The mammalian vomeronasal organ (VNO) is an accessory olfactory structure implicated in the sensing of pheromones. Although virtually nothing is known about sensory transduction in the mammalian VNO, recent findings have raised the possibility that it proceeds via a G-protein-coupled mechanism and involves a cyclic nucleotide-gated ion channel as in the nasal olfactory epithelium. To investigate this possibility, we cloned G-protein alpha subunits, adenylyl cyclases, and guanylyl cyclases that are expressed in the VNO and examined their patterns of expression. Of seven G alpha subunits identified as being expressed in the VNO, we found that mRNAs encoding only two, G alpha o and G alpha i2, are highly expressed in VNO neurons. Moreover, G alpha o and G alpha i2 are highly expressed by separate subsets of neurons that are located in different regions of the VNO neuroepithelium. Immunohistochemical studies show that both G alpha o and G alpha i2 are enriched in VNO microvilli, suggesting that G-proteins containing both of these alpha subunits may be involved in VNO sensory transduction. Of the adenylyl and guanylyl cyclases that we cloned, we found that only one, adenylyl cyclase type II, is highly expressed in VNO neurons; furthermore, it is expressed by both G alpha o+ and G alpha i2+ subsets. Our findings suggest that spatially segregated subsets of VNO neurons may use different, but related, sensory transduction pathways in which G-proteins and an adenylyl cyclase play major roles.

Several social and reproductive behaviors are under the influence of the vomeronasal (VN) organ; VN neurons detect odorous molecules emitted by individuals of the same species. There are two types of VN neurons, and these differ in their expression of chemosensory receptors and G protein subunits. The significance of this dichotomy is largely unknown. VN neurons express high levels of either G alpha i2 or G alpha o. A mouse line carrying a targeted disruption of the G alpha i2 gene offered the opportunity for studying the effects of a lack of receptor signaling through the heterotrimeric Gi2 protein in one VN cell type. As a consequence of this deficiency, the number of VN neurons that normally express G alpha i2 is decreased by half. These residual neurons are defective in eliciting a response in their target neurons in the accessory olfactory bulb. Moreover, G alpha i2 mutant mice show alterations in behaviors for which an intact VN organ is known to be important. Display of maternal aggressive behavior is severely blunted, and male mice show significantly less aggression toward an intruder. However, male mice show unaltered sexual-partner preference. This suggests that the two types of VN neurons may have separate functions in mediating behavioral changes in response to chemosensory information.

Signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) is mediated by the intracellular dioxin receptor which, in its dioxin-activated state, regulates transcription of target genes encoding drug-metabolizing enzymes, such as cytochrome P-45OIA1 and glutathione S-transferase Ya. Exposure of the dioxin receptor to dioxin leads to an apparent translocation of the receptor to the nucleus in vivo and to a rapid conversion of the receptor from a latent, non-DNA-binding form to a species that binds to dioxin-responsive positive control elements in vitro. This DNA-binding form of receptor appears to be a heterodimeric complex with the helix-loop-helix factor Arnt. In this study, we show that activation of the cytochrome P-450IA1 gene and minimal dioxin-responsive reporter constructs by the dioxin receptor was inhibited following prolonged treatment of human keratinocytes with the phorbol ester 12-0-tetradecanoylphorbol-13-acetate. Inhibition of the receptor-mediated activation response was also achieved by treatment of the cells with a number of protein kinase inhibitors, one of which, calphostin C, shows selectivity for protein kinase C. Taken together, these data suggest that protein kinase C-dependent phosphorylation may play an essential role in the dioxin signaling pathway. This hypothesis is supported by the observation that pretreatment of the cells with 12-0-tetradecanoylphorbol-13-acetate inhibited the DNA-binding activity of the dioxin receptor in vivo. In vivo, the dioxin receptor was found to be a phosphoprotein. In vitro, dephosphorylation of the ligand-activated, heteromeric dioxin receptor form or dephosphorylation of the individual ligand-binding and Arnt receptor subunits inhibited the xenobiotic response element-binding activity. Moreover, dephosphorylation experiments with the individual receptor subunits prior to assembly of the xenobiotic response element-binding receptor form indicated that phosphorylation seemed to be important for the DNA-binding activity per se of the receptor, whereas Arnt appeared to require phosphorylation to interact with the receptor. Finally, a protein kinase C inhibitor-sensitive cytosolic catalytic activity that could restore the DNA-binding activity of the dephosphorylated dioxin receptor form was identified.Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and several related halogenated aromatic hydrocarbons are members of a class of environmental pollutants that give rise to a plethora of teratogenic, tumorigenic, and toxic responses in rodents. In humans, a well-documented toxic effect of this class of compounds is a hyperkeratotic and metaplastic response of the hair follicles and interfollicular epidermis, leading to persistent acne-like lesions called chloracne (reviewed in reference 58). On a molecular level, dioxin induces transcription of genes encoding drug-metabolizing enzymes, such as glutathione S-transferase Ya, quinone reductase, and cytochrome P-450IA1, in both rodent and human cells (for a review, see reference 30).Signal transduction by dio...

A multitude of signalling pathways are involved in the process of forming an eye. Here we demonstrate that β-catenin is essential for eye development as inactivation of β-catenin prior to cellular specification in the optic vesicle caused anophthalmia in mice. By achieving this early and tissue-specific β-catenin inactivation we find that retinal pigment epithelium (RPE) commitment was blocked and eye development was arrested prior to optic cup formation due to a loss of canonical Wnt signalling in the dorsal optic vesicle. Thus, these results show that Wnt/β-catenin signalling is required earlier and play a more central role in eye development than previous studies have indicated. In our genetic model system a few RPE cells could escape β-catenin inactivation leading to the formation of a small optic rudiment. The optic rudiment contained several neural retinal cell classes surrounded by an RPE. Unlike the RPE cells, the neural retinal cells could be β-catenin-negative revealing that differentiation of the neural retinal cell classes is β-catenin-independent. Moreover, although dorsoventral patterning is initiated in the mutant optic vesicle, the neural retinal cells in the optic rudiment displayed almost exclusively ventral identity. Thus, β-catenin is required for optic cup formation, commitment to RPE cells and maintenance of dorsal identity of the retina.