Rare event of histone demethylation can initiate singular gene expression of olfactory receptors
ABSTRACT Mammals sense odors through the gene family of olfactory receptors (ORs). Despite the enormous number of OR genes (∼1,400 in mouse), each olfactory sensory neuron expresses one, and only one, of them. In neurobiology, it remains a long-standing mystery how this singularity can be achieved despite intrinsic stochasticity of gene expression. Recent experiments showed an epigenetic mechanism for maintaining singular OR expression: Once any ORs are activated, their expression inhibits further OR activation by down-regulating a histone demethylase Lsd1 (also known as Aof2 or Kdm1a), an enzyme required for the removal of the repressive histone marker H3K9me3 on OR genes. However, it remains unclear at a quantitative level how singularity can be initiated in the first place. In particular, does a simple activation/feedback scheme suffice to generate singularity? Here we show theoretically that rare events of histone demethylation can indeed produce robust singularity by separating two timescales: slow OR activation by stepwise H3K9me3 demethylation, and fast feedback to turn off Lsd1. Given a typical 1-h response of transcriptional feedback, to achieve the observed extent of singularity (only 2% of neurons express more than one ORs), we predict that OR activation must be as slow as 5-10 d-a timescale compatible with experiments. Our model further suggests H3K9me3-to-H3K9me2 demethylation as an additional rate-limiting step responsible for OR singularity. Our conclusions may be generally applicable to other systems where monoallelic expression is desired, and provide guidelines for the design of a synthetic system of singular expression.
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- "Thus, for most ORs, H3K9 demethylation is necessary, but not sufficient, for expression at high cellular levels. Although recent theoretical calculations support that a slow demethylation process combined with a fast feedback can explain singular OR expression (Tan et al., 2013), our experimental findings suggest that an additional regulatory layer may also contribute to the singularity of OR choice. Were the absence of H3K9 methylation sufficient for robust OR transcription, then most G9a/GLP KO OSNs should express multiple ORs at high levels. "
ABSTRACT: An astounding property of the nervous system is its cellular diversity. This diversity, which was initially realized by morphological and electrophysiological differences, is ultimately produced by variations in gene-expression programs. In most cases, these variations are determined by external cues. However, a growing number of neuronal types have been identified in which inductive signals cannot explain the few but decisive transcriptional differences that cause cell diversification. Here, we show that heterochromatic silencing, which we find is governed by histone methyltransferases G9a (KMT1C) and GLP (KMT1D), is essential for stochastic and singular olfactory receptor (OR) expression. Deletion of G9a and GLP dramatically reduces the complexity of the OR transcriptome, resulting in transcriptional domination by a few ORs and loss of singularity in OR expression. Thus, our data suggest that, in addition to its previously known functions, heterochromatin creates an epigenetic platform that affords stochastic, mutually exclusive gene choices and promotes cellular diversity.Cell Reports 11/2014; 9(3):884-92. DOI:10.1016/j.celrep.2014.10.001 · 8.36 Impact Factor
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ABSTRACT: Blood tests, which are commonly used for cancer screening, generally have low sensitivity. Here, we developed a novel rapid and simple method to generate silver nanoscale hexagonal columns (NHCs) for use in surface-enhanced Raman scattering (SERS). We reported that the intensity of SERS spectra of clinical serum samples obtained from gastrointestinal cancer patients is was significantly higher than that of SERS spectra of clinical serum samples obtained from non-cancer patients. We estimated the combined constituents on silver NHCs by using a field emission-type scanning electron microscope, Raman microscopes, and a 3D laser scanning confocal microscope. We obtained the Raman scattering spectra of samples of physically fractured cells and clinical serum. No spectra were obtained for chemically lysed cultured cells and DNA, RNA, and protein extracted from cultured cells. We believe that our method, which uses SERS with silver NHCs to detect circulating nucleosomes bound by methylated cell-free DNA, may be successfully implemented in blood tests for cancer screening.Scientific Reports 05/2015; 5:10455. DOI:10.1038/srep10455 · 5.58 Impact Factor
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ABSTRACT: This chapter describes the main characteristics of olfactory receptor (OR) genes of vertebrates, including generation of this large multigenic family and pseudogenization. OR genes are compared in relation to evolution and among species. OR gene structure and selection of a given gene for expression in an olfactory sensory neuron (OSN) are tackled. The specificities of OR proteins, their expression, and their function are presented. The expression of OR proteins in locations other than the nasal cavity is regulated by different mechanisms, and ORs display various additional functions. A conventional olfactory signal transduction cascade is observed in OSNs, but individual ORs can also mediate different signaling pathways, through the involvement of other molecular partners and depending on the odorant ligand encountered. ORs are engaged in constitutive dimers. Ligand binding induces conformational changes in the ORs that regulate their level of activity depending on odorant dose. When present, odorant binding proteins induce an allosteric modulation of OR activity. Since no 3D structure of an OR has been yet resolved, modeling has to be performed using the closest G-protein-coupled receptor 3D structures available, to facilitate virtual ligand screening using the models. The study of odorant binding modes and affinities may infer best-bet OR ligands, to be subsequently checked experimentally. The relationship between spatial and steric features of odorants and their activity in terms of perceived odor quality are also fields of research that development of computing tools may enhance. © 2015 Elsevier Inc. All rights reserved.Progress in molecular biology and translational science 01/2015; 130:1-36. DOI:10.1016/bs.pmbts.2014.11.001 · 3.49 Impact Factor