CELL ADHESION
Knowing when to stick together. A bodily organ is like a football quarterback: no matter how good it is at carrying out its assigned task, its efforts are for naught if there is no protection. That is why there are epithelia and offensive lines. In virtually every organ, from the eye to the intestine, a protective layer called an epithelium protects the inside of the organ from the brutal effects of the outside world. When this protection breaks down, the effects can be even more catastrophic than a fifteen-yard loss on third down. Over the years, researchers studying the cornea have determined that many different eye diseases with many different symptoms can all be traced back to the epithelium not doing its job.
The Sugrue lab has therefore chosen to study the mechanisms by which the epithelial cells cling to each other, as well as how they move around in response to wounding and other affronts. Because cells cling in a similar way in very different epithelia, Sugrue and his colleagues hope that their results will be instructive for many other tissues.
The stratified squamous epithelium of the cornea is the major refractive surface of the eye. Thus, a smooth, intact, and healthy corneal epithelium is requisite for normal vision. The corneal epithelium resists pressures and represents a barrier to fluid loss and pathogen entrance. These qualities require the epithelial cells to remain tightly adherent to the underlying matrix as well as to one another. In addition, because of its vulnerable position, the corneal epithelium must retain a rapid, highly efficient, and well developed wound healing capability. It is well recognized that corneal epithelial defects may serve as early signs of ocular surface disease. In addition, disparate diseases of the ocular surface present abnormalities of epithelial healing. While medical advances such as topical treatments, lenses, and grafting techniques (including keratoplasty, conjunctival transplants, and keratoepithelioplasty) have greatly improved prognosis, ocular surface disorders remain a major therapeutic challenge. Strikingly, whereas ocular surface diseases have etiologies that are quit variable, they all share the common pathogenic theme of failure in corneal epithelial adhesion.
My laboratory has focused on two distinguishing characteristics of epithelium: interaction with the basal lamina, and cell-cell adhesion. Our goal is to determine the role and regulation of specific adhesion molecules: in the maintenance of tissue integrity of corneal epithelium, in epithelial migration following wounding, and in the re-establishment of the competent epithelial barrier upon healing of the wound. Our hypothesis is that molecules in the sedentary epithelium that ensure stable cell-matrix and intercellular adhesion become involved in more supple adhesive interactions during epithelial migration. These changes may include the disassembly of dedicated adhesion structures, movement of adhesion molecules, and changes in their molecular associations and post-translational modifications. Finally, upon the completion of the wound closure, the epithelium must reverse all these changes to establish a competent epithelium.
My laboratory uses a combination of morphological, immunological, biochemical and molecular biological methods. We are examining how the epithelial cell can discriminate between adhesive interactions with other cells and interactions with the extracellular matrix. We are exmaining the distribution, movement, and metabolism of specific integrin heterodimers on migrating and quiescent cells. These data suggest that, while certain integrins are present on both quiescent and migrating epithelium, they may play distinct roles under different physiological conditions. For example, a6B4 lace at the hemidesmosome on quiescent epithelia, but it also mediates the adhesion of actively migrating epithelia to laminin. In addition, our resulls tempt one to speculate that the additional role for B1 integrins may be cell-cell adhesion of migrating epithelial cells.
A protein related to the desmosome has recently been discovered by Pin Ouyang, while a graduate student here. This phosphoprotein of 150 kDa, which we call 08L, is localized to the periphery of desmosomes. Assembly experiments have shown that 08L is the last (known) component to assemble to the desmosome and that 08L is the first to leave during desmosome disassembly. The presence of 08L at the desmosome is correlated with increased keratin filament organization.
The discovery of 08L has afforded us the opportunity to address some long-standing questions: How do intermediate filaments bind to desmosomes or other membrane locations? Is there direct association between the filaments and desmoplakin, or are there other intermediary proteins involved? And what are the molecular controls in the assembly and disassembly of the desmosomal complex?
Our working hypothesis is that 08L is not integral to the desmosome structure but may be a regulatory protein involved in assembly of the intermediate filament-desmosome complex. We have isolated cDNAs coding for 08L and have identified the human gene and chromosomal location. Functional studies of the 08L protein, which will initially be carried out in transient expression systems, will allow us to study the assembly of 08L to the desmosome and resolve the relationship of 08L with other adhesion related molecules, including cadherins, desmogleins, and integrins. We will also explore genetic diseases involving mutations within the 08L gene and the generation of transgenic animals expressing mutagenized proteins driven by promoters specific to the corneal epithelium, such as the keratin 12 promoter. We hope to shed some light on the function(s) of 08L and related proteins in the maintenance of a stable epithelium.