Abnormalities in the cerebellum and brainstem in homozygous lurcher mice

Smaller Absolute Quantities but Greater Relative Densities of Microvessels Are Associated with Cerebellar Degeneration in Lurcher Mice

Degenerative affections of nerve tissues are often accompanied by changes of vascularization. In this regard, not much is known about hereditary cerebellar degeneration. In this study, we compared the vascularity of the individual cerebellar components and the mesencephalon of 3-month-old wild type mice (n = 5) and Lurcher mutant mice, which represent a model of hereditary olivocerebellar degeneration (n = 5). Paraformaldehyde-fixed brains were processed into 18-μm thick serial sections with random orientation. Microvessels were visualized using polyclonal rabbit anti-laminin antibodies. Then, the stacks comprised of three 5-μm thick optical sections were recorded using systematic uniform random sampling. Stereological assessment was conducted based on photo-documentation. We found that each of the cerebellar components has its own features of vascularity. The greatest number and length of vessels were found in the granular layer; the number of vessels was lower in the molecular layer, and the lowest number of vessels was observed in the cerebellar nuclei corresponding with their low volume. Nevertheless, the nuclei had the greatest density of blood vessels. The reduction of cerebellum volume in the Lurcher mice was accompanied by a reduction in vascularization in the individual cerebellar components, mainly in the cortex. Moreover, despite the lower density of microvessels in the Lurcher mice compared with the wild type mice, the relative density of microvessels in the cerebellar cortex and nuclei was greater in Lurcher mice. The complete primary morphometric data, in the form of continuous variables, is included as a supplement. Mapping of the cerebellar and midbrain microvessels has explanatory potential for studies using mouse models of neurodegeneration.

Jan Evangelista Purkyně and the cerebellum then and now

The name of Jan Evangelista Purkyně and the cerebellum belong inseparably together. He was the first who saw and described the largest nerve cells in the brain, de facto in the cerebellum. The most distinguished researchers of the nervous system then showed him the highest recognition by naming these neurons as Purkinje cells. Through experiments by J. E. Purkyně and his followers properly functionally was attributed to the cerebellum share in precision of motor skills. Despite ongoing and fruitful research, after a relatively long time, especially in the last two decades, scientists had to constantly replenish and re-evaluate the traditional conception of the cerebellum and formulate a new one. It started in the early 1990s, when it was found that cerebellar cortex contains more neurons than the cerebral cortex. Shortly thereafter it was gradually revealed that such enormous numbers of neural cells are not without an impact on brain functions and that the cerebellum, except its traditional role in the motor skills, also participates in higher nervous activity. These new findings were obtained thanks to the introduction of modern methods of examination into the clinical praxis, and experimental procedures using animal models of cerebellar disorders described below.

Behavioral effects of neonatal lesions on the cerebellar system

Several rodent models with spontaneous mutations causing cerebellar pathology are impaired in motor functions during the neonatal period, including Grid2(Lc), Rora(sg), Dab1(scm), Girk2(Wv), Lmx1a(dr-sst), Myo5a(dn), Inpp4a(wbl), and Cacna1a(rol) mice as well as shaker and dystonic rats. Deficits are also evident in murine null mutants such as Zic1, Fgfr1/FgFr2, and Xpa/Ercc8. Behavioral deficits are time-dependent following X-irradiated- or aspiration-induced lesions of the cerebellum in rats. In addition, motor functions are deficient after lesions in cerebellar-related pathways. As in animal subjects, sensorimotor disturbances have been described in children with cerebellar lesions. These results underline the importance of the cerebellum and its connections in the development of motor functions.

Motor learning of mice lacking cerebellar Purkinje cells

The cerebellum plays a key role in the acquisition and execution of motor tasks whose physiological foundations were postulated on Purkinje cells' long-term depression (LTD). Numerous research efforts have been focused on understanding the cerebellum as a site of learning and/or memory storage. However, the controversy on which part of the cerebellum participates in motor learning, and how the process takes place, remains unsolved. In fact, it has been suggested that cerebellar cortex, deep cerebellar nuclei, and/or their combination with some brain structures other than the cerebellum are responsible for motor learning. Different experimental approaches have been used to tackle this question (cerebellar lesions, pharmacological agonist and/or antagonist of cerebellar neurotransmitters, virus tract tracings, etc.). One of these approaches is the study of spontaneous mutations affecting the cerebellar cortex and depriving it of its main input–output organizer (i.e., the Purkinje cell). In this review, we discuss the results obtained in our laboratory in motor learning of both Lurcher (Lc/+) and tambaleante (tbl/tbl) mice as models of Purkinje-cell-devoid cerebellum.

Intracerebellar application of P19-derived neuroprogenitor and naive stem cells to Lurcher mutant and wild type B6CBA mice

Background

Neurotransplantation has great potential for future treatments of various neurodegenerative disorders. Preclinically, the Lurcher mutant mouse represents an appropriate model of genetically-determined olivocerebellar degeneration. The aim of the present study was to assess survival of naïve and neurally differentiated P19 carcinoma stem cells following transplantation into the cerebellum of Lurcher mice and wild type littermates.

Material/Methods

Adult normal wild type (n=51) and Lurcher mutant mice (n=87) of the B6CBA strain were used. The mean age of the animals at the time of transplantation was 261.5 days. Suspension of naive and neurally differentiated P19 carcinoma stem cells was injected into the cerebellum of the mice. In the Lurcher mutants, 2 depths of graft injection were used. Three weeks after implantation the brains of experimental animals were examined histologically.

Results

Survival of neuroprogenitor grafts at a depth of 1.6 mm was significantly higher in wild type vs. Lurcher mutant mice. In wild type mice, the typical graft localization was in the middle of the cerebellum, whereas in Lurcher mice the graft was never found inside the degenerated cerebellum and was primarily localized in the mesencephalon.

Conclusions

We conclude that the appearance and low survival rate of cerebellar P19 carcinoma stem cell grafts in the Lurcher mutant mice weigh against the therapeutic value of this cell line in preclinical studies of neurodegeneration.

Heterochronic protein expression patterns in the developing embryonic chick cerebellum

The advantages of the embryonic chick as a model for studying neural development range from the relatively low cost of fertilized eggs to the rapid rate of development. We investigated in ovo cerebellar development in the chick, which has a nearly identical embryonic period as the mouse (19-22 days). We focused on three antigens: Calbindin (CB), Zebrin II (ZII), and Calretinin (CR), and our results demonstrate asynchronous expression patterns during cerebellar development. Presumptive CB+ Purkinje cells are first observed at embryonic day (E)10 in clusters in posterior cerebellum. At E12, corresponding with global expression of CB across the cerebellum, Purkinje cells began to express ZII. By E14-E16, Purkinje cells disperse into a monolayer and develop a pattern of alternating immunopositive and immunonegative ZII stripes. CR is initially expressed by clusters of presumptive Purkinje cells in the nodular zone at E8. However, this expression is transient and at later stages, CR is largely confined to the granule and molecular layers. Before hatch (E18-E20), Purkinje cells adopt a morphologically mature phenotype with complex dendritic arborizations. Comparing this data to that seen in mice, we found that the sequence of Purkinje cell formation, protein expression, and development is similar in both species, but these events consistently begin ∼5-7 days earlier in the precocial chick cerebellum, suggesting an important role for heterochrony in neurodevelopment.

Changes of motor abilities during ontogenetic development in Lurcher mutant mice

Lurcher mutant mice represent a natural model of olivocerebellar degeneration. This degeneration is caused by a mutation of the gene for the delta2 glutamate receptor. Lurcher mutants suffer from cerebellar ataxia and cognitive functions deficiency as a consequence of excitotoxic apoptosis of Purkinje cells in the cerebellar cortex and a secondary decrease of granule cells and inferior olive neurons. This process finishes by the 90th day of postnatal life, but already by 14 days, the Purkinje cells are damaged and the ataxia is fully developed. Purkinje cells die by apoptosis within the first 3 weeks of life. The aim of our work was to study the development of motor functions in the course of the ontogenetic development in Lurcher mutant mice of the B6CBA strain and to compare it with wild type mice of the same strain. Mice aged 2, 3, 6, 9, and 22 weeks were used in our experiment. Motor skills were examined using four standard tests: the horizontal wire, rotating cylinder, footbridge and slanting ladder. Our findings in Lurcher mutant mice show a significant increase of motor abilities up to the sixth postnatal week and selective decrease early after this period. This improvement of motor skills is caused by the physiological development of musculature and the nervous system, probably with some contribution of plasticity of the maturing brain. The cause of the decline of these abilities immediately after the completion of the development is unknown.

A preliminary study of solid embryonic cerebellar graft survival in adult B6CBA Lurcher mutant and wild type mice

Lurcher mutant mice represent a model of olivocerebellar degeneration. They suffer from complete loss of Purkinje cells and a reduction of granule cells and inferior olive neurons. Their wild type littermates serve as healthy controls. The aim of the work was to compare solid embryonic cerebellar graft survival within a period of 9 weeks after their transplantation in adult Lurcher mutant and wild type mice of the B6CBA strain. The solid grafts were injected through a hole in the occipital bone. Host mice were sacrificed 3, 6, or 9 weeks after the transplantation and their cerebella and brain-stems were examined histologically to assess graft presence and structure. We did not find significant differences in graft survival rates between Lurcher mutant and wild type mice. The frequency of graft presence did not differ between mice examined 3, 6, and 9 weeks after the transplantation, neither in Lurchers nor in wild type mice. The grafts were of various sizes. In some cases, only small residua of the grafts were found. Nerve fiber sprouting and cell migration from the graft to the host tissue were observed more often in wild type mice than in Lurchers when examined 6 weeks after surgery. In the period 3-9 weeks after transplantation, massive dying out of the grafts was not observed despite regressive processes in some of the grafts. The degenerative changes in the Lurcher mutant cerebellum do not have strong impact on the fate of the solid cerebellar graft.

The Lurcher mouse: Fresh insights from an old mutant

The Lurcher mouse was first discovered in 1954 as a spontaneously occurring autosomal dominant mutation that caused the degeneration of virtually all cerebellar Purkinje cells and most olivary neurons and granule cells. More recent molecular studies revealed that Lurcher is a gain of function mutation in the delta2 glutamate receptor (GluRdelta2) that converts an alanine to threonine in the highly conserved third hydrophobic segment of GluRdelta2. The mutation converts the receptor into a constitutively leaky cation channel. The GluRdelta2 receptor is predominantly expressed in cerebellar Purkinje cells and in the heterozygous Lurcher mutant (+/Lc). Purkinje cells die due to the mutation in the GluRdelta2 receptor, while olivary neurons and granule cells degenerate due to the loss of their Purkinje cell targets. The purpose of the review is to provide highlights from 5 decades of research on the Lurcher mutant that have provided insights into the developmental mechanisms that regulate cell number during development, cerebellar pattern formation, cerebellar physiology, and the role of the cerebellum in CNS function.

Spontaneous and induced mouse mutations with cerebellar dysfunctions: behavior and neurochemistry

Grid2(Lc) (Lurcher), Grid2(ho) (hot-foot), Rora(sg) (staggerer), nr (nervous), Agtpbp1(pcd) (Purkinje cell degeneration), Reln(rl) (reeler), and Girk2(Wv) (Weaver) are spontaneous mutations with cerebellar atrophy, ataxia, and deficits in motor coordination tasks requiring balance and equilibrium. In addition to these signs, the Dst(dt) (dystonia musculorum) spinocerebellar mutant displays dystonic postures and crawling. More recently, transgenic models with human spinocerebellar ataxia mutations and alterations in calcium homeostasis have been shown to exhibit cerebellar anomalies and motor coordination deficits. We describe neurochemical characteristics of these mutants with respect to regional brain metabolism as well as amino acid and biogenic amine concentrations, uptake sites, and receptors.

Development of Hsp25 expression compartments is not constrained by Purkinje cell defects in the Lurcher mouse mutant

Four transverse zones can be distinguished in the adult mouse cerebellar cortex based on differential expression of cell-specific antigens, termination patterns of mossy fiber afferents, and phenotypes of mouse mutants with cerebellar defects: the anterior zone (AZ), central zone (CZ), posterior zone (PZ), and nodular zone (NZ). In the heterozygous Lurcher (Lc/+) mouse a zonally restricted abnormality in Purkinje cell development is seen. The Purkinje cell-specific antigen zebrin II is normally differentially expressed in all four zones of the adult cerebellum, but in the Lc/+ mutant is confined to the PZ and NZ, caudal to a transverse boundary in the dorsal aspect of lobule VIII. In this study we wanted to understand why zebrin II expression is arrested at this boundary and whether the Lc mutation affects the differentiation of additional Purkinje cell antigens in a similar manner. To determine this, we took advantage of the dynamic developmental timetable of another Purkinje cell antigen, the small heat shock protein Hsp25. Using immunohistochemistry we demonstrate that cerebellar maturation anterior to the CZ/PZ transverse boundary appears to be unaffected by the Lc allele, in that initial progression of Hsp25 expression in the Lc/+ cerebellum was similar to controls. Double-labeling experiments with anti-Hsp25 and anti-calbindin suggest that characteristic banding patterns of Hsp25 in Lc/+ cerebellum develop and are preserved despite cell loss. Thus, since simple temporal or spatial models cannot account for the zonal restriction seen during Lc/+ cerebellar development, the abnormality may be zebrin II-specific.

Purkinje cell loss affects differentially the execution, acquisition and prepulse inhibition of skeletal and facial motor responses in Lurcher mice

Adult heterozygous Lurcher mice show a degeneration of almost all Purkinje cells and 90% of the granular cells of the cerebellum, resulting in ataxia or general deficits in motor coordination. These mice are therefore an excellent model for studying the role of the cerebellar cortex in motor performance, including the acquisition of new motor abilities. The performance of 3-month-old Lurcher mice was studied in various behavioural (fall, horizontal bar, rotating cylinder, and ladder), spatial orientation (water maze) and associative learning (eyelid classical conditioning) tasks and compared with that of wild-type mice. Behavioural tasks indicated a deficit for motor abilities in Lurcher mice but with some adaptation to the tests and improvement in performance. Wild-type and Lurcher mice performed swimming equally, but the latter learned the task significantly more slowly than the former. The late component of reflex blinks was smaller in amplitude and had a longer latency in Lurcher mice than in controls. Learning curves for Lurcher mice during classical conditioning of eyelid responses were similar to controls, but the amplitude of the learned response in Lurcher mice was significantly lower. The startle response to a severe tone was similar in both control and Lurcher mice but the latter were unable to produce prepulse inhibition. These results suggest that the cerebellar cortex is not indispensable for the performance of this complete set of skeletal and facial tasks, or for the acquisition of new motor abilities, but it is for the appropriate execution and adjustment of any of these motor activities.

The effects of cerebellar damage on maze learning in animals

The role of the cerebellum in spatial learning has recently been investigated in genetically and non-genetically lesioned animal models, particularly in water mazes, in view of the minimal impact such lesions exert on swimming movements. A dissociation between place and cued learning in the Morris water maze has been observed in several models, including cerebellar mutant mice (Rora(sg), Nna1(pcd-1J), nervous), rats with lesions of either the lateral cerebellar cortex or the dentate nucleus, and rats with selective Purkinje cell loss caused by intracerebroventricular injections of OX-7-saporin, confirming the hypothesis that cerebellar damage may cause a cognitive deficit independently of fine motor control. In addition, the results of hemicerebellectomized rats indicate the probable involvement of the cerebellum in working memory and the procedural aspect of maze learning. The findings of impaired maze learning in cerebellar-lesioned mice and rats are concordant with those of deficient visuospatial functions in patients with cerebellar atrophy. The spatial deficits may be ascribed to altered metabolic activity in cerebellar-related pathways.

Motor coordination in mice with hotfoot, Lurcher, and double mutations of the Grid2 gene encoding the delta-2 excitatory amino acid receptor

Grid2(ho/ho) is a loss of function gene mutation resulting in abnormal dendritic arborizations of Purkinje cells. These mutants were compared in a series of motor coordination tests requiring balance and equilibrium to nonataxic controls (Grid2(ho/+)) and to a double mutant (Grid2(ho/Lc)) with an inserted Lc mutation. The performance of Grid2(ho/ho) mutant mice was poorer than that of controls on stationary beam, coat hanger, unsteady platform, and rotorod tests. Grid2(ho/Lc) did not differ from Grid2(Lc/+) mice. However, the insertion of the Lc mutation in Grid2(ho/Lc) potentiated the deficits found in Grid2(ho/ho) in stationary beam, unsteady platform, and rotorod tests. These results indicate a deleterious effect of the Lc mutation on Grid2-deficient mice.

The delta2 glutamate receptor: 10 years later

The orphan glutamate receptor delta2 (GluRdelta2) is predominantly expressed in Purkinje cells and plays a crucial role in cerebellar functions: mice that lack the GluRdelta2 gene display ataxia and impaired synaptic plasticity. However, when expressed alone or with other glutamate receptors, GluRdelta2 does not form functional glutamate-gated ion channels nor does it bind to glutamate analogs. Therefore, the mechanisms by which GluRdelta2 participates in cerebellar functions have been elusive. Studies of mutant mice such as lurcher, hotfoot, and GluRdelta2 knockout mice have provided clues to the structure and function of GluRdelta2. GluRdelta2 has a channel pore similar to that of other glutamate receptors; the channel is functional at least when the lurcher mutation is present. GluRdelta2 must be transported to the Purkinje cell surface to function; the absence of surface GluRdelta2 causes the ataxic phenotype of hotfoot mice. In GluRdelta2-null mice, the presence of naked spines not innervated by parallel fibers may influence the sustained innervation of mutant Purkinje cells by multiple climbing fibers. From these results, several hypotheses about mechanisms by which GluRdelta2 functions are proposed in this article. Further characterization of GluRdelta2's functions will provide key insights into normal and abnormal cerebellar functions.

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.

Motor performance and regional brain metabolism of spontaneous murine mutations with cerebellar atrophy

Three spontaneous mutations with cerebellar atrophy were evaluated for motor coordination and regional brain metabolism, as assessed by cytochrome oxidase (CO) activity. Despite similar neuropathological characteristics, the behavioral phenotype of Lurcher is less severe than that of staggerer, possibly caused by the slower onset of their neuronal degeneration. Although fewer cerebellar cells degenerate in hot-foot than in Lurcher, their motor deficits are more severe, indicating the presence of dysfunctional cells. CO activity in the deep cerebellar nuclei was increased in Lurcher and staggerer but unchanged in hot-foot, probably due to the severe loss of GABAergic input from Purkinje cells in the first two mutants but not the third. Altered CO activity in cerebellar-related pathways was linearly correlated with motor performance, indicating that the activity of this enzyme is associated not only with neuronal activity but also with motor performance.

Distribution of serotonin, its metabolites and 5-HT transporters in the neostriatum of Lurcher and weaver mutant mice

Serotonin (5-HT) uptake sites, or transporters, were measured in the neostriatum (caudate putamen) of wild type (+/+) mice and heterozygous (wv/+) and homozygous (wv/wv) weaver, as well as in heterozygous Lurcher (Lc/+) mutants. These topological surveys were carried out by quantitative ligand binding autoradiography using the uptake site antagonist [3H]-citalopram as a probe of innervation densities in four quadrants of the rostral neostriatum and in two halves of the caudal neostriatum. In addition, tissue concentrations of 5-HT, 5-hydroxyindole-3-acetic acid and 5-hydroxytryptophol were measured by high-performance liquid chromatography with electrochemical detection in these neostriatal divisions. In +/+ mice and in Lc/+ mutants there was a dorso-ventral gradient of increasing 5-HT levels, and they exhibited a similar heterogeneity of [3H]-citalopram labeling. In contrast, the gradients of 5-HT concentrations and [3H]-citalopram binding disappeared in the weaver mutants, suggesting a rearrangement of the 5-HT innervation. This reorganization of the 5-HT system in the neostriatum was more obvious in the wv/wv and is compatible with the hypothesis that the postnatal dopaminergic deficiencies that characterize weaver mutants lead to a sprouting of fibers and thus constitute a genetic model of dopaminergic denervation that leads to a 5-HT hyperinnervation.

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.

Autoradiography of glutamate receptor binding in adult Lurcher mutant mice

The mutation Lurcher, resulting from a gain of malfunction of the delta2 glutamate receptor expressed specifically by cerebellar Purkinje cells, causes a primary total loss of these neurons of the cerebellar cortex, as well as the secondary degeneration of cerebellar granule and inferior olive neurons. The distributions of glutamate receptors sensitive to amino-methylisoxazole-propionic acid (AMPA), to kainic acid (KA), and to N-methyl-D-aspartic acid (NMDA) as well as metabotropic sites (MET1 and MET2) were examined in wild type and Lurcher mice by quantitative autoradiography. This study was undertaken to determine the gene effect on the distribution of the various glutamate receptor subtypes, as well as how the cerebellar lesion affects the glutamatergic system in other brain regions. In cerebellum, there were postsynaptic AMPA and metabotropic receptors on Purkinje cells, postsynaptic NMDA receptors on granule cells, as well as KA receptors on granule cells or on parallel fibers. Taking into account surface areas, binding to all receptor subtypes was lower in the cerebellar cortex of Lurcher mutants than in wild type mice, while in the deep cerebellar nuclei only KA receptors were diminished. In other brain regions, the alterations followed always the same pattern characterized by a decrease of NMDA and KA receptors but with an increase of AMPA sites; these reciprocal changes were seen in thalamus. neostriatum, limbic regions, and motor cerebral cortical regions. Comparisons of glutamate receptor distribution in Lurcher mutants and in human autosomal cerebellar ataxia may permit further understanding of the role of glutamate-induced toxicity on neuronal death in these heredo-degenerative diseases.

Effects of buspirone on brain indoleamines and catecholamines in wild-type mice and Lurcher mutants

The effects of a chronic serotoninergic stimulation on brain monoamine levels and metabolism were studied in wild-type (+/+) mice and Lurcher (Lc/+) mutants. Endogenous serotonin, dopamine, noradrenaline and some of their major metabolites were measured in the frontal cortex, neostriatum, thalamus, brainstem, cerebellum and spinal cord. In +/+ mice, buspirone (1 mg/kg; i.p.) treatment during 40 days increased indoleamines, albeit with moderate changes in the ratios between tissue serotonin metabolites and endogenous serotonin, augmented noradrenaline contents in the spinal cord, and caused elevations of dopamine metabolites in most regions. In Lc/+ mutants, the effects of buspirone were attenuated, but higher L-tryptophan and indoleamine levels, suggest a storage of serotonin in a non-releasable compartment. In the hypoplastic Lc/+ cerebellum, indoleamine content was accrued, but with a decreased [serotonin metabolites]/[serotonin] ratio, indicating that the reorganized nerve terminals in Lc/+ mutants although they can synthesize and accumulate serotonin, may not utilize it efficiently in synaptic transmission.

GluR delta 2 and the development and death of cerebellar Purkinje neurons in lurcher mice

Lurcher (Lc) is a spontaneous, semidominant mouse neurological mutation. Heterozygous lurcher mice (Lc/+) display ataxia due to a selective, cell-autonomous, apoptotic death of 90% of cerebellar Purkinje cells during postnatal development. Homozygous lurcher mice (Lc/Lc) die shortly after birth due to massive loss of mid- and hindbrain neurons during late embryogenesis. We identified the mutations responsible for neurodegeneration in two independent Lc alleles as identical G-to-A transitions that change a highly conserved alanine to a threonine residue in transmembrane domain III of the mouse delta 2 glutamate receptor gene (GluRE2). Lc/+ Purkinje cells displayed a very high membrane conductance and a depolarized resting potential, indicating the presence of a large, constitutive inward current. Expression of the mutant GluR delta 2Lc protein in Xenopus oocytes confirmed these results, demonstrating that lurcher is an inherited neurodegenerative disorder resulting from a gain-of-function mutation in a glutamate receptor gene. Further characterization of GluR delta 2 signaling and the activation of apoptotic death in Lc Purkinje cells have begun to yield mechanistic insights into this neurodegenerative disease, and to highlight its relationship to neuronal loss following ischemia.

The lurcher mutation and ionotropic glutamate receptors: contributions to programmed neuronal death in vivo

The recent positional cloning and physiological characterization of the lurcher mutation resulted in the identification of a novel stimulus that results in neurodegeneration. The catastrophic loss of cerebellar Purkinje cells in lurcher heterozygotes has now been strongly associated with a large constitutive inward current which ultimately activates a programmed form of neuronal death. The completely penetrant and focal nature of the lurcher phenotype gives us an opportunity to investigate the manner in which neurons respond to an aberrant signal in the context of the brain parenchyma. Although there is no human genetic disease that is equivalent to the lurcher mutation at this time, its triggering of programmed neuronal death enables us to pose and address questions that are relevant to a large number of human neurological diseases. The advantage of working in a genetically manipulable in vivo mammalian system is evident: we can address questions relating to gene function in the nervous system in a context that is physiological. Classical genetic analyses looking for molecules that suppress or modify the lurcher phenotype are under way and have now been supplemented with two novel techniques developed in our laboratory: biolistic transfection of cerebellar slices and Bacterial Artificial Chromosome modification. The integration of these novel and classical approaches will facilitate the testing of hypotheses, developed during the course of our study of the lurcher mutation, which explore the propagation of abnormal signals and the initiation of programmed neuronal death in neurons.

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

Cerebellar Purkinje cells in the heterozygous Lurcher mutant undergo cell autonomous degeneration beginning in the second week of postnatal development and becoming almost total around 30-45 days. The Lurcher mutation was recently identified as gain-of-function defect in the delta 2 glutamate receptor causing a constitutive current leak, suggesting that +/Lc Purkinje cells die by an excitotoxic mechanism. In previous studies we have shown that overexpression of bcl-2, a key regulator of cell death, in the heterozygous Lurcher mutant does not prevent +/Lc Purkinje cell death. To investigate further the mechanisms of +/Lc Purkinje cell death, we have crossed +/Lc mutants with a second line of Hu-bcl-2 transgenics (NSE73a) that shows an earlier onset of transgene expression and higher expression levels. Analysis of eight +/Lc-NSE73a mutants (4 at 2 months and 4 at 5-6 months) showed that Hu-bcl-2 overexpression delayed, but ultimately could not prevent +/Lc Purkinje cell death.

Brain dopamine and amino acid concentrations in Lurcher mutant mice

Lurcher mutant mice are characterized by massive degeneration of the cerebellum, including Purkinje cells and granule cells, as well as for the loss of neurons from the inferior olive. Concentrations of dopamine and two of its metabolites and of several amino acid neurotransmitters were determined in the cerebellum and in other brain regions of these mutants. By comparison to wild-type mice of the same background strain, glutamate and taurine concentrations were reduced in the Lurcher cerebellum. No decrease was found for aspartate, gamma-aminobutyric acid (GABA), glycine, as well as dopamine and its metabolites. Moreover, no neurochemical alterations occurred in the brain stem, thalamus, or neostriatum of Lurcher mutants. A selective reduction of glutamate concentration was found in the hippocampus, while all amino acids measured were decreased in the entorhinal-piriform areas. These results indicate region-selective reductions of neurotransmitter concentrations in a mouse mutant with a defined cerebellar cortical pathology.

Postnatal expression of Hu-bcl-2 gene in Lurcher mutant mice fails to rescue Purkinje cells but protects inferior olivary neurons from target-related cell death

The Lurcher mutant has been extensively studied as a model for cell-autonomous and target-related cell death, yet there are still many unknowns concerning the mechanisms of neuronal degeneration in this mutant. As a key regulator of apoptosis, a bcl-2 transgene has been overexpressed in the heterozygous Lurcher mutant to investigate the effects of BCL-2 on two types of in vivo neuronal cell loss in Lurcher: cell-autonomous Purkinje cell degeneration and target-related olivary neuron death. Six adult +/Lc mutants expressing a human bcl-2 transgene (Hu-bcl-2) were generated by crossing +/Lc mutants with NSE71 Hu-bcl-2 transgenic mice. Analysis of these brains showed that bcl-2 overexpression did not prevent +/Lc Purkinje cell degeneration, but it did rescue most olivary neurons from target-related cell death. Although the number of olivary neurons was equivalent to wild-type numbers, the inferior olive nucleus was significantly shorter in its rostrocaudal extent, suggesting that olivary neurons are atrophied. We propose that Lurcher gene action causes Purkinje cell degeneration independently of a BCL-2-mediated pathway. Furthermore, although bcl-2 overexpression rescues olivary neurons from target-related cell death, it does not prevent the atrophy associated with the loss of target-related trophic support.