Stomatin-related olfactory protein, SRO, specifically expressed in the murine olfactory sensory neurons

A Conserved Role for Stomatin Domain Genes in Olfactory Behavior

The highly-conserved stomatin domain has been identified in genes throughout all classes of life. In animals, different stomatin domain-encoding genes have been implicated in the function of the kidney, red blood cells, and specific neuron types, although the underlying mechanisms remain unresolved. In one well-studied example of stomatin domain gene function, the Caenorhabditis elegans gene mec-2 and its mouse homolog Stoml3 are required for the function of mechanosensory neurons, where they modulate the activity of mechanosensory ion channels on the plasma membrane. Here, we identify an additional shared function for mec-2 and Stoml3 in a very different sensory context, that of olfaction. In worms, we find that a subset of stomatin domain genes are expressed in olfactory neurons, but only mec-2 is strongly required for olfactory behavior. mec-2 acts cell-autonomously and multiple alternatively-spliced isoforms of mec-2 can be substituted for each other. We generate a Stoml3 knock-out (KO) mouse and demonstrate that, like its worm homolog mec-2, it is required for olfactory behavior. In mice, Stoml3 is not required for odor detection, but is required for odor discrimination. Therefore, in addition to their shared roles in mechanosensory behavior, mec-2 and Stoml3 also have a shared role in olfactory behavior.

Expression pattern of Stomatin-domain proteins in the peripheral olfactory system

Recent data show that Stomatin-like protein 3 (STOML3), a member of the stomatin-domain family, is expressed in the olfactory sensory neurons (OSNs) where it modulates both spontaneous and evoked action potential firing. The protein family is constituted by other 4 members (besides STOML3): STOM, STOML1, STOML2 and podocin. Interestingly, STOML3 with STOM and STOML1 are expressed in other peripheral sensory neurons: dorsal root ganglia. In here, they functionally interact and modulate the activity of the mechanosensitive Piezo channels and members of the ASIC family. Therefore, we investigated whether STOM and STOML1 are expressed together with STOML3 in the OSNs and whether they could interact. We found that all three are indeed expressed in ONSs, although STOML1 at very low level. STOM and STOML3 share a similar expression pattern and STOML3 is necessary for STOM to properly localize to OSN cilia. In addition, we extended our investigation to podocin and STOML2, and while the former is not expressed in the olfactory system, the latter showed a peculiar expression pattern in multiple cell types. In summary, we provided a first complete description of stomatin-domain protein family in the olfactory system, highlighting the precise compartmentalization, possible interactions and, finally, their functional implications.

Photocatalytic degradation of lignin by low content g-C3N4 modified TiO2 under visible light

TiO2/g-C3N4 photocatalysts efficiently degraded lignin to obtain small molecule aromatics, which facilitated the efficient utilization of biomass.

INPP5E controls ciliary localization of phospholipids and the odor response in olfactory sensory neurons

The lipid composition of the primary cilia membrane is emerging as a critical regulator of cilia formation, maintenance and function. Here, we show that conditional deletion of the phosphoinositide 5′-phosphatase gene Inpp5e, mutation of which is causative of Joubert syndrome, in terminally developed mouse olfactory sensory neurons (OSNs), leads to a dramatic remodeling of ciliary phospholipids that is accompanied by marked elongation of cilia. Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂], which is normally restricted to the proximal segment redistributed to the entire length of cilia in Inpp5e knockout mice with a reduction in phosphatidylinositol (3,4)-bisphosphate [PI(3,4)P₂] and elevation of phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P₃] in the dendritic knob. The redistribution of phosphoinositides impaired odor adaptation, resulting in less efficient recovery and altered inactivation kinetics of the odor-evoked electrical response and the odor-induced elevation of cytoplasmic Ca²⁺. Gene replacement of Inpp5e through adenoviral expression restored the ciliary localization of PI(4,5)P₂ and odor response kinetics in OSNs. Our findings support the role of phosphoinositides as a modulator of the odor response and in ciliary biology of native multi-ciliated OSNs.

Summary: Cilia of olfactory sensory neurons have a unique lipid composition. Localization of phospholipids is controlled by the INPP5E phosphatase and is involved in modulation of the odor response.

Gene Expression Profile of Olfactory Transduction Signaling in an Animal Model of Human Multiple Sclerosis

Olfactory dysfunction occurs in multiple sclerosis in humans, as well as in an animal model of experimental autoimmune encephalomyelitis (EAE). The aim of this study was to analyze differentially expressed genes (DEGs) in olfactory bulb of EAE-affected mice by next generation sequencing, with a particular focus on changes in olfaction-related signals. EAE was induced in C57BL/6 mice following immunization with myelin oligodendrocyte glycoprotein and adjuvant. Inflammatory lesions were identified in the olfactory bulbs as well as in the spinal cord of immunized mice. Analysis of DEGs in the olfactory bulb of EAE-affected mice revealed that 44 genes were upregulated (and which were primarily related to inflammatory mediators), while 519 genes were downregulated; among the latter, olfactory marker protein and stomatin-like 3, which have been linked to olfactory signal transduction, were significantly downregulated (log2 [fold change] >1 and p-value <0.05). These findings suggest that inflammation in the olfactory bulb of EAE-affected mice is associated with the downregulation of some olfactory signal transduction genes, particularly olfactory marker protein and stomatin-like 3, which may lead to olfactory dysfunction in an animal model of human multiple sclerosis.

Hierarchical deconstruction of mouse olfactory sensory neurons: from whole mucosa to single-cell RNA-seq

The mouse olfactory mucosa is a complex chemosensory tissue composed of multiple cell types, neuronal and non-neuronal. We have here applied RNA-seq hierarchically, in three steps of decreasing cellular heterogeneity: starting with crude tissue samples dissected from the nose, proceeding to flow-cytometrically sorted pools of mature olfactory sensory neurons (OSNs), and finally arriving at single mature OSNs. We show that 98.9% of intact olfactory receptor (OR) genes are expressed in mature OSNs. We uncover a hitherto unknown bipartition among mature OSNs. We find that 19 of 21 single mature OSNs each express a single intact OR gene abundantly, consistent with the one neuron-one receptor rule. For the 9 single OSNs where the two alleles of the abundantly expressed OR gene exhibit single-nucleotide polymorphisms, we demonstrate that monoallelic expression of the abundantly expressed OR gene is extremely tight. The remaining two single mature OSNs lack OR gene expression but express Trpc2 and Gucy1b2. We establish these two cells as a neuronal cell type that is fundamentally distinct from canonical, OR-expressing OSNs and that is defined by the differential, higher expression of 55 genes. We propose this tiered experimental approach as a paradigm to unravel gene expression in other cellularly heterogeneous systems.

Olfactory sensory neurons transiently express multiple olfactory receptors during development

In mammals, each olfactory sensory neuron randomly expresses one, and only one, olfactory receptor (OR)--a phenomenon called the "one-neuron-one-receptor" rule. Although extensively studied, this rule was never proven for all ~1,000 OR genes in one cell at once, and little is known about its dynamics. Here, we directly tested this rule by single-cell transcriptomic sequencing of 178 cells from the main olfactory epithelium of adult and newborn mice. To our surprise, a subset of cells expressed multiple ORs. Most of these cells were developmentally immature. Our results illustrated how the "one-neuron-one-receptor" rule may have been established: At first, a single neuron temporarily expressed multiple ORs--seemingly violating the rule--and then all but one OR were eliminated. This work provided experimental evidence that epigenetic regulation in the olfactory system selects a single OR by suppressing a few transiently expressed ORs in a single cell during development.

Differential dopamine receptor subtype regulation of adenylyl cyclases in lipid rafts in human embryonic kidney and renal proximal tubule cells

Dopamine D1-like receptors (D1R and D5R) stimulate adenylyl cyclase (AC) activity, whereas the D2-like receptors (D2, D3 and D4) inhibit AC activity. D1R, but not the D5R, has been reported to regulate AC activity in lipid rafts (LRs). We tested the hypothesis that D1R and D5R differentially regulate AC activity in LRs using human embryonic kidney (HEK) 293 cells heterologously expressing human D1 or D5 receptor (HEK-hD1R or HEK-hD5R) and human renal proximal tubule (hRPT) cells that endogenously express D1R and D5R. Of the AC isoforms expressed in HEK and hRPT cells (AC3, AC5, AC6, AC7, and AC9), AC5/6 was distributed to a greater extent in LRs than non-LRs in HEK-hD1R (84.5±2.3% of total), HEK-hD5R (68.9±3.1% of total), and hRPT cells (66.6 ± 2.2% of total) (P<0.05, n=4/group). In HEK-hD1R cells, the D1-like receptor agonist fenoldopam (1 μM/15 min) increased AC5/6 protein (+17.2 ± 3.9% of control) in LRs but decreased it in non-LRs (-47.3±5.3% of control) (P<0.05, vs. control, n=4/group). By contrast, in HEK-hD5R cells, fenoldopam increased AC5/6 protein in non-LRs (+67.1 ± 5.3% of control, P<0.006, vs. control, n=4) but had no effect in LRs. In hRPT cells, fenoldopam increased AC5/6 in LRs but had little effect in non-LRs. Disruption of LRs with methyl-β-cyclodextrin decreased basal AC activity in HEK-D1R (-94.5 ± 2.0% of control) and HEK-D5R cells (-87.1 ± 4.6% of control) but increased it in hRPT cells (6.8±0.5-fold). AC6 activity was stimulated to a greater extent by D1R than D5R, in agreement with the greater colocalization of AC5/6 with D1R than D5R in LRs. We conclude that LRs are essential not only for the proper membrane distribution and maintenance of AC5/6 activity but also for the regulation of D1R- and D5R-mediated AC signaling.

The membrane proteome of sensory cilia to the depth of olfactory receptors

In the nasal cavity, the nonmotile cilium of olfactory sensory neurons (OSNs) constitutes the chemosensory interface between the ambient environment and the brain. The unique sensory organelle facilitates odor detection for which it includes all necessary components of initial and downstream olfactory signal transduction. In addition to its function in olfaction, a more universal role in modulating different signaling pathways is implicated, for example, in neurogenesis, apoptosis, and neural regeneration. To further extend our knowledge about this multifunctional signaling organelle, it is of high importance to establish a most detailed proteome map of the ciliary membrane compartment down to the level of transmembrane receptors. We detached cilia from mouse olfactory epithelia via Ca(2+)/K(+) shock followed by the enrichment of ciliary membrane proteins at alkaline pH, and we identified a total of 4,403 proteins by gel-based and gel-free methods in conjunction with high resolution LC/MS. This study is the first to report the detection of 62 native olfactory receptor proteins and to provide evidence for their heterogeneous expression at the protein level. Quantitative data evaluation revealed four ciliary membrane-associated candidate proteins (the annexins ANXA1, ANXA2, ANXA5, and S100A5) with a suggested function in the regulation of olfactory signal transduction, and their presence in ciliary structures was confirmed by immunohistochemistry. Moreover, we corroborated the ciliary localization of the potassium-dependent Na(+)/Ca(2+) exchanger (NCKX) 4 and the plasma membrane Ca(2+)-ATPase 1 (PMCA1) involved in olfactory signal termination, and we detected for the first time NCKX2 in olfactory cilia. Through comparison with transcriptome data specific for mature, ciliated OSNs, we finally delineated the membrane ciliome of OSNs. The membrane proteome of olfactory cilia established here is the most complete today, thus allowing us to pave new avenues for the study of diverse molecular functions and signaling pathways in and out of olfactory cilia and thus to advance our understanding of the biology of sensory organelles in general.

The scaffold protein MUPP1 regulates odorant-mediated signaling in olfactory sensory neurons

The olfactory signal transduction cascade transforms odor information into electrical signals by a cAMP-based amplification mechanism. The mechanisms underlying the very precise temporal and spatial organization of the relevant signaling components remains poorly understood. Here, we identify, using co-immunoprecipitation experiments, a macromolecular assembly of signal transduction components in mouse olfactory neurons, organized through MUPP1. Disruption of the PDZ signaling complex, through use of an inhibitory peptide, strongly impaired odor responses and changed the activation kinetics of olfactory sensory neurons. In addition, our experiments demonstrate that termination of the response is dependent on PDZ-based scaffolding. These findings provide new insights into the functional organization, and regulation, of olfactory signal transduction.

Regulation of ASIC channels by a stomatin/STOML3 complex located in a mobile vesicle pool in sensory neurons

A complex of stomatin-family proteins and acid-sensing (proton-gated) ion channel (ASIC) family members participate in sensory transduction in invertebrates and vertebrates. Here, we have examined the role of the stomatin-family protein stomatin-like protein-3 (STOML3) in this process. We demonstrate that STOML3 interacts with stomatin and ASIC subunits and that this occurs in a highly mobile vesicle pool in dorsal root ganglia (DRG) neurons and Chinese hamster ovary cells. We identify a hydrophobic region in the N-terminus of STOML3 that is required for vesicular localization of STOML3 and regulates physical and functional interaction with ASICs. We further characterize STOML3-containing vesicles in DRG neurons and show that they are Rab11-positive, but not part of the early-endosomal, lysosomal or Rab14-dependent biosynthetic compartment. Moreover, uncoupling of vesicles from microtubules leads to incorporation of STOML3 into the plasma membrane and increased acid-gated currents. Thus, STOML3 defines a vesicle pool in which it associates with molecules that have critical roles in sensory transduction. We suggest that the molecular features of this vesicular pool may be characteristic of a ‘transducosome’ in sensory neurons.

A unique transcriptome at the brain-environment interface: local translation in the rat olfactory epithelium

All olfactory epithelium cells, including rapidly self-renewing olfactory sensory neurons (OSN), are continuously subjected to external airborne aggressions. We hypothesized that the apical part of rat olfactory epithelia (AOE) could be the site of a local translation to be able to respond rapidly to external stimuli. We purified significant amounts of mRNAs from AOE. Sequencing of the cDNA library identified 348 mRNA species. Of these, the 220 AOE transcripts encoding proteins with known biological functions were classified in functional groups. The main functional class (40%) coded for defense, detoxification, anti-oxidant stress and innate immunity. Other classes comprised mRNAs encoding functions for neuronal metabolism and life (19%), nuclear transcription control (15%), cell survival and proliferation (13%), RNA processing and translation (12%). They did not contain any known members of the olfactory transduction pathway. The expression of a sub-set of AOE transcripts was investigated in sub-cellular AOE fractions highly enriched in ciliated dendrites and in AOE fractions after forced hemilateral OSN-specific degeneration. All the mRNAs tested were found to be: i) present in enriched ciliated dendrite preparations ii) down-regulated after OSN degeneration iii) co-purified with polysomal fractions, suggesting their commitment to local translation. We provide strong evidence that the extreme apical side of the olfactory epithelium expresses a unique transcriptome, whose function is not related to olfaction but mainly to defense and survival. The possible local translation of this transcriptome is demonstrated, in supporting cells as well as in olfactory neuron ciliated dendrites.

Stomatin-like protein-1 interacts with stomatin and is targeted to late endosomes

The human stomatin-like protein-1 (SLP-1) is a membrane protein with a characteristic bipartite structure containing a stomatin domain and a sterol carrier protein-2 (SCP-2) domain. This structure suggests a role for SLP-1 in sterol/lipid transfer and transport. Because SLP-1 has not been investigated, we first studied the molecular and cell biological characteristics of the expressed protein. We show here that SLP-1 localizes to the late endosomal compartment, like stomatin. Unlike stomatin, SLP-1 does not localize to the plasma membrane. Overexpression of SLP-1 leads to the redistribution of stomatin from the plasma membrane to late endosomes suggesting a complex formation between these proteins. We found that the targeting of SLP-1 to late endosomes is caused by a GYXXPhi (Phi being a bulky, hydrophobic amino acid) sorting signal at the N terminus. Mutation of this signal results in plasma membrane localization. SLP-1 and stomatin co-localize in the late endosomal compartment, they co-immunoprecipitate, thus showing a direct interaction, and they associate with detergent-resistant membranes. In accordance with the proposed lipid transfer function, we show that, under conditions of blocked cholesterol efflux from late endosomes, SLP-1 induces the formation of enlarged, cholesterol-filled, weakly LAMP-2-positive, acidic vesicles in the perinuclear region. This massive cholesterol accumulation clearly depends on the SCP-2 domain of SLP-1, suggesting a role for this domain in cholesterol transfer to late endosomes.

The proteome of rat olfactory sensory cilia

Olfactory sensory neurons expose to the inhaled air chemosensory cilia which bind odorants and operate as transduction organelles. Odorant receptors in the ciliary membrane activate a transduction cascade which uses cAMP and Ca(2+) for sensory signaling in the ciliary lumen. Although the canonical transduction pathway is well established, molecular components for more complex aspects of sensory transduction, like adaptation, regulation, and termination of the receptor response have not been systematically identified. Moreover, open questions in olfactory physiology include how the cilia exchange solutes with the surrounding mucus, assemble their highly polarized set of proteins, and cope with noxious substances in the ambient air. A specific ciliary proteome would promote research efforts in all of these fields. We have improved a method to detach cilia from rat olfactory sensory neurons and have isolated a preparation specifically enriched in ciliary membrane proteins. Using LC-ESI-MS/MS analysis, we identified 377 proteins which constitute the olfactory cilia proteome. These proteins represent a comprehensive data set for olfactory research since more than 80% can be attributed to the characteristic functions of olfactory sensory neurons and their cilia: signal processing, protein targeting, neurogenesis, solute transport, and cytoprotection. Organellar proteomics thus yielded decisive information about the diverse physiological functions of a sensory organelle.

A single conserved proline residue determines the membrane topology of stomatin

Stomatin is an integral membrane protein which is widely expressed in many cell types. It is accepted that stomatin has a unique hairpin-loop topology: it is anchored to the membrane with an N-terminal hydrophobic domain and the N- and C-termini are cytoplasmically localized. Stomatin is a prototype for a family of related proteins, containing among others MEC-2 (mechanosensory protein 2) from Caenorhabditis elegans, SLP (stomatin-like protein)-3 and podocin, all of which interact with ion channels to regulate their activity. Members of the stomatin family partly localize in DRMs (detergent-resistant membrane domains) enriched in cholesterol and sphingolipids. It has been proposed that a highly conserved proline residue in the middle of the hydrophobic domain directly binds cholesterol and that cholesterol binding is necessary for the regulation of ion channels. In the present study we show that a small part of the stomatin pool exists as a single-pass transmembrane protein rather than a hairpin-loop protein. The highly conserved proline residue is crucial for adopting the hairpin-loop topology: substitution of this proline residue by serine transfers the whole stomatin pool to the single-pass transmembrane form, which no longer localizes to DRMs. These results suggest that formation of the hairpin loop is inefficient and that the conserved proline residue is indispensable for formation of the hairpin loop. The single-pass transmembrane form exists also for SLP-3 and it should be considered that it mediates part of the physiological functions of stomatin and related proteins.

Olfactory cilia: our direct neuronal connection to the external world

An organism's awareness of its surroundings is dependent on sensory function. As antennas to our external environment, cilia are involved in fundamental biological processes such as olfaction, photoreception, and touch. The olfactory system has adapted this organelle for its unique sensory function and optimized it for detection of external stimuli. The elongated and tapering structure of olfactory cilia and their organization into an overlapping meshwork bathed by the nasal mucosa is optimized to enhance odor absorption and detection. As many as 15-30 nonmotile, sensory cilia on dendritic endings of single olfactory sensory neurons (OSNs) compartmentalize signaling molecules necessary for odor detection allowing for efficient and spatially confined responses to sensory stimuli. Although the loss of olfactory cilia or deletion of selected components of the olfactory signaling cascade leads to anosmia, the mechanisms of ciliogenesis and the selected enrichment of signaling molecules remain poorly understood. Much of our current knowledge is the result of elegant electron microscopy studies describing the structure and organization of the olfactory epithelium and cilia. New genetic and cell biological approaches, which compliment these early studies, show promise in elucidating the mechanisms of olfactory cilia assembly, maintenance, and compartmentalization. Importantly, emerging evidence suggests that olfactory dysfunction represents a previously unrecognized clinical manifestation of multiple ciliary disorders. Future work investigating the mechanisms of olfactory dysfunction combining both clinical studies with basic science research will provide us important new information regarding the pathogenesis of human sensory perception diseases.

Molecular modeling of prohibitin domains

Prohibitins comprise a family of highly conserved ubiquitous eukaryotic proteins that mainly localize to the mitochondria. They have been implicated in important cellular processes such as cellular signaling and transcriptional control, apoptosis, cellular senescence, and mitochondrial biogenesis. Using molecular modeling techniques, we have generated structural models of human prohibitins BAP32 and BAP37, which have previously been shown to exist as large ringlike oligomers in the membrane-bound state. The middle domain of prohibitins is evolutionary conserved in the family of SPFH (PHB) domain proteins. On the basis of the known structure of flotillin-2, another member of the SPFH-domain family, we have generated homology models for BAP32 and BAP37, and elucidated the implications for formation of high molecular weight oligomers. A model for the dimeric-building block of BAP32: BAP37 for such assemblies was generated and its stability scrutinized by molecular dynamics simulations. The model of BAP32 was also analyzed as to potential ligand-binding sites and the previously identified ligand melanogenin was docked into a membrane-proximal cavity. The results are discussed in the context of prohibitin interactions with mitochondrial AAA-proteases and we suggest two possible interaction interfaces between the BAP32:BAP37 building block and the protease.

Calcium-signaling networks in olfactory receptor neurons

The olfactory neuroepithelium represents a unique interface between the brain and the external environment. Olfactory function comprises a distinct set of molecular tasks: sensory signal transduction, cytoprotection and adult neurogenesis. A multitude of biochemical studies has revealed the central role of Ca(2+) signaling in the function of olfactory receptor neurons (ORNs). We set out to establish Ca(2+)-dependent signaling networks in ORN cilia by proteomic analysis. We subjected a ciliary membrane preparation to Ca(2+)/calmodulin-affinity chromatography using mild detergent conditions in order to maintain functional protein complexes involved in olfactory Ca(2+) signaling. Thus, calmodulin serves as a valuable tool to gain access to novel Ca(2+)-regulated protein complexes. Tandem mass spectrometry (nanoscale liquid-chromatography-electrospray injection) identified 123 distinct proteins. Ninety-seven proteins (79%) could be assigned to specific olfactory functions, including 32 to sensory signal transduction and 40 to cytoprotection. We point out novel perspectives for research on the Ca(2+)-signaling networks in the olfactory system of the rat.

Ciliary band gene expression patterns in the embryo and trochophore larva of an indirectly developing polychaete

The trochophore larvae of indirectly developing spiralians have ciliary bands with motor and feeding functions. The preoral prototroch ciliary band is the first differentiating organ in annelid and mollusk embryos. Here we report the expression of several ciliary band markers during embryogenesis and early larval stages of the indirectly developing polychaete Hydroides elegans. Genes with similarity to caveolin, beta-tubulin, alpha-tubulin, and tektin are expressed in the eight primary prototroch precursors, 1q(221) and 1q(212). Blastomeres 1q(221) and 1q(212) locate at the same equatorial latitude after the complementary asymmetric division of their 1q(22) and 1q(21) precursors. In addition, caveolin and alpha-tubulin are expressed in the metatroch and adoral ciliary zone. Caveolin is expressed in foregut ciliated cells, and alpha-tubulin is expressed in apical tuft ciliated cells. The expression of a beta-thymosin homolog is restricted to 1q(122) and 1q(121) blastomeres, which locate just above and in close association with the eight primary prototroch cells 1q(221) and 1q(212). In addition, the beta-thymosin homolog has a transient expression in the hindgut and apical zone. The expression of all these genes provides a landmark for the early specification of ciliary bands and other ciliated organs.

Vomeronasal versus olfactory epithelium: is there a cellular basis for human vomeronasal perception?

The vomeronasal organ (VNO) constitutes an accessory olfactory organ that receives chemical stimuli, pheromones, which elicit behavioral, reproductive, or neuroendocrine responses among individuals of the same species. In many macrosmatic animals, the morphological substrate constitutes a separate organ system consisting of a vomeronasal duct (ductus vomeronasalis, VND), equipped with chemosensory cells, and a vomeronasal nerve (nervus vomeronasalis, VNN) conducting information into the accessory olfactory bulb (AOB) in the central nervous system (CNS). Recent data require that the long-accepted dual functionality of a main olfactory system and the VNO be reexamined, since all species without a VNO are nevertheless sexually active, and species possessing a VNO also can sense other than "vomeronasal" stimuli via the vomeronasal epithelium (VNE). The human case constitutes a borderline situation, as its embryonic VNO anlage exerts a developmental track common to most macrosmatics, but later typical structures such as the VNN, AOB, and probably most of the chemoreceptor cells within the still existent VND are lost. This review also presents recent information on the VND including immunohistochemical expression of neuronal markers, intermediate filaments, lectins, integrins, caveolin, CD44, and aquaporins. Further, we will address the issue of human pheromone candidates.

Flotillins and the PHB Domain Protein Family: Rafts, Worms and Anaesthetics

While our understanding of lipid microdomains has advanced in recent years, many aspects of their formation and dynamics are still unclear. In particular, the molecular determinants that facilitate the partitioning of integral membrane proteins into lipid raft domains are yet to be clarified. This review focuses on a family of raft-associated integral membrane proteins, termed flotillins, which belongs to a larger class of integral membrane proteins that carry an evolutionarily conserved domain called the prohibitin homology (PHB) domain. A number of studies now suggest that eucaryotic proteins carrying this domain have affinity for lipid raft domains. The PHB domain is carried by a diverse array of proteins including stomatin, podocin, the archetypal PHB protein, prohibitin, lower eucaryotic proteins such as the Dictyostelium discoideum proteins vacuolin A and vacuolin B and the Caenorhabditis elegans proteins unc-1, unc-24 and mec-2. The presence of this domain in some procaryotic proteins suggests that the PHB domain may constitute a primordial lipid recognition motif. Recent work has provided new insights into the trafficking and targeting of flotillin and other PHB domain proteins. While the function of this large family of proteins remains unclear, studies of the C. elegans PHB proteins suggest possible links to a class of volatile anaesthetics raising the possibility that these lipophilic agents could influence lipid raft domains. This review will discuss recent insights into the cell biology of flotillins and the large family of evolutionarily conserved PHB domain proteins.

Differentially expressed transcripts from phenotypically identified olfactory sensory neurons

In comparing purified mouse olfactory sensory neurons (OSNs) with neighboring cells, we identified 54 differentially expressed transcripts. One-third of the transcripts encode proteins with no known function, but the others have functions that correlate with challenges faced by OSNs. The OSNs expressed a diversity of signaling protein genes, including stomatin (Epb7.2), S100A5, Ddit3, Sirt2, CD81, Sdc2, Omp, and Ptpla. The elaboration of dendrites, cilia, and axons that places OSNs in contact with diverse cell types and signals presumably also requires large investments in cytoskeletal-associated proteins, lipid biosynthesis, and energy production. Several of the genes encode proteins that participate in these biological processes, including ATP5g3, Ndufa9, Sqrdl, Mdh1, Got1, beta-2 tubulin, Capza1, Bin3, Tom1, Acl6, and similar to O-MACS. Three transcripts had restricted expression patterns. Similar to O-MACS and Gstm2 had zonally restricted expression patterns in OSNs and sustentacular cells but not in Bowman's glands, suggesting that zonality can be differentially regulated by cell type. The mosaic expression pattern of S100A5 in approximately 70% of OSNs predicts that it is coexpressed with a subset of odorant receptors. We captured four abundant transcripts, Cyp2a4, similar to Cyp2g1, Gstm2, and Cbr2, that encode xenobiotic metabolizing enzymes expressed by sustentacular cells or Bowman's glands, reinforcing the interpretation that clearance of xenobiotic compounds is a major function of these cells. Within the olfactory epithelium, Cbr2 is a new anatomical marker for sustentacular cells. We also discovered that Reg3g is a marker for respiratory epithelium.

MEC-2 Is Recruited to the Putative Mechanosensory Complex in C. elegans Touch Receptor Neurons through Its Stomatin-like Domain

BACKGROUND

The response to gentle body touch in C. elegans requires a degenerin channel complex containing four proteins (MEC-2, MEC-4, MEC-6, and MEC-10). The central portion of the integral membrane protein MEC-2 contains a stomatin-like region that is highly conserved from bacteria to mammals. The molecular function of this domain in MEC-2, however, is unknown.

RESULTS

Here, we show that MEC-2 colocalizes with the degenerin MEC-4 in regular puncta along touch receptor neuron processes. This punctate localization requires the other channel complex proteins. The stomatin-like region of MEC-2 interacts with the intracellular cytoplasmic portion of MEC-4. Missense mutations in this region that destroy the interaction also disrupt the punctate localization and degenerin-regulating function of MEC-2. Missense mutations outside this region apparently have no effect on the punctate localization but significantly reduce the regulatory effect of MEC-2 on the MEC-4 degenerin channel. A second stomatin-like protein, UNC-24, colocalizes with MEC-2 in vivo and coimmunoprecipitates with MEC-2 and MEC-4 in Xenopus oocytes; unc-24 enhances the touch insensitivity of temperature-sensitive alleles of mec-4 and mec-6.

CONCLUSION

Two stomatin homologs, MEC-2 and UNC-24, interact with the MEC-4 degenerin through their stomatin-like regions, which act as protein binding domains. At least in the case of MEC-2, this binding allows its nonstomatin domains to regulate channel activity. Stomatin-like regions in other proteins may serve a similar protein binding function.

Trafficking prerogatives of olfactory receptors

Olfactory receptors lead lives of exclusivity and privilege, the monarchs of fiefdoms organized solely to carry out their instructions. Each olfactory sensory neuron expresses one allele of one of approximately 1000 olfactory receptor genes. It is thought that olfactory receptor diversity is critical for the ability of animals to detect many thousands of odorants, but supporting functional evidence has been difficult to obtain because olfactory receptors expressed in heterologous cells are typically retained in the endoplasmic reticulum. The membrane trafficking entitlements enjoyed by olfactory receptors appear to be available only in mature olfactory sensory neurons. Evidence is accumulating that cell-type-specific accessory proteins regulate first the exit of olfactory receptors from the endoplasmic reticulum, and then the trafficking of olfactory receptors from post-Golgi compartments to the plasma membrane of the olfactory cilia where the receptors gain access to odorants. Critical olfactory receptor accessory proteins are known only in the nematode Caenorhabditis elegans, where the absence of a novel protein called ODR-4 or a clathrin adaptor, UNC-101, interferes with proper trafficking. Similar functional specificity also occurs in a parallel chemosensory system, the mammalian vomeronasal organ. Trafficking of the V2R type of vomeronasal receptors is mediated by a vomeronasal-specific family of major histocompatibility complex proteins. Removal of olfactory receptors from the plasma membrane may be regulated in a more conventional fashion because odor stimulation has been linked to receptor phosphorylation, to the function of G-protein coupled receptor kinase 3, and to an increase in vesicles retrieved from the plasma membrane.

Age-related trends in gene expression in the chemosensory-nasal mucosae of senescence-accelerated mice

We have utilized high-density GeneChip oligonucleotide arrays to investigate the use of the senescence-accelerated mouse (SAM) as a biogerontological resource to identify patterns of gene expression in the chemosensory-nasal mucosa. Gene profiling in chronologically young and old mice of the senescence-resistant (SAMR) and senescence-prone (SAMP) strains revealed 133 known genes that were modulated by a three-fold or greater change either in one strain or the other or in both strains during aging. We also identified known genes in our study which based on their encoded proteins were identified as aging-related genes in the aging neocortex and cerebellum of mice as reported by Lee et al. (2000) [Nat. Genet. 25 (2000) 294]. Changes in gene profiles for chemosensory-related genes including olfactory and vomeronasal receptors, sensory transduction-associated proteins, and odor and pheromone transport molecules in the young SAMR and SAMP were compared with age-matched C57BL/6J mice. An analysis of known gene expression profiles suggests that changes in the expression of immune factor genes and genes associated with cell cycle progression and cell death were particularly prominent in the old SAM strains. A preliminary cellular validation study supported the dysregulation of cell cycle-related genes in the old SAM strains. The results of our initial study indicated that the use of the SAM models of aging could provide substantive information leading to a more fundamental understanding of the aging process in the chemosensory-nasal mucosa at the genomic, molecular, and cellular levels.