Identification of the PH Domain of Scaffold Protein SH2B1 as a Critical Regulator of Energy Balance and Glucose Metabolism

Abstract

Obesity is a growing epidemic but the cellular and molecular mechanisms underlying obesity are still not well understood. Mutations in the adaptor protein SH2B1 have been identified in individuals with severe early-onset childhood obesity, insulin resistance, and hyperphagia. Adaptor proteins have protein binding domains that link protein binding partners and therefore facilitate the creation of larger signaling complexes. This phenotype is also seen in mice lacking SH2B1 (SH2B1-KO mice). Furthermore, experiments using SH2B1-KO mice and cultured neurons suggest that neuronal SH2B1 is critical for this phenotype. Three of the human obesity-associated mutations are located in the pleckstrin homology (PH) domain of SH2B1, suggesting that the PH domain is important for the overall function of SH2B1. The aims of this thesis work were to gain insight into how SH2B1 regulates energy balance and glucose metabolism, and the contribution of its PH domain to these functions by using two novel mouse models. The first model contained one of the human obesity-associated mutations (P322S). Body weight, food intake, glucose tolerance and insulin tolerance were not substantially altered by the P322S mutation when mice were fed normal chow. However, the P322S mutation decreased glucose tolerance in mice challenged by a high fat diet. No effect on insulin tolerance was noted. In contrast, a second mouse model with a two-amino acid deletion (ΔP317,R318) in the PH domain of SH2B1 showed an obese phenotype. The ΔP317,R318 mice showed significantly increased body weight, adiposity, plasma leptin levels and plasma insulin levels, and decreased glucose and insulin tolerance. In females, the reduced glucose tolerance and increased plasma insulin levels appeared even before the onset of obesity, suggesting that the ΔP317, R318 mutation results in impaired glucose homeostasis independent of the obese phenotype. At the cellular level, deleting P317, R318 changed the localization of SH2B1 from being primarily in the cytoplasm and plasma membrane to being primarily in the nucleus. Deleting P317, R318 also impaired the ability of SH2B1 to enhance nerve growth factor-induced neurite outgrowth in preneuronal PC12 cells. This work provides evidence that the PH domain is a key regulator of SH2B1 subcellular localization, neurite outgrowth, and the control of energy balance and glucose homeostasis.