Ecdysteroid biosynthesis in the crustacean Y‐organ and control by an eyestalk neuropeptide

Abstract

AbstractThe endocrine axis is described that controls growth, molting, and regeneration in crustaceans: The ecdysial glands (Y‐organs), sources of ecdysteroid hormones, and eyestalk neurosecretory centers, sources of a putative peptide (molt‐inhibiting hormone, MIH) that exerts negative control of Y‐organ function. Current knowledge of the physiology of these components is briefly reviewed, and in this context Y‐organs are discussed as useful models for study of steroidogenesis generally in animals at the cellular and molecular levels. Data are emphasized that demonstrate the utilization of cholesterol by Y‐organs for biosynthesis of at least two ecdysteroid secretion products, and suppression of the process by MIH. Evidence from radioactive tracer experiments is presented to show that (a) cholesterol in circulating hemolymph and in vitro in the presence of hemolymph serum is quantitatively bound to high‐density lipoprotein (HDL), (b) Y‐organs take up cholesterol by an active process, and (c) uptake is greatly accelerated in the absence of MIH (eyestalks absent) or inhibited by eyestalk extract. Results of in vitro experiments with 125I‐labeled HDL suggest that the HDL‐cholesterol complex is taken into Y‐organ cells mediated by receptors for the HDL apoprotein. Measured as a function of HDL concentration in the medium, cellular acid‐precipitable radioactivity (HDL uptake), acid‐soluble radioactivity (HDL degradation), and ecdysone secretion each exhibited saturation kinetics. Also, at saturation levels of external HDL, uptake, degradation, and secretion were significantly higher in Y‐organs from de‐eyestalked crabs than in glands from intact crabs.