Oligodendrocyte plasticity with an intact cell body in vitro

Oligodendroglial macroautophagy is essential for myelin sheath turnover to prevent neurodegeneration and death

Although macroautophagy deficits are implicated across adult-onset neurodegenerative diseases, we understand little about how the discrete, highly evolved cell types of the central nervous system use macroautophagy to maintain homeostasis. One such cell type is the oligodendrocyte, whose myelin sheaths are central for the reliable conduction of action potentials. Using an integrated approach of mouse genetics, live cell imaging, electron microscopy, and biochemistry, we show that mature oligodendrocytes require macroautophagy to degrade cell autonomously their myelin by consolidating cytosolic and transmembrane myelin proteins into an amphisome intermediate prior to degradation. We find that disruption of autophagic myelin turnover leads to changes in myelin sheath structure, ultimately impairing neural function and culminating in an adult-onset progressive motor decline, neurodegeneration, and death. Our model indicates that the continuous and cell-autonomous maintenance of the myelin sheath through macroautophagy is essential, shedding insight into how macroautophagy dysregulation might contribute to neurodegenerative disease pathophysiology.

Oligodendrocytes assemble myelin and support the axons they myelinate. Aber et al. report that oligodendrocytes coordinate autophagy and endocytosis to turn over myelin. The absence of oligodendroglial autophagy causes myelin abnormalities, behavioral dysfunction, glial and neurodegeneration, and death, demonstrating the importance of this process for a healthy CNS.

An optimized method for adult zebrafish brain-tissue dissociation that allows access mitochondrial function under healthy and epileptic conditions

Mitochondria play key roles in brain metabolism. Not surprisingly, mitochondria dysfunction is a ubiquitous cause of neurodegenerative diseases. In turn, acquired forms of epilepsy etiology is specifically intriguing since mitochondria function and dysfunction remain not completely enlightened. Investigation in the field includes models of epileptic disorder using mainly rodents followed by mitochondrial function evaluation, which in general evidenced controversial data. So, we considered the efforts and limitations in this research field and we took into account that sample preparation and quality are critical for bioenergetics investigation. For these reasons the aim of the present study was to develop a thorough protocol for adult zebrafish brain-tissue dissociation to evaluate oxygen consumption flux and reach the bioenergetics profile in health and models of epileptic disorder in both, in vitro using pentylenetetrazole (PTZ) and N-methyl-D-Aspartic acid (NMDA), and in vivo after kainic acid (KA)-induced status epilepticus. In conclusion, we verify that fire-polished glass Pasteur pipette is eligible to brain-tissue dissociation and to study mitochondrial function and dysfunction in adult zebrafish. The results give evidence for large effect size in increase of coupling efficiency respiration (p/O₂) correlated to treatment with PTZ and spare respiratory capacity (SRC) in KA-induced model indicating oxidative phosphorylation (OXPHOS) variable alterations. Further investigation is needed in order to clarify the bioenergetics role as well as other mitochondrial functions in epilepsy.

The adult oligodendrocyte can participate in remyelination

Endogenous remyelination of the CNS can be robust and restore function, yet in multiple sclerosis it becomes less complete with time. Promoting remyelination is a major therapeutic goal, both to restore function and to protect axons from degeneration. Remyelination is thought to depend on oligodendrocyte progenitor cells, giving rise to nascent remyelinating oligodendrocytes. Surviving, mature oligodendrocytes are largely regarded as being uninvolved. We have examined this question using two large animal models. In the first model, there is extensive demyelination and remyelination of the CNS, yet oligodendrocytes survive, and in recovered animals there is a mix of remyelinated axons interspersed between mature, thick myelin sheaths. Using 2D and 3D light and electron microscopy, we show that many oligodendrocytes are connected to mature and remyelinated myelin sheaths, which we conclude are cells that have reextended processes to contact demyelinated axons while maintaining mature myelin internodes. In the second model in vitamin B₁₂-deficient nonhuman primates, we demonstrate that surviving mature oligodendrocytes extend processes and ensheath demyelinated axons. These data indicate that mature oligodendrocytes can participate in remyelination.

A proteomic analysis of contextual fear conditioned rats reveals dynamic modifications in neuron and oligodendrocyte protein expression in the dentate gyrus

Contextual memory is an intricate process involving synaptic plasticity and network rearrangement. Both are governed by many molecular processes including phosphorylation and modulation of protein expression. However, little is known about the molecules involved in it. Here, we exploited the advantages of a quantitative proteomic approach to identify a great number of molecules in the rat dentate gyrus after a contextual fear conditioning session. Our results allowed us to highlight protein expression patterns, not only related to neuroplasticity, but also to myelin structure, such as myelin basic protein and myelin proteolipid protein showing a decrease in expression. Validation of the modification in protein expression reveals a dynamic profile during the 48 h following the fear conditioning session. The expression of proteins involved in neurite outgrowth, such as BASP-1 and calcineurin B1, and in synaptic structure and function, VAMP2 and RAB3C, was increased in the dentate gyrus of rats submitted to fear conditioning compared to controls. We showed that the increase in BASP-1 protein was specific to fear conditioning learning as it was not present in immediate-shock rats, neither in rats exposed to a novel environment without being shocked. As myelin is known to stabilise synaptic network, the decrease in myelin proteins suggests a neuroglia interactive process taking place in the dentate gyrus in the 24 h following contextual fear learning, which has never been demonstrated before. These results therefore open the way to the study of new plasticity mechanisms underlying learning and memory.

Adaptive myelination from fish to man

Myelinated axons with nodes of Ranvier are an evolutionary elaboration common to essentially all jawed vertebrates. Myelin made by Schwann cells in our peripheral nervous system and oligodendrocytes in our central nervous system has been long known to facilitate rapid energy efficient nerve impulse propagation. However, it is now also clear, particularly in the central nervous system, that myelin is not a simple static insulator but that it is dynamically regulated throughout development and life. New myelin sheaths can be made by newly differentiating oligodendrocytes, and mature myelin sheaths can be stimulated to grow again in the adult. Furthermore, numerous studies in models from fish to man indicate that neuronal activity can affect distinct stages of oligodendrocyte development and the process of myelination itself. This begs questions as to how these effects of activity are mediated at a cellular and molecular level and whether activity-driven adaptive myelination is a feature common to all myelinated axons, or indeed all oligodendrocytes, or is specific to cells or circuits with particular functions. Here we review the recent literature on this topic, elaborate on the key outstanding questions in the field, and look forward to future studies that incorporate investigations in systems from fish to man that will provide further insight into this fundamental aspect of nervous system plasticity. This article is part of a Special Issue entitled SI: Myelin Evolution.

Mature oligodendrocytes actively increase in vivo cytoskeletal plasticity following CNS damage

BACKGROUND

Oligodendrocytes are myelinating cells of the central nervous system which support functionally, structurally, and metabolically neurons. Mature oligodendrocytes are generally believed to be mere targets of destruction in the context of neuroinflammation and tissue damage, but their real degree of in vivo plasticity has become a matter of debate. We thus investigated the in vivo dynamic, actin-related response of these cells under different kinds of demyelinating stress.

METHODS

We used a novel mouse model (oLucR) expressing luciferase in myelin oligodendrocyte glycoprotein-positive oligodendrocytes under the control of a β-actin promoter. Activity of this promoter served as surrogate for dynamics of the cytoskeleton gene transcription through recording of in vivo bioluminescence following diphtheria toxin-induced oligodendrocyte death and autoimmune demyelination. Cytoskeletal gene expression was quantified from mature oligodendrocytes directly isolated from transgenic animals through cell sorting.

RESULTS

Experimental demyelinating setups augmented oligodendrocyte-specific in vivo bioluminescence. These changes in luciferase signal were confirmed by further ex vivo analysis of the central nervous system tissue from oLucR mice. Increase in bioluminescence upon autoimmune inflammation was parallel to an oligodendrocyte-specific increased transcription of β-tubulin.

CONCLUSIONS

Mature oligodendrocytes acutely increase their cytoskeletal plasticity in vivo during demyelination. They are therefore not passive players under demyelinating conditions but can rather react dynamically to external insults.