Immunology, Endocrine & Metabolic Agents in Medicinal Chemistry (Formerly Current Medicinal Chemistry - Immunology, Endocrine and Metabolic Agents) - Volume 11, Issue 3, 2011

This Theme Issue entitled ‘Nongenomic effects of thyroid hormones in Central Nervous System and Skeletal Muscle: from zebrafish to man’, is focused on the effects of thyroid hormones that are not mediated by the classical nuclear receptor, but by other receptors located on the plasma membrane or in the cytoplasm, including integrin αvβ3 and the nuclear receptor β. The first contribution is from Paul J. Davis and his group with a paper on the acute modulation of neuronal excitability by thyroid hormones, particularly the sodium current (INa). The results show that T4 inhibits neuronal excitability in a large population of Pre-Frontal Cortex (PFC) pyramidal neurons of young adult rats. T4 at nanomolar concentrations inhibits excitability in rat brain slices with their neuronal circuit intact, as shown by Vm depolarization, spike number decrease, or both in the majority of the recorded neurons. The paper of Yonkers and Ribera shows that in zebrafish embryos thyroxine, T4, but not T3, rapidly upregulates the current amplitude of a specific sodium channel subtype, Nav1.6a. In mammals, the orthologous channel, Nav1.6, is expressed at the nodes of Ranvier. The regulation of Nav1.6a channels involves a plasma membrane receptor, integrin αvβ3, indicative of a nongenomic mechanism. The binding of T4 to this integrin is inhibited by the RGD (Arg-Gly-Asp) peptide, an important functional site recognized by several different integrins. Taken together, these studies show that genomic and nongenomic actions of thyroid hormones may work in concert to regulate the density and the activity of Na+ channels in neurons. The action of T3 is predominantly genomic in the regulation of the expression of Na+ channels in excitable cells, while T4 acts nongenomically to regulate the basal activity of Na+ channels. The modulation of ion channels by thyroid hormones mediated by αvβ3 integrin, G protein, or other receptor/transducer represents a metabotropic-type signal with the involvement of second messengers and/or phosphorylation-dephosphorylation mechanisms, whereas the interaction of thyroid hormones could also be a direct one, as observed with benzodiazepine or neurosteroids for the GABA receptors, and as it appears to be in another paper of this Theme Issue (Puia). The advantage of a direct modulation could be a faster neurotransmission, but this does not seem to be the case from the paper of Yonkers and Ribera, whereas a direct effect could be responsible for the fast modulation observed by Davis and coworkers. The contribution of Puia can be considered complementary, as far as mechanism is concerned, to both Yonkers and Ribera on one side and to that of Davis et al. on the other, since it focuses on fast neurotransmission mediated by glutamate (excitatory) and Gaba (inhibitory in adult, but excitatory during development) ionotropic receptors. The author shows for both types of channels a fast modulation by thyroid hormones with a decrease in both current and frequency of synaptic potentials. These fast nongenomic effects do not depend on kinase modulation or on the involvement of the αvβ3 integrin receptor. This could again be a direct effect: the only one really compatible with very fast neurotransmission that could be highly conserved and important. This direct effect was previously hypothesized by J.V. Martin in 1996 for recombinant GABA ionotropic receptor subunits expressed in synaptoneurosomes. Of course, we need to take into account that the differences in mechanism may depend on the complexity of the model under study, zebrafish vs rat, the brain area and the experimental conditions.....

Acute modulation of ion channel function by a nongenomic mechanism initiated at the plasma membrane is a newly appreciated action of thyroid hormone. However, whether thyroid hormones at physiological concentrations contribute to the setting and dynamic regulation of basal activity of neuron excitability in the mammalian central nervous system is an issue that is not yet well-understood. In this study, we used acute prefrontal cortex (PFC) slices of the young adult rat (6-8 weeks) to explore the action of L-thyroxine (T4) on the excitability of layer V-VI pyramidal neurons. In the basal state of the brain slice model, blood vessel thyroid hormone content was removed prior to study, so that exposure to a bath solution containing T4 may allow identification of rapid-onset effects of T4 on neuronal excitability. Such rapidonset effects are nongenomic in mechanism. We show that in whole-cell current clamp recordings, 18/23 pyramidal neurons responded electrophysiologically to a 3 min bath application of T4 (5-37.5 nM as directly-measured free hormone). Among the T4-responsive neurons (18/23 cells), 94% showed an average 12.4 mV membrane potential (Vm) depolarization and 72% showed an average spike of 45%. These results indicate that T4 can rapidly modulate neuronal excitability.

Thyroid hormones modulate ion flux across cellular membranes by effects on both transporters and channels. One such mechanism involves modulation of voltage-gated sodium current in spinal neurons by thyroxine. Experiments in zebrafish embryos have shown that thyroxine rapidly upregulates the current amplitude of a specific sodium channel subtype, nav1.6a. In mammals, the orthologous channel, Nav1.6, is expressed at nodes of Ranvier. In contrast to traditional thyroid hormone mechanisms that depend on nuclear receptors and regulation of gene expression, the regulation of nav1.6a channels involves a plasma membrane receptor, the integrin dimer αVβ3, indicative of a nongenomic mechanism. Although acute changes in thyroxine levels lead to rapid changes in nav1.6a function in vivo, the concentration of thyroxine in vivo is relatively stable in the absence of experimental intervention. On this basis, the endogenous concentration of thyroxine provides on-going regulation of voltage-gated sodium channel function through interaction with integrin αVβ3. This interaction is evident with blockade of the thyroxine-αVβ3 pathway, via antagonists of thyroid hormone (tetrac) or the integrin receptor (LM609 antibody). These interventions also produce significant changes in biologically relevant zebrafish behavior. In summary, thyroxine binding to integrin αVβ3 regulates voltage-gated sodium current in the embryonic nervous system and serves as an important mechanism by which thyroid hormones influence development and behavior.

Thyroid hormones 3,5,3'-triiodo-L-thyronine (T3) and thyroxine (T4) are important metabolic and physiological regulators of many tissues within the body during development and in adulthood. These effects have largely been attributed to the modulation of thyroid hormone receptor-dependent gene transcription. However, nongenomic actions of thyroid hormones are emerging in a number of cell types. These actions require a plasma membrane receptor or nuclear receptors located in cytoplasm. Several studies propose that this actions are initiated by activation of N-methyl-D-aspartate (NMDA)-, γ-amino butyric acid (GABA)- or G protein-coupled receptors. Recently, the receptor integrin αVβ3 at the Arg-Gly-Asp recognition site has been described to mediate actions of T3 and T4. The hormone-evoked signal is transduced downstream Ca2+ mobilization or monomeric GTPase activation through different kinase pathways, including protein kinase A, protein kinase C and the mitogen-activated protein kinases (MAPKs), into complex cellular/nuclear events. These findings support the contention that thyroid hormones have a variety of nongenomic pathways regulating many important aspects of cell physiology. Among the most relevant nongenomic effects of thyroid hormones is the regulation of the cytoskeleton. This review focuses on the current knowledge and important results on the mechanisms of nongenomic thyroid action with special emphasis on the cytoskeletal proteins and their differential regulation by kinases and Ca2+- mediated mechanisms in developmental rat brain. The roles of Ca2+ mobilization and subsequent cytoskeletal protein phosphorylation are crucial biological markers that have been shown to be the essence of brain development and preservation of cognitive functions.

Ligand gated ionotropic receptors are responsible for the fast neurotransmission in the brain and are the target of widely used drugs, such as anxiolytics, anticonvulsant and antidepressant, and of endogenously produced substances, e.g. hormones. In this review, the fast regulatory effects of thyroid hormones (THs; T3 and T4) on ligand gated ion channels will be analyzed and discussed. More precisely, the focus will be on receptors that mediate the majority of inhibitory and excitatory neurotransmission in the brain, respectively γ-aminobutyric acid (GABA) and glutamate ionotropic receptors. Brain is an important target for THs, as proved by the profound alterations in its structure and functionality related to their imbalance. Indeed, dysthyroidism is often associated with several neuropsychiatric disorders that derive also from a dysfunction of GABAergic and glutamatergic neurotransmission. The molecular mechanisms responsible for these changes are not completely clarified yet. THs, through the binding of nuclear receptors, can modulate the expression of proteins directly or indirectly involved in neurotransmission; in addition, their non genomic fast modulation of ionotropic receptors mediating excitatory and inhibitory neurotransmission could also play an important role. THs modulate recombinant and native receptors activated by exogenous GABA or glutamate as well as synaptic and extrasynaptic excitatory and inhibitory neurotransmission. These non genomic effects do not depend on protein phosphorylation or on the membrane integrin receptor αVβ3 activation. Keeping in mind that a local regulation of THs levels can occur in different brain regions the fast modulation of THs on synaptic activity could provide a rapid and efficacious control of the function of several brain circuitries.

The signals initiated by skeletal muscle contraction and exercise involve several biochemical mechanisms both in the short- and long-time range that are under the control of thyroid hormones as genomic as well as nongenomic mechanisms. The effect of thyroid hormones on skeletal muscle has been studied both in cells in culture and in vivo. In L- 6 myoblasts thyroid hormone increases intracellular calcium and pH. Administration of a single injection of thyroid hormone in rat i.p. leads to a rapid rise in T3 about 50-fold, that peaked at 2 hours. The effect of this treatment was evaluated on signaling kinases, in particular on the phosphorylation of the p38MAPK pathway, and AMP-activated protein kinase. Both were activated and both can be a downstream target of the integrin αvβ3, the reported plasma membrane receptor for thyroid hormones. Contractile activity leads to mitochondrial biogenesis within skeletal muscle, which gives rise to signaling pathways leading to the expression of transcription factors. These are activated by a Ca2+-dependent release, p38 and AMP-activated protein kinase. The activated transcription factors such as nuclear respiratory factor 1, interact with the coactivator PGC-1α to stimulate the expression of nuclear genes encoding mitochondrial proteins. This sequence of events can be induced not only by physical exercise, but also by thyroid hormone (T3) treatment in vivo, particularly in slow twitch skeletal muscle. In conclusion, the old statement that thyroid hormone helps to keep the steady state, is still valid, furthermore it contributes to a good performance of skeletal muscle as exercise does.

Antibodies for therapy and diagnostics make up one of the fastest growing segments of the biopharmaceutical market. This review provides an overview of methods and processes for the selection and production of therapeutic and diagnostic recombinant proteins, including monoclonal and polyclonal antibodies. This overview also includes a comparative economic summary of current and some potential expression systems for manufacture of these highly functional recombinant proteins and antibodies, and explores the future direction of the antibody industry.

Vitamin D is a hormone precursor, originally provided by ultraviolet light-induced skin synthesis of cholecalciferol but, with modern lifestyles, humans depend on dietary intake [mainly as ergocalciferol]; adequate intakes are essential for musculo-skeletal health, the ‘classical’ benefits of this vitamin, modulated by the activated hormonal metabolite, calcitriol. Most other tissues also contain vitamin D receptors and it's activating enzyme. Hypovitaminosis D is associated with many disease risks and many protective mechanisms are known that explain this; in vitro and in vivo repletion or supplementation activating such mechanisms [e.g. in inflammation, atherosclerosis & malignancy]. This review outlines knowledge on the roles of vitamin in reducing all-cause mortality and ameliorating disease [including metabolic syndrome, insulin resistance and secretion, diabetes, hypertension, cardiovascular disease, autoimmunity, arthritis, multiple sclerosis, periodontal disease, inflammatory bowel disease, asthma and respiratory infections, tuberculosis, nonalcoholic fatty liver disease, polycystic ovary syndrome and fertility, wound healing, psoriasis and sepsis]. Interactions of vitamin D with calcium & vitamin A and variation in effects with genetic polymorphisms are referred to. Reasons for the continuing high prevalences of vitamin D insufficiency world-wide and current difficulties in defining both vitamin D status and adequate repletion are discussed. It is suggested that whilst calcitriol analogues may have increasing roles in treatment, adequate vitamin D repletion at the population level is likely to provide a very cost-effective measure for health promotion. However, adequate randomized controlled trials are needed before larger doses than currently used are considered for long-term use at the population level.

Prolactin-releasing peptide (PrRP) was first isolated from bovine hypothalamus as an orphan G-protein-coupled receptor using the strategy of reverse pharmacology. PrRP is expressed specifically in the human pituitary and is identified in the hypothalamus as a potent prolactin-releasing factor (PRF) for anterior pituitary cells. The initial studies showed that PrRP was a potent and specific prolactin-releasing factor; however our studies in the pituitary demonstrated that PrRP is more important such as modulator of prolactin release mediated by thyroid-releasing hormone (TRH) than PRF itself. However, physiological studies indicated that PrRP might play a wide range of roles in neuroendocrinology other than prolactin release, i.e., sleep regulation, metabolic homeostasis, food intake, stress responses or cardiovascular regulation. Over 150 papers and several patents have been published on this subject since its initial discovery in 1998. Herein, I review the state of current knowledge of the PrRP system, especially its roles in brain functions and implications for therapy.