3^−/− mice will provide new insight regarding axonal degenerative processes.

Purkinje cell injury, structural plasticity and fusion in patients with Friedreich's ataxia

Purkinje cell pathology is a common finding in a range of inherited and acquired cerebellar disorders, with the degree of Purkinje cell injury dependent on the underlying aetiology. Purkinje cells have an unparalleled resistance to insult and display unique regenerative capabilities within the central nervous system. Their response to cell injury is not typical of most neurons and likely represents both degenerative, compensatory and regenerative mechanisms. Here we present a pathological study showing novel and fundamental insights into Purkinje cell injury, remodelling and repair in Friedreich’s ataxia; the most common inherited ataxia. Analysing post-mortem cerebellum tissue from patients who had Friedreich's ataxia, we provide evidence of significant injury to the Purkinje cell axonal compartment with relative preservation of both the perikaryon and its extensive dendritic arborisation. Axonal remodelling of Purkinje cells was clearly elevated in the disease. For the first time in a genetic condition, we have also shown a disease-related increase in the frequency of Purkinje cell fusion and heterokaryon formation in Friedreich's ataxia cases; with evidence that underlying levels of cerebellar inflammation influence heterokaryon formation. Our results together further demonstrate the Purkinje cell’s unique plasticity and regenerative potential. Elucidating the biological mechanisms behind these phenomena could have significant clinical implications for manipulating neuronal repair in response to neurological injury.

Essential Tremor: A Common Disorder of Purkinje Neurons?

Essential tremor (ET) is one of the most common neurological diseases, with an estimated 7 million affected individuals in the United States. Postmortem studies in the past few years have resulted in new knowledge as well as a new formulation of disease pathophysiology. This new formulation centers on the notion that ET might be a disease of the cerebellum and, more specifically, the Purkinje cell (PC) population. Indeed, several investigators have proposed that ET may be a "Purkinjopathy." Supporting this formulation are data from controlled postmortem studies demonstrating (1) a range of morphological changes in the PC axon, (2) abnormalities in the position and orientation of PC bodies, (3) reduction in the number of PCs in some studies, (4) morphological changes in and pruning of the PC dendritic arbor with loss of dendritic spines, and (5) alterations in both the PC-basket cell interface and the PC-climbing fiber interface in ET cases. This new formulation has engendered some controversy and raised additional questions. Whether the constellation of changes observed in ET differs from that seen in other degenerative disorders of the cerebellum remains to be determined, although initial studies suggest the likely presence of a distinct profile of changes in ET.

Torpedo formation and Purkinje cell loss: modeling their relationship in cerebellar disease

Torpedo formation and Purkinje cell (PC) loss represent standard and inter-related cerebellar responses to injury. Surprisingly, the nature of their relationship has not been carefully characterized across a range of normal and disease states. Are brains with more torpedoes expected to have fewer PCs? We quantified torpedoes and PCs in four groups: essential tremor (ET), spinocerebellar ataxia (SCA), multiple system atrophy-cerebellar (MSA-C), and controls. Brains from 100 individuals (58 ET, 27 controls, 7 SCA, 8 MSA-C) were available at the New York Brain Bank. After complete neuropathological assessment, a standard parasagittal neocerebellar block was harvested; a 7-μm thick section was stained with Luxol fast blue/hematoxylin and eosin; and torpedoes and PCs were quantified. For a given PC count, SCA and MSA-C cases often had higher torpedo counts than ET cases or controls. Furthermore, the relationship between torpedo and PC counts was complex. The correlation between torpedo and PC counts was negative in ET cases (i.e., individuals with more torpedoes had fewer PCs [i.e., more PC loss]) whereas the relationship was positive in MSA-C cases (i.e., individuals with fewer PCs [i.e., more PC loss] had fewer torpedoes). Patients with SCA showed both patterns. When all diagnostic groups were combined, the correlation was best fit by a quadratic (i.e., parabolic) model rather than a simple linear model; this model incorporated data on the negative correlation in ET cases, the mixed results in SCA cases, and the positive correlation in MSA-C cases (r = 0.636). The relationship between torpedo and PC counts was complex and heterogeneous across a range of cerebellar disease states, and was best characterized by a quadratic rather than a simple model. With more severe cerebellar disease, torpedoes can be quite numerous and are likely a common feature of surviving PCs, but eventually, dramatic loss of PC leads to a paradoxical reduction in observable torpedoes.

From neurons to neuron neighborhoods: the rewiring of the cerebellar cortex in essential tremor

Remarkably little has been written on the biology of essential tremor (ET), despite its high prevalence. The olivary model, first proposed in the 1970s, is the traditional disease model for ET; however, the model is problematic for a number of reasons. Recently, intensive tissue-based studies have identified a series of structural changes in the brains of most ET cases, and nearly all of the observed changes are located in the cerebellar cortex. These studies suggest that Purkinje cells are central to the pathogenesis of ET and may thus provide a focus for the development of novel therapeutic strategies. Arising from these studies, a new model of ET proposes that the population of Purkinje cells represents the site of the initial molecular/cellular events leading to ET. Furthermore, a number of secondary changes/remodeling observed in the molecular and granular layers (i.e., in the Purkinje cell "neighborhood") are likely to be of additional mechanistic importance. On a physiological level, the presence of remodeling indicates the likely formation of aberrant synapses and the creation of new/abnormal cortical circuits in ET. Specific efforts need to be devoted to understanding the cascade of biochemical and cellular events occurring in the Purkinje cell layer in ET and its neuron neighborhood, as well as the physiological effects of secondary remodeling/rewiring that are likely to be occurring in this brain region in ET.

Understanding essential tremor: progress on the biological front

For many years, little was written about the underlying biology of ET, despite its high prevalence. Discussions of disease mechanisms were dominated by a focus on tremor physiology. The traditional model of ET, the olivary model, was proposed in the 1970s. The model suffers from several critical problems, and its relevance to ET has been questioned. Recent mechanistic research has focused on the cerebellum. Clinical and neuroimaging studies strongly implicate the importance of this brain region in ET. Recent mechanistic research has been grounded more in tissue-based changes (i.e., postmortem studies of the brain). These studies have collectively and systematically identified a sizable number of changes in the ET cerebellum, and have led to a new model of ET, referred to as the cerebellar degenerative model. Hence, there is a renewed interest in the science behind the biology of ET. How the new understanding of ET will translate into treatment changes is an open question.

CHP1-mediated NHE1 biosynthetic maturation is required for Purkinje cell axon homeostasis

Axon degeneration is a critical pathological feature of many neurodegenerative diseases. Misregulation of local axonal ion homeostasis has been recognized as an important yet understudied mechanism for axon degeneration. Here we report a chemically induced, recessive mouse mutation, vacillator (vac), which causes ataxia and concomitant axon degeneration of cerebellar Purkinje cells. By positional cloning, we identified vac as a point mutation in the calcineurin-like EF hand protein 1 (Chp1) gene that resulted in the production of mutant CHP1 isoforms with an amino acid substitution in a functional EF-hand domain or a truncation of this motif by aberrant splicing and significantly reduced protein levels. CHP1 has been previously shown to interact with the sodium hydrogen exchanger 1 (NHE1), a major regulator of intracellular pH. We demonstrated that CHP1 assists in the full glycosylation of NHE1 that is necessary for the membrane localization of this transporter and that truncated isoforms of CHP1 were defective in stimulating NHE1 biosynthetic maturation. Consistent with this, membrane localization of NHE1 at axon terminals was greatly reduced in Chp1-deficient Purkinje cells before axon degeneration. Furthermore, genetic ablation of Nhe1 also resulted in Purkinje cell axon degeneration, pinpointing the functional convergence of the two proteins. Our findings clearly demonstrate that the polarized presynaptic localization of NHE/CHP1 is an important feature of neuronal axons and that selective disruption of NHE1-mediated proton homeostasis in axons can lead to degeneration, suggesting that local regulation of pH is pivotal for axon survival.

Purkinje cell axonal anatomy: quantifying morphometric changes in essential tremor versus control brains

Growing clinical, neuro-imaging and post-mortem data have implicated the cerebellum as playing an important role in the pathogenesis of essential tremor. Aside from a modest reduction of Purkinje cells in some post-mortem studies, Purkinje cell axonal swellings (torpedoes) are present to a greater degree in essential tremor cases than controls. Yet a detailed study of more subtle morphometric changes in the Purkinje cell axonal compartment has not been undertaken. We performed a detailed morphological analysis of the Purkinje cell axonal compartment in 49 essential tremor and 39 control brains, using calbindin D28k immunohistochemistry on 100-µm cerebellar cortical vibratome tissue sections. Changes in axonal shape [thickened axonal profiles (P = 0.006), torpedoes (P = 0.038)] and changes in axonal connectivity [axonal recurrent collaterals (P < 0.001), axonal branching (P < 0.001), terminal axonal sprouting (P < 0.001)] were all present to an increased degree in essential tremor cases versus controls. The changes in shape and connectivity were significantly correlated [e.g. correlation between thickened axonal profiles and recurrent collaterals (r = 0.405, P < 0.001)] and were correlated with tremor duration among essential tremor cases with age of onset >40 years. In essential tremor cases, thickened axonal profiles, axonal recurrent collaterals and branched axons were 3- to 5-fold more frequently seen on the axons of Purkinje cells with torpedoes versus Purkinje cells without torpedoes. We document a range of changes in the Purkinje cell axonal compartment in essential tremor. Several of these are likely to be compensatory changes in response to Purkinje cell injury, thus illustrating an important feature of Purkinje cells, which is that they are relatively resistant to damage and capable of mobilizing a broad range of axonal responses to injury. The extent to which this plasticity of the Purkinje cell axon is partially neuroprotective or ultimately ineffective at slowing further cellular changes and cell death deserves further study in essential tremor.

Mature Purkinje cells require the retinoic acid-related orphan receptor-α (RORα) to maintain climbing fiber mono-innervation and other adult characteristics

Neuronal maturation during development is a multistep process regulated by transcription factors. The transcription factor RORα (retinoic acid-related orphan receptor α) is necessary for early Purkinje cell (PC) maturation but is also expressed throughout adulthood. To identify the role of RORα in mature PCs, we used Cre-lox mouse genetic tools in vivo that delete it specifically from PCs between postnatal days 10-21. Up to 14 d of age, differences between mutant and control PCs were not detectable: both were mono-innervated by climbing fibers (CFs) extending along their well-developed dendrites with spiny branchlets. By week 4, mutant mice were ataxic, some PCs had died, and remaining PC soma and dendrites were atrophic, with almost complete disappearance of spiny branchlets. The innervation pattern of surviving RORα-deleted PCs was abnormal with several immature characteristics. Notably, multiple functional CF innervation was reestablished on these mature PCs, simultaneously with the relocation of CF contacts to the PC soma and their stem dendrite. This morphological modification of CF contacts could be induced even later, using lentivirus-mediated depletion of rora from adult PCs. These data show that the late postnatal expression of RORα cell-autonomously regulates the maintenance of PC dendritic complexity, and the CF innervation status of the PC (dendritic vs somatic contacts, and mono-innervation vs multi-innervation). Thus, the differentiation state of adult neurons is under the control of transcription factors; and in their absence, adult neurons lose their mature characteristics and acquire some characteristics of an earlier developmental stage.

Regrowth of transected retinal ganglion cell axons despite persistent astrogliosis in the lizard (Gallotia galloti)

We analysed the astroglia response that is concurrent with spontaneous axonal regrowth after optic nerve (ON) transection in the lizard Gallotia galloti. At different post-lesional time points (0.5, 1, 3, 6, 9 and 12 months) we used conventional electron microscopy and specific markers for astrocytes [glial fibrillary acidic protein (GFAP), vimentin (Vim), sex-determining region Y-box-9 (Sox9), paired box-2 (Pax2)¸ cluster differentiation-44 (CD44)] and for proliferating cells (PCNA). The experimental retina showed a limited glial response since the increase of gliofilaments was not significant when compared with controls, and proliferating cells were undetectable. Conversely, PCNA(+) cells populated the regenerating ON, optic tract (OTr) and ventricular wall of both the hypothalamus and optic tectum (OT). Subpopulations of these PCNA(+) cells were identified as GFAP(+) and Vim(+) reactive astrocytes and radial glia. Reactive astrocytes up-regulated Vim at 1 month post-lesion, and both Vim and GFAP at 12 months post-lesion in the ON-OTr, indicating long-term astrogliosis. They also expressed Pax2, Sox9 and CD44 in the ON, and Sox9 in the OTr. Concomitantly, persistent tissue cavities and disorganised regrowing fibre bundles reaching the OT were observed. Our ultrastructural data confirm abundant gliofilaments in reactive astrocytes joined by desmosomes. Remarkably, they also accumulated myelin debris and lipid droplets until late stages, indicating their participation in myelin removal. These data suggest that persistent mammalian-like astrogliosis in the adult lizard ON contributes to a permissive structural scaffold for long-term axonal regeneration and provides a useful model to study the molecular mechanisms involved in these beneficial neuron-glia interactions.

Profound morphological and functional changes of rodent Purkinje cells between the first and the second postnatal weeks: a metamorphosis?

Between the first and the second postnatal week, the development of rodent Purkinje cells is characterized by several profound transitions. Purkinje cells acquire their typical dendritic "espalier" tree morphology and form distal spines. During the first postnatal week, they are multi-innervated by climbing fibers and numerous collateral branches sprout from their axons, whereas from the second postnatal week, the regression of climbing fiber multi-innervation begins, and Purkinje cells become innervated by parallel fibers and inhibitory molecular layer interneurons. Furthermore, their periods of developmental cell death and ability to regenerate their axon stop and their axons become myelinated. Thus a Purkinje cell during the first postnatal week looks and functions differently from a Purkinje cell during the second postnatal week. These fundamental changes occur in parallel with a peak of circulating thyroid hormone in the mouse. All these features suggest to some extent an interesting analogy with amphibian metamorphosis.

GABAergic signaling and connectivity on Purkinje cells are impaired in experimental autoimmune encephalomyelitis

A significant proportion of multiple sclerosis (MS) patients have functionally relevant cerebellar deficits, which significantly contribute to disability. Although clinical and experimental studies have been conducted to understand the pathophysiology of cerebellar dysfunction in MS, no electrophysiological and morphological studies have investigated potential alterations of synaptic connections of cerebellar Purkinje cells (PC). For this reason we analyzed cerebellar PC GABAergic connectivity in mice with MOG((35-55))-induced experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. We observed a strong reduction in the frequency of the spontaneous inhibitory post-synaptic currents (IPSCs) recorded from PCs during the symptomatic phase of the disease, and in presence of prominent microglia activation not only in the white matter (WM) but also in the molecular layer (ML). The massive GABAergic innervation on PCs from basket and stellate cells was reduced and associated to a decrease of the number of these inhibitory interneurons. On the contrary no significant loss of the PCs could be detected. Incubation of interleukin-1beta (IL-1β) was sufficient to mimic the electrophysiological alterations observed in EAE mice. We thus suggest that microglia and pro-inflammatory cytokines, together with a degeneration of basket and stellate cells and their synaptic terminals, contribute to impair GABAergic transmission on PCs during EAE. Our results support a growing body of evidence that GABAergic signaling is compromised in EAE and in MS, and show a selective susceptibility to neuronal and synaptic degeneration of cerebellar inhibitory interneurons.

Lysosomal compromise and brain dysfunction: examining the role of neuroaxonal dystrophy

Lysosomal diseases are a family of over 50 disorders caused by defects in proteins critical for normal function of the endosomal/lysosomal system and characterized by complex pathogenic cascades involving progressive dysfunction of many organ systems, most notably the brain. Evidence suggests that compromise in lysosomal function is highly varied and leads to changes in multiple substrate processing and endosomal signalling, in calcium homoeostasis and endoplasmic reticulum stress, and in autophagocytosis and proteasome function. Neurons are highly vulnerable and show abnormalities in perikarya, dendrites and axons, often in ways seemingly unrelated to the primary lysosomal defect. A notable example is NAD (neuroaxonal dystrophy), which is characterized by formation of focal enlargements (spheroids) containing diverse organelles and other components consistent with compromise of retrograde axonal transport. Although neurons may be universally susceptible to NAD, GABAergic neurons, particularly Purkinje cells, appear most vulnerable and ataxia and related features of cerebellar dysfunction are a common outcome. As NAD is found early in disease and thus may be a contributor to Purkinje cell dysfunction and death, understanding its link to lysosomal compromise could lead to therapies designed to prevent its occurrence and thereby ameliorate cerebellar dysfunction.

Axotomy-induced neurotrophic withdrawal causes the loss of phenotypic differentiation and downregulation of NGF signalling, but not death of septal cholinergic neurons

BACKGROUND

Septal cholinergic neurons account for most of the cholinergic innervations of the hippocampus, playing a key role in the regulation of hippocampal synaptic activity. Disruption of the septo-hippocampal pathway by an experimental transection of the fimbria-fornix drastically reduces the target-derived trophic support received by cholinergic septal neurons, mainly nerve growth factor (NGF) from the hippocampus. Axotomy of cholinergic neurons induces a reduction in the number of neurons positive for cholinergic markers in the medial septum. In several studies, the reduction of cholinergic markers has been interpreted as analogous to the neurodegeneration of cholinergic cells, ruling out the possibility that neurons lose their cholinergic phenotype without dying. Understanding the mechanism of cholinergic neurodegeneration after axotomy is relevant, since this paradigm has been extensively explored as an animal model of the cholinergic impairment observed in neuropathologies such as Alzheimer's disease.The principal aim of this study was to evaluate, using modern quantitative confocal microscopy, neurodegenerative changes in septal cholinergic neurons after axotomy and to assess their response to delayed infusion of NGF in rats.

RESULTS

We found that there is a slow reduction of cholinergic cells labeled by ChAT and p75 after axotomy. However, this phenomenon is not accompanied by neurodegenerative changes or by a decrease in total neuronal number in the medial septum. Although the remaining axotomized-neurons appear healthy, they are unable to respond to delayed NGF infusion.

CONCLUSIONS

Our results demonstrate that at 3 weeks, axotomized cholinergic neurons lose their cholinergic phenotype without dying and down-regulate their NGF-receptors, precluding the possibility of a response to NGF. Therefore, the physiological role of NGF in the adult septal cholinergic system is to support phenotypic differentiation and not survival of neurons. This evidence raises questions about the relationship between transcriptional regulation of the cholinergic phenotype by retrograde-derived trophic signaling and the transcriptional changes experienced when retrograde transport is impaired due to neuropathological conditions.

Animal models of human cerebellar ataxias: a cornerstone for the therapies of the twenty-first century

Cerebellar ataxias represent a group of disabling neurological disorders. Our understanding of the pathogenesis of cerebellar ataxias is continuously expanding. A considerable number of laboratory animals with neurological mutations have been reported and numerous relevant animal models mimicking the phenotype of cerebellar ataxias are becoming available. These models greatly help dissecting the numerous mechanisms of cerebellar dysfunction, a major step for the assessment of therapeutics targeting a given deleterious pathway and for the screening of old or newly synthesized chemical compounds. Nevertheless, differences between animal models and human disorders should not be overlooked and difficulties in terms of characterization should not be occulted. The identification of the mutations of many hereditary ataxias, the development of valuable animal models, and the recent identifications of the molecular mechanisms underlying cerebellar disorders represent a combination of key factors for the development of anti-ataxic innovative therapies. It is anticipated that the twenty-first century will be the century of effective therapies in the field of cerebellar ataxias. The animal models are a cornerstone to reach this goal.

Overexpression of GAP-43 modifies the distribution of the receptors for myelin-associated growth-inhibitory proteins in injured Purkinje axons

Abstract Neurons with enhanced intrinsic growth capabilities can elongate their axons into non-permissive territories, but the mechanisms that enable the outgrowing processes to overcome environmental inhibition are largely unknown. To address this issue, we examined adult mouse Purkinje cells that overexpress the axonal growth-associated protein GAP-43. After injury, these neurons exhibit sprouting along the intracortical neuritic course and at the severed stump in the white matter. To determine whether GAP-43-overexpressing Purkinje cells are responsive to extrinsic inhibitory cues, we investigated the content and subcellular localization of major receptors for myelin-associated inhibitory proteins, PlexinB1 and the Nogo receptor (NgR) with the related co-receptors LINGO-1 and p75. Expression of these molecules, estimated by measuring perikaryal immunostaining intensity and Western blot, was not different in wild-type or transgenic mice, and it was not overtly modified after axotomy. Following injury, however, the content of PlexinB1 was significantly reduced in GAP-43-overexpressing neurites. Furthermore, in the same axons the distribution of both PlexinB1 and NgR was altered, being inverse to that of GAP-43. Labelling for the two receptors was conspicuously reduced on the axonal surface and it was almost undetectable in the outgrowing sprouts, which showed strong GAP-43 immunoreactivity. These observations indicate that although GAP-43 overexpression does not modify the expression of receptors for myelin-associated inhibitory factors, it interferes with their subcellular localization and exposure on the neuritic membrane. Therefore, GAP-43 promotes axon growth by multiple synergistic mechanisms that potentiate the intrinsic motility of the elongating processes, while reducing their sensitivity to environmental inhibition.

Distinct modes of neuritic growth in purkinje neurons at different developmental stages: axonal morphogenesis and cellular regulatory mechanisms

Background

During development, neurons modify their axon growth mode switching from an elongating phase, in which the main axon stem reaches the target territory through growth cone-driven extension, to an arborising phase, when the terminal arbour is formed to establish synaptic connections. To investigate the relative contribution of cell-autonomous factors and environmental signals in the control of these distinct axon growth patterns, we examined the neuritogenesis of Purkinje neurons in cerebellar cultures prepared at elongating (embryonic day 17) or arborising (postnatal day zero) stages of Purkinje axon maturation.

Methodology/Principal Findings

When placed in vitro, Purkinje cells of both ages undergo an initial phase of neurite elongation followed by the development of terminal ramifications. Nevertheless, elongation of the main axon stem prevails in embryonic Purkinje axons, and many of these neurons are totally unable to form terminal branches. On the contrary, all postnatal neurites switch to arbour growth within a few days in culture and spread extensive terminal trees. Regardless of their elongating or arborising pattern, defined growth features (e.g. growth rate and tree extension) of embryonic Purkinje axons remain distinct from those of postnatal neurites. Thus, Purkinje neurons of different ages are endowed with intrinsic stage-specific competence for neuritic growth. Such competence, however, can be modified by environmental cues. Indeed, while exposure to the postnatal environment stimulates the growth of embryonic axons without modifying their phenotype, contact-mediated signals derived from granule cells specifically induce arborising growth and modulate the dynamics of neuritic elongation.

Conclusions/Significance

Cultured Purkinje cells recapitulate an intrinsically coded neuritogenic program, involving initial navigation of the axon towards the target field and subsequent expansion of the terminal arborisation. The execution of this program is regulated by environmental signals that modify the growth competence of Purkinje cells, so to adapt their endogenous properties to the different phases of neuritic morphogenesis.

Models of traumatic cerebellar injury

Traumatic brain injury (TBI) is a major cause of morbidity and mortality worldwide. Studies of human TBI demonstrate that the cerebellum is sometimes affected even when the initial mechanical insult is directed to the cerebral cortex. Some of the components of TBI, including ataxia, postural instability, tremor, impairments in balance and fine motor skills, and even cognitive deficits, may be attributed in part to cerebellar damage. Animal models of TBI have begun to explore the vulnerability of the cerebellum. In this paper, we review the clinical presentation, pathogenesis, and putative mechanisms underlying cerebellar damage with an emphasis on experimental models that have been used to further elucidate this poorly understood but important aspect of TBI. Animal models of indirect (supratentorial) trauma to the cerebellum, including fluid percussion, controlled cortical impact, weight drop impact acceleration, and rotational acceleration injuries, are considered. In addition, we describe models that produce direct trauma to the cerebellum as well as those that reproduce specific components of TBI including axotomy, stab injury, in vitro stretch injury, and excitotoxicity. Overall, these models reveal robust characteristics of cerebellar damage including regionally specific Purkinje cell injury or loss, activation of glia in a distinct spatial pattern, and traumatic axonal injury. Further research is needed to better understand the mechanisms underlying the pathogenesis of cerebellar trauma, and the experimental models discussed here offer an important first step toward achieving that objective.

Anosmin-1 stimulates outgrowth and branching of developing Purkinje axons

During development, Purkinje axons elongate along precise trajectories and acquire stereotypic branching patterns to innervate targets in the deep nuclei and cerebellar cortex. These processes are accomplished through cell-intrinsic mechanisms, whose operation is regulated by environmental signaling cues. Here, we show that Anosmin-1, the protein defective in the X-linked form of Kallmann syndrome, is one among such cues. Anosmin-1, that stimulates axon elongation and branching in the olfactory system, is expressed by Purkinje cells and deep nuclear neurons of the rat cerebellum during the ontogenetic period when Purkinje axons acquire their mature pattern. These neurons also express the putative Anosmin-1 receptor, fibroblast growth factor receptor 1. Application of Anosmin-1 to dissociated cultures of embryonic (embryonic day 17, E17) or postnatal (postnatal day 0, P0) rat cerebellar cells enhances neuritic elongation and exerts a strong promoting action on the budding of collateral branches and on the extension of terminal arbors. Opposite effects are observed when neutralizing anti-Anosmin-1 antibodies are applied to the same cultures. Comparable results are obtained by administering the protein or the blocking antibodies to organotypic cultures of postnatal (P0) rat cerebellum. In P10 cerebellar slices, Anosmin-1 does not enhance the spontaneous regenerative capabilities of severed Purkinje axons, but promotes the terminal outgrowth of injured neurites into embryonic neocortical explants apposed to the axotomy site. Although Anosmin-1 is unable to change the overall intrinsic growth competence of Purkinje cells, it exerts a powerful stimulatory action on the budding and extension of collateral branches and terminal plexus, contributing to the patterning of Purkinje axons.

Axonal abnormalities in cerebellar Purkinje cells of the Ts65Dn mouse

Ts65Dn mice are a genetic model for Down syndrome. Among others, these mice have cerebellar pathology features which parallel those seen in Down syndrome patients. Both individuals with Down syndrome and Ts65Dn mice have reduced cerebellar volume and numbers of granule and Purkinje cells. In this report, we describe morphological abnormalities of axons of Purkinje cells in the cerebellum of Ts65Dn mice, by using anti-calbindin immunocytochemistry. A consistent number of Purkinje cells shows axons bearing giant varicosities along their transit through the granular layer. The cerebellar arbor vitae made by fasciculated Purkinje cell axons has a patchy appearance, some tracks being devoid of calbindin staining. The infraganglionic plexus, formed by recurrent collaterals of Purkinje cell axons, has enormously increased density, which is evidence for a compensatory reaction to degeneration of distal segments of axons. These alterations are accompanied by strong glial reaction as evidenced by GFAP immunocytochemistry. Moreover, the alterations are more consistent in the anterior lobules of the vermis and intermediate cortex. The axonal pathology of Purkinje cells may explain the impairment in cerebellar functions observed in Ts65Dn mice at the adulthood.

Expression of X-chromosome linked inhibitor of apoptosis protein in mature Purkinje cells and in retinal bipolar cells in transgenic mice induces neurodegeneration

Transgenic mice with overexpression of the caspase-inhibitor, X-chromosome-linked inhibitor of apoptosis protein (XIAP) in Purkinje cell (PC) and in retinal bipolar cells (RBCs) were produced to study the regulation of cell death. Unexpectedly, an increased neurodegeneration was observed in the PCs in these L7-XIAP mice after the third postnatal week with the mice exhibiting severe ataxia. The loss of PCs was independent of Bax as shown by crossing the L7-XIAP mice with Bax gene-deleted mice. Electron microscopy revealed intact organelles in PCs but with the stacking of ER cisterns indicative of cell stress. Immunostaining for cell death proteins showed an increased phosphorylation of c-Jun in the PCs, suggesting an involvement in cell degeneration. Apart from PCs, the number of RBCs was decreased in adult retina in line with the expression pattern for the L7 promoter. The data show that overexpression of the anti-apoptotic protein XIAP in vulnerable neurons leads to enhanced cell death. The mechanisms underlying this neurodegeneration can be related to the effects of XIAP on cell stress and altered cell signaling.

Why Is Wallerian Degeneration in the CNS So Slow?

Wallerian degeneration (WD) is the set of molecular and cellular events by which degenerating axons and myelin are cleared after injury. Why WD is rapid and robust in the PNS but slow and incomplete in the CNS is a longstanding mystery. Here we review current work on the mechanisms of WD with an emphasis on deciphering this mystery and on understanding whether slow WD in the CNS could account for the failure of CNS axons to regenerate.

Engineered expression of polysialic acid enhances Purkinje cell axonal regeneration in L1/GAP-43 double transgenic mice

Purkinje axons in adult mammals are generally unable to regenerate after axotomy. Our recent work has shown that over-expression of growth related genes, GAP-43 and L1, in Purkinje cells increased their axonal outgrowth into a predegenerated peripheral nerve graft, but not into a fresh graft [Zhang et al., (2005) Proc. Natl Acad. Sci. USA, 102, 14883-14888]. In the current study we investigated whether engineered expression of growth permissive molecule polysialic acid (PSA) in the glial scar or on transplanted Schwann cells could overcome the inhibitory environment and promote Purkinje axonal regeneration. A stab wound was introduced in the cerebellum of the L1/GAP-43 transgenic mice and a lentiviral vector (LV) carrying the polysialyltransferase (PST) cDNA (LV/PST) was injected into the lesion site to transduce the cells in the glial scar. Regenerating Purkinje axons were examined by calbindin immunostaining. There was increased Purkinje axonal sprouting in the area expressing high-level PSA. However, Purkinje axons were unable to grow into the lesion cavity. In the second set of experiments when LV/PST transduced Schwann cells were transplanted into the lesion site, the number of Purkinje axons growing into the transplant was nine times more than that growing into Schwann cell transplant expressing GFP two months post operation. Our result suggests that transplanted Schwann cells engineered to express PSA support axonal regeneration better than naïve Schwann cells.

The strange case of Purkinje axon regeneration and plasticity

In the last few years Purkinje cells have become a most interesting model to investigate cellular/molecular mechanisms of axon regeneration and plasticity. Adult Purkinje cells are most peculiar for their weak cell body response to axotomy, which is accompanied by a strong resistance to injury and a virtually absolute inability to regenerate severed neurites, even in the presence of favourable environmental conditions. The same neurons show a vigorous intrinsic inclination toward axonal sprouting and structural plasticity, which can be elicited by removing extrinsic growth-inhibitory cues. These features gradually develop during early postnatal life, but the underlying mechanisms and biological significance remain unclear. This article reviews recent studies aimed at addressing these questions with respect to the general issue of brain repair. Indeed, understanding the reasons for the extremely poor regenerative capacity of Purkinje cells will be most important to elucidate basic biological mechanisms of axon regeneration and plasticity, and to promote circuit rewiring in the adult CNS.

The response of magnocellular neurons of the hypothalamo-neurohyphyseal system to hypophysectomy, nitric oxide synthase expression as well as survival and regeneration in developing vs. adult rats

This study examined the age-related changes in nitric oxide synthase immunoreactivity (NOS-IR), survival and regeneration of magnocellular neurons in the hypothalamo-neurohypophyseal system (HNS) in rats following hypophysectomy. In adult animal, hypophysectomy induced a significant increase in NOS-IR in the supraoptic (SON), paraventricular nuclei (PVN) and median eminence (ME) by 3 days post-lesion. NOS sustained an increased level until 2 weeks after hypophysectomy and then returned to normal control level. In contrast, at postnatal day 7 (PN7), no obvious increase in NOS-IR was observed in the SON, PVN and ME following the injury compared with age-matched controls. At PN14, the same injury induced an increase in NOS-IR in SON, PVN and ME but the increase was more transient with peak NOS-IR at 3 days and returning to the corresponding control level at 1 week after hypophysectomy. In contrast to a striking age-dependent alteration in NOS-IR in the SON and PVN, hypophysectomy induced substantial degeneration of arginine vasopressin (AVP) and oxytocin (OT) neurons in the SON and PVN in both immature and adult rats and there was no obvious difference in neuronal survival after the same injury among these three groups of different ages by quantitative analysis. Following hypophysectomy, a large number of fibers were observed in the contact zone of the median eminence and the adjacent lumen of the third cerebral ventricle (V3) in adult rats, whereas few fibers could be found in the lumen of the V3 in the immature rats after the same injury. Relationships between NOS induction and magnocellular neuronal survival and regeneration were discussed.

Neurite-myelin interaction in the control of purkinje axon growth and regeneration

The limited regenerative capability of adult central neurons results from an interplay between the intrinsic growth potential of injured nerve cells and environmental growth-inhibitory signals, such as myelin-associated molecules. These molecules, which are known to inhibit axonal regeneration, are expressed in the adult intact central nervous system, suggesting that they also exert a constitutive regulatory function on uninjured neurites. However, most of the mechanisms underlying this function of myelin-associated factors are still unclear. To address this issue, we examined Purkinje cells, which show extremely poor regenerative capabilities. Such a weak intrinsic growth potential is attributed, at least in part, to inhibitory activity exerted by the myelin-associated molecule Nogo-A on growth-associated gene expression and axon plasticity of adult Purkinje cells. Indeed, the regenerative potential of these neurons declines during postnatal development in parallel with myelin deposition. In addition, myelin-associated factors regulate the development of the Purkinje axon intracortical plexus, contributing to the shaping of mature cerebellar connectivity. The action of environmental inhibitory factors can be partially counteracted by overexpression of neuronal growth-associated molecules. In fact, following axotomy, transgenic Purkinje cells that overexpress GAP-43 are able to sprout their axons into white matter and show a pronounced tendency to lose myelin. Altogether, these results suggest that the control exerted by myelin on Purkinje axon plasticity plays an important role in preventing aberrant growth and in stabilizing intracortical connectivity. Such a constitutive regulation may eventually hamper compensatory mechanisms set up by the same neurons in response to injury and thus determine their weak regenerative abilities.

Degradation of chondroitin sulfate proteoglycans induces sprouting of intact purkinje axons in the cerebellum of the adult rat

Chondroitin sulfate proteoglycans are major constituents of the extracellular matrix and form perineuronal nets. Information regarding the growth-inhibitory activity of these molecules after injury is rapidly expanding. However, less is known about their physiological role in the adult undamaged CNS. Here, we investigated the function of chondroitin sulfate proteoglycans in maintaining the proper structure of Purkinje axons in the cerebellum of adult rats. To this end, we examined the morphology and distribution of intracortical Purkinje neurites after intraparenchymal injection of chondroitinase ABC. Staining with the lectin Wisteria floribunda agglutinin or 2B6 antibodies showed that this treatment efficiently removed chondroitin sulfate proteoglycans from wide areas of the cerebellar cortex. In the same sites, there was a profuse outgrowth of terminal branches from the Purkinje infraganglionic plexus, which invaded the deeper regions of the granular layer. In contrast, myelinated axon segments were not affected and maintained their normal relationship with oligodendroglial sheaths. Purkinje axon sprouting was first evident at 4 d and increased further at 7 d after enzyme application. Within 42 d, the expression pattern of chondroitin sulfate proteoglycans gradually recovered, whereas axonal modifications progressively regressed. Our results show that, in the absence of injury or novel external stimuli, degradation of chondroitin sulfate proteoglycans is sufficient to induce Purkinje axon sprouting but not the formation of long-lasting synaptic contacts. Together with other growth-inhibitory molecules, such as myelin-associated proteins, chondroitin sulfate proteoglycans restrict structural plasticity of intact Purkinje axons to maintain normal wiring patterns in the adult cerebellar cortex.

Expression of netrin-1, slit-1 and slit-3 but not of slit-2 after cerebellar and spinal cord lesions

To determine whether members of the Netrin-1 and Slit families and their receptors are expressed after central nervous system (CNS) injury, we performed in situ hybridization for netrin-1, slit-1, 2 and 3, and their receptors (dcc, unc5h-1, 2 and 3, robo-1, 2 and 3) 8 days, 2-3 months and 12-18 months after traumatic lesions of rat cerebellum. The expression pattern of these molecules was unchanged in axotomized Purkinje cells, whereas unc5h3 expression was upregulated in deafferented granule cells. Cells expressing slit-2 or dcc were never detected at the lesion site. By contrast, cells expressing netrin-1, slit-1 and slit-3, unc5h-1, 2 and 3, and robo-1, 2 and 3 (rig-1) could be detected at the cerebellar lesion site as soon as 8 days after injury. Expression of unc5h-2, robo-1, robo-2, slit-1 and slit-3 at the lesion site was maintained until 3 months, and up to 12-18 months for unc5h-1 and 3 and robo-3. Likewise, in the mouse spinal cord, netrin-1, slit-1 and slit-3 were also expressed at the lesion site 8 days after injury. Most of the cells expressing these mRNAs were located at the centre of the lesions, suggesting that they are macrophages/activated microglial cells (macrophagic cells) or meningeal fibroblastic cells. The macrophagic nature of most Netrin-1-positive cells and the macrophagic or fibroblastic nature of Robo-1-positive cells were corroborated by double staining. Thus, Netrin-1, Slits and their receptors may contribute to the regenerative failure of axons in the adult CNS by inhibiting axon outgrowth or by participating in the formation of the CNS scar.

Growth-associated protein GAP-43 and L1 act synergistically to promote regenerative growth of Purkinje cell axons in vivo

Neuronal expression of growth-associated protein 43 (GAP-43) and the cell adhesion molecule L1 has been correlated with CNS axonal growth and regeneration, but it is not known whether expression of these molecules is necessary for axonal regeneration to occur. We have taken advantage of the fact that Purkinje cells do not express GAP-43 or L1 in adult mammals or regenerate axons into peripheral nerve grafts to test the importance of these molecules for axonal regeneration in vivo. Transgenic mice were generated in which Purkinje cells constitutively express L1 or both L1 and GAP-43 under the Purkinje cell-specific L7 promoter, and regeneration of Purkinje cell axons into peripheral nerve grafts implanted into the cerebellum was examined. Purkinje cells expressing GAP-43 or L1 showed minor enhancement of axonal sprouting. Purkinje cells expressing both GAP-43 and L1 showed more extensive axonal sprouting and axonal growth into the proximal portion of the graft. When a predegenerated nerve graft was implanted into double-transgenic mice, penetration of the graft by Purkinje cell axonal sprouts was strongly enhanced, and some axons grew along the entire intracerebral length of the graft (2.5-3.0 mm) and persisted for several months. The results demonstrate that GAP-43 and L1 coexpressed in Purkinje cells can act synergistically to switch these regeneration-incompetent CNS neurons into a regeneration-competent phenotype and show that coexpression of these molecules is a key regulator of the regenerative ability of intrinsic CNS neurons in vivo.

Cell-autonomous death of cerebellar purkinje neurons with autophagy in Niemann-Pick type C disease

Niemann-Pick type C is a neurodegenerative lysosomal storage disorder caused by mutations in either of two genes, npc1 and npc2. Cells lacking Npc1, which is a transmembrane protein related to the Hedgehog receptor Patched, or Npc2, which is a secreted cholesterol-binding protein, have aberrant organelle trafficking and accumulate large quantities of cholesterol and other lipids. Though the Npc proteins are produced by all cells, cerebellar Purkinje neurons are especially sensitive to loss of Npc function. Since Niemann-Pick type C disease involves circulating molecules such as sterols and steroids and a robust inflammatory response within the brain parenchyma, it is crucial to determine whether external factors affect the survival of Purkinje cells (PCs). We investigated the basis of neurodegeneration in chimeric mice that have functional npc1 in only some cells. Death of mutant npc1 cells was not prevented by neighboring wild-type cells, and wild-type PCs were not poisoned by surrounding mutant npc1 cells. PCs undergoing cell-autonomous degeneration have features consistent with autophagic cell death. Chimeric mice exhibited a remarkable delay and reduction of wasting and ataxia despite their substantial amount of mutant tissue and dying cells, revealing a robust mechanism that partially compensates for massive PC death.

Niemann-Pick disease type C is a deadly neurodegenerative disease that is most often due to mutations in a gene called npc1. As a consequence of intracellular lipid trafficking defects, patients with Niemann-Pick type C, and mice with the same disease, lose an important class of cerebellar neurons called Purkinje cells (PCs). Npc1 (the protein coded by npc1) might be needed in other cell types to produce substances that nourish PCs or within the PCs themselves. To see which is true, the researchers constructed genetically mosaic mice in which some cells have mutant Npc1 and some have normal Npc1 function. In the cerebella of these mosaic mice, PCs lacking Npc1 continued to die even while surrounded by normal cells, while normal PCs appeared unaffected by their partially mutant surroundings. From these findings, the researchers concluded that the neurodegeneration is due to a problem within PCs and not due to a lack of supporting factors provided by other cells or an extrinsic toxic or inflammatory insult. Npc1 probably functions within PCs to allow critical transport processes necessary for cell survival. The researchers also found that the degenerating PCs undergo a complex process called autophagy in which the cells sense a lack of key nutrients and start to break down their own structures to feed themselves. By identifying exactly which cells require Npc1 function, the researchers set the stage for investigating the exact molecular roles of Npc1 protein in the cells where it is most needed.

Effects of monocular enucleation on calbindin-D 28k and c-Fos expression in the lateral geniculate nucleus in rats

The present study was undertaken to evaluate the effects of monocular enucleation on the calbindin-D 28k (CB) and c-Fos immunoreactive (IR) neurons in the lateral geniculate nucleus (LGN) complex of adult rats. The enucleation resulted in neuronal degeneration and decrease of neurons in the LGN complex. Our study demonstrated a decrease of CB-IR neuronal density on the contralateral side of the ventral (vLGN) and dorsal LGN (dLGN) until 12 weeks post-enucleation (PE). On the ipsilateral side, CB-IR neuronal density in the dLGN and vLGN showed significant and continuous decrease until 48 and 12 weeks PE, respectively. In an additional experiment, c-Fos-IR neurons were increased at 2 days PE in the vLGN with ipsilateral predominance. At 7 days, c-Fos-IR neurons on the ipsilateral vLGN were still higher than those of pre-enucleated rats. The present results suggest that monocular enucleation affects the expression of the CB and c-Fos in the LGN complex. It is indicated that these may play an important role in the neuronal degeneration and neuroplasticity of the subvisual system in rats.

Cell death and axon regeneration of Purkinje cells after axotomy: challenges of classical hypotheses of axon regeneration

Although adult mammalian neurons are able to regenerate their axons in the peripheral nervous system under certain conditions, they are not able to do it in the central nervous system. The environment surrounding the severed axons appears to be a key factor for axon regeneration. Many studies aiming to enhance axon regeneration in the CNS of adult mammals have successfully manipulated this environment by adding growth permissive molecules and/or neutralizing growth inhibitory molecules. In both cases, the number of axons able to regenerate was low and the different neuronal populations were not equal in their regenerative response, suggesting that manipulation of the environment is not always sufficient. This is particularly well illustrated in the cerebellar system, in which axotomized inferior olivary neurons regenerate when confronted with a permissive environment, whereas mature Purkinje cells do not. The intrinsic ability of a neuron to regenerate its axon is generally correlated with the intensity of its reaction to axotomy (expression of molecules, probability to die). Furthermore, molecules such as GAP-43 (growth-associated molecule) and c-Jun are involved in both axon regeneration and cell death suggesting that these two processes are linked. Surprisingly, Purkinje cells lose their capacity to regenerate their axon (even in the absence of myelin) during development before losing their capacity to react to an axotomy by cell death. These results emphasize the different reactions to axotomy between neuron types and underline that in Purkinje cells, the two cell decisions (axon regeneration and cell death) are differently regulated and therefore not part of the same signaling pathway.

Cell death or survival: Molecular and connectional conditions for olivocochlear neurons after axotomy

We aimed to determine whether rat olivocochlear neurons survive axotomy inflicted through cochlear ablation, or if they degenerate. To estimate their intrinsic potential for axonal regeneration, we investigated the expression of the transcription factor c-Jun and the growth-associated protein-43 (GAP43). Axonal tracing studies based on application of Fast Blue into the cochlea and calcitonin gene-related peptide immunostaining revealed that many, but not all, lateral olivocochlear neurons in the ipsilateral lateral superior olive degenerated upon cochleotomy. A decrease of their number was noticed 2 weeks after the lesion, and 2 months postoperative the population was reduced to approximately one quarter (27-29%) of its original size. No further reduction took place at longer survival times up to 1 year. Most or all shell neurons and medial olivocochlear neurons survived axotomy. Following cochleotomy, 56-60% of the lateral olivocochlear neurons in the ipsilateral lateral superior olive were found to co-express c-Jun and GAP43. Only a small number of shell and medial olivocochlear neurons up-regulated c-Jun expression, and only a small number of shell neurons expressed GAP43. Up-regulation of c-Jun and GAP43 in lateral olivocochlear neurons upon axotomy suggests that they have an intrinsic potential to regenerate after axotomy, but cell counts based on the markers Fast Blue and calcitonin gene-related peptide indicate that this potential cannot be exploited and degeneration is induced instead. The survival of one quarter of the axotomized lateral olivocochlear neurons and of all, or almost all, shell and medial olivocochlear neurons appeared to depend on connections of these cells to other regions than the cochlea by means of axon collaterals, which remained intact after cochleotomy.

Cerebellar grafting in the oculomotor system as a model to study target influence on adult neurons

In the last decades, there have been many efforts directed to gain a better understanding on adult neuron-target cell relationships. Embryonic grafts have been used for the study of neural circuit rewiring. Thus, using several donor neuronal tissues, such as cerebellum or striatum, developing grafted cells have been shown to have the capability of substituting neural cell populations and establishing reciprocal connections with the host. In addition, different lesion paradigms have also led to a better understanding of target dependence in neuronal cells. Thus, for example, axotomy induces profound morphofunctional changes in adult neurons, including the loss of synaptic inputs and discharge alterations. These alterations are probably due to trophic factor loss in response to target disconnection. In this review, we summarize the different strategies performed to disconnect neurons from their targets, and the effects of target substitution, performed by tissue grafting, upon neural properties. Using the oculomotor system-and more precisely the abducens internuclear neurons-as a model, we describe herein the effects of disconnecting a population of central neurons from its natural target (i.e., the medial rectus motoneurons at the mesencephalic oculomotor nucleus). We also analyze target-derived influences in the structure and physiology of these neurons by using cerebellar embryonic grafts as a new target for the axotomized abducens internuclear neurons.

GAP-43 overexpression in adult mouse Purkinje cells overrides myelin-derived inhibition of neurite growth

Up-regulation of growth-associated proteins in adult neurons promotes axon regeneration and neuritic elongation onto nonpermissive substrates. To investigate the interaction between these molecules and myelin-related inhibitory factors, we examined transgenic mice in which overexpression of the growth-associated protein GAP-43 is driven by the Purkinje cell-specific promoter L7. Contrary to their wild-type counterparts, which have extremely poor regenerative capabilities, axotomized transgenic Purkinje cells exhibit profuse sprouting along the intracortical neurite and at the severed stump [Buffo et al. (1997) J. Neurosci., 17, 8778-8791]. Here, we investigated the relationship between such sprouting axons and oligodendroglia to ask whether GAP-43 overexpression enables Purkinje neurites to overcome myelin-derived inhibition. Intact transgenic Purkinje axons display normal morphology and myelination. Following injury, however, many GAP-43-overexpressing neurite stumps are devoid of myelin cover and sprout into white matter regions containing densely packed myelin and Nogo-A- or MAG-immunopositive oligodendrocytes. The intracortical segments of these neurites show focal accumulations of GAP-43, which are associated with disrupted or retracted myelin sheaths. Numerous sprouts originate from such demyelinated segments and spread into the granular layer. Some myelin loss, though not axon sprouting, is also evident in wild-type mice, but this phenomenon is definitely more rapid and extensive in transgenic cerebella. Thus, GAP-43-overexpressing Purkinje axons are endowed with enhanced capabilities for growing into nonpermissive territories and show a pronounced tendency to lose myelin. Our observations suggest that accumulation of GAP-43 along precise axon segments disrupts the normal axon-glia interaction and enhances the retraction of oligodendrocytic processes to facilitate the outgrowth of neuritic sprouts.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

Cerebellar Purkinje cell loss in aging Hu-Bcl-2 transgenic mice

The number of cerebellar Purkinje cells is increased by over 40% in young transgenic mice that overexpress a human Bcl-2 transgene (Hu-Bcl-2). To determine whether the Bcl-2-mediated rescue of Purkinje cells persists through life, the numbers of Purkinje cells were estimated in 6-, 12-, 18-, and 24-month-old Hu-Bcl-2 transgenic mice and age-matched controls. In addition, the expression of four markers for Purkinje cell differentiation, calbindin (CaBP), the 67-kDa isoform of glutamic acid decarboxylase (GAD67), gamma-aminobutyric acid transaminase (GABA-T), and the NMDA-R1 receptor subtype (NMDA-NR1) was analyzed in 6-month-old Hu-Bcl-2 transgenics and controls to determine whether overexpression of Bcl-2 and rescue from naturally occurring cell death affects the normal differentiation of Purkinje cells. The estimates of Purkinje cell numbers showed that the number of Purkinje cells in the Hu-Bcl-2 transgenics declines after 6 months to approach wild-type values by 18 months. Although the exogenous human BCL-2 is still expressed in Purkinje cells at 24 months, the expression levels of human BCL-2 appear to decline significantly after 6 months, suggesting that survival of the supernumary Purkinje cells depends on the sustained overexpression of Bcl-2. All the Purkinje cells in the Hu-Bcl-2 transgenic mice appeared to express normal levels of the differentiation markers analyzed so there was no evidence for a class of Purkinje cells that do not differentiate normally when rescued from naturally occurring cell death.

Axonal sprouting in the optic nerve is not a prerequisite for successful regeneration

Axonal sprouting, the production of axons additional to the parent one, occurs during optic nerve regeneration in goldfish and the frog Rana pipiens, with numbers of regenerate axons exceeding normal values four- to sixfold (Murray [1982] J. Comp. Neurol. 209:352-362; Stelzner and Strauss [1986] J. Comp. Neurol. 245:83-103). To determine whether axonal sprouting is a prerequisite for regeneration, the frog Litoria moorei was examined, a species that undergoes successful optic nerve regeneration but with a different time course compared with R. pipiens. Sprouting was assessed, as in goldfish and R. pipiens, from electron microscopic counts between the lesion and chiasm. However, disconnected axons that persist after axotomy would have falsely elevated the counts. The suspected overlap of these two axon populations was confirmed by labeling regenerate axons anterogradely with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) and disconnected ones retrogradely with DiA (4-4-dihexadecylaminostyrl 1-N methylpyridinium iodide). Numbers of disconnected axons were estimated after preventing regeneration and subtracted from numbers in regenerate nerves. Throughout, the total number of regenerate axons was approximately one third lower than normal (P < 0.05) supporting a previous finding of minimal axonal sprouting in L. moorei (Dunlop et al. [2002] J. Comp. Neurol. 446:276-287). The validity of the subtractive electron microscopic method was confirmed by retrograde labeling to estimate numbers of retinal ganglion cells whose axons had crossed the lesion; values were approximately one third lower than normal. The data suggest that sprouting is not essential for either axon outgrowth or topographic map refinement.

Extrinsic regulation of injury/growth-related gene expression in the inferior olive of the adult rat

Successful axon regeneration relies on the capability of the lesioned neurons to up-regulate a specific set of injury/growth-associated genes. In the adult central nervous system, the strength of the cell body response is generally related to the distance of the injury site from the perikaryon, being stronger for proximal lesions. Nevertheless, inferior olive (IO) cells react to injury and regenerate their axons even after distal transections. To investigate the mechanisms that regulate the IO growth properties, we examined the expression of injury/growth markers (nitric oxide synthase, growth-associated protein 43 and c-Jun) after target deletion or axotomy performed at different sites along the olivocerebellar pathway. Both axon injury and target loss disclose two subsets of IO neurons distributed within precise subnuclei: one subset up-regulates all markers in all conditions, whereas the other shows a mild c-Jun expression but remains unresponsive even after a very proximal axotomy. These observations indicate that distinct subpopulations of IO cells respond to different regulatory strategies. Unresponsive neurons appear insensitive to environmental positive or negative cues, suggesting that they are intrinsically unable to set up a cellular reaction to injury. In contrast, cell body changes in reactive neurons are elicited after the removal of retrogradely transported target-derived inhibitory signals. Target loss also induces degeneration of IO cells, whose survival remains partially dependent on Purkinje targets in adulthood. Thus, the intrinsic regenerative potential of a functionally homogeneous population is regulated by multiple mechanisms, specific for distinct neuronal subsets.

Cell-autonomous mechanisms and myelin-associated factors contribute to the development of Purkinje axon intracortical plexus in the rat cerebellum

The highly specific connection patterns of the mature CNS are shaped through finely regulated processes of axon growth and retraction. To investigate the relative contribution of cell-autonomous mechanisms and extrinsic cues in these events, we examined the development of Purkinje axon intracortical plexus in the rat cerebellum. During the first postnatal week, several new processes sprout from focal swellings along the initial portion of the Purkinje neurite and spread in the granular layer. Intense structural plasticity occurs during the following week, with pruning of collateral branches and remodeling of terminal arbors. The mature distribution of the Purkinje infraganglionic plexus, confined within the most superficial portion of the granular layer, is attained at approximately postnatal day 15. A similar neuritic branching pattern is also developed by Purkinje cells grown in cultures of dissociated cerebellar cells or transplanted to extracerebellar CNS regions, suggesting that cell-autonomous mechanisms contribute to determining the Purkinje axon phenotype. The structural remodeling of Purkinje intracortical plexus is concomitant with the development of cerebellar myelin. To ask whether myelin-associated factors contribute to the morphological maturation of Purkinje neurites, we prevented normal myelinogenesis by killing oligodendrocyte precursors with 5'-azacytidine or by applying neutralizing antibodies against the myelin-associated neurite growth inhibitor Nogo-A. In both conditions, Purkinje axons retained exuberant branches, and the terminal plexus spanned the entire extent of the granular layer. Thus, the formation of Purkinje axon collaterals is, in part, controlled by intrinsic determinants, but their growth and distribution are regulated by environmental signals, among which are myelin-derived cues.

Long-term injured purkinje cells are competent for terminal arbor growth, but remain unable to sustain stem axon regeneration

Long-distance axon regeneration requires the activation of a specific set of neuronal growth-associated genes. Adult Purkinje cells fail to upregulate these molecules in response to axotomy and show extremely weak regenerative properties. Nevertheless, starting from several months after injury, transected Purkinje axons undergo spontaneous sprouting. Here, we asked whether long-term injured Purkinje cells acquire novel intrinsic growth properties that enable them to upregulate growth-associated genes and sustain axon regeneration. To test this hypothesis, we examined axon growth and cell body changes in adult rat Purkinje neurons following axotomy and implantation of embryonic neocortical tissue or Schwann cells into the injury track. Purkinje cells that survived over 6 months after injury/transplantation displayed profuse sprouting in the injured cerebellum and developed extensive networks of terminal branches into embryonic neocortical grafts. In addition, severed Purkinje axons exposed to these transplants 6 months after injury grew faster than their counterparts confronted with the same environment immediately after axotomy. Nevertheless, long-term injured Purkinje cells failed to regenerate stem neurites into Schwann cell grafts, and, under all experimental conditions, they did not upregulate growth-associated molecules, including c-Jun, GAP-43, SNAP-25, and NADPH-diaphorase. These results indicate that the long-term injured Purkinje cells remain unable to activate the gene program required to sustain axon regeneration and their plasticity is restricted to terminal arbor remodeling. We propose that the delayed growth of injured Purkinje cells reflects an adaptive phenomenon by which the severed axon stump develops a new terminal arbor searching for alternative connections with local partners.

Regenerative and survival capabilities of Purkinje cells overexpressing c-Jun

Following axotomy, cerebellar Purkinje cells (PCs) do not elongate their axons, even in a favourable environment, and are resistant to death. They have no constitutive presence of common growth-associated proteins, such as GAP-43 and c-Jun. Previous experiments show that injured transgenic PCs overexpressing GAP-43 exhibit a profuse sprouting along the axon and at its severed end. Nevertheless, the lesioned axons are unable to regenerate either spontaneously or into growth-permissive environments. In addition, a considerable number of GAP-43 transgenic PCs degenerate after injury. c-Jun is an inducible transcription factor expressed in axotomized central neurons and regenerating peripheral neurons. It also contributes to programmed cell death during development. To test whether c-Jun could modify the response of PCs to axotomy or enhance the growth/death phenomena of GAP-43 Purkinje neurons, we generated transgenic mice overexpressing c-Jun in PCs. However, c-Jun upregulation did not affect the adult intact phenotype of these neurons and their regenerative and survival capabilities after axotomy. Also in the cross-bred GAP-43/c-Jun mice, c-Jun did not modify the response of GAP-43 PCs to axotomy. By contrast, in organotypic cultures of cerebellum taken from 9-day-old-pups, the survival capabilities of PCs overexpressing c-Jun decreased, in association with a consistent c-Jun phosphorylation. On the whole our data show that c-Jun alone is unable to trigger regenerative or degenerative phenomena in PCs and suggest that the cellular action of this early gene in developing and mature neurons strongly depends on interplaying intracellular signals.