Crosstalk of CGMP and CAMP in the vertebrate phototransduction cascade

Abstract

The conventional phototransduction cascade suggests that cyclic guanosine monophosphate (cGMP) functions as the chief secondary messenger while intracellular calcium concentration dominates as the central feedback modulator. Countless years and several scientific institutions have led to this conclusion, and it stands as the most extensively researched and explicit among all sensory transduction schemes. However, experimental evidence suggests that our understanding of the phototransduction cascade mechanisms remains significantly incomplete [1]. According to the canonical cascade scheme, all transients should be completed within a second once the light stimulus is no longer present. However, our data indicates that there are long-lasting changes in cell sensitivity and dark current parameters after the stimulus, which can last for more than 10 s. Phenomena that deviate from the standard phototransduction cascade behavior can potentially be clarified by an alternative regulatory mechanism that is based on cyclic adenosine monophosphate (cAMP). Previous research has provided convincing evidence that the intracellular levels of cAMP can significantly impact the functioning of the phototransduction cascade on both slow (day) [2] and relatively fast (minutes) [3] time scales. Additionally, there is phenomenological evidence indicating the existence of other regulatory signaling pathways in the phototransduction cascade without a corresponding mechanism in the classical phototransduction scheme, including inositol triphosphate (IP3) and diacylglycerol (DAG). We investigate whether cAMP, IP3, and DAG regulate the phototransduction cascade during photoresponse. For this regulatory effect to occur, there must be a change in the signaling molecule concentration during the process. These processes occur in less than a second, and it is crucial for the presence of the regulatory effect. Given that traditional fluorescence methods cannot measure the concentration of any signaling molecule in the retina, a hardware-software setup has been developed that allows cryofixation of retinal samples at the required speed. The setup permits fixing up to six samples in a series with a delay of no more than 80 milliseconds after light stimulation. The concentration of signal molecules is assessed using high-performance liquid chromatography coupled with high-resolution tandem mass spectrometry. The results demonstrate a 4.5-fold elevation in cAMP concentration 1.1 s after switching on a light with an intensity close to saturation. The concentration of cAMP is directly proportional to the intensity of the stimulating light; there is no increase in cAMP at lower light intensities. No noteworthy changes in IP3 and DAG concentration were detected in response to light stimulation. The findings align with existing literature on the kinetics of light-triggered protein kinase A (PKA) activity [3], which indicated an initial decrease followed by an increase in PKA activity. These results could potentially inform the revision and expansion of the phototransduction cascade model.