Protein zero, a nervous system adhesion molecule, triggers epithelial reversion in host carcinoma cells

Cingulin family: Structure, function and clinical significance

Cingulin and its paralog paracingulin are vital components of the apical junctional complex in vertebrate epithelial and endothelial cells. They are both found in tight junctions (TJ), and paracingulin is also detectable in adherens junctions (AJ) as TJ cytoplasmic plaque proteins. Cingulin and paracingulin interact with other proteins to perform functions. They interact with cytoskeletal proteins, modulate the activity of small GTPases, such as RhoA and Rac1, and regulate gene expression. In addition, cingulin and paracingulin regulate barrier function and many pathological processes, including inflammation and tumorigenesis. In this review, we summarize the discovery and structure, expression and subcellular distribution, and molecular interactions of cingulin family proteins and discuss their role in development, physiology, and pathological processes.

E-cadherin binds to desmoglein to facilitate desmosome assembly

Desmosomes are adhesive junctions composed of two desmosomal cadherins: desmocollin (Dsc) and desmoglein (Dsg). Previous studies demonstrate that E-cadherin (Ecad), an adhesive protein that interacts in both trans (between opposing cells) and cis (on the same cell surface) conformations, facilitates desmosome assembly via an unknown mechanism. Here we use structure-function analysis to resolve the mechanistic roles of Ecad in desmosome formation. Using AFM force measurements, we demonstrate that Ecad interacts with isoform 2 of Dsg via a conserved Leu-175 on the Ecad cis binding interface. Super-resolution imaging reveals that Ecad is enriched in nascent desmosomes, supporting a role for Ecad in early desmosome assembly. Finally, confocal imaging demonstrates that desmosome assembly is initiated at sites of Ecad mediated adhesion, and that Ecad-L175 is required for efficient Dsg2 and desmoplakin recruitment to intercellular contacts. We propose that Ecad trans interactions at nascent cell-cell contacts initiate the recruitment of Dsg through direct cis interactions with Ecad which facilitates desmosome assembly.

Fibronectin type III and intracellular domains of Toll-like receptor 4 interactor with leucine-rich repeats (Tril) are required for developmental signaling

Toll-like receptor 4 interactor with leucine-rich repeats (Tril) functions as a coreceptor for Toll-like receptors (Tlrs) to mediate innate immune responses in adults. In embryos, Tril signals to promote degradation of the Bmp inhibitor, Smad7, to allow for blood formation. It is not known whether this function requires, or is independent of, Tlrs. In the current studies, we performed a structure-function analysis, which indicated that the fibronectin type III (FN) domain and the intracellular domain of Tril are required to trigger Smad7 degradation in Xenopus embryos. Furthermore, we found evidence suggesting that a Tril deletion mutant lacking the FN domain (Tril∆FN) can dominantly inhibit signaling by endogenous Tril when overexpressed. This finding raises the possibility that the FN domain functions to bind endogenous Tril ligands. We also show that Tril cycles between the cell surface and endosomes and that the Tril extracellular domain, as well as cadherin based cell-cell adhesion, are required for cell surface retention, while the intracellular domain is required for internalization in Xenopus ectodermal explants. Using a CHO cell aggregation assay, we show that, unlike other transmembrane proteins that contain leucine-rich repeats, Tril is not sufficient to mediate homophilic adhesion.

Polarization and myelination in myelinating glia

Myelinating glia, oligodendrocytes in central nervous system and Schwann cells in peripheral nervous system, form myelin sheath, a multilayered membrane system around axons enabling salutatory nerve impulse conduction and maintaining axonal integrity. Myelin sheath is a polarized structure localized in the axonal side and therefore is supposed to be formed based on the preceding polarization of myelinating glia. Thus, myelination process is closely associated with polarization of myelinating glia. However, cell polarization has been less extensively studied in myelinating glia than other cell types such as epithelial cells. The ultimate goal of this paper is to provide insights for the field of myelination research by applying the information obtained in polarity study in other cell types, especially epithelial cells, to cell polarization of myelinating glia. Thus, in this paper, the main aspects of cell polarization study in general are summarized. Then, they will be compared with polarization in oligodendrocytes. Finally, the achievements obtained in polarization study for epithelial cells, oligodendrocytes, and other types of cells will be translated into polarization/myelination process by Schwann cells. Then, based on this model, the perspectives in the study of Schwann cell polarization/myelination will be discussed.

The molecular composition and function of desmosomes

Desmosomes are intercellular adhesive junctions that are particularly prominent in tissues experiencing mechanical stress, such as the heart and epidermis. Whereas the related adherens junction links actin to calcium-dependent adhesion molecules known as classical cadherins, desmosomes link intermediate filaments (IF) to the related subfamily of desmosomal cadherins. By tethering these stress-bearing cytoskeletal filaments to the plasma membrane, desmosomes serve as integrators of the IF cytoskeleton throughout a tissue. Recent evidence suggests that IF attachment in turn strengthens desmosomal adhesion. This collaborative arrangement results in formation of a supracellular network, which is critical for imparting mechanical integrity to tissues. Diseases and animal models targeting desmosomal components highlight the importance of desmosomes in development and tissue integrity, while the downregulation of individual protein components in cancer metastasis and wound healing suggests their importance in cell homeostasis. This chapter will provide an update on desmosome composition, function, and regulation, and will also discuss recent work which raises the possibility that desmosome proteins do more than play a structural role in tissues where they reside.

L-MPZ, a novel isoform of myelin P0, is produced by stop codon readthrough

Myelin protein zero (P0 or MPZ) is a major myelin protein (∼30 kDa) expressed in the peripheral nervous system (PNS) in terrestrial vertebrates. Several groups have detected a P0-related 36-kDa (or 35-kDa) protein that is expressed in the PNS as an antigen for the serum IgG of patients with neuropathy. The molecular structure and function of this 36-kDa protein are, however, still unknown. We hypothesized that the 36-kDa protein may be derived from P0 mRNA by stop codon readthrough. We found a highly conserved region after the regular stop codon in predicted sequences from the 3'-UTR of P0 in higher animals. MS of the 36-kDa protein revealed that both P0 peptides and peptides deduced from the P0 3'-UTR sequence were found among the tryptic fragments. In transfected cells and in an in vitro transcription/translation system, the 36-kDa molecule was also produced from the identical mRNA that produced P0. We designated this 36-kDa molecule as large myelin protein zero (L-MPZ), a novel isoform of P0 that contains an additional domain at the C terminus. In the PNS, L-MPZ was localized in compact myelin. In transfected cells, just like P0, L-MPZ was localized at cell-cell adhesion sites in the plasma membrane. These results suggest that L-MPZ produced by the stop codon readthrough mechanism is potentially involved in myelination. Since this is the first finding of stop codon readthrough in a common mammalian protein, detailed analysis of L-MPZ expression will help to understand the mechanism of stop codon readthrough in mammals.

Surface expression of precursor N-cadherin promotes tumor cell invasion

The expression of N-cadherin (NCAD) has been shown to correlate with increased tumor cell motility and metastasis. However, NCAD-mediated adhesion is a robust phenomenon and therefore seems to be inconsistent with the "release" from intercellular adhesion required for invasion. We show that in the most invasive melanoma and brain tumor cells, altered posttranslational processing results in abundant nonadhesive precursor N-cadherin (proNCAD) at the cell surface, although total NCAD levels remain constant. We demonstrate that aberrantly processed proNCAD promotes cell migration and invasion in vitro. Furthermore, in human tumor specimens, we find high levels of proNCAD as well, supporting an overall conclusion that proNCAD and mature NCAD coexist on these tumor cell surfaces and that it is the ratio between these functionally antagonistic moieties that directly correlates with invasion potential. Our work provides insight into what may be a widespread mechanism for invasion and metastasis and challenges the current dogma of the functional roles played by classic cadherins in tumor progression.

Pals1 is a major regulator of the epithelial-like polarization and the extension of the myelin sheath in peripheral nerves

Diameter, organization, and length of the myelin sheath are important determinants of the nerve conduction velocity, but the basic molecular mechanisms that control these parameters are only partially understood. Cell polarization is an essential feature of differentiated cells, and relies on a set of evolutionarily conserved cell polarity proteins. We investigated the molecular nature of myelin sheath polarization in connection with the functional role of the cell polarity protein pals1 (Protein Associated with Lin Seven 1) during peripheral nerve myelin sheath extension. We found that, in regard to epithelial polarity, the Schwann cell outer abaxonal domain represents a basolateral-like domain, while the inner adaxonal domain and Schmidt-Lanterman incisures form an apical-like domain. Silencing of pals1 in myelinating Schwann cells in vivo resulted in a severe reduction of myelin sheath thickness and length. Except for some infoldings, the structure of compact myelin was not fundamentally affected, but cells produced less myelin turns. In addition, pals1 is required for the normal polarized localization of the vesicular markers sec8 and syntaxin4, and for the distribution of E-cadherin and myelin proteins PMP22 and MAG at the plasma membrane. Our data show that the polarity protein pals1 plays an essential role in the radial and longitudinal extension of the myelin sheath, likely involving a functional role in membrane protein trafficking. We conclude that regulation of epithelial-like polarization is a critical determinant of myelin sheath structure and function.

Different cellular and molecular mechanisms for early and late-onset myelin protein zero mutations

Mutations in the gene MPZ, encoding myelin protein zero (MPZ), cause inherited neuropathies collectively called Charcot-Marie-Tooth type 1B (CMT1B). Based on the age of onset, clinical and pathological features, most MPZ mutations are separable into two groups: one causing a severe, early-onset, demyelinating neuropathy and a second, causing a late-onset neuropathy with prominent axonal loss. To investigate potential pathomechanisms underlying the two phenotypes, we transiently transfected HeLa cells with two late-onset (T95M, H10P) and two early-onset (H52R, S22_W28 deletion) mutations and analyzed their effects on intracellular protein trafficking, glycosylation, cell viability and intercellular adhesion. We found that the two late-onset mutations were both transported to the cell membrane and moderately reduced MPZ-mediated intercellular adhesion. The two early-onset mutations caused two distinct abnormalities. H52R was correctly glycosylated and trafficked to the plasma membrane, but strongly affected intercellular adhesion. When co-expressed with wild-type MPZ (wtMPZ), a functional dominant negative effect was observed. Alternatively, S22_W28 deletion was retained within the cytoplasm and reduced both adhesion caused by wtMPZ and cellular viability. Since the same trafficking patterns were observed in transfected murine Schwann cells, they are not an artifact of heterologous cell expression. Our results suggest that at least some late-onset mutations cause a partial loss of function in the transfected cells, whereas multiple abnormal gain of function pathways can result in early-onset neuropathy. Further characterization of these pathways will lead to a better understanding of the pathogenesis of CMT1B and a rational basis for treating these debilitating inherited neuropathies.

Myelin protein zero/P0 phosphorylation and function require an adaptor protein linking it to RACK1 and PKC alpha

Point mutations in the cytoplasmic domain of myelin protein zero (P0; the major myelin protein in the peripheral nervous system) that alter a protein kinase Cα (PKCα) substrate motif (198HRSTK201) or alter serines 199 and/or 204 eliminate P0-mediated adhesion. Mutation in the PKCα substrate motif (R198S) also causes a form of inherited peripheral neuropathy (Charcot Marie Tooth disease [CMT] 1B), indicating that PKCα-mediated phosphorylation of P0 is important for myelination. We have now identified a 65-kD adaptor protein that links P0 with the receptor for activated C kinase 1 (RACK1). The interaction of p65 with P0 maps to residues 179–197 within the cytoplasmic tail of P0. Mutations or deletions that abolish p65 binding reduce P0 phosphorylation and adhesion, which can be rescued by the substitution of serines 199 and 204 with glutamic acid. A mutation in the p65-binding sequence G184R occurs in two families with CMT, and mutation of this residue results in the loss of both p65 binding and adhesion function.

The neurobiology of Schwann cells

This selective review of Schwann cell biology focuses on questions relating to the origins, development and differentiation of Schwann cells and the signals that control these processes. The importance of neuregulins and their receptors in controlling Schwann cell precursor survival and generation of Schwann cells, and the role of these molecules in Schwann cell biology is addressed. The reciprocal signalling between peripheral glial cells and neurons in development and adult life revealed in recent years is highlighted, and the profound change in survival regulation from neuron-dependent Schwann cell precursors to adult Schwann cells that depend on autocrine survival signals is discussed. Besides providing neuronal and autocrine signals, Schwann cells signal to mesenchymal cells and influence the development of the connective tissue sheaths of peripheral nerves. The importance of Desert Hedgehog in this process is described. The control of gene expression during Schwann cell development and differentiation by transcription factors is reviewed. Knockout of Oct-6 and Krox-20 leads to delay or absence of myelination, and these results are related to morphological or physiological observations on knockout or mutation of myelin-related genes. Finally, the relationship between selected extracellular matrix components, integrins and the cytoskeleton is explored and related to disease.

Expression of P0 glycoprotein in CNS glia: Effects of overexpression in N20.1 cells

To examine effects of expression of the PNS myelin P0 glycoprotein in glial cells of CNS lineage, we transfected murine N20.1 glial cells with a rat P0 cDNA. A stably transfected cell line expressing high levels of P0 message showed P0 immunostaining, along with changes in morphology. Polymerase chain reaction (PCR) identified the predicted rat P0 sequence in the transfected N20.1 cells and further revealed low levels of mouse P0 message in the nontransfected cells and in primary mouse astrocytes. This is the first evidence of endogenous expression of message for P0 glycoprotein in CNS glia. Quantitative RT-PCR confirmed the expression of rat P0 mRNA in the transfected N20.1 cells, at levels about 400 times greater than murine P0 in nontransfected cells. A 27-kD band was detected in the transfected cells by Western blot with P0 antibody, but not in mock-transfected or nontransfected N20.1 cells. Immunocytochemistry following permeabilization showed intracellular vesicular localization of P0 in the cytoplasm and perinuclear rings in transfected cells, with a similar pattern but much lower levels in nontransfected cells. Faint surface staining for P0 protein without permeabilization was seen only on the transfected cells. A few transfected cells with membrane sheets stained more intensely for surface P0. Quantitative RT-PCR was used to determine if P0 overexpression altered expression of other myelin-related genes compared with glial fibrillary acidic protein (GFAP); the ratios of myelin basic protein (MBP)/GFAP and proteolipid protein (PLP)/GFAP were increased 2- to 3-fold in the P0-transfected cells. We conclude that P0 overexpression alters N20.1 gene expression and cell morphology, and shifts the cells from astroglial to oligodendroglial phenotype.

The Minimal Essential Unit for Cadherin-mediated Intercellular Adhesion Comprises Extracellular Domains 1 and 2

N-cadherin comprises five homologous extracellular domains, a transmembrane, and a cytoplasmic domain. The extracellular domains of N-cadherin play important roles in homophilic cell adhesion, but the contribution of each domain to this phenomenon has not been fully evaluated. In particular, the following questions remain unanswered: what is the minimal domain combination that can generate cell adhesion, how is domain organization related to adhesive strength, and does the cytoplasmic domain serve to facilitate extracellular domain interaction? To address these issues, we made serial constructs of the extracellular domains of N-cadherin and produced various cell lines to examine adhesion properties. We show that the first domain of N-cadherin alone on the cell surface fails to generate adhesive activity and that the first two domains of N-cadherin form the "minimal essential unit" to mediate cell adhesion. Cell lines expressing longer extracellular domains or N-cadherin wild type cells formed larger cellular aggregates than those expressing shorter aggregates. However, adhesion strength, as measured by a shearing test, did not reveal any differences among these aggregative cell lines, suggesting that the first two domains of N-cadherin cells generate the same strength of adhesive activity as longer extracellular domain cells. Furthermore, truncations of the first two domains of N-cadherin are also sufficient to form cisdimerization at an adhesive junction. Our findings suggest that the extracellular domains of N-cadherin have distinct roles in cell adhesion, i.e. the first two domains are responsible for homophilic adhesion activity, and the other domains promote adhesion efficiency most likely by positioning essential domains relatively far out from the cell surface.

Association of connexin36 with zonula occludens-1 in HeLa cells, betaTC-3 cells, pancreas, and adrenal gland

The PDZ domain-containing protein zonula occludens-1 (ZO-1), a well-established component of tight junctions, has recently been shown to interact with various connexin proteins that form gap junctions. We investigated the association of connexin36 (Cx36) with ZO-1 in various cultured cells and tissues. Punctate immunofluorescence labeling for Cx36 was detected in Cx36-transfected HeLa cells, betaTC-3 cells, pancreatic islets, and adrenal medulla. Immunofluorescence for ZO-1 was also punctate in cells and tissues, and was colocalized with Cx36 at points of cell-cell contact. Immunoprecipitation of either Cx36 or ZO-1 from cell lysates and tissue homogenates resulted in immunoblot detection of ZO-1 or Cx36, respectively, in immunoprecipitates. A 14-amino acid peptide corresponding to the carboxy-terminus of Cx36 showed binding capacity to the PDZ1 domain of ZO-1, which was eliminated after removal of the last 4 carboxy-terminus amino acids. Low micromolar concentrations of the 14-amino acid peptide produced up to 85% inhibition of Cx36 interaction with the PDZ1 domain of ZO-1. These results provide evidence for molecular interaction between Cx36 and ZO-1 in vitro, and in vivo, and suggest that the interference with Cx36/ZO-1 interaction by short carboxy-terminus peptides of Cx36 may be of value for functional studies of this interaction.

Phenotypic differences between peripheral myelin protein-22 (PMP22) and myelin protein zero (P0) mutations associated with Charcot-Marie-Tooth-related diseases

Mutations in the genes for peripheral myelin protein-22 (PMP22) and myelin protein zero (P0) cause human hereditary neuropathies with varying clinical and pathological phenotypes. In this study, we examine the effects of representative disease-causing mutations on the subcellular distribution of their corresponding PMP22- and P0-enhanced green fluorescent protein (EGFP) fusion proteins. In transiently transfected HeLa and 293 cells, we find that wild-type P0-EGFP and PMP22-EGFP are efficiently synthesized and transported through the secretory pathway to the plasma membrane. The P0-EGFP and PMP22-EGFP mutants can be classified into several groups: those that are transported to the plasma membrane as in the majority of P0 mutants; those that are retained in the endoplasmic reticulum as in the majority of PMP22 mutants; and those that are a mixture of the two. In addition, several of these disease-causing mutations are associated with the development of abnormal intracellular cytoplasmic structures that we have previously identified as either intracellular myelin figures or aggresomes. Our studies indicate that different types of PMP22 and P0 mutations are associated with specific intracellular chaperone proteins, including calnexin and BiP, and that these associations can be altered by glycosylation. These findings indicate that the various P0 and PMP22 mutants may exert their pathogenic effects in different subcellular compartments and by different mechanisms in the mammalian cell.

Tight junction proteins

A fundamental function of epithelia and endothelia is to separate different compartments within the organism and to regulate the exchange of substances between them. The tight junction (TJ) constitutes the barrier both to the passage of ions and molecules through the paracellular pathway and to the movement of proteins and lipids between the apical and the basolateral domains of the plasma membrane. In recent years more than 40 different proteins have been discovered to be located at the TJs of epithelia, endothelia and myelinated cells. This unprecedented expansion of information has changed our view of TJs from merely a paracellular barrier to a complex structure involved in signaling cascades that control cell growth and differentiation. Both cortical and transmembrane proteins integrate TJs. Among the former are scaffolding proteins containing PDZ domains, tumor suppressors, transcription factors and proteins involved in vesicle transport. To date two components of the TJ filaments have been identified: occludin and claudin. The latter is a protein family with more than 20 members. Both occludin and claudins are integral proteins capable of interacting adhesively with complementary molecules on adjacent cells and of co-polymerizing laterally. These advancements in the knowledge of the molecular structure of TJ support previous physiological models that exhibited TJ as dynamic structures that present distinct permeability and morphological characteristics in different tissues and in response to changing natural, pathological or experimental conditions.

Mutations in the cytoplasmic domain of P0 reveal a role for PKC-mediated phosphorylation in adhesion and myelination

Mutations in P0 (MPZ), the major myelin protein of the peripheral nervous system, cause the inherited demyelinating neuropathy Charcot-Marie-Tooth disease type 1B. P0 is a member of the immunoglobulin superfamily and functions as a homophilic adhesion molecule. We now show that point mutations in the cytoplasmic domain that modify a PKC target motif (RSTK) or an adjacent serine residue abolish P0 adhesion function and can cause peripheral neuropathy in humans. Consistent with these data, PKCalpha along with the PKC binding protein RACK1 are immunoprecipitated with wild-type P0, and inhibition of PKC activity abolishes P0-mediated adhesion. Point mutations in the RSTK target site that abolish adhesion do not alter the association of PKC with P0; however, deletion of a 14 amino acid region, which includes the RSTK motif, does abolish the association. Thus, the interaction of PKCalpha with the cytoplasmic domain of P0 is independent of specific target residues but is dependent on a nearby sequence. We conclude that PKC-mediated phosphorylation of specific residues within the cytoplasmic domain of P0 is necessary for P0-mediated adhesion, and alteration of this process can cause demyelinating neuropathy in humans.

In early development of the rat mRNA for the major myelin protein P(0) is expressed in nonsensory areas of the embryonic inner ear, notochord, enteric nervous system, and olfactory ensheathing cells

The myelin protein P(0) has a major structural role in Schwann cell myelin, and the expression of P(0) protein and mRNA in the Schwann cell lineage has been extensively documented. We show here, using in situ hybridization, that the P(0) gene is also activated in a number of other tissues during embryonic development. P(0) mRNA is first detectable in 10-day-old embryos (E10) and is at this time seen only in cells in the cephalic neural crest and in the otic placode/pit. P(0) expression continues in the otic vesicle and at E12 P(0) expression in this structure largely overlaps with expression of another myelin gene, proteolipid protein. In the developing ear at E14, P(0) expression is complementary to expression of serrate and c-ret mRNAs, which later are expressed in sensory areas of the inner ear, while expression of bone morphogenetic protein (BMP)-4 and P(0), though largely complementary, shows small areas of overlap. P(0) mRNA and protein are detectable in the notochord from E10 to at least E13. In addition to P(0) expression in a subpopulation of trunk crest cells at E11/E12 and in Schwann cell precursors thereafter, P(0) mRNA is also present transiently in a subpopulation of cells migrating in the enteric neural crest pathway, but is down-regulated in these cells at E14 and thereafter. P(0) is also detected in the placode-derived olfactory ensheathing cells from E13 and is maintained in the adult. No signal is seen in cells in the melanocyte migration pathway or in TUJ1 positive neuronal cells in tissue sections. The activation of the P(0) gene in specific tissues outside the nervous system was unexpected. It remains to be determined whether this is functionally significant, or whether it is an evolutionary relic, perhaps reflecting ancestral use of P(0) as an adhesion molecule.

Rapid functional analysis in Xenopus oocytes of Po protein adhesive interactions

We have developed a coupled Xenopus oocyte expression system for evaluating the functional effects of mutations in known or suspected adhesion molecules, which allows for a very rapid assessment of intercellular adhesion. As a model protein, we first used Protein zero (Po), an adhesion molecule that mediates self-adhesion of the Schwann cell plasma membrane to form compact myelin in the mammalian PNS. A wide variety of mutations in Po cause certain human peripheral neuropathies, such as the Charcot-Marie-Tooth disease (CMT) type 1B and Dejerine-Sottas syndrome (DSS). After wild-type Po mRNA is injected, the protein is synthesized and correctly targeted to the oocyte cell surface. When two oocytes are paired, wild-type Po redistributes and concentrates at the cell-cell apposition region, and by electron microscopy, the oocyte pairs show close cell-cell appositions and are devoid of the microvilli that are observed in uninjected oocyte pairs. These are hallmark features of highly adhesive cell:cell interfaces. Several point mutations in Po were engineered, corresponding to the molecular defects in the CMT type 1B or DSS. The proteins encoded by these mutations reached the cell surface but failed to concentrate at the oocyte interface. Po carrying a point mutation that is found in DSS is not targeted on the plasma membrane and fail to accumulate at the cell-cell contact site.

Truncated DCC reduces N-cadherin/catenin expression and calcium-dependent cell adhesion in neuroblastoma cells

The deleted in colorectal cancer (DCC) protein is important in the pathway guidance of cells and cell processes during neural development, and DCC has also been implicated in the aberrant cellular migrations of neuroblastoma dissemination. We attempted to further define DCC protein function by the overexpression of full-length and truncated DCC constructs in a human neuroblastoma cell line. Overexpression of the truncated DCC protein resulted in a less epithelioid morphology. This was accompanied by decreases in expression of N-cadherin and alpha- and beta-catenin by immunoblot and Northern blot analysis. Levels of desmoglein were relatively less affected, whereas endogenous DCC protein levels were increased in the truncated transfectants. N-cadherin immunofluorescence was consistent with the immunoblot studies and localized the protein to the cytoplasm and sites of cell-cell contact. Cell aggregation studies demonstrated diminished calcium-dependent aggregation in the truncated transfectants. In conclusion, overexpression of a truncated DCC protein in neuroblastoma cells resulted in the loss of an epithelioid morphology, diminished expression of N-cadherin and alpha- and beta-catenin, and diminished calcium-dependent cell adhesion. These studies provide the first evidence of an apparent functional link between DCC and N-cadherin/catenin-dependent cell adhesion.

Are desmosomes more than tethers for intermediate filaments?

Desmosomes are intercellular adhesive junctions that anchor intermediate filaments at membrane-associated plaques in adjoining cells, thereby forming a three-dimensional supracellular scaffolding that provides tissues with mechanical strength. But desmosomes have also recently been recognized as sensors that respond to environmental and cellular cues by modulating their assembly state and, possibly, their signalling functions.

Interaction of junctional adhesion molecule with the tight junction components ZO-1, cingulin, and occludin

Junctional adhesion molecule (JAM) is an integral membrane protein that has been reported to colocalize with the tight junction molecules occludin, ZO-1, and cingulin. However, evidence for the association of JAM with these molecules is missing. Transfection of Chinese hamster ovary cells with JAM (either alone or in combination with occludin) resulted in enhanced junctional localization of both endogenous ZO-1 and cotransfected occludin. Additionally, JAM was coprecipitated with ZO-1 in the detergent-insoluble fraction of Caco-2 epithelial cells. A putative PDZ-binding motif at the cytoplasmic carboxyl terminus of JAM was required for mediating the interaction of JAM with ZO-1, as assessed by in vitro binding and coprecipitation experiments. JAM was also coprecipitated with cingulin, another cytoplasmic component of tight junctions, and this association required the amino-terminal globular head of cingulin. Taken together, these data indicate that JAM is a component of the multiprotein complex of tight junctions, which may facilitate junction assembly.

Plakoglobin induces desmosome formation and epidermoid phenotype in N-cadherin-expressing squamous carcinoma cells deficient in plakoglobin and E-cadherin

Pg is a homologue of beta-catenin and Armadillo, the product of the Drosophila segment polarity gene and has been shown to have both adhesive and signaling functions. It interacts with both classic and desmosomal cadherins. Pg interaction with the desmosomal cadherins is essential for desmosome assembly. Its precise role in the classic cadherin complexes is unclear, although Pg-E-cadherin interaction appears to be necessary for the formation of desmosomes. In addition to cadherins in adhesion complexes, Pg interacts with a number of proteins involved in regulation of cell differentiation and proliferation such as Lef-1/Tcf-1 transcription factors and the tumor suppressor protein APC. In this study, we have introduced Pg cDNA into SCC9 cells, a Pg- and E-cadherin-deficient squamous cell carcinoma line, which also lacks desmosomes. These cells have both alpha-catenin and beta-catenin, display unusual expression of N-cadherin, and have the typical fibroblastic phenotype of transformed cells. Pg-expressing SCC9 cells (SCC9P) formed desmosomes. Desmosome formation coincided with the appearance of an epidermoid phenotype, with increased adhesiveness and a contact-dependent decrease in growth. Biochemical characterization of SCC9P cells showed an increase in the expression and stability of N-cadherin and a decrease in level and stability of beta-catenin, without any apparent effects on alpha-catenin. These results show that, in the absence of E-cadherin, Pg can efficiently use N-cadherin to induce desmosome formation and epidermoid phenotype. They also suggest a role for Pg as one of the regulators of the intracellular beta-catenin levels and underscore the pivotal role of this protein in regulating cell adhesion and differentiation.

Absence of P0 leads to the dysregulation of myelin gene expression and myelin morphogenesis

P0, the major peripheral nervous system (PNS) myelin protein, is a member of the immunoglobulin supergene family of membrane proteins and can mediate homotypic adhesion. P0 is an essential structural component of PNS myelin; mice in which P0 expression has been eliminated by homologous recombination (P0-/-) develop a severe dysmyelinating neuropathy with predominantly uncompacted myelin. Although P0 is thought to play a role in myelin compaction by promoting adhesion between adjacent extracellular myelin wraps, as an adhesion molecule it could also have a regulatory function. Consistent with this hypothesis, Schwann cells in adult P0-/- mice display a novel molecular phenotype: PMP22 expression is down-regulated, MAG and PLP expression are up-regulated, and MBP expression is unchanged. As in quaking viable mutant mice (qk(v)), which have uncompacted myelin morphologically similar to that found in P0-/- mice, neither the qKI-6 or qKI-7 proteins are expressed in P0-/- peripheral nerve. In addition to these changes in gene expression in the P0 knockout, PLP/DM-20 accumulates in the endoplasmic reticulum of P0-/- Schwann cells, whereas MAG accumulates in redundant loops of uncompacted myelin, not at nodes of Ranvier or Schmidt-Lantermann incisures. Taken together, these results demonstrate that P0 is involved, either directly or indirectly, in the regulation of both myelin gene expression and myelin morphogenesis.

Independent regulation of adherens and tight junctions by tyrosine phosphorylation in Caco-2 cells

To study the role of tyrosine phosphorylation in the control of intercellular adhesion of intestinal cells, we have generated several clones of Caco-2 cells that express high levels of pp60v-src only after addition of butyrate. Expression of this oncogene in cells 5 days after confluence induced beta-catenin and p120-ctn tyrosine phosphorylation, redistribution of E-cadherin to the cytosol and disassembly of adherens junctions. However, tight junctions of Caco-2 cells at 5 days after confluence were not altered by expression of pp60v-src. Similar results were obtained when Caco-2 cells were incubated with phosphotyrosine phosphatase inhibitor orthovanadate. Although addition of this compound to postconfluent cells disrupt adherens junctions, tight junctions remain unaltered, as determined measuring monolayer permeability to mannitol or hyperphosphorylation of Triton-insoluble occludin. Modifications in tight junction permeability of Caco-2 were only observed at high concentrations of orthovanadate (1 mM). Interestingly, this tyrosine phosphorylation-refractory state was achieved after confluence since early postconfluent cells (day 2) showed a limited but significant response to low doses of orthovanadate. These results suggest that tight junctions of differentiated Caco-2 cells are uncoupled from adherens junctions and are insensitive to regulation by tyrosine phosphorylation.

Peripheral myelin protein 22 and protein zero: a novel association in peripheral nervous system myelin

Mutations found in the two major glycosylated transmembrane proteins of the PNS myelin, the peripheral myelin protein zero (P0) and peripheral myelin protein 22 (PMP22), have been independently associated with the most common hereditary demyelinating peripheral neuropathies. Genotype-phenotype correlations in humans and transgenic animals have provided functional evidence that P0 and PMP22 are involved in formation and maintenance of compact myelin. Here, we demonstrate for the first time that P0 and PMP22 proteins form complexes in the myelin membrane, as shown by coimmunoprecipitation experiments, and that glycosylation is not involved in mediating these interactions. Complex formation was also detected when the two proteins were coexpressed in heterologous cells. In transfected cells, P0 and PMP22 are recruited and colocalize at the apposed plasma membranes of expressors as shown by confocal microscopy. These findings provide a new basis for a better understanding of myelin assembly and of the pathomechanisms involved in demyelinating peripheral neuropathies. Furthermore, these results propose a possible explanation why alterations in either of these molecules are sufficient to destabilize the myelin structure and cause a similar disease phenotype.

Peripheral myelin protein 22 and protein zero: a novel association in peripheral nervous system myelin

Mutations found in the two major glycosylated transmembrane proteins of the PNS myelin, the peripheral myelin protein zero (P0) and peripheral myelin protein 22 (PMP22), have been independently associated with the most common hereditary demyelinating peripheral neuropathies. Genotype-phenotype correlations in humans and transgenic animals have provided functional evidence that P0 and PMP22 are involved in formation and maintenance of compact myelin. Here, we demonstrate for the first time that P0 and PMP22 proteins form complexes in the myelin membrane, as shown by coimmunoprecipitation experiments, and that glycosylation is not involved in mediating these interactions. Complex formation was also detected when the two proteins were coexpressed in heterologous cells. In transfected cells, P0 and PMP22 are recruited and colocalize at the apposed plasma membranes of expressors as shown by confocal microscopy. These findings provide a new basis for a better understanding of myelin assembly and of the pathomechanisms involved in demyelinating peripheral neuropathies. Furthermore, these results propose a possible explanation why alterations in either of these molecules are sufficient to destabilize the myelin structure and cause a similar disease phenotype.

The structural analysis of adhesions mediated by Ep-CAM

The epithelial cell adhesion molecule Ep-CAM is capable of mediating Ca2+-independent homotypic cell-cell adhesion when introduced into cells lacking their own means of cell-cell interactions. We used (confocal) immunofluorescent and (immuno-) electron microscopy to investigate the structural organization of Ep-CAM-mediated adhesions and their relation to other types of intercellular adhesions. Ep-CAM-transfected cell lines, cells of epithelial origin, and epithelial tissues were analyzed. In transfected L cells Ep-CAM brings the opposing intercellular membranes into a close proximity (approximately 10-14 nm) at sporadic contacts; however, no structures resembling junctional complexes were observed. In L cells cotransfected with Ep-CAM and E-cadherin, both molecules localize at the sites of cell-cell contact, forming independent adhesion sites with no Ep-CAM detectable within the structurally distinguishable cadherin-mediated adherens junctions. In well-differentiated carcinoma cell lines Ep-CAM colocalized with E-cadherin practically along the whole lateral domain; however, no colocalization was observed between Ep-CAM and the components of the tight junction complex (occludin and ZO-1), desmosomes (desmoplakins I/II), or cell-substrate adhesions (beta1 integrins). This was confirmed by analysis of polarized epithelium of normal colon where Ep-CAM was present at the lateral membrane including the adherens junction areas, but was fully excluded from the apical cell membrane (microvilli), tight junctions, and desmosomes. We conclude that (1) Ep-CAM does not form junctional complexes in L cells, (2) in epithelial cells, cell surface Ep-CAM is present at the lateral cell membrane, but is excluded from tight junctions and desmosomes, and (3) in epithelial cells, Ep-CAM is present within adhesions mediated by the classic cadherins (especially E-cadherin) with both types of molecules remaining as independent clusters. The colocalization with cadherins might be important for the modulating effect of Ep-CAM on cadherin-mediated adhesions.

Suppression of tumorigenicity in an aggressive cervical carcinoma induced by protein zero, a nervous system IgCAM

In mammals, protein zero (P0), a neural IgCAM, is expressed solely in the peripheral nervous system where it mediates self-adhesion of Schwann cell membranes as compact myelin is generated. We show that when P0 is expressed in HeLa, a cervical carcinoma cell line, cells regain adhesion-mediated growth control, including the acquisition of contact inhibition and loss of anchorage-independent growth. Additionally, P0-expressing HeLa cells lose the ability to invade an artificial matrix, which correlates with decreased secretion of matrix-degrading enzymes. Lastly, and of great interest, unlike the aggressively metastatic cell line from which they were derived, P0-HeLa cells are neither tumorigenic nor metastatic when injected into athymic nude mice. By all these criteria, P0 expression appears to efficiently suppress in the long term, the transformed state of this carcinoma cell line. N-cadherin and its intracellular partners plakoglobin, alpha- and beta-catenin were significantly upregulated in the P0-HeLa cells. It appears therefore that P0 induces epithelialization and suppression of tumorigenicity in HeLa through the activation of the cadherin/catenin signaling systems. We conclude that the forced expression of bona fide adhesion molecules, such as P0, may serve as ‘upstream’ inducers of an essentially dormant but undamaged adhesion program in carcinoma cells that ultimately triggers the re-acquisition of normal epithelial characteristics, thereby suppressing tumorigenicity. Therapeutically, it may be that intercellular adhesion, no matter how it is induced, may serve as a single master event that is able to induce reversion of the carcinomatous state.

Protein zero, a myelin IgCAM, induces physiologically operative tight junctions in nonadhesive carcinoma cells

In the peripheral nervous system, protein zero (P0), a homophilic immunoglubulin cell adhesion molecule, mediates adhesion of Schwann cell membranes as they enwrap axons and generate compact myelin. Although P0 is naturally only expressed in peripheral myelin, it can behave as a vigorous adhesion molecule in a variety of cell types (Filbin et al. [1990] Nature 344:871-872; Schneider-Schaulies et al. [1990] J Neurosci Res 27:286-297; Doyle et al. [1995] J Cell Biol 131:465-482) and can thus be characterized as an obligatory adhesion molecule. Previously, we showed that when HeLa, a cervical carcinoma cell line devoid of epithelial junctions, is forced to express P0, strong cell-cell adhesion is induced, proteins associated with junctional elements are upregulated, and ultrastructurally tight junctions, adherens junctions, and desmosomes become apparent (Doyle et al., 1995). In this report, we assessed whether the tight junctions were physiologically operative in P0 HeLa expressors. Consistent with the presence of operative tight junctions, we found that P0 expressors in monolayers maintained endogenous proteins in their apical and basolateral plasma membrane subdomains. Furthermore, these cells generated a higher transepithelial resistance than did control HeLa cells, which is indicative of the formation of an effective intercellular permeability barrier.

Myelin protein zero and membrane adhesion

Protein zero (P0) is a member of the immunoglobulin gene superfamily (IgCAM) that is expressed at high levels in myelinated vertebrates in central (fish and amphibia) and peripheral (all species) myelin. This glycoprotein is the major adhesive component of peripheral myelin, where it mediates self-adhesion of the Schwann cell plasma membrane. Although the expression of P0 is naturally limited to Schwann cells, the molecular mechanisms of P0-mediated adhesion can be considered general and "obligatory" because, when expressed in a variety of cell lines, P0 induces strong intercellular adhesion. Modeling studies, X-ray crystallographic analysis, and experimental site-directed mutagenesis have provided excellent working models for understanding how P0 mediates adhesion at the atomic level. These models remain to be experimentally tested. However, in humans, certain mutations in P0 produce dysmyelinating disease, possibly due to disruptions in the predicted P0 lattice.

Tissue architecture: the ultimate regulator of epithelial function?

The architecture of a tissue is defined by the nature and the integrity of its cellular and extracellular compartments, and is based on proper adhesive cell-cell and cell-extracellular matrix interactions. Cadherins and integrins are major adhesion-mediators that assemble epithelial cells together laterally and attach them basally to a subepithelial basement membrane, respectively. Because cell adhesion complexes are linked to the cytoskeleton and to the cellular signalling pathways, they represent checkpoints for regulation of cell shape and gene expression and thus are instructive for cell behaviour and function. This organization allows a reciprocal flow of mechanical and biochemical information between the cell and its microenvironment, and necessitates that cells actively maintain a state of homeostasis within a given tissue context. The loss of the ability of tumour cells to establish correct adhesive interactions with their microenvironment results in disruption of tissue architecture with often fatal consequences for the host organism. This review discusses the role of cell adhesion in the maintenance of tissue structure and analyses how tissue structure regulates epithelial function.

Epithelial cell adhesion molecule (Ep-CAM) modulates cell-cell interactions mediated by classic cadherins

The contribution of noncadherin-type, Ca2+-independent cell-cell adhesion molecules to the organization of epithelial tissues is, as yet, unclear. A homophilic, epithelial Ca2+-independent adhesion molecule (Ep-CAM) is expressed in most epithelia, benign or malignant proliferative lesions, or during embryogenesis. Here we demonstrate that ectopic Ep-CAM, when expressed in cells interconnected by classic cadherins (E- or N-cadherin), induces segregation of the transfectants from the parental cell type in coaggregation assays and in cultured mixed aggregates, respectively. In the latter assay, Ep-CAM-positive transfectants behave like cells with a decreased strength of cell-cell adhesion as compared to the parental cells. Using transfectants with an inducible Ep-CAM-cDNA construct, we demonstrate that increasing expression of Ep-CAM in cadherin-positive cells leads to the gradual abrogation of adherens junctions. Overexpression of Ep-CAM has no influence on the total amount of cellular cadherin, but affects the interaction of cadherins with the cytoskeleton since a substantial decrease in the detergent-insoluble fraction of cadherin molecules was observed. Similarly, the detergent-insoluble fractions of alpha- and beta-catenins decreased in cells overexpressing Ep-CAM. While the total beta-catenin content remains unchanged, a reduction in total cellular alpha-catenin is observed as Ep-CAM expression increases. As the cadherin-mediated cell-cell adhesions diminish, Ep-CAM-mediated intercellular connections become predominant. An adhesion-defective mutant of Ep-CAM lacking the cytoplasmic domain has no effect on the cadherin-mediated cell-cell adhesions. The ability of Ep-CAM to modulate the cadherin-mediated cell-cell interactions, as demonstrated in the present study, suggests a role for this molecule in development of the proliferative, and probably malignant, phenotype of epithelial cells, since an increase of Ep-CAM expression was observed in vivo in association with hyperplastic and malignant proliferation of epithelial cells.

Border controls at the mammalian spinal cord: late-surviving neural crest boundary cap cells at dorsal root entry sites may regulate sensory afferent ingrowth and entry zone morphogenesis

Boundary caps (BCs) form when neural crest cells, migrating ventrally alongside the neural tube, arrest at sites where axons will enter and exit. However, nothing is known of their subsequent fate and functions. We have found late-surviving neural crest BC cell clusters at proximal dorsal root entry sites throughout rat spinal cord development. Sensory afferents cross BCs to enter the spinal cord, while exiting astrocyte processes, destined to form the dorsal root entry zone (DREZ) after birth, are temporarily stalled in their vicinity. To test whether contact with BC cells influences neurite outgrowth from dorsal root ganglia, neurons were cultured on embryonic dorsal root/spinal cord cryosections. Neurites that entered CNS territory preferentially extended over BC cells. Thus, BC cells could be instrumental in regulating afferent ingrowth and DREZ morphogenesis in mammalian spinal cord development.

Ins and outs of peripheral myelin protein-22: mapping transmembrane topology and intracellular sorting

Molecular genetic studies have shown that the peripheral myelin protein 22 (PMP22) is a key gene in hereditary peripheral neuropathies and appears to be essential for the formation and maintenance of myelin in the PNS. Based on the amino acid sequence the predicted structure of PMP22 protein contains two major distinct hydrophilic regions and four transmembrane domains. To analyze the cellular localization and membrane topology of PMP22 we inserted an octapeptide tag-sequence at the amino or at the carboxyl terminus of the PMP22 open reading frame and generated different chimeric constructs which were expressed in HeLa cells. The expression of the tagged PMP22 protein and its orientation with respect to the plasma membrane were analyzed using antibodies raised against specific PMP22 epitopes and the tag sequence. Combined indirect, double-immunofluorescence labeling and confocal microscopy showed that PMP22 is synthesized in the rough endoplasmic reticulum of transfected cells and passes through the Golgi apparatus to the cell surface. We determined the transmembrane organization of PMP22 providing the first experimental evidence that confirms the cytoplasmic disposition of its N and C termini and the extracellular localization of the two hydrophilic domains containing amino acids 28-40 and 118-131. This study provides the basis for further analysis aimed to identify functional domains of wild-type PMP22 and the cellular sorting of mutant forms of PMP22.

The epithelial cell default-phenotype hypothesis and its implications for cancer

The expression of epithelial cell adhesion and cytoskeletal genes is orchestrated by an apparently unique set of rules. No tissue-specific transactivator proteins have been found to drive them; only ubiquitous factors are utilized. In non-epithelial cells, they are actively repressed. Moreover, it was recently found that a single protein (adenovirus E1a) coordinately represses non-epithelial genes while inducing epithelial genes. A simple model is offered to explain how epithelial gene expression is coordinated. Under this model, the epithelial cell gene expression program is a transcriptional 'default'; that is, it occurs in the absence of tissue-specific transactivation. Conversion to this default requires only that mesenchymal transactivators are not expressed, or that central 'integrator' proteins are inactive. In their absence, mesenchymal gene expression cannot occur. Moreover, because the repressors cease to be expressed, the epithelial genes are induced. Oncogenes generally cause the breakdown of the epithelial phenotype--generating carcinomas--so genes such as E1a that cause epithelial conversion may prove useful for both understanding and controlling cancer.

Protein kinase C activation upregulates intercellular adhesion of alpha-catenin-negative human colon cancer cell variants via induction of desmosomes

The alpha-catenin molecule links E-cadherin/ beta-catenin or E-cadherin/plakoglobin complexes to the actin cytoskeleton. We studied several invasive human colon carcinoma cell lines lacking alpha-catenin. They showed a solitary and rounded morphotype that correlated with increased invasiveness. These round cell variants acquired a more normal epithelial phenotype upon transfection with an alpha-catenin expression plasmid, but also upon treatment with the protein kinase C (PKC) activator 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Video registrations showed that the cells started to establish elaborated intercellular junctions within 30 min after addition of TPA. Interestingly, this normalizing TPA effect was not associated with alpha-catenin induction. Classical and confocal immunofluorescence showed only minor TPA-induced changes in E-cadherin staining. In contrast, desmosomal and tight junctional proteins were dramatically rearranged, with a conversion from cytoplasmic clusters to obvious concentration at cell-cell contacts and exposition at the exterior cell surface. Electron microscopical observations revealed the TPA-induced appearance of typical desmosomal plaques. TPA-restored cell-cell adhesion was E-cadherin dependent as demonstrated by a blocking antibody in a cell aggregation assay. Addition of an antibody against the extracellular part of desmoglein-2 blocked the TPA effect, too. Remarkably, the combination of anti-E-cadherin and anti-desmoglein antibodies synergistically inhibited the TPA effect. Our studies show that it is possible to bypass the need for normal alpha-catenin expression to establish tight intercellular adhesion by epithelial cells. Apparently, the underlying mechanism comprises upregulation of desmosomes and tight junctions by activation of the PKC signaling pathway, whereas E-cadherin remains essential for basic cell-cell adhesion, even in the absence of alpha-catenin.

Reversion of the malignant phenotype of human breast cells in three-dimensional culture and in vivo by integrin blocking antibodies

In a recently developed human breast cancer model, treatment of tumor cells in a 3-dimensional culture with inhibitory beta1-integrin antibody or its Fab fragments led to a striking morphological and functional reversion to a normal phenotype. A stimulatory beta1-integrin antibody proved to be ineffective. The newly formed reverted acini re-assembled a basement membrane and re-established E-cadherin-catenin complexes, and re-organized their cytoskeletons. At the same time they downregulated cyclin D1, upregulated p21(cip,wat-1), and stopped growing. Tumor cells treated with the same antibody and injected into nude mice had significantly reduced number and size of tumors in nude mice. The tissue distribution of other integrins was also normalized, suggesting the existence of intimate interactions between the different integrin pathways as well as adherens junctions. On the other hand, nonmalignant cells when treated with either alpha6 or beta4 function altering antibodies continued to grow, and had disorganized colony morphologies resembling the untreated tumor colonies. This shows a significant role of the alpha6/beta4 heterodimer in directing polarity and tissue structure. The observed phenotypes were reversible when the cells were disassociated and the antibodies removed. Our results illustrate that the extracellular matrix and its receptors dictate the phenotype of mammary epithelial cells, and thus in this model system the tissue phenotype is dominant over the cellular genotype.

Studies on the effects of altered PMP22 expression during myelination in vitro

Severe inherited dysmyelinating diseases of the peripheral nervous system, the Charcot-Marie-Tooth type1A disease (CMT1A) and the hereditary neuropathy with liability to pressure palsies (HNPP) are associated with a large DNA duplication or deletion of a chromosomal region containing the peripheral myelin protein 22 (PMP22) gene. It has been suggested that a gene dosage effect involving PMP22 is responsible for the pathological phenotype. We investigated if altered PMP22 expression affects the onset of myelin formation and the ultrastructure of myelin. Rat Schwann cell cultures were stably infected with recombinant retrovirus vectors harboring the rat PMP22 cDNA in sense or antisense orientation. Schwann cells over- or underexpressing PMP22 were cocultured with purified DRG neurons under conditions that promote myelination. We examined PMP22 expression and localization in the myelin forming cultures by RT-PCR, immunohistochemistry and confocal microscopy, and we analyzed myelin ultrastructure by electron microscopy. Our results demonstrate that abnormal levels of PMP22 expression do not impair the early stages of myelination and membrane compaction and do not interfere with the expression of other myelin genes. Our observations further indicate that PMP22 is involved more in controlling myelin thickness and stability than in the events determining the initial steps of myelin formation.

Desmosomes: differentiation, development, dynamics and disease

Recent evidence on the distribution of desmosomal glycoprotein isoforms that shows their combined expression in individual desmosomes has strengthened the belief that the latter are involved in epithelial differentiation and morphogenesis. It has been shown that cellular interactions and protein kinase C can modulate the adhesive properties of desmosomes in epithelial cell sheets. Genetic studies indicate the involvement of desmosomal components in cancer and epidermal diseases.

Dominant-negative effect on adhesion by myelin Po protein truncated in its cytoplasmic domain

The myelin Po protein is believed to hold myelin together via interactions of both its extracellular and cytoplasmic domains. We have already shown that the extracellular domains of Po can interact in a homophilic manner (Filbin, M.T., F.S. Walsh, B.D. Trapp, J.A. Pizzey, and G.I. Tennekoon. 1990. Nature (Lond.). 344:871-872). In addition, we have shown that for this homophilic adhesion to take place, the cytoplasmic domain of Po must be intact and most likely interacting with the cytoskeleton; Po proteins truncated in their cytoplasmic domains are not adhesive (Wong, M.H., and M.T. Filbin, 1994. J. Cell Biol. 126:1089-1097). To determine if the presence of these truncated forms of Po could have an effect on the functioning of the full-length Po, we coexpressed both molecules in CHO cells. The adhesiveness of CHO cells expressing both full-length Po and truncated Po was then compared to cells expressing only full-length Po. In these coexpressors, both the full-length and the truncated Po proteins were glycosylated. They reached the surface of the cell in approximately equal amounts as shown by an ELISA and surface labeling, followed by immunoprecipitation. Furthermore, the amount of full-length Po at the cell surface was equivalent to other cell lines expressing only full-length Po that we had already shown to be adhesive. Therefore, there should be sufficient levels of full-length Po at the surface of these coexpressors to measure adhesion of Po. However, as assessed by an aggregation assay, the coexpressors were not adhesive. By 60 min they had not formed large aggregates and were indistinguishable from the control transfected cells not expressing Po. In contrast, in the same time, the cells expressing only the full-length Po had formed large aggregates. This indicates that the truncated forms of Po have a dominant-negative effect on the adhesiveness of the full-length Po. Furthermore, from cross-linking studies, full-length Po, when expressed alone but not when coexpressed with truncated Po, appears to cluster in the membrane. We suggest that truncated Po exerts its dominant-negative effect by preventing clustering of full-length Po. We also show that colchicine, which disrupts microtubules, prevents adhesion of cells expressing only the full-length Po. This strengthens our suggestion that an interaction of Po with the cytoskeleton, either directly or indirectly, is required for adhesion to take place.

Crystal structure of the extracellular domain from P0, the major structural protein of peripheral nerve myelin

P0, the major protein of peripheral nerve myelin, mediates membrane adhesion in the spiral wraps of the myelin sheath. We have determined the crystal structure of the extracellular domain from P0 (P0ex) at 1.9 A resolution. P0ex is folded like a typical immunoglobulin variable-like domain; five residues at the C-terminus are disordered, suggesting a flexible linkage to the membrane. The requirements for crystallization of P0ex are similar to those for maintaining the native extracellular spacing of adjacent myelin lamellae; thus, given the self-adhesive character of P0ex, the crystal itself may reveal some of the natural interactions that occur between P0 molecules in myelin. The structure leads to the suggestion that P0 extracellular domains may emanate from the membrane surface as tetramers that link to tetramers on the opposing membrane surface, to result in the formation of networks of molecules. We report analytical ultracentrifugation data for P0ex that support this idea.

Parallel evolution and coexpression of the proteolipid proteins and protein zero in vertebrate myelin

Vertebrate myelin contains two proteins that mediate compaction: protein zero (P0), an immunoglobulin gene superfamily member, or proteolipid proteins, 4-hydrophobic domain-motif proteins biogenetically unrelated to P0. The prevailing view has been that expression of P0 and proteolipid proteins is mutually exclusive; P0, which mediates myelin compaction in fish, is thought to be completely replaced by the newer proteolipid proteins in the terrestrial vertebrate CNS. However, we now find that proteolipid proteins are actually major myelin constituents in bony fish and amphibia, and so are coexpressed with P0. Clearly, myelin proteolipids are not new additions to the myelin protein repertoire, but instead were ancestral sheath components, expressed approximately 440 million years ago in the first myelinated fish that existed at least approximately 100 million years before the origin of amphibians. In conclusion, P0 and the proteolipid proteins are evolving in parallel in myelinating cells of most vertebrate species.