PubMed · 7397707
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M L Gutierrez, S T Crooke. 1980. Mitotane (o,p'-DDD).. https://doi.org/10.1016/s0305-7372(80)80025-9
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Morphological features of the goldfish caudal neurosecretory system were investigated by means of immunohistochemical localization of urotensins I and II (UI and UII) and electron microscopic examination of the caudal neurosecretory neurons, the urophysis, and the synaptic neuropil. The aim of the work is to provide a detailed morphological description of the afferent synapses to the caudal neurons and to analyze their distribution through the rostrocaudal extension of the caudal neurosecretory system. Three morphologically different types of neurosecretory cells have been identified according to size and shape: large, medium, and small Dahlgren cells. The three different-sized cells share similar patterns of immunoreactivity with the UI (or oCRF) and the UII antisera. Electron microscopic examination of the synaptic neuropil throughout the caudal system revealed the presence of four types of terminals: dense-cored-vesicle end bulbs (DC), spherical-vesicle end bulbs (S), flattened-vesicle end bulbs (F), and granular-vesicle end bulbs (G). The present study demonstrates that the small Dahlgren cells receive different synaptic inputs from the large and the medium neurosecretory cells. Indeed, G terminals are only found on the small Dahlgren cells, whereas DC, S, and F terminals are distributed on the large, medium, and small Dahlgren cell bodies and proximal processes.
The low-density lipoprotein (LDL) receptor plays a key role in maintaining circulating and cellular cholesterol homeostasis. The LDL receptor is a transmembrane glycoprotein whose biochemical and genetic properties have been extensively studied notably by Brown, Goldstein and colleagues [Brown, M. S., & Goldstein, J. L., (1986) Science 232, 34-47]. However, few if any structural studies of the LDL receptor have been reported, and details of its secondary and tertiary structure are lacking. In an attempt to determine the low-resolution structure of the LDL receptor, we have purified the receptor from bovine adrenal cortices using modifications of the method of Schneider et al. [Schneider, W. J., Goldstein, J. L., & Brown, M. S. (1985) Methods in Enzymol.109, 405-417]. Using circular dichroism, the secondary structure of the detergent-solubilized bovine LDL receptor at 25 degrees C was shown to be 19% alpha-helix, 42% beta-sheet, and 39% random coil. Interestingly, the detergent-solubilized receptor appeared to be quite resistant to changes in secondary structure over the temperature range 10-90 degrees C, with only minor but reversible changes being observed. In contrast, a more pronounced unfolding of the detergent-solubilized receptor was observed in the presence of guanidinium hydrochloride. Using the complete sequence of the human LDL receptor, sequence analysis by the Chou-Fasman prediction algorithm showed quite good agreement with the experimentally determined secondary structure of the bovine LDL receptor at 25 degrees C. Finally, the purified, bovine LDL receptor was reconstituted into large unilamellar vesicles of egg yolk phosphatidylcholine using a procedure exploiting preformed vesicles and detergent dialysis. We showed previously using negative stain electron microscopy that reconstituted vesicles bind LDL. Now, using cryoelectron microscopy of frozen hydrated reconstituted vesicles evidence of an extended, stick-like morphology (length approximately 120 A) for the extracellular domain of the LDL receptor has been obtained. Successful purification of the receptor, its incorporation into single bilayer vesicles, and its direct visualization by cryoelectron microscopy pave the way for more detailed structural studies of the LDL receptor and the receptor-LDL complex.
The class B, type I scavenger receptor, SR-BI, binds high density lipoprotein (HDL) and mediates the selective uptake of HDL cholesteryl ester (CE) by cultured transfected cells. The high levels of SR-BI expression in steroidogenic cells in vivo and its regulation by tropic hormones provides support for the hypothesis that SR-BI is a physiologically relevant HDL receptor that supplies substrate cholesterol for steroid hormone synthesis. This hypothesis was tested by determining the ability of antibody directed against murine (m) SR-BI to inhibit the selective uptake of HDL CE in Y1-BS1 adrenocortical cells. Anti-mSR-BI IgG inhibited HDL CE-selective uptake by 70% and cell association of HDL particles by 50% in a dose-dependent manner. The secretion of [3H]steroids derived from HDL containing [3H]CE was inhibited by 78% by anti-mSR-BI IgG. These results establish mSR-BI as the major route for the selective uptake of HDL CE and the delivery of HDL cholesterol to the steroidogenic pathway in cultured mouse adrenal cells.