The biological function of PANDER has remained elusive since its initial discovery in 2002. Our previous reports and a publication from another research group suggested a potential role for PANDER in glucose homeostasis; however, the lack of a knockout mouse has hindered further biological evaluation. Our investigation has attempted to directly address this limitation via creation of a PANDER−/− mouse and to further establish a functional role for PANDER in the maintenance of glucose homeostasis.
Targeted disruption of PANDER did not result in increased embryonic lethality or gross morphological differences between PANDER−/−
, or wild-type mice. However, PANDER−/−
male mice displayed glucose intolerance, whereas littermate and genotype-matched females were indistinguishable. However, it is not unusual for a glucose-intolerant phenotype to be more severe in male versus female mouse models (21
). Interestingly, our PANDER transgenic model also displays a male-specific phenotype (6
). Deletion of PANDER did not appear to alter fasting levels of various hormones (i.e., insulin, glucagon, amylin, or leptin), and the metabolic defect occurred in the PANDER−/−
mice during periods of nutrient challenge (i.e., glucose), suggesting that PANDER serves a putative role in regulating postprandial glucose homeostasis rather than maintaining basal normoglycemia.
male mice are glucose intolerant. In addition, isolated islets appear to exhibit an insulin secretory defect in response to glucose (in vitro) and arginine (in vivo) and display an abnormal calcium response to both glucose and KCl. Indeed, it has been proposed that decreased GSIS can be attributed to abnormal Ca2+
handling of pancreatic islets, as has been reported in the islets obtained from the type 2 diabetic models of the Goto-Kakizaki and neonatal streptozotocin rats (24
). An additional reported defect in Ca2+
handling has been the absence of the initial sequestration of Ca2+
by β-cell SERCA found in diabetic db/db
murine islets (26
). The initial Ca2+
dip after glucose stimulation is dependent on functional SERCA and appears to serve a critical role in GSIS. However, mRNA levels of SERCA 2/3 were similar between PANDER−/−
and wild-type mice, but activity has yet to be evaluated. Overall, the defective Ca2+
handling observed in the PANDER−/−
islets may provide a causative mechanism for the impaired GSIS.
A peculiar caveat to our results was discordance between the higher insulin levels found in vivo during the GTT and impaired insulin secretion in vitro in the PANDER−/−
mice. Protective and compensatory mechanisms may exist in vivo that preserve and maintain β-cell function, whereas these defects are substantially exaggerated and identified in isolated islets. Also, circulating insulin levels not only reflect secretion but also clearance, and therefore suggest PANDER−/−
mice may also have altered hepatic insulin clearance. Indeed, our model displayed decreased insulin clearance, which suggests that PANDER may have a role in impacting liver function. Alternatively, the decreased clearance may serve as a compensatory mechanism to counteract impaired insulin secretion. Interestingly, other knockout models have also demonstrated a similar phenotype of impaired β-cell function with in vivo hyperinsulinemia. The deletion of hepatocyte nuclear factor-4α (HNF-4α) in pancreatic β-cells resulted in hyperinsulinemia in fasted and fed mice but also demonstrated impaired glucose tolerance and abnormal responses of the isolated HNF-4α−/−
islets by both islet perifusion and calcium imaging (27
). In addition, the glucagon receptor knockout (Gcgr−/−
) mouse demonstrated higher insulin levels after a tail vein-injected glucose challenge, yet revealed a blunted glucose-stimulated insulin response in isolated islets at high glucose concentrations (28
). In Gcgr−/−
mice, the impaired GSIS was restored and enhanced due to increased levels of biologically active GLP-1. However, PANDER−/−
did not demonstrate increased postprandial serum GLP-1 levels compared with wild-type mice (data not shown), which suggests that the incongruity of the observed defect in GSIS with the otherwise higher levels of circulating insulin detected during GTT may be the result of altered hepatic function. Although PANDER−/−
mice display impaired GSIS, additional data are needed to substantiate that the deficiency in Ca2+
regulation of the pancreatic β-cell is causative of the insulin secretory defect, which is beyond the scope of this investigation. Further studies are needed to fully establish the role of PANDER in GSIS and the molecular mechanism responsible for impaired insulin secretion. The lack of our PANDER−/−
model to develop overt diabetes or chronic hyperglycemia may be potentially attributed to PANDER serving multiple functions. The hyperinsulinemic-euglycemic clamp studies showed significantly decreased HGP and a strong trend in increased HGP suppression compared with wild-type mice. This result would directly oppose the glucose intolerance observed during the GTT and could be limiting the higher glycemic values observed during this assay. Previous literature showing that PANDER binds to liver membranes and putatively suppresses insulin action in conjunction with the PANDER transgenic model demonstrating increased HGP is very consistent with our findings in the PANDER−/−
. Our surprising result in the PANDER−/−
mouse certainly suggests that PANDER may be serving multiple roles in regulating glucose levels via not only the liver but also through a direct involvement within the pancreatic islet in either regulation or facilitation of insulin secretion.