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Putting the model to the test: are APC proteins essential for neuronal polarity, axon outgrowth, and axon targeting?

Rusan NM, Akong K, Peifer M - J. Cell Biol. (2008)

Bottom Line: We completely removed both APCs from Drosophila melanogaster larval neural precursors and neurons, testing whether APCs play universal roles in neuronal polarity.Likewise, CB, lobular plug, and mushroom body neurons do not require APCs for polarization, axon outgrowth, or, in the latter two cases, axon targeting.These data suggest that proposed cytoskeletal roles for APCs in mammals should be reassessed using loss of function tools.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.

ABSTRACT
The highly polarized architecture of neurons is important for their function. Experimental data based on dominant-negative approaches suggest that the tumor suppressor adenomatous polyposis coli (APC), a regulator of Wnt signaling and the cytoskeleton, regulates polarity of neuroectodermal precursors and neurons, helping specify one neurite as the axon, promoting its outgrowth, and guiding axon pathfinding. However, such dominant-negative approaches might affect processes in which APC is not essential. We completely removed both APCs from Drosophila melanogaster larval neural precursors and neurons, testing whether APCs play universal roles in neuronal polarity. Surprisingly, APCs are not essential for asymmetric cell division or the stereotyped division axis of central brain (CB) neuroblasts, although they do affect cell cycle progression and spindle architecture. Likewise, CB, lobular plug, and mushroom body neurons do not require APCs for polarization, axon outgrowth, or, in the latter two cases, axon targeting. These data suggest that proposed cytoskeletal roles for APCs in mammals should be reassessed using loss of function tools.

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APCs are not required for MB axon targeting. (A and E) Posterior-dorsal (A) and anterior views (E) of third instar MB. Cell body cluster plus dendritic region (blue), peduncle (P, yellow), and medial (M, pink) and dorsal (D, green) axon projections. (B and F) Wild type. (C and G) Control wild-type clones. (D and H) APC2g10APC1Q8 clones. Asterisks, brain hemispheres where a clone was not induced. (I and I′) Pathfinding error. (I′) Enlargement of area is shown in dashed square. White arrow indicates error, and yellow arrow indicates normal axon trajectory. (J and K) Wild-type and APC double  medullar neurons. Arrow indicates axon outgrowth defect. (L and M) MBs expressing armS10. Bars, 50 μm.
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fig4: APCs are not required for MB axon targeting. (A and E) Posterior-dorsal (A) and anterior views (E) of third instar MB. Cell body cluster plus dendritic region (blue), peduncle (P, yellow), and medial (M, pink) and dorsal (D, green) axon projections. (B and F) Wild type. (C and G) Control wild-type clones. (D and H) APC2g10APC1Q8 clones. Asterisks, brain hemispheres where a clone was not induced. (I and I′) Pathfinding error. (I′) Enlargement of area is shown in dashed square. White arrow indicates error, and yellow arrow indicates normal axon trajectory. (J and K) Wild-type and APC double medullar neurons. Arrow indicates axon outgrowth defect. (L and M) MBs expressing armS10. Bars, 50 μm.

Mentions: Our final test used some of the best characterized larval neurons, the mushroom body (MB; Heisenberg, 2003). They have highly reproducible axon and dendrite projections with fasciculated axons that first project ventrally and then turn and bifurcate in separate dorsal and medial projections (we visualized MBs using the MB-specific GAL4201Y driver; Fig. 4, A, B, E, and F; Yang et al., 1995). We generated control and APC2g10 APC1Q8 double mutant clones and specifically visualized clones arising in MBs (see Materials and methods). In control clones, MB axon projections were normal (n = 12; Fig. 4, C and G). In APC2g10 APC1Q8 double mutant clones, MB neurons differentiated, polarized, and sent out axons and dendrites normally. 25/26 clones had normal axon architecture (Fig. 4, D and H). One clone made an axon outgrowth error (Fig. 4, I and I′), which may reflect a modest role for APCs in this complex guidance decision or may be a random error unrelated to APC.


Putting the model to the test: are APC proteins essential for neuronal polarity, axon outgrowth, and axon targeting?

Rusan NM, Akong K, Peifer M - J. Cell Biol. (2008)

APCs are not required for MB axon targeting. (A and E) Posterior-dorsal (A) and anterior views (E) of third instar MB. Cell body cluster plus dendritic region (blue), peduncle (P, yellow), and medial (M, pink) and dorsal (D, green) axon projections. (B and F) Wild type. (C and G) Control wild-type clones. (D and H) APC2g10APC1Q8 clones. Asterisks, brain hemispheres where a clone was not induced. (I and I′) Pathfinding error. (I′) Enlargement of area is shown in dashed square. White arrow indicates error, and yellow arrow indicates normal axon trajectory. (J and K) Wild-type and APC double  medullar neurons. Arrow indicates axon outgrowth defect. (L and M) MBs expressing armS10. Bars, 50 μm.
© Copyright Policy
Related In: Results  -  Collection

License 1 - License 2
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getmorefigures.php?uid=PMC2568018&req=5

fig4: APCs are not required for MB axon targeting. (A and E) Posterior-dorsal (A) and anterior views (E) of third instar MB. Cell body cluster plus dendritic region (blue), peduncle (P, yellow), and medial (M, pink) and dorsal (D, green) axon projections. (B and F) Wild type. (C and G) Control wild-type clones. (D and H) APC2g10APC1Q8 clones. Asterisks, brain hemispheres where a clone was not induced. (I and I′) Pathfinding error. (I′) Enlargement of area is shown in dashed square. White arrow indicates error, and yellow arrow indicates normal axon trajectory. (J and K) Wild-type and APC double medullar neurons. Arrow indicates axon outgrowth defect. (L and M) MBs expressing armS10. Bars, 50 μm.
Mentions: Our final test used some of the best characterized larval neurons, the mushroom body (MB; Heisenberg, 2003). They have highly reproducible axon and dendrite projections with fasciculated axons that first project ventrally and then turn and bifurcate in separate dorsal and medial projections (we visualized MBs using the MB-specific GAL4201Y driver; Fig. 4, A, B, E, and F; Yang et al., 1995). We generated control and APC2g10 APC1Q8 double mutant clones and specifically visualized clones arising in MBs (see Materials and methods). In control clones, MB axon projections were normal (n = 12; Fig. 4, C and G). In APC2g10 APC1Q8 double mutant clones, MB neurons differentiated, polarized, and sent out axons and dendrites normally. 25/26 clones had normal axon architecture (Fig. 4, D and H). One clone made an axon outgrowth error (Fig. 4, I and I′), which may reflect a modest role for APCs in this complex guidance decision or may be a random error unrelated to APC.

Bottom Line: We completely removed both APCs from Drosophila melanogaster larval neural precursors and neurons, testing whether APCs play universal roles in neuronal polarity.Likewise, CB, lobular plug, and mushroom body neurons do not require APCs for polarization, axon outgrowth, or, in the latter two cases, axon targeting.These data suggest that proposed cytoskeletal roles for APCs in mammals should be reassessed using loss of function tools.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.

ABSTRACT
The highly polarized architecture of neurons is important for their function. Experimental data based on dominant-negative approaches suggest that the tumor suppressor adenomatous polyposis coli (APC), a regulator of Wnt signaling and the cytoskeleton, regulates polarity of neuroectodermal precursors and neurons, helping specify one neurite as the axon, promoting its outgrowth, and guiding axon pathfinding. However, such dominant-negative approaches might affect processes in which APC is not essential. We completely removed both APCs from Drosophila melanogaster larval neural precursors and neurons, testing whether APCs play universal roles in neuronal polarity. Surprisingly, APCs are not essential for asymmetric cell division or the stereotyped division axis of central brain (CB) neuroblasts, although they do affect cell cycle progression and spindle architecture. Likewise, CB, lobular plug, and mushroom body neurons do not require APCs for polarization, axon outgrowth, or, in the latter two cases, axon targeting. These data suggest that proposed cytoskeletal roles for APCs in mammals should be reassessed using loss of function tools.

Show MeSH
Related in: MedlinePlus