Netrins are a family of secreted molecules that are important for axonal outgrowth and guidance in the developing nervous system. However, the signaling mechanisms that lie immediately downstream of netrin receptors remain poorly understood. Here we report that the netrin receptor DCC (deleted in colorectal cancer) interacts with the focal adhesion kinase (FAK), a kinase implicated in regulating cell adhesion and migration. FAK was expressed in developing brains and was localized with DCC in cultured neurons. Netrin-1 induced FAK and DCC tyrosine phosphorylation. Disruption of FAK signaling abolished netrin-1-induced neurite outgrowth and attractive growth cone turning. Taken together, these results indicate a new signaling mechanism for DCC, in which FAK is activated upon netrin-1 stimulation and mediates netrin-1 function; they also identify a critical role for FAK in axon navigation.

Netrins regulate axon path-finding during development, but the underlying mechanisms are not well understood. Here, we provide evidence for the involvement of the unconventional myosin X (Myo X) in netrin-1 function. We find that Myo X interacts with the netrin receptor deleted in colorectal cancer (DCC) and neogenin, a DCC-related protein. Expression of Myo X redistributes DCC to the cell periphery or to the tips of neurites, whereas its silencing prevents DCC distribution in neurites. Moreover, expression of DCC, but not neogenin, stimulates Myo X-mediated formation and elongation of filopodia, suggesting that Myo X function may be differentially regulated by DCC and neogenin. The involvement of Myo X in netrin-1 function was further supported by the effects of inhibiting Myo X function in neurons. Cortical explants derived from mouse embryos expressing a motor-less Myo X exhibit reduced neurite outgrowth in response to netrin-1 and chick commissural neurons expressing the motor-less Myo X, or in which Myo X is silenced using microRNA (miRNA), show impaired axon projection in vivo. Taken together, these results identify a novel role for Myo X in regulating netrin-1 function.

The Slit protein guides neuronal and leukocyte migration through the transmembrane receptor Roundabout (Robo). We report here that the intracellular domain of Robo interacts with a novel family of Rho GTPase activating proteins (GAPs). Two of the Slit-Robo GAPs (srGAPs) are expressed in regions responsive to Slit. Slit increased srGAP1-Robo1 interaction and inactivated Cdc42. A dominant negative srGAP1 blocked Slit inactivation of Cdc42 and Slit repulsion of migratory cells from the anterior subventricular zone (SVZa) of the forebrain. A constitutively active Cdc42 blocked the repulsive effect of Slit. These results have demonstrated important roles for GAPs and Cdc42 in neuronal migration. We propose a signal transduction pathway from the extracellular guidance cue to intracellular actin polymerization.

S U M M A R Y Primary cilia (PC) are solitary, sensory organelles that are critical for several signaling pathways. PC were detected by immunofluorescence of cultured cells and breast tissues. After growth for 7 days in vitro, PC were detected in ?70% of breast fibroblasts and in 7-19% of epithelial cells derived from benign breast (184A1 and MCF10A). In 11 breast cancer cell lines, PC were present at a low frequency in four (from 0.3% to 4% of cells), but were absent in the remainder. The cancer cell lines with PC were all of the basal B subtype, which is analogous to the clinical triple-negative breast cancer subtype. Furthermore, the frequency of PC decreased with increasing degree of transformation/progression in the MCF10 and MDA-MB-435/LCC6 isogenic models of cancer progression. In histologically normal breast tissues, PC were frequent in fibroblasts and myoepithelial cells and less common in luminal epithelial cells. Of 26 breast cancers examined, rare PC were identified in cancer epithelial cells of only one cancer, which was of the triple-negative subtype. These data indicate a decrease or loss of PC in breast cancer and an association of PC with the basal B subtype. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials. (J Histochem Cytochem 58:857-870, 2010)

The BRMS1 metastasis suppressor interacts with the protein AT-rich interactive domain 4A (ARID4A, RBBP1) as part of SIN3⅐histone deacetylase chromatin remodeling complexes. These transcriptional co-repressors regulate diverse cell phenotypes depending upon complex composition. To define BRMS1 complexes and their roles in metastasis suppression, we generated BRMS1 mutants (BRMS1 mut ) and mapped ARID4A interactions. BRMS1L174D disrupted direct interaction with ARID4A in yeast two-hybrid genetic screens but retained an indirect association with ARID4A in MDA-MB-231 and -435 human breast cancer cell lines by co-immunoprecipitation. Deletion of the first coiled-coil domain (BRMS1 ⌬CC1 ) did not disrupt direct interaction in yeast two-hybrid screens but did prevent association by co-immunoprecipitation. These results suggest altered complex composition with BRMS1 mut . Although basal transcription repression was impaired and the pro-metastatic protein osteopontin was differentially down-regulated by BRMS1 L174D and BRMS1 ⌬CC1 , both down-regulated the epidermal growth factor receptor and suppressed metastasis in MDA-MB-231 and -435 breast cancer xenograft models. We conclude that BRMS1 mut , which modifies the composition of a SIN3⅐histone deacetylase chromatin remodeling complex, leads to altered gene expression profiles. Because metastasis requires the coordinate expression of multiple genes, down-regulation of at least one important gene, such as the epidermal growth factor receptor, had the ability to suppress metastasis. Understanding which interactions are necessary for particular biochemical/ cellular functions may prove important for future strategies targeting metastasis.The ability of a cancer cell to complete all steps of the metastatic cascade requires diverse tumor-host interactions that are dependent on the coordinate expression of specific genes both intrinsically and extrinsically (1-3). The metastasis suppressor BRMS1 3 has been shown to regulate the expression of multiple genes leading to the suppression of metastasis in multiple model systems, including human breast carcinoma (4, 5), melanoma (6), and ovarian carcinoma (7), without preventing orthotopic tumor growth. Specifically, down-regulation of the pro-metastatic genes osteopontin (OPN) and urokinase-type plasminogen activator has been linked to BRMS1 expression (8, 9). Gap junctional intercellular communication is restored by BRMS1 through a change in connexin expression (10). Microarray and proteomic analyses have also been performed showing multiple changes in gene and protein expression when BRMS1 was introduced (11-13). Clinically, loss of BRMS1 protein has been correlated with progesterone receptor expression and inversely correlated with HER2 expression in breast cancer patients (14).BRMS1 has been proposed to regulate transcription of genes by interaction with a large SIN3⅐HDAC chromatin remodeling complex through interaction with the protein AT-rich interactive domain 4A (ARID4A) that suppresses basal transcription in vivo using a Gal...