All Publications

  • Functional Regeneration of the Sensory Root via Axonal Invasion CELL REPORTS Nichols, E. L., Smith, C. J. 2020; 30 (1): 9-+


    Regeneration following spinal root avulsion is broadly unsuccessful despite the regenerative capacity of other PNS-located nerves. By combining focal laser lesioning to model root avulsion in zebrafish, time-lapse imaging, and transgenesis, we identify that regenerating DRG neurons fail to recapitulate developmental paradigms of actin-based invasion after injury. We demonstrate that inducing actin reorganization into invasive components via pharmacological and genetic approaches in the regenerating axon can rescue sensory axon spinal cord entry. Cell-autonomous induction of invasion components using constitutively active Src induces DRG axon regeneration, suggesting an intrinsic mechanism can be activated to drive regeneration. Furthermore, analyses of neuronal activity and animal behavior show restoration of sensory circuit activity and behavior upon stimulating axons to re-enter the spinal cord via invasion. Altogether, our data identify induction of invasive components as sufficient for functional sensory root regeneration after injury.

    View details for DOI 10.1016/j.celrep.2019.12.008

    View details for Web of Science ID 000506009500002

    View details for PubMedID 31914401

    View details for PubMedCentralID PMC6996490

  • Synaptic-like Vesicles Facilitate Pioneer Axon Invasion. Current biology : CB Nichols, E. L., Smith, C. J. 2019


    Synaptic vesicles are indispensable for neuronal communication in mature circuits. Synaptic vesicle biogenesis must be concurrent with axon navigation for synaptogenesis, but whether synaptic vesicles are functionally employed in circuit formation before synaptogenesis is poorly understood. Here, we use time-lapse imaging and transgenesis in zebrafish to visualize the role of synaptic-like vesicles in navigation of dorsal root ganglia pioneer axons. We identify that synaptic-like vesicles accumulate in the central growth cone as the pioneer axon breaches the spinal boundary at the dorsal root entry zone. Inhibition of vesicle release with cell-specific tetanus toxin expression results in pioneer axon pathfinding defects and altered spinal entry. We further show that the matrix metalloproteinase (MMP) mmp14a is required in pioneer axons to navigate across the boundary of the spinal cord and, with super-resolution microscopy, is positioned with synaptic vesicles at the boundary. Manipulations of concurrent actin reorganization reveal that actin remodeling drives vesicle release and subsequent MMP activity. Together, these data point to an indispensable role for synaptic-like vesicles at specific points in axon navigation as regulators of growth cone microenvironment.

    View details for DOI 10.1016/j.cub.2019.06.078

    View details for PubMedID 31378609

  • Generating intravital super-resolution movies with conventional microscopy reveals actin dynamics that construct pioneer axons DEVELOPMENT Zhang, Y., Nichols, E. L., Zellmer, A. M., Guldner, I. H., Kankel, C., Zhang, S., Howard, S. S., Smith, C. J. 2019; 146 (5)


    Super-resolution microscopy is broadening our in-depth understanding of cellular structure. However, super-resolution approaches are limited, for numerous reasons, from utilization in longer-term intravital imaging. We devised a combinatorial imaging technique that combines deconvolution with stepwise optical saturation microscopy (DeSOS) to circumvent this issue and image cells in their native physiological environment. Other than a traditional confocal or two-photon microscope, this approach requires no additional hardware. Here, we provide an open-access application to obtain DeSOS images from conventional microscope images obtained at low excitation powers. We show that DeSOS can be used in time-lapse imaging to generate super-resolution movies in zebrafish. DeSOS was also validated in live mice. These movies uncover that actin structures dynamically remodel to produce a single pioneer axon in a 'top-down' scaffolding event. Further, we identify an F-actin population - stable base clusters - that orchestrate that scaffolding event. We then identify that activation of Rac1 in pioneer axons destabilizes stable base clusters and disrupts pioneer axon formation. The ease of acquisition and processing with this approach provides a universal technique for biologists to answer questions in living animals.

    View details for DOI 10.1242/dev.171512

    View details for Web of Science ID 000461331900006

    View details for PubMedID 30760484

    View details for PubMedCentralID PMC6432666

  • Pioneer axons employ Cajal's battering ram to enter the spinal cord NATURE COMMUNICATIONS Nichols, E. L., Smith, C. J. 2019; 10: 562


    Sensory axons must traverse a spinal cord glia limitans to connect the brain with the periphery. The fundamental mechanism of how these axons enter the spinal cord is still debatable; both Ramon y Cajal's battering ram hypothesis and a boundary cap model have been proposed. To distinguish between these hypotheses, we visualized the entry of pioneer axons into the dorsal root entry zone (DREZ) with time-lapse imaging in zebrafish. Here, we identify that DRG pioneer axons enter the DREZ before the arrival of neural crest cells at the DREZ. Instead, actin-rich invadopodia in the pioneer axon are necessary and sufficient for DREZ entry. Using photoactivable Rac1, we demonstrate cell-autonomous functioning of invasive structures in pioneer axon spinal entry. Together these data support the model that actin-rich invasion structures dynamically drive pioneer axon entry into the spinal cord, indicating that distinct pioneer and secondary events occur at the DREZ.

    View details for DOI 10.1038/s41467-019-08421-9

    View details for Web of Science ID 000457582900006

    View details for PubMedID 30718484

    View details for PubMedCentralID PMC6362287

  • Ensheathing cells utilize dynamic tiling of neuronal somas in development and injury as early as neuronal differentiation NEURAL DEVELOPMENT Nichols, E. L., Green, L. A., Smith, C. J. 2018; 13: 19


    Glial cell ensheathment of specific components of neuronal circuits is essential for nervous system function. Although ensheathment of axonal segments of differentiated neurons has been investigated, ensheathment of neuronal cell somas, especially during early development when neurons are extending processes and progenitor populations are expanding, is still largely unknown.To address this, we used time-lapse imaging in zebrafish during the initial formation of the dorsal root ganglia (DRG).Our results show that DRG neurons are ensheathed throughout their entire lifespan by a progenitor population. These ensheathing cells dynamically remodel during development to ensure axons can extend away from the neuronal cell soma into the CNS and out to the skin. As a population, ensheathing cells tile each DRG neuron to ensure neurons are tightly encased. In development and in experimental cell ablation paradigms, the oval shape of DRG neurons dynamically changes during partial unensheathment. During longer extended unensheathment neuronal soma shifting is observed. We further show the intimate relationship of these ensheathing cells with the neurons leads to immediate and choreographed responses to distal axonal damage to the neuron.We propose that the ensheathing cells dynamically contribute to the shape and position of neurons in the DRG by their remodeling activity during development and are primed to dynamically respond to injury of the neuron.

    View details for DOI 10.1186/s13064-018-0115-8

    View details for Web of Science ID 000441983300001

    View details for PubMedID 30121077

    View details for PubMedCentralID PMC6098834