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Huntington’s disease (HD) is a hereditary neurodegenerative disorder of the basal ganglia for which disease-modifying treatments are not yet available. Although gene-silencing therapies are currently being tested, further molecular mechanisms must be explored to identify druggable targets for HD. Cytoplasmic polyadenylation element binding proteins 1 to 4 (CPEB1 to CPEB4) are RNA binding proteins that repress or activate translation of CPE-containing transcripts by shortening or elongating their poly(A) tail. Here, we found increased CPEB1 and decreased CPEB4 protein in the striatum of patients and mouse models with HD. This correlated with a reprogramming of polyadenylation in 17.3% of the transcriptome, markedly affecting neurodegeneration-associated genes including PSEN1 , MAPT , SNCA , LRRK2 , PINK1 , DJ1 , SOD1 , TARDBP , FUS , and HTT and suggesting a new molecular mechanism in neurodegenerative disease etiology. We found decreased protein content of top deadenylated transcripts, including striatal atrophy–linked genes not previously related to HD, such as KTN1 and the easily druggable SLC19A3 (the ThTr2 thiamine transporter). Mutations in SLC19A3 cause biotin-thiamine–responsive basal ganglia disease (BTBGD), a striatal disorder that can be treated with a combination of biotin and thiamine. Similar to patients with BTBGD, patients with HD demonstrated decreased thiamine in the cerebrospinal fluid. Furthermore, patients and mice with HD showed decreased striatal concentrations of thiamine pyrophosphate (TPP), the metabolically active form of thiamine. High-dose biotin and thiamine treatment prevented TPP deficiency in HD mice and attenuated the radiological, neuropathological, and motor HD-like phenotypes, revealing an easily implementable therapy that might benefit patients with HD.

Correction of mis-splicing events is a growing therapeutic approach for neurological diseases such as spinal muscular atrophy or neuronal ceroid lipofuscinosis 7, which are caused by splicing-affecting mutations. Mis-spliced effector genes that do not harbour mutations are also good candidate therapeutic targets in diseases with more complex aetiologies such as cancer, autism, muscular dystrophies or neurodegenerative diseases. Next-generation RNA sequencing (RNA-seq) has boosted investigation of global mis-splicing in diseased tissue to identify such key pathogenic mis-spliced genes. Nevertheless, while analysis of tumour or dystrophic muscle biopsies can be informative on early stage pathogenic mis-splicing, for neurodegenerative diseases, these analyses are intrinsically hampered by neuronal loss and neuroinflammation in post-mortem brains. To infer splicing alterations relevant to Huntington's disease pathogenesis, here we performed intersect-RNA-seq analyses of human post-mortem striatal tissue and of an early symptomatic mouse model in which neuronal loss and gliosis are not yet present. Together with a human/mouse parallel motif scan analysis, this approach allowed us to identify the shared mis-splicing signature triggered by the Huntington's disease-causing mutation in both species and to infer upstream deregulated splicing factors. Moreover, we identified a plethora of downstream neurodegeneration-linked mis-spliced effector genes that-together with the deregulated splicing factors-become new possible therapeutic targets. In summary, here we report pathogenic global mis-splicing in Huntington's disease striatum captured by our new intersect-RNA-seq approach that can be readily applied to other neurodegenerative diseases for which bona fide animal models are available., (© The Author(s) (2021). Published by Oxford University Press on behalf of the Guarantors of Brain.)

Huntington's disease and X-linked dystonia parkinsonism are two monogenic basal ganglia model diseases. Huntington's disease is caused by a polyglutamine-encoding CAG repeat expansion in the Huntingtin (HTT) gene leading to several toxic interactions of both the expanded CAG-containing mRNA and the polyglutamine-containing protein, while X-linked dystonia parkinsonism is caused by a retrotransposon insertion in the TAF1 gene, which decreases expression of this core scaffold of the basal transcription factor complex TFIID. SRSF6 is an RNA-binding protein of the serine and arginine-rich (SR) protein family that interacts with expanded CAG mRNA and is sequestered into the characteristic polyglutamine-containing inclusion bodies of Huntington's disease brains. Here we report decreased levels of the SRSF6 interactor and regulator SREK1-another SR protein involved in RNA processing-which includes TAF1 as one of its targets. This led us to hypothesize that Huntington's disease and X-linked dystonia parkinsonism pathogeneses converge in TAF1 alteration. We show that diminishing SRSF6 through RNA interference in human neuroblastoma cells leads to a decrease in SREK1 levels, which, in turn, suffices to cause diminished TAF1 levels. We also observed decreased SREK1 and TAF1 levels in striatum of Huntington's disease patients and transgenic model mice. We then generated mice with neuronal transgenic expression of SREK1 (TgSREK1 mice) that, interestingly, showed transcriptomic alterations complementary to those in Huntington's disease mice. Most importantly, by combining Huntington's disease and TgSREK1 mice we verify that SREK1 overexpression corrects TAF1 deficiency and attenuates striatal atrophy and motor phenotype of Huntington's disease mice. Our results therefore demonstrate that altered RNA processing upon SREK1 dysregulation plays a key role in Huntington's disease pathogenesis and pinpoint TAF1 as a likely general determinant of selective vulnerability of the striatum in multiple neurological disorders., (© The Author(s) (2020). Published by Oxford University Press on behalf of the Guarantors of Brain.)

Huntington's disease (HD) is an inherited progressive neurodegenerative disease characterized by brain atrophy particularly in the striatum that produces motor impairment, and cognitive and psychiatric disturbances. Multiple pathogenic mechanisms have been proposed including dysfunctions in neurotrophic support and calpain-overactivation, among others. Kinase D-interacting substrate of 220 kDa (Kidins220), also known as ankyrin repeat-rich membrane spanning (ARMS), is an essential mediator of neurotrophin signaling. In adult brain, Kidins220 presents two main isoforms that differ in their carboxy-terminal length and critical protein-protein interaction domains. These variants are generated through alternative terminal exon splicing of the conventional exon 32 (Kidins220-C32) and the recently identified exon 33 (Kidins220-C33). The lack of domains encoded by exon 32 involved in key neuronal functions, including those controlling neurotrophin pathways, pointed to Kidins220-C33 as a form detrimental for neurons. However, the functional role of Kidins220-C33 in neurodegeneration or other pathologies, including HD, has not been explored. In the present work, we discover an unexpected selective downregulation of Kidins220-C33, in the striatum of HD patients, as well as in the R6/1 HD mouse model starting at early symptomatic stages. These changes are C33-specific as Kidins220-C32 variant remains unchanged. We also find the early decrease in Kidins220-C33 levels takes place in neurons, suggesting an unanticipated neuroprotective role for this isoform. Finally, using ex vivo assays and primary neurons, we demonstrate that Kidins220-C33 is downregulated by mechanisms that depend on the activation of the protease calpain. Altogether, these results strongly suggest that calpain-mediated Kidins220-C33 proteolysis modulates onset and/or progression of HD., (© 2019 International Society of Neuropathology.)

Activating transcription factor-5 (ATF5) is a stress-response transcription factor induced upon different cell stressors like fasting, amino-acid limitation, cadmium or arsenite. ATF5 is also induced, and promotes transcription of anti-apoptotic target genes like MCL1, during the unfolded protein response (UPR) triggered by endoplasmic reticulum stress. In the brain, high ATF5 levels are found in gliomas and also in neural progenitor cells, which need to decrease their ATF5 levels for differentiation into mature neurons or glia. This initially led to believe that ATF5 is not expressed in adult neurons. More recently, we reported basal neuronal ATF5 expression in adult mouse brain and its neuroprotective induction during UPR in a mouse model of status epilepticus. Here we aimed to explore whether ATF5 is also expressed by neurons in human brain both in basal conditions and in Huntington's disease (HD), where UPR has been described to be partially impaired due to defective ATF6 processing. Apart from confirming that ATF5 is present in human adult neurons, here we report accumulation of ATF5 within the characteristic polyglutamine-containing neuronal nuclear inclusions in brains of HD patients and mice. This correlates with decreased levels of soluble ATF5 and of its antiapoptotic target MCL1. We then confirmed the deleterious effect of ATF5 deficiency in a Caenorhabditis elegans model of polyglutamine-induced toxicity. Finally, ATF5 overexpression attenuated polyglutamine-induced apoptosis in a cell model of HD. These results reflect that decreased ATF5 in HD-probably secondary to sequestration into inclusions-renders neurons more vulnerable to mutant huntingtin-induced apoptosis and that ATF5-increasing interventions might have therapeutic potential for HD.

Huntington's disease (HD) is caused by a CAG-repeat encoding a polyglutamine (polyQ) tract in the huntingtin protein. There is plenty of evidence of polyQ-driven toxicity. However, CAG repeat RNA-driven alteration of splicing has recently been proposed in analogy to CUG-repeat diseases. Here we review the reported alteration of the CAG-repeat associated splicing factor SRSF6 in brains of HD patients and mouse models and how this correlates with altered splicing of, at least, two microtubule-associated proteins in HD, namely MAPT (tau) and MAP2. Regarding tau, altered splicing of exon 10 has been reported, along with increased levels and 4R/3R-tau ratio and detection of tau in a new nuclear rod-shaped histopathological hallmark termed tau nuclear rod (TNR) or tau nuclear indentation (TNI). These findings, together with an attenuation of HD phenotype in R6/1 mice with tau deficiency and subsequent studies showing increased phosphorylation in mouse models and increased levels in CSF of patients, has led to proposing HD as a tauopathy. Regarding MAP2, an increase in its juvenile form and a decrease in total MAP2 together with redistribution from dendrites to soma is observed in HD patients, which may contribute to the dendritic atrophy in HD. Furthermore, MAP2 positive structures filling nuclear indentations have occasionally been found and co-localized with tau. Therefore, altered MAP function with imbalance in tau/MAP2 content could contribute to HD striatal atrophy and dysfunction. Besides, TNIs might be indicative of such MAP abnormalities. TNIs are also found in early pathology Alzheimer's disease and in tauopathy mice over-expressing mutant 4R-tau. This indicates that tau alteration is sufficient for TNI detection, which becomes a marker of increased total tau and/or altered 4R/3R-tau ratio and reporter of pathology-associated nuclear indentations. Altogether, these recent studies suggest that correcting the SRSF6-driven missplicing and/or microtubule-associated imbalance might be of therapeutic value in HD., (© 2016 International Society of Neuropathology.)

An imbalance of tau isoforms containing either three or four microtubule-binding repeats causes frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) in families with intronic mutations in the MAPT gene. Here we report equivalent imbalances at the mRNA and protein levels and increased total tau levels in the brains of subjects with Huntington's disease (HD) together with rod-like tau deposits along neuronal nuclei. These tau nuclear rods show an ordered filamentous ultrastructure and can be found filling the neuronal nuclear indentations previously reported in HD brains. Finally, alterations in serine/arginine-rich splicing factor-6 coincide with tau missplicing, and a role of tau in HD pathogenesis is evidenced by the attenuation of motor abnormalities of mutant HTT transgenic mice in tau knockout backgrounds.

Correction of mis-splicing events is a growing therapeutic approach for neurological diseases such as spinal muscular atrophy or neuronal ceroid lipofuscinosis 7, which are caused by splicing-affecting mutations. Mis-spliced effector genes that do not harbour mutations are also good candidate therapeutic targets in diseases with more complex aetiologies such as cancer, autism, muscular dystrophies or neurodegenerative diseases. Next-generation RNA sequencing (RNA-seq) has boosted investigation of global mis-splicing in diseased tissue to identify such key pathogenic mis-spliced genes. Nevertheless, while analysis of tumour or dystrophic muscle biopsies can be informative on early stage pathogenic mis-splicing, for neurodegenerative diseases, these analyses are intrinsically hampered by neuronal loss and neuroinflammation in post-mortem brains. To infer splicing alterations relevant to Huntington's disease pathogenesis, here we performed intersect-RNA-seq analyses of human post-mortem striatal tissue and of an early symptomatic mouse model in which neuronal loss and gliosis are not yet present. Together with a human/mouse parallel motif scan analysis, this approach allowed us to identify the shared mis-splicing signature triggered by the Huntington's disease-causing mutation in both species and to infer upstream deregulated splicing factors. Moreover, we identified a plethora of downstream neurodegeneration-linked mis-spliced effector genes that-together with the deregulated splicing factors-become new possible therapeutic targets. In summary, here we report pathogenic global mis-splicing in Huntington's disease striatum captured by our new intersect-RNA-seq approach that can be readily applied to other neurodegenerative diseases for which bona fide animal models are available. [ABSTRACT FROM AUTHOR]

Huntington's disease (HD) is a fatal degenerative disorder affecting the nervous system. It is characterized by motor, cognitive, and psychiatric dysfunctions, with a late onset and an autosomal dominant pattern of inheritance. HD-causing mutation consists in an expansion of repeated CAG triplets in the huntingtin gene (HTT), encoding for an expanded polyglutamine (polyQ) stretch in the huntingtin protein (htt). The mutation causes neuronal dysfunction and loss through multiple mechanisms, affecting both the nucleus and cytoplasm. P2X7 receptor (P2X7R) emerged as a major player in neuroinflammation, since ATP – its endogenous ligand – is massively released under this condition. Indeed, P2X7R stimulation in the central nervous system (CNS) is known to enhance the release of pro-inflammatory cytokines from microglia and of neurotransmitters from neuronal presynaptic terminals, as well as to promote apoptosis. Previous experiments performed with neurons expressing the mutant huntingtin and exploiting HD mouse models demonstrated a role of P2X7R in HD. On the basis of those results, here, we explore for the first time the status of P2X7R in HD patients' brain. We report that in HD postmortem striatum, as earlier observed in HD mice, the protein levels of the full-length form of P2X7R, also named P2X7R-A, are upregulated. In addition, the exclusively human naturally occurring variant lacking the C-terminus region, P2X7R-B, is upregulated as well. As we show here, this augmented protein levels can be explained by elevated mRNA levels. Furthermore, in HD patients' striatum, P2X7R shows not only an augmented total transcript level but also an alteration of its splicing. Remarkably, P2X7R introns 10 and 11 are more retained in HD patients when compared with controls. Taken together, our data confirm that P2X7R is altered in brains of HD subjects and strengthen the notion that P2X7R may represent a potential therapeutic target for HD. [ABSTRACT FROM AUTHOR]

Huntington's disease and X-linked dystonia parkinsonism are two monogenic basal ganglia model diseases. Huntington's disease is caused by a polyglutamine-encoding CAG repeat expansion in the Huntingtin (HTT) gene leading to several toxic interactions of both the expanded CAG-containing mRNA and the polyglutamine-containing protein, while X-linked dystonia parkinsonism is caused by a retrotransposon insertion in the TAF1 gene, which decreases expression of this core scaffold of the basal transcription factor complex TFIID. SRSF6 is an RNA-binding protein of the serine and arginine-rich (SR) protein family that interacts with expanded CAG mRNA and is sequestered into the characteristic polyglutamine-containing inclusion bodies of Huntington's disease brains. Here we report decreased levels of the SRSF6 interactor and regulator SREK1-another SR protein involved in RNA processing-which includes TAF1 as one of its targets. This led us to hypothesize that Huntington's disease and X-linked dystonia parkinsonism pathogeneses converge in TAF1 alteration. We show that diminishing SRSF6 through RNA interference in human neuroblastoma cells leads to a decrease in SREK1 levels, which, in turn, suffices to cause diminished TAF1 levels. We also observed decreased SREK1 and TAF1 levels in striatum of Huntington's disease patients and transgenic model mice. We then generated mice with neuronal transgenic expression of SREK1 (TgSREK1 mice) that, interestingly, showed transcriptomic alterations complementary to those in Huntington's disease mice. Most importantly, by combining Huntington's disease and TgSREK1 mice we verify that SREK1 overexpression corrects TAF1 deficiency and attenuates striatal atrophy and motor phenotype of Huntington's disease mice. Our results therefore demonstrate that altered RNA processing upon SREK1 dysregulation plays a key role in Huntington's disease pathogenesis and pinpoint TAF1 as a likely general determinant of selective vulnerability of the striatum in multiple neurological disorders. [ABSTRACT FROM AUTHOR]

Increased incidence of neuronal nuclear indentations is a well-known feature of the striatum of Huntington's disease (HD) brains and, in Alzheimer's disease (AD), neuronal nuclear indentations have recently been reported to correlate with neurotoxicity caused by improper cytoskeletal/nucleoskeletal coupling. Initial detection of rod-shaped tau immunostaining in nuclei of cortical and striatal neurons of HD brains and in hippocampal neurons of early Braak stage AD led us to coin the term 'tau nuclear rods (TNRs).' Although TNRs traverse nuclear space, they in fact occupy narrow cytoplasmic extensions that fill indentations of the nuclear envelope and we will here refer to this histological hallmark as Tau-immunopositive nuclear indentations (TNIs). We reasoned that TNI formation is likely secondary to tau alterations as TNI detection in HD correlates with an increase in total tau, particularly of the isoforms with four tubulin binding repeats (4R-tau). Here we analyze transgenic mice that overexpress human 4R-tau with a frontotemporal lobar degeneration-tau point mutation (P301S mice) to explore whether tau alteration is sufficient for TNI formation. Immunohistochemistry with various tau antibodies, immunoelectron microscopy and double tau-immunofluorescence/DAPI-nuclear counterstaining confirmed that excess 4R-tau in P301S mice is sufficient for the detection of abundant TNIs that fill nuclear indentations. Interestingly, this does not correlate with an increase in the number of nuclear indentations, thus suggesting that excess total tau or an isoform imbalance in favor of 4R-tau facilitates tau detection inside preexisting nuclear indentations but does not induce formation of the latter. In summary, here we demonstrate that tau alteration is sufficient for TNI detection and our results suggest that the neuropathological finding of TNIs becomes a possible indicator of increased total tau and/or increased 4R/3R-tau ratio in the affected neurons apart from being an efficient way to monitor pathology-associated nuclear indentations. [ABSTRACT FROM AUTHOR]