Post-translational modifications on RNA-binding proteins: accelerators, brakes, or passengers in neurodegeneration?

RNA-binding proteins (RBPs) are critical players in RNA expression and metabolism, thus, the proper regulation of this class of proteins is critical for cellular health. Regulation of RBPs often occurs through post-translational modifications (PTMs), which allow the cell to quickly and efficiently respond to cellular and environmental stimuli. PTMs have recently emerged as important regulators of RBPs implicated in neurodegenerative disorders, in particular amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here, we summarize how disease-associated PTMs influence the biophysical properties, molecular interactions, subcellular localization, and function of ALS/FTD-linked RBPs, such as FUS and TDP-43. We will discuss how PTMs are believed to play pathological, protective, or ambiguous roles in these neurodegenerative disorders.

Disease-linked TDP-43 hyperphosphorylation suppresses TDP-43 condensation and aggregation.

Post-translational modifications (PTMs) have emerged as key modulators of protein phase separation and have been linked to protein aggregation in neurodegenerative disorders. The major aggregating protein in amyotrophic lateral sclerosis and frontotemporal dementia, the RNA-binding protein TAR DNA-binding protein (TDP-43), is hyperphosphorylated in disease on several C-terminal serine residues, a process generally believed to promote TDP-43 aggregation. Here, we however find that Casein kinase 1δ-mediated TDP-43 hyperphosphorylation or C-terminal phosphomimetic mutations reduce TDP-43 phase separation and aggregation, and instead render TDP-43 condensates more liquid-like and dynamic. Multi-scale molecular dynamics simulations reveal reduced homotypic interactions of TDP-43 low-complexity domains through enhanced solvation of phosphomimetic residues. Cellular experiments show that phosphomimetic substitutions do not affect nuclear import or RNA regulatory functions of TDP-43, but suppress accumulation of TDP-43 in membrane-less organelles and promote its solubility in neurons. We speculate that TDP-43 hyperphosphorylation may be a protective cellular response to counteract TDP-43 aggregation.

Global Approaches in Studying RNA-Binding Protein Interaction Networks.

RNA-binding proteins (RBPs) play crucial roles in almost all aspects of cellular biology. RBP binding at specific target sites impacts expression of functionally coordinated sets of mRNAs and involves combinatorial and dynamic interactions with other RBPs. The complexity and principles of these regulatory networks are only beginning to be understood. In recent years, transcriptome-wide experimental and computational methods to study RBPs and their interactions with RNA provided new insights into their function. Here, we review the approaches used in examining RBPs and their networks and the concepts that have been developed. We emphasize studies focusing on RBP-RNA interactions and higher-order RBP coregulation and describe approaches that integrate multiple types of transcriptome-wide data to form a global picture of these regulatory pathways.

The RNA-binding protein FUS is chaperoned and imported into the nucleus by a network of import receptors.

Fused in sarcoma (FUS) is a predominantly nuclear RNA-binding protein with key functions in RNA processing and DNA damage repair. Defects in nuclear import of FUS have been linked to severe neurodegenerative diseases; hence, it is of great interest to understand this process and how it is dysregulated in disease. Transportin-1 (TNPO1) and the closely related transportin-2 have been identified as major nuclear import receptors of FUS. They bind to the C-terminal nuclear localization signal of FUS and mediate the protein's nuclear import and at the same time also suppress aberrant phase transitions of FUS in the cytoplasm. Whether FUS can utilize other nuclear transport receptors for the purpose of import and chaperoning has not been examined so far. Here, we show that FUS directly binds to different import receptors in vitro. FUS formed stable complexes not only with TNPO1 but also with transportin-3, importin ß, importin 7, or the importin ß/7 heterodimer. Binding of these alternative import receptors required arginine residues within FUS-RG/RGG motifs and was weakened by arginine methylation. Interaction with these importins suppressed FUS phase separation and reduced its sequestration into stress granules. In a permeabilized cell system, we further showed that transportin-3 had the capacity to import FUS into the nucleus, albeit with lower efficiency than TNPO1. Our data suggest that aggregation-prone RNA-binding proteins such as FUS may utilize a network of importins for chaperoning and import, similar to histones and ribosomal proteins.

Immunoblot screening of CRISPR/Cas9-mediated gene knockouts without selection.

BACKGROUND: Targeted genomic editing using the CRISPR/Cas9 methodology has opened exciting new avenues in probing gene function in virtually any model system, including cultured mammalian cells. Depending on the desired mutation, several experimental options exist in the isolation of clonal lines, such as selection with introduced markers, or screening by PCR amplification of genomic DNA. However, streamlined approaches to establishing deletion and tagging mutants with minimal genomic perturbation are of interest in applying this methodology. RESULTS: We developed a procedure for rapid screening of clonal cell lines for the deletion of a protein of interest following CRISPR/Cas9 targeting in the absence of selective pressure based on dot immunoblots. To assess the technique, we probed clonal isolates of 293-TREx cells that were targeted with three separate sgRNAs against the HuR gene. Validation of knockout candidates by western blot indicated that the normalized protein abundances indicated by the dot blot serve as accurate predictors of deletion. In total, 32 independent biallelic deletion lines out of 248 screened clones were isolated, and recovery of null mutants ranged from 6 to 36% for the individual sgRNAs. Genomic sequencing verified small deletions at the targeted locus. CONCLUSIONS: Clonal screening for CRISPR/Cas9-mediated editing events using dot immunoblot is a straightforward and efficient approach that facilitates rapid generation of genomic mutants to study gene function.

Mammalian pumilio proteins control cellular morphology, migration, and adhesion.

Pumilio proteins are RNA-binding proteins that control mRNA translation and stability by binding to the 3' UTR of target mRNAs. Mammals have two canonical Pumilio proteins, PUM1 and PUM2, which are known to act in many biological processes, including embryonic development, neurogenesis, cell cycle regulation and genomic stability. Here, we characterized a new role of both PUM1 and PUM2 in regulating cell morphology, migration, and adhesion in T-REx-293 cells, in addition to previously known defects in growth rate. Gene ontology analysis of differentially expressed genes in PUM double knockout (PDKO) cells for both cellular component and biological process showed enrichment in categories related to adhesion and migration. PDKO cells had a collective cell migration rate significantly lower than that of WT cells and displayed changes in actin morphology. In addition, during growth, PDKO cells aggregated into clusters (clumps) due to an inability to escape cell-cell contacts. Addition of extracellular matrix (Matrigel) alleviated the clumping phenotype. Collagen IV (ColIV), a major component of Matrigel, was shown to be the driving force in allowing PDKO cells to monolayer appropriately, however, ColIV protein levels remained unperturbed in PDKO cells. This study characterizes a novel cellular phenotype associated with cellular morphology, migration, and adhesion which can aid in developing better models for PUM function in both developmental processes and disease.

Analysis of RBP Regulation and Co-regulation of mRNA 3' UTR Regions in a Luciferase Reporter System.

The luciferase reporter assay is a widely used tool to study the cis and trans factors controlling regulation of gene expression. In this assay, regulatory elements can be fused to the luciferase gene, and as a result effect protein output by changing rates of transcription, rates of translation, or mRNA stability. This protocol focuses on probing the function of RNA-binding proteins (RBPs) through their interactions with the 3' untranslated region (UTR), thus examining gene expression regulation on the mRNA level. Assessment of 3' UTR sequence requirements, as well as single and co-regulatory roles of RBPs in regulation of mRNAs will be discussed.

Antagonistic and cooperative AGO2-PUM interactions in regulating mRNAs.

Approximately 1500 RNA-binding proteins (RBPs) profoundly impact mammalian cellular function by controlling distinct sets of transcripts, often using sequence-specific binding to 3' untranslated regions (UTRs) to regulate mRNA stability and translation. Aside from their individual effects, higher-order combinatorial interactions between RBPs on specific mRNAs have been proposed to underpin the regulatory network. To assess the extent of such co-regulatory control, we took a global experimental approach followed by targeted validation to examine interactions between two well-characterized and highly conserved RBPs, Argonaute2 (AGO2) and Pumilio (PUM1 and PUM2). Transcriptome-wide changes in AGO2-mRNA binding upon PUM knockdown were quantified by CLIP-seq, and the presence of PUM binding on the same 3'UTR corresponded with cooperative and antagonistic effects on AGO2 occupancy. In addition, PUM binding sites that overlap with AGO2 showed differential, weakened binding profiles upon abrogation of AGO2 association, indicative of cooperative interactions. In luciferase reporter validation of candidate 3'UTR sites where AGO2 and PUM colocalized, three sites were identified to host antagonistic interactions, where PUM counteracts miRNA-guided repression. Interestingly, the binding sites for the two proteins are too far for potential antagonism due to steric hindrance, suggesting an alternate mechanism. Our data experimentally confirms the combinatorial regulatory model and indicates that the mostly repressive PUM proteins can change their behavior in a context-dependent manner. Overall, the approach underscores the importance of further elucidation of complex interactions between RBPs and their transcriptome-wide extent.

Selection-dependent and Independent Generation of CRISPR/Cas9-mediated Gene Knockouts in Mammalian Cells.

The CRISPR/Cas9 genome engineering system has revolutionized biology by allowing for precise genome editing with little effort. Guided by a single guide RNA (sgRNA) that confers specificity, the Cas9 protein cleaves both DNA strands at the targeted locus. The DNA break can trigger either non-homologous end joining (NHEJ) or homology directed repair (HDR). NHEJ can introduce small deletions or insertions which lead to frame-shift mutations, while HDR allows for larger and more precise perturbations. Here, we present protocols for generating knockout cell lines by coupling established CRISPR/Cas9 methods with two options for downstream selection/screening. The NHEJ approach uses a single sgRNA cut site and selection-independent screening, where protein production is assessed by dot immunoblot in a high-throughput manner. The HDR approach uses two sgRNA cut sites that span the gene of interest. Together with a provided HDR template, this method can achieve deletion of tens of kb, aided by the inserted selectable resistance marker. The appropriate applications and advantages of each method are discussed.