Analysis of peroxisomal turnover and myelin maintenance in mice with oligodendrocyte-specific MFP2-deficiency
by Sarah Richert
Date of Examination:2016-10-17
Date of issue:2016-11-02
Advisor:Dr. Celia Kassmann
Referee:Prof. Dr. Klaus-Armin Nave
Referee:Prof. Dr. Ernst A. Wimmer
Referee:PD Dr. Sven Thoms
Persistent Address:
http://dx.doi.org/10.53846/goediss-5949
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Abstract
English
Although caused by distinct genetic mutations, the peroxisomal disorders X-linked adrenoleukodystrophy (X-ALD), pseudoneonatal adrenoleukodystrophy (pseudo NALD), and neonatal adrenoleukodystrophy (NALD) share several disease hallmarks. Strikingly, the underlying dysfunctions are either related to generalized peroxisomal defects or to defects in peroxisomal β-oxidation. In contrast, leukodystrophy is not a feature of other peroxisomal disorders e.g. in α-oxidation or plasmalogen-synthesis defects. This indicates a common pathomechanism for β-oxidation and generalized peroxisomal defects. Further indications for this derive from mice lacking complete peroxisomal function only in oligodendrocytes. The pattern of inflammatory subcortical demyelination in these CnpCre/Wt*Pex5-/- mutants is reminiscent of the cerebral pathology known from X-ALD patients lacking peroxisomal β-oxidation of very long chain fatty acids. This led to the hypothesis that impaired β-oxidation may culminate in secondary loss of further peroxisomal functions. So far, several mouse models with defective peroxisomal β-oxidation were generated, but failed to develop cerebral demyelination. In search for an appropriate model with impaired peroxisomal β-oxidation to investigate possible secondary peroxisomal defects and subsequent pathology, aged CnpCre/Wt*Mfp2-/- mice were analyzed. Indeed, CnpCre/Wt*Mfp2-/- mice developed demyelinating lesions in the frontal corpus callosum when aged ≥ 16 months. This was accompanied by reactive gliosis, lymphocyte infiltration, and behavioral alterations. Thus CnpCre/Wt*Mfp2-/- mice proved to be suitable to study demyelination and possibly preceding effects of impaired peroxisomal β-oxidation on peroxisomes. To facilitate oligodendrocyte specific analysis of oligodendroglial peroxisomes novel transgenic mice with fluorescently labeled peroxisomes in oligodendrocytes (Cnp-mEos2-PTS1) were generated. Employing a photo-convertible fluorescent protein enabled ‘pulse-chase’ experiments to provide insight into peroxisomal biogenesis and degradation. Brain sections from double-transgenic Cnp-mEos2-PTS1*CnpCre/Wt*Mfp2-/- mice revealed a progressively decreasing number and increased size of peroxisomes. Alterations were observed already at 2 months, preceding disease onset by approximately one year. At 16 months of age only 50% of peroxisomes were preserved. In vitro experiments using primary MFP2-deficient oligodendrocytes proved a dramatically reduced peroxisomal turnover by both, decreased degradation of pulse-labeled peroxisomes, i.e. enhanced organelle aging, and diminished appearance of new peroxisomes. The mechanism of this decreased organelle turnover remains elusive. Interestingly, inhibiting pexophagy in control oligodendrocytes by use of 3-Methyladenine also blocked peroxisomal biogenesis, which indicates tight coupling between peroxisomal biogenesis and degradation. Together the data suggest that perturbation of peroxisomal β-oxidation in oligodendrocytes causes secondary impairment of peroxisomal functions, which precedes and possibly triggers cerebral demyelination.
Keywords: Peroxisome; Oligodendrocyte; Myelin; MFP2; Peroxisome turnover; mEos2