Only multimeric hensin located in the extracellular matrix can induce apical endocytosis and reverse the polarity of intercalated cells

Hypokalemic Distal Renal Tubular Acidosis

Distal renal tubular acidosis (DRTA) is defined as hyperchloremic, non-anion gap metabolic acidosis with impaired urinary acid excretion in the presence of a normal or moderately reduced glomerular filtration rate. Failure in urinary acid excretion results from reduced H⁺ secretion by intercalated cells in the distal nephron. This results in decreased excretion of NH₄⁺ and other acids collectively referred as titratable acids while urine pH is typically above 5.5 in the face of systemic acidosis. The clinical phenotype in patients with DRTA is characterized by stunted growth with bone abnormalities in children as well as nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria, hypocitraturia, and relatively alkaline urine. Hypokalemia is a striking finding that accounts for muscle weakness and requires continued treatment together with alkali-based therapies. This review will focus on the mechanisms responsible for impaired acid excretion and urinary potassium wastage, the clinical features, and diagnostic approaches of hypokalemic DRTA, both inherited and acquired.

Renal acid-base regulation: new insights from animal models

Because majority of biological processes are dependent on pH, maintaining systemic acid-base balance is critical. The kidney contributes to systemic acid-base regulation, by reabsorbing HCO3 (-) (both filtered by glomeruli and generated within a nephron) and acidifying urine. Abnormalities in those processes will eventually lead to a disruption in systemic acid-base balance and provoke metabolic acid-base disorders. Research over the past 30 years advanced our understanding on cellular and molecular mechanisms responsible for those processes. In particular, a variety of transgenic animal models, where target genes are deleted either globally or conditionally, provided significant insights into how specific transporters are contributing to the renal acid-base regulation. Here, we broadly overview the mechanisms of renal ion transport participating to acid-base regulation, with emphasis on data obtained from transgenic mice models.

Cell biology of the intercalated cell in the kidney

The intercalated cell of the collecting tubule of the mammalian kidney is specialized for the transport of H(+) and HCO3. They exist in two forms; one specialized for acid secretion and the other secretes HCO3 into the urine. We discovered many years ago that feeding animals an acid diet converts the HCO3 secreting form to an acid secreting type. Here I discuss the molecular basis of this transformation. The conversion of the cell types is mediated by an extracellular matrix protein hensin (also known as DMBT1). However much remains to be identified in the differentiation of these cells.

Terminal differentiation in epithelia: the role of integrins in hensin polymerization

Epithelia, the most abundant cell type, differentiate to protoepithelia from stem cells by developing apical and basolateral membrane domains and form sheets of cells connected by junctions. Following this differentiation step, the cells undergo a second step (terminal differentiation), during which they acquire a mature phenotype, which unlike the protoepithelial one is tissue and organ specific. An extracellular matrix (ECM) protein termed hensin (DMBT1) mediates this differentiation step in the kidney intercalated cells. Although hensin is secreted as a soluble monomer, it requires polymerization and deposition in the ECM to become active. The polymerization step is mediated by the activation of inside-out signaling by integrins and by the secretion of two proteins: cypA (a cis-trans prolyl isomerase) and galectin 3.

Effects of DMTB1 over-expression on the biological behavior of esophagus carcinoma cell line EC9706

AIM: To establish the cell line EC9706 of esophagus carcinoma with stable expression of a new candidate tumor suppressor gene DMBT1 and to analyze cell invasiveness and cell proliferation compared to the control group so as to provide any insights into the function of the newly-discovered tumor suppressor gene.

METHODS: The full-length DMBT1 expression plasmid pTRexDest30_DMBT18kb.2 was transfected into esophagus carcinoma cell line EC9706 with Lipofectamine 2000 which was subsequently screened with G418. DMBT1 transcript and protein were determined by semi-quantitative RT-PCR and Western blot; then cell growth and proliferation was evaluated with MTT; finally transwell assay was applied to evaluate the ability of its migration before and after the transfection.

RESULTS: After transfection, the levels of DMBT1 protein and mRNA were 3.2 times higher than control group. There was significant difference between transfection group and the control group. The growth curves mapped with MTT indicated that the group transfected with pTRexDest30_DMBT1 showed slower speed of proliferation versus the vector control and the untreated group. In addition, cell survival in serum-free medium was markedly inhibited compared to the vector control. Transwell assay further confirmed conspicuous discrepancy in migration (206 ± 25 vs 367 ± 42, P < 0.01), the experimental group versus the control group.

CONCLUSION: The biological model that stably expresses DMBT1 was successfully established; DMBT1 over-expression significantly inhibits the proliferation and invasive process of esophagus carcinoma cell line in vitro.

Secreted cyclophilin A, a peptidylprolyl cis-trans isomerase, mediates matrix assembly of hensin, a protein implicated in epithelial differentiation

Hensin is a rabbit ortholog of DMBT1, a multifunctional, multidomain protein implicated in the regulation of epithelial differentiation, innate immunity, and tumorigenesis. Hensin in the extracellular matrix (ECM) induced morphological changes characteristic of terminal differentiation in a clonal cell line (clone C) of rabbit kidney intercalated cells. Although hensin is secreted in monomeric and various oligomeric forms, only the polymerized ECM form is able to induce these phenotypic changes. Here we report that hensin secretion and matrix assembly were inhibited by the peptidylprolyl cis-trans isomerase (PPIase) inhibitors cyclosporin A (CsA) and a derivative of cyclosporin A with modifications in the d-Ser side chain (Cs9) but not by the calcineurin pathway inhibitor FK506. PPIase inhibition led to failure of hensin polymerization in the medium and ECM, plus the loss of apical cytoskeleton, apical microvilli, and the columnar epithelial shape of clone C cells. Cyclophilin A was produced and secreted into the media to a much greater extent than cyclophilins B and C. Our results also identified the direct CsA-sensitive interaction of cyclophilin A with hensin, suggesting that cyclophilin A is the PPIase that mediates the polymerization and matrix assembly of hensin. These results are significant because this is the first time a direct role of peptidylprolyl cis-trans isomerase activity has been implicated in the process of epithelial differentiation.

Role of integrins in the assembly and function of hensin in intercalated cells

Epithelial differentiation proceeds in at least two steps: Conversion of a nonepithelial cell into an epithelial sheet followed by terminal differentiation into the mature epithelial phenotype. It was recently discovered that the extracellular matrix (ECM) protein hensin is able to convert a renal intercalated cell line from a flat, squamous shape into a cuboidal or columnar epithelium. Global knockout of hensin in mice results in embryonic lethality at the time that the first columnar cells appear. Here, antibodies that either activate or block integrin beta1 were used to demonstrate that activation of integrin alpha v beta 1 causes deposition of hensin in the ECM. Once hensin polymerizes and deposits into the ECM, it binds to integrin alpha 6 and mediates the conversion of epithelial cells to a cuboidal phenotype capable of apical endocytosis; therefore, multiple integrins play a role in the terminal differentiation of the intercalated cell: alpha v beta 1 generates polymerized hensin, and another set of integrins (containing alpha 6) mediates signals between hensin and the interior of the cells.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

Differentiation of columnar epithelia: the hensin pathway

Epithelia, the most common variety of cells in complex organisms exist in many shapes. They are sheets of polarized cells that separate two compartments and selectively transport materials from one to the other. After acquiring these general characteristics, they differentiate to become specialized types such as squamous columnar or transitional epithelia. High density seeding converts a kidney-derived cell line from flat ;generic' epithelial cells to columnar cells. The cells acquire all the characteristics of differentiated columnar cells, including microvilli, and the capacity for apical endocytosis. The high seeding density induces the deposition of a new protein termed hensin and polymerization of hensin is the crucial event that dictates changes in epithelial phenotype. Hensin is widely expressed in most epithelia. Its deletion in mice leads to embryonic lethality at the time of generation of the first columnar epithelium, the visceral endoderm. Moreover many human cancers have deletions in the hensin gene, which indicates that it is a tumor suppressor.

Galectin-3 expression is induced in renal β-intercalated cells during metabolic acidosis

The adaptation of the cortical collecting duct (CCD) to metabolic acidosis requires the polymerization and deposition in the extracellular matrix of the novel protein hensin. HCO3−-secreting β-intercalated cells remove apical Cl−:HCO3−exchangers and may reverse functional polarity to secrete protons. Using intercalated cells in culture, we found that galectin-3 facilitated hensin polymerization, thereby causing their differentiation into the H+-secreting cell phenotype. We examined the expression of galectin-3 in the rabbit kidney and its relationship to hensin during metabolic acidosis. In control kidneys, galectin-3 was expressed in the cortical and medullary collecting ducts. In the outer cortex 26 ± 3% of CCD cells expressed galectin-3 compared with 64 ± 3% of the cells of the inner cortex. In the CCD, galectin-3 was rarely expressed in β-intercalated cells, being primarily present in α-intercalated and principal cells. During metabolic acidosis, the intensity of cellular staining for galectin-3 increased and more cells began to express it; the percentage of CCD cells expressing galectin-3 increased from 26 ± 3 to 66 ± 3% in the outer cortex and from 64 ± 3 to 78 ± 4% in the inner cortex. This was particularly evident in β-intercalated cells where expression was found in only 8 ± 2% in control animals but in 75 ± 2% during metabolic acidosis in the outer cortex and similarly for the inner cortex (26 ± 6 to 90 ± 7%). Importantly, both galectin-3 and hensin were found in the extracellular matrix of microdissected CCDs; and during metabolic acidosis, many more cells exhibited this extracellular colocalization. Thus galectin-3 may play several important roles in the CCD, including mediating the adaptation of β-intercalated cells during metabolic acidosis.

Zona pellucida domain proteins

Many eukaryotic proteins share a sequence designated as the zona pellucida (ZP) domain. This structural element, present in extracellular proteins from a wide variety of organisms, from nematodes to mammals, consists of approximately 260 amino acids with eight conserved cysteine (Cys) residues and is located close to the C terminus of the polypeptide. ZP domain proteins are often glycosylated, modular structures consisting of multiple types of domains. Predictions can be made about some of the structural features of the ZP domain and ZP domain proteins. The functions of ZP domain proteins vary tremendously, from serving as structural components of egg coats, appendicularian mucous houses, and nematode dauer larvae, to serving as mechanotransducers in flies and receptors in mammals and nonmammals. Generally, ZP domain proteins are present in filaments and/or matrices, which is consistent with the role of the domain in protein polymerization. A general mechanism for assembly of ZP domain proteins has been presented. It is likely that the ZP domain plays a common role despite its presence in proteins of widely diverse functions.

Role of hensin in mediating the adaptation of the cortical collecting duct to metabolic acidosis

PURPOSE OF REVIEW

The cortical collecting duct is able to secrete HCO3-, a state that can be converted to acid secretion during metabolic acidosis. Bicarbonate secretion in this segment is mediated by beta-intercalated cells whereas alpha-intercalated cells perform acid secretion. During metabolic acidosis, the number of beta-intercalated cells is reduced while that of alpha-intercalated cells increases without a change in the total number of intercalated cells, suggesting conversion of one cell type to another. Using an immortalized intercalated cell line we found that this adaptation is mediated by an extracellular protein named hensin. Hensin is secreted as a monomer which is then polymerized in the extracellular environment by a complex process requiring at least three other proteins.

RECENT FINDINGS

We describe that a cyclophilin, via its cis/trans prolyl isomerase activity, is required for this polymerization. This may explain the distal renal tubular acidosis observed with cyclosporin A therapy. In addition, galectin-3 is needed to aggregate the protein. Finally, we recently found that activation of integrins is also necessary for the development of the hensin fiber. Hensin is expressed in all epithelia and deletion of its gene is embryonic lethal at an early stage when the first columnar epithelia develop.

SUMMARY

These studies suggest that the response of intercalated cells to metabolic acidosis uses a pathway that is involved in terminal differentiation of columnar epithelia.

Conversion of ES cells to columnar epithelia by hensin and to squamous epithelia by laminin

Single-layered epithelia are the first differentiated cell types to develop in the embryo, with columnar and squamous types appearing immediately after blastocyst implantation. Here, we show that mouse embryonic stem cells seeded on hensin or laminin, but not fibronectin or collagen type IV, formed hemispheric epithelial structures whose outermost layer terminally differentiated to an epithelium that resembled the visceral endoderm. Hensin induced columnar epithelia, whereas laminin formed squamous epithelia. At the egg cylinder stage, the distal visceral endoderm is columnar, and these cells begin to migrate anteriorly to create the anterior visceral endoderm, which assumes a squamous shape. Hensin expression coincided with the dynamic appearance and disappearance of columnar cells at the egg cylinder stage of the embryo. These expression patterns, and the fact that hensin null embryos (and those already reported for laminin) die at the onset of egg cylinder formation, support the view that hensin and laminin are required for terminal differentiation of columnar and squamous epithelial phenotypes during early embryogenesis.

Cyclosporin A produces distal renal tubular acidosis by blocking peptidyl prolyl cis-trans isomerase activity of cyclophilin

Cyclosporin A (CsA), a widely used immunosuppressant, causes distal renal tubular acidosis (dRTA). It exerts its immunosuppressive effect by a calcineurin-inhibitory complex with its cytosolic receptor, cyclophilin A. However, CsA also inhibits the peptidyl prolyl cis-trans isomerase (PPIase) activity of cyclophilin A. We studied HCO(3)(-) transport and changes in beta-intercalated cell pH on luminal Cl(-) removal in isolated, perfused rabbit cortical collecting tubules (CCDs) before and after exposure to media pH 6.8 for 3 h. Acid incubation causes adaptive changes in beta-intercalated cells by extracellular deposition of hensin (J Clin Invest 109: 89, 2002). Here, CsA prevented this adaptation. The unidirectional HCO(3)(-) secretory flux, estimated as the difference between net flux and that after Cl(-) removal from the lumen, was -6.7 +/- 0.2 pmol.min(-1).mm(-1) and decreased to -1.3 +/- 0.2 after acid incubation. CsA in the bath prevented the adaptive decreases in HCO(3)(-) secretion and apical Cl(-):HCO(3)(-) exchange. To determine the mechanism, we incubated CCDs with FK-506, which inhibits calcineurin activity independently of the host cell cyclophilin. FK-506 did not prevent the acid-induced adaptive decrease in unidirectional HCO(3)(-) secretion. However, [AD-Ser](8) CsA, a CsA derivative, which does not inhibit calcineurin but inhibits PPIase activity of cyclophilin A, completely blocked the effect of acid incubation on apical Cl(-):HCO(3)(-) exchange. Acid incubation resulted in prominent "clumpy" staining of extracellular hensin and diminished apical surface of beta-intercalated cells [smaller peanut agglutinin (PNA) caps]. CsA and [AD-Ser](8) CsA prevented most hensin staining and the reduction of apical surface; PNA caps were more prominent. We suggest that hensin polymerization around adapting beta-intercalated cells requires the PPIase activity of cyclophilins. Thus CsA is able to prevent this adaptation by inhibition of a peptidyl prolyl cis-trans isomerase activity. Such inhibition may cause dRTA during acid loading.

A family of genes encoding zona pellucida (ZP) domain proteins is expressed in various epithelial tissues during Drosophila embryogenesis

Zona pellucida (ZP) domain proteins have been identified in various species from worms to humans. Most of the characterized ZP family members are secreted or remain anchored to the plasma membrane where they play a structural role and/or act as receptors. In humans, several ZP proteins attracted attention because of their abundant expression in certain organs and their relation to various diseases. Here, we compare the molecular architecture and embryonic expression pattern of the 18 genes encoding ZP proteins in Drosophila melanogaster. Only five of these genes have been genetically characterized. All ZP genes are expressed in the embryo in epithelial tissues, such as the foregut, the hindgut, the Malpighian tubules, the salivary glands, the tracheal system, sensory organs and epidermis. Five genes are expressed during oogenesis; two of them are transcribed in the follicular epithelium, but not in the germ line cells.

Terminal differentiation of intercalated cells: the role of hensin

During the response to metabolic acidosis, the intercalated cell of the collecting tubule converts from one that secretes HCO3(-) to one that absorbs HCO3(-) by H(+) secretion. The molecular basis of this complex change in phenotype was studied in an immortalized intercalated cell line. We found that it was induced by secretion, polymerization, and deposition of a protein, which we termed hensin, into the extracellular matrix. Surprisingly, this change in phenotype is identical to terminal differentiation of epithelial cells in that it recapitulated all the characteristics of terminal differentiation, including a change in cell shape, acquisition of specialized apical structures (microvilli and ruffles), and the ability to secrete and endocytose materials in a regulated manner from the apical membrane. Hensin is expressed in most epithelia, and others have discovered that it is deleted in a large number of epithelial tumors. These results suggest that conversion of polarity of the intercalated cells represents a process of terminal differentiation.

Terminal differentiation of epithelia from trophectoderm to the intercalated cell: the role of hensin

The intercalated cells of the collecting tubules of mammalian kidneys were discovered by Haggege and Richet to change their morphology in response to a variety of physiologic stimuli related to changes in acid base status. Recent studies showed that the conversion of beta to alpha intercalated cell under the influence of acidification of the medium is due to the deposition of hensin in the extracellular matrix of these cells and activation of a novel inductive signal transduction pathway. The conversion of beta to alpha cells is shown to be a process of terminal differentiation. Hensin is secreted as a monomer, and activation of the cell induces two activities that convert it to a dimer by folding and into a fiber by bundling of the folded dimers by galectin 3. Only the fiber is functional. Hensin is expressed in most epithelial cells, and its staining pattern suggests that it might be involved in the terminal differentiation of most epithelia. There is loss of heterozygosity of hensin in a large number of epithelial and neural tumors, making it likely that it is a tumor suppressor gene.

Terminal Differentiation of Epithelia

All epithelia form sheets of cells connected by tight and adherent junctions and exhibit polarized distribution of membrane proteins and lipids. During their development, epithelia progress from this 'generic' phenotype to terminally differentiated states characterized by the development of apical structures such as microvilli, apical endocytosis and regulated exocytosis as well as characteristic cell shapes. We have identified an extracellular matrix protein, hensin, which when polymerized into a fiber induces the terminal differentiation of renal cells. Hensin is expressed in most epithelia where it exists in tissue-specific alternately spliced forms. Many epithelial tumors have deletions in the human ortholog of hensin. We propose that hensin mediates terminal differentiation of these epithelia.

Acid incubation reverses the polarity of intercalated cell transporters, an effect mediated by hensin

Metabolic acidosis causes a reversal of polarity of HCO(3)(-) flux in the cortical collecting duct (CCD). In CCDs incubated in vitro in acid media, beta-intercalated (HCO(3)(-)-secreting) cells are remodeled to functionally resemble alpha-intercalated (H(+)-secreting) cells. A similar remodeling of beta-intercalated cells, in which the polarity of H(+) pumps and Cl(-)/HCO(3)(-) exchangers is reversed, occurs in cell culture and requires the deposition of polymerized hensin in the ECM. CCDs maintained 3 h at low pH ex vivo display a reversal of HCO(3)(-) flux that is quantitatively similar to an effect previously observed in acid-treated rabbits in vivo. We followed intracellular pH in the same beta-intercalated cells before and after acid incubation and found that apical Cl/HCO(3) exchange was abolished following acid incubation. Some cells also developed basolateral Cl(-)/HCO(3)(-) exchange, indicating a reversal of intercalated cell polarity. This adaptation required intact microtubules and microfilaments, as well as new protein synthesis, and was associated with decreased size of the apical surface of beta-intercalated cells. Addition of anti-hensin antibodies prevented the acid-induced changes in apical and basolateral Cl(-)/HCO(3)(-) exchange observed in the same cells and the corresponding suppression of HCO(3)(-) secretion. Acid loading also promoted hensin deposition in the ECM underneath adapting beta-intercalated cells. Hence, the adaptive conversion of beta-intercalated cells to alpha-intercalated cells during acid incubation depends upon ECM-associated hensin.

Plasticity of intercalated cell polarity: effect of metabolic acidosis

The cortical collecting duct (CCD) is capable of secreting H(+) or HCO3(-) depending on the acid-base status in vivo. Transport is a function of two types of intercalated cells in the CCD: A-intercalated cells secrete H(+) and B-intercalated cells secrete HCO3(-). Metabolic acidosis results in a decrease in HCO3(-) secretion and an increase in H(+) secretion by the respective cells. Using a model of metabolic acidosis in vitro, we have shown that the down-regulation of HCO3(-) secretion occurs by endocytosis of apical anion exchangers in B-intercalated cells. The finding of basolateral anion exchangers in some adapted B-intercalated cells is consistent with a reversal of functional epithelial polarity. Plasticity of polarity is also observed in cultured intercalated cells: high-density plating results in converting B- to A-intercalated cells via the deposition of the novel protein hensin in the extracellular matrix. A key problem in renal physiology is to investigate the role of hensin in mediating the adaptation of the CCD to acidosis in vitro and in vivo.

Induction of terminal differentiation in epithelial cells requires polymerization of hensin by galectin 3

During terminal differentiation, epithelia become columnar and develop specialized apical membrane structures (microvilli) and functions (regulated endocytosis and exocytosis). Using a clonal intercalated epithelial cell line, we found that high seeding density induced these characteristics, whereas low density seeding maintained a protoepithelial state. When cells were plated at low density, but on the extracellular matrix of high density cells, they converted to the more differentiated phenotype. The extracellular matrix (ECM) protein responsible for this activity was purified and found to be a large 230-kD protein, which we termed hensin. High density seeding caused hensin to be polymerized and deposited in the extracellular matrix, and only this form of hensin was able to induce terminal differentiation. Antibodies to hensin blocked the change in phenotype. However, its purification to homogeneity resulted in loss of activity, suggesting that an additional protein might be necessary for induction of terminal differentiation. Here, we found that a 29-kD protein specifically associates with hensin in the ECM. Addition of purified p29 restored the activity of homogenously purified hensin. Mass fingerprinting identified p29 as galectin 3. Purified recombinant galectin 3 was able to bind to hensin and to polymerize it in vitro. Seeding cells at high density induced secretion of galectin 3 into the ECM where it bundled hensin. Hence, the high density state causes a secretion of a protein that acts on another ECM protein to allow the new complex to signal the cell to change its phenotype. This is a new mechanism of inside-out signaling.

Phenotypic plasticity and terminal differentiation of the intercalated cell: the hensin pathway

The intercalated cell of the collecting tubule exists in a spectrum of types. The alpha form secretes acid by an apical H(+) ATPase and a basolateral Cl:HCO(3) exchanger which is an alternatively spliced form of the red cell band 3 (kAE1), while the beta form secretes HCO(3) by having these transporters on the reverse membranes. In a clonal cell line of the beta form we found that seeding density causes this conversion. A new protein, termed hensin, was deposited in the extracellular matrix of high-density cells which on purification reversed the polarity of the transporters. Hensin also induced the expression of the microvillar protein, villin, and caused the appearance of the apical terminal web proteins, cytokeratin 19 and actin, all of which led to the development of an exuberant microvillar structure. In addition, hensin caused the beta cells to assume a columnar shape. All of these studies demonstrate that the conversion of polarity in the intercalated cell, at least in vitro, represents terminal differentiation and that hensin is the first protein in a new pathway that mediates this process. Hensin, DMBT1, CRP-ductin, and ebnerin are alternately spliced products from a single gene located on human chromosome 10q25-26, a region often deleted in several cancers, especially malignant gliomas. Hensin is expressed in many epithelial cell types, and it is possible that it plays a similarly important role in the differentiation of these epithelia as well.