MRI of spontaneous and therapy-induced vascular remodeling after experimental stroke

Abstract

Although advances have been made toward elucidating the molecular mechanisms of stroke pathology, there are still no clinically effective strategies for poststroke brain repair. A potential therapeutic target, the peri-infarct area angiogenesis, is linked to neuronal survival, neurogenesis, and plasticity. The aim of this thesis was to shed light on vascular remodeling in stroke recovery and pave the way to an MRI-guided therapy that promotes neurorestoration.

1. Following a stroke, the primary injury site can disrupt functional connections in nearby and remotely associated brain regions, resulting in the development of secondary injuries. In Chapter II, we found that cerebral blood flow (CBF), quantified by arterial spin labeling (ASL) MRI was decreased in the thalamus 2 days after focal cerebral ischemia in rats. Partial thalamic CBF recovery occurred by day 7, after which the ipsilateral thalamus was chronically hyperperfused up to 3 months post-stroke. Endothelial cell immunohistochemistry showed increased angiogenesis in the ipsilateral thalamus at the end of the follow-up. Thus, thalamic pathology involved long-term hemodynamic changes and angiogenesis, which may support the removal of necrotic brain tissue, offset secondary neuronal degeneration, and contribute to tissue remodeling.

2. The pattern of vascular remodeling in relation to stroke recovery remains largely unclear. In Chapter III we used steady-state contrast-enhanced (ssCE)-MRI to assess the development of cerebral blood volume and microvessel density (MVD) in perilesional and exofocal areas from (sub)acute to chronic time points in a rat stroke model. In remote ipsilateral areas, the thalamus and substantia nigra – not part of the ischemic territory – MVD gradually increased between days 1 and 70, which was confirmed by histology. Therefore, initial microvascular collapse, with maintained collateral flow in larger vessels, is followed by dynamic revascularization in peri-lesional tissue, as well as remote subcortical nuclei.

3. Characterization of controlled release of (therapeutic) protein(s) from hydrogel drug-delivery systems under in vivo conditions is critical, as the in vivo environment introduces many additional variables and factors that cannot be effectively simulated under in vitro conditions. In Chapter IV we developed a non-invasive in vivo MRI method to monitor local protein release from biodegradable thermosensitive hydrogels injected into rat brain by utilizing gadolinium-labeled albumin as a model protein. Our results demonstrate the potential of MRI to non-invasively monitor in vivo intracerebral protein release from an in situ-forming hydrogel while simultaneously evaluating tissue status, which could aid in the development and optimization of such drug-delivery systems for brain disorders.

4. Vascular endothelial growth factor (VEGF) and Angiopoietin-1 (Ang1) are potent pro-angiogenic peptides, with suggested neuroprotective role in cerebral stroke. In Chapter V the effects of delayed, prolonged administration of VEGF and Ang1 on cerebrovascular remodeling and functional neurological recovery was determined in a rat stroke model. We injected an in situ-forming peptide hydrogel with controlled release of VEGF/Ang1, directly into the cortical infarction area. Injection was guided and release kinetics were measured with our newly developed MRI method in Chapter IV. The treatment resulted in significant recovery from sensorimotor deficits, accompanied by significantly increased vascularization in the perilesional cortex. Histology confirmed (re)vascularization of perilesional areas and neuronal sparing or neurogenesis. Increased vascular density was also measured in remote subcortical areas connected to the infarct (thalamus) in all groups, but significantly elevated number of neurons was only present in the treatment group. Thus, late treatment with intralesionally injected hydrogel carrying VEGF/Ang1 offers an effective strategy to support prolonged brain tissue regeneration and neurological recovery after stroke.

The experimental results in this thesis support the concept that brain recovery is consistent with coordinated neovascularization both proximal and distal to the lesion, which may be exploited for post-stroke brain repair therapies.