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Living organisms can anticipate predictable environmental cycles using self-sustained pacemakers found in the brain and in a variety of tissues. This internal clock generates rhythms that can persist even in the absence of environmental time cues. For the circadian clock to be functional it must have the ability to entrain, i.e. to respond to environmental cues such as the 24 h light-dark cycle. The clock mechanism involves negative feedback loops associated with daily oscillations of several genes expression at both the RNA and protein levels. The mammalian Aryl Hydrocarbon Receptor Nuclear Translocator (ARNT) homologous gene tango (tgo) is essential for normal Drosophila melanogaster development. TANGO (TGO) is a transcription factor, which belongs to the PAS (Per-Arnt-Sim) superfamily and, in the yeast-two-hybrid system, physically interacts with the circadian proteins CLOCK (CLK) and CRYPTOCHROME (CRY), suggesting a possible role for tgo in the circadian clock. Miss-expression of tgo does not interfere with the 24h period of locomotor activity but causes abnormal behavioural responses to light. However, the generalised reduction of TGO promotes an internal desynchronization between the morning and evening clock oscillators, suggesting that TGO might be involved in neuronal crosstalk. Moreover, miss-expression of tgo seems to promote an advance in the phase of expression of the TIMELESS protein, a key regulator of circadian light-responses in Drosophila. Finally, TGO was shown to be involved in sleep regulation, as revealed by a reduction in the amount of sleep in flies overexpressing or down-regulating this protein. In conclusion, this study indicates that TGO does not control the inner molecular cogs of the clock but strongly suggests a potential function for TGO in the input and/or output mechanisms and also a role in the sleep regulation process.

Melanopsin containing intrinsically photosensitive retinal ganglion cells (ipRGCs) are primarily sensitive blue light (- 480 nm) irradiance detectors that mediate non-visual responses (NVRs) such as melatonin suppression by light and alteration of mood and alertness levels. Also, ipRGCs integrate inputs from rods and cones that are content dependent and enable fine-tuning of light-dependent NVR.

Spatiotemporal increase of the intracellular Ca2+ is the most universal way to regulate the function of a eukaryotic cell. Owing to a host of actin-binding proteins and enzymes whose activities are modulated by the local concentration of Ca2+, free Ca2+ in cytosol serves as a pivotal second messenger in a variety of cell functions. The rise and fall of intracellular Ca2+ wave has been best illustrated in eggs at fertilization. However, the molecular mechanism by which intracellular Ca2+ is increased in the fertilized egg is largely unknown despite the discoveries of the distinct Ca2+-mobilizing second messengers in the past 30 years. In this thesis, I have used the starfish oocytes to study how Ca2+ signaling can be modulated by the actin cytoskeleton, which is known to be dynamically remodelled during meiotic maturation and fertilization of the egg. The principal issues of my experimental work are: (i) to establish the role of actin-binding proteins and PIP2 in the regulation of the Ca2+ signaling; (ii) to study the effect of the Ca2+ -store depletion on Ca2+ signaling and on the structure and function of the actin cytoskeleton, and (iii) to study the role of the actin-cytoskeleton in establishing the block to polyspermy. Microinjected into starfish eggs, actin-binding protein gelsolin, function- blocking antibody to depactin, and the PIP2-sequestering fusion protein that indirectly alters the actin cytoskeleton, all changed a certain aspect of Ca2+ signaling. Depletion of the Ca2+ store with ionomycin in turn drastically changed the cortical structure and the actin cytoskeleton of the eggs, eventually leading to a deleterious effect on egg activation and early development. Finally, the alteration of the actin cytoskeleton led to failure to establish a fast and slow block to polyspermy. Taken together, this study indicated that the actin cytoskeleton is an important factor that optimizes the Ca2+ response at egg activation and guides monospermic fertilization.