Biology subjects

Sohn, E. J.

Publications and source records attributed to Sohn, E. J..

5 recordsLinked to original sources

bioRxiv2025

Elevation of the mechanically-sensitive protein emerin links nuclear mechanotransduction to tau-induced cytoskeletal remodeling in neurons

Tauopathies are a group of neurodegenerative disorders, including Alzheimers disease, that are neuropathologically defined by deposition of pathological forms of tau in the brain. While tau is reported to drive neurotoxicity by negatively affecting cytoskeletal, nucleoskeletal, and genomic architecture, the mechanisms mediating tau-induced dysfunction of the cytoskeleton and nucleoskeleton are incompletely understood. Based on proteomic profiling, we identify a suite of cytoskeletal and nucleoskeletal proteins with differing abundance in a cellular model of tauopathy, iTau. Building upon previous findings that pathogenic forms of tau reduce nuclear tension, we find that protein levels of emerin, a central regulator of nuclear mechanotransduction, are significantly elevated in iTau cells and in induced pluripotent stem cell (iPSC)-derived neurons carrying a mutation in the microtubule-associated protein tau (MAPT) gene that causes autosomal dominant frontotemporal dementia. We find that neuronal emerin overexpression is sufficient to drive neurotoxicity, increase overall levels of filamentous actin (F-actin), and induce nuclear invagination, cellular phenotypes that also occur in settings of tauopathy. Mass spectrometry-based identification of emerin-interacting proteins in iTau-derived neurons reveals increased interactions with cytoskeletal proteins and reduced interactions with nuclear proteins. Indeed, we find that emerin relocalizes from the nucleus to the cytosol in the setting of tauopathy, suggesting that pathogenic tau impacts nuclear mechanotransduction pathways. Overall, we identify emerin as a mediator of cytoskeletal remodeling in tauopathy and provide a foundation for future studies into the mechanosensitive function of emerin in neurons. SIGNIFICANCE STATEMENTCells experience and respond to diverse mechanical forces that shape their morphology, function, and survival through a process termed "mechanotransduction." While well studied in non-neuronal cells, neuronal mechanotransduction remains poorly understood despite exposure of the brain to vascular flow, movement, injury, and disease. We identify the mechanosensitive protein emerin as a key regulator of nuclear mechanotransduction in neurons. Emerin overexpression is sufficient to increase filamentous actin, induce nuclear invagination, and drive neurotoxicity, revealing a novel function for emerin in neurons. In cellular models of tauopathy, emerin is elevated and relocalizes from the nucleus to the cytoplasm, where it alters cytoskeletal structure. These findings establish emerin as a mechanosensitive regulator in neurons and link disrupted nuclear neuronal mechanotransduction to neurodegeneration.

cell biology↗

bioRxiv2025

Molecular Basis of EWS Interdomain Self-Association and Its Role in Condensate Formation

Ewing sarcoma, the second most common pediatric bone and soft tissue cancer, is caused by aberrant fusion of the EWS low-complexity domain (EWSLCD) to the DNA-binding domain of the transcription factor FLI1. The resulting fusion, EWS::FLI1, directly interacts with and engages in a dynamic interplay with EWS that drives tumorigenesis and regulates the function of both proteins. While EWSLCD is known to promote self-association, the role of the RNA-binding domains (RBDs) of EWS, which include RGG repeat regions and a structured RNA-recognition motif (RRM), remains less well understood. Here, we investigate the interplay between EWSLCD and RBDs using biomolecular condensation assays, microscopy, NMR spectroscopy, and molecular simulations. Our studies reveal that RBDs differentially influence EWSLCD condensate formation and suggest that electrostatics and polypeptide-chain length likely contribute to this interaction. NMR spectroscopy and molecular dynamics simulations further demonstrate that EWSLCD and the central RNA-binding region, comprising the RRM and RGG2 domains, engage in transient, non-specific interactions that are broadly distributed across both regions and involve diverse residue types. Specifically, tyrosine, polar residues, and proline within EWSLCD preferentially interact with arginine, glycine, and proline residues in the RBD. Atomistic simulations of EWS confirm that the full-length protein exhibits a similar interaction profile with conserved chemical specificity, supporting a model in which a network of weak, distributed interdomain contacts underlies EWS self-association. Together, these findings provide molecular insight into the mechanisms of EWS condensate formation and lay the groundwork for understanding how interdomain interactions regulate EWS and EWS::FLI1 function. SIGNIFICANCEThis study shows that the RNA-binding regions of the RNA-binding protein EWS subtly tune biomolecular condensation through numerous transient contacts with the low complexity domain that is common to both EWS and the oncogenic fusion EWS::FLI1. These results provide insight into condensate formation of EWS, which may be useful for understanding the oncogenic mechanisms of EWS::FLI1 and broadly, into pathogenic biomolecular condensate formation.

biophysics↗

bioRxiv2025

Distinct mechanisms underlying extrachromosomal telomere DNA generation in ALT cancers

Alternative lengthening of telomeres (ALT) is a telomerase-independent telomere maintenance mechanism observed in 15% of human cancers. A hallmark of ALT cancers is the presence of C-circles, circular single-stranded DNAs (ssDNAs) enriched with cytosine-rich telomere (C-rich, CCCTAA) sequences. G-circles, containing guanosine-rich telomere (G-rich, GGGTTA) ssDNAs, also exist but are much less abundant. Recent studies indicate that excessive displacement of Okazaki fragments during lagging-strand synthesis is a unique feature of ALT telomeres and responsible for generating C-circles/C-rich ssDNAs. However, the distinct characteristics of C-circles compared to G-circles remain unclear. Here, we demonstrate that co-deficiency of the DNA translocases SMARCAL1 and FANCM leads to abundant generation of G-circle/G-rich ssDNAs. These G-rich ssDNAs mainly exist in linear form, ranging in size from 500 to 3000 nucleotides, which differs significantly from the structure and size of C-circle/C-rich ssDNAs. Mechanistically, both C-rich and G-rich ssDNAs originate from BLM/POLD-mediated excessive strand displacement; however, they differ in their origins and initiation mechanisms. Specifically, C-rich ssDNAs arise from lagging daughter strands initiated by the CST complex, whereas G-rich ssDNAs originate from leading daughter strands through RAD51-dependent G-strand synthesis. Our findings propose two distinct mechanisms for generating two different extrachromosomal telomere DNAs, C-and G-circles, during ALT-mediated telomere elongation.

biochemistry↗

bioRxiv2023

Insights into Molecular Diversity within the FET Family: Unraveling Phase Separation of the N-Terminal Low Complexity Domain from RNA-Binding Protein EWS

The FET family proteins, which includes FUS, EWS, and TAF15, are RNA chaperones instrumental in processes such as mRNA maturation, transcriptional regulation, and the DNA damage response. These proteins have clinical significance: chromosomal rearrangements in FET proteins are implicated in Ewing family tumors and related sarcomas. Furthermore, point mutations in FUS and TAF15 are associated with neurodegenerative conditions like amyotrophic lateral sclerosis and frontotemporal lobar dementia. The fusion protein EWS::FLI1, the causative mutation of Ewing sarcoma, arises from a genomic translocation that fuses the low-complexity domain (LCD) of EWS (EWSLCD) with the DNA binding domain of the ETS transcription factor FLI1. This fusion not only alters transcriptional programs but also hinders native EWS functions like splicing. However, the precise function of the intrinsically disordered EWSLCD is still a topic of active investigation. Due to its flexible nature, EWSLCD can form transient interactions with itself and other biomolecules, leading to the formation of biomolecular condensates through phase separation - a mechanism thought to be central to the oncogenicity of EWS::FLI1. In our study, we used paramagnetic relaxation enhancement NMR, analytical ultracentrifugation, light microscopy, and all-atom molecular dynamics (MD) simulations to better understand the self-association and phase separation tendencies of EWSLCD. Our aim was to elucidate the molecular events that underpin EWSLCD-mediated biomolecular condensation. Our NMR data suggest tyrosine residues primarily drive the interactions vital for EWSLCD phase separation. Moreover, a higher density and proximity of tyrosine residues amplify the likelihood of condensate formation. Atomistic MD simulations and hydrodynamic experiments revealed that the tyrosine-rich N and C-termini tend to populate compact conformations, establishing unique contact networks, that are connected by a predominantly extended, tyrosine-depleted, linker region. MD simulations provide critical input on the relationship between contacts formed within a single molecule (intramolecular) and inside the condensed phase (intermolecular), and changes in protein conformations upon condensation. These results offer deeper insights into the condensate-forming abilities of the FET proteins and highlights unique structural and functional nuances between EWS and its counterparts, FUS and TAF15.

biophysics↗

bioRxiv2023

Extrachromosomal Telomeres Derived from Excessive Strand Displacements

Alternative Lengthening of Telomeres (ALT) is a telomere maintenance mechanism mediated by break-induced replication (BIR), evident in approximately 15% of human cancers. A characteristic feature of ALT cancers is the presence of C-circles, circular single-stranded telomeric DNAs composed of C-rich sequences. Despite the fact that extrachromosomal C-rich single-stranded DNAs (ssDNAs), unique to ALT cells, are considered potential precursors of C-circles, their generation process remains undefined. Here, we introduce a highly sensitive method to detect single stranded telomeric DNA, called 4SET (Strand-Specific Southern-blot for Single-stranded Extrachromosomal Telomeres) assay. Utilizing 4SET, we are able to capture C-rich single stranded DNAs that are near 200 to 1500 nucleotides in size. Both linear C-rich ssDNAs and C-circles are abundant in the fractions of cytoplasm and nucleoplasm, which supports the idea that linear C-rich ssDNA accumulation may indeed precede C-circle formation. We also found that C-rich ssDNAs originate during Okazaki fragment processing during lagging strand DNA synthesis. The generation of C-rich ssDNA requires CST-PP (CTC1/STN1/TEN1-PRIMASE-Polymerase alpha) complex-mediated priming of the C-strand DNA synthesis and subsequent excessive strand displacement of the C-rich strand mediated by the DNA Polymerase delta and the BLM helicase. Our work proposes a new model for the generation of C-rich ssDNAs and C-circles during ALT-mediated telomere elongation.

biochemistry↗