SummaryThe radical response of peripheral nerves to injury (Wallerian degeneration) is the cornerstone of nerve repair. We show that activation of the transcription factor c-Jun in Schwann cells is a global regulator of Wallerian degeneration. c-Jun governs major aspects of the injury response, determines the expression of trophic factors, adhesion molecules, the formation of regeneration tracks and myelin clearance and controls the distinctive regenerative potential of peripheral nerves. A key function of c-Jun is the activation of a repair program in Schwann cells and the creation of a cell specialized to support regeneration. We show that absence of c-Jun results in the formation of a dysfunctional repair cell, striking failure of functional recovery, and neuronal death. We conclude that a single glial transcription factor is essential for restoration of damaged nerves, acting to control the transdifferentiation of myelin and Remak Schwann cells to dedicated repair cells in damaged tissue.

Mutations of the LGI1 (leucine-rich, glioma-inactivated 1) gene underlie autosomal dominant lateral temporal lobe epilepsy, a focal idiopathic inherited epilepsy syndrome. The LGI1 gene encodes a protein secreted by neurons, one of the only non-ion channel genes implicated in idiopathic familial epilepsy. While mutations probably result in a loss of function, the role of LGI1 in the pathophysiology of epilepsy remains unclear. Here we generated a germline knockout mouse for LGI1 and examined spontaneous seizure characteristics, changes in threshold for induced seizures and hippocampal pathology. Frequent spontaneous seizures emerged in homozygous LGI1−/− mice during the second postnatal week. Properties of these spontaneous events were examined in a simultaneous video and intracranial electroencephalographic recording. Their mean duration was 120 ± 12 s, and behavioural correlates consisted of an initial immobility, automatisms, sometimes followed by wild running and tonic and/or clonic movements. Electroencephalographic monitoring indicated that seizures originated earlier in the hippocampus than in the cortex. LGI1−/− mice did not survive beyond postnatal day 20, probably due to seizures and failure to feed. While no major developmental abnormalities were observed, after recurrent seizures we detected neuronal loss, mossy fibre sprouting, astrocyte reactivity and granule cell dispersion in the hippocampus of LGI1−/− mice. In contrast, heterozygous LGI1+/− littermates displayed no spontaneous behavioural epileptic seizures, but auditory stimuli induced seizures at a lower threshold, reflecting the human pathology of sound-triggered seizures in some patients. We conclude that LGI1+/− and LGI1−/− mice may provide useful models for lateral temporal lobe epilepsy, and more generally idiopathic focal epilepsy.

Using a combination of exome sequencing and linkage analysis, we investigated an English family with two affected siblings in their 40s with recessive Charcot-Marie Tooth disease type 2 (CMT2). Compound heterozygous mutations in the immunoglobulin-helicase-μ-binding protein 2 (IGHMBP2) gene were identified. Further sequencing revealed a total of 11 CMT2 families with recessively inherited IGHMBP2 gene mutations. IGHMBP2 mutations usually lead to spinal muscular atrophy with respiratory distress type 1 (SMARD1), where most infants die before 1 year of age. The individuals with CMT2 described here, have slowly progressive weakness, wasting and sensory loss, with an axonal neuropathy typical of CMT2, but no significant respiratory compromise. Segregating IGHMBP2 mutations in CMT2 were mainly loss-of-function nonsense in the 5' region of the gene in combination with a truncating frameshift, missense, or homozygous frameshift mutations in the last exon. Mutations in CMT2 were predicted to be less aggressive as compared to those in SMARD1, and fibroblast and lymphoblast studies indicate that the IGHMBP2 protein levels are significantly higher in CMT2 than SMARD1, but lower than controls, suggesting that the clinical phenotype differences are related to the IGHMBP2 protein levels.

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In genital mucosa, different fates are described for HIV according to the subtype of dendritic cells (DCs) involved in its recognition. This notably depends on the C-type lectin receptor, langerin or DC-SIGN, involved in gp120 interaction. Langerin blocks HIV transmission by its internalization in specific organelles of Langerhans cells. On the contrary, DC-SIGN enhances HIV trans-infection of T lymphocytes. Thus, approaches aiming to inhibit DC-SIGN, without blocking langerin, represent attractive anti-HIV strategies. We previously demonstrated that dendrons bearing multiple copies of glycomimetic compounds were able to block DC-SIGN-dependent HIV infection in cervical explant models. Optimization of such ligand requires detailed characterization of its binding mode. In the present work, we determined the first high-resolution structure of a glycomimetic/DC-SIGN complex by X-ray crystallography. This glycomimetic, pseudo-1,2-mannobioside, shares shape and conformational properties with Manα1-2Man, its natural counterpart. However, it uses the binding epitope previously described for Lewis X, a ligand specific for DC-SIGN among the C-type lectin family. Thus, selectivity gain for DC-SIGN versus langerin is observed with pseudo-1,2-mannobioside as shown by surface plasmon resonance analysis. In parallel, ligand binding was also analyzed by TR-NOESY and STD NMR experiments, combined with the CORCEMA-ST protocol. These studies demonstrate that the complex, defined by X-ray crystallography, represents the unique binding mode of this ligand as opposed to the several binding orientations described for the natural ligand. This exclusive binding mode and its selective interaction properties position this glycomimetic as a good lead compound for rational improvement based on a structurally driven approach.

Langerin is a C-type lectin specifically expressed in Langerhans cells. As recently shown for HIV, Langerin is thought to capture pathogens and mediate their internalisation into Birbeck Granules for elimination. However, the precise functions of Langerin remain elusive, mostly because of the lack of information on its binding properties and physiological ligands. Based on recent reports that Langerin binds to sulfated sugars, we conducted here a comparative analysis of Langerin interaction with mannose-rich HIV glycoprotein gp120 and glycosaminoglycan (GAGs), a family of sulfated polysaccharides expressed at the surface of most mammalian cells. Our results first revealed that Langerin bound to these different glycans through very distinct mechanisms and led to the identification of a novel, GAG-specific binding mode within Langerin. In contrast to the canonical lectin domain, this new binding site showed no Ca2+-dependency, and could only be detected in entire, trimeric extracellular domains of Langerin. Interestingly binding to GAGs, did not simply rely on a net charge effect, but rather on more discrete saccharide features, such as 6-O-sulfation, or iduronic acid content. Using molecular modelling simulations, we proposed a model of Langerin/heparin complex, which located the GAG binding site at the interface of two of the three Carbohydrate-recognition domains of the protein, at the edge of the a-helix coiled-coil. To our knowledge, the binding properties that we have highlighted here for Langerin, have never been reported for C-type lectins before. These findings provide new insights towards the understanding of Langerin biological functions.

Langerin is a C-type lectin present on Langerhans cells that mediates capture of pathogens in a carbohydrate-dependent manner, leading to subsequent internalization and elimination in the cellular organelles called Birbeck granules. This mechanism mediated by langerin was shown to constitute a natural barrier for HIV-1 particle transmission. Besides interacting specifically with high mannose and fucosylated neutral carbohydrate structures, langerin has the ability to bind sulfated carbohydrate ligands as 6-sulfated galactosides in the Ca(2+)-dependent binding site. Very recently langerin was demonstrated to interact with sulfated glycosaminoglycans (GAGs), in a Ca(2+)-independent way, resulting in the proposal of a new binding site for GAGs. On the basis of those results, we have conducted a structural study of the interactions of small heparin (HEP)-like oligosaccharides with langerin in solution. Heparin bead cross-linking experiments, an approach specifically designed to identify HEP/heparan sulfate binding sites in proteins were first carried out and experimentally validated the previously proposed model for the interaction of langerin extracellular domain with 6 kDa HEP. High-resolution NMR studies of a set of eight synthetic HEP-like trisaccharides harboring different sulfation patterns demonstrated that all of them bound to langerin in a Ca(2+)-dependent way. The binding epitopes were determined by saturation transfer difference NMR and the bound conformations by transferred NOESY experiments. These experimental data were combined with docking and molecular dynamics and resulted in the proposal of a binding mode characterized by the coordination of calcium by the two equatorial hydroxyl groups, OH3 and OH4, at the non-reducing end. The binding also includes the carboxylate group at the adjacent iduronate residue. This epitope is shared by all eight ligands, explaining the absence of any impact on binding from differences in their substitution patterns. Finally, in contrast to the small trisaccharides, we demonstrated that a longer HEP-like hexasaccharide, bearing an additional O-sulfate group at the non-reducing end, which precludes binding to the Ca(2+) site, interacts with langerin in the previously identified Ca(2+)-independent binding site.

At the surface of dendritic cells, C-type lectin receptors (CLRs) allow the recognition of carbohydrate-based PAMPS or DAMPS (pathogen- or danger-associated molecular patterns, respectively) and promote immune response regulation. However, some CLRs are hijacked by viral and bacterial pathogens. Thus, the design of ligands able to target specifically one CLR, to either modulate an immune response or to inhibit a given infection mechanism, has great potential value in therapeutic design. A case study is the selective blocking of DC-SIGN, involved notably in HIV trans-infection of T lymphocytes, without interfering with langerin-mediated HIV clearance. This is a challenging task due to their overlapping carbohydrate specificity. Toward the rational design of DC-SIGN selective ligands, we performed a comparative affinity study between DC-SIGN and langerin with natural ligands. We found that GlcNAc is recognized by both CLRs; however, selective sulfation are shown to increase the selectivity in favor of langerin. With the combination of site-directed mutagenesis and X-ray structural analysis of the langerin/GlcNS6S complex, we highlighted that 6-sulfation of the carbohydrate ligand induced langerin specificity. Additionally, the K313 residue from langerin was identified as a critical feature of its binding site. Using a rational and a differential approach in the study of CLR binding sites, we designed, synthesized, and characterized a new glycomimetic, which is highly specific for DC-SIGN vs langerin. STD NMR, SPR, and ITC characterizations show that compound 7 conserved the overall binding mode of the natural disaccharide while possessing an improved affinity and a strict specificity for DC-SIGN.