Control of endocrine pancreatic beta-cell fate, function, and proliferation

胰岛是胰腺内的类器官，其是高功能的、严格调节的机器，具有从各种内分泌细胞类型分泌的特定激素，其彼此以及交感神经和副交感神经网络相互通信。与毛细血管床的接触允许对来自血流的葡萄糖或其他调节剂做出快速反应。成熟胰岛中的主要内分泌类型为α（α;分泌胰高血糖素）、β（β;胰岛素）、δ（δ;生长抑素）和PP（胰多肽）。由于胰岛素（Ins）在血糖控制中的核心作用，β细胞是最重要的。胰岛内分泌细胞与自己沟通。β细胞使用间隙连接偶联进行有效的葡萄糖刺激胰岛素分泌（GSIS）。胰高血糖素（Gcg）参与对葡萄糖的反调节反应，而生长抑素（Som）对β细胞具有重要的限制作用，可防止胰岛素过度分泌和低血糖症。这最大限度地减少了B细胞在重复释放刺激下的耗竭，并允许正常功能的寿命。正常β细胞在有丝分裂方面非常静止，并且在β细胞应激或衰竭（表型丧失、死亡）下，它们的最小复制能力不能跟上身体的生理需要。我们最近发表的β细胞特异性转录因子（TF）基因Mnx 1的条件失活（Pan et al.，Development 2016）提供了剖析将内分泌祖细胞分配到β细胞谱系的机制以及通过阻断转分化为Som+ δ样细胞来维持命运的新方法。在大多数辟田胞经历Mnxl-失活并转分化为Som+细胞的情况下，少数“逃逸者”辟田胞保持Mnxl+。在附近存在大量Som+Hhex+转分化β细胞的情况下，这些逃逸者进入过度增殖状态，似乎在动物的整个生命中持续存在，导致增生的胰岛充满明显正常的成熟β细胞。我们解决了Mnx 1在控制早期β细胞谱系分配和后期维持β细胞命运和功能中的作用。我们将确定诱导β细胞增殖的信号是否来源于转分化的δ或δ样细胞，它是否在胰岛局部起作用，它是否需要与其他信号协同作用，以及它是否可以类似地作用于非常老的β细胞。我们将研究Mnx 1在人β细胞、β样细胞系和正常组织样本中的表达和耗竭效应。我们将阐明Mnx 1靶基因的各种集合和成熟和老年β细胞中的表观遗传指导作用。我们测试了我们的新假设，即存在“谱系/命运巩固检查点”，在该检查点处，基因调控网络（GRNs）开始在正常组织中进一步优化（扩增/修剪），或者在该检查点处，神秘缺陷的GRNs进行重新配置以导致明确的谱系/命运转换。我们对δ和转化的δ样细胞有了更严格的理解，这是及时的，因为该领域现在更接近于使动物真实，导致增生的胰岛充满了明显正常的成熟β细胞。我们解决了Mnx 1在体外控制多能细胞的早期细胞中的作用，我们需要了解胰岛微器官内精细调节细胞增殖的内部信号作用，以及它们如何在功能上恢复以抵消压力。

The islets of Langerhans are organoids within the pancreas that are high functioning, tightly regulated machines with specific hormones secreted from various endocrine cell-types, which intercommunicate with each other and the sympathetic and parasympathetic neural network. Contact with the capillary bed allows rapid response to glucose or other regulators from the bloodstream. The major endocrine types in the mature islet are alpha (α; secreting glucagon), beta (β; insulin), delta (δ; somatostatin), and PP (pancreatic polypeptide). The β cells are most important because of the central role of insulin (Ins) in glycemic control. Islet endocrine cells communicate with themselves. The β cells use gap-junction coupling for efficient glucose-stimulated insulin secretion (GSIS). Glucagon (Gcg) is involved in the counter-regulatory response to glucose, and somatostatin (Som) is an essential limiting influence on β cells, preventing insulin over-secretion and hypoglycemia. This minimizes the exhaustion of b cells under a repeated-release stimulus and allows a lifetime of normal function. Normal β cells are very quiescent with respect to mitosis, and under β-cell stress or failure (loss of phenotype, death), their minimal replicative capacity cannot keep up with the body's physiological needs. Our recently published conditional inactivation of the β-cell-specific transcription factor (TF) gene Mnx1 (Pan et al., Development 2016) provides new ways of dissecting the mechanisms allocating endocrine progenitors to the β-cell lineage, and maintaining fate by blocking transdifferentiation to Som+ δ-like cells. In a context wherein most β cells undergo Mnx1-inactivation and transdifferentiation to Som+ cells, a few “escaper” β cells remain Mnx1+. In the presence of a great number of nearby Som+Hhex+ transdifferentiated β cells, these escapers enter a hyperproliferative state that seems to persist over the entire life of the animal, leading to hyperplastic islets filled with apparently normal, mature β cells. We address the role of Mnx1 in controlling early β-cell lineage allocation and later-stage maintenance of β-cell fate and function. We will determine if the signal inducing β-cell proliferation is derived from the transdifferentiated δ or δ-like cells, if it works islet-locally, if it requires synergy with other signals, and if it can work similarly on very old β cells. We will examine Mnx1 expression and depletion effects in human β cells, β-like cell lines, and normal tissue samples. We will clarify the various sets of Mnx1 target genes and epigenetic guidance effects in maturing and older β cells. We test our novel hypothesis that there are “lineage/fate consolidation checkpoints” at which gene regulatory networks (GRNs) begin further optimization (augmentation/pruning) in normal tissue, or at which cryptically deficient GRNs undergo reconfiguration to lead to explicit lineage/fate conversion. Our generation of a more rigorous understanding of δ and converted δ-like cells is timely, because the field is now much closer to making authentic the animal, leading to hyperplastic islets filled with apparently normal, mature β cells. We address the role of Mnx1 in controlling early cells from pluripotent cells in vitro, and we need to understand the internal signaling effects within the islet mini-organs that finely regulate glycemia, and how they can be functionally restored to offset stress.