Fig. 8: Effect of brain ischemia on GA stability, MTFL₄₅₇ regulation, and proposed model.

A Strong association between neuronal degeneration and serum protein leakage after ischemic damage. Immunohistochemistry of brain coronal sections from animals sacrificed 5 h after insult was performed with an antibody recognizing a calpain-generated neoepitope in spectrin N-terminal fragment (SNTF, magenta), labeling cells where this protease is overactive, and a mouse antibody recognizing GM130 (red). Neurodegeneration was also detected by Fluoro-Jade C (FJC) staining (green). Three different tissue areas were compared: the ischemic core, an area peripheral to the infarct core, and the equivalent area of the contralateral hemisphere. Leakage of mouse immunoglobulins due to early blood-brain barrier (BBB) breakage after the ischemic insult, detected by the secondary anti-mouse antibody (see Fig. S5), was observed in the neurodegenerating tissue and strongly interfered with GM130 detection. B, C GM130 staining after preincubation of coronal sections with an anti-mouse IgG (Fab specific) antibody to improve detection. Animals were retro-orbitally injected with peptides MTMyc or MTFL₄₅₇ (10 nmol/g) 10 min after damage initiation and sacrificed 5 h later. Comparison of the contralateral (B) and the ischemic peripheral areas (C). Representative images correspond to single sections. Scale bar: 10 µm. D Model of TrkB-FL regulation in excitotoxicity and MTFL₄₅₇ action. Endocytosis of neurotrophin receptor TrkB-FL is promoted by excitotoxicity in neurons treated with control peptide MTMyc or without treatment (left panel). In endosomes, TrkB-FL interacts with the protein Hrs and is retrogradely transported to the Golgi apparatus (GA), where activation of organelle-associated proteinases would be responsible for receptor processing by calpain and regulated intramembrane proteolysis (RIP). Although partial recycling back to the membrane might occur via mechanisms similar to those found after BDNF activation, there is a strong decrease in BDNF/TrkB-FL signaling and CREB/MEF2 promoter activities, causing transcriptional changes that favor neuronal death. In parallel, the GA is disrupted, a hallmark common to many neurodegenerative diseases (NDDs). The neuroprotective peptide MTFL₄₅₇ interferes with the TrkB-FL/Hrs interaction induced by excitotoxicity, receptor retrograde transport and processing, as well as GA fragmentation (right panel). We propose that interference by MTFL₄₅₇ with the TrkB-FL/Hrs interaction might favor rapid recycling back to the membrane, similar to that of isoform TrkB-T1, sustained BDNF/TrkB-FL/PLCγ signaling, and promotion of neuronal survival.