Mitochondrial dynamics and bioenergetics regulated by netrin-1 in oligodendrocytes.

Mitochondria are dynamic organelles that produce energy and molecular precursors that are essential for myelin synthesis. Unlike in neurons, mitochondria in oligodendrocytes increase intracellular movement in response to glutamatergic activation and are more susceptible to oxidative stress than in astrocytes or microglia. The signaling pathways that regulate these cell type-specific mitochondrial responses in oligodendrocytes are not understood. Here, we visualized mitochondria migrating through thin cytoplasmic channels crossing myelin basic protein-positive compacted membranes and localized within paranodal loop cytoplasm. We hypothesized that local extracellular enrichment of netrin-1 might regulate the recruitment and function of paranodal proteins and organelles, including mitochondria. We identified rapid recruitment of mitochondria and paranodal proteins, including neurofascin 155 (NF155) and the netrin receptor deleted in colorectal carcinoma (DCC), to sites of contact between oligodendrocytes and netrin-1-coated microbeads in vitro. We provide evidence that Src-family kinase activation and Rho-associated protein kinase (ROCK) inhibition downstream of netrin-1 induces mitochondrial elongation, hyperpolarization of the mitochondrial inner membrane, and increases glycolysis. Our findings identify a signaling mechanism in oligodendrocytes that is sufficient to locally recruit paranodal proteins and regulate the subcellular localization, morphology, and function of mitochondria.

Distinct age and differentiation-state dependent metabolic profiles of oligodendrocytes under optimal and stress conditions.

Within the microenvironment of multiple sclerosis lesions, oligodendrocytes are subject to metabolic stress reflecting effects of focal ischemia and inflammation. Previous studies have shown that under optimal conditions in vitro, the respiratory activity of human adult brain-derived oligodendrocytes is lower and more predominantly glycolytic compared to oligodendrocytes differentiated in vitro from post natal rat brain oligodendrocyte progenitor cells. In response to sub-lethal metabolic stress, adult human oligodendrocytes reduce overall energy production rate impacting the capacity to maintain myelination. Here, we directly compare the metabolic profiles of oligodendrocytes derived from adult rat brain with oligodendrocytes newly differentiated in vitro from oligodendrocyte progenitor cells obtained from the post natal rat brain, under both optimal culture and metabolic stress (low/no glucose) conditions. Oxygen consumption and extracellular acidification rates were measured using a Seahorse extracellular flux analyzer. Our findings indicate that under optimal conditions, adult rat oligodendrocytes preferentially use glycolysis whereas newly differentiated post natal rat oligodendrocytes, and the oligodendrocyte progenitor cells from which they are derived, mainly utilize oxidative phosphorylation to produce ATP. Metabolic stress increases the rate of ATP production via oxidative phosphorylation and significantly reduces glycolysis in adult oligodendrocytes. The rate of ATP production was relatively unchanged in newly differentiated post natal oligodendrocytes under these stress conditions, while it was significantly reduced in oligodendrocyte progenitor cells. Our study indicates that both age and maturation influence the metabolic profile under optimal and stressed conditions, emphasizing the need to consider these variables for in vitro studies that aim to model adult human disease.

Sublethal oligodendrocyte injury: A reversible condition in multiple sclerosis?

OBJECTIVE: Degeneration of oligodendroglial distal processes has been identified as an early event in multiple sclerosis (MS) lesion development. Our objective was to further define the development of the "dying-back" oligodendrocyte lesion in situ and to model the development and potential reversibility of such responses using dissociated cultures of adult human brain-derived oligodendrocytes. METHODS: In situ analyses were performed on glutaraldehyde-fixed thin sections of clinically acute and pathologically active cases of MS. In vitro studies were conducted using adult human brain-derived oligodendrocytes challenged by metabolic stress conditions (low nutrient/glucose). RESULTS: In situ analyses indicated a spectrum of myelin changes in the presence of morphologically intact oligodendrocytes; these included degeneration of the inner cytoplasmic tongue with increasing sizes of intramyelinic bleb formation that could result in radial fractures of the myelin sheath. Macrophages with ingested myelin fragments were identified only once the fragmentation was established. In vitro studies indicated that oligodendrocyte process retraction, which was linked to reduced glycolytic respiratory activity, is reversible until a critical time point. Subsequent cell death was not linked to caspase-3-dependent programs. Gene expression studies conducted at the latest reversible time point revealed reduced expression of pathways associated with cell process outgrowth and myelination, as well as with metabolic activity. INTERPRETATION: Our findings reveal the potential to protect and possibly restore myelin elaborated by existent oligodendrocytes in early and evolving MS lesions, and suggest the necessity of ongoing studies of the mechanisms underlying subsequent adult human oligodendrocyte cell death. Ann Neurol 2017;81:811-824.

Potential Benefit of the Charge-Stabilized Nanostructure Saline RNS60 for Myelin Maintenance and Repair.

Myelin injury in multiple sclerosis (MS) has been attributed both to "outside-in" primary immune mediated and "inside-out" metabolic stress of oligodendrocyte (OL) related mechanisms. Subsequent remyelination is dependent on recruitment and differentiation of oligodendrocyte progenitor cells (OPCs). RNS60 is a physically-modified saline containing charge-stabilized nanobubbles generated by subjecting normal saline to Taylor-Couette-Poiseuille (TCP) flow under elevated oxygen pressure. Administration of RNS60 has been shown to reduce the severity of EAE by dampening the immune response and myelin loss. Additionally, RNS60 has been demonstrated to enhance mitochondrial ATP synthesis in neurons. Here, we used post-natal rat derived OLs and OPCs to assess the impact of RNS60 on the response of OLs to metabolic stress in vitro (glucose-nutrient deprivation, referred to as 'NG') and on OPC differentiation capacity. Under the NG condition, our findings indicate that RNS60 decreases caspases 3/7 activation. Respirometric analyses revealed that RNS60 increased spare glycolytic capacity (SGC) under normal culture conditions. However, RNS60 enhanced OL spare respiratory capacity (SRC) when a metabolic stress was present. Furthermore, we show that RNS60 promotes OPC differentiation under physiological conditions. Our findings provide evidence for the potential therapeutic efficacy of RNS60 through the promotion of OL survival and OPC differentiation.

Oligodendrogliopathy in Multiple Sclerosis: Low Glycolytic Metabolic Rate Promotes Oligodendrocyte Survival.

UNLABELLED: Multiple sclerosis (MS) lesions feature demyelination with limited remyelination. A distinct injury phenotype of MS lesions features dying back of oligodendrocyte (OL) terminal processes, a response that destabilizes myelin/axon interactions. This oligodendrogliopathy has been linked with local metabolic stress, similar to the penumbra of ischemic/hypoxic states. Here, we developed an in vitro oligodendrogliopathy model using human CNS-derived OLs and related this injury response to their distinct bioenergetic properties. We determined the energy utilization properties of adult human surgically derived OLs cultured under either optimal or metabolic stress conditions, deprivation of growth factors, and glucose and/or hypoxia using a Seahorse extracellular flux analyzer. Baseline studies were also performed on OL progenitor cells derived from the same tissue and postnatal rat-derived cells. Under basal conditions, adult human OLs were less metabolically active than their progenitors and both were less active than the rat cells. Human OLs and progenitors both used aerobic glycolysis for the majority of ATP production, a process that contributes to protein and lipid production necessary for myelin biosynthesis. Under stress conditions that induce significant process retraction with only marginal cell death, human OLs exhibited a significant reduction in overall energy utilization, particularly in glycolytic ATP production. The stress-induced reduction of glycolytic ATP production by the human OLs would exacerbate myelin process withdrawal while favoring cell survival, providing a potential basis for the oligodendrogliopathy observed in MS. The glycolytic pathway is a potential therapeutic target to promote myelin maintenance and enhance repair in MS. SIGNIFICANCE STATEMENT: The neurologic deficits that characterize multiple sclerosis (MS) reflect disruption of myelin (demyelination) within the CNS and failure of repair (remyelination). We define distinct energy utilization properties of human adult brain-derived oligodendrocytes and oligodendrocyte progenitor cells under conditions of metabolic stress that model the initial relapsing and subsequent progressive phases of MS. The observed changes in energy utilization affect both cell survival and myelination capacity. These processes may be amenable to therapeutic interventions to limit the extent of cumulative tissue injury and to promote repair in MS.

Microglial VPAC1R mediates a novel mechanism of neuroimmune-modulation of hippocampal precursor cells via IL-4 release.

Neurogenesis, the production of new neurons from neural stem/progenitor cells (NSPCs), occurs throughout adulthood in the dentate gyrus of the hippocampus, where it supports learning and memory. The innate and adaptive immune systems are increasingly recognized as important modulators of hippocampal neurogenesis under both physiological and pathological conditions. However, the mechanisms by which the immune system regulates hippocampal neurogenesis are incompletely understood. In particular, the role of microglia, the brains resident immune cell is complex, as they have been reported to both positively and negatively regulate neurogenesis. Interestingly, neuronal activity can also regulate the function of the immune system. Here, we show that depleting microglia from hippocampal cultures reduces NSPC survival and proliferation. Furthermore, addition of purified hippocampal microglia, or their conditioned media, is trophic and proliferative to NSPCs. VIP, a neuropeptide released by dentate gyrus interneurons, enhances the proliferative and pro-neurogenic effect of microglia via the VPAC1 receptor. This VIP-induced enhancement is mediated by IL-4 release, which directly targets NSPCs. This demonstrates a potential neuro-immuno-neurogenic pathway, disruption of which may have significant implications in conditions where combined cognitive impairments, interneuron loss, and immune system activation occurs, such as temporal lobe epilepsy and Alzheimer's disease.