The cellular form of the prion protein (PrPC) has been widely investigated since its alternative folded isoform is the causative agent of prion disorders. PrPC is highly expressed in the nervous system, where it is involved in many physiological processes such as the maintenance of peripheral nervous system (PNS) myelination. A similar role in the central nervous system (CNS) is still controversial, since PrPC absence affects proliferation and maturation of oligodendrocyte precursor cells without affecting myelination. On the other hand, PrPC is involved in metal homeostasis and modulates oxidative stress, two processes influencing myelin formation and maintenance. In light of these considerations, we took advantage of wild-type (WT, Prnp+/+) and PrPC knock-out (KO, Prnp0/0) mice to investigate PrPC role in CNS myelination. Myelin composition was examined in mouse brains at different developmental stages, from early postnatal days to aging, through cholesterol content measurement and through myelin protein and gene expression evaluation. Furthermore, peripheral myelin status was also investigated in the sciatic nerve (SN) in order to have a comparative analysis between the CNS and PNS myelin. Finally, an ex vivo model based on organotypic hippocampal cultures (OHC) was established to better investigate PrPC role in CNS myelin.
Concerning the PNS, most of our results are consistent with the PNS myelin degeneration observed in literature. Accordingly, results show a 50% decrease in SN cholesterol content, a strong reduction of myelin genes transcription and an altered expression level of some myelin proteins during Prnp0/0 mouse aging. Moreover, some changes in the early postnatal period were also detected. Focusing on the CNS, a small reduction in cholesterol content was observed at postnatal day 1 and in aging brains of Prnp0/0 mice, suggesting an alteration in CNS myelin status. Differently from PNS, CNS myelin proteins are slightly upregulated in Prnp0/0 brains during the whole life. In particular, in Prnp0/0 brains a higher amount of myelin proteolipid protein, myelin basic protein, myelin oligodendrocyte glycoprotein and the glycosylated form of myelin associated glycoprotein was detected. This myelin protein upregulation correlates with a higher gene expression in Prnp0/0 mouse brains. To better investigate PrPC role in CNS myelination, an ex vivo model based on OHC was established. In accordance with in vivo experiments, after 28 days in vitro the amount of myelin proteins and myelin gene expression is higher in Prnp0/0 than in Prnp+/+ OHC. Furthermore, the copper chelator cuprizone (CZ) was administered to OHC to investigate its effect in PrPC presence or absence. CZ treatment did not trigger the expected myelin degeneration. Differently it induced an increase in myelin protein percentage particularly in Prnp+/+ OHC, while the effect on Prnp0/0 OHC was milder. Thus, different response of Prnp+/+ and Prnp0/0 OHC to CZ exposure denotes a distinct CNS susceptibility to this kind of stimulus in the presence or absence of PrPC.
Altogether, these results support the hypothesis that PrPC is involved in CNS myelination, although in a different manner compared to the PNS. In fact, the differences observed in CNS lipid composition, protein expression and gene transcription are less pronounced compared to PNS, but are extended for the whole lifetime. Since electron microscopy on CNS myelinated regions did not show gross morphological changes between Prnp+/+ and Prnp0/0 mice, it is possible that myelin protein overexpression could be the result of a compensatory mechanism to prevent functional abnormalities. Furthermore, since PrPC plays neuroprotective roles, it is possible that it could protect myelin under stress conditions.
