- Email: [email protected]
Off-track gets axons on track
16
News & Comment
TRENDS in Neurosciences Vol.25 No.1 January 2002
Journal Club
Off-track gets axons on track In the developing nervous system, growing axons are guided to their destinations by molecules in their environment that bind to receptors on the axon surface, transducing signals that are relayed inside neurons to govern axon steering. A prominent family of axon-guidance molecules is the Semaphorins (Semas), which can mediate the repulsion of axons, deflecting them from inappropriate territory. The molecular chain of command underlying Sema function remains poorly understood, but now, another piece of this puzzle has been put in place, with the discovery by Winberg et al. [1] of the transmembrane molecule Off-track. Previous studies have shown that Semas bind to a receptor complex that includes neuropilins and plexins, but interestingly, both of these molecules lack the tyrosine kinase activity that is used commonly by axon-guidance receptors to relay signals into the cell interior. However, plexins were known to become phosphorylated in cell extracts and they could be co-purified with a 160-kDa protein, which might provide the tyrosine kinase activity necessary for downstream signalling. Winberg and colleagues have now identified this protein as the previously known Drosophila transmembrane protein Dtrk, now renamed Off-track (OTK). Co-transfection of COS cells with the genes encoding OTK and various
Drosophila plexins showed that, as expected, the two proteins formed complexes together, suggesting a role for OTK in Sema signaling. In vivo, OTK is expressed in axon projections in the central nervous system (CNS) and in motor-axon pathways in the periphery. In flies lacking OTK, a subset of CNS axons appears wavy and defasciculated, rather than forming the parallel tracks observed in wild-type embryos, and motor-axon pathways are aberrant, with axons failing to diverge from the main nerve at several choice-points. These defects are very similar to those observed in flies with mutant Sema1a or PlexinA genes. The involvement of OTK in semaphorin signalling was demonstrated further by reducing the gene dosage for Otk and Sema1, or Otk and PlexinA together. In this case, the motor-axon guidance defects in flies lacking one copy of each gene resembled strongly the defects in flies lacking two copies of a single gene – good (though not definitive) evidence that the two genes function in the same pathway. The scenario proposed here for OTK function is, therefore, that OTK associates with plexins on the axon surface (although this remains to be demonstrated) and operates downstream of the Semaphorins to transduce signals. An alternative
hypothesis, that OTK interacts with Sema1a on the cell surface, is probably not the case, because when Sema1A is expressed aberrantly on muscles, motor neurons fail to innervate them correctly and this phenotype can be suppressed by removing one copy of Otk. A fly in the ointment is that OTK transpires to be a kinase ‘dead’ receptor, which is unlikely to be capable of transducing a signal, owing to the alteration of some crucial residues within its catalytic domain. This situation might be resolved if OTK and plexin recruited an additional active kinase to the receptor complex, a situation that has precedent in the mode of action of some other receptors. There are also hints of further diversity in Sema signaling, because both plexins and OTK can bind homophilically, and some Semas might themselves be capable of transducing signals, leaving many aspects of these molecular systems as topics for future research. 1 Winberg, M.L. et al. (2001) The transmembrane protein Off-track associates with plexins and functions downstream of semaphorin signaling during axon guidance. Neuron 32, 53–62
Sarah Guthrie [email protected]
Cell migration within the cortex – and what cell surface receptors have to say about it Cell migration is an important factor in the development of the adult central nervous system (CNS). It is one of the mechanisms that nature has adopted to create highly ordered and complex brains. However, it remains one of the least well understood of all cell behaviors known to neurobiologists, partly because of its complexity. Cells migrate at specific times and take characteristic routes to travel long distances and reach many distinct places, to create a highly organized CNS. This is likely to require the co-operation of multiple intrinsic and extrinsic signaling mechanisms. Caric et al. [1] now describe a recent attempt at gaining a more http://tins.trends.com
mechanistic understanding of this cell migration. During normal development of the CNS, neurons and glial cells are generated in the ventricular zone (VZ), located on the inner surface of the developing neural tube. At specific times in development, these cells follow characteristic routes to distinct positions within the forming brain. Epidermal growth factor receptor (EGFR) has been implicated in the control of cell migration in the telencephalon. For example, in EGFR-null mice, neural cells fail to reach their final positions because they cannot leave the VZ, and levels of EGFR expression can affect the timing of
migration and the final position of migrating cells. Although these data suggest a role for EGFRs in cell migration, the underlying mechanism could be explained by either chemoattraction or chemorepulsion. Caric et al. use a clever combination of cortical explants and in utero infections to better understand the mechanisms underlying cell migration in response to changes in the expression level of EGFRs. The explant assay allows them to control carefully the location and concentration of the ligand for EGFR. Using this assay, they can distingish clearly between the two mechanisms of attraction and repulsion,
0166-2236/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.
News & Comment
TRENDS in Neurosciences Vol.25 No.1 January 2002
Journal Club
Off-track gets axons on track In the developing nervous system, growing axons are guided to their destinations by molecules in their environment that bind to receptors on the axon surface, transducing signals that are relayed inside neurons to govern axon steering. A prominent family of axon-guidance molecules is the Semaphorins (Semas), which can mediate the repulsion of axons, deflecting them from inappropriate territory. The molecular chain of command underlying Sema function remains poorly understood, but now, another piece of this puzzle has been put in place, with the discovery by Winberg et al. [1] of the transmembrane molecule Off-track. Previous studies have shown that Semas bind to a receptor complex that includes neuropilins and plexins, but interestingly, both of these molecules lack the tyrosine kinase activity that is used commonly by axon-guidance receptors to relay signals into the cell interior. However, plexins were known to become phosphorylated in cell extracts and they could be co-purified with a 160-kDa protein, which might provide the tyrosine kinase activity necessary for downstream signalling. Winberg and colleagues have now identified this protein as the previously known Drosophila transmembrane protein Dtrk, now renamed Off-track (OTK). Co-transfection of COS cells with the genes encoding OTK and various
Drosophila plexins showed that, as expected, the two proteins formed complexes together, suggesting a role for OTK in Sema signaling. In vivo, OTK is expressed in axon projections in the central nervous system (CNS) and in motor-axon pathways in the periphery. In flies lacking OTK, a subset of CNS axons appears wavy and defasciculated, rather than forming the parallel tracks observed in wild-type embryos, and motor-axon pathways are aberrant, with axons failing to diverge from the main nerve at several choice-points. These defects are very similar to those observed in flies with mutant Sema1a or PlexinA genes. The involvement of OTK in semaphorin signalling was demonstrated further by reducing the gene dosage for Otk and Sema1, or Otk and PlexinA together. In this case, the motor-axon guidance defects in flies lacking one copy of each gene resembled strongly the defects in flies lacking two copies of a single gene – good (though not definitive) evidence that the two genes function in the same pathway. The scenario proposed here for OTK function is, therefore, that OTK associates with plexins on the axon surface (although this remains to be demonstrated) and operates downstream of the Semaphorins to transduce signals. An alternative
hypothesis, that OTK interacts with Sema1a on the cell surface, is probably not the case, because when Sema1A is expressed aberrantly on muscles, motor neurons fail to innervate them correctly and this phenotype can be suppressed by removing one copy of Otk. A fly in the ointment is that OTK transpires to be a kinase ‘dead’ receptor, which is unlikely to be capable of transducing a signal, owing to the alteration of some crucial residues within its catalytic domain. This situation might be resolved if OTK and plexin recruited an additional active kinase to the receptor complex, a situation that has precedent in the mode of action of some other receptors. There are also hints of further diversity in Sema signaling, because both plexins and OTK can bind homophilically, and some Semas might themselves be capable of transducing signals, leaving many aspects of these molecular systems as topics for future research. 1 Winberg, M.L. et al. (2001) The transmembrane protein Off-track associates with plexins and functions downstream of semaphorin signaling during axon guidance. Neuron 32, 53–62
Sarah Guthrie [email protected]
Cell migration within the cortex – and what cell surface receptors have to say about it Cell migration is an important factor in the development of the adult central nervous system (CNS). It is one of the mechanisms that nature has adopted to create highly ordered and complex brains. However, it remains one of the least well understood of all cell behaviors known to neurobiologists, partly because of its complexity. Cells migrate at specific times and take characteristic routes to travel long distances and reach many distinct places, to create a highly organized CNS. This is likely to require the co-operation of multiple intrinsic and extrinsic signaling mechanisms. Caric et al. [1] now describe a recent attempt at gaining a more http://tins.trends.com
mechanistic understanding of this cell migration. During normal development of the CNS, neurons and glial cells are generated in the ventricular zone (VZ), located on the inner surface of the developing neural tube. At specific times in development, these cells follow characteristic routes to distinct positions within the forming brain. Epidermal growth factor receptor (EGFR) has been implicated in the control of cell migration in the telencephalon. For example, in EGFR-null mice, neural cells fail to reach their final positions because they cannot leave the VZ, and levels of EGFR expression can affect the timing of
migration and the final position of migrating cells. Although these data suggest a role for EGFRs in cell migration, the underlying mechanism could be explained by either chemoattraction or chemorepulsion. Caric et al. use a clever combination of cortical explants and in utero infections to better understand the mechanisms underlying cell migration in response to changes in the expression level of EGFRs. The explant assay allows them to control carefully the location and concentration of the ligand for EGFR. Using this assay, they can distingish clearly between the two mechanisms of attraction and repulsion,
0166-2236/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.