Dynamic balance between vesicle transport and microtubule growth enables neurite outgrowth
Fig 1
Identification of different level SCPs involved in neurite outgrowth and their incorporation into multicompartmental ODE model.
(A) Based on our observations dystrophic bulbs (right panel) revealed by tubulin staining are 8–10 times larger than physiological growth cones (left panel). Characteristics of dystrophic bulbs are an accumulation of anterograde vesicles [35] as well as disorganized microtubules [3]. (B) The three major SCP types, Production of Membrane Components, Vesicle Transport & Exocytosis and Microtubule Growth, describe essential sub-cellular functions involved in neurite outgrowth. We labeled them as Level-1 SCPs and identified their Level-2 children and Level-3 grandchildren SCPs. (C) To model membrane production at the Trans-Golgi network (TGN) and delivery to the growth cone plasma membrane (GC-PM) via vesicle transport, we extended a dynamical model that simulates vesicle transport between the endoplasmic reticulum and the Golgi [8]. In our version, new synthesized membrane is added to the Trans-Golgi network from where it is transported to the growth cone plasma membrane by vesicular transport. Vesicles bud from the TGN, move along the microtubules via active kinesin mediated transport and fuse with the growth cone plasma membrane, leading to neurite shaft growth. The vesicles pass through three intermediate compartments: the cell body cytoplasm (CBC), the neurite shaft cytoplasm (NSC) and the growth cone cytoplasm (GCC). In each compartment kinesin has a different affinity for microtubules, leading to varying fractions of vesicles that are actively transported via kinesin along the MT. Retrograde vesicles are generated via endocytosis at the growth cone plasma membrane and move along the microtubule towards TGN through dynein mediated active transport. Vesicle budding at the TGN or GC-PM is mediated by the interaction of recruitment factors and coat proteins, and vesicle fusion with the TGN or the growth cone membrane by the formation of SNARE complexes between vesicle(v)-SNAREs and target(t)-SNAREs, catalyzed by local tethering machineries. Motor proteins are bound to vesicles through motor protein receptors. AG/BG label vesicles that bud from the TGN with coat protein A/B, APM/BPM vesicles that bud from the GC-PM with coat protein A/B. Since at steady state almost all anterograde vesicles bud with coat protein B from the TGN, and almost all retrograde vesicles bud with coat protein A from the GC-PM, we highlighted their transport routes in brighter colors. (D) To simulate MT growth, we consider two different MT pools, stable and dynamicMTs. After nucleation new MTs are added to the pool of dynamic MTs that is characterized by alternating phases of growth and catastrophic breakdown. The frequency and duration of these phases depend on the tubulin concentration (regulated by the SCP Tubulin Sequestration) and GTP hydrolysis rate (SCP GTP Hydrolysis). Consequently, the length distribution of the dynamic MT pool and the degradation rate of dynamic MTs depends on the activity of both SCPs (S2 Fig). Dynamic MTs are either degraded or converted into stable MTs that form the growing neurite scaffold.