Intracellular spreading of second messengers.

In the process of sensing their surroundings, cells constantly convert environmental signals. In order to achieve the transduction of incoming signals, it is quite common for cells to use intracellular second messengers. One reason for employing second messengers is to amplify weak signals above the threshold for detection. Another reason is to mediate various additional responses, like adaptation to the presence of a stimulus. Thus, the spreading of second messenger signals within a cell is an important aspect of signal transduction. In this context, Allbritton et al. (1992) measured the cytoplasmic diffusion coefficients of IP3 and Ca2+ and considered the implications for the effective ranges of second messenger action: the IP3 signal would spread to large distances, while the Ca2+ signal would be restricted. However, since IP3 can release Ca2+ from internal stores, IP3 can also function as the messenger for the rapid and extensive propagation of Ca2+ signals. In this issue of The Journal of Physiology, Gray-Keller et al. reveal an important additional twist to the spreading of second messenger signals by considering the situation of coupled second messengers, where there is a reciprocal influence between the second messengers and an ensuing interdependence of the spreading of the second messenger signals. Specifically, the authors examined the diffusion of second messengers and the spread of excitation and adaptation in vertebrate rod outer segments. Rod outer segments are the site of phototransduction, which is the process of conversion of incoming photons to an electrical signal by the rod photoreceptor cells. Rods have achieved the ultimate in sensitivity by being able to detect single photons. Rods also exhibit adaptation, which allows them to operate in the presence of background light spanning more than five orders of magnitude of intensity. The mechanism through which rods achieve this high sensitivity and large operating range is known in considerable detail, making phototransduction one of the best understood signal transduction processes. The intracellular transmitter mediating phototransduction is cGMP, which in the dark binds to and keeps open cationic channels located on the plasma membrane of the rod outer segment. Incoming photons trigger a biochemical amplification cascade, resulting in the hydrolysis of cGMP and closure of the cGMP-gated channels. Ca2+ is another intracellular transmitter involved in phototransduction and has been shown to mediate the adaptation of the cell to background light. Ca2+ enters the rod outer segment through the cGMP-gated channels and is extruded by a Na+-Ca2+, K+ exchanger. Thus, changes in the cGMP concentration can affect the Ca2+ concentration by changing the influx of Ca2+ through the channels. In turn, Ca2+ modulates the cGMP-metabolizing enzymes and so changes in the Ca2+ concentration affect the cGMP concentration. In short, the two transmitters are coupled. The detailed understanding of phototransduction makes it an attractive system for studying the diffusion of second messengers and the spread of the cellular signals associated with them. Moreover, the light stimulus can be controlled with high precision. In their study, Gray-Keller et al. (1999) have taken full advantage of the opportunities afforded by the system. They employed two-photon excitation for light stimulation, which results in a highly localized stimulus. They then followed the spatiotemporal response to the stimulus by combining electrical recording and fluorescence imaging to measure the spread of excitation and adaptation and the Ca2+ concentration profile. In good agreement with the previous measurements of Lamb et al. (1981), the present work finds that excitation and adaptation spread along the length of the rod outer segment with length constants of 3.5 and 8 μm, respectively. However, the new measurements also show that Ca2+ spreads with the same length constant as excitation. The results can be understood at least partly by considering the diffusion of cGMP and Ca2+, the respective turnover rates, and the pathways responsible for the coupling between the two: cGMP diffuses much faster than Ca2+, but Ca2+ is turned over much faster than cGMP. The result is that the spread of cGMP is slowed down through the coupling with Ca2+, and the observed spread of excitation is essentially the spread of the Ca2+-coupled cGMP. On the other hand, the spread of Ca2+ follows the spread of cGMP, resulting in similar length constants for Ca2+ and excitation. It is unclear what we can make of the discrepancy between the length constants for the spread of Ca2+ and adaptation. The difference may perhaps indicate the presence of additional adaptation pathway(s), or it may simply be the manifestation of the way in which adaptation depends on Ca2+. Additional mathematical modelling and experiments would be necessary in order to resolve this issue. An interesting prediction borne out by the mathematical model advanced by Gray-Keller et al. (1999) is that increases in the turnover rate of the transmitter should decrease its length constant and thereby restrict its range of action. In the case of rod photoreceptors, for example, adaptation to background light should be accompanied by a decrease in the spread of excitation. There are similar implications for other signal transduction systems, but it should be kept in mind that the particular pathways responsible for the coupling between the second messengers have a very significant influence on the actual spreading of the messengers and associated signals. Thus, it would be interesting to explore the situation in olfactory signal transduction, where the coupling between the two messengers, cAMP and Ca2+, is radically different from that in phototransduction. Another interesting situation would arise in systems manifesting cAMP oscillations through coupling between cAMP and Ca2+ (Cooper et al. 1995).