SS18 regulates pluripotent-somatic transition through phase separation

Abstract

The transition from pluripotent to somatic states marks a critical event in mammalian development, but remains largely unresolved. Here we report the identification of SS18 as a regulator for pluripotent to somatic transition or PST by CRISPR-based whole genome screens. Mechanistically, SS18 forms microscopic condensates in nuclei through a C-terminal intrinsically disordered region (IDR) rich in tyrosine, which, once mutated, no longer form condensates nor rescue SS18−/− defect in PST. Yet, the IDR alone is not sufficient to rescue the defect even though it can form condensates indistinguishable from the wild type protein. We further show that its N-terminal 70aa is required for PST by interacting with the Brg/Brahma-associated factor (BAF) complex, and remains functional even swapped onto unrelated IDRs or even an artificial 24 tyrosine polypeptide. Finally, we show that SS18 mediates BAF assembly through phase separation to regulate PST. These studies suggest that SS18 plays a role in the pluripotent to somatic interface and undergoes liquid-liquid phase separation through a unique tyrosine-based mechanism.

Phase transition and remodeling complex assembly are important for SS18-SSX oncogenic activity in synovial sarcomas

“…With this in consideration, we speculated that SS18 acquired a characteristic of phase separation to participate in specific physiological functions in Chordata in the process of evolution. Interestingly, a recent study showed that SS18 mediates CBAF assembly through phase separation to regulate pluripotent-somatic transition, consistent with our speculation 32 . In their study, the authors found that a C-terminal intrinsically disordered region of SS18 regulates its forming microscopic condensates under the condition of ectopic overexpression, and that the N-terminal 70aa of SS18 is required for its binding to CBAF.…”

“…3 – 5 ). In addition, we noted that the endogenous SS18 or SS18-SSX1 shows as a similar puncta-like distribution in mouse embryonic stem or synovial sarcoma cells as that of overexpression in HeLa, HEK293T, or synovial sarcoma cells 30 , 32 (Figs. 3 – 5 ).…”

Hedgehog-responsive PDGFRa(+) fibroblasts maintain a unique pool of alveolar epithelial progenitor cells during alveologenesis

“… SUPPORTING CITATIONS The following references are cited in the supplemental information : Cassandras et al, 2020 ; Chen et al, 2017 ; Frazier et al, 2019 ; Hagan et al, 2020 ; Huaqi et al, 2019 ; Jin et al, 2018 ; Kuang et al, 2021 ; Liu et al, 2018 ; Manchado et al, 2016 ; Maynard et al, 2020 ; Nakamura et al, 2015 ; Park et al, 2019 ; Treutlein et al, 2014 ; Wang et al, 2019 ; Wei et al, 2021 ; Wimmer et al, 2019 ; Wong et al, 2018 ; Zhang et al, 2021 . …”

H3K27ac mediated SS18/BAFs relocation regulates JUN induced pluripotent-somatic transition

“…Based on these findings, we construct a robust and fast PST system, JUN TetON ESC [13]. Compared to other PST models (2-3 days) [9][10][11]14], JUN-induced PST takes within only 8 h (less than one cell cycle), more than 6 h JUN induction will lead an irreversible differentiation of mESCs [15], thus offering us a good opportunity to capture the key events of PST occurred in the early stage. Recently, we reported that SS18/BAFs play a critical role in JUN-induced PST [15].…”

“…Compared to other PST models (2-3 days) [9][10][11]14], JUN-induced PST takes within only 8 h (less than one cell cycle), more than 6 h JUN induction will lead an irreversible differentiation of mESCs [15], thus offering us a good opportunity to capture the key events of PST occurred in the early stage. Recently, we reported that SS18/BAFs play a critical role in JUN-induced PST [15]. BAFs, also known as mammalian switch/sucrose nonfermentable(mSWI/SNF), initially found in S. cerevisiae, contain an enzyme subunit BRG1 or BRM serving the ATP-dependent chromatin remodeling function, as well as other regulatory subunits, like SS18 [16].…”