Alterations in the Establishment and Maintenance of Epithelial Cell Polarity As a Basis for Disease Processes

Alterations in the establishment and maintenance of epithelial cell polarity as a basis for disease processes.

B A Molitoris, W J Nelson

J Clin Invest. 1990;85(1):3-9. https://doi.org/10.1172/JCI114427.

Research Article

Find the latest version: https://jci.me/114427/pdf Perspectives

Alterations in the Establishment and Maintenance of Epithelial Cell Polarity as a Basis for Disease Processes Bruce A. Molitons* and W. James Nelsont *Department ofMedicine, Veterans Administration Medical Center, University ofColorado School ofMedicine, Denver, Colorado 80220; and tInstitutefor Cancer Research, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111

Polarized epithelial cells play fundamental roles in the vecto- internal milieu and has a compliment of intrinsic and extrinsic rial movement of ions, water, and macromolecules between membrane proteins found over the entire surface membrane biological compartments. These vectorial processes include of nonpolarized cells. These basolateral membrane proteins absorption (enterocytes and renal proximal tubule cells), se- maintain the normal physiologic state of the cell and also par- cretion (hepatocytes, endocrine and exocrine cells), and ex- ticipate in signal recognition and transduction. The lipid com- change (alveolar cells and capillary endothelium). The ability position of apical and basolateral membranes also differs to conduct these vectorial processes is dependent on the struc- markedly (Table I; for review, see reference 3). This difference tural and functional organization of epithelial cells into: (a) is responsible for large physiochemical differences between the structurally, biochemically, and physiologically distinct apical two membrane domains (4) and influences the function of and basolateral plasma membrane domains that contain dif- numerous membrane proteins. The establishment and main- ferent ion channels, transport proteins, enzymes, and lipids; tenance of these specific apical and basolateral membrane and (b) cell-cell junctional complexes that integrate cells into a protein and lipid differences is essential for the efficient func- monolayer forming a cellular barrier between biological com- tioning of polarized epithelial cells. For example, sodium partments. reabsorption by renal proximal tubular cells is dependent on The establishment and maintenance of this specialized cel- the polarized localization of specific carrier proteins such as lular organization is a multistage process involving the forma- the Na+/H' antiporter and glucose, amino acid- and phos- tion of cell-cell and cell-substratum contacts, and the estab- phate Na'-dependent cotransporters to the apical membrane, lishment and maintenance of the polarized distributions of and the polarized distribution of Na',K+-ATPase to the baso- plasma membrane and cytoplasmic components. Aberrations lateral membrane. in any stage of this process could result in the development of Junctional complex. The lateral membrane domain pos- cell- and tissue-specific abnormalities, and ultimately a patho- sesses a specialized intercellular junctional complex composed logic state. of zonula occludens (tight junction), zonula adherens (inter- In this review we describe recent findings regarding basic mediate junction), desmosomes, gap junctions, and cell adhe- cellular mechanisms involved in the organization of polarized sion proteins (Fig. 1). The components of the junctional com- epithelial cells. These fundamental cellular concepts are then plex have four important functions: (a) regulating initial cell- used as a foundation for discussing potential alterations in- cell recognition and adhesion (cell adhesion proteins); (b) volved in disease processes in epithelial cells. maintaining the cohesive structural integrity of the epithelial monolayer (intermediate junctions and desmosomes); (c) al- Characteristics ofpolarized epithelial cells lowing for intercellular communication (gap junctions); and Distinct surface membrane domains. Polarized epithelial cells (d) (the tight junction) regulating the permeability of the para- have a characteristic cellular organization that includes a sur- cellular pathway ("gate" function) and maintaining the pro- face plasma membrane organized into distinct apical and ba- tein and lipid differences between apical and basolateral solateral domains. Within these surface membrane domains membrane domains ("fence" function) (5, 6). enzymes, transport proteins, hormone receptors, and lipids are The basal membrane contains specialized proteins that reg- localized in a polarized fashion (Table I; for review, see refer- ulate cell-substratum interactions. Cell attachment to the sub- ences 1 and 2). Apical membranes face the external compart- stratum occurs via specific receptors to type IV collagen, pro- ment and are composed of membrane proteins with special- teoglycan, and laminin (7, 8), is tissue specific, and is impor- ized properties related to the cell's primary function (e.g., ab- tant for epithelial morphogenesis, maintenance of the sorption). The basolateral membrane domain faces the differentiated state, and tissue-specific gene expression (7, 8). Cytoskeletal organization. The structural organization of epithelial cells is dependent on the polarized nature of the Address correspondence to Dr. Bruce A. Molitoris, Department of cytoskeleton and its interaction with the surface membrane Medicine (I 1 IC), V.A. Medical Center, 1055 Clermont St., Denver, (Fig. 1; 9-1 1). Actin microfilaments and their associated pro- CO 80220. teins interact with surface membrane proteins via direct or Received for publication 11 October 1989 and in revised form 16 October 1989. indirect linkages to fodrin (termed spectrin in erythrocytes), ankyrin, protein 4.1, vinculin, the ll0-kD protein (brush J. Clin. Invest. border myosin I) of the microvillus, and talin (1 1). For exam- © The American Society for Clinical Investigation, Inc. ple, ankyrin binds with high affinity to Na+,K+-ATPase (12), 0021-9738/90/01/0003/07 $2.00 and a complex containing Na+,K+-ATPase, ankyrin, and fo- Volume 85, January 1990, 3-9 drin has been detected in extracts of Madin-Darby canine kid-

Epithelial Cell Polarity and Disease Processes 3 Table . Asymmetry of the Surface Membrane ofPolarized Subsequently, cells develop biochemically and physiologically Epithelial Cells distinct apical and basolateral membrane domains, and cytoplasmic and cytoskeletal organization (2). In the mammalian Basolateral Characteristic Apical membrane membrane embryo, formation of a polarized epithelium (trophectoderm) occurs at the 8 cell stage (morula) during induction of exten- Proteins sive cell-cell contact (compaction) (17). Cell-cell and cell- Enzymes Leucine aminopeptidase Adenylate cyclase substratum contact also induces development of cell polarity Maltase in freshly isolated hepatocytes and MDCK cells previously GPI-linked proteins grown under suspension conditions. Before this cellular reor- (alkaline phosphatase) ganization, surface membrane proteins and lipids are randomly distributed over the entire surface membrane (18). Po- Receptors Insulin larization of the apical membrane follows either cell substra- Parathyroid tum attachment or cell-cell contact, but basolateral hormone membrane polarization requires extensive cell-substratum Epidermal and cell-cell contact ( 19). growth factor Cell-cell contact and the establishment of polarity. Cell- Laminin cell recognition and contact between individual cells is initi- ATPases H+-ATPase Na',K+-ATPase ated by cell adhesion molecules (CAMs). CAMs are cell sur- Mg2+-ATPase Ca2+-ATPase face membrane glycoproteins that are highly conserved, bind to each other in a homophilic manner and mediate calcium- Carriers Amiloride-sensitive Na' C1-/HCO3 dependent aggregation of epithelial cells (20-22). Cell-cell channel exchanger contact results in the rapid assembly of other components of Na'-dependent Na'-independent the junctional complex (e.g., tight junctions and desmosomes) cotransporters glucose carrier and the subsequent demarcation of apical and basolateral Na+/K+/Cl membrane domains. After cell-cell contact there is a redistri- cotransporter bution of surface membrane and cytoskeletal proteins to the Na+/H+ antiporter basolateral membrane. For example, after cell-cell and cell- Lipids substratum contact, Na',K+-ATPase, ankyrin, and fodrin, Cholesterol High Low which are distributed randomly over the entire surface mem- Sphingomyelin High Low brane, undergo a spatial reorganization resulting in the forma- Phosphatidylcholine Low High tion of metabolically stable, nonextractable cytoskeletal- Phosphatidylinositol Low High membrane protein complexes localized to the basolateral membrane (23, 24). Physical properties This process of recruitment and stabilization of surface Electrical resistance High Low membrane proteins may be initiated by cytoskeletal-CAM in- Membrane fluidity Low High teractions. As noted earlier, Na+,K+-ATPase forms a metabolically stable detergent-insoluble complex with actin, fodrin, and ankyrin (12, 13; Fig. 1, inset). Uvomorulin (E-cadherin) may also form complexes with ankyrin and fodrin (25). Inter- ney (MDCK)' epithelial cells (13), demonstrating a direct link- estingly, deletion of part of the cytoplasmic domain of uvo- age of cytoskeletal proteins to a specific integral membrane morulin inhibits cell-cell binding in transfected fibroblasts protein. Cytokeratin intermediate filaments, composed of (26), perhaps because of loss of linkage to cytoplasmic proteins bundles of protein filaments, attach to the surface membrane (25). It is, therefore, possible that uvomorulin-cytoskeletal in- at maculae adherens (desmosomes) and hemidesmosomes, teractions form a nucleation site for recruitment of other cyto- and provide a structural continuum between adjacent cells and skeletal-membrane protein complexes to the basolateral with the substratum (1 1). Microtubules are aligned with the membrane (e.g., Na+,K+-ATPase). apico-basal axis of the cell, and they determine the spatial Polarized targeting to distinct membrane domains. Further orientation of the endoplasmic reticulum, Golgi apparatus, genesis and maintenance of the polarized organization of epi- and lysosomes (14, 15), and the distribution of actin microfila- thelial cells is dependent on the targeting of proteins and lipids ments within the cell (16). to specific membrane domains. The observation that enveloped RNA viruses bud vectorially in MDCK cells permitted Establishment and maintenance the dissection of intracellular targeting of individual enveloped of the polarized epithelial cell glycoproteins to different plasma membrane domains (27). In The first step toward the establishment of a polarized epithe- polarized epithelial cells, influenza virus hemagglutinin (HA) lium is the organization of individual cells, through cell-cell is transported to the apical membrane, and the G protein of and cell-substratum contacts, into a cohesive monolayer. vesicular stomatitis virus (VSV) is transported to the basolateral membrane (27). These studies demonstrated that both glycoproteins were cotransported through the endoplasmic re- 1. Abbreviations used in this paper: CAM, cell adhesion molecules; ticulum and Golgi complex. However, in the trans-cisternae of GPI, glycosylphosphatidylinositol; HA, hemagglutinin; MDCK, the Golgi complex (TGN), the pathways of these proteins sepa- Madin-Darby canine kidney cells; TGN, trans-Golgi network; VSV, rated and HA and G proteins were transported via different vesicular stomatitis virus. vesicles to their target membrane domains (28) (Fig. 1). More

4 B. A. Molitoris and W J. Nelson APICAL cessesFigureinvolved1. Cellularin componentsthe establishmentand pro-and 'z tjN. -' / a', !' 1-lL in' 14 red maintenance of epithelial cell polarity. On the left, apical and basolateral protein syn- t\I r 0 0 ._ __ thesis, sorting, and targeting pathways are ACTIN shown. Basolateral membrane (A) and api- t2 ' u'-~ A/Hically d sUp(C)Ww4destinedV protein vesicles move ZO Hi / b TW MF MFt / I) C from the TGN to their respective surface ZA - -2/ - membrane domains. In hepatocytes, apical C 4 proteins may first be delivered to the baso- I B\ »4CT'1\A\|TGN 9 VLS MT \ \ 31 e ¢XJA TPase Apicalredistributedendocytosisto is also shown. Recep- tor-mediated endocytosis starts (a) with MA 4,/tANKRYNd - sthea\formation\\ of a clatherin-coated vesicle GOLGI &/-> C; Of' g (2) from a clatherin-coated pit (1). Clath- LATERAL ICOMPLE erin then disassembles leaving primary or early endosomes. From primary endo- IAOMP/N GJ|4/ 1/ i9//>i somes, membrane components can either ;J ER recycle back to the surface membrane of origin (b), be transferred (c) to lysosomes BASAL~)D'--L~~ 1____*1 -~ ~r -. ~ (L) via multivesicular bodies (4), or un- EC dergo transcytosis (d). On the right, components of the cytoskeletal network are shown, including actin microfilaments (MF), which form the structural core within each microvillus and are held together by the "bundling" proteins villin and fimbrin (0). Microtu- bules (MT) form along the apico-basal axis of the cell and intermediate filaments (IF) interconnect adjacent cells at macula adherens (MA) and the substratum at hemi-desmosomes (HD). Inset, The interactions of actin microfilaments and their associated cytoskeletal proteins. The junctional complex along the lateral membrane includes the zonula occludens (ZO), zonula adherens (ZA), maculae adherens (MA), gap junctions (GJ),(GJ)and CIAMs.IL

recent studies of the targeting of endogenous apical and baso- The mechanisms involved in sorting and targeting proteins lateral membrane proteins in MDCK cells have yielded similar to different plasma membrane domains remain poorly under- conclusions. For example, Na',K+-ATPase and other glyco- stood. It has been proposed that within the Golgi complex proteins were targeted directly to the basolateral membrane specific receptors target proteins to their final destination. For (29) and apical membrane (30), respectively, in MDCK cells. example, mannose-6-phosphate receptors in the Golgi com- An additional intracellular pathway for the delivery of api- plex recognize phosphorylated mannose side chains on lyso- cal proteins has been recently described. Hubbard and co- somal hydrolases and then target these proteins to lysosomes workers showed that both apical and basolateral membrane (38). Sorting signals for the apically destined influenza virus proteins in hepatocytes were first transported from the TGN to HA protein and basolaterally targeted VSV G protein have the basolateral membrane (sinusoidal) surface (31). Thereaf- been localized to the external domain of each protein (39). ter, proteins destined for the apical domain were specifically However, receptor proteins that recognize these signals have retrieved from the membrane (endocytosis) and redistributed not yet been demonstrated. Recently it has been suggested that to the apical surface (Fig. 1). Similar results have been ob- the targeting of apical membrane glycoproteins may be me- tained in studies of protein targeting in the small intestine (32), diated through the covalent linkage to a glycolipid, glycosyl- although conflicting reports using a similar model, in the small phosphatidylinositol (GPI). In MDCK cells, LLC-PK1 cells, intestine, have been reported by two different laboratories (33, and two human intestinal cell lines, endogenous GPI-linked 34). Clearly, a basolateral to apical membrane "tract" exists as glycoproteins localized preferentially to the apical surface hepatocytes, enterocytes, and transfected MDCK cells use this (40, 41). transport pathway for the transcytosis of polymeric Igs (35). The role of microtubules in the movement of surface Further studies, therefore, will be necessary to define the ex- membrane-destined vesicles has been demonstrated in several tent to which this pathway is involved in the targeting of spe- systems. While the polarized basolateral delivery of VSV G cific membrane components to the apical surface. protein in MDCK cells (42), and Na+,K+-ATPase in intestinal Understanding the intracellular transport of lipids to the epithelium (43) was unaffected by microtubule depolymeriz- surface membrane (establishment phase) has been difficult be- ing agents, the apical membrane delivery of endogenous cause of the lack of immunologic techniques and domain-spe- membrane proteins in intestinal epithelial cells, and influenza cific markers. Like proteins, phospholipids are synthesized in HA in MDCK cells was dependent on the presence of intact the endoplasmic reticulum, move to the cis-Golgi, and migrate microtubules (42, 43). Disruption of microtubules both in to the TGN (3). However, the mechanisms involved in the vitro and in vivo led to the accumulation of apical markers transport of phospholipids from the TGN to the surface mem- and microvilli-like structures in the basolateral membrane brane remain in question. While it is generally felt that phos- (43). This microtubule-directed transport of vesicles to the pholipids travel in protein-shuttling, domain-specific vesicles apical membrane may be mediated by two soluble cytoplasmic (3), two new lines of evidence indicate that additional protein- ATPases, kinesin and dynein (44). independent pathways may also be involved (36, 37). Epithelial cells can also qualitatively and quantitatively

Epithelial Cell Polarity and Disease Processes 5 Figure 2. Freeze-fracture electron micrograph of the apical portion of rat renal proximal tubule cells after 50 min of ischemia. X23,000. alter the protein composition of plasma membrane domains the segregation of these proteins. This cytoskeletal tethering by endocytosis and exocytosis. This has been demonstrated in mechanism has been studied in erythrocytes where the cyto- epithelial cells, such as the intercalated cells of the cortical skeletal matrix is bound to Band III (49) by ankryn and spec- collecting tubule, which amplify or diminish apical membrane trin. Using fluorescence recovery after photobleaching, the H+-ATPases in response to changes in Pco2 and pH (45). diffusion coefficient of Band III increased 50-fold in spectrin- These cells are also capable of reversing surface membrane deficient spherocytes or under conditions favoring spectrin polarity in response to environmental stimuli (46). dissociation (50). Salas et al. (51), using fluorescence recovery Epithelial cells that conduct both apical and basolateral after photobleaching and Triton X-100 extractability tech- endocytosis and transcytosis must limit the intracellular mix- niques in MDCK cells, demonstrated that both apical and ing of surface membrane proteins and lipids from different basolateral membrane domains contain mobile and immobile domains. A highly efficient sorting mechanism is essential if membrane protein fractions. The immobile membrane pro- distinct apical and basolateral membrane domains are to be teins were not extracted with Triton X-100, indicating that maintained, since the quantity of surface membrane internal- they were linked to the cytoskeleton. Significantly, when cyto- ized via endocytosis per hour can greatly exceed the entire skeletal associated membrane proteins were localized to the membrane surface area (47). Sorting of endocytosed surface alternate (incorrect) surface membrane domain, the proteins membrane occurs in the endosomal compartment (Fig. 1) were mobile and extractable (51). This suggests the presence of where three subsequent processing pathways are available: re- domain-specific cytoskeletal matrices that bind to and stabilize cycling to the original surface membrane (b); delivery to lyso- specific membrane proteins. somes (c); and delivery to the alternate membrane domain Since some basolateral and apical domain-specific proteins (transcytosis; d) (for review, see reference 47). Recycling of demonstrate free rotational diffusion and no cytoskeletal asso- membrane components to the original membrane is an effi- ciations (under the above criteria), other mechanism(s) must cient process (48); however, the mechanisms responsible for operate to maintain their high degree of polarity. It has been intracellular sorting of endocytic vesicles remain largely un- proposed that the tight junction may form a diffusion barrier known. (fence) in the membrane and thus limit diffusion between Cytoskeleton and tight junctions in the maintenance ofpo- membrane domains. This hypothesis is supported for proteins larity. Once membrane proteins and lipids have been delivered by several lines of indirect evidence (for review, see references and incorporated into specific membrane domains, it is essen- 13 and 14). Direct evidence, however, indicates that surface tial that their polarized distribution be maintained. Two cellu- membrane lipid polarity requires intact tight junctions. When lar mechanisms are thought to be involved: the tight junction a fluorescent phosphatidylethanolamine derivative was intro- and the actin-associated cytoskeleton. The actin-associated cy- duced into the external leaflet of the apical membrane of toskeleton (Fig. 1, inset) may function to reduce the lateral MDCK cells, the probe remained localized within the apical mobility of specific membrane proteins and hence maintain domain unless cell-cell contacts were disrupted (52).

6 B. A. Molitoris and W. J. Nelson Altered epithelial cell polarity in pathologic states vasive in collagen gels and embryonal heart tissue (59). To- The establishment and maintenance of polarity in epithelial gether these data suggest that epithelial cell invasiveness, and cells is a complex multistage process. It is probable, therefore, therefore carcinogenesis, may in part be related to loss of cell- that alterations in one or more of these cellular processes play a cell adhesion and surface membrane polarity. The cellular role in the etiology of specific disease states. If this is the case, event(s) resulting in diminished expression of uvomorulin and then pathologic processes in epithelial cells could be classified the mechanism(s) leading to cellular invasiveness remain un- according to an abnormality in one or more of these processes. known. Here we will examine four disease states in that context. Ischemia in proximal tubule cells and bile duct ligation Microvillus inclusion disease (Davidson's disease) is an (hepatocytes) both result in loss ofapical microvilli with cellu- autosomal recessive disorder characterized clinically by pro- lar internalization and blebbing of the apical membrane into tracted diarrhea and high mortality (53). Morphologically, the the lumen, loss ofsurface membranepolarity ofboth apical and proximal enterocytes and colonic cells have scanty, disorgan- basolateral membrane domain-specific markers (62-64), alter- ized, short microvilli, and numerous cytoplasmic vesicular ations in apical terminal web actin microfilaments (65, 66), bodies that consist of a complete apical membrane with mi- and reduction in vectorial transport functions (63, 67, 68). The crovilli and an associated terminal web (54). Because of these alterations in vectorial transport functions were related to the alterations, "mature" enterocytes are unable to absorb ions, redistribution of surface membrane phospholipids (67) and nutrients, and water. Adjacent Goblet, Paneth, and enteroen- domain-specific apical and basolateral membrane enzymes docrine cells appear normal (54). The mechanism(s) that re- (63, 68). For example, ischemia-induced redistribution of sults in the formation of these abnormal vesicular bodies and Na',K+-ATPase to the apical membrane, in renal proximal apical surface membranes is unknown. However, recent in tubule cells, was associated with reduced Na+ reabsorption, vitro cell culture studies have demonstrated the existence of probably secondary to Na',K+-ATPase pumping Na' back similar vesicular bodies in MDCK epithelial cells grown at low into the urinary lumen (68). Furthermore, reestablishment of densities in the absence of cell-cell contact (55). These intra- surface membrane Na',K+-ATPase polarity was essential for cellular vesicular structures, termed vacuolar apical compart- restoration of normal cellular Na+ transport (69). ments, possess microvilli and apical marker proteins but ex- The cellular events occurring during cell injury remain clude basolateral membrane proteins (55). Treatment of intes- largely unknown. However, ischemia in renal proximal tubule tinal cells with microtubule depolymerizing drugs also leads to cells results in the rapid duration-dependent opening of tight the occurrence of similar structures, which were localized to junctions, which precedes or occurs concurrently with the re- the basolateral membrane (43). After removal of the colchi- distribution of surface membrane phospholipids and Na',K+- cine, these aberrant structures were shuttled to their correct ATPase to the alternate surface membrane domain (70). Mi- apical domain, presumably via a microtubule-dependent pro- crofilament disruption may also be involved in ischemic cell cess. Taken together, these data suggest that a defect in micro- injury, as disruption of terminal web and microvillar actin tubule-mediated delivery of apically destined vesicles may be microfilaments occurs early and in a time-dependent fashion playing a role in the etiology of Davidson's disease. Other (65). Disruption of microvillar core microfilaments may be potential mechanisms, however, include the inability of api- responsible for loss of apical membrane and the markedly cally destined vesicles to fuse with the apical surface, or ineffi- altered morphology of apical microvilli seen after ischemic cient recycling of endocytic membrane components. injury (Fig. 2). Microfilament disruption during ischemia may Carcinogenesis is a complex multistage process character- be the result of cellular ATP depletion as metabolic inhibitors ized by loss ofgrowth regulation, invasiveness, angiogenesis, result in breakdown of actin microfilaments in cell culture and metastasis. 80-90%o of human tumors are of epithelial studies (71). Taken together, these data indicate that the cell's origin (carcinomas). Many carcinomas are characterized by inability to maintain surface membrane polarity may correlate alterations in surface membrane morphology, cell polarity, with the disruption of lateral cell-cell attachments and the gap junctions, tight junctions, and cell-cell adhesiveness (56, actin cytoskeleton. Under these aberrant conditions, apical 57). Furthermore, tumor invasiveness is inversely related to and basolateral membrane proteins and lipids may be capable cellular differentiation (58). of lateral diffusion, within the plane of the membrane bilayer, Recent cell culture evidence indicates that the invasive into the alternate domain, resulting in the loss of surface aspect of this process may be related to loss of cell-cell adhe- membrane polarity. For repolarization of surface membrane sion. First, transformation of MDCK cells with either Mo- lipids and proteins, and therefore the restoration of normal loney or Harvey sarcoma viruses resulted in the formation of cellular function, cell-cell contacts and cytoskeletal organiza- invasive cells with fibroblastic morphology that lacked surface tion must be reestablished, and the abnormal membrane com- membrane expression of the cell adhesion protein, uvomoru- ponents must be removed. lin. On the other hand, transformed cells that maintained epi- In summary, the establishment and maintenance of epithe- thelial characteristics and a high surface membrane uvomoru- lial polarity is essential for normal cell structure and function. lin content were noninvasive (59). Reductions in cell-cell Many pathologic processes may be secondary to either funda- contacts had been previously reported when pp60 v-src was mental errors in the complex processes involved in the estab- expressed in MDCK cells (60), and when SV-40-DNA and lishment and maintenance of epithelial polarity, or loss of the ras-DNA were used to transform primary rabbit mammary cell's ability to maintain the polar state secondary to stresses epithelial cells (61). placed on the cell. Continued detailed analysis of these patho- Second, treatment of confluent monolayers of MDCK cells logic states and of the mechanisms that are involved in the with an anti-uvomorulin MAb resulted in dissociation of cells, establishment and maintenance of polarity in epithelial cells loss of surface expression of uvomorulin, and development of may further our understanding of the etiology of specific dis- a fibroblastic phenotype. Concurrently, these cells became in- ease processes.

Epithelial Cell Polarity and Disease Processes 7 phosphatase and glutamyl transpeptidase as polarization markers during the organization of LLC-PKI cells into an epithelial membrane. J. H. Dahl and the laboratory of Dr. L. Andrew Biol. Chem. 259:574-582. The authors thank Rolf E., P. J. I. Salas, D. Gundersen, and E. Rodri- for assistance with the freeze fracture electron micrograph 19. Vega-Salas, D. Staehelin guez-Boulan. 1987. Formation of the apical pole of epithelial (Madin- (cover photo). is independent to Dr. Molitoris from the Na- Darby canine kidney) cells. Polarity ofan apical protein This work was supported by grants basolateral marker requires tional Institutes of Health (NIH) DK-41126 and the Veterans Admin- of tight junctions while segregation of a Research Service; grants to Dr. Nelson from the National cell-cell interactions. J. Cell Biol. 104:905-916. istration 20. Edelman, G. M. 1986. Cell adhesion molecules in the regula- Science Foundation (DCB-8609091) and the NIH (GM-35527); grants 2:81-116. (CA-06927 and tion ofanewal form and tissue pattern. Annu. Rev. Cell Biol. to the Institute for Cancer Research from the NIH and M. Takeichi. 1988. Expressed appropriation from the Commonwealth of Penn- 21. Nose, A., A. Nagafuchi, RR-05539); and an recombinant cadherins mediate cell sorting in model systems. Cell. sylvania. Dr. Molitoris is a recipient of a Clinical Investigator Award Service. Dr. Nelson is a 54:993-1001. from the Veterans Administration Research A. Grimaldi. 1988. The role Investigator Award from the American 22. Gumbiner, B., B. Stevenson, and recipient of an Established the molecule uvomorulin in the formation and main- Heart Association. of cell adhesion tenance of the epithelial junctional complex. J. Cell Biol. 107:1575- 1587. 23. Nelson, W. J., and P. J. Veshnock. 1986. 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