Biochemistry and physiology of hexose-6-phosphate knockout mice

Hexose-6-phosphate dehydrogenase (H6PDH) has emerged as an important factor in setting the redox status of the endoplasmic reticulum (ER) lumen. An important role of H6PDH is to generate a high NADPH/NADP+ ratio which permits 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) to act as an oxo-reductase, catalyzing the activation of glucocorticoids (GCs). In H6PDH knockout mice 11β-HSD1 assumes dehydrogenase activity and inactivates GCs, rendering the target cell relatively GC insensitive. Consequently, H6PDHKO mice have a phenotype consistent with defects in the permissive and adaptive actions of GCs upon physiology. H6PDHKO mice have also offered an insight into muscle physiology as they also present with a severe vacuolating myopathy, abnormalities of glucose homeostasis and activation of the unfolded protein response due to ER stress, and a number of mechanisms driving this phenotype are thought to be involved. This article will review what we understand of the redox control of GC hormone metabolism regulated by H6PDH, and how H6PDHKO mice have allowed an in-depth understanding of its potentially novel, GC-independent roles in muscle physiology.