Epsilon cells are endocrine cells located in the islets o...

Epsilon cells are endocrine cells located in the islets of Langerhans that express ghrelin (GHRL) (reviewed in Dassaye et al. 2016, Sakata et al. 2019, Alvarez Fallas et al. 2021). GHRL is a peptide hormone that in adults is secreted mainly from enteroendocrine cells, known as ghrelin cells, in the lining of the stomach and duodenum (reviewed in Kulkarni et al. 2024). Epsilon cells in the pancreas may be an important source of GHRL during human embryonic development, as they reach their maximum number at gestational week 23, constituting ~30% of islet cells (Andralojc et al. 2009). In the adult human pancreas, epsilon cells are rare, constituting less than 1% of islet cells (Wierup et al. 2002, Andralojc et al. 2009, Baron et al. 2016, Dominguez Gutierrez et al. 2018), and had even been suggested to be absent in adults (Mastracci and Sussel 2012). Although GHRL produced by epsilon cells does not significantly contribute to the total level of GHRL in the blood, it may have an important paracrine effect on neighboring islet cells that express high levels of GHRL receptor GHSR (growth hormone secretagogue receptor) (reviewed in Kulkarni et al. 2024). High GHSR levels are seen on human delta cells (DiGruccio et al. 2016; Gupta et al. 2021) and mouse delta cells (Gupta et al. 2021). GHSR is also expressed on human beta cells (DiGruccio et al. 2016, Gupta et al. 2021) and mouse and human PP cells (Gupta et al. 2021). Some of the GHSR-positive cells in both mouse and human express markers of both delta cells (SST) and PP cells (PPY) (Gupta et al. 2021). In total, GHSR is expressed in ~2% human pancreatic islet cells, but is also detectable in acinar, ductal, and stellate cells (Gupta et al. 2021). Human and mouse epsilon cells do not express GHSR (Gupta et al. 2021). GHRL is known to stimulates somatostatin secretion and suppresses glucose-stimulated insulin secretion (Wierup et al. 2004, Tong et al. 2010, DiGruccio et al. 2016).

Epsilon cells are derived from endocrine precursor cells. By immunohistochemical analysis of fetal and adult human pancreas, epsilon cells are usually round or ovoid and are often located at the periphery of islets of Langerhans as single cells or small clusters of cells (Wierup et al. 2002, Andralojc et al. 2009). Occasionally, GHRL-positive cells can be observed in the duct epithelium of the adult human pancreas, but it is not known whether these cells are related to epsilon cells or to enteroendocrine cells (Wierup et al. 2002, Andralojc et al. 2009). There is no co-localization of GHRL with insulin, glucagon (GCG), or somatostatin at any stage of human pancreatic development nor in the adult pancreas (Wierup et al. 2002, Andralojc et al. 2009). No co-localization of GHRL with the pancreatic peptide (PP) was reported by Wierup et al. 2002, but Andralojc et al. 2009 reported that GHRL occasionally co-localized with PP in islet cells during late gestation (Andralojc et al. 2009). In the developing mouse pancreas at E18.5, GHRL is detectable in a portion of mouse alpha cells, where it is co-expressed with GCG, and in epsilon cells (Prado et al. 2004, Heller et al. 2005). The percentage of GHRL-positive cells markedly decreases after birth in human (Wierup et al. 2002), rat (Wierup et al. 2004), and mouse (Yu et al. 2021) pancreas. GHRL expression is not affected in epsilon cells isolated from type II diabetes mellitus donors (Gupta et al. 2021).

The number of epsilon cells in the adult human pancreas is not affected by age or sex, but is significantly inversely correlated with the body mass index (BMI) (Andralojc et al. 2009). In mice, plasma GHRL level does not correlate with plasma PPY level (Gupta et al. 2021).

Pancreatic epsilon cells are characterized by expression of the following markers:

For potential additional epsilon cell markers that are supported by a single study in humans, please refer to Baron et al. 2016, Muraro et al. 2016 (Supplementary Table S3) and Dominguez Gutierrez et al. 2018.

The scRNA-seq study published by Muraro et al. 2016 did not identify ISL1 mRNA, NEUROD1 mRNA, and NKX2.2 mRNA as significantly differentially expressed in human pancreatic epsilon cells. In human fetal pancreas, all epsilon cells are positive for ISL1 and NKX2-2 by immunohistochemistry (Andralojc et al. 2009).

Human pancreatic epsilon cells do not express PAX6 or NKX6-1 by immunohistochemistry (Andralojc et al. 2009). Specific downregulation of PAX6 mRNA and the mRNA of PAX6 interactor MAFB is seen during differentiation of human pancreatic epsilon cells (Yu et al. 2021). Only low level of PAX6 mRNA has been reported in human pancreatic epsilon cells by scRNA-seq (Baron et al. 2016). Transcription factors Nkx2-2 (Prado et al. 2004) and Pax6 (Heller et al. 2005) negatively regulate formation of epsilon cells in the developing mouse pancreas. NKX2-2 was, however, reported to directly activate GHRL gene transcription in both mouse (Hill et al. 2010) and human (Shiimura et al. 2015). Further studies are needed to delineate the role of NKX2-2 in differentiation of pancreatic epsilon cells more precisely.

The importance of TM4SF4 for the pancreatic epsilon cell fate is supported by a mouse study in which it was shown that expression of Tm4sf4 is elevated in the pancreas of Nkx2-2 knockout mice, which have an abundance of epsilon cells (Anderson et al. 2011). Knockdown of tm4sf4 in zebrafish leads to a decrease in the epsilon cell population (Anderson et al. 2011).

In mouse, epsilon cells develop from Ngn3-positive pancreatic endocrine precursors that give rise to all pancreatic endocrine cell types (Prado et al. 2004).

In the developing mouse pancreas, the Arx gene is expressed in epsilon and alpha cells (Lange et al. 2022), but epsilon cells are not affected by deletion of the Arx gene (Heller et al. 2005). Deletion of the Pax4 gene has been reported both to increase the number of epsilon cells in mouse pancreas (Prado 2004) and to have no effect on epsilon cells (Heller et al. 2005). Irx2 gene is upregulated and expressed in mouse but not human pancreatic epsilon cells (Yu et al. 2021). Irs4 was also reported to be expressed in mouse but not in human epsilon cells (Lange et al. 2022).

Lineage tracing in mouse (Arnes et al. 2012) and prediction of human pancreatic endocrine lineages from scRNA-seq data using Slingshot (Olaniru et al. 2022) suggests that a subset of GHRL-positive epsilon cells may be pluripotent and contributing to ductal, exocrine, and endocrine lineages.