Absence of 2-Hydroxylated Sphingolipids Is Compatible with Normal Neural Development But Causes Late-Onset Axon and Myelin Sheath Degeneration

Institute of Physiological Chemistry, University of Bonn, 53115 Bonn, Germany.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 10/2008; 28(39):9741-54. DOI: 10.1523/JNEUROSCI.0458-08.2008
Source: PubMed


Sphingolipids containing 2-hydroxylated fatty acids are among the most abundant lipid components of the myelin sheath and therefore are thought to play an important role in formation and function of myelin. To prove this hypothesis, we generated mice lacking a functional fatty acid 2-hydroxylase (FA2H) gene. FA2H-deficient (FA2H(-/-)) mice lacked 2-hydroxylated sphingolipids in the brain and in peripheral nerves. In contrast, nonhydroxylated galactosylceramide was increased in FA2H(-/-) mice. However, oligodendrocyte differentiation examined by in situ hybridization with cRNA probes for proteolipid protein and PDGFalpha receptor and the time course of myelin formation were not altered in FA2H(-/-) mice compared with wild-type littermates. Nerve conduction velocity measurements of sciatic nerves revealed no significant differences between FA2H(-/-) and wild-type mice. Moreover, myelin of FA2H(-/-) mice up to 5 months of age appeared normal at the ultrastructural level, in the CNS and peripheral nervous system. Myelin thickness and g-ratios were normal in FA2H(-/-) mice. Aged (18-month-old) FA2H(-/-) mice, however, exhibited scattered axonal and myelin sheath degeneration in the spinal cord and an even more pronounced loss of stainability of myelin sheaths in sciatic nerves. These results show that structurally and functionally normal myelin can be formed in the absence of 2-hydroxylated sphingolipids but that its long-term maintenance is strikingly impaired. Because axon degeneration appear to start rather early with respect to myelin degenerations, these lipids might be required for glial support of axon function.

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    • "The additional deletion of PLP in these mutants substantially aggravates the axonal demise, but not myelin defects (Edgar et al., 2009). Finally, grossly normal myelin can be formed in the absence of enzymes important for synthesis of specific myelin lipids, but surprisingly this results in axon degeneration (Sheikh et al., 1999; Zoller et al., 2008). The mechanisms of how absence of these myelin proteins and lipids causes axonal degeneration have not been identified, but biochemical screening studies suggest that some of these proteins may facilitate the transport of trophic substances from OLGs that could be important for axonal integrity (Werner et al., 2007). "
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    ABSTRACT: 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).
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    • "V. Kota, H. Hama / Advances in Biological Regulation 54 (2014) 223–230 225 indicate that 2 0 -hydroxy GalCer is dispensable for the myelination process but critical for the long-term stability of myelin. A timely report on Fa2h knockout mice corroborated the findings in FA2H deficiency (Zoller et al., 2008). Myelin in Fa2h knockout mice were devoid of 2 0 -hydroxy GalCer. "
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    • "On the other hand the fa2h−/− mice with intact CGT showed absence of 2-OH galactosylceramide and sulfatide in normal myelin.48 However, myelin sheath degeneration was observed with aging.48,55 These studies of different murine knockout models deprived of specific myelin sphingolipids demonstrate that myelin biogenesis in the nervous system remains intact, likely due to the redundancies of these myelin components. "
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