Quantitative analyses upon the dual inactivation of <i>Arx</i> and <i>Pax4</i> in glucagon-producing cells.

(A–E) Quantitative comparison of the number of insulin- (A), glucagon- (B) and somatostatin- (C) expressing cells between 4 month-old Glu-ArxKO/Pax4KO animals and age-/sex-matched WT controls. A significant increase in the numbers of insulin- and somatostatin-expressing cells was observed in Glu-ArxKO/Pax4KO animals, whilst no significant variation in glucagon+ cells was noted. Interestingly, both islet count (D) and size (E) were significantly increased in these animals compared to their WT counterparts, suggesting a process of islet neogenesis in addition to an increased insulin+ cell mass. (F) 4 month-old Glu-ArxKO/Pax4KO animals (and age/sex-matched WT controls) were challenged with glucose. Double-mutant animals displayed an increased capacity to counteract the glucose bolus with a lower peak in glycemia, suggestive of a functional increased β-cell mass. n≥3 in all experiments, ** p<0.01, * p<0.05 using ANOVA. (G) Schematic detailing the consequences of Arx (and Pax4) inactivation triggered in α-cells. Following the inactivation of Arx (and Pax4), α-cells are converted into β-like cells (1–2). The resulting shortage in glucagon (and/or putative additional signals - 3) promotes the proliferation of duct-lining cells, some of which re-express the developmental factor Ngn3. Our results indicate that such Ngn3+ cells adopt an endocrine cell identity, the glucagon+ cell fate being clearly favored (4). Whether neo-generated somatostatin+ cells contribute to the supplementary β-like cell mass remains to be determined (“?”). Similarly, one could assume that Ngn3+ cells could directly give rise to β-like cells, but additional experiments and mouse lines would be required to address this question (“?”). Subsequently, neo-formed glucagon+ cells are, yet again, turned into β-like cells upon the inactivation of Arx (and Pax4) (5). Such repeated cycles of neogenesis/double conversion (3 to 5) eventually result in an islet hypertrophy caused by a β-like cell hyperplasia (6).