Role of MASTL in mammals: Molecular functions and physiological relevance

Abstract

Role of MASTL in mammals: molecular functions and physiological relevance Belén Sanz Castillo, May 2017 MASTL is a Ser/Thr kinase first identified in Drosophila and Xenopus as a protein required for mitosis. MASTL inhibits, through phosphorylation of ENSA and ARPP19, the phosphatase PP2A in complex with the B55 family of regulatory subunits. In this work, we describe that Mastl is essential for mitotic progression in mouse embryonic fibroblasts (MEFs) derived from a conditional loss-of-function mouse model for Mastl, through inhibition of PP2A/B55 complexes. Mastl mediated inhibition of PP2A/B55 prevents premature dephosphorylation of CDK1 phosphosubstrates that are essential for chromosome condensation and segregation, and proper progression through mitosis. This proliferative role of Mastl is essential in vitro in cell culture and in vivo. Work done in a mouse model for Mastl shows that ablation of Mastl in young mice results in mitotic aberrations, severe proliferative defects and rapid death after Mastl depletion. Deletion of Mastl in elder mice, in contrast, causes less severe proliferative problems and improved survival. Nevertheless, Mastl depletion in adult mice also leads, in the long term, to impaired tissue regeneration, altered tissue homeostasis and eventual death of the mice. Although PP2A/B55 is a phosphatase that regulates multiple cellular processes, the only function attributed so far to the PP2A/B55-inhibitory kinase MASTL in mammals is its role in mitosis. Aiming to identify new functions of MASTL, we have explored other potential cell cycledependent and independent functions of MASTL. Whereas Mastl was not required for S-phase entry or maintenance of the quiescence state, we have found a new and unexplored role for the MASTL-PP2A/B55 pathway in the control of insulin signaling downstream of mTORC1. The mTORC1/S6K1 axis triggers a negative feedback loop that inhibits the upstream PI3K pathway, which in turn, is also important for controlling mTORC1 activity. We have found that MASTL-mediated PP2A/B55 inhibition prevents the activating dephosphorylation of the feedback target proteins, IRS1 and GRB10, and is required to fine-tune the feedback-mediated inhibition of the PI3K/AKT pathway and its metabolic consequences. Interestingly, MASTL activity is also modulated downstream of the mTORC1/S6K1 axis in conditions of feedback activity. This feedback loop is physiologically relevant in vivo for metabolic diseases involving insulin resistance, such as obesity or type 2 diabetes. Importantly, we have found that depletion of Mastl improves glucose tolerance in a mouse model of high fat diet-induced obesity, and lowers basal glycaemia in old mice. This data together indicate that MASTL, through the inhibition of PP2A/B55, has a new mitotic-independent function in mammals in the control of PI3K/AKTmTORC1 signaling and cooperates to control glucose homeostasis in vivo.