Rameshwar K Sharma
Salus University, USA
After his undergraduate work in the Birla Institute Pilani, India, Dr. Sharma received his Ph.D. from the University of Connecticut. His research is dedicated to the advancement of the field of membrane guanylate cyclases (MGCs). Discovery of the first membrane guanylate cyclase, ANF-RGC, established cyclic GMP as an intracellular hormonal second messenger. Discovery of others demonstrated that the operational principles of the MGCs transduction mechanisms are complex and unique to each form. Contrary to the other two signaling systems, cyclic AMP and IP3, cyclic GMP pathway dispenses with intermediaries. The hormone binds to an MGC and catalyzes its activity directly. The pathway is multifaceted, its molecular design incorporates multiple regulations: by ATP, Ca2+-sensing subunits and atmospheric carbon dioxide. These diverse effectors confer MGC cellular specificity and multi-switching signaling attributes linked with the physiology of cardiac vasculature, smooth muscle relaxation, blood pressure regulation, cellular growth, sensory transductions, neural plasticity and memory.
This presentation covers the latest state-of-the art developments on the biochemistry and physiology of the Membrane Guanylate Cyclase (MGC) field at the basic molecular, technological and clinical levels. (1) The membrane guanylate cyclase transduction system differs from the three-component design of its predecessor cyclic AMP second messenger system: adenylate cyclase, G-protein and G-protein coupled receptors, in that it consists of a single entity, a membrane-spanning protein that serves as both receptor and signal transducer. Notably, it also differs structurally from its soluble form, which is hetero-dimeric and requires heme for its activity. (2) Upon purification and characterization of the first MGC, several major surprises emerged. The MGC is also a surface receptor of the polypeptide hormone, Atrial Natriuretic Factor (ANF). This feature changes the paradigms of the modes of formation and function of the second messenger in cell signaling. Because ANF is the most hypotensive hormone, it paves new ways to understand its mechanism of action at the most basic level and to develop drugs against irregularities of the cardiac vasculature. Application of the mouse genetics exposes a remarkable 7-amino acid residue encoded motif of ANF-RGC that controls all ANF-modulated blood pressure regulated activity. Furthermore, the ANF-RGC discovery creates the impression that the hormonal signal transduction is its exclusive physiological function. (3) The impression crumbles with the discovery of ROS-GC MGC. ROS-GC expands the family; links it with the physiology of photo-transduction; and transcends a strictly neural function. The ROS-GC subfamily represents a new template of MGC signal transduction system. The novel feature is that in a feedback mechanism it interlocks with the light-induced fall in [Ca2+]i occurring in the outer segments of the rods and cones. (4) In contrast to the ANF-RGC, the ROS-GC is two-component transduction machinery. It employs its sensor components, GCAPs and S100B (a CD-GCAP), to capture [Ca2+]I signals and utilizes ROS-GC as a transducer component. (5) Accordingly, the first ROS-GC-modulated photo-transduction molecular model is proposed. It plays a pivotal role in the discovery and therapeutically diagnosed analysis of the ROS-GC gene-linked dystrophies. (6) An extraordinary feature specific for cone photo-transduction is uncovered where the ROS-GC is a bimodal Ca2+ transduction switch. It turns “OFF” as intracellular [Ca2+] rises above 75 nM, but then turns back “ON” when [Ca2+] exceeds 345 nM. The “OFF” mode is controlled by GCAPs and the “ON” mode by S100B. These modes occur uniquely in the outer segments and their occurrence in synapses of the cones in rodent retinas extends the role of ROS-GC beyond photo-transduction to the signaling processes between the photoreceptor and cone ON-bipolar cells. (7) The finding that another Ca2+-sensor CD-GCAP, Neurocalcin (NC), is expressed in the Inner Plexiform Layer (IPL) of the retina establishes linkage with signaling by additional neurons. (8) The presence of ROS-GC transduction system in the olfactory bulb and of a variant form of ROS-GC, ONE-GC, in the “necklace ONE-GC” neurons links it with the odorant transduction. Here, four amazing features of the Ca2+-modulated ONE-GC transduction system are disclosed. (i) In addition to ROS-GC1, GCAP1 is also a Ca2+-sensor component of ONE-GC. (ii) Yet, in contrast to the ROS-GC, it modulates ONE-GC catalytic activity only in the higher range of Ca2+, K1/2 of 700 nM. (iii) Instead of inhibiting, it stimulates the guanylate cyclase activity. (iv) The ONE-GC captures odorant signals at its extracellular domain and amplifies them at multiple intracellular domains, incorporating features of both ANF-RGC and ROS-GC transduction systems. (9) The presence of GCAP1- and S100B-modulated ROS-GC1 transduction systems in pinealocytes and of the Ca2+-sensor frequenin-modulated ROS-GC-like in hippocampal layers links the Ca2+-modulated MGC transduction system with “Other than Vision-Linked” Neurons. (10) In a total paradigm change, the dogma is shattered that ANF-RGC, is the specific transducer of ANF alone. It is now known that ANF-RGC also transduces a Ca2+ signal. Ca2+ captured by its sensor NCδ directly activates the catalytic module of ANF-RGC. Accordingly, and impressively, targeted gene-deletion mouse model studies demonstrate that both pathways are linked with blood pressure regulation. Their disruption causes hypertension. Thus the ANF-RGC embodies the combined features of hormone receptor and ROS-GC forms of membrane guanylate cyclases. These studies also broaden classification of the Ca2+ sensors. NCδ, originally classified as a neuronal calcium sensor, serves more widespread functions. (11) With the discovery that the Ca2+-bimodal modulated ROS-GC1 transduction switch occurs in the male gonads, the MGC network extends to the fertility field. Finally, (12) in an extraordinary development photoreceptor ROS-GC via a Ca2+-independent mechanism interlinks its CO2 and the Ca2+-modulated signaling modes. Yet, the Ca2+-modulated modes remain segregated. The domain targeted by CO2 is conserved in the family, thus regulation by CO2 may turn out to be a universal property of the membrane guanylate cyclase family.