Axonal response of mitochondria to demyelination and complex IV activity within demyelinated axons in experimental models of multiple sclerosis

Hypoxia in multiple sclerosis

Low oxygen availability (hypoxia) is a prominent but poorly understood feature in multiple sclerosis (MS). Whether hypoxia causes or drives MS pathology and symptoms or whether it is a consequence of other pathological events, such as inflammation and vascular dysfunction, is unknown. Here, we summarize the available literature on the interplay between hypoxia and both pathological and symptomatic features of MS. Severe environmental hypoxia (i.e., altitude) may trigger or facilitate MS-related events, possibly by exacerbating tissue hypoxia in the central nervous system. Accordingly, increasing oxygen supply can mitigate pathological and clinical parameters in MS models. In contrast, stimulating the endogenous hypoxia response and adaptation systems by controlled exposure to hypoxia (hypoxia conditioning) renders the central nervous system more resistant to hypoxic insults, thereby attenuating pathology and symptomatology in MS models. Overlapping mechanisms likely play a role in the benefits conferred by physical activity in MS. We provide an integrative model to explain the paradoxically beneficial outcomes of both increased and decreased ambient oxygen conditions. In conclusion, controlled exposure to hypoxia, perhaps in combination with exercise, is a promising, possibly disease-course modifying therapeutic approach for MS. However, many open questions remain.

Single intracerebroventricular TNFR2 agonist injection impacts remyelination in the cuprizone model

The development of therapeutics that enhances the regeneration of myelin sheaths following demyelination is predicted to prevent neurodegeneration. A promising target to enhance remyelination is the immunomodulatory cytokine tumor necrosis factor alpha (TNFα) and its receptors TNFR1 and TNFR2. TNFR2 on oligodendrocyte lineage cells and microglia coordinates different protective functions, such as proliferation of oligodendrocyte progenitor cells, survival of mature oligodendrocytes, and release of anti-inflammatory cytokines, in animal models of inflammation and demyelination. Here, we find in the cuprizone model that following demyelination, fewer axons are unmyelinated in the corpus callosum at an early stage of remyelination after single TNFR2 agonist delivery in the lateral ventricle, while astrocyte and microglia number and coverage are unchanged. Towards later stages of remyelination, TNFR2 agonist treatment maintains the number of oligodendrocyte lineage cells, and large caliber axons have thinner myelin. Hence, even short-term stimulation of TNFR2 has a positive impact on the remyelination processes. This study informs further on the beneficial implications of TNFR2 signaling on oligodendrocyte lineage cells and remyelination, emphasizing its potential therapeutic value for demyelinating diseases, including multiple sclerosis. KEY MESSAGES: Single TNFR2 agonist treatment in the lateral ventricle following cuprizone-induced demyelination impacts remyelination by: Leading to a lower percentage of unmyelinated axons at early stages. Preserving the number of oligodendrocyte lineage cells in the corpus callosum at later stages. Covering large calibre axons with thinner myelin sheaths at later stages.

Rescue of lysosomal acid lipase deficiency in mice by rAAV8 liver gene transfer

BACKGROUND

Lysosomal acid lipase deficiency (LAL-D) is an autosomal recessive disorder caused by mutations in the LIPA gene, which results in lipid accumulation leading to multi-organ failure. If left untreated, the severe form of LAL-D results in premature death within the first year of life due to failure to thrive and hepatic insufficiency. Weekly systemic injections of recombinant LAL protein, referred as enzyme replacement therapy, is the only available supportive treatment.

METHOD

Here, we characterized a novel Lipa-/- mouse model and developed a curative gene therapy treatment based on the in vivo administration of recombinant (r)AAV8 vector encoding the human LIPA transgene under the control of a hepatocyte-specific promoter.

RESULTS

Here we define the minimal rAAV8 dose required to rescue disease lethality and to correct cholesterol and triglyceride accumulation in multiple organs and blood. Finally, using liver transcriptomic and biochemical analysis, we show mitochondrial impairment in Lipa-/- mice and its recovery by gene therapy.

CONCLUSIONS

Overall, our in vivo gene therapy strategy achieves a stable long-term LAL expression sufficient to correct the disease phenotype in the Lipa-/- mouse model and offers a new therapeutic option for LAL-D patients.

Na+/K+-ATPase: ion pump, signal transducer, or cytoprotective protein, and novel biological functions

Na+/K+-ATPase is a transmembrane protein that has important roles in the maintenance of electrochemical gradients across cell membranes by transporting three Na+ out of and two K+ into cells. Additionally, Na+/K+-ATPase participates in Ca2+-signaling transduction and neurotransmitter release by coordinating the ion concentration gradient across the cell membrane. Na+/K+-ATPase works synergistically with multiple ion channels in the cell membrane to form a dynamic network of ion homeostatic regulation and affects cellular communication by regulating chemical signals and the ion balance among different types of cells. Therefore, it is not surprising that Na+/K+-ATPase dysfunction has emerged as a risk factor for a variety of neurological diseases. However, published studies have so far only elucidated the important roles of Na+/K+-ATPase dysfunction in disease development, and we are lacking detailed mechanisms to clarify how Na+/K+-ATPase affects cell function. Our recent studies revealed that membrane loss of Na+/K+-ATPase is a key mechanism in many neurological disorders, particularly stroke and Parkinson's disease. Stabilization of plasma membrane Na+/K+-ATPase with an antibody is a novel strategy to treat these diseases. For this reason, Na+/K+-ATPase acts not only as a simple ion pump but also as a sensor/regulator or cytoprotective protein, participating in signal transduction such as neuronal autophagy and apoptosis, and glial cell migration. Thus, the present review attempts to summarize the novel biological functions of Na+/K+-ATPase and Na+/K+-ATPase-related pathogenesis. The potential for novel strategies to treat Na+/K+-ATPase-related brain diseases will also be discussed.

How does Nogo-A signalling influence mitochondrial function during multiple sclerosis pathogenesis?

Multiple sclerosis (MS) is a severe neurological disorder that involves inflammation in the brain, spinal cord and optic nerve with key disabling neuropathological outcomes being axonal damage and demyelination. When degeneration of the axo-glial union occurs, a consequence of inflammatory damage to central nervous system (CNS) myelin, dystrophy and death can lead to large membranous structures from dead oligodendrocytes and degenerative myelin deposited in the extracellular milieu. For the first time, this review covers mitochondrial mechanisms that may be operative during MS-related neurodegenerative changes directly activated during accumulating extracellular deposits of myelin associated inhibitory factors (MAIFs), that include the potent inhibitor of neurite outgrowth, Nogo-A. Axonal damage may occur when Nogo-A binds to and signals through its cognate receptor, NgR1, a multimeric complex, to initially stall axonal transport and limit the delivery of important growth-dependent cargo and subcellular organelles such as mitochondria for metabolic efficiency at sites of axo-glial disintegration as a consequence of inflammation. Metabolic efficiency in axons fails during active demyelination and progressive neurodegeneration, preceded by stalled transport of functional mitochondria to fuel axo-glial integrity.

Propionic acid promotes neurite recovery in damaged multiple sclerosis neurons

Neurodegeneration in the autoimmune disease multiple sclerosis still poses a major therapeutic challenge. Effective drugs that target the inflammation can only partially reduce accumulation of neurological deficits and conversion to progressive disease forms. Diet and the associated gut microbiome are currently being discussed as crucial environmental risk factors that determine disease onset and subsequent progression. In people with multiple sclerosis, supplementation of the short-chain fatty acid propionic acid, as a microbial metabolite derived from the fermentation of a high-fiber diet, has previously been shown to regulate inflammation accompanied by neuroprotective properties. We set out to determine whether the neuroprotective impact of propionic acid is a direct mode of action of short-chain fatty acids on CNS neurons. We analysed neurite recovery in the presence of the short-chain fatty acid propionic acid and butyric acid in a reverse-translational disease-in-a-dish model of human-induced primary neurons differentiated from people with multiple sclerosis-derived induced pluripotent stem cells. We found that recovery of damaged neurites is induced by propionic acid and butyric acid. We could also show that administration of butyric acid is able to enhance propionic acid-associated neurite recovery. Whole-cell proteome analysis of induced primary neurons following recovery in the presence of propionic acid revealed abundant changes of protein groups that are associated with the chromatin assembly, translational, and metabolic processes. We further present evidence that these alterations in the chromatin assembly were associated with inhibition of histone deacetylase class I/II following both propionic acid and butyric acid treatment, mediated by free fatty acid receptor signalling. While neurite recovery in the presence of propionic acid is promoted by activation of the anti-oxidative response, administration of butyric acid increases neuronal ATP synthesis in people with multiple sclerosis-specific induced primary neurons.

How to Use the Cuprizone Model to Study De- and Remyelination

Multiple sclerosis (MS) is an autoimmune and inflammatory disorder affecting the central nervous system whose cause is still largely unknown. Oligodendrocyte degeneration results in demyelination of axons, which can eventually be repaired by a mechanism called remyelination. Prevention of demyelination and the pharmacological support of remyelination are two promising strategies to ameliorate disease progression in MS patients. The cuprizone model is commonly employed to investigate oligodendrocyte degeneration mechanisms or to explore remyelination pathways. During the last decades, several different protocols have been applied, and all have their pros and cons. This article intends to offer guidance for conducting pre-clinical trials using the cuprizone model in mice, focusing on discovering new treatment approaches to prevent oligodendrocyte degeneration or enhance remyelination.

How neurons maintain their axons long-term: an integrated view of axon biology and pathology

Axons are processes of neurons, up to a metre long, that form the essential biological cables wiring nervous systems. They must survive, often far away from their cell bodies and up to a century in humans. This requires self-sufficient cell biology including structural proteins, organelles, and membrane trafficking, metabolic, signalling, translational, chaperone, and degradation machinery-all maintaining the homeostasis of energy, lipids, proteins, and signalling networks including reactive oxygen species and calcium. Axon maintenance also involves specialised cytoskeleton including the cortical actin-spectrin corset, and bundles of microtubules that provide the highways for motor-driven transport of components and organelles for virtually all the above-mentioned processes. Here, we aim to provide a conceptual overview of key aspects of axon biology and physiology, and the homeostatic networks they form. This homeostasis can be derailed, causing axonopathies through processes of ageing, trauma, poisoning, inflammation or genetic mutations. To illustrate which malfunctions of organelles or cell biological processes can lead to axonopathies, we focus on axonopathy-linked subcellular defects caused by genetic mutations. Based on these descriptions and backed up by our comprehensive data mining of genes linked to neural disorders, we describe the 'dependency cycle of local axon homeostasis' as an integrative model to explain why very different causes can trigger very similar axonopathies, providing new ideas that can drive the quest for strategies able to battle these devastating diseases.

Parvalbumin basket cell myelination accumulates axonal mitochondria to internodes

Parvalbumin-expressing (PV⁺) basket cells are fast-spiking inhibitory interneurons that exert critical control over local circuit activity and oscillations. PV⁺ axons are often myelinated, but the electrical and metabolic roles of interneuron myelination remain poorly understood. Here, we developed viral constructs allowing cell type-specific investigation of mitochondria with genetically encoded fluorescent probes. Single-cell reconstructions revealed that mitochondria selectively cluster to myelinated segments of PV⁺ basket cells, confirmed by analyses of a high-resolution electron microscopy dataset. In contrast to the increased mitochondrial densities in excitatory axons cuprizone-induced demyelination abolished mitochondrial clustering in PV⁺ axons. Furthermore, with genetic deletion of myelin basic protein the mitochondrial clustering was still observed at internodes wrapped by noncompacted myelin, indicating that compaction is dispensable. Finally, two-photon imaging of action potential-evoked calcium (Ca²⁺) responses showed that interneuron myelination attenuates both the cytosolic and mitochondrial Ca²⁺ transients. These findings suggest that oligodendrocyte ensheathment of PV⁺ axons assembles mitochondria to branch selectively fine-tune metabolic demands.