Autoradiography of progesterone and model compound entry and distribution in Xenopus laevis oocytes

Subcellular distribution of normal and mutant vitamin D receptors in living cells. Studies with a novel fluorescent ligand

To understand the subcellular localization of the vitamin D receptor (VDR) and to measure VDR content in single cells, we recently developed a fluorescent labeled ligand, 4,4-difluoro-4-bora-3a, 4a-diaza-s-indacene (BODIPY)-calcitriol. This tagged hormone has intact biological activity, high affinity and specific binding to the receptor, and enhanced fluorescent emission upon receptor binding. Using BODIPY-calcitriol, here we monitored the subcellular distribution of VDR in living cultured cells by microscopy. Time course studies showed that an equilibrium between the cytoplasmic and nuclear hormone binding developed within 5 min and was maintained thereafter. We found a substantial proportion of VDR residing in the cytoplasm, colocalized with endoplasmic reticulum, the Golgi complex, and microtubules. Confocal microscopy clarified the presence of VDR within discrete regions of the nucleus and along the nuclear envelope. There was no VDR in the plasma membrane. Low affinity BODIPY-calcitriol binding sites were in the mitochondria. Mutations in the VDR gene selectively and specifically altered BODIPY-calcitriol distribution. Defects in the hormone binding region of VDR prevented both nuclear and cytoplasmic hormone binding. Defects in the DNA binding region decreased the nuclear retention of VDR and prevented localization to nuclear foci. These results with BODIPY-calcitriol reveal cytoplasmic VDR localization in living cells and open the possibility of studying the three-dimensional architecture of intranuclear target sites.

Stimulation of Xenopus oocyte maturation by inhibition of the G-protein alpha S subunit, a component of the plasma membrane and yolk platelet membranes

Oocytes of Xenopus laevis undergo maturation when injected with an affinity-purified antibody against the COOH-terminal decapeptide of the alpha subunit of the G-protein Gs, an antibody that inhibits Gs activity. Germinal vesicle breakdown, chromosome condensation, and polar body formation occur, with a time course similar to that for oocytes treated with progesterone. The alpha S antibody-injected oocytes also acquire the ability to be activated by sperm. Coinjection of the catalytic subunit of cAMP-dependent protein kinase, or incubation with cycloheximide, inhibits maturation in response to injection of the alpha S antibody; these experiments show that the alpha S antibody acts at an early point in the pathway leading to oocyte maturation, before formation of maturation promoting factor, and like progesterone, its action requires protein synthesis. Immunogold electron microscopy shows that alpha S is present in the yolk platelet membranes as well as the plasma membrane. These results support the hypothesis that progesterone acts by inhibiting alpha S, and suggest that the target of progesterone could include yolk platelet membranes as well as the plasma membrane.

Developmental changes in the presence of ecdysteroid receptors in the central nervous system of third instar larvae of Sarcophaga bullata

Radiolabeled ponasterone A, a high affinity ligand for ecdysteroid receptors which agonises the effects of 20-hydroxyecdysone, was used in combination with thaw-mount autoradiography to study the stage-specific presence of ecdysteroid receptors in the central nervous system of Sarcophaga bullata. In third instar larvae, nuclear high affinity binding of tritiated or iodinated ponasterone A occurs in the same target cells and both radioligands were displaced by an 100-fold excess of unlabeled ponasterone A or an 500-fold excess of 20-hydroxyecdysone. Target neurons for ponasterone A appear first in the third instar larvae on day 4.0 (early wandering stage) where many cells of the perineurium, ring gland, lateral neurosecretory cells in the brain and certain neurons in abdominal ganglia exhibit nuclear high-affinity binding for ponasterone A. At day 5.5 after larviposition, less binding is present in the perineurium but many neurons, including certain neurosecretory cells in the pars intercerebralis, pars lateralis, tritocerebrum, and neurosecretory cell groups 7, 8, 9, and 10 of the dipteran suboesophageal and abdominal ganglia show increased nuclear ecdysteroid binding. At this stage nuclear binding also occurs in the ring gland except in the central corpus allatum and for the first time in the neurons of the inner optic lobes. The results show that ecdysteroid receptors are present in distinct cerebral neurons and that their expression or ecdysteroid-binding capability is under developmental control.

Uptake, distribution and binding of vertebrate and invertebrate steroid hormones and time-dependence of ponasterone A binding in Calliphora vicina. Comparisons among cholesterol, corticosterone, cortisol, dexamethasone, 5 alpha-dihydrotestosterone, 1,25-dihydroxyvitamin D3, ecdysone, estradiol-17 beta, ponasterone A, progesterone, and testosterone

The presence of specific binding sites for radiolabelled vertebrate-type and arthropod-type steroid hormones was investigated in several organs including salivary gland, and central nervous system of third instar Calliphora vicina larvae by thaw-mount autoradiography. Ponasterone A, a 20-hydroxyecdysone agonist and 20-hydroxyecdysone are the only steroids which bind to nuclear high affinity binding sites. These binding sites are DNA associated while nucleoli show no tracer binding. Ecdysone, an endogenous 20-hydroxyecdysone precursor, is taken up by target cells but no significant nuclear binding occurs. 1,25-Dihydroxyvitamin D3 concentrates in cytoplasm only and its uptake is highest compared to all other steroids. Progesterone and testosterone show weak accumulation in the cytoplasm, while for cholesterol, corticosterone, cortisol, dexamethasone, dihydrotestosterone and estradiol-17 beta, no noticeable uptake occurs. For ponasterone A, a clear time dependence of uptake and intracellular distribution is visible, suggesting the existence and involvement of specific ecdysteroid uptake and transport mechanisms. These results suggest the presence of binding sites for various mammalian steroids in insects. Whether vertebrate steroid hormones or metabolites of them play a role in insects or whether the uptake and binding is based on chemical similarities alone without biological significance remains to be further investigated.