We have quantitated mRNA levels for the non-odorant G protein-coupled receptors encoded in the mouse genome in 41 tissues and provide this data set as a resource for predicting roles for incompletely characterized GPCRs, exploring tissue-specific gene expression, and other purposes. The fact that tissues that comprise classical physiological systems (cardiovascular, gastrointestinal, etc.) were clustered together simply on the basis of their GPCR repertoires speaks to the key roles that GPCRs play in homeostatic regulation.
Our anatomic expression profiling yielded a large amount of information consistent with known physiology, and high-level expression of a GPCR in a particular tissue cluster or specific tissue correlated well with its physiological role. While this result is not surprising, it does provide confidence that roles for orphan receptors or GPCRs not known to play a role in a particular physiological process might be predicted by presence in a given cluster. Our demonstration that Gpr91 expression pointed to a role for extracellular succinate in regulating lipolysis in adipocytes validates this notion and is also of intrinsic interest.
The concentration of succinate in plasma has been reported at 5−125 μM, a range that surrounds the EC50
for Gpr91 activation (He et al., 2004
) and the IC50
for inhibition of lipolysis in WAT (). Succinate concentrations increase during exercise and metabolic acidosis and, in rodents, in hyperglycemic metabolic states (Forni et al., 2005
; Hochachka and Dressendorfer, 1976
; Krebs, 1950
; Kushnir et al., 2001
; Nordmann and Nordmann, 1961
; Sadagopan et al., 2007
). Thus, excursions in the levels of extracellular succinate do occur and might regulate adipocyte function in vivo. Adipocyte function was not investigated in mice lacking Gpr91, which are grossly healthy (He et al., 2004
). Overall, a physiological role for succinate in regulating adipocyte metabolism is plausible, but when and how such a system might be important and/or redundant with other systems that govern adipocyte function remains unknown.
49 of the 353 GPCRs profiled were expressed in only one or two of the 41 tissues examined (see Table 2 in the Supplement
). Such confined expression might point up targets of pharmaceutical interest. For example, testes showed a GPCR expression pattern very distinct from that of other tissues. Gpr150, Gpr66 and Gpr15, Mtnr1a, Pgr23 were almost perfectly specific to testes. Whether such receptors play a role in spermatogenesis or other testicular functions and their potential utility as targets for drugs aimed at controlling fertility is unknown.
A comparison of each receptor's expression pattern with that of every other () provided a means of pointing up possible roles for a given receptor outside its main physiological cluster. Hm74(Gpr109a), the ketone body receptor that is activated therapeutically by niacin to treat dyslipidemias (Soga et al., 2003
; Tunaru et al., 2003
; Wise et al., 2003
) provides an interesting example. By traditional clustering analysis Hm74(Gpr109a) is placed in the “adipose” cluster (), and activation of adipocyte Hm74 likely mediates the anti-lipolytic actions of niacin (Tunaru et al., 2003
). It was recently shown that the skin flushing side effect of niacin (Carlson, 2005
) is mediated by Hm74 expression by bone-marrow derived epidermal Langerhans cells that release of vasodilatory prostanoids (Benyo et al., 2006
; Benyo et al., 2005
). By quantitative profiling across tissues, Hm74 was noted to be expressed relatively highly in skin and other ”barrier-cluster” tissues as well as adipose (). By expression correlation analysis, Hm74 was not found with the adipose cluster on the diagonal but instead clustered with receptors with more widespread expression (Supplemental Figures S4
). A search of off-diagonal interactions revealed that Hm74 interacts not only with the adipose cluster, but also with receptors in both immune and barrier clusters (Supplemental Figure S4
). Thus, analysis of GPCR expression data from these different perspectives may generate hypotheses regarding on-target side effects of drugs.
GPCR genes are usually relatively small, often intronless, and range from closely to distantly related. These features plus the availability of quantitative expression data across multiple tissues for hundreds of related genes that show clusters based on shared tissue-specific patterns may provide a resource for those interested in identifying the combinations of cis-acting elements that specify gene expression in a given cell type.
Some GPCRs are thought to function as heterodimers, and cluster analysis for co-expression might point to potential receptor pairs. For example, Gabbr1 is currently thought to function as an obligate heterodimer with Gpr51(Gabbr2) (Jones et al., 1998
; Kaupmann et al., 1998
; Kuner et al., 1999
; White et al., 1998
). Our data (Supplement S2
) and that of others (Calver et al., 2000
) demonstrate similar expression patterns for these receptors in CNS and divergent expression in the periphery. These results are consistent with Gabbr1 and Gabbr2 heterodimer formation in the CNS but raise the possibility that in the periphery, expression of one or the other partner is regulated, that these receptors may use other partners, and/or that heterodimerization may not be required in all settings (Cheng et al., 2007
Several caveats should be stated regarding interpreting our expression data. 1) Relative mRNA levels, of course, will not always reflect relative protein expression levels nor the relative importance of a particular receptor in a particular tissue. Indeed, some important receptors such as adrenergic receptors were expressed at relatively low levels. 2) Tissues are comprised of multiple cell types, and receptor expression can be restricted to a minority cell type. In the extreme, receptor expression in a minority population can be missed by whole tissue analysis. For example, neither our qPCR data (supplemental S2
) nor those of others (Liberles and Buck, 2006
) detected trace amine associated receptor 1 (Taar1) expression in brain, but a Taar 1 Lac-Z knock-in mouse revealed Taar1 expression in discrete neuronal populations and Taar1-dependent regulation of dopaminergic activity (Lindemann et al., 2008
). 3) The presence of a receptor within a particular anatomical cluster does not exclude important functions in tissues outside that cluster. For example, the D2 dopamine receptor (Drd2) is recognized as the major dopamine receptor subtype in pituitary (Kelly et al., 1997
; Saiardi et al., 1997
), but Drd2 was in the CNS cluster while the D3 dopamine receptor (Drd3) was in the pituitary cluster (). The cluster analysis presented here used the Pearson correlation, which normalizes expression levels (see Methods) to focus on gaining information from the pattern of gene expression at the cost of ignoring absolute levels of receptor expression, and the Drd cluster results are presumably a result of the fact that Drd2 is expressed at moderate levels in numerous CNS structures while Drd3 is expressed at very low levels in only a few structures (supplemental S2
). However, our quantitative expression data (supplemental S2
) reveals that Drd2 is expressed approximately 100-fold higher than Drd3 in pituitary, consistent with the dominant role of Drd2 in pituitary function. Thus, this resource is best utilized when the data are analyzed from several perspectives and should be viewed as a means of generating hypotheses to be tested experimentally. Raw expression data for the 353 GPCRs in the 41 organ samples are available online at http://pdsp.med.unc.edu/apGPCRe/
. for those who wish to perform their own analyses.
Lastly, expression of individual GPCRs in specific tissues can be different in human and mouse. When used together with GPCR expression patterns in human (SymAtlas, SAGEmap), our dataset should facilitate rational use of mouse to model the roles of GPCRs in human physiology and disease.