To measure the curviness of terminal branchlets we calculated convex hulls ( supplementary material Fig. At least five animals per genotype were used for quantification. For time-lapse analysis, every time point was analyzed separately.
To analyze the branch distribution in class IV neurons we used Sholl analysis ( Sholl, 1953) (ImageJ). In class III neurons, spiked protrusions were defined as unbranched terminal processes shorter than 30 μm.
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Images were analyzed with ImageJ software (NIH), using the NeuronJ plug-in ( Meijering et al., 2004) to trace the dendritic branches. The identification of these transcription factor programs has revealed part of the developmental logic used to create neuron class-specific dendrite shape yet the molecular programs that translate this logic into the branching and dynamic properties of each neuronal type remain unknown. In Drosophila, candidate-prompted analysis as well as large-scale screens have led to the identification of a number of transcription factors that are capable of modulating the morphology of peripheral nervous system (PNS) dendrites ( Crozatier and Vincent, 2008 Grueber et al., 2003a Hattori et al., 2007 Jinushi-Nakao et al., 2007 Li et al., 2004 Moore et al., 2002 Parrish et al., 2006 Sugimura et al., 2004). For example, in the rodent cortex, the bHLH transcription factor neurogenin 2, which is involved in the specification of neuronal subtypes, defines the unipolar dendritic morphology of pyramidal neurons ( Hand et al., 2005). The morphology of dendrites is defined by transcriptional codes ( Jan and Jan, 2010). We propose that dedicated molecular mechanisms underlie the formation and dynamics of distinct dendrite branch types to elicit the accurate establishment of neuronal circuits.
In fact, loss of fascin function leads to a partial conversion of class III neurons to class IV characteristics, while the reverse effect is obtained by fascin overexpression in class IV neurons. Furthermore, fascin defines the morphological distinction between class III and class IV neurons. Previous studies have shown that transcription factor combinations define dendrite patterns we find that fascin represents a downstream component of such programs, as it is a major effector of the transcription factor Cut in defining class III-specific dendrite morphology. In class III neurons, fascin controls the formation and dynamics of terminal branchlets. We found that the terminal branchlets of different classes of neurons have distinctive dynamics and are formed on the basis of molecularly separable mechanisms in particular, class III neurons require fascin for terminal branching whereas class IV neurons do not. We addressed the function of fascin, a conserved actin-bundling protein involved in filopodium formation, in class III and class IV sensory neurons.
We investigated specific classes of sensory neurons of Drosophila larvae to address the fundamental mechanisms underlying the formation of distinct branch types. Nonetheless, how diverse neuronal class-specific dendrite branches are generated is not understood. The branched morphology of dendrites represents a functional hallmark of distinct neuronal types.
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