Particular aspects of myelin-axon interactions in health and disease : the expression of myelin-associated glycoprotein isoforms in CNS and PNS. Early axonal pathology in the dysmyelinating peripheral neuropathy CMT1A

An intact myelin sheath is crucial for the rapid propagation of action potentials along
myelinated axons. There are many neurodegenerative diseases associated with defect myelin
sheaths resulting in severe clinical symptoms such as multiple sclerosis and a group of hereditary
neuropathies called “Charcot Marie Tooth diseases”. For the maintenance of both, the integrity
of the axon and the myelin sheath, reciprocal signaling between the axon and glia is required.
Some molecular components involved in these interactions between glial cells and the axons
have been investigated. One of these molecules is MAG, the myelin associated glycoprotein,
inserted into the periaxonal glial membrane. MAG is expressed as a large and a small isoform (LMAG,
S-MAG) that display a common extracellular but different intracellular domains that
cannot be discriminated by antibody staining. In the first part of the thesis, the question how Land
S-MAG are differentially expressed in the CNS and PNS was addressed. For this study a
transgenic mouse that expresses the small MAG isoform tagged with green fluorescent protein
(GFP) was previously generated (Erb. et al). In the CNS, L- and S-MAG were differentially
expressed in certain brain regions such as the corpus callosum and the perforant pathway. In
some myelinated fibers L-MAG was predominantly expressed, in others only S-MAG. In the PNS,
S-MAG was the predominant isoform; L-MAG was only weakly detectable very early during
development. In the PNS, S-MAG-GFP was localized in the expected compartments such as
periaxonal membranes, paranodes and Schmidt-Lanterman incisures. In addition, S-MAG was
expressed in ring- or disc-like compartments surrounding axons suggesting that there are
incisure-like structures distinct from classical Schmidt-Lanterman incisures. The S-MAG-GFP
mouse will be a valuable animal model to study the dynamic processes during the formation of
Schmidt-Lanterman incisures or paranodal structures in vitro.
“Charcot Marie Tooth” diseases are classified into demyelinating and axonal forms and
subdivided into different subtypes according to their genetic backgrounds. CMT1A has been
classified as primary demyelinating disease and is caused by a duplication of the DNA region
encoding the compact myelin protein PMP22 (peripheral myelin protein 22), which results in an
overexpression of PMP22. The effects of PMP22 overexpression on the Schwann cells have been
well studied. Overexpression of PMP22 impairs Schwann cell differentiation and myelination,
and results in accumulations of PMP22. In the second part of this thesis, the question how
abnormal Schwann cells influence the development of axons, especially the axonal cytoskeleton,
in an animal model for CMT1A disease was addressed. The analysis was focused on the
establishment of the neurofilament system during development. In particular, the neurofilament
subunit composition (content of heavy, medium and light chain) and phosphorylation were
investigated. In CMT1A mice, there were early changes in the subunit composition and
phosphorylation. The axonal pathology in CMT1A mice is marked by a strong increase of the
non-phosphorylated neurofilament heavy chains (NF-H) relative to NF-M and NF-L. As
unbalanced neurofilament subunits stoichiometries have been associated with reduced axonal
calibers, they may account for the predominance of small caliber myelinated axons observed in
CMT1A mice. The precise molecular mechanisms by which Schwann cells influence the
neurofilament system are not known. However, it was hypothesized that MAG regulates
neurofilament phosphorylation interacting with its axonal receptor that induces the activity of
Cdk5 or ERK1/2 kinases in the axons. Therefore, the expression of MAG was investigated
CMT1A mice to see whether it may correlate with the neurofilament phosphorylation pattern.