Neuronal ER Stress Impedes Myeloid-Cell-Induced Vascular Regeneration through IRE1α Degradation of Netrin-1

Abstract

In stroke and proliferative retinopathy, despite hypoxia driven angiogenesis, delayed revascularization of ischemic tissue aggravates the loss of neuronal function. What hinders vascular regrowth in the ischemic central nervous system remains largely unknown. Using the ischemic retina as a model of neurovascular interaction in the CNS, we provide evidence that the failure of reparative angiogenesis is temporally and spatially associated with endoplasmic reticulum (ER) stress. The canonical ER stress pathways of protein kinase RNA-like ER kinase (PERK) and inositol-requiring enzyme-1α (IRE1α) are activated within hypoxic/ischemic retinal ganglion neurons, initiating a cascade that results in angiostatic signals. Our findings demonstrate that the endoribonuclease IRE1α degrades the classical guidance cue netrin-1. This neuron-derived cue triggers a critical reparative-angiogenic switch in neural macrophage/microglial cells. Degradation of netrin-1, by persistent neuronal ER stress, thereby hinders vascular regeneration. These data identify a neuronal-immune mechanism that directly regulates reparative angiogenesis.

Nrf2 in ischemic neurons promotes retinal vascular regeneration through regulation of semaphorin 6A

“…To identify potential candidate mediators, we first analyzed the available Nrf2 Chip-seq databases (25,26) and found five axon guidance genes (ephrinA5, ephrinB2, Sema4A, Sema6D, Sema3E) that might represent the direct candidate targets of Nrf2. We therefore investigated this potential regulatory relationship, using quantitative RT-PCR, and also included analysis of Sema3A, Sema6A, Slit1, and Netrin1, all of which have been shown to regulate angiogenesis in previous studies (1,2,27). The result showed that Sema6A mRNA was significantly increased in Nrf2 −/− retinas at P15 of OIR, whereas no significant changes in expression were observed for the other genes (Fig.…”

“…The Nrf2 OIR. The OIR model was described previously (16,27,51,52). Mice pups were exposed to 75% (vol/vol) oxygen for 5 d from P7 to P12.…”

Assessment of Vascular Regeneration in the CNS Using the Mouse Retina

“…In the presented study, we demonstrate that sustained neuronal ischemia activates endoplasmic reticulum (ER) stress and via one of its effector endoribonucleases IRE1α, cleaves the mRNA of the classical neuronal guidance cue netrin-1. We show that intra-ocular delivery of the Netrin-1, stimulates a program of reparative angiogenesis in retinal myeloid cells and thus accelerates neural tissue revascularization after OIR 25 . Moreover, we provide an example of accelerated vascular regeneration using lentiviral mediated silencing of IRE1α.…”

“…When assessing effects of a viral vector, one must consider tropism of the virus and allow sufficient time for full expression of virally-delivered transgenes. Rapidly expressing viral vectors such as 3 rd generation lentiviruses 24,25,33 are recommended.…”

“…For example, enhancing "physiological-like" vascular regrowth in ischemic retinopathies has been elegantly demonstrated through introduction of endothelial precursor cells 17 , inhibition of Müller cell-expressed VEGF induced downregulation of other angiogenic factors 18 , injection of myeloid progenitors 19 , inhibition of NADPH oxidase induced apoptosis 20 , increasing dietary ω-3 polyunsaturated fatty acid intake 21 , treatment with a carboxyl-terminal fragment of tryptophan tRNA synthetase 22 , and direct administration of VEGF or FGF-2 for protection of glial cells 23 . Moreover, we have demonstrated that modulating classical neuronal guidance cues such as Semaphorins or Netrins in ischemic retinopathies accelerates vascular regeneration of healthy vessels within the retina and consequently reduces pathological angiogenesis 24,25 . Of direct clinical relevance, several of the aforementioned animal studies provide evidence that promoting vascular regeneration during the early ischemic phase of retinopathies can significantly reduce sight-threatening pre-retinal neovascularization .…”