Age Mosaicism across Multiple Scales in Adult Tissues

Most neurons are not replaced during an animal's lifetime. This nondividing state is characterized by extreme longevity and age-dependent decline of key regulatory proteins. To study the lifespans of cells and proteins in adult tissues, we combined isotope labeling of mice with a hybrid imaging method (MIMS-EM). Using ¹⁵N mapping, we show that liver and pancreas are composed of cells with vastly different ages, many as old as the animal. Strikingly, we also found that a subset of fibroblasts and endothelial cells, both known for their replicative potential, are characterized by the absence of cell division during adulthood. In addition, we show that the primary cilia of beta cells and neurons contains different structural regions with vastly different lifespans. Based on these results, we propose that age mosaicism across multiple scales is a fundamental principle of adult tissue, cell, and protein complex organization.

Visualization of the intrarenal distribution of capillary blood flow

This study describes a modified technique to fill the renal vasculature with a silicon rubber (Microfil) compound and obtain morphologic information about the intrarenal distribution of capillary blood flow under a variety of conditions. Kidneys and cremaster muscles of rats were perfused in vivo with Microfil using a perfusion pressure equal to the animal's mean arterial pressure at body temperature. Microfil did not alter arteriolar diameter or the pattern of flow in the microcirculation of the cremaster muscle. The modified protocol reproducibly filled the renal vasculature, including; glomerular, peritubular, and vasa recta capillaries. We compared the filling of the renal circulation in control rats with that seen in animals subjected to maneuvers reported to alter the intrarenal distribution of blood flow. Infusion of angiotensin II, hypotension, volume expansion, and mannitol- or furosemide-induced diuresis redistributed flow between renal cortical and medullary capillaries. The advantage of the current technique is that it provides anatomical information regarding the number, diameter, and branching patterns of capillaries in the postglomerular circulation critical in determining the intrarenal distribution of cortical and medullary blood flow.

Visualization of long-lived proteins reveals age mosaicism within nuclei of postmitotic cells

Toyama et al. monitor the replacement of long-lived components of nuclear pore complexes (NPCs) and nucleosomes in postmitotic cells. They describe age mosaicism at the level of chromatin organization and find that NPCs are maintained by piecemeal replacement in postmitotic nondividing cells but by entire complex replacement in an ESCRT-dependent manner in nondividing, starved quiescent cells.

Many adult tissues contain postmitotic cells as old as the host organism. The only organelle that does not turn over in these cells is the nucleus, and its maintenance represents a formidable challenge, as it harbors regulatory proteins that persist throughout adulthood. Here we developed strategies to visualize two classes of such long-lived proteins, histones and nucleoporins, to understand the function of protein longevity in nuclear maintenance. Genome-wide mapping of histones revealed specific enrichment of long-lived variants at silent gene loci. Interestingly, nuclear pores are maintained by piecemeal replacement of subunits, resulting in mosaic complexes composed of polypeptides with vastly different ages. In contrast, nondividing quiescent cells remove old nuclear pores in an ESCRT-dependent manner. Our findings reveal distinct molecular strategies of nuclear maintenance, linking lifelong protein persistence to gene regulation and nuclear integrity.

Effect of an anti-depressant on mouse hippocampus protein turnover using MIMS

Although antidepressants have been used in the treatment of affective disorders for over fifty years, the precise mechanism of their action remains unknown. Treatment regimens are based by and large on empirical parameters and characterized by a trial and error scheme. A better understanding of the mechanisms involved in antidepressant drug response is of fundamental importance for the development of new compounds that have a higher success rate and specificity. In order to elucidate the molecular pathways involved in the action of antidepressants, we wish to identify brain areas, cell types, and organelles that are targeted by antidepressant treatment in mice. Multi-isotope Imaging Mass Spectrometry (MIMS) allows a quantitative approach to this analysis, allowing us to delineate antidepressant effect on protein synthesis in the brain at single cell and organelle resolution. In these experiments, we obtained a global analysis of protein turnover in the hippocampus dentate gyrus (DG) and in the Cornu Ammonis (CA) regions, together with a subcellular analysis in the granular cells and others.

Brain stem cell division and maintenance studied using multi-isotope imaging mass spectrometry (MIMS)

New neurons are continuously produced from neural stem cells in specific regions of the adult brain of animals and humans. In the hippocampus, a region crucial for cognitive function, neurogenesis responds to a multitude of extrinsic stimuli; emerging evidence indicates that it may be important for behavior, pathophysiology, brain repair, and response to drugs. We have developed an approach to identify and quantify the cellular targets of pro- and anti-neurogenic stimuli, based on reporter transgenic mouse lines in which neural stem and progenitor cells or their progeny are marked by fluorescent proteins. Here, we demonstrate the feasibility of using MIMS for studying adult neurogenesis.

Study of protein and RNA in dendritic spines using multi-isotope imaging mass spectrometry (MIMS)

The classical view of neuronal protein synthesis is that proteins are made in the cell body and then transported to their functional sites in the dendrites and the dendritic spines. Indirect evidence, however, suggests that protein synthesis can directly occur in the distal dendrites, far from the cell body. We are developing protocols for dual labeling of RNA and proteins using 15N-uridine and 18O- or 13C-leucine pulse chase in cultured neurons to identify and localize both protein synthesis and fate of newly synthesized proteins. Pilot experiments show discrete localization of both RNA and newly synthesized proteins in dendrites, close to dendritic spines. We have for the first time directly imaged and measured the production of proteins at the subcellular level in the neuronal dendrites, close to the functional sites, the dendritic spines. This will open a powerful way to study neural growth and synapse plasticity in health and disease.

Quantitative imaging of inositol distribution in yeast using multi-isotope imaging mass spectrometry (MIMS)

Despite the widely recognized importance of the several species of inositol polyphosphates in cell biology, inositol has not been successfully imaged and quantified inside cells using traditional spectrophotometry. Multi-isotope imaging mass spectrometry (MIMS) technology, however, has facilitated direct imaging and measurement of cellular inositol. After pulsing cells with inositol labeled with the stable isotope Carbon-13 (¹³C), the label was detected in subcellular volumes by MIMS. The tridimensional localization of ¹³C within the cell illustrated cellular distribution and local accumulation of inositol. In parallel, we performed control experiments with ¹³C-Glucose to compare a different ¹³C distribution pattern. Because many functions recently attributed to inositol polyphosphates are localized in the nucleus, we analyzed its relative nuclear concentration. We engineered yeast with human thymidine permease and viral thymidine kinase, then fed them with ¹⁵N-thymidine. This permitted direct analysis of the nuclear DNA through the detection of the ¹⁵N isotopic signal. We found practically no co-localization between inositol signal (¹³C-isotope) and nuclear signal (¹⁵N-isotope). The ¹³C-tag (inositol) accumulation was highest at the plasma membrane and in cytoplasmic domains. In time-course labeling experiments performed with wild type yeast (WT) or modified yeast unable to synthesize inositol from glucose (ino1Δ), the half-time of labeled inositol accumulation was ~1 hour in WT and longer in ino1Δ. These studies should serve as a template to study metabolism and physiological role of inositol using genetically modified yeasts.

Quantitative imaging of selenoprotein with multi-isotope imaging mass spectrometry (MIMS)

Multi-isotope imaging mass spectrometry (MIMS) allows high resolution quantitative imaging of protein and nucleic acid synthesis at the level of a single cell using stable isotope labels. We employed MIMS to determine the compartmental localization of selenoproteins tagged with stable isotope selenium compounds in human aortic endothelial cells (HAEC), and to compare the efficiency of labeling (to determine the ideal selenium source) from these compounds: [⁸²Se]-selenite, [⁷⁷Se]-seleno-methionine, and [⁷⁶Se]-methyl-selenocysteine. We found that all three selenium sources appear to be localized in the nucleus as well as in the cytoplasm in HAEC. Seleno-methionine appears to be a better source for (seleno)protein synthesis. For MIMS detection, we compared freeze-drying to thin layer vs. thin sectioning for sample preparation. MIMS provides a unique and novel way to dissect selenoprotein synthesis in cells.

Detection of immunolabels with multi-isotope imaging mass spectrometry (MIMS)

We have developed a method that combines the use of stable isotopes, MIMS and antibody. We began with using well-established antibodies, anti-actin and anti-synaptophysin, in mouse intestinal cells. We extended the method to an immunogold assay to specifically localize Ribeye, a major protein component of retina synaptic ribbons, or to localize a synaptic vesicle-containing protein, synaptophysin. Both are localized in presynaptic nerve terminal of photoreceptors cells in retina. Our results show that by MIMS analysis of the Au signal we can directly identify antibodies tagged with non amplified 1.4 nm gold nanoparticles. They also demonstrate that the gold nanoparticle-tagged antibodies do not dilute the ¹⁵N/¹⁴N signal used for measuring protein turnover. Thus we can simultaneously and directly use MIMS to measure protein turnover and to identify cell type or specific protein.

Segmentation of multi-isotope imaging mass spectrometry data for semi-automatic detection of regions of interest

Multi-isotope imaging mass spectrometry (MIMS) associates secondary ion mass spectrometry (SIMS) with detection of several atomic masses, the use of stable isotopes as labels, and affiliated quantitative image-analysis software. By associating image and measure, MIMS allows one to obtain quantitative information about biological processes in sub-cellular domains. MIMS can be applied to a wide range of biomedical problems, in particular metabolism and cell fate [1], [2], [3]. In order to obtain morphologically pertinent data from MIMS images, we have to define regions of interest (ROIs). ROIs are drawn by hand, a tedious and time-consuming process. We have developed and successfully applied a support vector machine (SVM) for segmentation of MIMS images that allows fast, semi-automatic boundary detection of regions of interests. Using the SVM, high-quality ROIs (as compared to an expert's manual delineation) were obtained for 2 types of images derived from unrelated data sets. This automation simplifies, accelerates and improves the post-processing analysis of MIMS images. This approach has been integrated into "Open MIMS," an ImageJ-plugin for comprehensive analysis of MIMS images that is available online at http://www.nrims.hms.harvard.edu/NRIMS_ImageJ.php.

High-resolution quantitative imaging of mammalian and bacterial cells using stable isotope mass spectrometry

Background

Secondary-ion mass spectrometry (SIMS) is an important tool for investigating isotopic composition in the chemical and materials sciences, but its use in biology has been limited by technical considerations. Multi-isotope imaging mass spectrometry (MIMS), which combines a new generation of SIMS instrument with sophisticated ion optics, labeling with stable isotopes, and quantitative image-analysis software, was developed to study biological materials.

Results

The new instrument allows the production of mass images of high lateral resolution (down to 33 nm), as well as the counting or imaging of several isotopes simultaneously. As MIMS can distinguish between ions of very similar mass, such as ¹²C¹⁵N^- and ¹³C¹⁴N^-, it enables the precise and reproducible measurement of isotope ratios, and thus of the levels of enrichment in specific isotopic labels, within volumes of less than a cubic micrometer. The sensitivity of MIMS is at least 1,000 times that of ¹⁴C autoradiography. The depth resolution can be smaller than 1 nm because only a few atomic layers are needed to create an atomic mass image. We illustrate the use of MIMS to image unlabeled mammalian cultured cells and tissue sections; to analyze fatty-acid transport in adipocyte lipid droplets using ¹³C-oleic acid; to examine nitrogen fixation in bacteria using ¹⁵N gaseous nitrogen; to measure levels of protein renewal in the cochlea and in post-ischemic kidney cells using ¹⁵N-leucine; to study DNA and RNA co-distribution and uridine incorporation in the nucleolus using ¹⁵N-uridine and ⁸¹Br of bromodeoxyuridine or ¹⁴C-thymidine; to reveal domains in cultured endothelial cells using the native isotopes ¹²C, ¹⁶O, ¹⁴N and ³¹P; and to track a few ¹⁵N-labeled donor spleen cells in the lymph nodes of the host mouse.

Conclusion

MIMS makes it possible for the first time to both image and quantify molecules labeled with stable or radioactive isotopes within subcellular compartments.

CN- secondary ions form by recombination as demonstrated using multi-isotope mass spectrometry of 13C- and 15N-labeled polyglycine

We have studied the mechanism of formation CN- secondary ions under Cs+ primary ion bombardment. We have synthesized 13C and 15N labeled polyglycine samples with the distance between the two labels and the local atomic environment of the 13C label systematically varied. We have measured four masses in parallel: 12C, 13C, and two of 12C14N, 13C14N, 12C15N, and 13C15N. We have calculated the 13C/12C isotope ratio, and the different combinations of the CN isotope ratios (27CN/26CN, 28CN/27CN, and 28CN/26CN). We have measured a high 13C15N- secondary ion current from the 13C and 15N labeled polyglycines, even when the 13C and 15N labels are separated. By comparing the magnitude of the varied combinations of isotope ratios among the samples with different labeling positions, we conclude the following: CN- formation is in large fraction due to recombination of C and N; the CO double bond decreases the extent of CN- formation compared to the case where carbon is singly bonded to two hydrogen atoms; and double-labeling with 13C and 15N allows us to detect with high sensitivity the molecular ion 13C15N-.

Transport of 13C-oleate in adipocytes measured using multi imaging mass spectrometry

The mechanism of long chain free fatty acid (FFA) transport across cell membranes is under active investigation. Here we describe the use of multi imaging mass spectrometry (MIMS) to monitor intracellular concentrations of FFA and provide new insight into FFA transport in cultured adipocytes. Cells were incubated with 13C-oleate:BSA and either dried directly or dried after washing with a medium deprived of 13C-oleate:BSA. Cells were analyzed with MIMS using a scanning primary Cs+ ion beam and 12C-, 13C-, 12C14N-, 13C14N-) (or 12C 15N-) were imaged simultaneously. From these quantitative images the values of the 13C/ 12C ratios were determined in the intracellular lipid droplets, in the cytoplasm and outside the 3T3F442A adipocytes. The results indicate that after incubation with 13C-oleate:BSA the droplet 13C/ 12C ratio was 15 +/- 6%. This value is about 14-fold higher than the 13C/ 12C terrestrial ratio (1.12%). After washing the 13C-oleate:BSA, the droplet 13C/ 12C ratios decreased to 1.6 +/- 0.1%, about 40% greater than the natural abundance. Results for washed cells indicate that relatively little FFA was esterified. The unwashed cell results, together with the value of the lipid water partition coefficient, reveal that intracellular unbound FFA (FFAu) concentrations were on average about 4.5-fold greater than the extracellular FFAu concentrations. These results are consistent with the possibility that FFA may be pumped into adipocytes against their electro-chemical potential. This work demonstrates that MIMS can be used to image and quantitate stable isotope labeled fatty acid in intracellular lipid droplets.

In-situ imaging mass spectrometry analysis of melanin granules in the human hair shaft

The elemental composition of melanin granules and other components of the hair shaft was determined by multi-isotope imaging mass spectrometry, a method with unique advantages for the visualization and quantification of stable isotopes and the elemental composition in study of the fine structure of biologic samples. We mapped and quantified the chemical composition of hair cross-sections using secondary ions generated from naturally occurring 16O, 12C14N, 32S, and 34S with a maximum lateral resolution of 35 nm. Based on these elemental maps of unprecedented resolution we obtained simultaneously the chemical fingerprints and the structural features, such as cuticle, melanin granules, the macro fibrils of the cortex, and small sulfur-rich domains in the medulla, in the hair cross-section. We found an intriguing distribution of 16O, 12C14N, and 32S in melanin granules that we interpret as a highly anisotropic pattern of oxidation.

Measure of carbon and nitrogen stable isotope ratios in cultured cells

We report the measurement of the natural isotope ratios of nitrogen and carbon in subcellular volumes of individual cells among a population of cultured cells using a multi-isotope imaging mass spectrometer (MIMS), [MIMS is the prototype of the NanoSIMS 50, Cameca, France.] We also measured the nitrogen and carbon isotope ratio in cells after they had been cultured in media enriched with the amino acid glycine labeled with either 13C or 15N. The results demonstrate that 13C/12C and 15N/14N isotope ratios can be measured directly on a subcellular scale. This opens the way for the use of stable isotopes, in particular 15N, as labels to measure the intracellular turnover of biomolecules. Such a capability should help resolve a wide range of biomedical problems.

Intracellular ion concentrations and their maintenance by Na+/K(+)-ATPase in preimplantation mouse embryos

We have measured the amounts of Na+, K+ and C- in preimplantation mouse embryos (1-cell, 2-cell and morula) using electron probe X-ray microanalysis. The levels of these ions do not vary much over this period, and are approximately the same as those found in other mammalian cells, contrary to previous reports. We have confirmed that preimplantation embryos exhibit Na+/K(+)-ATPase activity at all stages examined, and have shown that the ATPase maintains high K+/Na+ ratios (12-16) in all these embryonic stages, comparable to those seen in other healthy cells; this is in contrast to the low ratios reported in earlier work. Inhibition of the Na+/K(+)-ATPase results in the slow exchange of intracellular K+ for extracellular Na+ (half-time approximately 5 h), indicating that Na+/K(+)-ATPase activity maintains steep Na+ and K+ gradients in preimplantation mouse embryos as it does in most other cells.

Calcium and magnesium transport by isolated goldfish hair cells

We used electron-probe analysis (EPA) to investigate the transport of the divalent cations calcium and magnesium across the plasma membranes of hair cells. Unlike ion-sensitive fluorescent dyes, EPA detects these ions regardless of the state of chemical combination inside the cell; changes in these cell ions determined by EPA indicate net transport across the cell membrane. Raising or lowering either extracellular divalent cation within 1 mM of its control level raised or lowered its cell contents, but further increases in extracellular concentration of either ion had little additional effect on the cell content of that ion. New steady-state contents could be obtained within minutes, but the net divalent cation currents required to account for the observed changes would have been smaller than most currents recorded electrophysiologically, less than 1 pA. The effects of replacing extracellular Na+ with other ions were consistent with the presence in hair cells of exchangers for divalent cations thought to occur in other tissues: electrically neutral sodium/magnesium exchange (2 Na+ per Mg2+) and electrogenic sodium/calcium exchange (at least 3 Na+ per Ca2+). The increase in cell Ca after 1 minute of potassium-depolarization was similar to that expected from electrophysiological studies of voltage-sensitive calcium currents in goldfish hair cells. After that time in elevated potassium, however, either calcium-entry pathways were inhibited or calcium-export mechanisms were enhanced.

Extracellular N-methyl-D-glucamine leads to loss of hair-cell sodium, potassium, and chloride

The organic cation N-methyl-D-glucamine (NMDG) is often used to replace extracellular sodium in experimental studies. Replacing 100 mM of Na+ with NMDG+ in the fluid bathing isolated goldfish hair cells led to a rapid loss not only of cell sodium, but also of cell potassium and chloride. The loss of inorganic cell solutes was accompanied by acidification of the cells. Cell volume did not change significantly. These results are consistent with passage of the cationic form of NMDG, a titratable amine with a pKa of 9.6, across the hair-cell membrane. These results should have bearing in interpreting results of experiments in which this cation is used to replace extracellular sodium, particularly for periods of time longer than 3 min.

Electron-probe analysis of isolated goldfish hair cells: implications for preparing healthy cells

Electron-probe analysis provides an objective criterion for the physiological status of cells: whether they show the high potassium and low sodium that are expected of healthy animal cells. Preparing isolated goldfish hair cells that were healthy by this criterion required several precautions, including: limited exposure to enzymes and to simple salt solutions, a rest period between enzyme treatment and mechanical disruption of the tissue, and presence of bovine albumin in the medium both during the rest period and during mechanical dispersion and plating. Cells prepared with these precautions from the saccule and lagena and kept in an enriched medium had the following elemental composition (mole percentages with respect to phosphorus): K, 103; Na, 18; Cl, 23; S, 13; Mg, 8; Ca, 1.5. These mole percentages were close to these elements' total millimolar concentrations in the cells. If the precautions were not taken, cells with intact surface membranes (as assessed by exclusion and retention of dyes) could be obtained, but the cells had elevated cell sodium and low cell potassium.

Rapid resting ion fluxes in goldfish hair cells are balanced by (Na+,K+)-ATPase

Inhibition of sodium/potassium pumping by isolated goldfish hair cells led to a rapid gain of sodium and loss of potassium. Half-times for turnover were about 10 min, among the fastest of any cell type examined by electron-probe analysis. Pumping was inhibited by removal of extracellular potassium or by treatment with 1 mM ouabain, as expected of a classical (Na+,K+)-ATPase. The initial rate of entry of sodium after inhibition, about 4 mM/min, provided an estimate of resting sodium-entry and sodium-pumping rates. After return to control medium, cells loaded with sodium by removal of extracellular potassium could recover their normal high-potassium/low-sodium status. The initial rate of recovery (an estimate of the cells' maximum sodium-pumping rate) was sufficient to lower cell sodium by 10 mM/min. This functional estimate of hair-cell (Na+,K+)-ATPase activity was of the same order of magnitude as the biochemical activity of (Na+,K+)-ATPase previously reported for sensory epithelia of other species. The balance between sodium entry and sodium pumping determines hair-cell ionic composition, and thus the resting potential and the driving forces for sodium-coupled transport processes. Imbalance due to excess sodium entry or loss of pump capacity could have significant consequences for hair-cell function and integrity.

A novel H+ permeability dominating intracellular pH in the early mouse embryo

Most cell types are relatively impermeant to H+ and are able to regulate their intracellular pH by means of plasma membrane proteins, which transport H+ or bicarbonate across the membrane in response to perturbations of intracellular pH. Mouse preimplantation embryos at the 2-cell stage, however, do not appear to possess specific pH-regulatory mechanisms for relieving acidosis. They are, instead, highly permeable to H+, so that the intracellular pH in the acid and neutral range is determined by the electrochemical equilibrium of H+ across the plasma membrane. When intracellular pH is perturbed, the rate of the ensuing H+ flux across the plasma membrane is determined by the H+ electrochemical gradient: its dependence on external K+ concentration indicates probable dependence on membrane potential and the rate depends on the H+ concentration gradient across the membrane. The large permeability at the 2-cell stage is absent or greatly diminished in the trophectoderm of blastocysts, but still present in the inner cell mass. Thus, the permeability to H+ appears to be developmentally regulated.

Adhesion is required for protein kinase C-dependent activation of the Na+/H+ antiporter by platelet-derived growth factor

Adhesion of normal, anchorage-dependent cells to a solid substratum leads to activation of the Na+/H+ antiporter and elevation of intracellular pH. These effects are mediated by extracellular matrix proteins, such as fibronectin, and their receptors, the integrins. Experiments using pharmacological inhibition and down-regulation of protein kinase C (PKC) in C3H 10T1/2 cells show that platelet-derived growth factor induces activation of the Na+/H+ antiporter by means of a PKC-dependent pathway in adherent cells but cannot do so in poorly adherent cells. Poorly adherent cells are, however, able to elevate intracellular pH in response to a phorbol ester, indicating that PKC and subsequent steps in the pathway are functional. These results indicate that coupling of platelet-derived growth factor to PKC activation requires cell adhesion.

cAMP activates the sodium pump in cultured cells of the elasmobranch rectal gland

The inorganic ion content of rectal gland cells cultured from Squalus acanthias was studied by electron probe analysis in order to determine the effect of stimulation by cAMP. Cell sodium was reduced by 30% (P less than 0.01) at 8 min after exposure to dibutyryl cAMP and theophylline and remained low at 25 and 33 min. Chloride content also fell significantly with stimulation. Although cAMP may activate several transport sites, the results are consistent with a direct effect of stimulation to increase the activity of the sodium pump in shark rectal gland.

Cellular electrolyte and volume changes induced by acidosis in the rabbit proximal straight tubule

Cellular acidosis induced either by high Pco2 or by low HCO3- concentrations has been shown to cause cell swelling in isolated, lumen-collapsed, S2 segments of the rabbit proximal tubule (Sullivan et al., Am J Physiol 1990; 258: F831-F839). The swelling is not followed by a volume regulatory response. The ionic basis of the swelling has been investigated by measurement of the cellular K+, Na+, and Cl- content (electron probe) and HCO3- concentration (pH-sensitive fluorescent dye). Cell content of K+, Na+, and Cl- was expressed as a ratio to P content. Exposure to 15% CO2 increased K/P from 0.98 to 1.16, Cl/P from 0.14 to 0.20, and Na/P from 0.09 to 0.11. Cell (HCO3-) increased from 22 to 32 mM. Reduction in bath (HCO3-) from 25 to 5 mM reduced cell (HCO3-) from 24 to 8 mM and increased K/P from 0.75 to 0.90. Na/P fell from 0.13 to 0.09, and Cl/P fell from 0.15 to 0.12. Thus, swelling resulting from acidosis induced by high CO2 was accompanied by an accumulation of K+, Cl-, and HCO3-; that resulting from acidosis induced by a fall in (HCO3-) was combined with an accumulation of K+ and an unidentified anion. To determine if the swelling induced by a fall in pH might be coupled with depolarization of the basolateral membrane, the effect of 1 mM barium was tested. Barium caused cell volume to increase 10.2%. Cell pH rose from 7.38 to 7.56, K/P increased from 0.63 to 0.73, Na/P did not change, and Cl/P rose from 0.17 to 0.20. Cell (HCO3-) increased 10.4 mM. When the pH of the barium-treated tissue was reduced to 7.02 by raising Pco2, additional cell swelling and accumulation of K+ occurred. The effect on cell volume of a reduction of bath (HCO3-) from 25 to 5 mM at constant bath pH was determined. Cell pH was not altered. Cell volume decreased 3% initially and then returned to the control level. When the bath (HCO3-) was restored to 25 mM, cell volume increased 3.9% and then returned to the baseline. Thus, volume regulation was not impaired. It was concluded that a fall in cell pH induces swelling, and this is coupled with an accumulation of K+. This is probably the result of a pH effect on barium-sensitive and barium-insensitive K+ conductance pathways. The nature of the anions that balance the gain in K+ depends on the means used to induce acidosis.

Relief from alkaline load in two-cell stage mouse embryos by bicarbonate/chloride exchange

Mouse embryos at the two-cell stage are able to recover from an alkaline load. We found that this recovery is mediated by sodium-independent bicarbonate/chloride exchange: intracellular pH (pHi) recovery from alkaline load is inhibited by the anion exchange inhibitor 4,4'-diisothiocyanostilbene disulfonic acid, lack of bicarbonate, or lack of chloride. The dependence of the pHi recovery on extracellular chloride concentration exhibits Michaelis-Menten kinetics. Furthermore, uptake of chloride is inhibited in a dose-dependent manner by extracellular bicarbonate. The Km for external chloride was found to be about 3 mM, with a Ki for external bicarbonate of about 2 mM. The exchanger is active above approximately pH 7.15. These results demonstrate that mouse embryos at the two-cell stage possess a sodium-independent bicarbonate/chloride exchange mechanism that is similar to that found in other mammalian cells. This bicarbonate/chloride exchanger appears to be the sole pHi-regulatory mechanism in the two-cell stage mouse embryo, since our previous results have shown that there are apparently no specific mechanisms active in these cells for relieving acid loads.

Insoluble fibronectin activates the Na/H antiporter by clustering and immobilizing integrin alpha 5 beta 1, independent of cell shape

Growth of anchorage-dependent cells requires both soluble mitogens and insoluble extracellular matrix molecules such as fibronectin. Soluble growth factors activate chemical signaling pathways and stimulate proliferation by binding to transmembrane receptors. Insoluble fibronectin also binds to cell-surface receptors; however, it is thought to act primarily via effects on the cytoskeleton and cell shape. We recently demonstrated that cell spreading on surface-adsorbed fibronectin activates the Na/H antiporter and that inhibition of this chemical-signaling pathway suppresses growth. We now show that insoluble fibronectin activates the Na/H antiporter by clustering and immobilizing integrin alpha 5 beta 1, independent of effects on cell shape. These results show that an extracellular matrix receptor can behave similarly to a growth factor receptor to activate a signaling pathway implicated in growth control.

Multiple integrins share the ability to induce elevation of intracellular pH

Previous work has shown that adhesion of anchorage-dependent cells to fibronectin via integrin alpha 5 beta 1 leads to activation of the Na-H antiporter and a rise in intracellular pH (pHi). We now show that adhesion of bovine capillary endothelial cells (BCE) to fibrinogen; collagens type III, IV, and V; laminin; and vitronectin; ligands that bind other members of the integrin family, resulted in significant elevations in pHi. Other ligands (basic fibroblast growth factor, concanavalin A, and thrombin), which bind cells when immobilized on plastic, but that do not bind integrins and do not support cell growth, do not elevate pHi. Adhesion to an antibody against integrin alpha v beta 3 also elevates pHi. Adhesion of peripheral human T lymphocytes to an antibody against the integrin LFA-1 induced a rise in pHi. Antibodies to CD2 or ICAM-2 had only slight effects on pHi, whereas an antibody to the T cell receptor complex that strongly activates T cells induced a large increase in pHi. We conclude that elevation of pHi by integrins is specific and is a property shared by many members of the integrin family.

Direct determination of free fatty acid transport across the adipocyte plasma membrane using quantitative fluorescence microscopy

Movement of free fatty acids (FFA) across the plasma membrane has been directly measured for the first time, using fluorescent FFA analogs and quantitative fluorescence microscopy. The rate of short chain FFA (less than or equal to 12 carbons) transport from the extracellular medium into intracellular lipid droplets of 3T3F442A adipocytes was more than 40-fold faster than long chain FFA (16 and 18 carbons). The membrane-impermeable amino reagent 4,4'-diisothiocyanostilbene-2,2'-disulfonate, inhibited greater than or equal to 50% of the long chain FFA transport but had no effect on short chain FFA transport. Oleic acid (2 microM) inhibited 90% of the fluorescent oleate transport but had no effect on the 11-carbon analog. These results indicate that a large fraction of long chain FFA uptake is mediated by a plasma membrane protein (s).

Two-cell stage mouse embryos appear to lack mechanisms for alleviating intracellular acid loads

Mouse embryos at the two-cell stage, like other cells, can recover from an intracellular acid-load. Our previous work has shown, surprisingly, that there is no contribution to this recovery by Na+/H+ antiport activity. Here we show that the recovery similarly is not affected by inhibition of other known intracellular pH (pHi) regulatory mechanisms. Specifically, the recovery is unaffected by lack of external Na+, inhibition of anion exchange, or lack of bicarbonate, which eliminates the Na(+)-dependent HCO3-/Cl- exchanger as a possible mechanisms. These conditions also eliminate any possible Na+,HCO3- cotransporter operating to relieve acid-loading. Recovery is unaffected similarly by nonspecific inhibitors of H(+)-ATPase activity. These observations lead to the conclusion that recovery from acid-load is a passive process in the two-cell mouse embryo. Similarly, the mean base-line pHi (6.84) is not dependent on known pHi regulatory mechanisms. The embryos exhibit a marked intracellular alkalinization when exposed to Cl(-)-free medium in the presence of bicarbonate. This response is eliminated by an inhibitor of anion exchange and by lack of bicarbonate, but is independent of Na+. These results indicate that there is probably a Na(+)-independent HCO3-/Cl- exchanger active in these cells, presumably functioning to alleviate alkaline loads.

Proximal tubule volume regulation in hypo-osmotic media: intracellular K+, Na+, and Cl-

This study sought to measure the net loss of intracellular K+, Na+, and Cl- that accompanied isosmotic cell volume regulation in hypotonic media and to determine if electrolyte loss depended on the rate at which the extracellular osmolality was reduced. Isolated nonperfused proximal S2 segments from rabbit kidney cortex were studied in vitro. Gradual lowering of osmolality from 295 to 150 mOsm/kg at a rate of 2 mOsm/kg/min did not cause an increase in tubule cell volume until the medium osmolality decreased below 190 mOsm/kg. By contrast, tubules rapidly bathed in low osmolality media exhibited classical osmometric swelling followed by incomplete volume regulatory decrease. Volume regulation associated with gradual and rapid lowering of osmolality was accompanied by the net loss of intracellular K+, Na+, and Cl- (measured by electron probe); however, the temporal pattern of electrolyte loss depended on the rate of osmotic change. With gradual lowering of osmolality, cell K+ content did not decrease significantly until osmolality was lowered below 200 mOsm/kg, whereas Cl- was lost at the 200 mOsm/kg level and below. With rapid lowering of osmolality, cell K+ content was strikingly decreased at the 200 mOsm/kg level, but Cl- did not change appreciably until osmolality was decreased to 150 mOsm/kg. Cell Na+ content decreased in hypo-osmotic media, but the magnitude was relatively small. During volume regulation that accompanied either gradual or rapid lowering of medium osmolality from 295 to 150 mOsm/kg, intracellular osmolal gap, the difference between medium osmolality and the sum of intracellular concentrations of K+, Na+, and Cl- decreased 87 and 58 mOsm/kg, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of intracellular pH and growth by fibronectin in capillary endothelial cells

The aim of this work was to analyze the mechanism by which fibronectin (FN) regulates capillary endothelial cell proliferation. Endothelial cell growth can be controlled in chemically-defined medium by varying the density of FN coated on the substratum (Ingber, D. E., and J. Folkman. J. Cell Biol. 1989. 109:317-330). In this system, DNA synthetic rates are stimulated by FN in direct proportion to its effect on cell extension (projected cell areas) both in the presence and absence of saturating amounts of basic FGF. To investigate direct growth signaling by FN, we carried out microfluorometric measurements of intracellular pH (pHi), a cytoplasmic signal that is commonly influenced by soluble mitogens. pHi increased 0.18 pH units as FN coating densities were raised and cells progressed from round to spread. Intracellular alkalinization induced by attachment to FN was rapid and followed the time course of cell spreading. When measured in the presence and absence of FGF, the effects of FN and FGF on pHi were found to be independent and additive. Furthermore, DNA synthesis correlated with pHi for all combinations of FGF and FN. Ethylisopropylamiloride, a specific inhibitor of the plasma membrane Na+/H+ antiporter, completely suppressed the effects of FN on both pHi and DNA synthesis. However, cytoplasmic pH per se did not appear to be a critical determinant of growth since DNA synthesis was not significantly inhibited when pHi was lowered over the physiological range by varying the pH of the medium. We conclude that FN and FGF exert their growth-modulating effects in part through activation of the Na+/H+ exchanger, although they appear to trigger this system via separate pathways.

Apparent absence of Na+/H+ antiport activity in the two-cell mouse embryo

We have used the pH-sensitive dye BCECF to investigate the regulation of intracellular pH (pHi) by two-cell stage mouse embryos in bicarbonate-free medium. There is no indication of a Na+/H+ antiport active in regulating pHi, as recovery from acid-loading was insensitive to amiloride, ethylisopropylamiloride, or the absence of extracellular Na+. Instead, protons appear to be in equilibrium across the plasma membrane, as indicated by the response of pHi to changes in external K+. The embryos have an intracellular buffering power in the normal range (25.3 mM/pH); their apparent permeability to protons is, however, very high (0.22 cm/sec).

Alanine stimulation of passive potassium efflux in hepatocytes is independent of Na(+)-K+ pump activity

We have studied the effects of alanine on electrolyte content and ion transport in rat hepatocytes in primary culture. Application of 10 mM alanine is followed by 1) an increase in the rate of sodium entry; 2) an increase in intracellular sodium content; 3) an increase in ouabain-inhibitable rubidium uptake, a measure of Na(+)-K+ pump rate; 4) an increase in unidirectional potassium efflux, whether or not the Na(+)-K+ pump was inhibited; and 5) an increase in the initial rate of potassium loss after Na(+)-K+ pump inhibition. This increase occurred even when alanine was presented in Ringer made hypertonic by the addition of sucrose. Application of hypotonic solution led to a significant net loss of potassium, but no net loss of potassium was observed after alanine application. Thus alanine stimulates the Na(+)-K+ pump by increasing intracellular sodium secondary to an increase in the rate of sodium entry. Passive potassium efflux is stimulated by a mechanism that is independent of the stimulation of the Na(+)-K+ pump. The stimulated potassium efflux does not appear to be a response to cell swelling.

Proximal tubule volume regulation in hyperosmotic media: intracellular K+, Na+, and Cl-

Nonperfused proximal S2 segments from rabbit kidney cortex have been shown to keep cell volume constant as medium osmolality is slowly raised but to shrink and not exhibit regulatory volume increase (RVI) if medium osmolality is abruptly elevated (J. Lohr and J. Grantham. J. Clin. Invest. 78: 1165-1172, 1986). In the current study, 0.5 mM butyrate in the medium 1) extended the range from 361 to 450 mosmol/kgH2O over which cells maintained volume constant as osmolality was gradually raised and 2) restored RVI after cell shrinkage when osmolality was rapidly raised from 295 to 400 mosmol/kgH2O. Volume regulation was associated with net increases in intracellular Na+ and Cl- but no change in K+ (measured by electron probe). The increments in Na+ and Cl- were insufficient to account for the total addition of osmolytes required for volume maintenance or restoration. The fraction of the expected increase in intracellular osmoles accounted for by the increase in [(K+)i + (Na+)i + (Cl-)i] was 52 and 21% for gradual and rapid osmotic changes, respectively. We conclude that butyrate enhances the capacity of S2 segments to regulate volume in hyperosmotic medium by promoting addition of Na+ and Cl- and by other undermined factors.

Renal epithelial cyst formation and enlargement in vitro: dependence on cAMP

Cysts, a common abnormality of kidneys, are collections of urine-like fluid enclosed by a continuous layer of epithelial cells. Renal cysts derive from nephrons and collecting ducts and progressively enlarge as a consequence of epithelial proliferation and transepithelial fluid secretion. The initiation of cyst formation and the factors that control cyst enlargement are unknown. We used an in vitro model of renal cysts to explore the role of the cAMP signal transduction system in the formation and expansion of cysts. MDCK cells, cultured in hydrated-collagen gel, produced polarized monolayered epithelial cysts when intracellular cAMP was increased by prostaglandin E1, arginine vasopressin, cholera toxin, forskolin, or 8-bromoadenosine 3',5'-cyclic monophosphate. All agonists were potentiated by 3-isobutyl-1-methylxanthine, a nucleotide phosphodiesterase inhibitor. The cell proliferation component of cyst enlargement was accelerated by cAMP agonists, as shown by the increased growth of MDCK cells in subconfluent monolayers. The fluid secretion component, reflected by the transepithelial movement of fluid across polarized monolayers of MDCK cells grown on permeable supports, was stimulated by cAMP agonists in the basolateral medium. Chloride levels were higher in the cyst fluid and the secreted fluid than in the bathing medium. We conclude that the development of MDCK cysts is dependent on cAMP. This signal transduction system may be an important modulator of epithelial cell proliferation and transepithelial fluid secretion in the kidney.

Studies of terminal differentiation of electrolyte transport in the renal proximal tubule using short-term primary cultures

There are several lines of indirect evidence suggesting that the renal tubule cells have not yet reached terminal differentiation at birth. Methods used in cell biology can now be applied to study renal ontogeny. This review describes how primary cultures of proximal tubule cells from rats can be used to investigate developmental changes in Na permeability and Na-K-ATPase-mediated transport.

Effect of cell spreading on cytoplasmic pH in normal and transformed fibroblasts

Growth of normal fibroblasts requires an adhesive substratum on which to spread, whereas transformed cells can grow in suspension. Since an alkaline cytoplasm has been shown to be required for growth, we measured cytoplasmic pH in individual cells as a function of spreading. The degree of spreading was controlled by coating tissue culture plastic with varying amounts of the nonadhesive polymer polyHEMA. Completely round BALB/c 3T3 cells were 0.15 pH unit more acidic than spread cells. In short-term experiments, cells were treated with the peptide Gly-Arg-Gly-Asp-Ser-Pro to induce rounding or by plating on fibronectin to induce spreading. When cells were induced to change shape, pH changed rapidly and reversibly. All of the anchorage-dependent cell lines tested behaved similarly, but 3T3 cells transformed by the plasma membrane oncogene src or ras were able to maintain a relatively alkaline pH even when completely round. Cells transformed by the nuclear oncogene myc behaved like normal cells. Our results suggest that the requirement for spreading may in part be mediated by cytoplasmic pH. Anchorage-independent growth due to oncogenes that localize to the plasma membrane is associated with loss of this control mechanism.

Noninvasive measurement of glucose uptake by two populations of murine embryos

Glucose uptake was measured by a noninvasive fluorescence technique on a total of 165 morula- and blastocyst-stage murine embryos in two different culture media. Eighty-four embryos were tested in M2 medium, and the remaining 81 embryos were tested in M16. Embryos assayed in M2 took up significantly less glucose over the 4-h assay period than did embryos assayed in M16. The lower uptake of glucose by embryos in M2 corresponded with a decrease in the quality of embryos cultured overnight in M2 as judged by morphological criteria. Embryos that were judged to be degenerate or had gross abnormalities took up significantly less glucose than did normal embryos. Glucose uptake in both populations of embryos covered a wide range of values and was normally distributed. A significant effect between mothers was noted in glucose uptake for embryos assayed in both M16 and M2 media. The possible uses of noninvasive measures of glucose uptake as a test of embryo viability or for optimizing culture conditions are discussed.

Acute renal failure with selective medullary injury in the rat

Since human acute renal failure (ARF) is frequently the result of multiple rather than single insults, we used a combination of treatments to induce ARF in rats. Uninephrectomized, salt-depleted rats injected with indomethacin developed ARF after administration of radiocontrast. After 24 h, the plasma creatine rose from 103 +/- 3 to 211 +/- 22 mumol/liter (mean +/- SE) and the creatinine clearance dropped from 0.7 +/- 0.1 to 0.2 +/- 0.04 ml/min (P less than 0.001). Severe injury was confined to the outer medulla and comprised necrosis of medullary thick ascending limbs (mTALs), tubular collapse, and casts. Other nephron segments were free of damage except for the proximal convoluted tubules which showed vacuole formation originating from lateral limiting membranes that resembled changes reported in human contrast nephropathy. Cell damage to mTALs included mitochondrial swelling, nuclear pyknosis, and cytoplasmic disruption with superimposed calcification; these changes were most severe in the deepest areas of the outer medulla, away from vasa recta in zones remote from oxygen supply. The fraction of mTALs with severe damage was 30 +/- 7% (range 2-68) and the extent of injury was correlated with a rise in plasma creatinine (r = 0.8, P less than 0.001). Thus, the nature of mTAL injury was similar to the selective lesions observed in isolated kidneys perfused with cell-free medium and was shown to derive from an imbalance between high oxygen demand by actively transporting mTALs and the meager oxygen supply to the renal medulla. Combined multiple renal insults in the rat produce ARF that resembles the clinical syndrome of contrast nephropathy and is characterized by selective mTAL injury conditioned by medullary hypoxia.

Ouabain-resistant, amiloride-sensitive Na+-K+ pumping activity and morphological changes are inducible

The functional expression of a ouabain-resistance gene has been studied in a transfected cell line, OR-6 cells. These cells possess a ouabain-inhibitable, amiloride-resistant Na+-K+ pump under control conditions. They can be induced by brief culture in the presence of low concentrations of ouabain to express a ouabain-resistant, amiloride-sensitive Na+-K+ pumping activity. They revert to the uninduced phenotype shortly after removal of ouabain from the culture medium. When placed in low-potassium, low-serum, or dibutyryl adenosine 3'-5'-cyclic monophosphate-containing media, these cells produce long cellular extensions. Thus a single inducible gene appears to regulate sensitivity of the Na+-K+ pumping activity to cardiac glycosides and to amiloride, and it may be related to important characteristics of cellular morphology.

Studies on terminal differentiation of rat renal proximal tubular cells in culture: ouabain-sensitive K and Na transport

We have studied the ontogeny of Na-K ATPase-mediated Na and K transport in rat renal proximal tubular cells using electron probe analysis. The cells were cultured from kidneys of 10-day-old, young (Y), and 40-day-old, adult (A) rats. Before an experiment cells were Na-loaded and K-depleted by incubation in K-free medium. The maximum rate of ouabain-sensitive Na and K transport was measured after reactivating the Na-K pump by transferring the cells from K-free medium to medium containing 5 mM K. In cells cultured for 2 days, ouabain-sensitive Na and K net initial transport rates were significantly higher in A than in Y cells. Between 2 and 4 days in culture there was a significant decrease in ouabain-sensitive Na and K transport rates in both Y and A cells. From 2 to 4 days of culture there was, in Y but not in A cells, a significant decrease in K/Na ratio. The decrease in K/Na ratio was due to a significant increase in Na content. After incubation in K-free medium, net intracellular solute accumulation was observed in A and Y cells cultured for 4 days but not in A and Y cells cultured for 2 days. In conclusion, maximal Na- and K-pump-mediated transport increases during terminal differentiation. This increase can be measured in cells cultured for 2 days. With longer time in culture, Na-K pump activity decreases and the difference between A and Y cells is not measurable.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of experimental models to study the development of renal function

By use of animal experiments it has been demonstrated that renal cells are immature at birth. Membrane transport is quantitatively and qualitatively different. The control of fluid and electrolyte balance is therefore different in infancy. The increased filtered load at birth as well as hormones will induce tubule maturation. The use of animal experiments will in the future give an insight into how external factors influence growth and maturation.

Rescue of a wild-type MDCK cell by a ouabain-resistant mutant

When wild-type MDCK cells (W-MDCK) were cocultured in mixed monolayers with a ouabain-resistant mutant (R-MDCK), the wild-type cells were protected from the effect of ouabain up to concentrations as high as 100 microM. Rescue depended on the dose of ouabain and on the proportion of each cell type in the coculture. The survival of R-MDCK cells at 1 microM ouabain was not endangered by varying from 1:9 to 9:1 the proportion of W-MDCK cells to be rescued. Ouabain binding revealed two kinds of binding sites in R-MDCK cells, one with high and the other with low affinity. Only the high affinity site was present in W-MDCK cells. Electron probe analysis of individual cells revealed that rescued cells kept a high K and a low Na intracellular contents, similar to control cells. Histograms of intracellular K/Na in cocultured cells treated with ouabain were unimodal. Using microinjection of Lucifer yellow or electrophysiological techniques we estimated that at most 13% of the R-MDCK and W-MDCK cells may be connected at a given time through cell-to-cell junctions. Therefore permanent cell-to-cell communication did not seem to play a central role in the rescue. W-MDCK cells cocultured with R-MDCK cells and subsequently separated, were not rescued. Thus rescue did not seem to depend on the transfer from R-MDCK to W-MDCK cells of either ouabain-resistant Na-K pumps or of information to synthesize them. It is speculated that intercellular communications were sporadic events, so that all cells may become intermittently connected and rescued.