Light-driven calcium signals in mouse cone photoreceptors.

ABSTRACT

Calcium mediates various neuronal functions. The complexity of neuronal Ca²âº signaling is well exemplified by retinal cone photoreceptors, which, with their distinct compartmentalization, offer unique possibilities for studying the diversity of Ca²âº functions in a single cell. Measuring subcellular Ca²âº signals in cones under physiological conditions is not only fundamental for understanding cone function, it also bears important insights into pathophysiological processes governing retinal neurodegeneration. However, due to the proximity of light-sensitive outer segments to other cellular compartments, optical measurements of light-evoked Ca²âº responses in cones are challenging. We addressed this problem by generating a transgenic mouse (HR2.1TN-XL) in which both short- and middle-wavelength-sensitive cones selectively express the genetically encoded ratiometric Ca²âº biosensor TN-XL. We show that HR2.1TN-XL allows recording of light-evoked Ca²âº responses using two-photon imaging in individual cone photoreceptor terminals and to probe phototransduction and its diverse regulatory mechanisms with pharmacology at subcellular resolution. To further test this system, we asked whether the classical, nitric oxide (NO)-soluble guanylyl-cyclase (sGC)-cGMP pathway could modulate Ca²âº in cone terminals. Surprisingly, NO reduced Ca²âº resting levels in mouse cones, without evidence for direct sGC involvement. In conclusion, HR2.1TN-XL mice offer unprecedented opportunities to elucidate light-driven Ca²âº dynamics and their (dys)regulation in cone photoreceptors.