Traversing the basement membrane in vivo: a diversity of strategies

Abstract

The basement membrane is a dense, highly cross-linked, sheet-like extracellular matrix that underlies all epithelia and endothelia in multicellular animals. During development, leukocyte trafficking, and metastatic disease, cells cross the basement membrane to disperse and enter new tissues. Based largely on in vitro studies, cells have been thought to use proteases to dissolve and traverse this formidable obstacle. Surprisingly, recent in vivo studies have uncovered a remarkably diverse range of cellular- and tissue-level strategies beyond proteolysis that cells use to navigate through the basement membrane. These fascinating and unexpected mechanisms have increased our understanding of how cells cross this matrix barrier in physiological and disease settings.

Figure 1.

Basement membrane removal during uterine–vulval attachment in C. elegans. (A) Schematic diagram of basement membrane remodeling. The gonadal anchor cell (AC) initiates the basement membrane (BM) breach with invadopodia. The DCC (deleted in colon cancer) receptor enriches at the site of breach and through its effectors generates F-actin that builds a large protrusion. This invasive process physically displaces the basement membrane and directs invasion through a single basement membrane gap into the underlying dividing vulval precursor cells (VPCs). The basement membrane gap then expands through basement membrane sliding (black arrows in right panel) by sliding over the underlying VPCs as they invaginate. (B) Confocal imaging over the course of anchor cell (green) invasion from a lateral view with fluorescent images overlaid on a DIC micrograph. The basement membrane (magenta) ultimately stabilizes over a specific vulval cell, named the vulD cell (white dotted lines in the far right image). Image reproduced from Hagedorn and Sherwood (2011) with permission from Elsevier. (C) A lateral-view confocal time series showing the growth of the invasive membrane protrusion of the anchor cell (cyan) as it advances through the basement membrane (magenta). (D) A ventral-view time series shows the expanding hole in the basement membrane that forms during anchor cell invasion. Yellow arrows point to physically displaced basement membrane. C and D reproduced from Hagedorn et al. (2013). Bars, 5 µm.

Figure 2.

Basement membrane breaching is triggered by mechanical force in mouse embryos. (A) Schematic model of distal visceral endoderm (DVE) formation resulting from constrained growth by the maternal tissue. The growing embryo expands from day 5.0 to 5.5 and becomes restricted laterally (red arrows) by the maternal uterine tissue (uterine epithelium; light pink). Forced to elongate in the proximal–distal axis direction (large yellow arrow), the distal basement membrane (green) breach is triggered by mechanical strain. Epiblast cells (light blue) exit through the basement membrane gaps into the visceral endoderm (VE) layer (yellow), forming the DVE (white). (B and C) Fluorescence microscopy of explanted embryos expressing Cer1 (cerberus-related cytokine 1)-EGFP (a DVE marker, which is induced after the cells breach the basement membrane; green) that were cultured in narrow (growth restricting) and wide (nongrowth restricting) cavity devices and immunostained with anti-collagen IV (labeling the basement membrane; magenta) and TOTO3 (nuclei; green). The basement membrane is breached and newly induced DVE cells transmigrate from the VE layer in embryos cultured in the narrow, but not wide, cavities. (D and E) 3D rendering of the intensity of the collagen IV signal in B and C. The loss of collagen signal can be seen at the distal tip of the embryo. (F) Confocal imaging of a cellular protrusion (arrow) forming from Sox2-Venus (epiblast lineage; yellow) into the VE layer through a breach in the basement membrane (arrowhead; anti-collagen IV, magenta). Nuclei are stained with DAPI (blue). Bar, 50 µm. Images adapted from Hiramatsu et al., (2013) with permission from Elsevier.

Figure 3.

Basement membrane disassembly during EMT in the gastrulating epiblast cells of the chick embryo. (A) Before EMT, the epiblast cells (light purple; lateral cells) interact with the underlying basement membrane (green) through CLASP-mediated (blue) and dystroglycan-mediated (red) cortical anchoring of microtubules (black). At the initiation of EMT, basement membrane is lost under the epiblast cells at the streak midline (epiblast medial cells). This loss is associated with the down-regulation of CLASP proteins leading to the destabilization of microtubules and a loss of dystroglycan localization at the cell–basement membrane interface. The invading epiblast cells de-epithelialize, ingress, and form into mesoderm and endoderm precursors (dark pink) during EMT. (B) Fluorescence microscopy showing that laminin (marking basement membrane; red) and dystroglycan (green) are lost in the primitive streak (PS; arrowheads indicate streak midline) during chick gastrulation EMT. Images adapted from Nakaya et al. (2013). (C) Treatment of embryos with taxol, a microtubule-stabilizing agent, results in retention of the basement membrane (laminin; green) in the medial epiblasts (arrowheads indicate streak midline). Numerous gaps in the basement membrane, however, are still present (arrows). Images adapted from Nakaya et al. (2008) with permission from Macmillan Publishers Ltd.: Nature Cell Biology, copyright 2008. Bar, 20 µm.

Figure 4.

Crossing of vascular endothelial basement membrane. Leukocytes readily traverse vessel walls during immune surveillance by entering the lymph (1) through gaps in the basement membrane (2) and exiting through venules (3) at low expression regions (LERs) of basement membrane in capillary beds (4). Tumor cells migrate away from the primary tumor and intravasate via the blood (5) or lymphatic system (6), perhaps aided by activated fibroblasts and/or macrophages. During metastatic dissemination, circulating tumor cells ultimately become lodged in capillary beds and extravasate to colonize distant organs (7). In addition, activated endothelial cells might break down basement membrane during vascular sprouting at the initiation of angiogenesis (8). See text for additional details.