The DNA methylome-based regulation of functional beta-cell mass

功能β细胞团不足可引起2型糖尿病（T2D）。该应用将阐明胚胎多能胰腺祖细胞（mPPCs）中的DNA甲基化组如何预先决定这种肿块。在胚胎发生过程中，mPPCs激活促内分泌转录因子（TF） Ngn3，产生内分泌祖细胞（EPCs），并由此衍生β细胞。DNA甲基化是一个相对稳定的抑制标记，由包括DNA甲基转移酶1 （Dnmt1）在内的几种酶奠定。DNMT1多态性与β细胞功能减弱和人T2D相关。我们发现胎儿暴露于母体低蛋白饮食，已知会损害功能性β细胞质量，增强小鼠mPPCs中Dnmt1的表达。我们最近发表的研究结果表明，mPPC和EPC基因库都可以分成不同的亚群，这些亚群在可能的指导基因增强子上携带不同的DNA甲基化水平，这可能会影响胰岛细胞的命运选择。此外，Dnmt1在mPPCs/EPCs中的高表达有利于β细胞的分化；然而，EPC向胰岛细胞分化涉及Dnmt1的快速和实质性下调，以及在调节β细胞增殖和功能的基因增强子中一致的去甲基化（称为适应度）。后者包括突触联合蛋白7 (Syt7)，这是一种促进葡萄糖刺激胰岛素分泌的基因。我们的模型是mPPCs中的甲基化组可以影响其后代EPCs和β细胞中的甲基化组进化。因此，具有不同甲基组的mPPCs可以通过预先设定基因的表达水平来产生具有不同适应度的β细胞亚群，这些基因将在随后被激活以获得高β细胞适应度。通过改变Dnmt1表达和/或母体饮食操纵来扰乱早期甲基化，将改变β细胞亚群的部分，从而改变β细胞的功能质量和对T2D的易感性。与该模型一致，我们发现一部分（约55%）来自Ngn3+ EPCs共表达TF Myt1（即Myt1+Ngn3+或“M+N+”）的β细胞比正常生理条件下的M-N+细胞具有更高的出生后适应性。在这里，我们将首先定义M+N+或M-N+祖细胞衍生的β细胞亚群在代谢应激（如衰老、高血糖或胰岛素抵抗）下的适应度，以阐明其生理意义（Am 1）。然后，我们将检查两个亚群的转录组/甲基组，以确定能够解释其不同β细胞适应性的关键甲基化位点，包括测试两个β细胞亚群中Syt7的差异表达（我们观察到）是否有助于其不同的胰岛素分泌活性（目的2）。最后，我们将研究母体低蛋白饮食暴露如何影响M+N+和M-N+祖细胞衍生的β细胞亚群的衍生和适应度，重点关注Dnmt1表达增强在mPPCs中的作用（目的3）。我们希望通过建立母体因素和调节功能β细胞质量和T2D易感性的遗传网络之间的直接机制联系来影响该领域。我们创新的β细胞亚群组合谱系标记使我们处于研究这些问题的独特位置。

Insufficient functional β-cell mass can cause type 2 diabetes (T2D). This application will elucidate how the DNA methylomes in embryonic multipotent pancreatic progenitor cells (mPPCs) pre-determine this mass. During embryogenesis, mPPCs activate pro-endocrine transcription factor (TF) Ngn3 to give rise to endocrine progenitor cells (EPCs), from which β cells are derived. DNA methylation is a relatively stable repressive mark that is laid down by several enzymes including DNA methyl-transferase 1 (Dnmt1). DNMT1 polymorphisms are associated with attenuated β-cell function and human T2D. We show that fetal exposure to maternal low-protein diet, known to compromise functional β-cell mass, enhances Dnmt1 expression in mouse mPPCs. We recently published that both mPPC and EPC pools can be split into subsets that carry different DNA methylation levels at likely instructive gene enhancers, which can bias islet-cell fate choice. Moreover, higher Dnmt1 expression in mPPCs/EPCs favors β-cell differentiation; yet EPC differentiation toward islet cells involves a rapid and substantial downregulation of Dnmt1 and concordant de-methylation in putative enhancers of genes that regulate β-cell proliferation and function (referred to as fitness). The latter includes Synaptotagmin 7 (Syt7), a gene that promotes glucose-stimulated insulin secretion. Our model is that the methylomes in mPPCs can influence the evolving methylomes in their descendant EPCs and β cells. Thus, mPPCs with distinct methylomes can give rise to β-cell subsets with different fitness by pre-setting the expression levels of genes that will be activated later for high β-cell fitness qualities. Perturbing the early-stage methylomes, by changed Dnmt1 expression and/or maternal diet manipulation, will shift the portions of β-cell subsets and consequently the functional β-cell mass and the susceptibility to T2D. Consistent with this model, we showed that a portion (~55%) of β cells derived from a subset of Ngn3+ EPCs that co-express TF Myt1 (i.e., Myt1+Ngn3+ or “M+N+”) has higher postnatal fitness than those from M-N+ cells under normal physiology. Here, we will first define the fitness of M+N+ or M-N+ progenitor-derived β-cell subsets under metabolic stresses such as aging, hyperglycemia, or insulin resistance to elucidate their physiological significance (Am 1). We will then examine the transcriptomes/methylomes of the two subsets to define the key methylated loci that can account for their distinct β-cell fitness, including testing if the differential Syt7 expression in the two β-cell subsets (observed by us) contributes to their different insulin secretion activities (Aim 2). Lastly, we will examine how maternal low-protein diet exposure impacts the derivation and fitness of M+N+ and M-N+ progenitor-derived β-cell subsets, focusing on the roles of enhanced Dnmt1 expression in mPPCs (Aim 3). We expect to impact the field by establishing direct mechanistic links between maternal factors and genetic networks that regulate functional β-cell mass and susceptibility to T2D. Our innovative combinatorial lineage marking of β-cell subsets put us in a unique position to study these issues.