INNATE AND GLIAL CELLS ORCHESTRATE MYELIN INJURY AND REPAIR IN LPC-INDUCED DEMYELINATION MOUSE MODEL IN VIVO

Abstract

Myelination of axons by mature oligodendrocytes (OLs) enables saltatory conduction and provides trophic support to maintain axonal integrity. The loss of myelin sheath observed in multiple sclerosis (MS) trigger microglia and astroglial cell activation, thereby releasing pro- and anti-inflammatory factor or signal modulating cell responses, and participating in myelin and cellular debris removal. Thus, glial cells regulate the fate of OL lineage by influencing the survival of myelinating OLs or promoting remyelination by oligodendrocyte precursor cells (OPCs). Hence, we investigated glial dynamics, signaling, and cross-talk during de- and remyelination using the lysophosphatidylcholine (LPC)-induced demyelination model in the dorsal mouse spinal cord. To this end, we combined in vivo longitudinal two-photon and CARS microscopies, as well as histopathological and qR-PCR analysis. Furthermore, we performed RNA sequencing using cell-specific RiboTag expression in OPCs, microglia and astrocytes to decipher their transcriptomic gene profiles. LPC triggered rapid myelin loss through its action on mOLs following time-dependent breakdown cellular patterns, with concomitant axonal degeneration. In parallel, we observed the transient recruitment of CD11c+ microglia to the lesion site, while GFAP+ astrocytes were secondary recruited and sustained until complete remyelination. Conversely, LysM+ innate immune cells invaded the spinal tissue in two distinct waves. Furthermore, while both microglia and infiltrating immune cells exhibited pro-inflammatory polarization consistent with the loss of OLs, the induction of anti-inflammatory microglia likely induced similar phenotypic shift in immune cells during remyelination and axon repair. Additionally, the transcriptomic analysis of glial cells revealed that OPCs, microglia, and astrocytes were rapidly proliferating in response demyelination. Moreover, OPCs seemed to engage differentiation and maturation in late demyelination while preventing axonal degeneration. In contrast, microglia behaved as antigen-presenting cells to recruit immune cells, whereas its phagocytic and lysosomal activities, as well as anti-inflammatory gene expression later supported efficient remyelination. Astroglia immediately after LPC prevented excitotoxicity by enhancing buffering functions, ensuring neurotransmission continuity. They as well exhibited a balanced inflammatory polarization, alternately promoting neuroinflammation or remyelination and CNS repair. Finally, we identified genes suggesting glial intercommunication and signaling highlighting their reciprocal regulation in de-and remyelination, in addition to their close commitment to the neuronal compartment during the whole process. In conclusion, these data demonstrate the complexity of cellular dynamics underlying myelin injury, where de- and remyelination trigger an intricate combination of time-dependent, coordinated glial, immune, and neuronal signaling. Moreover, understanding each genetic cell activation cascade could pave the way for new therapeutic perspectives.