The navigation of commissural axons in the developing spinal cord has attracted multiple studies over the years. Many important concepts emerged from these studies which have enlighten the general ...mechanisms of axon guidance. The navigation of commissural axons is regulated by a series of cellular territories which provides the diverse guidance information necessary to ensure the successive steps of their pathfinding towards, across, and away from the ventral midline. In this review, we discuss how repulsive forces, by propelling, channelling, and confining commissural axon navigation, bring key contributions to the formation of this neuronal projection.
Accurate perception of guidance cues is crucial for cell and axon migration. During initial navigation in the spinal cord, commissural axons are kept insensitive to midline repellents. Upon midline ...crossing in the floor plate, they switch on responsiveness to Slit and Semaphorin repulsive signals and are thus propelled away and prevented from crossing back. Whether and how the different midline repellents control specific aspects of this navigation remain to be elucidated. We set up a paradigm for live-imaging and super-resolution analysis of PlexinA1, Neuropilin-2, and Robo1/2 receptor dynamics during commissural growth cone navigation in chick and mouse embryos. We uncovered a remarkable program of sensitization to midline cues achieved by unique spatiotemporal sequences of receptor allocation at the growth-cone surface that orchestrates receptor-specific growth-cone behavior changes. This reveals post-translational mechanisms whereby coincident guidance signals are temporally resolved to allow the generation of specific guidance responses.
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•Live monitoring of guidance receptors reveals specific dynamics in commissural axons•Slit and Semaphorin receptors have different temporal patterns of membrane insertion•A front-rear polarity organizes PlxnA1 and Robo1 receptors at the growth-cone surface•Robo1, but not Robo2, is sorted during midline crossing
Pignata et al. developed a setup to monitor the dynamics of receptors for midline repellents during commissural axon navigation across the floor plate. They find specific spatiotemporal patterns of receptor insertion at the growth-cone membrane. This suggests that differences of receptor dynamics can elicit functional specificities of midline repellents.
The mechanisms governing the navigation of commissural axons during embryonic development have been extensively investigated in the past years, often using the drosophila ventral nerve cord and the ...spinal cord as model systems. Similarities but also specificities in the general strategies, the molecular signals as well as in the regulatory pathways controlling the response of commissural axons to the guidance cues have been found between species. Whether the semaphorin signaling contributes to midline crossing in the fly nervous system remains unknown, while in contrast, it does play a prominent contribution in vertebrates. In this review we discuss the functions of the semaphorins during commissural axon guidance in the developing spinal cord, focusing on the family member semaphorin 3B (Sema3B) in the context of midline crossing in the spinal cord.
Spinal commissural axon navigation across the midline in the floor plate requires repulsive forces from local Slit repellents. The long-held view is that Slits push growth cones forward and prevent ...them from turning back once they became sensitized to these cues after midline crossing. We analyzed with fluorescent reporters Slits distribution and FP glia morphology. We observed clusters of Slit-N and Slit-C fragments decorating a complex architecture of glial basal process ramifications. We found that PC2 proprotein convertase activity contributes to this pattern of ligands. Next, we studied Slit-C acting via PlexinA1 receptor shared with another FP repellent, the Semaphorin3B, through generation of a mouse model baring PlexinA1
mutation abrogating SlitC but not Sema3B responsiveness, manipulations in the chicken embryo, and ex vivo live imaging. This revealed a guidance mechanism by which SlitC constantly limits growth cone exploration, imposing ordered and forward-directed progression through aligned corridors formed by FP basal ramifications.
The mechanisms underlying the chronicity of autoimmune diseases of the central nervous system (CNS) are largely unknown. In particular, it is unclear whether tissue-resident memory T cells (T
) ...contribute to lesion pathogenesis during chronic CNS autoimmunity. Here, we observed that a high frequency of brain-infiltrating CD8
T cells exhibit a T
-like phenotype in human autoimmune encephalitis. Using mouse models of neuronal autoimmunity and a combination of T single-cell transcriptomics, high-dimensional flow cytometry, and histopathology, we found that pathogenic CD8
T cells behind the blood-brain barrier adopt a characteristic T
differentiation program, and we revealed their phenotypic and functional heterogeneity. In the diseased CNS, autoreactive tissue-resident CD8
T cells sustained focal neuroinflammation and progressive loss of neurons, independently of recirculating CD8
T cells. Consistently, a large fraction of autoreactive tissue-resident CD8
T cells exhibited proliferative potential as well as proinflammatory and cytotoxic properties. Persistence of tissue-resident CD8
T cells in the CNS and their functional output, but not their initial differentiation, were crucially dependent on CD4
T cells. Collectively, our results point to tissue-resident CD8
T cells as essential drivers of chronic CNS autoimmunity and suggest that therapies targeting this compartmentalized autoreactive T cell subset might be effective for treating CNS autoimmune diseases.
The outcome of cancer and autoimmunity is often dictated by the effector functions of CD4+ conventional T cells (Tconv). Although activation of the NF-κB signaling pathway has long been implicated in ...Tconv biology, the cell-autonomous roles of the separate NF-κB transcription-factor subunits are unknown. Here, we dissected the contributions of the canonical NF-κB subunits RelA and c-Rel to Tconv function. RelA, rather than c-Rel, regulated Tconv activation and cytokine production at steady-state and was required for polarization toward the TH17 lineage in vitro. Accordingly, RelA-deficient mice were fully protected against neuroinflammation in a model of multiple sclerosis due to defective transition to a pathogenic TH17 gene-expression program. Conversely, Tconv-restricted ablation of c-Rel impaired their function in the microenvironment of transplanted tumors, resulting in enhanced cancer burden. Moreover, Tconv required c-Rel for the response to PD-1-blockade therapy. Our data reveal distinct roles for canonical NF-κB subunits in different disease contexts, paving the way for subunit-targeted immunotherapies.
Les commissures forment un ensemble de connexions nerveuses assurant la communication entre les neurones de chaque hémi partie du système nerveux central des bilatériens. Au cours du développement ...embryonnaire, les axones des neurones commissuraux sont guidés au travers de la ligne médiane délimitant ces deux parties. Plusieurs sources de signaux de guidage attractifs et répulsifs agissent de concert pour organiser les trajectoires de ces axones. Dans la moelle épinière, les axones commissuraux traversent la ligne médiane dans un territoire ventral, la plaque du plancher (PP). Au cours de la traversée de la PP, ils acquièrent une sensibilité à des signaux répulsifs exprimés par ce territoire qui leur empêchent de rebrousser le chemin et qui les poussent hors de la PP. Plusieurs couples ligands/récepteurs médient ces forces répulsives mais les mécanismes qui sous-tendent l'acquisition de la sensibilité aux signaux répulsifs restent encore peu connus. Par exemple on ignore si les axons se sensibilisent à tous les signaux répulsifs en même temps, quand précisément ce switch de réponse se fait, et les contributions précises de chacun de ces signaux. Une spécificité fonctionnelle est suggérée par l'analyse des phénotypes d'invalidation des gènes codant pour ces récepteurs chez la souris ou encore par des manipulations d'expression chez l'embryon de poulet. L'objectif de mes travaux de thèse a été de tester l'hypothèse selon laquelle la génération de spécificités fonctionnelles pourrait résulter de contrôles précis et distincts de la dynamique spatiale et temporelle des récepteurs de guidage à la surface du cône de croissance. J'ai tout d'abord développé un dispositif de vidéomicroscopie adapté à l'enregistrement de cônes de croissance accomplissant la traversée de la PP, sur des moelles épinières en configuration de «livre ouvert». Afin de visualiser l'adressage à la surface du cône de croissance, j'ai exploité une forme de GFP sensible au pH, dont les propriétés de fluorescence à pH neutre permettent un suivi spécifique du pool de surface des protéines (Nawabi et al., 2010; Delloye-Bourgeois et al, 2014). J'ai utilisé ce paradigme pour comparer la dynamique temporelle de 4 récepteurs médiant les réponses aux divers signaux répulsifs de la PP: Nrp2, Robo1, Robo2 et PlxnA1. Les vecteurs d'expression de ces formes pHLuo de récepteurs ont été introduits dans les neurones commissuraux de la moelle épinière d'embryon de poulet par électroporation in ovo. Par des approches de microscopie à super-résolution sur les livres-ouverts, j'ai aussi étudié la distribution spatiale des récepteurs répulsifs à la surface des cônes de croissances au cours de la traversée. L'ensemble de ces expériences a pu démontrer que les récepteurs sont adressés à la membrane à différents temps de la navigation de la PP et occupent, de plus, des domaines distincts du cône de croissance. J'ai ensuite adapté la technique d'électroporation à la moelle épinière d'embryon de souris. Ces expériences ont montré que les séquences temporelles observées chez le poulet sont conservées chez la souris. J'ai également réintroduit le récepteur Robo1 dans une lignée de souris présentant une invalidation des récepteurs Robo1/2 et montré que l'altération de la traversée de la PP caractéristique de cette lignée est abolie dans la population d'axones capables d'adresser le récepteur Robo1 à la membrane. Au final, mes résultats démontrent que les axones commissuraux ne sont pas sensibilisés aux signaux répulsifs par la mise en œuvre d'un programme général. Au contraire, les récepteurs de guidage possèdent des profils de dynamiques temporelles spécifiques, et des domaines de distribution distincts dans le cône de croissance. Le contrôle de la dynamique d'adressage représente ainsi un mécanisme permettant de discriminer des signaux concomitants, en les fonctionnalisant à différents temps de la navigation de la moelle épinière
During embryonic development, commissural axons are guided through the midline, crossing from one side of the CNS to the other one at specific time points and positions to project onto contralateral neurons. Several sources of attractive cues regulate their navigation. In addition, repulsive forces act at different steps to keep the axons along their path. In the developing spinal cord, commissural axons cross the midline in a ventral territory, the floor plate (FP). Commissural axons gain sensitivity to repellents present in the FP after their crossing. The setting of these novel properties is necessary for preventing the axons to cross back and also for pushing them towards FP exit. Various ligand/receptor couples have been reported to mediate these repulsive forces. Whether commissural axons gain response to all the repulsive cues at the same time is not known. Whether these repulsive cascades have specific functions is suggested by different outcome of their invalidation in mouse models, but how are set these differences also remains unknown. We hypothesized that the generation of functional specificities could be achieved though specific controls of the spatial and temporal dynamics of guidance receptors at the growth cone surface. During my PhD, I developed a set up for time-lapse imaging of “open book” spinal cords, to monitor the dynamics of guidance receptors in axons experiencing native guidance decisions across the midline. To visualize their cell surface sorting, receptors were fused to the pH-sensitive GFP, pHLuorin, whose fluorescence at neutral pH reports membrane protein pools (Nawabi et al, 2010; Delloye-Bourgeois et al, 2014), and were expressed in spinal commissural neurons through in ovo electroporation. This paradigm revealed striking differences in the temporal dynamics of Nrp2, Robo1, Robo2 and PlexinA1, the receptors known to mediate the responsiveness to the major midline repellents referenced in vertebrates: Slit-Ns, Slit-Cs and Semaphorin3B. Moreover, using super-resolution microscopy, I could evidence that PlexinA1 and Robo1 are sorted in distinct subdomains of commissural growth cones navigating the floor plate. I also introduced the pHLuo-tagged receptors in the mouse embryo. These experiments showed that the temporal sequences established in the chick are conserved in the mouse, and that FP crossing in Robo1/2 mutant embryos was rescued in growth cones that could achieve cell surface sorting of Robo1. Thus, my results show that guidance receptors for midline repellents have highly specific spatial and temporal dynamics. The generation of a temporal sequences of cell surface sorting thus represents a mechanism whereby commissural growth cones discriminate concomitant signals by functionalizing them at different timing of their spinal cord navigation
AIM To present the current state-of-the art of molecular imaging in the management of patients affected by inflammatory bowel disease(IBD).METHODS A systematic review of the literature was performed ...in order to find important original articles on the role of molecular imaging in the management of patients affected by IBD. The search was updated until February 2016 and limited to articles in English.RESULTS Fifty-five original articles were included in this review, highlighting the role of single photon emission tomography and positron emission tomography. CONCLUSION To date, molecular imaging represents a useful tool to detect active disease in IBD. However, the available data need to be validated in prospective multicenter studies on larger patient samples.
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