ATXN2-CAG42 sequesters PABPC1 into insolubility and induces FBXW8 in cerebellum of old ataxic knock-in mice.

Spinocerebellar Ataxia Type 2 (SCA2) is caused by expansion of a polyglutamine encoding triplet repeat in the human ATXN2 gene beyond (CAG)(31). This is thought to mediate toxic gain-of-function by protein aggregation and to affect RNA processing, resulting in degenerative processes affecting preferentially cerebellar neurons. As a faithful animal model, we generated a knock-in mouse replacing the single CAG of murine Atxn2 with CAG42, a frequent patient genotype. This expansion size was inherited stably. The mice showed phenotypes with reduced weight and later motor incoordination. Although brain Atxn2 mRNA became elevated, soluble ATXN2 protein levels diminished over time, which might explain partial loss-of-function effects. Deficits in soluble ATXN2 protein correlated with the appearance of insoluble ATXN2, a progressive feature in cerebellum possibly reflecting toxic gains-of-function. Since in vitro ATXN2 overexpression was known to reduce levels of its protein interactor PABPC1, we studied expansion effects on PABPC1. In cortex, PABPC1 transcript and soluble and insoluble protein levels were increased. In the more vulnerable cerebellum, the progressive insolubility of PABPC1 was accompanied by decreased soluble protein levels, with PABPC1 mRNA showing no compensatory increase. The sequestration of PABPC1 into insolubility by ATXN2 function gains was validated in human cell culture. To understand consequences on mRNA processing, transcriptome profiles at medium and old age in three different tissues were studied and demonstrated a selective induction of Fbxw8 in the old cerebellum. Fbxw8 is encoded next to the Atxn2 locus and was shown in vitro to decrease the level of expanded insoluble ATXN2 protein. In conclusion, our data support the concept that expanded ATXN2 undergoes progressive insolubility and affects PABPC1 by a toxic gain-of-function mechanism with tissue-specific effects, which may be partially alleviated by the induction of FBXW8.

Dysregulated expression of lipid storage and membrane dynamics factors in Tia1 knockout mouse nervous tissue.

During cell stress, the transcription and translation of immediate early genes are prioritized, while most other messenger RNAs (mRNAs) are stored away in stress granules or degraded in processing bodies (P-bodies). TIA-1 is an mRNA-binding protein that needs to translocate from the nucleus to seed the formation of stress granules in the cytoplasm. Because other stress granule components such as TDP-43, FUS, ATXN2,SMN, MAPT, HNRNPA2B1, and HNRNPA1 are crucial for the motor neuron diseases amyotrophic lateral sclerosis (ALS)/spinal muscular atrophy (SMA) and for the frontotemporal dementia(FTD), here we studied mouse nervous tissue to identify mRNAs with selective dependence on Tia1 deletion. Transcriptome profiling with oligonucleotide microarrays in comparison of spinal cord and cerebellum, together with independent validation in quantitative reverse transcriptase PCR and immunoblots demonstrated several strong and consistent dysregulations. In agreement with previously reported TIA1 knock down effects, cell cycle and apoptosis regulators were affected markedly with expression changes up to +2-fold, exhibiting increased levels for Cdkn1a, Ccnf, and Tprkb vs.decreased levels for Bid and Inca1 transcripts. Novel and surprisingly strong expression alterations were detected for fat storage and membrane trafficking factors, with prominent +3-fold upregulations of Plin4, Wdfy1, Tbc1d24, and Pnpla2 vs. a −2.4-fold downregulation of Cntn4 transcript, encoding an axonal membrane adhesion factor with established haploinsufficiency.In comparison, subtle effects on the RNA processing machinery included up to 1.2-fold upregulations of Dcp1b and Tial1. The effect on lipid dynamics factors is noteworthy, since also the gene deletion of Tardbp (encoding TDP-43) and Atxn2 led to fat metabolism phenotypes in mouse. In conclusion, genetic ablation of the stress granule nucleator TIA-1 has a novel major effect on mRNAs encoding lipid homeostasis factors in the brain, similar to the fasting effect.

Insulin receptor and lipid metabolism pathology in ataxin-2 knock-out mice.

Ataxin-2 is a cytoplasmic protein, product of the SCA2 gene. Expansion of the normal polyglutamine tract in the protein leads to the neurodegenerative disorder Spino-Cerebellar Ataxia type 2 (SCA2). Although ataxin-2 has been related to polyribosomes, endocytosis and actin-cytoskeleton organization, its biological function remains unknown. In the present study, an ataxin-2 deficient mouse (Sca2(-/-)) was generated to investigate the functional role of this protein. Homozygous mice exhibited reduced fertility and locomotor hyperactivity. In analyses up to the age of 6 months, the absence of ataxin-2 led to abdominal obesity and hepatosteatosis. This was associated with reduced insulin receptor expression in liver and cerebellum, although the mRNA levels were increased indicating a post-transcriptional effect of ataxin-2 on the insulin receptor status. As in insulin resistance syndromes, insulin levels were increased in pancreas and blood serum. In the cerebellum, increased levels of gangliosides and sulfatides, as well as decreased cholesterol dynamics, may be relevant for cellular membrane functions, and alterations in the sphingomyelin cycle may affect second messengers. Thus, the data suggest altered signaling in ataxin-2 deficient organisms.

Parkinson phenotype in aged PINK1-deficient mice is accompanied by progressive mitochondrial dysfunction in absence of neurodegeneration.

BACKGROUND: Parkinson's disease (PD) is an adult-onset movement disorder of largely unknown etiology. We have previously shown that loss-of-function mutations of the mitochondrial protein kinase PINK1 (PTEN induced putative kinase 1) cause the recessive PARK6 variant of PD. METHODOLOGY/PRINCIPAL FINDINGS: Now we generated a PINK1 deficient mouse and observed several novel phenotypes: A progressive reduction of weight and of locomotor activity selectively for spontaneous movements occurred at old age. As in PD, abnormal dopamine levels in the aged nigrostriatal projection accompanied the reduced movements. Possibly in line with the PARK6 syndrome but in contrast to sporadic PD, a reduced lifespan, dysfunction of brainstem and sympathetic nerves, visible aggregates of alpha-synuclein within Lewy bodies or nigrostriatal neurodegeneration were not present in aged PINK1-deficient mice. However, we demonstrate PINK1 mutant mice to exhibit a progressive reduction in mitochondrial preprotein import correlating with defects of core mitochondrial functions like ATP-generation and respiration. In contrast to the strong effect of PINK1 on mitochondrial dynamics in Drosophila melanogaster and in spite of reduced expression of fission factor Mtp18, we show reduced fission and increased aggregation of mitochondria only under stress in PINK1-deficient mouse neurons. CONCLUSION: Thus, aging Pink1(-/-) mice show increasing mitochondrial dysfunction resulting in impaired neural activity similar to PD, in absence of overt neuronal death.

Parkinson patient fibroblasts show increased alpha-synuclein expression.

Parkinson's disease (PD) is a neurodegenerative movement disorder of advanced age with largely unknown etiology, but well documented tissue damage from oxidative stress. Increased alpha-synuclein (SNCA) expression is known to cause a rare form of PD, early-onset autosomal dominant PARK4. We have previously shown that loss-of-function mutations of the mitochondrial kinase PINK1 which cause the early-onset recessive PARK6 variant result in oxidative damage in patient fibroblasts. We now investigated the molecular chain of events from mitochondrial dysfunction to cell death which is largely unknown. Primary skin fibroblast cultures from patients were analysed for gene expression anomalies. In G309D-PINK1 patient fibroblasts, mainly genes regulated by oxidative stress, as well as genes encoding synaptic proteins such as SNCA showed altered expression. The induction of SNCA was also observed in control fibroblasts with knock-down of PINK1. The induction of SNCA expression was found to constitute a specific disease biomarker in sporadic PD patient fibroblasts. To understand the mechanism of this induction, we exposed control fibroblasts to oxidative, proteasomal and endoplasmic reticulum stress and were able to trigger the SNCA expression upregulation. Our data indicate that loss-of-function of PINK1 leads to enhanced alpha-synuclein expression and altered cell-cell contact. Alpha-synuclein induction might represent a common event for different variants of PD as well as a PD-specific trigger of neurodegeneration. We propose that the expression changes described might potentially serve as biomarkers that allow objective PD patient diagnosis in an accessible, peripheral tissue.