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

Abstract

RBM45 is a multifunctional RNA binding protein (RBP) found in cytoplasmic and nuclear inclusions in amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), and Alzheimers 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.