Characterization of LKB1 and SIK family kinases in drosophila

Abstract

Liver kinase B1 (LKB1) has important roles in governing energy homeostasis by regulating AMP-activated protein kinase (AMPK) and other AMPK-related kinases, including salt-inducible kinases (SIKs). However, the in vivo functions of LKB1 and SIKs in metabolism are poorly understood. Therefore I decide to study two SIK family kinases, SIK and SIK3, in Drosophila. First, I generate Drosophila SIK mutant to examine the role of SIK in vivo. I observe that the mutant flies have higher amounts of lipid and glycogen stores and are resistant to starvation. Interestingly, SIK transcripts are highly enriched in the brain, and neuronal-specific expression of exogenous SIK fully rescues lipid and glycogen storage phenotypes as well as starvation resistance of the mutant. Using genetic and biochemical analyses, I demonstrate that S157 phosphorylation of CREB-regulated transcription coactivator (CRTC) by SIK is critical for inhibiting CRTC activity in vivo. Furthermore, double mutants of SIK and CRTC become sensitive to starvation, and the S157 phosphomimic mutation of CRTC reduces lipid and glycogen levels in SIK mutant, suggesting that CRTC mediates the effects of SIK signaling. These results strongly support the importance of SIK-CRTC signaling axis that functions in the brain to maintain energy homeostasis in Drosophila. To examine the function of SIK3 in vivo, I generate Drosophila SIK3 mutant and find that the mutant flies have reduced lipid stores with severe lipodystrophy and increased brummer, the Drosophila homolog of adipose triglyceride lipase (ATGL), gene expression. These phenotypes are highly consistent with LKB1 mutants. The expression of constitutively active SIK3 in the fat body rescue the phenotypes of LKB1 mutant flies, suggesting SIK3 as the key downstream of LKB1. Using genetic and biochemical analyses, I identify HDAC4, a class IIa histone deacetylase, as a lipolytic target of LKB1-SIK3 pathway. Also, the Drosophila insulin-like peptides (DILPs) and adipokinetic hormone (AKH) pathways related to the mammalian insulin and glucagon pathways control LKB1-SIK3-HDAC4 signaling. These data establish LKB1-SIK3 pathway as a critical regulatory mechanism to control lipid homeostasis in Drosophila. Collectively, the present studies on LKB1 and the SIK family kinases in Drosophila provide a new paradigm for understanding complex molecular mechanisms in maintaining animal energy homeostasis.