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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.

Duflocq A, Chareyre F, Giovannini M, Couraud F, Davenne M - BMC Biol. (2011)

Bottom Line: 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.Protein 4.1B plays a key role in ensuring the proper molecular compartmentalization of this hemi-node-type region.

View Article: PubMed Central - HTML - PubMed

Affiliation: INSERM UMRS 952, 9 Quai St Bernard, F-75005, Paris, France.

ABSTRACT

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.

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PSD-93 is not required for voltage-gated potassium (Kv)1 channels expression at the axon initial segment (AIS) and juxtapara (JXP)-AIS. Triple immunostaining of ankyrin G (AnkG) (A, D), Kv1.1 (B, E) and PSD-93 (C, F) in motor neurons (MNs) labeled with the anti-Peripherin antibody (data not shown), in wild-type (WT) (A-C) and PSD-93-/- (D-F) mice. Inset in (E): Mean Kv1.1 immunofluorescence intensity at the AIS and JXP-AIS in WT and KO mice. Brackets indicate Kv1.1+ and PSD-93+ domains in the AIS and JXP-AIS. Scale bar = 5 μm.
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Figure 9: PSD-93 is not required for voltage-gated potassium (Kv)1 channels expression at the axon initial segment (AIS) and juxtapara (JXP)-AIS. Triple immunostaining of ankyrin G (AnkG) (A, D), Kv1.1 (B, E) and PSD-93 (C, F) in motor neurons (MNs) labeled with the anti-Peripherin antibody (data not shown), in wild-type (WT) (A-C) and PSD-93-/- (D-F) mice. Inset in (E): Mean Kv1.1 immunofluorescence intensity at the AIS and JXP-AIS in WT and KO mice. Brackets indicate Kv1.1+ and PSD-93+ domains in the AIS and JXP-AIS. Scale bar = 5 μm.

Mentions: We then tested the second candidate, PSD-93, suspected to control or contribute to the clustering of Kv1 channels at the AIS [15,19] but not at JXP-nodes [39]. In line with the idea of PSD-93 playing a role in the clustering of Kv1 channels at the AIS, we found PSD-93 being restricted to the distal part of adult MN AISs, overlapping exactly with Kv1.1 distribution (Figure 9A-C). PSD-93 was also expressed at the MN AIS at the early stage of Kv1 channels expression at the AIS (data not shown). We thus analyzed PSD-93- mice [40], in which we found a normal distribution of Kv1.1 in 100% of MN AISs, similar to WT littermate controls (Figure 9B, E): Kv1.1 at the MN distal AIS in PSD-93-/- mice displayed no statistically significant difference of immunofluorescence intensity compared to WT mice (Figure 9E; respectively 14 and 11 MN AISs analyzed from 3 PSD-93-/- and 3 WT mice). This result indicates that the clustering of Kv1 channels at the MN AIS does not require PSD-93.


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.

Duflocq A, Chareyre F, Giovannini M, Couraud F, Davenne M - BMC Biol. (2011)

PSD-93 is not required for voltage-gated potassium (Kv)1 channels expression at the axon initial segment (AIS) and juxtapara (JXP)-AIS. Triple immunostaining of ankyrin G (AnkG) (A, D), Kv1.1 (B, E) and PSD-93 (C, F) in motor neurons (MNs) labeled with the anti-Peripherin antibody (data not shown), in wild-type (WT) (A-C) and PSD-93-/- (D-F) mice. Inset in (E): Mean Kv1.1 immunofluorescence intensity at the AIS and JXP-AIS in WT and KO mice. Brackets indicate Kv1.1+ and PSD-93+ domains in the AIS and JXP-AIS. Scale bar = 5 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3198992&req=5

Figure 9: PSD-93 is not required for voltage-gated potassium (Kv)1 channels expression at the axon initial segment (AIS) and juxtapara (JXP)-AIS. Triple immunostaining of ankyrin G (AnkG) (A, D), Kv1.1 (B, E) and PSD-93 (C, F) in motor neurons (MNs) labeled with the anti-Peripherin antibody (data not shown), in wild-type (WT) (A-C) and PSD-93-/- (D-F) mice. Inset in (E): Mean Kv1.1 immunofluorescence intensity at the AIS and JXP-AIS in WT and KO mice. Brackets indicate Kv1.1+ and PSD-93+ domains in the AIS and JXP-AIS. Scale bar = 5 μm.
Mentions: We then tested the second candidate, PSD-93, suspected to control or contribute to the clustering of Kv1 channels at the AIS [15,19] but not at JXP-nodes [39]. In line with the idea of PSD-93 playing a role in the clustering of Kv1 channels at the AIS, we found PSD-93 being restricted to the distal part of adult MN AISs, overlapping exactly with Kv1.1 distribution (Figure 9A-C). PSD-93 was also expressed at the MN AIS at the early stage of Kv1 channels expression at the AIS (data not shown). We thus analyzed PSD-93- mice [40], in which we found a normal distribution of Kv1.1 in 100% of MN AISs, similar to WT littermate controls (Figure 9B, E): Kv1.1 at the MN distal AIS in PSD-93-/- mice displayed no statistically significant difference of immunofluorescence intensity compared to WT mice (Figure 9E; respectively 14 and 11 MN AISs analyzed from 3 PSD-93-/- and 3 WT mice). This result indicates that the clustering of Kv1 channels at the MN AIS does not require PSD-93.

Bottom Line: 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.Protein 4.1B plays a key role in ensuring the proper molecular compartmentalization of this hemi-node-type region.

View Article: PubMed Central - HTML - PubMed

Affiliation: INSERM UMRS 952, 9 Quai St Bernard, F-75005, Paris, France.

ABSTRACT

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.

Show MeSH
Related in: MedlinePlus