Current Biology . . . Cao Yujie and others reveal the mechanism of microtube microtube microtube skeleton regulation mediated by the negative end of the microtube.

In Fig. 1, the left is Luban Road overpass in Shanghai, and the right is an ultra-high-resolution image of the microtubule skeleton of neurons.red is acetylated microtubules, green is tyrosine microtubules. The brain is an important organ to coordinate movement, control other organs of the body, and is closely related to advanced cognitive function. Neurons are the basic unit of the brain.in order to adapt to its special functions, nerve cells have highly specialized morphology and structure.generally speaking, neurons are composed of many short dendrites and a long axon, which perform the function of receiving and transmitting signals respectively.because the brain needs to establish connections with various parts of the body, morphologically, nerve cells are the longest cells in the human body.studies have shown that cytoskeleton, especially microtubules and microtubule related proteins, play an important role in regulating the structure and function of neurons.neurons are highly polarized cells with asymmetry in morphology and function (Fig. 2).the microtubule in cytoskeleton is a hollow cylinder formed by the polymerization of tubulin dimer, which polymerizes from the negative end to the positive end.the positive and negative ends of microtubules are also different in structure and properties.the positive end of the microtubule (MT plus tip) is more dynamic and active, while the negative end (MT minus tip) is more stable.based on the polarity of microtubules, there are two distribution patterns in axons and dendrites, that is, the positive end (plus end-out MT) or the negative end-out MT (minus end-out MT) are toward the distal end of axons and dendrites, which are also referred to as positive microtubules and negative microtubules.as early as 1981, people have observed the uniform and positive microtubules in axons by means of electron microscopy imaging technology.later, the research group of Peter baas found in 1988 that the arrangement of microtubules in dendrites is heterogeneous, and there are both positive and negative microtubules (Fig. 2, Fig. 3).in axons, parallel and co oriented microtubules are more conducive to various intracellular transport dominated by motor protein.on the contrary, this reverse parallel microtubule structure in dendrites is not conducive to transport, and the contradiction between this structure and function could not be explained at that time.with the improvement of scientific research and technology, a lot of progress and breakthroughs have been made in related fields. However, up to now, the specific regulatory mechanism of specialized microtubule structure in axons and dendrites of nerve cells has not been fully studied.Fig. 2, the microtubule structure of hippocampal neurons cultured in vitro. Fig. 3, the cross-sectional electron microscopy of dendrites (hooking buffer treatment) by Peter baas research group in 1988 is shown on the left, and the microtubule structure after treatment is shown on the right (refer to [4]). Purple and dark blue are the inherent dimers of microtubules, and yellow and light blue represent the side chains formed after buffer treatment. on February 20, 2020, Casper C. hoogenraad team of Utrecht University in the Netherlands (the first author is Cao Yujie) published articles on Current Biology Magazine: microtubule min end binding protein camsap2 and kinesin-14 motor kifc3 control dentritic microtubule Organization revealed that camsap2 and kifc3 of kinesin14 family are involved in the regulation of microtubule structure in neurons. the researchers found that kifc3 of the kinesin 14 family plays an important role in the development of neurons. down regulating the expression of kifc3 protein through mRNA interference, dendrite branches of neurons were affected. specifically, the number of dendrites, the average length and the total length of dendrites decreased in different degrees. the previous phenotype was verified again in the mouse brain slice culture system. When kifc3 expression was down regulated, neurons could still migrate smoothly, but dendrites could not develop normally. the researchers further examined the expression of kifc3 in neurons. the overexpressed kifc3 protein was enriched in dendrites and could specifically bind acetylated microtubules. Kifc1, another protein of kinesin 14 family, promotes axonal growth and extension through sliding microtubules in axons. However, different from kifc1, kifc3 has only one MT binding domain, that is, motor domain. In addition to the motor domain, kifc1 also has an N-terminal microtubule binding domain. Therefore, kifc1 can bind multiple microtubules at the same time and slide each other. in order to further explore the mechanism of kifc3 regulating microtubule structure, immunoprecipitation and mass spectrometry were used to detect the possible binding proteins of kifc3 in the nervous system, and then camsap2 was found. according to the existing literature, camsap2 can stabilize the microtubule structure by stabilizing the negative end of microtubule. through biochemical experiments, it was found that camsap2 binds to kifc3 dimer through two coiled coil domains. at the cellular level, the researchers observed that kifc3 and camsap2 could bind to the negative end of microtubules using total internal reflection fluorescence microscopy and laser cutting technology. based on the interaction between kifc3 and camsap2, the researchers speculate that kifc3 can bind to additional microtubules through camsap2, thus regulating microtubule organization and structure. next, the relationship between kifc3-camsap2 and microtubule dynamics in dendrites was examined by short-term and long-term perspectives. the dynamic of microtubules can be detected in a short time (5 minutes) by using the positive end marker of microtubule and live imaging technology. the researchers found that when kifc3 and camsap2 were down regulated, the dynamics and stability of negative end out MT were significantly increased. then, the stability of microtubule structure was detected in a long time range (3 hours) by using laser activated labeling and living cell imaging technology. It was found that the microtubule structure in dendrites became more loose after the down-regulation of kifc3 and camsap2, and the microtubules in the experimental group slipped and moved towards the cell body. combined with the above data, the researchers speculate that kifc3 can recognize the negative end of microtubules through camsap2, and anchor and cross link different microtubules to form a more stable microtubule structure. this stable microtubule structure plays an important role in the development of neurons. the latest research [2] shows that negative microtubules are more stable and acetylated in neuronal dendrites. while the positive microtubules are more dynamic and tyrosine. microtubules in the same direction are closer to each other in space. this study demonstrated that kifc3-camsap2 protein complex could recognize and bind acetylated microtubules through the motor domain of kifc3, and then stabilize and cross link different microtubules through camsap2, so as to specifically aggregate and bind negative microtubules. the aggregation of microtubules can promote the transport efficiency of vesicles, organelles and proteins in dendrites. most of motor proteins play more important roles in transport. Only a few motor proteins, including kifc1 of kinesin 14 family, can participate in the regulation of microtubule structure and function. this study revealed that kifc3, as a kinesin 14 family, can specifically recognize and regulate the structure of negative terminal outward microtubules through binding proteins, and also reveal a new mechanism of dendritic microtubule regulation. at present, most of the related researches mainly focus on the positive end related proteins of microtubules, but the negative end also plays an important role in the development of neurons, and most of the molecular mechanism has not been clearly studied, which is worthy of further study in the future. Fig. 4, structural pattern of microtubule regulated by kifc3 and camsap2. < br / < br / < br / original link: plate maker: plate maker: Reference: 1. TAs, R.P., chazeau, A., closin, B.M.C., lambers, m.l.a., hoogenaad, C.C., and kapitein, L.C., and kapitein, L.C. (2017). Differentiation betweenopositiproperly oriented microtubles controls polarized neural transport. Neuron 96, 1264-1271 e12655.3. Baas, P.W., deatch, J.S., black, M.M., and banker, bank, P.W., deatch, J.S., black, M.M., and banker, L.C., and banker, L.C., and kapitein, l and, G.A. (1988). Polarity orientation of microtubules in hippocampal neurons:uniformity in the axon and nonuniformity in the dendrite. Proc Natl Acad Sci US A 85, 8335-8339.4. Baas,P.W., and Lin, S. (2011). Hooks and comets: The story of microtubule polarityorientation in the neuron. Dev Neurobiol 71, 403-418.5.Muralidharan, H., and Baas, P.W. (2019). Mitotic Motor KIFC1 Is an Organizer ofMicrotubules in the Axon. J Neurosci 39,3792-3811.