Melatonin regulates splenocytes proliferation via IP3-dependent intracellular Ca2+release in seasonally breeding bird,Perdicula asiatica

Elevation of intracellular free Ca(2+) is one of the key triggering signals for T-cell activation by antigen. A remarkable variety of Ca(2+) signals in T cells, ranging from infrequent spikes to sustained oscillations and plateaus, derives from the interactions of multiple Ca(2+) sources and sinks in the cell. Following engagement of the T cell receptor, intracellular channels (IP3 and ryanodine receptors) release Ca(2+) from intracellular stores, and by depleting the stores trigger prolonged Ca(2+) influx through store-operated Ca(2+) (CRAC) channels in the plasma membrane. The amplitude and dynamics of the Ca(2+) signal are shaped by several mechanisms, including K(+) channels and membrane potential, slow modulation of the plasma membrane Ca(2+)-ATPase, and mitochondria that buffer Ca(2+) and prevent the inactivation of CRAC channels. Ca(2+) signals have a number of downstream targets occurring on multiple time scales. At short times, Ca(2+) signals help to stabilize contacts between T cells and antigen-presenting cells through changes in motility and cytoskeletal reorganization. Over periods of minutes to hours, the amplitude, duration, and kinetic signature of Ca(2+) signals increase the efficiency and specificity of gene activation events. The complexity of Ca(2+) signals contains a wealth of information that may help to instruct lymphocytes to choose between alternate fates in response to antigenic stimulation.

The cytoskeleton is a phylogenetically well-preserved structure that plays a key role in cell physiology. Dynamic and differential changes in cytoskeletal organization occur in cellular processes according to the cell type and the specific function. In neurons, microtubules, microfilaments and intermediate filament (IF) rearrangements occur during axogenesis, and neurite formation which eventually differentiate into axons and dendrites to constitute synaptic patterns of connectivity. In epithelial cells, dynamic modifications occur in the three main cytoskeletal components and phosphorylation of cytoskeletal associated proteins takes place during the formation of the epithelial cell monolayer that eventually will transport water. In pathological processes such as neurodegenerative and psychiatric diseases an abnormal cytoskeletal organization occurs. Melatonin, the main product secreted by pineal gland during dark phase of the photoperiod, is capable of influencing microfilament, microtubule and IF organization by acting as a cytoskeletal modulator. In this paper we will summarize the evidence which provides the data that melatonin regulates cytoskeletal organization and we describe recent findings, which indicate that melatonin effects on microfilament rearrangements in stress fibers are involved in the mechanism by which the indole synchronizes water transport in kidney-derived epithelial cells. In addition, we review recent data, which indicates that melatonin protects the neuro-cytoskeletal organization from damage caused by free radicals contributing to cell survival, in addition to the already described mechanism elicited by the indole to prevent apoptosis and to scavenge free radicals. Moreover, we discuss the implications of an altered cytoskeletal organization for neurodegenerative and psychiatric illnesses and its re-establishment by melatonin.

Mel 1a melatonin receptors belong to the super-family of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors. So far, interest in Mel 1a receptor signaling has focused mainly on the modulation of the adenylyl cyclase pathway via pertussis toxin (PTX)-sensitive G proteins. To further investigate signaling of the human Mel 1a receptor, we have developed an antibody directed against the C terminus of this receptor. This antibody detected the Mel 1a receptor as a protein with an apparent molecular mass of approximately 60 kDa in immunoblots after separation by SDS-PAGE. It also specifically precipitated the 2-[125I]iodomelatonin (125I-Mel)-labeled receptor from Mel 1a-transfected HEK 293 cells. Coprecipitation experiments showed that G(i2), G(i3), and G(q/11) proteins couple to the Mel 1a receptor in an agonist-dependent and guanine nucleotide-sensitive manner. Coupling was selective since other G proteins present in HEK 293 cells, (G(i1), G(o), G(s), G(z), and G12) were not detected in receptor complexes. Coupling of the Mel 1a receptor to G(i) and G(q) was confirmed by inhibition of high-affinity 125I-Mel binding to receptors with subtype-selective G protein alpha-subunit antibodies. G(i2) and/or G(i3) mediated adenylyl cyclase inhibition while G(q/11) induced a transient elevation in cytosolic calcium concentrations in HEK 293 cells stably expressing Mel 1a receptors. Melatonin-induced cytosolic calcium mobilization via PTX-insensitive G proteins was confirmed in primary cultures of ovine pars tuberalis cells endogenously expressing Mel 1a receptors. In conclusion, we report the development of the first antibody recognizing the cloned human Mel 1a melatonin receptor protein. We show that Mel 1a receptors functionally couple to both PTX-sensitive and PTX-insensitive G proteins. The previously unknown signaling of Mel 1a receptors through G(q/11) widens the spectrum of potential targets for melatonin.