Vascular Remodeling After Cerebral Ischemia

Neovascularization After Cerebral Ischemia and Hypoxia

Although the adult brain vascular network becomes quiescent after the completion of brain development, the remodeling of existing blood vessels or de novo vessel formation has been well documented after cerebral ischemia in both humans and laboratory animals as one of the CNS responses to ischemic injury or hypoxia. Because the mechanisms and mediators involved in the above two processes are complex and partially overlap, “neovascularization” has been the commonly recognized term to describe such phenomenon. As in other organs, neovascularization in the brain requires an orchestrated interplay among the immune, endocrine, and vascular systems, which can be categorized into three distinct mechanisms, namely angiogenesis, vasculogenesis, and arteriogenesis.

Arteriogenesis is a crucial process for maintaining bulk blood supply in the event of abrupt obstruction of blood flow like an ischemic stroke, or a chronic adaptation in response to progressive narrowing of the vasculature occurring in human carotid stenoocclusive diseases. Arteriogenesis, enabling blood flowing through the preexisting arterial anastomosis virtually instantaneously after cerebral ischemia, is not only the most immediate neovascularization process but also the most effective flow compensation mechanism in restoring ischemia-induced perfusion deficit and in salvaging potential tissue loss. Unlike arteriogenesis, vascular neogenesis via angiogenesis and vasculogenesis involves cell proliferation and thus requires time; consequently they are unlikely to deliver functional blood flow soon enough to prevent ischemic cell death. It is generally accepted that when capillaries develop in previously avascular tissue, it is called vasculogenesis, a repair process initiated by the recruitment of endothelial progenitor cells (EPCs). In contrast, angiogenesis often refers to the formation of new capillaries by sprouting from the existing venules.

With respect to the direct functional output of neovascularization processes in restoring blood flow, experimental evidence suggests that the early stage of cerebral blood volume (CBV) increase is predominantly due to the recruitment of collateral flow, which is indispensible in reducing ischemic cell death, whereas the later phase of CBV increase is likely attributed to the emergence of angiogenesis and/or vasculogenesis, which may influence recovery and long-term functional outcome . The mechanism and contribution of each neovascularization process following cerebral ischemia is discussed further in this chapter from the perspectives of human and experimental brain ischemia and hypoxia.

Angiogenesis After Ischemia and Hypoxia

Ample structural evidence supports the occurrence of angiogenesis and induction of angiogenetic factors following both human stroke and experimental cerebral ischemia or hypoxia . Stroke patients with the higher blood vessel counts in the infarcted brain tissue correlated with longer survival, suggesting that angiogenesis might play a beneficial role in stroke outcome . Angiogenesis is reported to act as a route for infiltrating macrophages that clean up the necrotic debris and thus promote tissue remodeling, adding to the list of benefits. Stroke-induced angiogenesis appears to be transient and restricted to the border of the infarct, suggesting that it is a spatially and temporally self-contained process. However, due to the limited size of small capillaries, angiogenesis is unable to fully restore the function of larger vessels. Besides, factors produced during angiogenesis such as vascular endothelial growth factor (VEGF) and matrix metalloproteinase 9 have the potential to worsen vasogenic edema by increasing blood–brain barrier (BBB) leakage or by increasing the risk of hemorrhagic transformation.

The main stimulus of angiogenesis both in development and following ischemia is hypoxia, activating hypoxia-inducing factor (HIF) and downstream signaling events. HIF led to the increased expression of VEGF and activation of the receptor pathways, which are known as the earliest and the most crucial players in angiogenesis. The other important early regulators of angiogenesis include the Notch–Jagged/Delta-like pathways, whereas later vascular development and lumen formation is controlled by the ephrin–Eph, Wnt, and those involved in axonal guidance, such as the semaphorins, netrins, and ROBO/Slits signaling pathways. The angiopoietin–Tie, platelet-derived growth factor, and transforming growth factor beta families are also known to regulate additional aspects of angiogenesis that may include, but not limited to, vessel maturation. To form mature and functional blood vessels, pericytes and vascular smooth muscle cells are required to stabilize capillaries and control vessel conductance, respectively. To that end, PDGF is most instrumental in recruiting pericytes to cap the newly formed vascular tube and Ang-1 in reducing vascular leakage.

VEGF-A expression is upregulated by hypoxia via the hypoxia response element in the promoter region of the vegf gene. The tip cells of growing vessels express a high level of VEGFR-2 and organize a filopodia to migrate toward the area of higher VEGF-A expression, whereas the stalk cells proliferate, adding to the length of the sprouting vessels. Using two-photon longitudinal in vivo imaging technique, a detailed spatiotemporal dynamics of angiogenesis was witnessed in the mouse cerebral cortex after continuous exposure to hypoxia. After 7–14 days of living in the hypoxia chamber, capillary vessels located on average 60 μm away from penetrating arterioles in the cortex began to form sprouts, although red blood cells were still stagnant inside the sprouts at this stage. After 14–21 days of hypoxia, functional blood flow was established once the sprouting vessel made connection with an existing capillary. The maturation of the newly born vessels is evident by the wrapping of vessels with neighboring astrocyte processes, forming a “neuron-glia-vascular” unit . However, emerging evidence suggests that microglia-secreted factors may also interact with VEGF in promoting or guiding vessel sprouting after chronic hypoxia.

In response to mild ischemia such as unilateral common carotid artery occlusion, it appears that vascular remodeling including pial arteries and veins, and capillary dilation in the parenchyma, and collateral growth ( Fig. 19.1 ) are sufficient to restore blood flow without angiogenesis by using repeated in vivo optical imaging of cerebral microvasculature in living mice. Other studies using the permanent middle cerebral artery occlusion (MCAO) mouse models (distal occlusion) also consistently showed various remodeling of the vasculature on the ischemic cortical surface. Whether this remodeling of the vasculature involves capillary angiogenesis triggered by the ischemic insult is not supported by in vivo microscopic imaging studies. As expected, a rapid deterioration of capillary networks in the ischemic cortex was evidenced ( Fig. 19.2 ; ). Failure of in vivo detection of angiogenic capillaries in the experimental ischemia models could be due to spatiotemporal heterogeneity of the tissue remodeling after ischemic events that may limit the field of view during the microscopic imaging, or lack of the hypoxia-induced angiogenesis.

Apart from the cerebral cortex in which the vascular remodeling processes were most reported, the thalamus was also known for angiogenesis after middle cerebral artery (MCA) stroke, likely related to local hemodynamic impairment and its synaptic connections to the cortex. Following experimental stroke, the ipsilateral thalamus suffered from temporary hypoperfusion during the first week after MCAO, followed by hyperperfusion chronically for 1–3 months. At the end of 3 months, angiogenesis was detected as increased blood vessel branching in the ipsilateral thalamus, representing a novel type of remote plasticity that may support the removal of necrotic brain tissue and aid functional recovery .