Review www.cell-stress.com Role of RNA Binding Proteins with prion-like domains in muscle and neuromuscular diseases Gina Picchiarelli1 and Luc Dupuis1,* 1 Université de Strasbourg, INSERM, Mécanismes Centraux et Périphériques de la Neurodégénérescence, UMR_S 1118, Strasbourg, France. * Corresponding Author: Luc Dupuis, Université de Strasbourg, INSERM, Mécanismes Centraux et Périphériques de la Neurodégénérescence, UMR_S 1118, Strasbourg, France; E-mail: [email protected] ABSTRACT A number of neuromuscular and muscular diseases, including am- doi: 10.15698/cst2020.04.217 yotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA) and several Received originally: 28.11.2019 myopathies, are associated to mutations in related RNA-binding proteins in revised form: 24.02.2020, (RBPs), including TDP-43, FUS, MATR3 or hnRNPA1/B2. These proteins harbor Accepted 02.03.2020, Published 10.03.2020. similar modular primary sequence with RNA binding motifs and low complexi- ty domains, that enables them to phase separate and create liquid microdo- mains. These RBPs have been shown to critically regulate multiple events of Keywords: RNA-Binding Protein (RBP), RNA lifecycle, including transcriptional events, splicing and RNA trafficking Muscle, Dystrophy, Amyotrophic lateral and sequestration. Here, we review the roles of these disease-related RBPs in sclerosis (ALS), Spinal muscular atrophy (SMA), Inclusion body myopathy (IBM), muscle and motor neurons, and how their dysfunction in these cell types Fragile X-associated tremor / ataxia might contribute to disease. syndrome (FXTAS), Multisystem proteinopathy (MSP), Huntington’s disease. Abbreviatons: AchR – acetylcholine receptor; ALS – KEY CONCEPTS amyotrophic lateral sclerosis; FTD – fronto-temporal dementia; hnRNP – heterogenous nuclear ribo- • Mutations in RNA-binding proteins with prion like domains cause neu- nucleoprotein; IBM – Inclusion body romuscular diseases. myositis; NMJ – neuromuscular junction; OPMD – oculopharyngeal • RNA-binding proteins with “prion-like domains” are involved in multiple muscular dystrophy; PrLD – prion-like steps of RNA metabolism. domain; RBP – RNA binding protein; RGG – Arginine-Glycine-Glycine rich • RNA-binding proteins with prion like domains regulate muscular devel- domain; SMA – spinal muscular opment and function. atrophy; SMN – survival of motor neurons. INTRODUCTION distal myopathies. Although the distinction between pri- Neuromuscular diseases collectively affect muscle function, mary muscle and primary neuronal neuromuscular diseas- either by directly impairing muscle structure or function, or es might a priori seem obvious, there are significant clinical by affecting muscle control by motor neurons. As a conse- and genetic overlaps between these diseases [2–4]. In this quence of impaired muscle function, patients develop review, we describe how mutations in functionally related weakness that can be progressive and lead to paralysis and RNA-binding proteins (RBPs) are associated with both mus- early death. Amyotrophic lateral sclerosis (ALS) and spinal cle and motor neuron diseases, and how these mutations muscular atrophy (SMA) are two typical diseases of the participate in compromising the neuromuscular system. motor neurons, in which muscle weakness is primarily The most important neuromuscular diseases considered caused by the degeneration of motor neurons [1]. In con- are presented in BOX 1. trast, myopathies primarily affect muscle structure and/or In recent years, genetics uncovered a large number of function with clinically affected muscles either proximal, causes of neuromuscular diseases. Interestingly, a subset such as in limb girdle muscle dystrophy, and/or distal in of genes causing either motor neuron diseases or myopa- OPEN ACCESS | www.cell-stress.com 76 Cell Stress | APRIL 2020 | Vol. 4 No. 4 G. Picchiarelli and L. Dupuis (2020) RBPs in neuromuscular diseases BOX 1 | Clinical definition of the major neuromuscular diseases reviewed here. Amyotrophic lateral sclerosis ALS: is a neurodegenerative disease whose first symptoms generally occur around 60 years of age and which is characterized by progressive paralysis leading to death within a few years after onset. ALS is associated with the degeneration of lower (spinal and bulbar) and upper (cortical) motor neurons. Fronto-temporal dementia FTD: is a form of dementia involving progressive atrophy of frontal and temporal lobes of the cerebral cortex. FTD affects individuals usually between 55 and 65 years, and is characterized by alterations in cognitive functions (behavior, language, sociability, personality) and loss of memory. Inclusion body myositis IBM: is a subtype of inflammatory myopathy, usually occurring in patients older than 50 years of age. IBM leads to progressive muscle weakness and histopathological features such as atrophic muscle fibers, infiltration of non-necrotic myofibers, rimmed vacuoles or congophilic inclusions. Multisystem proteinopathy: is a degenerative disorder that can manifest either as IBM, FTD, ALS, or can be associated with Paget disease of the bone or combinations between these different diseases. Spinal muscular atrophy SMA: is a juvenile neuromuscular disorder characterized by loss of motor neurons, muscular weakness and muscle wasting. Four types of SMA are defined according to disease onset and severity: type I is the most thies encode proteins that bind RNA (hence RNA-binding to 3’ untranslated regions (UTR) of mRNAs [15]. Besides proteins,severe formRBPs) and and begins share before a number the age of ofbiochemical six months, and type II beginsmRNAs, between a number the ages of theseof six andRBPs 18 are months, also critical type IIIin beginsthe life functional properties. RBPs associated to neuromuscular cycle of small RNA species, in particular microRNA biogen- in childhood or adolescence and type IV which begins in adulthood and is the least severe form. SMA is caused by loss of diseases are part of a large group of proteins involved in esis [16–18]. mRNAsexpression lifecycle, of the that SMN are protein. collectively termed heterogene- The so-called PrLD is typically found in most disease as- ous nuclear ribonucleoproteins (hnRNPs). Many of these sociated RBPs. It consists of a domain of low primary se- hnRNPs also display a low complexity domain that resem- quence complexity, rich in uncharged polar amino acids bles yeast prions and is called “prion-like domain” (PrLD). (asparagine, glutamine, and tyrosine) and in glycine [19, Most of these PrLD containing RBPs are associated with 20] and displays high similarity to yeast proteins with prion human diseases [5–9], in particular neuromuscular diseas- properties [21]. In the human genome, more than 200 en- es. coded proteins display a PrLD, and a large proportion of In this review, we describe the general properties of these also include RNA binding motifs [14, 19, 20]. The disease associated RBPs. We then provide specific exam- combination of RNA binding properties with PrLDs allows ples for the involvement of RBPs in neuromuscular diseas- RBPs to phase separate in liquid compartments. Liquid es. Liquid Phase Separation (LLPS) is a disassembly mechanism of two liquids resulting in the appearance of two phases MODULAR STRUCTURE AND GENERAL CELLULAR [22]. This leads to the rapid and reversible creation of liq- FUNCTIONS OF RBPs uid microdomains (so called membrane-less organelles), RBPs associated to neuromuscular diseases display a mod- physically separated from the rest of the cell, and allowing ular structure with well identified subdomains. First, inter- specialized functions. [14]. In this respect, RBPs are re- action of RBPs with RNA occurs through relatively limited quired for the generation and maintenance of key nuclear sets of protein modules [10] in particular RNA recognition subdomains such as nucleoli, paraspeckles, gems, Cajal domains (RRM) and/or hnRNPK homology domain (KH). bodies, P-bodies or cytoplasmic stress granules through Other protein domains are variably present in RBPs and LLPS. Disease associated mutations in RBPs compromise include Arginine-Glycine-Glycine rich domain (RGG), dou- LLPS, leading to the appearance of solids aggregates [23– ble-stranded RNA binding motifs (dsRBM), DEAD box, A2 26]. recognition element (A2RE), AU rich element (ARE), Zinc In the next sections, major RBPs are reviewed for their fingers domain (Zn), Zn-knuckle motifs, S1 domain, PAZ and involvement in neuromuscular diseases (Table 1). PIWI domains [10, 11]. (Figure 1). This interaction with various RNA species, as well as TDP43 their capacity to shuttle between nucleus and cytoplasm TAR DNA-binding protein of 43 kDa (TDP43) is an RBP able allow RBPs to participate in all steps of the mRNA cycle, to bind to single stranded DNA and RNA in order to modu- from transcription, maturation, transport, translation, sta- late splicing, RNA stability and biogenesis [27–29]. TDP43 bility to degradation [11–14]. RBPs also contribute to trans- was initially characterized as a protein binding to the retro- lational and post translational regulation through binding viral protein Tar [30] and later shown to modulate the OPEN ACCESS | www.cell-stress.com 77 Cell Stress | APRIL 2020 | Vol. 4 No. 4 G. Picchiarelli and L. Dupuis (2020) RBPs in neuromuscular diseases FIGURE 1: Domain organization of RBPs with prion like domains. NLS: nuclear localization signal, NES: nuclear export signal, RRM: RNA recognition motif, RGG:arginine/glycine-rich region, G-rich: glycine-rich region, ZNF: Zinc finger