The dynamics of revascularization after white matter infarction monitored in Flt1-tdsRed and Flk1-GFP mice

Pericyte-derived fibrotic scarring is conserved across diverse central nervous system lesions

Fibrotic scar tissue limits central nervous system regeneration in adult mammals. The extent of fibrotic tissue generation and distribution of stromal cells across different lesions in the brain and spinal cord has not been systematically investigated in mice and humans. Furthermore, it is unknown whether scar-forming stromal cells have the same origin throughout the central nervous system and in different types of lesions. In the current study, we compared fibrotic scarring in human pathological tissue and corresponding mouse models of penetrating and non-penetrating spinal cord injury, traumatic brain injury, ischemic stroke, multiple sclerosis and glioblastoma. We show that the extent and distribution of stromal cells are specific to the type of lesion and, in most cases, similar between mice and humans. Employing in vivo lineage tracing, we report that in all mouse models that develop fibrotic tissue, the primary source of scar-forming fibroblasts is a discrete subset of perivascular cells, termed type A pericytes. Perivascular cells with a type A pericyte marker profile also exist in the human brain and spinal cord. We uncover type A pericyte-derived fibrosis as a conserved mechanism that may be explored as a therapeutic target to improve recovery after central nervous system lesions.

Transient Receptor Potential Melastatin 3 Is Functionally Expressed in Oligodendrocyte Precursor Cells and Is Upregulated in Ischemic Demyelinated Lesions

Oligodendrocyte precursor cells (OPCs) are glial cells that differentiate into oligodendrocytes and myelinate axons. The number of OPCs is reportedly increased in brain lesions in some demyelinating diseases and during ischemia; however, these cells also secrete cytokines and elicit both protective and deleterious effects in response to brain injury. The mechanism regulating the behaviors of OPCs in physiological and pathological conditions must be elucidated to control these cells and to treat demyelinating diseases. Here, we focused on transient receptor potential melastatin 3 (TRPM3), a Ca²⁺-permeable channel that is activated by the neurosteroid pregnenolone sulfate (PS) and body temperature. Trpm3⁺/Pdgfra⁺ OPCs were detected in the cerebral cortex (CTX) and corpus callosum (CC) of P4 and adult rats by in situ hybridization. Trpm3 expression was detected in primary cultured rat OPCs and was increased by treatment with tumor necrosis factor α (TNFα). Application of PS (30-100 µM) increased the Ca²⁺ concentration in OPCs and this effect was inhibited by co-treatment with the TRP channel blocker Gd³⁺ (100 µM) or the TRPM3 inhibitor isosakuranetin (10 µM). Stimulation of TRPM3 with PS (50 µM) did not affect the differentiation or migration of OPCs. The number of Trpm3⁺ OPCs was markedly increased in demyelinated lesions in an endothelin-1 (ET-1)-induced ischemic rat model. In conclusion, TRPM3 is functionally expressed in OPCs in vivo and in vitro and is upregulated in inflammatory conditions such as ischemic insults and TNFα treatment, implying that TRPM3 is involved in the regulation of specific behaviors of OPCs in pathological conditions.

Acute Ablation of Cortical Pericytes Leads to Rapid Neurovascular Uncoupling

Pericytes are perivascular mural cells that enwrap brain capillaries and maintain blood-brain barrier (BBB) integrity. Most studies suggest that pericytes regulate cerebral blood flow (CBF) and oxygen delivery to activated brain structures, known as neurovascular coupling. While we have previously shown that congenital loss of pericytes leads over time to aberrant hemodynamic responses, the effects of acute global pericyte loss on neurovascular coupling have not been studied. To address this, we used our recently reported inducible pericyte-specific Cre mouse line crossed to iDTR mice carrying Cre-dependent human diphtheria toxin (DT) receptor, which upon DT treatment leads to acute pericyte ablation. As expected, DT led to rapid progressive loss of pericyte coverage of cortical capillaries up to 50% at 3 days post-DT, which correlated with approximately 50% reductions in stimulus-induced CBF responses measured with laser doppler flowmetry (LDF) and/or intrinsic optical signal (IOS) imaging. Endothelial response to acetylcholine, microvascular density, and neuronal evoked membrane potential responses remained, however, unchanged, as well as arteriolar smooth muscle cell (SMC) coverage and functional responses to adenosine, as we previously reported. Together, these data suggest that neurovascular uncoupling in this model is driven by pericyte loss, but not other vascular deficits or neuronal dysfunction. These results further support the role of pericytes in CBF regulation and may have implications for neurological conditions associated with rapid pericyte loss such as hypoperfusion and stroke, as well as conditions where the exact time course of global regional pericyte loss is less clear, such as Alzheimer's disease (AD) and other neurogenerative disorders.

Transplantation of iPS-derived vascular endothelial cells improves white matter ischemic damage

White matter infarct induces demyelination and brain dysfunction. We previously reported that transplantation of brain microvascular endothelial cells improved the behavioral outcome and promoted remyelination by increasing the number of oligodendrocyte precursor cells in the rat model of white matter infarct. In this study, we investigated the effects of transplantation of vascular endothelial cells generated from human induced pluripotent stem cells (iPSCs) on the rat model of white matter infarct. Seven days after induction of ischemic demyelinating lesion by injection of endothelin‐1 into the internal capsule of a rat brain, iPSC‐derived vascular endothelial cells (iVECs) were transplanted into the site of demyelination. The majority of iVECs transplanted into the internal capsule survived for 14 days after transplantation when traced by immunohistochemistry for a human cytoplasmic protein. iVEC transplantation significantly recovered hind limb rotation angle as compared to human iPSC or rat meningeal cell transplantation when evaluated using footprint test. Fourteen days after iVEC transplantation, the infarct area remarkably decreased as compared to that just before the transplantation when evaluated using magnetic resonance imaging or luxol fast blue staining, and remyelination was promoted dramatically in the infarct when assessed using luxol fast blue staining. Transplantation of iVECs increased the number of oligodendrocyte lineage cells and suppressed the inflammatory response and reactive astrocytogenesis. These results suggest that iVEC transplantation may prove useful in treatment for white matter infarct.