Osteogenesis associates with inflammation in early-stage atherosclerosis evaluated by molecular imaging in vivo.
BACKGROUND
Arterial calcification is associated with cardiovascular events; however, mechanisms of calcification in atherosclerosis remain obscure.


RESULTS
We tested the hypothesis that inflammation promotes osteogenesis in atherosclerotic plaques using in vivo molecular imaging in apolipoprotein E-/- mice (20 to 30 weeks old, n=35). A bisphosphonate-derivatized near-infrared fluorescent imaging agent (excitation 750 nm) visualized osteogenic activity that was otherwise undetectable by x-ray computed tomography. Flow cytometry validated the target specifically in osteoblast-like cells. A spectrally distinct near-infrared fluorescent nanoparticle (excitation 680 nm) was coinjected to simultaneously image macrophages. Fluorescence reflectance mapping demonstrated an association between osteogenic activity and macrophages in aortas of apolipoprotein E-/- mice (R2=0.93). Intravital dual-channel fluorescence microscopy was used to further monitor osteogenic changes in inflamed carotid arteries at 20 and 30 weeks of age and revealed that macrophage burden and osteogenesis concomitantly increased during plaque progression (P<0.01 and P<0.001, respectively) and decreased after statin treatment (P<0.0001 and P<0.05, respectively). Fluorescence microscopy on cryosections colocalized near-infrared fluorescent osteogenic signals with alkaline phosphatase activity, bone-regulating protein expression, and hydroxyapatite nanocrystals as detected by electron microscopy, whereas von Kossa and alizarin red stains showed no evidence of calcification. Real-time reverse-transcription polymerase chain reaction revealed that macrophage-conditioned media increased alkaline phosphatase mRNA expression in vascular smooth muscle cells.


CONCLUSIONS
This serial in vivo study demonstrates the real-time association of macrophage burden with osteogenic activity in early-stage atherosclerosis and offers a cellular-resolution tool to identify preclinical microcalcifications.
Aikawa
Elena
E
Center for Molecular Imaging Research, Massachusetts General Hospital, Harvard Medical School, 149 13th St, Room 5420, Charlestown, MA 02129, USA. eaikawa@mgh.harvard.edu
Nahrendorf
Matthias
M
Figueiredo
Jose-Luiz
JL
Swirski
Filip K
FK
Shtatland
Timur
T
Kohler
Rainer H
RH
Jaffer
Farouc A
FA
Aikawa
Masanori
M
Weissleder
Ralph
R
D000818:Animals; D001057:Apolipoproteins E / Q000172:deficiency* / Q000235:genetics; D050197:Atherosclerosis / Q000473:pathology / Q000503:physiopathology*; D002114:Calcinosis / Q000209:etiology / Q000473:pathology; D004195:Disease Models, Animal; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007249:Inflammation / Q000209:etiology* / Q000517:prevention & control; D051379:Mice; D018345:Mice, Knockout; D010012:Osteogenesis / Q000502:physiology*
D001057:Apolipoproteins E