Specific combinations of G-protein subunits discriminate hormonal signalling in rat pituitary (GH(3)) cells in culture

Biochemical and physiological insights into TRH receptor-mediated signaling

Thyrotropin-releasing hormone (TRH) is an important endocrine agent that regulates the function of cells in the anterior pituitary and the central and peripheral nervous systems. By controlling the synthesis and release of thyroid hormones, TRH affects many physiological functions, including energy homeostasis. This hormone exerts its effects through G protein-coupled TRH receptors, which signal primarily through G_q/11 but may also utilize other G protein classes under certain conditions. Because of the potential therapeutic benefit, considerable attention has been devoted to the synthesis of new TRH analogs that may have some advantageous properties compared with TRH. In this context, it may be interesting to consider the phenomenon of biased agonism and signaling at the TRH receptor. This possibility is supported by some recent findings. Although knowledge about the mechanisms of TRH receptor-mediated signaling has increased steadily over the past decades, there are still many unanswered questions, particularly about the molecular details of post-receptor signaling. In this review, we summarize what has been learned to date about TRH receptor-mediated signaling, including some previously undiscussed information, and point to future directions in TRH research that may offer new insights into the molecular mechanisms of TRH receptor-triggered actions and possible ways to modulate TRH receptor-mediated signaling.

Inhibitory effect of somatostatin-14 on L-type voltage-gated calcium channels in cultured cone photoreceptors requires intracellular calcium

The inhibitory effects of somatostatin have been well documented for many physiological processes. The action of somatostatin is through G-protein-coupled receptor-mediated second-messenger signaling, which in turn affects other downstream targets including ion channels. In the retina, somatostatin is released from a specific class of amacrine cells. Here we report that there was a circadian phase-dependent effect of somatostatin-14 (SS14) on the L-type voltage-gated calcium channels (L-VGCCs) in cultured chicken cone photoreceptors, and our study reveals that this process is dependent on intracellular calcium stores. Application of 500 nM SS14 for 2 h caused a decrease in L-VGCC currents only during the subjective night but not the subjective day. We then explored the cellular mechanisms underlying the circadian phase-dependent effect of SS14. The inhibitory effect of SS14 on L-VGCCs was mediated through the pertussis-toxin-sensitive G-protein-dependent somatostatin receptor 2 (sst2). Activation of sst2 by SS14 further activated downstream signaling involving phospholipase C and intracellular calcium stores. Mobilization of intracellular Ca2+ was required for somatostatin induced inhibition of photoreceptor L-VGCCs, suggesting that somatostatin plays an important role in the modulation of photoreceptor physiology.

An analog of thyrotropin-releasing hormone (TRH) is neuroprotective against glutamate-induced toxicity in fetal rat hippocampal neurons in vitro

TRH has been found to be efficacious in treating certain neurodegenerative disorders such as epilepsy, Alzheimer's disease, neurotrauma and depression, however, its mechanism of action is poorly understood. Since glutamate (Glu) toxicity has been implicated in these disorders, we utilized primary enriched cultures of rat fetal (E 17) hippocampal neurons to test the hypothesis that an analog of TRH, 3-Methyl-Histidine TRH (3Me-H TRH), given concurrently with Glu would protect such neurons against cell damage and cell death. Cell viability was assessed via Trypan Blue exclusion cell counts, and neuronal damage was determined by assaying lactic acid dehydrogenase (LDH) released in the conditioned media. Fetal hippocampal neurons were cultured in neurobasal media for 7 days. On day 7, neurons (10(6)/well) were treated with: control media, 10 microM 3Me-H TRH, 500 microM Glu or 500 microM Glu with either 10, 1, 0.1, 0.01 or 0.001 microM 3Me-H TRH. Both media and neurons were harvested 16 h after treatment. Prolonged exposure to 10 microM 3Me-H TRH was not toxic to the cells, whereas neurons exposed to 500 microM Glu resulted in maximal cell death. Notably, 10, 1 and 0.1 microM 3Me-H TRH, when co-treated with 500 microM Glu, protected fetal neurons against cell death in a concentration-dependent manner. These results provide support for an important neuroprotective effect of TRH/analogs against glutamate toxicity in primary hippocampal neuronal culture and implicate a potentially beneficial role of TRH/analogs in neurodegenerative diseases.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

Cross signaling, cell specificity, and physiology

The literature on intracellular signal transduction presents a confusing picture: every regulatory factor appears to be regulated by all signal transduction cascades and to regulate all cell processes. This contrasts with the known exquisite specificity of action of extracellular signals in different cell types in vivo. The confusion of the in vitro literature is shown to arise from several causes: the inevitable artifacts inherent in reductionism, the arguments used to establish causal effect relationships, the use of less than adequate models (cell lines, transfections, acellular systems, etc.), and the implicit assumption that networks of regulations are universal whereas they are in fact cell and stage specific. Cell specificity results from the existence in any cell type of a unique set of proteins and their isoforms at each level of signal transduction cascades, from the space structure of their components, from their combinatorial logic at each level, from the presence of modulators of signal transduction proteins and of modulators of modulators, from the time structure of extracellular signals and of their transduction, and from quantitative differences of expression of similar sets of factors.