SRF'ing the actin cytoskeleton with no destrin.

ALL LIVING CELLS, INCLUDING bacteria, require an intricate actin cytoskeleton for normal homeostasis. Once thought to be a static scaffold functioning solely to maintain cell shape, the actin cytoskeleton is now known to be a dynamic menagerie of proteins controlling such processes as migration and contraction, endocytosis and exocytosis, apoptosis, intracellular trafficking, and signal transduction. Many of the component parts of the actin cytoskeleton are also found in the so-called nucleoskeleton, which appears to play an important role in gene transcription (14). For example, nuclear actin and myosin have recently been implicated in interchromosomal associations that bring co-regulated genes in close proximity following hormone stimulation (10). In this manner, coexpressed gene loci may be regionalized to nuclear centers where key transcription factors are concentrated. There is also evidence for nucleoskeletal proteins directly mediating gene expression (14). The actin cytoskeleton appears to effect changes in gene expression indirectly through physical associations with transcription factors held in abeyance within the cytosol. One of the better understood transcription factors under such control is myocardin-related transcription factor A (MRTF-A). The myocardin family of coregulators binds serum response factor (SRF) over a 10-base pair element called the CArG box (11). Based on the binding rules of SRF, there are 1,216 permutations of the CArG box with the vast majority of functional elements found in close proximity to 200 target genes (7). SRF is only weakly active in stimulating CArGdependent target genes. However, the CArG-SRF-MRTF-A ternary complex comprises a potent transcriptional switch for gene expression (11). Whereas the founding member, myocardin, is constitutively nuclear, MRTF-A undergoes signal-dependent changes in cellular localization (4). Thus, under normal (e.g., quiescent) conditions, MRTF-A (aka MAL) physically associates with globular (G) actin in the cytosol and is unable to bind nuclear SRF. However, following agonist stimulation (e.g., serum), G-actin is polymerized into filamentous (F) actin, thereby releasing MRTF-A, allowing for nuclear translocation where MRTF-A engages SRF bound to CArG boxes in and around key target genes (8). The phenomenon of G-actin polymerization into F-actin, and the latter’s depolymerization or severing back into G-actin, is known as actin dynamics or actin treadmilling (12). Many proteins are required to coordinate the waves of G-actin polymerization (e.g., profilin) and subsequent F-actin depolymerization (e.g., cofilin). Loss of function in any number of actin treadmilling proteins has severe consequences for normal cell biology and organismal life (7). Despite the wealth of data describing actin dynamics, there has been no genome-wide investigation of changes in gene expression linked to actin dynamics, nor have there been many studies validating in vivo actin dynamics. Now, in this issue of Physiological Genomics, Verdoni et al. (18) provide startling new in vivo data demonstrating a link between defective actin dynamics and global changes in gene expression that include an enrichment of cytoskeletal genes under direct control of SRF (18). To study the role of actin dynamics on gene expression control in vivo, Verdoni et al. (18) exploited two mouse strains exhibiting variations in expression of a critical actin-binding protein. The corneal disease-1 (corn1) mouse carries a 35-kb chromosomal deletion of the entire coding region of destrin (Dstn) (5). Dstn (aka actin depolymerizing factor, ADF) is a 19-kDa protein that depolymerizes F-actin. Dstn is related in primary amino acid sequence and function to cofilin and is expressed in essentially all mammalian tissues (9). Surprisingly, mice lacking Dstn (Dstn) are viable and exhibit little pathology in most organ systems, suggesting compensation from other ADFs (e.g., cofilin). The Dstn mouse does, however, display autosomal recessive traits that manifest initially as corneal epithelial hyperproliferation and neovascularization followed by frank cataract formation (15). In a second mouse model (Dstn), a milder corneal phenotype is associated with a putative hypomorphic Dstn allele stemming from a nonsynonymous mutation (Pro106Ser) (5). Verdoni et al. first showed elevated phalloidin and beta actin staining within the epithelial layer of the cornea in both mutant mouse strains, consistent with Dstn’s major function as an F-actin depolymerizing factor; though both mutants display F-actin accumulation, the Dstn mutant showed stronger staining. They then performed well-controlled microarrays from 14-dayold Dstn or Dstn corneas, a time coinciding with the emergence of F-actin accumulation and epithelial cell hyperproliferation. Consistent with the differential phalloidin staining, hierarchical clustering of array data revealed greater changes (log2-fold compared with control, wild-type mouse) in corneal gene expression from Dstn mice with 599 genes significantly elevated (compared with 150 in Dstn) and 627 genes reduced (compared with 52 in Dstn). Thus, the extent of F-actin accumulation appears to correlate with the level of gene expression providing compelling support for the actin cytoskeleton coordinating gene expression control. Using the database for annotation, visualization, and integrative discovery (DAVID), Verdoni et al. found cell cycle-associated genes to be enriched in the corneas of Dstn mice, but not the Dstn mice. These findings are congruent with the hyperproliferation that is more evident in the Dstn mouse (15). On the other hand, both mutants showed significant enrichment for gene ontology-annotated genes related to the actin cytoskeleton or components of the cell (e.g., adherens junction) directly connected to the cytoskeleton. Historically, while the cell biology of actin cytoskeletal proteins has been studied in great detail, there is a paucity of