Obesity-related insulin resistance is a highly prevalent and growing health concern, which places stress on the pancreatic islets of Langerhans by increasing insulin secretion to lower blood glucose levels. The glucose transporters GLUT1 and GLUT3 play a key role in glucose-stimulated insulin secretion in human islets, while GLUT2 is the key isoform in rodent islets. However, it is unclear whether other glucose transporters also contribute to insulin secretion by pancreatic islets. Herein, we show that SLC2A6 (GLUT6) is markedly upregulated in pancreatic islets from genetically obese leptin-mutant (ob/ob) and leptin receptor-mutant (db/db) mice, compared to lean controls. Furthermore, we observe that islet SLC2A6 expression positively correlates with body mass index in human patients with type 2 diabetes. To investigate whether GLUT6 plays a functional role in islets, we crossed GLUT6 knockout mice with C57BL/6 ob/ob mice. Pancreatic islets isolated from ob/ob mice lacking GLUT6 secreted more insulin in response to high-dose glucose, compared to ob/ob mice that were wild type for GLUT6. The loss of GLUT6 in ob/ob mice had no adverse impact on body mass, body composition, or glucose tolerance at a whole-body level. This study demonstrates that GLUT6 plays a role in pancreatic islet insulin secretion in vitro but is not a dominant glucose transporter that alters whole-body metabolic physiology in ob/ob mice.

‘Inner mitochondrial membrane peptidase 2 like’ (IMMP2L) is a nuclear-encoded mitochondrial peptidase that has been conserved through evolutionary history, as has its target enzyme, ‘mitochondrial glycerol phosphate dehydrogenase 2′ (GPD2). IMMP2L is known to cleave the mitochondrial transit peptide from GPD2 and another nuclear-encoded mitochondrial respiratory-related protein, cytochrome C1 (CYC1). However, it is not known whether IMMP2L peptidase activates or alters the activity or respiratory-related functions of GPD2 or CYC1. Previous investigations found compelling evidence of behavioural change in the Immp2l^KD−/− KO mouse, and in this study, EchoMRI analysis found that the organs of the Immp2l^KD−/− KO mouse were smaller and that the KO mouse had significantly less lean mass and overall body weight compared with wildtype littermates (p < 0.05). Moreover, all organs analysed from the Immp2l^KD−/− KO had lower relative levels of mitochondrial reactive oxygen species (mitoROS). The kidneys of the Immp2l^KD−/− KO mouse displayed the greatest decrease in mitoROS levels that were over 50% less compared with wildtype litter mates. Mitochondrial respiration was also lowest in the kidney of the Immp2l^KD−/− KO mouse compared with other tissues when using succinate as the respiratory substrate, whereas respiration was similar to the wildtype when glutamate was used as the substrate. When glycerol-3-phosphate (G3P) was used as the substrate for Gpd2, we observed ~20% and ~7% respective decreases in respiration in female and male Immp2l^KD−/− KO mice over time. Together, these findings indicate that the respiratory-related functions of mGpd2 and Cyc1 have been compromised to different degrees in different tissues and genders of the Immp2l^KD−/− KO mouse. Structural analyses using AlphaFold2-Multimer further predicted that the interaction between Cyc1 and mitochondrial-encoded cytochrome b (Cyb) in Complex III had been altered, as had the homodimeric structure of the mGpd2 enzyme within the inner mitochondrial membrane of the Immp2l^KD−/− KO mouse. mGpd2 functions as an integral component of the glycerol phosphate shuttle (GPS), which positively regulates both mitochondrial respiration and glycolysis. Interestingly, we found that nonmitochondrial respiration (NMR) was also dramatically lowered in the Immp2l^KD−/− KO mouse. Primary mouse embryonic fibroblast (MEF) cell lines derived from the Immp2l^KD−/− KO mouse displayed a ~27% decrease in total respiration, comprising a ~50% decrease in NMR and a ~12% decrease in total mitochondrial respiration, where the latter was consistent with the cumulative decreases in substrate-specific mediated mitochondrial respiration reported here. This study is the first to report the role of Immp2l in enhancing Gpd2 structure and function, mitochondrial respiration, nonmitochondrial respiration, organ size and homeostasis.

‘Inner mitochondrial membrane peptidase 2 like’ (IMMP2L) is a nuclear encoded mitochondrial peptidase that has been conserved through evolutionary history as has its target enzyme ‘mitochondrial glycerol phosphate dehydrogenase 2’ (GPD2). IMMP2L is known to cleave the mitochondrial transit peptide from GPD2 and another nuclear encoded mitochondrial respiratory related protein cytochrome C1 (CYC1). However, it is not known whether IMMP2L peptidase activates or alters the activity or respiratory related functions of GPD2 or CYC1. Previous investigations found compelling evidence of behavioural change in the Immp2lKD-/- KO mouse and in this study EchoMRI analysis found that many organs of the Immp2lKD-/- KO mouse were smaller and that the KO mouse had significantly less lean mass and overall body weight compared with wildtype littermates (p<0.05). Moreover, all of the organs analysed from the Immp2lKD-/- KO had lower relative levels of mitochondrial reactive oxygen species (mitoROS). The kidneys of the Immp2lKD-/- KO mouse displayed the greatest decrease in mitoROS levels that were over 50% less compared with wildtype litter mates. Mitochondrial respiration was also lower in the kidney of the Immp2lKD-/- KO mouse compared with other tissues when using succinate as the respiratory substrate, whereas respiration was similar to wildtype when glutamate was used as substrate. When glycerol-3-phosphate (G3P) was used as the substrate for Gpd2 we observed ~20% and ~7% respective decreases in respiration in female and male Immp2lKD-/- KO mice over time. Together these findings suggest that the respiratory related functions of mGpd2 and Cyc1 have been compromised to different degrees in different tissues in the Immp2lKD-/- KO mouse. Structural analyses using AlphaFold2-Multimer further predicted that the interaction between Cyc1 and mitochondrial encoded cytochrome b (Cyb) in Complex III had been altered as had the homodimeric structure of the mGpd2 enzyme within the inner mitochondrial membrane of the Immp2lKD-/- KO mouse. mGpd2 functions as an integral component of the glycerol phosphate shuttle (GPS) which positively regulates both mitochondrial respiration and glycolysis. Interestingly, we found that nonmitochondrial respiration (NMR) was also dramatically lowered in the Immp2lKD-/- KO mouse. Primary mouse embryonic fibroblast (MEF) cell lines derived from the Immp2lKD-/- KO mouse displayed a ~27% decrease in total respiration comprised of a ~50% decrease in NMR and a ~12% decrease in total mitochondrial respiration where the latter was consistent with the decreases in substrate specific mitochondrial respiration reported here. This study is the first to report the role of Immp2l in enhancing Gpd2 structure and function, mitochondrial respiration, nonmitochondrial respiration, organ size and homeostasis.