The N-myc downstream-regulated gene (NDRG) family consists of four related proteins, NDRG1-NDRG4, in mammals. We previously generated NDRG1-deficient mice that were unable to maintain myelin sheaths in peripheral nerves. This condition was consistent with human hereditary motor and sensory neuropathy, Charcot-Marie-Tooth disease type 4D, caused by a nonsense mutation of NDRG1. In contrast, the effects of genetic defects of the other NDRG members remain unknown. In this study, we focused on NDRG4, which is specifically expressed in the brain and heart. In situ mRNA hybridization on the brain revealed that NDRG4 was expressed in neurons of various areas. We generated NDRG4-deficient mice that were born normally with the expected Mendelian frequency. Immunochemical analysis demonstrated that the cortex of the NDRG4-deficient mice contained decreased levels of brain-derived neurotrophic factor (BDNF) and normal levels of glial cell line-derived neurotrophic factor, NGF, neurotrophin-3, and TGF-1. Consistent with BDNF reduction, NDRG4-deficient mice had impaired spatial learning and memory but normal motor function in the Morris water maze test. When temporary focal ischemia of the brain was induced, the sizes of the infarct lesions were larger, and the neurological deficits were more severe in NDRG4-deficient mice compared with the control mice. These findings indicate that NDRG4 contributes to the maintenance of intracerebral BDNF levels within the normal range, which is necessary for the preservation of spatial learning and the resistance to neuronal cell death caused by ischemic stress.
N-myc downstream-regulated gene (NDRG)3 family members NDRG1-NDRG4 are intracellular proteins, consist of 340 -394 amino acid residues, and share 53-65% sequence identity with each other. Furthermore, accumulating evidence implicates their roles in development, cancer metastasis, and the immune system (1-5).We originally identified RTP (NDRG1) as a homocysteineresponsive gene in human umbilical vein endothelial cells (6), which is also called DRG1, Cap43, Rit42, Ndr1, and PROXY-1. NDRG1 expression is induced by a number of conditions, such as DNA damage, hypoxia, and intracellular calcium ion elevation (4). Overexpression of NDRG1 suppresses the metastatic potency of some types of cancer cells (4) and enhances the degranulation of mast cells in response to various stimuli (7). A nonsense mutation of NDRG1 causes hereditary motor and sensory neuropathy, Charcot-Marie-Tooth disease type 4D, which presents as distal muscle wasting and atrophy, foot and hand deformities, tendon areflexia, sensory loss, and deafness in afflicted individuals (8). We previously generated NDRG1-deficient mice and revealed the essential role of NDRG1 in the cytoplasm of Schwann cells for the maintenance of myelin sheaths in peripheral nerves (9). A frame shift deletion of Ndrg1 in Greyhounds also causes polyneuropathy (10).Similar to NDRG1, the expression of NDRG2 is induced by stress conditions such as hypoxia (1). NDRG2 expression is upregulated in cort...