Magnetic resonance imaging contrast agent targeted toward activated platelets allows in vivo detection of thrombosis and monitoring of thrombolysis.

BACKGROUND: Platelets are the key to thrombus formation and play a role in the development of atherosclerosis. Noninvasive imaging of activated platelets would be of great clinical interest. Here, we evaluate the ability of a magnetic resonance imaging (MRI) contrast agent consisting of microparticles of iron oxide (MPIOs) and a single-chain antibody targeting ligand-induced binding sites (LIBS) on activated glycoprotein IIb/IIIa to image carotid artery thrombi and atherosclerotic plaques. METHODS AND RESULTS: Anti-LIBS antibody or control antibody was conjugated to 1-microm MPIOs (LIBS MPIO/control MPIO). Nonocclusive mural thrombi were induced in mice with 6% ferric chloride. MRI (at 9.4 T) was performed once before and repeatedly in 12-minute-long sequences after LIBS MPIO/control MPIO injection. After 36 minutes, a significant signal void, corresponding to MPIO accumulation, was observed with LIBS MPIOs but not control MPIOs (P<0.05). After thrombolysis, in LIBS MPIO-injected mice, the signal void subsided, indicating successful thrombolysis. On histology, the MPIO content of the thrombus, as well as thrombus size, correlated significantly with LIBS MPIO-induced signal void (both P<0.01). After ex vivo incubation of symptomatic human carotid plaques, MRI and histology confirmed binding to areas of platelet adhesion/aggregation for LIBS MPIOs but not for control MPIOs. CONCLUSIONS: LIBS MPIOs allow in vivo MRI of activated platelets with excellent contrast properties and monitoring of thrombolytic therapy. Furthermore, activated platelets were detected on the surface of symptomatic human carotid plaques by ex vivo MRI. This approach represents a novel noninvasive technique allowing the detection and quantification of platelet-containing thrombi.

Fourier transform infrared analysis of purified lactose permease: a monodisperse lactose permease preparation is stably folded, alpha-helical, and highly accessible to deuterium exchange.

The lactose permease, encoded by the lacY gene of Escherichia coli, is an integral membrane protein that functions as a proton and lactose symporter. In this study, we have characterized a novel monodisperse, purified preparation of lactose permease, as well as functionally reconstituted lactose permease, using spectroscopic techniques. The purification of monodisperse lactose permease has been aided by the development of a lacY gene product containing an amino-terminal six histidine affinity tag. In the novel purification method described here, lactose permease is purified from beta-dodecyl maltoside-solubilized membrane vesicles using three sequential column steps: hydroxyapatite, nickel-nitriloacetic acid (Ni-NTA) affinity, and cation-exchange chromatography. The hydroxyapatite step was shown to be essential in reducing aggregation of the final purified protein. Amino acid composition analysis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis support the conclusion that the protein has been purified to greater than 90% homogeneity. The protein has been successfully reconstituted and has been shown to be active for lactose transport. Fourier transform infrared (FT-IR) spectroscopy has been performed on monodisperse lactose permease and on proteoliposomes containing functional lactose permease. FT-IR spectroscopy supports the conclusion that the monodisperse lactose permease preparation is 80% alpha-helical and stably folded at 20 degreesC; thermal denaturation is first detected at 70 degreesC. Because the purified protein is also readily susceptible to 2H exchange, these results suggest that the protein is conformationally flexible and that 2H exchange is facilitated as the result of conformational fluctuations from the folded state.