Olfactory receptor accessory proteins play crucial roles in receptor function and gene choice

Abstract

Each of the olfactory sensory neurons (OSNs) chooses to express a single G protein-coupled olfactory receptor (OR) from a pool of hundreds. Here, we show the receptor transporting protein (RTP) family members play a dual role in both normal OR trafficking and determining OR gene choice probabilities. Rtp1 and Rtp2 double knockout mice (RTP1,2DKO) show OR trafficking defects and decreased OSN activation. Surprisingly, we discovered a small subset of the ORs are expressed in larger numbers of OSNs despite the presence of fewer total OSNs in RTP1,2DKO. Unlike typical ORs, some overrepresented ORs show robust cell surface expression in heterologous cells without the co-expression of RTPs. We present a model in which developing OSNs exhibit unstable OR expression until they choose to express an OR that exits the ER or undergo cell death. Our study sheds light on the new link between OR protein trafficking and OR transcriptional regulation.DOI: http://dx.doi.org/10.7554/eLife.21895.001

Structural instability and divergence from conserved residues underlie intracellular retention of mammalian odorant receptors

“…To gain a more comprehensive understanding of OR cell surface expression, we selected 210 ORs (76 oORs and 134 uORs in vivo) and tested which ORs exhibit cell surface expression in heterologous cells. Consistent with previous findings (21), oORs, as a group, showed more robust OR surface expression than uORs (p<0.05, U test) ( Fig. 3A).…”

“…In addition to RTP1 and RTP2, a previous study showed that an Hsp70 homolog enhanced the cell surface expression of an OR, suggesting the importance of chaperones for the cell surface expression of ORs (45). A possibility is that the unstable nature of ORs may be integral to proper OSN development (21,46) . Initiation of OR expression induces unfolded protein response (UPR) in developing OSNs, suggesting inefficient folding and ER accumulation of ORs in these cells (46,47).…”

“…Initiation of OR expression induces unfolded protein response (UPR) in developing OSNs, suggesting inefficient folding and ER accumulation of ORs in these cells (46,47). RTP1 and RTP2 are induced by UPR signaling in developing OSNs, presumably allowing OR proteins to exit the ER and downregulate UPR (21,46,47) . There is also the possibility that OSNs lack specific quality control proteins that are common in other cell types, allowing less stable ORs to be trafficked to the cell membrane where they are finally functional.…”

“…Receptor transporting protein (RTP) 1 and RTP2, which are proposed to act as chaperones in the olfactory sensory neurons, enhance the cell surface expression of many ORs when co-transfected in heterologous cells (15)(16)(17)(18)(19)(20). In RTP1 and RTP2 double knockout mice (RTP DKO), the majority of ORs are significantly underrepresented (uORs) due to the absence of mature OSNs expressing them, suggesting that these ORs require RTP1 and RTP2 in order to function (21).…”

A Testis ER Chaperone Marks Mature Olfactory and Vomeronasal Sensory Neurons

“…A number of factors have been identified that can aid in ER exit and plasma membrane expression of ORs and VRs. Among these are receptor transporting protein 1 and 2 (Rtp1/2) (Saito et al, 2004;Bush and Hall, 2008;Sharma et al, 2017), receptor expression enhancing protein 1 (REEP1), and Hsc70t (Neuhaus, 2006), a testisenriched member of the Hsp70 family of chaperones. These factors appear to increase cell surface expression for some, but not all, ORs.…”