Ca2+ Sensor Neurocalcin Signal Transduction Site of ROS-GC1 Membrane Guanylate Cyclase in the Ganglion Cells

Abstract

Purpose:: ROS-GC1 is a bimodal Ca²⁺ switch in sensory neurons, including retina. This switch is operated by two distinct calcium signals. One signal is a transition of Ca²⁺ from semimicromolar to nanomolar levels and leads to activation via GCAPs; the second signal involves rise in Ca²⁺ into the semi- to micromolar range, and activation via CD-GCAPs. Neurocalcin Δ is one of the three CD-GCAPs. It modulates ROS-GC1 in certain large ganglion cells. The present study maps the neurocalcin Δ-modulated core signal transduction site in ROS-GC1.

Methods:: Multiple techniques involving surface plasmon resonance binding studies, functional reconstitution studies with numerous ROS-GC1 constructs, deletion analyses, peptide competition assays, immunohistochemical techniques and computational three-dimensional modeling were used.

Results:: The findings revealed that the neurocalcin Δ signal transduction site is defined by the ROS-GC1 domain M817-V852. The site is distinct from the previously characterized signal transduction sites for GCAP1, GCAP2, and S100B. The site is also unusual because it is housed within the core catalytic domain, which is conserved among all membrane guanylate cyclases. To date no signal transduction site has been mapped to this conserved catalytic module region of any member of the membrane guanylate cyclase family. Initial modeling studies support the biochemical findings. Immunohistochemical analyses show that the ROS-GC1-neurocalcin Δ signaling system is present in large ganglion cells across the tested species: rat, guinea pig, and bovine.

Conclusions:: The findings provide evidence for a new Ca²⁺ signaling mechanism of the ROS-GC1 transduction machinery and indicate how this machinery may transduce diverse Ca²⁺ signals within a sub-population of ganglion cells.

Keywords: ganglion cells • calcium • signal transduction