The unique property of the pancreatic β cell is its ability to secrete insulin to allow circulating glucose levels to be maintained within a slim physiologic range, despite wide fluctuations in power consumption and expenditure. It is able to sense the glucose concentration in the extracellular milieu, and adapt its insulin secretion fee via a complex interplay between nutrients, hormones, and neuronal signals. Whereas the minute-to-min regulation of insulin secretion occurs at the degree of exocytosis of preformed insulin, adaptation to lasting adjustments in the environment additionally includes regulated adjustments in the transcription fee of the insulin gene, translation of the mRNA, and processing of the proinsulin molecule in to fully mature insulin. These procedures are coordinately regulated by glucose (Figure 6.1) under typical circumstances, and their perturbation leads to β-cell dysfunction and type 2 diabetes.
Insulin gene expression
Structure of the insulin gene
The insulin gene is specifically expressed in pancreatic β cells, even though reasonable levels of expression have actually been detected in the brain [1], the thymus [2], and in the yolk sac throughout fetal progress [3]. The gene’s sequence is highly conserved throughout evolution, and is present as a single copy in a lot of species, including humans, where it is located on chromosome 11 between the genes for tyrosine hydroxylase and insulin-love growth factor 2. In rodents, there are two nonallelic insulin genes (I and II), resulting from duplication of the original insulin II gene [4]. The human insulin gene contains 3 exons and two introns. The very first intron is within the 5′-untranslated region, whereas the second intron interrupts the C-peptide coding sequence. The mature preproinsulin mRNA is 446 base pairs (bp) long.
Pancreatic β-cell-enriched gene transcription is controlled by a variety of proteins, including the 2 positive-acting islet- enriched transcription factors including Pax6, Pancreatic and duodenal homeobox-1 (Pdx-1), Neurogenic differentiation 1 (NeuroD1/Beta2), and v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (MafA) [5,6], and ubiquitously distributed transcriptional activators responsive to personal signaling pathways (e.g. activating transcription factor-2 responsive to Ca+2 signaling [7]; nuclear factor of activated T cells (NFAT) responsive to Ca+2 [8]; kinases such as extracellular-regulated kinases (ERK)1/2 [9]; the transcriptional repressors CCAAT/enhancer-binding healthy protein beta (C/EBPβ) [10] and c-Jun [11]; phosphatases (e.g. calcineurin [12]); and coactivators such as p300 [13]. Insulin gene expression is principally controlled by a highly conserved region lying around 340bp upstream of the transcription initiation start, termed the enhancer/promoter manage region [14,15]. Consid- erable improvement has actually been earned in defining the lots of various cis– and trans-acting factors that make certain precise transcriptional regulation, along with the focus listed here on describing the β-cell-enriched transcription factors a lot of pertinent to metabolically regulated expression, specifically Pdx-1, NeuroD1/Beta2, and MafA.
Pdx-1 is a homeodomain healthy protein that plays a serious role in pancreatic β-cell progress and function [16,17]. It primarily binds as a monomer to the conserved AT-rich A3 box (-201/-196 bp) and activates insulin transcription, even though this healthy protein additionally appears to act as a repressor in various other gene contexts [18]. Pdx-1 is developed early in rodent pancreatic progenitors, and is necessary to acinar, ductal, and islet endocrine cell formation [5]. the 2 homozygous and heterozygous mutations in the PDX-1 gene have actually been identified in humans, which lead, respectively, to finish agenesis of the pancreas and a form of maturity-onset diabetes in the young known as MODY 4 [5].
NeuroD1/Beta2 is a simple helix-loop-helix (bHLH) transcription factor, which binds at the conserved insulin E1 (-100/-91bp) site in a complex along with ubiquitously expressed E-box proteins. NeuroD1-null mice die of serious diabetes shortly after birth because of its essential role in β-cell formation [19]. Moreover, deletion of NeuroD1 specifically in adult β cells in vivo induces glucose intolerance and loss of expression of lots of genes associated along with cell maturation and function [20]. Mutations in human NEUROD1 predisposes one to one more form of maturity-onset diabetes in the young, MODY 6 [21], presumably due to its importance in the production and maintenance of fully functional glucose-responsive β cells.
The MafA activator is a simple leucine zipper protein, which binds as a dimer to the conserved insulin C1/RIPE3b1 (-118/-107bp) element. MafA, and the just added islet synthesized large Maf family member, MafB, are expressed unusually late in pancreatic cell progress in relation to various other islet-enriched transcription factors. In rodents, MafB is principally present in producing α cells and β cells, then comes to be restricted to α cells soon after birth [22,23]. In contrast, MafA is just discovered in β cells, along with expression very first detected throughout the principal wave of insulin-positive cell production at embryonic day 13.5 in mice [24]. In human islets, MafB is not just present in α cells yet additionally co-developed along with MafA in β cells [25]. A novel role for MafA and MafB in β-cell maturation and function was revealed upon comparing their properties to various other islet-enriched transcription factor mutant mice. While islet cell populations are either lost or re-mentioned in a lot of transcription factor knockout mice [5], the principal defect in MafB-/- embryos was low insulin and glucagon hormone expression, along with no adjustment in endocrine cell numbers or islet cell identity [22,23]. In contrast, islet cell progress was unaffected in MafA-/- [26] and MafAΔPanc [27] mice, even though glucose-regulated insulin secretion was compromised in adults.
Overall, a highly sophisticated network of transcription factors provides the infrastructure for exactly regulating insulin gene transcription, which relies on the cooperative and synergistic interactions between transcription factors and recruited coactivators. Significantly, these factors are additionally essential in regulating lots of various other β-cell genes, resulting in a lot of distinct developmental and adult phenotypes in studies of total and conditional transcription factor knockout mice. In the adhering to section, the effect of increasing glucose levels, the a lot of physiologically impactful mediator of β-cell function on Pdx-1, NeuroD1, and MafA will certainly be presented (Figure 6.1).
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