Purification and assays of vasodilator-stimulated phosphoprotein

Carbon monoxide and nitric oxide mediate cytoskeletal reorganization in microvascular cells via vasodilator-stimulated phosphoprotein phosphorylation: evidence for blunted responsiveness in diabetes

OBJECTIVE

We examined the effect of the vasoactive agents carbon monoxide (CO) and nitric oxide (NO) : n the phosphorylation and intracellular redistribution of vasodilator-stimulated phosphoprotein (VASP), a critical actin motor protein required for cell migration that also controls vasodilation and platelet aggregation.

RESEARCH DESIGN AND METHODS

We examined the effect of donor-released CO and NO in endothelial progenitor cells (EPCs) and platelets from nondiabetic and diabetic subjects and in human microvascular endothelial cells (HMECs) cultured under low (5.5 mmol/l) or high (25 mmol/l) glucose conditions. VASP phosphorylation was evaluated using phosphorylation site-specific antibodies.

RESULTS

In control platelets, CO selectively promotes phosphorylation at VASP Ser-157, whereas NO promotes phosphorylation primarily at Ser-157 and also at Ser-239, with maximal responses at 1 min with both agents on Ser-157 and at 15 min on Ser-239 with NO treatment. In diabetic platelets, neither agent resulted in VASP phosphorylation. In nondiabetic EPCs, NO and CO increased phosphorylation at Ser-239 and Ser-157, respectively, but this response was markedly reduced in diabetic EPCs. In endothelial cells cultured under low glucose conditions, both CO and NO induced phosphorylation at Ser-157 and Ser-239; however, this response was completely lost when cells were cultured under high glucose conditions. In control EPCs and in HMECs exposed to low glucose, VASP was redistributed to filopodia-like structures following CO or NO exposure; however, redistribution was dramatically attenuated under high glucose conditions.

CONCLUSIONS

Vasoactive gases CO and NO promote cytoskeletal changes through site- and cell type-specific VASP phosphorylation, and in diabetes, blunted responses to these agents may lead to reduced vascular repair and tissue perfusion.

VASP protects actin filaments from gelsolin: an in vitro study with implications for platelet actin reorganizations

An initial step in platelet shape change is disassembly of actin filaments, which are then reorganized into new actin structures, including filopodia and lamellipodia. This disassembly is thought to be mediated primarily by gelsolin, an abundant actin filament-severing protein in platelets. Shape change is inhibited by VASP, another abundant actin-binding protein. Paradoxically, in vitro VASP enhances formation of actin filaments and bundles them, activities that would be expected to increase shape change, not inhibit it. We hypothesized that VASP might inhibit shape change by stabilizing filaments and preventing their disassembly by gelsolin. Such activity would explain VASP's known physiological role. Here, we test this hypothesis in vitro using either purified recombinant or endogenous platelet VASP by fluorescence microscopy and biochemical assays. VASP inhibited gelsolin's ability to disassemble actin filaments in a dose-dependent fashion. Inhibition was detectable at the low VASP:actin ratio found inside the platelet (1:40 VASP:actin). Gelsolin bound to VASP-actin filaments at least as well as to actin alone. VASP inhibited gelsolin-induced nucleation at higher concentrations (1:5 VASP:actin ratios). VASP's affinity for actin (K(d) approximately 0.07 microM) and its ability to promote polymerization (1:20 VASP actin ratio) were greater with Ca(++)-actin than with Mg(++)-actin (K(d) approximately 1 microM and 1:1 VASP), regardless of the presence of gelsolin. By immunofluorescence, VASP and gelsolin co-localized in the filopodia and lamellipodia of platelets spreading on glass, suggesting that these in vitro interactions could take place within the cell as well. We conclude that VASP stabilizes actin filaments to the severing effects of gelsolin but does not inhibit gelsolin from binding to the filaments. These results suggest a new concept for actin dynamics inside cells: that bundling proteins protect the actin superstructure from disassembly by severing, thereby preserving the integrity of the cytoskeleton.