Science for Health
12 July 2012
NIMR scientists have uncovered a highly precise strategy that enables axons to target to a distinct synaptic layer using the visual system of Drosophila as a model. This study is published in Neuron.
Neural circuits in many brain areas of vertebrates and invertebrates share a striking organization into parallel synaptic layers. These are essential units for both development and function, as they bring potential synaptic partners into close vicinity and enable parallel processing of sensory input information. However, our understanding of the molecular mechanisms that control the formation of layer-specific connections in the developing brain remains limited.
In this study, Katarina Timofeev, Willy Joly, Dafni Hadjieconomou and Iris Salecker (pictured) from NIMR's Division of Molecular Neurobiology, have uncovered an unexpected mechanism required for layer-specific targeting in the developing visual system of Drosophila. Netrins are highly conserved guidance cues, known to elicit an attractive growth cone response by engaging the Frazzled receptor, the homolog of Unc-40 in worms and DCC in vertebrates. Using concise genetic manipulations the authors showed that this guidance system plays a central role in directing one photoreceptor subtype to its synaptic target layer.
In the Drosophila visuaI system, eight photoreceptor subtypes (R-cells, R1-R8) extend axons from the retina into the optic lobe, which consists of four main areas: the lamina, medulla, lobula and lobula plate. R8 photoreceptor axons terminate in one of ten synaptic layers in the medulla, called M3. The Frazzled receptor is specifically expressed in R8 photoreceptors during mid-pupal development and, in its absence, R8 axons fail to target to the M3 layer. Intriguingly, Netrins - the activating ligands - are localized in this single layer from early developmental stages onwards. Loss of Netrins in the target area results in similar R8-axon targeting defects as loss of frazzled in photoreceptors. Moreover, provision of membrane-tethered instead of diffusible Netrins can substitute for secreted ligands, suggesting that these act at short-range.
But why are Netrins, which are highly diffusible, localized in a single layer instead of forming a gradient? This is achieved by two mechanisms - local release primarily from axon terminals of intermediate targets, the lamina neurons L3, and ligand capture by the Frazzled receptor associated with target neurons in the medulla. This paper demonstrates that Netrins play an instructive role, and that their localization is essential ito direct specific cell types expressing the receptor to a single target layer.
Our findings provide insights into a surprisingly straightforward mechanism to control the development of laminated circuits in the brain - the use of a localized, short-range chemoattractant guidance cue to provide distinct positional information about a target layer. This mechanism likely works in concert with other previously uncovered strategies and highlights that layer-specific targeting must be seen as a dynamic process controlled by multiple coordinated cell-type specific programs.
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(A) Wild type optic lobe at 55 hours after puparium formation. Netrin-B (blue) is localized in the medulla (me) neuropil layer M3, the target layer of R8 photoreceptor axons (red, double arrowhead in inset).
(B, C) Analysis of an enhancer trap insertion into the Netrin-B locus in conjunction with membrane-bound GFP (B) and Flybow (C) reporter transgenes identifies lamina neurons L3 within the population of Netrin-releasing neurons (arrowhead).
(D) Loss of Netrin-A and Netrin-B causes many R8 axons, labeled with the marker Rh6-lacZ (green), to stall at the medulla neuropil border (arrowhead) or to stop in interim layers (arrow).
La, lamina; Lo, lobula. Scale bars, 20 μm.
Katarina Timofeev, Willy Joly, Dafni Hadjieconomou and Iris Salecker (2012)
Localized Netrins act as positional cues to control layer-specific targeting of photoreceptor axons in Drosophila
Neuron, 75:80-93. Full-text
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