Amino Acid Sensing by the α-Cell Mitochondrial Phosphoenolpyruvate Cycle Regulates Intracellular Ca2+ Levels Without...

Pancreatic islet α-cells are increasingly recognized as amino acid sensors for the organism. Building on our prior work in β-cells, we sought to determine whether the mitochondrial phosphoenolpyruvate (PEP) cycle is involved in α-cell amino acid sensing. Three different methods were used to probe the PEP cycle, including pyruvate kinase activators (TEPP-46), and mice with α-cell-specific deletion (KO) of pyruvate kinase M (PKM1/2-αKO) or mitochondrial PEP carboxykinase (PCK2-αKO). The mitochondrial fuel leucine, in the presence of glutamine, antagonized alanine/arginine-stimulated Ca2+ influx and glucagon secretion under hypoglycemic conditions. Both PKM1/2 and PCK2 deletion prevented leucine from closing α-cell KATP channels. The Ca2+ response to amino acids was suppressed by pyruvate kinase activation with TEPP-46 and enhanced by α-cell deletion of PKM1/2 or PCK2-all without changing glucagon secretion. Using diazoxide/KCl to probe the pathways downstream of membrane depolarization, we identified a further role of the PEP cycle in homeostatically regulating Ca2+ levels. In sum, α-cell pyruvate kinase and the mitochondrial PEP cycle senses leucine and inhibits KATP channels similarly to β-cells, while restricting amino acid-stimulated membrane depolarization and Ca2+ influx. However, none of the amino acids tested, including alanine/arginine, regulate glucagon secretion by modulating membrane depolarization or Ca2+ influx.

Our studies identify a role for the α-cell phosphoenolpyruvate cycle in sensing amino acids under hypoglycemic conditions. Leucine, in the presence of glutamine, opposes alanine/arginine-stimulated Ca2+ influx and glucagon secretion. Pyruvate kinase and phosphoenolpyruvate carboxykinase 2 are required for leucine to close α-cell KATP channels and limit Ca2+ influx. All of the amino acids tested regulate glucagon secretion, but none do so by modulating membrane depolarization or intracellular Ca2+ levels.