Zita Peterlin

Olfactory receptor responding to gut microbiota-derived signals plays a role in renin secretion and blood pressure regulation

Olfactory receptors are G protein-coupled receptors that mediate olfactory chemosensation and serve as chemosensors in other tissues. We find that Olfr78, an olfactory receptor expressed in the kidney, responds to short chain fatty acids (SCFAs). Olfr78 is expressed in the renal juxtaglomerular apparatus, where it mediates renin secretion in response to SCFAs. In addition, both Olfr78 and G protein-coupled receptor 41 (Gpr41), another SCFA receptor, are expressed in smooth muscle cells of small resistance vessels. Propionate, a SCFA shown to induce vasodilation ex vivo, produces an acute hypotensive response in wild-type mice. This effect is differentially modulated by disruption of Olfr78 and Gpr41 expression. SCFAs are end products of fermentation by the gut microbiota and are absorbed into the circulation. Antibiotic treatment reduces the biomass of the gut microbiota and elevates blood pressure in Olfr78 knockout mice. We conclude that SCFAs produced by the gut microbiota modulate blood pressure via Olfr78 and Gpr41.

A pharmacological profile of the aldehyde receptor repertoire in rat olfactory epithelium

Several lines of evidence suggest that odorants are recognized through a combinatorial process in the olfactory system; a single odorant is recognized by multiple receptors and multiple odorants are recognized by the same receptor. However few details of how this might actually function for any particular odour set or receptor family are available. Approaching the problem from the ligands rather than the receptors, we used the response to a common odorant, octanal, as the basis for defining multiple receptor profiles. Octanal and other aldehydes induce large EOG responses in the rodent olfactory epithelium, suggesting that these compounds activate a large number of odour receptors (ORs). Here, we have determined and compared the pharmacological profile of different octanal receptors using Ca2+ imaging in isolated olfactory sensory neurones (OSNs). It is believed that each OSN expresses only one receptor, thus the response profile of each cell corresponds to the pharmacological profile of one particular receptor. We stimulated the cells with a panel of nine odorants, which included octanal, octanoic acid, octanol and cinnamaldehyde among others (all at 30μm). Cluster analysis revealed several distinct pharmacological profiles for cells that were all sensitive to octanal. Some receptors had a broad molecular range, while others were activated only by octanal. Comparison of the profiles with that of the one identified octanal receptor, OR‐I7, indicated several differences. While OR‐I7 is activated by low concentrations of octanal and blocked by citral, other receptors were less sensitive to octanal and not blocked by citral. A lower estimate for the maximal number of octanal receptors is between 33 and 55. This large number of receptors for octanal suggests that, although the peripheral olfactory system is endowed with high sensitivity, discrimination among different compounds probably requires further central processing.

A G protein/cAMP signal cascade is required for axonal convergence into olfactory glomeruli

The mammalian odorant receptors (ORs) comprise a large family of G protein-coupled receptors that are critical determinants of both the odorant response profile and the axonal identity of the olfactory sensory neurons in which they are expressed. Although the pathway by which ORs activate odor transduction is well established, the mechanism by which they direct axons into proper glomerular relationships remains unknown. We have developed a gain-of-function approach by using injection of retroviral vectors into the embryonic olfactory epithelium to study the ORs′ contribution to axon guidance. By ectopically expressing ORs, we demonstrate that functional OR proteins induce axonal coalescence. Furthermore, ectopic expression of Gα mutants reveals that activation of the signal transduction cascade is sufficient to cause axonal convergence into glomeruli. Analysis of Gα subunit expression indicates that development and odorant transduction use separate transduction pathways. Last, we establish that the generation of cAMP through adenylyl cyclase 3 is necessary to establish proper axonal identity. Our data point to a model in which axonal sorting is accomplished by OR stimulation of cAMP production by coupling to Gαs.

Quantitative morphologic classification of layer 5 neurons from mouse primary visual cortex

The understanding of any neural circuit requires the identification and characterization of all its components. Morphologic classifications of neurons are, therefore, of central importance to neuroscience. We use a quantitative method to classify neurons from layer 5 of mouse primary visual cortex, based on multidimensional clustering. To reconstruct neurons, we used Golgi impregnations and biocytin injections, as well as DiOlistics, a novel technique of labeling neurons with lipophilic dyes. We performed computerized 3‐D reconstructions of 158 layer 5 cells to measure a series of morphologic variables. Principal component analysis and cluster analysis were used for the classification of cell types. Five major classes of cells were found: group 1 includes large pyramidal neurons with apical dendrites that reach layer 1 with an apical tuft; group 2 consists of short pyramidal neurons and large multipolar cells with “polarized” dendritic trees; group 3 is composed of less extensive pyramidal neurons; group 4 includes small cells; and group 5 includes another set of short pyramidal neurons in addition to “atypically oriented” cells. Our sample included a relatively homogeneous group of 27 neurons that project to the superior colliculus, which clustered mainly in group 1, thus supporting the validity of the classification. Cluster analysis of neuronal morphologies provides an objective method to quantitatively define different neuronal phenotypes and may serve as a basis for describing neocortical circuits. J. Comp. Neurol. 461:415–428, 2003.

The Importance of Odorant Conformation to the Binding and Activation of a Representative Olfactory Receptor

Olfactory receptors (ORs) form a large family of G protein-coupled receptor proteins (GPCRs) responsible for sensing the ambient chemical environment. The molecular recognition strategies used by ORs to detect and distinguish odorant molecules are unclear. Here, we investigated the variable of odorant carbon chain conformation for an established odorant-OR pair: n-octanal and rat OR-I7. A series of conformationally restricted octanal mimics were tested on live olfactory sensory neurons (OSNs). Our results support a model in which unactivated OR-I7 binds aliphatic aldehydes indiscriminately, and then applies conformational and length filters to distinguish agonists from antagonists. Specific conformers are proposed to activate OR-I7 by steric buttressing of an OR activation pocket. Probing endogenously expressed rat OSNs with octanal and constrained mimics furnished evidence that odorant conformation contributes to an odorants unique olfactory code signature.

The state of the art of odorant receptor deorphanization: A report from the orphanage

The odorant receptors (ORs) provide our main gateway to sensing the world of volatile chemicals. This involves a complex encoding process in which multiple ORs, each of which detects its own set of odorants, work as an ensemble to produce a distributed activation code that is presumably unique to each odorant. One marked challenge to decoding the olfactory code is OR deorphanization, the identification of a set of activating odorants for a particular receptor. Here, we survey various methods used to try to express defined ORs of interest. We also suggest strategies for selecting odorants for test panels to evaluate the functional expression of an OR. Integrating these tools, while retaining awareness of their idiosyncratic limitations, can provide a multi-tiered approach to OR deorphanization, spanning the initial discovery of a ligand to vetting that ligand in a physiologically relevant setting.

Discrimination of Saturated Aldehydes by the Rat I7 Olfactory Receptor

The discrimination of n-alkyl-saturated aldehydes during the early stage of odorant recognition by the rat I7 olfactory receptor (OR-I7) is investigated. The concentrations of odorants necessary for 50% activation (or inhibition) of the OR-I7 are measured by calcium imaging recordings of dissociated rat olfactory sensory neurons, expressing the recombinant OR-I7 from an adenoviral vector. These are correlated with the corresponding binding free energies computed for a homology structural model of the OR-I7 built from the crystal structure of bovine visual rhodopsin at 2.2 A resolution.

A Painful Trp Can Be a Bonding Experience

The receptive field of the TRPA1 nociceptor is remarkably expansive when compared to other chemodetectors such as odorant receptors. The identification of a unique mechanism utilized by TRPA1 helps clarify how this protein can efficiently alert the cell to an array of reactive chemical agents, regardless of their structure.

Selective activation of G-protein coupled receptors by volatile anesthetics

Ion channels and ionotropic neurotransmitter receptors have long been investigated as the principle targets of inhaled volatile anesthetics (VAs), but emerging evidence suggests that G-protein coupled receptors (GPCRs) might also directly interact with VAs. To survey the extent of interaction between VAs and diverse GPCRs, we have turned to the 1000+ member family of olfactory receptors (ORs), taking advantage of their unique expression pattern of a single OR per neuron. Through optical imaging and electrophysiological recordings, we show that different VAs trigger the normal transduction cascade in distinct subsets of cells in a dose-dependant manner. Together with evidence of antagonism by odorants, this selective activation strongly implicates a direct action of VAs upon particular olfactory receptors. The finding that VAs stimulate nearly 8% of olfactory GPCRs suggests that probing related Class A GPCRs may reveal a pool of VA targets whose altered signaling contributes to anesthetic effects.

A Renal Olfactory Receptor Aids in Kidney Glucose Handling

Olfactory receptors (ORs) are G protein-coupled receptors which serve important sensory functions beyond their role as odorant detectors in the olfactory epithelium. Here we describe a novel role for one of these ORs, Olfr1393, as a regulator of renal glucose handling. Olfr1393 is specifically expressed in the kidney proximal tubule, which is the site of renal glucose reabsorption. Olfr1393 knockout mice exhibit urinary glucose wasting and improved glucose tolerance, despite euglycemia and normal insulin levels. Consistent with this phenotype, Olfr1393 knockout mice have a significant decrease in luminal expression of Sglt1, a key renal glucose transporter, uncovering a novel regulatory pathway involving Olfr1393 and Sglt1. In addition, by utilizing a large scale screen of over 1400 chemicals we reveal the ligand profile of Olfr1393 for the first time, offering new insight into potential pathways of physiological regulation for this novel signaling pathway.