Astrocytes Are Required for Oligodendrocyte Survival and Maintenance of Myelin Compaction and Integrity.

Mohammad Abu-Rub,Reshmi Tognatta,Eric D. Garrison,Molly T. Karl,Anastas Popratiloff,Jessica K. Schenck,Robert H. Miller,Sharyl L. Fyffe-Maricich

doi:10.3389/fncel.2020.00074

Abstract

Astrocytes have been implicated in regulating oligodendrocyte development and myelination in vitro, although their functions in vivo remain less well defined. Using a novel approach to locally ablate GFAP+ astrocytes, we demonstrate that astrocytes are required for normal CNS myelin compaction during development, and for maintaining myelin integrity in the adult. Transient ablation of GFAP+ astrocytes in the mouse spinal cord during the first postnatal week reduced the numbers of mature oligodendrocytes and inhibited myelin formation, while prolonged ablation resulted in myelin that lacked compaction and structural integrity. Ablation of GFAP+ astrocytes in the adult spinal cord resulted in the rapid, local loss of myelin integrity and regional demyelination. The loss of myelin integrity induced by astrocyte ablation was greatly reduced by NMDA receptor antagonists, both in vitro and in vivo, suggesting that myelin stability was affected by elevation of local glutamate levels following astrocyte ablation. Furthermore, targeted delivery of glutamate into adult spinal cord white matter resulted in reduction of myelin basic protein expression and localized disruption of myelin compaction which was also reduced by NMDA receptor blockade. The pathology induced by localized astrocyte loss and elevated exogenous glutamate, supports the concept that astrocytes are critical for maintenance of myelin integrity in the adult CNS and may be primary targets in the initiation of demyelinating diseases of the CNS, such as Neuromyelitis Optica (NMO).

Highlights

Astrocytes perform multiple functions in the developing and adult CNS associated with maintaining brain and spinal cord integrity and homeostasis including the release, uptake, and sequestration of neurotransmitters such as glutamate (Abbott et al, 2006; Parpura and Zorec, 2010; Fyffe-Maricich et al, 2013; Lundgaard et al, 2014)
To assay the effects of local depletion of GFAP+ astrocytes, transgenic animals were generated in which an inducible caspase 9 construct (Caprariello et al, 2012) was expressed under the transcriptional control of a mouse glial fibrillary acidic protein promoter (GFAP-inducible Caspase 9 (iCP9)) (Casper and McCarthy, 2006), resulting in astrocyte-specific expression of the iCP9 construct
The iCP9 gene was linked by an internal ribosome entry site (IRES) sequence to a DsRed reporter, thereby identifying DsRed-expressing astrocytes in the CNS as susceptible to CIDinduced apoptosis (Figure 1A)

Summary

Introduction

Astrocytes perform multiple functions in the developing and adult CNS associated with maintaining brain and spinal cord integrity and homeostasis including the release, uptake, and sequestration of neurotransmitters such as glutamate (Abbott et al, 2006; Parpura and Zorec, 2010; Fyffe-Maricich et al, 2013; Lundgaard et al, 2014). Cell culture studies suggest astrocytes influence the initial generation of oligodendrocytes and their precursors (Noble and Murray, 1984). Other astrocytic signals influence CNS myelination through the modulation of OPC proliferation (Richardson et al, 1988), migration (Tsai et al, 2002), differentiation, the induction of myelination (Ishibashi et al, 2006), and modulation of myelin thickness (Fyffe-Maricich et al, 2013). Formation of myelin wraps depends on the disassembly of actin filaments (Zuchero et al, 2015), and subsequent compaction of the cytoplasmic leaflets. Sensory enrichment enhances oligodendrocyte integration and myelination in the mature cortex (Hughes et al, 2018)

Methods

Results

Conclusion