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Casein Kinase 1δ Stabilizes Mature Axons by Inhibiting Transcription Termination of Ankyrin

by Labella, Hujber, Moore, Rawson, Merrill, Allaire, Ailion, Hollien, Bastiani and Jorgensen

Developmental Cell 2020, 52, pp. 88-103

During animal development, the nervous system develops by neurons sending out long axonal processes using structures called growth cones. When the growth cone reaches its target, it collapses and forms a synapse; a cell-cell contact allowing signaling across the nervous system. Once the growth cone has collapsed and the synapse is formed, the axon is stabilized and the neuron becomes a mature cell with a more stable morphology. Immature neurons are dynamic and constantly send out axons and retract them, while mature neurons are stable and can maintain synaptic connectivity and architecture throughout the life of the animal. What the intrinsic program within the cell that tells the cell when to switch from an immature (developing and highly dynamic) state to a mature (stable) state is still not clear.

The authors identified novel genes involved in the transition of neurons from immature to the stable state in the nematode Caenorhabditis elegans. They found that knockouts of casein kinase 1δ caused continued axonal outgrowth even into adulthood, in other words, their neurons never matured to the stable state. The authors performed a screen to find mutations that suppressed the casein kinase 1δ outgrowth phenotype; from this information, they discovered that casein kinase 1δ promotes the expression of the giant form of the ankyrin gene. Loss of the giant form of ankyrin had the same phenotype as casein kinase 1δ mutants suggesting it was the causative gene.

One of the critical components of the paper was showing that the casein kinase 1δ mutants specifically affect axon outgrowth, but not synapse formation. Here the authors used the Vutara 352's unique ability to perform super-resolution imaging in thick samples to image C. elegans neurons in vivo in intact animals. Without super-resolution imaging it is not possible to identify individual synapse because of their density and small size.

Further, the authors were able to easily quantify the number of synapses, the size of the synapse, and the density of the synapse, all markers for the functionality of the synapse, using the cluster analysis package in the Vutara SRX software. The authors, through a combination of clever genetic screens and powerful super-resolution imaging in thick samples, were able to tease apart one of the genetic programs controlling neuronal development.