Ephrins regulate the formation of terminal axonal arbors during the development of thalamocortical projections

Multiple roles of ephrins during the formation of thalamocortical projections: Maps and more

The functional architecture of the cerebral cortex is based on intrinsic connections that precisely link neurons from distinct cortical laminae as well as layer-specific afferent and efferent projections. Experimental strategies using in vitro assays originally developed by Friedrich Bonhoeffer have suggested that positional cues confined to individual layers regulate the assembly of local cortical circuits and the formation of thalamocortical projections. One of these wiring molecules is ephrinA5, a ligand for Eph receptor tyrosine kinases. EphrinA5 and Eph receptors exhibit highly dynamic expression patterns in distinct regions of the cortex and thalamus during early and late stages of thalamocortical and cortical circuit formation. In vitro assays suggest that ephrinA5 is a multifunctional wiring molecule for different populations of cortical and thalamic axons. Additionally, the expression patterns of ephrinA5 during cortical development are consistent with this molecule regulating, in alternative ways, specific components of thalamic and cortical connectivity. To test this directly, the organization of thalamocortical projections was examined in mice lacking ephrinA5 gene expression. The anatomical studies in ephrinA5 knockout animals revealed a miswiring of limbic thalamic projections and changes in neocortical circuits that were predicted from the expression pattern and the in vitro analysis of ephrinA5 function.

Constructing the mammalian neocortex: the role of intrinsic factors

The mammalian neocortex is subdivided into regions that are specialised for the processing of particular forms of information. These regions are distinct in terms of their cytoarchitecture, electrophysiology, and connectivity. How this regional diversity is generated through development is currently a topic of considerable interest and has centered upon two main issues. First, to what extent are these regions prespecified by intrinsic genetic mechanisms? Second, what is the influence of extrinsic activity in transmitting signals that ultimately shape functional regions? Historically, experimental evidence has tended to emphasise the role of extrinsic influences, but the identification and analysis of several genes that are expressed asymmetrically in the developing neocortex have tempered this viewpoint. We review current literature from the standpoint that intrinsic influences act early in neocortical development to generate molecular patterning whose main role is the guidance of long-range projections from the dorsal thalamus. Extrinsic influences appear to generate receptive fields for peripheral input, the summation of which determines the areal extent of particular neocortical region.

Integrin antagonists affect growth and pathfinding of ventral motor nerves in the trunk of embryonic zebrafish

Integrins are thought to be important receptors for extracellular matrix (ECM) components on growing axons. Ventral motor axons in the trunk of embryonic zebrafish grow in a midsegmental pathway through an environment rich in ECM components. To test the role of integrins in this process, integrin antagonists (the disintegrin echistatin in native and recombinant form, as well as the Arg-Gly-Asp-Ser peptide) were injected into embryos just prior to axon outgrowth at 14-16 h postfertilization (hpf). All integrin antagonists affected growth of ventral motor nerves in a similar way and native echistatin was most effective. At 24 hpf, when only the three primary motor axons per trunk hemisegment had grown out, 80% (16 of 20) of the embryos analyzed had abnormal motor nerves after injection of native echistatin, corresponding to 19% (91 of 480) of all nerves. At 33 hpf, when secondary motor axons were present in the pathway, 100% of the embryos were affected (24 of 24), with 20% of all nerves analyzed (196 of 960) being abnormal. Phenotypes comprised abnormal branching (64% of all abnormal nerves) and truncations (36% of all abnormal nerves) of ventral motor nerves at 24 hpf and mostly branching of the nerves at 33 hpf (94% of all abnormal nerves). Caudal branches were at least twice as frequent as rostral branches. Surrounding trunk tissue and a number of other axon fascicles were apparently not affected by the injections. Thus integrin function contributes to both growth and pathfinding of axons in ventral motor nerves in the trunk of zebrafish in vivo.