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L1CAM/Neuroglian controls the axon-axon interactions establishing layered and lobular mushroom body architecture.

Siegenthaler D, Enneking EM, Moreno E, Pielage J - J. Cell Biol. (2015)

Bottom Line: We demonstrate that the Drosophila melanogaster L1CAM homologue Neuroglian mediates adhesion between functionally distinct mushroom body axon populations to enforce and control appropriate projections into distinct axonal layers and lobes essential for olfactory learning and memory.For functional cluster formation, intracellular Ankyrin2 association is sufficient on one side of the trans-axonal complex whereas Moesin association is likely required simultaneously in both interacting axonal populations.Together, our results provide novel mechanistic insights into cell adhesion molecule-mediated axon-axon interactions that enable precise assembly of complex neuronal circuits.

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Affiliation: Friedrich Miescher Institute for Biomedical Research, 4058 Basel, Switzerland University of Basel, 4003 Basel, Switzerland.

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Nrg controls MB axon tract choice. (A–I) MARCM analysis of nrg14 mutants. Bars, 10 µm. (A–C) Frontal projections of control and nrg14 mutant single-cell clones. Absence of nrg14 in individual MB neurons does not cause obvious alteration of axonal projections. (D–F) Frontal projections of control and nrg14 αβ NB clones (NBc). The majority of nrg14 αβ NBc do not show an axonal phenotype (E). (F) Example of an nrg14 αβ NBc in which axons fail to project into the pedunculus and form circular projections in the posterior brain. (G–I) Large control and nrg14 NBc that include either α’β’ and αβ (H) or all three MB subtypes (G and I). Top panels show frontal projections of the entire MB. Bottom panels show medial (side) views of the NBc marked by GFP (green) and Dlg (blue; G and H) or FasII (red; I). In contrast to controls, nrg14 mutant α’β’ and αβ axons but not γ axons (I) project aberrantly straight to the α lobe tip, bypassing the MB pedunculus and lobes (asterisks). (J) Quantification of MARCM phenotypes (n = 31, 11, 5, 131, 9, and 14, in the respective order of the genotypes indicated). (K) Schematic drawing of wild-type and nrg14 mutant axon trajectories in G–I.
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fig2: Nrg controls MB axon tract choice. (A–I) MARCM analysis of nrg14 mutants. Bars, 10 µm. (A–C) Frontal projections of control and nrg14 mutant single-cell clones. Absence of nrg14 in individual MB neurons does not cause obvious alteration of axonal projections. (D–F) Frontal projections of control and nrg14 αβ NB clones (NBc). The majority of nrg14 αβ NBc do not show an axonal phenotype (E). (F) Example of an nrg14 αβ NBc in which axons fail to project into the pedunculus and form circular projections in the posterior brain. (G–I) Large control and nrg14 NBc that include either α’β’ and αβ (H) or all three MB subtypes (G and I). Top panels show frontal projections of the entire MB. Bottom panels show medial (side) views of the NBc marked by GFP (green) and Dlg (blue; G and H) or FasII (red; I). In contrast to controls, nrg14 mutant α’β’ and αβ axons but not γ axons (I) project aberrantly straight to the α lobe tip, bypassing the MB pedunculus and lobes (asterisks). (J) Quantification of MARCM phenotypes (n = 31, 11, 5, 131, 9, and 14, in the respective order of the genotypes indicated). (K) Schematic drawing of wild-type and nrg14 mutant axon trajectories in G–I.

Mentions: To address potential functions of Nrg that may be masked by the hypomorphic nature of our extra- and intracellular mutations, we next analyzed MB neurons lacking all Nrg using the mosaic analysis with a repressible cell marker (MARCM) technique (Lee and Luo, 1999). Axons of nrg14 single cell mutant clones never failed to grow out or to enter the pedunculus, and we observed only minor branching defects in agreement with prior observations (Fig. 2, A–C, J; Goossens et al., 2011). To address whether Nrg is potentially required for the coordination of larger population of axons or for the interaction between axons of different identity, we next generated NB clones that either included only αβ neurons, both α′β′ and αβ neurons, or all three subtypes of MB neurons (Fig. 1 A). Interestingly, the majority of αβ NB clones did not show any alteration of axonal projections (Fig. 2, D, E, and J). However, in ∼20% of these clones, we observed defects including the formation of ball-like structures below the calyx resembling the phenotype of hypomorphic nrg mutations (Fig. 2, F and J). In contrast, 78% of NB clones that included α′β′ neurons in addition to αβ neurons showed striking defects in MB development, with mutant α′β′ and αβ axons projecting straight to the tip of the α-lobes. In these cases, axons take a shortcut and circumvent their normal path through the pedunculus and the lobes (Fig. 2, G–K). Interestingly, mutant γ axons projected appropriately through the pedunculus to the lobes (Fig. 2 I). These data indicate that Nrg is not required in single axons navigating into the MB structure but is required within populations of α′β′ and/or pioneering αβ axons that likely mediate an interaction between these two distinct axonal populations.


L1CAM/Neuroglian controls the axon-axon interactions establishing layered and lobular mushroom body architecture.

Siegenthaler D, Enneking EM, Moreno E, Pielage J - J. Cell Biol. (2015)

Nrg controls MB axon tract choice. (A–I) MARCM analysis of nrg14 mutants. Bars, 10 µm. (A–C) Frontal projections of control and nrg14 mutant single-cell clones. Absence of nrg14 in individual MB neurons does not cause obvious alteration of axonal projections. (D–F) Frontal projections of control and nrg14 αβ NB clones (NBc). The majority of nrg14 αβ NBc do not show an axonal phenotype (E). (F) Example of an nrg14 αβ NBc in which axons fail to project into the pedunculus and form circular projections in the posterior brain. (G–I) Large control and nrg14 NBc that include either α’β’ and αβ (H) or all three MB subtypes (G and I). Top panels show frontal projections of the entire MB. Bottom panels show medial (side) views of the NBc marked by GFP (green) and Dlg (blue; G and H) or FasII (red; I). In contrast to controls, nrg14 mutant α’β’ and αβ axons but not γ axons (I) project aberrantly straight to the α lobe tip, bypassing the MB pedunculus and lobes (asterisks). (J) Quantification of MARCM phenotypes (n = 31, 11, 5, 131, 9, and 14, in the respective order of the genotypes indicated). (K) Schematic drawing of wild-type and nrg14 mutant axon trajectories in G–I.
© Copyright Policy - openaccess
Related In: Results  -  Collection

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Show All Figures
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fig2: Nrg controls MB axon tract choice. (A–I) MARCM analysis of nrg14 mutants. Bars, 10 µm. (A–C) Frontal projections of control and nrg14 mutant single-cell clones. Absence of nrg14 in individual MB neurons does not cause obvious alteration of axonal projections. (D–F) Frontal projections of control and nrg14 αβ NB clones (NBc). The majority of nrg14 αβ NBc do not show an axonal phenotype (E). (F) Example of an nrg14 αβ NBc in which axons fail to project into the pedunculus and form circular projections in the posterior brain. (G–I) Large control and nrg14 NBc that include either α’β’ and αβ (H) or all three MB subtypes (G and I). Top panels show frontal projections of the entire MB. Bottom panels show medial (side) views of the NBc marked by GFP (green) and Dlg (blue; G and H) or FasII (red; I). In contrast to controls, nrg14 mutant α’β’ and αβ axons but not γ axons (I) project aberrantly straight to the α lobe tip, bypassing the MB pedunculus and lobes (asterisks). (J) Quantification of MARCM phenotypes (n = 31, 11, 5, 131, 9, and 14, in the respective order of the genotypes indicated). (K) Schematic drawing of wild-type and nrg14 mutant axon trajectories in G–I.
Mentions: To address potential functions of Nrg that may be masked by the hypomorphic nature of our extra- and intracellular mutations, we next analyzed MB neurons lacking all Nrg using the mosaic analysis with a repressible cell marker (MARCM) technique (Lee and Luo, 1999). Axons of nrg14 single cell mutant clones never failed to grow out or to enter the pedunculus, and we observed only minor branching defects in agreement with prior observations (Fig. 2, A–C, J; Goossens et al., 2011). To address whether Nrg is potentially required for the coordination of larger population of axons or for the interaction between axons of different identity, we next generated NB clones that either included only αβ neurons, both α′β′ and αβ neurons, or all three subtypes of MB neurons (Fig. 1 A). Interestingly, the majority of αβ NB clones did not show any alteration of axonal projections (Fig. 2, D, E, and J). However, in ∼20% of these clones, we observed defects including the formation of ball-like structures below the calyx resembling the phenotype of hypomorphic nrg mutations (Fig. 2, F and J). In contrast, 78% of NB clones that included α′β′ neurons in addition to αβ neurons showed striking defects in MB development, with mutant α′β′ and αβ axons projecting straight to the tip of the α-lobes. In these cases, axons take a shortcut and circumvent their normal path through the pedunculus and the lobes (Fig. 2, G–K). Interestingly, mutant γ axons projected appropriately through the pedunculus to the lobes (Fig. 2 I). These data indicate that Nrg is not required in single axons navigating into the MB structure but is required within populations of α′β′ and/or pioneering αβ axons that likely mediate an interaction between these two distinct axonal populations.

Bottom Line: We demonstrate that the Drosophila melanogaster L1CAM homologue Neuroglian mediates adhesion between functionally distinct mushroom body axon populations to enforce and control appropriate projections into distinct axonal layers and lobes essential for olfactory learning and memory.For functional cluster formation, intracellular Ankyrin2 association is sufficient on one side of the trans-axonal complex whereas Moesin association is likely required simultaneously in both interacting axonal populations.Together, our results provide novel mechanistic insights into cell adhesion molecule-mediated axon-axon interactions that enable precise assembly of complex neuronal circuits.

View Article: PubMed Central - HTML - PubMed

Affiliation: Friedrich Miescher Institute for Biomedical Research, 4058 Basel, Switzerland University of Basel, 4003 Basel, Switzerland.

Show MeSH
Related in: MedlinePlus