Rules for nuclear localization sequence recognition by karyopherin beta 2

Multiple types of nuclear localization signals in Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite that belongs to the Amoebozoa supergroup whose study related to the nucleocytoplasmic transport of proteins through the nucleus is poorly studied. In this work, we have performed in silico predictions of the potential nuclear localization signals (NLS) corresponding to the proteome of 8201 proteins from Entamoeba histolytica annotated in the AmoebaDB database. We have found the presence of monopartite nuclear localization signals (MNLSs), bipartite nuclear localization signals (BNLSs), and non-canonical monopartite NLSs with lengths exceeding 20 amino acid residues. Additionally, we detected a new type of NLS consisting of multiple juxtaposed bipartite NLSs (JNLSs) that have not been described in any eukaryotic organism. Also, we have generated consensus sequences for the nuclear import of proteins with the NLSs obtained. Docking experiments between EhImportin α and an MNLS, BNLS, and JNLS outlined the interacting residues between the Importin and cargo proteins, emphasizing their putative roles in nuclear import. By transfecting HA-tagged protein constructs, we assessed the nuclear localization of MNLS (U1A and U2AF1), JMNLS (U2AF2), and non-canonical NLS (N-terminus of Pol ll) in vivo. Our data provide the basis for understanding the nuclear transport process in E. histolytica.

Early Stages of FUS Droplet Formation via Liquid-Liquid Phase Separation

The phase behavior of complex biomolecular solutions may explain different cellular processes, including the organization of cells by membraneless organelles. The early stages of phase separation are crucial to understanding the underlying mechanism and identifying biomolecules that trigger or drive the transition. Here, we analyze the early events of liquid-liquid phase separation (LLPS) of FUS by multiangle time-resolved static and dynamic light scattering. LLPS was triggered by TEV-catalyzed cleavage of the MBP-tag from FUS-MBP. The light scattering measurements revealed the existence of at least two fractions of FUS-MBP aggregates already prior to the onset of LLPS. The orders of magnitude of the aggregate size in these two fractions are 10 and 100 nm, respectively. LLPS started after an induction period, which depended on the concentration of FUS-MBP. The data from time-dependent light scattering revealed a coalescence of droplets also denoted as a step growth process. A step growth process instead of nucleation and growth via monomer addition suggests that LLPS takes place within the spinodal rather than between the binodal and the spinodal.

The interferon-regulated host factor hnRNPA0 modulates HIV-1 production by interference with LTR activity, mRNA trafficking, and programmed ribosomal frameshifting

The interplay between host factors and viral components impacts viral replication efficiency profoundly. Members of the cellular heterogeneous nuclear ribonucleoprotein family (hnRNPs) have been extensively studied as HIV-1 host dependency factors, but whether they play a role in innate immunity is currently unknown. This study aimed to identify hnRNPA0 as a type I interferon (IFN)-repressed host factor in HIV-1-infected cells. Knockdown of hnRNPA0, a situation that mirrors conditions under IFN stimulation, increased LTR activity, export of unspliced HIV-1 mRNA, viral particle production, and thus, increased infectivity. Conversely, hnRNPA0 overexpression primarily reduced plasmid-driven and integrated HIV-1 long terminal repeat (LTR) activity, significantly decreasing total viral mRNA and protein levels. In addition, high levels of hnRNPA0 significantly reduced the HIV-1 programmed ribosomal frameshifting efficiency, resulting in a shift in the HIV-1 p55/p15 ratio. The HIV-1 alternative splice site usage remained largely unaffected by altered hnRNPA0 levels suggesting that the synergistic inhibition of the LTR activity and viral mRNA transcription, as well as impaired ribosomal frameshifting efficiency, are critical factors for efficient HIV-1 replication regulated by hnRNPA0. The pleiotropic dose-dependent effects under high or low hnRNPA0 levels were further confirmed in HIV-1-infected Jurkat cells. Finally, our study revealed that hnRNPA0 levels in PBMCs were lower in therapy-naive HIV-1-infected individuals compared to healthy controls. Our findings highlight a significant role for hnRNPA0 in HIV-1 replication and suggest that its IFN-I-regulated expression levels are critical for viral fitness allowing replication in an antiviral environment.IMPORTANCERNA-binding proteins, in particular, heterogeneous nuclear ribonucleoproteins (hnRNPs), have been extensively studied. Some act as host dependency factors for HIV-1 since they are involved in multiple cellular gene expression processes. Our study revealed hnRNPA0 as an IFN-regulated host factor, that is differently expressed after IFN-I treatment in HIV-1 target cells and lower expressed in therapy-naïve HIV-1-infected individuals. Our findings demonstrate the significant pleiotropic role of hnRNPA0 in viral replication: In high concentrations, hnRNPA0 limits viral replication by negatively regulating Tat-LTR transcription, retaining unspliced mRNA in the nucleus, and significantly impairing programmed ribosomal frameshifting. Low hnRNPA0 levels as observed in IFN-treated THP-1 cells, particularly facilitate HIV LTR activity and unspliced mRNA export, suggesting a role in innate immunity in favor of HIV replication. Understanding the mode of action between hnRNPA0 and HIV-1 gene expression might help to identify novel therapeutically strategies against HIV-1 and other viruses.

Kapβ2 Inhibits Perioperative Neurocognitive Disorders in Rats with Mild Cognitive Impairment by Reversing the Nuclear-Cytoplasmic Mislocalization of hnRNPA2/B1

Harmful stimuli trigger mutations lead to uncontrolled accumulation of hnRNPA2/B1 in the cytoplasm, exacerbating neuronal damage. Kapβ2 mediates the bidirectional transport of most substances between the cytoplasm and the nucleus. Kapβ2 guides hnRNPA2/B1 back into the nucleus and restores its function, alleviating related protein toxicity. Here, we aim to explore the involvement of Kapβ2 in neurodegeneration in rats with MCI following sevoflurane anesthesia and surgery. Firstly, novel object recognition test and Barnes maze were conducted to assess behavioral performances, and we found Kapβ2 positively regulated the recovery of memory and cognitive function. In vivo electrophysiological experiments revealed that the hippocampal theta rhythm energy distribution was disrupted, coherence was reduced, and long-term potentiation was attenuated in MCI rats. LTP was greatly improved with positive modulation of Kapβ2. Next, functional MRI and BOLD imaging will be employed to examine the AFLL and FC values of dynamic connectivity between the cortex and hippocampus of the brain. The findings show that regulating Kapβ2 in the hippocampus region enhances functional activity and connections between brain regions in MCI rats. WB results showed that increasing Kapβ2 expression improved the expression and recovery of cognitive-related proteins in the hippocampus of MCI rats. Finally, WB and immunofluorescence were used to examine the changes in hnRNPA2/B1 expression in the nucleus and cytoplasm after overexpression of Kapβ2, and it was found that nucleocytoplasmic mis location was alleviated. Overall, these data show that Kapβ2 reverses the nucleoplasmic misalignment of hnRNPA2/B1, which slows neurodegeneration towards dementia in MCI after sevoflurane anesthesia and surgery. Our findings may lead to new approaches for perioperative neuroprotection of MCI patients.

Targeting colorectal cancer at the level of nuclear pore complex

BACKGROUND

Nuclear pore complexes (NPCs) are the architectures entrenched in nuclear envelop of a cell that regulate the nucleo-cytoplasmic transportation of materials, such as proteins and RNAs for proper functioning of a cell. The appropriate localization of proteins and RNAs within the cell is essential for its normal functionality. For such a complex transportation of materials across the NPC, around 60 proteins are involved comprising nucleoporins, karyopherins and RAN system proteins that play a vital role in NPC's structure formation, cargo translocation across NPC, and cargoes' rapid directed transportation respectively. In various cancers, the structure and function of NPC is often exaggerated, following altered expressions of its nucleoporins and karyopherins, affecting other proteins of associated signaling pathways. Some inhibitors of karyopherins at present, have potential to regulate the altered level/expression of these karyopherin molecules.

AIM OF REVIEW

This review summarizes the data from 1990 to 2023, mainly focusing on recent studies that illustrate the structure and function of NPC, the relationship and mechanisms of nucleoporins and karyopherins with colorectal cancer, as well as therapeutic values, in order to understand the pathology and underlying basis of colorectal cancer associated with NPC. This is the first review to our knowledge elucidating the detailed updated studies targeting colorectal cancer at NPC. The review also aims to target certain karyopherins, Nups and their possible inhibitors and activators molecules as a therapeutic strategy.

KEY SCIENTIFIC CONCEPTS OF REVIEW

NPC structure provides understanding, how nucleoporins and karyopherins as key molecules are responsible for appropriate nucleocytoplasmic transportation. Many studies provide evidences, describing the role of disrupted nucleoporins and karyopherins not only in CRC but also in other non-hematological and hematological malignancies. At present, some inhibitors of karyopherins have therapeutic potential for CRC, however development of more potent inhibitors may provide more effective therapeutic strategies for CRC in near future.

Strategies for the Nuclear Delivery of Metal Complexes to Cancer Cells

The nucleus is an essential organelle for the function of cells. It holds most of the genetic material and plays a crucial role in the regulation of cell growth and proliferation. Since many antitumoral therapies target nucleic acids to induce cell death, tumor-specific nuclear drug delivery could potentiate therapeutic effects and prevent potential off-target side effects on healthy tissue. Due to their great structural variety, good biocompatibility, and unique physico-chemical properties, organometallic complexes and other metal-based compounds have sparked great interest as promising anticancer agents. In this review, strategies for specific nuclear delivery of metal complexes are summarized and discussed to highlight crucial parameters to consider for the design of new metal complexes as anticancer drug candidates. Moreover, the existing opportunities and challenges of tumor-specific, nucleus-targeting metal complexes are emphasized to outline some new perspectives and help in the design of new cancer treatments.

The molecular basis for cellular function of intrinsically disordered protein regions

Intrinsically disordered protein regions exist in a collection of dynamic interconverting conformations that lack a stable 3D structure. These regions are structurally heterogeneous, ubiquitous and found across all kingdoms of life. Despite the absence of a defined 3D structure, disordered regions are essential for cellular processes ranging from transcriptional control and cell signalling to subcellular organization. Through their conformational malleability and adaptability, disordered regions extend the repertoire of macromolecular interactions and are readily tunable by their structural and chemical context, making them ideal responders to regulatory cues. Recent work has led to major advances in understanding the link between protein sequence and conformational behaviour in disordered regions, yet the link between sequence and molecular function is less well defined. Here we consider the biochemical and biophysical foundations that underlie how and why disordered regions can engage in productive cellular functions, provide examples of emerging concepts and discuss how protein disorder contributes to intracellular information processing and regulation of cellular function.

Molecular consequences of PQBP1 deficiency, involved in the X-linked Renpenning syndrome

Mutations in the PQBP1 gene (polyglutamine-binding protein-1) are responsible for a syndromic X-linked form of neurodevelopmental disorder (XL-NDD) with intellectual disability (ID), named Renpenning syndrome. PQBP1 encodes a protein involved in transcriptional and post-transcriptional regulation of gene expression. To investigate the consequences of PQBP1 loss, we used RNA interference to knock-down (KD) PQBP1 in human neural stem cells (hNSC). We observed a decrease of cell proliferation, as well as the deregulation of the expression of 58 genes, comprising genes encoding proteins associated with neurodegenerative diseases, playing a role in mRNA regulation or involved in innate immunity. We also observed an enrichment of genes involved in other forms of NDD (CELF2, APC2, etc). In particular, we identified an increase of a non-canonical isoform of another XL-NDD gene, UPF3B, an actor of nonsense mRNA mediated decay (NMD). This isoform encodes a shorter protein (UPF3B_S) deprived from the domains binding NMD effectors, however no notable change in NMD was observed after PQBP1-KD in fibroblasts containing a premature termination codon. We showed that short non-canonical and long canonical UPF3B isoforms have different interactomes, suggesting they could play distinct roles. The link between PQBP1 loss and increase of UPF3B_S expression was confirmed in mRNA obtained from patients with pathogenic variants in PQBP1, particularly pronounced for truncating variants and missense variants located in the C-terminal domain. We therefore used it as a molecular marker of Renpenning syndrome, to test the pathogenicity of variants of uncertain clinical significance identified in PQPB1 in individuals with NDD, using patient blood mRNA and HeLa cells expressing wild-type or mutant PQBP1 cDNA. We showed that these different approaches were efficient to prove a functional effect of variants in the C-terminal domain of the protein. In conclusion, our study provided information on the pathological mechanisms involved in Renpenning syndrome, but also allowed the identification of a biomarker of PQBP1 deficiency useful to test variant effect.

Cryo-EM structures of the D290V mutant of the hnRNPA2 low-complexity domain suggests how D290V affects phase separation and aggregation

Heterogeneous nuclear ribonucleoprotein A2 (hnRNPA2) is a human ribonucleoprotein that transports RNA to designated locations for translation via its ability to phase separate. Its mutated form, D290V, is implicated in multisystem proteinopathy known to afflict two families, mainly with myopathy and Paget's disease of bone. Here, we investigate this mutant form of hnRNPA2 by determining cryo-EM structures of the recombinant D290V low complexity domain. We find that the mutant form of hnRNPA2 differs from the WT fibrils in four ways. In contrast to the WT fibrils, the PY-nuclear localization signals in the fibril cores of all three mutant polymorphs are less accessible to chaperones. Also, the mutant fibrils are more stable than WT fibrils as judged by phase separation, thermal stability, and energetic calculations. Similar to other pathogenic amyloids, the mutant fibrils are polymorphic. Thus, these structures offer evidence to explain how a D-to-V missense mutation diverts the assembly of reversible, functional amyloid-like fibrils into the assembly of pathogenic amyloid, and may shed light on analogous conversions occurring in other ribonucleoproteins that lead to neurological diseases such as amyotrophic lateral sclerosis and frontotemporal dementia.

Nuclear-import receptors as gatekeepers of pathological phase transitions in ALS/FTD

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are fatal neurodegenerative disorders on a disease spectrum that are characterized by the cytoplasmic mislocalization and aberrant phase transitions of prion-like RNA-binding proteins (RBPs). The common accumulation of TAR DNA-binding protein-43 (TDP-43), fused in sarcoma (FUS), and other nuclear RBPs in detergent-insoluble aggregates in the cytoplasm of degenerating neurons in ALS/FTD is connected to nuclear pore dysfunction and other defects in the nucleocytoplasmic transport machinery. Recent advances suggest that beyond their canonical role in the nuclear import of protein cargoes, nuclear-import receptors (NIRs) can prevent and reverse aberrant phase transitions of TDP-43, FUS, and related prion-like RBPs and restore their nuclear localization and function. Here, we showcase the NIR family and how they recognize cargo, drive nuclear import, and chaperone prion-like RBPs linked to ALS/FTD. We also discuss the promise of enhancing NIR levels and developing potentiated NIR variants as therapeutic strategies for ALS/FTD and related neurodegenerative proteinopathies.

Nuclear localization signal-tagged systems: relevant nuclear import principles in the context of current therapeutic design

Nuclear targeting of therapeutics provides a strategy for enhancing efficacy of molecules active in the nucleus and minimizing off-target effects. 'Active' nuclear-directed transport and efficient translocations across nuclear pore complexes provide the most effective means of maximizing nuclear localization. Nuclear-targeting systems based on nuclear localization signal (NLS) motifs have progressed significantly since the beginning of the current millennium. Here, we offer a roadmap for understanding the basic mechanisms of nuclear import in the context of actionable therapeutic design for developing NLS-therapeutics with improved treatment efficacy.

TAF15 amyloid filaments in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) causes frontotemporal dementia (FTD), the most common form of dementia after Alzheimer's disease, and is often also associated with motor disorders1. The pathological hallmarks of FTLD are neuronal inclusions of specific, abnormally assembled proteins2. In the majority of cases the inclusions contain amyloid filament assemblies of TAR DNA-binding protein 43 (TDP-43) or tau, with distinct filament structures characterizing different FTLD subtypes3,4. The presence of amyloid filaments and their identities and structures in the remaining approximately 10% of FTLD cases are unknown but are widely believed to be composed of the protein fused in sarcoma (FUS, also known as translocated in liposarcoma). As such, these cases are commonly referred to as FTLD-FUS. Here we used cryogenic electron microscopy (cryo-EM) to determine the structures of amyloid filaments extracted from the prefrontal and temporal cortices of four individuals with FTLD-FUS. Surprisingly, we found abundant amyloid filaments of the FUS homologue TATA-binding protein-associated factor 15 (TAF15, also known as TATA-binding protein-associated factor 2N) rather than of FUS itself. The filament fold is formed from residues 7-99 in the low-complexity domain (LCD) of TAF15 and was identical between individuals. Furthermore, we found TAF15 filaments with the same fold in the motor cortex and brainstem of two of the individuals, both showing upper and lower motor neuron pathology. The formation of TAF15 amyloid filaments with a characteristic fold in FTLD establishes TAF15 proteinopathy in neurodegenerative disease. The structure of TAF15 amyloid filaments provides a basis for the development of model systems of neurodegenerative disease, as well as for the design of diagnostic and therapeutic tools targeting TAF15 proteinopathy.

Intrinsically disordered regions and RNA binding domains contribute to protein enrichment in biomolecular condensates in Xenopus oocytes

Proteins containing both intrinsically disordered regions (IDRs) and RNA binding domains (RBDs) can phase separate in vitro, forming bodies similar to cellular biomolecular condensates. However, how IDR and RBD domains contribute to in vivo recruitment of proteins to biomolecular condensates remains poorly understood. Here, we analyzed the roles of IDRs and RBDs in L-bodies, biomolecular condensates present in Xenopus oocytes. We show that a cytoplasmic isoform of hnRNPAB, which contains two RBDs and an IDR, is highly enriched in L-bodies. While both of these domains contribute to hnRNPAB self-association and phase separation in vitro and mediate enrichment into L-bodies in oocytes, neither the RBDs nor the IDR replicate the localization of full-length hnRNPAB. Our results suggest a model where the additive effects of the IDR and RBDs regulate hnRNPAB partitioning into L-bodies. This model likely has widespread applications as proteins containing RBD and IDR domains are common biomolecular condensate residents.

Nuclear localization of HD-Zip IV transcription factor GLABRA2 is driven by Importin α

GLABRA2 (GL2), a class IV homeodomain leucine-zipper (HD-Zip IV) transcription factor (TF) from Arabidopsis , is a developmental regulator of specialized cell types in the epidermis. GL2 contains a putative monopartite nuclear localization sequence (NLS) partially overlapping with its homeodomain (HD). We demonstrate that NLS deletion or alanine substitution of its basic residues (KRKRKK) affects nuclear localization and results in a loss-of-function phenotype. Fusion of the predicted NLS (GTNKRKRKKYHRH) to the fluorescent protein EYFP is sufficient for its nuclear localization in roots and trichomes. The functional NLS is evolutionarily conserved in a distinct subset of HD-Zip IV members including PROTODERMAL FACTOR2 (PDF2). Despite partial overlap of the NLS with the HD, genetic dissection of the NLS from PDF2 indicates that nuclear localization and DNA binding are separable functions. Affinity purification of GL2 from plant tissues followed by mass spectrometry-based proteomics identified Importin α (IMPα) isoforms as potential GL2 interactors. NLS structural prediction and molecular docking studies with IMPα-3 revealed major interacting residues. Split-ubiquitin cytosolic yeast two-hybrid assays suggest interaction between GL2 and four IMPα isoforms from Arabidopsis. Direct interactions were verified in vitro by co-immunoprecipitation with recombinant proteins. IMPα triple mutants ( impα- 1,2,3 ) exhibit defects in EYFP:GL2 nuclear localization in trichomes but not in roots, consistent with tissue-specific and redundant functions of IMPα isoforms in Arabidopsis . Taken together, our findings provide mechanistic evidence for IMPα-dependent nuclear localization of GL2 and other HD-Zip IV TFs in plants.

One sentence summary

GLABRA2, a representative HD-Zip IV transcription factor from Arabidopsis , contains an evolutionarily conserved monopartite nuclear localization sequence that is recognized by Importin α for translocation to the nucleus, a process that is necessary for cell-type differentiation of the epidermis.

Investigation of a Fused in Sarcoma Splicing Mutation in a Chinese Amyotrophic Lateral Sclerosis Patient

OBJECTIVE

Genetic mutations of fused in sarcoma (FUS) causing amyotrophic lateral sclerosis (ALS) may disrupt mRNA splicing events. For example, the FUS c.1394-2delA variant was reported in two western ALS patients, but its molecular mechanism is unclear. In this study, we aim to investigate FUS splice site mutations in Chinese ALS patients.

METHODS

Sanger sequencing was used to identify FUS splicing mutations in Chinese ALS patients. We combined a deep learning tool (SpliceAI), RNA sequencing, and RT-PCR/RT-qPCR to analyze the effect of FUS c.1394-2delA mutation on RNA splicing and expression. AlphaFold was used to predict the protein structure of mutant FUS. In transfected cell lines, we used immunofluorescence to assess cytoplasmic mislocalization of mutant FUS protein.

RESULTS

We identified a de novo FUS splice acceptor site mutation (c.1394-2delA, p. Gly466Valfs*14) in one Chinese sporadic ALS patient, which is linked to exon 14 skipping, and upregulated total FUS mRNA expression. The FUS splice site mutation was predicted to be translated into a truncated protein product at C-terminal. In vitro studies revealed that the FUS mutation increased cytoplasmic mislocalization in both HEK293T and SH-SY5Y cells.

CONCLUSIONS

We identified a de novo FUS splicing mutation (c.1394-2delA, p. Gly466Valfs*14) in 1 out of 233 Chinese ALS patients. It caused abnormal RNA splicing, upregulated gene expression, truncated FUS translation, and cytosolic mislocalization. Our findings suggested that FUS splice site mutation is rare in Chinese ALS patients and extended our knowledge of molecular mechanisms of the FUS c.1394-2delA mutation.

Why to target G-quadruplexes using peptides: Next-generation G4-interacting ligands

Guanine-rich oligonucleotides existing in both DNA and RNA are able to fold into four-stranded DNA secondary structures via Hoogsteen type hydrogen-bonding, where four guanines self-assemble into a square planar arrangement, which, when stacked upon each other, results in the formation of higher-order structures called G-quadruplexes. Their distribution is not random; they are more frequently present at telomeres, proto-oncogenic promoters, introns, 5'- and 3'-untranslated regions, stem cell markers, ribosome binding sites and so forth and are associated with various biological functions, all of which play a pivotal role in various incurable diseases like cancer and cellular ageing. Several studies have suggested that G-quadruplexes could not regulate biological processes by themselves; instead, various proteins take part in this regulation and can be important therapeutic targets. There are certain limitations in using whole G4-protein for therapeutics purpose because of its high manufacturing cost, laborious structure prediction, dynamic nature, unavailability for oral administration due to its degradation in the gut and inefficient penetration to reach the target site because of the large size. Hence, biologically active peptides can be the potential candidates for therapeutic intervention instead of the whole G4-protein complex. In this review, we aimed to clarify the biological roles of G4s, how we can identify them throughout the genome via bioinformatics, the proteins interacting with G4s and how G4-interacting peptide molecules may be the potential next-generation ligands for targeting the G4 motifs located in biologically important regions.

Cryo-EM Structure of the Full-length hnRNPA1 Amyloid Fibril

Heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) is a multifunctional RNA-binding protein that is associated with neurodegenerative diseases, such as amyotrophic lateral sclerosis and multisystem proteinopathy. In this study, we have used cryo-electron microscopy to investigate the three-dimensional structure of amyloid fibrils from full-length hnRNPA1 protein. We find that the fibril core is formed by a 45-residue segment of the prion-like low-complexity domain of the protein, whereas the remaining parts of the protein (275 residues) form a fuzzy coat around the fibril core. The fibril consists of two fibril protein stacks that are arranged into a pseudo-21 screw symmetry. The ordered core harbors several of the positions that are known to be affected by disease-associated mutations, but does not encompass the most aggregation-prone segments of the protein. These data indicate that the structures of amyloid fibrils from full-length proteins may be more complex than anticipated by current theories on protein misfolding.

To RNA-binding and beyond: Emerging facets of the role of Rbfox proteins in development and disease

The RNA-binding Fox-1 homologue (Rbfox) proteins represent an ancient family of splicing factors, conserved through evolution. All members share an RNA recognition motif (RRM), and a particular affinity for the GCAUG signature in target RNA molecules. The role of Rbfox, as a splice factor, deciding the tissue-specific inclusion/exclusion of an exon, depending on its binding position on the flanking introns, is well known. Rbfox often acts in concert with other splicing factors, and forms splicing regulatory networks. Apart from this canonical role, recent studies show that Rbfox can also function as a transcription co-factor, and affects mRNA stability and translation. The repertoire of Rbfox targets is vast, including genes involved in the development of tissue lineages, such as neurogenesis, myogenesis, and erythropoeiesis, and molecular processes, including cytoskeletal dynamics, and calcium handling. A second layer of complexity is added by the fact that Rbfox expression itself is regulated by multiple mechanisms, and, in vertebrates, exhibits tissue-specific expression. The optimum dosage of Rbfox is critical, and its misexpression is etiological to various disease conditions. In this review, we discuss the contextual roles played by Rbfox as a tissue-specific regulator for the expression of many important genes with diverse functions, through the lens of the emerging data which highlights its involvement in many human diseases. Furthermore, we explore the mechanistic details provided by studies in model organisms, with emphasis on the work with Drosophila. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Turnover and Surveillance > Regulation of RNA Stability RNA Processing > Splicing Regulation/Alternative Splicing.

The HNRNPF/H RNA binding proteins and disease

The members of the HNRNPF/H family of heterogeneous nuclear RNA proteins-HNRNPF, HNRNPH1, HNRNPH2, HNRNPH3, and GRSF1, are critical regulators of RNA maturation. Documented functions of these proteins include regulating splicing, particularly alternative splicing, 5' capping and 3' polyadenylation of RNAs, and RNA export. The assignment of these proteins to the HNRNPF/H protein family members relates to differences in the amino acid composition of their RNA recognition motifs, which differ from those of other RNA binding proteins (RBPs). HNRNPF/H proteins typically bind RNA sequences enriched with guanine (G) residues, including sequences that, in the presence of a cation, have the potential to form higher-order G-quadruplex structures. The need to further investigate members of the HNRNPF/H family of RBPs has intensified with the recent descriptions of their involvement in several disease states, including the pediatric tumor Ewing sarcoma and the hematological malignancy mantle cell lymphoma; newly described groups of developmental syndromes; and neuronal-related disorders, including addictive behavior. Here, to foster the study of the HNRNPF/H family of RBPs, we discuss features of the genes encoding these proteins, their structures and functions, and emerging contributions to disease. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Liquid-liquid Phase Separation in Viral Function

An emerging set of results suggests that liquid-liquid phase separation (LLPS) is the basis for the formation of membrane-less compartments in cells. Evidence is now mounting that various types of virus-induced membrane-less compartments and organelles are also assembled via LLPS. Specifically, viruses appear to use intracellular phase transitions to form subcellular microenvironments known as viral factories, inclusion bodies, or viroplasms. These compartments - collectively referred to as viral biomolecular condensates - can be used to concentrate replicase proteins, viral genomes, and host proteins that are required for virus replication. They can also be used to subvert or avoid the intracellular immune response. This review examines how certain DNA or RNA viruses drive the formation of viral condensates, the possible biological functions of those condensates, and the biophysical and biochemical basis for their assembly.

A new Karyopherin-β2 binding PY-NLS epitope of HNRNPH2 linked to neurodevelopmental disorders

The HNRNPH2 proline-tyrosine nuclear localization signal (PY-NLS) is mutated in HNRNPH2-related X-linked neurodevelopmental disorder, causing the normally nuclear HNRNPH2 to accumulate in the cytoplasm. We solved the cryoelectron microscopy (cryo-EM) structure of Karyopherin-β2/Transportin-1 bound to the HNRNPH2 PY-NLS to understand importin-NLS recognition and disruption in disease. HNRNPH2 206RPGPY210 is a typical R-X2-4-P-Y motif comprising PY-NLS epitopes 2 and 3, followed by an additional Karyopherin-β2-binding epitope, we term epitope 4, at residues 211DRP213; no density is present for PY-NLS epitope 1. Disease variant mutations at epitopes 2-4 impair Karyopherin-β2 binding and cause aberrant cytoplasmic accumulation in cells, emphasizing the role of nuclear import defect in disease. Sequence/structure analysis suggests that strong PY-NLS epitopes 4 are rare and thus far limited to close paralogs of HNRNPH2, HNRNPH1, and HNRNPF. Epitope 4-binidng hotspot Karyopherin-β2 W373 corresponds to close paralog Karyopherin-β2b/Transportin-2 W370, a pathological variant site in neurodevelopmental abnormalities, suggesting that Karyopherin-β2b/Transportin-2-HNRNPH2/H1/F interactions may be compromised in the abnormalities.

Defective recognition of a nonclassical nuclear localization signal in neurodevelopmental disorders

In this issue of Structure, Gonzalez et al. present the cryo-EM structure of Karyopherin-β2 bound to the proline-tyrosine nuclear localization signal (PY-NLS) of heterogeneous nuclear ribonucleoprotein H2 (HNRNPH2). The structure advances our understanding of not only the diversity of PY-NLSs but also the pathogenic mechanisms arising from HNRNPH2 variants.

Mechanisms of NURR1 Regulation: Consequences for Its Biological Activity and Involvement in Pathology

NURR1 (Nuclear receptor-related 1 protein or NR4A2) is a nuclear protein receptor transcription factor with an essential role in the development, regulation, and maintenance of dopaminergic neurons and mediates the response to stressful stimuli during the perinatal period in mammalian brain development. The dysregulation of NURR1 activity may play a role in various diseases, including the onset and progression of neurodegenerative diseases, and several other pathologies. NURR1 is regulated by multiple mechanisms, among which phosphorylation by kinases or SUMOylation are the best characterized. Both post-translational modifications can regulate the activity of NURR1, affecting its stability and transcriptional activity. Other non-post-translational regulatory mechanisms include changes in its subcellular distribution or interaction with other protein partners by heterodimerization, also affecting its transcription activity. Here, we summarize the currently known regulatory mechanisms of NURR1 and provide a brief overview of its participation in pathological alterations.

A murine model of hnRNPH2-related neurodevelopmental disorder reveals a mechanism for genetic compensation by Hnrnph1

Mutations in HNRNPH2 cause an X-linked neurodevelopmental disorder with features that include developmental delay, motor function deficits, and seizures. More than 90% of patients with hnRNPH2 have a missense mutation within or adjacent to the nuclear localization signal (NLS) of hnRNPH2. Here, we report that hnRNPH2 NLS mutations caused reduced interaction with the nuclear transport receptor Kapβ2 and resulted in modest cytoplasmic accumulation of hnRNPH2. We generated 2 knockin mouse models with human-equivalent mutations in Hnrnph2 as well as Hnrnph2-KO mice. Knockin mice recapitulated clinical features of the human disorder, including reduced survival in male mice, impaired motor and cognitive functions, and increased susceptibility to audiogenic seizures. In contrast, 2 independent lines of Hnrnph2-KO mice showed no detectable phenotypes. Notably, KO mice had upregulated expression of Hnrnph1, a paralog of Hnrnph2, whereas knockin mice failed to upregulate Hnrnph1. Thus, genetic compensation by Hnrnph1 may counteract the loss of hnRNPH2. These findings suggest that HNRNPH2-related disorder may be driven by a toxic gain of function or a complex loss of HNRNPH2 function with impaired compensation by HNRNPH1. The knockin mice described here are an important resource for preclinical studies to assess the therapeutic benefit of gene replacement or knockdown of mutant hnRNPH2.

Interaction of hnRNPB1 with Helix-12 of hHOTAIR Reveals the Distinctive Mode of RNA Recognition That Enables the Structural Rearrangement by LCD

The lncRNA human Hox transcript antisense intergenic RNA (hHOTAIR) regulates gene expression by recruiting chromatin modifiers. The prevailing model suggests that hHOTAIR recruits hnRNPB1 to facilitate intermolecular RNA-RNA interactions between the lncRNA HOTAIR and its target gene transcripts. This B1-mediated RNA-RNA interaction modulates the structure of hHOTAIR, attenuates its inhibitory effect on polycomb repression complex 2, and enhances its methyl transferase activity. However, the molecular details by which the nuclear hnRNPB1 protein assembles on the lncRNA HOTAIR have not yet been described. Here, we investigate the molecular interactions between hnRNPB1 and Helix-12 (hHOTAIR). We show that the low-complexity domain segment (LCD) of hnRNPB1 interacts with a strong affinity for Helix-12. Our studies revealed that unbound Helix-12 folds into a specific base-pairing pattern and contains an internal loop that, as determined by thermal melting and NMR studies, exhibits hydrogen bonding between strands and forms the recognition site for the LCD segment. In addition, mutation studies show that the secondary structure of Helix-12 makes an important contribution by acting as a landing pad for hnRNPB1. The secondary structure of Helix-12 is involved in specific interactions with different domains of hnRNPB1. Finally, we show that the LCD unwinds Helix-12 locally, indicating its importance in the hHOTAIR restructuring mechanism.

Cyclic AMP (cAMP)-dependent proteolysis of GATA6 by proteasome: Zinc-finger domain of GATA6 has signals for nuclear export and proteolysis, both of which are responsive to cAMP

Transcription factor GATA6 stably expressed in Chinese hamster ovary (CHO)-K1 cells is exported from the nucleus to the cytoplasm and degraded there by proteasome upon treatment with dibutylyl-cyclic AMP (dbcAMP), which is a membrane-permeable cyclic AMP (cAMP) analogue. The cAMP-dependent proteolysis of GATA6 was characterized by dissection of the GATA6 protein into a zinc-finger domain (Zf) and the surrounding region (ΔZf). These segments were separately expressed in CHO-K1 cells stably, and followed by treatment with dbcAMP. The nuclear localized Zf was degraded by proteasome similarly to the full-length GATA6. Site-directed mutants of nuclear localizing signal (NLS) (³⁴⁵RKRKPK³⁵⁰ → AAAAPK and AAAAPA) and closely related GATA4 showed the same behavior. Although nuclear-localized ΔZf was degraded by proteasome, the cytoplasmic-located ΔZf was resistant to proteolysis in contrast to the NLS mutants. We also searched for a potential NLS and nuclear export signal (NES) with computational prediction programs and compared the results with ours. All these results suggest that the amino acid sequence(s) of the Zf of GATA6 is responsive to cAMP-dependent nuclear export and proteolysis.

A minimal construct of nuclear-import receptor Karyopherin-β2 defines the regions critical for chaperone and disaggregation activity

Karyopherin-β2 (Kapβ2) is a nuclear-import receptor that recognizes proline-tyrosine nuclear localization signals of diverse cytoplasmic cargo for transport to the nucleus. Kapβ2 cargo includes several disease-linked RNA-binding proteins with prion-like domains, such as FUS, TAF15, EWSR1, hnRNPA1, and hnRNPA2. These RNA-binding proteins with prion-like domains are linked via pathology and genetics to debilitating degenerative disorders, including amyotrophic lateral sclerosis, frontotemporal dementia, and multisystem proteinopathy. Remarkably, Kapβ2 prevents and reverses aberrant phase transitions of these cargoes, which is cytoprotective. However, the molecular determinants of Kapβ2 that enable these activities remain poorly understood, particularly from the standpoint of nuclear-import receptor architecture. Kapβ2 is a super-helical protein comprised of 20 HEAT repeats. Here, we design truncated variants of Kapβ2 and assess their ability to antagonize FUS aggregation and toxicity in yeast and FUS condensation at the pure protein level and in human cells. We find that HEAT repeats 8 to 20 of Kapβ2 recapitulate all salient features of Kapβ2 activity. By contrast, Kapβ2 truncations lacking even a single cargo-binding HEAT repeat display reduced activity. Thus, we define a minimal Kapβ2 construct for delivery in adeno-associated viruses as a potential therapeutic for amyotrophic lateral sclerosis/frontotemporal dementia, multisystem proteinopathy, and related disorders.

A new Karyopherin-β2 binding PY-NLS epitope of HNRNPH2 is linked to neurodevelopmental disorders

The normally nuclear HNRNPH2 is mutated in HNRNPH2 -related X-linked neurodevelopmental disorder causing the protein to accumulate in the cytoplasm. Interactions of HNRNPH2 with its importin Karyopherin-β2 (Transportin-1) had not been studied. We present a structure that shows Karyopherin-β2 binding HNRNPH2 residues 204-215, a proline-tyrosine nuclear localization signal or PY-NLS that contains a typical R-X _2-4 -P-Y motif, ²⁰⁶ RPGPY ²¹⁰ , followed a new Karyopherin-β2 binding epitope at ²¹¹ DRP ²¹³ that make many interactions with Karyopherin-β2 W373. Mutations at each of these sites decrease Karyopherin-β2 binding affinities by 70-100 fold, explaining aberrant accumulation in cells and emphasizing the role of nuclear import defects in the disease. Sequence/structure analysis suggests that the new epitope C-terminal of the PY-motif, which binds Karyopherin-β2 W373, is rare and thus far limited to close paralogs HNRNPH2, HNRNPH1 and HNRNPF. Karyopherin-β2 W373, a HNRNPH2-binding hotspot, corresponds to W370 of close paralog Transportin-2, a site of pathological variants in patients with neurodevelopmental abnormalities, suggesting that Transportin-2-HNRNPH2/H1/F interactions may be compromised in the abnormalities.

Summary

HNRNPH2 variants in HNRNPH2 -related X-linked neurodevelopmental disorder aberrantly accumulate in the cytoplasm. A structure of Karyopherin-β2•HNRNPH2 explains nuclear import defects of the variants, reveals a new NLS epitope that suggests mechanistic changes in pathological variants of Karyopherin-β2 paralog Transportin-2.

Transportin 1 is a major nuclear import receptor of the nitric oxide synthase interacting protein

The nitric oxide synthase interacting protein (NOSIP), an E3-ubiquitin ligase, is involved in various processes like neuronal development, craniofacial development, granulopoiesis, mitogenic signaling, apoptosis, and cell proliferation. The best-characterized function of NOSIP is the regulation of endothelial nitric oxide synthase activity by translocating the membrane-bound enzyme to the cytoskeleton, specifically in the G2 phase of the cell cycle. For this, NOSIP itself has to be translocated from its prominent localization, the nucleus, to the cytoplasm. Nuclear import of NOSIP was suggested to be mediated by the canonical transport receptors importin α/β. Recently, we found NOSIP in a proteomic screen as a potential importin 13 cargo. Here, we describe the nuclear shuttling characteristics of NOSIP in living cells and in vitro and show that it does not interact directly with importin α. Instead, it formed stable complexes with several importins (-β, -7, -β/7, -13, and transportin 1) and was also imported into the nucleus in digitonin-permeabilized cells by these factors. In living HeLa cells, transportin 1 seems to be the major nuclear import receptor for NOSIP. A detailed analysis of the NOSIP-transportin 1 interaction revealed a high affinity and an unusual binding mode, involving the N-terminal half of transportin 1. In contrast to nuclear import, nuclear export of NOSIP seems to occur mostly by passive diffusion. Thus, our results uncover additional layers in the larger process of endothelial nitric oxide synthase regulation.

DNA damage stress-induced translocation of mutant FUS proteins into cytosolic granules and screening for translocation inhibitors

Fused in sarcoma/translated in liposarcoma (FUS) is an RNA-binding protein, and its mutations are associated with neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), through the DNA damage stress response, aberrant stress granule (SG) formation, etc. We previously reported that translocation of endogenous FUS into SGs was achieved by cotreatment with a DNA double-strand break inducer and an inhibitor of DNA-PK activity. In the present study, we investigated cytoplasmic SG formation using various fluorescent protein-tagged mutant FUS proteins in a human astrocytoma cell (U251) model. While the synergistic enhancement of the migration of fluorescent protein-tagged wild-type FUS to cytoplasmic SGs upon DNA damage induction was observed when DNA-PK activity was suppressed, the fluorescent protein-tagged FUS^P525L mutant showed cytoplasmic localization. It migrated to cytoplasmic SGs upon DNA damage induction alone, and DNA-PK inhibition also showed a synergistic effect. Furthermore, analysis of 12 sites of DNA-PK-regulated phosphorylation in the N-terminal LC region of FUS revealed that hyperphosphorylation of FUS mitigated the mislocalization of FUS into cytoplasmic SGs. By using this cell model, we performed screening of a compound library to identify compounds that inhibit the migration of FUS to cytoplasmic SGs but do not affect the localization of the SG marker molecule G3BP1 to cytoplasmic SGs. Finally, we successfully identified 23 compounds that inhibit FUS-containing SG formation without changing normal SG formation. Highlights Characterization of DNA-PK-dependent FUS stress granule localization.A compound library was screened to identify compounds that inhibit the formation of FUS-containing stress granules.

Identification of the Karyopherin Superfamily in Maize and Its Functional Cues in Plant Development

Appropriate nucleo-cytoplasmic partitioning of proteins is a vital regulatory mechanism in phytohormone signaling and plant development. However, how this is achieved remains incompletely understood. The Karyopherin (KAP) superfamily is critical for separating the biological processes in the nucleus from those in the cytoplasm. The KAP superfamily is divided into Importin α (IMPα) and Importin β (IMPβ) families and includes the core components in mediating nucleocytoplasmic transport. Recent reports suggest the KAPs play crucial regulatory roles in Arabidopsis development and stress response by regulating the nucleo-cytoplasmic transport of members in hormone signaling. However, the KAP members and their associated molecular mechanisms are still poorly understood in maize. Therefore, we first identified seven IMPα and twenty-seven IMPβ genes in the maize genome and described their evolution traits and the recognition rules for substrates with nuclear localization signals (NLSs) or nuclear export signals (NESs) in plants. Next, we searched for the protein interaction partners of the ZmKAPs and selected the ones with Arabidopsis orthologs functioning in auxin biosynthesis, transport, and signaling to predict their potential function. Finally, we found that several ZmKAPs share similar expression patterns with their interacting proteins, implying their function in root development. Overall, this article focuses on the Karyopherin superfamily in maize and starts with this entry point by systematically comprehending the KAP-mediated nucleo-cytoplasmic transport process in plants, and then predicts the function of the ZmKAPs during maize development, with a perspective on a closely associated regulatory mechanism between the nucleo-cytoplasmic transport and the phytohormone network.

Non-transport roles of nuclear import receptors: In need of the right balance

Nuclear import receptors ensure the recognition and transport of proteins across the nuclear envelope into the nucleus. In addition, as diverse processes as mitosis, post-translational modifications at mitotic exit, ciliogenesis, and phase separation, all share a common need for regulation by nuclear import receptors - particularly importin beta-1 and importin beta-2/transportin - independent on nuclear import. In particular, 1) nuclear import receptors regulate the mitotic spindle after nuclear envelope breakdown, 2) they shield cargoes from unscheduled ubiquitination, regulating their timely proteolysis; 3) they regulate ciliary factors, crucial to cell communications and tissue architecture during development; and 4) they prevent phase separation of toxic proteins aggregates in neurons. The balance of nuclear import receptors to cargoes is critical in all these processes, albeit in opposite directions: overexpression of import receptors, as often found in cancer, inhibits cargoes and impairs downstream processes, motivating the therapeutic design of specific inhibitors. On the contrary, elevated expression is beneficial in neuronal contexts, where nuclear import receptors are regarded as potential therapeutic tools in counteracting the formation of aggregates that may cause neurodegeneration. This paradox demonstrates the amplitude of nuclear import receptors-dependent functions in different contexts and adds complexity in considering their therapeutic implications.

Kap-β2/Transportin mediates β-catenin nuclear transport in Wnt signaling

Wnt signaling is essential for many aspects of embryonic development including the formation of the primary embryonic axis. In addition, excessive Wnt signaling drives multiple diseases including cancer, highlighting its importance for disease pathogenesis. β-catenin is a key effector in this pathway that translocates into the nucleus and activates Wnt responsive genes. However, due to our lack of understanding of β-catenin nuclear transport, therapeutic modulation of Wnt signaling has been challenging. Here, we took an unconventional approach to address this long-standing question by exploiting a heterologous model system, the budding yeast Saccharomyces cerevisiae, which contains a conserved nuclear transport machinery. In contrast to prior work, we demonstrate that β-catenin accumulates in the nucleus in a Ran-dependent manner, suggesting the use of a nuclear transport receptor (NTR). Indeed, a systematic and conditional inhibition of NTRs revealed that only Kap104, the ortholog of Kap-β2/Transportin-1 (TNPO1), was required for β-catenin nuclear import. We further demonstrate direct binding between TNPO1 and β-catenin that is mediated by a conserved PY-NLS. Finally, using Xenopus secondary axis and TCF/LEF (T Cell factor/lymphoid enhancer factor family) reporter assays, we demonstrate that our results in yeast can be directly translated to vertebrates. By elucidating the nuclear localization signal in β-catenin and its cognate NTR, our study suggests new therapeutic targets for a host of human diseases caused by excessive Wnt signaling. Indeed, we demonstrate that a small chimeric peptide designed to target TNPO1 can reduce Wnt signaling as a first step toward therapeutics.

Phase Separation of Heterogeneous Nuclear Ribonucleoprotein A1 upon Specific RNA-Binding Observed by Magnetic Resonance

Interaction of heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) with specific single-stranded RNA and its relation to liquid-liquid phase separation (LLPS) were studied in vitro by magnetic resonance based on site-directed spin labelling. An ensemble model of dispersed hnRNP A1 in the absence of RNA was derived from distance distributions between spin labelled sites and small angle X-ray scattering. This model revealed a compact state of the low-complexity domain and its interaction with the RNA recognition motifs. Paramagnetic relaxation enhancement NMR spectroscopy confirmed this interaction. Addition of RNA to dispersed hnRNP A1 induced liquid-droplet formation. Such LLPS depended on RNA concentration and sequence, with continuous wave EPR spectroscopy showing an influence of RNA point mutations on local protein dynamics. We propose that an interplay of sequence-specific RNA binding and LLPS contributes to regulation of specific RNA segregation during stress response.

Nuclear entry and egress of parvoviruses

Parvoviruses are small non-enveloped single-stranded DNA viruses, which depend on host cell nuclear transcriptional and replication machinery. After endosomal exposure of nuclear localization sequence and a phospholipase A₂ domain on the capsid surface, and escape into the cytosol, parvovirus capsids enter the nucleus. Due to the small capsid diameter of 18-26 nm, intact capsids can potentially pass into the nucleus through nuclear pore complexes (NPCs). This might be facilitated by active nuclear import, but capsids may also follow an alternative entry pathway that includes activation of mitotic factors and local transient disruption of the nuclear envelope. The nuclear entry is followed by currently undefined events of viral genome uncoating. After genome release, viral replication compartments are initiated and infection proceeds. Parvoviral genomes replicate during cellular S phase followed by nuclear capsid assembly during virus-induced S/G2 cell cycle arrest. Nuclear egress of capsids occurs upon nuclear envelope degradation during apoptosis and cell lysis. An alternative pathway for nuclear export has been described using active transport through the NPC mediated by the chromosome region maintenance 1 protein, CRM1, which is enhanced by phosphorylation of the N-terminal domain of VP2. However, other alternative but not yet uncharacterized nuclear export pathways cannot be excluded.

Bioinformatics and Functional Analysis of a New Nuclear Localization Sequence of the Influenza A Virus Nucleoprotein

Influenza viruses deliver their genome into the nucleus of infected cells for replication. This process is mediated by the viral nucleoprotein (NP), which contains two nuclear localization sequences (NLSs): NLS1 at the N-terminus and a recently identified NLS2 (²¹²GRKTR²¹⁶). Through mutagenesis and functional studies, we demonstrated that NP must have both NLSs for an efficient nuclear import. As with other NLSs, there may be variations in the basic residues of NLS2 in different strains of the virus, which may affect the nuclear import of the viral genome. Although all NLS2 variants fused to the GFP mediated nuclear import of GFP, bioinformatics showed that 98.8% of reported NP sequences contained either the wild-type sequence ²¹²GRKTR²¹⁶ or ²¹²GRRTR²¹⁶. Bioinformatics analyses used to study the presence of NLS2 variants in other viral and nuclear proteins resulted in very low hits, with only 0.4% of human nuclear proteins containing putative NLS2. From these, we studied the nucleolar protein 14 (NOP14) and found that NLS2 does not play a role in the nuclear import of this protein but in its nucleolar localization. We also discovered a functional NLS at the C-terminus of NOP14. Our findings indicate that NLS2 is a highly conserved influenza A NP sequence.

Nuclear Effectors in Plant Pathogenic Fungi

The nuclear import of proteins is a fundamental process in the eukaryotes including plant. It has become evident that such basic process is exploited by nuclear effectors that contain nuclear localization signal (NLS) and are secreted into host cells by fungal pathogens of plants. However, only a handful of nuclear effectors have been known and characterized to date. Here, we first summarize the types of NLSs and prediction tools available, and then delineate examples of fungal nuclear effectors and their roles in pathogenesis. Based on the knowledge on NLSs and what has been gleaned from the known nuclear effectors, we point out the gaps in our understanding of fungal nuclear effectors that need to be filled in the future researches.

Emerging roles of hnRNP A2B1 in cancer and inflammation

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are a group of RNA-binding proteins with important roles in multiple aspects of nucleic acid metabolism, including the packaging of nascent transcripts, alternative splicing, transactivation of gene expression, and regulation of protein translation. As a core component of the hnRNP complex in mammalian cells, heterogeneous nuclear ribonucleoprotein A2B1 (hnRNP A2B1) participates in and coordinates various molecular events. Given its regulatory role in inflammation and cancer progression, hnRNP A2B1 has become a novel player in immune response, inflammation, and cancer development. Concomitant with these new roles, a surprising number of mechanisms deemed to regulate hnRNP A2B1 functions have been identified, including post-translational modifications, changes in subcellular localization, direct interactions with multiple DNAs, RNAs, and proteins or the formation of complexes with them, which have gradually made hnRNP A2B1 a molecular target for multiple drugs. In light of the rising interest in the intersection between cancer and inflammation, this review will focus on recent knowledge of the biological roles of hnRNP A2B1 in cancer, immune response, and inflammation.

Inhibition of Karyopherin β1-Mediated Nuclear Import Disrupts Oncogenic Lineage-Defining Transcription Factor Activity in Small Cell Lung Cancer

Genomic studies support the classification of small cell lung cancer (SCLC) into subtypes based on the expression of lineage-defining transcription factors ASCL1 and NEUROD1, which together are expressed in ∼86% of SCLC. ASCL1 and NEUROD1 activate SCLC oncogene expression, drive distinct transcriptional programs, and maintain the in vitro growth and oncogenic properties of ASCL1 or NEUROD1-expressing SCLC. ASCL1 is also required for tumor formation in SCLC mouse models. A strategy to inhibit the activity of these oncogenic drivers may therefore provide both a targeted therapy for the predominant SCLC subtypes and a tool to investigate the underlying lineage plasticity of established SCLC tumors. However, there are no known agents that inhibit ASCL1 or NEUROD1 function. In this study, we identify a novel strategy to pharmacologically target ASCL1 and NEUROD1 activity in SCLC by exploiting the nuclear localization required for the function of these transcription factors. Karyopherin β1 (KPNB1) was identified as a nuclear import receptor for both ASCL1 and NEUROD1 in SCLC, and inhibition of KPNB1 led to impaired ASCL1 and NEUROD1 nuclear accumulation and transcriptional activity. Pharmacologic targeting of KPNB1 preferentially disrupted the growth of ASCL1+ and NEUROD1+ SCLC cells in vitro and suppressed ASCL1+ tumor growth in vivo, an effect mediated by a combination of impaired ASCL1 downstream target expression, cell-cycle activity, and proteostasis. These findings broaden the support for targeting nuclear transport as an anticancer therapeutic strategy and have implications for targeting lineage-transcription factors in tumors beyond SCLC.

SIGNIFICANCE

The identification of KPNB1 as a nuclear import receptor for lineage-defining transcription factors in SCLC reveals a viable therapeutic strategy for cancer treatment.

Interactions of amyloid coaggregates with biomolecules and its relevance to neurodegeneration

The aggregation of amyloidogenic proteins is a pathological hallmark of various neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. In these diseases, oligomeric intermediates or toxic aggregates of amyloids cause neuronal damage and degeneration. Despite the substantial effort made over recent decades to implement therapeutic interventions, these neurodegenerative diseases are not yet understood at the molecular level. In many cases, multiple disease-causing amyloids overlap in a sole pathological feature or a sole disease-causing amyloid represents multiple pathological features. Various amyloid pathologies can coexist in the same brain with or without clinical presentation and may even occur in individuals without disease. From sparse data, speculation has arisen regarding the coaggregation of amyloids with disparate amyloid species and other biomolecules, which are the same characteristics that make diagnostics and drug development challenging. However, advances in research related to biomolecular condensates and structural analysis have been used to overcome some of these challenges. Considering the development of these resources and techniques, herein we review the cross-seeding of amyloidosis, for example, involving the amyloids amyloid β, tau, α-synuclein, and human islet amyloid polypeptide, and their cross-inhibition by transthyretin and BRICHOS. The interplay of nucleic acid-binding proteins, such as prions, TAR DNA-binding protein 43, fused in sarcoma/translated in liposarcoma, and fragile X mental retardation polyglycine, with nucleic acids in the pathology of neurodegeneration are also described, and we thereby highlight the potential clinical applications in central nervous system therapy.

Nuclear import of PTPN18 inhibits breast cancer metastasis mediated by MVP and importin β2

Distant metastasis is the primary cause of breast cancer-associated death. The existing information, such as the precise molecular mechanisms and effective therapeutic strategies targeting metastasis, is insufficient to combat breast cancer. This study demonstrates that the protein tyrosine phosphatase PTPN18 is downregulated in metastatic breast cancer tissues and is associated with better metastasis-free survival. Ectopic expression of PTPN18 inhibits breast cancer cell metastasis. PTPN18 is translocated from the cytoplasm to the nucleus by MVP and importin β2 in breast cancer. Then, nuclear PTPN18 dephosphorylates ETS1 and promotes its degradation. Moreover, nuclear PTPN18 but not cytoplasmic PTPN18 suppresses transforming growth factor-β signaling and epithelial-to-mesenchymal transition by targeting ETS1. Our data highlight PTPN18 as a suppressor of breast cancer metastasis and provide an effective antimetastatic therapeutic strategy.

FUS-P525L Juvenile Amyotrophic Lateral Sclerosis and Intellectual Disability

Background and Objectives

Amyotrophic lateral sclerosis (ALS) is characterized by upper and lower motor neuron degeneration, with juvenile ALS (jALS) defined as disease with age at onset (AAO) before 25 years. We aimed to identify the genetic basis of 2 unrelated patients with jALS with very rapid deterioration and early age intellectual disability (ID) and to assess association of genetic findings with both phenotypes in a large cohort of patients with ALS and controls, and in the literature.

Methods

Exome sequencing was performed in 2 unrelated probands and their parents. Trio analyses included de novo, rare homozygosity, and compound heterozygosity analyses. A TaqMan genotyping assay was used to genotype ALS cohorts. A systematic literature review was conducted and additional information from authors obtained to assess prevalence of fused in sarcoma (FUS)-ALS associated with ID.

Results

A de novo mutation FUS-P525L was identified in both patients. Additional variations were identified in other genes related to intellectual disabilities. Among 8 additional unrelated juvenile patients, one carried the same FUS mutation and had a similar medical history of mild ID and fulminant ALS, whereas the others did not carry any FUS coding mutations and had no reported learning or intellectual disabilities (p = 0.0083). In addition, 486 patients with ALS with AAO ≥25 years were negative for this mutation. An extensive literature review showed that among all patients with FUS-related ALS with full phenotype reports, 10.3% exhibited additional learning/intellectual disabilities.

Discussion

FUS-P525L mutation was identified in 3 among 10 patients with jALS (30%) in our clinical cohort, all with a very aggressive disease course and ID. Together with literature reports, these results support a novel association between mutations in FUS and early life ID. Additional variations identified in genes related to ID and brain development in our patients (GPT2, DNAH10, and SCUBE2) may suggest a complex oligogenic inheritance for this phenotype. We propose that this mutation should be screened in patients with ALS with very early AAO, aggressive disease course, and sporadic occurrence, especially when ALS is accompanied by ID.

Regulating Phase Transition in Neurodegenerative Diseases by Nuclear Import Receptors

RNA-binding proteins (RBPs) with a low-complexity prion-like domain (PLD) can undergo aberrant phase transitions and have been implicated in neurodegenerative diseases such as ALS and FTD. Several nuclear RBPs mislocalize to cytoplasmic inclusions in disease conditions. Impairment in nucleocytoplasmic transport is another major event observed in ageing and in neurodegenerative disorders. Nuclear import receptors (NIRs) regulate the nucleocytoplasmic transport of different RBPs bearing a nuclear localization signal by restoring their nuclear localization. NIRs can also specifically dissolve or prevent the aggregation and liquid–liquid phase separation of wild-type or disease-linked mutant RBPs, due to their chaperoning activity. This review focuses on the LLPS of intrinsically disordered proteins and the role of NIRs in regulating LLPS in neurodegeneration. This review also discusses the implication of NIRs as therapeutic agents in neurogenerative diseases.

Recognition of the TDP-43 nuclear localization signal by importin α1/β

Cytoplasmic mislocalization of the TAR-DNA binding protein of 43 kDa (TDP-43) leads to large, insoluble aggregates that are a hallmark of amyotrophic lateral sclerosis and frontotemporal dementia. Here, we study how importin α1/β recognizes TDP-43 bipartite nuclear localization signal (NLS). We find that the NLS makes extensive contacts with importin α1, especially at the minor NLS-binding site. NLS binding results in steric clashes with the C terminus of importin α1 that disrupts the TDP-43 N-terminal domain (NTD) dimerization interface. A putative phosphorylation site in the proximity of TDP-43 R83 at the minor NLS site destabilizes binding to importins by reducing the NLS backbone dynamics. Based on these data, we explain the pathogenic role of several post-translational modifications and mutations in the proximity of TDP-43 minor NLS site that are linked to disease and shed light on the chaperone activity of importin α1/β.

40S hnRNP particles are a novel class of nuclear biomolecular condensates

Heterogenous nuclear ribonucleoproteins (hnRNPs) are abundant proteins implicated in various steps of RNA processing that assemble on nuclear RNA into larger complexes termed 40S hnRNP particles. Despite their initial discovery 55 years ago, our understanding of these intriguing macromolecular assemblies remains limited. Here, we report the biochemical purification of native 40S hnRNP particles and the determination of their complete protein composition by label-free quantitative mass spectrometry, identifying A-group and C-group hnRNPs as the major protein constituents. Isolated 40S hnRNP particles dissociate upon RNA digestion and can be reconstituted in vitro on defined RNAs in the presence of the individual protein components, demonstrating a scaffolding role for RNA in nucleating particle formation. Finally, we revealed their nanometer scale, condensate-like nature, promoted by intrinsically disordered regions of A-group hnRNPs. Collectively, we identify nuclear 40S hnRNP particles as novel dynamic biomolecular condensates.

Tumor suppressor BAP1 nuclear import is governed by transportin-1

Subcellular localization of the deubiquitinating enzyme BAP1 is deterministic for its tumor suppressor activity. While the monoubiquitination of BAP1 by an atypical E2/E3-conjugated enzyme UBE2O and BAP1 auto-deubiquitination are known to regulate its nuclear localization, the molecular mechanism by which BAP1 is imported into the nucleus has remained elusive. Here, we demonstrated that transportin-1 (TNPO1, also known as Karyopherin β2 or Kapβ2) targets an atypical C-terminal proline-tyrosine nuclear localization signal (PY-NLS) motif of BAP1 and serves as the primary nuclear transporter of BAP1 to achieve its nuclear import. TNPO1 binding dissociates dimeric BAP1 and sequesters the monoubiquitination sites flanking the PY-NLS of BAP1 to counteract the function of UBE2O that retains BAP1 in the cytosol. Our findings shed light on how TNPO1 regulates the nuclear import, self-association, and monoubiquitination of BAP1 pertinent to oncogenesis.

Reovirus μ2 protein modulates host cell alternative splicing by reducing protein levels of U5 snRNP core components

Mammalian orthoreovirus (MRV) is a double-stranded RNA virus from the Reoviridae family presenting a promising activity as an oncolytic virus. Recent studies have underlined MRV’s ability to alter cellular alternative splicing (AS) during infection, with a limited understanding of the mechanisms at play. In this study, we investigated how MRV modulates AS. Using a combination of cell biology and reverse genetics experiments, we demonstrated that the M1 gene segment, encoding the μ2 protein, is the primary determinant of MRV’s ability to alter AS, and that the amino acid at position 208 in μ2 is critical to induce these changes. Moreover, we showed that the expression of μ2 by itself is sufficient to trigger AS changes, and its ability to enter the nucleus is not required for all these changes. Moreover, we identified core components of the U5 snRNP (i.e. EFTUD2, PRPF8, and SNRNP200) as interactors of μ2 that are required for MRV modulation of AS. Finally, these U5 snRNP components are reduced at the protein level by both MRV infection and μ2 expression. Our findings identify the reduction of U5 snRNP components levels as a new mechanism by which viruses alter cellular AS.

Heterozygous frameshift variants in HNRNPA2B1 cause early-onset oculopharyngeal muscular dystrophy

Missense variants in RNA-binding proteins (RBPs) underlie a spectrum of disease phenotypes, including amyotrophic lateral sclerosis, frontotemporal dementia, and inclusion body myopathy. Here, we present ten independent families with a severe, progressive muscular dystrophy, reminiscent of oculopharyngeal muscular dystrophy (OPMD) but of much earlier onset, caused by heterozygous frameshift variants in the RBP hnRNPA2/B1. All disease-causing frameshift mutations abolish the native stop codon and extend the reading frame, creating novel transcripts that escape nonsense-mediated decay and are translated to produce hnRNPA2/B1 protein with the same neomorphic C-terminal sequence. In contrast to previously reported disease-causing missense variants in HNRNPA2B1, these frameshift variants do not increase the propensity of hnRNPA2 protein to fibrillize. Rather, the frameshift variants have reduced affinity for the nuclear import receptor karyopherin β2, resulting in cytoplasmic accumulation of hnRNPA2 protein in cells and in animal models that recapitulate the human pathology. Thus, we expand the phenotypes associated with HNRNPA2B1 to include an early-onset form of OPMD caused by frameshift variants that alter its nucleocytoplasmic transport dynamics.

Missense variants in RNA-binding proteins underlie many diseases. Here the authors report an oculopharyngeal muscular dystrophy caused by heterozygous frameshift mutations in HNRNPA2B1 that alter its nucleocytoplasmic transport dynamics and result in cytoplasmic accumulation of hnRNPA2 protein.

Valosin-containing protein (VCP) regulates the stability of fused in sarcoma (FUS) granules in cells by changing ATP concentrations

Fused in sarcoma (FUS), a DNA/RNA-binding protein, undergoes liquid-liquid phase separation to form granules in cells. Aberrant FUS granulation is associated with neurodegenerative diseases, including amyotrophic lateral sclerosis and frontotemporal lobar degeneration. We found that FUS granules contain a multifunctional AAA ATPase, valosin-containing protein (VCP), which is known as a key regulator of protein degradation. FUS granule stability depends on ATP concentrations in cells. VCP ATPase changes the FUS granule stability time-dependently by consuming ATP to reduce its concentrations in the granules: VCPs in de novo FUS granules stabilize the granules, while long-lasting VCP colocalization destabilizes the granules. The proteolysis-promoting function of VCP may subsequently dissolve the unstabilized granules. We propose that VCP colocalized to the FUS granules acts as a timer to limit the residence time of the granules in cells.