Bernard Zalc

The central role of microglia in CNS function in health and disease has resulted in large interest in targeting microglial as treatments for neurodegenerative disease; understanding the factors that regulate microglial gene expression will be crucial to this goal. microRNAs (miRNAs) are among the most abundant post transcriptional regulators of gene expression. miRNAs suggests miRNA were likely key to significant evolutionary events as regulators of gene expression. The miRNAome of microglia is critical to their correct functioning but the miRNA that define microglia identity and regulate key functions have not been fully defined. In this study we performed a detailed analysis of the microglial miRNAome to identify miRNA enriched in microglia that are conserved across species (human, mouse, and xenopus). We further characterised the expression of these conserved miRNAs during demyelination and remyelination and identified conserved function of a microglial–enriched miRNA across species. These findings reveal evolutionary conservation of specific miRNAs, suggesting an important role in establishing and maintaining microglial identity. They also highlight miRNAs that may be critical for microglial function in the central nervous system in both health and disease. Overall, this work advances our understanding of the factors that regulate microglial gene expression.

Background: To recover normal functions, remyelination in multiple sclerosis is crucial. Although endogenous remyelination occurs, it is often insufficient, and finding molecules promoting repair of demyelinated lesions is needed. Objectives: To compare the remyelination potential of evobrutinib, an inhibitor of Bruton's tyrosine kinase and clemastine, an antagonist of M1 muscarinic acetylcholine receptor. Methods: Remyelination was investigated in lysolecithin demyelinated organotypic mouse cerebellar slices and a transgenic Xenopus model of inducible-demyelination. Results: Evobrutinib (100 nM) and clemastine (200 nM) potentiated remyelination of mouse cerebellar slices by a factor of 2.9 and 1.76, respectively. In conditionally demyelinated Xenopus , evobrutinib and clemastine increased remyelination by a factor of 1.61 and 1.92, respectively. Evobrutinib targets Bruton's tyrosine kinase expressed by microglia, and we showed that the increase in number of myeloid cells following demyelination is due to an extravasation from nearby vessels of macrophages migrating toward the optic nerve. In contrast, clemastine is expected to antagonize muscarinic receptor 1 expressing cells of the oligodendroglial lineage. We investigated a possible synergistic effect on remyelination by adding simultaneously both molecules. In both experimental models tested no significative improvement on remyelination of co-treatment with evobrutinib plus clemastine was observed. Discussion: While evobrutinib increased 1.59 fold the number of microglia/macrophages, in the presence of clemastine the number of innate immune cells was decreased by 0.39 fold, therefore counteracting the beneficial effect of microglia/macrophages on remyelination.

Background: Microglia, an immune cell found exclusively within the CNS, initially develop from haematopoietic stem cell precursors in the yolk sac and colonise all regions of the CNS early in development. Microglia have been demonstrated to play an important role in the development of oligodendrocytes, the myelin producing cells in the CNS, as well as in myelination. Mertk is a receptor expressed on microglia that mediates immunoregulatory functions, including myelin efferocytosis. Findings: Here we demonstrate an unexpected role for Mertk-expressing microglia in both oligodendrogenesis and myelination. The selective depletion of Mertk from microglia resulted in reduced oligodendrocyte production in early development and the generation of pathological myelin. During demyelination, mice deficient in microglial Mertk had thinner myelin and showed signs of impaired OPC differentiation. We established that Mertk signalling inhibition impairs oligodendrocyte repopulation in Xenopus tadpoles following demyelination. Conclusion: These data highlight the importance of microglia in myelination and are the first to identify Mertk as a regulator of oligodendrogenesis and myelin ultrastructure.

Background and Objectives To test whether low concentrations of teriflunomide (TF) could promote remyelination, we investigate the effect of TF on oligodendrocyte in culture and on remyelination in vivo in 2 demyelinating models. Methods The effect of TF on oligodendrocyte precursor cell (OPC) proliferation and differentiation was assessed in vitro in glial cultures derived from neonatal mice and confirmed on fluorescence-activated cell sorting–sorted adult OPCs. The levels of the 8,9-unsaturated sterols lanosterol and zymosterol were quantified in TF- and sham-treated cultures. In vivo, TF was administered orally, and remyelination was assessed both in myelin basic protein–GFP-nitroreductase (Mbp:GFP-NTR) transgenic Xenopus laevis demyelinated by metronidazole and in adult mice demyelinated by lysolecithin. Results In cultures, low concentrations of TF down to 10 nM decreased OPC proliferation and increased their differentiation, an effect that was also detected on adult OPCs. Oligodendrocyte differentiation induced by TF was abrogated by the oxidosqualene cyclase inhibitor Ro 48-8071 and was mediated by the accumulation of zymosterol. In the demyelinated tadpole, TF enhanced the regeneration of mature oligodendrocytes up to 2.5-fold. In the mouse demyelinated spinal cord, TF promoted the differentiation of newly generated oligodendrocytes by a factor of 1.7-fold and significantly increased remyelination. Discussion TF enhances zymosterol accumulation in oligodendrocytes and CNS myelin repair, a beneficial off-target effect that should be investigated in patients with multiple sclerosis.

Myelin, rather than being a static insulator of axons, is emerging as an active participant in circuit plasticity. This requires precise regulation of oligodendrocyte numbers and myelination patterns. Here, by devising a laser ablation approach of single oligodendrocytes, followed by in vivo imaging and correlated ultrastructural reconstructions, we report that in mouse cortex demyelination as subtle as the loss of a single oligodendrocyte can trigger robust cell replacement and remyelination timed by myelin breakdown. This results in reliable reestablishment of the original myelin pattern along continuously myelinated axons, while in parallel, patchy isolated internodes emerge on previously unmyelinated axons. Therefore, in mammalian cortex, internodes along partially myelinated cortical axons are typically not reestablished, suggesting that the cues that guide patchy myelination are not preserved through cycles of de- and remyelination. In contrast, myelin sheaths forming continuous patterns show remarkable homeostatic resilience and remyelinate with single axon precision.

Ubiquitous exposure to endocrine-disrupting chemicals (EDCs) has caused serious concerns about the ability of these chemicals to affect neurodevelopment, among others. Since endocrine disruption (ED)-induced developmental neurotoxicity (DNT) is hardly covered by the chemical testing tools that are currently in regulatory use, the Horizon 2020 research and innovation action ENDpoiNTs has been launched to fill the scientific and methodological gaps related to the assessment of this type of chemical toxicity. The ENDpoiNTs project will generate new knowledge about ED-induced DNT and aims to develop and improve in vitro, in vivo, and in silico models pertaining to ED-linked DNT outcomes for chemical testing. This will be achieved by establishing correlative and causal links between known and novel neurodevelopmental endpoints and endocrine pathways through integration of molecular, cellular, and organismal data from in vitro and in vivo models. Based on this knowledge, the project aims to provide adverse outcome pathways (AOPs) for ED-induced DNT and to develop and integrate new testing tools with high relevance for human health into European and international regulatory frameworks.

Animals are able to move and react in numerous ways to external stimuli. Thus, environmental stimuli need to be detected, information must be processed and finally an output decision must be transmitted to the musculature to get the animal moving. All these processes depend on the nervous system which comprises an intricate neuronal network and many glial cells. In the last decades, a neurono‐centric view on nervous system function channeled most of the scientific interest toward the analysis of neurons and neuronal functions. Neurons appeared early in animal evolution and the main principles of neuronal function from synaptic transmission to propagation of action potentials are conserved during evolution. In contrast, not much is known on the evolution of glial cells that were initially considered merely as static support cells. Although it is now accepted that glial cells have an equally important contribution as their neuronal counterpart to nervous system function, their evolutionary origin is unknown. Did glial cells appear several times during evolution? What were the first roles glial cells had to fulfil in the nervous system? What triggered the formation of the amazing diversity of glial morphologies and functions? Is there a possible mechanism that might explain the appearance of complex structures such as myelin in vertebrates? Here, we postulate a common evolutionary origin of glia and depict a number of selective forces that might have paved the way from a simple supporting cell to a wrapping and myelin forming glial cell.