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1. Axonal growth on surfaces with periodic geometrical patterns

   https://doi.org/10.1371/journal.pone.0257659  

   Sunnerberg, Jacob P. ; Descoteaux, Marc ; Kaplan, David L. ; Staii, Cristian ( September 2021 , PLOS ONE)

   Dague, Etienne (Ed.)

   The formation of neuron networks is a complex phenomenon of fundamental importance for understanding the development of the nervous system, and for creating novel bioinspired materials for tissue engineering and neuronal repair. The basic process underlying the network formation is axonal growth, a process involving the extension of axons from the cell body towards target neurons. Axonal growth is guided by environmental stimuli that include intercellular interactions, biochemical cues, and the mechanical and geometrical features of the growth substrate. The dynamics of the growing axon and its biomechanical interactions with the growing substrate remains poorly understood. In this paper, we develop a model of axonal motility which incorporates mechanical interactions between the axon and the growth substrate. We combine experimental data with theoretical analysis to measure the parameters that describe axonal growth on surfaces with micropatterned periodic geometrical features: diffusion (cell motility) coefficients, speed and angular distributions, and axon bending rigidities. Experiments performed on neurons treated Taxol (inhibitor of microtubule dynamics) and Blebbistatin (disruptor of actin filaments) show that the dynamics of the cytoskeleton plays a critical role in the axon steering mechanism. Our results demonstrate that axons follow geometrical patterns through a contact-guidance mechanism, in which high-curvature geometrical features impart high traction forces to the growth cone. These results have important implications for our fundamental understanding of axonal growth as well as for bioengineering novel substrates that promote neuronal growth and nerve repair. 

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2. A new view of axon growth and guidance grounded in the stochastic dynamics of actin networks

   https://doi.org/10.1098/rsob.220359  

   Forghani, Rameen ; Chandrasekaran, Aravind ; Papoian, Garegin ; Giniger, Edward ( June 2023 , Open Biology)

   The mechanism of axon growth and guidance is a core, unsolved problem in neuroscience and cell biology. For nearly three decades, our view of this process has largely been based on deterministic models of motility derived from studies of neurons cultured in vitro on rigid substrates. Here, we suggest a fundamentally different, inherently probabilistic model of axon growth, one that is grounded in the stochastic dynamics of actin networks. This perspective is motivated and supported by a synthesis of results from live imaging of a specific axon growing in its native tissue in vivo , together with single-molecule computational simulations of actin dynamics. In particular, we show how axon growth arises from a small spatial bias in the intrinsic fluctuations of the axonal actin cytoskeleton, one that produces net translocation of the axonal actin network by differentially modulating local probabilities of network expansion versus compaction. We discuss the relationship between this model and current views of axon growth and guidance mechanism and demonstrate how it offers explanations for various longstanding puzzles in this field. We further point out the implications of the probabilistic nature of actin dynamics for many other processes of cell morphology and motility. 

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3. Spatiotemporal analysis of axonal autophagosome–lysosome dynamics reveals limited fusion events and slow maturation

   https://doi.org/10.1091/mbc.E22-03-0111  

   Cason, Sydney E. ; Mogre, Saurabh S. ; Holzbaur, Erika L. ; Koslover, Elena F. ( November 2022 , Molecular Biology of the Cell)

   Edelstein-Keshet, Leah (Ed.)

   Macroautophagy is a homeostatic process required to clear cellular waste. Neuronal autophagosomes form constitutively in the distal tip of the axon and are actively transported toward the soma, with cargo degradation initiated en route. Cargo turnover requires autophagosomes to fuse with lysosomes to acquire degradative enzymes; however, directly imaging these fusion events in the axon is impractical. Here we use a quantitative model, parameterized and validated using data from primary hippocampal neurons, to explore the autophagosome maturation process. We demonstrate that retrograde autophagosome motility is independent of fusion and that most autophagosomes fuse with only a few lysosomes during axonal transport. Our results indicate that breakdown of the inner autophagosomal membrane is much slower in neurons than in nonneuronal cell types, highlighting the importance of this late maturation step. Together, rigorous quantitative measurements and mathematical modeling elucidate the dynamics of autophagosome–lysosome interaction and autophagosomal maturation in the axon. 

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4. Sequential dynein effectors regulate axonal autophagosome motility in a maturation-dependent pathway

   https://doi.org/10.1083/jcb.202010179  

   Cason, Sydney E. ; Carman, Peter J. ; Van Duyne, Claire ; Goldsmith, Juliet ; Dominguez, Roberto ; Holzbaur, Erika L. F. ( May 2021 , Journal of Cell Biology)

   Autophagy is a degradative pathway required to maintain homeostasis. Neuronal autophagosomes form constitutively at the axon terminal and mature via lysosomal fusion during dynein-mediated transport to the soma. How the dynein–autophagosome interaction is regulated is unknown. Here, we identify multiple dynein effectors on autophagosomes as they transit along the axons of primary neurons. In the distal axon, JIP1 initiates autophagosomal transport. Autophagosomes in the mid-axon require HAP1 and Huntingtin. We find that HAP1 is a dynein activator, binding the dynein–dynactin complex via canonical and noncanonical interactions. JIP3 is on most axonal autophagosomes, but specifically regulates the transport of mature autolysosomes. Inhibiting autophagosomal transport disrupts maturation, and inhibiting autophagosomal maturation perturbs the association and function of dynein effectors; thus, maturation and transport are tightly linked. These results reveal a novel maturation-based dynein effector handoff on neuronal autophagosomes that is key to motility, cargo degradation, and the maintenance of axonal health.

    

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5. A Protein Composite Neural Scaffold Modulates Astrocyte Migration and Transcriptome Profile

   https://doi.org/10.1002/mabi.202100406  

   Yao, Li ; Brice, Ryan ; Shippy, Teresa ( January 2022 , Macromolecular Bioscience)

   Bioscaffold implantation is a promising approach to facilitate the repair and regeneration of wounded neural tissue after injury to the spinal cord or peripheral nerves. However, such bioscaffold grafts currently result in only limited functional recovery. The generation of a neural scaffold using a combination of collagen and glutenin is reported. The conduit material and mechanical properties, as well as its effect on astrocyte behavior is tested. After neural injuries, astrocytes move into the lesion and participate in the process of remodeling the micro‐architecture of the wounded neural tissue. In this study, human astrocytes grown on glutenin‐collagen scaffolds show higher motility and a lower proliferation rate compared with those grown on collagen scaffolds. RNA sequencing reveals that astrocytes grown on the two types of scaffolds show differentially expressed genes in Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways such as actin cytoskeleton and focal adhesion that regulate astrocyte migration on scaffolds. The gene expression of aggrecan and versican, chondroitin sulfate proteoglycans that inhibit axonal growth, is down‐regulated in astrocytes grown on glutenin‐collagen scaffolds. These outcomes indicate that the implantation of glutenin‐collagen scaffolds may promote astrocyte function in the neural regeneration process by enhanced cell migration and reduced glial scar formation.

    

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