Non-lysosomal degradation of misfolded human lysozymes with and without an asparagine-linked glycosylation site

Human lysozyme is a monomeric secretory protein composed of 130 amino acid residues, with four intramolecular disulfide bonds and no oligosaccharides. In this study, a mutant protein, [Ala128] lysozyme, which cannot fold because it lacks a disulfide bond, Cys6-Cys128, was expressed in mouse fibroblasts and was found to be mostly degraded in the cells, whereas the control wild-type lysozyme was quantitatively secreted into the media. The degradation of [Ala128]lysozyme was independent of the transport from the endoplasmic reticulum to the Golgi apparatus. The degradation was greatly inhibited by incubation of cells at 15 degrees C, but was minimally affected by treatment of cells with the lysosomotropic agent, chloroquine, implying a non-lysosomal process. Additional mutations (Gly48-->Ser or Met29-->Thr) were created to make asparagine-linked (N-linked) glycosylation site in the [Ala128]lysozyme, and the resultant double mutants, [Ser48, Ala128]lysozyme and [Thr29, Ala128]lysozyme, were analyzed with respect to their intracellular degradation. These mutant proteins were susceptible to N-linked glycosylation, and were degraded in a similar manner to that of [Ala128] lysozyme, except that the onset of degradation of [Ser48, Ala128]lysozyme and [Thr29, Ala128] lysozyme, but not of [Ala128]lysozyme, was preceded by a lag period of up to 60 min. Furthermore, the degradative double mutants, [Ser48, Ala128]lysozyme and [Thr29, Ala128]lysozyme, were glycosylated post-translationally as well as co-translationally. These observations suggest that there is some interaction between the mechanisms of glycosylation and degradation.

Quantitative immunogold localization of Na, K-ATPase along rat nephron

Ultrastructural localization of Na, K-ATPase alpha-subunit along rat nephron segments was investigated quantitatively by immunogold electron microscopy on LR-White ultrathin sections using affinity-purified antibody against alpha-subunit of the enzyme. Ultrathin sections were incubated with the antibody at a saturation level and the number of gold particles bound per micron of the plasma membrane (particle density) of the tubular epithelial cells from the proximal tubule to the collecting duct was determined. In all the tubular epithelial cells, gold particles were located exclusively on the basolateral surface, and no significant binding of gold particles to the apical surface was observed. Distribution of gold particles on the basolateral membranes was quite heterogeneous; lateral membranes and infolded basal membranes were highly labeled, whereas the basal membranes which are in direct contact with the basal lamina were scarcely labeled. The average particle density on the basal surface was highest in the distal straight tubule cells (11.4 units), very high in the distal convoluted tubule cells (9.8 units), intermediate in the proximal tubule cells (3.3 units), in the connecting tubule cells (4.3 units), and in the principal cells of the collecting duct (5.6-3.8 units), low in the thin limb of Henle's loop (1.0 unit), and at the control level in the intercalated cells in the connecting and collecting duct. The relative number of gold particles/mm nephron segment and the relative number of gold particles in the various nephron segments were calculated using quantitative morphological data. The estimated distribution profile of the former was in good agreement with the Na, K-ATPase activity profile in rat nephron, which was determined biochemically with a microenzymatic method.

Effect of dietary fat and fiber on fecal flora, bacterial metabolites, and fecal properties in Japanese volunteers

The effects of dietary fat and dietary fiber (DF) levels in diet on fecal flora, activities of three fecal enzymes, putrefactive metabolites, fecal mutagenicity and fecal properties were studied in eight healthy volunteers. They were given low fat and low DF diet (LF: fat energy ratio was 13.9%, and DF intake was 9.0 g/day) for 10 days, high fat and low DF diet (HF: fat energy ratio was 52.7%, and DF intake was 7.1 g/day) for 10 days, and high fat and high DF diet (HFF: fat energy ratio was 52.0%, and DF intake was 24.8 g/day) for 10 days. No change of fecal flora at the bacterial group level was observed throughout the experimental period, except that the population of lactobacilli showed a tendency to increase in HF period. Fecal activities of beta-glucuronidase, beta-glucosidase and nitroreductase and some putrefactive products were unchanged between LF and HF, while these values decreased in HFF period. No significant change of fecal properties was observed between LF and HF, while by HFF supplementation fecal weight increased and fecal pH value was lower than that in LF and HF. Excretions of iron, zinc and calcium in feces did not increase by high DF supplementation.

Is there any effect of volcanic eruptions of Mount Sakurajima on canine lungs exposed naturally?--Morphometric analysis of intrapulmonary particulate deposit amount and histopathological investigations

In order to see whether any effect of inhalation of volcanic ash and gases from Mt. Sakurajima on canine lungs is observed or not, we examined the amount of intrapulmonary particulate deposits (IPD) and histopathological changes. Twenty-five abandoned or stray dogs (group A) in the areas affected enormously by volcanic ash and gases were examined in comparison with 13 abandoned or stray dogs (group B) in the area scarcely influenced. The amount of IPD was measured by using an image analyzer combined with a microscope. Age-associated increase of IPD values was noted, but mean IPD values were not different between groups A and B. Incidence of goblet cell hyperplasia was not different between the two groups. In none of the cases examined, squamous metaplasia of respiratory epithelia, pulmonary fibrosis, silicotic nodules, emphysematous change, or histopathological findings, which are indicative of bronchial asthma, were observed. In conclusion, obvious effect of volcanic eruption on canine lungs was not observed through both the measurement of IPD value and the histopathological evaluation.

Heavy chain binding protein (BiP/GRP78) and endoplasmin are exported from the endoplasmic reticulum in rat exocrine pancreatic cells, similar to protein disulfide-isomerase

Previously we found that in rat exocrine pancreatic cells, protein disulfide-isomerase (PDI), one of the major resident proteins in the lumen of the endoplasmic reticulum (ER) of many cells, is localized not only in the ER but also in the Golgi apparatus, secretory granules, plasma membranes, and even in the glandular lumens, despite possessing the ER retention signal KDEL (Lys-Asp-Glu-Leu) at the carboxyl terminus. In this report, we examined whether other ER luminal proteins bearing the KDEL signal at their C-termini, such as BiP/GRP78 and endoplasmin/GRP94 are also exported from the ER. We prepared two kinds of affinity-purified polyclonal antibodies; one against a synthetic peptide with 12 amino acids which is identical to the carboxyl terminus of BiP and another against purified endoplasmin. Immunoblot analysis using these two antibodies showed that BiP and endoplasmin exist in both the plasma membrane and the microsomal fractions, similar to the intracellular distribution of PDI in rat exocrine pancreas. The ratios of the amount of the three proteins in the two fractions, however, were variable, suggesting that the KDEL-bearing proteins such as PDI, BiP, and endoplasmin are exported from the ER with different efficiencies. Postembedding protein A-immunogold electron microscopy revealed that endoplasmin was exported from the ER and secreted to the extracellular space. The secretion of PDI in rat pancreatic lobules was inhibited by Brefeldin A (BFA) and by guanidino acid esters (FOY-305), which are known to be the inhibitors of the intracellular transport. Taken together with the previous immunogold electron microscopic analyses by Akagi et al. (1988), it is strongly suggested that in rat exocrine pancreatic cells PDI and the other KDEL-bearing proteins found in the extracellular space were not artificially released by cell damage during incubation but were secreted via the normal secretory pathway.

Two forms of the low-affinity Fc receptor for IgE differentially mediate endocytosis and phagocytosis: identification of the critical cytoplasmic domains

We have previously identified two species of the low-affinity human Fc receptor for IgE, Fc epsilon RIIa and Fc epsilon RIIb, which differ only in a short stretch of amino acids at the N-terminal cytoplasmic end. Their differential expressions on B cells and monocytes suggest that Fc epsilon RIIa and Fc epsilon RIIb are involved in B-cell function and IgE-mediated immunity, respectively. Here we show that Fc epsilon RII-mediated endocytosis is observed only in Fc epsilon RIIa-expressing cells, whereas IgE-dependent phagocytosis is observed only in Fc epsilon RIIb-expressing cells, demonstrating the functional difference between Fc epsilon RIIa and Fc epsilon RIIb. Furthermore, site-directed mutagenesis revealed that the tyrosine residue in the Fc epsilon RIIa-specific region is important for endocytosis, and the Asn-Pro residues in the Fc epsilon RIIb-specific region are required for phagocytosis. These findings suggest that endocytosis and phagocytosis are functionally separable phenomena involving distinct amino acid residues.

Quantitative immunoelectron microscopic localization of Na, K-ATPase alpha-subunit in the epithelial cells of rat vestibular apparatus

Na, K-ATPase was quantitatively localized in the epithelial cells of rat vestibular apparatus such as macula utriculi, macula sacculi and crista ampullaris. Immunogold localization method was carried out at the saturation level of antibody using an affinity purified antibody against the alpha-subunit of rat kidney Na, K-ATPase. Numerous gold particles were found on the basolateral membrane of the dark cells, a small number of gold particles were found on the basolateral membrane of the transitional epithelium cells and hair cells, but the luminal surface membranes of the hair cells, transitional epithelium cells, planum semilunatum cells and dark cells were rarely labeled by gold particles. Significance of the abundant localization of Na, K-ATPase on the basolateral surface of the dark cells in the production of endolymph was discussed.

Localization of nonspecific lipid transfer protein (nsLTP = sterol carrier protein 2) and acyl-CoA oxidase in peroxisomes of pigment epithelial cells of rat retina

We investigated the localization of nonspecific lipid transfer protein (nsLTP) in rat retina, especially in the pigment epithelial (RPE) cells, by the avidin-biotin-peroxidase complex method on cryosections for light microscopy and by the cryoimmunogold method for electron microscopy. Light microscopic observation revealed that the RPE, inner segment layer, nerve fiber layer, and Müller cells contain nsLTP. In the RPE cells gold particles were exclusively concentrated in the small peroxisomes (microperoxisomes; 0.1-0.3 micron in diameter), which were identified by double staining using anti-nsLTP and anti-catalase antibodies. In the peroxisomes gold particles were distributed homogeneously in the matrices and no preferential binding to the limiting membrane was observed. Acyl-CoA oxidase was also localized in the matrices of the peroxisomes. We suggest that the peroxisomes in RPE cells play important roles in the metabolism of lipids of the outer segment disk membranes, especially in the beta-oxidation of polyunsaturated long-chain and very long-chain fatty acids, such as docosahexaenoic acid which is composed of approximately one third of fatty acids in the disk membranes.

Different intracellular localization of peroxisomal proteins in fibroblasts from patients with aberrant peroxisome assembly

We investigated intracellular localization of peroxisomal proteins in fibroblasts from patients with Zellweger syndrome and neonatal adrenoleukodystrophy in whom peroxisomes were morphologically deficient or severely decreased. Indirect immunofluorescence staining revealed that catalase was mainly detected in the cytosol of fibroblasts from these patients, but a small amount of catalase was detected in granular pattern in a small percentage of cells. Double immunofluorescence staining revealed that catalase-containing particles in these patients also contained acyl-CoA oxidase and nonspecific lipid transfer protein. However, a 70 kD integral membrane protein and 3-ketoacyl-CoA thiolase were detected in all cells in granular pattern. Subcellular fractionation using digitonin after cell labeling revealed that a small amount of acyl-CoA oxidase and about half of thiolase in the precursor form were detected in the particulate fraction. These data suggest that the mechanisms of the transport and processing of catalase, acyl-CoA oxidase and nonspecific lipid transfer protein are different from those of the 70 kD integral membrane protein and 3-ketoacyl-CoA thiolase.

Cascade-connective optical parallel logic processor using electrophotonic devices

To perform image or arithmetic processing optically, it is necessary that a large number of required optical processors be connected in a series. For such cascade connections, however, coding and decoding processes are undesirable because of the necessity for the complicated hardware that often results. We propose here a new logic algorithm for application to cascade connections, which utilizes, in place of coding and decoding processes, true logic signals and their complements. Its optical implementation is also discussed. A processing module is constructed from electro-photonic devices referred to as vertical to surface transmission electro-photonic devices, a ferroelectrical liquid-crystal spatial light modulator, and a planar microlens array. Its logic operations are successfully demonstrated.

Quantitative immunocytochemical analysis of the induction of cytochrome P450IIB in rat hepatocytes

We examined whether induction of the phenobarbital (PB)-inducible form of cytochrome P450 (P450IIB) in rat hepatocytes could be analyzed quantitatively by immunogold electron microscopy. Rats received intraperitoneal injections of PB every 24 hr and livers at the various stages of PB induction were fixed by perfusion with a mixture of paraformaldehyde (4%) and glutaraldehyde (0.1%) and embedded in LR White. Ultra-thin sections were cut and labeled by the protein A-gold procedure using affinity-purified anti-P450IIB antibody which was previously immunoabsorbed with liver microsomes from a control rat (not treated with PB). We counted the number of gold particles per micron of the rough ER membranes (particle density). Before PB treatment, the particle density of the rough ER in rat hepatocytes was practically zero and increased markedly at 48 and 72 hr after PB treatment. The rough microsomes were prepared from these PB-treated rat livers. The amount of P450IIB was estimated by immunoblot analysis and the number of gold particles bound to the rough microsomal membrane was determined by the same post-embedding immunogold procedure. The particle density of the rough microsomes increased in parallel with the increase in the amount of P450IIB, indicating good correlation of the two variables. Thus, the induction of cytochrome P450IIB can be quantitatively and reliably investigated by immunogold electron microscopy.

Ultrastructural localization of Na+, K(+)-ATPase in the exorbital lacrimal gland of rat

Ultrastructural localization of Na+, K(+)-ATPase in the exorbital lacrimal gland of rat was investigated quantitatively by protein A-gold technique. Na+, K(+)-ATPase was purified from the rat kidney, and anti-holo Na+, K(+)-ATPase antibody was obtained from the rabbit by injecting the purified enzyme. A specific antibody against the alpha-subunit of Na+, K(+)-ATPase was affinity purified. Immunoblot analysis revealed that the antibody bound specifically to the alpha-subunit of Na+, K(+)-ATPase of the lacrimal gland. Rats were fixed by perfusion with 4% paraformaldehyde containing 1% glutaraldehyde, and the lacrimal glands were embedded in LR White resin. Ultrathin sections were incubated with affinity purified antibody against the alpha-subunit of Na+, K(+)-ATPase, and then with protein A-gold complex. The sections were observed under an electron microscope. Light microscopy with silver enhancement procedure revealed that Na+, K(+)-ATPase was located mainly on the basal region of the cells of intralobular and interlobular ducts. Quantitative immunoelectron microscopic analysis showed that gold particles were found on the basolateral surfaces of the interlobular and intralobular ducts cells and on the basolateral surface of the acinar cells, whereas no significant binding was observed on any part of the apical surfaces of these cells. Labeling density of gold particles was highest on the basolateral surface of the interlobular duct cells, secondarily highest on the basolateral surface of the intralobular duct cells, and lowest on the basolateral surface of the acinar cells. The distribution pattern of Na+, K(+)-ATPase in the acinar cells and the duct cells suggest that this enzyme may play an important role in primary secretion and in determining the composition of electrolytes in the final secretion, respectively.

Molecular cloning, sequencing, and expression of cDNA for rat liver microsomal aldehyde dehydrogenase

The cDNA clone for rat liver microsomal aldehyde dehydrogenase (msALDH) was isolated and sequenced. The deduced amino acid sequence consisting of 484 amino acid residues revealed that the carboxyl-terminal region of msALDH has a hydrophobic segment, which is probably important for the insertion of this enzyme into the endoplasmic reticulum membrane. COS-1 cells transfected with the expression vector pcD containing the full-length cDNA showed that the active enzyme was expressed and localized mainly on the cytoplasmic surface of the endoplasmic reticulum membranes. It has been proposed that ALDH isozymes form a superfamily consisting of class 1, 2, and 3 ALDHs (Hempel, J., Harper, K., and Lindahl, R., (1989) Biochemistry 28, 1160-1167). Comparison of the amino acid sequence of rat liver msALDH with those of rat other class ALDHs showed that msALDH was 24.2, 24.0, and 65.5% identical to phenobarbital-inducible ALDH (variant class 1), mitochondrial ALDH (class 2), and tumor-associated ALDH (class 3), respectively. Several amino acid residues common to the other known ALDHs, however, were found to be conserved in msALDH. Based on these results, we proposed to classify msALDH as a new type, class 4 ALDH.

Chromaffin granule H(+)-ATPase has F1-like structure

Isolated H(+)-ATPase from chromaffin granules was reconstituted into liposomes and the resultant proteoliposomes were further purified by Ficoll density gradient centrifugation. Studies by electron microscopy showed that proteoliposomes had particle structures (average diameter, about 10 nm) on their outer surface. These particles could be removed from the proteoliposomes by cold treatment. Immuno-electron microscopy showed that these particles were recognized by antibodies against the hydrophilic sector of the enzyme. These results indicate that the H(+)-ATPase has a peripheral membrane structure similar to that of F1-ATPase.

Dynamics of Ca2+ transients in norepinephrine-stimulated individual H-35 hepatoma cells: fura-2 digital imaging microscopy and high time-resolution microspectrofluorometry

We investigated spatiotemporal changes in cytoplasmic free Ca2+ concentration ([Ca2+]i) in norepinephrine (NE)-stimulated and fura-2-loaded individual H-35 rat hepatoma cells, using digital imaging microscopy and high time-resolution microspectrofluorometry. Application of NE (5 x 10(-6) M) resulted in an initial transient increase in [Ca2+]i, followed by a small sustained [Ca2+]i plateau above the pre-stimulation level. The initial peak and the small sustained plateau originated from intracellular stores and the extracellular space, respectively. The initial transient evoked by NE was totally blocked by phentolamine, an alpha-adrenergic antagonist, but was not blocked by either pre-incubation with nominally Ca(2+)-free medium or by pre-treatment of cells with La3+. On the other hand, the sustained plateau was eliminated by Ca(2+)-free medium or La3+. Therefore, H-35 cells have a Ca(2+)-signaling pathway which is activated via alpha-adrenergic receptors. Mn2+ entered the cytosol after NE stimulation, as shown by quenching of fura-2. This indicates that H-35 hepatoma cells possess Mn(2+)-permeable Ca2+ channels at the plasma membrane. In addition, the Ca2+ efflux pattern from H-35 cells to the extracellular space during NE stimulation was visualized by digital imaging microscopy when free fura-2 was equilibrated between the cells and the extracellular space. The efflux of Ca2+ from H-35 begins between the initial [Ca2+]i transient and the sustained [Ca2+]i plateau.

Ca2+ dynamics in rat pancreatic AR-42J and AR-IP cells

Spatiotemporal change of the cytosolic free Ca2+ concentration ([Ca2+]i) in response to a variety of secretagogues was examined in rat pancreatoma AR-42J and AR-IP cells by microspectroflurometry and digital imaging microscopy after loading with fura-2. In the presence of external Ca2+, carbachol, CCK-OP (cholecystokinin-octapeptide), gastrin, norepinephrine or high K+ evoked a large transient increase in [Ca2+]i in AR-42J cells which declined to a sustained level before slowly declining towards the resting level. In the absence of external Ca2+, a transient increase in [Ca2+]i were evoked by all the ligands except for high K+ stimulation, which declined rapidly towards the resting level. The [Ca2+]i increase caused by carbachol and high K+ treatment was inhibited by muscarinic receptor antagonist, atropine, and by L-type Ca2+ channel blocker, nifedipine, respectively. The transient [Ca2+]i increase induced by gastrin stimulation was not blocked by Ca2+ channel blocker, lanthanum. In the AR-IP cells, which are non-differentiated pancreatoma cell line, all stimulations including high K+ treatment have failed to evoke [Ca2+]i response. These intracellular Ca2+ mobilizations in response to ligands in AR-42J cells were displayed by digital imaging microscopy. From these results we conclude that AR-42J cells has an alpha-adrenergic receptor, in addition to muscarinic acetylcholine receptor, CCK-OP receptor, gastrin receptor and voltage dependent Ca2+ channel. In marked contrast, AR-IP cells have neither any hormone receptor for the above ligands nor voltage dependent Ca2+ channel.

Stacks of flattened smooth endoplasmic reticulum highly enriched in inositol 1,4,5-trisphosphate (InsP3) receptor in mouse cerebellar Purkinje cells

By immunogold electron microscopy we have shown that in mouse cerebellar Purkinje cells fixed by perfusion with formaldehyde-glutaraldehyde solution, the InsP3 receptor are numerously detected on the stacks of flattened cisterns (OTSU et al, (1990) Cell Struct. Funct., 15: 163-173). In the present experiment we investigated distribution, structure and properties of the stacks by conventional electronmicroscopy, lectin cytochemistry and immunoelectron microscopy. The size and number of stacks were variable depending on their intracellular localization; short stacks with 2-4 parallel cisterns predominate in the perikaryon, long stacks with 4-15 cisterns in the proximal dendrite, and long stacks with 3-4 cisterns in the distal dendrites. The flattened cisterns bind with concanavalin A but not with wheat-germ agglutinin and may contain KDEL proteins loaded with Lys-Asp-Glu-Leu at their C-terminin in their lumens, indicating that the cisterns are derived from ER membranes. The electron dense materials sandwiched between the cisternal membranes are composed of small particles, short cylindrical in shape and approximately 20 nm in diameter, and markedly labeled with anti InsP3R antibody. We suggest that they correspond to the tetramer of the InsP3R or their related molecules. It is not clear whether the stacks of flattened cisterns exist per se in the Purkinje cells or smooth ER existing in singlet in vivo in the Purkinje cells forms stacks during fixation. It is strongly suggested, however, that the smooth ER membranes covered by the InsP3R or their related molecules can easily interact and stack each other in the Purkinje cells.

Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, inhibits acidification and protein degradation in lysosomes of cultured cells

Bafilomycin A1 is known as a strong inhibitor of the vacuolar type H(+)-ATPase in vitro, whereas other type ATPases, e.g. F1,F0-ATPase, are not affected by this antibiotic (Bowman, E.M., Siebers, A., and Altendorf, K. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 7972-7976). Effects of this inhibitor on lysosomes of living cultured cells were tested. The acidification of lysosomes revealed by the incubation with acridine orange was completely inhibited when BNL CL.2 and A431 cells were treated with 0.1-1 microM bafilomycin A1. The effect was revealed by washing the cells. Both studies using 3-(2,4-dinitroanilino)-3'-amino-N-methyldipropylamine and fluorescein isothiocyanate-dextran showed that the intralysomal pH of A431 cells increased from about 5.1-5.5 to about 6.3 in the presence of 1 microM bafilomycin A1. The pH increased gradually in about 50 min. In the presence of 1 microM bafilomycin A1, 125I-labeled epidermal growth factor (EGF) bound to the cell surface at 4 degrees C was internalized normally into the cells at 37 degrees C but was not degraded at all, in marked contrast to the rapid degradation of 125I-EGF in the control cells without the drug. Immunogold electron microscopy showed that EGF was transported into lysosomes irrespective of the addition of bafilomycin A1. These results suggest that the vacuolar type H(+)-ATPase plays a pivotal role in acidification and protein degradation in the lysosomes in vivo.

Molecular cloning and deduced amino acid sequence of nonspecific lipid transfer protein (sterol carrier protein 2) of rat liver: a higher molecular mass (60 kDa) protein contains the primary sequence of nonspecific lipid transfer protein as its C-terminal part

Two types of cDNA for nonspecific lipid transfer protein (nsLTP), identical to sterol carrier protein 2, of rat liver were cloned; one was 787 base pairs (bp) long containing a 429-bp open reading frame of 143 amino acids, with a mass of 15,303 Da (15-kDa protein). The cDNA from the other type was 1966 bp long, including a 1641-bp open reading frame of 547 amino acids, giving a mass of 59,002 Da (60-kDa protein). The deduced primary sequence for the 15-kDa protein was exactly the same as the published sequence of purified nsLTP, except for an extra N-terminal sequence of 20 amino acids, consistent with the finding that nsLTP is synthesized as a larger precursor and processed to a mature form. The sequence for the 60-kDa protein contained, at the 3' end, the full sequence of the 15-kDa protein, a larger precursor to nsLTP. The 15- and 60-kDa proteins, synthesized in vitro from the respective cDNAs, were both immunoprecipitated by rabbit anti-rat liver nsLTP antibody and comigrated in SDS/PAGE with the proteins made in vitro from total liver RNA. These results shed new light on the dispute among several groups of investigators about the crossreactivity of anti-nsLTP antibody with a higher molecular mass, 60-kDa protein. In Northern blot analysis, two major RNA bands, 0.85 and 2.2 kilobases (kb) long, were detected together with two minor bands of 1.6 and 2.9 kb. The 0.85- and 2.2-kb RNAs most likely encode the 15-and 60-kDa proteins, respectively.

Cytosolic acidification leads to Ca2+ mobilization from intracellular stores in single and populational parietal cells and platelets

Regulatory relationship and gain control between cytosolic free Ca2+ concentration (Cai) and cytosolic pH (pHi) were evaluated by two different cell types, gastric parietal cells, and blood platelets. Studies were carried out in both single cells and populations of cells, using Ca2(+)-indicative probe fura-2 (1-(2-(5'-carboxyoxazol-2'-yl)-6-aminobenzofuran-5-oxy)-2-(2 '-amino-5'- methylphenoxy)ethane-N,N,N',N'-tetraacetic acid) and pH-indicative probe BCECF (2',7'-bis(carboxyethyl)carboxyfluorescein). Stimulation of single and populational parietal cells and platelets with gastrin and thrombin, respectively, resulted in an increase in Cai. In both populational cell types, an initial change in pHi during agonist stimulation occurred almost simultaneously with the mobilization of Ca2+; an initial transient decrease in pHi was followed by a slower increase in pHi above the prestimulation level. When populational platelets were preloaded with the Ca2+ chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), the thrombin-induced initial large increase in Cai was apparently inhibited, whereas the pHi decrease induced by thrombin was not altered. This suggests that the initial Cai change is not a prerequisite for the pHi change. The effect of pHi on Cai was examined next. In both single and populational cell types, application of the K(+)-H+ ionophore nigericin, which induced a transient decrease in pHi, led to the release of Ca2+ from intracellular stores. In single parietal cells double-labeled with fura-2 and BCECF, a temporal decrease in pHi preceded the rise in Cai after stimulation with nigericin. A decrease in pHi and an increase in Cai occurred at 1.5 and 4 s, respectively. In single parietal cells, replacement of medium Na+ with N-methyl-D-glucamine (NMG+), which also induced a decrease in pHi, resulted in repetitive Ca2+ spike oscillations. The source of Ca2+ utilized for the Ca2+ oscillation that was induced by NMG+ originated from the agonist-sensitive pool. Thus, several maneuvers, which were capable of decreasing pHi, led to an increase in Cai. Cytosolic acidification may be a part of the trigger for Ca2+ mobilization from intracellular stores in both parietal cells and platelets.

Expression, localization, and function of an N-terminal half fragment of the rat Na,K-ATPase beta-subunit in HeLa cells

The N-terminal half of the beta-subunit of rat brain Na,K-ATPase was expressed in HeLa cells transfected with the plasmid pSV2TKneo beta N containing the truncated beta-subunit cDNA to study the assembly and transport of alpha-beta complex. Immunoprecipitation from extracts of metabolically labeled transformed cells demonstrated that the truncated beta-subunit polypeptide (beta N) was neither transported to the plasma membrane nor assembled into an alpha-beta complex with the endogenous alpha-subunit. Cell fractionation experiments showed that the beta N truncated subunit remained unassembled within rough microsomes, suggesting that it never exited from the endoplasmic reticulum (ER). The assembly of the endogenous alpha-and beta-subunits in the beta N-expressing cells was significantly inhibited compared with control cells or with the transformants that did not express the beta N. These results suggest that the N-terminal portion of the beta-subunit interferes with the normal assembly of the endogenous complex which normally takes place in the ER.

Calcium distribution and mobilization in single rat parotid acinar cells investigated by digital imaging microscopy

The intracellular distribution and mobilization of cytosolic free calcium in single rat parotid acinar cells was analyzed by a digital imaging microscope equipped with a microspectrofluorometer, using calcium-sensitive dye fura-2. In the resting state, intracellular distribution of cytosolic free calcium concentration ([Ca2+]i) was heterogeneous: [Ca2+]i in the nuclear and perinuclear region was usually higher than that in the cytoplasm. By Ca(2+)-ionophore ionomycin and muscarinic agonist carbachol stimulation in the presence of 1 mM extracellular Ca2+, [Ca2+]i increased markedly and the gradient of [Ca2+]i between the nuclear region and the cytoplasm decreased. In ionomycin stimulation, [Ca2+]i increased homogeneously and this homogeneous increase was irreversible. In carbachol stimulation the gradient of [Ca2+]i between the nuclear region and the cytoplasm obviously reappeared within 2 min. By carbachol stimulation in the absence of extracellular Ca2+ (added 1 mM EGTA), [Ca2+]i returned to the prestimulation level after the initial transient increase. The distribution of [Ca2+]i also returned approximately to the prestimulation state. The gradient of [Ca2+]i between the nuclear region and the cytoplasm did not disappear even when [Ca2+]i elevated at the peak value.