Molecular Magnetic Resonance Imaging of Endothelial Activation in the Central Nervous System

Abstract

Endothelial cells of the central nervous system over-express surface proteins during neurological disorders, either as a cause, or a consequence, of the disease. Since the cerebral vasculature is easily accessible by large contrast-carrying particles, it constitutes a target of choice for molecular magnetic resonance imaging (MRI). In this review, we highlight the most recent advances in molecular MRI of brain endothelial activation and focus on the development of micro-sized particles of iron oxide (MPIO) targeting adhesion molecules including intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1), P-Selectin and E-Selectin. We also discuss the perspectives and challenges for the clinical application of this technology in neurovascular disorders (ischemic stroke, intracranial hemorrhage, subarachnoid hemorrhage, diabetes mellitus), neuroinflammatory disorders (multiple sclerosis, brain infectious diseases, sepsis), neurodegenerative disorders (Alzheimer's disease, vascular dementia, aging) and brain cancers (primitive neoplasms, metastasis).

Keywords: antibodies; leucocytes; lymphocytes; neuroinflammation; ultra-small particles of iron oxide (USPIO).

Figure 1

Schematic representation of the multistep process of leukocyte diapedesis. In inflammatory conditions, leukocytes first interact with the selectins expressed by the CNS endothelium, allowing their rolling to the vessel wall. Rolling leukocytes sense chemokines that induce their activation, thereby triggering the activation of their integrins and subsequent interactions with adhesion molecules (VCAM-1 and ICAM-1). Lastly, the leukocytes transmigrate across the endothelial layer, typically by a paracellular route, by a PECAM-1 dependent mechanism. LFA-1: lymphocyte function-associated antigen-1; VLA-4: very late antigen-4; PSGL-1: P-selectin glycoprotein ligand-1; VCAM-1: vascular cell adhesion molecule-1; ICAM-1: intercellular adhesion molecule-1; PECAM-1: platelet endothelial cell adhesion molecule-1.

Figure 2

Schematic representation of the selectin and adhesion molecule expression profiles of quiescent, type I activated and type II activated endothelial cells. Type I activated endothelial cells present high surface levels of P-selectin. Type II activated endothelial cells present high surface levels of VCAM-1, ICAM-1 and E-selectin. Each of these four proteins are suitable targets for molecular MRI of endothelial activation. VCAM-1: vascular cell adhesion molecule-1; ICAM-1: intercellular adhesion molecule-1; PECAM-1: platelet endothelial cell adhesion molecule-1.

Figure 3

Schematic representation of the structure of a typical targeted microsized particle of iron oxide (MPIO) for molecular MRI. Targeted MPIOs are made of two parts: the first, the contrastophore, is the part responsible for the changes in the MRI contrast. The second, the pharmacophore, is the part responsible for the binding of the contrastophore to its target. In this figure are listed the most common contrastophores and pharmacophores. In the lower part of the figure is a simplified view of the biological function of the targets of the most popular pharmacophores for molecular MRI of endothelial activation.

Figure 4

Molecular MRI of VCAM-1 in the subacute phase of ischemic stroke. (A) Schematic representation of the experiment. MPIO-αVCAM-1 were injected 24 hours after stroke onset (induced by electrocoagulation of the middle cerebral artery). (B) In vivo MRI before and after MPIO-αVCAM-1 injection showing cortical signal voids extending outside the ischemic lesion (as revealed by a hypersignal on T2-weighted imaging), revealing the inflammatory penumbra, which is defined by the mismatch between the VCAM-1 overexpressing region and the ischemic lesion. (C) Schematic representation (left) and immunohistological images of the activated brain vessels in the inflammatory penumbra with numerous bound MPIO-αVCAM-1 visible as white dots on the darkfield image. Further details on this experimental data are available in the paper by Gauberti et al. .

Figure 5

Molecular MRI of P-selectin in the brain and spinal cord of experimental autoimmune encephalomyelitis (EAE) mice. (A) Schematic representation of the experiment. MPIO-αP-Selectin were injected several days after EAE induction and MRI of the brain and the spinal cord were acquired. (B) Immunohistological images revealing numerous MPIO-αP-Selectin on an activated endothelium in the brain of an EAE mouse. (C) In vivo MRI of the brain and spinal cord of an EAE mouse before and after MPIO-αP-Selectin injection. Molecular MRI reveals strong endothelial activation in the whole CNS, predominating in the spinal cord. Further details on this experiment are available in the paper by Fournier et al. .

Figure 6

Molecular MRI of VCAM-1 in a model of vascular dementia induced by chronic cerebral hypoperfusion. (A) Schematic representation of the experimental model. The right common carotid artery (CCA) was ligated (after its isolation from the jugular vein, JV), inducing chronic hypoperfusion of the right brain hemisphere. (B) Immunohistological images revealing VCAM-1 expression by the endothelium of the hypoperfused brain. (C) In vivo MRI of the brain before and after MPIO-αVCAM-1 injection 48 hours after CCA ligation revealing significant endothelial activation in the hypoperfused hemisphere. Further details on this experiment are available in Montagne et al. .

Figure 7

Schematic representation of the relationship between spatial resolution and MPIO detection sensitivity. Given the heterogenous repartition of targeted MPIOs in the inflamed CNS, increase in imaging resolution allows isolation of MPIO-labeled (MPIO+) from non-labeled tissue (MPIO-) and therefore increases the sensitivity of MRI. This comes at the price of a lower signal to noise ratio and a slower image acquisition.