The intracellular region of truncated neurotrophin receptor TrkB-T1 promotes stroke-related effects in glial reactivity and neurotoxicity
The development of advanced therapies for stroke, spinal cord injury or neurodegenerative diseases -main causes of death, disability and dementia- requires a profound understanding of the complex interactions established among excitotoxic neuronal death, aberrant neurotrophic-signaling, glial reactivity, and neuroinflammation. However, the master proteins coordinating these mechanisms have not been yet defined. Different evidence suggests that the truncated form of the neurotrophin tyrosine kinase receptor, TrkB-T1, might play such a key role. The levels of this TrkB isoform increase in stroke while those of the full-length pro-survival isoform (TrkB-FL) are reduced. Additionally, ischemic stroke and, specifically, excitotoxicity induce TrkB-T1 regulated intramembrane proteolysis (RIP), a process releasing a receptor ectodomain able to bind the brain-derived neurotrophic factor (BDNF) and leading to decreased BDNF-signaling. We hypothesize that the second RIP product, TrkB-T1 intracellular domain (TrkB-T1-ICD), might similarly contribute to neurotoxicity but also reactive gliosis and neuroinflammation. Herein, we first demonstrate migration of the cytoplasmic TrkB-T1-ICD to the nuclei of neurons undergoing excitotoxicity, suggesting a possible role in the transcriptional control induced by injury. Then, taking advantage of cell-penetrating peptides (CPPs), we produce a TrkB-T1-ICD mock peptide (Bio-LTT1Ct) containing the short TrkB-T1 intracellular region (23 amino acids) and test it in vitro and in vivo. Notably, this peptide migrates to the nucleus of both neurons and astrocytes cultured in vitro and provokes cell death. Additionally, Bio-LTT1Ct induces early transcriptional changes in neurons resembling those triggered by excitotoxicity such as the inhibition of the promoter activity of pro-survival transcription factors CREB and MEF2, and altered mRNA levels of their regulated genes. In vivo, Bio-LTT1Ct is accessible to the brain cortex after intranasal delivery, being efficiently distributed into cortical neurons and astrocytes of both hemispheres. Moreover, peptide administration is sufficient to promote important pathological hallmarks of stroke such as the imbalance of the TrkB isoforms, and the reactivity of astrocyte and microglia, cells that acquire proinflammatory profiles. Altogether, these results establish TrkB-T1 RIP as a central mechanism of ischemic damage and demonstrate that the receptor intracellular region is sufficient to recapitulate stroke-like effects on neurotoxicity, glial reactivity and neuroinflammation.