From sticky zippers to morphology

Hyper-adhesion: a unique property of desmosomes

Hyper-adhesion is a unique, strongly adhesive form of desmosomal adhesion that functions to maintain tissue integrity. In this short review, we define hyper-adhesion, summarise the evidence for it in culture and in vivo, discuss its role in development, wound healing, and skin disease, and speculate about its molecular and cellular basis.

Desmosomes in vivo

The structure, function, and regulation of desmosomal adhesion in vivo are discussed. Most desmosomes in tissues exhibit calcium-independent adhesion, which is strongly adhesive or “hyperadhesive”. This is fundamental to tissue strength. Almost all studies in culture are done on weakly adhesive, calcium-dependent desmosomes, although hyperadhesion can be readily obtained in confluent cell culture. Calcium dependence is a default condition in vivo, found in wounds and embryonic development. Hyperadhesion appears to be associated with an ordered arrangement of the extracellular domains of the desmosomal cadherins, which gives rise to the intercellular midline identified in ultrastructural studies. This in turn probably depends on molecular order in the desmosomal plaque. Protein kinase C downregulates hyperadhesion and there is preliminary evidence that it may also be regulated by tyrosine kinases. Downregulation of desmosomes in vivo may occur by internalisation of whole desmosomes rather than disassembly. Hyperadhesion has implications for diseases such as pemphigus.

PEP-19, an intrinsically disordered regulator of calmodulin signaling

PEP-19 is a small calmodulin (CaM)-binding protein that greatly increases the rates of association and dissociation of Ca(2+) from the C-domain of CaM, an effect that is mediated by an acidic/IQ sequence in PEP-19. We show here using NMR that PEP-19 is an intrinsically disordered protein, but with residual structure localized to its acidic/IQ motif. We also show that the k(on) and k(off) rates for binding PEP-19 to apo-CaM are at least 50-fold slower than for binding to Ca(2+)-CaM. These data indicate that intrinsic disorder confers plasticity that allows PEP-19 to bind to either apo- or Ca(2+)-CaM via different structural modes, and that complex formation may be facilitated by conformational selection of residual structure in the acidic/IQ sequence.

Hyper-adhesion: a new concept in cell–cell adhesion

We have developed a new concept of cell–cell adhesion termed ‘hyper-adhesion’, the very strong adhesion adopted by desmosomes. This uniquely desmosomal property accounts for their ability to provide the intercellular links in the desmosome–intermediate filament complex. These links are targeted by diseases, resulting in disruption of the complex with severe consequences. Hyper-adhesion is characteristic of desmosomes in tissues and is believed to result from a highly ordered arrangement of the extracellular domains of the desmosomal cadherins that locks their binding interaction so that it is highly resistant to disruption. This ordered arrangement may be reflected by and dependent upon a similarly ordered molecular structure of the desmosomal plaque. Hyper-adhesion can be down-regulated to a more weakly adhesive state by cell signalling involving protein kinase C, which translocates to the desmosomal plaque. Down-regulation takes place in wound edge epithelium and appears to be accompanied by loss of the ordered arrangement causing desmosomes to adopt the type of weaker adhesion characteristic of adherens junctions. We review the evidence for hyper-adhesion and speculate on the molecular basis of its mechanism.

Hyper-adhesion in desmosomes: its regulation in wound healing and possible relationship to cadherin crystal structure

The resistance of tissues to physical stress is dependent upon strong cell-cell adhesion in which desmosomes play a crucial role. We propose that desmosomes fulfil this function by adopting a more strongly adhesive state, hyper-adhesion, than other junctions. We show that the hyper-adhesive desmosomes in epidermis resist disruption by ethylene glycol bis(2-aminoethyl ether)-N,N,N′N′-tetraacetic acid (EGTA) and are thus independent of Ca2+. We propose that Ca2+ independence is the normal condition for tissue desmosomes. Ca2+ independence is associated with an organised arrangement of the intercellular adhesive material exemplified by a dense midline. When epidermis is wounded, desmosomes in the wound-edge epithelium lose hyper-adhesiveness and become Ca2+ dependent, i.e. readily dissociated by EGTA. Ca2+-dependent desmosomes lack a midline and show narrowing of the intercellular space. We suggest that this indicates a less-organised, weakly adhesive arrangement of the desmosomal cadherins, resembling classical cadherins in adherens junctions. Transition to Ca2+ dependence on wounding is accompanied by relocalisation of protein kinase C α to desmosomal plaques suggesting that an `inside-out' transmembrane signal is responsible for changing desmosomal adhesiveness. We model hyper-adhesive desmosomes using the crystal packing observed for the ectodomain of C-cadherin and show how the regularity of this 3D array provides a possible explanation for Ca2+ independence.

Aspects of the structure and assembly of desmosomes

Desmosomes are found principally in epithelial cells and consist of disc-like plaques, the extracellular face of which is paired with that of a neighbouring cell. There is increasing evidence that desmosomes are adhesive structures, and that two types of desmosomal glycoproteins, the desmogleins (Dsg) and desmocollins (Dsc) both Ca(2+)-binding cadherin-like molecules, perform this role in adhesion through interaction of their extracellular domains. A number of isoforms of Dsg and Dsc are present in specific tissues. The cytoplasmic side of the plaque is attached to intermediate filaments through desmoplakin, a major plaque protein. Also associated with desmosomes are plakoglobin and beta-catenin, suggesting that the adhesive function of desmosomes might be mediated by signal transduction. Formation of desmosomes can be studied by growing epithelial cells in low-Ca2+ medium (LCM, < 0.1 mM), where desmosomal proteins are either synthesized but not assembled, or form partially assembled but unstable half-desmosomes. Addition of Ca2+ (to about 2mM) initiates cell contact and, in the case of half-desmosomes, leads to stabilization by incorporation into membranes and formation of typical paired structures. In cases where such pre-assembled structures are not formed, recruitment of desmosomal proteins appears to occur by vesicular transport of desmocollins and desmogleins to the cell surface, where association is made with plakoglobin and later, with desmoplakin. Although much remains to be learned of the assembly process, specific interacting domains of the molecular components are being recognized. Desmosome assembly is part of a coordinated pattern of junction formation which accompanies the establishment of cell polarity, resulting in differentiation of apical and basolateral cell surfaces. Desmosomes are now being regarded, not as static and inert structures, but as membrane specializations linked to systems involved in cell-cell communication as well as adhesion.

Dual sets of chimeric alleles identify specificity sequences for the bE and bW mating and pathogenicity genes of Ustilago maydis

The b mating-type locus of the fungal plant pathogen Ustilago maydis encodes two multiallelic gene products, bE and bW, that control the formation and maintenance of the infectious cell type. Dimerization via the N-terminal regions of bE and bW proteins encoded by alleles of different specificities establishes a homeodomain-containing transcription factor. The bE and bW products encoded by alleles of like specificities fail to dimerize. We constructed sets of chimeric alleles for the bE1 and bE2 genes and for the bW1 and bW2 genes to identify sequences that control specificity. The mating behavior of strains carrying chimeric alleles identified three classes of specificity: b2 (class I), specificity different from either parental type (class II), and b1 (class III). Crosses between strains carrying bE and bW chimeric alleles identified two short blocks of amino acids that influence specificity and that are located in the N-terminal variable regions of the b proteins. Comparisons of pairs of chimeric alleles encoding polypeptides differing in specificity and differing at single amino acid positions identified 16 codon positions that influence the interaction between bE and bW. Fifteen of these positions lie within the blocks of amino acids identified by crosses between the strains carrying chimeric alleles. Overall, this work provides insight into the organization of the regions that control recognition.