Wadley, M. | bioRxiv Science

bioRxiv2025

53BP1 condensates function as bioreactors for NHEJ directed DNA repair and insulators to determine pathway choice

Genomic integrity requires efficient resolution of DNA damage. Non-homologous end joining (NHEJ) is the primary mechanism of DNA double strand break (DSB) repair in mammalian cells and is mediated by 53BP1, a tumor suppressor involved in preventing DSB end-resection and homologous recombination. NHEJ repair foci form following DSB formation, however how mesoscale assembly occurs and whether 53BP1 is the driver of this process are unknown. Also, despite knowledge of the identify of key pathway molecules, the specific functions of mesoscale repair condensates in DNA repair and pathway selectivity are unknown. To address these gaps, we determined the minimal domain of 53BP1 sufficient for phase separation in vitro and identified the key residues that governing its condensation. Utilizing a separation-of-function mutant, we demonstrate that 53BP1and its protein condensation is the core driver of NHEJ foci formation. 53BP1 condensates function as bioreactor compartments, increasing the effective concentration of substrates near double strand breaks and are essential for efficient DNA damage resolution. Additionally, we show that 53BP1 condensates function as insulators around DSB sites to prevent end-resection and direct repair pathway selectivity. Collectively our work reveals a specialized compartment for DNA repair through the spatial clustering of 53BP1 molecules into repair foci essential to maintain genome integrity.

cell biology↗

bioRxiv2023

Unexpected roles for AMPK in suppression of autophagy and reactivation of mTORC1 signaling during prolonged amino acid deprivation

AMPK promotes catabolic and suppresses anabolic cell metabolism to promote cell survival during energetic stress, in part by inhibiting mTORC1, an anabolic kinase requiring sufficient levels of amino acids. We found that cells lacking AMPK displayed increased apoptotic cell death during nutrient stress caused by prolonged amino acid deprivation. We presumed that impaired autophagy explained this phenotype, as a prevailing view posits that AMPK initiates autophagy (often a pro-survival response) through phosphorylation of ULK1. Unexpectedly, however, autophagy remained unimpaired in cells lacking AMPK, as monitored by several autophagic readouts in several cell lines. More surprisingly, the absence of AMPK increased ULK1 signaling and LC3b lipidation during amino acid deprivation while AMPK-mediated phosphorylation of ULK1 S555 (a site proposed to initiate autophagy) decreased upon amino acid withdrawal or pharmacological mTORC1 inhibition. In addition, activation of AMPK with compound 991, glucose deprivation, or AICAR blunted autophagy induced by amino acid withdrawal. These results demonstrate that AMPK activation and glucose deprivation suppress autophagy. As AMPK controlled autophagy in an unexpected direction, we examined how AMPK controls mTORC1 signaling. Paradoxically, we observed impaired reactivation of mTORC1 in cells lacking AMPK upon prolonged amino acid deprivation. Together these results oppose established views that AMPK promotes autophagy and inhibits mTORC1 universally. Moreover, they reveal unexpected roles for AMPK in the suppression of autophagy and the support of mTORC1 signaling in the context of prolonged amino acid deprivation. These findings prompt a reevaluation of how AMPK and its control of autophagy and mTORC1 impact health and disease.

cell biology↗