Neuronal membrane sensitivity to a salmon calcitonin analogue with negligible ability to lower serum calcium

Signal transduction by calcitonin Multiple ligands, receptors, and signaling pathways

Calcitonin (CT) is a peptide hormone that is secreted by the parafollicular cells of the thyroid in response to elevated serum calcium levels. It acts to reduce serum calcium by inhibiting bone resorption and promoting renal calcium excretion. In addition to this hypocalcemie effect, calcitonin modulates the renal transport of water and several ions other than calcium and acts on the central nervous system to induce analgesia, anorexia, and gastric secretion. The CT receptor, a member of a newly described family of serpentine G protein-coupled receptors, has recently been shown to couple to multiple trimeric G proteins, thereby activating several signaling proteins, including protein kinase C, cAMP-dependent protein kinase and calcium/calmodulin-dependent protein kinase. In kidney proximal tubule cells (LLC-PK1), the CT-activated signaling mechanisms vary in a cell cycle-dependent manner, with the receptor coupling through a G(s) protein during G(2) phase and through a G(i) protein and possibly a G(q) protein during S phase. These signaling mechanisms differentially modulate the activities of Na(+)/K(+)-ATPase and the apical Na(+)/H(+) exchanger, effector molecules that play important roles in transepithelial Na(+) transport. Cloning of CT receptors has revealed the presence of alternatively spliced cassettes, resulting in the expression of different isoforms of the receptor. The availability of these recombinant CT receptors has allowed preliminary characterization of the effects of changes in the receptor's structure on its ligand binding and signal transduction properties. Thus, the cellular and molecular biology of CT is complex, with several structurally related peptide ligands and multiple isoforms of the CT receptor that can independently activate diverse signaling pathways. As the recent exciting results in this field are extended, we can expect rapid progress in understanding the molecular basis of the diverse effects of CT and, possibly, of the CT-related peptides CGRP and amylin.

Molecular physiology of amylin

Amylin is a 37-amino acid peptide first isolated, purified, and characterized from the amyloid deposits in the pancrease of type 2 diabetics. It is synthesized and secreted primarily from pancreatic beta cells along with insulin. The ability of amylin to potently reduce insulin-stimulated incorporation of glucose into glycogen in skeletal muscle requires both an intact 2Cys-7Cys disulfide bond and a COOH-terminal amide. Amylin has structural and functional relationships to two other messenger proteins, calcitonin and CGRP. Amylin has relatively potent calcitonin-like activity on bone metabolism and weaker CGRP-like activity on the vasculature. CGRP is a slightly weaker agonist than amylin for metabolic responses. Although rat calcitonins are weak, teleost fish calcitonins are very potent agonists for amylin's metabolic effects. This group of peptides appears to act on a family of related G protein-coupled receptors; several variant calcitonin receptors have recently been cloned and expressed. These receptors appear to be coupled to adenylyl cyclase in many instances; recent evidence supports the view that amylin's effects on skeletal muscle occur, at least in large part, through activation of the cAMP pathway.

Molecular cloning of two receptors from rat brain with high affinity for salmon calcitonin

Two receptors with high affinity for salmon calcitonin were cloned from the nucleus accumbens region of rat brain. The deduced 479 amino acid sequence of cDNA clone L2175-D20 (designated C1a receptor) is 78% and 66% identical with those reported for human and porcine calcitonin receptors, respectively. Clone U3237-A2 codes for a receptor (designated C1b) that is identical to C1a except for a 37 amino acid insert in the second extracellular domain. COS-7 cells transfected with either transcript bound [125I]salmon calcitonin with high affinity (Kd = 8 pM for C1a; Kd = 48 pM for C1b) and responded to salmon calcitonin with increases in cAMP. Tissue distribution studies revealed C1a transcript in rat brain, skeletal muscle, kidney and lung, whereas C1b was predominantly found in brain.

Analogue separates biological effects of salmon calcitonin on brain and renal cortical membranes

The conformation-activity relationship of salmon calcitonin in kidney and brain was investigated with regard to effects on membrane binding and adenylate cyclase activity. Since an amphipathic alpha-helical conformation on the calcitonin molecule is associated with high potency in lowering serum calcium, the activity of the parent peptide was compared to that of [Gly8, D-Arg24]des-Leu16-salmon calcitonin, a calcitonin analogue (CTA) with less helix forming potential. The results indicate that while salmon calcitonin possesses similar potency in brain and kidney, CTA is effective only in brain. Furthermore, CTA did not inhibit the binding of 125I-labeled human calcitonin gene-related peptide (HCGRP) to brain membranes. Our findings suggest that the specific binding and effects of salmon calcitonin on adenylate cyclase activity in brain do not depend on conformational features in the middle region of the molecule, although the alpha-helical structure in this region does appear to be an important property for salmon calcitonin binding to renal cortical membranes.