Ischemic stroke constitutes the major cause of death and disability in the industrialized world. The interest in microglia arose from the evidence outlining the role of neuroinflammation in ischemic stroke pathobiology. Microglia constitute the powerhouse of innate immunity in the brain. Microglial cells are highly ramified, and use these ramifications as sentinels to detect changes in brain homeostasis. Once a danger signal is recognized, cells become activated and mount specialized responses that range from eliminating cell debris to secreting inflammatory signals and trophic factors. Originally, it was suggested that microglia play essentially a detrimental role in ischemic stroke. However, recent reports are providing evidence that the role of these cells is more complex than what was originally thought. Although these cells play detrimental role in the acute phase, they are required for tissue regeneration in the post-acute phases. This complex role of microglia in ischemic stroke pathobiology constitutes a major challenge for the development of efficient immunomodulatory therapies. This review aims at providing an overview regarding the role of resident microglia and peripherally recruited macrophages in ischemic pathobiology. Furthermore, the review will highlight future directions towards the development of novel fine-tuning immunomodulatory therapeutic interventions.

We recently described a novel ribosome-based regulatory mechanism/checkpoint that controls innate immune gene translation and microglial activation in non-sterile inflammation orchestrated by RNA binding protein SRSF3. Here we describe a role of SRSF3 in the regulation of microglia/macrophage activation phenotypes after experimental stroke. Using a model-system for analysis of the dynamic translational state of microglial ribosomes we show that 24 hrs after stroke highly upregulated immune mRNAs are not translated resulting in a marked dissociation of mRNA and protein networks in activated microglia/macrophages. Next, microglial activation after stroke was characterized by a robust increase in pSRSF3/SRSF3 expression levels. Targeted knockdown of SRSF3 using intranasal delivery of siRNA 24 hrs after stroke caused a marked knockdown of endogenous protein. Further analyses revealed that treatment with SRSF3-siRNA alleviated translational arrest of selected genes and induced a transient but significant increase in innate immune signaling and IBA1+ immunoreactivity peaking 5 days after initial injury. Importantly, delayed SRSF3-mediated increase in immune signaling markedly reduced the size of ischemic lesion measured 7 days after stroke. Together, our findings suggest that targeting SRSF3 and immune mRNAs translation may open new avenues for molecular/therapeutic reprogramming of innate immune response after ischemic injury.

The heterogeneity of receptor isoforms can cause an apparent paradox where each isoform can promote different or even opposite biological pathways. One example is the neurotrophin receptor TrkC. The trkC mRNA translates a full-length receptor tyrosine kinase (TrkC-FL) whose activation by the growth factor NT3 promotes neuronal survival. In some diseases, the trkC mRNA is spliced to a kinase-truncated isoform (TrkC.T1) whose activation by NT3 up-regulates tumor necrosis factor alpha (TNF-α) causing neurotoxicity. Since TrkC.T1 expression is significantly increased at the onset of neurodegeneration, we hypothesized that in disease TrkC.T1-mediated toxicity prevails over TrkC-FL-mediated survival. To study this, we developed small molecules that selectively antagonize NT3-driven TrkC.T1 neurotoxicity without compromising TrkC-FL survival. In a genetic mouse model of retinitis pigmentosa, therapeutic administration of TrkC.T1 antagonists prevents elevation of TNF-α and reduces photoreceptor neuronal death. This work demonstrates the importance of accounting for functional and structural heterogeneity in receptor–ligand interactions, illustrates chemical biology strategies to develop isoform-selective agents, validates TrkC.T1 as a druggable target, and expands the therapeutic concept of reducing neurotoxicity as a strategy to achieve neuroprotection.