Prominent roles for odorant receptor coding sequences in allelic exclusion

Abstract

Mammalian odorant receptors (ORs) are crucial for establishing the functional organization of the olfactory system, but the mechanisms controlling their expression remain largely unexplained. Here, we utilized a transgenic approach to explore OR gene regulation. We determined that although olfactory sensory neurons (OSNs) are capable of supporting expression of multiple functional ORs, several levels of control ensure that each neuron normally expresses only a single odorant receptor. Surprisingly, this regulation extends beyond endogenous ORs even preventing expression of transgenes consisting of OR-coding sequences driven by synthetic promoters. Thus, part of the intrinsic feedback system must rely on elements present in the OR-coding sequence. Notably, by expressing the same transgenic ORs precociously in immature neurons, we have overcome this suppression and established a generic method to express any OR in approximately 90% of OSNs. These results provide important insights into the hierarchy of OR gene expression and the vital role of the OR-coding sequence in this regulation.

Figure 1. Strategies used to investigate OR expression

(A) Transgenic lines were generated using OMP and Gγ8 promoters directly upstream of three different OR coding sequences. Surprisingly, none of these 31 lines expressed detectable transgenic-mRNA in the MOE. (B) In an alternative approach, OR expression in OSNs was driven indirectly from olfactory promoters using a tetracycline-transactivator based system. We generated transgenic lines where the Gγ8 promoter drives expression of TTA in immature OSNs (i) and used two knockin lines: one driving TTA under the control of the OMP promoter in most mature OSNs (ii) and the other designed to express rTTA in a subset of mature OSNs that also express the P2 receptor (iii). These tetracycline transactivator lines were crossed with multiple lines of TetO-OR mice (iv) to express transgenic OR and bicistronically encoded GFP (or LacZ) in OSNs.

Figure 2. TetO-OR transgenes driven by OMP-TTA are broadly expressed in the MOE

3-week old mice carrying the OMP-TTA knockin express TetO-OR-IRES-GFP transgenes but only in subsets of sensory neurons of the MOE (A-D). GFP fluorescence (A-D, G) and immunohistochemistry (D) demonstrate expression of a TetO-M72 (A, G), TetO-rI7 (B) and two TetO-MOR28 lines (C, D) in 10–30% of OSNs; note that as expected the cells expressing GFP are immunoreactive for the co-expressed transgenic OR (D). OMP is expressed in a much larger number of OSNs as indicated by an OMP-GFP knockin (E). In contrast to their limited expression in the MOE most TetO-OR transgenes are expressed in the majority of VNO sensory neurons (with 5 of 6 lines showing expression in >80% of neurons) as indicated for a TetO-M72 line (F). Expression of transgenic and endogenous ORs are mutually exclusive in OSNs (G) as indicated by the lack of cellular overlap between GFP fluorescence (TetO-M72, green) and endogenous MOR28 (immunofluorescence, red). Scale bars, MOE = 20 μm, VNO = 100 μm.

Figure 3. Suppression of TetO-OR transgenes in cells expressing endogenous ORs

In situ hybridization using an rTTA probe demonstrates that 6 week old mice carrying the P2-rTTA knockin and TetO-rI7 transgene express rTTA in an OR like pattern after doxycycline treatment (A). In contrast, GFP-fluorescence revealed that very few, basally-localized, OSNs express the TetO-rI7 transgene (B, see text for more detail). Broken lines indicate the base of the MOE; scale bars, A: 200 μm; B: 10 μm.

Figure 4. Precocious expression of TetO-OR using Gγ8-TTA overcomes suppression of the TetO-transgene

In situ hybridization of the MOE of 1–2 week old mice with Gγ8 (A) and TTA (B) probes indicate that TTA is expressed in a large subset (>50%) of Gγ8 positive neurons in Gγ8-TTA animals. GFP fluorescence demonstrates that in 2–3 week-old double transgenic mice (carrying Gγ8-TTA and TetO-OR transgenes) many OSNs express TetO-M72 (C) and TetO-rI7 (D). GFP positive OSNs (C-D) extend dendrites to the surface of the MOE with the more apically located neurons co-expressing OMP (red immunofluorescence, D). Electrophysiological recordings from OSNs expressing the M72-transgene or rI7-transgene of 1–2 week old mice (E) demonstrate that transgenic receptors are functional and confer selective responses to acetophenone (ACE-M72, n=24 cells) and octanal (OCT-rI7, n=7 cells). Arrowheads mark onset of odor pulses that selectively induced rapid depolarization of OSNs triggering trains of action potentials (inset). (F) Representative confocal images of the MOE from 3-week old mice carrying both Gγ8-TTA and TetO-OR transgenes as well as a P2-lacZ knockin construct to mark cells expressing the endogenous P2-receptor show that cells expressing transgenic ORs (GFP-fluorescence) do not co-express the endogenous receptor P2 (red, β-galactosidase-immunofluorescence). Similarly, there was no overlap between transgenic (green, GFP) and the endogenous OR, MOR28 (red, immunofluorescence) in Gγ8-TTA, TetO-M72 double-transgenic animals (G). Note that the different levels of GFP-fluorescence reflect regional differences in Gγ8-TTA expression, with higher expression in dorsal regions (F, left panel) compared to ventral regions where MOR28 is predominantly expressed (G). Scale bar, 20 μm.

Figure 5. Expression of transgenic receptor in most OSNs

(A) MOE from a 6 week old Gγ8-TTA, TetO-M72 double transgenic animal showing transgene expression (GFP, green) in significantly fewer OSNs than in younger mice (see Fig. 4). Note these GFP-positive cells show a markedly basal localization that corresponds with Gγ8-expression in immature OSNs. However, the expression pattern of TetO-M72 driven by OMP-TTA in 6 week old mice (B) remains similar to that observed in 3 week old animals (Fig. 2) i.e. restricted to 10–30% of OSNs. Remarkably, in combination Gγ8-TTA and OMP-TTA are capable of driving TetO-OR transgene expression in virtually all OSNs as shown for 6-week old mice expressing TetO-M72 (C, E) and TetO-rI7 (D). Such mice retain a very small number of OSNs expressing endogenous receptors as indicated by staining for a P2-lacZ reporter (red, E). Dotted lines indicate the base of the MOE; scale bar, 20 μm (A-E). (F) Diagram summarizing these results depicts expression of endogenous receptors (red) and TetO-OR transgenes driven by Gγ8-TTA and/or OMP-TTA (green) in non-overlapping subsets of OSNs. When TTA expression is driven by Gγ8 (left lane), only a small subset of relatively immature neurons express the OR-transgene reflecting the limited expression of Gγ8 in 6 week old mice. Indirect expression of OR-transgenes using OMP-TTA (middle) results in endogenous receptor mediated silencing of the OR-transgene in the majority of OSNs. In contrast, by combining OMP-TTA and Gγ8-TTA (right lane), precocious expression of the transgene under the control of Gγ8-TTA is maintained in mature neurons under the control of OMP-TTA silencing expression of endogenous ORs in the vast majority of OSNs.

Figure 6. GPCR function is not required for OR suppression and OSNs are competent to express two functional receptors

(A) Diagram indicates that mutation of the highly conserved “DRY” sequence of M72 to “ALE” (red residues) prevents G-protein coupling (Acharya and Karnik, 1996). However, this mutation has no influence on the mutually exclusive expression of the transgenic (TetO-M72_ALE) and endogenous receptors driven by OMP-TTA (B, left panel) or Gγ8-TTA (B, right panel). Shown are TetO-M72_ALE expression (green) with endogenous MOR28 immunofluorescence (B, left panel, red) and β-galactosidase immunostaining in 3 week old mice also carrying a P2-lacZ allele (B, right panel, red). The transgene used to co-express two odorant receptors in an OSN is schematically shown (C). In situ hybridization and GFP fluorescence (D) indicate that both ORs (M72 and rI7) and GFP are expressed in the MOE of 3 week old mice when driven by OMP-TTA. Moreover, functional recordings (E) from GFP-labeled OSNs (1-2 week old animals) demonstrate that these neurons co-express M72 and rI7 and respond to both octanal (OCT) and acetophenone (ACE); insets show that both odorants trigger action potentials (n=17 cells). Scale bar, 10 μm (B), 20 μm (D).