The importance of MafA for glucose homeostasis was shown in MafA-deficient mice, which displayed glucose intolerance after weaning due to impaired glucose-stimulated insulin secretion (GSIS) and age-dependent diabetes progression [
11,
12]. MafA-deficient mice had decreased mRNA levels of
Ins1,
Ins2,
Pdx1,
Neurod1 and
Slc2a2, suggesting that MafA is essential for the transcriptional identity of β-cells. MafA works synergistically with Neurod1 and Pdx1 to transactivate the insulin promoter, which is an effect that could not be fully replicated with MafB nor Maf [
13]. Along these lines, MafA promotes insulin transcription when overexpressed in rat islets [
14] and when ectopically expressed in chick embryonic endoderm [
15] or in other non-β-cell lines of endodermal origin [
16,
17].
Several studies suggest that MafA is dispensable for β-cell development but plays an essential role in β-cell maturation and glucose responsiveness of adult β-cells. In rodents,
Mafa expression is low at birth, increasing during the postnatal period [
18,
19]. Interestingly, premature expression of MafA in pancreatic endocrine progenitors prevents differentiation into hormone-expressing cells [
20,
21], suggesting that the induction of MafA expression in β-cells is time sensitive. Consistently, in mice with whole pancreas MafA knockout (
Mafa∆panc), the effects of MafA loss are not apparent until three weeks of age, when the KO mice showed lower β-cell mass, decreased insulin expression, and impaired glucose tolerance compared to controls [
22]. Similarly, at twelve weeks of age in whole-body MafA knockout mice, there is a reduced β-cell:α-cell ratio, decreased islet insulin content and β-cell dedifferentiation into MafB-expressing, progenitor-like cells [
12]. These results may recapitulate some aspects of normal islet development. MafB is actually the predominant large MAF protein expressed during pancreatic development in both α- and β-cells [
4]. MafB can upregulate
Ins1 and
Ins2 transcription [
8], but in adult mice, MafB is mainly expressed in α-cells and upregulates
Gcg gene expression [
4,
23]. The decrease in β-cell MafB expression after birth is at least partly due to methylation, specifically by DNA methyltransferase 3a (Dnmt3a) which binds and represses at a region −1032 to −838 upstream of the
Mafb transcriptional start site [
24].
Mafb∆panc mice have higher blood glucose levels at P1, but two weeks after birth, blood glucose levels normalize. Likewise, at E15.5,
Mafb∆panc mice have a lower number of insulin
+ and glucagon
+ cells compared to controls, but two weeks after birth, cell numbers became roughly the same [
25]. MafB does seem to have a role in enhancing postnatal β-cell function under metabolic stress (pregnancy, high-fat diet) [
24,
26], but MafA remains critical for proper β-cell function and glucose homeostasis in mature β-cells, with forced MafB expression unable to compensate for MafA loss in adult mice [
24].
Reduced MafA expression is associated with diabetes progression in mice and human patients. In islets isolated from diabetic
db/db mice, MafA is decreased due to hyperglycemia-associated oxidative stress and c-Jun activity [
9]. Similarly, islets from subjects with type 2 diabetes (T2D) display a marked decrease in
MAFA mRNA levels and protein expression [
10,
29,
30]. Several studies have shown possible mechanisms for this phenomenon. In β-cells from subjects with T2D [
31] and in in vitro studies mimicking oxidative stress [
10], MafA was found primarily in the cytoplasm rather than properly localized in the nucleus. However, when MafA levels were “rescued” in
db/db mice, GSIS and β-cell mass improved, suggesting that preserving MafA expression in β-cells can still mitigate diabetes progression [
32]. scRNA-seq analysis of human β-cells supports this concept as well, wherein metabolically inflexible β-cells show lower MAFA activity as compared to healthy β-cells [
33]. This finding is thought to align with previous patterns of T2D-induced oxidative stress, in which case MafA activity is high under healthy islet conditions but can also increase as a protective mechanism against acute oxidative stress [
34]. In a different scRNA-seq analysis of human islets, β-cells co-expressing
MAFA/MAFB had increased expression of genes related to β-cell identity, glucose metabolism and exocytosis compared to β-cells that only expressed one or neither TF [
35]. Since MAFA and MAFB are capable of forming heterodimers [
8,
36], it is possible that the co-expression of both can enhance the β-cell maturity transcriptional program. These and other studies have documented the functional and transcriptional heterogeneity of β-cells [
33,
35,
37].