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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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Identification of a para-axon initial segment (AIS). Triple immunostaining of Peripherin (A, E), ankyrin G (AnkG) (B, F) and contactin-associated protein (Caspr) (C, G) (merged in (D, H)) showing a Caspr-positive segment at the distal end of the AIS (D, H). (E-H) Magnified views of the frames in (A-D). Brackets indicate the AnkG+ AIS and the Caspr+ para-AIS. Scale bar = 10 μm (A-D), 2 μm (E-H).
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Figure 5: Identification of a para-axon initial segment (AIS). Triple immunostaining of Peripherin (A, E), ankyrin G (AnkG) (B, F) and contactin-associated protein (Caspr) (C, G) (merged in (D, H)) showing a Caspr-positive segment at the distal end of the AIS (D, H). (E-H) Magnified views of the frames in (A-D). Brackets indicate the AnkG+ AIS and the Caspr+ para-AIS. Scale bar = 10 μm (A-D), 2 μm (E-H).

Mentions: Recent observations that some neurons could change their excitability properties as a mechanism of homeostatic plasticity, by either moving their AIS away from the soma [3] or increasing the length of their AIS [4], asks the question as to how far the AIS could respectively move or extend, or as a corollary where exactly beyond the AIS does the axon start being myelinated. Given the many similar features shared by AISs and nodes of Ranvier (including expression of AnkG, Nav and KCNQ2 channels), we analyzed whether AISs would, like nodes of Ranvier, be immediately followed by a myelinated axon and thus flanked by both a paranode-like and a contiguous JXP-node-like compartment. We therefore first analyzed the expression of the paranode marker, Caspr (also called Paranodin). Caspr is known to interact in cis with another paranodal cell-recognition protein, contactin, and their interaction allows the tight attachment of the myelin sheath paranodal loops to the axonal membrane. In 100% of somatic α MNs (n = 162), along their Peripherin+ axon, we found Caspr expression directly contiguous to the AnkG+ AIS (Figure 5A-H). This result suggests the presence of a paranode-like compartment immediately after the AIS. Given the differences between nodes of Ranvier and AISs, we refer here to this compartment as the 'para-AIS' (see Discussion).


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)

Identification of a para-axon initial segment (AIS). Triple immunostaining of Peripherin (A, E), ankyrin G (AnkG) (B, F) and contactin-associated protein (Caspr) (C, G) (merged in (D, H)) showing a Caspr-positive segment at the distal end of the AIS (D, H). (E-H) Magnified views of the frames in (A-D). Brackets indicate the AnkG+ AIS and the Caspr+ para-AIS. Scale bar = 10 μm (A-D), 2 μm (E-H).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 5: Identification of a para-axon initial segment (AIS). Triple immunostaining of Peripherin (A, E), ankyrin G (AnkG) (B, F) and contactin-associated protein (Caspr) (C, G) (merged in (D, H)) showing a Caspr-positive segment at the distal end of the AIS (D, H). (E-H) Magnified views of the frames in (A-D). Brackets indicate the AnkG+ AIS and the Caspr+ para-AIS. Scale bar = 10 μm (A-D), 2 μm (E-H).
Mentions: Recent observations that some neurons could change their excitability properties as a mechanism of homeostatic plasticity, by either moving their AIS away from the soma [3] or increasing the length of their AIS [4], asks the question as to how far the AIS could respectively move or extend, or as a corollary where exactly beyond the AIS does the axon start being myelinated. Given the many similar features shared by AISs and nodes of Ranvier (including expression of AnkG, Nav and KCNQ2 channels), we analyzed whether AISs would, like nodes of Ranvier, be immediately followed by a myelinated axon and thus flanked by both a paranode-like and a contiguous JXP-node-like compartment. We therefore first analyzed the expression of the paranode marker, Caspr (also called Paranodin). Caspr is known to interact in cis with another paranodal cell-recognition protein, contactin, and their interaction allows the tight attachment of the myelin sheath paranodal loops to the axonal membrane. In 100% of somatic α MNs (n = 162), along their Peripherin+ axon, we found Caspr expression directly contiguous to the AnkG+ AIS (Figure 5A-H). This result suggests the presence of a paranode-like compartment immediately after the AIS. Given the differences between nodes of Ranvier and AISs, we refer here to this compartment as the 'para-AIS' (see Discussion).

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