Acta Neuropathologica https://doi.org/10.1007/s00401-020-02203-0 REVIEW The role of hnRNPs in frontotemporal dementia and amyotrophic lateral sclerosis Alexander Bampton1,2 · Lauren M. Gittings3 · Pietro Fratta4 · Tammaryn Lashley1,2 · Ariana Gatt1,2 Received: 25 June 2020 / Revised: 27 July 2020 / Accepted: 27 July 2020 © The Author(s) 2020 Abstract Dysregulated RNA metabolism is emerging as a crucially important mechanism underpinning the pathogenesis of fron- totemporal dementia (FTD) and the clinically, genetically and pathologically overlapping disorder of amyotrophic lateral sclerosis (ALS). Heterogeneous nuclear ribonucleoproteins (hnRNPs) comprise a family of RNA-binding proteins with diverse, multi-functional roles across all aspects of mRNA processing. The role of these proteins in neurodegeneration is far from understood. Here, we review some of the unifying mechanisms by which hnRNPs have been directly or indirectly linked with FTD/ALS pathogenesis, including their incorporation into pathological inclusions and their best-known roles in pre-mRNA splicing regulation. We also discuss the broader functionalities of hnRNPs including their roles in cryptic exon repression, stress granule assembly and in co-ordinating the DNA damage response, which are all emerging patho- genic themes in both diseases. We then present an integrated model that depicts how a broad-ranging network of pathogenic events can arise from declining levels of functional hnRNPs that are inadequately compensated for by autoregulatory means. Finally, we provide a comprehensive overview of the most functionally relevant cellular roles, in the context of FTD/ALS pathogenesis, for hnRNPs A1-U. Keywords hnRNP · Frontotemporal dementia · Amyotrophic lateral sclerosis · RNA · Autoregulation Introduction RNA and protein homeostasis have been identifed as con- verging mechanisms of neurotoxicity in both disorders. Frontotemporal lobar degeneration (FTLD) is an umbrella RNA-binding proteins (RBPs) play a central role in regulat- pathological term that encompasses a group of heterogene- ing all aspects of gene expression, hence their dysfunction is ous neurodegenerative disorders known to cause frontotem- likely to be a key contributing feature of disrupted RNA and poral dementia (FTD) [108]. FTLD is believed to lie on a protein homeostasis in these diseases [118]. The heterogene- single disease continuum with the neuromuscular disease ous nuclear ribonucleoprotein (hnRNP) family is a family amyotrophic lateral sclerosis (ALS) [52]. Indeed, disrupted of RBPs containing one or more RNA-binding domains that facilitate their extensive and divergent functionality across all stages of nucleic acid metabolism [59]. More recently, Tammaryn Lashley and Ariana Gatt: Joint senior authors. hnRNPs have also been shown to play a role in the orches- tration of the DNA damage in response to genotoxicity and * Tammaryn Lashley [email protected] assembly of stress granules in response to other cellular stresses. Here, we review some of the common themes by 1 The Queen Square Brain Bank for Neurological Disorders, which hnRNPs function to maintain homeostasis within cells Department of Clinical and Movement Neuroscience, UCL and, by extension, highlight potentially vulnerable pathways Queen Square Institute of Neurology, London, UK by which neurotoxicity can be induced or exacerbated fol- 2 Department of Neurodegenerative Diseases, UCL Queen lowing their dysregulation during FTLD/ALS pathogenesis. Square Institute of Neurology, London, UK 3 Department of Neurobiology, Barrow Neurological Institute, 350 W Thomas Road, Phoenix, AZ 85013, USA 4 Department of Neuromuscular Diseases, UCL Queen Square Institute of Neurology, London, UK Vol.:(0123456789)1 3 Acta Neuropathologica Structure and function of hnRNP proteins on its own. Whilst others can override potential export signals to prevent shuttling and promote complete nuclear Early studies using nucleoplasm immunopurifications retention [148]. reported three novel hnRNPs (A, B, C) to be highly Functionally, hnRNP proteins have been implicated at all abundant polypeptide components of mRNA-bound com- stages of mRNA maturation including transcriptional regu- plexes [32, 163]. The hnRNP family has since expanded lation, capping, alternative splicing, polyadenylation, trans- to include at least twenty other closely related and ubiqui- port and stability [49]. HnRNP localisation is predominantly tously expressed RBPs named alphabetically from hnRNP nuclear, however, several hnRNPs can shuttle between the A1 to hnRNP U [49] (Table 1). Structurally, hnRNPs are nucleus and the cytoplasm to regulate additional cytoplas- best defned by their modular structure consisting of one mic functions such as mRNA nucleocytoplasmic transport or more RNA-binding domains (Fig. 1). These domains, and translation [134]. Indeed, hnRNPs form highly dynamic which include the most frequently found RNA-recogni- complexes with RNA and other RBPs to regulate these pro- tion motif (RRM), K homology (KH) domain and RGG cesses. They are able to successfully associate and interact box motif, confer hnRNPs with the ability to bind a large with an array of diferent mRNA processing machinery by number of RNA targets, within a vast RNA-binding inter- virtue of constant remodelling of their mRNA-protein com- actome. Notably, as with other RBPs, hnRNPs can also plex compositions [48]. Hence, hnRNPs bind RNA in a com- bind RNA through their intrinsically disordered regions binatorial arrangement according to their relative afnities (IDRs) or low complexity domains (LCDs) as they are for specifc sequence elements and their relative abundances more commonly referred. These are regions of low amino in a spatial and temporal manner [48, 80]. The uniquely acid complexity which facilitates the formation of higher- assembled constellation of potentially synergistic or antago- order ribonucleoprotein complexes via LCD-driven phase nistically acting hnRNPs may in-turn enhance or suppress separation [20, 74]. Hence, whilst hnRNPs can interact the recruitment of further RBPs to the complex which ulti- with RNA binding partners in a sequence-specifc manner, mately dictates its precise functionality. Post-translational nonspecifc interactions are also prevalent among hnRNPs modifcations are also likely to modulate hnRNP functioning consistent with their observed overlapping, as well as dis- in diferent cellular contexts which represents another layer tinct functionalities [27]. Several hnRNPs also contain of regulatory control over an extensive hnRNP protein-RNA nuclear localisation sequences to ensure a predominantly network [74]. nuclear subcellular localisation or nuclear export signals Notably, despite their many structural and functional sim- which mediates their shuttling to and from the cytoplasm. ilarities, the distinction of hnRNP proteins from other RBPs Intriguingly, some nuclear localisation sequences (e.g., including SR splicing factors and messenger RNPs (mRNPs) m9) can also serve as a bi-directional import/export signal proteins is largely a historic one based on old nomencla- ture [48]. Indeed, the well characterised TAR DNA binding Table 1 The hnRNP family and their common aliases HnRNP protein Alternative protein names A1, A2/B1, A3, A/B hnRNP A1; hnRNP A2/B1; HnRNP A3, HNRPA3; hnRNP A/B, ABBP-1 C hnRNP C, hnRNP C1/C2 D (D0, DL) hnRNP D0, AUF1; hnRNP D-like, laAUF1, JKT41-binding protein E (E1, E2) hnRNP E1, PCBP1, Alpha-CP1; hnRNP E2, PCBP2, Alpha-CP2 F hnRNP F, nucleolin-like protein mcs94-1 G hnRNP G, RNA-binding motif protein, X chromosome (RBMX), Glycoprotein p43 H (H1, H2, H3) hnRNP H1; hnRNP H2, FTP-3, hnRNP H’; hnRNP H3, hnRNP 2H9 I hnRNP I, PTB, PPTB-1 K hnRNP K, TUNP L (L, LL) hnRNP L; hnRNP LL, SRRF M hnRNP M P hnRNP P, FUS, 75 kDA DNA-pairing protein, oncogene TLS, POMp75 Q hnRNP Q, SYNCRIP, GRY-RBP, NS1-associated protein R hnRNP R U hnRNP U, GRIP120, SAF-A, Nuclear p120 ribonucleoprotein Each hnRNP protein’s most commonly used protein name is highlighted in bold text 1 3 Acta Neuropathologica Fig. 1 The hnRNP family: composition and structure. The hnRNP tifed domain in this category. Several hnRNPs also possess a nuclear family are named alphabetically from A1 to U, with hnRNP U being import/export signal to enable them to perform both nuclear and the largest protein (120 kDa) in the class. The proteins all contain cytoplasmic functions. RRM RNA recognition motif, KH K-homol- varying combinations and quantities of RNA-binding domains which ogy domain, RGG Arg-Gly-Gly repeat domain, NLS nuclear locali- facilitate their myriad functional roles in pre-mRNA processing. sation signal. Number in the bottom right corner of each schematic RNA-recognition motifs (RRMs) are by far the most commonly iden- indicates amino acid length 1 3 Acta Neuropathologica protein 43 (TDP-43) is frequently categorised as a member classical inclusions but also pathologies associated with the of the hnRNP family but is not named as such due to being C9orf72 expansion mutation, as reviewed below. missed by the initial 2-dimensional gel and immunopurifca- tion experiments. For reasons of clarity and conciseness we TDP‑43 and FUS pathologies focus the second half of this review on the original hnRNP (A1-U) proteins, noting that more recently added members TDP-43 and FUS are probably the most well-known hnRNPs of the hnRNP family, including TDP-43, have been reviewed in the feld of neurodegeneration. Their accumulation in extensively elsewhere [90, 110]. pathological inclusions