PPAR-γ activation enhances myelination and neurological recovery in premature rabbits with intraventricular hemorrhage

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Significance

Intraventricular hemorrhage (IVH) is the most common neurological disorder of premature infants and remains a major cause of white matter injury in the survivors. No therapeutic strategy exists to prevent IVH-induced hypomyelination and cerebral palsy in these infants. We showed that pharmacological or genetic activation of peroxisome proliferator activated receptor-γ (PPAR-γ) ameliorated inflammation, oligodendrocyte progenitor cell (OPC) maturation, myelination, and motor function in rabbit kits with IVH. PPAR-γ activation also promoted microglial phagocytosis but did not reduce hydrocephalus. Transcriptomic analyses identified previously unrecognized PPAR-γ–induced genes in the isolated OPCs and microglia, which would reprogram these cells toward reducing inflammation and promoting myelination. The study highlights PPAR-γ–induced reversal of white matter injury in premature newborns with IVH and the underlying mechanisms.

Abstract

Intraventricular hemorrhage (IVH) results in periventricular inflammation, hypomyelination of the white matter, and hydrocephalus in premature infants. No effective therapy exists to prevent these disorders. Peroxisome proliferator activated receptor-γ (PPAR-γ) agonists reduce inflammation, alleviate free radical generation, and enhance microglial phagocytosis, promoting clearance of debris and red blood cells. We hypothesized that activation of PPAR-γ would enhance myelination, reduce hydrocephalus, and promote neurological recovery in newborns with IVH. These hypotheses were tested in a preterm rabbit model of IVH; autopsy brain samples from premature infants with and without IVH were analyzed. We found that IVH augmented PPAR-γ expression in microglia of both preterm human infants and rabbit kits. The treatment with PPAR-γ agonist or PPAR-γ overexpression by adenovirus delivery further elevated PPAR-γ levels in microglia, reduced proinflammatory cytokines, increased microglial phagocytosis, and improved oligodendrocyte progenitor cell (OPC) maturation in kits with IVH. Transcriptomic analyses of OPCs identified previously unrecognized PPAR-γ–induced genes for purinergic signaling, cyclic adenosine monophosphate generation, and antioxidant production, which would reprogram these progenitors toward promoting myelination. RNA-sequencing analyses of microglia revealed PPAR-γ–triggered down-regulation of several proinflammatory genes and transcripts having roles in Parkinson’s disease and amyotrophic lateral sclerosis, contributing to neurological recovery in kits with IVH. Accordingly, PPAR-γ activation enhanced myelination and neurological function in kits with IVH. This also enhanced microglial phagocytosis of red blood cells but did not reduce hydrocephalus. Treatment with PPAR-γ agonist might enhance myelination and neurological recovery in premature infants with IVH.

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Most cited references 52

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Microglia development follows a stepwise program to regulate brain homeostasis.

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Microglia, the resident myeloid cells of the central nervous system, play important roles in life-long brain maintenance and in pathology. Despite their importance, their regulatory dynamics during brain development have not been fully elucidated. Using genome-wide chromatin and expression profiling coupled with single-cell transcriptomic analysis throughout development, we found that microglia undergo three temporal stages of development in synchrony with the brain--early, pre-, and adult microglia--which are under distinct regulatory circuits. Knockout of the gene encoding the adult microglia transcription factor MAFB and environmental perturbations, such as those affecting the microbiome or prenatal immune activation, led to disruption of developmental genes and immune response pathways. Together, our work identifies a stepwise microglia developmental program integrating immune response pathways that may be associated with several neurodevelopmental disorders.

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Interferon regulatory factor 7 (IRF7) was originally identified in the context of Epstein-Barr virus (EBV) infection, and has since emerged as the crucial regulator of type I interferons (IFNs) against pathogenic infections, which activate IRF7 by triggering signaling cascades from pathogen recognition receptors (PRRs) that recognize pathogenic nucleic acids. Moreover, IRF7 is a multifunctional transcription factor, underscored by the fact that it is associated with EBV latency, in which IRF7 is induced as well as activated by the EBV principal oncoprotein latent membrane protein-1 (LMP1). Aberrant production of type I IFNs is associated with many types of diseases such as cancers and autoimmune disorders. Thus, tight regulation of IRF7 expression and activity is imperative in dictating appropriate type I IFN production for normal IFN-mediated physiological functions. Posttranslational modifications have important roles in regulation of IRF7 activity, exemplified by phosphorylation, which is indicative of its activation. Furthermore, mounting evidence has shed light on the importance of regulatory ubiquitination in activation of IRF7. Albeit these exciting findings have been made in the past decade since its discovery, many questions related to IRF7 remain to be addressed.

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Intraventricular hemorrhage in premature infants: mechanism of disease.

Praveen Ballabh (2010)

Intraventricular hemorrhage (IVH) is a major complication of prematurity. IVH typically initiates in the germinal matrix, which is a richly vascularized collection of neuronal-glial precursor cells in the developing brain. The etiology of IVH is multifactorial and is primarily attributed to the intrinsic fragility of the germinal matrix vasculature and the disturbance in the cerebral blood flow (CBF). Although this review broadly describes the pathogenesis of IVH, the main focus is on the recent development in molecular mechanisms that elucidates the fragility of the germinal matrix vasculature. The microvasculature of the germinal matrix is frail because of an abundance of angiogenic blood vessels that exhibit paucity of pericytes, immaturity of basal lamina, and deficiency of glial fibrillary acidic protein (GFAP) in the ensheathing astrocytes endfeet. High VEGF and angiopoietin-2 levels activate a rapid angiogenesis in the germinal matrix. The elevation of these growth factors may be ascribed to a relative hypoxia of the germinal matrix perhaps resulting from high metabolic activity and oxygen consumption of the neural progenitor cells. Hence, the rapid stabilization of the angiogenic vessels and the restoration of normal CBF on the first day of life are potential strategies to prevent IVH in premature infants.