Cathepsin D deficiency delays central nervous system myelination by inhibiting proteolipid protein trafficking from late endosome/lysosome to plasma membrane

Abstract

This study aimed to investigate the role of cathepsin D (CathD) in central nervous system (CNS) myelination and its possible mechanism. By using CathD knockout mice in conjunction with immunohistochemistry, immunocytochemistry and western blot assays, the myelination of the CNS and the development of oligodendrocyte lineage cells in vivo and in vitro were observed. Endocytosis assays, real-time-lapse experiments and total internal reflection fluorescence microscopy were used to demonstrate the location and movement of proteolipid protein in oligodendrocyte lineage cells. In addition, the relevant molecular mechanism was explored by immunoprecipitation. The increase in Fluoromyelin Green staining and proteolipid protein expression was not significant in the corpus callosum of CathD−/− mice at the age of P11, P14 and P24. Proteolipid protein expression was weak at each time point and was mostly accumulated around the nucleus. The number of oligodendrocyte lineage cells (olig2+) and mature oligodendrocytes (CC1+) significantly decreased between P14 and P24. In the oligodendrocyte precursor cell culture of CathD−/− mice, the morphology of myelin basic protein-positive mature oligodendrocytes was simple while oligodendrocyte precursor cells showed delayed differentiation into mature oligodendrocytes. Moreover, more proteolipid protein gathered in late endosomes/lysosomes (LEs/Ls) and fewer reached the plasma membrane. Immunohistochemistry and immunoelectron microscopy analysis showed that CathD, proteolipid protein and VAMP7 could bind with each other, whereas VAMP7 and proteolipid protein colocalized with CathD in late endosome/lysosome. The findings of this paper suggest that CathD may have an important role in the myelination of CNS, presumably by altering the trafficking of proteolipid protein. Cathepsin D regulates the transport of the major component of myelin, proteolipid protein (PLP), to the cell membrane. The enzyme cathepsin D is involved in the breakdown of proteins but little is known about how mutations in cathepsin D are associated with fatal neurological diseases. A team of Chinese researchers led by Shu-Yi Pan at the Navy General Hospital of PLA in Beijing examined the effects of cathepsin D deficiency on the formation of a protective sheath around nerves, a process known as myelination, in mice. They found a marked decrease in the number of mature myelin-producing cells in the brains of mice lacking cathepsin D. Furthermore, in these cells PLP-containing vesicles were transported more slowly towards the plasma membrane than in the controls, highlighting a potential mechanism through which cathepsin D regulates myelination.