No abstract

Both cell-intrinsic and extrinsic pathways govern axon regeneration, but only a limited number of factors have been identified and it is not clear to what extent axon regeneration is evolutionarily conserved. Whether dendrites also regenerate is unknown. Here we report that, like the axons of mammalian sensory neurons, the axons of certain Drosophila dendritic arborization (da) neurons are capable of substantial regeneration in the periphery but not in the CNS, and activating the Akt pathway enhances axon regeneration in the CNS. Moreover, those da neurons capable of axon regeneration also display dendrite regeneration, which is cell type-specific, developmentally regulated, and associated with microtubule polarity reversal. Dendrite regeneration is restrained via inhibition of the Akt pathway in da neurons by the epithelial cell-derived microRNA bantam but is facilitated by cell-autonomous activation of the Akt pathway. Our study begins to reveal mechanisms for dendrite regeneration, which depends on both extrinsic and intrinsic factors, including the PTEN-Akt pathway that is also important for axon regeneration. We thus established an important new model system-the fly da neuron regeneration model that resembles the mammalian injury model-with which to study and gain novel insights into the regeneration machinery.

Axon extension during development of the nervous system is guided by many factors, but the signalling mechanisms responsible for triggering this extension remain mostly unknown. Here we have examined the role of Rho family small guanosine triphosphatases (GTPases) in mediating axon guidance by diffusible factors. Expression of either dominant-negative or constitutively active Cdc42 in cultured Xenopus laevis spinal neurons, at a concentration that does not substantially affect filopodial formation and neurite extension, abolishes the chemoattractive growth cone turning induced by a gradient of brain-derived neurotrophic factor that can activate Cdc42 and Rac in cultured neurons. Chemorepulsion induced by a gradient of lysophosphatidic acid is also abolished by the expression of dominant-negative RhoA. We also show that an asymmetry in Rho kinase or filopodial initiation across the growth cone is sufficient to trigger the turning response and that there is a crosstalk between the Cdc42 and RhoA pathways through their converging actions on the myosin activity essential for growth cone chemorepulsion.

Infrared fluorescent proteins (IFPs) provide an additional color to GFP and its red homologs in protein labeling. Based on structural analysis of the dimer interface, a monomeric bateriophytochrome is identified from a sequence database, and is engineered into a naturally-monomeric IFP (mIFP). We demonstrate that mIFP correctly labels proteins in live Drosophila and zebrafish requiring no exogenous cofactor, and will thus be useful in molecular, cell and developmental biology.

SUMMARY During developmental remodeling, neurites destined for pruning often degenerate on-site. Physical injury also induces degeneration of neurites distal to the injury site. Prompt clearance of degenerating neurites is important for maintaining tissue homeostasis and preventing inflammatory responses. Here we show that in both dendrite pruning and dendrite injury of Drosophila sensory neurons, epidermal cells rather than hemocytes are the primary phagocytes in clearing degenerating dendrites. Epidermal cells act via Draper-mediated recognition to facilitate dendrite degeneration and to engulf and degrade degenerating dendrites. Using multiple dendritic membrane markers to trace phagocytosis, we show that two members of the CD36 family, croquemort (crq) and debris buster (dsb), act at distinct stages of phagosome maturation for dendrite clearance. Our finding reveals the physiological importance of coordination between neurons and their surrounding epidermis, for both dendrite fragmentation and clearance.

Highlights d Piezo cell autonomously inhibits axon regeneration in Drosophila sensory neurons d Piezo channels are activated after axon injury or during axon regeneration d Piezo inhibits axon regeneration through calcium signaling, Nos, and PKG d Piezo1 also inhibits axon regeneration in mammals in vitro and in vivo

No abstract

Little is known about the spatial and temporal dynamics of presynaptic and postsynaptic specializations that culminate in synaptogenesis. Here, we imaged presynaptic vesicle clusters in motor axons and postsynaptic acetylcholine receptor (AChR) clusters in embryonic zebrafish to study the earliest events in synaptogenesis in vivo. Prepatterned AChR clusters are present on muscle fibers in advance of motor axon outgrowth from the spinal cord. Motor axon growth cones and filopodia are selectively extended toward and contact prepatterned AChR clusters, followed by the rapid clustering of presynaptic vesicles and insertion of additional AChRs, hallmarks of synaptogenesis. All initially formed neuromuscular synapses contain AChRs that were inserted into the membrane at the time the prepattern is present. Examination of embryos in which AChRs were blocked or clustering is absent showed that neither receptor activity or receptor protein is required for these events to occur. Thus, during initial synaptogenesis, postsynaptic differentiation precedes presynaptic differentiation, and prepatterned neurotransmitter clusters mark sites destined for synapse formation.