To ensure a proper neuronal development and synaptic plasticity, the brain contains the highest amount of transcriptional and post-transcriptional mechanisms described thus far (Pilaz and Silver
2015). Following RNA splicing and the first step of quality control in the nucleus, mRNAs will be exported through the nuclear pore into the cytoplasm, where the interaction with different RBPs ensures proper control of localization, stability, and translation (Darnell and Richter
2012; Soheilypour and Mofrad
2018). The neuronal transcriptome is enormously diverse due to alternative splicing, polyadenylation, intron retention, and the occurrence of non-canonical coding sequences (Sibley et al.
2016). To guide the expression of the transcriptome, multiple RBPs will dynamically interact in a spatially and temporally defined as well as cell type-specific manner which explains the great variety of RBPs in cells (Schieweck et al.
2021). RBPs bound to mRNAs and are associated with motor proteins to specific localizations in
membrane-less and
shape high molecular weight complexes (mRNP). The kind of RBP determines the subcellular location of their components including mRNAs. In the brain, the axonal cone growth has a differential translational where the RBPs CPEB1 (Richter
2007) and ZBP1 (Huttelmaier et al.
2005) are critical mediators of mRNA transport and its translation. ZBP1 assembly β-actin mRNA to direct localization in axons and dendrites (Song et al.
2015).
Staufen is another RBP involved with RNA granules moving along microtubules into dendrites of hippocampal neurons in a bidirectional manner (Kohrmann et al.
1999). The fragile X mental retardation protein (FMRP) interacts with kinesin to dendritic mRNA localization and regulates the local translation in these sites (Wang et al.
2016; Dictenberg et al.
2008). Pumilio 2 (Pum2) acts like FMRP as a translational regulator, and its specific localization is related to repressor function by inhibiting translation and promoting mRNA decay (Goldstrohm et al.
2018).
RBPs Dysregulation: Triggering the Disease
From the progress in genetic studies, it has been established that RBP dysregulation or mutation can trigger loss of neurons, neuronal function, and neurodegeneration (Kapeli et al.
2017). Nowadays, more and more RBPs are being recognized as causal factors or associated with neurological diseases, autoimmunity, and cancer (Wolozin and Ivanov
2019; Van Nostrand et al.
2020) reinforcing the importance of RBPs in the maintenance of the normal physiology of the CNS. A considerable amount of RBPs have LCR, so these proteins are prone to structural modifications and consequently trigger the loss or gain of function, contributing to the severity of neurodegeneration (Kapeli et al.
2017). Despite the complexities that RBPs currently represent, the binding sites in RNAs could provide more detailed information on the development of neuronal diseases that involve RBPs, which would undoubtedly be an important contribution to this public health problem (Pan et al.
2020).
Mutations in genes encoding RBP have been observed in patients with motor neuron disorders such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), multisystem proteinopathy (MSP), and frontotemporal degeneration (FTD); of all these, ALS is the most common motor neuron disorder in adults; this condition is characterized by the progressive loss of motor neurons triggering fatal paralysis (Robberecht and Eykens
2015).
In neurons, mRNAs can be transported to and from axons and dendrites as mRNA can be translated locally. In neuronal processes, such as dendrites, there are a great variety of mRNAs and a large part of the transcriptome is present both in dendrites and in axons. In the vertebrate brain, mRNAs containing localization elements or
“zip codes” have been identified in neuronal processes, including those encoding structural proteins (Tiruchinapalli et al.
2003), receptors (Grooms et al.
2006), and signaling molecules (Martin and Ephrussi
2009; Terenzio et al.
1421). These
cis-acting sequences located in the 3′UTR are generally recognized by RBPs and will assemble into an RNP (Turner-Bridger et al.
2020). The
cis information contained in the 3′UTR mRNAs is essential to the RBPs recognition; the activated leukocyte cell adhesion molecule (ALCAM) mediates homophilic adhesion of axons from the same neuronal subtype and is required for the formation of axon bundles. The lack of its 3′UTR results in overexpression and induces axon bundle aggregation and prevents axonal growth, whereas a decreased expression results in fasciculation (Thelen et al.
2012), accordingly. The full length of ALCAM maintains the right amount.
The RNP complex can mediate interactions with the translation machinery and self-assemble into transport granules (Eliscovich and Singer
2017). One of the most abundant RNPs in both axons and dendrites is β-actin, and its association with the
zip code binding protein 1 (ZBP1) is essential for transport and proper localization (Biswas et al.
2019; Das and Yoon
2019).
Late endosomes serve as a platform for local axonal translation by binding to RBP, ribosomes, and mRNA (Cioni et al.
2019). Another interesting cellular component that regulates neuronal protein synthesis are RNA granules. Neurons contain various RNA granules, including SG and PB (Thelen and Kye
2020). Importantly, it has been shown that RNP granules located in dendrites can disassemble and mRNAs can be used as a template to produce synaptic proteins (Schieweck et al.
2021; Krichevsky and Kosik
2001). This process is crucial for neuronal health and function, as it is a cellular homeostatic mechanism for managing external stress and controlling synaptic plasticity. These mechanisms will directly impact the composition of the local proteome (Das and Yoon
2019; Jung et al.
2014).
As RBPs determine the axonal or dendritic mRNA
repertoire as well as proteomes by trafficking mRNAs and regulating local protein synthesis, RBP plays a crucial role in neuronal function. Dysfunctional RNA processing in neuronal tissue plays a crucial role in neuronal pathology and is often observed in neurodegenerative diseases (Thelen and Kye
2020). Moreover, RNP plays an important role in RNA metabolism, regulating ribosome formation, spliceosomes, and silencing complexes. When gene mutations or deletions occur in neurons or a well-
misregulated assembly of RNP occurs, it results in neuron degeneration which can lead to SMA, ALS, fragile X syndrome (FXS), among other pathologies (Shukla and Parker
2016).
Due to complex neuronal compartmentalization, it is indispensable to provide local mRNA transcripts and make neurons vulnerable to any change and loss of RNPs complex. Neurons can synthesize proteins at the synaptic compartment in response to many stimuli. For example, the cellular components necessary to produce proteins such as ribosomes and mRNAs are detected at the synaptic area (Ainsley et al.
2014; Poulopoulos et al.
2019). Thus, RNPs deliver specific sets of mRNAs and produce different proteins in particular subcellular compartments, due to the motifs that provide an accurate function of RBP and RNPs at local mRNA translation. These processes are crucial for neuronal development and function (Jung et al.
2014).
Role of RBPs in Neurotoxicity
The brain is susceptible to damage by several toxic agents such as metals, microorganisms, persistent organic pollutants, and high levels of glutamate. RBPs as key regulators of transcriptome are found deregulated in many neurotoxic diseases. In FTD and ALS, there are abnormal controls of mRNA translation by TDP-43 and FUS accumulation in SGs. The motor neuron degeneration in these diseases is related to mutations in RBPs genes (Gowell et al.
2021; Zhou et al.
2020; Vance et al.
2009).
The abnormal cytoplasmic accumulation of TDP-43, correctly called TDP-43 proteinopathy, contributes to neurotoxicity and the oligonucleotides treatment composed of TDP-43 target sequences rescues neurotoxicity (Schieweck et al.
2021; Mann et al.
2019). ALS-associated mutations in TDP-43 are frequently found in LCR Gly-rich domains that regulate phosphorylation and ubiquitination sites (Pesiridis et al.
2009). The prion-related domains rich in glutamine(Q) and asparagine (N) present in TDP-43, TIA-1, and FUS are associated with a highly prone to aggregation (Udan and Baloh
2011). As described above, the cytosolic accumulation of almost any RBPs and the disruption of their nuclear functions is a triggering feature of neurotoxicity. For example, wild-type human TDP-43 can be toxic when expressed in a heterologous
C. elegans system or overexpressed in a cell culture model (Ash et al.
2010). In mice, TDP-43 mutant alleles cause dose-dependent asymmetrical motor axon withdrawal and the lethality and cognitive dysfunction are rescued with functional TDP-43 (Ebstein et al.
2019). The wild-type human TDP-43 expression causes mitochondrial aggregation, motor deficits, and early mortality in transgenic mice (Xu et al.
2010). In chick embryo models, TDP-43
Q331K and TDP-43
M337V showed a dramatic reduction in maturation compared to TDP-43WT with a failure to develop normal limbs and tail buds (Sreedharan et al.
2008). The lacking TDP-43 in flies results in deficient locomotive behaviors, life span reduction, and anatomical defects at the neuromuscular junctions (Feiguin et al.
2009). Some studies report that TDP-43 mutations are more neurotoxic compared to wild-type TDP-43; however, it is necessary to emphasize that a mutation in this RBP is not necessary to promote ALS (Gregory et al.
2020; Wegorzewska et al.
2009). Although some studies report neurodegeneration in the absence of cytosolic aggregation how consequence from TDP-43 specific localization to motor neuron nuclei (Hanson et al.
2010).
From RNA interference screening, the inositol-1,4,5-triphosphate receptor type 1 (ITPR1, mediator of Ca
2+ efflux) was identified as a new regulator of nucleocytoplasmic transport of TDP-43 since the silencing of this receptor promotes the cytosolic accumulation of TDP-43. Therefore, these findings also suggest that the expression and localization of TDP-43 are regulated by Ca
2+ (Kim et al.
2012). Duan et al. (
2019) revealed that PARylation levels are an important regulator of assembly and disassembly dynamics of RNP granules containing hnRNP A1 and TDP-43. They also showed that both genetic and pharmacological inhibition of PARP mitigates neurotoxicity mediated by hnRNP A1 and TDP-43 in cellular and Drosophila models of ALS. At the same time, PAR binding through the hnRNP A1 PAR-binding motif regulates its association with stress granules (Duan et al.
2019).
Mutations in Matrin 3 (MATR3), a DNA and RNA-binding protein little studied so far, have also been described as causing ALS and FTD. Using a primary neuron model to evaluate MATR3-mediated neurotoxicity, Malik et al. (
2018) showed that neurons were bidirectionally vulnerable to MATR3 levels. In addition, the ZnF MATR3 domains partially modulated toxicity; however, the elimination of their motifs for RNA recognition did not affect neuronal survival. On the other hand, contrary to other RBPs related to ALS, the cytoplasmic redistribution of MATR3 mitigated neurodegeneration, suggesting that nuclear MATR3 mediates toxicity (Malik et al.
2018).
Another of the main cause of ALS and FTD is the expanded GGGGCC (G
4C
2)
n repeats in the first intron of the
C9orf72 gene; this repetition promotes a gain of function that undoubtedly alters the homeostasis of post-transcriptional processes. Celona et al. (
2017) identified Zfp106, a ZnF domain RBP, as a specific 4G RNA repeat binding protein. Furthermore, the authors showed that Zfp106 interacts with other RBPs. Zfp106 potently suppresses neurotoxicity in a
Drosophila model of ALS C9orf72 (Celona et al.
2017). Another RBP in these diseases is the RNA editing enzyme adenosine deaminase acting on RNA 2 (ADAR2), which is mislocalized in C9orf72 repeat expansion = mediated ALS/FTD. Because of this mislocalization, severe RNA editing alterations were observed in multiple brain regions. The mislocalization of ADAR2 in C9orf72-mediated ALS/FTD is responsible for the alteration of RNA processing events that may impact vast cellular functions, including the integrated stress response (ISR) and protein translation (Moore et al.
2019).
In autism disorder (ASD), CPEB4 regulates the translation of specific mRNAs by modulating their poly(A)-tails, and it was found to bind transcripts of most high-confidence ASD risk genes. Individuals with idiopathic ASD show imbalances in CPEB4 transcript isoforms, and 9% of the transcriptome shows reduced poly(A)-tail length (Parras et al.
2018). In the same disease, functional defects of the cerebral cortex contribute to the clinical symptoms of ASD, and impairment of Rbfox1-iso1 is a main effector. The Rbfox1-iso1 knockdown in hippocampal neurons resulted in the reduction of primary axon length, total length of dendrites, spine density, and mature spine number with an important impact on neuronal migration and synapse network formation during corticogenesis (Hamada et al.
2016).
Taken together, the literature supports that RBPs are key regulators in many neurotoxicology diseases; in Table
1, we summarized the association of different RBPs dysfunction and process altered in neurological disorders.
Table 1
RBP dysfunction and process altered in neurological disorders
FUS: RRM, G rich, Q/G/S/Y, ZnF, RGG | ALS FTD | Alternative splicing Transport | |
Rbfox: RRM | Epilepsy, ASD, and mental retardation | Alternative splicing Polyadenylation | |
PABP: RRM | MD | Alternative splicing Stability mRNA | Banerjee et al. 2013; Schoser and Timchenko 2010) |
HuR/ELAVL1: RRM | SSN, diabetic nephropathy, glioma progression and PEM | Stability, alternative splicing, polyadenylation, 3′UTR binding and transport | Zhu et al. 2007; Filippova et al. 2017; Ince-dunn et al. 2012) |
U1A: RRM | SMA | Inhibits polyadenylation upon direct binding to mRNA | |
TDP-43: RRMG rich | ALS and FTD | Alternative splicing, miRNA biogenesis, stability, and transport | |
CPEB: RRM | ASD | Polyadenylation | |
TIA-1: RRM/KH | ALS | Alternative splicing Apoptosis promotor via FAST-K | Wang et al. 2014; Rayman and Kandel 2017) |
ZBP1: RRM/KH | Guide, growth and branched axon, dendritic development, synaptogenesis, and regeneration | 3′UTR binding and stability, translational repression Axonal mRNA transport, localization, and degradation | Gallagher and Ramos 2018; Bryant and Yazdani 2016) |
Nova: KH | FXS | Alternative splicing Polyadenylation | |
FMRP: KH/RGG | FXS | Alternative splicing, mRNA stability, dendritic mRNA transport, and local postsynaptic protein synthesis | Burd and Dreyfusst 1994; Yang et al. 2018; Hall and Berry-Kravis 2018; Telias 2019) |
hnRNP: KH RGG | ALS, FTD, Kabulki syndrome, and Au-Kline syndrome | Transcription, silencing 3′UTR binding and stability | |
QK1: KH | Schizophrenia and ataxia | Stability, translation, alternative splicing, and localization | |
STAU1: dsRBD | MD and AD | Alternative splicing and 3′UTR binding | Zhong et al. 2020; Yu et al. 2015; Bondy-Chorney et al. 2016) |
Adar 1/2: dsRBD | ALS, FTD, and IPF | miRNA processing and alternative splicing | Moore et al. 2019; Bryant and Yazdani 2016; Barraud and Allain 2012) |
EWS: RGG | ALS, FTD, and Ewing sarcoma | Alternative splicing | Shaw et al. 2010; Selvanathan et al. 2015) |
ATX2: RRM | ALS, SCA2, ELA, and FTD | Polyadenylation mRNA stability SG and PB formation | Zhou et al. 2014; Ostrowski et al. 2017; Watanabe et al. 2020; Nonhoff et al. 2007) |