Ndrg1-deficient mice exhibit a progressive demyelinating disorder of peripheral nerves

Autosomal-recessive forms of demyelinating Charcot-Marie-Tooth disease

Autosomal-recessive forms of Charcot-Marie-Tooth (ARCMT) account for less than 10% of the families in the European CMT population but are more frequent in the Mediterranean basin and the Middle East because of more widespread consanguinity. Until now, demyelinating ARCMT was more extensively studied at the genetic level than the axonal form. Since 1999, the number of localized or identified genes responsible for demyelinating ARCMT has greatly increased. Eight genes, EGR2, GDAP1, KIAA1985, MTMR2, MTMR13, NDRG1, PRX, and CTDP1, have been identified and two new loci mapped to chromosomes 10q23 and 12p11-q13. In this review, we will focus on the particular clinical and/or neuropathological features of the phenotype caused by mutations in each of these genes, which might guide molecular diagnosis.

Pathomechanisms of mutant proteins in Charcot-Marie-Tooth disease

We review the putative functions and malfunctions of proteins encoded by genes mutated in Charcot-Marie-Tooth disease (CMT; inherited motor and sensory neuropathies) in normal and affected peripheral nerves. Some proteins implicated in demyelinating CMT, peripheral myelin protein 22, protein zero (P0), and connexin32 (Cx32/GJB1) are crucial components of myelin. Periaxin is involved in connecting myelin to the surrounding basal lamina. Early growth response 2 (EGR2) and Sox10 are transcriptional regulators of myelin genes. Mutations in the small integral membrane protein of lysosome/late endosome, the myotubularin-related protein 2 (MTMR2), and MTMR13/set-binding factor 2 are involved in vesicle and membrane transport and the regulation of protein degradation. Pathomechanisms related to alterations of these processes are a widespread phenomenon in demyelinating neuropathies because mutations of myelin components may also affect protein biosynthesis, transport, and/or degradation. Related disease mechanisms are also involved in axonal neuropathies although there is considerably more functional heterogeneity. Some mutations, most notably in P0, GJB1, ganglioside-induced differentiation-associated protein 1 (GDAP1), neurofilament light chain (NF-L), and dynamin 2 (DNM2), can result in demyelinating or axonal neuropathies introducing additional complexity in the pathogenesis. Often, this relates to the intimate connection between Schwann cells and neurons/axons leading to axonal damage even if the mutation-caused defect is Schwann-cell-autonomous. This mechanism is likely for P0 and Cx32 mutations and provides the basis for the unifying hypothesis that also demyelinating neuropathies develop into functional axonopathies. In GDAP1 and DNM2 mutants, both Schwann cells and axons/neurons might be directly affected. NF-L mutants have a primary neuronal defect but also cause demyelination. The major challenge ahead lies in determining the individual contributions by neurons and Schwann cells to the pathology over time and to delineate the detailed molecular functions of the proteins associated with CMT in health and disease.

NDRG1-linked Charcot-Marie-Tooth disease (CMT4D) with central nervous system involvement

Charcot-Marie-Tooth disease type 4D (CMT4D) is an autosomal recessive demyelinating polyneuropathy, associated with deafness exclusively found in Gypsies and resulting from a homozygous R148X mutation in the N-myc downstream-regulated gene 1 (NDRG1). We report the detailed phenotypic study of a family without Gypsy ancestry, who presented with severe demyelinating polyneuropathy, deafness, subcortical white matter abnormalities on brain magnetic resonance imaging studies, and the R148X mutation in NDRG1. For the first time, central nervous system white matter lesions are demonstrated in CMT4D. This report extends the clinical knowledge of CMT4D and indicates that the role of the R148X mutation in NDRG1 in the central nervous system should be further studied.

The repression of human differentiation-related gene NDRG2 expression by Myc via Miz-1-dependent interaction with the NDRG2 core promoter

The N-myc downstream-regulated gene 1 (ndrg1) is highly expressed in N-myc knock-out mice through an unknown regulatory mechanism. As one member of the human NDRG gene family, NDRG2 encodes a protein highly homologous to Ndrg1. However, it is uncertain whether the expression of human NDRG2 is regulated by Myc because mouse ndrg2 and -3 are not affected by Myc. In this study, we provide the novel evidence that the expression of human NDRG2 is down-regulated by Myc via transcriptional repression. A high level of NDRG2 was observed as Myc expression was reduced in differentiated cells, whereas a low level of NDRG2 was shown following increased Myc expression upon serum stimulation. The ectopic expression of c-Myc dramatically reduces the cellular Ndrg2 protein and mRNA level. We further identified the core promoter region of NDRG2 that is required for Myc repression on NDRG2 transcription, and we verified the interaction of Myc with the core promoter region both in vitro and in vivo. Moreover, the c-Myc-mediated repression of NDRG2 requires association with Miz-1, and possibly the recruitment of other epigenetic factors, such as histone deacetylases, to the promoter. The regulatory function of Myc on NDRG2 gene expression implicated the role of the Ndrg2 in regulating cell differentiation.

Peripheral myelin maintenance is a dynamic process requiring constant Krox20 expression

Onset of myelination in Schwann cells is governed by several transcription factors, including Krox20/Egr2, and mutations affecting Krox20 result in various human hereditary peripheral neuropathies, including congenital hypomyelinating neuropathy (CHN) and Charcot-Marie-Tooth disease (CMT). Similar molecular information is not available on the process of myelin maintenance. We have generated conditional Krox20 mutations in the mouse that allowed us to develop models for CHN and CMT. In the latter case, specific inactivation of Krox20 in adult Schwann cells results in severe demyelination, involving rapid Schwann cell dedifferentiation and increased proliferation, followed by an attempt to remyelinate and a block at the promyelinating stage. These data establish that Krox20 is not only required for the onset of myelination but that it is also crucial for the maintenance of the myelinating state. Furthermore, myelin maintenance appears as a very dynamic process in which Krox20 may constitute a molecular switch between Schwann cell myelination and demyelination programs.

Tumor growth suppression in pancreatic cancer by a putative metastasis suppressor gene Cap43/NDRG1/Drg-1 through modulation of angiogenesis

Cap43 has been identified as a nickel- and calcium-induced gene, and is also known as N-myc downstream-regulated gene 1 (NDRG1), Drg-1 and rit42. It is also reported that overexpression of Cap43 suppresses metastasis of some malignancies, but its precise role remains unclear. In this study, we asked how Cap43 could modulate the tumor growth of pancreatic cancer. Stable Cap43 cDNA transfectants of pancreatic cancer cells with Cap43 overexpression showed similar growth rates in culture as their control counterparts with low Cap43 protein level. By contrast, Cap43 overexpression showed a marked decrease in tumor growth rates in vivo. Moreover, a marked reduction in tumor-induced angiogenesis was observed. Gelatinolytic activity by matrix metalloproteinase-9 and invasive ability in Matrigel invasion activity were markedly decreased in pancreatic cancer cell lines with high Cap43 expression. Cellular expression of matrix metalloproteinase-9 and two major angiogenic factors, vascular endothelial growth factor and interleukin-8, were also significantly decreased in cell lines with Cap43 overexpression as compared with their parental counterparts. Immunohistochemical analysis of specimens from 65 patients with pancreatic ductal adenocarcinoma showed a significant association between Cap43 expression and tumor microvascular density (P = 0.0001) as well as depth of invasion (P = 0.0003), histopathologic grading (P = 0.0244), and overall survival rates for patients with pancreatic cancer (P = 0.0062). Thus, Cap43 could play a key role in the angiogenic on- or off-switch of tumor stroma in pancreatic ductal adenocarcinoma.

Comparative gene expression analysis reveals a characteristic molecular profile of the superior olivary complex

The superior olivary complex (SOC) is a very conspicuous structure in the mammalian auditory brainstem. It represents the first binaural processing center and is important for sound localization in the azimuth and in feedback regulation of cochlear function. In order to define molecular determinants of the SOC, which are of potential functional relevance, we have performed a comprehensive analysis of its transcriptome by serial analysis of gene expression in adult rats. Here, we performed a detailed analysis of the SOC's gene expression profile compared to that of two other neural tissues, the striatum and the hippocampus, and with extraocular muscle tissue. This tested the hypothesis that SOC-specific or significantly upregulated transcripts provide candidates for the specific function of auditory neurons. Thirty-three genes were significantly upregulated in the SOC when compared to the two other neural tissues. Thirteen encoded proteins involved in neurotransmission, including action potential propagation, exocytosis, and myelination; five genes are important for the energy metabolism, and five transcripts are unknown or poorly characterized and have yet to be described in the nervous system. The comparison of functional gene classes indicates that the SOC has the highest energy demand of the three neural tissues, yet protein turnover is apparently not increased. This suggests a high energy demand for fueling auditory neurotransmission. Such a demand may have implications on auditory-specific tasks and relate to central auditory processing disorders. Ultimately, these data provide new avenues to foster investigations of auditory function and to advance molecular physiology in the central auditory system.

17β-Estradiol Induces Down-Regulation ofCap43/NDRG1/Drg-1, a Putative Differentiation-Related and Metastasis Suppressor Gene, in Human Breast Cancer Cells

PURPOSE

Cap43 is known as a nickel- and calcium-inducible gene. In the present study, we examined whether 17beta-estradiol (E2) could affect the expression of Cap43 in breast cancer.

EXPERIMENTAL DESIGN

Real-time PCR, immunoblotting, and immunocytochemistry were used to examine the expression of Cap43 and estrogen receptor-alpha (ER-alpha) in breast cancer cell lines. MDA-MB-231 and SK-BR-3 cell lines were transfected with ER-alpha cDNA to establish cells overexpressing ER-alpha. Immunohistochemistry was used to evaluate the expression of the Cap43 protein in breast cancer patients (n = 96), and the relationship between Cap43 expression and clinicopathologic findings was examined.

RESULTS

Of the eight cell lines, four expressed higher levels of Cap43 with very low levels of ER-alpha, whereas the other four expressed lower levels of Cap43 with high ER-alpha levels. Treatment with E2 decreased the expression of Cap43 dose-dependently in ER-alpha-positive cell lines but not in ER-alpha-negative lines. Administration of antiestrogens, tamoxifen and ICI 182780, abrogated the E2-induced down-regulation of Cap43. Overexpression of ER-alpha in both ER-alpha-negative cell lines, SK-BR-3 and MDA-MB-231, resulted in down-regulation of Cap43. Immunostaining studies showed a significant correlation between Cap43 expression and the histologic grade of tumors (P = 0.0387). Furthermore, Cap43 expression was inversely correlated with the expression of ER-alpha (P = 0.0374).

CONCLUSIONS

E2-induced down-regulation of Cap43 seems to be mediated through ER-alpha-dependent pathways in breast cancer cells both in culture and in patients. Cap43 has potential as a molecular marker to determine the therapeutic efficacy of antiestrogenic anticancer agents in breast cancer.

N-Myc Down-regulated Gene 1 Modulates the Response of Term Human Trophoblasts to Hypoxic Injury

The placenta is susceptible to diverse insults during human pregnancy. The expression of the protein N-myc down-regulated gene 1 (NDRG1) is regulated during cell proliferation, differentiation, and in response to stress. Nevertheless, the function of this protein in humans remains unknown. We tested the hypothesis that NDRG1 is up-regulated in hypoxic primary human trophoblasts and that NDRG1 modulates trophoblast response to hypoxia. We initially demonstrated that the expression of NDRG1 is enhanced in primary human trophoblasts exposed to hypoxia. Importantly, we found a similar increase in NDRG1 expression in placental samples derived from either singleton gestations complicated by intrauterine growth restriction or from dizygotic twin gestation where one twin exhibited growth restriction. Having established efficient lentivirus-mediated transfection of primary human trophoblasts, we overexpressed NDRG1 in trophoblasts, which resulted in enhanced trophoblast differentiation. In contrast, lentivirus-driven short interfering RNA-mediated silencing of NDRG1 diminished trophoblast viability and differentiation. Consistent with these results, NDRG1 reduced the expression level of p53 in trophoblasts cultured in standard or hypoxic conditions. Furthermore, NDRG1 expression was regulated by the activity of SIRT1 (Sir2-like protein 1), which promotes cell survival. Together, our data indicate that NDRG1 interacts with SIRT1/p53 signaling to attenuate hypoxic injury in human trophoblasts.

Early clinical and electrophysiologic features of the two most common autosomal recessive forms of Charcot-Marie-Tooth disease in the Roma (Gypsies)

Our recent studies of the genetic epidemiology of neuromuscular disorders in Gypsies in Bulgaria have revealed that two private disorders, hereditary motor and sensory neuropathy type Lom and hereditary motor and sensory neuropathy type Russe, account for most cases of Charcot-Marie-Tooth disease in this population. In this study, we examined the clinical and electrophysiologic manifestations of the two disorders in childhood, aiming to identify the distinctive features that allow early differential diagnosis. The study included 13 patients, aged between 2 and 15 years. The childhood clinical manifestations of both neuropathies were similar, although they tended to be more severe in hereditary motor and sensory neuropathy type Lom. The nerve conduction velocities in hereditary motor and sensory neuropathy type Lom were lower than in hereditary motor and sensory neuropathy type Russe. Brainstem auditory evoked potentials were abnormal in hereditary motor and sensory neuropathy type Lom, even at an early age, and normal in hereditary motor and sensory neuropathy type Russe. Although electrophysiologic data provide a more reliable differentiation than clinical data, the definitive diagnosis should rely on genetic testing. (J Child Neurol 2006;21:20-25).

Schwann cells and the pathogenesis of inherited motor and sensory neuropathies (Charcot-Marie-Tooth disease)

Over the last 15 years, a number of mutations in a variety of genes have been identified that lead to inherited motor and sensory neuropathies (HMSN), also called Charcot-Marie-Tooth disease (CMT). In this review we will focus on the molecular and cellular mechanisms that cause the Schwann cell pathologies observed in dysmyelinating and demyelinating forms of CMT. In most instances, the underlying gene defects alter primarily myelinating Schwann cells followed by secondary axonal degeneration. The first set of proteins affected by disease-causing mutations includes the myelin components PMP22, P0/MPZ, Cx32/GJB1, and periaxin. A second group contains the regulators of myelin gene transcription EGR2/Krox20 and SOX10. A third group is composed of intracellular Schwann cells proteins that are likely to be involved in the synthesis, transport and degradation of myelin components. These include the myotubularin-related lipid phosphatase MTMR2 and its regulatory binding partner MTMR13/SBF2, SIMPLE, and potentially also dynamin 2. Mutations affecting the mitochondrial fission factor GDAP1 may indicate an important contribution of mitochondria in myelination or myelin maintenance, whereas the functions of other identified genes, including NDRG1, KIAA1985, and the tyrosyl-tRNA synthase YARS, are not yet clear. Mutations in GDAP1, YARS, and the pleckstrin homology domain of dynamin 2 lead to an intermediate form of CMT that is characterized by moderately reduced nerve conduction velocity consistent with minor myelin deficits. Whether these phenotypes originate in Schwann cells or in neurons, or whether both cell types are directly affected, remains a challenging question. However, based on the advances in systematic gene identification in CMT and the analyses of the function and dysfunction of the affected proteins, crucially interconnected pathways in Schwann cells in health and disease have started to emerge. These networks include the control of myelin formation and stability, membrane trafficking, intracellular protein sorting and quality control, and may extend to mitochondrial dynamics and basic protein biosynthesis.

Pathomechanisms of mutant proteins in Charcot-Marie-Tooth disease

We review the putative functions and malfunctions of proteins encoded by genes mutated in Charcot-Marie-Tooth disease (CMT; inherited motor and sensory neuropathies) in normal and affected peripheral nerves. Some proteins implicated in demyelinating CMT, peripheral myelin protein 22, protein zero (P0), and connexin32 (Cx32/GJB1) are crucial components of myelin. Periaxin is involved in connecting myelin to the surrounding basal lamina. Early growth response 2 (EGR2) and Sox10 are transcriptional regulators of myelin genes. Mutations in the small integral membrane protein of lysosome/late endosome, the myotubularin-related protein 2 (MTMR2), and MTMR13/set-binding factor 2 are involved in vesicle and membrane transport and the regulation of protein degradation. Pathomechanisms related to alterations of these processes are a widespread phenomenon in demyelinating neuropathies because mutations of myelin components may also affect protein biosynthesis, transport, and/or degradation. Related disease mechanisms are also involved in axonal neuropathies although there is considerably more functional heterogeneity. Some mutations, most notably in P0, GJB1, ganglioside-induced differentiation-associated protein 1 (GDAP1), neurofilament light chain (NF-L), and dynamin 2 (DNM2), can result in demyelinating or axonal neuropathies introducing additional complexity in the pathogenesis. Often, this relates to the intimate connection between Schwann cells and neurons/axons leading to axonal damage even if the mutation-caused defect is Schwann-cell-autonomous. This mechanism is likely for P0 and Cx32 mutations and provides the basis for the unifying hypothesis that also demyelinating neuropathies develop into functional axonopathies. In GDAP1 and DNM2 mutants, both Schwann cells and axons/neurons might be directly affected. NF-L mutants have a primary neuronal defect but also cause demyelination. The major challenge ahead lies in determining the individual contributions by neurons and Schwann cells to the pathology over time and to delineate the detailed molecular functions of the proteins associated with CMT in health and disease.

NDRG1 interacts with APO A-I and A-II and is a functional candidate for the HDL-C QTL on 8q24

Hereditary Motor and Sensory Neuropathy Lom (HMSNL) is a severe autosomal recessive peripheral neuropathy, the most common form of demyelinating Charcot-Marie-Tooth (CMT) disease in the Roma (Gypsy) population. The mutated gene, N-myc downstream-regulated gene 1 (NDRG1), is widely expressed and has been implicated in a range of processes and pathways. To gain an insight into NDRG1 function we performed yeast two-hybrid screening and identified interacting proteins whose known functions suggest involvement in cellular trafficking. Further analyses, focusing on apolipoproteins A-I and A-II, confirmed their interaction with NDRG1 in mammalian cells and suggest a defect in Schwann cell lipid trafficking as a major pathogenetic mechanism in HMSNL. At the same time, the chromosomal location of NDRG1 coincides with a reported HDL-C QTL in humans and in mice. A putative role of NDRG1 in the general mechanisms of HDL-mediated cholesterol transport was supported by biochemical studies of blood lipids, which revealed an association between the Gypsy founder mutation, R148X, and decreased HDL-C levels.

Exploitation of KESTREL to identify NDRG family members as physiological substrates for SGK1 and GSK3

We detected a protein in rabbit skeletal muscle extracts that was phosphorylated rapidly by SGK1 (serum- and glucocorticoid-induced kinase 1), but not by protein kinase Ba, and identified it as NDRG2 (N-myc downstream-regulated gene 2). SGK1 phosphorylated NDRG2 at Thr330, Ser332 and Thr348 in vitro. All three residues were phosphorylated in skeletal muscle from wild-type mice, but not from mice that do not express SGK1. SGK1 also phosphorylated the related NDRG1 isoform at Thr328, Ser330 and Thr346 (equivalent to Thr330, Ser332 and Thr348 of NDRG2), as well as Thr356 and Thr366. Residues Thr346, Thr356 and Thr366 are located within identical decapeptide sequences GTRSRSHTSE, repeated three times in NDRG1. These threonines were phosphorylated in NDRG1 in the liver, lung, spleen and skeletal muscle of wild-type mice, but not in SGK1-/- mice. Knock-down of SGK1 in HeLa cells using small interfering RNA also suppressed phosphorylation of the threonine residues in the repeat region of NDRG1. The phosphorylation of NDRG1 by SGK1 transformed it into an excellent substrate for GSK3 (glycogen synthase kinase 3), which could then phosphorylate Ser342, Ser352 and Ser362 in the repeat region. Incubation of HeLa cells with the specific GSK3 inhibitor CT 99021 increased the electrophoretic mobility of NDRG1 in HeLa cells, demonstrating that this protein is phosphorylated by GSK3 in cells. Our results identify NDRG1 and NDRG2 as physiological substrates for SGK1, and demonstrate that phosphorylation of NDRG1 by SGK1 primes it for phosphorylation by GSK3.

NDRG1 is necessary for p53-dependent apoptosis

Although a number of target genes for the tumor suppressor p53 have been described, the mechanism of p53-dependent apoptosis is incompletely understood. Thus, it is essential to identify and characterize additional target genes that could mediate apoptosis. In the study reported here, we isolated a p53-regulated gene named NDRG1 (N-Myc down-regulated gene 1). Its expression is induced by DNA damage in a p53-dependent fashion. The promoter region of the NDRG1 gene contains a p53 binding site that confers p53-dependent transcriptional activation via a heterologous reporter. RNA interference and inducible gene expression approaches suggest that NDRG1 is necessary but not sufficient for p53-mediated caspase activation and apoptosis. This report further supports the notion that p53 controls a network of genes that are required for its apoptotic function.

Function of Drg1/Rit42 in p53-dependent mitotic spindle checkpoint

Mutations in the Drg1/RTP/Rit42 gene are commonly identified in hereditary neuropathies of the motor and sensory systems. This gene was also identified as a p53 target gene and a differentiation-related, putative metastatic suppressor gene in human colon and prostate cancer. In this study, we show that the Rit42 protein is a microtubule-associated protein that localizes to the centrosomes and participates in the spindle checkpoint in a p53-dependent manner. When ectopically expressed and exposed to spindle inhibitors, Rit42 inhibited polyploidy in several p53-deficient tumor cell lines and increased the population of cells in mitotic arrest. Blocking endogenous Rit42 expression by small interfering RNA in normal human mammary epithelial cells resulted in the disappearance of astral microtubules, and dividing spindle fiber formation was rarely detected. Moreover, these cells underwent microtubule inhibitor-induced reduplication, leading to a polyploidy state. Our findings imply that Rit42 plays a role in the regulation of microtubule dynamics and the maintenance of euploidy.