Pathophysiological implications of RNP granules in frontotemporal dementia and ALS

Neurodegenerative diseases are a group of chronic, progressive, age-related disorders that are becoming increasingly prevalent in the ageing population. Despite the variety of clinical features observed, neurodegenerative diseases are characterised by protein aggregation and deposition at the molecular level. The nature of such intracellular protein aggregates is dependent on disease type and specific to disease subtype. Frontotemporal dementia and amyotrophic lateral sclerosis (ALS) are two overlapping neurodegenerative diseases, exhibiting pathological aggregates commonly composed of the proteins: Fused in Sarcoma (FUS) or Transactive Response DNA Binding Protein of 43 KDa (TDP-43). The presence of these protein aggregates in late disease stages is suggestive of a converging underlying mechanism of pathology across diseases involving disrupted proteostasis. Despite this, at present there are no effective therapeutics for the diseases, with current treatment strategies generally tending to be only for symptom management. An area of research that has gained increased interest in recent years is the formation and maintenance of ribonucleoprotein (RNP) granules. These are membraneless organelles that consist of RNA and protein elements, which can be either constitutively expressed (such as nuclear paraspeckles) or upregulated under conditions of cellular stress as an adaptive response (such as cytoplasmic stress granules). RNA-binding proteins are a key component of RNP granules, and crucially some of which, for example FUS and TDP-43, are also neurodegenerative disease-associated proteins. Therefore, a better understanding of RNA-binding proteins in RNP granule formation and the regulation and maintenance of RNP granule biophysical properties and dynamics may provide insights into mechanisms contributing to disrupted proteostasis in neurodegenerative pathology; and thus open up new avenues for therapeutic discovery and development. This review will focus on stress granule and paraspeckle RNP granules, and discuss their possible contribution to pathology in cases of frontotemporal dementia and ALS.