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Opposing roles of PlexinA and PlexinB in axonal branch and varicosity formation.

Neufeld SQ, Hibbert AD, Chen BE - Mol Brain (2011)

Bottom Line: However, very few molecules have been identified that are involved in determining where and how many synapses form.In contrast, knocking down PlexinB expression decreased morphological complexity by decreasing the number of branches and the overall size of the axonal arbor, but did not reduce the number of varicosities.Our results demonstrate opposing roles for PlexinA and PlexinB in local wiring within a target region, where PlexinA functions to suppress excessive axonal branches and synapses and PlexinB facilitates axonal growth.

View Article: PubMed Central - HTML - PubMed

Affiliation: Research Institute of the McGill University Health Centre, Centre for Research in Neuroscience, Montréal, Québec, Canada.

ABSTRACT
Establishing precise synaptic connectivity during development is crucial for neural circuit function. However, very few molecules have been identified that are involved in determining where and how many synapses form. The Plexin cell-surface molecules are a conserved family of axon guidance receptors that mediate axon fasciculation and repulsion during neural development, and later in development PlexinA receptors are involved in eliminating axonal branches and synapse numbers. Here we investigate the roles of PlexinA and PlexinB receptors in axonal branch and varicosity formation in Drosophila. We knocked down PlexinA or PlexinB expression using RNAi in identified mechanosensory neurons and analyzed axonal branching patterns and varicosity formations. Reducing PlexinA expression increased the axonal arbor complexity by increasing the number of branches and varicosities along the axon. In contrast, knocking down PlexinB expression decreased morphological complexity by decreasing the number of branches and the overall size of the axonal arbor, but did not reduce the number of varicosities. Our results demonstrate opposing roles for PlexinA and PlexinB in local wiring within a target region, where PlexinA functions to suppress excessive axonal branches and synapses and PlexinB facilitates axonal growth.

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Reducing PlexinA in axons produces opposite effects from reducing PlexinB. A, The pSc axonal arbors in PlexinA mutants have significantly more branches compared to controls. RNAi of PlexinB in pSc neurons using the 455-Gal4 driver significantly reduced the number of branches compared to 455-Gal4 controls. B, PlexinA RNAi in pSc neurons increased the total axonal arbor size, and PlexinB RNAi in pSc neurons decreased the arbor size. Combining the RNAi in pSc neurons with the heterozygous mutant enhanced the RNAi phenotype compared to 455-Gal4 controls, but was not significantly different from RNAi in pSc neurons alone. 1 asterisk indicates statistically significant differences with p values less than 0.05, 2 asterisks represent p values less than 0.01, and 3 asterisks represent p values less than 0.001. Measurements in all PlexinA mutants are significantly greater than measurements in all PlexinB mutants in both graphs, and are not indicated for clarity.
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Figure 2: Reducing PlexinA in axons produces opposite effects from reducing PlexinB. A, The pSc axonal arbors in PlexinA mutants have significantly more branches compared to controls. RNAi of PlexinB in pSc neurons using the 455-Gal4 driver significantly reduced the number of branches compared to 455-Gal4 controls. B, PlexinA RNAi in pSc neurons increased the total axonal arbor size, and PlexinB RNAi in pSc neurons decreased the arbor size. Combining the RNAi in pSc neurons with the heterozygous mutant enhanced the RNAi phenotype compared to 455-Gal4 controls, but was not significantly different from RNAi in pSc neurons alone. 1 asterisk indicates statistically significant differences with p values less than 0.05, 2 asterisks represent p values less than 0.01, and 3 asterisks represent p values less than 0.001. Measurements in all PlexinA mutants are significantly greater than measurements in all PlexinB mutants in both graphs, and are not indicated for clarity.

Mentions: When we combined PlexinA RNAi and PlexALOF/+ flies, we found that all 455-Gal4; UAS-PlexA-dsRNA/+; PlexALOF/+ flies had routing errors and excessive branches in the pSc axon (Figure 1E). The enhancement of the excessive branching and misrouting phenotypes was so severe that only 10 out of 18 flies were analyzable due to the number of overlapping and intertwined branches. The number of pSc axonal branches and total arbor size (31 ± 2.0 and 1305 ± 49 μm, respectively) in these flies were significantly greater than control, but were not significantly different from PlexinA RNAi within the pSc alone (Figure 2). The 455-Gal4; UAS-PlexA-dsRNA/+; PlexALOF/+ flies did have a substantial increase in axon guidance errors with 56% entering the CNS incorrectly. The pSc axon guidance errors entered into the haltere nerve pathway at a posterior entry point in the metathoracic neuromere (arrowheads, Figure 1E) instead of into the mesothoracic neuromere. These results indicate that the finer scale branching decisions are more sensitive to PlexinA expression than axon guidance decisions, because we observed an increase in branch number even in heterozygous PlexinA mutants. As PlexinA levels are progressively reduced, axonal arbor complexity increases, demonstrating its critical role in suppressing or destabilizing inappropriate branches.


Opposing roles of PlexinA and PlexinB in axonal branch and varicosity formation.

Neufeld SQ, Hibbert AD, Chen BE - Mol Brain (2011)

Reducing PlexinA in axons produces opposite effects from reducing PlexinB. A, The pSc axonal arbors in PlexinA mutants have significantly more branches compared to controls. RNAi of PlexinB in pSc neurons using the 455-Gal4 driver significantly reduced the number of branches compared to 455-Gal4 controls. B, PlexinA RNAi in pSc neurons increased the total axonal arbor size, and PlexinB RNAi in pSc neurons decreased the arbor size. Combining the RNAi in pSc neurons with the heterozygous mutant enhanced the RNAi phenotype compared to 455-Gal4 controls, but was not significantly different from RNAi in pSc neurons alone. 1 asterisk indicates statistically significant differences with p values less than 0.05, 2 asterisks represent p values less than 0.01, and 3 asterisks represent p values less than 0.001. Measurements in all PlexinA mutants are significantly greater than measurements in all PlexinB mutants in both graphs, and are not indicated for clarity.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3094289&req=5

Figure 2: Reducing PlexinA in axons produces opposite effects from reducing PlexinB. A, The pSc axonal arbors in PlexinA mutants have significantly more branches compared to controls. RNAi of PlexinB in pSc neurons using the 455-Gal4 driver significantly reduced the number of branches compared to 455-Gal4 controls. B, PlexinA RNAi in pSc neurons increased the total axonal arbor size, and PlexinB RNAi in pSc neurons decreased the arbor size. Combining the RNAi in pSc neurons with the heterozygous mutant enhanced the RNAi phenotype compared to 455-Gal4 controls, but was not significantly different from RNAi in pSc neurons alone. 1 asterisk indicates statistically significant differences with p values less than 0.05, 2 asterisks represent p values less than 0.01, and 3 asterisks represent p values less than 0.001. Measurements in all PlexinA mutants are significantly greater than measurements in all PlexinB mutants in both graphs, and are not indicated for clarity.
Mentions: When we combined PlexinA RNAi and PlexALOF/+ flies, we found that all 455-Gal4; UAS-PlexA-dsRNA/+; PlexALOF/+ flies had routing errors and excessive branches in the pSc axon (Figure 1E). The enhancement of the excessive branching and misrouting phenotypes was so severe that only 10 out of 18 flies were analyzable due to the number of overlapping and intertwined branches. The number of pSc axonal branches and total arbor size (31 ± 2.0 and 1305 ± 49 μm, respectively) in these flies were significantly greater than control, but were not significantly different from PlexinA RNAi within the pSc alone (Figure 2). The 455-Gal4; UAS-PlexA-dsRNA/+; PlexALOF/+ flies did have a substantial increase in axon guidance errors with 56% entering the CNS incorrectly. The pSc axon guidance errors entered into the haltere nerve pathway at a posterior entry point in the metathoracic neuromere (arrowheads, Figure 1E) instead of into the mesothoracic neuromere. These results indicate that the finer scale branching decisions are more sensitive to PlexinA expression than axon guidance decisions, because we observed an increase in branch number even in heterozygous PlexinA mutants. As PlexinA levels are progressively reduced, axonal arbor complexity increases, demonstrating its critical role in suppressing or destabilizing inappropriate branches.

Bottom Line: However, very few molecules have been identified that are involved in determining where and how many synapses form.In contrast, knocking down PlexinB expression decreased morphological complexity by decreasing the number of branches and the overall size of the axonal arbor, but did not reduce the number of varicosities.Our results demonstrate opposing roles for PlexinA and PlexinB in local wiring within a target region, where PlexinA functions to suppress excessive axonal branches and synapses and PlexinB facilitates axonal growth.

View Article: PubMed Central - HTML - PubMed

Affiliation: Research Institute of the McGill University Health Centre, Centre for Research in Neuroscience, Montréal, Québec, Canada.

ABSTRACT
Establishing precise synaptic connectivity during development is crucial for neural circuit function. However, very few molecules have been identified that are involved in determining where and how many synapses form. The Plexin cell-surface molecules are a conserved family of axon guidance receptors that mediate axon fasciculation and repulsion during neural development, and later in development PlexinA receptors are involved in eliminating axonal branches and synapse numbers. Here we investigate the roles of PlexinA and PlexinB receptors in axonal branch and varicosity formation in Drosophila. We knocked down PlexinA or PlexinB expression using RNAi in identified mechanosensory neurons and analyzed axonal branching patterns and varicosity formations. Reducing PlexinA expression increased the axonal arbor complexity by increasing the number of branches and varicosities along the axon. In contrast, knocking down PlexinB expression decreased morphological complexity by decreasing the number of branches and the overall size of the axonal arbor, but did not reduce the number of varicosities. Our results demonstrate opposing roles for PlexinA and PlexinB in local wiring within a target region, where PlexinA functions to suppress excessive axonal branches and synapses and PlexinB facilitates axonal growth.

Show MeSH
Related in: MedlinePlus