Regulation of cellular membrane transport by the exocytotic insertion and endocytic retrieval of transporters

Constitutive apical membrane recycling in Aplysia enterocytes

In Aplysia californica enterocytes, alanine-stimulated Na+ absorption increases both apical membrane exocytosis and fractional capacitance (fCa; a measure of relative apical membrane surface area). These increases are thought to reduce membrane tension during periods of nutrient absorption that cause the enterocytes to swell osmotically. In the absence of alanine, exocytosis and fCa are constant. These findings imply equal rates of constitutive endocytosis and exocytosis and constitutive recycling of the apical plasma membrane. Thus, the purpose of this study was to confirm and determine the relative extent of constitutive apical membrane recycling in Aplysia enterocytes. Biotinylated lectins are commonly used to label plasma membranes and to investigate plasma membrane recycling. Of fourteen biotinylated lectins tested, biotinylated wheat germ agglutinin (bWGA) bound preferentially to the enterocytes apical surface. Therefore, we used bWGA, avidin D (which binds tightly to biotin), and the UV fluorophore 7-amino-4-methylcoumarin-3-acetic acid (AMCA)-conjugated avidin D to assess the extent of constitutive apical membrane recycling. A temperature-dependent (20 vs. 4 degrees C) experimental protocol employed the use of two tissues from each of five snails and resulted in a approximately 60% difference in apical surface fluorescence intensity. Because the extent of membrane recycling is proportional to the difference in surface fluorescence intensity, this difference reveals a relatively high rate of constitutive apical membrane recycling in Aplysia enterocytes.

Vesicle targeting to the apical domain regulates bile excretory function in isolated rat hepatocyte couplets

BACKGROUND & AIMS

Plasma membrane solute transport may be regulated in many epithelial cells by vesicle traffic to and from the site of residence of the transporter. The aim of this study was to determine if this phenomenon may also play a role in the regulation of canalicular transport of bile acids.

METHODS

Confocal microscopy and image analysis were performed to quantitatively assess changes in secretory capacity and vesicle targeting in isolated rat hepatocyte couplets that had been exposed to fluorescent bile acid after pretreatment with dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) and/or nocodazole.

RESULTS

DBcAMP stimulated bile acid secretion by 240% while significantly increasing canalicular circumference. Nocodazole decreased secretion by 410% and significantly decreased canalicular circumference. When DBcAMP was added to nocodazole-treated couplets, a slight but significant increase was found in both fluorescent bile acid secretion and canalicular circumference as compared with nocodazole alone. Finally, DBcAMP stimulated translocation of vesicles to the canalicular membrane as determined by immunocytochemical localization of a putative bile acid transporter, Ca2+, Mg2+-ecto-adenosine triphosphatase.

CONCLUSIONS

The findings support the view that apical membrane transport activity in the rat hepatocyte is highly regulated by the insertion of vesicles into this domain and that this process involves both microtubule-dependent and -independent mechanisms.

Frequency modulation of transmitter release

In 1952 Fatt and Katz recorded at a frog neuromuscular junction while stimulating the nerve and found "... that successive endplate potential responses varied in a step-like manner, corresponding to units of miniature endplate potentials" (J Physiol 117, 109-128). This led them to propose that fast neuromuscular transmission is 'quantal'. Quantal release is now commonly ascribed to a vesicular form of neurosecretion since vesicles have routinely been visualized in presynaptic terminals. The vesicular hypothesis (Del Castillo and Katz, 1955) assumes that quanta, or 'transmitter packets of standard size', are assembled and stored in the numerous vesicles routinely identified in micrographs of virtually all central and peripheral presynaptic nerve terminals. Simply stated, this model predicts that each one of the miniature synaptic signals (MSSs) follows from the exocytosis of one vesicle's contents. However, the time required for membrane fusion preceding exocytosis (Almers and Tse, 1990) and the variability in MSS amplitude and time course (Vautrin et al, 1992a,b) cannot readily be reconciled by a simple, exocytotic model of quantal release from preloaded vesicles. These difficulties with the original model have led us to re-evaluate MSSs generated at the classical peripheral synapse, the cholinergic neuromuscular junction of the mouse diaphragm, as well as at central synapses between embryonic hippocampal neurons mediated by gamma-aminobutyric acid (GABA). At these synapses, the release of GABA is also assumed to have classical quantal properties like peripheral acetylcholine release (Edwards et al, 1990). Our results show that at both synapses, progressive alterations in elementary signal properties can be induced in a remarkably rapid manner. The original report of preferred amplitudes and intervals in the spontaneous miniature signals (Fatt and Katz, 1952) has repeatedly been confirmed and is here incorporated into a dynamic model of fast synaptic transmission. Although MSSs exhibit variable rise-times and peak amplitudes, they can both be described in terms of synchronization of transmitter release. We have reviewed many experimental findings, which together strongly suggest that the original interpretation of Fatt and Katz (1952) regarding MSSs as reflecting the non-propagated 'neurogenic' activity of 'terminal spots' may be a useful concept to pursue since it may help to explain part of the underlying molecular basis of quantal release.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium-dependent phosphate transport in a rat kidney endosomal fraction

An endosome-enriched fraction was prepared from rat kidney cortex by a standard procedure employing centrifugation on a Percoll gradient. This fraction showed time-dependent accumulation of inorganic phosphate (Pi) which was stimulated two- to threefold during the initial phase by an inwardly directed Na+ gradient. Na+ gradient-dependent Pi accumulation decreased with increasing medium osmolality and Pi binding accounted for only 16% of the total accumulation at two minutes. Like the Pi transporter in the brush border membrane (BBM), the Na+ gradient-dependent Pi uptake (but not the Na(+)-independent component) by the endosomal fraction was stimulated by intravesicular Pi and by an outwardly directed proton gradient, and was inhibited by extravesicular arsenate. Unlike the Pi transporter in BBM, the endosomal Pi transporter was not changed by acidic pH under non-gradient conditions. Activation of the endosomal proton pump by extravesicular ATP, leading to acidification of the vesicle interior, was accompanied by stimulation of endosomal Na+ gradient-dependent Pi transport. Inhibition of the proton pump by deletion of chloride or addition of N-ethylmaleimide abolished the stimulation of Pi uptake by ATP. The data indicate that the Na(+)-dependent Pi transporter in renal endosomal fractions is an intrinsic endosomal component. It remains to be determined if the endosomal Pi transporter plays a role in regulation of renal Pi transport.

Effect of phenylarsine oxide on fluid phase endocytosis: further evidence for activation of the glucose transporter

We have shown previously that insulin stimulates fluid phase endocytosis in 3T3-L1 adipocytes (Gibbs et al., 1986). Using [14C]sucrose as an endocytotic marker, we show here that phenylarsine oxide, a trivalent arsenical which binds neighboring dithiols, blocked not only insulin-stimulated fluid phase endocytosis, but basal endocytosis as well. The Ki for this process was 6 microM in the presence or absence of insulin and the time required for inhibition was less than 2.5 min, the limit of detection in our assay system. These results can be compared with the inhibitory effect of phenylarsine oxide on insulin-stimulated glucose transport. Although the Ki for insulin-stimulated transport (7 microM) was similar to that for inhibition of endocytosis, basal glucose transport was not affected by the inhibitor. Further, when cells were prestimulated with insulin causing maximal stimulation of the glucose transport rate, phenylarsine oxide induced a time-dependent reduction to the basal rate (t 1/2 of 10 min), despite the fact that endocytosis was blocked immediately. This observation suggests that if the transporter is recycled by an exocytotic/endocytotic mechanism, it is distinct from fluid-phase endocytosis/exocytosis, which is a vesicle-mediated process, and provides further evidence that the transporter may undergo intrinsic activation/inactivation which does not require vesicle movement.

Cellular aspects of proximal tubular phosphate reabsorption

In vivo manipulations to alter renal Pi reabsorption and the subsequent isolation of proximal tubular brush border membrane vesicles have greatly increased our knowledge about the regulation of renal Pi reabsorption via the Na+/Pi cotransport system. Only recently, direct biochemical and cell-biological access has become possible by the use of established and primary cell cultures. Based on the results obtained with isolated brush border membranes and cultured cells, a model has been presented, which might serve as a basis for future research of the regulatory control mechanisms of the renal Na+/Pi cotransport. At present, a major drawback is the fact that the molecular identity of the Na+/Pi cotransport system is still unknown. The identification of this transport system would certainly be a great step and would allow to verify or falsify one or the other hypotheses postulated in the past few years for the regulatory control mechanism(s) of the renal Na+/Pi cotransport.

Regulatory volume decrease in alveolar macrophages: cation loss is not correlated with changes in membrane recycling

Alveolar macrophages regain their normal volume after swelling in hypo-osmotic solutions. This process, termed regulatory volume decrease (RVD), is initiated 3-5 minutes after exposure of cells to hypo-osmotic solutions, and by 30 min, near-normal volumes are attained. Volume decrease does not occur at 0 degrees C or in solutions in which Na+ has been replaced by K+, or Cl- by the impermeant anion gluconate. These results, as well as direct measurement of intracellular cations, indicate that decreases in cell volume result primarily from the loss of K+ and Cl- and are similar to RVD in lymphocytes. Kinetic analysis of cation loss, both by directly measuring changes in intracellular cation content and by assaying rubidium efflux, showed that cation loss occurred immediately upon media dilution. The rate of cation loss fit first-order kinetics and preceded both the initiation of volume decrease and the maximum increase in surface receptor number. These results suggest that the cation transporters responsible for RVD are located at the cell surface and that regulation of activity is not dependent on alterations in membrane movement.

Dipeptide metalloendoprotease substrates are glucose transport inhibitors and membrane structure perturbants

Dipeptide substrates for metalloendoproteases have previously been shown to block biological processes requiring membrane fusion. Thus, we employed such compounds as potential inhibitors of the insulin-dependent activation of glucose transport in fat cells. This event is thought to involve vesicle movement from an intracellular site to the cell surface and would therefore require membrane fusion during the activation step. We find that synthetic dipeptides which are metalloendoprotease substrates rapidly and reversibly inhibit insulin-activated glucose oxidation in a dose-dependent manner but exhibit essentially no effect on basal levels. A similar result is obtained when glucose transport is measured directly in intact fat cells, in metabolically poisoned cells, and in isolated membrane vesicles derived from insulin-activated or untreated fat cells. That is, the dipeptide substrates inhibit insulin-activated glucose uptake to a greater extent than basal transport, and they do so even when vesicle translocation and fusion have already taken place as in ATP-depleted cells and isolated vesicles. Onset of transport inhibition after dipeptide addition is rapid, but not instantaneous, with a t 1/2 of 15-30 s. The metalloendoprotease substrates also inhibit glucose uptake and cytochalasin B binding in human erythrocytes but not in human placental microsomes. Finally, light microscopic examination of substrate-treated red cells reveals marked cupping and/or echinolation of the cell membrane. We conclude the following from these observations: Metalloendoprotease substrates are inhibitors of adipocyte glucose transport.(ABSTRACT TRUNCATED AT 250 WORDS)

The number of chloride-cation cotransport sites on Ehrlich ascites cells measured with [3H]bumetanide

Simultaneous measurements were made of net Cl influxes and [3H]bumetanide binding to Ehrlich ascites tumor cells in which the chloride-cation cotransport pathway had been activated by hypertonic challenge. There was a good linear correlation between inhibition of Cl influx during regulatory volume increase and numbers of bumetanide molecules bound per cell, consistent with high specificity of bumetanide binding to cotransport sites. The extrapolation to the number of bumetanide binding sites per cell at maximal inhibition of Cl transport thus gives the number of cotransport sites per cell as 2.0 X 10(6). From this, and the fluxes measured (not necessarily maximum fluxes), the turnover number is calculated at 50 Cl ions per site per second. Unstimulated cells in isotonic medium, with negligible bumetanide-inhibitable fluxes, have the same number of bumetanide binding sites as the activated cells undergoing volume regulation.

Current concepts of regulation of phosphate transport in renal proximal tubules

The wealth of new information on BBM transport of Pi which has accumulated in recent years gives an indication of the importance and intellectual challenge that the mechanism of this process poses to investigators. In this brief reflection on the field, we have tried to draw attention to some general principles and features which may be helpful as working hypotheses in the development of the field. To date, a disproportionate amount of effort may have been spent on deciphering putative intracellular regulatory mechanisms, without knowing some essential fundamental properties of the Na+-Pi-COT. We suggest that a major effort should be exerted towards elucidating biogenesis of the Na+-Pi-COT, the possible existence of a membrane cycling mechanism, and a refined analysis of the Na+-Pi-COT in specific subsegments of proximal tubules. Advances in these areas together with studies of both the rapid and long-term adaptive regulation of Pi transport are needed, given the central role of the kidney in total body Pi homeostasis both in health and disease.

Na-Pi cotransport in LLC-PK1 cells: fast adaptive response to Pi deprivation

A short-term regulation of the Na-dependent transport of Pi in LLC-PK1 cells by the extracellular concentration of Pi is described. Pi deprivation of the cells for 10 min led to an increase (30%) of the Na-Pi cotransport activity, whereas the Na-dependent D-glucose transport system was not affected. This regulatory phenomenon was not affected by the presence of cycloheximide. The same adaptive response was observed in apical membrane vesicles isolated from rapidly adapted cells. In the isolated membranes, increased Na-Pi cotransport is characterized by an increase of the maximal rate of uptake (control: 193 +/- 15; adapted: 306 +/- 51 pmol X mg-1 X 15 s-1), whereas the apparent Km for Pi remained constant. The results suggest that LLC-PK1 cells possess a mechanism or mechanisms that allow a rapid "activation" and "inactivation" of Na-dependent Pi transport systems as a function of the concentration of the extracellular Pi.

Phosphorylation of the glucose transporter in vitro and in vivo by protein kinase C

The Ca2+- and phospholipid-dependent protein kinase (protein kinase C) is present in many mammalian tissues, and its important physiological protein substrates are only now beginning to be identified. A useful advance in identifying these intracellular substrates has been the recognition that the kinase is the receptor for phorbol esters, which stimulate phosphotransferase activity. Phorbol ester-induced changes in protein phosphorylation in intact cells may thus be taken, in part, as a probable indication of protein kinase C activation. The many cellular effects of phorbol esters include the stimulation of glucose uptake, although the response of glucose uptake to phorbol esters appears to be complex, apparently varying in response time and requirement for protein synthesis. Such observations prompted us to explore one possible explanation for the alteration of glucose uptake, namely, phosphorylation of the glucose transporter by protein kinase C. We report here that incubation of purified human erythrocyte glucose transporter with rat brain protein kinase C results in the phosphorylation of a protein of relative molecular mass (Mr) 50,000-60,000 which has subsequently been identified as the glucose transporter by specific immunoprecipitation with a monoclonal antibody. Immunoprecipitation of membrane proteins from 32P-labelled human erythrocytes revealed a phorbol ester-stimulated phosphorylation of the transporter. This covalent modification of the glucose transporter may thus, in part, underlie the ability of phorbol esters and certain hormones to stimulate glucose uptake.

Effect of insulin on the activity of amino acid transport systems in cultured human fibroblasts

The regulation of amino acid transport by insulin has been studied in cultured human fibroblasts. Among the six amino acid transport systems operating in cultured human fibroblasts, two systems (A and X-C) are strongly stimulated by insulin and four (ASC, X-AG, y+ and L) are essentially not sensitive to the presence of the hormone in the incubation medium. The hormonal stimulation of system A and system X-C became significant after 3 h of incubation and increased up to 12 h. The stimulatory effect was related to insulin concentration, with a half-maximal stimulation at 10(-9) M hormone concentration. Insulin enhanced transport activity by increasing the maximal velocity (Vmax) of transport, without significant changes in Km values.

Involvement of calcium and cytoskeleton in gallbladder epithelial cell volume regulation

The importance of calcium and cellular cytoskeletal elements in the activation or control of volume regulation by epithelial cells was explored in Necturus gallbladder. Gallbladder cells have been previously shown to rapidly readjust their volumes to control size after osmotic perturbation of the mucosal bathing solution. Removal of calcium from the perfusates caused dramatic morphological changes that prevented assessment of the role of extracellular calcium in volume regulation. The regulatory volume increase (RVI) that follows shrinkage of the cell due to perfusion of a hypertonic mannitol solution is insensitive to agents that interfere with cell calcium- or calmodulin-mediated events (quinidine, trifluoperazine) and is not blocked by agents that cause changes in the cytoskeleton (colchicine, cytochalasin B). Osmotically induced cell swelling is followed by regulatory volume decrease (RVD), which is inhibited by agents that interfere with calcium-dependent processes (quinidine, trifluoperazine) and by the microfilament inhibitor, cytochalasin B. These results indicate that RVD depends on calcium, calmodulin, and an intact microfilament network, whereas RVI is independent of these factors.

Maximal flux responses after multiple challenges with vasopressin

Antidiuretic hormone (ADH) increases transepithelial flux of water and particular solutes across the amphibian urinary bladder and mammalian collecting duct by increasing the permeability of the apical surface. We find that if each challenge with ADH is ended by replacing the medium bathing both the mucosal and serosal surfaces of the toad bladder, then rechallenge with the same supramaximal dose of ADH 36-100 min later produces flux equivalent to or greater than the original response, but rechallenge after 15 min produces only 68% of the original response. If the medium bathing the mucosal surface is neither replaced nor returned to its original volume, complete recovery of the osmotic flux response to ADH does not occur. Maximal restimulation by ADH occurs with transepithelial osmotic gradients between 119 and 180 mosmol/kg during both challenges (the serosal bath is always isotonic amphibian Ringers). In addition, ADH-containing serosal baths that have maximally activated transport across bladders for 30-60 min can be reused and again produce maximal activation of ADH responses in fresh bladders or in the original bladders after washing. These results are in contradistinction to reports of desensitization of transepithelial flux upon rechallenge with ADH after an initial stimulation under many conditions. Our findings suggest that desensitization in vitro may result from experimental design rather than intrinsic biological characteristics of the system.

Post-translational control by carrier availability of amino acid transport in fetal human fibroblasts

The action of serum on the expression of the starvation-enhanced amino acid transport by System A (as a part of the adaptive regulation mechanism) has been studied in cultured fetal human fibroblasts. Serum enhanced L-proline uptake of cells starved in serum-free medium. This effect was rapid, proportional to the amount of pre-existing transporters, insensitive to cycloheximide and kinetically characterized by an increase of transport Vmax. These results can be interpreted to indicate that serum is essential for a vectorial post-translational event leading to insertion of transport proteins into the cell membrane.

Do hydrophobic sequences cleaved from cellular polypeptides induce membrane fusion reactions in vivo?

The concept that a direct interaction between Ca2+ and phospholipids is a major factor in membrane fusion reactions is questioned. Attention is drawn to a number of findings on associations between fusion and the proteolysis of membrane proteins. It is proposed that hydrophobic polypeptides, which are functionally comparable to the fusogenic proteins of certain viruses but which are produced in cells by the endogenous proteolysis of membrane and cellular proteins, may induce membrane fusion reactions in vivo.

Endocytosis and exocytosis: current concepts of vesicle traffic in animal cells

Animal cells have specific pathways to transport macromolecules from their surrounding environment to their interior, and from internal compartments to the cell surface or other intracellular locations. Many of these movements appear to be receptor-dependent processes in which specific membrane receptors bind macromolecules, segregate them into discrete membrane-limited compartments, and move the molecules to new locations. Such processes include the clustering and internalization of receptor-bound ligands at the cell surface in clathrin-coated pits, the formation of endocytic vesicles (receptosomes) from coated pits, the movement of receptosomes by saltatory motion to the Golgi system, the concentration of materials in the coated pits of the Golgi system that are destined for delivery to lysosomes, and the directed traffic of materials destined for exocytosis out of the Golgi to the cell surface. This review describes some of the experiments which have led to our current understanding of the various organelles involved in this traffic and some of the biochemical mechanisms involved.