In this study, we successfully generated Ndrg1−/− mice. The Ndrg1−/− mice exhibited a progressive demyelinating disorder of the peripheral nerves. Histological and quantitative analyses revealed that Schwann cell proliferation and the initial myelination of Ndrg1−/− mice were normal after birth (Fig. and Table ). However, sporadic degeneration began by 5 weeks of age (Fig. ). These results strongly suggest that the ability to form myelin sheaths is retained but some defect in the maintenance of the myelin sheath is present in the Schwann cells of Ndrg1−/− mice. Therefore, NDRG1 is essential for maintenance of the myelin sheath.
It has been reported that NDRG1 expression is induced by differentiation or stress stimuli (21
). NDRG1 has also been proposed to shuttle between the cytoplasm and the nucleus in cells (14
). Furthermore, phosphorylation of NDRG1 depends on extracellular stimuli (2
). These observations imply that NDRG1 may have a role in signal transduction. Recently, it was reported that rat NDRG1 is expressed in astrocytes only in the regions where neurons existed (28
). This observation suggests that NDRG1 may also play a similar role in neuronal survival in the brain. We demonstrated that NDRG1 was abundantly expressed in the Schwann cell cytoplasm rather than in myelin sheaths (Fig. ). This expression pattern is unique compared to that of other Charcot-Marie-Tooth disease-responsible proteins, such as peripheral myelin protein 22, myelin protein zero, connexin 32, and L-periaxin (4
). These proteins are localized to the plasma membrane of Schwann cells and are thought to have a role in the formation and/or stabilization of the myelin sheaths. Cytoplasmic expression and phosphorylation of NDRG1 implies its association with intracellular signal transduction in Schwann cells. The NDRG1-mediated signals in Schwann cells related to axonal cross talk could be important for the maintenance of myelin sheaths and axonal survival.
mice exhibited muscle weakness, whereas the complicated motor abilities were relatively retained (Fig. ). These results indicate that NDRG1 deficiency causes peripheral nerve degeneration leading to muscle weakness. This suggests that peripheral nerves may be quite vulnerable to NDRG1 deficiency but that some degree of functional redundancy for NDRG1 may exist within the central nervous system. NDRG1 is one of four NDRG family members exhibiting different expression patterns (20
). We previously demonstrated that NDRG4 is abundantly expressed in neurons in the brain but not in the peripheral nerves (32
). NDRG4 expression is induced by homocysteine and reduced both the proliferation and migration rates of cultured cells (19
), suggesting that NDRG4 could play a role similar to that of NDRG1 in the brain. NDRG2 and NDRG3 were expressed less in the sciatic nerve than in the brain (Fig. ). Indeed, no apparent morphological abnormality of the brain was detected in Ndrg1−/−
mice (data not shown). NDRG1 deficiency may be compensated for by other NDRG members in the brain.
Although the Ndrg1−/−
mice exhibited reductive depletion of NDRG1, a nonsense mutation of human NDRG1
(R148X) is responsible for Charcot-Marie-Tooth disease type 4D (9
). The phenotypes of patients with this disease (10
) and of Ndrg1−/−
mice in peripheral nerves were similar. This suggests that the C-terminal region of NDRG1 may be essential for NDRG1 function.
In conclusion, we found that NDRG1 deficiency leads to a peripheral neuropathy characterized by demyelination, though the initial formation of the myelin sheaths was normal. NDRG1 is abundantly expressed in the cytoplasm of Schwann cells and plays an essential role in maintenance of myelin sheaths. Although the exact molecular functions of NDRG1 are still under investigation, the Ndrg1−/− mouse will be a good model for Charcot-Marie-Tooth disease type 4D and may be used for future analysis of human peripheral nerve neuropathy as well as provide insight into potential therapies.