Dscam-mediated self- versus non-self-recognition by individual neurons.
Explore the source record for details and available documents.
Alternative Splicing↗
Biomedical subjects
A Chess
Publications and source records attributed to A Chess.
Asynchronous replication and allelic exclusion in the immune system.
The development of mature B cells involves a series of molecular decisions which culminate in the expression of a single light-chain and heavy-chain antigen receptor on the cell surface. There are two alleles for each receptor locus, so the ultimate choice of one receptor type must involve a process of allelic exclusion. One way to do this is with a feedback mechanism that downregulates rearrangement after the generation of a productive receptor molecule, but recent work suggests that monoallelic epigenetic changes may also take place even before rearrangement. To better understand the basis for distinguishing between alleles, we have analysed DNA replication timing. Here we show that all of the B-cell-receptor loci (mu, kappa and lambda) and the TCRbeta locus replicate asynchronously. This pattern, which is established randomly in each cell early in development and maintained by cloning, represents an epigenetic mark for allelic exclusion, because it is almost always the early-replicating allele which is initially selected to undergo rearrangement in B cells. These results indicate that allelic exclusion in the immune system may be very similar to the process of X chromosome inactivation.
Alleles↗
Allele-specific expression patterns of interleukin-2 and Pax-5 revealed by a sensitive single-cell RT-PCR analysis.
Autosomal genes that are subject to random allelic inactivation (RAI), like imprinted genes [1] and genes subject to X-inactivation [2], require mechanisms that dictate the differential transcriptional regulation of two sequence-identical alleles. RAI genes include olfactory receptor genes [3], and the various genes encoding antigen receptors on lymphocytes (immunoglobulin genes, T cell receptor genes and NK receptor genes [4] [5] [6] [7]). These observations raise the possibility that other genes might be similarly regulated. Moreover, an interesting possibility is that certain genes might be monoallelically expressed in some cells and biallelically expressed in others. Recently, reports of monoallelic expression of interleukin-2 (IL-2) [8] [9] and IL-4 [10] [11] have raised the possibility that the cytokine gene family may be subject to monoallelic expression. Another report suggests that the gene encoding the transcription factor Pax-5, which is involved in B-cell (and cerebellar) development [12] [13], is also subject to monoallelic expression [14]. Using a novel single-cell reverse transcription-polymerase chain reaction (RT-PCR) approach, we have analyzed the IL-2 and Pax-5 genes in mice. We found that IL-2 is monoallelically transcribed in some T cells and biallelically transcribed in others, raising interesting questions regarding cytokine gene regulation. Additionally, our analyses suggest that Pax-5 is consistently biallelically transcribed. Thus, the IL-2 gene and other cytokine genes may be regulated in a stochastic manner that results in 0, 1 or 2 alleles of a given cytokine gene expressed in each T cell. This type of regulation could account for the wide variety of different combinations of cytokine genes expressed in individual T cells and therefore plays a role in the generation of T cells with a range of different effector functions.
Alleles↗
Odorant receptors: axon contact-mediated diversity.
To make the most of a small number of neurons, the nematode olfactory system includes neurons that are bilaterally symmetrical in morphology, but differ in the sets of genes they express. An intriguing recent example is the axon contact-mediated asymmetry in expression of the str-2 odorant receptor gene.
Animals↗
Olfactory neurons are interdependent in maintaining axonal projections.
In mice, individual olfactory neurons express one of the thousand distinct olfactory receptor genes [1] [2] [3]. Neurons that express a given receptor converge on distinct loci in the olfactory bulb to form structures called glomeruli [4] [5] [6]. The olfactory receptor is involved in an instructive manner in this axonal convergence [6] [7] but little is known about the mechanisms involved in maintaining convergence. We have previously created a transgenic olfactory receptor locus that functions independently of the endogenous loci [8]. Here, we show that, although the projections of neurons expressing this ectopic transgenic olfactory receptor always converge in newborn mice, surprisingly, in adult mice, convergence is not always maintained. Moreover, in adult mice there is a positive correlation between the number of neurons expressing the transgenic receptor and the probability of maintaining convergence. These observations, taken together with the variability observed in wild-type [4] [6] and genetically manipulated mice ([6] and our unpublished observations), suggest that olfactory neurons require the presence of other similar axons to maintain a glomerulus. We call this phenomenon interdependence.
Animals↗
YAC transgene-mediated olfactory receptor gene choice.
In the mouse, individual olfactory neurons express one of a thousand distinct olfactory receptor genes. Furthermore, only one allele of the expressed gene is transcribed. This phenomenon, random allelic inactivation, along with the observation that the olfactory receptor genes reside in large chromosomal arrays, suggests a role for long-range gene regulation in olfactory receptor gene choice. We have constructed a 300-kb yeast artificial chromosome (YAC) transgene in which a single receptor gene is marked while maintaining its coding region. This 300-kb piece of DNA functions as an independent olfactory receptor gene locus in directing olfactory receptor gene choice in both the olfactory system and the accessory olfactory system (vomeronasal organ, VNO). Furthermore, the transgene, like endogenous olfactory receptor loci, is subject to allelic inactivation.
Alleles↗
Convergent projections of Drosophila olfactory neurons to specific glomeruli in the antennal lobe.
Candidate Drosophila olfactory receptors (ORs) provide molecular tools to investigate how the organization of the Drosophila olfactory system determines the coding of olfactory stimuli. Neurons in the third antennal segment and maxillary palp appear to express different ORs. Individual olfactory neurons send axonal projections to glomeruli in the antennal lobe. Using transgenic flies, we provide evidence that the neurons expressing a given OR gene, which have cell bodies distributed among neurons expressing other ORs, converge in their projections to topographically fixed glomeruli in the antennal lobe. This convergence allows for the formation of an odotopic map in the antennal lobe whose organization could provide a basis for olfactory discrimination in Drosophila.
Animals↗
Identification of candidate Drosophila olfactory receptors from genomic DNA sequence.
We have taken advantage of the availability of a large amount of Drosophila genomic DNA sequence in the Berkeley Drosophila Genome Project database ( approximately 1/5 of the genome) to identify a family of novel seven transmembrane domain encoding genes that are putative Drosophila olfactory receptors. Members of the family are expressed in distinct subsets of olfactory neurons, and certain family members are restricted to distinct portions of the olfactory system. This pattern of expression has interesting similarities to and differences from the expression patterns observed for olfactory receptors in vertebrates. The Drosophila olfactory system is simpler than mammalian systems, yet it is complex enough to present a fascinating system in which to study neural information processing. Moreover, the powerful genetic manipulations available in Drosophila, when combined with electrophysiological and behavioral analyses, make this an attractive model system in which to study olfactory discrimination.
Amino Acid Sequence↗
Kappa chain monoallelic demethylation and the establishment of allelic exclusion.
Allelic exclusion in kappa light-chain synthesis is thought to result from a feedback mechanism by which the expression of a functional kappa light chain on the surface of the B cell leads to an intracellular signal that down-regulates the V(D)J recombinase, thus precluding rearrangement of the other allele. Whereas such a feedback mechanism clearly plays a role in the maintenance of allelic exclusion, here we provide evidence suggesting that the initial establishment of allelic exclusion involves differential availability of the two kappa alleles for rearrangement. Analysis of kappa+ B-cell populations and of individual kappa+ B cells that have rearranged only one allele demonstrates that in these cells, critical sites on the rearranged allele are unmethylated, whereas the nonrearranged allele remains methylated. This pattern is apparently generated by demethylation that is initiated at the small pre-B cell stage, on a single allele, in a process that occurs prior to rearrangement and requires the presence in cis of both the intronic and 3' kappa enhancers. Taken together with data demonstrating that undermethylation is required for rearrangement, these results indicate that demethylation may actually underly the process of allelic exclusion by directing the initial choice of a single kappa allele for rearrangement.
Alleles↗
Olfactory G proteins: simple and complex signal transduction.
In both vertebrates and invertebrates, olfactory perception is mediated by G-protein-coupled receptors. Recent work, in both mouse and Caenorhabditis elegans, sheds light on the role of specific G proteins in olfactory signal transduction, neuronal morphology and axon guidance.
Animals↗
Expansion of the allelic exclusion principle?
Explore the source record for details and available documents.
Alleles↗
Olfactory receptor gene regulation.
Explore the source record for details and available documents.
Animals↗
The specification of olfactory neurons.
Recent studies have shed light on the different relationships between odorant receptor expression and the specification of neural identity in the olfactory systems of vertebrates and invertebrates. In mice, neuronal identity and axon guidance are specified by the single expressed olfactory receptor, whereas in C. elegans, neuronal identity appears to be independent of receptor expression.
Animals↗
Allelic inactivation regulates olfactory receptor gene expression.
We suggest a model in which a hierarchy of controls is exerted on the family of odorant receptor genes to assure that a sensory neuron expresses a single receptor from a family of 1000 genes. We propose that a cis-regulatory element directs the stochastic expression of only one gene from a large array of linked receptor genes. Moreover, only one allelic array encoding multiple receptor genes is active in an individual neuron. We demonstrate that in a neuron expressing a given receptor, expression derives exclusively from one allele. In addition, we observe that alleles encoding the odorant receptors are replicated asynchronously, a phenomenon consistently associated with allelic inactivation. This model, involving inactivation of one allelic array and cis control of the active array, provides a mechanism such that individual neurons express one or a small number of receptors.
Alleles↗
The family of genes encoding odorant receptors in the channel catfish.
The anatomical and numerical simplicity of the fish olfactory system has led us to examine the family of olfactory receptors expressed in the catfish. We have identified a family of genes encoding seven transmembrane domain receptors that share considerable homology with the odorant receptors of the rat. The size of the catfish receptor repertoire appears to be far smaller than in mammals. Analysis of the nucleotide sequences suggests that these receptor genes have undergone positive Darwinian selection to generate enhanced diversity within the putative odorant-binding domains. Individual receptor clones anneal with 0.5%-2% of the olfactory neurons, suggesting that a single cell expresses only a small subset of distinct odorant receptors. Each cell, therefore, possesses a unique identity defined by the receptors it expresses. These data suggest that the brain may discriminate among odors by determining which neurons have been activated.
Amino Acid Sequence↗
Coding of olfactory information: topography of odorant receptor expression in the catfish olfactory epithelium.
Discrimination among the vast array of odors requires that the brain discern which of the numerous odorant receptors have been activated. If individual olfactory neurons express only a subset of the odorant receptor repertoire, then the nature of a given odorant can be discerned by identifying which cells have been activated. We performed in situ hybridization experiments demonstrating that individual olfactory neurons express different complements of odorant receptors and are therefore functionally distinct. Thus, a topographic map, defining either the positions of specific neurons in the epithelium or the positions of their projections, may be employed to determine the quality of an olfactory stimulus. Neurons expressing specific receptors appear to be randomly distributed within the olfactory epithelium. These data are consistent with a model in which randomly dispersed olfactory neurons with common receptor specificities project to common glomeruli in the olfactory bulb.
Animals↗
Molecular cloning and single-channel properties of the cyclic nucleotide-gated channel from catfish olfactory neurons.
We have cloned a functional cDNA encoding the cyclic nucleotide-gated channel selectively expressed in catfish olfactory sensory neurons. The cyclic nucleotide-gated channels share sequence and structural features with the family of voltage-gated ion channels. This homology is most evident in transmembrane region S4, the putative voltage sensor domain, and the H5 domain, thought to form the channel pore. We have characterized the single-channel properties of the cloned catfish channel and compared these properties with the channel in native catfish olfactory sensory neurons. The channel is activated equally well by cAMP and cGMP, shows only a slight voltage dependence of gating, and exhibits a pH- and voltage-dependent subconductance state similar to that observed for the voltage-gated L-type calcium channel.
Amino Acid Sequence↗
Molecular biology of smell: expression of the multigene family encoding putative odorant receptors.
Explore the source record for details and available documents.
Amino Acid Sequence↗