A novel hypothesis about mechanisms affecting conduction velocity of central myelinated fibers

Myelin: A possible proton capacitor for energy storage during sleep and energy supply during wakefulness

There are several physiological reasons why biological organisms sleep. One key one concerns brain metabolism. In our article we discuss the role of metabolism in myelin, based on the recent discovery that myelin contains mitochondrial components that enable the production of adenosine triphosphate (ATP) via oxidative phosphorylation (OXPHOS). These mitochondrial components in myelin probably originate from vesiculation of the mitochondrial membranes in form from mitochondrial derived vesicles (MDVs). We hypothesize that myelin acts as a proton capacitor, accumulating energy in the form of protons during sleep and converting it to ATP via OXPHOS during wakefulness. Empirical evidence supporting our hypothesis is discussed, including data on myelin metabolic activity, MDVs, and allometric scaling between white matter volume and sleep duration in mammals.

Exploring the Therapeutic Potential: Bioactive Molecules and Dietary Interventions in Multiple Sclerosis Management

Multiple sclerosis (MS) is a chronic autoimmune demyelinating disease of the central nervous system, the etiology of which is still unclear. Its hallmarks are inflammation and axonal damage. As a disease primarily impacting younger individuals, the social cost of MS is high. It has been proposed that environmental factors, smoking, and dietary habits acting on a genetic susceptibility play a role in MS. Recent studies indicate that diet can significantly influence the onset and progression of MS. This review delves into the impact of natural bioactive molecules on MS development and explores the dietary interventions that hold promise in managing the disease. Dietary patterns, including ketogenic and Mediterranean diets, are discussed. Theories about the potential mechanistic associations beneath the noted effects are also proposed. Several dietary components and patterns demonstrated the potential for a significant impact on MS. However, extensive prospective clinical trials are necessary to fully understand the role of natural bioactive molecules as disease modifiers in MS.

The Significance of Lipids for the Absorption and Release of Oxygen in Biological Organisms

A critically important step for the uptake and transport of oxygen (O2) in living organisms is the crossing of the phase boundary between gas (or water) and lipid/proteins in the cell. Classically, this transport across the phase boundary is explained as a transport by proteins or protein-based structures. In our contribution here, we want to show the significance of passive transport of O2 also (and in some cases probably predominantly) through lipids in many if not all aerobic organisms. In plants, the significance of lipids for gas exchange (absorption of CO2 and release of O2) is well recognized. The leaves of plants have a cuticle layer as the last film on both sides formed by polyesters and lipids. In animals, the skin has sebum as its last layer consisting of a mixture of neutral fatty esters, cholesterol and waxes which are also at the border between the cells of the body and the air. The last cellular layers of skin are not vascularized therefore their metabolism totally depends on this extravasal O2 absorption, which cannot be replenished by the bloodstream. The human body absorbs about 0.5% of O2 through the skin. In the brain, myelin, surrounding nerve cell axons and being formed by oligodendrocytes, is most probably also responsible for enabling O2 transport from the extracellular space to the cells (neurons). Myelin, being not vascularized and consisting of water, lipids and proteins, seems to absorb O2 in order to transport it to the nerve cell axon as well as to perform extramitochondrial oxidative phosphorylation inside the myelin structure around the axons (i.e., myelin synthesizes ATP) - similarly to the metabolic process occurring in concentric multilamellar structures of cyanobacteria. Another example is the gas transport in the lung where lipids play a crucial role in the surfactant ensuring incorporation of O2 in the alveoli where there are lamellar body and tubular myelin which form multilayered surface films at the air-membrane border of the alveolus. According to our view, the role played by lipids in the physical absorption of gases appears to be crucial to the existence of many, if not all, of the living aerobic species.

Myelination increases chemical energy support to the axon without modifying the basic physicochemical mechanism of nerve conduction

The existence of different conductive patterns in unmyelinated and myelinated axons is uncertain. It seems that considering exclusively physical electrical phenomena may be an oversimplification. A novel interpretation of the mechanism of nerve conduction in myelinated nerves is proposed, to explain how the basic mechanism of nerve conduction has been adapted to myelinated conditions. The neurilemma would bear the voltage-gated channels and Na⁺/K⁺-ATPase in both unmyelinated and myelinated conditions, the only difference being the sheath wrapping it. The dramatic increase in conduction speed of the myelinated axons would essentially depend on an increment in ATP availability within the internode: myelin would be an aerobic ATP supplier to the axoplasm, through connexons. In fact, neurons rely on aerobic metabolism and on trophic support from oligodendrocytes, that do not normally duplicate after infancy in humans. Such comprehensive framework of nerve impulse propagation in axons may shed new light on the pathophysiology of nervous system disease in humans, seemingly strictly dependent on the viability of the pre-existing oligodendrocyte.

Oligodendrocyte Physiology Modulating Axonal Excitability and Nerve Conduction

Oligodendrocytes enable saltatory conduction by forming a myelin sheath around axons, dramatically boosts action potential conduction velocity. In addition to this canonical function of oligodendrocytes, it is now known that oligodendrocytes can respond to neuronal activity and regulate axonal conduction. Importantly, white matter plasticity, including adaptive responses by oligodendrocytes, has been shown to be involved in learning and memory. In this chapter, the role of oligodendrocytes in axonal conduction and axonal excitability will be reviewed. Focus will be paid to the mechanisms through which oligodendrocytes, including perineuronal oligodendrocytes, facilitate and suppress axonal conduction.

Extramitochondrial energy production in platelets

BACKGROUND INFORMATION

Energy demand in human platelets is very high, to carry out their functions. As for most human cells, the aerobic metabolism represents the primary energy source in platelets, even though mitochondria are negligibly represented. Following the hypothesis that other structures could be involved in chemical energy production, in this work, we have investigated the functional expression of an extramitochondrial aerobic metabolism in platelets.

RESULTS

Oximetric and luminometric analyses showed that platelets consume large amounts of oxygen and produce ATP in the presence of common respiring substrates, such as pyruvate + malate or succinate, although morphological electron microscopy analysis showed that these contain few mitochondria. However, evaluation of the anaerobic glycolytic metabolism showed that only 13% of consumed glucose was converted to lactate. Interestingly, the highest OXPHOS activity was observed in the presence of NADH, not a readily permeant respiring substrate for mitochondria. Also, oxygen consumption and ATP synthesis fuelled by NADH were not affected by atractyloside, an inhibitor of the adenine nucleotide translocase, suggesting that these processes may not be ascribed to mitochondria. Functional data were confirmed by immunofluorescence microscopy and Western blot analyses, showing a consistent expression of the β subunit of F₁ F_o -ATP synthase and COXII, a subunit of Complex IV, but a low signal of translocase of the inner mitochondrial membrane (a protein not involved in OXPHOS metabolism). Interestingly, the NADH-stimulated oxygen consumption and ATP synthesis increased in the presence of the physiological platelets agonists, thrombin or collagen.

CONCLUSIONS

Data suggest that in platelets, aerobic energy production is mainly driven by an extramitochondrial OXPHOS machinery, originated inside the megakaryocyte, and that this metabolism plays a pivotal role in platelet activation.

SIGNIFICANCE

This work represents a further example of the existence of an extramitochondrial aerobic metabolism, which can contribute to the cellular energy balance.

Oligodendrocyte Precursor Cell Viability, Proliferation, and Morphology is Dependent on Mesh Size and Storage Modulus in 3D Poly(ethylene glycol)-Based Hydrogels

Oligodendrocytes in the central nervous system (CNS) are responsible for generating myelin, an electrically insulating layer around neuronal axons. When myelin is damaged, neurons are incapable of sustaining normal communications, which can manifest in patients as pain and loss of mobility and vision. A plethora of research has used biomaterials to promote neuronal regeneration, but despite the wide implications of a disrupted myelin sheath, very little is known about how biomaterial environments impact proliferation of oligodendrocyte precursor cells (OPCs) or their differentiation into myelinating oligodendrocytes. This work investigates how the storage modulus and mesh size of a polyethylene glycol (PEG)-based hydrogel, varied via two different mechanisms, directly affect the proliferation of two OPC lines encapsulated and cultured in 3D. Viability and proliferation of both OPC lines was dependent on hydrogel swelling and stiffness, where the concentration of ATP increased more in the more compliant gels. OPCs multiplied in the 3D hydrogels, creating significantly larger spheroids in the less cross-linked conditions. Stiffer, more highly cross-linked materials lead to greater expression of PDGFRα, an OPC receptor, indicating that fewer cells were committed to the oligodendrocyte lineage or had dedifferentiated in compliant materials. Laminin incorporation in the 3D matrix was found to have little effect on viability or proliferation. These findings provide valuable information on how mesh size and stiffness affect OPCs where more compliant materials favor proliferation of OPCs with less commitment to a mature oligodendrocyte lineage. Such information will be useful in the development of translational biomaterials to stimulate oligodendrocyte maturation for neural regeneration.

Oligodendrocyte Precursor Cells in Spinal Cord Injury: A Review and Update

Spinal cord injury (SCI) is a devastating condition to individuals, families, and society. Oligodendrocyte loss and demyelination contribute as major pathological processes of secondary damages after injury. Oligodendrocyte precursor cells (OPCs), a subpopulation that accounts for 5 to 8% of cells within the central nervous system, are potential sources of oligodendrocyte replacement after SCI. OPCs react rapidly to injuries, proliferate at a high rate, and can differentiate into myelinating oligodendrocytes. However, posttraumatic endogenous remyelination is rarely complete, and a better understanding of OPCs' characteristics and their manipulations is critical to the development of novel therapies. In this review, we summarize known characteristics of OPCs and relevant regulative factors in both health and demyelinating disorders including SCI. More importantly, we highlight current evidence on post-SCI OPCs transplantation as a potential treatment option as well as the impediments against regeneration. Our aim is to shed lights on important knowledge gaps and to provoke thoughts for further researches and the development of therapeutic strategies.

Functional Expression of Electron Transport Chain and FoF1-ATP Synthase in Optic Nerve Myelin Sheath

Our previous studies reported evidence for aerobic ATP synthesis by myelin from both bovine brainstem and rat sciatic nerve. Considering that the optic nerve displays a high oxygen demand, here we evaluated the expression and activity of the five Respiratory Complexes in myelin purified from either bovine or murine optic nerves. Western blot analyses on isolated myelin confirmed the expression of ND4L (subunit of Complex I), COX IV (subunit of Complex IV) and β subunit of F1Fo-ATP synthase. Moreover, spectrophotometric and in-gel activity assays on isolated myelin, as well as histochemical activity assays on both bovine and murine transversal optic nerve sections showed that the respiratory Complexes are functional in myelin and are organized in a supercomplex. Expression of oxidative phosphorylation proteins was also evaluated on bovine optic nerve sections by confocal and transmission electron microscopy. Having excluded a mitochondrial contamination of isolated myelin and considering the results form in situ analyses, it is proposed that the oxidative phosphorylation machinery is truly resident in optic myelin sheath. Data may shed a new light on the unknown trophic role of myelin sheath. It may be energy supplier for the axon, explaining why in demyelinating diseases and neuropathies, myelin sheath loss is associated with axonal degeneration.

Support of Nerve Conduction by Respiring Myelin Sheath: Role of Connexons

Recently, we have demonstrated that myelin conducts an extramitochondrial oxidative phosphorylation, hypothesizing a novel supportive role for myelin in favor of the axon. We have also hypothesized that the ATP produced in myelin could be transferred thought gap junctions. In this work, by biochemical, immunohistochemical, and electrophysiological techniques, the existence of a connection among myelin to the axon was evaluated, to understand how ATP could be transferred from sheath to the axoplasm. Data confirm a functional expression of oxidative phosphorylation in isolated myelin. Moreover, WB and immunohistochemistry on optic nerve slices show that connexins 32 and 43 are present in myelin and colocalize with myelin basic protein. Interestingly, addition of carbenoxolone or oleamide, two gap junction blockers, causes a decrease in oxidative metabolism in purified myelin, but not in mitochondria. Similar effects were observed on conduction speed in hippocampal Schaffer collateral, in the presence of oleamide. Confocal analysis of optic nerve slices showed that lucifer yellow (that only passes through aqueous pores) signal was found in both the sheath layers and the axoplasma. In the presence of oleamide, but not with oleic acid, signal significantly decreased in the sheath and was lost inside the axon. This suggests the existence of a link among myelin and axons. These results, while supporting the idea that ATP aerobically synthesized in myelin sheath could be transferred to the axoplasm through gap junctions, shed new light on the function of the sheath.

Age-related visual signal changes induced by hypoxemic hypoxia: a study on aircraft pilots of different ages

PURPOSE

Exposure to high altitude leads to a series of alterations of higher nervous functions because of hypobaric hypoxia. Sensory systems, mainly the visual one, seem to be particularly involved. This study aimed to assess the effects of hypoxemic hypoxia on the transmission of the visual stimulus simulating a condition of breathing at an altitude of 18,000 feet (5,486 m) through the administration of an air mixture with 10% O2.

METHODS

The subjects involved in the study were 98 pilots of military aircraft (male, acclimated, healthy, 20/20 Uncorrected Visual Acuity (UCVA)/Best Corrected Visual Acuity (BCVA), and aged between 26 and 49 years) divided into 2 groups according to age (A: 26-36 years; B: 37-49 years). The visual evoked potentials were initially recorded at sea level (760 mm Hg) and subsequently at a simulated altitude of 18,000 feet (5,486 m) through the administration of an air mixture with 10% O2 that induced a blood saturation of 80% O2 after 15 minutes. The analysis was carried out using two different kinds of stimulus (15' and 60' of arc). The latency and the amplitude of N-75 (N1) and P-100 (P1) waves have been evaluated. Results obtained from visual evoked potentials were analyzed with Student t-test.

RESULTS

In the first group (pilots aged 26-36 years), an increase in both latency and amplitude of P-100 wave was observed and in the second group (pilots aged 37-49 years), an increase was found in latency and a significant reduction in amplitude.

CONCLUSIONS

The study suggests the existence of a mechanism or a particular anatomic and physiologic condition (probably the neurovascular coupling) that connects the local neuronal activity and the resulting changes in cerebral perfusion. This complex series of events binds together different structures and cell types, and it seems that younger people have a better resistance against the hypoxic insult to the central nervous system because of more efficient compensatory mechanisms.

Concepts for regulation of axon integrity by enwrapping glia

Long axons and their enwrapping glia (EG; Schwann cells (SCs) and oligodendrocytes (OLGs)) form a unique compound structure that serves as conduit for transport of electric and chemical information in the nervous system. The peculiar cytoarchitecture over an enormous length as well as its substantial energetic requirements make this conduit particularly susceptible to detrimental alterations. Degeneration of long axons independent of neuronal cell bodies is observed comparatively early in a range of neurodegenerative conditions as a consequence of abnormalities in SCs and OLGs . This leads to the most relevant disease symptoms and highlights the critical role that these glia have for axon integrity, but the underlying mechanisms remain elusive. The quest to understand why and how axons degenerate is now a crucial frontier in disease-oriented research. This challenge is most likely to lead to significant progress if the inextricable link between axons and their flanking glia in pathological situations is recognized. In this review I compile recent advances in our understanding of the molecular programs governing axon degeneration, and mechanisms of EG’s non-cell autonomous impact on axon-integrity. A particular focus is placed on emerging evidence suggesting that EG nurture long axons by virtue of their intimate association, release of trophic substances, and neurometabolic coupling. The correction of defects in these functions has the potential to stabilize axons in a variety of neuronal diseases in the peripheral nervous system and central nervous system (PNS and CNS).

Oxydative phosphorylation in sciatic nerve myelin and its impairment in a model of dysmyelinating peripheral neuropathy

Myelin sheath is the proteolipid membrane wrapping the axons of CNS and PNS. We have shown data suggesting that CNS myelin conducts oxidative phosphorylation (OXPHOS), challenging its role in limiting the axonal energy expenditure. Here, we focused on PNS myelin. Samples were: (i) isolated myelin vesicles (IMV) from sciatic nerves, (ii) mitochondria from primary Schwann cell cultures, and (iii) sciatic nerve sections, from wild type or Charcot-Marie-Tooth type 1A (CMT1A) rats. The latter used as a model of dys-demyelination. O₂ consumption and activity of OXPHOS proteins from wild type (Wt) or CMT1A sciatic nerves showed some differences. In particular, O₂ consumption by IMV from Wt and CMT1A 1-month-old rats was comparable, while it was severely impaired in IMV from adult affected animals. Mitochondria extracted from CMT1A Schwann cell did not show any dysfunction. Transmission electron microscopy studies demonstrated an increased mitochondrial density in dys-demyelinated axons, as to compensate for the loss of respiration by myelin. Confocal immunohistochemistry showed the expression of OXPHOS proteins in the myelin sheath, both in Wt and dys-demyelinated nerves. These revealed an abnormal morphology. Taken together these results support the idea that also PNS myelin conducts OXPHOS to sustain axonal function.