New issues in phototransduction

Light and dark cycles are one of the principal driving forces for the metabolism of both eukaryotic and prokaryotic organisms, which are the evolutionary closest one to the first living beings. Therefore, is reasonable to think that the alternation between light and dark has influenced the evolution of all the organisms on Earth. Cyanobacteria as well as yeasts have evolved oscillating rhythms of gene expression, also known as circadian rhythms, in order to synchronize their physiological processes with Earth’s day/night cycles. Light cues can also drive unicellular organisms’ motility and growth. However, the light perception of these organisms is limited, and the vast majority of them can express a small number of molecules that can detect a limited range of the light spectrum. On the other hand, metazoan and in particular vertebrates have evolved very specialized and sensitive neurons, rods and cones, which enable visual perception in almost all the lightning conditions that occur during the 24 hours of the day. It has been hypothesized that vertebrates have evolved firstly cone photoreceptors, to perceive light changes during the day (photopic vision). However, cones’ sensitivity is very limited at night and in low illumination conditions their functions as photons detectors are drastically reduced. Rods instead can perceive very dim light signals at night (scotopic vision), even single photons, ensuring an approximate visual stimulus also in poor lighting conditions. It has been proposed that rods are the latest evolved photoreceptors, which have conferred to vertebrates an extraordinary sensitive visual system. The cellular and molecular mechanisms of light and dark adaptation in vertebrates’ photoreceptors have been widely studied and accurately described since the early seventies of the last century. Nonetheless, the study of these fascinating and complex dynamics has left some open questions, regarding both physiological and pathological aspects of photoreceptors metabolism as well as the influence of these alternating mechanisms on phototransduction, the set of enzymatic reactions that transform light stimuli into electrical signals. During the course of my PhD, I have focused my attention on rod’s physiology, in particular on the mechanisms that govern light induced degeneration of photoreceptors, in a Xenopus laevis model of retinitis pigmentosa. Frogs carrying a specific genetic mutation displayed an altered turnover of their cellular body as well as some deficits of phototransduction signaling cascade. In our work, we attempted to estimate how many light induced photoisomerization were necessary to see these alterations. Moreover, I have partially continued the assessment of the response of rods photoreceptors to very localized light stimuli, elicited by mean of a special type of metal coated, tapered optical fibers. Through this technique, we have studied light adaptation of Xenopus laevis’ rods to very confined light stimuli, in order to understand if variations of this process may occur along the rod cells bodies. Furthermore, I have assessed the possible contribution of the circadian rhythms on the phototransduction machinery, by altering the light and dark adaptation cycles of Xenopus frogs. Finally, I have initiated the study of the possible coupling between mechano and phototransduction in rod photoreceptors. This last part is the most fascinating one and made me embrace the idea that sensory neurons are surely specialized cells for one sensory stimulus such as light, sound or chemicals molecules, but others sensory stimuli like temperature variations and small mechanical forces could modulate significantly the perception of the principal stimulus.