ScienceWeek

DEVELOPMENT: ON THE GUIDANCE OF RETINAL NEURONS

The following points are made by Liqun Luo (Nature 2006 439:23):

1) Our brain is made up of maps that organize what we sense. In the visual system, for example, an object is represented by the spatial activation pattern of retinal ganglion cells (RGCs), which form a two-dimensional sheet in the retina. RGC nerve fibers (axons) project into the brain in an orderly manner along both the x and y axes, such that the two-dimensional image is recapitulated in the optic tectum region of the brain. But how do maps like this form during development? Although the wiring diagram for how the RGC axons connect up with the tectum -- the retinotopic map -- has been extensively studied, it is still not completely understood[1]. New work[2] identifies one of the signals that direct nerve fibers from the retina to their destination in the brain.

2) The retinotopic map was first elaborated by Roger Sperry 42 years ago[3]. By following point-to-point connections made as frog RGC axons regenerated between the retina and tectum, Sperry postulated that individual RGC axons must carry chemical tags that allow them to read the positional information in the tectum, also of a chemical nature. To limit the number of different tags needed to specify the connections, Sperry further proposed that the chemicals on RGC axons and in the tectum form gradients, such that the amounts of a tag could specify different positions. These ideas have been borne out spectacularly by experiment: first in the anterior posterior axis, where gradients of a family of molecules called EphrinA in the tectum specify where RGC axons will end up[4,5]; and more recently in the medial lateral axis, where gradients of EphrinB molecules organize how the RGCs are wired up.

3) In the chick and the mouse, RGC axons home in on their exact targets along the medial lateral axis in the tectum primarily by regulating the direction of branches that extend from the primary axons[1]. When the primary axon ends up in a position lateral to where it should be, it projects branches medially to link up with its "termination zone"; conversely, if the primary axon is medial to the termination zone, it branches out laterally. Graded expression of EphrinB molecules in the tectum and their receptors, the EphBs, in RGCs regulate this direction of branching. RGCs that originate from ventral-most retina end up at the highest concentration of EphrinB because these RGCs have the most receptors, and therefore receive the most of the attractive EphrinB signal. In mutant mice that lack EphB2 and EphB3, individual axon branches preferentially extend laterally regardless of the position of primary axons relative to their termination zone, causing a lateral shift in RGC axon targeting.

4) The directions provided by EphrinB/EphB alone, however, are not sufficient to account for the medial lateral map. If they were, all RGC axons would head for the medial-most tectum, which is most attractive, and leave the lateral tectum unoccupied. Modelling studies suggest that an additional activity, most likely a repellent gradient in the same direction as the EphrinB attractive gradient, is necessary to counterbalance the medial-directing activity of EphrinB. Schmitt et al[2] now show that a gradient of the Wnt3 molecule is a strong candidate for this other directional signal.

References (abridged):

1. McLaughlin, T. & O'Leary, D. D. Annu. Rev. Neurosci. 28, 327 355 (2005)

2. Schmitt, A. M. et al. Nature 439, 31 37 (2006)

3. Sperry, R. W. Proc. Natl Acad. Sci. USA 50, 703 710 (1963)

4. Drescher, U. et al. Cell 82, 369 370 (1995)

5. Cheng, H. -J. , Nakamoto, M. , Bergemann, A. D. & Flanagan, J. G. Cell 82, 371 381 (1995)

Nature http://www.nature.com/nature

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NEUROSCIENCE: ON AXON GROWTH TARGETING

The following points are made by Timothy Gomez (Nature 2005 434:835):

1) The adult nervous system is characterized by a complex network of neurons and their target cells, which are wired up during development when neurons extend processes called axons. The paths taken by axons depend on the movement of their terminal "growth cones" -- structures that control the direction and rate of axon extension in response to molecular cues in the environment. For example, some cues, known as chemotropic factors, can orient the growth cones towards or away from the source of the cue. Growth cones sense external cues through receptors on their surface, and then generate intracellular signals that determine the direction in which the axon grows. One such signal is ionic calcium (Ca2+), through which several environmental guidance cues influence the orientation of growth cones. But exactly how these cues promote the appropriate changes in Ca2+ levels is poorly defined. New work [1,2] describes an important and unexpected role for a channel protein.

2) The concentration of intracellular Ca2+ in growth cones is tightly controlled by a variety of known channels, pumps and buffers. Fluctuations in intracellular Ca2+ concentrations occur when channels in the plasma membrane, or in the membranes of intracellular stores, open to let Ca2+ flow into the cytosol. Receptors for external molecular cues can activate channels that raise Ca2+ levels in this way. For example, two normally chemoattractive factors -- netrin and brain-derived neurotrophic factor (BDNF) -- are believed to increase Ca2+ levels in neurons by triggering influx of the ion and its release from intracellular stores[1-3].

3) Previous work had suggested that voltage-dependent Ca2+ channels (VDCCs) on the plasma membrane are partly, but not completely, responsible for the Ca2+ elevations induced by netrin[3]. However, it was uncertain how netrin depolarizes neurons sufficiently to activate these channels. Wang and Poo[1] and Li et al[2] have provided conclusive evidence that transient receptor potential (TRP) channels are -- either with or without VDCCs -- involved in the chemotropic turning of growth cones in response to netrin and BDNF.

4) TRP channels constitute a diverse family of cation channels that are activated by a variety of means to regulate many biological functions[4]. Although they have been shown to function in nerve growth cones[5], the new studies[1,2] are the first to demonstrate that they have a role in chemotropic axon guidance. Together, these studies define a signalling pathway from receptor activation to changes in the concentration of Ca2+ in the cytosol that lead to growth-cone turning.

References (abridged):

1. Wang, G. X. & Poo, M. Nature 434, 898-904 (2005)

2. Li, Y. et al. Nature 434, 894-898 (2005)

3. Hong, K., Nishiyama, M., Henley, J., Tessier-Lavigne, M. & Poo, M. Nature 403, 93-98 (2000)

4. Montell, C., Birnbaumer, L. & Flockerzi, V. Cell 108, 595-598 (2002)

5. Greka, A., Navarro, B., Oancea, E., Duggan, A. & Clapham, D. E. Nature Neurosci. 6, 837-845 (2003)

Nature http://www.nature.com/nature

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DEVELOPMENTAL NEUROBIOLOGY: ON AXON GUIDANCE

The following points are made by Sarah Guthrie (Current Biology 2004 14:R632):

1) The central nervous system develops as an axon scaffold arranged fairly symmetrically around a ventral midline. As axons grow, they maintain strict relationships to the midline in order to form correct pathways and functional connections. Some axons grow either away from or parallel to the midline, and never cross. Other axons cross the midline once, forming a commissure, and then continue to grow on the other side without recrossing. In humans and other mammals, many major axon tracts have crossed projections. These include the corticospinal tract, which descends from the cerebral cortex, crosses the midline in the brainstem and innervates motor neurons in the brainstem and spinal cord. Two large sensory tracts conveying information from the periphery also cross the midline as they ascend via the spinal cord to the brain. The dorsal column-medial lemniscus pathway crosses within the brainstem, while the spinothalamic tract contains commissural neurons which send axons across the midline at spinal levels.

2) A very similar projection pattern is seen in the fruitfly Drosophila, and many salient features of midline guidance appear to be conserved. For example, in both vertebrates and invertebrates, the attractant protein netrin serves to guide commissural axons towards the midline [1]. In Drosophila, axonal projections away from the midline depend on the presence at the midline of the repellent molecule Slit, which binds axonal Robo receptors [2,3]. As they approach the midline, axons are attracted by netrin, and express only a low level of Robo [4]. Commissureless (Comm) protein at the midline acts to keep Robo receptors localized cytoplasmically, away from the cell surface [5]. When axons cross the midline this inhibition ceases, allowing Robo to appear on cell surfaces and mediate repulsion. This mechanism ensures that axons are not repelled prematurely by the midline, which would result in an absence of crossing.

3) This web of interactions has been a prototype for those interested in studying midline guidance in vertebrates. The discovery of a number of vertebrate Robo and Slits, in species including rodents and humans, has sparked intense research activity to understand the role of Robo-Slit signalling in vertebrates. While two of the vertebrate Robos -- Robo1 and Robo2 -- are highly similar to the fruitfly Robo1, a third, Robo3/Rig1, is more distantly related. Three vertebrate Slits are widely expressed in the midline of the nervous system, as well as in other regions, while spinal commissural axons express Robos. Recent studies have shown that commissural axons fail to exit the midline in slit1, slit2, slit3 triple mutants, while in vitro data show that commissural axons become insensitive to floor plate attraction and sensitive to Slit-mediated repulsion only after crossing the midline. This modulation of repulsion at the midline is reminiscent of the situation in Drosophila. But are the same mechanisms at work? So far no vertebrate homologues of Comm have been identified.

References (abridged):

1 Varela-Echavarri a, A. and Guthrie, S. (1997). Molecules making waves in axon guidance. Genes Dev. 11, 545-557

2 Kidd, T., Brose, K., Mitchell, K.J., Fetter, R.D., Tessier-Lavigne, M., Goodman, C.S. and Tear, G. (1998). Roundabout controls axon crossing of the CNS midline and defines a novel subfamily of evolutionarily conserved guidance receptors. Cell 92, 205-215

3 Kidd, T., Bland, K.S. and Goodman, C.S. (1999). Slit is the midline repellent for the Robo receptor in Drosophila. Cell 96, 785-794

4 Kidd, T., Russell, C., Goodman, C.S. and Tear, G. (1998). Dosage-sensitive and complementary functions of roundabout and commissureless control axon crossing of the CNS midline. Cell 92, 205-215

5 Keleman, K., Rajagopalan, S., Cleppien, D., Teis, D., Paiha, K., Huber, L.A., Technau, G.M. and Dickson, B.J. (2002). Comm sorts robo to control axon guidance at the Drosophila midline. Cell 110, 415-427

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