Role of large MAF transcription factors in the mouse endocrine pancreas

Structure of the Eurasian moorhen spleen: A comprehensive study using gross anatomy, light, and transmission electron microscopy

The spleen is the largest secondary lymphoid organ with major roles in the removal of blood-borne antigens. The Eurasian moorhen is a wild aquatic bird that revealed the adaptation to harsh environmental conditions. Information regarding the structural features of moorhen's spleen is lacking. The present study aimed to describe the composition of moorhen's spleen using anatomical dissection, histology, histochemistry, immunohistochemistry, and transmission electron microscopy. The spleen appeared as a brownish red sickle-shaped organ close to the proventriculus and gizzard. The splenic capsule was very thin, and the trabeculae were poorly developed. The white pulp formed of the periarteriolar lymphoid sheath, lymphoid follicles, and periellipsoidal lymphatic sheath. The red pulp composed of sinusoids and cords and contained various types of blood cells. Blood vessels were observed within the splenic capsule and inside the parenchyma. Notably, penicillar capillaries (PCs) appeared branched and surrounded by well-developed ellipsoids. Direct connections were observed between PCs and splenic sinusoids suggesting a closed type of circulation. Ultrastructurally, intercellular spaces and vascular channels were evident in the wall of PCs. Ellipsoid-associated cells, lymphocytes, monocytes, and heterophils were observed within splenic ellipsoids. Ellipsoids were thus suggested as a crucial component of moorhen's spleen. Numerous MafB-immunoreactive (IR) macrophages were seen within the red pulp. Splenic cords contained the highest number of PHH3-IR cells, while CCASP3-IR cells were exclusive to follicles of the white pulp. In conclusion, the structure of moorhen's spleen revealed species-specific features. The findings of the present study could help to improve the immunity of domestic birds.

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 β-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 (A0B^Δpanc), 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 A0B^Δpanc 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 A0B^Δpanc mice can be an efficient inducible model for diabetes research.

Phenotypic analysis of mice carrying human-type MAFB p.Leu239Pro mutation

The transcription factor, MafB, plays important role in the differentiation and functional maintenance of various cells and tissues, such as the inner ear, kidney podocyte, parathyroid gland, pancreatic islet, and macrophages. The rare heterozygous substitution (p.Leu239Pro) of the DNA binding domain in MAFB is the cause of Focal Segmental Glomerulosclerosis associated with Duane Retraction Syndrome, which is characterized by impaired horizontal eye movement due to cranial nerve maldevelopment in humans. In this research, we generated mice carrying MafB p.Leu239Pro (Mafb^mt/mt) and retrieved their tissues for analysis. As a result, we found that the phenotype of Mafb^mt/mt mouse was similar to that of the conventional Mafb deficient mouse. This finding suggests that the Leucine residue at 239 in the DNA binding domain plays a key role in MafB function and could contribute to the diagnosis or development of treatment for patients carrying the MafB p.Leu239Pro missense variant.

Whole-Genome Sequencing of Childhood Cancer Survivors Treated with Cranial Radiation Therapy Identifies 5p15.33 Locus for Stroke: A Report from the St. Jude Lifetime Cohort Study

PURPOSE

To identify genetic factors associated with risk of stroke among survivors of childhood cancer treated with cranial radiotherapy (CRT).

EXPERIMENTAL DESIGN

We analyzed whole-genome sequencing (36.8-fold) data of 686 childhood cancer survivors of European ancestry [median (range), 40.4 (12.4-64.7) years old; 54% male] from the St. Jude Lifetime Cohort study treated with CRT, of whom 116 (17%) had clinically diagnosed stroke. Association analyses (single-variant and Burden/SKAT tests) were performed, adjusting for demographic characteristics and childhood cancer treatment exposures.

RESULTS

We identified a genome-wide significant association between 5p15.33 locus and stroke [rs112896372: HR = 2.55; P = 1.42 × 10^-8], with a stronger association (HR = 3.68) among survivors treated with CRT dose 25-50 Gray (Gy) and weaker associations among those treated with CRT doses <20 or 20-25 or >50 Gy (HRs = 2.14, 2.40, and 2.28). The association was replicated in 90 CRT-exposed African survivors (HR = 3.05; P = 0.034). In CRT-exposed Europeans, rs112896372 significantly (P < 0.001) improved predictive ability (AUC = 0.717) for determining stroke risk than nongenetic factors alone (AUC = 0.663) at 30 years since diagnosis, with significant improvement among African survivors (P = 0.047). SNP rs112896372 was further evaluated in three independent datasets including 1,641 European (HR = 1.54; P = 0.055) and 316 African survivors (HR = 1.88; P = 0.283) not treated with CRT, and 166,988 males in the UK Biobank (OR = 1.0012; P = 0.042).

CONCLUSIONS

A novel locus 5p15.33 is associated with stroke risk among childhood cancer survivors, with a possible CRT dose-specific effect. The locus is of potential clinical utility in characterizing individuals who may benefit from surveillance and intervention strategies.

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

The pancreatic-islet-enriched transcription factors MafA and MafB have unique expression patterns in β cells in rodents. MafA is specifically expressed in β cells and is a key regulatory factor for maintaining adult β-cell function, whereas MafB plays an essential role in β-cell development during embryogenesis, and its expression in β cells gradually decreases and is restricted to α cells after birth in rodents.

The pancreatic-islet-enriched transcription factors MafA and MafB have unique expression patterns in β cells in rodents. MafA is specifically expressed in β cells and is a key regulatory factor for maintaining adult β-cell function, whereas MafB plays an essential role in β-cell development during embryogenesis, and its expression in β 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⁺) β cells in MafA-deficient adult mice. To elucidate how MafB functions in the adult β cell under MafA-deficient conditions, we generated MafA and MafB double-knockout (A0B0) mice in which MafB was specifically deleted from β 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⁺ β cells because of increased β-cell apoptosis, indicating MafB can take part in the maintenance of adult β cells under certain pathological conditions.

A New Way for Beta Cell Neogenesis: Transdifferentiation from Alpha Cells Induced by Glucagon-Like Peptide 1

Recent studies showed that alpha cells, especially immature cells and proalpha cells, might be the precursors of beta cells. Exposure to glucagon-like peptide 1 (GLP1) can ameliorate hyperglycemia in diabetic mice and restore the beta cell mass. In the present study, we adopted single high-dose (60 mg/kg, i.p.) streptozotocin (STZ) to model diabetes mellitus (DM) and randomly assigned short-tail (SD) rats to a normal group, a diabetic group, GLP1 groups (50 μg/kg, 100 μg/kg, and 200 μg/kg), a GLP1 (200 μg/kg) with exendin (9-39) group, and a GLP1 with LY294002 group. We found that the pancreatic insulin-glucagon-positive cell populations increased according to the increase in GLP1 exposure. By contrast, no insulin-amylase-positive cell populations or insulin/pan-cytokeratin cells were observed in the pancreatic sections. The GLP1 receptor antagonist exendin (9-39) and the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) family inhibitor LY294002 not only suppressed protein kinase B (Akt), pancreatic and duodenal homeobox 1 (Pdx1), forkhead box O 1 (FoxO1), and mast cell function-associated antigen A (MafA) mRNA expression but also increased MAFB expression. We concluded that treatment with GLP1 might result in beta cell neogenesis by promoting the transdifferentiation of alpha cells but not by pancreatic acinar cells, ductal cells, or the self-replication of beta cells. The regulation on the GLP1 receptor and its downstream transcription factor PI3K/AKT/FOXO1 pathway, which causes increased pancreatic and duodenal homeobox 1 (Pdx1) and MafA mRNA expression but causes decreased MAFB expression, may be the mechanism involved in this process.

A synopsis of factors regulating beta cell development and beta cell mass

The insulin-secreting beta cells in the endocrine pancreas regulate blood glucose levels, and loss of functional beta cells leads to insulin deficiency, hyperglycemia (high blood glucose) and diabetes mellitus. Current treatment strategies for type-1 (autoimmune) diabetes are islet transplantation, which has significant risks and limitations, or normalization of blood glucose with insulin injections, which is clearly not ideal. The type-1 patients can lack insulin counter-regulatory mechanism; therefore, hypoglycemia is a potential risk. Hence, a cell-based therapy offers a better alternative for the treatment of diabetes. Past research was focused on attempting to generate replacement beta cells from stem cells; however, recently there has been an increasing interest in identifying mechanisms that will lead to the conversion of pre-existing differentiated endocrine cells into beta cells. The goal of this review is to provide an overview of several of the key factors that regulate new beta cell formation (neogenesis) and beta cell proliferation.

Transcription factor regulation of pancreatic organogenesis, differentiation and maturation

Lineage tracing studies have revealed that transcription factors play a cardinal role in pancreatic development, differentiation and function. Three transitions define pancreatic organogenesis, differentiation and maturation. In the primary transition, when pancreatic organogenesis is initiated, there is active proliferation of pancreatic progenitor cells. During the secondary transition, defined by differentiation, there is growth, branching, differentiation and pancreatic cell lineage allocation. The tertiary transition is characterized by differentiated pancreatic cells that undergo further remodeling, including apoptosis, replication and neogenesis thereby establishing a mature organ. Transcription factors function at multiple levels and may regulate one another and auto-regulate. The interaction between extrinsic signals from non-pancreatic tissues and intrinsic transcription factors form a complex gene regulatory network ultimately culminating in the different cell lineages and tissue types in the developing pancreas. Mutations in these transcription factors clinically manifest as subtypes of diabetes mellitus. Current treatment for diabetes is not curative and thus, developmental biologists and stem cell researchers are utilizing knowledge of normal pancreatic development to explore novel therapeutic alternatives. This review summarizes current knowledge of transcription factors involved in pancreatic development and β-cell differentiation in rodents.