In vivo multicolor molecular MR imaging using diamagnetic chemical exchange saturation transfer liposomes.

A variety of (super)paramagnetic contrast agents are available for enhanced MR visualization of specific tissues, cells, or molecules. To develop alternative contrast agents without the presence of metal ions, liposomes were developed containing simple bioorganic and biodegradable compounds that produce diamagnetic chemical exchange saturation transfer MR contrast. This diamagnetic chemical exchange saturation transfer contrast is frequency-dependent, allowing the unique generation of "multicolor" images. The contrast can be turned on and off at will, and standard images do not show the presence of these agents. As an example, glycogen, L-arginine, and poly-L-lysine were encapsulated inside liposomes and injected intradermally into mice to image the lymphatic uptake of these liposomes. Using a frequency-dependent acquisition scheme, it is demonstrated that multicolor MRI can differentiate between different contrast particles in vivo following their homing to draining lymph nodes. Being nonmetallic and bioorganic, these diamagnetic chemical exchange saturation transfer liposomes form an attractive novel platform for multicolor imaging in vivo.

Tie2-dependent knockout of HIF-1 impairs burn wound vascularization and homing of bone marrow-derived angiogenic cells.

AIMS: Hypoxia-inducible factor 1 (HIF-1) is a heterodimer composed of HIF-1α and HIF-1ß subunits. HIF-1 is known to promote tissue vascularization by activating the transcription of genes encoding angiogenic factors, which bind to receptors on endothelial cells (ECs) and bone marrow-derived angiogenic cells (BMDACs). In this study, we analysed whether HIF-1 activity in the responding ECs and BMDACs is also required for cutaneous vascularization during burn wound healing. METHODS AND RESULTS: We generated mice with floxed alleles at the Hif1a or Arnt locus encoding HIF-1α and HIF-1ß, respectively. Expression of Cre recombinase was driven by the Tie2 gene promoter, which is expressed in ECs and bone marrow cells. Tie2Cre(+) and Tie2Cre(-) mice were subjected to burn wounds of reproducible diameter and depth. Deficiency of HIF-1α or HIF-1ß in Tie2-lineage cells resulted in delayed wound closure, reduced vascularization, decreased cutaneous blood flow, impaired BMDAC mobilization, and decreased BMDAC homing to burn wounds. CONCLUSION: HIF-1 activity in Tie2-lineage cells is required for the mobilization and homing of BMDACs to cutaneous burn wounds and for the vascularization of burn wound tissue.