Breaching the basement membrane: who, when and how?

The basement membrane (BM), a specialized network of extracellular matrix macromolecules, surrounds epithelial, endothelial, muscle, fat and nerve cells. During development, immune surveillance and disease states ranging from cancer to fibrosis, host cells penetrate the BM by engaging tissue-invasive programs, the identity of which remain largely undefined. Although it is commonly assumed that all cells employ similar mechanisms to cross BM barriers, accumulating evidence indicates that cells might selectively mobilize protease-dependent or -independent invasion programs. New data indicate that protease-dependent transmigration is largely reliant on a group of membrane-anchored metalloenzymes, termed the membrane-type matrix metalloproteinases, which irreversibly remodel BM structure. By contrast, mechanisms that enable protease-independent transmigration remain undefined and potentially involve the reversible disassembly of the BM network. Further characterization of the molecular mechanisms underlying BM transmigration should provide important insights into pathophysiologic tissue remodeling events and also enable the development of novel therapeutics.

Introduction

Early in development, animals ranging from flies to humans direct the embryonic epithelium to orchestrate the organization of an extracellular, supramolecular network of proteins, glycoproteins and proteoglycans, termed the basement membrane (BM) 1, 2, 3. This conglomerate of structural macromolecules coalesces to form a dense, 100–300 nm-thick lamina that underlies all epithelia and, in higher organisms, ensheaths endothelial cells, nerves, smooth muscle cells and adipocytes 1, 4, 5. The assembled BM provides adherent cells with structural support and functional cues by virtue of its biomechanical properties, display of adhesion receptor ligands and repertoire of matrix-bound growth factors 4, 6. With a pore size in the order of 50 nm, only small molecules are able to passively diffuse across this thin, but structurally rugged, barrier 6, 7, 8. Nonetheless, normal cells are able to traffic freely and rapidly across BMs by activating tissue-invasive programs during morphogenesis and immune surveillance 9, 10, 11, 12, 13. Furthermore, in a repetitive theme familiar to biologists, cell populations participating in pathologic events such as cancer can inappropriately co-opt ‘normal’ BM-transmigration programs to dire and, most often, lethal consequences by driving the metastatic process [14].

The transmigration of cells across the BM has an unquestionably important role in normal and neoplastic events and has been the subject of thousands of reports – not to mention innumerable reviews – in the literature 1, 2, 3, 6, 14. Nevertheless, it probably comes as a surprise that the mechanisms that enable cells to cross this structural barrier remain largely unknown and the subject of considerable debate. Whereas the ability of a migrating cell to perforate the BM has been almost uniformly ascribed to proteolytic events, >500 proteinases are encoded within the mammalian genome, thus complicating efforts to identify a subset of crucial, matrix-degrading enzymes [15]. As such, using various model constructs designed to recapitulate BM structure in vitro, most investigators have, over the past 20 years, accepted a largely circumstantial premise that secreted proteinases directly regulate transmigration in both physiologic and pathologic states 16, 17, 18. Very recently, however, evidence has begun to accumulate indicating that accepted dogma might now be ripe for revisiting 1, 19, 20, 21. New insights into BM assembly and structure have raised serious concerns regarding the use of most of the in vitro models used for analyzing invasion 22, 23, 24. Furthermore, in vivo studies of mice harboring inactivating mutations of proteinases commonly implicated as the ‘usual suspects’ in BM transmigration seldom seem to exhibit frank defects in BM invasion-associated events 25, 26, 27, 28. Instead, recent studies support the contention that a small subset of membrane-anchored metalloproteases assumes a previously unrecognized role in this process [21]. Although we would be pleased to inform readers that solutions to all queries regarding BM invasion programs lie within this perspective, this is far from the case and continued efforts are required to build a definitive model of BM transmigration. Rather, it is our intent to highlight existing conundrums and caveats in the field, to pose possible solutions as to the means by which normal and neoplastic cells traverse BM barriers and to outline experimental systems in which these hypotheses might be evaluated and tested in rigorous fashion.