Sodium/calcium exchanger expression in the mouse and rat olfactory systems

Sodium/calcium (Na(+)/Ca(2+)) exchangers are membrane transport systems that regulate Ca(2+)-homeostasis in many eukaryotic cells. In olfactory and vomeronasal sensory neurons ligand-induced olfactory signal transduction is associated with influx and elevation of intracellular Ca(2+), [Ca(2+)](i). While much effort has been devoted to the characterization of Ca(2+)-related excitation and adaptation events of olfactory chemosensory neurons (OSNs), much less is known about mechanisms that return [Ca(2+)](i) to the resting state. To identify proteins participating in the poststimulus Ca(2+)-clearance of mouse OSNs, we analyzed the expression of three potassium (K(+))-independent (NCX1, 2, 3) and three K(+)-dependent (NCKX1, 2, 3) Na(+)/Ca(2+) exchangers. In situ hybridization showed that mRNAs of all six Na(+)/Ca(2+) exchangers coexist in neurons of the olfactory and vomeronasal systems, and that some are already detectable in the embryo. Of these, NCX1 and NCKX1 represent the most and least abundant mRNAs, respectively. Moreover, immunohistochemistry revealed that the NCX1, 2, and 3 proteins are expressed in nearly all neurons of the olfactory epithelium, the vomeronasal organ, the septal organ of Masera, and the Grueneberg ganglion. These three exchanger proteins display different expression profiles in dendrites, knobs, and plasma membranes of OSNs and in sustentacular cells. Furthermore, we show that NCX1 mRNA in rat olfactory mucosa is expressed as 8 alternative splice variants. This is the first comprehensive analysis of Na(+)/Ca(2+) exchanger expression in the mammalian olfactory system. Our results suggest that Ca(2+)-extrusion by OSNs utilizes multiple different Na(+)/Ca(2+) exchangers and that different subtypes are targeted to different subcellular compartments.

Backbone dynamics of the olfactory marker protein as studied by 15N NMR relaxation measurements

Nuclear magnetic resonance (NMR) (15)N relaxation measurements of the olfactory marker protein (OMP) including longitudinal relaxation (T(1)), transverse relaxation (T(2)), and (15)N-{(1)H} NOE data were collected at low protein concentrations (<or=100 microM) and at two field strengths (14.4 and 18.8 T) for 135 of 162 backbone amide groups. Rotational diffusion of the OMP was found to be axially symmetric with D( parallel)/D( perpendicular) = 1.20 +/- 0.02 with an overall global correlation time of 8.93 +/- 0.03 ns. Model-free internal dynamic analyses of these data provided a description of the protein's dynamics on multiple time scales. The results of these studies indicate that there is a large degree of conformational flexibility for alpha-helix 1 (alpha1), loop 1, and the conserved Omega-loop (loop 3). The functional significance that these dynamic regions of OMP have in modulating olfactory signal transduction is discussed.

The interaction of Bex and OMP reveals a dimer of OMP with a short half-life

Olfactory marker protein (OMP) participates in the olfactory signal transduction pathway. This is evident from the behavioral and electrophysiological deficits of OMP-null mice, which can be reversed by intranasal infection of olfactory sensory neurons with an OMP-expressing adenovirus. Bex, brain expressed X-linked protein, has been identified as a protein that interacts with OMP. We have now further characterized the interaction of OMP and Bex1/2 by in vitro binding assays and by immuno-coprecipitation experiments. OMP is a 19 kDa protein but these immunoprecipitation studies have revealed the unexpected presence of a 38 kDa band in addition to the expected 19 kDa band. Furthermore, the 38 kDa form was preferentially co-immunoprecipitated with Bex from cell extracts. In-gel tryptic digestion, mass spectrometry, and two-dimensional gel electrophoresis indicate that the 38 kDa protein behaves as a covalently cross-linked OMP-homodimer. The 38 kDa band was also identified in western blots of olfactory epithelium demonstrating its presence in vivo. The stabilities and subcellular localizations of the OMP-monomer and -dimer were studied in transfected cells. These results demonstrated that the OMP-dimer is much less stable than the monomer, and that while the monomer is present both in the nuclear and cytosolic compartments, the dimer is preferentially located in a Triton X-100 insoluble cytoskeletal fraction. These novel observations led us to hypothesize that regulation of the level of the rapidly turning-over OMP-dimer and its interaction with Bex1/2 is critical for OMP function in sensory transduction.

Identification of members of the Bex gene family as olfactory marker protein (OMP) binding partners

Olfactory marker protein (OMP) expression is a hallmark of mature vertebrate olfactory receptor neurons (ORNs). Evidence for OMP function derives from altered behavioral and electrophysiological activities of OMP-KO mice. The molecular basis for the altered phenotype following the deletion of OMP is still unclear. Recent structural studies predict the involvement of OMP in protein-protein interaction. Here we report the identification of an OMP partner, Bex2, by phage-display screening of an olfactory mucosal cDNA-library. In situ hybridization demonstrates cellular co-localization of OMP mRNA with mRNAs for Bex1, Bex2, and Bex3 in ORNs of olfactory tissue of the mouse. The OMP/Bex interaction has been confirmed by demonstrating the chemical cross-linking of recombinant rat OMP with a synthetic peptide derived from the Bex amino acid sequence. The subcellular localization of Bex and OMP proteins was evaluated in transfected HEK293 cells. Bex is visualized in the nucleus and cytoplasm. Following co-transfection we observed the unexpected presence of some OMP in the nucleus along with Bex. Together, these data argue convincingly that we have identified Bex as an OMP partner whose further characterization will provide insight to the role of OMP and to the mechanism of the OMP/Bex interaction in ORN differentiation and function.

Olfactory marker protein (OMP) exhibits a beta-clam fold in solution: implications for target peptide interaction and olfactory signal transduction

Olfactory marker protein (OMP) is a ubiquitous, cytoplasmic protein found in mature olfactory receptor neurons of all vertebrates. Electrophysiological and behavioral studies demonstrate that it is a modulator of the olfactory signal transduction pathway. Here, we demonstrate that the solution structure of OMP, as determined by NMR studies, is a single globular domain protein comprised of eight beta-strands forming two beta-sheets oriented orthogonally to one another, thus exhibiting a "beta-clam" or "beta-sandwich" fold: beta-sheet 1 is comprised of beta3-beta8-beta1-beta2 and beta-sheet 2 contains beta6-beta5-beta4-beta7. Insertions include two, long alpha-helices located on opposite sides of the beta-clam and three flexible loops. The juxtaposition of beta-strands beta6-beta5-beta4-beta7-beta2-beta1-beta8-beta3 forms a continuously curved surface and encloses one side of the beta-clam. The "cleft" formed by the two beta-sheets is opposite to the closed end of the beta-clam. Using a peptide titration series, we have identified this cleft as the binding surface for a peptide derived from the Bex1 protein. The highly conserved Omega-loop structure adjacent to the Bex1 peptide-binding surface found in OMP may be the site of additional OMP-protein interactions related to its role in modulating olfactory signal transduction. Thus, the interaction between the OMP and Bex1 proteins could facilitate the interaction between OMP and other components of the olfactory signaling pathway.

Adenoviral vector-mediated rescue of the OMP-null phenotype in vivo

The use of gene deletion by homologous recombination to determine gene or protein function has wide application in vertebrate neurobiology. An ideal complement to gene deletion would be subsequent gene replacement to demonstrate re-acquisition of function. Here we used an adenoviral vector to replace the olfactory marker protein (OMP) gene in olfactory receptor neurons of adult OMP-null mice and demonstrated the subsequent re-acquisition of function. Our results show that short-term expression of OMP restores the kinetics of electrophysiological responses of OMP-null mice to those of the control phenotype. This adenoviral-mediated rescue of the OMP-null phenotype is consistent with involvement of OMP in olfactory transduction.

Targeted deletion of a cyclic nucleotide-gated channel subunit (OCNC1): biochemical and morphological consequences in adult mice

The olfactory cyclic nucleotide-gated channel subunit 1 (OCNC1) is required for signal transduction in olfactory receptor cells. To further investigate the role of this channel in the olfactory system, the biochemical and morphological consequences of targeted disruption of OCNC1 were investigated in adult mice. Null as compared to wild-type mice had smaller olfactory bulbs, suggesting compromised development of the central target of the receptor cells. Ectopic olfactory marker protein (OMP)-stained fibers localized to the external plexiform layer reflected the relative immaturity of the olfactory bulb in the null mice. The olfactory epithelium of the knock-out mouse was thinner and showed lower expression of olfactory marker protein and growth-associated protein 43, indicating decreases in both generation and maturation of receptor cells. Tyrosine hydroxylase (TH) expression in the olfactory bulb, examined as a reflection of afferent activity, was reduced in the majority of periglomerular neurons but retained in atypical or "necklace" glomeruli localized to posterior aspects of the olfactory bulb. Double label studies demonstrated that the remaining TH-immunostained neurons received their innervation from a subset of receptor cells previously shown to express a phosphodiesterase that differs from that found in most receptor cells. These data indicate that expression of OCNC1 is required for normal development of the olfactory epithelium and olfactory bulb. The robust expression of TH in some periglomerular cells in the OCNC1-null mice suggests that receptor cells innervating these glomeruli may use an alternate signal transduction pathway.

Age-dependent association of isolated bovine lens multicatalytic proteinase complex (proteasome) with heat-shock protein 90, an endogenous inhibitor

The multicatalytic proteinase complex (MPC) (proteasome) is a high-molecular-weight proteolytic enzyme found in eukaryotic cells and archaebacteria. Regulatory proteins that inhibit or activate the MPC have been described. Association with an ATPase complex alters the specificity of the multicatalytic proteinase complex to permit cleavage of ubiquitinylated proteins. Unidentified proteins have been observed in highly purified preparations of the multicatalytic proteinase complex. Based on immunoreactivity and N-terminal sequencing, we have identified heat-shock protein 90 as a major component of the multicatalytic proteinase complex prepared from 1-month, but not 2-year bovine lenses. alpha-Crystallin, a lens structural protein with chaperone activity, is also found in multicatalytic proteinase complex preparations. Both heat-shock protein 90 and alpha-crystallin inhibit hydrolysis of Cbz-Leu-Leu-Leu-MCA by the multicatalytic proteinase complex at a stoichiometry of 1 mol heat-shock protein per mole of MPC. Heat-shock proteins may interact with denatured proteins and facilitate their degradation. These studies give evidence for the involvement of heat-shock proteins in proteolysis by direct interaction with the multicatalytic proteinase complex.

Thermal stability and activation of bovine lens multicatalytic proteinase complex (proteasome)

The multicatalytic proteinase complex (MPC; proteasome) can be isolated in a latent form which then can be activated for protein hydrolysis by physiological and nonphysiological treatments, including high temperature. In this study, the temperature dependency profiles for the hydrolysis of Cbz-Gly-Gly-Leu-pNA and Cbz-Val-Gly-Arg-pNA by bovine lens MPC are found to be those expected for a thermostable enzyme, with single optima above 50 degrees C. In contrast, hydrolyses of Cbz-Leu-Leu-Glu-2NNp and alpha 2-crystallin, a lens structural protein, show two temperature transitions, indicating that hydrolysis of these substrates can be activated by elevated temperature. Temperature dependency profiles of peptidase activity in Tris-HCl compared to Hepes buffer suggest that Tris decreases the thermal stability of MPC. After 10 min preincubation in Tris-HCl at 53 degrees C, lens MPC activities are reduced by 50-60% and loss of the major MPC band can be seen on nondenaturing gels. The presence of alpha 2-crystallin during preincubation partially prevents the loss of activity. Although alpha-crystallin has been reported to function as a molecular chaperone, similar protection by other MPC substrates suggests that alpha 2-crystallin stabilized the MPC as a substrate. Our findings indicate both activation and inactivation of the enzyme at elevated temperatures. It is proposed therefore that high temperature activates the MPC but to a more labile form which can be partially stabilized by protein substrates.

Bovine lens multicatalytic proteinase complex

The ocular lens grows by laying down new cells on top of old in a differentiation process that results in loss of protein-synthesizing capacity, but preservation of the cells themselves for the lifetime of the organism. The transparency and refractive index of the lens depend on protein integrity and longevity, yet proteolysis is needed for normal growth and development. Therefore, control of proteolysis must be stringent. Here we review the structural features and major proteolytic enzymes of the lens and the properties of the bovine lens multicatalytic proteinase complex, including native and SDS-PAGE patterns, and activation and inhibition by cations, amphiphilic molecules and temperature.

3,4-dichloroisocoumarin-induced activation of the degradation of beta-casein by the bovine pituitary multicatalytic proteinase complex

The breakdown of beta-casein (caseinolytic activity) by the bovine pituitary multicatalytic proteinase complex (MPC) is initiated by a fourth active site different from the previously described chymotrypsin-like activity (cleavage of Cbz-Gly-Gly-Leu-p-nitroanilide, where Cbz is benzyloxycarbonyl), trypsin-like activity (cleavage of Cbz-D-Ala-Leu-Arg-2-naphthylamide), and peptidylglutamyl peptide bond-hydrolyzing (PGP) activity (cleavage of Cbz-Leu-Leu-Glu-2-naphthylamide) (Yu, B., Pereira, M. E., and Wilk, S. (1991) J. Biol. Chem. 266, 17396-17400). 3,4-Dichloroisocoumarin, a serine proteinase inhibitor, stimulated the caseinolytic activity of bovine pituitary or lens MPC, 3-18-fold under conditions under which the other three catalytic activities were inactivated. Addition of hydroxylamine to the modified enzyme did not reverse the effects of the inhibitor. A form of the proteinase exhibiting only 2-4% of control chymotrypsin-like, trypsin-like, and PGP activities degraded beta-casein with no accumulation of intermediate peptides. 3,4-Dichloroisocoumarin, by reacting with the chymotrypsin-like, trypsin-like, and/or PGP-active sites, may promote a conformational change of MPC, rendering the caseinolytic active site accessible to the substrate. Once bound to the active site, beta-casein is rapidly degraded either by the caseinolytic component itself or by a cooperative interaction with catalytic centers that are not affected by the serine proteinase inhibitor. These results imply that the caseinolytic component does not belong to the class of serine proteinases. Other proteins tested were not degraded by the 3,4-dichloroisocoumarin-treated enzyme, suggesting that the conformation of beta-casein may be more adequate for degradation by the caseinolytic component.

Post-translational arginylation in the bovine lens

This study demonstrates post-translational arginylation of bovine serum albumin and endogenous lens proteins by bovine lens arginyl-tRNA:protein transferase. This reaction has been proposed to be the first step in marking specific proteins for degradation by the non-lysosomal, ATP-dependent, ubiquitin-mediated proteolytic pathway. The transferase was obtained by the method used for isolation of the same enzyme from reticulocytes (Ferber and Ciechanover, 1987, Nature 326, 808-11). Incorporation of [3H]Arg was linear for at least 2 hr at 37 degrees C. The amount of incorporation was directly proportional to the amount of lens enzyme or substrate added. Arginylation was ATP-dependent. A requirement for tRNA was demonstrated by inhibition upon pretreatment of the enzyme preparation with nuclease to hydrolyse endogenous tRNA, and restoration of activity upon replacement of tRNA. [3H]Leu, [3H]Lys and [3H]His were not incorporated, demonstrating specificity of the reaction for arginine. This is the first demonstration of post-translational modification of proteins by arginylation in the lens.

Conserved N-terminal sequences in homologous subunits of the multicatalytic proteinase complex (proteasome)

The bovine lens multicatalytic proteinase complex (MPC) (MW 700 kDa) comprises at least twelve subunits in the molecular mass range 22-35 kDa. Three of the subunits, L1 (27 kDa), L2 (24 kDa) and L3 (29 kDa), were purified by reverse phase HPLC. Their amino acid composition and N-terminal sequences indicate that they are not identical. L1 and L2 subunits show very high (greater than 90%) sequence homology with specific subunits of rat liver and human reticulocyte MPC and these are considered to be homologous components of the MPC which are highly conserved in evolution.

Covalent labelling of bovine lens multicatalytic proteinase complex with [3H]di-isopropyl fluorophosphate

Enzymatically active lens multicatalytic proteinase complex bound [3H]iPr2P-F after incubation for 3 hours at ambient temperature. Label was associated with the lowest molecular weight band (Mr 22,000) on sodium dodecyl sulfate polyacrylamide gels. This binding was inhibited by preincubation of the enzyme with the cysteine-directed reagent, p-chloromercuribenzoate, which inhibits all three hydrolytic activities of the enzyme. Leupeptin, which inhibits the arginyl-hydrolyzing component, but not the iPr2P-F-inhibitable leucyl-hydrolyzing component of the enzyme, does not inhibit [3H]iPr2P-F binding. These data suggest that the leucy-hydrolyzing component of the lens multicatalytic proteinase complex is localized to the 22,000 Mr subunit and is a member of the thiol-dependent subclass of serine proteinases.

Common epitopes of bovine lens multicatalytic-proteinase-complex subunits

Component polypeptides of both the bovine lens and pituitary multicatalytic proteinase complexes demonstrate different immunoreactivities with a polyclonal antiserum raised against the purified pituitary enzyme. Four (Mr 24000, 26000, 34000 and 38000) of eight bands that have been resolved by SDS/polyacrylamide-gel electrophoresis are stained in immunoblot experiments. Monospecific antibodies obtained from this antiserum by affinity purification from the 38000- and 34000-Mr bands of the lens enzyme bound equally well to either band, but showed little or no binding to the 26000- and 24000-Mr bands upon immunoblotting. Antibody affinity-purified from the 24000-Mr band showed comparable binding to the 24000-, 34000- or 38000-Mr band. One explanation of these results is that the 24000-Mr polypeptide is derived from the higher-Mr polypeptide(s) and has lost some of the common immunodeterminants.

The bovine lens neutral proteinase comprises a family of cysteine-dependent proteolytic activities

Inhibitor studies with peptide substrates demonstrate that bovine lens neutral proteinase comprises three distinct activities. Diisopropylfluorophosphate distinguishes the activity hydrolyzing carbobenzoxy-Gly-Gly-Leu-p-nitroanilide (inhibited) from that hydrolyzing carbobenzoxy-Leu-Leu-Glu-2-naphthylamide (not inhibited). Leupeptin inhibits hydrolysis of the substrate carbobenzoxy-Leu-Leu-Arg-2-naphthylamide, but not hydrolysis of carbobenzoxy-Gly-Gly-Leu-p-nitroanilide or carbobenzoxy-Leu-Leu-Glu-2-naphthylamide, demonstrating the presence of the third activity. Inhibition of the three activities by thiol reagents suggests that each activity may be dependent on an active-site cysteine residue.

Differential inhibition of two proteolytic activities in bovine lens neutral-proteinase preparations

Hydrolysis of carbobenzoxy-Leu-Leu-Glu 2-naphthylamide by bovine lens neutral-proteinase preparations is not affected by the esterase inhibitor di-isopropyl fluorophosphate, whereas hydrolysis of carbobenzoxy-Gly-Gly-Leu p-nitroanilide is completely inhibited. Hydrolysis of alpha-crystallin, a lens structural protein, can be inhibited by only 50% after prolonged treatment with di-isopropyl fluorophosphate. These data suggest that the lens neutral-proteinase preparation contains at least two enzymes, one of which may be a serine proteinase. This may account, in part, for the previously observed complex response of the preparation to inhibitors.

Age changes in bovine lens endopeptidase activity

Lens endopeptidase activity and thermal stability have been determined as a function of cell development, cell age, and animal age. Lenses from animals aged 3 months to 15 years (lens weights 1.15-2.80 g) were divided into epithelial (outermost), cortical (peripheral), and nuclear (central) regions. Changes accompanying cell development were determined by measuring specific activity in epithelial (undifferentiated), outer cortical (differentiating), inner cortical (mature) and nuclear (aged) regions of individual lenses. Thermal stability of the enzyme activity obtained from the outer cortical and nuclear regions of the same lenses was also determined. Specific activity and thermal stability were found to decrease as a function of lens cell development. Changes with cell development represent the effects of both differentiation and increasing cell age. To determine the effects of cell age alone, activity was determined in the same population of aged, fully differentiated cells in lenses of different ages. Specific activity decreased as a function of cell age alone. Changes with animal age were determined by comparing cells of the same developmental stage from animals of different ages (e.g., differentiating cells of the cortex in animals 3 months to 15 years old). Specific activity for the cortical region increased with animal age while specific activity in the nuclear region appeared to remain constant or decrease slightly with increasing animal age. Thermal stability of the enzyme activity from the cortex was different in young and adult lenses. The change in stability occurred early in the lifespan and was therefore more closely related to animal development than to aging.

A synthetic endopeptidase substrate hydrolyzed by the bovine lens neutral proteinase preparation

Lens neutral proteinase is thought to exhibit primarily endopeptidase activity. We have identified a synthetic endopeptidase substrate which is hydrolyzed by the bovine lens neutral proteinase preparation. Among 11 fluoro- and chromogenic endopeptidase substrates, only carbobenzoxy-glycylglycyl-L-leucyl-p-nitroanilide is effectively hydrolyzed. The activity hydrolyzing this substrate co-elutes with neutral proteinase activity upon gel filtration and specifically attacks the leucyl-p-nitroaniline bond. Optimal hydrolysis of the synthetic substrate is at neutral pH and high temperature (53 degrees C), analogous to the alpha-crystallin protein substrate obtained from lens. The rate of hydrolysis of the synthetic substrate increased proportionally with temperature between 20 and 60 degrees C, in contrast to alpha-crystallin. The rate of hydrolysis was linear for at least 1 h at 37 degrees C and there was no evidence of enzyme activation at high temperature.