Aims/hypothesis: C57BL/6J mice exhibit impaired glucose tolerance. The aims of this study were to map the genetic loci underlying this phenotype, to further characterise the physiological defects and to identify candidate genes. Methods: Glucose tolerance was measured in an intraperitoneal glucose tolerance test and genetic determinants mapped in an F2 intercross. Insulin sensitivity was measured by injecting insulin and following glucose disposal from the plasma. To measure beta cell function, insulin secretion and electrophysiological studies were carried out on isolated islets. Candidate genes were investigated by sequencing and quantitative RNA analysis. Results: C57BL/6J mice showed normal insulin sensitivity and impaired insulin secretion. In beta cells, glucose did not stimulate a rise in intracellular calcium and its ability to close K ATP channels was impaired. We identified three genetic loci responsible for the impaired glucose tolerance. Nicotinamide nucleotide transhydrogenase (Nnt) lies within one locus and is a nuclear-encoded mitochondrial proton pump. Expression of Nnt is more than sevenfold and fivefold lower respectively in C57BL/6J liver and islets. There is a missense mutation in exon 1 and a multi-exon deletion in the C57BL/6J gene. Glucokinase lies within the Gluchos2 locus and shows reduced enzyme activity in liver. Conclusions/interpretation: The C57BL/6J mouse strain exhibits plasma glucose intolerance reminiscent of human type 2 diabetes. Our data suggest a defect in beta cell glucose metabolism that results in reduced electrical activity and insulin secretion. We have identified three loci that are responsible for the inherited impaired plasma glucose tolerance and identified a novel candidate gene for contribution to glucose intolerance through reduced beta cell activity.

Neonatal diabetes is a rare monogenic form of diabetes that usually presents within the first six months of life. It is commonly caused by gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of the plasmalemmal ATP-sensitive K + (K ATP ) channel. To better understand this disease, we generated a mouse expressing a Kir6.2 mutation (V59M) that causes neonatal diabetes in humans and we used Cre-lox technology to express the mutation specifically in pancreatic β cells. These β-V59M mice developed severe diabetes soon after birth, and by 5 weeks of age, blood glucose levels were markedly increased and insulin was undetectable. Islets isolated from β-V59M mice secreted substantially less insulin and showed a smaller increase in intracellular calcium in response to glucose. This was due to a reduced sensitivity of K ATP channels in pancreatic β cells to inhibition by ATP or glucose. In contrast, the sulfonylurea tolbutamide, a specific blocker of K ATP channels, closed K ATP channels, elevated intracellular calcium levels, and stimulated insulin release in β-V59M β cells, indicating that events downstream of K ATP channel closure remained intact. Expression of the V59M Kir6.2 mutation in pancreatic β cells alone is thus sufficient to recapitulate the neonatal diabetes observed in humans. β-V59M islets also displayed a reduced percentage of β cells, abnormal morphology, lower insulin content, and decreased expression of Kir6.2, SUR1, and insulin mRNA. All these changes are expected to contribute to the diabetes of β-V59M mice. Their cause requires further investigation.

Physiological studies of mice are facilitated by normal plasma and 24-hour urinary reference ranges, but variability of these parameters may increase due to stress that is induced by housing in metabolic cages. We assessed daily weight, food and water intake, urine volume and final day measurements of the following: plasma sodium, potassium, chloride, urea, creatinine, calcium, phosphate, alkaline phosphatase, albumin, cholesterol and glucose; and urinary sodium, potassium, calcium, phosphate, glucose and protein in 24- to 30-week-old C3H/HeH, BALB/cAnNCrl and C57BL/6J mice. Between 15 and 20 mice of each sex from all three strains were individually housed in metabolic cages with ad libitum feeding for up to seven days. Acclimatization was evaluated using general linear modelling for repeated measures and comparison of biochemical data was by unpaired t-test and analysis of variance (SPSS version 12.0.1). Following an initial 5-10% fall in body weight, daily dietary intake, urinary output and weight in all three strains reached stable values after 3-4 days of confinement. Significant differences in plasma glucose, cholesterol, urea, chloride, calcium and albumin, and urinary glucose, sodium, phosphate, calcium and protein were observed between strains and genders. Thus, these results provide normal reference values for plasma and urinary biochemistry in three strains housed in metabolic cages and demonstrate that 3-4 days are required to reach equilibrium in metabolic cage studies. These variations due to strain and gender have significant implications for selecting the appropriate strain upon which to breed genetically-altered models of metabolic and renal disease.

Corrochano Sanchez et al. identify a novel mutation (I588V) in SCN4A, which encodes the Nav1.4 voltage-gated sodium channel, in a patient with myotonia and periodic paralysis. By generating and characterizing a mouse model (‘draggen’) carrying the equivalent point mutation (I582V), they uncover novel pathological and metabolic features of SCN4A channelopathies.

A serious adverse clinical effect of glucocorticoid steroid treatment is secondary osteoporosis, enhancing fracture risk in bone. This rapid increase in bone fracture risk is largely independent of bone loss (quantity), and must therefore arise from degradation of the quality of the bone matrix at the micro- and nanoscale. However, we lack an understanding of both the specific alterations in bone quality n steroid-induced osteoporosis as well as the mechanistic effects of these changes. Here we demonstrate alterations in the nanostructural parameters of the mineralized fibrillar collagen matrix, which affect bone quality, and develop a model linking these to increased fracture risk in glucocorticoid induced osteoporosis. Using a mouse model with an N-ethyl-N-nitrosourea (ENU)-induced corticotrophin releasing hormone promoter mutation (Crh− 120/+) that developed hypercorticosteronaemia and osteoporosis, we utilized in situ mechanical testing with small angle X-ray diffraction, synchrotron micro-computed tomography and quantitative backscattered electron imaging to link altered nano- and microscale deformation mechanisms in the bone matrix to abnormal macroscopic mechanics. We measure the deformation of the mineralized collagen fibrils, and the nano-mechanical parameters including effective fibril modulus and fibril to tissue strain ratio. A significant reduction (51%) of fibril modulus was found in Crh− 120/+ mice. We also find a much larger fibril strain/tissue strain ratio in Crh− 120/+ mice (~ 1.5) compared to the wild-type mice (~ 0.5), indicative of a lowered mechanical competence at the nanoscale. Synchrotron microCT show a disruption of intracortical architecture, possibly linked to osteocytic osteolysis. These findings provide a clear quantitative demonstration of how bone quality changes increase macroscopic fragility in secondary osteoporosis.