RBM45 associates with nuclear stress bodies and forms nuclear inclusions during chronic cellular stress and in neurodegenerative diseases

RBM45 is a multifunctional RNA binding protein (RBP) found in cytoplasmic and nuclear inclusions in amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), and Alzheimer’s disease (AD). While cytoplasmic RBM45 inclusions contain other disease-associated proteins, nuclear RBM45 inclusions are morphologically and biochemically distinct from previously described nuclear inclusion pathology in these diseases. To better understand nuclear RBM45 aggregation and inclusion formation, we evaluated the association of RBM45 with a variety of membraneless nuclear organelles, including nuclear speckles, Cajal bodies, and nuclear gems. Under basal conditions, RBM45 is diffusely distributed throughout the nucleus and does not localize to a specific nuclear organelle. During cellular stress, however, the nuclear RBM45 distribution undergoes an RNA-binding dependent rearrangement wherein RBM45 coalesces into a small number of nuclear puncta. These puncta contain the nuclear stress body (NSB) markers heat shock factor 1 (HSF1) and scaffold attachment factor B (SAFB). During chronic stress, the persistent association of RBM45 with NSBs leads to the formation of large, insoluble nuclear RBM45 inclusions. RBM45 nuclear inclusions persist after stressor removal and NSB disassembly and the inclusions resemble the nuclear RBM45 pathology seen in ALS, FTLD, and AD. We also quantified the cell type- and disease-specific patterns of RBM45 pathology in ALS, FTLD, AD, and non-neurologic disease control subjects. RBM45 nuclear and cytoplasmic inclusions are found in neurons and glia in ALS, FTLD, and AD but not in controls. Across diseases, RBM45 nuclear inclusion pathology occurs more frequently than cytoplasmic RBM45 inclusion pathology and exhibits cell type-specific variation. Collectively, our results define new stress-associated functions of RBM45, a mechanism for its nuclear aggregation and inclusion formation, a role for NSBs in the pathogenesis of diseases such as ALS, FTLD, and AD, and further underscore the importance of self-association to both the normal and pathological functions of RBPs in these diseases.

Fragile X-associated tremor ataxia syndrome with co-occurrent progressive supranuclear palsy-like neuropathology

Co-occurrence of multiple neuropathologic changes is a common phenomenon, most prominently seen in Alzheimer’s disease (AD) and Parkinson’s disease (PD), complicating clinical diagnosis and patient management. Reports of co-occurring pathological processes are emerging in the group of genetically defined repeat-associated non-AUG (RAN)-translation related diseases. Here we report a case of Fragile X-associated tremor-ataxia syndrome (FXTAS) with widespread and abundant nuclear inclusions of the RAN-translation related FMRpolyG-peptide. In addition, we describe prominent neuronal and glial tau pathology representing changes seen in progressive supranuclear palsy (PSP). The highest abundance of the respective pathological changes was seen in distinct brain regions indicating an incidental, rather than causal correlation.

Poly(Q) Expansions in ATXN7 Affect Solubility but Not Activity of the SAGA Deubiquitinating Module

Spinocerebellar ataxia type 7 (SCA7) is a debilitating neurodegenerative disease caused by expansion of a polyglutamine [poly(Q)] tract in ATXN7, a subunit of the deubiquitinase (DUB) module (DUBm) in the SAGA complex. The effects of ATXN7-poly(Q) on DUB activity are not known. To address this important question, we reconstituted the DUBmin vitrowith either wild-type ATXN7 or a pathogenic form, ATXN7-92Q NT, with 92 Q residues at the N terminus (NT). We found that both forms of ATXN7 greatly enhance DUB activity but that ATXN7-92Q NT is largely insoluble unless it is incorporated into the DUBm. Cooverexpression of DUBm components in human astrocytes also promoted the solubility of ATXN7-92Q, inhibiting its aggregation into nuclear inclusions that sequester DUBm components, leading to global increases in ubiquitinated H2B (H2Bub) levels. Global H2Bub levels were also increased in the cerebellums of mice in a SCA7 mouse model. Our findings indicate that although ATXN7 poly(Q) expansions do not change the enzymatic activity of the DUBm, they likely contribute to SCA7 by initiating aggregates that sequester the DUBm away from its substrates.