Collateral branches from axons are key components of functional neural circuits that allow neurons to connect with multiple synaptic targets. Like axon growth and guidance, formation of collateral branches depends on the regulation of microtubules, but how such regulation is coordinated to ensure proper circuit development is not known. Based on microarray analysis, we have identified a role for microtubule-associated protein 7 (MAP7) during collateral branch development of dorsal root ganglia (DRG) sensory neurons. We show that MAP7 is expressed at the onset of collateral branch formation. Perturbation of its expression by overexpression or shRNA knockdown alters axon branching in cultured DRG neurons. Localization and time-lapse imaging analysis reveals that MAP7 is enriched at branch points and co-localizes with stable microtubules but enters the new branch with a delay, suggesting a role in branch maturation. We have also investigated a spontaneous mutant mouse that expresses a truncated MAP7 and found a gain-of-function phenotype both in vitro and in vivo. Further domain analysis suggests that the amino half of MAP7 is responsible for branch formation, suggesting a mechanism independent of its known interaction with kinesin. Moreover this mouse exhibits increased pain sensitivity, a phenotype that is consistent with increased collateral branch formation. Thus, our study not only uncovers the first neuronal function of MAP7, but also demonstrates the importance of proper microtubule regulation in neural circuit development. Furthermore, our data provide new insights into microtubule regulation during axonal morphogenesis and may shed light on MAP7 function in neurological disorders.
Neurons communicate with multiple targets by forming axonal branches. In search of intrinsic factors that control collateral branch development, we identified a role for microtubule-associated protein 7 (MAP7) in dorsal root ganglia sensory neurons. We show that MAP7 expression is developmentally regulated and perturbation of this expression alters branch formation. Cell biological analysis indicates that MAP7 promotes branch maturation. Analysis of a spontaneous mouse mutant suggests a molecular mechanism for branch regulation and the potential influence of collateral branches on pain sensitivity. Our studies thus establish the first neuronal function of MAP7 and demonstrate its role in branch morphogenesis and neural circuit function. These findings may help understand the contribution of MAP7 to neurological disorders and nerve regeneration.
The authors declare no competing financial interests.
We thank Zhen Zhao, Zheng Wang, and Joe Hacia for microarray analysis, and Zhen Zhao, Caihong Xia, and Muye Zhu for preliminary studies, and Zongxiu Zhang for in situ analysis. We thank members of the Ma lab for helpful discussion. We also thank Matthew Dalva for helpful discussion, and Erik Dent and Peter Baas for comments on an early version of the manuscript. This work was supported by a grant from NIH (NS062047) to LM.