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Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: Protection and replenishment of a functional pancreatic -cell mass (BCM) are key goals of all diabetes therapies. Apelin, a small regulatory peptide, is the endogenous ligand for the apelin receptor (APJ) receptor. The apelin-APJ signaling system is expressed in rodent and human islet cells. Apelin exposure has been shown to inhibit and to stimulate insulin secretion. Our aim was to assess the influence of a selective APJ deletion in pancreatic islet cells on islet homeostasis and glucose tolerance in mice. Cre-LoxP strategy was utilized to mediate islet APJ deletion. APJ deletion in islet cells (APJ(islet)) resulted in a significantly reduced islet size, density and BCM. An ip glucose tolerance test showed significantly impaired glucose clearance in APJ(islet) mice. APJ(islet) mice were not insulin resistant and in vivo glucose-stimulated insulin secretion was reduced modestly. In vitro glucose-stimulated insulin secretion showed a significantly reduced insulin secretion by islets from APJ(islet) mice. Glucose clearance in response to ip glucose tolerance test in obese APJ(islet) mice fed a chronic high-fat (HF) diet, but not pregnant APJ(islet) mice, was impaired significantly. In addition, the obesity-induced adaptive elevations in mean islet size and fractional islet area were reduced significantly in obese APJ(islet) mice when compared with wild-type mice. Together, these findings demonstrate a stimulatory role for the islet cell apelin-APJ signaling axis in regulation of pancreatic islet homeostasis and in metabolic induced -cell hyperplasia. The results indicate the apelin-APJ system can be exploited for replenishment of BCM.
Question: What is apelin?
Output:
apelin, a small regulatory peptide, is the endogenous ligand for the apelin receptor (apj) receptor.