A common genetic variant of a mitochondrial RNA processing enzyme predisposes to insulin resistance.

Mitochondrial energy metabolism plays an important role in the pathophysiology of insulin resistance. Recently, a missense N437S variant was identified in the MRPP3 gene, which encodes a mitochondrial RNA processing enzyme within the RNase P complex, with predicted impact on metabolism. We used CRISPR-Cas9 genome editing to introduce this variant into the mouse Mrpp3 gene and show that the variant causes insulin resistance on a high-fat diet. The variant did not influence mitochondrial gene expression markedly, but instead, it reduced mitochondrial calcium that lowered insulin release from the pancreatic islet ß cells of the Mrpp3 variant mice. Reduced insulin secretion resulted in lower insulin levels that contributed to imbalanced metabolism and liver steatosis in the Mrpp3 variant mice on a high-fat diet. Our findings reveal that the MRPP3 variant may be a predisposing factor to insulin resistance and metabolic disease in the human population.

An Inducible Diabetes Mellitus Murine Model Based on MafB Conditional Knockout under MafA-Deficient Condition.

Diabetes mellitus is an increasingly severe chronic metabolic disease that is occurring at an alarming rate worldwide. Various diabetic models, including non-obese diabetic mice and chemically induced diabetic models, are used to characterize and explore the mechanism of the disease's pathophysiology, in hopes of detecting and identifying novel potential therapeutic targets. However, this is accompanied by disadvantages, such as specific conditions for maintaining the incidence, nonstable hyperglycemia induction, and potential toxicity to other organs. Murine MAFA and MAFB, two closely-linked islet-enriched transcription factors, play fundamental roles in glucose sensing and insulin secretion, and maintenance of pancreatic ß-cell, respectively, which are highly homologous to human protein orthologs. Herein, to induce the diabetes mellitus model at a specific time point, we generated Pdx1-dependent Mafb-deletion mice under Mafa knockout condition (A0BΔpanc), via tamoxifen-inducible Cre-loxP system. After 16 weeks, metabolic phenotypes were characterized by intraperitoneal glucose tolerance test (IPGTT), urine glucose test, and metabolic parameters analysis. The results indicated that male A0BΔpanc mice had obvious impaired glucose tolerance, and high urine glucose level. Furthermore, obvious renal lesions, impaired islet structure and decreased proportion of insulin positive cells were observed. Collectively, our results indicate that A0BΔpanc mice can be an efficient inducible model for diabetes research.

MafB Is Important for Pancreatic ß-Cell Maintenance under a MafA-Deficient Condition.

The pancreatic-islet-enriched transcription factors MafA and MafB have unique expression patterns in ß cells in rodents. MafA is specifically expressed in ß cells and is a key regulatory factor for maintaining adult ß-cell function, whereas MafB plays an essential role in ß-cell development during embryogenesis, and its expression in ß cells gradually decreases and is restricted to α cells after birth in rodents. However, it was previously observed that MafB started to be reexpressed in insulin-positive (insulin+) ß cells in MafA-deficient adult mice. To elucidate how MafB functions in the adult ß cell under MafA-deficient conditions, we generated MafA and MafB double-knockout (A0B0) mice in which MafB was specifically deleted from ß cells. As a result, the A0B0 mice became more vulnerable to diabetes under a high-fat diet (HFD) treatment, with impaired islet formation and a decreased number of insulin+ ß cells because of increased ß-cell apoptosis, indicating MafB can take part in the maintenance of adult ß cells under certain pathological conditions.

Role of large MAF transcription factors in the mouse endocrine pancreas.

The members of the MAF family of transcription factors are homologs of v-Maf -the oncogenic component of the avian retrovirus AS42. The MAF family is subdivided into 2 groups, small and large MAFs. To elucidate the role of the large MAF transcription factors in the endocrine pancreas, we analyzed large MAF gene knockout mice. It has been shown that Mafa(-/-) mice develop phenotypes including abnormal islet structure soon after birth. This study revealed that Ins1 and Ins2 transcripts and the protein contents were significantly reduced in Mafa(-/-) mice at embryonic day 18.5. In addition, Mafa(-/-);Mafb(-/-) mice contained less than 10% of the insulin transcript and protein of those of wild-type mice, suggesting that Mafa and Mafb cooperate to maintain insulin levels at the embryonic stage. On the other hand, the number of insulin-positive cells in Mafa(-/-) mice was comparable to that of wild-type mice, and even under a Mafb-deficient background the number of insulin-positive cells was not decreased, suggesting that Mafb plays a dominant role in embryonic ß-cell development. We also found that at 20 weeks of age Mafa(-/-);Mafb(+/-) mice showed a higher fasting blood glucose level than single Mafa(-/-) mice. In summary, our results indicate that Mafa is necessary for the maintenance of normal insulin levels even in embryos and that Mafb is important for the maintenance of fasting blood glucose levels in the Mafa-deficient background in adults.