Investigating hexose-6-phosphate dehydrogenase in the control of skeletal muscle function and carbohydrate metabolism

This proposal aims to understand more about the metabolic pathways that mammals use to derive energy from the glucose found in food and utilise it in muscle. Muscle is a vital tissue for controlling body glucose levels, which when elevated can be detrimental. We currently live in a time when glucose and calorie rich food is widely available (the consequences of too much glucose and calories are seen in obesity and diabetes). However, the underlying biological consequences our changing diets has are poorly understood so it is becoming increasingly important to gain a better understanding of the fundamental physiological processes that regulate glucose and energy metabolism in muscle. Our recent work has identified two enzymes in muscle that control important aspects of glucose metabolism. The first enzyme called 11b-HSD1 controls steroid hormone levels, which have potent effects on glucose usage in response to stress. The second enzyme called H6PDH metabolises glucose. What is interesting is that H6PDH can control 11b-HSD1 depending upon the supply of glucose, however, steroid hormones can control glucose levels, highlighting that these enzymes can control each others functions. This interaction between glucose and steroid hormone metabolism is very novel and our recent preliminary findings indicate that it plays an important role, therefore we wish to explain the underlying biology in muscle. To achieve this, we will manipulate H6PDH and 11b-HSD1 levels in muscle to varying degrees (e.g. simultaneously trying to increase steroid hormone metabolism, decrease glucose metabolism) using cells in culture and observe the consequences on cellular metabolism. These experiments will help to guide our use of mice with genetically modified H6PDH and 11b-HSD1 genes, which will allow a true assessment of their roles in a more physiologically relevant system. The experiments will test how steroid hormones and glucose metabolism can be manipulated and derive the true function of these systems in helping to control muscle tissues energy requirements. Ultimately, we hope these basic and fundamental studies will help other scientists, pharmaceutical companies and medical practitioners to improve their scientific knowledge and research. Understanding how energy metabolism is controlled will also aid improvements society's knowledge and hopefully create future health benefits.

Mammalian carbohydrate homeostasis is of primary importance to energy provision and the action of insulin on metabolism. In muscle, glucose is phosphorylated to glucose-6-phosphate (G6P) and used by multiple pathways including ATP generation. We have identified the enzyme H6PDH, within the sarcoplasmic reticulum (SR) of muscle which can metabolise G6P and may have an important role in its utilisation. One known function of H6PDH is to generate NADPH cofactor which allows the enzyme 11b-HSD1 to produce intracellular glucocorticoids (GCs), hormones that control metabolism by regulating gene transcription. H6PDH knockout mice (H6PDHKO) show the obligate nature of 11b-HSD1 for H6PDH. In the absence of H6PDH, 11b-HSD1 inactivates GCs, leading to blunted responses to GCs. Interestingly H6PDHKO mice also develop a vacuolating myopathy associated with activation of ER stress pathways and alterations in local and global glucose homeostasis. We hypothesise that changes in muscle H6PDH expression can modulate G6P and ATP levels that affect local and whole body metabolism which can be independent of GCs and 11b-HSD1. Aims - Determine the contributions of H6PDH mediated G6P and GC metabolism to glucose homeostasis - Define the contribution of H6PDH G6P metabolism to myopathy and glucose homeostasis in H6PDKO mice - Elucidate the relationship between H6PDH expression in muscle and the ER stress response This study will define the metabolic role of H6PDH in muscle and how it relates to GC metabolism, insulin sensitivity and glucose homeostasis and explore the observations linking aberrant ER metabolism to defective muscle structure and function. The proposed research will determine new and fundamental mechanisms of carbohydrate metabolism in highly relevant systems that have important benefits for our understanding of mammalian metabolic physiology.