BDNF signaling in Hebbian and Stentian structural plasticity in the developing visual system

During development, patterned neural activity in input neurons innervating their target, instructs topographic map refinement. Axons from adjacent neurons, firing with similar patterns of neural activity, converge onto target neurons and stabilize their synapses with these postsynaptic partners (Hebbian plasticity). On the other hand, non-correlated firing of inputs promotes synaptic weakening and exploratory axonal growth (Stentian plasticity). We used visual stimulation to control the visually-evoked correlation structure of neural activity in ectopic ipsilaterally projecting (ipsi) retinal ganglion cell axons with respect to their neighboring contralateral eye inputs in the optic tectum of albino Xenopus laevis tadpoles. Multiphoton imaging of the ipsi axons in the live tadpole, combined with manipulation of brain-derived neurotrophic factor (BDNF) signaling, revealed that presynaptic p75NTR and TrkB both promoted axonal branch addition during Stentian plasticity, whereas predominantly postsynaptic BDNF signaling mediated activity-dependent Hebbian suppression of axon branch addition. Additionally, we found that BDNF signaling is required for local suppression of branch loss induced by correlated firing.

In situ cryo-electron tomography reveals local cellular machineries for axon branch development

Neurons are highly polarized cells forming an intricate network of dendrites and axons. They are shaped by the dynamic reorganization of cytoskeleton components and cellular organelles. Axon branching allows to form new paths and increases circuit complexity. However, our understanding of branch formation is sparse due to technical limitations. Using in situ cellular cryo-electron tomography on primary mouse neurons, we directly visualized the remodeling of organelles and cytoskeleton structures at axon branches. Strikingly, branched areas functioned as hotspots concentrating organelles to support dynamic activities. Unaligned actin filaments assembled at the base of premature branches and remained while filopodia diminished. Microtubules and ER co-migrated into preformed branches to support outgrowth together with accumulating compact ~500 nm mitochondria and locally clustered ribosomes. We obtained a roadmap of events and present the first direct evidence of local protein synthesis selectively taking place at axon branches, allowing to serve as unique control hubs for axon development and downstream neural network formation.

Completion of neuronal remodeling prompts myelination along developing motor axon branches

Neuronal remodeling and myelination are two fundamental processes during neurodevelopment. How they influence each other remains largely unknown, even though their coordinated execution is critical for circuit function and often disrupted in neuropsychiatric disorders. It is unclear whether myelination stabilizes axon branches during remodeling or whether ongoing remodeling delays myelination. By modulating synaptic transmission, cytoskeletal dynamics, and axonal transport in mouse motor axons, we show that local axon remodeling delays myelination onset and node formation. Conversely, glial differentiation does not determine the outcome of axon remodeling. Delayed myelination is not due to a limited supply of structural components of the axon–glial unit but rather is triggered by increased transport of signaling factors that initiate myelination, such as neuregulin. Further, transport of promyelinating signals is regulated via local cytoskeletal maturation related to activity-dependent competition. Our study reveals an axon branch–specific fine-tuning mechanism that locally coordinates axon remodeling and myelination.

Branch-restricted localization of phosphatase Prl-1 specifies axonal synaptogenesis domains

Central nervous system (CNS) circuit development requires subcellular control of synapse formation and patterning of synapse abundance. We identified the Drosophila membrane-anchored phosphatase of regenerating liver (Prl-1) as an axon-intrinsic factor that promotes synapse formation in a spatially restricted fashion. The loss of Prl-1 in mechanosensory neurons reduced the number of CNS presynapses localized on a single axon collateral and organized as a terminal arbor. Flies lacking all Prl-1 protein had locomotor defects. The overexpression of Prl-1 induced ectopic synapses. In mechanosensory neurons, Prl-1 modulates the insulin receptor (InR) signaling pathway within a single contralateral axon compartment, thereby affecting the number of synapses. The axon branch–specific localization and function of Prl-1 depend on untranslated regions of the prl-1 messenger RNA (mRNA). Therefore, compartmentalized restriction of Prl-1 serves as a specificity factor for the subcellular control of axonal synaptogenesis.

Regulation of branching dynamics by axon-intrinsic asymmetries in Tyrosine Kinase Receptor signaling

Axonal branching allows a neuron to connect to several targets, increasing neuronal circuit complexity. While axonal branching is well described, the mechanisms that control it remain largely unknown. We find that in the Drosophila CNS branches develop through a process of excessive growth followed by pruning. In vivo high-resolution live imaging of developing brains as well as loss and gain of function experiments show that activation of Epidermal Growth Factor Receptor (EGFR) is necessary for branch dynamics and the final branching pattern. Live imaging also reveals that intrinsic asymmetry in EGFR localization regulates the balance between dynamic and static filopodia. Elimination of signaling asymmetry by either loss or gain of EGFR function results in reduced dynamics leading to excessive branch formation. In summary, we propose that the dynamic process of axon branch development is mediated by differential local distribution of signaling receptors.