Cholecystokinin 1 receptor activation restores normal mTORC1 signaling and is protective to Purkinje cells of SCA mice

Spinocerebellar ataxias (SCAs) are a group of genetic diseases characterized by progressive ataxia and neurodegeneration, often in cerebellar Purkinje neurons. A SCA1 mouse model, Pcp2-ATXN1[30Q]D776, has severe ataxia in absence of progressive Purkinje neuron degeneration and death. Previous RNA-seq analyses identify cerebellar upregulation of the peptide hormone cholecystokinin (Cck) in Pcp2-ATXN1[30Q]D776 mice. Importantly, absence of Cck1 receptor (Cck1R) in Pcp2-ATXN1[30Q]D776 mice confers a progressive disease with Purkinje neuron death. Administration of a Cck1R agonist, A71623, to Pcp2-ATXN1[30Q]D776;Cck-/- and Pcp2-AXTN1[82Q] mice dampens Purkinje neuron pathology and associated deficits in motor performance. In addition, A71623 administration improves motor performance of Pcp2-ATXN2[127Q] SCA2 mice. Moreover, the Cck1R agonist A71623 corrects mTORC1 signaling and improves expression of calbindin in cerebella of AXTN1[82Q] and ATXN2[127Q] mice. These results indicate that manipulation of the Cck-Cck1R pathway is a potential therapeutic target for treatment of diseases involving Purkinje neuron degeneration.

Receptor protein tyrosine phosphatases control Purkinje neuron firing

Spinocerebellar ataxias (SCA) are a genetically heterogeneous family of cerebellar neurodegenerative diseases characterized by abnormal firing of Purkinje neurons and degeneration. We recently demonstrated the slowed firing rates seen in several SCAs share a common etiology of hyper-activation of the Src family of non-receptor tyrosine kinases (SFKs). However, the lack of clinically available neuroactive SFK inhibitors lead us to investigate alternative mechanisms to modulate SFK activity. Previous studies demonstrate that SFK activity can be enhanced by the removal of inhibitory phospho-marks by receptor-protein-tyrosine phosphatases (RPTPs). In this Extra View we show that MTSS1 inhibits SFK activity through the binding and inhibition of a subset of the RPTP family members, and lowering RPTP activity in cerebellar slices with peptide inhibitors increases the suppressed Purkinje neuron basal firing rates seen in two different SCA models. Together these results identify RPTPs as novel effectors of Purkinje neuron basal firing, extending the MTSS1/SFK regulatory circuit we previously described and expanding the therapeutic targets for SCA patients.

The clinical and genetic spectrum of spinocerebellar ataxia 14

Spinocerebellar ataxia 14 (SCA14) is associated with missense mutations in the protein kinase C gamma gene (PRKCG), rather than a nucleotide repeat expansion. In this large-scale study of PRKCG in patients with ataxia, two new missense mutations, an in-frame deletion, and a possible splice site mutation were found and can now be added to the four previously described missense mutations. The genotype/phenotype correlations in these families are described.

A novel H101Q mutation causes PKCγ loss in spinocerebellar ataxia type 14

Spinocerebellar ataxia type 14 (SCA14) is an autosomal dominant neurodegenerative disorder, first described in a Japanese family, showing linkage to chromosome 19q13.4-qter. Recently, mutations have been identified in the PRKCG gene in families with SCA14. The PRKCG gene encodes the protein kinase Cgamma (PKCgamma), a member of a serine/threonine kinase family involved in signal transduction important for several cellular processes, including cell proliferation and synaptic transmission. To identify the disease-causing mutation in a large group of ataxia patients, we searched for mutations in the PRKCG gene. We ascertained 366 unrelated patients with spinocerebellar ataxia, either pure or with associated features such as epilepsy, mental retardation, seizures, paraplegia, and tremor. A C-to-G transversion in exon 4, resulting in a histidine-to-glutamine change at codon 101 of the PKCgamma protein, was identified in patients from a family with slowly progressive pure cerebellar ataxia. Functional studies performed in HEK293 cells transfected with normal or mutant construct showed that this mutation affects PKCgamma stability or solubility, verified by time-dependent decreased protein levels in cell culture. In conclusion, the H101Q mutation causes slowly progressive uncomplicated ataxia by interfering with PKCgamma stability or solubility, which consequently may cause in either case a decrease in the overall PKCgamma-dependent phosphorylation.

Mutant protein kinase Cgamma found in spinocerebellar ataxia type 14 is susceptible to aggregation and causes cell death

Spinocerebellar ataxia type 14 (SCA14) is an autosomal dominant neurodegenerative disease characterized by various symptoms including cerebellar ataxia. Recently, several missense mutations in the protein kinase Cgamma (gammaPKC) gene have been found in different SCA14 families. To elucidate how the mutant gammaPKC causes SCA14, we examined the molecular properties of seven mutant (H101Y, G118D, S119P, S119F, Q127R, G128D, and F643L) gammaPKCs fused with green fluorescent protein (gammaPKC-GFP). Wild-type gammaPKC-GFP was expressed ubiquitously in the cytoplasm of CHO cells, whereas mutant gammaPKC-GFP tended to aggregate in the cytoplasm. The insolubility of mutant gammaPKC-GFP to Triton X-100 was increased and correlated with the extent of aggregation. gammaPKC-GFP in the Triton-insoluble fraction was rarely phosphorylated at Thr(514), whereas gammaPKC-GFP in the Triton-soluble fraction was phosphorylated. Furthermore, the stimulation of the P2Y receptor triggered the rapid aggregation of mutant gammaPKC-GFP within 10 min after transient translocation to the plasma membrane. Overexpression of the mutant gammaPKC-GFP caused cell death that was more prominent than wild type. The cytotoxicity was exacerbated in parallel with the expression level of the mutant. These results indicate that SCA14 mutations make gammaPKC form cytoplasmic aggregates, suggesting the involvement of this property in the etiology of SCA14.

Deficiency in 3'-phosphoglycolate processing in human cells with a hereditary mutation in tyrosyl-DNA phosphodiesterase (TDP1)

Tyrosyl-DNA phosphodiesterase (TDP1) is a DNA repair enzyme that removes peptide fragments linked through tyrosine to the 3′ end of DNA, and can also remove 3′-phosphoglycolates (PGs) formed by free radical-mediated DNA cleavage. To assess whether TDP1 is primarily responsible for PG removal during in vitro end joining of DNA double-strand breaks (DSBs), whole-cell extracts were prepared from lymphoblastoid cells derived either from spinocerebellar ataxia with axonal neuropathy (SCAN1) patients, who have an inactivating mutation in the active site of TDP1, or from closely matched normal controls. Whereas extracts from normal cells catalyzed conversion of 3′-PG termini, both on single-strand oligomers and on 3′ overhangs of DSBs, to 3′-phosphate termini, extracts of SCAN1 cells did not process either substrate. Addition of recombinant TDP1 to SCAN1 extracts restored 3′-PG removal, allowing subsequent gap filling on the aligned DSB ends. Two of three SCAN1 lines examined were slightly more radiosensitive than normal cells, but only for fractionated radiation in plateau phase. The results suggest that the TDP1 mutation in SCAN1 abolishes the 3′-PG processing activity of the enzyme, and that there are no other enzymes in cell extracts capable of processing protruding 3′-PG termini. However, the lack of severe radiosensitivity suggests that there must be alternative, TDP1-independent pathways for repair of 3′-PG DSBs.

Protein kinase C gamma mutations in spinocerebellar ataxia 14 increase kinase activity and alter membrane targeting

The protein kinase C gamma (PKCgamma) gene is mutated in spinocerebellar ataxia type 14 (SCA14). In this study, we investigated the effects of two SCA14 missense mutations, G118D and C150F, on PKCgamma function. We found that these mutations increase the intrinsic activity of PKCgamma. Direct visualization of labelled PKCgamma in living cells demonstrates that the mutant protein translocates more rapidly to selected regions of the plasma membrane in response to Ca2+ influx. These results point to specific alterations in mutant PKCgamma function that could lead to the selective neuronal degeneration of SCA14.

Mutation in the catalytic domain of protein kinase C gamma and extension of the phenotype associated with spinocerebellar ataxia type 14

BACKGROUND

Autosomal dominant cerebellar ataxias comprise a clinically, neuropathologically, and genetically heterogeneous group of neurodegenerative disorders. The vast majority of cases are caused by trinucleotide or pentanucleotide repeat expansions in 9 different genes. Spinocerebellar ataxia type 14 (SCA14) is a relatively pure form of autosomal dominant cerebellar ataxia mapped to chromosome 19q and caused by missense mutations in the gene encoding protein kinase C gamma (PRKCG), which are all located in the regulatory domain.

OBJECTIVES

To identify new SCA14 families and to describe the associated phenotype.

METHODS

We describe a new SCA14 family of French ancestry with 14 patients and 4 probably affected individuals. Linkage to the SCA14 locus was evaluated according to standard procedures using 5 markers covering the SCA14 candidate interval. All 18 exons of the PRKCG gene and splice junctions were screened with direct sequencing in the index patient.

RESULTS

Linkage to the SCA14 locus was established with lod scores greater than 3 in the interval between DNA segments D19S571 and D19S926. Direct sequencing of the PRKCG gene revealed a T-to-C transition in exon 18 responsible for a novel missense mutation, F643L, which mapped to a highly conserved amino acid of the catalytic domain of protein kinase C gamma. The mutation showed complete segregation with the disease phenotype, was present in all affected and probably affected individuals, and was not observed on 410 control chromosomes from healthy white subjects. Age at onset, assessed in 14 affected individuals, was broader than in previous reports and ranged from childhood to age 60 years. All affected patients had slowly progressive cerebellar ataxia frequently associated with brisk reflexes. Cognitive impairment was also a striking feature in this family and has not been reported previously. Interestingly, there was no axial myoclonus as reported in a Japanese SCA14 family, but electrophysiological recordings in a single patient showed diffuse myoclonus in the arms and legs.

CONCLUSIONS

We have identified a new SCA14 family with the first mutation (F643L) located in the catalytic domain of the enzyme. The wide range of ages at onset, the presence of myoclonus in the limbs, and the presence of cognitive impairment extend the phenotype associated with this genetic entity.

USP7, a ubiquitin-specific protease, interacts with ataxin-1, the SCA1 gene product

Spinocerebellar ataxia type 1 (SCA1) is an autosomal-dominant neurodegenerative disorder characterized by ataxia and progressive motor deterioration. SCA1 has been known to associate with elongated polyglutamine tract in ataxin-1, the SCA1 gene product. Using the yeast two-hybrid system, we have found that USP7, a ubiquitin-specific protease, binds to ataxin-1. Further experiments with deletion mutants indicated that the C-terminal region of ataxin-1 was essential for the interaction. Liquid beta-galactosidase assay and coimmunoprecipitation experiments revealed that the strength of the interaction between USP7 and ataxin-1 is influenced by the length of the polyglutamine tract in the ataxin-1; weaker interaction was observed in mutant ataxin-1 with longer polyglutamine tract and USP7 was not recruited to the mutant ataxin-1 aggregates in the Purkinje cells of SCA1 transgenic mice. Our results suggest that altered function of the ubiquitin system can be involved in the pathogenesis of spinocerebellar ataxia type 1.

Abnormal activity of membrane phospholipid synthetic enzymes in the brain of patients with Friedreich's ataxia and spinocerebellar atrophy type-1

Much evidence, derived from biochemical studies of both blood and autopsied brain, has suggested that phospholipid metabolism is abnormal in patients with Friedreich's ataxia (FA), a disorder characterized by severe neuronal loss in the spinal cord and lower brain stem with no, or only modest, damage in other brain regions. To establish the cause of our recent finding of reduced brain levels of phospholipids in FA, we assayed activities of 10 phospholipid-metabolizing enzymes in the autopsied cerebellar cortex of patients with the disorder and, for comparison, in a group of patients with spinocerebellar ataxia type 1 (SCA-1), a disease characterized, unlike FA, by marked neuronal loss in the cerebellar cortex. Enzyme activities were also measured in four brain areas which are relatively unaffected morphologically in both FA and SCA-1. We found that ethanolamine kinase activity was increased in multiple brain regions of patients with FA (increased 31%-137%) and, more modestly, in SCA-1 (increased 39%-60%), suggesting a nonspecific enhancement of phosphoethanolamine production in both disorders. In contrast, the activity of phosphoethanolamine cytidylyltransferase (PECT), the rate-limiting enzyme of phosphatidylethanolamine synthesis, was significantly and markedly decreased by 35%-78% in the cerebellar, frontal, and occipital cortices of patients with FA but was normal in SCA-1. Reduced PECT activity in FA may explain the lower brain levels of phosphatidylethanolamine in the disorder. Moreover, because decreased PECT activity in FA occurs in brain regions having no, or only modest, morphologic damage, this may represent a systemic change consequent to the frataxin gene defect. Our data also suggest that therapeutic intervention in FA designed to increase synthesis of membrane phospholipids may warrant further investigation.