Deep Intronic FGF14 GAA Repeat Expansion in Late-Onset Cerebellar Ataxia.

BACKGROUND: The late-onset cerebellar ataxias (LOCAs) have largely resisted molecular diagnosis. METHODS: We sequenced the genomes of six persons with autosomal dominant LOCA who were members of three French Canadian families and identified a candidate pathogenic repeat expansion. We then tested for association between the repeat expansion and disease in two independent case-control series - one French Canadian (66 patients and 209 controls) and the other German (228 patients and 199 controls). We also genotyped the repeat in 20 Australian and 31 Indian index patients. We assayed gene and protein expression in two postmortem cerebellum specimens and two induced pluripotent stem-cell (iPSC)-derived motor-neuron cell lines. RESULTS: In the six French Canadian patients, we identified a GAA repeat expansion deep in the first intron of FGF14, which encodes fibroblast growth factor 14. Cosegregation of the repeat expansion with disease in the families supported a pathogenic threshold of at least 250 GAA repeats ([GAA]≥250). There was significant association between FGF14 (GAA)≥250 expansions and LOCA in the French Canadian series (odds ratio, 105.60; 95% confidence interval [CI], 31.09 to 334.20; P<0.001) and in the German series (odds ratio, 8.76; 95% CI, 3.45 to 20.84; P<0.001). The repeat expansion was present in 61%, 18%, 15%, and 10% of French Canadian, German, Australian, and Indian index patients, respectively. In total, we identified 128 patients with LOCA who carried an FGF14 (GAA)≥250 expansion. Postmortem cerebellum specimens and iPSC-derived motor neurons from patients showed reduced expression of FGF14 RNA and protein. CONCLUSIONS: A dominantly inherited deep intronic GAA repeat expansion in FGF14 was found to be associated with LOCA. (Funded by Fondation Groupe Monaco and others.).

Machine learning nominates the inositol pathway and novel genes in Parkinson's disease.

There are 78 loci associated with Parkinson's disease in the most recent genome-wide association study (GWAS), yet the specific genes driving these associations are mostly unknown. Herein, we aimed to nominate the top candidate gene from each Parkinson's disease locus and identify variants and pathways potentially involved in Parkinson's disease. We trained a machine learning model to predict Parkinson's disease-associated genes from GWAS loci using genomic, transcriptomic and epigenomic data from brain tissues and dopaminergic neurons. We nominated candidate genes in each locus and identified novel pathways potentially involved in Parkinson's disease, such as the inositol phosphate biosynthetic pathway (INPP5F, IP6K2, ITPKB and PPIP5K2). Specific common coding variants in SPNS1 and MLX may be involved in Parkinson's disease, and burden tests of rare variants further support that CNIP3, LSM7, NUCKS1 and the polyol/inositol phosphate biosynthetic pathway are associated with the disease. Functional studies are needed to further analyse the involvements of these genes and pathways in Parkinson's disease.

NPTX1 mutations trigger endoplasmic reticulum stress and cause autosomal dominant cerebellar ataxia.

With more than 40 causative genes identified so far, autosomal dominant cerebellar ataxias exhibit a remarkable genetic heterogeneity. Yet, half the patients are lacking a molecular diagnosis. In a large family with nine sampled affected members, we performed exome sequencing combined with whole-genome linkage analysis. We identified a missense variant in NPTX1, NM_002522.3:c.1165G>A: p.G389R, segregating with the phenotype. Further investigations with whole-exome sequencing and an amplicon-based panel identified four additional unrelated families segregating the same variant, for whom a common founder effect could be excluded. A second missense variant, NM_002522.3:c.980A>G: p.E327G, was identified in a fifth familial case. The NPTX1-associated phenotype consists of a late-onset, slowly progressive, cerebellar ataxia, with downbeat nystagmus, cognitive impairment reminiscent of cerebellar cognitive affective syndrome, myoclonic tremor and mild cerebellar vermian atrophy on brain imaging. NPTX1 encodes the neuronal pentraxin 1, a secreted protein with various cellular and synaptic functions. Both variants affect conserved amino acid residues and are extremely rare or absent from public databases. In COS7 cells, overexpression of both neuronal pentraxin 1 variants altered endoplasmic reticulum morphology and induced ATF6-mediated endoplasmic reticulum stress, associated with cytotoxicity. In addition, the p.E327G variant abolished neuronal pentraxin 1 secretion, as well as its capacity to form a high molecular weight complex with the wild-type protein. Co-immunoprecipitation experiments coupled with mass spectrometry analysis demonstrated abnormal interactions of this variant with the cytoskeleton. In agreement with these observations, in silico modelling of the neuronal pentraxin 1 complex evidenced a destabilizing effect for the p.E327G substitution, located at the interface between monomers. On the contrary, the p.G389 residue, located at the protein surface, had no predictable effect on the complex stability. Our results establish NPTX1 as a new causative gene in autosomal dominant cerebellar ataxias. We suggest that variants in NPTX1 can lead to cerebellar ataxia due to endoplasmic reticulum stress, mediated by ATF6, and associated to a destabilization of NP1 polymers in a dominant-negative manner for one of the variants.

Local Acceleration of Neurofilament Transport at Nodes of Ranvier.

Myelinated axons are constricted at nodes of Ranvier. These constrictions are important physiologically because they increase the speed of saltatory nerve conduction, but they also represent potential bottlenecks for the movement of axonally transported cargoes. One type of cargo are neurofilaments, which are abundant space-filling cytoskeletal polymers that function to increase axon caliber. Neurofilaments move bidirectionally along axons, alternating between rapid movements and prolonged pauses. Strikingly, axon constriction at nodes is accompanied by a reduction in neurofilament number that can be as much as 10-fold in the largest axons. To investigate how neurofilaments navigate these constrictions, we developed a transgenic mouse strain that expresses a photoactivatable fluorescent neurofilament protein in neurons. We used the pulse-escape fluorescence photoactivation technique to analyze neurofilament transport in mature myelinated axons of tibial nerves from male and female mice of this strain ex vivo Fluorescent neurofilaments departed the activated region more rapidly in nodes than in flanking internodes, indicating that neurofilament transport is faster in nodes. By computational modeling, we showed that this nodal acceleration can be explained largely by a local increase in the duty cycle of neurofilament transport (i.e., the proportion of the time that the neurofilaments spend moving). We propose that this transient acceleration functions to maintain a constant neurofilament flux across nodal constrictions, much as the current increases where a river narrows its banks. In this way, neurofilaments are prevented from piling up in the flanking internodes, ensuring a stable neurofilament distribution and uniform axonal morphology across these physiologically important axonal domains.SIGNIFICANCE STATEMENT Myelinated axons are constricted at nodes of Ranvier, resulting in a marked local decrease in neurofilament number. These constrictions are important physiologically because they increase the efficiency of saltatory nerve conduction, but they also represent potential bottlenecks for the axonal transport of neurofilaments, which move along axons in a rapid intermittent manner. Imaging of neurofilament transport in mature myelinated axons ex vivo reveals that neurofilament polymers navigate these nodal axonal constrictions by accelerating transiently, much as the current increases where a river narrows its banks. This local acceleration is necessary to ensure a stable axonal morphology across nodal constrictions, which may explain the vulnerability of nodes of Ranvier to neurofilament accumulations in animal models of neurotoxic neuropathies and neurodegenerative diseases.

Sacsin, mutated in the ataxia ARSACS, regulates intermediate filament assembly and dynamics.

Loss of sacsin, a large 520 kDa multidomain protein, causes autosomal recessive spastic ataxia of the Charlevoix-Saguenay, one of the most common childhood-onset recessive ataxias. A prominent feature is abnormal bundling of neurofilaments in many neuronal populations. This study shows the direct involvement of sacsin domains in regulating intermediate filament assembly and dynamics and identifies important domains for alleviating neurofilament bundles in neurons lacking sacsin. Peptides encoding sacsin internal repeat (SIRPT) 1, J-domains, and ubiquitin-like domain modified neurofilament assembly in vivo. The domains with chaperone homology, the SIRPT and the J-domain, had opposite effects, promoting and preventing filament assembly, respectively. In cultured Sacs-/- motor neurons, both the SIRPT1 and J-domain resolved preexisting neurofilament bundles. Increasing expression of heat shock proteins also resolved neurofilament bundles, indicating that this endogenous chaperone system can compensate to some extent for sacsin deficiency.-Gentil, B. J., Lai, G.-T., Menade, M., Larivière, R., Minotti, S., Gehring, K., Chapple, J.-P., Brais, B., Durham, H. D. Sacsin, mutated in the ataxia ARSACS, regulates intermediate filament assembly and dynamics.

Altered organization of the intermediate filament cytoskeleton and relocalization of proteostasis modulators in cells lacking the ataxia protein sacsin.

Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS) is caused by mutations in the gene SACS, encoding the 520 kDa protein sacsin. Although sacsin's physiological role is largely unknown, its sequence domains suggest a molecular chaperone or protein quality control function. Consequences of its loss include neurofilament network abnormalities, specifically accumulation and bundling of perikaryal and dendritic neurofilaments. To investigate if loss of sacsin affects intermediate filaments more generally, the distribution of vimentin was analysed in ARSACS patient fibroblasts and in cells where sacsin expression was reduced. Abnormal perinuclear accumulation of vimentin filaments, which sometimes had a cage-like appearance, occurred in sacsin-deficient cells. Mitochondria and other organelles were displaced to the periphery of vimentin accumulations. Reorganization of the vimentin network occurs in vitro under stress conditions, including when misfolded proteins accumulate. In ARSACS patient fibroblasts HSP70, ubiquitin and the autophagy-lysosome pathway proteins Lamp2 and p62 relocalized to the area of the vimentin accumulation. There was no overall increase in ubiquitinated proteins, suggesting the ubiquitin-proteasome system was not impaired. There was evidence for alterations in the autophagy-lysosome pathway. Specifically, in ARSACS HDFs cellular levels of Lamp2 were elevated while levels of p62, which is degraded in autophagy, were decreased. Moreover, autophagic flux was increased in ARSACS HDFs under starvation conditions. These data show that loss of sacsin effects the organization of intermediate filaments in multiple cell types, which impacts the cellular distribution of other organelles and influences autophagic activity.

Altered synaptic and firing properties of cerebellar Purkinje cells in a mouse model of ARSACS.

KEY POINTS: Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is an early-onset neurodegenerative human disease characterized in part by ataxia and Purkinje cell loss in anterior cerebellar lobules. A knock-out mouse model has been developed that recapitulates several features of ARSACS. Using this ARSACS mouse model, we report changes in synaptic input and intrinsic firing in cerebellar Purkinje cells, as well as in their synaptic output in the deep cerebellar nuclei. Changes in firing are observed in anterior lobules that later exhibit Purkinje cell death, but not in posterior lobules that do not. Our results show that both synaptic and intrinsic alterations in Purkinje cell properties likely contribute to disease manifestation in ARSACS; these findings resemble pathophysiological changes reported in several other ataxias. ABSTRACT: Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is an early-onset neurodegenerative disease that includes a pronounced and progressive cerebellar dysfunction. ARSACS is caused by an autosomal recessive loss-of-function mutation in the Sacs gene that encodes the protein sacsin. To better understand the cerebellar pathophysiology in ARSACS, we studied synaptic and firing properties of Purkinje cells from a mouse model of ARSACS, Sacs-/- mice. We found that excitatory synaptic drive was reduced onto Sacs-/- Purkinje cells, and that Purkinje cell firing rate, but not regularity, was reduced at postnatal day (P)40, an age when ataxia symptoms were first reported. Firing rate deficits were limited to anterior lobules that later display Purkinje cell death, and were not observed in posterior lobules where Purkinje cells are not lost. Mild firing deficits were observed as early as P20, prior to the manifestation of motor deficits, suggesting that a critical level of cerebellar dysfunction is required for motor coordination to emerge. Finally, we observed a reduction in Purkinje cell innervation onto target neurons in the deep cerebellar nuclei (DCN) in Sacs-/- mice. Together, these findings suggest that multiple alterations in the cerebellar circuit including Purkinje cell input and output contribute to cerebellar-related disease onset in ARSACS.

Vapb/Amyotrophic lateral sclerosis 8 knock-in mice display slowly progressive motor behavior defects accompanying ER stress and autophagic response.

Missense mutations (P56S) in Vapb are associated with autosomal dominant motor neuron diseases: amyotrophic lateral sclerosis and lower motor neuron disease. Although transgenic mice overexpressing the mutant vesicle-associated membrane protein-associated protein B (VAPB) protein with neuron-specific promoters have provided some insight into the toxic properties of the mutant proteins, their role in pathogenesis remains unclear. To identify pathological defects in animals expressing the P56S mutant VAPB protein at physiological levels in the appropriate tissues, we have generated Vapb knock-in mice replacing wild-type Vapb gene with P56S mutant Vapb gene and analyzed the resulting pathological phenotypes. Heterozygous P56S Vapb knock-in mice show mild age-dependent defects in motor behaviors as characteristic features of the disease. The homozygous P56S Vapb knock-in mice show more severe defects compared with heterozygous mice reflecting the dominant and dose-dependent effects of P56S mutation. Significantly, the knock-in mice demonstrate accumulation of P56S VAPB protein and ubiquitinated proteins in cytoplasmic inclusions, selectively in motor neurons. The mutant mice demonstrate induction of ER stress and autophagic response in motor neurons before obvious onset of behavioral defects, suggesting that these cellular biological defects might contribute to the initiation of the disease. The P56S Vapb knock-in mice could be a valuable tool to gain a better understanding of the mechanisms by which the disease arises.

Sacs knockout mice present pathophysiological defects underlying autosomal recessive spastic ataxia of Charlevoix-Saguenay.

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS [MIM 270550]) is an early-onset neurodegenerative disorder caused by mutations in the SACS gene. Over 170 SACS mutations have been reported worldwide and are thought to cause loss of function of sacsin, a poorly characterized and massive 520 kDa protein. To establish an animal model and to examine the pathophysiological basis of ARSACS, we generated Sacs knockout (Sacs(-/-)) mice. Null animals displayed an abnormal gait with progressive motor, cerebellar and peripheral nerve dysfunctions highly reminiscent of ARSACS. These clinical features were accompanied by an early onset, progressive loss of cerebellar Purkinje cells followed by spinal motor neuron loss and peripheral neuropathy. Importantly, loss of sacsin function resulted in abnormal accumulation of non-phosphorylated neurofilament (NF) bundles in the somatodendritic regions of vulnerable neuronal populations, a phenotype also observed in an ARSACS brain. Moreover, motor neurons cultured from Sacs(-/-) embryos exhibited a similar NF rearrangement with significant reduction in mitochondrial motility and elongated mitochondria. The data points to alterations in the NF cytoskeleton and defects in mitochondrial dynamics as the underlying pathophysiological basis of ARSACS.

Mitochondrial dysfunction and Purkinje cell loss in autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS).

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a childhood-onset neurological disease resulting from mutations in the SACS gene encoding sacsin, a 4,579-aa protein of unknown function. Originally identified as a founder disease in Québec, ARSACS is now recognized worldwide. Prominent features include pyramidal spasticity and cerebellar ataxia, but the underlying pathology and pathophysiological mechanisms are unknown. We have generated an animal model for ARSACS, sacsin knockout mice, that display age-dependent neurodegeneration of cerebellar Purkinje cells. To explore the pathophysiological basis for this observation, we examined the cell biological properties of sacsin. We show that sacsin localizes to mitochondria in non-neuronal cells and primary neurons and that it interacts with dynamin-related protein 1, which participates in mitochondrial fission. Fibroblasts from ARSACS patients show a hyperfused mitochondrial network, consistent with defects in mitochondrial fission. Sacsin knockdown leads to an overly interconnected and functionally impaired mitochondrial network, and mitochondria accumulate in the soma and proximal dendrites of sacsin knockdown neurons. Disruption of mitochondrial transport into dendrites has been shown to lead to abnormal dendritic morphology, and we observe striking alterations in the organization of dendritic fields in the cerebellum of knockout mice that precedes Purkinje cell death. Our data identifies mitochondrial dysfunction/mislocalization as the likely cellular basis for ARSACS and indicates a role for sacsin in regulation of mitochondrial dynamics.

Reversal of neuropathy phenotypes in conditional mouse model of Charcot-Marie-Tooth disease type 2E.

Mutations in the gene encoding for the neurofilament light subunit (NF-L) are responsible for Charcot-Marie-Tooth (CMT) neuropathy type 2E. To address whether CMT2E disease is potentially reversible, we generated a mouse model with conditional doxycycline-responsive gene system that allows repression of mutant hNF-LP22S transgene expression in adult neurons. The hNF-LP22S;tTa transgenic (tg) mice recapitulated key features of CMT2E disease, including aberrant hindlimb posture, motor deficits, hypertrophy of muscle fibres and loss of muscle innervation without neuronal loss. Remarkably, a 3-month treatment of hNF-LP22S;tTa mice with doxycycline after onset of disease efficiently down-regulated expression of hNF-LP22S and it caused reversal of CMT neurological phenotypes with restoration of muscle innervation and of neurofilament protein distribution along the sciatic nerve. These data suggest that therapeutic approaches aimed at abolishing expression or neutralizing hNF-L mutants might not only halt the progress of CMT2E disease, but also revert the disabilities.

A NUDEL-dependent mechanism of neurofilament assembly regulates the integrity of CNS neurons.

The cytoskeleton controls the architecture and survival of central nervous system (CNS) neurons by maintaining the stability of axons and dendrites. Although neurofilaments (NFs) constitute the main cytoskeletal network in these structures, the mechanism that underlies subunit incorporation into filaments remains a mystery. Here we report that NUDEL, a mammalian homologue of the Aspergillus nidulans nuclear distribution molecule NudE, is important for NF assembly, transport and neuronal integrity. NUDEL facilitates the polymerization of NFs through a direct interaction with the NF light subunit (NF-L). Knockdown of NUDEL by RNA interference (RNAi) in a neuroblastoma cell line, primary cortical neurons or post-natal mouse brain destabilizes NF-L and alters the homeostasis of NFs. This results in NF abnormalities and morphological changes reminiscent of neurodegeneration. Furthermore, variations in levels of NUDEL correlate with disease progression and NF defects in a mouse model of neurodegeneration. Thus, NUDEL contributes to the integrity of CNS neurons by regulating NF assembly.

Neuronal over-expression of chromogranin A accelerates disease onset in a mouse model of ALS.

Recent studies provided evidence that chromogranins can interact with mutant superoxide dismutase 1 (SOD1) and that chromogranin B (CgB) may act as a susceptibility gene and modifier of onset in amyotrophic lateral sclerosis (ALS). To further investigate the role of chromogranins in ALS pathogenesis, we generated SOD1(G37R) mice that over-express CgA under the control of Thy1 promoter. Here, we report that neuronal over-expression of CgA in SOD1(G37R) mice caused acceleration of onset of motor impairment and exacerbation of motor neuron degeneration. The use of monoclonal antibody specific to misfolded mutant SOD1 demonstrated a higher level of misfolded SOD1 species in double transgenic mice compared to SOD1(G37R) mice, suggesting a stabilization of pathogenic SOD1 species by excess CgA. These results suggest a role of chromogranins as modulators of disease onset in ALS pathogenesis.

Intracerebroventricular infusion of monoclonal antibody or its derived Fab fragment against misfolded forms of SOD1 mutant delays mortality in a mouse model of ALS.

The finding of a secretion pathway and toxicity for mutant superoxide dismutase 1 (SOD1) raised up the possibility of using immunization approaches to reduce or neutralize the burden of toxic SOD1 species in the nervous system. Here we tested a passive immunization approach based on intracerebroventricular infusion in G93A-SOD1 mice of monoclonal antibodies specific to misfolded forms of SOD1 (mSOD1). We tested two monoclonal antibodies that bind distinct epitopes in mSOD1 and that do not bind to intact wild-type (WT) SOD1. One antibody succeeded in reducing the level of mSOD1 by 23% in the spinal cord and in prolonging the lifespan of G93A-SOD1 mice in proportion to the duration of treatment. However, another monoclonal antibody binding to a different SOD1 epitope failed to confer protection indicating that not all anti-SOD1 antibodies might be suitable for immunotherapy. Interestingly, the variable Fab fragment of an anti-SOD1 antibody was sufficient to confer some protection in G93A-SOD1 mice. The partial dispensability of Fc region should offer some advantages for development of immunotherapy with antibodies of smaller molecular size and low immunogenicity. From these results, we propose that passive immunization strategies should be considered as potential avenues for treatment of familial amyotrophic lateral sclerosis caused by SOD1 mutations.

Sacs R272C missense homozygous mice develop an ataxia phenotype.

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS [MIM 270550]) is an early-onset neurodegenerative disorder caused by mutations in the SACS gene. Over 200 SACS mutations have been identified. Most mutations lead to a complete loss of a sacsin, a large 520 kD protein, although some missense mutations are associated with low levels of sacsin expression. We previously showed that Sacs knock-out mice demonstrate early-onset ataxic phenotype with neurofilament bundling in many neuronal populations. To determine if the preservation of some mutated sacsin protein resulted in the same cellular and behavioral alterations, we generated mice expressing an R272C missense mutation, a homozygote mutation found in some affected patients. Though SacsR272C mice express 21% of wild type brain sacsin and sacsin is found in many neurons, they display similar abnormalities to Sacs knock-out mice, including the development of an ataxic phenotype, reduced Purkinje cell firing rates, and somatodendritic neurofilament bundles in Purkinje cells and other neurons. Together our results support that Sacs missense mutation largely lead to loss of sacsin function.

The leukodystrophy mutation Polr3b R103H causes homozygote mouse embryonic lethality and impairs RNA polymerase III biogenesis.

Recessive mutations in the ubiquitously expressed POLR3A and POLR3B genes are the most common cause of POLR3-related hypomyelinating leukodystrophy (POLR3-HLD), a rare childhood-onset disorder characterized by deficient cerebral myelin formation and cerebellar atrophy. POLR3A and POLR3B encode the two catalytic subunits of RNA Polymerase III (Pol III), which synthesizes numerous small non-coding RNAs. We recently reported that mice homozygous for the Polr3a mutation c.2015G > A (p.Gly672Glu) have no neurological abnormalities and thus do not recapitulate the human POLR3-HLD phenotype. To determine if other POLR3-HLD mutations can cause a leukodystrophy phenotype in mouse, we characterized mice carrying the Polr3b mutation c.308G > A (p.Arg103His). Surprisingly, homozygosity for this mutation was embryonically lethal with only wild-type and heterozygous animals detected at embryonic day 9.5. Using proteomics in a human cell line, we found that the POLR3B R103H mutation severely impairs assembly of the Pol III complex. We next generated Polr3aG672E/G672E/Polr3b+/R103Hdouble mutant mice but observed that this additional mutation was insufficient to elicit a neurological or transcriptional phenotype. Taken together with our previous study on Polr3a G672E mice, our results indicate that missense mutations in Polr3a and Polr3b can variably impair mouse development and Pol III function. Developing a proper model of POLR3-HLD is crucial to gain insights into the pathophysiological mechanisms involved in this devastating neurodegenerative disease.

The type III neurofilament peripherin is expressed in the tuberomammillary neurons of the mouse.

BACKGROUND: Peripherin, a type III neuronal intermediate filament, is widely expressed in neurons of the peripheral nervous system and in selected central nervous system hindbrain areas with projections towards peripheral structures, such as cranial nerves and spinal cord neurons. Peripherin appears to play a role in neurite elongation during development and axonal regeneration, but its exact function is not known. We noticed high peripherin expression in the posterior hypothalamus of mice, and decided to investigate further the exact location of expression and function of peripherin in the mouse posterior hypothalamus. RESULTS: In situ hybridization indicated expression of peripherin in neurons with a distribution reminiscent of the histaminergic neurons, with little signal in any other part of the forebrain. Immunocytochemical staining for histidine decarboxylase and peripherin revealed extensive colocalization, showing that peripherin is produced by histaminergic neurons in all parts of the tuberomammillary nucleus. We next used histamine immunostaining in peripherin knockout, overexpressing and wild type mice to study if altered peripherin expression affects these neurons, but could not detect any visible difference in the appearance of these neurons or their axons. Peripherin knockout mice and heterozygotic littermates were used for measurement of locomotor activity, feeding, drinking, and energy expenditure. Both genotypes displayed diurnal rhythms with all the parameters higher during the dark period. The respiratory quotient, an indicator of the type of substrate being utilized, also exhibited a significant diurnal rhythm in both genotypes. The diurnal patterns and the average values of all the recorded parameters for 24 h, daytime and night time were not significantly different between the genotypes, however. CONCLUSION: In conclusion, we have shown that peripherin is expressed in the tuberomammillary neurons of the mouse hypothalamus. Monitoring of locomotor activity, feeding, drinking, and energy expenditure in mice either lacking or overexpressing peripherin did not reveal any difference, so the significance of peripherin in these neurons remains to be determined. The complete overlap between histidine decarboxylase and peripherin, both the protein and its mRNA, renders peripherin a useful new marker for histaminergic neurons in the hypothalamus.

Absence of neurological abnormalities in mice homozygous for the Polr3a G672E hypomyelinating leukodystrophy mutation.

Recessive mutations in the ubiquitously expressed POLR3A gene cause one of the most frequent forms of childhood-onset hypomyelinating leukodystrophy (HLD): POLR3-HLD. POLR3A encodes the largest subunit of RNA Polymerase III (Pol III), which is responsible for the transcription of transfer RNAs (tRNAs) and a large array of other small non-coding RNAs. In order to study the central nervous system pathophysiology of the disease, we introduced the French Canadian founder Polr3a mutation c.2015G > A (p.G672E) in mice, generating homozygous knock-in (KI/KI) as well as compound heterozygous mice for one Polr3a KI and one null allele (KI/KO). Both KI/KI and KI/KO mice are viable and are able to reproduce. To establish if they manifest a motor phenotype, WT, KI/KI and KI/KO mice were submitted to a battery of behavioral tests over one year. The KI/KI and KI/KO mice have overall normal balance, muscle strength and general locomotion. Cerebral and cerebellar Luxol Fast Blue staining and measurement of levels of myelin proteins showed no significant differences between the three groups, suggesting that myelination is not overtly impaired in Polr3a KI/KI and KI/KO mice. Finally, expression levels of several Pol III transcripts in the brain showed no statistically significant differences. We conclude that the first transgenic mice with a leukodystrophy-causing Polr3a mutation do not recapitulate the childhood-onset HLD observed in the majority of human patients with POLR3A mutations, and provide essential information to guide selection of Polr3a mutations for developing future mouse models of the disease.

Defective axonal transport of neurofilament proteins in neurons overexpressing peripherin.

Peripherin is a type III neuronal intermediate filament detected in motor neuron inclusions of amyotrophic lateral sclerosis (ALS) patients. We previously reported that overexpression of peripherin provokes late-onset motor neuron dysfunction in transgenic mice. Here, we show that peripherin overexpression slows down axonal transport of neurofilament (NF) proteins, and that the transport defect precedes by several months the appearance of axonal spheroids in adult mice. Defective NF transport by peripherin up-regulation was further confirmed with dorsal root ganglia (DRG) neurons cultured from peripherin transgenic embryos. Immunofluorescence microscopy and western blotting revealed that excess peripherin provokes reduction in levels of hyperphosphorylated NF-H species in DRG neurites. Similarly the transport of a green fluorescent protein (GFP)-tagged NF-M, delivered by means of a lentiviral construct, was impaired in DRG neurites overexpressing peripherin. These results demonstrate that peripherin overexpression can cause defective transport of type IV NF proteins, a phenomenon that may account for the progressive formation of ALS-like spheroids in axons.

Functions of intermediate filaments in neuronal development and disease.

Five major types of intermediate filament (IF) proteins are expressed in mature neurons: the three neurofilament proteins (NF-L, NF-M, and NF-H), alpha-internexin, and peripherin. While the differential expression of IF genes during embryonic development suggests potential functions of these proteins in axogenesis, none of the IF gene knockout experiments in mice caused gross developmental defects of the nervous system. Yet, deficiencies in neuronal IF proteins are not completely innocuous. Substantial developmental loss of motor axons was detected in mice lacking NF-L and in double knockout NF-M;NF-H mice, supporting the view of a role for IFs in axon stabilization. Moreover, the absence of peripherin resulted in approximately 30% loss of small sensory axons. Mice lacking NF-L had a scarcity of IF structures and exhibited a severe axonal hypotrophy, causing up to 50% reduction in conduction velocity, a feature that would be very detrimental for large animal species. Unexpectedly, the NF-M rather than NF-H protein turned out to be required for proper radial growth of large myelinated axons. Studies with transgenic mice suggest that some types of IF accumulations, reminiscent of those found in amyotrophic lateral sclerosis (ALS), can have deleterious effects and even cause neurodegeneration. Additional evidence for the involvement of IFs in pathogenesis came from the recent discovery of neurofilament gene mutations linked to ALS and Charcot-Marie-Tooth disease (CMT2E). Conversely, we discuss how certain types of perikaryal neurofilament aggregates might confer protection in motor neuron disease.