ABSTRACT
BACKGROUND: Spontaneous deep intracerebral hemorrhage (ICH) is a devastating subtype of stroke without specific treatments. It has been thought that smooth muscle cell (SMC) degeneration at the site of arteriolar wall rupture may be sufficient to cause hemorrhage. However, deep ICHs are rare in some aggressive small vessel diseases that are characterized by significant arteriolar SMC degeneration. Here we hypothesized that a second cellular defect may be required for the occurrence of ICH. METHODS: We studied a genetic model of spontaneous deep ICH using Col4a1+/G498V and Col4a1+/G1064D mouse lines that are mutated for the α1 chain of collagen type IV. We analyzed cerebroretinal microvessels, performed genetic rescue experiments, vascular reactivity analysis, and computational modeling. We examined postmortem brain tissues from patients with sporadic deep ICH. RESULTS: We identified in the normal cerebroretinal vasculature a novel segment between arterioles and capillaries, herein called the transitional segment (TS), which is covered by mural cells distinct from SMCs and pericytes. In Col4a1 mutant mice, this TS was hypermuscularized, with a hyperplasia of mural cells expressing more contractile proteins, whereas the upstream arteriole exhibited a loss of SMCs. TSs mechanistically showed a transient increase in proliferation of mural cells during postnatal maturation. Mutant brain microvessels, unlike mutant arteries, displayed a significant upregulation of SM genes and Notch3 target genes, and genetic reduction of Notch3 in Col4a1+/G498V mice protected against ICH. Retina analysis showed that hypermuscularization of the TS was attenuated, but arteriolar SMC loss was unchanged in Col4a1+/G498V, Notch3+/- mice. Moreover, hypermuscularization of the retinal TS increased its contractility and tone and raised the intravascular pressure in the upstream feeding arteriole. We similarly found hypermuscularization of the TS and focal arteriolar SMC loss in brain tissues from patients with sporadic deep ICH. CONCLUSIONS: Our results suggest that hypermuscularization of the TS, through increased Notch3 activity, is involved in the occurrence of ICH in Col4a1 mutant mice, by raising the intravascular pressure in the upstream feeding arteriole and promoting its rupture at the site of SMC loss. Our human data indicate that these 2 mutually reinforcing vascular defects may represent a general mechanism of deep ICH.
Subject(s)
Cerebral Hemorrhage/etiology , Cerebral Hemorrhage/prevention & control , Microvessels/metabolism , Muscle, Smooth, Vascular/metabolism , Animals , Biomarkers , Cerebral Hemorrhage/diagnosis , Cerebral Hemorrhage/metabolism , Collagen Type IV/genetics , Collagen Type IV/metabolism , Disease Models, Animal , Disease Susceptibility , Gene Expression , Genotype , Humans , Immunohistochemistry , Mice , Mice, Knockout , Microvessels/physiopathology , Molecular Imaging , Mutation , Myocytes, Smooth Muscle/metabolism , Receptor, Notch3/metabolism , Retina/metabolism , Retina/pathology , Retinal Neovascularization/genetics , Retinal Neovascularization/metabolism , Retinal Neovascularization/pathologyABSTRACT
Microglia are known to regulate several aspects of the development of the central nervous system. When microglia colonize the spinal cord, from E11.5 in the mouse embryo, they interact with growing central axons of dorsal root ganglion sensory neurons (SNs), which suggests that they may have some functions in SN development. To address this issue, we analyzed the effects of embryonic macrophage ablation on the early development of SNs using mouse embryo lacking embryonic macrophages (PU.1 knock-out mice) and immune cell ablation. We discovered that, in addition to microglia, embryonic macrophages contact tropomyosin receptor kinase (Trk) C+ SN, TrkB+ SN, and TrkA+ SN peripheral neurites from E11.5. Deprivation of immune cells resulted in an initial reduction of TrkC+ SN and TrkB+ SN populations at E11.5 that was unlikely to be related to an alteration in their developmental cell death (DCD), followed by a transitory increase in their number at E12.5. It also resulted in a reduction of TrkA+ SN number during the developmental period analyzed (E11.5-E15.5), although we did not observe any change in their DCD. Proliferation of cells negative for brain fatty acid-binding protein (BFABP- ), which likely correspond to neuronal progenitors, was increased at E11.5, while their proliferation was decreased at E12.5, which could partly explain the alterations of SN subtype production observed from E11.5. In addition, we observed alterations in the proliferation of glial cell progenitors (BFABP+ cells) in the absence of embryonic macrophages. Our data indicate that embryonic macrophages and microglia ablation alter the development of SNs.
