Overexpression of a Hu-bcl-2 transgene in Lurcher mutant mice delays Purkinje cell death

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.

Mechanisms of compartmental purkinje cell death and survival in the lurcher mutant mouse

The Lurcher mutant mouse is characterized by its ataxic gait and loss of cerebellar Purkinje cells and their afferents, granule cells and olivary neurons, during the first weeks of postnatal development. For the 50 years since its discovery, the heterozygous Lurcher mutant has served as an important model system for studying neuron-target interactions in the developing cerebellum and cerebellar function. The identification of the Lurcher (Lc) gene over 10 years ago as a gain-of-function mutation in the δ2 glutamate receptor (GluRδ2) led to extensive studies of cell death mechanisms in the Lc/+ cerebellum. The advantage of this model system is that GluRδ2(+) receptors and GluRδ2(Lc) channels are expressed predominantly in Purkinje cells, making it possible to study the effects of a well-characterized leak current in a well-defined cell type during a critical phase of neuronal development. Yet there is still controversy surrounding the mechanisms of neuronal death in Lc/+ Purkinje cells with competing hypotheses for necrotic, apoptotic, and autophagic cell death pathways as a consequence of the excitotoxic stress caused by the GluRδ2(Lc) leak current. The goal of this review is to summarize recent studies that critically test the role of various cell death pathways in Lc/+ Purkinje cell degeneration with respect to evidence for the molecular heterogeneity of Purkinje cells. We propose that the expression of putative survival factors, such as heat shock proteins, in a subset of cerebellar Purkinje cells may affect cell death pathways and account for the pattern and diverse mechanisms of Lc/+ Purkinje degeneration.

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.

Hrk/DP5 contributes to the apoptosis of select neuronal populations but is dispensable for haematopoietic cell apoptosis

The pro-apoptotic BH3-only members of the Bcl2 family, crucial initiators of cell death, are activated by a diverse array of developmental cues or experimentally applied stress stimuli. We have investigated, through gene targeting in mice, the biological roles for the BH3-only family member HRK (also known as DP5) in apoptosis regulation. Hrk gene expression was found to be restricted to cells and tissues of the central and peripheral nervous systems. Sensory neurons from mice lacking Hrk were less sensitive to apoptosis induced by nerve growth factor (NGF) withdrawal, consistent with the induction of Hrk following NGF deprivation. By contrast, cerebellar granule neurons that upregulate Hrk upon transfer to low-K+ medium underwent apoptosis normally under these conditions in the absence of Hrk. Furthermore, loss of Hrk was not sufficient to rescue the neuronal degeneration in lurcher mutant mice. Despite previous reports, no evidence was found for Hrk expression or induction in growth-factor-dependent haematopoietic cell lines following withdrawal of their requisite cytokine, and haematopoietic progenitors lacking HRK died normally in response to cytokine deprivation. These results demonstrate that HRK contributes to apoptosis signalling elicited by trophic factor withdrawal in certain neuronal populations but is dispensable for apoptosis of haematopoietic cells.

On the variety of cell death pathways in the Lurcher mutant mouse

Apoptosis as well as autophagy have been implicated in the death of cerebellar Purkinje cells (PCs) in the Lurcher (Lc/+) mutant mouse and at least two different apoptotic pathways participate in the transsynaptic death of granule cells (GC) and inferior olivary (IO) neurones. The relative contribution of these pathways can only be assessed from their momentary involvement at any stage of the complete course of neurodegeneration. Here we used quantitative labelling for activated caspase-3 (Casp-3) and Fluoro-Jade B (FJ-B) to investigate the spatio-temporal pattern of neuronal death from P6 to P67 in Lc/+ mutants. Activated Casp-3 was present only in narrow time intervals (P14 to P22 in PCs; P14 to P28 in GCs) and in small subpopulations of PCs, GCs, and IO neurones. FJ-B positive PCs were detected during a broader period (P14 to P28), and outnumbered Casp-3 labelled PCs by a factor exceeding eight. Nevertheless, FJ-B labelling was restricted to PCs and never found in either GC or IO neurones. In conclusion, we present the first complete time course and extent of Casp-3 activation in Lc/+ mutants and show that the majority of dying neurones in Lc/+ mutants undergo Casp-3 independent cell death. The cellular overload produced by the initial gene defect in Lc/+ mutants apparently activates a variety of apoptotic and non-apoptotic pathways within the same neuronal population. Moreover, we present the first evidence for the ability of FJ-B to selectively label a discrete population of dying PCs, implying a higher selectivity of FJ-B than previously supposed.

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.

Loss of pro-apoptotic BH3-only Bcl-2 family member bim does not protect mutantLurcher mice from neurodegeneration

Lurcher (lc) mice have a semi-dominant mutation in the gene encoding the delta2 glutamate receptor (GRID2). The resulting constitutive activity of this receptor in heterozygous +/lc (grid(+/lc)) and homozygous (grid(lc/lc)) mice leads to the death of all cerebellar Purkinje cells and most afferent granule neurons. Some studies have indicated that the death of Purkinje cells occurs by apoptosis, and the secondary loss of granule neurons has been shown to require the pro-apoptotic Bcl-2 family member Bax. The BH3-only protein Bim has been shown to contribute to cytokine withdrawal-induced apoptosis of sympathetic neurons and to be responsible for the kidney degeneration in mice lacking the pro-survival protein Bcl-2. Because Bim is expressed strongly in cerebellar Purkinje cells, we have examined whether it has a role in their death in mutant Lurcher mice. Our studies show that Bim deficiency does not modify the Lurcher phenotype, ruling out an indispensable role for Bim in this neurodegenerative disease.

Lurcher GRID2-induced death and depolarization can be dissociated in cerebellar Purkinje cells

The Lurcher mutation transforms the GRID2 receptor into a constitutively opened channel. In Lurcher heterozygous mice, cerebellar Purkinje cells are permanently depolarized, a characteristic that has been thought to be the primary cause of their death, which occurs from the second postnatal week onward. The more dramatic phenotype of Lurcher homozygotes is thought to be due to a simple gene dosage effect of the mutant allele. We have analyzed the phenotype of Lurcher/hotfoot heteroallelic mutants bearing only one copy of the Lurcher allele and no wild-type Grid2. Our results show that the absence of wild-type GRID2 receptors in these heteroallelic mutants induces an early and massive Purkinje cell death that is correlated with early signs of autophagy. This neuronal death is independent of depolarization and can be explained by the direct activation of autophagy by Lurcher GRID2 receptors through the recently discovered signaling pathway formed by GRID2, n-PIST, and Beclin1.

Bax and p53 are differentially involved in the regulation of caspase-3 expression and activation during neurodegeneration in Lurcher mice

Intrinsic Purkinje cell death in heterozygous Lurcher (Grid2Lc/+) mice is accompanied by the target-related death of granule cells and olivary neurons. The expression of pro-caspase-3 is increased in Grid2Lc/+ Purkinje cells and activated caspase-3 is detected in all three cell types before their death. Bax inactivation in Grid2Lc/+ mutants rescues granule cells but not Purkinje cells. Here, we show that, while Bax inactivation inhibits caspase-3 activation in both cell types, p53 inactivation does not affect caspase-3 activation and neuronal loss in Grid2Lc/+ mice. The up-regulation of pro-caspase-3 in Grid2Lc/+ Purkinje cells is Bax and p53 independent. These results suggest that Grid2Lc/+ granule cell death is dependent on Bax and caspase-3 activation, whereas several pathways can mediate Grid2Lc/+ Purkinje cell death.

Insights from mouse models into the molecular basis of neurodegeneration

Thanks largely to cloning the genes for several neurodegenerative diseases over the past decade and the existence of mouse mutants, the molecular basis of neurodegeneration is finally beginning to yield some of its secrets. We discuss what has been learned about the pathogenesis of "triplet repeat" diseases through mouse models for spinocerebellar ataxia types 1 and 3 and Huntington disease, including the roles of nuclear aggregates and protein cleavage. We also discuss the neurologic phenotypes that arise from mutations in neurotransmitter receptors (lurcher mice) and ion channels (weaver, leaner, and tottering mice), drawing parallels between ischemic cell death and the neurodegeneration that occurs in the lurcher mouse. Finally, we discuss common mechanisms of cell death and lessons learned from these mouse models that might have broader relevance to other neurologic disorders.

Bax inactivation in lurcher mutants rescues cerebellar granule cells but not purkinje cells or inferior olivary neurons

Lurcher is a gain-of-function mutation in the delta2 glutamate receptor gene (Grid2) that turns the receptor into a leaky ion channel. The expression of the Lurcher gene in heterozygous (Grid2(Lc/+)) mutants induces the death of almost all Purkinje cells starting from the second postnatal week. Ninety percent of the granule cells and 60-75% of the inferior olivary neurons die because of the loss of their target neurons, the Purkinje cells. The apoptotic nature of the neurodegeneration has been demonstrated previously by the presence of activated caspase-3 and DNA fragmentation. Bax, a pro-apoptotic gene of the Bcl-2 family, has been shown to be involved in developmental neuronal death. To study the role of Bax in Grid2(Lc/+) neurodegeneration, double mutants with Grid2(Lc/)+ mice and Bax knock-out mice (Bax-/-) were generated. Bax deletion had no effect on the death of Purkinje cells and inferior olivary neurons, although a temporary rescue of some Purkinje cells could be detected in P15 Grid2(Lc/)+;Bax-/- animals. From postnatal day 15 (P15) to P60, the number of granule cells in Grid2(Lc/)+;Bax-/-mice did not significantly change and was significantly increased compared with the number found in Grid2(Lc/)+;Bax+/+ mice. Granule cell number in P60 Grid2(Lc/)+;Bax-/- mice corresponded to 70% of the number found in wild-type mice. Our results show that Bax inactivation in Grid2(Lc/+) mice does not rescue intrinsic Purkinje cell death or the target-related cell death of olivary neurons, but Bax inactivation does inhibit persistently target-related cell death in cerebellar granule cells.

Target-related and intrinsic neuronal death in Lurcher mutant mice are both mediated by caspase-3 activation

The Lurcher (Lc) mutation in the delta2 glutamate receptor gene leads to the presence of a constitutive inward current in the cerebellar Purkinje cells of Lurcher heterozygous mice and to the postnatal degeneration of these neurons. In addition, cerebellar granule cells and olivary neurons of Lc/+ mice die as an indirect effect of the mutation after the loss of their target Purkinje cells. The apoptotic nature of Lc/+ Purkinje cell death remains controversial. To address this question, we studied the involvement of caspase-3, a key effector of apoptosis, in the neurodegenerative processes occurring in Lc/+ cerebellum. Several antibodies recognizing different regions of caspase-3 were used in immunoblotting and immunohistochemical experiments. We demonstrate that pro-caspase-3 is specifically upregulated in the dying Lc/+ Purkinje cells, but not in granule cells and olivary neurons, suggesting that different death-inducing signals trigger variant apoptotic pathways in the CNS. The subcellular localization of pro-caspase-3 was shown to be cytoplasmic and mitochondrial. Active caspase-3 as well as DNA fragmentation was found in numerous granule cells and some Purkinje cells of the Lc/+ cerebellum. Thus, caspase-3 activation is involved in both the direct and indirect neuronal death induced by the Lurcher mutation, strongly supporting the idea that the Lc/+ Purkinje cell dies by apoptosis.