The Brain-Specific Alternatively Spliced Isoforms of Adapter Protein SH2B1 Regulate Energy Balance and Neuronal Morphology and Function

Abstract

Obesity currently afflicts over 750 million people worldwide and represents a major risk factor for diabetes, cardiovascular disease, and other health issues. Because few effective treatments for obesity exist, we need to better understand the molecular mechanisms underlying obesity to identify novel therapeutic targets. Monogenic obesity results from a variant or deficiency in a single gene. Humans with genetic mutations in the SH2B1 gene display severe obesity and insulin resistance, the latter a hallmark symptom of diabetes. Mice lacking Sh2b1 are similarly obese and insulin-resistant. SH2B1 is an adapter protein that is recruited to the receptors of multiple hormones and neurotrophic factors. In humans and mice, alternative splicing generates four known SH2B1 isoforms (alpha, beta, gamma, delta) that differ only in their C-terminal tails. SH2B1-beta and SH2B1-gamma are expressed in all tissues, whereas SH2B1-alpha and SH2B1-delta are expressed almost exclusively in brain tissue.

It was unknown how the different isoforms of SH2B1 contribute to SH2B1 regulation of energy balance and glucose homeostasis. Because the brain is the organ primarily responsible for regulating energy balance and both SH2B1-alpha and SH2B1-delta are expressed primarily in brain tissue, I investigated the contributions of the brain-specific isoforms of SH2B1 to energy balance and glucose metabolism. Using a novel mouse model, I demonstrate that deletion of SH2B1-alpha and SH2B1-delta suppresses appetite and body weight, and, indirectly, improves glucose control. My research suggests that the different SH2B1 isoforms perform unique, non-redundant functions in the context of body weight regulation. Because deletion of SH2B1-alpha and SH2B1-delta protects mice from diet-induced obesity, my research presents potential targets for obesity therapeutics.

SH2B1 is thought to regulate body weight primarily through its activity in neurons. However, it was unknown how the different isoforms of SH2B1 regulate the morphology and function of neurons. I therefore investigated the contributions of all four SH2B1 isoforms, with a particular focus on the brain-specific delta isoform, to neuronal morphology and function. My results show that, unlike other SH2B1 isoforms that localize primarily to the cytoplasm and the plasma membrane, SH2B1-delta localizes to the plasma membrane and the nucleolus of neurons and neuron-like PC12 cells. Nucleolar localization of SH2B1-delta is directed by two highly basic regions that are unique to the C-terminal tail of SH2B1-delta. Using PC12 cells, I demonstrate that SH2B1-delta promotes neurotrophic factor-induced signaling events and gene expression. Using primary hippocampal neurons, I show that neurons lacking SH2B1 exhibit less outgrowth and complexity than control neurons and that SH2B1 is critical for BDNF-induced expression of two immediate early genes that serve important roles in synaptic plasticity. Reintroduction of each SH2B1 isoform into neurons lacking endogenous SH2B1 increases their neurite outgrowth and complexity, and SH2B1-delta causes the most robust increase among the four isoforms. For SH2B1-delta to maximally increase neurite outgrowth and complexity, it must have a functional SH2 domain and localize both to the nucleolus and plasma membrane. Thus, SH2B1 isoforms, and particularly the brain-specific delta isoform, enhance the development and/or maintenance of neurons and neuronal synapses.

To summarize, this dissertation advances our understanding of the functions of different isoforms of adapter protein SH2B1 in the context of body weight regulation and neuronal morphology and function. Thus, this work advances our knowledge of molecular components, at the whole-animal and cellular levels, underlying obesity.