Architecture of androgen receptor pathways amplifying glucagon-like peptide-1 insulinotropic action in male pancreatic β cells

Male mice lacking the androgen receptor (AR) in pancreatic β cells exhibit blunted glucose-stimulated insulin secretion (GSIS), leading to hyperglycemia. Testosterone activates an extranuclear AR in β cells to amplify glucagon-like peptide-1 (GLP-1) insulinotropic action. Here, we examined the architecture of AR targets that regulate GLP-1 insulinotropic action in male β cells. Testosterone cooperates with GLP-1 to enhance cAMP production at the plasma membrane and endosomes via: (1) increased mitochondrial production of CO2, activating the HCO3--sensitive soluble adenylate cyclase; and (2) increased Gαs recruitment to GLP-1 receptor and AR complexes, activating transmembrane adenylate cyclase. Additionally, testosterone enhances GSIS in human islets via a focal adhesion kinase/SRC/phosphatidylinositol 3-kinase/mammalian target of rapamycin complex 2 actin remodeling cascade. We describe the testosterone-stimulated AR interactome, transcriptome, proteome, and metabolome that contribute to these effects. This study identifies AR genomic and non-genomic actions that enhance GLP-1-stimulated insulin exocytosis in male β cells.

A plasma membrane-associated glycolytic metabolon is functionally coupled to KATP channels in pancreatic α and β cells from humans and mice

The ATP-sensitive K+ (KATP) channel is a key regulator of hormone secretion from pancreatic islet endocrine cells. Using direct measurements of KATP channel activity in pancreatic β cells and the lesser-studied α cells, from both humans and mice, we provide evidence that a glycolytic metabolon locally controls KATP channels on the plasma membrane. The two ATP-consuming enzymes of upper glycolysis, glucokinase and phosphofructokinase, generate ADP that activates KATP. Substrate channeling of fructose 1,6-bisphosphate through the enzymes of lower glycolysis fuels pyruvate kinase, which directly consumes the ADP made by phosphofructokinase to raise ATP/ADP and close the channel. We further show the presence of a plasma membrane-associated NAD+/NADH cycle whereby lactate dehydrogenase is functionally coupled to glyceraldehyde-3-phosphate dehydrogenase. These studies provide direct electrophysiological evidence of a KATP-controlling glycolytic signaling complex and demonstrate its relevance to islet glucose sensing and excitability.

β-cell deletion of the PKm1 and PKm2 isoforms of pyruvate kinase in mice reveals their essential role as nutrient sensors for the KATP channel

Pyruvate kinase (PK) and the phosphoenolpyruvate (PEP) cycle play key roles in nutrient-stimulated K_ATP channel closure and insulin secretion. To identify the PK isoforms involved, we generated mice lacking β-cell PKm1, PKm2, and mitochondrial PEP carboxykinase (PCK2) that generates mitochondrial PEP. Glucose metabolism was found to generate both glycolytic and mitochondrially derived PEP, which triggers K_ATP closure through local PKm1 and PKm2 signaling at the plasma membrane. Amino acids, which generate mitochondrial PEP without producing glycolytic fructose 1,6-bisphosphate to allosterically activate PKm2, signal through PKm1 to raise ATP/ADP, close K_ATP channels, and stimulate insulin secretion. Raising cytosolic ATP/ADP with amino acids is insufficient to close K_ATP channels in the absence of PK activity or PCK2, indicating that K_ATP channels are primarily regulated by PEP that provides ATP via plasma membrane-associated PK, rather than mitochondrially derived ATP. Following membrane depolarization, the PEP cycle is involved in an 'off-switch' that facilitates K_ATP channel reopening and Ca²⁺ extrusion, as shown by PK activation experiments and β-cell PCK2 deletion, which prolongs Ca²⁺ oscillations and increases insulin secretion. In conclusion, the differential response of PKm1 and PKm2 to the glycolytic and mitochondrial sources of PEP influences the β-cell nutrient response, and controls the oscillatory cycle regulating insulin secretion.

The role of potassium and host calcium signaling in Toxoplasma gondii egress

Toxoplasma gondii is an obligate intracellular parasite and replicates inside a parasitophorous vacuole (PV) within the host cell. The membrane of the PV (PVM) contains pores that permits for equilibration of ions and small molecules between the host cytosol and the PV lumen. Ca²⁺ signaling is universal and both T. gondii and its mammalian host cell utilize Ca²⁺ signals to stimulate diverse cellular functions. Egress of T. gondii from host cells is an essential step for the infection cycle of T. gondii, and a cytosolic Ca²⁺ increase initiates a Ca²⁺ signaling cascade that culminates in the stimulation of motility and egress. In this work we demonstrate that intracellular T. gondii tachyzoites are able to take up Ca²⁺ from the host cytoplasm during host cell signaling events. Both intracellular and extracellular Ca²⁺ sources are important in reaching a threshold of parasite cytosolic Ca²⁺ needed for successful egress. Two peaks of Ca²⁺ were observed in egressing single parasites with the second peak resulting from Ca²⁺ entry. We patched infected host cells to allow the delivery of precise concentrations of Ca²⁺ for the stimulation of motility and egress. Using this approach of patching infected host cells, allowed us to determine that increasing the host cytosolic Ca²⁺ to a specific concentration can trigger egress, which is further accelerated by diminishing the concentration of potassium (K⁺).

Pyruvate Kinase Controls Signal Strength in the Insulin Secretory Pathway

Pancreatic β cells couple nutrient metabolism with appropriate insulin secretion. Here, we show that pyruvate kinase (PK), which converts ADP and phosphoenolpyruvate (PEP) into ATP and pyruvate, underlies β cell sensing of both glycolytic and mitochondrial fuels. Plasma membrane-localized PK is sufficient to close K_ATP channels and initiate calcium influx. Small-molecule PK activators increase the frequency of ATP/ADP and calcium oscillations and potently amplify insulin secretion. PK restricts respiration by cyclically depriving mitochondria of ADP, which accelerates PEP cycling until membrane depolarization restores ADP and oxidative phosphorylation. Our findings support a compartmentalized model of β cell metabolism in which PK locally generates the ATP/ADP required for insulin secretion. Oscillatory PK activity allows mitochondria to perform synthetic and oxidative functions without any net impact on glucose oxidation. These findings suggest a potential therapeutic route for diabetes based on PK activation that would not be predicted by the current consensus single-state model of β cell function.

The acidocalcisome inositol-1,4,5-trisphosphate receptor of Trypanosoma brucei is stimulated by luminal polyphosphate hydrolysis products

Acidocalcisomes are acidic calcium stores rich in polyphosphate (polyP) and are present in trypanosomes and also in a diverse range of other organisms. Ca²⁺ is released from these organelles through a channel, inositol 1,4,5-trisphosphate receptor (TbIP₃R), which is essential for growth and infectivity of the parasite Trypanosoma brucei However, the mechanism by which TbIP₃R controls Ca²⁺ release is unclear. In this work, we expressed TbIP₃R in a chicken B lymphocyte cell line in which the genes for all three vertebrate IP₃Rs were stably ablated (DT40-3KO). We show that IP₃-mediated Ca²⁺ release depends on Ca²⁺ but not on ATP concentration and is inhibited by heparin, caffeine, and 2-aminomethoxydiphenyl borate (2-APB). Excised patch clamp recordings from nuclear membranes of DT40 cells expressing only TbIP₃R disclosed that luminal inorganic orthophosphate (P_i) or pyrophosphate (PP_i), and neutral or alkaline pH can stimulate IP₃-generated currents. In contrast, polyP or acidic pH did not induce these currents, and nuclear membranes obtained from cells expressing rat IP₃R were unresponsive to polyP or its hydrolysis products. Our results are consistent with the notion that polyP hydrolysis products within acidocalcisomes or alkalinization of their luminal pH activate TbIP₃R and Ca²⁺ release. We conclude that TbIP₃R is well-adapted to its role as the major Ca²⁺ release channel of acidocalcisomes in T. brucei.

5-Diphosphoinositol pentakisphosphate (5-IP7) regulates phosphate release from acidocalcisomes and yeast vacuoles

Acidocalcisomes of Trypanosoma brucei and the acidocalcisome-like vacuoles of Saccharomyces cerevisiae are acidic calcium compartments that store polyphosphate (polyP). Both organelles possess a phosphate-sodium symporter (TbPho91 and Pho91p in T. brucei and yeast, respectively), but the roles of these transporters in growth and orthophosphate (P_i) transport are unclear. We found here that Tbpho91 ^-/- trypanosomes have a lower growth rate under phosphate starvation and contain larger acidocalcisomes that have increased P_i content. Heterologous expression of TbPHO91 in Xenopus oocytes followed by two-electrode voltage clamp recordings disclosed that myo-inositol polyphosphates stimulate both sodium-dependent depolarization of the oocyte membrane potential and P_i conductance. Deletion of the SPX domain in TbPho91 abolished this stimulation. Inositol pyrophosphates such as 5-diphosphoinositol pentakisphosphate generated outward currents in Na⁺/P_i-loaded giant vacuoles prepared from WT or from TbPHO91-expressing pho91Δ strains but not from the pho91Δ yeast strains or from the pho91Δ strains expressing PHO91 or TbPHO91 with mutated SPX domains. Our results indicate that TbPho91 and Pho91p are responsible for vacuolar P_i and Na⁺ efflux and that myo-inositol polyphosphates stimulate the Na⁺/P_i symporter activities through their SPX domains.

Alkalinization-Induced Changes in Intracellular Calcium in Rat Spinal Cord Neurons

It is well-known that pH changes can influence a lot of cellular processes. In this work, we have specifically studied the influence of alkalinization, which can be developed in spinal cord neurons during hyperventilation (respiratory alkalosis) and chronic renal failure (metabolic alkalosis) on calcium homeostasis. Application of Tyrode solution with increased pH (pH = 8.8) to secondary sensory neurons isolated from rat spinal dorsal horn induced elevation of intracellular free calcium concentration in the cytosol ([Ca2+]i) if applied after membrane depolarization. Repetitive application of alkaline solution led to disappearance of such elevations. Depletion of endoplasmic reticulum (ER) calcium stores by 30 mM caffeine almost completely blocked the effect of elevated extracellular pH. If caffeine-induced [Ca2+]i transients were evoked during alkalinization, their amplitudes were decreased by 41%. Preapplication of 500 nM ionomycin resulted in disappearance of alkalinization-induced [Ca2+]i transients, whereas prolonged applications (for 20 min) of 200 nM thapsigargin, a blocker of Ca2+ ATPase of the endoplasmic reticulum, resulted in disappearance of the rapid phase of the [Ca2+]i transients induced by alkalinization. Preapplication of the mitochondrial protonophore CCCP (10 microM) also induced changes in the alkalinization-induced calcium response--it lost its peak and was transformed into an irregular wave terminating in several seconds. The data obtained indicate that alkalinization induces an increase of [Ca2+]i level in the investigated neurons via a combined action of both intracellular Ca2+-accumulating structures--the endoplasmic reticulum and mitochondria. This suggestion was supported by morphological data that both structures in these neurons are tightly connected and may interact during release of accumulated calcium ions.

Intracellular calcium homeostasis changes induced in rat spinal cord neurons by extracellular acidification

Changes in intracellular Ca2+ induced by extracellular acidification to pH = 6 were studied in isolated rat spinal dorsal horn neurons using indo-1 fluorescent technique. In all neurons such treatment induced a decrease of basal [Ca2+]i level by 20.8%, preceded in some of them by temporary increase. The changes were completely reversible. The depolarization-induced [Ca2+]i transients became strongly and also reversibly depressed. If tested after termination of acidification, they demonstrated substantial prolongation of their decay phase, reaching 310% at 120 sec after the application of depolarization. To analyze the mechanisms of such changes, mitochondrial protonophore CCCP has been applied between the end of acidification and the depolarizing pulse. This completely eliminated the described slowing of the transients' decay. To the contrary, application of caffeine to induce Ca2+ release from the endoplasmic reticulum did not show significant changes in the corresponding [Ca2+]i transients. A conclusion is made that in mammalian neurons extracellular acidification, apart from inhibiting voltage-operated Ca2+ channels, also substantially alters the Ca2+ exchange function of mitochondria responsible for rapid accumulation of ions and their delayed release back into the cytosol.

Role of mitochondria in intracellular calcium signaling in primary and secondary sensory neurones of rats

The participation of different calcium-regulated mechanisms in the generation of cytosolic Ca(2+) transients during neuronal excitation has been compared in isolated large and small primary (dorsal root ganglia (DRG)) and secondary (spinal dorsal horn (DH)) rat sensory neurones. As it was shown before in murine primary sensory neurones the application of mitochondrial protonophore CCCP by itself induced only small elevation of [Ca(2+)](i). However, its preceding application substantially increased the peak amplitude of depolarization-induced transients. Application of CCCP immediately after termination of the depolarizing pulse induced in both types of primary neurones a massive release of Ca(2+) from mitochondria into the cytosol. In secondary neurones application of CCCP by itself induced a substantial release of Ca(2+) from the mitochondria, but its preceding application resulted in only an insignificant increase in the peak amplitude of depolarization-triggered calcium transients. Application of CCCP immediately after termination of depolarization elicited a small release of Ca(2+), which became more pronounced when the application was delayed. Preceding application of CCCP increased the amplitude of the transients induced by caffeine-triggered Ca(2+) release from the endoplasmic reticulum in secondary neurones and did not affect those in large primary neurones. These findings may be explained by substantial differences in the density and distribution of mitochondria in the cytosol of primary and secondary sensory neurones. This suggestion was confirmed electronmicroscopically, showing a much lower density of mitochondria near plasmalemma in secondary sensory neurones and predominant clustered location of mitochondria beneath the plasmalemma in the primary cells. The possible functional importance of these differences is discussed.

Metabotropic purinoreceptors in rat dorsal horn neurones: predominant dendritic location

Elevations of cytosolic free Ca2+ concentration ([Ca2+]i) induced by addition of ATP have been compared in rat dorsal horn neurones in slices and after their isolation. ATP application induced in neurones in situ a rise of [Ca2+]i by 201 +/- 12 nM. In Ca2+-free external solution the rise was 156 +/- 14 nM (n = 45 of 76), indicating the presence of active purinergic metabotropic receptors in about 59% of neurones. [Ca2+]i transients induced by 2MeSATP in Ca2+-free external solution were completely abolished by 10 microM PPADS, indicating that some of the corresponding receptors are of the P2Y1 type. In acutely isolated neurones which lost their dendrites, there were no metabotropic response. The results confirm the presence of metabotropic postsynaptic purinoreceptors located in the dendritic tree of dorsal horn neurones.