Limits...
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.

Show MeSH

Related in: MedlinePlus

Localization of axon initial segments (AISs) in motor neurons (MNs). Triple immunostaining of Peripherin (A, F), ankyrin G (AnkG) (B, G) (brackets indicate the AIS) and microtubule-associated protein 2 (MAP2) (C, H) (AnkG and MAP2 are merged in (D, I)) in the adult mouse lumbar spinal cord showing two populations of MN AISs: emanating directly from the soma (A-E) or from a primary MAP2+ dendrite (F-J). (E, J) Simplified drawing of the merged image, respectively in (D) and (I). Note that the green MAP2 staining progressively decreases within the red AnkG+ AIS. Scale bar = 10 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3198992&req=5

Figure 2: Localization of axon initial segments (AISs) in motor neurons (MNs). Triple immunostaining of Peripherin (A, F), ankyrin G (AnkG) (B, G) (brackets indicate the AIS) and microtubule-associated protein 2 (MAP2) (C, H) (AnkG and MAP2 are merged in (D, I)) in the adult mouse lumbar spinal cord showing two populations of MN AISs: emanating directly from the soma (A-E) or from a primary MAP2+ dendrite (F-J). (E, J) Simplified drawing of the merged image, respectively in (D) and (I). Note that the green MAP2 staining progressively decreases within the red AnkG+ AIS. Scale bar = 10 μm.

Mentions: We first analyzed the localization of the AIS in somatic MNs. For this we used an anti-Peripherin antibody, which in the spinal ventral horn specifically labels somatic MNs [32]. We used this antibody throughout our study instead of the more common MN marker, the anti-choline acetyltransferase (ChAT) antibody [33], not compatible with the low tissue fixation conditions (required for analyzing the expression of Nav and Kv channels: see below). We verified that all anti-ChAT labeled somatic MNs were also labeled by the anti-Peripherin antibody (data not shown). AISs were labeled with an antibody against AnkG. In the majority of MNs analyzed throughout the ventral horn of the lumbar spinal cord, the AIS was located on a neurite originating directly from the soma (Figure 2A-E), while other MNs had their AIS located on a secondary branch (Figure 2F-J). We used a somatodendritic marker, an antibody against microtubule-associated protein 2 (MAP2), to show that this secondary branch emanated from a dendrite (Figure 2F-J). Interestingly, even for AISs located on a branch that emanates directly from the soma (Figure 2A-E), MAP2 was expressed in their proximal portion, between the soma and the AnkG-labeled AIS, showing that the axon hillock has a somatodendritic identity. MAP2 staining progressively faded away at some distance within the AIS (Figure 2C, H), which reflects the barrier formed by the AIS between the somatodendritic and axonal compartments [34].


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)

Localization of axon initial segments (AISs) in motor neurons (MNs). Triple immunostaining of Peripherin (A, F), ankyrin G (AnkG) (B, G) (brackets indicate the AIS) and microtubule-associated protein 2 (MAP2) (C, H) (AnkG and MAP2 are merged in (D, I)) in the adult mouse lumbar spinal cord showing two populations of MN AISs: emanating directly from the soma (A-E) or from a primary MAP2+ dendrite (F-J). (E, J) Simplified drawing of the merged image, respectively in (D) and (I). Note that the green MAP2 staining progressively decreases within the red AnkG+ AIS. Scale bar = 10 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: Localization of axon initial segments (AISs) in motor neurons (MNs). Triple immunostaining of Peripherin (A, F), ankyrin G (AnkG) (B, G) (brackets indicate the AIS) and microtubule-associated protein 2 (MAP2) (C, H) (AnkG and MAP2 are merged in (D, I)) in the adult mouse lumbar spinal cord showing two populations of MN AISs: emanating directly from the soma (A-E) or from a primary MAP2+ dendrite (F-J). (E, J) Simplified drawing of the merged image, respectively in (D) and (I). Note that the green MAP2 staining progressively decreases within the red AnkG+ AIS. Scale bar = 10 μm.
Mentions: We first analyzed the localization of the AIS in somatic MNs. For this we used an anti-Peripherin antibody, which in the spinal ventral horn specifically labels somatic MNs [32]. We used this antibody throughout our study instead of the more common MN marker, the anti-choline acetyltransferase (ChAT) antibody [33], not compatible with the low tissue fixation conditions (required for analyzing the expression of Nav and Kv channels: see below). We verified that all anti-ChAT labeled somatic MNs were also labeled by the anti-Peripherin antibody (data not shown). AISs were labeled with an antibody against AnkG. In the majority of MNs analyzed throughout the ventral horn of the lumbar spinal cord, the AIS was located on a neurite originating directly from the soma (Figure 2A-E), while other MNs had their AIS located on a secondary branch (Figure 2F-J). We used a somatodendritic marker, an antibody against microtubule-associated protein 2 (MAP2), to show that this secondary branch emanated from a dendrite (Figure 2F-J). Interestingly, even for AISs located on a branch that emanates directly from the soma (Figure 2A-E), MAP2 was expressed in their proximal portion, between the soma and the AnkG-labeled AIS, showing that the axon hillock has a somatodendritic identity. MAP2 staining progressively faded away at some distance within the AIS (Figure 2C, H), which reflects the barrier formed by the AIS between the somatodendritic and axonal compartments [34].

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