Longitudinal monitoring of microglial/ macrophage activation in ischemic rat brain using Iba-1-specific nanoparticle enhanced MRI

Significance 

After a stroke, the brain’s resident immune cells, the microglia/macrophages, become activated with one phenotype predominating over another in a time-dependent manner. Microglia rapidly develop a pro-inflammatory phenotype in response to acute stroke injury; meanwhile, activation of microglia also present reparative and anti-inflammatory roles through a regulatory/homeostatic phenotype, which facilitates recovery. Previous studies have shown that the position of the active microglia/macrophages determined via dynamic analyses in relation to the infarct in the ischemic brain at different times is an important feature of these different phenotypes. To develop treatment approaches that will both suppress the harmful effects of microglial/macrophage activation while still maintaining neurovascular remodeling and repair, it is necessary to understand the dynamics in vivo of stroke-induced activation of cerebral microglial/ macrophages.

To determine the spatiotemporal distribution of microglial/macrophage activation in the brain of a living rat, University of New Mexico Health Sciences Center scientists:  Professor Laurel Sillerud, Dr. Yirong Yang, Lisa Yang, Kelsey Duval, Jeffrey Thompson, and Professor Yi Yang used nanoparticle-enhanced magnetic resonance imaging (MRI) to visualize neuroinflammation occurring in the living brain after an ischemic stroke. The original research article is published in the Journal of Cerebral Blood Flow & Metabolism1.  Up to four weeks post-stroke, MRI with the aid of superparamagnetic iron–platinum (FePt) nanoparticles, containing surface anti-Iba-1 antibodies specific for activated microglia/macrophages, revealed the positions and amounts of microglia/macrophages using T2*-weighted imaging. The team found that the nanoparticles accumulated in the regions of the brain (Figure 1) that coincided with areas where active microglia/macrophages were detected by post-mortem immunohistochemistry.

They also observed a good correlation in morphological and distributive dynamic changes between the Fe+-cells and the Iba-1+-microglia/macrophages. The microglial/macrophage activation and phenotypic changes that were assessed by post-mortem immunohistochemistry over the four weeks post-stroke were found to parallel the spatiotemporal changes of nanoparticle-induced image contrast detected by T2*-weighted MRI. Using these techniques, the research team concluded that the maximum level of microglial/macrophage activation occurred 7- days post-stroke; the activation then diminished after two weeks and continued to fall at four weeks.  Professor Sillerud said that it was “now possible to essentially count the microglial cells responsible for stroke-induced neuroinflammation in the living brain.”

In this molecular imaging study, the authors were able to successfully demonstrate the enhancement of MRI using anti-Iba-1conjugated FePt nanoparticles and to visualize the longitudinal time-course and spatial distribution of the inflammation induced by stroke and to show the precise and localized changes in microglial/macrophage activation. Their results provide evidence that the progression and treatment of stroke in living animals and the dynamic development of the associated neuroinflammation can be monitored non-invasively using this novel application of targeted nanoparticles. It will be interesting in the future to modify the FePt nanoparticles and adapt them to develop a theranostic agent where both detection and therapeutic molecules can be combined for better stroke therapy.

Longitudinal monitoring of microglial/ macrophage activation in ischemic rat brain using Iba-1-specific nanoparticle enhanced magnetic resonance imaging - Medicine Innovates
Figure 1. T2*-weighted MRI of the brain of a living rat one week after a middle cerebral artery occlusion (A) prior to and (B) subsequent to the intravenous injection of Iba-1-targeted FePt nanoparticles. Note the darkened areas in the ischemic hemisphere (white arrow) in B that reflect the labeling of microglia/macrophages by the FePt nanoparticles (yellow Arrow). (C) A post-mortem formalin-fixed brain slice from the same rat stained for Iba-1 (green) shows that the lesion area in the ischemic hemisphere (arrow) coincides with that seen in the T2*-w MR image in (A). “J Cereb Blood Flow Metab. 2020 Dec;40(1_suppl):S117-S133. Copyright ©2020 Sage Publications Inc. DOI: 10.1177/0271678X20953913.

About the author

Professor Laurel Sillerud is a Research Professor in the Department of Neurology at the University of New Mexico School of Medicine. He has devoted his career to the application of magnetic resonance and nanotechnology to the diagnosis and characterization of disease, most recently to Stroke and Alzheimer’s dementia.  He obtained his Ph.D. in Biophysics and Physiology from the University of Minnesota School of Medicine.  His postdoctoral research in the laboratory of Robert G. Shulman at Yale University emphasized non-invasive magnetic resonance investigations of metabolism in living systems, including humans.  He subsequently was director of the Biomedical NMR Center at Los Alamos National Laboratory where he developed magnetic resonance methods for the diagnosis of prostate cancer.  Upon joining the faculty at the University of New Mexico, he specialized in the application of magnetic nanotechnology and MRI to the detection cancer and brain diseases. He served for several years as Director of the MRI Core and the UNM BRaIN Center.  He was elected as a Fellow of the American Association for the Advancement of Science in 2014.

Selected publications can be found at https://pubmed.ncbi.nlm.nih.gov/?term=Sillerud+L

About the author

Dr. Yi Yang is a Professor of Neurology at the University of New Mexico School of Medicine. She holds an MD and a PhD in medicine and neuroscience from the Chongqing Medical University in China and received her first post-doctoral training at Bejing Institute of Basic Medical Sciences/the University of Hong Kong, and then at Department of Neuroscience, University of New Mexico. She joined the Department of Neurology in University of New Mexico in 2004 as a basic science faculty. The research of Dr. Yang’s laboratory is focused on the understanding and translational potential of the cellular and molecular mechanisms of brain injury and the neurovascular remodeling associated with ischemic cerebral stroke, vascular cognitive impairment, and dementia (). This project of “Longitudinal monitor of microglia activation after stroke with SPION­enhanced MRI” was supported by a NIH/NINDS grant, 1R21NS091710k, to Dr. Yi Yang.

Selected publications can be found at https://www.researchgate.net/profile/Yi-Yang-234

Reference

Sillerud LO, Yang Y, Yang LY, Duval KB, Thompson J, Yang Y. Longitudinal monitoring of microglial/macrophage activation in ischemic rat brain using Iba-1-specific nanoparticle-enhanced magnetic resonance imaging. J Cereb Blood Flow Metab. 2020 Dec;40(1_suppl):S117-S133.

Go To J Cereb Blood Flow Metab

About Priti