Internodal myelination during development quantitated using X-ray diffraction

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Abstract

Characterizing the formation, accretion, and stability of myelin during development, maturation, and senescence is important for better understanding critical periods in the function of the nervous system in normal growth and following environmental insult or genetic mutation. Although there are numerous studies on the ultrastructural, biochemical, and genetic aspects of myelin development and maturation, few have used X-ray diffraction (XRD), which can rapidly provide unique metrics about internodal myelin based on measurements from whole, unfixed tissue. Besides periodicity (the classic attribute of internodal myelin measured by XRD), other parameters include: relative amount of myelin, membrane dimensions, and packing disorder. To provide a baseline for future experiments on myelin structural integrity, we used XRD to characterize internodal myelin as a function of age (from 5 to 495 days) in the mouse, a species increasingly used for developing transgenic models of human neurological diseases. As expected, the relative amount of myelin increased with age in both PNS and CNS, with the most rapid accumulation occurring in the youngest age group. Changes in rate of myelin accretion yielded three distinct age brackets during which small but significant changes in structural parameters were detected: in PNS, myelin period increased, packing distortion decreased, width of extracellular apposition (EXT) decreased, and widths of cytoplasmic apposition (CYT) and lipid bilayer (LPG) increased; in CNS, myelin period decreased, packing distortion decreased, EXT and CYT decreased, and LPG increased. We propose that the data obtained here can serve as a basis for rapidly detecting abnormal pathologies during myelination.

Introduction

Nerve conduction in vertebrates is greatly speeded up by the ensheathment of its axons with myelin, an insulating, multilamellar membrane assembly (Lazzarini, 2004). Internodal myelin, which is the portion of the sheath between nodes of Ranvier, consists of membrane pairs that are closely apposed, adhering at their cytoplasmic and extracellular appositions. Compared to other membranes that are loci for enzymatic activities, internodal myelin has a high lipid-to-protein ratio (Norton and Poduslo, 1973). The proteins in compact myelin include P0, MBP, PMP-22, and P2 in PNS, and PLP and MBP in the CNS (Kursula, 2008, Simons and Trotter, 2007, Trapp and Kidd., 2004). The recent co-localization of MBP and ATP synthase in optic nerve myelin (Ravera et al., 2009) may indicate a more active physiological role for compact myelin. The protein composition is the predominant determinant of myelin’s periodicity, as the thickness of the membrane bilayer is fairly constant across species whereas the extent of close apposition between bilayers—or membrane packing—varies considerably (Kirschner and Blaurock, 1992, Kirschner et al., 1989). Depending on animal species the periodicity ranges from 160 to 185 Å in the PNS, and 150 to 170 Å in the CNS.

Both ultrastructural and biochemical approaches have been used to follow myelination. For example, electron microscopy reveals that for PNS myelin the ratio of the number of myelin layers to the axon area rapidly increases during the first 75 days (Friede and Samorajski, 1967; Low, 1976). Based on biochemical metrics of brain weight, myelin yield, and accumulation of MBP and specific myelin lipids, CNS myelination shows an initial rapid increase followed by a more gradual increase (Barbarese et al., 1978, Morell et al., 1972, Norton and Poduslo, 1973, Uzman and Rumley, 1958). The few XRD studies that have been undertaken to characterize myelin accretion and changes in myelin structure with age show that within the first few weeks there is an initial rapid accumulation of PNS and CNS myelin and small changes in their periods (Kirschner and Sidman, 1976, Mateu et al., 1990, Mateu et al., 1991, Vargas et al., 2000).

To quantitate more thoroughly the formation and development of internodal myelin and its growth trends, we used XRD with an electronic detector to analyze whole nerves in both the PNS and CNS as a function of age. The advantage of this technique is the rapidity of the measurements, the large sampling volume due to the use of intact nerves, and the fact that the tissue is unfixed (Avila et al., 2005). We found that the most rapid rate of myelin accretion occurred during the first 3–4 weeks in the PNS and the first 5 weeks in the CNS, and this was followed by a dramatic and progressive decrease in rate. In parallel, the myelin period increased in the PNS and decreased in the CNS. The mean distortion in period decreased with age, suggesting greater structural stability in membrane packing. Underlying the periodicity changes, the membrane bilayer structure showed small changes: whereas the lipid bilayer width increased with age and the width of the extracellular apposition decreased in both PNS and CNS myelins, the cytoplasmic apposition width increased in the PNS but decreased in the CNS. The detailed characteristics quantified here may serve as a benchmark for understanding the dynamic interactions that occur in this multilamellar membrane assembly in diseases where genetic or acquired abnormalities underlie a defect in the myelination program.

Section snippets

Materials and methods

The ages of the mice (DDY strain; originally obtained from Clea Japan, Inc.) ranged from 5 to 495 days. Mice were sacrificed by cervical dislocation. Optic and sciatic nerves were maintained moist with Ringers solution during dissection, tied off using surgical silk, and immediately placed in physiological saline (pH 7.4). The nerves, devoid of adhering blood vessels, etc., were inserted into quartz capillaries (0.7 mm for sciatic nerves and 0.5 mm for optic nerves) which were filled with saline

Myelin diffraction was stronger from sciatic nerves than from optic nerves

To quantitate the gradual accumulation of internodal myelin with murine development, we analyzed the X-ray diffraction patterns recorded from intact, mouse sciatic and optic nerves from the postnatal period (4 days for PNS and 15 days for CNS) to about 1.5 years of age. From the youngest to the oldest, the myelin diffraction patterns for both PNS and CNS samples showed a progressive increase in the intensity of the peaks above background, which is the X-ray scatter originating from internodal

Discussion

Electron micrographs of myelin sheaths during the early postnatal period show changes in the membrane packing. For example, in the PNS there is a residual layer of cytoplasm present at the same time that the extracellular surfaces are closely apposed (see Figs. 6 and 7 in Peters et al. (1991), and Fig. 1.7 in Landon and Hall (1976)). Given that the inter-membrane interactions in internodal myelin during development are dynamic, it is important to have a quantitative benchmark of myelin

Acknowledgments

This research was supported by the European Leukodystrophy Association (ELA), Boston College Institutional Research Funds, and the Undergraduate Research Fellowship Program at Boston College. We thank the anonymous referees for their constructive comments on this manuscript.

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