Organic anion and cation transporter expression and function during embryonic kidney development and in organ culture models

Organic anion and cation transporters (OATs, OCTs, and OCTNs) mediate the proximal tubular secretion of numerous clinically important compounds, including various commonly prescribed pharmaceuticals. Here, we report determination of the ontogeny of these transporters and of NaP(i)2 and SGLT1, using quantitative polymerase chain reaction (QPCR) to determine expression levels of transporter genes in rat embryonic kidneys on each day of gestation from embryonic day (ed) 13 to ed18, in cultures of induced and uninduced metanephric mesenchyme (MM), and on each day of 1 week of whole embryonic kidney (WEK) culture. We also examined ontogeny of Oat1 protein expression in rat embryonic kidney by immunohistochemistry. Finally, we used uptake of fluorescein (FL) as a novel in vitro functional assay of OAT expression in WEK and MM. Developmental induction of OAT and OCT genes does not occur uniformly: some genes are induced early (e.g., Oat1 and Oat3, potential early markers of proximal tubulogenesis), and others after kidney development is relatively advanced (e.g., Oct1, a potential marker of terminal differentiation). The ontogeny of transporter genes in WEK and MM is similar to that observed in vivo, indicating that these organ culture systems may represent convenient in vitro models to study the developmental induction of OATs, OCTs, and OCTNs. Functional transport was evidenced by accumulation of FL in the developing tubule in WEK and MM organ cultures. Our findings on the renal ontogeny of OATs and OCTs could carry implications both for the development of more rational therapeutics for premature infants, as well as for our understanding of proximal tubule differentiation.

Multiple fibroblast growth factors support growth of the ureteric bud but have different effects on branching morphogenesis

Together with glial-derived neurotrophic factor (GDNF), soluble factors present in a metanephric mesenchyme (MM) cell conditioned medium (BSN-CM) are necessary to induce branching morphogenesis of the isolated ureteric bud (UB) in vitro (Proc. Natl. Acad. Sci. USA 96 (1999) 7330). Several lines of evidence are presented here in support of a modulating role for fibroblast growth factors (FGFs) in this process. RT-PCR revealed the expression of two FGF receptors, FGFR1(IIIc) and FGFR2(IIIb), in isolated embryonic day 13 rat UBs, which by indirect immunofluorescence displayed a uniform distribution. Rat kidney organ culture experiments in the presence of a soluble FGFR2(IIIb) chimera or a neutralizing antibody to FGF7 suggested an important contribution of FGFs other than FGF7 to the branching program. Several FGFs, including FGF1, FGF2, FGF7 and FGF10, in combination with GDNF and BSN-CM were found to affect growth and branching of the isolated UB, albeit with very different effects. FGF1 and FGF7 were at extreme ends of the spectrum, with FGF10 (more FGF1-like) and FGF2 (more FGF7-like) falling in between. FGF1 induced the formation of elongated UB branching stalks with distinct proliferative ampullary tips, whereas FGF7 induced amorphous buds displaying nonselective proliferation with little distinction between stalks and ampullae. Electron microscopic examination demonstrated that FGF1 treatment induced cytoskeletal organization, intercellular junctions and lumens along the stalk portion of the developing tubules, while the ampullary regions contained 'less differentiated' cells with an abundant secretory apparatus. In contrast, FGF7-induced UBs displayed this 'less differentiated' morphology regardless of position on the structure and were virtually indistinguishable from FGF1-induced ampullae. Consistent with this, GeneChip array analysis (employing a novel nanogram-scale assay consisting of two rounds of amplification and in vitro transcription for analyzing small quantities of RNA) revealed that FGF7-induced UBs expressed more markers of cell proliferation than FGF1, which caused the UB to express cytoskeletal proteins, extracellular matrix proteins, and at least one integrin, some of which may be important in UB branch elongation. Thus, while the various FGFs examined all support UB growth, FGF1 and FGF10 appear to be more important for branching and branch elongation, and may thus play a role in determination of nephron number and patterning in the developing kidney. These in vitro data may help to explain results from knockout and transgenic studies and suggest how different FGFs may, together with GDNF and other factor(s) secreted by MM cells, regulate branching morphogenesis of the UB by their relative effects on its growth, branching and branch elongation and differentiation, thereby affecting patterning in the developing kidney.

Endostatin regulates branching morphogenesis of renal epithelial cells and ureteric bud

Endostatin (ES) inhibits endothelial cell migration and has been found to bind to glypicans (Gpcs) on both endothelial cells and renal epithelial cells. We examined the possibility that ES might regulate epithelial cell morphogenesis. The addition of ES to cultured epithelial cells causes an inhibition of both hepatocyte growth factor- and epidermal growth factor-dependent process formation and migration. In contrast, ES does not inhibit epidermal growth factor-dependent morphogenesis in renal epithelial cells derived from Gpc-3 -/mice, whereas expression of Gpc-1 in these cells reconstitutes ES responsiveness. Gpc-3 -/mice have been shown to display enhanced ureteric bud (UB) branching early in development, and cultured UB cells release ES into the media, suggesting that ES binding to Gpcs may regulate UB branching. The addition of ES inhibits branching of the explanted UB, whereas a neutralizing Ab to ES enhances UB outgrowth and branching. Thus, local expression of ES at the tips of the UB may play a role in the regulation of UB arborization.

Involvement of laminin binding integrins and laminin-5 in branching morphogenesis of the ureteric bud during kidney development

Branching morphogenesis of the ureteric bud (UB) [induced by the metanephric mesenchyme (MM)] is necessary for normal kidney development. The role of integrins in this complex developmental process is not well understood. However, the recent advent of in vitro model systems to study branching of UB cells and isolated UB tissue makes possible a more detailed analysis of the integrins involved. We detected integrin subunits alpha3, alpha6, beta1, and beta4 in both the UB and cells derived from the early UB. Blocking the function of each of these integrin subunits individually markedly inhibited branching morphogenesis in cell culture models. However, inhibiting individual integrin function with blocking antibodies in whole kidney and isolated UB culture only partially inhibited UB branching morphogenesis, suggesting that, in these more complex in vitro systems, multiple integrins are involved in the branching program. In whole organ and isolated bud culture, marked retardation of UB branching was observed only when both alpha3 and alpha6 integrin subunits were inhibited. The alpha6 integrin subunit can be expressed as both alpha6beta1 and alpha6beta4, and both of these beta subunits are important for UB branching morphogenesis in both cell and organ culture. Furthermore, laminin-5, a common ligand for integrins alpha3beta1 and alpha6beta4, was detected in the developing UB and shown to be required for normal UB branching morphogenesis in whole embryonic kidney organ culture as well as isolated UB culture. Together, these data from UB cell culture, organ culture, and isolated UB culture models indicate that both integrin alpha3 and alpha6 subunits play a direct role in UB branching morphogenesis, as opposed to being modulators of the inductive effects of mesenchyme on UB development. Furthermore the data are consistent with a role for laminin-5, acting through its alpha3beta1 and/or alpha6beta4 integrin receptors, in UB branching during nephrogenesis. These data may help to partially explain the renal phenotype seen in integrin alpha3 and alpha3/alpha6 subunit-deficient animals.

Identification of pleiotrophin as a mesenchymal factor involved in ureteric bud branching morphogenesis

Branching morphogenesis is central to epithelial organogenesis. In the developing kidney, the epithelial ureteric bud invades the metanephric mesenchyme, which directs the ureteric bud to undergo repeated branching. A soluble factor(s) in the conditioned medium of a metanephric mesenchyme cell line is essential for multiple branching morphogenesis of the isolated ureteric bud. The identity of this factor had proved elusive, but it appeared distinct from factors such as HGF and EGF receptor ligands that have been previously implicated in branching morphogenesis of mature epithelial cell lines. Using sequential column chromatography, we have now purified to apparent homogeneity an 18 kDa protein, pleiotrophin, from the conditioned medium of a metanephric mesenchyme cell line that induces isolated ureteric bud branching morphogenesis in the presence of glial cell-derived neurotrophic factor. Pleiotrophin alone was also found to induce the formation of branching tubules in an immortalized ureteric bud cell line cultured three-dimensionally in an extracellular matrix gel. Consistent with an important role in ureteric bud morphogenesis during kidney development, pleiotrophin was found to localize to the basement membrane of the developing ureteric bud in the embryonic kidney. We suggest that pleiotrophin could act as a key mesenchymally derived factor regulating branching morphogenesis of the ureteric bud and perhaps other embryonic epithelial structures.

The organic anion transporter family: from physiology to ontogeny and the clinic

The organic anion transporter (OAT) family handles a wide variety of clinically important compounds (antibiotics, nonsteriodal anti-inflammatory drugs, etc.) and toxins. However, little is known about their appearance during development despite documented differences in the handling of anionic drugs among neonates, children, and adults. A similar spatiotemporal pattern of mRNA expression of the OATs (OAT1-4) during kidney development suggests that OAT genes may be useful in understanding the mechanisms of proximal tubule maturation. Moreover, OAT expression in unexpected extrarenal sites (e.g., spinal cord, bone, skin) has also been detected during development, possibly indicating a role for these transporters in the formation or preservation of extrarenal tissues. The cloning of these transporters also paves the way for computer-based modeling of drug-transporter interactions at the molecular level, potentially aiding in the design and assessment of new drugs. Additionally, increased understanding of single nucleotide polymorphisms in OATs and other transporters may eventually allow the use of a patient's expression profile and polymorphisms to individualize drug therapy.

Changes in global gene expression patterns during development and maturation of the rat kidney

We set out to define patterns of gene expression during kidney organogenesis by using high-density DNA array technology. Expression analysis of 8,740 rat genes revealed five discrete patterns or groups of gene expression during nephrogenesis. Group 1 consisted of genes with very high expression in the early embryonic kidney, many with roles in protein translation and DNA replication. Group 2 consisted of genes that peaked in midembryogenesis and contained many transcripts specifying proteins of the extracellular matrix. Many additional transcripts allied with groups 1 and 2 had known or proposed roles in kidney development and included LIM1, POD1, GFRA1, WT1, BCL2, Homeobox protein A11, timeless, pleiotrophin, HGF, HNF3, BMP4, TGF-alpha, TGF-beta2, IGF-II, met, FGF7, BMP4, and ganglioside-GD3. Group 3 consisted of transcripts that peaked in the neonatal period and contained a number of retrotransposon RNAs. Group 4 contained genes that steadily increased in relative expression levels throughout development, including many genes involved in energy metabolism and transport. Group 5 consisted of genes with relatively low levels of expression throughout embryogenesis but with markedly higher levels in the adult kidney; this group included a heterogeneous mix of transporters, detoxification enzymes, and oxidative stress genes. The data suggest that the embryonic kidney is committed to cellular proliferation and morphogenesis early on, followed sequentially by extracellular matrix deposition and acquisition of markers of terminal differentiation. The neonatal burst of retrotransposon mRNA was unexpected and may play a role in a stress response associated with birth. Custom analytical tools were developed including "The Equalizer" and "eBlot," which contain improved methods for data normalization, significance testing, and data mining.

Matrix metalloproteinases and their inhibitors regulate in vitro ureteric bud branching morphogenesis

Mammalian kidney development is initiated by the mutual interaction between embryonic metanephric mesenchyme (MM) and the ureteric bud (UB), leading to tightly controlled UB branching morphogenesis. In a three-dimensional cell culture model, which employs MM cell-derived conditioned medium (BSN-CM) to induce UB cell branching morphogenesis in extracellular matrix (ECM) gels (Sakurai H, Barros EJ, Tsukamoto T, Barasch J, and Nigam SK. Proc Natl Acad Sci USA 94: 6279-6284, 1997), branching morphogenesis was inhibited by both chemical agents (ilomastat and 1,10-orthophenanthroline) and a physiological protein factor [tissue inhibitor of metalloproteinases (TIMP)-2], known to act as matrix metalloproteinase (MMP) inhibitors. In addition, UB branching was inhibited in isolated UB culture (Qiao J, Sakurai H, and Nigam SK. Proc Natl Acad Sci USA 96: 7330-7335, 1999) by TIMP-2 and ilomastat, suggesting a direct role for MMPs in UB branching. Gelatin zymography and enzymatic measurement of MMP activity revealed that MMPs could originate from at least three different sources: the conditioned medium, the ECM, and the UB cells themselves. In the UB cells, transcription of several MMPs [gelatinase A (MMP2) and B (MMP9), stromelysin (MMP3), MT1-MMP] and TIMPs was altered by BSN-CM and changed as more complex branching structures formed. The ECM appeared to serve as both a reservoir for MMPs and modulated their expression because different ECM compositions altered the total MMP activity as well as specific subsets of MMPs expressed by the UB cells (as determined by zymography and Northern analysis). In the context of UB branching morphogenesis during kidney development, our data suggest a complex model in which soluble factors produced by the MM, in the context of specific ECM components, modulate the expression of specific subsets of MMPs and TIMPs in the UB, which alter as structures develop and the matrix environment changes. This suggests distinct roles for different subsets of MMPs and their inhibitors during different phases of branching morphogenesis.

A role for Timeless in epithelial morphogenesis during kidney development

Central to the process of epithelial organogenesis is branching morphogenesis into tubules and ducts. In the kidney, this can be modeled by a very simple system consisting of isolated ureteric bud (UB) cells, which undergo branching morphogenesis in response to soluble factors present in the conditioned medium of a metanephric mesenchyme cell line. By employing a targeted screen to identify transcription factors involved early in the morphogenetic program leading to UB branching, we identified the mammalian ortholog of Timeless (mTim) as a potential immediate early gene (IEG) important in this process. In the embryo, mTim was found to be expressed in patterns very suggestive of a role in epithelial organogenesis with high levels of expression in the developing lung, liver, and kidney, as well as neuroepithelium. In the embryonic kidney, the expression of mTim was maximal in regions of active UB branching, and a shift from the large isoform of mTim to a smaller isoform occurred as the kidney developed. Selective down-regulation of mTim resulted in profound inhibition of embryonic kidney growth and UB morphogenesis in organ culture. A direct effect on the branching UB was supported by the observation that down-regulation of mTim in the isolated UB (cultured in the absence of mesenchyme) resulted in marked inhibition of morphogenesis, suggesting a key role for Tim in the epithelial cell morphogenetic pathway leading to the formation of branching tubules.

Selective degradation of E-cadherin and dissolution of E-cadherin-catenin complexes in epithelial ischemia

Ischemic epithelial cells are characterized by disruption of intercellular junctions and loss of apical-basolateral protein polarity, which are normally dependent on the integrity of the adherens junction (AJ). Biochemical analysis of both whole ischemic kidneys and ATP-depleted Madin-Darby canine kidney (MDCK) cells demonstrated a striking loss of E-cadherin (the transmembrane protein of the AJ) with the appearance and accumulation of an approximately 80-kDa fragment reactive with anti-E-cadherin antibodies on Western blots of ATP-depleted MDCK cells. This apparent ischemia-induced degradation of E-cadherin was not blocked by either inhibitors of the major proteolytic pathways (i.e., proteasome, lysosome, or calpain), or by chelation of intracellular calcium, suggesting the involvement of a protease capable of functioning at low ATP and low calcium levels. Immunocytochemistry revealed the movement of several proteins normally comprising the AJ, including E-cadherin and beta-catenin, away from lateral portions of the plasma membrane to intracellular sites. Moreover, rate-zonal centrifugation and immunoprecipitation with anti-E-cadherin and anti-beta-catenin antibodies indicated that ATP depletion disrupted normal E-cadherin-catenin interactions, resulting in the dissociation of alpha- and gamma-catenin from E-cadherin and beta-catenin-containing complexes. Because the generation and maintenance of polarized epithelial cells are dependent upon E-cadherin-mediated cell-cell adhesion and normal AJ function, we propose that the rapid degradation of E-cadherin and dissolution of the AJ is a key step in the development of the ischemic epithelial cell phenotype. Furthermore, we hypothesize that the reassembly of the AJ after ischemia/ATP depletion may require a novel bioassembly mechanism involving recombination of newly synthesized and sorted E-cadherin with preexisting pools of catenins that have (temporally) redistributed intracellularly.

Pretreatment with inducers of ER molecular chaperones protects epithelial cells subjected to ATP depletion

We have investigated the potential cytoprotective role of endoplasmic reticulum (ER) molecular chaperones in a cultured cell model of renal ischemia. Madin-Darby canine kidney (MDCK) cells were pretreated with tunicamycin (an inducer of ER but not cytosolic molecular chaperones) for 12-16 h, followed by 6 h of ATP depletion. A rapid and severe depletion of cellular ATP was noted in both control and tunicamycin-treated cells. Trypan blue exclusion assays indicated that pretreatment of MDCK cells with tunicamycin reduced ATP depletion-induced cell damage by approximately 80% compared with nonpretreated controls. This apparent cytoprotective effect was also found following pretreatment with another inducer of ER molecular chaperones (i.e., A23187). For example, A23187 was found to reduce lactate dehydrogenase release by approximately 50% compared with untreated controls, whereas E-64, a cysteine protease inhibitor which may affect degradation of some proteins in the ER, had little or no effect on cell injury. Moreover, a fluorescent assay confirmed the marked reduction in cell damage following ATP depletion (up to 80% reduction in tunicamycin-pretreated cells). Together, these findings are consistent with the notion that induction of ER molecular chaperones leads to the acquisition of cytoprotection in the face of ATP depletion. However, inhibition of protein translation by cycloheximide was found to only partially attenuate the observed cytoprotective effect, raising the possibility that other, as yet to be identified, nonprotein synthesis-dependent mechanisms may also play a role in the observed cytoprotection.

Proteasome inhibition leads to a heat-shock response, induction of endoplasmic reticulum chaperones, and thermotolerance

The accumulation of misfolded proteins in the cytosol leads to increased expression of heat-shock proteins, while accumulation of such proteins in the endoplasmic reticulum (ER) stimulates the expression of many ER resident proteins, most of which function as molecular chaperones. Recently, inhibitors of the proteasome have been identified that can block the rapid degradation of abnormal cytosolic and ER-associated proteins. We therefore tested whether these agents, by causing the accumulation of abnormal proteins, might stimulate the expression of cytosolic heat-shock proteins and/or ER molecular chaperones and thereby induce thermotolerance. Exposure of Madin-Darby canine kidney cells to various proteasome inhibitors, including the peptide aldehydes (MG132, MG115, N-acetyl-leucyl-leucyl-norleucinal) and lactacystin, inhibited the degradation of short-lived proteins and increased markedly the levels of mRNAs encoding cytosolic heat-shock proteins (Hsp70, polyubiquitin) and ER chaperones (BiP, Grp94, ERp72), as shown by Northern blot analysis. However, inhibitors of cysteine proteases (E64), serine proteases (leupeptin), or metalloproteases (1, 10-phenanthroline) had no effect on the levels of these mRNAs. The relative efficacies of the peptide aldehyde inhibitors in inducing these mRNAs correlated with their potencies against the proteasome. Furthermore, reduction of the aldehyde group of MG132 decreased its inhibitory effect on proteolysis and largely prevented the induction of these mRNAs. Although treatment with the proteasome inhibitors caused rapid increases in mRNA levels (as early as 2 h after treatment with MG132), the inhibitors did not detectably affect total protein synthesis, total protein secretion, ER morphology, or the retention of ER-lumenal proteins, even after 18 h of treatment. Together, the findings suggest that inhibition of proteasome function induces heat-shock proteins and ER chaperones due to the accumulation of sufficient amounts of abnormal proteins and/or the inhibition of degradation of a key regulatory factor (e.g. heat-shock factor). Since expression of heat-shock proteins can protect cells from thermal injury, we tested whether the proteasome inhibitors might also confer thermotolerance. Treatment of cells with MG132 for as little as 2 h, markedly increased the survival of cells subjected to high temperatures (up to 46 degrees C). Thus, these agents may have applications in protecting against cell injury.

Molecular cloning and characterization of NKT, a gene product related to the organic cation transporter family that is almost exclusively expressed in the kidney

We have identified a gene product (NKT) encoding an apparently novel transcript that appears to be related to the organic ion transporter family and is expressed almost exclusively in the kidney. Analysis of the deduced 546-amino acid protein sequence indicates that NKT is a unique gene product which shares a similar transmembrane domain hydropathy profile as well as transporter-specific amino acid motifs with a variety of bacterial and mammalian nutrient transporters. Nevertheless, the overall homology of NKT to two recently cloned organic ion transport proteins (NLT and OCT-1) is significantly greater; together these three gene products may represent a new subgroup of transporters. The NKT was characterized further with respect to its tissue distribution and its expression during kidney development. A 2.5-kilobase transcript was found in kidney and at much lower levels in brain, but not in a number of other tissues. Studies on the embryonic kidney indicate that the NKT transcript is developmentally regulated with significant expression beginning at mouse gestational day 18 and rising just before birth, consistent with a role in differentiated kidney function. Moreover, in situ hybridization detected specific signals in mouse renal proximal tubules. NKT was mapped by linkage disequilibrium to mouse chromosome 19, the same site to which several mouse mutations localize, including that for osteochondrodystrophy (ocd). Although initial experiments in a Xenopus oocyte expression system failed to demonstrate transport of known substrates for OCT-1, the homology to OCT-1 and other transporters, along with the proximal tubule localization, raise the possibility that this gene may play a role in organic solute transport or drug elimination by the kidney.

Perturbations in maturation of secretory proteins and their association with endoplasmic reticulum chaperones in a cell culture model for epithelial ischemia

The effects of ischemia on the maturation of secretory proteins are not well understood. Among several events that occur during ischemia-reperfusion are a rapid and extensive decrease in ATP levels and an alteration of cellular oxidative state. Since the normal folding and assembly of secretory proteins are mediated by endoplasmic reticulum (ER) molecular chaperones, the function of which depends on ATP and maintenance of an appropriate redox environment, ischemia might be expected to perturb folding of secretory proteins. In this study, whole animal and cultured cell models for the epithelial ischemic state were used to examine this possibility. After acute kidney ischemia, marked increases in the mRNA levels of the ER chaperones glucose-regulated protein (grp)78/immunoglobulin-binding protein (BiP), grp94, and ER protein (ERp)72 were noted. Likewise, when cellular ATP was depleted to less than 10% of control with antimycin A, mRNA levels of BiP, ERp72, and grp94 were increased in kidney and thyroid epithelial cell culture models. Since the signal for the up-regulation of these stress proteins is believed to be the accumulation of misfolded/misassembled secretory proteins in the ER, their induction after ischemia in vivo and antimycin treatment of cultured cells suggests that maturation of secretory proteins in the ER lumen might indeed be perturbed. To analyze the effects of antimycin A on the maturation of secretory proteins, we studied the fate of thyroglobulin (Tg), a large oligomeric secretory glycoprotein, the folding and assembly of which seems to require a variety of ER chaperones. Treatment of cultured thyroid epithelial cells with antimycin A greatly inhibited ( > 90%) the secretion of Tg. Sucrose density gradient analysis revealed that in antimycin A-treated cells Tg associates into large macromolecular complexes which, by immunofluorescence, appeared to localize to the ER. Furthermore, coimmunoprecipitation studies after antimycin A treatment demonstrated that Tg stably associates with BiP, grp94, and ERp72. Together, our results suggest that a key cellular lesion in ischemia is the misfolding of secretory proteins as they transit the ER, and this leads not only to increased expression of ER chaperones but also to their stable association with and the subsequent retention of at least some misfolded secretory proteins.

Dependence of epithelial intercellular junction biogenesis on thapsigargin-sensitive intracellular calcium stores

Perturbation of potentially regulatable endoplasmic reticulum (ER) calcium stores with the Ca-ATPase inhibitor, thapsigargin (TG), perturbs the formation of desmosomes and tight junctions during polarized epithelial cell biogenesis, despite the development of cell contact. In a Madin-Darby canine kidney cell model for intercellular junction assembly, TG treatment inhibited the development of transepithelial electrical resistance (TER), a measure of tight junction assembly, in a dose-dependent manner. The TG-induced inhibition of tight junction assembly was paralleled by a defect in the sorting of the tight junction protein, ZO-1. An even more dramatic delay in sorting of the desmosomal protein, desmoplakin, was observed in the presence of TG. In addition, while both ZO-1 and desmoplakin-I in control cells were shown to become associated with the Triton X-100 insoluble cytoskeleton during intercellular junction assembly, prior treatment with 100 nM TG diminished this biochemical stabilization into the detergent-insoluble fraction, particularly in the case of ZO-1. Although spectrofluorimetric measurements in fura-2 loaded Madin-Darby canine kidney cells confirmed the occurrence of TG-mediated release of calcium from internal stores, total cytosolic calcium during junction assembly remained similar to untreated cells. Therefore, the presence of cytosolic calcium alone is not sufficient for normal intercellular junction biogenesis if intracellular stores are perturbed by TG. The results indicate the presence of calcium-sensitive intracellular mechanisms involved in the sorting and cytoskeletal stabilization of both tight junction and desmosomes and suggest a role for calcium-dependent signaling pathways at an early (possibly common) step in polarized epithelial biogenesis.

Induction of the FK506-binding protein, FKBP13, under conditions which misfold proteins in the endoplasmic reticulum

In order to determine whether the endoplasmic reticulum (ER) luminal FK506-binding protein, FKBP13, shares properties of ER molecular chaperones, MDCK cells were treated with either tunicamycin or Ca2+ ionophores. By Northern-blot analysis, tunicamycin resulted in a 2-fold rise in FKBP13 mRNA, whereas ionophores (A23187 and ionomycin) caused a more impressive rise in FKBP13 mRNA (up to 5-fold with ionomycin). Actinomycin D chase experiments in ionomycin-treated cells revealed no change in the half-life of FKBP13 mRNA, indicating that the increase in FKBP13 mRNA observed was not due to greater message stability. Moreover, sequencing of the 5' flanking region of the gene for murine FKBP13 revealed significant similarity to similar regions in human BiP (immunoglobulin-binding protein) and the human glucose-regulated protein grp94, including a 37 bp sequence in FKBP13 with approximately 50% identity with the unfolded protein response element of the BiP gene. Together, these data suggest a role for FKBP13 in ER protein folding.

Epithelial inositol 1,4,5-trisphosphate receptors. Multiplicity of localization, solubility, and isoforms

In an earlier subcellular fractionation study of epithelial tissue (liver and pancreas), we demonstrated that the inositol 1,4,5-trisphosphate receptor (IP3R) is found in association with biochemically distinct cellular membranes, including the endoplasmic reticulum (ER) and plasma membrane (Sharp, A. H., Snyder, S. H., and Nigam, S. K. (1992) J. Biol. Chem. 267, 7444-7449). To further characterize epithelial IP3Rs, we have now employed cultured Madin-Darby canine kidney (MDCK) cells, a well studied tight polarized epithelial cell type. Indirect immunofluorescence with an antiserum which specifically recognizes IP3R in MDCK cells by immunoblotting and immunoprecipitation gave an ER-like staining pattern as well as a basolateral plasma membrane-like staining pattern, the latter being particularly evident in highly confluent monolayers. In sections of adult rat kidney tubules a similar staining pattern was observed. Interestingly, whereas known basolateral proteins (Na+,K(+)-ATPase and the facilitated glucose transporter) gave a continuous basolateral staining pattern, that seen for IP3R was discontinuous (punctate). A highly similar staining pattern was observed for the caveolar protein, caveolin, suggesting that the punctate basolateral plasma membrane-like staining pattern observed for IP3R reflects its localization to basolateral caveolae. Biotinylation of non-permeabilized and permeabilized MDCK cells, followed by immunoprecipitation of IP3R and detection with streptavidin, indicated that while most IP3R is localized to biotin-inaccessible compartments (i.e. ER), a fraction (10-20%) of IP3R was accessible to externally added biotin primarily from the basolateral side. This result is compatible with the dual ER and basolateral caveolar localization suggested by immunocytochemistry, although it does not exclude the presence of some IP3R in the basolateral plasma membrane as well. Solubility studies revealed IP3R to be considerably more insoluble than the basolateral proteins, Na+,K(+)-ATPase and the liver cell adhesion molecule, as well as the cytoskeletal proteins, ankyrin and fodrin. In the most insoluble fraction, IP3R was found along with caveolin, further supporting the notion that part of the cellular IP3R pool associates with caveolae. Since multiple localizations of IP3R within a cell might reflect the existence of multiple isoforms, polymerase chain reaction amplification of first strand cDNA with primers specific for the three isotypes of IP3R was performed. All three isoforms of IP3R were expressed in the homogeneous population of MDCK cells. The existence of distinct membrane localizations and multiple isoforms of IP3R within the same cell type suggests an explanation for the complex spatiotemporal patterns of Ca2+ release from inositol 1,4,5-trisphosphate-sensitive Ca2+ pools in epithelial and other cells.

A set of endoplasmic reticulum proteins possessing properties of molecular chaperones includes Ca(2+)-binding proteins and members of the thioredoxin superfamily

The major proteins in the lumen of the endoplasmic reticulum (ER) are thought to function in Ca2+ sequestration or as "molecular chaperones" in the folding and assembly of membrane or secreted proteins. Based on the ability of many chaperones to bind selectively to unfolded proteins and to dissociate from them upon ATP hydrolysis, we developed an affinity chromatography method to isolate proteins with these characteristics from pancreatic or liver ER. Seven ER proteins bound selectively to denatured protein columns and were specifically eluted by ATP (10(-6) M) but not by a nonhydrolyzable ATP analog. These proteins were identified with antibodies and microsequencing as the ER chaperone BiP (grp78), grp94, calreticulin, a novel 46-kDa protein that binds azido-ATP, as well as three members of the thioredoxin superfamily: protein-disulfide isomerase, ERp72, and a previously reported 50-kDa protein (p50). This set of seven proteins bound to and was eluted with ATP from a variety of denatured proteins, including histone, gelatin, alpha fetoprotein, thyroglobulin, lysozyme, casein, and IgG. The release of grp94, protein-disulfide isomerase, ERp72, calreticulin, and p50 was stimulated by Ca2+ in the presence of ATP. These proteins thus appear to function as Ca(2+)-dependent chaperones, which may account for the Ca2+ and ATP requirement for protein folding in the ER.

Localization of the FK506-binding protein, FKBP 13, to the lumen of the endoplasmic reticulum

The function of the immunophilins, FKBP 12 and FKBP 13, which are binding proteins for the immunosuppressant drug FK506 and rapamycin, remains poorly defined, although it has been suggested that immunophilins and immunophilin-like proteins may play a role in protein sorting/folding and intracellular calcium ion regulation. As a first step towards understanding the function of FKBP 13, we studied its subcellular localization by immunoblotting of well-defined subcellular fractions from a canine pancreatic homogenate and immunocytochemical analysis of an overexpressed cloned cDNA for FKBP 13. Whereas FKBP 12 fractionated entirely into the cytosol, virtually all FKBP 13 was found in the rough microsomal fraction which consisted of highly purified rough endoplasmic reticulum (ER), along with several well-characterized ER markers [the immunoglobulin heavy-chain binding protein (BiP), grp 94 and ribophorin I]. Moreover, FKBP 13 co-banded with the ER markers on isopycnic sucrose gradients. By immunofluorescence, the overexpressed cDNA for FKBP 13 in Hela cells gave an ER-staining pattern highly similar to that of known ER proteins. Addition of the ligand FK506 did not appear to alter the distribution of FKBP 13. Separation of the ER luminal contents and membrane revealed FKBP 13 to be a luminal ER protein. Since the lumen of the ER is where the folding of membrane and secreted proteins occurs, as well as a major site of intracellular calcium storage, it seems possible that FKBP 13 may be involved in one of these functions.

Effect of total parenteral nutrition, systemic sepsis, and glutamine on gut mucosa in rats

The effect of the combination of total parenteral nutrition (TPN) and systemic sepsis on mucosal morphology and protein synthesis was investigated. Rats were given a standard TPN mixture consisting of glucose (216 kcal.kg-1.day-1), lipid (24 kcal.kg-1.day-1), and amino acids (1.5 g N.kg-1.day-1) for 5 days. On the 5th day the rats (n = 37) were randomized into four groups according to diet as follows: 1) control nonseptic on standard TPN, 2) control nonseptic on TPN with glutamine, 3) septic on standard TPN, and 4) septic with the TPN supplemented with glutamine. Twenty hours after the injection of Escherichia coli, the rats were given a 4-h constant infusion of [U-14C]leucine to determine the mucosal fractional protein synthesis rates. The following results were obtained. 1) Histological examination showed that systemic sepsis caused tissue damage to the ileum and jejunum. 2) Glutamine supplementation attenuated these changes. 3) There were no visible changes to the colon either from glutamine supplementation or sepsis. 4) Sepsis was associated with an increase in mucosal protein synthesis and decreased muscle synthesis. 5) Addition of glutamine to the TPN mix further increased protein synthesis in the intestinal mucosa of septic rats.

Lipid droplets of neuroepithelial cells are a major calcium storage site during neural tube formation in chick and mouse embryos

In situ precipitation of calcium (Ca2+) with fluoride and antimonate shows that Ca(2+)-specific precipitate is localized almost exclusively within lipid droplets of neuroepithelial cells during neural tube formation in chick and mouse embryos. The density of Ca2+ precipitate within lipid droplets is generally greater in the apical ends of cells situated in regions of the neuroepithelium that are actively engaged in bending. These findings suggest that lipid droplets, in addition to providing a source of metabolic fuel for developing neuroepithelial cells, also serve as Ca(2+)-storage and -releasing sites during neurulation.

A morphometric and computer-assisted three-dimensional reconstruction study of neural tube formation in chick embryos

The origin of the driving forces for neural tube formation remains uncertain but is currently thought to involve the participation of microfilament bundles situated in the apical ends of neuroepithelial cells. In the work presented here, we show how morphometric measurements that map local variations in the apical geometry of neuroepithelial cells (especially apical constriction) can provide information on the distribution of motive forces within the neuroepithelium during neural tube formation. When used in combination with computer-assisted, three-dimensional reconstruction, it becomes possible to analyze the morphometric data from a dynamic, three-dimensional perspective. As an example application of this method, we have used morphometry to evaluate the effects of ionomycin on the developing neuroepithelium. Treatment of early (stages 6-8) chick embryos with 5 microM ionomycin was found to cause rapid bending of the neuroepithelium within 1 min of exposure and a dramatic acceleration of the normal sequence of neural tube formation. Electron microscopy and morphometry revealed that this acceleration was coincident with a marked increase in the local degree of apical constriction of neuroepithelial cells, presumably a consequence of enhanced contractile activity of apical microfilament bundles. This work shows that transient elevation of free calcium levels can accelerate the usual sequential phases of NT formation. The rapidity of the response (hours of normal development reduced to minutes), increased prominence of apical microfilament bundles, and the enhanced degree of apical constriction strongly support a direct causal role for apical microfilament bundles and apical constriction of neuroepithelial cells in bending of the neuroepithelium.

Neural tube formation in the mouse: a morphometric and computerized three-dimensional reconstruction study of the relationship between apical constriction of neuroepithelial cells and the shape of the neuroepithelium

Morphometry and computerized three-dimensional reconstruction were used to study the relationship between apical constriction of neuroepithelial cells and the pattern of bending of the neuroepithelium in the developing neural tube of the 12-somite mouse embryo. The neuroepithelium of the mouse exhibits prominent regional variations in size and shape along the embryo axis. The complex shape of most of the cephalic neural tube (e.g., forebrain and midbrain) is due to the coexistence of concave and convex bending sites whereas more caudal regions (e.g., hindbrain and spinal cord) generally lack sites of convex bending and have a relatively simple shape. The apical morphology of neuroepithelial cells was found to be correlated more closely with the local status of bending of the neuroepithelium than with the specific region of the neural tube in which they are located. In areas of enhanced apical constriction, microfilament bundles were particularly prominent. Morphometry revealed that patterns of bending of the neuroepithelium were correlated almost exactly with those of apical constriction throughout the forming neural tube. These findings support the idea that apical constriction of neuroepithelial cells, resulting from tension generated by microfilament bundles, plays a major role in bending of the neuroepithelium during neural tube formation in the mouse.

Intrinsic and extrinsic factors collaborate to generate driving forces for neural tube formation in the chick: a study using morphometry and computerized three-dimensional reconstruction

The formation of the neural tube, the rudiment of the entire central nervous system, is one of the earliest morphogenetic movements. The origin of the driving forces for this process remains uncertain, but recent studies suggest the involvement of both intrinsic and extrinsic factors. In the present study, we have used morphometry, analysis of stereopair photographs of whole embryos, and computerized three-dimensional reconstruction to investigate the factors which constitute the bulk of the driving forces for neural tube formation in the developing midbrain of Hamburger and Hamilton stages 5-9 chick embryos. Results support the notion that neural tube formation is driven by a coordinated interplay of intrinsic and extrinsic forces. Initial bending of the neural plate along the midline of the embryo and uplifting of the neural folds is accomplished primarily through the combined action of intrinsic forces (resulting from apical constriction of neuroepithelial cells) and extrinsic forces (mostly a passive consequence of head-fold formation). However, once in the uplifted position, curling over of neural folds and closure of the neural tube is driven largely by apical constriction-mediated (intrinsic) forces that are generated by cells in the midlateral walls of the forming neural tube.

Biomechanical basis of diazepam-induced neural tube defects in early chick embryos: a morphometric study

The biomechanical basis of diazepam (Valium/Roche)-induced neural tube defects in the chick was investigated using a combination of electron microscopy and morphometry. Embryos at stage 8 (four-somite stage) of development were explanted and grown for 6 hr in nutrient medium containing 400 micrograms/ml diazepam. Nearly 80% of these embryos exhibited neural tube defects that were most pronounced in the forming midbrain region and typified by a "relaxation" or "collapse" of neural folds. The hindbrain and spinal cord regions were less affected. Electron microscopy revealed that neuroepithelial cells in diazepam-treated embryos had smoother apical surfaces and broader apical widths than did controls. Morphometric measurements supported this observation and further showed that these effects were focused at sites within the wall of the forming neural tube that typically exhibit the greatest degree of bending and apical constriction (i.e., the floor and midlateral walls). Overall results indicate that neural tube defects associated with exposure to diazepam are due largely to a general inhibition of the contractile activity of apical microfilament bundles in neuroepithelial cells. These findings 1) emphasize the important contribution of microfilament-mediated apical constriction of neuroepithelial cells in providing the driving forces for bending of the neuroepithelium during neural tube formation and 2) suggest that agents or conditions that impair their contractile activity could play a role in the pathogenesis of certain types of neural tube defects.

Immunoelectron microscopic localization of actin in neurites of cultured embryonic chick dorsal root ganglia: actin is a component of granular, microtubule-associated crossbridges

Axons have been shown to contain substantial quantities of actin distributed along their length. However, the general lack of information on the structure and organizational state of this protein in axons has made it difficult to assign it a functional role. In the present study, we used electron microscopic immunocytochemistry (immunogold labeling) on neurites from cultured embryonic chick dorsal root ganglia to: (1) determine the distribution of actin in neurites: (2) identify actin-containing structures; and (3) reveal whether or not actin is associated preferentially with microtubules. Results show that actin is present throughout neurites but is organized primarily into short filaments that are localized almost exclusively to granular, microtubule-associated crossbridges. We propose that these short actin filaments are part of the framework of a supramolecular 'carrier complex' for the slow component b polypeptides. In addition, actin-containing crossbridges are often closely associated with the surfaces of membrane-bound organelles. This suggests that actin and microtubule-associated crossbridges are involved somehow in fast axonal transport, although the nature of their participation in this process still needs to be resolved.

A morphometric study of cytoskeletal reorganization in rat sciatic nerve axons following beta,beta'-iminodipropionitrile (IDPN) treatment

Electron microscopy and morphometry were used to study changes in the organization of cytoskeletal elements (microtubules (MTs) and neurofilaments (NFs] in myelinated rat sciatic nerve axons resulting from exposure to beta,beta'-iminodipropionitrile (IDPN). Adult male rats were injected subepineurially with IDPN and sacrificed at intervals. Electron micrographs of thin sections through injected nerve segments revealed that MTs segregated from NFs in axons and formed prominent clusters. Morphometric measurements of intermicrotubular distances within these clusters showed that adjacent MTs had a fairly uniform spatial separation that appeared to be maintained by granular MT-associated cross-bridges. In addition, MT clusters underwent a progressive, but reversible, compaction during the first 12 h of IDPN treatment.

Time-lapse photographic study of neural tube closure defects caused by xylocaine in the chick

Sequential changes in the morphology of early chick embryos were, for the first time, photographically recorded. Embryos were explanted at stage 8 (four-somite) or 9- (six-somite) of development using New's technique and grown in nutrient medium (thin albumen) with or without a teratologic dose (200 micrograms/ml) of xylocaine. They were photographed using a Nikon Diaphot inverted microscope equipped with both phase-contrast optics and photomicrographic accessories maintained in an incubator. It was found, among other things, that a characteristic neural tube closure defect often seen in the midbrain and anterior portion of the hindbrain of xylocaine (200 micrograms/ml)-treated chick embryos was a consequence of failure of the neural tube to withstand the tension generated by the rapidly expanding cephalic region, which occurred, regardless of the stage at explanation, when corresponding control embryos had advanced to stage 10+ (11-somite) of development.