Olfactory receptors: transduction, diversity, human psychophysics and genome analysis

The emerging understanding of the molecular basis of olfactory mechanisms allows one to answer some long-standing questions regarding the complex recognition machinery involved. The ability of the olfactory system to detect chemicals at sub-nanomolar concentrations is explained by a plethora of amplification devices, including the coupling of receptors to second messenger generation through GTP-binding proteins. Specificity and selectivity may be understood in terms of a diverse repertoire of olfactory receptors of the seven-transmembrane-domain receptor superfamily, which are probably disposed on olfactory sensory neurons according to a clonal exclusion rule. Signal termination may be related to sets of biotransformation enzymes that process odorant molecules, as well as to receptor desensitization. Many of the underlying molecular components show specific expression in olfactory epithelium, with a well-orchestrated developmental sequence of emergence, possibly related to sensory neuronal function and connectivity requirements. A general model for molecular recognition in biological receptor repertoires allows a prediction of the number of olfactory receptors necessary to achieve efficient detection and sheds light on the analogy between the immune and olfactory systems. The molecular cloning and mapping of a human genomic olfactory receptor cluster on chromosome 17 provides insight into olfactory receptor diversity, polymorphism and evolution. Combined with future genotype-phenotype correlation, with particular reference to specific anosmia, as well as with computer-based molecular modelling, these studies may provide insight into the odorant specificity of olfactory receptors.

Nato3 is an evolutionarily conserved bHLH transcription factor expressed in the CNS of Drosophila and mouse

The evolutionarily conserved basic helix-loop-helix (bHLH) transcription factors play important roles during development. Here we report the identification of Nato3 (nephew of atonal fer3) orthologs in Drosophila, C. elegans, mouse, and man, all of which share a high degree of similarity within the bHLH domain. Expression analysis revealed Nato3 transcripts in the central nervous system of both fly and mouse embryos. In the fly, Dnato3 is highly expressed in 9-15h embryos in a few ventral nerve cord cells and a subset of neurons in the brain. In mouse, the MNato3 transcripts were detected from embryonic day 7 until 5 weeks postnatally, with highest levels in the midbrain, thalamus, hypothalamus, pons, and medulla oblongata. In contrast to the brain, expression in the spinal cord was limited to the embryonic stages.

Autoregulation and multiple enhancers control Math1 expression in the developing nervous system

Development of the vertebrate nervous system requires the actions of transcription factors that establish regional domains of gene expression, which results in the generation of diverse neuronal cell types. MATH1, a transcription factor of the bHLH class, is expressed during development of the nervous system in multiple neuronal domains, including the dorsal neural tube, the EGL of the cerebellum and the hair cells of the vestibular and auditory systems. MATH1 is essential for proper development of the granular layer of the cerebellum and the hair cells of the cochlear and vestibular systems, as shown in mice carrying a targeted disruption of Math1. Previously, we showed that 21 kb of sequence flanking the Math1-coding region is sufficient for Math1 expression in transgenic mice. Here we identify two discrete sequences within the 21 kb region that are conserved between mouse and human, and are sufficient for driving a lacZ reporter gene in these domains of Math1 expression in transgenic mice. The two identified enhancers, while dissimilar in sequence, appear to have redundant activities in the different Math1 expression domains except the spinal neural tube. The regulatory mechanisms for each of the diverse Math1 expression domains are tightly linked, as separable regulatory elements for any given domain of Math1 expression were not found, suggesting that a common regulatory mechanism controls these apparently unrelated domains of expression. In addition, we demonstrate a role for autoregulation in controlling the activity of the Math1 enhancer, through an essential E-box consensus binding site.

Functional conservation of atonal and Math1 in the CNS and PNS

To determine the extent to which atonal and its mouse homolog Math1 exhibit functional conservation, we inserted (beta)-galactosidase (lacZ) into the Math1 locus and analyzed its expression, evaluated consequences of loss of Math1 function, and expressed Math1 in atonal mutant flies. lacZ under the control of Math1 regulatory elements duplicated the previously known expression pattern of Math1 in the CNS (i.e., the neural tube, dorsal spinal cord, brainstem, and cerebellar external granule neurons) but also revealed new sites of expression: PNS mechanoreceptors (inner ear hair cells and Merkel cells) and articular chondrocytes. Expressing Math1 induced ectopic chordotonal organs (CHOs) in wild-type flies and partially rescued CHO loss in atonal mutant embryos. These data demonstrate that both the mouse and fly homologs encode lineage identity information and, more interestingly, that some of the cells dependent on this information serve similar mechanoreceptor functions.

Math1: an essential gene for the generation of inner ear hair cells

The mammalian inner ear contains the cochlea and vestibular organs, which are responsible for hearing and balance, respectively. The epithelia of these sensory organs contain hair cells that function as mechanoreceptors to transduce sound and head motion. The molecular mechanisms underlying hair cell development and differentiation are poorly understood. Math1, a mouse homolog of the Drosophila proneural gene atonal, is expressed in inner ear sensory epithelia. Embryonic Math1-null mice failed to generate cochlear and vestibular hair cells. This gene is thus required for the genesis of hair cells.

Organization and evolution of olfactory receptor genes on human chromosome 11

Olfactory receptors (OR) are encoded by a large multigene family including hundreds of members dispersed throughout the human genome. Cloning and mapping studies have determined that a large proportion of the olfactory receptor genes are located on human chromosomes 6, 11, and 17, as well as distributed on other chromosomes. In this paper, we describe and characterize the organization of olfactory receptor genes on human chromosome 11 by using degenerate PCR-based probes to screen chromosome 11-specific and whole genome clone libraries for members of the OR gene family. OR genes were identified by DNA sequencing and then localized to regions of chromosome 11. Physical maps of several gene clusters were constructed to determine the chromosomal relationships between various members of the family. This work identified 25 new OR genes located on chromosome 11 in at least seven distinct regions. Three of these regions contain gene clusters that include additional members of this gene family not yet identified by sequencing. Phylogenetic analysis of the newly described OR genes suggests a mechanism for the generation of genetic diversity.

Math1 is essential for genesis of cerebellar granule neurons

The cerebellum is essential for fine motor control of movement and posture, and its dysfunction disrupts balance and impairs control of speech, limb and eye movements. The developing cerebellum consists mainly of three types of neuronal cells: granule cells in the external germinal layer, Purkinje cells, and neurons of the deep nuclei. The molecular mechanisms that underlie the specific determination and the differentiation of each of these neuronal subtypes are unknown. Math1, the mouse homologue of the Drosophila gene atonal, encodes a basic helix-loop-helix transcription factor that is specifically expressed in the precursors of the external germinal layer and their derivatives. Here we report that mice lacking Math1 fail to form granule cells and are born with a cerebellum that is devoid of an external germinal layer. To our knowledge, Math1 is the first gene to be shown to be required in vivo for the genesis of granule cells, and hence the predominant neuronal population in the cerebellum.

Cerebellar deficient folia (cdf): a new mutation on mouse chromosome 6

Cerebellar deficient folia, cdf, is a spontaneous autosomal recessive mutation in the mouse with unique pathology; the cerebellar cortex of the cdf/cdf mouse has only 7 folia instead of 10, which is the normal count for the C3H/HeJ strain in which this mutation arose. The cerebellum of the cdf/cdf mouse is hypoplastic and contains mineral deposits in the ventral vermis that are not present in controls. We used an intersubspecific intercross between C3H/HeSnJ-cdf/+ and Mus musculus castaneus (CAST/Ei) to map the cdf mutation to Chromosome (Chr) 6. The most likely gene order is D6Mit16-(cdf, D6Mit3)-D6Mit70-D6Mit29-D6Mit32, which positions cdf distal to lurcher (Lc) and proximal to motor neuron degeneration 2 (mnd2). The definitive visible phenotypes and histopathologies of cdf, Lc, and mnd2 support our mapping evidence that cdf is a distinct gene. The novel pathology of cdf should help elucidate the complicated process of cerebellar folia patterning and development. cdf recombined with mouse atonal homolog 1, Math1, the mouse homolog of the Drosophila atonal gene.

Evolutionary conservation of sequence and expression of the bHLH protein Atonal suggests a conserved role in neurogenesis

atonal is a Drosophila proneural gene that belongs to the family of basic helix-loop-helix (bHLH)- containing proteins. It is expressed in the chordotonal organs and photoreceptor cells, and flies that lack Atonal protein are ataxic and blind. Here we report the cloning of atonal homologs from red flour beetle, puffer fish, chicken, mouse, and human. The bHLH domain is conserved throughout evolution, while the entire coding region is highly similar in mammals. Both the chicken and the mouse homologs are expressed early in embryogenesis in the hind brain, and specifically in cells predicted to give rise to the external granular layer of the cerebellum. In addition, these genes are expressed throughout the dorsal part of the spinal cord, in patterns different from those found for other genes, like LH-2 and wnt-1. The mouse homolog (Math1) maps to mouse chromosome 6, and the human homolog (HATH1) to human chromosome 4q22. Two neurological mouse mutants, Lc and chp, were found to map to the vicinity of Math1, but are not caused by mutations in Math1. The evolutionary conservation of this gene and its mRNA expression patterns during embryogenesis suggests that it plays a key role in the development of the vertebrate central nervous system.

Olfactory receptor gene cluster on human chromosome 17: possible duplication of an ancestral receptor repertoire

A gene superfamily of olfactory receptors (ORs) has recently been identified in a number of species. These receptors share a seven transmembrane domain structure with many neurotransmitter and hormone receptors, and are likely to underlie the recognition and G-protein-mediated transduction of odorant signals. Previously, OR genes cloned in different species were from random locations in the respective genomes. We report here the cloning of 16 human OR genes, all from chromosome 17 (17p13.3). The intronless coding regions are physically mapped (on 35 cosmids) in one 0.35Mb long contiguous cluster, with an average intergenic separation of 15kb. The human OR genes in the cluster belong to four different gene subfamilies, displaying as much sequence variability as any randomly selected group of ORs. This suggests that the cluster identified may be one of several copies of an ancestral OR gene repertoire whose existence may predate the divergence of mammals. The latter may have duplicated in some species to form the present mammalian OR gene repertoire, with several hundred genes. The human chromosome 17 OR gene cluster may thus be a good model for understanding human olfaction, as well as the ontogeny and phylogeny of the OR gene superfamily.

Glutathione S-transferases in rat olfactory epithelium: purification, molecular properties and odorant biotransformation

The olfactory epithelium is exposed to a variety of xenobiotic chemicals, including odorants and airborne toxic compounds. Recently, two novel, highly abundant, olfactory-specific biotransformation enzymes have been identified: cytochrome P-450olf1 and olfactory UDP-glucuronosyltransferase (UGT(olf)). The latter is a phase II biotransformation enzyme which catalyses the glucuronidation of alcohols, thiols, amines and carboxylic acids. Such covalent modification, which markedly affects lipid solubility and agonist potency, may be particularly important in the rapid termination of odorant signals. We report here the identification and characterization of a second olfactory phase II biotransformation enzyme, a glutathione S-transferase (GST). The olfactory epithelial cytosol shows the highest GST activity among the extrahepatic tissues examined. Significantly, olfactory epithelium had an activity 4-7 times higher than in other airway tissues, suggesting a role for this enzyme in chemoreception. The olfactory GST has been affinity-purified to homogeneity, and shown by h.p.l.c. and N-terminal amino acid sequencing to constitute mainly the Yb1 and Yb2 subunits, different from most other tissues that have mixtures of more enzyme classes. The identity of the olfactory enzymes was confirmed by PCR cloning and restriction enzyme analysis. Most importantly, the olfactory GSTs were found to catalyse glutathione conjugation of several odorant classes, including many unsaturated aldehydes and ketones, as well as epoxides. Together with UGT(olf), olfactory GST provides the necessary broad coverage of covalent modification capacity, which may be crucial for the acuity of the olfactory process.

Interaction of the beta-adrenergic receptor with Gs following delipidation. Specific lipid requirements for Gs activation and GTPase function

Preparations of beta-adrenergic receptor and Gs from turkey erythrocytes were delipidated by previously developed procedures. Three synthetic phospholipids, dioleoylglycerophosphoethanolamine, dioleoylglycerophosphocholine and dioleoylglycerophosphoserine plus an unphosphorylated lipid, were all required to restore receptor-mediated activation of Gs by GTP[gamma S]. The same lipids were necessary for the reconstitution of the isoproterenol-enhanced GTPase. The requirement for the unphosphorylated lipid could be fulfilled by 1-mono-oleoyl glycerol, alpha-tocopherol or oleic acid. Cholesterol hemisuccinate further enhanced the receptor-mediated activity of the relipidated system when present in addition to the lipids specified above. Cholesterol hemisuccinate had no effect on the basal rate of Gs activation and depressed the basal GTPase. It is therefore suggested that cholesterol hemisuccinate affects the receptor or the coupling of the receptor to Gs. In the system relipidated with the three dioleoyl phospholipids, plus alpha-tocopherol and cholesterol hemisuccinate, the initial rate of Gs activation per mole receptor appeared to be considerably higher than in the native turkey erythrocyte membrane.