Evolution of the Axon Initial Segment: Finding the Membrane Diffusion Barrier in Cnidarians
Open Access
- Author:
- Trigg, Liana Marie
- Area of Honors:
- Interdisciplinary in Biochemistry and Molecular Biology and Neuroscience
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Timothy J Jegla, Thesis Supervisor
David Scott Gilmour, Thesis Honors Advisor
Dr. Bernhard Lüscher, Faculty Reader
Ping Li, Thesis Honors Advisor - Keywords:
- neuronal polarity
membrane diffusion barrier
axon initial segment - Abstract:
- Polar neurons with functionally distinct axons (to send info) and dendrites (to receive info) were a key innovation for directional signaling in complex nervous systems. There are two key features of polarity in neurons – distinct microtubule polarity that allows differential trafficking and the axon initial segment (AIS) and its membrane diffusion barrier. Axon initial segments are specialized for action potential initiation. This is accomplished by the requirement of ion channels for action potentials and a membrane diffusion barrier in order to keep other somatodendritic ion channels into the axon. The evolutionary origins of neuronal polarity are largely unknown. It is not clear whether polarity evolved with centralization, or whether it preexisted in simpler nervous systems such as those of Cnidarians that have traditionally been viewed as being comprised of unpolarized neurons. The anthozoan (Cnidarian) Nematostella vectensis is a sea anemone whose primitive net-like nervous system predates complex central nervous systems present in bilateral metazoans. Cnidarian neurons have traditionally been viewed as unpolarized, but differentiated axons in Nematostella were recently identified. If the axons of Nematostella in polarized cells are functionally analogous to the axons in vertebrates, then the AIS should be present. This would be the first evidence to suggest that axon specialization occurred before nervous system centralization. Nematostella have the gene that encodes an ortholog of ankyrin-G, a protein that allows for the organization and functionality of the barrier in humans, leading to the assumption that they likely have axon initial segments and therefore the membrane diffusion barriers characteristic of axons. We hypothesize that the diffusion barrier will be present in some classes of Nematostella neurons because Nematostella have many of the signaling genes used differentially in axons and dendrites. In this project we addressed the hypothesis by looking for a diffusion barrier characteristic of the AIS. There exist multiple classifications of neurons including branched and unbranched neurons. Through experimentation we determined that the unbranched neurons do not contain membrane diffusion barriers. We hypothesize that further analysis of the branched neurons with multiple neurites will exhibit the same mechanism of establishing functional polarity as more complex bilaterians by means of the diffusion barrier and presence of the axon initial segment.