Olfactory receptor–dependent receptor repression in Drosophila

Abstract

In olfactory systems across phyla, most sensory neurons express a single olfactory receptor gene selected from a large genomic repertoire. We describe previously unknown receptor gene–dependent mechanisms that ensure singular expression of receptors encoded by a tandem gene array [Ionotropic receptor 75c (Ir75c), Ir75b, and Ir75a, organized 5′ to 3′] in Drosophila melanogaster. Transcription from upstream genes in the cluster runs through the coding region of downstream loci and inhibits their expression in cis, most likely via transcriptional interference. Moreover, Ir75c blocks accumulation of other receptor proteins in trans through a protein-dependent, posttranscriptional mechanism. These repression mechanisms operate in endogenous neurons, in conjunction with cell type–specific gene regulatory networks, to ensure unique receptor expression. Our data provide evidence for inter-olfactory receptor regulation in invertebrates and highlight unprecedented, but potentially widespread, mechanisms for ensuring exclusive expression of chemosensory receptors, and other protein families, encoded by tandemly arranged genes.

Olfactory Receptor Gene Regulation in Insects: Multiple Mechanisms for Singular Expression

“…(C) Feedback mechanisms contributing to the refinement and/or stability of receptor expression. Transcriptional interference by OrX of OrY might occur when inefficient transcriptional termination at the 3′ end of the former gene leads to the RNA polymerase II impeding transcription initiation at OrY (solid wavy orange and purple lines represent protein coding transcripts from OrX and OrY , respectively; the dashed purple line represents the 3′UTR of OrX transcripts that incorporate sequences encoded by OrY that are not translated into OrY) ( Mika et al, 2021 ). Receptor protein-dependent feedback on transcript or protein levels of other (not necessarily closely linked) receptors occurs through unknown mechanisms ( Maguire et al, 2020 ; Jafari et al, 2021 ; Mika et al, 2021 ).…”

“…Several TFs required for the correct expression of receptor genes in specific populations of neurons have been identified in D. melanogaster through loss-of-function genetic screens ( Jafari et al, 2012 ; Chai et al, 2019 ; Mika et al, 2021 ), candidate approaches ( Tichy et al, 2008 ; Li et al, 2013 ), and expression screens ( Li et al, 2020 ) [reviewed in Fuss and Ray (2009) ; Barish and Volkan (2015) , and Yan et al (2020) ]. Analogous to contributions of CREs, TFs can promote or repress receptor expression (and can have different roles for different genes), and unique combinations of these factors are required for individual receptors ( Figure 2A ).…”

“…Analogous to contributions of CREs, TFs can promote or repress receptor expression (and can have different roles for different genes), and unique combinations of these factors are required for individual receptors ( Figure 2A ). The convergence of several genetic screens on the same TFs (e.g., Pdm3 and E93) ( Jafari et al, 2012 ; Chai et al, 2019 ; Mika et al, 2021 ) suggests that a majority of the core trans -acting regulatory proteins have been identified. These TFs contain diverse types of DNA binding domains and while some orthologous proteins might have similar roles in other insects (e.g., Acj6) ( Clyne et al, 1999a ; Fujii et al, 2011 ), they are not obviously related to key TFs functioning in OR expression in mice ( Dalton and Lomvardas, 2015 ; Monahan and Lomvardas, 2015 ).…”

“…The lack of correlation between the presence of a TF binding motif in a CRE and TF requirement for a given receptor might reflect differences in in vitro and in vivo binding specificities for TFs and/or an indirect requirement for trans -acting factors in controlling receptor expression. Indeed, temporal manipulation of TF function indicates that several of these proteins have multiple roles in OSN development, for example, during SOP lineage specification ( Bai and Carlson, 2010 ; Jafari et al, 2012 ; Chai et al, 2019 ; Arguello et al, 2021 ; Mika et al, 2021 ). Moreover, many TFs are expressed and required in late pupal/adult stages implying roles in both initiation and maintenance of correct receptor expression ( Bai and Carlson, 2010 ; Jafari et al, 2012 ; Arguello et al, 2021 ; Mika et al, 2021 ).…”

“…Indeed, temporal manipulation of TF function indicates that several of these proteins have multiple roles in OSN development, for example, during SOP lineage specification ( Bai and Carlson, 2010 ; Jafari et al, 2012 ; Chai et al, 2019 ; Arguello et al, 2021 ; Mika et al, 2021 ). Moreover, many TFs are expressed and required in late pupal/adult stages implying roles in both initiation and maintenance of correct receptor expression ( Bai and Carlson, 2010 ; Jafari et al, 2012 ; Arguello et al, 2021 ; Mika et al, 2021 ). The biochemical properties of TF/CRE interactions that promote stable receptor expression in a given OSN type remain, however, largely elusive.…”