Dendrite pruning is critical for sculpting the final connectivity of neural circuits as it removes inappropriate projections, yet how neurons can selectively eliminate unnecessary dendritic branches remains elusive. Here, we show that calcium transients that are compartmentalized in specific dendritic branches act as temporal and spatial cues to trigger pruning in Drosophila sensory neurons. Calcium transients occurred in local dendrites at ~3 hours before branch elimination. In dendritic branches, intrinsic excitability increased locally to activate calcium influx via the voltage-gated calcium channels (VGCCs), and blockade of the VGCC activities impaired pruning. Further genetic analyses suggest that the calcium-activated protease calpain functions downstream of the calcium transients. Our findings reveal the importance of the compartmentalized subdendritic calcium signaling in spatiotemporally selective elimination of dendritic branches.

To cover the receptive field completely and non-redundantly, neurons of certain functional groups arrange tiling of their dendrites. In Drosophila class IV dendrite arborization (da) neurons, the NDR family kinase Tricornered (Trc) is required for homotypic repulsion of dendrites that facilitates dendritic tiling. We here report that Sin1, Rictor, and target of rapamycin (TOR), components of the TOR complex 2 (TORC2), are required for dendritic tiling of class IV da neurons. Similar to trc mutants, dendrites of sin1 and rictor mutants show inappropriate overlap of the dendritic fields. TORC2 components physically and genetically interact with Trc, consistent with a shared role in regulating dendritic tiling. Moreover, TORC2 is essential for Trc phosphorylation on a residue that is critical for Trc activity in vivo and in vitro. Remarkably, neuronal expression of a dominant active form of Trc rescues the tiling defects in sin1 and rictor mutants. These findings suggest that TORC2 likely acts together with the Trc signalling pathway to regulate the dendritic tiling of class IV da neurons, and thus uncover the first neuronal function of TORC2 in vivo.

In response to changes in the environment, dendrites from certain neurons change their shape, yet the mechanism remains largely unknown. Here we show that dendritic arbors of adult Drosophila sensory neurons are rapidly reshaped from a radial shape to a lattice-like shape within 24 hr after eclosion. This radial-to-lattice reshaping arises from rearrangement of the existing radial branches into the lattice-like pattern, rather than extensive dendrite pruning followed by regrowth of the lattice-shaped arbors over the period. We also find that the dendrite reshaping is completely blocked in mutants for the matrix metalloproteinase (Mmp) 2. Further genetic analysis indicates that Mmp2 promotes the dendrite reshaping through local degradation of the basement membrane upon which dendrites of the sensory neurons innervate. These findings suggest that regulated proteolytic alteration of the extracellular matrix microenvironment might be a fundamental mechanism to drive a large-scale change of dendritic structures during reorganization of neuronal circuits.

The refinement of neural circuits involves dendrite pruning, a process that removes inappropriate projections that are formed during development. In Drosophila sensory neurons, compartmentalized calcium (Ca 2 þ ) transients in dendrites act as spatiotemporal cues to trigger pruning, yet how neurons define the dendrites with Ca 2 þ transients remains elusive. Here we report that local elevation of endocytic activity contributes to defining dendrites that generate Ca 2 þ transients, triggering pruning. In vivo imaging of single dendrites reveals an increase of endocytosis in proximal dendrites that spatially and temporally correlates with dendrite thinning, an early step in pruning tightly coupled with compartmentalized Ca 2 þ transients. Two GTPases, Rab5 and dynamin, are required for both the increased endocytic activity and compartmentalized Ca 2 þ transients. Further genetic analyses suggest that local endocytosis in proximal dendrites functions cooperatively with global endocytosis-mediated protein degradation pathways to promote dendrite pruning.

The brain changes in response to experience and altered environment. To do that, the nervous system often remodels the structures of neuronal circuits. This structural plasticity of the neuronal circuits appears to be controlled not only by intrinsic factors, but also by extrinsic mechanisms including modification of the extracellular matrix. Recent studies employing a range of animal models implicate that matrix metalloproteinases regulate multiple aspects of the neuronal development and remodeling in the brain. This paper aims to summarize recent advances of our knowledge on the neuronal functions of matrix metalloproteinases and discuss how they might relate in neuronal disease.

The axonal projection pattern of sensory neurons typically is regulated by environmental signals, but how different sensory afferents can establish distinct projections in the same environment remains largely unknown. Drosophila class IV dendrite arborization (C4da) sensory neurons project subtype-specific axonal branches in the ventral nerve cord, and we show that the Tripartite motif protein, Anomalies in sensory axon patterning (Asap) is a critical determinant of the axonal projection patterns of different C4da neurons. Asap is highly expressed in C4da neurons with both ipsilateral and contralateral axonal projections, but the Asap level is low in neurons that have only ipsilateral projections. Mutations in asap cause a specific loss of contralateral projections, whereas overexpression of Asap induces ectopic contralateral projections in C4da neurons. We also show by biochemical and genetic analysis that Asap regulates Netrin signaling, at least in part by linking the Netrin receptor Frazzled to the downstream effector Pico. In the absence of Asap, the sensory afferent connectivity within the ventral nerve cord is disrupted, resulting in specific larval behavioral deficits. These results indicate that different levels of Asap determine distinct patterns of axonal projections of C4da neurons by modulating Netrin signaling and that the Asap-mediated axonal projection is critical for assembly of a functional sensory circuit.

Sensory neurons with common functions are often nonrandomly arranged and form dendritic territories in stereotypic spatial patterns throughout the nervous system, yet molecular mechanisms of how neurons specify dendritic territories remain largely unknown. In Drosophila larvae, dendrites of class IV sensory (C4da) neurons completely but nonredundantly cover the whole epidermis, and the boundaries of these tiled dendritic fields are specified through repulsive interactions between homotypic dendrites. Here we report that, unlike the larval C4da neurons, adult C4da neurons rely on both dendritic repulsive interactions and external positional cues to delimit the boundaries of their dendritic fields. We identify Wnt5 derived from sternites, the ventral-most part of the adult abdominal epidermis, as the critical determinant for the ventral boundaries. Further genetic data indicate that Wnt5 promotes dendrite termination on the periphery of sternites through the Ryk receptor family kinase Derailed (Drl) and the Rho GTPase guanine nucleotide exchange factor Trio in C4da neurons. Our findings thus uncover the dendritic contact-independent mechanism that is required for dendritic boundary specification and suggest that combinatory actions of the dendritic contact-dependent and -independent mechanisms may ensure appropriate dendritic territories of a given neuron.

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