Ependyma: a new target for autoantibodies in neuromyelitis optica?

Abstract

Neuromyelitis Optica (NMO) is an autoimmune demyelinating disease of the central nervous system characterized by the presence of autoantibodies (called NMO-IgG) targeting aquaporin-4. Aquaporin-4 is expressed at the perivascular foot processes of astrocytes, in the glia limitans but also at the ependyma. Most studies have focus on studying the pathogenicity of NMO-IgG on astrocytes and NMO is now considered an astrocytopathy. However, periependymal lesions are observed in NMO suggesting that ependymal cells could also be targeted by NMO-IgG. Ependymal cells regulate CSF-parenchyma molecular exchanges, CSF flow and is a niche for subventricular neural stem cells. Our aim was to examine the effect of antibodies from NMO patients on ependymal cells. We exposed two models, i.e., primary culture of rat ependymal cells and explant cultures of rat lateral ventricular wall wholemounts, to purified IgG of NMO patients (NMO-IgG) for 24 hours. We then evaluated the treatment effect using immunolabeling, functional assays, ependymal flow analysis and bulk RNA sequencing. For each experiment, the effects were compared to purified IgG of healthy donor and to non-treated cells. We found that: i) NMO-IgG patients induced AQP4 agglomeration at the surface of ependymal cells, and induced cell enlargement in comparison to controls. In parallel, it induced an increase in gap junction connexin-43 plaque size; ii) NMO-IgG altered the orientation of ciliary basal bodies and functionally impaired cilia motility; iii) NMO-IgG activated the proliferation of subventricular neural stem cells; and iv) treatment with NMO-IgG upregulated the expression of pro-inflammatory cytokines and chemokines in the transcriptomic analysis. Our study showed that NMO-IgG can trigger an early and specific reactive phenotype in ependymal cells, with functional alterations of intercellular communication and cilia, activation of the subventricular stem cell proliferation and the secretion of pro-inflammatory cytokines. These findings suggest a key role for ependymal cells in the early phase of NMO lesion formation.

Pathogenic role of autoantibodies at the ependyma in autoimmune disorders of the central nervous system

“…In NMO and MS, the ependymal cell lining is mostly preserved even if small portions are denuded (Guo et al, 2017). Indeed, as shown by Bigotte et al (2022), the ependymal layer is preserved following NMO-IgG exposure, but ependymal cells become dysfunctional. Dysfunction of ependymal cells can lead to a decreased capacity to exchange nutrients but also to clear toxic waste and adequately propel CSF.…”

“…A clear loss of AQP4/GFAP in astrocytes and a disruption in myelin integrity was observed in the periventricular region but no specific analysis on ependymal cells was performed. More recently, high-resolution microscopy and functional assays have been applied to investigate the pathogenic role of NMO-IgG on ependymal cells ( Bigotte et al, 2022 ). Indeed, 24 h of exposure to human NMO-IgGs induced alterations in rat ependymal cells in vitro and ex vivo , including mis-localization of AQP4 and gap junction protein expression, as well as altered cell morphology.…”

“…Post-translational modifications of GlialCAM expression in ependymal cells are not known but, if present, could facilitate the targeting of ependymal cells by cross-reactivity with anti-EBNA1 antibodies ( Lanz et al, 2022 ). Given the recent discovery that ependymal cells are highly sensitive to anti-AQP4 from NMO patients ( Bigotte et al, 2022 ), it is probable that ependymal cells are also sensitive to antibodies present in MS. Indeed, ependymal cells are altered in MS ( Guo et al, 2017 ; Hatrock et al, 2020 ) and undergo gliosis, channel dysregulation, and cilia polarity disruption in experimental autoimmune encephalomyelitis (EAE), an animal model used to study MS ( Pourabdolhossein et al, 2017 ).…”

“…A variety of evidence suggests that metabolic balance and toxin clearance functions are dysregulated in ependymal cells in NMO and MS. Aquaporin-4 and connexin-43 alterations have been reported in NMO and could contribute to reduced metabolic support for periventricular cells ( Richard et al, 2020 ; Bigotte et al, 2022 ). Metal ion transport is also one major and highly conserved transport function orchestrated by ependymal cells ( MacDonald et al, 2021 ).…”

“…Thus, as with astrocytes, ependymal cells targeted by autoantibodies could ultimately contribute to adjacent tissue damage by glutamate release. Similarly, the production of pro-inflammatory cytokines and chemokines by ependymal cells could also participate in local tissue damage, the recruitment of immune cells, and an increase in the CSF load of cytokines ( Bigotte et al, 2022 ). Indeed, the presence of proinflammatory cytokines in the CSF has been correlated to the CSF titer of NMO-IgGs and the presence of B-cells in NMO, and to CSF oligoclonal bands in MS ( Sato et al, 2014 ; Farina et al, 2017 ).…”