Characterization of the axon initial segment (AIS) of motor neurons and identification of a para-AIS and a juxtapara-AIS, organized by protein 4.1B.

BACKGROUND: The axon initial segment (AIS) plays a crucial role: it is the site where neurons initiate their electrical outputs. Its composition in terms of voltage-gated sodium (Nav) and voltage-gated potassium (Kv) channels, as well as its length and localization determine the neuron's spiking properties. Some neurons are able to modulate their AIS length or distance from the soma in order to adapt their excitability properties to their activity level. It is therefore crucial to characterize all these parameters and determine where the myelin sheath begins in order to assess a neuron's excitability properties and ability to display such plasticity mechanisms. If the myelin sheath starts immediately after the AIS, another question then arises as to how would the axon be organized at its first myelin attachment site; since AISs are different from nodes of Ranvier, would this particular axonal region resemble a hemi-node of Ranvier? RESULTS: We have characterized the AIS of mouse somatic motor neurons. In addition to constant determinants of excitability properties, we found heterogeneities, in terms of AIS localization and Nav composition. We also identified in all α motor neurons a hemi-node-type organization, with a contactin-associated protein (Caspr)+ paranode-type, as well as a Caspr2+ and Kv1+ juxtaparanode-type compartment, referred to as a para-AIS and a juxtapara (JXP)-AIS, adjacent to the AIS, where the myelin sheath begins. We found that Kv1 channels appear in the AIS, para-AIS and JXP-AIS concomitantly with myelination and are progressively excluded from the para-AIS. Their expression in the AIS and JXP-AIS is independent from transient axonal glycoprotein-1 (TAG-1)/Caspr2, in contrast to juxtaparanodes, and independent from PSD-93. Data from mice lacking the cytoskeletal linker protein 4.1B show that this protein is necessary to form the Caspr+ para-AIS barrier, ensuring the compartmentalization of Kv1 channels and the segregation of the AIS, para-AIS and JXP-AIS. CONCLUSIONS: α Motor neurons have heterogeneous AISs, which underlie different spiking properties. However, they all have a para-AIS and a JXP-AIS contiguous to their AIS, where the myelin sheath begins, which might limit some AIS plasticity. Protein 4.1B plays a key role in ensuring the proper molecular compartmentalization of this hemi-node-type region.

Nav1.1 is predominantly expressed in nodes of Ranvier and axon initial segments.

Aggregation of voltage-gated sodium (Nav) channels in the axon initial segment (AIS) and nodes of Ranvier is essential for the generation and propagation of action potentials. From the three Nav channel isoforms (Nav1.1, Nav1.2 and Nav1.6) expressed in the adult CNS, Nav1.1 appears to play an important function since numerous mutations in its coding sequence cause epileptic syndromes. Yet, its distribution is still controversial. Here we demonstrate for the first time that in the adult CNS Nav1.1 is expressed in nodes of Ranvier throughout the mouse spinal cord and in many brain regions. We identified three populations of nodes: expressing Nav1.1, Nav1.6 or both. We also found Nav1.1 expression concentrated in a proximal AIS subcompartment in spinal cord neurons including 80% of motor neurons and in multiple brain areas. This novel distribution suggests that Nav1.1 is involved in the control of action potential generation and propagation.