Functional Recovery of Regenerating Motor Axons is Delayed in Mice Heterozygously Deficient for the Myelin Protein P0 Gene
Mice with a heterozygous knock-out of the myelin protein P0 gene (P0+/−) develop a neuropathy similar to human Charcot–Marie–Tooth disease. They are indistinguishable from wild-types (WT) at birth and develop a slowly progressing demyelinating neuropathy. The aim of this study was to investigate whether the regeneration capacity of early symptomatic P0+/− is impaired as compared to age matched WT. Right sciatic nerves were lesioned at the thigh in 7–8 months old mice. Tibial motor axons at ankle were investigated by conventional motor conduction studies and axon excitability studies using threshold tracking. To evaluate regeneration we monitored the recovery of motor function after crush, and then compared the fiber distribution by histology. The overall motor performance was investigated using Rotor-Rod. P0+/− had reduced compound motor action potential amplitudes and thinner myelinated axons with only a borderline impairment in conduction and Rotor-Rod. Plantar muscle reinnervation occurred within 21 days in all mice. Shortly after reinnervation the conduction of P0+/− regenerated axons was markedly slower than WT, however, this difference decayed with time. Nevertheless, after 1 month, regenerated P0+/− axons had longer strength-duration time constant, larger threshold changes during hyperpolarizing electrotonus and longer relative refractory period. Their performance at Rotor-Rod remained also markedly impaired. In contrast, the number and diameter distribution of regenerating myelinated fibers became similar to regenerated WT. Our data suggest that in the presence of heterozygously P0 deficient Schwann cells, regenerating motor axons retain their ability to reinnervate their targets and remyelinate, though their functional recovery is delayed.
KeywordsNerve activity Regeneration Ion channels Excitability Node of Ranvier Internode Mouse model Demyelination
The project was supported by Lundbeck Foundation, the Novo Nordisk Foundation, the Danish Medical Research Council, the Ludvig and Sara Elsass Foundation, the Foundation for Research in Neurology and Jytte and Kaj Dahlboms Foundation. We would like to thank Lis Hansen for expert technical assistance with histological preparations.
- 1.Sorensen J, Fugleholm K, Moldovan M, Schmalbruch H, Krarup C (2001) Axonal elongation through long acellular nerve segments depends on recruitment of phagocytic cells from the near-nerve environment. Electrophysiological and morphological studies in the cat. Brain Res 903:185–197PubMedCrossRefGoogle Scholar
- 11.Lupski JR, Reid JG, Gonzaga-Jauregui C, Rio DD, Chen DC, Nazareth L, Bainbridge M, Dinh H, Jing C, Wheeler DA, McGuire AL, Zhang F, Stankiewicz P, Halperin JJ, Yang C, Gehman C, Guo D, Irikat RK, Tom W, Fantin NJ, Muzny DM, Gibbs RA (2010) Whole-genome sequencing in a patient with Charcot–Marie–Tooth neuropathy. N Engl J Med 362:1181–1191PubMedCrossRefGoogle Scholar
- 13.Warner LE, Hilz MJ, Appel SH, Killian JM, Kolodry EH, Karpati G, Carpenter S, Watters GV, Wheeler C, Witt D, Bodell A, Nelis E, Van BC, Lupski JR (1996) Clinical phenotypes of different MPZ (P0) mutations may include Charcot–Marie–Tooth type 1B, Dejerine-Sottas, and congenital hypomyelination. Neuron 17:451–460PubMedCrossRefGoogle Scholar
- 14.Wrabetz L, D’Antonio M, Pennuto M, Dati G, Tinelli E, Fratta P, Previtali S, Imperiale D, Zielasek J, Toyka K, Avila RL, Kirschner DA, Messing A, Feltri ML, Quattrini A (2006) Different intracellular pathomechanisms produce diverse myelin protein zero neuropathies in transgenic mice. J Neurosci 26:2358–2368PubMedCrossRefGoogle Scholar
- 32.Boërio D, Kalmar B, Greensmith L, Bostock H (2010) Excitability properties of mouse motor axons in the mutant SOD1(G93A) model of amyotrophic lateral sclerosis. Muscle Nerve 41:774–784Google Scholar
- 42.Fabrizi GM, Simonati A, Morbin M, Cavallaro T, Taioli F, Benedetti MD, Edomi P, Rizzuto N (1998) Clinical and pathological correlations in Charcot–Marie–Tooth neuropathy type 1A with the 17p11.2p12 duplication: a cross-sectional morphometric and immunohistochemical study in twenty cases. Muscle Nerve 21:869–877PubMedCrossRefGoogle Scholar
- 59.Goldman L, Albus JS (1968) Computation of impulse conduction in myelinated fibers: theoretical basis of the velocity diameter relation. Biophys J 8:596–607Google Scholar
- 60.Erlanger J, Schoepfle GM (1946) A study of nerve degeneration and regeneration. Am J Physiol 147:550–581Google Scholar
- 62.Sanders FK, Whitteridge D (1946) Conduction velocity and myelin thickness in regenerating nerve fibers. J Physiol 105:152–174Google Scholar
- 63.Vizoso AD, Young JZ (1948) Internode length and fibre diameter in developing and regenerating nerves. J Anat 82:110–134Google Scholar