Biosynthesis and axonal transport of rat neurohypophysial proteins and peptides.

Abstract

~S-cysteine injected adjacent to the supraoptic nucleus (SON) of the rat is rapidly incorporated into proteins. These 35S-cysteine-labeled proteins in the SON (1-24 h after injection) were separated by polyacrylamide gel electrophoresis, and the distribution of radioactive proteins on the gels was analyzed. 1 h after injection, about 73% of the radioactivity appeared in two peaks (both about 20,000 mol wt). With time, these peaks (putative precursors of neurophysin) decreased, as a 12,000 mol wt peak (containing two distinct neurophysins) increased in radioactivity. Both the 20,000-and 12,000-mol wt proteins are transported into the axonal (median eminence) and nerve terminal (posterior pituitary) regions of the rat hypothalamo-neurohypophysial system. Conversion of the larger precursor protein to the smaller neurophysin appears to occur, in large part, intra-axonally during axonal transport. Six distinct aSS-cysteine-labeled peptides (< 2500 mol wt), in addition to arginine vasopressin and oxytocin, are also synthesized in the SON and transported to the posterior pituitary where they are released together with labeled neurophysin by potassium depolarization in the presence of extracellular calcium. These data provide support for the hypothesis that the neurohypophysial peptides (vasopressin and oxytocin) and neurophysins are derived from the post-translational cleavage of protein precursors synthesized in the SON, and that the conversion process can occur in the neurosecretory granule during axonal transport.The question whether neuronal peptides are synthesized by ribosomal mechanisms as protein precursors and then transformed to biologically active peptides by post-translational cleavage mechanisms, similar to those for insulin (47), or by direct enzymic (synthetase) mechanisms has received considerable attention in recent years (38). The answer to this question has several implications with regard to the cell biology of "peptidergic" neurons, i.e., neurons which synthesize and store specific peptides for release as intercellular messengers (3,13,30). Since ribosomes appear to be restricted to the neuron perikaryon (21), ribosomal mechanisms of peptide biosynthesis and their regulation would necessarily have to occur at this site in the neuron. In contrast, an enzymic mechanism of biosynthesis would allow these processes to occur in any part of the neuron (e.g., the axon terminal), analogous to the synthesis of conventional neurotransmitters (e.g., acetylcholine, gamma aminobutyric acid, catecholamines, etc.).

Neurophysin biosynthesis in normal rats and in rats with hereditary diabetes insipidus.

“…Sachs and his colleagues (4)(5)(6)(7)(8) have hypothesized that vasopressin and its neurophysin come from a common precursor protein that is synthesized in the hypothalamus by a ribosomal mechanism. We have already presented some evidence consistent with this hypothesis (9,10 sopressin and its associated neurophysin (11,12) and hence have diabetes insipidus-to determine which of the putative precursors, intermediates, and neurophysins are related to vasopressin and which to oxytocin. MATERIALS AND METHODS Animals and Operative Procedures.…”

Immunological evidence for two common precursors to corticotropins, endorphins, and melanotropin in the neurointermediate lobe of the toad pituitary.

“…In addition, the synthesis of another peptide with endorphin-like immuno- The immunoprecipitated peaks are illustrated in Figs. [1][2][3]. RF represents the mobility of each peak on acid/urea gels relative to cytochrome c. Mr was determined by elution of peaks from acid/urea gels and subsequent electrophoresis on NaDodSO4 gels and by direct application of immunoprecipitates to NaDodSO4 gels and correlation of the peaks with the acid/urea gel profile of the same aliquot.…”

Hyponatremia in rats induces downregulation of vasopressin synthesis.

“…Vasopressin is synthesized in the magnocellular neurons of the supraoptic and paraventricular nuclei of the hypothalamus and transported to the posterior pituitary via microtubules (15). Colchicine is well known to disrupt microtubular transport and we recently reported that low doses of colchicine injected into the third ventricle of the hypothalamus of rats blocked transport of vasopressin while synthesis continued (16 7 ,ug, was injected in 1 M1 0.15 M NaCl at a rate of 0.1 l every 10 s. After injection, the hole in the skull was sealed with bone wax (Ethicon, Somerville, NJ), the skin was closed with clips, and the animals were kept warm until they recovered from anesthesia when they were returned to their cages.…”