Microglia as target for anti-inflammatory approaches to prevent secondary brain injury after subarachnoid hemorrhage (SAH)

Abstract

Background Microglia-driven cerebral spreading inflammation is a key contributor to secondary brain injury after SAH. Genetic depletion or deactivation of microglia has been shown to ameliorate neuronal cell death. Therefore, clinically feasible anti-inflammatory approaches counteracting microglia accumulation or activation are interesting targets for SAH treatment. Here, we tested two different methods of interference with microglia-driven cerebral inflammation in a murine SAH model: (i) inflammatory preconditioning and (ii) pharmacological deactivation. Methods 7T-MRI-controlled SAH was induced by endovascular perforation in four groups of C57Bl/6 mice: (i) Sham-operation, (ii) SAH naïve, (iii) SAH followed by inflammatory preconditioning (LPS intraperitoneally), and (iv) SAH followed by pharmacological microglia deactivation (colony-stimulating factor-1 receptor-antagonist PLX3397 intraperitoneally). Microglia accumulation and neuronal cell death (immuno-fluorescence), as well as activation status (RT-PCR for inflammation-associated molecules from isolated microglia) were recorded at day 4 and 14. Toll-like receptor4 (TLR4) status was analyzed using FACS. Results Following SAH, significant cerebral spreading inflammation occurred. Microglia accumulation and pro-inflammatory gene expression were accompanied by neuronal cell death with a maximum on day 14 after SAH. Inflammatory preconditioning as well as PLX3397-treatment resulted in significantly reduced microglia accumulation and activation as well as neuronal cell death. TLR4 surface expression in preconditioned animals was diminished as a sign for receptor activation and internalization. Conclusions Microglia-driven cerebral spreading inflammation following SAH contributes to secondary brain injury. Two microglia-focused treatment strategies, (i) inflammatory preconditioning with LPS and (ii) pharmacological deactivation with PLX3397, led to significant reduction of neuronal cell death. Increased internalization of inflammation-driving TLR4 after preconditioning leaves less receptor molecules on the cell surface, providing a probable explanation for significantly reduced microglia activation. Our findings support microglia-focused treatment strategies to overcome secondary brain injury after SAH. Delayed inflammation onset provides a valuable clinical window of opportunity.

Neuroprotective Strategies in Aneurysmal Subarachnoid Hemorrhage (aSAH)

“…Microglia accumulation plays a significant role in the cerebral spreading of inflammation [ 29 ] as it can be a significant target for treatment strategies. The amount of neuronal cell death can be reduced by microglia depletion.…”

Toll-Like Receptor Signaling Pathways: Novel Therapeutic Targets for Cerebrovascular Disorders

“…Several experimental investigations have shown an up-regulation of the TLR2/4-MyD88-NF-κB signaling pathway in early brain injury following SAH [ 213 , 214 , 215 ]. Preconditioning with LPS resulted in a decreased number of microglia expressing TLR4 on their surface in a mice model of SAH [ 216 ]. It has been suggested that TLR4 antagonists with appropriate BBB penetration could be the potential therapeutic candidates for SAH [ 217 ].…”

Involvement of Microglia in the Pathophysiology of Intracranial Aneurysms and Vascular Malformations—A Short Overview

“…The M1 phenotype of microglia is associated with the increase and expansion of cerebral inflammation following SAH, leading to secondary brain injury [ 9 , 10 , 43 , 44 ]. Microglia activation is especially prevalent within the regions in proximity to the hematoma or the SAH inundated basal cisterns and as early as 24 h after the initial rupture.…”

“…This could have widespread preclinical and clinical implications, including a better understanding of numerous pathologies of the CNS and more specific ways to treat them. Microglia-targeted therapies may also become a valuable treatment method, wherein either inflammatory preconditioning with lipopolysaccharides or pharmacological deactivation with colony-stimulating factor-1 (CSF-1) receptor-antagonist Perxidartinib (PLX3397) can alleviate neuronal death after SAH [ 9 ]. Additional studies utilizing the aforementioned markers and protocol would indubitably enhance our knowledge regarding the implications of microglia and their implied neuroinflammatory processes in SAH pathophysiology.…”

“…Hemodynamic alterations and vascular wall degeneration have both been incriminated in the etiology of aneurysms, as hemodynamic and histological studies show their preferential placement where the blood flow is more turbulent, as well as the absence of the tunica media in their walls [ 6 , 7 , 8 ]. However, more and more evidence supports an important contribution of inflammatory processes leading to their occurrence [ 9 , 10 ].…”