Nerve growth factor affects Ca2+ currents via the p75 receptor to enhance prolactin mRNA levels in GH3 rat pituitary cells

Nerve growth factor: a neuroimmune crosstalk mediator for all seasons

Neurotrophic factors comprise a broad family of biomolecules - most of which are peptides or small proteins - that support the growth, survival and differentiation of both developing and mature neurons. The prototypical example and best-characterized neurotrophic factor is nerve growth factor (NGF), which is widely recognized as a target-derived factor responsible for the survival and maintenance of the phenotype of specific subsets of peripheral neurons and basal forebrain cholinergic nuclei during development and maturation. In addition to being active in a wide array of non-nervous system cells, NGF is also synthesized by a range of cell types not considered as classical targets for innervation by NGF-dependent neurons; these include cells of the immune-haematopoietic lineage and populations in the brain involved in neuroendocrine functions. NGF concentrations are elevated in numerous inflammatory and autoimmune states such as multiple sclerosis, chronic arthritis, systemic lupus erythematosus and mastocytosis, in conjunction with increased accumulation of mast cells. Intriguingly, NGF seems to be linked also with diabetic pathology and insulin homeostasis. Mast cells and NGF appear involved in neuroimmune interactions and tissue inflammation. As mast cells are capable of producing and responding to NGF, this suggests that alterations in mast cell behaviour could provoke maladaptive neuroimmune tissue responses, including those of an autoimmune nature. Moreover, NGF exerts a modulatory role on sensory nociceptive nerve physiology in the adult, which appears to correlate with hyperalgesic phenomena occurring in tissue inflammation. NGF can therefore be viewed as a multifactorial modulator of neuro-immune-endocrine functions.

Chronic atrial ionic remodeling by aldosterone: potentiation of L-type Ca2+ channels and its arrhythmogenic significance

It is widely accepted that aldosterone induces atrial fibrillation (AF) by promoting structural changes, but its effects on the function of primary atrial myocytes remain unknown. We have investigated this point in adult rat atrial myocytes, chronically exposed to the hormone. This treatment produced larger amplitude of Ca²⁺ transients, longer action potential (AP) duration, and higher incidence of unsynchronized Ca²⁺ oscillations. Moreover, it also gave rise to increases in both cell membrane capacitance (C_m, 30 %) and activity of L-type Ca²⁺ channels (LTCCs, 100 %). Concerning K⁺ currents, a twofold increase was also observed, but only in a delayed rectifier component (I_Ksus). Interestingly, the maximal conductance (G_max) of Na⁺ channels was also enhanced, but it occurred in the face of a negative shift in the voltage dependence of inactivation. Thus, at physiological potentials, a decreased fraction of available channels neutralized the effect on G_Na-max. With regard to the effects on both C_m and LTCCs, they involved activation of mineralocorticoid receptors (MRs), were dose-dependent (EC₅₀ ∼20-130 nM), and developed and recovered in days. Neither gating currents nor protein levels of LTCCs were altered. Instead, the effect on LTCCs was mimicked by cAMP, reverted by a PKA inhibitor, and attenuated by a nitric oxide donor (short-term exposures). Both EGTA and the antioxidant NAC prevented the increase in C_m, without significantly interfering with the upregulation of LTCCs. Overall, these results show that chronic exposures to aldosterone result in dire functional changes at the single myocyte level, which may explain the link between aldosteronism and AF.

Nerve growth factor sensitizes adult sympathetic neurons to the proinflammatory peptide bradykinin

Levels of nerve growth factor (NGF) are elevated in inflamed tissues. In sensory neurons, increases in NGF augment neuronal sensitivity (sensitization) to noxious stimuli. Here, we hypothesized that NGF also sensitizes sympathetic neurons to proinflammatory stimuli. We cultured superior cervical ganglion (SCG) neurons from adult male Sprague Dawley rats with or without added NGF and compared their responsiveness to bradykinin, a proinflammatory peptide. The NGF-cultured neurons exhibited significant depolarization, bursts of action potentials, and Ca(2+) elevations after bradykinin application, whereas neurons cultured without NGF showed only slight changes in membrane potential and cytoplasmic Ca(2+) levels. The NGF effect, which requires trkA receptors, takes hours to develop and days to reverse. We addressed the ionic mechanisms underlying this sensitization. NGF did not alter bradykinin-induced M-current inhibition or phosphatidylinositol 4,5-bisphosphate hydrolysis. Maxi-K channel-mediated current evoked by depolarizations was reduced by 50% by culturing neurons in NGF. Application of iberiotoxin or paxilline, blockers of Maxi-K channels, mimicked NGF treatment and sensitized neurons to bradykinin application. A calcium channel blocker also mimicked NGF treatment. We found that NGF reduces Maxi-K channel opening by decreasing the activity of nifedipine-sensitive calcium channels. In conclusion, culture in NGF reduces the activity of L-type calcium channels, and secondarily, the calcium-sensitive activity of Maxi-K channels, rendering sympathetic neurons electrically hyper-responsive to bradykinin.

Upregulation of voltage-gated calcium channel cav1.3 in bovine somatotropes treated with ghrelin

Activation of the growth hormone (GH) secretagogue receptor (GHS-R) by synthetic GH releasing peptides (GHRP) or its endogenous ligand (Ghrelin) stimulates GH release. Though much is known about the signal transduction underlying short-term regulation, there is far less information on the mechanisms that produce long-term effects. In the current report, using an enzyme-linked immunosorbent assay for GH detection and whole-cell patch-clamp recordings, we assessed the long-term actions of such regulatory factors on voltage-activated Ca²⁺ currents in bovine somatotropes (BS) separated on a Percoll gradient and detected by immunohistochemistry. After 24 h of treatment with Ghrelin (10 nM) or GHRP-6 (100 nM) enhanced BS secretory activity; GH secretion stimulated by GHS through the activation of GHS-R because treatment with the antagonist of GHS-R (D-Lys3-GHRP-6, 10 μM) blocked the GH secretion, and the effect was dose and time dependent (24, 48, and 72 h). GH secretion stimulated by GHRP-6 was abolished by nifedipine (0.5 μM), a blocker of L-type HVA Ca²⁺ channels, and KN-62 (10 μM), an inhibitor of Ca²⁺/CaM-KII. After 72 h in culture, all recorded BS exhibited two main Ca²⁺ currents: a low voltage-activated (LVA; T-type) and a high voltage-activated (HVA; mostly dihydropyridine-sensitive L-type) current. Interestingly, HVA and LVA channels were differentially upregulated by Ghrelin. Chronic treatment with the GHS induced a significant selective increase on the Ba²⁺ current through HVA Ca²⁺ channels, and caused only a small increase of currents through LVA channels. The stimulatory effect on HVA current density was accompanied by an augment in maximal conductance with no apparent changes in the kinetics and the voltage dependence of the Ca²⁺ currents, suggesting an increase in the number of functional channels in the cell membrane. Lastly, in consistency with the functional data, quantitative real-time RT-PCR revealed transcripts encoding for the Ca_v1.2 and Ca_v1.3 pore-forming subunits of L-type channels. The treatment with Ghrelin significantly increased the Ca_v1.3 subunit expression, suggeting that the chronic stimulation of the GHS receptor with Ghrelin or GHRP-6 increases the number of voltage-gated Ca²⁺ channels at the cell surface of BS.

Postsynaptic TRPC1 function contributes to BDNF-induced synaptic potentiation at the developing neuromuscular junction

Brain-derived neurotrophic factor (BDNF) induces synaptic potentiation at both neuromuscular junctions (NMJs) and synapses of the CNS through a Ca2+ -dependent pathway. The molecular mechanism underlying BDNF-induced synaptic potentiation, especially the regulation of Ca2+ dynamics, is not well understood. Using the Xenopus NMJ in culture as a model system, we show that pharmacological inhibition or morpholino-mediated knockdown of Xenopus TRPC1 (XTRPC1) significantly attenuated the BDNF-induced potentiation of the frequency of spontaneous synaptic responses at the NMJ. Functionally, XTRPC1 was required specifically in postsynaptic myocytes for BDNF-induced Ca2+ elevation and full synaptic potentiation at the NMJ, suggesting a previously underappreciated postsynaptic function of Ca2+ signaling in neurotrophin-induced synaptic plasticity, in addition to its well established role at presynaptic sites. Mechanistically, blockade of the p75 neurotrophin receptor abolished BDNF-induced postsynaptic Ca2+ elevation and restricted BDNF-induced synaptic potentiation, while knockdown of the TrkB receptor in postsynaptic myocytes had no effect. Our study suggests that BDNF-induced synaptic potentiation involves coordinated presynaptic and postsynaptic responses and identifies TRPC1 as a molecular mediator for postsynaptic Ca2+ elevation required for BDNF-induced synaptic plasticity.

Regulation of Ca v 3.1 channels by glucocorticoids

The activity of low voltage-activated Ca(2+) (Ca(V)3) channels is tightly coupled to neurotransmitter and hormone secretion. Previous studies have shown that Ca(V)3 channels are regulated by glucocorticoids (GCs), though the mechanism underlying channel regulation remains unclear. Here, using the pituitary GH(3) cell line as a model, we investigated whether Ca(V)3 channel expression is under the control of GCs, and if their actions are mediated by transcriptional and/or post-transcriptional mechanisms. RT-PCR and western blot analyses showed that Ca(V)3.1 but not Ca(V)3.2 and Ca(V)3.3 channels is expressed in the GH(3) cells, and patch clamp recordings confirmed that Ca(2+) currents through low voltage-activated channels were decreased after chronic treatment with GCs. Consistent with this, total plasma membrane expression of Ca(V)3.1 protein as analyzed by cell-surface biotinylation assays and semi-quantitative western blotting was also down-regulated, while quantitative real-time RT-PCR analysis revealed a significant decrease of Ca(V)3.1 mRNA expression in the treated cells. In contrast, patch-clamp recordings on HEK-293 cells stably expressing recombinant Ca(V)3.1 channels showed that Ca(2+) currents were not affected by GC treatment. These results suggest that decreased transcription is a likely mechanism to explain the inhibitory actions of GCs on the functional expression of native Ca(V)3.1 channels.

Up-regulation of high voltage-activated Ca(2+) channels in GC somatotropes after long-term exposure to ghrelin and growth hormone releasing peptide-6

Activation of the growth hormone (GH)-secretagogue receptor (GHS-R) by synthetic GH-releasing peptides (GHRP) or its endogenous ligand (ghrelin) stimulates GH release. Though much is known about the signal transduction underlying short-term regulation, there is far less information on mechanisms that produce long-term effects. In the current report, using whole-cell patch-clamp recordings, we assessed the long-term actions of such regulatory factors on voltage-activated Ca(2+) currents in GH-secreting cells derived from a rat pituitary tumour (GC cell line). After 96 h in culture, all recorded GC somatotropes exhibited two main Ca(2+) currents: a medium voltage-activated (MVA; T/R-type) and a high voltage-activated (HVA; mostly dihydropyridine-sensitive L-type) current. Interestingly, L- and non-L-type channels were differentially up-regulated by GHRP-6 and ghrelin. Chronic treatment with the GHS induced a significant selective increase on Ba(2+) current through HVA Ca(2+) channels, and caused only a modest increase of currents through MVA channels. Consistent with this, in presence of D-(Lys(3))-GHRP-6, a specific antagonist of the GHS-R, the increase in HVA Ca(2+) channel activity after chronic treatment with the GHS was abolished. The stimulatory effect on HVA current density evoked by the secretagogues was accompanied by an augment in maximal conductance with no apparent changes in the kinetics and the voltage dependence of the Ca(2+) currents, suggesting an increase in the number of functional channels in the cell membrane. Lastly, in consistency with the functional data, quantitative real-time RT-PCR revealed that the expression level of transcripts encoding for the Ca(V)1.3 pore-forming subunit of the L-type channels was significantly increased after chronic treatment of the GC cells with ghrelin.

Paracrinicity: the story of 30 years of cellular pituitary crosstalk

Living organisms represent, in essence, dynamic interactions of high complexity between membrane-separated compartments that cannot exist on their own, but reach behaviour in co-ordination. In multicellular organisms, there must be communication and co-ordination between individual cells and cell groups to achieve appropriate behaviour of the system. Depending on the mode of signal transportation and the target, intercellular communication is neuronal, hormonal, paracrine or juxtacrine. Cell signalling can also be self-targeting or autocrine. Although the notion of paracrine and autocrine signalling was already suggested more than 100 years ago, it is only during the last 30 years that these mechanisms have been characterised. In the anterior pituitary, paracrine communication and autocrine loops that operate during fetal and postnatal development in mammals and lower vertebrates have been shown in all hormonal cell types and in folliculo-stellate cells. More than 100 compounds have been identified that have, or may have, paracrine or autocrine actions. They include the neurotransmitters acetylcholine and gamma-aminobutyric acid, peptides such as vasoactive intestinal peptide, galanin, endothelins, calcitonin, neuromedin B and melanocortins, growth factors of the epidermal growth factor, fibroblast growth factor, nerve growth factor and transforming growth factor-beta families, cytokines, tissue factors such as annexin-1 and follistatin, hormones, nitric oxide, purines, retinoids and fatty acid derivatives. In addition, connective tissue cells, endothelial cells and vascular pericytes may influence paracrinicity by delivering growth factors, cytokines, heparan sulphate proteoglycans and proteases. Basement membranes may influence paracrine signalling through the binding of signalling molecules to heparan sulphate proteoglycans. Paracrine/autocrine actions are highly context-dependent. They are turned on/off when hormonal outputs need to be adapted to changing demands of the organism, such as during reproduction, stress, inflammation, starvation and circadian rhythms. Specificity and selectivity in autocrine/paracrine interactions may rely on microanatomical specialisations, functional compartmentalisation in receptor-ligand distribution and the non-equilibrium dynamics of the receptor-ligand interactions in the loops.