Paola Rebellato

Local Control of Nuclear Calcium Signaling in Cardiac Myocytes by Perinuclear Microdomains of Sarcolemmal Insulin-Like Growth Factor 1 Receptors

Rationale: The ability of a cell to independently regulate nuclear and cytosolic Ca2+ signaling is currently attributed to the differential distribution of inositol 1,4,5-trisphosphate receptor channel isoforms in the nucleoplasmic versus the endoplasmic reticulum. In cardiac myocytes, T-tubules confer the necessary compartmentation of Ca2+ signals, which allows sarcomere contraction in response to plasma membrane depolarization, but whether there is a similar structure tunneling extracellular stimulation to control nuclear Ca2+ signals locally has not been explored. Objective: To study the role of perinuclear sarcolemma in selective nuclear Ca2+ signaling. Methods and Results: We report here that insulin-like growth factor 1 triggers a fast and independent nuclear Ca2+ signal in neonatal rat cardiac myocytes, human embryonic cardiac myocytes, and adult rat cardiac myocytes. This fast and localized response is achieved by activation of insulin-like growth factor 1 receptor signaling complexes present in perinuclear invaginations of the plasma membrane. The perinuclear insulin-like growth factor 1 receptor pool connects extracellular stimulation to local activation of nuclear Ca2+ signaling and transcriptional upregulation through the perinuclear hydrolysis of phosphatidylinositol 4,5-biphosphate inositol 1,4,5-trisphosphate production, nuclear Ca2+ release, and activation of the transcription factor myocyte-enhancing factor 2C. Genetically engineered Ca2+ buffers—parvalbumin—with cytosolic or nuclear localization demonstrated that the nuclear Ca2+ handling system is physically and functionally segregated from the cytosolic Ca2+ signaling machinery. Conclusions: These data reveal the existence of an inositol 1,4,5-trisphosphate–dependent nuclear Ca2+ toolkit located in direct apposition to the cell surface, which allows the local control of rapid and independent activation of nuclear Ca2+ signaling in response to an extracellular ligand.

Local Control of Nuclear Ca2+ Signalling in Cardiac Myocytes by Perinuclear Microdomains of Sarcolemmal IGF-1 Receptors

Rationale: The ability of a cell to independently regulate nuclear and cytosolic Ca2+ signaling is currently attributed to the differential distribution of inositol 1,4,5-trisphosphate receptor channel isoforms in the nucleoplasmic versus the endoplasmic reticulum. In cardiac myocytes, T-tubules confer the necessary compartmentation of Ca2+ signals, which allows sarcomere contraction in response to plasma membrane depolarization, but whether there is a similar structure tunneling extracellular stimulation to control nuclear Ca2+ signals locally has not been explored. Objective: To study the role of perinuclear sarcolemma in selective nuclear Ca2+ signaling. Methods and Results: We report here that insulin-like growth factor 1 triggers a fast and independent nuclear Ca2+ signal in neonatal rat cardiac myocytes, human embryonic cardiac myocytes, and adult rat cardiac myocytes. This fast and localized response is achieved by activation of insulin-like growth factor 1 receptor signaling complexes present in perinuclear invaginations of the plasma membrane. The perinuclear insulin-like growth factor 1 receptor pool connects extracellular stimulation to local activation of nuclear Ca2+ signaling and transcriptional upregulation through the perinuclear hydrolysis of phosphatidylinositol 4,5-biphosphate inositol 1,4,5-trisphosphate production, nuclear Ca2+ release, and activation of the transcription factor myocyte-enhancing factor 2C. Genetically engineered Ca2+ buffers—parvalbumin—with cytosolic or nuclear localization demonstrated that the nuclear Ca2+ handling system is physically and functionally segregated from the cytosolic Ca2+ signaling machinery. Conclusions: These data reveal the existence of an inositol 1,4,5-trisphosphate–dependent nuclear Ca2+ toolkit located in direct apposition to the cell surface, which allows the local control of rapid and independent activation of nuclear Ca2+ signaling in response to an extracellular ligand.

Neural progenitors organize in small-world networks to promote cell proliferation

Significance Synchronized activity among groups of interconnected cells is essential for diverse functions in the brain. Most studies on neuronal networks have been performed in the mature brain when chemical synapses have been established. However, less is known about networking during embryonic development. We have studied neural progenitors and found that they form gap junction-mediated small-world networks, which, via electrical depolarization, drive spontaneous calcium activity to stimulate cell proliferation. Our data underscore the critical role of intricate cell signaling during embryonic development and show that complex networks of immature cells exist in the brain before birth. Coherent network activity among assemblies of interconnected cells is essential for diverse functions in the adult brain. However, cellular networks before formations of chemical synapses are poorly understood. Here, embryonic stem cell-derived neural progenitors were found to form networks exhibiting synchronous calcium ion (Ca2+) activity that stimulated cell proliferation. Immature neural cells established circuits that propagated electrical signals between neighboring cells, thereby activating voltage-gated Ca2+ channels that triggered Ca2+ oscillations. These network circuits were dependent on gap junctions, because blocking prevented electrotonic transmission both in vitro and in vivo. Inhibiting connexin 43 gap junctions abolished network activity, suppressed proliferation, and affected embryonic cortical layer formation. Cross-correlation analysis revealed highly correlated Ca2+ activities in small-world networks that followed a scale-free topology. Graph theory predicts that such network designs are effective for biological systems. Taken together, these results demonstrate that immature cells in the developing brain organize in small-world networks that critically regulate neural progenitor proliferation.

Perfluorooctane sulfonate induces neuronal and oligodendrocytic differentiation in neural stem cells and alters the expression of PPARγ in vitro and in vivo

Perfluorinated compounds are ubiquitous chemicals of major concern for their potential adverse effects on the human population. We have used primary rat embryonic neural stem cells (NSCs) to study the effects of perfluorooctane sulfonate (PFOS) on the process of NSC spontaneous differentiation. Upon removal of basic fibroblast growth factor, NSCs were exposed to nanomolar concentrations of PFOS for 48 h, and then allowed to differentiate for additional 5 days. Exposure to 25 or 50 nM concentration resulted in a lower number of proliferating cells and a higher number of neurite-bearing TuJ1-positive cells, indicating an increase in neuronal differentiation. Exposure to 50 nM also significantly increased the number of CNPase-positive cells, pointing to facilitation of oligodendrocytic differentiation. PPAR genes have been shown to be involved in PFOS toxicity. By q-PCR we detected an upregulation of PPARγ with no changes in PPARα or PPARδ genes. One of the downstream targets of PPARs, the mitochondrial uncoupling protein 2 (UCP2) was also upregulated. The number of TuJ1- and CNPase-positive cells increased after exposure to PPARγ agonist rosiglitazone (RGZ, 3 μM) and decreased after pre-incubation with the PPARγ antagonist GW9662 (5 μM). RGZ also upregulated the expression of PPARγ and UCP2 genes. Meanwhile GW9662 abolished the UCP2 upregulation and decreased Ca²⁺ activity induced by PFOS. Interestingly, a significantly higher expression of PPARγ and UCP3 genes was also detected in mouse neonatal brain after prenatal exposure to PFOS. These data suggest that PPARγ plays a role in the alteration of spontaneous differentiation of NSCs induced by nanomolar concentrations of PFOS.

RET PLCγ Phosphotyrosine Binding Domain Regulates Ca2+ Signaling and Neocortical Neuronal Migration

The receptor tyrosine kinase RET plays an essential role during embryogenesis in regulating cell proliferation, differentiation, and migration. Upon glial cell line-derived neurotrophic factor (GDNF) stimulation, RET can trigger multiple intracellular signaling pathways that in concert activate various downstream effectors. Here we report that the RET receptor induces calcium (Ca2+) signaling and regulates neocortical neuronal progenitor migration through the Phospholipase-C gamma (PLCγ) binding domain Tyr1015. This signaling cascade releases Ca2+ from the endoplasmic reticulum through the inositol 1,4,5-trisphosphate receptor and stimulates phosphorylation of ERK1/2 and CaMKII. A point mutation at Tyr1015 on RET or small interfering RNA gene silencing of PLCγ block the GDNF-induced signaling cascade. Delivery of the RET mutation to neuronal progenitors in the embryonic ventricular zone using in utero electroporation reveal that Tyr1015 is necessary for GDNF-stimulated migration of neurons to the cortical plate. These findings demonstrate a novel RET mediated signaling pathway that elevates cytosolic Ca2+ and modulates neuronal migration in the developing neocortex through the PLCγ binding domain Tyr1015.

Non–Dioxin-like Polychlorinated Biphenyls Interfere with Neuronal Differentiation of Embryonic Neural Stem Cells

Developmental exposure to food contaminants, such as polychlorinated biphenyls (PCBs), has been considered as a possible cause of neurodevelopmental disorders. We have investigated the effects of noncytotoxic concentrations of PCBs 153 and 180 on spontaneous differentiation of rat embryonic neural stem cells (NSCs). Upon removal of basic fibroblast growth factor to induce spontaneous differentiation, cells were exposed to 100 nM of the selected PCBs for 48 h and analyzed after 5 days. Both PCBs 153 and 180 induced a significant increase in the number of neurite-bearing Tuj1-positive cells with a concomitant decrease in proliferating cells, as detected by FUCCI transfection and EdU staining. Measurements of spontaneous Ca²⁺ oscillations showed a decreased number of cells with Ca²⁺ activity after PCB exposure, further confirming the increase in neuronal cells. Conversely, exposure to methylmercury (MeHg), which we evaluated in parallel, led to an increased number of cells with Ca²⁺ activity, in agreement with the previously observed inhibition of neuronal differentiation. Analysis with quantitative PCR of the Notch pathway revealed that PCBs have a repressive action on Notch signaling, whereas MeHg activates it. Altogether, the data indicate that nanomolar concentrations of the selected non-dioxin-like PCBs and MeHg interfere in opposite directions with neuronal spontaneous differentiation of NSCs through Notch signaling. Combined exposures to PCBs and MeHg resulted in an induction of apoptosis and an antagonistic interaction on spontaneous neuronal differentiation. NSCs are further proven to be a valuable in vitro model to identify potential developmental neurotoxicants.

Small-world networks of spontaneous Ca2+ activity

Synchronized network activity among groups of interconnected cells is essential for diverse functions in the brain. However, most studies have been made on cellular networks in the mature brain when chemical synapses have been formed. Much less is known about the situation earlier in development. When studying neural progenitors derived from embryonic stem cells and neural progenitors from mice embryos, we found networks of gap junction coupled cells with vivid spontaneous non-random calcium (Ca2+) activity driven by electrical depolarization that stimulated cell growth. Network activity was revealed by single-cell live Ca2+ imaging and further analyzed for correlations and network topology. The analysis revealed the networks to have small-world characteristics with scale-free properties. Taken together, these results demonstrate that immature cells in the developing brain organize in small-world networks that critically regulate neural progenitor proliferation.

[6]-Shogaol Induces Ca2+ Signals by Activating the TRPV1 Channels in the Rat Insulinoma INS-1E Cells

CONTEXT [6]-shogaol is a vanilloid compound present in steamed ginger (Zingiber officinale), a commonly used spice. Pancreatic beta-cells respond to nutrients like glucose, amino acids and fatty acids, by an increase in the cytoplasmic free Ca²⁺ concentration ([Ca²⁺](i)), which mediates diverse cellular processes in these cells. Some vanilloid compounds activate the transient receptor potential vanilloid receptor type 1 (TRPV1) channel. OBJECTIVE We investigated whether [6]-shogaol could trigger Ca²⁺ signals in the beta-cell. METHODS [Ca²⁺](i) was measured from single INS-1E cells by microscope-based fluorometry using fura-2 as the Ca²⁺ indicator. RESULTS In fura-2 loaded single rat insulinoma INS-1E cells, a widely used model of beta-cell, [6]-shogaol increased [Ca²⁺](i) in a concentration-dependent manner. [Ca²⁺](i) increase by [6]-shogaol was completely blocked when Ca²⁺ was omitted from the extracellular medium. Capsazepine, an inhibitor of the TRPV1 ion channel completely inhibited the [6]-shogaol-induced [Ca²⁺](i) increase. [Ca²⁺](i) increase obtained by 1 µM [6]-shogaol was greater than that obtained by 10 mM glucose. Moreover, a sub-stimulatory concentration of [6]-shogaol (300 nM), significantly enhanced the glucose-induced [Ca²⁺](i) increase in these cells. CONCLUSION We conclude that [6]-shogaol induces Ca²⁺ signals in the beta-cell by activating the TRPV1 channels, and it sensitizes the beta-cells to stimulation by glucose.

Reply to Comments on the article "[6]-Shogaol induced calcium signal in rat insulinoma cells".

The [Ca]i increase obtained by 1 μM [6]-shogaol was on the average greater than that obtained by 10 mM glucose. However, this observation per se is not important because [Ca]i increases by both [6]shogaol and glucose were dose dependent. Thus, comparing the effect of one particular concentration of [6]-shogaol with the effect of one particular concentration of glucose is not so useful. It should be noted that we used rat insulinoma cells which are less glucose responsive compared to the primary β-cells.