5-Hydroxytryptamine receptor "families"

The identification of multiple receptor subtypes for 5-hydroxytryptamine (5-HT) made by using radioligand binding techniques proliferated at a brisk rate in the 1980s. The application of molecular biological techniques to 5-HT receptor studies is likely to lead to an expansion rather than a reduction in the number of distinct 5-HT receptor subtypes. Although the current status of 5-HT receptor pharmacology may appear to be overwhelmingly confusing to most investigators, the evolving data suggest that 5-HT receptor subtypes can be categorized into three major families. Each family consists of multiple receptor subtypes that share similarities in their molecular biological, pharmacological, biochemical, and/or physiological properties. This review provides a summary of recent data as well as a framework for the classification of 5-HT receptor subtypes.     

High androgen receptor immunoexpression in human "Sertoli cell only" testis

Our purpose was to evaluate cellular androgen receptor (AR) distribution and intensity of immunostaining in the human azoospermic testis. Thirty six biopsy specimens from azoospermic men were immunostained, using a monoclonal antibody of human AR. The localization and the intensity of AR immunostaining was evaluated in Sertoli Cell Only (SCO) testis (G1, n = 21), in spermatogenesis arrest testis (G2, n = 11) and in histologically normal testis (G3, n = 4). We found an AR immunostaining in Sertoli, peritubular myoid and Leydig cells, but not in germ cells. The intensity of the immunostaining varied substantially between biopsy specimens of different patients. Sertoli and Leydig cells AR immunostaining (score and intensity) in SCO group was higher than in the other groups. For Sertoli cells, the score means of AR immunoreactivity were 20 +/- 2.36, 10.18 +/- 1.0 and 1 +/- 1, for G1, G2 and G3 groups, respectively. For Leydig cells, the score means were 10.24 +/- 1.37, 6 +/- 0.71 and 0, for G1, G2 and G3 groups, respectively. We found significant differences between G1 and G2 (p = 0.0008), between G1 and G3 (p = 1.54 10-7) and G2 and G3 (p = 0.00032). These results suggest that in the testis AR is located exclusively in somatic cells and its expression is higher in SCO syndrome than in normal and in arrest spermatogenesis testes.     

The "peripheral-type" benzodiazepine (omega 3) receptor in hyperammonemic disorders

Increased levels of brain ammonia occur in both congenital and acquired hyperammonemic syndromes including hepatic encephalopathy, fulminant hepatic failure, Reye's syndrome and congenital urea cycle disorders. In addition to its effect on neurotransmission and energy metabolism, ammonia modulates the expression of various genes including the astrocytic "peripheral-type" benzodiazepine (or omega 3) receptor (PTBR). Increased expression of the isoquinoline carboxamide binding protein (IBP), one of the components of the PTBR complex, is observed in brain and peripheral tissues following chronic liver failure as well as in cultured astrocytes exposed to ammonia. Increased densities of binding sites for the PTBR ligand [3H]-PK11195 are also observed in these conditions as well as in brains of animals with acute liver failure, congenital urea cycle disorders and in patients who died in hepatic coma. The precise role of PTBR in brain function has not yet fully elucidated, but among other functions, PTBR mediates the transport of cholesterol across the mitochondrial membrane and thus plays a key role in the biosynthesis of neurosteroids some of which modulate major neurotransmitter systems such as the gamma-aminobutyric acid (GABA(A)) and glutamate (N-methyl-D-aspartate (NMDA)) receptors. Activation of PTBR in chronic and acute hyperammonemia results in increased synthesis of neurosteroids which could lead to an imbalance between excitatory and inhibitory neurotransmission in the CNS. Preliminary reports suggest that positron emission tomography (PET) studies using [11C]-PK11195 may be useful for the assessment of the neurological consequences of chronic liver failure.     

Deceiving appearances: signaling by "dead" and "fractured" receptor protein-tyrosine kinases

The mechanisms by which most receptor protein-tyrosine kinases (RTKs) transmit signals are now well established. Binding of ligand results in the dimerization of receptor monomers followed by transphosphorylation of tyrosine residues within the cytoplasmic domains of the receptors. This tidy picture has, however, some strange characters lurking around the edges. Cases have now been identified in which RTKs lack kinase activity, but, despite being "dead" appear to have roles in signal transduction. Even stranger are the cases in which genes encoding RTKs produce protein products consisting of only a portion of the kinase domain. At least one such "fractured" RTK appears to be involved in signal transduction. Here we describe how these strange molecules might function and discuss the questions associated with their evolution. BioEssays 23:69-76, 2001.     

The occurrence of steroid-free, "activated" estrogen receptor in target cell nuclei

New techniques for the adrenalectomy of pigs and for the isolation of pig uterus nuclei are described. The isolated nuclei were analysed for their content of estradiol and of estradiol receptor. The concentration of the latter exceeded that of the hormone in extracts of uterine nuclei from ovariectomized pigs by ratios of 1.8--10.3. Substantial amounts of both monomer and "activated" dimer receptor but no estradiol at all were extracted from uterine nuclei of ovariectomized/adrenalectomized pigs. The mechanism of action of steroid hormones is discussed on the basis of these results.     

Transferrin receptor polarity and recycling accuracy in "tight" and "leaky" strains of Madin-Darby canine kidney cells

We have characterized the polarity of the transferrin receptor in the epithelial Madin-Darby canine kidney (MDCK) cell line. The receptor is present in approximately 165,000 copies per cell, migrates as a diffuse band upon SDS gel electrophoresis with Mr 90,000, displays a dissociation constant for diferritransferrin at neutral pH of approximately 2 nM, and is active in essentially all of the cells of the population. Transferrin-mediated 55Fe uptake was used to measure the polarity of active transferrin receptors in filter-grown MDCK cells. The ratio of basolateral to apical receptors was approximately 800:1 for the high resistance strain I MDCK cells (typically greater than 2,000 ohm X cm2) and approximately 300:1 for the lower resistance strain II cells (less than 350 ohm X cm2). In combination with morphometric data this shows that a difference in resistance between these two strains is not reflected in a significant difference in cell surface polarity. We used the recycling of transferrin receptor in filter-grown MDCK cells to evaluate the accuracy of the sorting of a basolateral protein during endocytosis. Monitoring the amount of apically released 125I-labeled transferrin after application of 55Fe- and 125I-labeled transferrin to the basolateral surface provided a sensitive assay of the accuracy of sorting during recycling of the receptor from endosomes to the plasma membrane. The accuracy of transferrin receptor sorting (greater than 99.88%) during a single cycle of transit between the endosome and the plasma membrane is sufficient to maintain the high level of polarity of the cell.     

A cholinergic receptor 12-hour interval-timer displaying "circa" periodicities

The finding of brief pulses of receptor escape from atropine blocking of acetylcholine paralleling moonset and moonset-plus-12-hr led to a search for similar pulses paralleling sunset. Pulses occurring in the hour before sunset as well as in the hour before sunset-plus-12-hr were found. The pulses of receptor escape associated with sunset and sunset-plus-12-hr were in the opposite direction from those associated with moonset and moonset-plus-12-hr. The direction of receptor escape in all cases reversed with the winter solstice. These findings appear to provide evidence for an endogenous 12-hr interval-timer responding to external 24- and 24.8-hr oscillating systems which interact to yield a family of frequencies including those designated "circa".     

Progesterone involvement in breast development and tumorigenesis--as revealed by progesterone receptor "knockout" and "knockin" mouse models

In light of recent clinical trials, the debate concerning the risks and benefits of progestin-based postmenopausal hormone replacement therapy (HRT) has reached a renewed level of urgency. Irrespective of the position taken, the consensus is that more basic research needs to be performed to address progesterone's fundamental role in mammary development and tumorigenesis. Towards this end, the progesterone receptor knockout (PRKO) mouse demonstrated that progesterone is essential for pregnancy-associated mammary gland ductal side-branching and alveologenesis and that these morphological changes are dependent on progesterone-induced mammary epithelial proliferation. Importantly, the PRKO mouse showed that the progesterone-proliferative signal significantly contributes to mammary tumor susceptibility in an established mammary tumor model. Insight into the cellular mechanism(s) by which progesterone affects mammary morphogenesis has been disclosed by a new PR-LacZ knockin mouse, which revealed that PR's spatial expression pattern undergoes precise choreographed distributional changes that precede key stages in postnatal mammary development. In the case of early pregnancy, the segregation of cells undergoing progesterone-induced proliferation from those that express PR implicates a paracrine mode of action for progesterone-induced mammary epithelial proliferation, whereas the preparturient decline of PR expression underscores the need to remove this signal for full functional differentiation of this tissue. Our findings support the proposal that the mammary gland's normal response to the progesterone-signal is dependent upon specific spatial organizational patterns of PR expression and that derailment in these cellular processes may contribute to abnormal mammary development, including cancer. This review concludes by emphasizing the need to identify the downstream molecular targets that mediate progesterone's effects in this tissue. Identification of such targets will not only enhance our mechanistic understanding of progesterone's role in mammary development and cancer, but may also facilitate the formulation of new design strategies in breast cancer diagnosis and/or treatment.     

A "chimeric" C57l-derived Ly49 inhibitory receptor resembling the Ly49D activation receptor.

Ly49D is a natural killer (NK) cell activation receptor that is responsible for differential mouse inbred strain-determined lysis of Chinese hamster ovary (CHO) cells. Whereas C57BL/6 NK cells kill CHO, BALB/c-derived NK cells cannot kill because they lack expression of Ly49D. Furthermore, the expression of Ly49D, as detected by monoclonal antibody 4E4, correlates well with CHO lysis by NK cells from different inbred strains. However, one discordant mouse strain was identified; C57L NK cells express the mAb 4E4 epitope but fail to lyse CHO cells. Herein we describe a Ly49 molecule isolated from C57L mice that is recognized by mAb 4E4 (anti-Ly49D). Interestingly, this molecule shares extensive similarity to Ly49D(B6) in its extracellular domain, but its cytoplasmic and transmembrane domains are identical to the inhibitory receptor Ly49A(B6), including a cytoplasmic ITIM. This molecule bears substantial overall homology to the previously cloned Ly49O molecule from 129 mice the serologic reactivity and function of which were undefined. Cytotoxicity experiments revealed that 4E4(+) LAK cells from C57L mice failed to lyse CHO cells and inhibited NK cell function in redirected inhibition assays. MHC class I tetramer staining revealed that the Ly49O(C57L)-bound H-2D(d) and lysis by 4E4(+) C57L LAK cells is inhibited by target H-2D(d). The structural basis for ligand binding was also examined in the context of the recent crystallization of a Ly49A-H-2D(d) complex. Therefore, this apparently "chimeric" Ly49 molecule serologically resembles an NK cell activation receptor but functions as an inhibitory receptor.     

Increased "peripheral-type" benzodiazepine receptor sites and mRNA in thalamus of thiamine-deficient rats.

"Peripheral-type" benzodiazepine receptors (PTBRs) are highly expressed on the outer mitochondrial membrane of several types of glial cells. In order to further elucidate the nature of the early glial cell changes in thiamine deficiency, PTBR sites and PTBR mRNA were measured in thalamus, a brain structure which is particularly vulnerable to thiamine deficiency, of thiamine-deficient rats at presymptomatic and symptomatic stages of deficiency. PTBR sites were measured using an in vitro binding technique and the selective radio ligand [3H]-PK11195. PTBR gene expression was measured by RT-PCR using oligonucleotide primers based upon the published sequence of the cloned rat PTBR. Microglial and astrocytic changes in thalamus due to thiamine deficiency were assessed using immunohistochemistry and antibodies to specific microglial (ED-1) and astrocytic (GFAP) proteins respectively. Significant increases of [3H]-PK11195 binding sites and concomitantly increased PTBR mRNA were observed in thalamus at the symptomatic stage of thiamine deficiency, coincident with severe neuronal cell loss and increased GFAP-immunolabelling (indicative of reactive gliosis). Positron Emission Tomography using 11C-PK11195 could provide a novel approach to the diagnosis and assessment of the extent of thalamic damage due to thiamine deficiency in humans with Wernicke's Encephalopathy.     

A virus takes an "L" turn to find its receptor

Virus-receptor interaction represents a crucial step during virus entry. In this issue of Cell Host & Microbe, Neu et?al. (2010) identify a receptor motif that engages JC virus, a human polyomavirus known to cause progressive multifocal leukoencephalopathy in immunocompromised individuals.