Exploring neuronatin as a novel SERCA regulator and its role in murine metabolism and skeletal and cardiac muscle function

Abstract

Neuronatin (NNAT) is a paternally expressed gene and protein found to be diversely involved in metabolic homeostasis. The mechanism underlying its metabolic contributions is speculated to be through regulation of the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) pump given NNAT’s sequence homology with two SERCA regulators, phospholamban and sarcolipin (SLN). Furthermore, previous work has shown that mice lacking NNAT (NnatKO) demonstrate altered glycemic control, reduced energy expenditure, and a propensity towards obesity. The purpose of this thesis was therefore to test the hypothesis that NNAT can act as a SERCA regulator and subsequently influence whole-body metabolism and muscle function. Using a cell model, we demonstrate that NNAT can negatively regulate and uncouple the SERCA pump. In our NnatKO mouse model, we find evidence of NNAT acting as a negative regulator of SERCA in the soleus muscle, though the effects on SERCA function appear to be dependent on housing conditions and biological sex. Under standard housing conditions, no effects of NnatKO were detected on body composition or metabolism, but under translatable conditions, NnatKO mice displayed more fat mass and less lean mass relative to total body weight. Under both housing conditions NnatKO mice have reduced soleus muscle force production with RNA sequencing data suggesting biological sex-dependent changes to structural genes and signaling pathways. When given access to a running wheel, NnatKO mice present with a low exercise volume, though this low volume was still able to increase glucose tolerance and soleus force production, however the genetic reduction of glycogen synthase kinase 3 in NnatKO skeletal muscle had little to no effect on soleus function. Finally, we found NnatKO mice to show signs of structural remodeling to the left ventricle, reductions to cardiac output, and stroke volume, though these effects are not present under translatable conditions. Together, this thesis provides data supporting NNAT as a negative regulator of SERCA and that it is an important component for skeletal and cardiac muscle function. Future work should investigate the mechanisms underlying these changes to muscle function as well as the potential to target NNAT as a therapeutic in aging and disease.