Effects of Human Obesity-associated Mutations on Cellular Actions of the Adapter Protein SH2B1.

Abstract

Src homology 2 B adapter protein 1 (SH2B1) modulates signaling by a variety of ligands that bind to receptor tyrosine kinases or JAK-associated cytokine receptors, including leptin, insulin, growth hormone (GH), and nerve growth factor (NGF). Targeted deletion of Sh2b1 in mice results in increased food intake, obesity, and insulin resistance. SH2B1 is expressed as four different isoforms: alpha, beta, gamma, and delta. SH2B1 loss-of-function mutations were identified in a cohort of patients with severe early-onset obesity. Mutation carriers exhibit hyperphagia, childhood-onset obesity, disproportionate insulin resistance, and reduced final height as adults. Unexpectedly, mutation carriers also exhibit a spectrum of behavioral abnormalities that were not reported for controls, including social isolation and aggression. The work in this thesis characterizes cellular functions of SH2B1 alpha, beta, and delta and examines how human mutations in SH2B1 affect their functions. SH2B1beta with a human mutation exhibits an impaired ability to enhance NGF-induced neurite outgrowth, accumulate in the nucleus, and enhance GH-induced macrophage migration. In contrast to SH2B1beta, SH2B1alpha does not enhance NGF-induced neurite outgrowth or uPAR mRNA expression and does not cycle through the nucleus. SH2B1delta has a sub-cellular localization at the plasma membrane and nucleolus. A bipartite nuclear localization signal (NLS) in the unique C-terminal tail of SH2B1delta is important for its nucleolar localization and enhancement of NGF-induced neurite outgrowth. SH2B1delta with a disease-causing human mutation does not enhance NGF-induced neurite outgrowth to the same extent as SH2B1delta WT. Thus, the ability of SH2B1 isoforms to promote neuronal differentiation and cell migration may contribute to the control of human food intake and body weight and is implicated in maladaptive human behavior. To translate results from PC12 cells to a GH context, SH2B1, including its NLS region, was shown to be required for the enhancement of expression of a sub-set of GH-induced genes. Taken together, these results highlight the importance of understanding how the different isoforms of SH2B1 regulate gene transcription, cell motility, and neuronal differentiation to promote whole-body energy homeostasis and insulin sensitivity, and affect behavior.