R-spondin-1 (Rspo1) is an intestinal growth factor known to exert its effects through activation of the canonical Wnt (cWnt) pathway, but its function in the β-cell had not been explored. In Chapter 2, Rspo1 mRNA was found to be expressed in murine islets and the murine MIN6 and βTC β-cell lines, and Rspo1 protein was detected in MIN6 β-cells. Rspo1 activated cWnt signaling and induced insulin mRNA expression in MIN6 β-cells. Analysis of MIN6 and mouse β-cell proliferation revealed that Rspo1 stimulated cell growth and significantly abolished cytokine-induced cellular apoptosis. Rspo1 also stimulated insulin secretion in a glucose-independent fashion. Chapter 2 further demonstrated that the glucagon-like peptide-1 receptor agonist, exendin-4 (EX4), stimulated Rspo1 mRNA transcript levels in MIN6 cells in a glucose-, time-, dose- and PI3-kinase-dependent fashion. Together, these studies demonstrate that Rspo1 is a novel β-cell growth factor and insulin secretagogue that is regulated by EX4. In Chapter 3, the role of Rspo1 in β-cells in vivo was explored using Rspo1 knock-out (Rspo1-/-) mice. Rspo1-/- mice had normal fasting glycemia but an improved glycemic control after an oral glucose challenge compared to Rspo1+/+ mice, with no difference in insulin sensitivity but an enhanced insulin response over 30 min; glucagon responses were normal. Rspo1 deficiency also resulted in an increase in β-cell mass in association with an increase in Ki67-positive β-cells, a marker of proliferation, relative to Rspo1+/+ mice. Rspo1-/- pancreatic tissues also demonstrated a significant increase in the number of insulin-positive ductal cells, suggestive of β-cell neogenesis. Rspo1-/- islets displayed no changes in glucose-induced insulin secretion but showed a complete absence of glucose-induced suppression glucagon secretion. Treatment of Rspo1-/- mice for 2 wk with EX4 resulted in a similar glycemic profile to EX4-treated Rspo1+/+ mice after an oral glucose challenge, with no changes in insulin sensitivity. Interestingly, EX4 administration to Rspo1-/- normalized β-cell mass to a level comparable to that in Rspo1+/+ mice. Although further studies are required, the findings in this thesis reveal a novel role for Rspo1 as a regulator of β-cell behaviour in vivo, and suggest novel roles for Rspo1 in both a- and ductal-cells.

R-spondin-1 (Rspo1) is an intestinal growth factor known to exert its effects through activation of the canonical Wnt (cWnt) pathway, but its function in the β-cell had not been explored. In Chapter 2, Rspo1 mRNA was found to be expressed in murine islets and the murine MIN6 and βTC β-cell lines, and Rspo1 protein was detected in MIN6 β-cells. Rspo1 activated cWnt signaling and induced insulin mRNA expression in MIN6 β-cells. Analysis of MIN6 and mouse β-cell proliferation revealed that Rspo1 stimulated cell growth and significantly abolished cytokine-induced cellular apoptosis. Rspo1 also stimulated insulin secretion in a glucose-independent fashion. Chapter 2 further demonstrated that the glucagon-like peptide-1 receptor agonist, exendin-4 (EX4), stimulated Rspo1 mRNA transcript levels in MIN6 cells in a glucose-, time-, dose- and PI3-kinase-dependent fashion. Together, these studies demonstrate that Rspo1 is a novel β-cell growth factor and insulin secretagogue that is regulated by EX4. In Chapter 3, the role of Rspo1 in β-cells in vivo was explored using Rspo1 knock-out (Rspo1-/-) mice. Rspo1-/- mice had normal fasting glycemia but an improved glycemic control after an oral glucose challenge compared to Rspo1+/+ mice, with no difference in insulin sensitivity but an enhanced insulin response over 30 min; glucagon responses were normal. Rspo1 deficiency also resulted in an increase in β-cell mass in association with an increase in Ki67-positive β-cells, a marker of proliferation, relative to Rspo1+/+ mice. Rspo1-/- pancreatic tissues also demonstrated a significant increase in the number of insulin-positive ductal cells, suggestive of β-cell neogenesis. Rspo1-/- islets displayed no changes in glucose-induced insulin secretion but showed a complete absence of glucose-induced suppression glucagon secretion. Treatment of Rspo1-/- mice for 2 wk with EX4 resulted in a similar glycemic profile to EX4-treated Rspo1+/+ mice after an oral glucose challenge, with no changes in insulin sensitivity. Interestingly, EX4 administration to Rspo1-/- normalized β-cell mass to a level comparable to that in Rspo1+/+ mice. Although further studies are required, the findings in this thesis reveal a novel role for Rspo1 as a regulator of β-cell behaviour in vivo, and suggest novel roles for Rspo1 in both a- and ductal-cells.