Slit-Robo signalling prevents sensory cells from crossing the midline in Drosophila

Slit and Robo regulate dendrite branching and elongation of space-filling neurons in Drosophila

Space-filling neurons extensively sample their receptive fields with fine dendritic branches. In this study we show that a member of the conserved Robo receptor family, Robo, and its ligand Slit regulate the dendritic differentiation of space-filling neurons. Loss of Robo or Slit function leads to faster elongating and less branched dendrites of the complex and space-filling class IV multi-dendritic dendrite-arborization (md-da) neurons in the Drosophila embryonic peripheral nervous system, but not of the simpler class I neurons. The total dendrite length of Class IV neurons is not modified in robo or slit mutant embryos. Robo mediates this process cell-autonomously. Upon Robo over-expression in md-da neurons the dendritic tree is simplified and time-lapse analysis during larval stages indicates that this is due to reduction in the number of newly formed branches. We propose that Slit, through Robo, provides an extrinsic signal to coordinate the growth rate and the branching level of space-filling neurons, thus allowing them to appropriately cover their target field.

Molecular mechanisms controlling midline crossing by precerebellar neurons

Precerebellar neurons of the inferior olive (IO) and lateral reticular nucleus (LRN) migrate tangentially from the rhombic lip toward the floor plate following parallel pathways. This process is thought to involve netrin-1 attraction. However, whereas the cell bodies of LRN neurons cross the midline, IO neurons are unable to do so. In many systems and species, axon guidance and cell migration at the midline are controlled by Slits and their receptor Robos. We showed previously that precerebellar axons and neurons do not cross the midline in the absence of the Robo3 receptor. To determine whether this signaling by Slits and the two other Robo receptors, Robo1 and Robo2, also regulates precerebellar neuron behavior at the floor plate, we studied the phenotype of Slit1/2 and Robo1/2/3 compound mutants. Our results showed that many IO neurons can cross the midline in absence of Slit1/2 or Robo1/2, supporting a role for midline repellents in guiding precerebellar neurons. We also show that these molecules control the development of the lamellation of the inferior olivary complex. Last, the analysis of Robo1/2/3 triple mutants suggests that Robo3 inhibits Robo1/2 repulsion in precrossing LRN axons but not in IO axons in which it has a dominant and distinct function.

Knot/Collier and cut control different aspects of dendrite cytoskeleton and synergize to define final arbor shape

In a complex nervous system, neuronal functional diversity is reflected in the wide variety of dendritic arbor shapes. Different neuronal classes are defined by class-specific transcription factor combinatorial codes. We show that the combination of the transcription factors Knot and Cut is particular to Drosophila class IV dendritic arborization (da) neurons. Knot and Cut control different aspects of the dendrite cytoskeleton, promoting microtubule- and actin-based dendritic arbors, respectively. Knot delineates class IV arbor morphology by simultaneously synergizing with Cut to promote complexity and repressing Cut-mediated promotion of dendritic filopodia/spikes. Knot increases dendritic arbor outgrowth through promoting the expression of Spastin, a microtubule-severing protein disrupted in autosomal dominant hereditary spastic paraplegia (AD-HSP). Knot and Cut may modulate cellular mechanisms that are conserved between Drosophila and vertebrates. Hence, this study gives significant general insight into how multiple transcription factors combine to control class-specific dendritic arbor morphology through controlling different aspects of the cytoskeleton.

Selective expression of Knot/Collier, a transcriptional regulator of the EBF/Olf-1 family, endows the Drosophila sensory system with neuronal class-specific elaborated dendritic patterns

Dendritic tree morphology is a hallmark of cellular diversity in the nervous system, and Drosophila dendritic arborization (da) neurons provide an excellent model system to study its molecular basis. The da neurons are classified into four classes I-IV in the order of increasing branching complexity. A transcriptional regulator of the early B-cell factor (EBF)/olfactory 1 (Olf-1) family, Knot (Kn)/Collier (Col) is expressed selectively in class IV neurons, which generate the most expansive and complicated dendritic trees in the four classes. Loss of kn function in class IV neurons greatly reduced the number of their dendritic branches. Conversely mis-expression of kn in classes I and II produced supernumerary higher-order branches, whereas class III-specific short and straight terminal branches was hardly formed by kn mis-expression. Neither kn loss of function nor mis-expression were associated with dramatic alterations in the expression patterns of two other transcriptional regulators, Abrupt (Ab) and Cut (Ct), which play important roles in shaping dendritic trees with distinct class specificity from Kn. In contrast, Kn was necessary and sufficient to drive expression of a gene that encodes a class IV-specific channel protein. Collectively, all of our results suggest that Kn exerts its cell-autonomous function to control the formation, and possibly the function, of class IV-like elaborated dendritic arbors.

Neurogenesis in the adult ischemic brain: generation, migration, survival, and restorative therapy

This article reviews current data on the induction of neurogenesis after stroke in the adult brain. The discussion of neurogenesis is divided into production, migration, and survival of these newly formed cells. For production, the subpopulations and the types of cell division are presented. Discussion of cell migration entails presenting data on both the pathways as well as the molecular targeting of newly formed neural progenitor cells to sites of injury. The role of the vascular and the astrocytic microenvironment in promoting the survival and integration of progenitor cells is also presented. Cell-based and pharmacological therapies designed to restore neurological function that promote neurogenesis are described. These therapies also induce angiogenesis and astrocytic changes that brain tissue, which prime the ischemic brain to foster the survival of the newly formed progenitor cells. Signaling pathways that regulate neurogenesis and angiogenesis are also addressed. This review summarizes recent data on neurogenesis and provides insight into the potential for restorative treatments of stroke.

FlyPNS, a database of the Drosophila embryonic and larval peripheral nervous system

Background

The embryonic and larval peripheral nervous system of Drosophila melanogaster is extensively studied as a very powerful model of developmental biology. One main advantage of this system is the ability to study the origin and development of individual sensory cells. However, there remain several discrepancies regarding the organization of sensory organs in each abdominal segment A1-A7.

Description

We have constructed a web site called FlyPNS (for Fly Peripheral Nervous System) that consolidates a wide range of published and unpublished information about the embryonic and larval sensory organs. It communicates (1) a PNS pattern that solves the discrepancies that have been found in the recent literature, (2) the correspondence between the different nomenclatures that have been used so far, (3) a comprehensive description of each sensory organ, and (4) a list of both published and unpublished markers to reliably identify each PNS cell.

Conclusions

The FlyPNS database integrates disparate data and nomenclature and thus helps understanding the conflicting observations that have been published recently. Furthermore, it is designed to provide assistance in the identification and study of individual sensory cells. We think it will be a useful resource for any researcher with interest in Drosophila sensory organs.