Abstract
Neuronal cell morphogenesis depends on proper regulation of microtubule-based transport, but the underlying mechanisms are not well understood. Here, we report our study of MAP7, a unique microtubule-associated protein that interacts with both microtubules and the motor protein kinesin-1. Structure-function analysis in rat embryonic sensory neurons shows that the kinesin-1 interacting domain in MAP7 is required for axon and branch growth but not for branch formation. Also, two unique microtubule binding sites are found in MAP7 that have distinct dissociation kinetics and are both required for branch formation. Furthermore, MAP7 recruits kinesin-1 dynamically to microtubules, leading to alterations in organelle transport behaviors, particularly pause/speed switching. As MAP7 is localized to branch sites, our results suggest a novel mechanism mediated by the dual interactions of MAP7 with microtubules and kinesin-1 in the precise control of microtubule-based transport during axon morphogenesis.
Highlights
Neuronal cell functions, including axon morphogenesis, require proper regulation of intracellular transport on microtubules via a variety of motor and non-motor microtubule-associated proteins (MAPs) (Armijo-Weingart and Gallo, 2017; Kalil and Dent, 2014; Kapitein and Hoogenraad, 2015; Kevenaar and Hoogenraad, 2015)
To understand the contribution of MAP7 domains in axon morphogenesis, we used E14 rat dorsal root ganglion (DRG) neurons that lack endogenous MAP7 expression at this time point and which exhibit a simple morphology when grown in culture (Tymanskyj et al, 2017; Wang et al, 1999; Zhao et al, 2009) (Figure 1B, and Figure 1—figure supplement 1)
Expression of an EGFP fusion protein of full length (FL) MAP7 (MAP7-FL-EGFP) resulted in a threefold increase in the number of branches produced per cell and a fivefold increase in the number of interstitial branches, defined as those arising from 90% of the axon length proximal to the soma, when compared with the EGFP control (Figure 1B,C, Figure 1—figure supplement 1)
Summary
Neuronal cell functions, including axon morphogenesis, require proper regulation of intracellular transport on microtubules via a variety of motor and non-motor microtubule-associated proteins (MAPs) (Armijo-Weingart and Gallo, 2017; Kalil and Dent, 2014; Kapitein and Hoogenraad, 2015; Kevenaar and Hoogenraad, 2015). Neurons are enriched with lattice-binding non-motor MAPs that are often associated with stable microtubules in axons and dendrites (Kapitein and Hoogenraad, 2015). Some of these proteins can influence motor protein functions (Encalada and Goldstein, 2014; Fu and Holzbaur, 2014; Nirschl et al, 2017). The mechanism and the functional role of the interaction between motor and non-motor MAPs in neurons remain poorly understood
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