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.

The kidney papilla is a stem cells niche

Stem cells are characterized by low cycle time, which has allowed us to identify such cells in the mature kidney. These putative stem cells are located mostly outside the renal tubule and are concentrated in the papilla of the kidney potentially under the urinary epithelium of the papilla. Clonal analysis of these cells shows that they can differentiate into epithelial, neuronal, and other uncharacterized cells. Induction of ischemic renal failure resulted in increased proliferation of these papillary cells. Injection of these cells under the renal capsule led to their incorporation into various tubule segments. It is likely that these stem cells sense a "damage" signal from the cortex resulting in proliferation followed by migration to the site of injury.

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.

Showing Some Skin

Skin . A Natural History. By Nina G. Jablonski . University of California Press, Berkeley, CA, 2006. 293 pp. $24.95, £15.95. ISBN 9780520242814.

The author discusses the characteristics, functions, evolutionary history, and possible future of the human integument.

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.