Filamin A regulates the organization and remodeling of the pericellular collagen matrix.

Extracellular matrix remodeling by cell adhesion-related processes is critical for proliferation and tissue homeostasis, but how adhesions and the cytoskeleton interact to organize the pericellular matrix (PCM) is not understood. We examined the role of the actin-binding protein, filamin A (FLNa), in pericellular collagen remodeling. Compared with wild-type (WT), mice with fibroblast-specific deletion of FLNa exhibited higher density but reduced organization of collagen fibers after increased loading of the periodontal ligament for 2 wk. In cultured fibroblasts, FLNa knockdown (KD) did not affect collagen mRNA, but after 24 h of culture, FLNa WT cells exhibited ∼2-fold higher cell-surface collagen KD cells and 13-fold higher levels of activated ß1 integrins. In FLNa WT cells, there was 3-fold more colocalization of talin with pericellular cleaved collagen than in FLNa KD cells. MMP-9 mRNA and protein expression were >2-fold higher in FLNa KD cells than in WT cells. Cathepsin B, which is necessary for intracellular collagen digestion, was >3-fold higher in FLNa WT cells than in KD cells. FLNa WT cells exhibited 2-fold more collagen phagocytosis than KD cells, which involved the FLNa actin-binding domain. Evidently, FLNa regulates PCM remodeling through its effects on degradation pathways that affect the abundance and organization of collagen.-Mezawa, M., Pinto, V. I., Kazembe, M. P., Lee, W. S., McCulloch, C. A. Filamin A regulates the organization and remodeling of the pericellular collagen matrix.

The role of homeobox genes in retinal development and disease.

Homeobox genes are an evolutionarily conserved class of transcription factors that are critical for development of many organ systems, including the brain and eye. During retinogenesis, homeodomain-containing transcription factors, which are encoded by homeobox genes, play essential roles in the regionalization and patterning of the optic neuroepithelium, specification of retinal progenitors and differentiation of all seven of the retinal cell classes that derive from a common progenitor. Homeodomain transcription factors control retinal cell fate by regulating the expression of target genes required for retinal progenitor cell fate decisions and for terminal differentiation of specific retinal cell types. The essential role of homeobox genes during retinal development is demonstrated by the number of human eye diseases, including colobomas and anophthalmia, which are attributed to homeobox gene mutations. In the following review, we highlight the role of homeodomain transcription factors during retinogenesis and regulation of their gene targets. Understanding the complexities of vertebrate retina development will enhance our ability to drive differentiation of specific retinal cell types towards novel cell-based replacement therapies for retinal degenerative diseases.

Filamin A protects cells against force-induced apoptosis by stabilizing talin- and vinculin-containing cell adhesions.

In mechanically loaded tissues such as weight-bearing joints, myocardium, and periodontal ligament, pathophysiological forces can disrupt cell-matrix contacts, which can induce cell death, leading to tissue and organ dysfunction. Protection against force-induced cell death may be mediated by filamin A (FLNa), an actin-binding protein that regulates ß1 integrin-mediated cell adhesion. We examined the affect of filamin expression on collagen distribution and cell death in the periodontal ligament, a force-loaded tissue. Conditional deletion of FLNa in fibroblasts was associated with 2-fold increase of acellular areas in periodontal ligament and 7-fold higher proportions of apoptotic cells. In cultured fibroblasts with FLNa knockdown, we examined the affect of supraphysiological forces (1 pN/µm(2) cell area; applied through the ß1 integrin) on recruitment of talin and vinculin to focal adhesions and on apoptosis. Compared with the wild type, FLNa-knockdown cells exhibited 3-fold increases in floating cells after overnight force application and a 2-fold increase in cell detachment. Force induced time-dependent reductions (P<0.05) in the numbers of activated ß1 integrin-, talin-, and vinculin-stained adhesions in FLNa-knockdown compared with those in wild-type cells. We conclude that FLNa protects against apoptosis in force-loaded cells, and this protection is mediated by enhanced formation and maturation of matrix adhesions.

Force-induced apoptosis mediated by the Rac/Pak/p38 signalling pathway is regulated by filamin A.

Cells in mechanically challenged environments cope with high-amplitude exogenous forces that can lead to cell death, but the mechanisms that mediate force-induced apoptosis and the identity of mechanoprotective cellular factors are not defined. We assessed apoptosis in NIH 3T3 and HEK (human embryonic kidney)-293 cells exposed to tensile forces applied through ß1-integrins. Apoptosis was mediated by Rac-dependent activation of p38α. Depletion of Pak1 (p21-activated kinase 1), a downstream effector of Rac, prevented force-induced p38 activation and apoptosis. Rac was recruited to sites of force transfer by filamin A, which inhibited force-induced apoptosis mediated by Rac and p38α. We conclude that, in response to tensile force, filamin A regulates Rac-dependent signals, which induce apoptosis through Pak1 and p38.

Filamin A Mediates Wound Closure by Promoting Elastic Deformation and Maintenance of Tension in the Collagen Matrix.

Cell-mediated remodeling and wound closure are critical for efficient wound healing, but the contribution of actin-binding proteins to contraction of the extracellular matrix is not defined. We examined the role of filamin A (FLNa), an actin filament cross-linking protein, in wound contraction and maintenance of matrix tension. Conditional deletion of FLNa in fibroblasts in mice was associated with ~4 day delay of full-thickness skin wound contraction compared with wild-type (WT) mice. We modeled the healing wound matrix using cultured fibroblasts plated on grid-supported collagen gels that create lateral boundaries, which are analogues to wound margins. In contrast to WT cells, FLNa knockdown (KD) cells could not completely maintain tension when matrix compaction was resisted by boundaries, which manifested as relaxed matrix tension. Similarly, WT cells on cross-linked collagen, which requires higher levels of sustained tension, exhibited approximately fivefold larger deformation fields and approximately twofold greater fiber alignment compared with FLNa KD cells. Maintenance of boundary-resisted tension markedly influenced the elongation of cell extensions: in WT cells, the number (~50%) and length (~300%) of cell extensions were greater than FLNa KD cells. We conclude that FLNa is required for wound contraction, in part by enabling elastic deformation and maintenance of tension in the matrix.