Ionomycin induced changes in intracellular free calcium in SH-SY5Y human neuroblastoma cells: sources of calcium and effects on [3H]-noradrenaline release

PERK activation at low glucose concentration is mediated by SERCA pump inhibition and confers preemptive cytoprotection to pancreatic β-cells

A decrease in the ATP/energy status of the cell in response to a decrease in glucose concentration results in SERCA pump inhibition, the efflux of Ca²⁺ from the ER and the activation of PERK, which plays an important role in both pancreatic β-cell function and survival.

Protein kinase R-like ER kinase (PERK) is activated at physiologically low glucose concentrations in pancreatic β-cells. However, the molecular mechanisms by which PERK is activated under these conditions and its role in β-cell function are poorly understood. In this report, we investigated, in dispersed rat islets of Langerhans and mouse insulinoma-6 (MIN6) cells, the relationship between extracellular glucose concentration, the free endoplasmic reticulum (ER) calcium concentration ([Ca²⁺]_ER) measured directly using an ER targeted fluorescence resonance energy transfer-based calcium sensor, and the activation of PERK. We found that a decrease in glucose concentration leads to a concentration-dependent reduction in [Ca²⁺]_ER that parallels the activation of PERK and the phosphorylation of its substrate eukaryotic initiation factor-2α. We provide evidence that this decrease in [Ca²⁺]_ER is caused by a decrease in sarcoplasmic/ER Ca²⁺-ATPase pump activity mediated by a reduction in the energy status of the cell. Importantly, we also report that PERK-dependent eukaryotic initiation factor-2α phosphorylation at low glucose concentration plays a significant role in 1) the regulation of both proinsulin and global protein synthesis, 2) cell viability, and 3) conferring preemptive cytoprotection against ER stress. Taken together, these results provide evidence that a decrease in the ATP/energy status of the cell in response to a decrease in glucose concentration results in sarcoplasmic/ER Ca²⁺-ATPase pump inhibition, the efflux of Ca²⁺ from the ER, and the activation of PERK, which plays an important role in both pancreatic β-cell function and survival.

Activation of Na+/H+ exchanger NHE3 by angiotensin II is mediated by inositol 1,4,5-triphosphate (IP3) receptor-binding protein released with IP3 (IRBIT) and Ca2+/calmodulin-dependent protein kinase II

Angiotensin II (ANG II) stimulates renal tubular reabsorption of NaCl by targeting Na(+)/H(+) exchanger NHE3. We have shown previously that inositol 1,4,5-triphosphate receptor-binding protein released with inositol 1,4,5-triphosphate (IRBIT) plays a critical role in stimulation of NHE3 in response to elevated intracellular Ca(2+) concentration ([Ca(2+)](i)). In this study, we investigated the role of IRBIT in mediating NHE3 activation by ANG II. IRBIT is abundantly expressed in the proximal tubules where NHE3 is located. ANG II at physiological concentrations stimulates NHE3 transport activity in a model proximal tubule cell line. ANG II-induced activation of NHE3 was abrogated by knockdown of IRBIT, whereas overexpression of IRBIT enhanced the effect of ANG II on NHE3. ANG II transiently increased binding of IRBIT to NHE3 at 5 min but became dissociated by 45 min. In comparison, it took at least 15 min of ANG II treatment for an increase in NHE3 activity and NHE3 surface expression. The stimulation of NHE3 by ANG II was dependent on changes in [Ca(2+)](i) and Ca(2+)/calmodulin-dependent protein kinases II. Inhibition of CaMKII completely blocked the ANG II-induced binding of IRBIT to NHE3 and the increase in NHE3 surface abundance. Several serine residues of IRBIT are thought to be important for IRBIT binding. Mutations of Ser-68, Ser-71, and Ser-74 of IRBIT decreased binding of IRBIT to NHE3 and its effect on NHE3 activity. In conclusion, our current findings demonstrate that IRBIT is critically involved in mediating activation of NHE3 by ANG II via a Ca(2+)/calmodulin-dependent protein kinases II-dependent pathway.

Inositol 1,4,5-trisphosphate-independent calcium signalling by platelet-derived growth factor in the human SH-SY5Y neuroblastoma cell

In adherent SH-SY5Y human neuroblastoma cells, activation of G-protein-coupled muscarinic M3 receptors evoked a biphasic elevation of both intracellular [Ca(2+)] ([Ca(2+)]i) and inositol-1,4,5-trisphosphate (D-Ins(1,4,5)P3) mass. In both cases, temporal profiles consisted of rapid transient elevations followed by a decline to a lower, yet sustained level. In contrast, platelet-derived growth factor (PDGF), a receptor tyrosine kinase agonist acting via PDGF receptor b chains in these cells, elicited a slow and transient elevation of [Ca(2+)]i that returned to basal levels within 5 to 10 min with no evidence of inositol phosphate generation. Full responses for either receptor type required intracellular and extracellular Ca(2+) and mobilization of a shared thapsigargin-sensitive intracellular Ca(2+) store. Strategies that affected the ability of D-Ins(1,4,5)P3 to interact with the Ins(1,4,5)P3-receptor demonstrated an Ins(1,4,5)P3-dependency of the muscarinic receptor-mediated elevation of [Ca(2+)]i but showed that PDGF-mediated elevations of [Ca(2+)]i are Ins(1,4,5)P3-independent in these cells.

Clostridium botulinum neurotoxins act with a wide range of potencies on SH-SY5Y human neuroblastoma cells

We have described, in undifferentiated SH-SY5Y human neuroblastoma cells, the relative potency of Clostridium botulinum neurotoxin (BoNT) serotypes A-F Sensitivity of stimulated [3H]-noradrenaline ([3H]-NA) release to the toxins had a rank order of potency of: C > D > A > B > F after 3 days exposure. The difference between the most potent (BoNT/C: IC50 0.54 nM) and the least (BoNT/F: IC50 > 300 nM) was approximately 1,000-fold. Though fluid phase endocytosis may have been the mechanism of entry for low potency toxins the far higher potency of BoNT/C would suggest receptor-driven entry. Potency was not a reflection of the dependence of the release mechanism on a particular SNARE since the substrate specificities were mixed throughout the potency order. This indicated that the toxins differed in their efficiency of binding/endocytosis or enzymatic activity inside the cell. The serotypes that cleaved vesicle-associated membrane protein (VAMP) isoforms (BoNT/B, D and F) did not fully inhibit [3H]-NA release. Cleavage of the appropriate substrate proteins was observed for all serotypes. SNAP-25 cleavage by BoNT/A was shown to be a dose-dependent and correlated closely with reduction of release, supporting proteolysis as the mechanism by which toxin inhibited secretion. Comparison of the SH-SY5Y cell line sensitivity to BoNT/A with glycine releasing rat primary spinal cord neuron cultures, revealed a massive difference in potency; the primary cultures being approximately 200,000-fold more sensitive. The demonstration, using BoNTs, of the crucial role of SNAP-25, VAMP and syntaxin in SH-SY5Y cells suggests the use of this neuroblastoma as a model in the study of these proteins in neurotransmitter release.

Mitochondrial DNA-depleted neuroblastoma (Rho degrees) cells exhibit altered calcium signaling

To investigate the role of chronic mitochondrial dysfunction on intracellular calcium signaling, we studied basal and stimulated cytosolic calcium levels in SH-SY5Y cells and a derived cell line devoid of mitochondrial DNA (Rho degrees ). Basal cytosolic calcium levels were slightly but significantly reduced in Rho degrees cells. The impact of chronic depletion of mitochondrial DNA was more evident following exposure of cells to carbachol, a calcium mobilizing agent. Calcium transients generated in Rho degrees cells following application of carbachol were more rapid than those in SH-SY5Y cells. A plateau phase of calcium recovery during calcium transients was present in SH-SY5Y cells but absent in Rho degrees cells. The rapid calcium transients in Rho degrees cells were due, in part, to increased reliance on Na(+)/Ca(2+) exchange activity at the plasma membrane and the plateau phase in calcium recovery in SH-SY5Y cells was dependent on the presence of extracellular calcium. We also examined whether mitochondrial DNA depletion influenced calcium responses to release of intracellular calcium stores. Rho degrees cells showed reduced responses to the uncoupler, FCCP, and the sarcoplasmic reticulum calcium ATPase inhibitor, thapsigargin. Acute exposure of SH-SY5Y cells to mitochondrial inhibitors did not mimic the results seen in Rho degrees cells. These results suggest that cytosolic calcium homeostasis in this neuron-like cell line is significantly altered as a consequence of chronic depletion of mitochondrial DNA.

Regulation of Ca2+ influx by a protein kinase C activator in chromaffin cells: differential role of P/Q- and L-type Ca2+ channels

Phorbol esters reduce depolarization-evoked Ca2+ influx in adrenal chromaffin cells, suggesting that voltage-sensitive Ca2+ channels (VSCCs) are inhibited by protein kinase C-mediated phosphorylation. We now address the possibility that L- and P/Q-type Ca2+ channel subtypes might be differentially involved in phorbol ester action. In bovine chromaffin cells, short-term (10 min) incubations with phorbol 12-myristate 13-acetate (PMA) inhibited early high K+-evoked rises in cytosolic free Ca2+ concentration ([Ca2+]i) and the early component of the depolarization-evoked Mn2+ quenching of fura-2 fluorescence in a dose-dependent manner (IC50: 18 and 7 nM; maximal inhibitions: 45 and 48%, respectively). The protein kinase C inhibitor staurosporine (100 nM) reverted the inhibitory action of PMA. PMA (0.1-1 microM) inhibited the early and late phases of the ionomycin (2 microM)-evoked [Ca2+]i transients by 14-23%. Omega-agatoxin IVA, a blocker of P/Q-type Ca2+ channels, inhibited high K+-evoked [Ca2+]i rises in a dose-dependent fashion (IC50 = 50 nM). In contrast, 0.1 microM omega-conotoxin GVIA, a blocker of N-type channels, was without effect. A sizeable (< 45%) component of early Ca2+ influx persisted in the combined presence of omega-agatoxin IVA (100 nM) and nitrendipine (1 microM). Simultaneous exposure to omega-agatoxin IVA and PMA inhibited both the early [Ca2+]i transients and Mn2+ quenching to a much greater extent than each drug separately. Inhibition of the [Ca2+]i transients by nitrendipine and PMA did not significantly exceed that produced by PMA alone. It is concluded that phorbol ester-mediated activation of protein kinase C inhibits preferentially L-type VSCCs over P/Q type channels in adrenal chromaffin cells. However, the possibility cannot be ruled out that dihydropyridine-resistant, non-P/Q type channels might also be negatively regulated by protein kinase C. This may represent an important pathway for the specific control of VSCCs by protein kinase C-linked receptors, not only in paraneurones but presumably also in neurones and other excitable cells.

Simultaneous measurements of cytosolic and mitochondrial Ca2+ transients in HT29 cells

Loading of HT29 cells with the Ca2+ dye fura-2/AM resulted in an nonhomogeneous intracellular distribution of the dye. Cellular compartments with high fura-2 concentrations were identified by correlation with mitochondrial markers, cellular autofluorescence induced by UV, and dynamic measurement of autofluorescence after inhibition of oxidative phosphorylation. Stimulation with carbachol (10(-4) mol/liter) increased cytosolic, nuclear, and mitochondrial Ca2+ activity ([Ca2+]c, [Ca2+]n, and [Ca2+]m, respectively) measured by UV confocal and conventional imaging. Similar results were obtained with a prototype two-photon microscope (Zeiss, Jena, Germany) allowing for fura-2 excitation. The increase of [Ca2+]m lagged behind that of [Ca2+]c and [Ca2+]n by 10-20 s, and after removing the agonist, [Ca2+]m also decreased with a delay. A strong increase of [Ca2+]m occurred only when a certain threshold of [Ca2+]c (around 1 micromol/liter) was exceeded. In a very similar way, ATP, neurotensin, and thapsigargin increased [Ca2+]c and [Ca2+]m. Carbonyl cyanide p-trifluoromethoxyphenylhyrdrazone reversibly reduced the increase of [Ca2+]m. The source of the mitochondrial Ca2+ increase had intra- and extracellular components, as revealed by experiments in low extracellular Ca2+. We conclude that agonist-induced Ca2+ signals are transduced into mitochondria. 1) Mitochondria could serve as a Ca2+ sink, 2) mitochondria could allow the modulation of [Ca2+]c and [Ca2+]n signals, and 3) [Ca2+]m may serve as a stimulatory metabolic signal when a cell is highly stimulated.