Implication of Bcl-2 and Caspase-3 in age-related Purkinje cell death in murine organotypic culture: an in vitro model to study apoptosis

HSV-1 cellular model reveals links between aggresome formation and early step of Alzheimer's disease

Many studies highlight the potential link between the chronic degenerative Alzheimer's disease and the infection by the herpes simplex virus type-1 (HSV-1). However, the molecular mechanisms making possible this HSV-1-dependent process remain to be understood. Using neuronal cells expressing the wild type form of amyloid precursor protein (APP) infected by HSV-1, we characterized a representative cellular model of the early stage of the sporadic form of the disease and unraveled a molecular mechanism sustaining this HSV-1- Alzheimer's disease interplay. Here, we show that HSV-1 induces caspase-dependent production of the 42 amino-acid long amyloid peptide (Aβ42) oligomers followed by their accumulation in neuronal cells. Aβ42 oligomers and activated caspase 3 (casp3A) concentrate into intracytoplasmic structures observed in Alzheimer's disease neuronal cells called aggresomes. This casp3A accumulation in aggresomes during HSV-1 infection limits the execution of apoptosis until its term, similarly to an abortosis-like event occurring in Alzheimer's disease neuronal cells patients. Indeed, this particular HSV-1 driven cellular context, representative of early stages of the disease, sustains a failed apoptosis mechanism that could explain the chronic amplification of Aβ42 production characteristic of Alzheimer's disease patients. Finally, we show that combination of flurbiprofen, a non-steroidal anti-inflammatory drug (NSAID), with caspase inhibitor reduced drastically HSV-1-induced Aβ42 oligomers production. This provided mechanistic insights supporting the conclusion of clinical trials showing that NSAIDs reduced Alzheimer's disease incidence in early stage of the disease. Therefore, from our study we propose that caspase-dependent production of Aβ42 oligomers together with the abortosis-like event represents a vicious circle in early Alzheimer's disease stages leading to a chronic amplification of Aβ42 oligomers that contributes to the establishment of degenerative disorder like Alzheimer's disease in patients infected by HSV-1. Interestingly this process could be targeted by an association of NSAID with caspase inhibitors.

Co-culture of exogenous oligodendrocytes with unmyelinated cerebella: Revisiting ex vivo models and new tools to study myelination

Common in vitro models used to study the mechanisms regulating myelination rely on co-cultures of oligodendrocyte precursor cells (OPCs) and neurons. In such models, myelination occurs in an environment that does not fully reflect cell-cell interactions and environmental cues present in vivo. To avoid these limitations while specifically manipulating oligodendroglial cells, we developed a reliable ex vivo model of myelination by seeding OPCs on cerebellar slices, deprived of their endogenous oligodendrocytes. We showed that exogenous OPCs seeded on unmyelinated cerebella, efficiently differentiate and form compact myelin. Spectral confocal reflectance microscopy and electron microscopy analysis revealed that the density of compacted myelin sheaths highly increases all along the culture. Importantly, we defined the appropriate culture time frame to study OPC differentiation and myelination, using accurate quantification resources we generated. Thus, this model is a powerful tool to study the cellular and molecular mechanisms of OPC differentiation and myelination. Moreover, it is suitable for the development and validation of new therapies for myelin-related disorders such as multiple sclerosis and psychiatric diseases.

Utilizing an Animal Model to Identify Brain Neurodegeneration-Related Biomarkers in Aging

Glycine N-methyltransferase (GNMT) regulates S-adenosylmethionine (SAMe), a methyl donor in methylation. Over-expressed SAMe may cause neurogenic capacity reduction and memory impairment. GNMT knockout mice (GNMT-KO) was applied as an experimental model to evaluate its effect on neurons. In this study, proteins from brain tissues were studied using proteomic approaches, Haemotoxylin and Eosin staining, immunohistochemistry, Western blotting, and ingenuity pathway analysis. The expression of Receptor-interacting protein 1(RIPK1) and Caspase 3 were up-regulated and activity-dependent neuroprotective protein (ADNP) was down-regulated in GNMT-KO mice regardless of the age. Besides, proteins related to neuropathology, such as excitatory amino acid transporter 2, calcium/calmodulin-dependent protein kinase type II subunit alpha, and Cu-Zn superoxide dismutase were found only in the group of aged wild-type mice; 4-aminobutyrate amino transferase, limbic system-associated membrane protein, sodium- and chloride-dependent GABA transporter 3 and ProSAAS were found only in the group of young GNMT-KO mice and are related to function of neurons; serum albumin and Rho GDP dissociation inhibitor 1 were found only in the group of aged GNMT-KO mice and are connected to neurodegenerative disorders. With proteomic analyses, a pathway involving Gonadotropin-releasing hormone (GnRH) signal was found to be associated with aging. The GnRH pathway could provide additional information on the mechanism of aging and non-aging related neurodegeneration, and these protein markers may be served in developing future therapeutic treatments to ameliorate aging and prevent diseases.

Roles of Progesterone, Testosterone and Their Nuclear Receptors in Central Nervous System Myelination and Remyelination

Progesterone and testosterone, beyond their roles as sex hormones, are neuroactive steroids, playing crucial regulatory functions within the nervous system. Among these, neuroprotection and myelin regeneration are important ones. The present review aims to discuss the stimulatory effects of progesterone and testosterone on the process of myelination and remyelination. These effects have been demonstrated in vitro (i.e., organotypic cultures) and in vivo (cuprizone- or lysolecithin-induced demyelination and experimental autoimmune encephalomyelitis (EAE)). Both steroids stimulate myelin formation and regeneration by acting through their respective intracellular receptors: progesterone receptors (PR) and androgen receptors (AR). Activation of these receptors results in multiple events involving direct transcription and translation, regulating general homeostasis, cell proliferation, differentiation, growth and myelination. It also ameliorates immune response as seen in the EAE model, resulting in a significant decrease in inflammation leading to a fast recovery. Although natural progesterone and testosterone have a therapeutic potential, their synthetic derivatives—the 19-norprogesterone (nestorone) and 7α-methyl-nortestosterone (MENT), already used as hormonal contraception or in postmenopausal hormone replacement therapies, may offer enhanced benefits for myelin repair. We summarize here a recent advancement in the field of myelin biology, to treat demyelinating disorders using the natural as well as synthetic analogs of progesterone and testosterone.

Secretin Prevents Apoptosis in the Developing Cerebellum Through Bcl-2 and Bcl-xL

Secretin (SCT) is involved in a variety of physiological processes and has been implicated in preventing apoptosis during brain development. However, little is known about the molecular mechanism underlying its neuroprotective effects. The B cell lymphoma 2 (Bcl-2) family proteins, such as Bcl-2 and Bcl-xL, determine the commitment of neurons to apoptosis. In SCT knockout mice, we found reduced transcript levels of anti-apoptotic genes Bcl-2 and Bcl-xL, but not of pro-apoptotic gene Bax, in the developing cerebellum. SCT treatment on ex vivo cultured cerebellar slices triggered a time-dependent increase of Bcl-2 and Bcl-xL expression. This SCT-induced transcriptional regulation of Bcl-2 and Bcl-xL was dependent on the cyclic AMP (cAMP) response element-binding protein (CREB), which is a key survival factor at the convergence of multiple signaling cascades. We further demonstrated that activation of CREB by SCT was mediated by cAMP/protein kinase A (PKA) and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase 1/2 (ERK1/2) cascades. These findings, collectively, provide an uncharacterized signaling cascade for SCT-mediated neuronal survival, in which SCT promotes the key anti-apoptotic elements Bcl-2 and Bcl-xL in the intrinsic death pathway through PKA- and ERK-regulated CREB phosphorylation.

Sil1-Mutant Mice Elucidate Chaperone Function in Neurological Disorders

Chaperone dysfunction leading to the build-up of misfolded proteins could frequently be linked to clinical manifestations also affecting the nervous system and the skeletal muscle. In addition, increase in chaperone function is beneficial to antagonize protein aggregation and thus represents a promising target for therapeutic concepts for many genetic and acquired chaperonopathies. However, little is known on the precise molecular mechanisms defining the cell and tissue abnormalities in the case of impaired chaperone function as well as on underlying effects in the case of compensatory up-regulation of chaperones. This scarcity of knowledge often arises from a lack of appropriate animal models that mimic closely the human molecular, cellular, and histological characteristics. Here, we introduce the Sil1-mutant woozy mouse as a suitable model to investigate molecular and cellular mechanisms of impaired ER-chaperone function affecting the integrity of nervous system and skeletal muscle. The overlapping clinical findings in man and mouse indicate that woozy is a good copy of a human phenotype called Marinesco-Sjögren syndrome. We confirm the presence of ER-stress and expand the biochemical knowledge of altered nuclear envelope in muscle, a hallmark of SIL1-disease. In addition, our data suggest that impaired excitation-contraction coupling might be part of the SIL1-pathophysiology. Our results moreover indicate that divergent expression of pro- and anti-survival proteins is decisive for Purkinje cell survival. By summarizing the current knowledge of woozy, we focus on the suitability of this animal model to study neuroprotective co-chaperone function and to investigate the involvement of co-chaperones in the predisposition of other disorders such as diabetic neuropathy.

Ephrin-B3 supports glioblastoma growth by inhibiting apoptosis induced by the dependence receptor EphA4

EphA4, an Ephrins tyrosine kinase receptor, behaves as a dependence receptor (DR) by triggering cell apoptosis in the absence of its ligand Ephrin-B3. DRs act as conditional tumor suppressors, engaging cell death based on ligand availability; this mechanism is bypassed by overexpression of DRs ligands in some aggressive cancers. The pair EphA4/Ephrin-B3 favors survival of neuronal progenitors of the brain subventricular zone, an area where glioblastoma multiform (GBM) are thought to originate. Here, we report that Ephrin-B3 is highly expressed in human biopsies and that it inhibits EphA4 pro-apoptotic activity in tumor cells. Angiogenesis is directly correlated with GBM aggressiveness and we demonstrate that Ephrin-B3 also supports the survival of endothelial cells in vitro and in vivo. Lastly, silencing of Ephrin-B3 decreases tumor vascularization and growth in a xenograft mice model. Interference with EphA4/Ephrin-B3 interaction could then be envisaged as a relevant strategy to slow GBM growth by enhancing EphA4-induced cell death.

The cell adhesion molecule CHL1 interacts with patched-1 to regulate apoptosis during postnatal cerebellar development

The immunoglobulin superfamily adhesion molecule close homolog of L1 (CHL1) plays important roles during nervous system development. Here, we identified the hedgehog receptor patched-1 (PTCH1) as a novel CHL1-binding protein and showed that CHL1 interacts with the first extracellular loop of PTCH1 via its extracellular domain. Colocalization and co-immunoprecipitation of CHL1 with PTCH1 suggest an association of CHL1 with this major component of the hedgehog signaling pathway. The trans-interaction of CHL1 with PTCH1 promotes neuronal survival in cultures of dissociated cerebellar granule cells and of organotypic cerebellar slices. An inhibitor of the PTCH1-regulated hedgehog signal transducer, smoothened (SMO), and inhibitors of RhoA and Rho-associated kinase (ROCK) 1 and 2 prevent CHL1-dependent survival of cultured cerebellar granule cells and survival of cerebellar granule and Purkinje cells in organotypic cultures. In histological sections from 10- and 14-day-old CHL1-deficient mice, enhanced apoptosis of granule, but not Purkinje, cells was observed. The results of the present study indicate that CHL1 triggers PTCH1-, SMO-, RhoA- and ROCK-dependent signal transduction pathways to promote neuronal survival after cessation of the major morphogenetic events during mouse cerebellar development.

Exogenous Sonic hedgehog modulates the pool of GABAergic interneurons during cerebellar development

All cerebellar GABAergic interneurons were derived from a common pool of precursor cells residing in the embryonic ventricular zone (VZ) and migrating in the prospective white matter (PWM) after birth, where both intrinsic and extrinsic factors contribute to regulate their amplification. Among the environmental factors, we focused on Sonic hedgehog (Shh), a morphogen well known to regulate neural progenitor cell proliferation. We asked if and how exogenous Shh treatment affects the lineage of cerebellar GABAergic interneurons. To address these issues, exogenous Shh was administered to embryonic and postnatal organotypic slices. We found that Shh is able to expand the pool of interneuron progenitors residing in the embryonic epithelium and in the postnatal PWM. In particular, Shh signalling pathway was highly mitogenic at early developmental stages of interneuron production, whereas its effect decreased after the first postnatal week. Gene expression analysis of sorted cells and in situ hybridization further showed that immature interneurons express both the Shh receptor patched and the Shh target gene Gli1. Thus, within the interneuron lineage, Shh might exert regulatory functions also in postmitotic cells. On the whole, our data enlighten the role of Shh during cerebellar maturation and further broaden our knowledge on the amplification mechanisms of the interneuron progenitor pool.

Enhanced survival of wild-type and Lurcher Purkinje cells in vitro following inhibition of conventional PKCs or stress-activated MAP kinase pathways

Recent studies using both dissociated and organotypic cell cultures have shown that heterozygous Lurcher (Lc/+) Purkinje cells (PCs) grown in vitro share many of the same survival and morphological characteristics as Lc/+ PCs in vivo. We have used this established tissue culture system as a valuable model for studying cell death mechanisms in a relatively simple system where neurodegeneration is induced by a constitutive cation leak mediated by the Lurcher mutation in the δ2 glutamate receptor (GluRδ2). In this study, Ca(++) imaging and immunocytochemistry studies indicate that intracellular levels of Ca(++) are chronically increased in Lc/+ PCs and the concentration and/or distribution of the conventional PKCγ isoform is altered in degenerating Lc/+ PCs. To begin to characterize the molecular mechanisms that regulate Lc/+ PC death, the contributions of conventional PKC pathways and of two MAP kinase family members, JNK and p38, were examined in slice cultures from wild-type and Lc/+ mutant mouse cerebellum. Cerebellar slice cultures from P0 pups were treated with either a conventional PKC inhibitor, a JNK inhibitor, or a p38 inhibitor either from 0 to 14 or 7 to 14 DIV. Treatment with either of the three inhibitors from 0 DIV significantly increased wild type and Lc/+ PC survival through 14 DIV, but only Lc/+ PC survival was significantly increased following treatments from 7 to 14 DIV. The results suggest that multiple PC death pathways are induced by the physical trauma of making organotypic slice cultures, naturally-occurring postnatal cell death, and the GluRδ2 (Lc) mutation.

Sonic Hedgehog promotes tumor cell survival by inhibiting CDON pro-apoptotic activity

The Hedgehog signaling is a determinant pathway for tumor progression. However, while inhibition of the Hedgehog canonical pathway-Patched-Smoothened-Gli-has proved efficient in human tumors with activating mutations in this pathway, recent clinical data have failed to show any benefit in other cancers, even though Sonic Hedgehog (SHH) expression is detected in these cancers. Cell-adhesion molecule-related/down-regulated by Oncogenes (CDON), a positive regulator of skeletal muscle development, was recently identified as a receptor for SHH. We show here that CDON behaves as a SHH dependence receptor: it actively triggers apoptosis in the absence of SHH. The pro-apoptotic activity of unbound CDON requires a proteolytic cleavage in its intracellular domain, allowing the recruitment and activation of caspase-9. We show that by inducing apoptosis in settings of SHH limitation, CDON expression constrains tumor progression, and as such, decreased CDON expression observed in a large fraction of human colorectal cancer is associated in mice with intestinal tumor progression. Reciprocally, we propose that the SHH expression, detected in human cancers and previously considered as a mechanism for activation of the canonical pathway in an autocrine or paracrine manner, actually provides a selective tumor growth advantage by blocking CDON-induced apoptosis. In support of this notion, we present the preclinical demonstration that interference with the SHH-CDON interaction triggers a CDON-dependent apoptosis in vitro and tumor growth inhibition in vivo. The latter observation qualifies CDON as a relevant alternative target for anticancer therapy in SHH-expressing tumors.

Possible role of pineal allopregnanolone in Purkinje cell survival

It is believed that neurosteroids are produced in the brain and other nervous systems. Here, we show that allopregnanolone (ALLO), a neurosteroid, is exceedingly produced in the pineal gland compared with the brain and that pineal ALLO acts on the Purkinje cell, a principal cerebellar neuron, to prevent apoptosis in the juvenile quail. We first demonstrated that the pineal gland is a major organ of neurosteroidogenesis. A series of experiments using molecular and biochemical techniques has further demonstrated that the pineal gland produces a variety of neurosteroids de novo from cholesterol in the juvenile quail. Importantly, ALLO was far more actively produced in the pineal gland than in the brain. Pinealectomy (Px) decreased ALLO concentration in the cerebellum and induced apoptosis of Purkinje cells, whereas administration of ALLO to Px quail chicks prevented apoptosis of Purkinje cells. We further found that Px significantly increased the number of Purkinje cells that expressed active caspase-3, a key protease in apoptotic pathway, and daily injection of ALLO to Px quail chicks decreased the number of Purkinje cells expressing active caspase-3. These results indicate that the neuroprotective effect of pineal ALLO is associated with the decrease in caspase-3 activity during the early stage of neuronal development. We thus provide evidence that the pineal gland is an important neurosteroidogenic organ and that pineal ALLO may be involved in Purkinje cell survival during development. This is an important function of the pineal gland in the formation of neuronal circuits in the developing cerebellum.

Klf9 is necessary and sufficient for Purkinje cell survival in organotypic culture

During their phase of developmental programmed cell death (PCD), neurons depend on target-released trophic factors for survival. After this period, however, they critically change as their survival becomes target-independent. The molecular mechanisms underlying this major transition remain poorly understood. Here, we investigated, which transcription factors (TFs) might be responsible for the closure of PCD. We used Purkinje cells as a model since their PCD is restricted to the first postnatal week in the mouse cerebellum. Transcriptome analysis of Purkinje cells during or after PCD allowed the identification of Krüppel like factor 9 (Klf9) as a candidate for PCD closure, given its high increase of expression at the end of the 1st postnatal week. Klf9 function was tested in organotypic cultures, through lentiviral vector-mediated manipulation of Klf9 expression. In absence of trophic factors, the Purkinje cell survival rate is of 40%. Overexpression of Klf9 during PCD dramatically increases the Purkinje cell survival rate from 40% to 88%, whereas its down-regulation decreases it to 14%. Accordingly, in organotypic cultures of Klf9 knockout animals, Purkinje cell survival rate is reduced by half as compared to wild-type mice. Furthermore, the absence of Klf9 could be rescued by Purkinje cell trophic factors, Insulin growth factor-1 and Neurotrophin3. Altogether, our results ascribe a clear role of Klf9 in Purkinje cell survival. Thus, we propose that Klf9 might be a key molecule involved in turning off the phase of Purkinje PCD.

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.

DCC constrains tumour progression via its dependence receptor activity

The role of deleted in colorectal carcinoma (DCC) as a tumour suppressor has been a matter of debate for the past 15 years. DCC gene expression is lost or markedly reduced in the majority of advanced colorectal cancers and, by functioning as a dependence receptor, DCC has been shown to induce apoptosis unless engaged by its ligand, netrin-1 (ref. 2). However, so far no animal model has supported the view that the DCC loss-of-function is causally implicated as predisposing to aggressive cancer development. To investigate the role of DCC-induced apoptosis in the control of tumour progression, here we created a mouse model in which the pro-apoptotic activity of DCC is genetically silenced. Although the loss of DCC-induced apoptosis in this mouse model is not associated with a major disorganization of the intestines, it leads to spontaneous intestinal neoplasia at a relatively low frequency. Loss of DCC-induced apoptosis is also associated with an increase in the number and aggressiveness of intestinal tumours in a predisposing APC mutant context, resulting in the development of highly invasive adenocarcinomas. These results demonstrate that DCC functions as a tumour suppressor via its ability to trigger tumour cell apoptosis.

Antiapoptotic protein Lifeguard is required for survival and maintenance of Purkinje and granular cells

Lifeguard (LFG) is an inhibitor of Fas-mediated cell death and is highly expressed in the cerebellum. We investigated the biological role of LFG in the cerebellum in vivo, using mice with reduced LFG expression generated by shRNA lentiviral transgenesis (shLFG mice) as well as LFG null mice. We found that LFG plays a role in cerebellar development by affecting cerebellar size, internal granular layer (IGL) thickness, and Purkinje cell (PC) development. All these features are more severe in early developmental stages and show substantial recovery overtime, providing a remarkable example of cerebellar plasticity. In adult mice, LFG plays a role in PC maintenance shown by reduced cellular density and abnormal morphology with increased active caspase 8 and caspase 3 immunostaining in shLFG and knockout (KO) PCs. We studied the mechanism of action of LFG as an inhibitor of the Fas pathway and provided evidence of the neuroprotective role of LFG in cerebellar granule neurons (CGNs) and PCs in an organotypic cerebellar culture system. Biochemical analysis of the Fas pathway revealed that LFG inhibits Fas-mediated cell death by interfering with caspase 8 activation. This result is supported by the increased number of active caspase 8-positive PCs in adult mice lacking LFG. These data demonstrate that LFG is required for proper development and survival of granular and Purkinje cells and suggest LFG may play a role in cerebellar disorders.

Novel protective effect of mifepristone on detrimental GABAA receptor activity to immature Purkinje neurons

Immature Purkinje neurons are particularly vulnerable cells. They survive in cerebellar slice cultures under treatment by the synthetic steroid mifepristone (RU486) that depolarizes them at this age. The present study aims at understanding the mechanism underlying this neuroprotective effect. In the developing cerebellum, the role of γ-aminobutyric acid (GABA) in neuron survival is unknown. In 3-d-old mouse cerebellar slice cultures, we show that GABA(A) receptor activation is depolarizing and excitatory. Antagonists of GABA(A) receptors rescue Purkinje neurons, demonstrating that GABA is endogenously released in this preparation and is toxic. Mifepristone likely protects these neurons by reversing GABA(A) receptor-mediated chloride fluxes and reducing their driving force. Neuroprotection by mifepristone is dose-dependently decreased by the agonist of GABA(A) receptors muscimol and by caffeine, an agonist of internal calcium store release. Moreover, the survival induced by neomycin, an inhibitor of calcium release, is partially reversed by muscimol. The p38 mitogen-activated protein kinase (MAPK) inhibitor SB239063 also rescues Purkinje neurons. In summary, we propose that when GABA is depolarizing, mifepristone protects Purkinje neurons by shunting GABA responses and probably chloride fluxes, by inhibiting p38 MAPK activity and likely internal calcium store release. A new and nonhormonal effect of mifepristone is thus revealed.

Foxp4 is essential in maintenance of Purkinje cell dendritic arborization in the mouse cerebellum

Purkinje cells (PCs) are one of the principal neurons in the cerebellar cortex that play a central role in the coordination of fine-tuning body movement and balance. To acquire normal cerebellum function, PCs develop extensive dendritic arbors that establish synaptic connections with the parallel fibers of granule cells to form the proper neuronal circuitry. Therefore, dendritic arborization of PCs is an important developmental step to construct the mature neural network in the cerebellum. However, the genetic control of this process is not fully understood. In this study, Foxp4, a forkhead transcription factor that is expressed specifically in migrating and mature PCs of cerebellum from embryonic stages to adulthood, was knocked down by small interfering RNA (siRNA) in organotypic cerebellar slice culture. When Foxp4 expression was knocked down at postnatal day 5 (P5), no abnormalities for early dendritic remodeling of PCs were observed. However, when Foxp4 was knocked down in P10 cerebellar slices, the organization of PC dendritic arbors was highly impaired, leaving hypoplastic but non-apoptotic cell bodies. The radial alignment of Bergmann glial fibers that associated with PC dendrites was also lost. These results suggest that Foxp4 is dispensable for the early PC dendrite outgrowth, but is essential for the maintenance of PC dendritic arborization and subsequent association with Bergmann glial fibers.

Neurotrophin-3 production promotes human neuroblastoma cell survival by inhibiting TrkC-induced apoptosis

Tropomyosin-related kinase receptor C (TrkC) is a neurotrophin receptor with tyrosine kinase activity that was expected to be oncogenic. However, it has several characteristics of a tumor suppressor: its expression in tumors has often been associated with good prognosis; and it was recently demonstrated to be a dependence receptor, transducing different positive signals in the presence of ligand but inducing apoptosis in the absence of ligand. Here we show that the TrkC ligand neurotrophin-3 (NT-3) is upregulated in a large fraction of aggressive human neuroblastomas (NBs) and that it blocks TrkC-induced apoptosis of human NB cell lines, consistent with the idea that TrkC is a dependence receptor. Functionally, both siRNA knockdown of NT-3 expression and incubation with a TrkC-specific blocking antibody triggered apoptosis in human NB cell lines. Importantly, disruption of the NT-3 autocrine loop in malignant human neuroblasts triggered in vitro NB cell death and inhibited tumor growth and metastasis in both a chick and a mouse xenograft model. Thus, we believe that our data suggest that NT-3/TrkC disruption is a putative alternative targeted therapeutic strategy for the treatment of NB.

Death and survival of heterozygous Lurcher Purkinje cells in vitro

The differentiation and survival of heterozygous Lurcher (+/Lc) Purkinje cells in vitro was examined as a model system for studying how chronic ionic stress affects neuronal differentiation and survival. The Lurcher mutation in the delta2 glutamate receptor (GluRdelta2) converts an orphan receptor into a membrane channel that constitutively passes an inward cation current. In the GluRdelta2(+/Lc) mutant, Purkinje cell dendritic differentiation is disrupted and the cells degenerate following the first week of postnatal development. To determine if the GluRdelta2(+/Lc) Purkinje cell phenotype is recapitulated in vitro, +/+, and +/Lc Purkinje cells from postnatal Day 0 pups were grown in either isolated cell or cerebellar slice cultures. GluRdelta2(+/+) and GluRdelta2(+/Lc) Purkinje cells appeared to develop normally through the first 7 days in vitro (DIV), but by 11 DIV GluRdelta2(+/Lc) Purkinje cells exhibited a significantly higher cation leak current. By 14 DIV, GluRdelta2(+/Lc) Purkinje cell dendrites were stunted and the number of surviving GluRdelta2(+/Lc) Purkinje cells was reduced by 75% compared to controls. However, treatment of +/Lc cerebellar cultures with 1-naphthyl acetyl spermine increased +/Lc Purkinje cell survival to wild type levels. These results support the conclusion that the Lurcher mutation in GluRdelta2 induces cell autonomous defects in differentiation and survival. The establishment of a tissue culture system for studying cell injury and death mechanisms in a relatively simple system like GluRdelta2(+/Lc) Purkinje cells will provide a valuable model for studying how the induction of a chronic inward cation current in a single cell type affects neuronal differentiation and survival.

Netrin-1 acts as a survival factor for aggressive neuroblastoma

Neuroblastoma (NB), the most frequent solid tumor of early childhood, is diagnosed as a disseminated disease in >60% of cases, and several lines of evidence support the resistance to apoptosis as a prerequisite for NB progression. We show that autocrine production of netrin-1, a multifunctional laminin-related molecule, conveys a selective advantage in tumor growth and dissemination in aggressive NB, as it blocks the proapoptotic activity of the UNC5H netrin-1 dependence receptors. We show that such netrin-1 up-regulation is a potential marker for poor prognosis in stage 4S and, more generally, in NB stage 4 diagnosed infants. Moreover, we propose that interference with the netrin-1 autocrine loop in malignant neuroblasts could represent an alternative therapeutic strategy, as disruption of this loop triggers in vitro NB cell death and inhibits NB metastasis in avian and mouse models.

Interference with netrin-1 and tumor cell death in non-small cell lung cancer

BACKGROUND

Netrin-1 may promote colorectal and breast tumorigenesis, by inhibiting apoptosis induced by its dependence receptors, deleted in colorectal cancer (DCC) and uncoordinated-5-homolog (UNC5H). The status of netrin-1 and its receptors in non-small cell lung cancer (NSCLC) was unknown.

METHODS

The levels of netrin-1 and its receptors were analyzed in a panel of 92 NSCLC and 25 human lung cancer cell lines by quantitative reverse transcription-polymerase chain reaction and immunohistochemistry. In lung cancer cell lines that express netrin-1, the expression of netrin-1 was inhibited by using small interfering RNA (siRNA), or interference with netrin-1 was performed by treatment with a decoy recombinant DCC ectodomain protein (DCC-5Fbn). Cell death was monitored with a trypan blue exclusion assay or by measuring caspase-3 activity. The effect of netrin-1 interference on tumor growth was analyzed by DCC-5Fbn intratumoral or netrin-1 siRNA intraperitoneal injection in mice engrafted with lung cancer cell lines. All statistical tests were two-sided.

RESULTS

High levels of netrin-1 were found in 43 of the 92 NSCLC tumor samples (47%). Interference with netrin-1 in human lung cancer cell lines was associated with UNC5H-mediated cell death in vitro (percentage of cell death in untreated and in DCC-5Fbn-treated cells = 8% and 26%, respectively, difference = 18%, 95% confidence interval [CI] = 10% to 26%; P = .049) and with lung tumor growth inhibition and/or regression in xenografted nude mice (12 mice in DCC-5Fbn-treated group and 13 mice in control group). Mean volume of control and DCC-5Fbn-treated tumors on day 46 was 489 and 84 mm(3), respectively (difference = 404 mm(3), 95% CI = 145 to 664 mm(3); P < .001).

CONCLUSIONS

Almost half of the NSCLC tissue samples examined expressed high levels of netrin-1. Extracellular targeting of the interaction between netrin-1 and UNC5H may be a promising therapeutic approach for NSCLCs that express netrin-1.

Intrinsic versus extrinsic determinants during the development of Purkinje cell dendrites

The peculiar shape and disposition of Purkinje cell (PC) dendrites, planar and highly branched, offers an optimal model to analyze cellular and molecular regulators for the acquisition of neuronal dendritic trees. During the first 2 weeks after the end of the proliferation period, PCs undergo a 2-phase remodeling process of their dendrites. The first phase consists in the complete retraction of the primitive but extensive dendritic tree, together with the formation of multiple filopodia-like processes arising from the cell body. In the second phase, there is a progressive disappearance of the somatic processes along with rapid growth and branching of the mature dendrite. Mature Purkinje cell dendrites bear two types of spiny protrusions, named spine and thorn. The spines are numerous, elongated, located at the distal dendritic compartment and form synapses with parallel fibers, whereas the thorns are shorter, rounded, emerge from the proximal compartment and synapse with climbing fibers. Different culture models and mutant mice analyses suggest the identification of intrinsic versus extrinsic determinants of the Purkinje cell dendritic development. The early phase of dendritic remodeling might be cell autonomous and regulated by specific transcription factors such as retinoid-related orphan receptor alpha (RORalpha). Afferent fibers, trophic factors and hormones regulate the orientation and growth of the mature dendritic tree contributing, with still unknown intrinsic factors, to sculpt its general architecture. The formation of spines appears as an intrinsic phenomenon independent of their presynaptic partner, the parallel fibers, and confined to the distal compartment by inhibitory influences of the climbing fibers along the proximal compartment.

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.

Purkinje cell survival in organotypic cultures: Implication of Rho and its downstream effector ROCK

Organotypic cultures of postnatal day 1 (P1) to P7 mouse cerebella are well-established models for studying cell survival. In the present work, we investigate the involvement of the Rho/ROCK intracellular pathway in Purkinje cell survival by using organotypic cultures of P3 Swiss mice. Specific inhibitors of Rho or ROCK were applied at different concentrations to the slice cultures, which were maintained for 5 days in vitro. We show that the bacterial exoenzyme C3 transferase, a specific inhibitor of the small GTPase Rho, increases Purkinje cell survival. There is a 4.5- and 2.5-fold increase in Purkinje cell survival when C3 intracellular uptake is promoted either by the PEP-1 peptide or by the C2IN carrier protein, respectively, and not with the commonly used TAT peptide. Moreover, treatment with Y27632 and H-1152, two specific inhibitors of the Rho kinase ROCK, also strongly reduces apoptotic cell death and results in 6.5- and 8.5-fold increases in cell survival, respectively. In immunohistochemical analysis, we also show that H-1152 did not change either glial fibrillary acidic protein or isolectin-B4 staining, indicating that this compound did not alter the cellular composition in our cultures. Thus, our data demonstrate that inhibition of Rho and its downstream effector ROCK may be used to enhance cell survival in neurodegenerative diseases.

Cell cycle inhibition and retinoblastoma protein overexpression prevent Purkinje cell death in organotypic slice cultures

Purkinje cells are vulnerable to a number of physical, chemical, and genetic insults during development and maturity. Normal development of these cells depends on the cell-cell interactions between granule and astroglial cell populations. Apoptotic death in Purkinje neurons had been shown to be associated with cell cycle activation, and new DNA synthesis is associated with Purkinje cell death in staggerer and lurcher mutant mice. Here using an in vitro organotypic slice culture model from 9 (P9) and 4 days (P4) old postnatal rats we show that the cyclin dependent kinase (cdk) inhibitors (roscovitine, olomoucine, and flavopiridol) protect the Purkinje cells from cell death. The results are more pronounced in the cerebellar sections from P4 rats. Analysis of Purkinje neurons in sections from P4 rats after 1 week of culturing showed that while there were very limited calbindin positive neurons in the untreated sections the cdk inhibitor treated sections had a notably higher number. Although treatment with cdk inhibitors inhibited Purkinje cell loss significantly, the morphology of these neurons was abnormal, with stunted dendrites and axons. Since the retinoblastoma protein (Rb) is the major pocket protein involved in determining the differentiated state of neurons we examined the effect of over-expressing Rb in the organotypic cultures. Rb overexpression significantly inhibited the Purkinje cell death and these neurons maintained their normal morphology. Thus our studies show that the cell death in Purkinje neurons observed in organotypic cultures is cell cycle dependent and the optimal survival requires Rb.

Developmental changes in Ca2+-regulated functions of early postnatal Purkinje neurons

Ca(2+) influx through L-type Ca(2+) channels regulates several different cellular processes in developing Purkinje neurons, including activation of transcription factors and expression of cellular proteins. In the current studies, we examined the age dependence of these actions of Ca(2+) during the early developmental period. Purkinje neurons acutely isolated from postnatal day 4-8 rat pups were studied. We also examined the sensitivity of the Ca(2+)-regulated processes to a toxic environmental factor (ethanol) known to show age-dependent actions on developing Purkinje neurons. Results show that Ca(2+) activation of the transcription factor cAMP-responsive element binding protein (CREB) and Ca(2+)-induced alterations in the level of the apoptotic enzyme caspase 3 show both dose and age dependence in the early-developing Purkinje neurons. Interestingly, the age dependence was opposite for the two proteins. Ca(2+) regulation of calbindin, a Ca(2+) binding protein, was dose dependent but showed little age dependence. Exposure to ethanol altered Ca(2+) activation of pCREB in an age-dependent manner but did not alter Ca(2+) regulation of caspase 3 or calbindin levels. Taken together, these results show that the downstream effects of Ca(2+) signaling have age-dependent components during early Purkinje neuron development. This age dependence may play an important role in the normal developmental program and could contribute to the critical window of sensitivity observed for certain toxic agents during early development.

RORalpha, a pivotal nuclear receptor for Purkinje neuron survival and differentiation: from development to ageing

RORalpha (Retinoid-related Orphan Receptor) is a transcription factor belonging to the superfamily of nuclear receptors. The spontaneous staggerer (sg) mutation, which consists of a deletion in the Rora gene, has been shown to cause the loss of function of the RORalpha protein. The total loss of RORalpha expression leads to cerebellar developmental defects, particularly to a dramatic decreased survival of Purkinje cells and an early block in the differentiation process. This review focuses on recent studies which position RORalpha as a pivotal factor controlling Purkinje cell survival and differentiation, from development to ageing.

Purkinje cell death: differences between developmental cell death and neurodegenerative death in mutant mice

This review is devoted to Purkinje cell death occurring during development and in spontaneous cerebellar mutations of the mouse. We first present evidence in favor of an apoptotic developmental Purkinje cell death. Then, the different types of Purkinje cell degeneration occurring in mutant mice primarily affecting this neuronal population (nervous, purkinje cell degeneration, Lurcher, toppler, and woozy) are described and discussed. In addition, we show, by reporting new data, that cell death in tambaleante mutant mice can be related to autophagy. Last, we discuss the fact that the cell death pathways in mutant mice are more complex than the three types of developmental death generally described (apoptosis, autophagy, necrosis), since they share often characteristics of more than one type of these developmental cell deaths, particularly autophagy and apoptosis.

Neuroprotective effect of mifepristone involves neuron depolarization

In several regions of the developing nervous system, neurons undergo programmed cell death. In the rat cerebellum, Purkinje cell apoptosis is exacerbated when cerebellar slices are cultured during the first postnatal week. To understand the mechanism of this developmental apoptosis, we took advantage of its inhibition by the steroid analog mifepristone. This effect did not involve the classical steroid nuclear receptors. Microarray analysis revealed that mifepristone down-regulated mRNA levels of the Na+/K+-ATPase alpha3 subunit more than three times. Consistent with the down-regulation of the Na+/K+-ATPase, mifepristone caused Purkinje cell membrane depolarization. Depolarizing agents like ouabain (1 microM), tetraethylammonium (2 mM), and veratridine (2 microM) protected Purkinje cells from apoptosis. These results suggest a role of excitatory inputs in Purkinje cell survival during early postnatal development. Indeed, coculturing cerebellar slices with glutamatergic inferior olivary neuron preparations allowed rescue of Purkinje cells. These findings reveal a new neuroprotective mechanism of mifepristone and support a pivotal role for excitatory inputs in the survival of Purkinje neurons. Mifepristone may be a useful lead compound in the development of novel therapeutic approaches for maintaining the resting potential of neurons at values favorable for their survival under neuropathological conditions.

Robust axonal sprouting and synaptogenesis in organotypic slice cultures of rat cerebellum exposed to increased potassium chloride

Organotypic slices of the rat cerebellum, cultured in physiological levels [K+]o (5 mM) for 14 days, loose the majority of granule cells in the anterior lobe resulting in few axons and atypical Purkinje cell dendrites with vacant spines. When the culture medium was switched from 5 mM to 20, 30 or 40 mM [K+]o during the last 7 days of cultures, slices developed axons with numerous vesicle-filled boutons that made synaptic contact with Purkinje cell spines. Most boutons had one or two spine profile contacts, while some were unusually large. Enlarged boutons abutted Purkinje cell somata or their dendrites, causing intervening spines to invaginate terminals to form rosette synaptic complexes. Calbindin immuno-labeling excluded Purkinje cell axonal collaterals as the source of rosette boutons and suggested a granule cell origin. Quantification of vacant spines as compared to those on boutons revealed a threshold for potassium, between 10 and 20 mM, where the number of synaptic spines increased and vacant spines decreased drastically. These findings suggest that elevated [K+]o triggers an activity-dependent plasticity in rat cerebellar slice cultures by promoting axonal sprouting with formation of vesicle-filled boutons and synaptogenesis on open receptor sites of Purkinje cell spines.

Progesterone increases oligodendroglial cell proliferation in rat cerebellar slice cultures

We have previously demonstrated that progesterone significantly increases the rate of myelination in organotypic slice cultures of 7-day-old rat and mouse cerebellum. Here, we show that progesterone (20microM) stimulates the proliferation of oligodendrocyte precursors in cultured cerebellar slices of 7-day-old rats. The steroid increased the number of pre-oligodendrocytes (NG2(+), O4(+)) and to some extent of oligodendrocyte precursors, corresponding to an earlier developmental stage (nestin(+), PDGFalphaR(+), NG2(+), O4(-)). Progesterone stimulated the proliferation of both NG2(+) and O4(+) cells as shown by increased double-immunolabeling with the cell proliferation marker Ki67. The mitogenic effect of progesterone was inhibited by the progesterone receptor antagonist mifepristone (10microM) and could not be mimicked by its GABA-active metabolite 3alpha,5alpha-tetrahydroprogesterone (allopregnanolone), even at the high concentration of 50microM. Results indicate that progesterone first strongly and transiently stimulates the proliferation of oligodendrocyte precursors, and that it may thereafter accelerate their maturation into myelinating oligodendrocytes. Although oligodendrocyte precursors may be a direct target for the actions of progesterone, their number may also be indirectly influenced by the effects of the steroid on neurons and microglial cells, since treatment of the cerebellar slices with progesterone enhanced staining of the neuronal cytoskeleton marker microtubule-associated protein-2 and increased the number of OX-42(+) microglia. A small percentage (about 0.1%) of the NG2(+) cells transiently became OX-42(+) in response to progesterone. These results point to novel mechanisms by which progesterone may promote myelination in the CNS, specifically by stimulating the proliferation and maturation of oligodendrocyte precursors into myelinating oligodendrocytes.

Developmental shift in the apostat: comparison of neurones and astrocytes

The intrinsic pathway of apoptosis was investigated in cell-free extracts of neurones and astrocytes at various stages of maturation. Neuronal extracts were activated 65-fold after 3 days, 9-fold after 7 days, and were not activated after 10 days in culture. In contrast, astrocyte extracts were activated to a similar extent at all stages, up to 60 days in culture. The co-incubation of neuronal and astrocyte extracts followed by addition of cytochrome c/2'-deoxyadenosine 5'-triphosphate led to a 40-fold activation, suggesting that the development-associated neuronal shift does not involve the appearance of a dominant inhibitor, but rather downregulation of some key component(s) involved in caspase activation.

Naturally occurring neuronal death during the postnatal development of Purkinje cells and their precerebellar afferent projections

Naturally occurring neuronal death plays a substantial developmental role in the building of the neural circuitries. The neuronal death caused by different cerebellar mutations is mostly of an apoptotic nature. Apart from the identity of the intrinsic mechanisms of the mutations, adult cerebellar mutants are a powerful tool to causally study the development of the cerebellar connectivity. Thus, studies on adult cerebellar neuronal cell death occurring in mouse mutants elucidate: (i) the dependence of the postsynaptic neurons on their partners, (ii) the 'en cascade' postsynaptic transneuronal degeneration after target-deprivation, and (iii) the close relationship between the molecular modular organization of the cerebellar cortex and dying Purkinje cells. Neuronal cell death has been extensively studied in developing olivocerebellar system. However, less data are available on the occurrence of naturally occurring neuronal death during the in vivo normal development of the Purkinje cells and the mossy fiber system neurons. The developmental role of neuronal death during the establishment and refinement of the olivocerebellar projection is currently discussed. Moreover, the occurrence of neuronal death during the development of the basilar pontine nuclei and its role in the acquisition of the adult pontocerebellar projection is still poorly understood. In the present review, we correlate the dates of Purkinje cells death with the inferior olivary and basilar pontine neuronal apoptosis, discussing their developmental relationships during the elaboration of the fine-grained maps of the cerebellar afferent connections.

Lack of NMDA receptor subunit exchange alters Purkinje cell dendritic morphology in cerebellar slice cultures

Early postnatal developmental changes in N-methyl-d-aspartate (NMDA) receptor (NR) subunits regulate cerebellar granule cell maturation and potentially Purkinje cell development. We therefore investigated Purkinje cell morphology in slice cultures from mice with genetic subunit exchange from NR2C to NR2B (NR2C-2B). NR2C-2B Purkinje cells after 12 days in vitro showed a significantly impaired dendritic arbour complexity with reduced branching density as compared to wild-type cells, a phenotype that was reversed by NMDA treatment. These data support the concept that in cerebellar slice cultures, Purkinje cell dendritic outgrowth is regulated by granule cell inputs.

Detection of Purkinje cell loss following drug exposures to developing rat pups using reverse transcriptase-polymerase chain reaction (RT-PCR) analysis for calbindin-D28k mRNA expression

A technique is described that allows for the identification and quantification of Purkinje cell loss in cerebellum subsequent to developmental toxic exposures. This technique relies upon the extensively validated findings that the Purkinje cell is the only site of expression in the cerebellum of the calcium binding protein calbindin-D28k. Thus, analysis of mRNA expression specific to this protein by comparison to matched controls provides a reliable means of determining whether cell loss has occurred. Purkinje cell loss was induced in rat pups by ethanol exposure on postnatal day (PN) 4 or valproic acid administration to pregnant dams on gestational day 13. Analysis was conducted on PN5 or PN10 and the results compared to parallel groups of pups where the Purkinje cells were counted by traditional means. When compared to matched control rat pups the decrease in calbindin-D28k mRNA expression indicates Purkinje cell loss regardless of whether the cell loss was induced by prenatal valproic acid or postnatal ethanol exposure. The availability of a biochemical alternative to histological cell counting allows for more detailed analyses of the mechanisms of Purkinje cell death induced by these two toxicants, including analyses of the early alterations in signal transduction proteins.

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.

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.

The kiss of death

The programmed cell death (PCD) of neurons is generally thought to be cell autonomous and not to require a death signal from other cells. A recent study by Marín-Teva et al., in this issue of Neuron, brings this theory into question and suggests that neighboring microglia actively participate in the PCD of Purkinje cells in the cerebellum.

Microglia Promote the Death of Developing Purkinje Cells

The loss of neuronal cells, a prominent event in the development of the nervous system, involves regulated triggering of programmed cell death, followed by efficient removal of cell corpses. Professional phagocytes, such as microglia, contribute to the elimination of dead cells. Here we provide evidence that, in addition to their phagocytic activity, microglia promote the death of developing neurons engaged in synaptogenesis. In the developing mouse cerebellum, Purkinje cells die, and 60% of these neurons that already expressed activated caspase-3 were engulfed or contacted by spreading processes emitted by microglial cells. Apoptosis of Purkinje cells in cerebellar slices was strongly reduced by selective elimination of microglia. Superoxide ions produced by microglial respiratory bursts played a major role in this Purkinje cell death. Our study illustrates a mammalian form of engulfment-promoted cell death that links the execution of neuron death to the scavenging of dead cells.

Patterned Purkinje cell death in the cerebellum

The object of this review is to assemble much of the literature concerning Purkinje cell death in cerebellar pathology and to relate this to what is now known about the complex topography of the cerebellar cortex. A brief introduction to Purkinje cells, and their regionalization is provided, and then the data on Purkinje cell death in mouse models and, where appropriate, their human counterparts, have been arranged according to several broad categories--naturally-occurring and targeted mutations leading to Purkinje cell death, Purkinje cell death due to toxins, Purkinje cell death in ischemia, Purkinje cell death in infection and in inherited disorders, etc. The data reveal that cerebellar Purkinje cell death is much more topographically complex than is usually appreciated.

Engrailed-2 negatively regulates the onset of perinatal Purkinje cell differentiation

The transcription factor Engrailed-2 is expressed in cerebellar Purkinje cells (PCs) throughout embryonic development but is downregulated in PCs after birth. Since the onset of PC differentiation coincides with this change of gene expression, we asked whether downregulation of Engrailed-2 is necessary for proper timing of PC differentiation. To investigate this, we used an L7En-2 transgenic mouse model in which Engrailed-2 expression in PCs is maintained beyond the day of birth. In these L7En-2 mice the onset of parvalbumin expression was delayed in all PCs by about 3 days; the spatial expression pattern, however, remained comparable to wildtype cerebella. Furthermore, parvalbumin expression resembled the known pattern of normal PC maturation, suggesting a direct link between parvalbumin expression and PC differentiation. Consistent with a delay of PC differentiation, we found that PCs of L7En-2 cerebella displayed a reduced tendency to align in the typical monolayer. The average size of L7En-2 PCs was reduced and the dendritic arbor developed more slowly than in wildtype PCs. In contrast, major morphological features of PCs were comparable in L7En-2 and wildtype cerebella after postnatal day 11. In addition, we observed a transient reduction of PC survival in organotypic slice cultures of L7En-2 cerebella in comparison with wildtype slice cultures. Since PC survival parallels PC differentiation in vitro, we propose that the observed delay in PC differentiation upon Engrailed-2 overexpression is an intrinsic property of Engrailed-2 activity, and that downregulation of Engrailed-2 in wildtype PCs around the day of birth is critical for the timing of distinct steps of PC differentiation.