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SPINOPHILIN SIGNALING: IMPACTS ON BODY WEIGHT, OBESITY, AND BETA-CELL FUNCTION
Obesity is a worldwide epidemic that is partially linked to changing lifestyles within the modern world, including increased access to calorically dense foods and decreased energy output due to more sedentary jobs. Obesity can lead to many different health complications, such as cardiovascular diseases or Type 2 Diabetes (T2D). Obesity-induced T2D is caused by dysfunction of the insulin-producing beta cells of the pancreas. However, mechanisms that promote obesity and the mechanisms by which obesity leads to beta cell dysfunction are not fully known.
Spinophilin is a filamentous (F)-actin binding, protein scaffolding, and protein phosphatase 1 (PP1)-targeting protein that can regulate protein. Spinophilin has multiple actions. Spinophilin can bundle filamentous actin to modulate the cellular cytoskeleton. Spinophilin also mediates substrate phosphorylation by targeting and modulating PP1 activity. In addition, spinophilin interacts with multiple proteins, including certain G-protein coupled receptors and can scaffold them with F-actin and/or PP1. Previous studies established that spinophilin KO mice have decreased fat mass, increased lean mass, and improved glucose tolerance. Yet, how spinophilin modulates the above metabolic parameters is unclear. We found that spinophilin is expressed in hypothalamic tissue and appears to also be expressed in the feeding center of the hypothalamus, as well as in other glucose-sensing cells known as tanycytes that neighbor the arcuate nucleus and the third ventricle. We found that loss of spinophilin limited weight gain observed in both a leptin receptor db/db mouse line (Leprdb/db) and mice fed a high-fat diet. Moreover, we found that the decreased fat mass seen in global spinophilin KO mice, at least in the Leprdb/db mice, was not due to major differences in feeding behaviors, consistent with what was observed by other groups using high-fat diet-fed mice.
As spinophilin was not associated with alterations in feeding, we posited that its ability to modulate glucose homeostasis may be linked to non-neuronal actions of the protein. Previous studies have found that spinophilin may regulate adipose tissue function and in vitro pancreatic beta cell function; however, its role in the pancreas and beta cells in vivo is not well characterized. We found that spinophilin is expressed in mouse pancreas. Using proteomics-based approaches we identified multiple putative spinophilin interacting proteins isolated from intact pancreas, including: PP1, the spinophilin homolog neurabin, and myosin-9. KEGG pathway analysis of proteomic proteins identified multiple pathways regulating ER stress, such as the unfolded protein response, and cytoskeletal arrangement. We observed decreased associations of spinophilin with PP1 and neurabin and increased association with myosin-9 in obese, Leprdb/db mice as early as 6 weeks, as well as significant decreases in body weight when spinophilin was knocked out in Leprdb/db mice. Moreover, we confirmed a robust and specific increased interaction of spinophilin with myosin-9, and other cytoskeletal proteins. Additionally, we found specific spinophilin interactions with ribosomal proteins, and exocrine and digestion proteins in high-fat diet-fed mice. Using our recently generated pancreatic beta cell-specific spinophilin KO mice, we found that loss of spinophilin in mice on a high-fat diet significantly reduces weight gain and improves whole- body glucose tolerance, and loss of spinophilin specifically within the beta cells also improves whole-body glucose tolerance, with no effect on body weight, further suggesting cell type-specific and independent roles for spinophilin on body weight and glucose homeostasis.
- Doctor of Philosophy
- Biological Sciences