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

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Abstract

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.

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Abbreviations

ACSF:

Artificial cerebrospinal fluid

ADP:

Adenosine diphosphate

ANT:

Adenosine nucleotide translocase

ATP:

Adenosine triphosphate

EDTA:

Ethylenediaminetetraacetic acid

EGTA:

Ethylene glycol tetraacetic acid

HEPES:

4-(2-Hydroxyethyl)-1-piperazineethanesulfonic acid

IM:

Isolated myelin

MBP:

Myelin basic protein

NADH:

Nicotinamide adenine dinucleotide reduced form

OXPHOS:

Oxidative phosphorylation

PBS:

Phosphate buffer saline

PV:

Presynaptic volley

R.O.D.:

Relative optical density

SDS-PAGE:

Sodium dodecyl sulfate polyacrylamide gel electrophoresis

TIM:

Transport inner membrane

WB:

Western blot

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Acknowledgments

This study was supported by a Grant from the “Compagnia di San Paolo”-Neuroscience Program 2008, for the research project entitled: “Energetic metabolism in myelinated axon: a new trophic role of myelin sheath,” and from the “Fondazione Giuseppe Levi–Accademia Nazionale dei Lincei” for the research project entitled: “Produzione extramitocondriale di ATP in mielina: localizzazione dei complessi della catena respiratoria e possible ruolo nella degenerazione assonale in Sclerosi Multipla.”

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Authors and Affiliations

  1. Biochemistry Lab, Department of Pharmacy, University of Genova, Viale Benedetto XV 3, 16132, Genova, Italy

    Silvia Ravera, Martina Bartolucci, Daniela Calzia, Alessandro Morelli & Isabella Panfoli

  2. Department of Neuroscience, Rehabilitation, Ophthalmology, Genetics, Maternal and Child Health, University of Genova, Via de Toni 5, 16132, Genova, Italy

    Enrico Adriano, Patrizia Garbati & Maurizio Balestrino

  3. DISTAV, University of Genova, C.so Europa 26, 16132, Genova, Italy

    Sara Ferrando & Paola Ramoino

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  1. Silvia Ravera
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  2. Martina Bartolucci
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Correspondence to Silvia Ravera.

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Ravera, S., Bartolucci, M., Adriano, E. et al. Support of Nerve Conduction by Respiring Myelin Sheath: Role of Connexons. Mol Neurobiol 53, 2468–2479 (2016). https://doi.org/10.1007/s12035-015-9216-0

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