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Embolus extravasation is an alternative mechanism for cerebral microvascular recanalization

Abstract

Cerebral microvascular occlusion is a common phenomenon throughout life1,2 that might require greater recognition as a mechanism of brain pathology. Failure to recanalize microvessels promptly may lead to the disruption of brain circuits and significant functional deficits3. Haemodynamic forces and the fibrinolytic system4 are considered to be the principal mechanisms responsible for recanalization of occluded cerebral capillaries and terminal arterioles. Here we identify a previously unrecognized cellular mechanism that may also be critical for this recanalization. By using high-resolution fixed-tissue microscopy and two-photon imaging in living mice we observed that a large fraction of microemboli infused through the internal carotid artery failed to be lysed or washed out within 48 h. Instead, emboli were found to translocate outside the vessel lumen within 2–7 days, leading to complete re-establishment of blood flow and sparing of the vessel. Recanalization occurred by a previously unknown mechanism of microvascular plasticity involving the rapid envelopment of emboli by endothelial membrane projections that subsequently form a new vessel wall. This was followed by the formation of an endothelial opening through which emboli translocated into the perivascular parenchyma. The rate of embolus extravasation was significantly decreased by pharmacological inhibition of matrix metalloproteinase 2/9 activity. In aged mice, extravasation was markedly delayed, resulting in persistent tissue hypoxia, synaptic damage and cell death. Alterations in the efficiency of the protective mechanism that we have identified may have important implications in microvascular pathology, stroke recovery and age-related cognitive decline.

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Figure 1: Emboli that fail to be washed out undergo extravasation leading to re-establishment of blood flow.
Figure 2: Focal endothelial remodelling underlies the translocation of emboli.
Figure 3: MMP-2/9 activity is induced around the occlusion site.
Figure 4: Delayed extravasation in ageing contributes to persistent hypoxia, synaptic injury and cell death.

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Acknowledgements

We thank S. Lo, C. Freitas, D. Choi and C. Whiteus for help with experiments; A. Schain for suggesting the use of ACTB–eGFP mice; E. Mugnaini for advice with transmission electron microscopy experiments; and G. D’avossa, J. García-Añoveros, J. Kessler and P. Opal for helpful discussions and critical review of the manuscript. This work was supported by National Institutes of Health/National Institute on Aging grant AG027855 (J.G.) and a Howard Hughes Medical Institute medical student research fellowship (C.K.L.).

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Authors

Contributions

J.G., C.K.L. and T.Y. conceived and designed the project. C.K.L., T.Y., B.H. and J.G. performed in vivo two-photon imaging. C.K.L., B.H. and J.G. adapted the technique and performed electron microscopy experiments. C.K.L. and T.Y. performed in situ zymography, ageing mice and SB-3CT experiments. T.Y. performed cell culture and the imaging experiment on human umbilical-vein endothelial cells. Z.L., C.K.L., T.Y., B.H. and J.G. performed histological and confocal microscopy experiments. J.G. wrote the manuscript with significant input from C.K.L. and T.Y.

Corresponding author

Correspondence to Jaime Grutzendler.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1-14 with legends and legends for Supplementary Movies 1-5. (PDF 1611 kb)

Supplementary Movie 1

This movie shows embolus in the process of extravasation – see Supplementary Information file for full legend. (MOV 22465 kb)

Supplementary Movie 2

This movie shows embolus extravasation and reestablishment of blood flow – see Supplementary Information file for full legend. (MOV 2156 kb)

Supplementary Movie 3

This movie shows microsphere extravasation – see Supplementary Information file for full legend. (MOV 6848 kb)

Supplementary Movie 4

This movie shows early extravasation and reestablishment of blood flow – see Supplementary Information file for full legend. (MOV 1378 kb)

Supplementary Movie 5

This movie shows cholesterol embolus occlusion – see Supplementary Information file for full legend. (MOV 778 kb)

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Lam, C., Yoo, T., Hiner, B. et al. Embolus extravasation is an alternative mechanism for cerebral microvascular recanalization. Nature 465, 478–482 (2010). https://doi.org/10.1038/nature09001

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