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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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Developmental expression of voltage-gated potassium (Kv)1 channels at the axon initial segment (AIS) and juxtapara (JXP)-AIS. (A-T) Triple immunostaining of ankyrin G (AnkG) (A, E, I, M, Q), contactin-associated protein (Caspr) (B, F, J, N, R) and Kv1.1 channels (C, G, K, O, S) (Caspr and Kv1.1 are merged in D, H, L, P, T) along the axon of motor neurons (MNs) labeled with the anti-Peripherin antibody (data not shown), from P1 to P14. Brackets indicate the Caspr+ domain (I-K, M-O, Q-S). (A'-T') Triple immunostaining of AnkG (A', E', I', M'), MBP (B', F', J', N') and Kv1.1 channels (C', G', K', O') (Caspr and MBP are merged in D', H', L', P') along the axon of MNs labeled with Peripherin (data not shown), from P3 to P14. Brackets indicate the MBP+ domain (E'-F', I'-K', M'-O'). Scale bar = 5 μm.
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Figure 7: Developmental expression of voltage-gated potassium (Kv)1 channels at the axon initial segment (AIS) and juxtapara (JXP)-AIS. (A-T) Triple immunostaining of ankyrin G (AnkG) (A, E, I, M, Q), contactin-associated protein (Caspr) (B, F, J, N, R) and Kv1.1 channels (C, G, K, O, S) (Caspr and Kv1.1 are merged in D, H, L, P, T) along the axon of motor neurons (MNs) labeled with the anti-Peripherin antibody (data not shown), from P1 to P14. Brackets indicate the Caspr+ domain (I-K, M-O, Q-S). (A'-T') Triple immunostaining of AnkG (A', E', I', M'), MBP (B', F', J', N') and Kv1.1 channels (C', G', K', O') (Caspr and MBP are merged in D', H', L', P') along the axon of MNs labeled with Peripherin (data not shown), from P3 to P14. Brackets indicate the MBP+ domain (E'-F', I'-K', M'-O'). Scale bar = 5 μm.

Mentions: Despite their crucial role in modulating the AIS spike-generating properties [12-14], the developmental time course of expression of Kv1 channels at the AIS has to date never been studied. Therefore, and as a first step towards investigating the mechanisms that could control the clustering of Kv1 channels at the AIS and JXP-AIS, we analyzed their developmental expression pattern in both compartments. We examined expression of Kv1.1 and Kv1.2 subunits in the ventral horn presumptive grey matter of mouse lumbar spinal cords at postnatal day 1 (P1), P3, P5, P7 and P14 (Figure 7A-T). We found an identical developmental distribution for Kv1.1 and Kv1.2 and thus restrict our following description to Kv1.1.


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)

Developmental expression of voltage-gated potassium (Kv)1 channels at the axon initial segment (AIS) and juxtapara (JXP)-AIS. (A-T) Triple immunostaining of ankyrin G (AnkG) (A, E, I, M, Q), contactin-associated protein (Caspr) (B, F, J, N, R) and Kv1.1 channels (C, G, K, O, S) (Caspr and Kv1.1 are merged in D, H, L, P, T) along the axon of motor neurons (MNs) labeled with the anti-Peripherin antibody (data not shown), from P1 to P14. Brackets indicate the Caspr+ domain (I-K, M-O, Q-S). (A'-T') Triple immunostaining of AnkG (A', E', I', M'), MBP (B', F', J', N') and Kv1.1 channels (C', G', K', O') (Caspr and MBP are merged in D', H', L', P') along the axon of MNs labeled with Peripherin (data not shown), from P3 to P14. Brackets indicate the MBP+ domain (E'-F', I'-K', M'-O'). Scale bar = 5 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 7: Developmental expression of voltage-gated potassium (Kv)1 channels at the axon initial segment (AIS) and juxtapara (JXP)-AIS. (A-T) Triple immunostaining of ankyrin G (AnkG) (A, E, I, M, Q), contactin-associated protein (Caspr) (B, F, J, N, R) and Kv1.1 channels (C, G, K, O, S) (Caspr and Kv1.1 are merged in D, H, L, P, T) along the axon of motor neurons (MNs) labeled with the anti-Peripherin antibody (data not shown), from P1 to P14. Brackets indicate the Caspr+ domain (I-K, M-O, Q-S). (A'-T') Triple immunostaining of AnkG (A', E', I', M'), MBP (B', F', J', N') and Kv1.1 channels (C', G', K', O') (Caspr and MBP are merged in D', H', L', P') along the axon of MNs labeled with Peripherin (data not shown), from P3 to P14. Brackets indicate the MBP+ domain (E'-F', I'-K', M'-O'). Scale bar = 5 μm.
Mentions: Despite their crucial role in modulating the AIS spike-generating properties [12-14], the developmental time course of expression of Kv1 channels at the AIS has to date never been studied. Therefore, and as a first step towards investigating the mechanisms that could control the clustering of Kv1 channels at the AIS and JXP-AIS, we analyzed their developmental expression pattern in both compartments. We examined expression of Kv1.1 and Kv1.2 subunits in the ventral horn presumptive grey matter of mouse lumbar spinal cords at postnatal day 1 (P1), P3, P5, P7 and P14 (Figure 7A-T). We found an identical developmental distribution for Kv1.1 and Kv1.2 and thus restrict our following description to Kv1.1.

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