MNX1 prevents somatostatin expression in human beta cells by repressing PERCC1

A highly specific mosaic transcriptional environment guides the fate of pancreatic progenitors during development and maintains the identity of mature endocrine cells. In mice, one such transcription factor is MNX1 (motor neuron and pancreas homeobox1). It is essential for beta cell development and identity, as its deletion in mature beta cells increases their somatostatin expression via an unknown mechanism. Here, we determined whether MNX1 plays a similar role in human beta cells. and dissected the molecular players involved in its function.

We used the human beta cell line EndoC-BH1, knocked down MNX1 expression using siRNA, and performed bulk RNA sequencing searching for MNX1 targets. We next performed loss-of-function (siRNA) and gain-of-function (transduction with lentivectors and lipofections) experiments to determine how MNX1 regulates gene expression in human beta cells.

siRNA-mediated downregulation of MNX1 in EndoC-BH1 cells induced the upregulation of somatostatin as well as its transcriptional activator haematopoietically expressed homeobox (HHEX) by more than fivefold. RNA-seq revealed PERCC1 (encoding proline and glutamate rich with coiled coil 1) to be among one of the highest upregulated genes, with eightfold induction. siRNA-mediated partial loss of PERCC1 upon MNX1 knockdown moderately impaired the upregulation of both SST and HHEX expression, while PERCC1 overexpression upregulated the expression of both HHEX and SST. PERCC1 expression was tightly regulated by HHEX and by its own expression in a positive feedback mechanism. Moreover, downregulation of NKX2.2, whose knockout in mice induces somatostatin upregulation, led to the upregulation of PERCC1, HHEX and SST in EndoC-BH1 cells. Finally, overexpression of PERCC1 in primary mouse beta cells revealed upregulation of both mouse Hhex and Sst.

We discovered a novel MNX1-PERCC1-HHEX regulatory axis in human beta cells. In these cells, MNX1 protects beta cell identity by inhibiting PERCC1, an activator of HHEX and SST expression. PERCC1 may thus be considered as a sharply tuned central hub for somatostatin regulation. Our results increase our knowledge of beta cell plasticity, and may have implications for therapeutic strategies aimed at protecting beta cell identity.