IL-1β impairs retrograde flow of BDNF signaling by attenuating endosome trafficking

Background: Pro-inflammatory cytokines accumulate in the brain with age and Alzheimer's disease and can impair neuron health and cognitive function. Brain-derived neurotrophic factor (BDNF) is a key neurotrophin that supports neuron health, function, and synaptic plasticity. The pro-inflammatory cytokine interleukin-1 beta (IL-1 beta) impairs BDNF signaling but whether it affects BDNF signaling endosome trafficking has not been studied. Methods: This study uses an in vitro approach in primary hippocampal neurons to evaluate the effect of IL-1 beta on BDNF signaling endosome trafficking. Neurons were cultured in microfluidic chambers that separate the environments of the cell body and its axon terminal, enabling us to specifically treat in axon compartments and trace vesicle trafficking in real-time. Results: We found that IL-1 beta attenuates BDNF signaling endosomes throughout networks in cultures. In IL-1 beta-treated cells, overall BDNF endosomal density was decreased, and the colocalization of BDNF endosomes with presynaptic terminals was found to be more than two times higher than in control cultures. Selective IL-1 beta treatment to the presynaptic compartment in microfluidic chamber attenuated BDNF endosome flux, as measured by reduced BDNF-GFP endosome counts in the somal compartment. Further, IL-1 beta decreased the BDNF-induced phosphorylation of Erk5, a known BDNF retrograde trafficking target. Mechanistically, the deficiency in trafficking was not due to impaired endocytosis of the BDNF-TrkB complex, or impaired transport rate, since BDNF endosomes traveled at the same rate in both control and IL-1 beta treatment groups. Among the regulators of presynaptic endosome sorting is the post-translational modification, ubiquitination. In support of this possibility, the IL-1 beta-mediated suppression of BDNF-induced Erk5 phosphorylation can be rescued by exogenous ubiquitin C-terminal hydrolase L1 (UCH-L1), a deubiquitinating enzyme that regulates ubiquitin and endosomal trafficking. Conclusions: We observed a state of neurotrophic resistance whereby, in the prolonged presence of IL-1 beta, BDNF is not effective in delivering long-distance signaling via the retrograde transport of signaling endosomes. Since IL-1 beta accumulation is an invariant feature across many neurodegenerative diseases, our study suggest that compromised BDNF retrograde transport-dependent signaling may have important implications in neurodegenerative diseases.