Large-scale genetic screens identify BET-1 as a cytoskeleton regulator promoting actin 2 health and lifespan

Abstract 21 22 The actin cytoskeleton is a three-dimensional scaffold of proteins that is a regulatory, energy23 consuming network with dynamic properties to shape the structure and function of the cell. Proper 24 actin function is required for many cellular pathways, including cell division, autophagy, chaperone 25 function, endocytosis, and exocytosis. Deterioration of these processes manifests during aging 26 and exposure to stress, which is in part due to the breakdown of the actin cytoskeleton. However, 27 the regulatory mechanisms involved in preservation of cytoskeletal form and function are not well 28 understood. Here, we performed a multi-pronged, cross-organismal screen combining a whole29 genome CRISPR-Cas9 screen in human fibroblasts with in vivo C. elegans synthetic lethality 30 screening. We identified the bromodomain protein, BET-1, as a key regulator of actin health and 31 longevity. Overexpression of bet-1 preserves actin health at late age and promotes lifespan and 32 healthspan in C. elegans. These beneficial effects are mediated through actin preservation by the 33 transcriptional regulator function of BET-1. Together, our discovery assigns a key role for BET-1 34 in cytoskeletal health, highlighting regulatory cellular networks promoting cytoskeletal 35 homeostasis. 36 37 INTRODUCTION 38 39 The actin cytoskeleton is composed of a complex network of filaments held together by actin40 binding proteins. Historically, the actin cytoskeleton has been viewed as merely the structural 41 framework of the cell, with its primary function being ascribed to cell division and the sorting and 42 transport of cellular cargo. However, actin function is also required for many other cellular 43 pathways, including autophagy, chaperone function, and transcriptional regulation (Balch et al., 44 2008; Blanpied et al., 2003; Caviston and Holzbaur, 2006; Higuchi-Sanabria et al., 2014; McCray 45 and Taylor, 2008). The functional significance of actin in cellular health and disease is highlighted 46 by clinical data, showing that loss of function of actin is observed in clinical manifestations, 47 including neurodegeneration and muscle myopathies (Acsadi et al., 1991; Alim et al., 2002, 2004), 48 and during aging (Baird et al., 2014; Higuchi-Sanabria et al., 2018; Sing et al., 2021). Specifically, 49 actin filaments show loss of stability and marked deterioration during aging, in both single-celled 50 yeast (Sing et al., 2021) and multiple cell-types of the multicellular nematode C. elegans (Higuchi51 Sanabria et al., 2018), whereas changes in β-actin expression hav