Contactin 2 homophilic adhesion structure and conformational plasticity

The cell-surface attached glycoprotein contactin 2 is ubiquitously expressed in the nervous system and mediates homotypic cell-cell interactions to organize cell guidance, differentiation, and adhesion. Contactin 2 consists of six Ig and four fibronectin type III domains (FnIII) of which the first four Ig domains form a horseshoe structure important for homodimerization and oligomerization. Here we report the crystal structure of the six-domain contactin 2Ig1-6 and show that the Ig5-Ig6 combination is oriented away from the horseshoe with flexion in interdomain connections. Two distinct dimer states, through Ig1-Ig2 and Ig3-Ig6 interactions, together allow formation of larger oligomers. Combined size exclusion chromatography with multiangle light scattering (SEC-MALS), small-angle X-ray scattering (SAXS) and native MS analysis indicates contactin 2Ig1-6 oligomerizes in a glycan dependent manner. SAXS and negative-stain electron microscopy reveals inherent plasticity of the contactin 2 full-ectodomain. The combination of intermolecular binding sites and ectodomain plasticity explains how contactin 2 can function as a homotypic adhesion molecule in diverse intercellular environments.

Phosphacan acts as a repulsive cue for cerebellar granule cells in a TAG-1/GD3 raft-dependent manner

Phosphacan, a chondroitin sulfate proteoglycan, is a repulsive cue of cerebellar granule cells. This study aims to explore the molecular mechanism. The glycosylphosphatidylinositol-anchored neural adhesion molecule TAG-1 is a binding partner of phosphacan, suggesting that the repulsive effect of phosphacan is possibly due to its interaction with TAG-1. The repulsive effect was greatly reduced on primary cerebellar granule cells of TAG-1-deficient mice. Surface plasmon resonance analysis confirmed the direct interaction of TAG-1 with chondroitin sulfate C. On postnatal days 1, 4, 7, 11, 15, and 20 and in adulthood, phosphacan was present in the molecular layer and internal granular layer, but not in the external granular layer. In contrast, transient TAG-1 expression was observed exclusively within the premigratory zone of the external granular layer on postnatal days 1, 4, 7, and 11. Boyden chamber cell migration assay demonstrated that phosphacan exerted its repulsive effect on the spontaneous and brain-derived neurotrophic factor (BDNF)-induced migration of cerebellar granule cells. The BDNF-induced migration was inhibited by MK-2206, an Akt inhibitor. The pretreatment with a raft-disrupting agent, methyl-β-cyclodextrin, also inhibited the BDNF-induced migration, suggesting that lipid rafts are involved in the migration of cerebellar granule cells. In primary cerebellar granule cells obtained on postnatal day 7 and cultured for 7 days, the ganglioside GD3 and TAG-1 preferentially localized in the cell body, whereas the ganglioside GD1b and NB-3 localized in not only the cell body but also neurites. Pretreatment with the anti-GD3 antibody R24, but not the anti-GD1b antibody GGR12, inhibited the spontaneous and BDNF-induced migration, and attenuated BDNF-induced Akt activation. These findings suggest that phosphacan is responsible for the repulsion of TAG-1-expressing cerebellar granule cells via GD3 rafts to attenuate BDNF-induced migration signaling.

TAG-1 Multifunctionality Coordinates Neuronal Migration, Axon Guidance, and Fasciculation

Neuronal migration, axon fasciculation, and axon guidance need to be closely coordinated for neural circuit assembly. Spinal motor neurons (MNs) face unique challenges during development because their cell bodies reside within the central nervous system (CNS) and their axons project to various targets in the body periphery. The molecular mechanisms that contain MN somata within the spinal cord while allowing their axons to exit the CNS and navigate to their final destinations remain incompletely understood. We find that the MN cell surface protein TAG-1 anchors MN cell bodies in the spinal cord to prevent their emigration, mediates motor axon fasciculation during CNS exit, and guides motor axons past dorsal root ganglia. TAG-1 executes these varied functions in MN development independently of one another. Our results identify TAG-1 as a key multifunctional regulator of MN wiring that coordinates neuronal migration, axon fasciculation, and axon guidance.

Suter et al. demonstrate that the motor neuron cell surface molecule TAG-1 confines motor neurons to the central nervous system, promotes motor axon fasciculation, and steers motor axons past inappropriate targets. This study highlights how a single cell adhesion molecule coordinates multiple steps in neuronal wiring through partially divergent mechanisms.

Impact of anti-CASPR2 autoantibodies from patients with autoimmune encephalitis on CASPR2/TAG-1 interaction and Kv1 expression

Autoantibodies against CASPR2 (contactin-associated protein-like 2) have been linked to autoimmune limbic encephalitis that manifests with memory disorders and temporal lobe seizures. According to the growing number of data supporting a role for CASPR2 in neuronal excitability, CASPR2 forms a molecular complex with transient axonal glycoprotein-1 (TAG-1) and shaker-type voltage-gated potassium channels (Kv1.1 and Kv1.2) in compartments critical for neuronal activity and is required for Kv1 proper positioning. Whereas the perturbation of these functions could explain the symptoms observed in patients, the pathogenic role of anti-CASPR2 antibodies has been poorly studied. In the present study, we find that patient autoantibodies alter Caspr2 distribution at the cell membrane promoting cluster formation. We confirm in a HEK cellular model that the anti-CASPR2 antibodies impede CASPR2/TAG-1 interaction and we identify the domains of CASPR2 and TAG-1 taking part in this interaction. Moreover, introduction of CASPR2 into HEK cells induces a marked increase of the level of Kv1.2 surface expression and in cultures of hippocampal neurons Caspr2-positive inhibitory neurons appear to specifically express high levels of Kv1.2. Importantly, in both cellular models, anti-CASPR2 patient autoAb increase Kv1.2 expression. These results provide new insights into the pathogenic role of autoAb in the disease.

Selective Axonal Expression of the Kv1 Channel Complex in Pre-myelinated GABAergic Hippocampal Neurons

In myelinated fibers, the voltage-gated sodium channels Nav1 are concentrated at the nodal gap to ensure the saltatory propagation of action potentials. The voltage-gated potassium channels Kv1 are segregated at the juxtaparanodes under the compact myelin sheath and may stabilize axonal conduction. It has been recently reported that hippocampal GABAergic neurons display high density of Nav1 channels remarkably in clusters along the axon before myelination (Freeman et al., 2015). In inhibitory neurons, the Nav1 channels are trapped by the ankyrinG scaffold at the axon initial segment (AIS) as observed in pyramidal and granule neurons, but are also forming “pre-nodes,” which may accelerate conduction velocity in pre-myelinated axons. However, the distribution of the Kv1 channels along the pre-myelinated inhibitory axons is still unknown. In the present study, we show that two subtypes of hippocampal GABAergic neurons, namely the somatostatin and parvalbumin positive cells, display a selective high expression of Kv1 channels at the AIS and all along the unmyelinated axons. These inhibitory axons are also highly enriched in molecules belonging to the juxtaparanodal Kv1 complex, including the cell adhesion molecules (CAMs) TAG-1, Caspr2, and ADAM22 and the scaffolding protein 4.1B. Here, taking advantage of hippocampal cultures from 4.1B and TAG-1 knock-out mice, we observed that 4.1B is required for the proper positioning of Caspr2 and TAG-1 along the distal axon, and that TAG-1 deficiency induces alterations in the axonal distribution of Caspr2. However, the axonal expression of Kv1 channels and clustering of ankyrinG were not modified. In conclusion, this study allowed the analysis of the hierarchy between channels, CAMs and scaffolding proteins for their expression along hippocampal inhibitory axons before myelination. The early steps of channel compartmentalization preceding myelination may be crucial for stabilizing nerve impulses switching from a continuous to saltatory conduction during network development.

Contactin-associated protein-like 2, a protein of the neurexin family involved in several human diseases

Contactin-associated protein-like 2 (CASPR2) is a cell adhesion protein of the neurexin family. Proteins of this family have been shown to play a role in the development of the nervous system, in synaptic functions, and in neurological diseases. Over recent years, CASPR2 function has gained an increasing interest as demonstrated by the growing number of publications. Here, we gather published data to comprehensively review CASPR2 functions within the nervous system in relation to CASPR2-related diseases in humans. On the one hand, studies on Cntnap2 (coding for CASPR2) knockout mice revealed its role during development, especially, in setting-up the inhibitory network. Consistent with this result, mutations in the CNTNAP2 gene coding for CASPR2 in human have been identified in neurodevelopmental disorders such as autism, intellectual disability, and epilepsy. On the other hand, CASPR2 was shown to play a role beyond development, in the localization of voltage-gated potassium channel (VGKC) complex that is composed of TAG-1, Kv1.1, and Kv1.2. This complex was found in several subcellular compartments essential for action potential propagation: the node of Ranvier, the axon initial segment, and the synapse. In line with a role of CASPR2 in the mature nervous system, neurological autoimmune diseases have been described in patients without neurodevelopmental disorders but with antibodies directed against CASPR2. These autoimmune diseases were of two types: central with memory disorders and temporal lobe seizures, or peripheral with muscular hyperactivity. Overall, we review the up-to-date knowledge on CASPR2 function and pinpoint confused or lacking information that will need further investigation.

Inhibitory axons are targeted in hippocampal cell culture by anti-Caspr2 autoantibodies associated with limbic encephalitis

Contactin-associated protein-like 2 (Caspr2), also known as CNTNAP2, is a cell adhesion molecule that clusters voltage-gated potassium channels (Kv1.1/1.2) at the juxtaparanodes of myelinated axons and may regulate axonal excitability. As a component of the Kv1 complex, Caspr2 has been identified as a target in neuromyotonia and Morvan syndrome, but also in some cases of autoimmune limbic encephalitis (LE). How anti-Caspr2 autoimmunity is linked with the central neurological symptoms is still elusive. In the present study, using anti-Caspr2 antibodies from seven patients affected by pure LE, we determined that IgGs in the cerebrospinal fluid of four out seven patients were selectively directed against the N-terminal Discoïdin and LamininG1 modules of Caspr2. Using live immunolabeling of cultured hippocampal neurons, we determined that serum IgGs in all patients strongly targeted inhibitory interneurons. Caspr2 was highly detected on GAD65-positive axons that are surrounding the cell bodies and at the VGAT-positive inhibitory presynaptic contacts. Functional assays indicated that LE autoantibodies may induce alteration of Gephyrin clusters at inhibitory synaptic contacts. Next, we generated a Caspr2-Fc chimera to reveal Caspr2 receptors on hippocampal neurons localized at the somato-dendritic compartment and post-synapse. Caspr2-Fc binding was strongly increased on TAG-1-transfected neurons and conversely, Caspr2-Fc did not bind hippocampal neurons from TAG-1-deficient mice. Our data indicate that Caspr2 may participate as a cell recognition molecule in the dynamics of inhibitory networks. This study provides new insight into the potential pathogenic effect of anti-Caspr2 autoantibodies in central hyperexcitability that may be related with perturbation of inhibitory interneuron activity.