Kathryn Volkening

Divergent patterns of cytosolic TDP-43 and neuronal progranulin expression following axotomy: Implications for TDP-43 in the physiological response to neuronal injury

We have performed sciatic axotomies in adult C57BL/6 mice and observed TDP-43 and progranulin (PGRN) expression patterns over 28 days. TDP-43 expression was markedly upregulated in axotomized motor neurons, with prominent cytosolic immunoreactivity becoming maximal by post-injury day 7 and returning to baseline levels by post-injury day 28. Increased TDP-43 expression was confirmed by western blot. TDP-43 mRNA expression was also increased. This was inversely correlated with neuronal PGRN expression which was clearly reduced by day 7 with a return to baseline by post-injury day 28. In contrast, microglial PGRN expression was dramatically increased, and correlated with the inflammatory response to axotomy. Cytosolic TDP-43 colocalized with Staufen and TIA-1, markers for RNA transport and stress granules respectively. We did not observe colocalization of TDP-43 or PGRN with degradative granules (P-bodies) or activated caspase 3. These results indicate that TDP-43 expression is altered in response to neuronal injury and that normal expression is restored following recovery. These findings suggest that the upregulation of TDP-43 expression with prominent cytosolic localization in motor neurons injured by degenerative processes such as ALS may actually represent an appropriate response to neuronal injury.

Ataxin-2 repeat-length variation and neurodegeneration

Expanded glutamine repeats of the ataxin-2 (ATXN2) protein cause spinocerebellar ataxia type 2 (SCA2), a rare neurodegenerative disorder. More recent studies have suggested that expanded ATXN2 repeats are a genetic risk factor for amyotrophic lateral sclerosis (ALS) via an RNA-dependent interaction with TDP-43. Given the phenotypic diversity observed in SCA2 patients, we set out to determine the polymorphic nature of the ATXN2 repeat length across a spectrum of neurodegenerative disorders. In this study, we genotyped the ATXN2 repeat in 3919 neurodegenerative disease patients and 4877 healthy controls and performed logistic regression analysis to determine the association of repeat length with the risk of disease. We confirmed the presence of a significantly higher number of expanded ATXN2 repeat carriers in ALS patients compared with healthy controls (OR = 5.57; P= 0.001; repeat length >30 units). Furthermore, we observed significant association of expanded ATXN2 repeats with the development of progressive supranuclear palsy (OR = 5.83; P= 0.004; repeat length >30 units). Although expanded repeat carriers were also identified in frontotemporal lobar degeneration, Alzheimers and Parkinsons disease patients, these were not significantly more frequent than in controls. Of note, our study identified a number of healthy control individuals who harbor expanded repeat alleles (31-33 units), which suggests caution should be taken when attributing specific disease phenotypes to these repeat lengths. In conclusion, our findings confirm the role of ATXN2 as an important risk factor for ALS and support the hypothesis that expanded ATXN2 repeats may predispose to other neurodegenerative diseases, including progressive supranuclear palsy.

Cytosolic TDP-43 expression following axotomy is associated with caspase 3 activation in NFL-/- mice: support for a role for TDP-43 in the physiological response to neuronal injury.

TAR DNA binding protein (TDP-43) mislocalization has been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). We have recently reported that TDP-43 and PGRN expression is altered in response to axotomy in C57BL6 mice and that normal expression is restored following recovery. We have performed axotomies in two different presymptomatic models of motor neuron degeneration, low molecular weight neurofilament knockout (NFL(-/-)) mice and mutant SOD1(G93A) transgenic (mtSOD1(G93A)) mice aged 6 weeks, and observed TDP-43 and PGRN expression patterns in axotomized spinal motor neurons over 28 days. In contrast to both C57BL6 mice and mtSOD1(G93A) mice, behavioural deficits in NFL(-/-) mice were sustained. We did not observe differences in TDP-43 or PGRN expression between C57BL6 mice and mtSOD1(G93A) mice throughout the observation period. However, compared to C57BL6 mice and mtSOD1(G93A) mice, NFL(-/-) mice exhibited late upregulation of cytosolic TDP-43 expression and persistent downregulation of neuronal PGRN expression accompanied by caspase 3 activation on post-injury day 28. By post-injury day 42, no cytosolic TDP-43-positive neurons remained in NFL(-/-) mice, suggesting that they had undergone apoptotic cell death. These findings suggest that whereas TDP-43 expression is normally upregulated transiently following axotomy, in the absence of NFL this response is delayed and associated with caspase 3 activation and neuronal death. These results further support that TDP-43 is involved in neurofilament mRNA metabolism and transport, and provide insight into the pathogenesis of motor neuron death in ALS in which NFL mRNA levels are selectively suppressed.

Altered microRNA expression profile in amyotrophic lateral sclerosis: a role in the regulation of NFL mRNA levels

BackgroundAmyotrophic Lateral Sclerosis (ALS) is a progressive, adult onset, fatal neurodegenerative disease of motor neurons. There is emerging evidence that alterations in RNA metabolism may be critical in the pathogenesis of ALS. MicroRNAs (miRNAs) are small non-coding RNAs that are key determinants of mRNA stability. Considering that miRNAs are increasingly being recognized as having a role in a variety of neurodegenerative diseases, we decided to characterize the miRNA expression profile in spinal cord (SC) tissue in sporadic ALS (sALS) and controls. Furthermore, we performed functional analysis to identify a group of dysregulated miRNAs that could be responsible for the selective suppression of low molecular weight neurofilament (NFL) mRNA observed in ALS.ResultsUsing TaqMan arrays we analyzed 664 miRNAs and found that a large number of miRNAs are differentially expressed in ventral lumbar SC in sALS compared to controls. We observed that the majority of dysregulated miRNAs are down-regulated in sALS SC tissues. Ingenuity Pathway Analysis (IPA) showed that dysregulated miRNAs are linked with nervous system function and cell death. We used two prediction algorithms to develop a panel of miRNAs that have recognition elements within the human NFL mRNA 3′UTR, and then we performed functional analysis for these miRNAs. Our results demonstrate that three miRNAs that are dysregulated in sALS (miR-146a*, miR-524-5p and miR-582-3p) are capable of interacting with NFL mRNA 3′UTR in a manner that is consistent with the suppressed steady state mRNA levels observed in spinal motor neurons in ALS.ConclusionsThe miRNA expression profile is broadly altered in the SC in sALS. Amongst these is a group of dysregulated miRNAs directly regulate the NFL mRNA 3′UTR, suggesting a role in the selective suppression of NFL mRNA in the ALS spinal motor neuron neurofilamentous aggregate formation.

Length of normal alleles of C9ORF72 GGGGCC repeat do not influence disease phenotype

Expansions of the noncoding GGGGCC hexanucleotide repeat in the Chromosome 9 open reading frame 72 (C9ORF72) gene cause frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). In this study we aimed to determine whether the length of the normal-unexpanded-allele of the GGGGCC repeat in C9ORF72 plays a role in the presentation of disease or affects age at onset in C9ORF72 mutation carriers. We also studied whether the GGGGCC repeat length confers risk or affects age at onset in FTD and ALS patients without C9ORF72 repeat expansions. C9ORF72 genotyping was performed in 580 FTD, 995 ALS, and 160 FTD-ALS patients, and 1444 controls, leading to the identification of 211 patients with pathogenic C9ORF72 repeat expansions. No meaningful association between the repeat length of the normal alleles of the GGGGCC repeat in C9ORF72 and disease phenotype or age at onset was observed in C9ORF72 mutation carriers or nonmutation carriers.

RNA metabolism in ALS: when normal processes become pathological.

Abstract Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease caused by the death of motor neurons. While the exact molecular and cellular basis for motor neuron death is not yet fully understood, the current conceptualization is that multiple aberrant biological processes contribute. Among these, one of the most compelling is based on alterations of RNA metabolism. In this review, we examine how the normal process of cellular response to stress leading to RNA stress granule formation might become pathological, resulting in the formation of stable protein aggregates. We discuss the emerging roles of post-translational modifications of RNA binding proteins in the genesis of these aggregates. We also review the contemporary literature regarding the potential role for more widespread alterations in RNA metabolism in ALS, including alterations in miRNA biogenesis, spliceosome integrity and RNA editing. A hypothesis is presented in which aberrant RNA processing, modulated through pathological stress granule formation as a reflection of either mutations within intrinsically disordered or prion-like domains of critical RNA binding proteins, or the post-translational modification of RNA binding proteins, contributes directly to motor neuron death.

The complement factor C5a receptor is upregulated in NFL-/- mouse motor neurons.

In NFL-/- mice, a model of motor neuron degeneration in ALS, degenerating spinal motor neurons express high levels of the receptor for the C5a anaphylatoxin (C5aR) early in the disease process. C5a is a potent in vitro neurotoxin for both Neuro2A and NGF-differentiated PC12 cells. While no interaction was observed between glutamate and C5a, both C5a and kainate upregulated the expression of activated C5aR. C5aR expression was increased in motor neurons in ALS. This data suggests that the early upregulation of C5aR may contribute to motor neuron damage that potentiates excitotoxicity in ALS.

TDP-43 and FUS/TLS: sending a complex message about messenger RNA in amyotrophic lateral sclerosis?

TAR DNA binding protein of 43 kDa (TDP‐43) and fused in sarcoma/translocated in liposarcoma (FUS/TLS) have recently been linked to the pathology of amyotrophic lateral sclerosis (ALS). These proteins share many common features that include interaction with either DNA or RNA, participation in the formation of RNP complexes, the formation of pathological aggregates in degenerating motor neurons in ALS, and the ability to impact the RNA metabolism pathway at multiple levels from transcription to translation. Coupled with the observation that mutations in either TDP‐43 or FUS/TLS are associated with ALS, this provides further support for the integral role of altered RNA metabolism in ALS.

Human low molecular weight neurofilament (NFL) mRNA interacts with a predicted p190RhoGEF homologue (RGNEF) in humans

In the mouse, p190RhoGEF is a low molecular weight neurofilament (NFL) mRNA stability factor that is involved in NF aggregate formation in neurons. A human homologue of this protein has not been described. Our objective was to identify a human homologue of p190RhoGEF, and to determine its interaction with human NFL mRNA. We used sequence homology searches to predict a human homologue (RGNEF), and RT-PCR to determine the expression of mRNA in ALS and neuropathologically normal control tissues. Gel shift assays determined the interaction of RGNEF with human NFL mRNA in vitro, while IP-RT-PCR and gel shift assays were used to confirm the interaction in tissue lysates. We determined that RGNEF is a human homologue of p190RhoGEF, and that its RNA is expressed in both brain and spinal cord. While RGNEF and NFL mRNA interact directly in vitro, interestingly they only appear to interact in ALS lysates and not in controls. These data add another player to the family of NFL mRNA stability regulators, and raise the intriguing possibility that the mechanism by which p190RhoGEF contributes to murine neuronal NF aggregate formation may be important to human ALS NF aggregate formation.

Detection of a novel frameshift mutation and regions with homozygosis within ARHGEF28 gene in familial amyotrophic lateral sclerosis.

Abstract Rho guanine nucleotide exchange factor (RGNEF) is a novel NFL mRNA destabilizing factor that forms neuronal cytoplasmic inclusions in spinal motor neurons in both sporadic (SALS) and familial (FALS) ALS patients. Given the observation of genetic mutations in a number of mRNA binding proteins associated with ALS, including TDP-43, FUS/TLS and mtSOD1, we analysed the ARHGEF28 gene (approx. 316 kb) that encodes for RGNEF in FALS cases to determine if mutations were present. We performed genomic sequencing, copy number variation analysis using TaqMan real-time PCR and spinal motor neuron immunohistochemistry using a novel RGNEF antibody. In this limited sample of FALS cases (n=7) we identified a heterozygous mutation that is predicted to generate a premature truncated gene product. We also observed extensive regions of homozygosity in the ARHGEF28 gene in two FALS patients. In conclusion, our findings of genetic alterations in the ARHGEF28 gene in cases of FALS suggest that a more comprehensive genetic analysis would be warranted.