Despite many years of in vitro research confirming the effectiveness of RGD in promoting cell attachment to a wide variety of biomaterials, animal studies evaluating tissue responses to implanted RGD-functionalized substrates have yielded more variable results The goals of this report are to present some of the reasons why cell culture studies may not always reliably predict in vivo responses, and more importantly, to highlight potential applications that may benefit from the use of RGD peptides.

SUMMARY Exomeres are a recently discovered type of extracellular nanoparticle with no known biological function. Herein, we describe a simple ultracentrifugation-based method for separation of exomeres from exosomes. Exomeres are enriched in Argonaute 1–3 and amyloid precursor protein. We identify distinct functions of exomeres mediated by two of their cargo, the β-galactoside α2,6-sialyltransferase 1 (ST6Gal-I) that α2,6- sialylates N-glycans, and the EGFR ligand, amphiregulin (AREG). Functional ST6Gal-I in exomeres can be transferred to cells, resulting in hypersialylation of recipient cell-surface proteins including β1-integrin. AREG-containing exomeres elicit prolonged EGFR and downstream signaling in recipient cells, modulate EGFR trafficking in normal intestinal organoids, and dramatically enhance the growth of colonic tumor organoids. This study provides a simplified method of exomere isolation and demonstrates that exomeres contain and can transfer functional cargo. These findings underscore the heterogeneity of nanoparticles and should accelerate advances in determining the composition and biological functions of exomeres.

The concomitant tyrosine phosphorylation of the focal adhesion protein, paxillin, and the tyrosine kinase, focal adhesion kinase (FAK), in response to multiple stimuli including integrin-mediated cell adhesion suggests that paxillin phosphorylation is closely coupled to FAK activity. In the present study, we have identified a specific tyrosine residue within paxillin, tyrosine 118 (Tyr-118), that represents the principle site of phosphorylation by FAK in vitro. The identification of this site as a target for FAK phosphorylation was accomplished by immunoprecipitating FAK and performing in vitro kinase assays, using as substrate either glutathione S-transferase (GST)-paxillin fusion proteins containing truncations in paxillin sequence or fusion proteins with phenylalanine substitutions for tyrosine residues. GST-paxillin containing a phenylalanine substitution at Tyr-118 (Y118F) was not phosphorylated by FAK immunoprecipitates; however, this mutant was shown to bind FAK equally as well as the wild type fusion protein. As a first step toward assessing the function of paxillin phosphorylation on Tyr-118, a Y118F paxillin cDNA construct was transiently transfected into NIH 3T3 cells. Similar to wild type paxillin, mutated paxillin localized to focal adhesions, indicating that the phosphorylation of paxillin on Tyr-118 is not essential for the recruitment of paxillin to sites of cell adhesion.

Colon adenocarcinomas are known to express elevated levels of A2-6 sialylation and increased activity of ST6Gal-I, the Golgi glycosyltransferase that creates A2-6 linkages. Elevated ST6Gal-I positively correlates with metastasis and poor survival, and therefore ST6Gal-I-mediated hypersialylation likely plays a role in colorectal tumor invasion. Previously we found that oncogenic ras (present in roughly 50% of colon adenocarcinomas) up-regulates ST6Gal-I and, in turn, increases sialylation of B 1 integrin adhesion receptors in colon epithelial cells. However, we wanted to know if this pattern held true in vivo and, if so, how B 1 hypersialylation might contribute to colon tumor progression. In the present study, we find that B 1 integrins from colon adenocarcinomas consistently carry higher levels of A2-6 sialic acid. To explore the effects of increased A2-6 sialylation on B 1 -integrin function, we stably expressed ST6Gal-I in a colon epithelial cell line lacking endogenous ST6Gal-I. ST6Gal-I expressors (with A2-6 sialylated B 1 integrins) exhibited up-regulated attachment to collagen I and laminin and increased haptotactic migration toward collagen I, relative to parental cells (with completely unsialylated B 1 integrins). Blockade of ST6Gal-I expression with short interfering RNA reversed collagen binding back to the level of ST6Gal-I nonexpressors, confirming that a2-6 sialylation regulates B 1 integrin function. Finally, we show that B 1 integrins from ST6Gal-I expressors have increased association with talin, a marker for integrin activation. Collectively, these findings suggest that B 1 hypersialylation may augment colon tumor progression by altering cell preference for certain extracellular matrix milieus, as well as by stimulating cell migration. (Cancer Res 2005; 65(11): 4645-52)

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