Evaluation of the Acquisition of the Aerobic Metabolic Capacity by Myelin, during its Development

Should the standard model of cellular energy metabolism be reconsidered? Possible coupling between the pentose phosphate pathway, glycolysis and extra-mitochondrial oxidative phosphorylation

The process of cellular respiration occurs for energy production through catabolic reactions, generally with glucose as the first process step. In the present work, we introduce a novel concept for understanding this process, based on our conclusion that glucose metabolism is coupled to the pentose phosphate pathway (PPP) and extra-mitochondrial oxidative phosphorylation in a closed-loop process. According to the current standard model of glycolysis, glucose is first converted to glucose 6-phosphate (glucose 6-P) and then to fructose 6-phosphate, glyceraldehyde 3-phosphate and pyruvate, which then enters the Krebs cycle in the mitochondria. However, it is more likely that the pyruvate will be converted to lactate. In the PPP, glucose 6-P is branched off from glycolysis and used to produce NADPH and ribulose 5-phosphate (ribulose 5-P). Ribulose 5-P can be converted to fructose 6-P and glyceraldehyde 3-P. In our view, a circular process can take place in which the ribulose 5-P produced by the PPP enters the glycolysis pathway and is then retrogradely converted to glucose 6-P. This process is repeated several times until the complete degradation of glucose 6-P. The role of mitochondria in this process is to degrade lipids by beta-oxidation and produce acetyl-CoA; the function of producing ATP appears to be only secondary. This proposed new concept of cellular bioenergetics allows the resolution of some previously unresolved controversies related to cellular respiration and provides a deeper understanding of metabolic processes in the cell, including new insights into the Warburg effect.

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.

The Flavone Cirsiliol from Salvia x jamensis Binds the F1 Moiety of ATP Synthase, Modulating Free Radical Production

Several studies have shown that mammalian retinal rod outer segments (OS) are peculiar structures devoid of mitochondria, characterized by ectopic expression of the molecular machinery for oxidative phosphorylation. Such ectopic aerobic metabolism would provide the chemical energy for the phototransduction taking place in the OS. Natural polyphenols include a large variety of molecules having pleiotropic effects, ranging from anti-inflammatory to antioxidant and others. Our goal in the present study was to investigate the potential of the flavonoid cirsiliol, a trihydroxy-6,7-dimethoxyflavone extracted from Salvia x jamensis, in modulating reactive oxygen species production by the ectopic oxidative phosphorylation taking place in the OS. Our molecular docking analysis identified cirsiliol binding sites inside the F1 moiety of the nanomotor F₁F_o-ATP synthase. The experimental approach was based on luminometry, spectrophotometry and cytofluorimetry to evaluate ATP synthesis, respiratory chain complex activity and H₂O₂ production, respectively. The results showed significant dose-dependent inhibition of ATP production by cirsiliol. Moreover, cirsiliol was effective in reducing the free radical production by the OS exposed to ambient light. We report a considerable protective effect of cirsiliol on the structural stability of rod OS, suggesting it may be considered a promising compound against oxidative stress.

Adenosine Blood Level: A Biomarker of White Matter Damage in Very Low Birth Weight Infants

BACKGROUND

Very low birth weight infants are at risk of developing periventricular white matter lesions. We previously reported high blood adenosine levels in premature infants and infants with low birth weight. We asked whether blood adenosine levels could be related to the vulnerability of the maturing white matter to develop lesions. The present study aims at finding a biomarker for the early detection of brain white matter lesions that can profoundly influence the neurodevelopmental outcome, whose pathophysiology is still unclear.

METHODS

Dried blood spots were prospectively collected for the newborn screening program and adenosine concentration measurements. Fifty-six newborns who tested four times for blood adenosine concentration (at days 3, 15, 30, and 40 post-birth) were included in the program. All infants underwent brain MRI at term equivalent age. Neurodevelopmental outcomes were studied with Griffiths Mental Development Scales (GMDS) at 12 ± 2 months corrected age.

RESULTS

Blood adenosine concentration increased over time from a median of 0.75 μM at Day 3 to 1.46 μM at Day 40. Adenosine blood concentration >1.58 μM at Day 15 was significantly associated with brain white matter lesions at MRI (OR (95 % CI) of 50.0 (3.6-688.3), p-value < 0.001). A moderate negative correlation between adenosine at 15 days of life and GMDS at 12 ± 2 months corrected age was found.

CONCLUSION

These findings suggest a potential role for blood adenosine concentration as a biomarker of creberal white matter lesions in very low birth weight infants.

Efficient extra-mitochondrial aerobic ATP synthesis in neuronal membrane systems

The nervous system displays high energy consumption, apparently not fulfilled by mitochondria, which are underrepresented therein. The oxidative phosphorylation (OxPhos) activity, a mitochondrial process that aerobically provides ATP, has also been reported also in the myelin sheath and the rod outer segment (OS) disks. Thus, commonalities and differences between the extra-mitochondrial and mitochondrial aerobic metabolism were evaluated in bovine isolated myelin (IM), rod OS, and mitochondria-enriched fractions (MIT). The subcellular fraction quality and the absence of contamination fractions have been estimated by western blot analysis. Oxygen consumption and ATP synthesis were stimulated by conventional (pyruvate + malate or succinate) and unconventional (NADH) substrates, observing that oxygen consumption and ATP synthesis by IM and rod OS are more efficient than by MIT, in the presence of both kinds of respiratory substrates. Mitochondria did not utilize NADH as a respiring substrate. When ATP synthesis by either sample was assayed in the presence of 10-100 µM ATP in the assay medium, only in IM and OS it was not inhibited, suggesting that the ATP exportation by the mitochondria is limited by extravesicular ATP concentration. Interestingly, IM and OS but not mitochondria appear able to synthesize ATP at a later time with respect to exposure to respiratory substrates, supporting the hypothesis that the proton gradient produced by the electron transport chain is buffered by membrane phospholipids. The putative transfer mode of the OxPhos molecular machinery from mitochondria to the extra-mitochondrial structures is also discussed, opening new perspectives in the field of neurophysiology.

The aerobic mitochondrial ATP synthesis from a comprehensive point of view

Most of the ATP to satisfy the energetic demands of the cell is produced by the F₁F_o-ATP synthase (ATP synthase) which can also function outside the mitochondria. Active oxidative phosphorylation (OxPhos) was shown to operate in the photoreceptor outer segment, myelin sheath, exosomes, microvesicles, cell plasma membranes and platelets. The mitochondria would possess the exclusive ability to assemble the OxPhos molecular machinery so to share it with the endoplasmic reticulum (ER) and eventually export the ability to aerobically synthesize ATP in true extra-mitochondrial districts. The ER lipid rafts expressing OxPhos components is indicative of the close contact of the two organelles, bearing different evolutionary origins, to maximize the OxPhos efficiency, exiting in molecular transfer from the mitochondria to the ER. This implies that its malfunctioning could trigger a generalized oxidative stress. This is consistent with the most recent interpretations of the evolutionary symbiotic process whose necessary prerequisite appears to be the presence of the internal membrane system inside the eukaryote precursor, of probable archaeal origin allowing the engulfing of the α-proteobacterial precursor of mitochondria. The process of OxPhos in myelin is here studied in depth. A model is provided contemplating the biface arrangement of the nanomotor ATP synthase in the myelin sheath.

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.

Potential role of endothelial cell surface ectopic redox complexes in COVID-19 disease pathogenesis

The novel coronavirus infectious disease (COVID-19) has rapidly spread and poses a great challenge to researchers, both in elucidating its pathogenic mechanism and developing effective treatments. It has been recently proposed that COVID-19 is an endothelial disease. Indeed, the COVID-19 virus binds to angiotensin-converting enzyme type 2 (ACE2), which is expressed in endothelial cells. ACE2 could be implicated in the production of reactive oxygen species (ROS) caused by endothelial dysfunction due to viral damage. Consequently, oxidative stress could prime these cells to acquire a pro-thrombotic and pro-inflammatory phenotype, predisposing patients to thromboembolic and vasculitic events and to disseminated intravascular coagulopathy (DIC). This implies a pivotal role played by oxygen in the pathogenetic mechanism of COVID-19 disease, in that its availability would tune the oxidant state and consequent damage.

Brain metabolic DNA: recent evidence for a mitochondrial connection

This review highlights recent data concerning the synthesis of brain metabolic DNA (BMD) by cytoplasmic reverse transcription and the prompt acquisition of the double-stranded configuration that allows its partial transfer to nuclei. BMD prevails in the mitochondrial fraction and is present in presynaptic regions and astroglial processes where it undergoes a turnover lasting a few weeks. Additional data demonstrate that BMD sequences are modified by learning, thus indicating that the modified synaptic activity allowing proper brain responses is encoded in learning BMD. In addition, several converging observations regarding the origin of BMD strongly suggest that BMD is reverse transcribed by mitochondrial telomerase.

An update of the chemiosmotic theory as suggested by possible proton currents inside the coupling membrane

Understanding how biological systems convert and store energy is a primary purpose of basic research. However, despite Mitchell's chemiosmotic theory, we are far from the complete description of basic processes such as oxidative phosphorylation (OXPHOS) and photosynthesis. After more than half a century, the chemiosmotic theory may need updating, thanks to the latest structural data on respiratory chain complexes. In particular, up-to date technologies, such as those using fluorescence indicators following proton displacements, have shown that proton translocation is lateral rather than transversal with respect to the coupling membrane. Furthermore, the definition of the physical species involved in the transfer (proton, hydroxonium ion or proton currents) is still an unresolved issue, even though the latest acquisitions support the idea that protonic currents, difficult to measure, are involved. Moreover, F_oF₁-ATP synthase ubiquitous motor enzyme has the peculiarity (unlike most enzymes) of affecting the thermodynamic equilibrium of ATP synthesis. It seems that the concept of diffusion of the proton charge expressed more than two centuries ago by Theodor von Grotthuss is to be taken into consideration to resolve these issues. All these uncertainties remind us that also in biology it is necessary to consider the Heisenberg indeterminacy principle, which sets limits to analytical questions.

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.