Identification of Scrt1, a new transcriptional regulator of beta-cell maturation using chromatin accessibility variations

Pancreatic beta-cells are highly specialized cells that produce and release insulin in response to nutrients, hormones and neurotransmitters. Glucose-induced insulin secretion, a unique feature of fully differentiated beta-cells, is only acquired after birth and is preceded by a phase of intense beta-cell proliferation. These events occurring in the neonatal period are critical for the establishment of an appropriate functional beta-cell mass covering the insulin needs throughout life. However, key regulators of gene expression and cis-regulatory elements involved in the cellular reprogramming along maturation remain to be elucidated. This project addresses this issue by using ATAC-seq (Assay for Transposase-Accessible Chromatin with high throughput sequencing) permitting a fine genome-wide mapping of chromatin accessibility. This approach is used to compare open chromatin regions in newborn and adult rat beta-cells. We obtained a genome-wide picture of chromatin accessible sites (100000) among which 20% were differentially accessible during maturation. Nearly 60% of these sites were in the proximity of significantly differentially expressed genes. An analysis of transcription factor binding sites revealed key known and unforeseen transcription factors which could explain these changes. We validated a transcriptional repressor named SCRT1, that depicted a significant effect on beta-cell proliferation and targeted several genes implicated in the acquisition of glucose-stimulated insulin secretion function. Thus, we were able to find several known and unforeseen key transcriptional regulators acting at cis-regulatory sites and promoters which depicted a differential accessibility and induced differential gene expression along maturation. These findings could be of interest to induce maturation of surrogate insulin-producing cells.胰腺β细胞（pancreatic beta-cells）是一类高度特化的细胞，可响应营养物质、激素及神经递质的刺激，合成并分泌胰岛素。葡萄糖诱导的胰岛素分泌是完全分化β细胞的独特特征，该功能仅在出生后获得，且在此之前会经历一段活跃的β细胞增殖阶段。新生期发生的这些事件，对于建立足以满足终生胰岛素需求的功能性β细胞团块至关重要。然而，参与β细胞成熟过程中细胞重编程的基因表达关键调控因子及顺式调控元件，其具体功能机制仍有待阐明。本研究借助ATAC-seq（转座酶可及性染色质高通量测序法，Assay for Transposase-Accessible Chromatin with high throughput sequencing）实现染色质可及性的高精度全基因组图谱绘制，以此解决上述问题。本研究采用该方法对比新生与成年大鼠β细胞的开放染色质区域，最终获得了覆盖约10万个染色质可及位点的全基因组图谱，其中20%的位点在β细胞成熟过程中呈现可及性差异。这些差异可及位点中，近60%位于显著差异表达基因的邻近区域。对转录因子结合位点的分析筛选出了一批已知及全新的关键转录因子，可解释上述染色质可及性的变化特征。我们验证了名为SCRT1的转录抑制因子，该因子可显著影响β细胞增殖，并靶向调控多个参与葡萄糖刺激胰岛素分泌功能获得的基因。综上，本研究鉴定出一批作用于顺式调控位点及启动子区域的关键转录调控因子（涵盖已知及全新类型），这些因子在β细胞成熟过程中呈现可及性差异，并诱导了差异基因表达。本研究发现可为诱导替代胰岛素分泌细胞的成熟提供参考依据。