EndophilinAs regulate endosomal sorting of BDNF-TrkB to mediate survival signaling in hippocampal neurons

The sorting of activated receptors into distinct endosomal compartments is essential to activate specific signaling cascades and cellular events including growth and survival. However, the proteins involved in this sorting are not well understood. We discovered a novel role of EndophilinAs in sorting of activated BDNF-TrkB receptors into late endosomal compartments. Mice lacking all three EndophilinAs accumulate Rab7-positive late endosomes. Moreover, EndophilinAs are differentially localized to, co-traffic with, and tubulate, distinct endosomal compartments: In response to BDNF, EndophilinA2 is recruited to both early and late endosomes, EndophilinA3 is recruited to Lamp1-positive late endosomes, and co-trafficks with Rab5 and Rab7 in both the presence and absence of BDNF, while EndophilinA1 colocalizes at lower levels with endosomes. The absence of all three EndophilinAs caused TrkB to accumulate in EEA1 and Rab7-positive endosomes, and impaired BDNF-TrkB-dependent survival signaling cascades. In addition, EndophilinA triple knockout neurons exhibited increased cell death which could not be rescued by exogenous BDNF, in a neurotrophin-dependent survival assay. Thus, EndophilinAs differentially regulate activated receptor sorting via distinct endosomal compartments to promote BDNF-dependent cell survival.peerReviewe

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-1β (IL-1β) 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-1β 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-1β attenuates BDNF signaling endosomes throughout networks in cultures. In IL-1β-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-1β 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-1β 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-1β treatment groups. Among the regulators of presynaptic endosome sorting is the post-translational modification, ubiquitination. In support of this possibility, the IL-1β-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-1β, BDNF is not effective in delivering long-distance signaling via the retrograde transport of signaling endosomes. Since IL-1β 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

Schwann cell myelination requires Dynein function

<p>Abstract</p> <p>Background</p> <p>Interaction of Schwann cells with axons triggers signal transduction that drives expression of Pou3f1 and Egr2 transcription factors, which in turn promote myelination. Signal transduction appears to be mediated, at least in part, by cyclic adenosine monophosphate (cAMP) because elevation of cAMP levels can stimulate myelination in the absence of axon contact. The mechanisms by which the myelinating signal is conveyed remain unclear.</p> <p>Results</p> <p>By analyzing mutations that disrupt myelination in zebrafish, we learned that Dynein cytoplasmic 1 heavy chain 1 (Dync1h1), which functions as a motor for intracellular molecular trafficking, is required for peripheral myelination. In <it>dync1h1</it> mutants, Schwann cell progenitors migrated to peripheral nerves but then failed to express Pou3f1 and Egr2 or make myelin membrane. Genetic mosaic experiments revealed that robust Myelin Basic Protein expression required Dync1h1 function within both Schwann cells and axons. Finally, treatment of <it>dync1h1</it> mutants with a drug to elevate cAMP levels stimulated myelin gene expression.</p> <p>Conclusion</p> <p>Dync1h1 is required for retrograde transport in axons and mutations of Dync1h1 have been implicated in axon disease. Our data now provide evidence that Dync1h1 is also required for efficient myelination of peripheral axons by Schwann cells, perhaps by facilitating signal transduction necessary for myelination.</p