The human chromosomal gene for necdin, a neuronal growth suppressor, in the Prader-Willi syndrome deletion region

Deficiency in Prader-Willi syndrome gene necdin leads to attenuated cardiac contractility

Prader-Willi syndrome (PWS) is a genetic disorder characterized by behavioral disturbances, hyperphagia, and intellectual disability. Several surveys indicate that PWS is also associated with cardiac abnormalities, possibly contributing to a high incidence of sudden death. However, the pathological mechanisms underlying cardiac dysfunction in PWS remain unclear. In this study, we found that deficiency in necdin, an intronless gene within PWS region, led to heart systolic and diastolic dysfunction in mice. Through yeast two-hybrid screening, we identified an interaction between necdin and non-muscle myosin regulatory light chain 12a/b (MYL12 A/B). We further showed that necdin stabilized MYL12 A/B via SGT1-heat shock protein 90 (HSP90) chaperone machinery. The zebrafish lacking the MYL12 A/B analog, MYL12.1, exhibited impaired heart function, while cardiac-specific overexpression of MYL12A normalized the heart dysfunction in necdin-deficient mice. Our findings revealed necdin dysfunction as a contributing factor to cardiomyopathy in PWS patients and emphasized the importance of HSP90 chaperone machinery and non-muscle myosin in heart fitness.

Cloning, tissue expression and imprinting status analysis of the NDN gene in Dolang sheep

BACKGROUND

Genomic imprinting refers to expressing parent-specific genes in mammalian diploid cells. The NDN gene is maternally imprinted in humans and mice and correlates with the timing of puberty. This study aimed to investigate its imprinting status and its relationship with the onset of puberty in Dolang sheep.

METHODS AND RESULTS

In this study, cloning and sequencing obtained the NDN gene cDNA sequence of 1082 bp of Dolang sheep, coding for 325 amino acids. Similarity analysis and phylogenetic tree showed that the NDN gene conformed to the law of speciation and was highly conserved among mammals. RT-qPCR results showed the highest expression of NDN mRNA was found in the hypothalamus at puberty, and the expression was significantly increased and then significantly decreased from prepuberty to postpuberty in the hypothalamus, pituitary, and ovary and oviduct. Based on expressed single nucleotide polymorphism (SNP), the NDN gene was expressed monoallelically in the tissues of adult and neonatal umbilical cords, and the expressed allele was paternally inherited. The NDN promoter region of 3400 bp was obtained by cloning and identified in monoallelic-expressing tissues (hypothalamus, ovary, spleen) as a differentially methylated region (DMR).

CONCLUSION

These findings will enrich the number of imprinted genes in sheep and suggest that the NDN gene could be a candidate gene for studying puberty initiation in Dolang sheep.

Necdin: A purposive integrator of molecular interaction networks for mammalian neuron vitality

Necdin was originally found in 1991 as a hypothetical protein encoded by a neural differentiation‐specific gene transcript in murine embryonal carcinoma cells. Virtually all postmitotic neurons and their precursor cells express the necdin gene (Ndn) during neuronal development. Necdin mRNA is expressed only from the paternal allele through genomic imprinting, a placental mammal‐specific epigenetic mechanism. Necdin and its homologous MAGE (melanoma antigen) family, which have evolved presumedly from a subcomplex component of the SMC5/6 complex, are expressed exclusively in placental mammals. Paternal Ndn‐mutated mice totally lack necdin expression and exhibit various types of neuronal abnormalities throughout the nervous system. Ndn‐null neurons are vulnerable to detrimental stresses such as DNA damage. Necdin also suppresses both proliferation and apoptosis of neural stem/progenitor cells. Functional analyses using Ndn‐manipulated cells reveal that necdin consistently exerts antimitotic, anti‐apoptotic and prosurvival effects. Necdin interacts directly with a number of regulatory proteins including E2F1, p53, neurotrophin receptors, Sirt1 and PGC‐1α, which serve as major hubs of protein–protein interaction networks for mitosis, apoptosis, differentiation, neuroprotection and energy homeostasis. This review focuses on necdin as a pleiotropic protein that integrates molecular interaction networks to promote neuronal vitality in modern placental mammals.

Necdin regulates BMAL1 stability and circadian clock through SGT1-HSP90 chaperone machinery

Circadian clocks are endogenous oscillators that control ∼24-hour physiology and behaviors in virtually all organisms. The circadian oscillator comprises interconnected transcriptional and translational feedback loops, but also requires finely coordinated protein homeostasis including protein degradation and maturation. However, the mechanisms underlying the mammalian clock protein maturation is largely unknown. In this study, we demonstrate that necdin, one of the Prader-Willi syndrome (PWS)-causative genes, is highly expressed in the suprachiasmatic nuclei (SCN), the pacemaker of circadian clocks in mammals. Mice deficient in necdin show abnormal behaviors during an 8-hour advance jet-lag paradigm and disrupted clock gene expression in the liver. By using yeast two hybrid screening, we identified BMAL1, the core component of the circadian clock, and co-chaperone SGT1 as two necdin-interactive proteins. BMAL1 and SGT1 associated with the N-terminal and C-terminal fragments of necdin, respectively. Mechanistically, necdin enables SGT1-HSP90 chaperone machinery to stabilize BMAL1. Depletion of necdin or SGT1/HSP90 leads to degradation of BMAL1 through the ubiquitin-proteasome system, resulting in alterations in both clock gene expression and circadian rhythms. Taken together, our data identify the PWS-associated protein necdin as a novel regulator of the circadian clock, and further emphasize the critical roles of chaperone machinery in circadian clock regulation.

Exome sequencing identifies novel NTRK1 mutations in patients with HSAN-IV phenotype

Hereditary sensory autonomic neuropathy type IV (HSAN-IV) is a rare autosomal recessive disorder that usually begins in infancy and is characterized by anhidrosis, insensitivity to noxious stimuli leading to self-mutilating behavior, and intellectual disability. HSAN-IV is caused by mutations in the neurotrophic tyrosine kinase receptor type 1 gene, NTRK1, encoding the high-affinity receptor of nerve growth factor (NGF) which maps to chromosome 1q21-q22. Patients with HSAN-IV lack all NGF-dependent neurons, the primary afferents and sympathetic postganglionic neurons leading to lack of pain sensation and the presence of anhidrosis, respectively. Herein, we report nine patients from nine unrelated families with HSAN-IV due to various mutations in NTRK1, five of which are novel. These are three missense and two nonsense mutations distributed in various domains of NTRK1 involved in binding of NGF. The affected patients had variable intellectual deficits, and some had delayed diagnosis of HSAN-IV. In addition to being the first report of HSAN-IV from the Arabian Peninsula, this report expands the mutational spectrum of patients with NTRK1 mutations and provides further insights for molecular and clinical diagnosis.

Necdin controls EGFR signaling linked to astrocyte differentiation in primary cortical progenitor cells

Cellular signaling mediated by the EGF receptor (EGFR) plays a key role in controlling proliferation and differentiation of cortical progenitor cells (CPCs). However, regulatory mechanisms of EGFR signaling in CPCs remain largely unknown. Here we demonstrate that necdin, a MAGE (melanoma antigen) family protein, interacts with EGFR in primary CPCs and represses its downstream signaling linked to astrocyte differentiation. EGFR was autophosphorylated and interacted with necdin in EGF-stimulated CPCs. Necdin bound to autophosphorylated EGFR via its tyrosine kinase domain. EGF-induced phosphorylation of ERK was enhanced in necdin-null CPCs, where the interaction between EGFR and the adaptor protein Grb2 was strengthened, suggesting that endogenous necdin suppresses the EGFR/ERK signaling pathway in CPCs. In necdin-null CPCs, astrocyte differentiation induced by the gliogenic cytokine cardiotrophin-1 was significantly accelerated in the presence of EGF, and inhibition of EGFR/ERK signaling abolished the acceleration. Furthermore, necdin strongly suppressed astrocyte differentiation induced by overexpression of EGFR or its ligand binding-defective mutant equivalent to a glioblastoma-associated EGFR variant. These results suggest that necdin acts as an intrinsic suppressor of the EGFR/ERK signaling pathway in EGF-responsive CPCs to restrain astroglial development in a cell-autonomous manner.

Novel NTRK1 Frameshift Mutation in Congenital Insensitivity to Pain With Anhidrosis

Congenital insensitivity to pain with anhidrosis is a rare autosomal recessive disorder. It has been reported that the defect in the NTRK1 gene encoding tropomyosin-related kinase A (TrkA) can cause congenital insensitivity to pain with anhidrosis. Nerve growth factor (NGF), the product of NGFB, mediates biological effects by binding to and activating tropomyosin-related kinase A. In addition, necdin (encoded by NDN) is also essential in nerve growth factor-tropomyosin-related kinase A pathway. We performed mutation analysis in NTRK1, NGFB, and NDN genes in a Chinese Han 17-year-old female patient with congenital insensitivity to pain with anhidrosis and her healthy family members. As a result, the patient was found to have a novel insertion in exon 7 (c.727insT) of NTRK1, which causes premature termination, and a single nucleotide polymorphism (rs2192206 G>A) in NDN. Our findings imply that the genetic variations of the nerve growth factor-tropomyosin-related kinase A pathway play an important role in congenital insensitivity to pain with anhidrosis.

Necdin promotes ubiquitin-dependent degradation of PIAS1 SUMO E3 ligase

Necdin, a pleiotropic protein that promotes differentiation and survival of mammalian neurons, is a member of MAGE (melanoma antigen) family proteins that share a highly conserved MAGE homology domain. Several MAGE proteins interact with ubiquitin E3 ligases and modulate their activities. However, it remains unknown whether MAGE family proteins interact with SUMO (small ubiquitin-like modifier) E3 ligases such as PIAS (protein inhibitor of activated STAT) family, Nsmce2/Mms21 and Cbx4/Pc2. In the present study, we examined whether necdin interacts with these SUMO E3 ligases. Co-immunoprecipitation analysis revealed that necdin, MAGED1, MAGEF1 and MAGEL2 bound to PIAS1 but not to Nsmce2 or Cbx4. These SUMO E3 ligases bound to MAGEA1 but failed to interact with necdin-like 2/MAGEG1. Necdin bound to PIAS1 central domains that are highly conserved among PIAS family proteins and suppressed PIAS1-dependent sumoylation of the substrates STAT1 and PML (promyelocytic leukemia protein). Remarkably, necdin promoted degradation of PIAS1 via the ubiquitin-proteasome pathway. In transfected HEK293A cells, amino- and carboxyl-terminally truncated mutants of PIAS1 bound to necdin but failed to undergo necdin-dependent ubiquitination. Both PIAS1 and necdin were associated with the nuclear matrix, where the PIAS1 terminal deletion mutants failed to localize, implying that the nuclear matrix is indispensable for necdin-dependent ubiquitination of PIAS1. Our data suggest that necdin suppresses PIAS1 both by inhibiting SUMO E3 ligase activity and by promoting ubiquitin-dependent degradation.

Genetic diseases: congenital central hypoventilation, Rett, and Prader-Willi syndromes

The present review summarizes current knowledge on three rare genetic disorders of respiratory control, congenital central hypoventilation syndrome (CCHS), Rett syndrome (RTT), and Prader-Willi syndrome (PWS). CCHS is characterized by lack of ventilatory chemosensitivity caused by PHOX2B gene abnormalities consisting mainly of alanine expansions. RTT is associated with episodes of tachypneic and irregular breathing intermixed with breathholds and apneas and is caused by mutations in the X-linked MECP2 gene encoding methyl-CpG-binding protein. PWS manifests as sleep-disordered breathing with apneas and episodes of hypoventilation and is caused by the loss of a group of paternally inherited genes on chromosome 15. CCHS is the most specific disorder of respiratory control, whereas the breathing disorders in RTT and PWS are components of a more general developmental disorder. The main clinical features of these three disorders are reviewed with special emphasis on the associated brain abnormalities. In all three syndromes, disease-causing genetic defects have been identified, allowing the development of genetically engineered mouse models. New directions for future therapies based on these models or, in some cases, on clinical experience are delineated. Studies of CCHS, RTT, and PWS extend our knowledge of the molecular and cellular aspects of respiratory rhythm generation and suggest possible pharmacological approaches to respiratory control disorders. This knowledge is relevant for the clinical management of many respiratory disorders that are far more prevalent than the rare diseases discussed here.

Necdin promotes tangential migration of neocortical interneurons from basal forebrain

Necdin is a pleiotropic protein that promotes neuronal differentiation and survival. In mammals, the necdin gene on the maternal chromosome is silenced by genomic imprinting, and only the paternal necdin gene is expressed in virtually all postmitotic neurons. Necdin forms a complex with the homeodomain protein Dlx2 to enhance its transcriptional activity. Dlx2 plays a major role in controlling tangential migration of GABAergic interneurons from the basal forebrain to the neocortex. Here, we examined whether Dlx2-expressing interneurons migrate properly in vivo in mutant mice lacking the paternal necdin gene. In necdin-deficient mice at birth, the population of Dlx2-expressing cells significantly decreased in the neocortex but increased in the preoptic area. DiI-labeled cell migration assay using organotypic forebrain slice cultures revealed that the number of cells migrating from the medial ganglionic eminence into the neocortex was significantly reduced in necdin-deficient embryos. Furthermore, necdin-deficient mice had a decreased population of neocortical GABA-containing neurons and were highly susceptible to pentylenetetrazole-induced seizures. These results suggest that necdin promotes tangential migration of neocortical GABAergic interneurons during mammalian forebrain development.

Necdin restricts proliferation of hematopoietic stem cells during hematopoietic regeneration

Hematopoietic stem cell (HSC) proliferation is tightly regulated by a poorly understood complex of positive and negative cell-cycle regulatory mechanisms. Necdin (Ndn) is an evolutionally conserved multifunctional protein that has been implicated in cell-cycle regulation of neuronal cells. Here, we provide evidence that necdin plays an important role in restricting excessive HSC proliferation during hematopoietic regeneration. We identify Ndn as being preferentially expressed in the HSC population on the basis of gene expression profiling and demonstrate that mice deficient in Ndn show accelerated recovery of the hematopoietic system after myelosuppressive injury, whereas no overt abnormality is seen in steady-state hematopoiesis. In parallel, after myelosuppression, Ndn-deficient mice exhibit an enhanced number of proliferating HSCs. Based on these findings, we propose that necdin functions in a negative feedback loop that prevents excessive proliferation of HSCs during hematopoietic regeneration. These data suggest that the inhibition of necdin after clinical myelosuppressive treatment (eg, chemotherapy, HSC transplantation) may provide therapeutic benefits by accelerating hematologic recovery.

EBNA3C can modulate the activities of the transcription factor Necdin in association with metastasis suppressor protein Nm23-H1

Previous studies have demonstrated the interaction between the Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) and the metastatic suppressor Nm23-H1 both in vitro and in vivo (C. Subramanian, M. A. Cotter II, and E. S. Robertson, Nat. Med. 7:350-355, 2001). Importantly EBNA3C can reverse the ability of Nm23-H1 to suppress migration of human cells in vitro. EBNA3C contributes to EBV-associated human cancers by regulating transcription of a number of cellular and viral promoters as well as targeting and altering the transcription activities of the metastasis suppressor Nm23-H1. Furthermore, Necdin is a cellular protein which is highly induced in terminally differentiated cells; it contributes to the regulation of cell growth and is also known to interact with viral oncoproteins. In this report, we show that Nm23-H1 and EBNA3C can modulate the biological functions of Necdin in the context of EBV infection and transformation. The levels of Necdin were consistently lower in EBV-positive cells, and EBNA3C could change the subcellular localization of Necdin as well as rescue cells from the antiangiogenic and antiproliferative effects mediated by Necdin. We also show that Necdin directly interacts with Nm23-H1, resulting in modulation of the biochemical function of Nm23-H1 as well as the biological function of Necdin. Both EBNA3C and Nm23-H1 were able to rescue not only Necdin-mediated transcriptional repression of the downstream vascular endothelial growth factor promoter but also Necdin-mediated growth suppression and antiangiogenic effects on cancer cells. The majority of this response was mediated through amino acid residues 191 to 222 of Necdin, which are also known to be important for nuclear matrix targeting. These studies suggest a role for Necdin in the regulation of downstream cellular targets in a hypoxic environment in virus-associated human cancers.

Necdin regulates p53 acetylation via Sirtuin1 to modulate DNA damage response in cortical neurons

Sirtuin1 (Sirt1), a mammalian homolog of yeast Sir2, deacetylates the tumor suppressor protein p53 and attenuates p53-mediated cell death. Necdin, a p53-interacting protein expressed predominantly in postmitotic neurons, is a melanoma antigen family protein that promotes neuronal differentiation and survival. In mammals, the necdin gene (Ndn) is maternally imprinted, and mutant mice carrying mutated paternal Ndn show abnormalities of neuronal development. Here we report that necdin regulates the acetylation status of p53 via Sirt1 to suppress p53-dependent apoptosis in postmitotic neurons. Double-immunostaining analysis demonstrated that necdin colocalizes with Sirt1 in postmitotic neurons of mouse embryonic forebrain in vivo. Coimmunoprecipitation and in vitro binding analyses revealed that necdin interacts with both p53 and Sirt1 to potentiate Sirt1-mediated p53 deacetylation by facilitating their association. Primary cortical neurons prepared from paternal Ndn-deficient mice have high p53 acetylation levels and are sensitive to the DNA-damaging compounds camptothecin and hydrogen peroxide. Moreover, DNA transfection per se increases p53 acetylation and apoptosis in paternal Ndn-deficient neurons, whereas small interfering RNA-mediated p53 knockdown completely blocks these changes. However, Sirt1 knockdown increases both acetylated p53 level and apoptosis in wild-type neurons but fails to affect them in paternal Ndn-deficient neurons. In organotypic forebrain slice cultures treated with hydrogen peroxide, p53 is accumulated and colocalized with necdin and Sirt1 in cortical neurons. These results suggest that necdin downregulates p53 acetylation levels by forming a stable complex with p53 and Sirt1 to protect neurons from DNA damage-induced apoptosis.

cDNA cloning and tissue expression analyses of the encoding regions for three novel porcine genes- MJD1, CDC42 and NECD

The cDNAs for Machado-Joseph disease protein 1 homolog (MJD1), cell division control protein 42 homolog precursor(CDC42) and necdin (NECD) genes of pig were amplified using the reverse transcriptase polymerase chain reaction (RT-PCR) based on the conserved coding sequence information of the MJD1, CDC42, and NECD genes from mouse and other mammals and the referenced porcine EST sequence information. Tissue expression analysis showed the swine MJD1, CDC42, and NECD genes were obviously differentially expressed in different tissues including muscle, heart, liver, backfat, kidney, lung, small intestine, and large intestine. Our experiment established the primary foundation for further research on these three swine genes.

Identification of candidate genes involved in the radiosensitivity of esophageal cancer cells by microarray analysis

Radiotherapy plays a key role in the control of tumor growth in esophageal cancer patients. To identify the patients who will benefit most from radiation therapy, it is important to know the genes that are involved in the radiosensitivity of esophageal cancer cells. Hence, we examined the global gene expression in radiosensitive and radioresistant esophageal squamous cell carcinoma cell lines. Radiosensitivities of 13 esophageal cancer cell lines were measured. RNA was extracted from each esophageal cancer cell line and a normal esophageal epithelial cell line, and the global gene expression profiles were analyzed using a 34 594-spot oligonucleotide microarray. In the clonogenic assay, one cell line (TE-11) was identified to be highly sensitive to radiation, while the other cell lines were found to be relatively radioresistant. We identified 71 candidate genes that were differentially expressed in TE-11 by microarray analysis. The up-regulated genes included CABPR, FABP5, DSC2, GPX2, NME, CBR3, DOCK8, and ABCC5, while the down-regulated genes included RPA1, LDOC1, NDN, and SKP1A. Our investigation provided comprehensive information on genes related to radiosensitivity of esophageal cancer cells; this information can serve as a basis for further functional studies.

Neural control of breathing: insights from genetic mouse models

Recent studies described the in vivo ventilatory phenotype of mutant newborn mice with targeted deletions of genes involved in the organization and development of the respiratory-neuron network. Whole body flow barometric plethysmography is the noninvasive method of choice for studying unrestrained newborn mice. Breathing-pattern abnormalities with apneas occur in mutant newborn mice that lack genes involved in the development and modulation of rhythmogenesis. Studies of deficits in ventilatory responses to hypercapnia and/or hypoxia helped to identify genes involved in chemosensitivity to oxygen and carbon dioxide. Combined studies in mutant newborn mice and in humans have shed light on the pathogenesis of genetically determined respiratory-control abnormalities such as congenital central hypoventilation syndrome, Rett syndrome, and Prader-Willi syndrome. The development of mouse models has opened up the field of research into new treatments for respiratory-control disorders in humans.

Necdin downregulates CDC2 expression to attenuate neuronal apoptosis

The cell cycle-regulatory transcription factor E2F1 induces apoptosis of postmitotic neurons in developmental and pathological situations. E2F1 transcriptionally activates many proapoptotic genes including the cyclin-dependent protein kinase cell division cycle 2 (Cdc2). Necdin is a potent mitotic suppressor expressed predominantly in postmitotic neurons and interacts with E2F1 to suppress E2F1-mediated gene transcription. The necdin gene NDN is maternally imprinted and expressed only from the paternal allele. Deletion of the paternal NDN is implicated in the pathogenesis of Prader-Willi syndrome, a genomic imprinting-associated neurodevelopmental disorder. Here, we show that paternally expressed necdin represses E2F1-dependent cdc2 gene transcription and attenuates apoptosis of postmitotic neurons. Necdin was abundantly expressed in differentiated cerebellar granule neurons (CGNs). Neuronal activity deprivation elevated the expression of both E2F1 and Cdc2 in primary CGNs prepared from mice at postnatal day 6, whereas the necdin levels remained unchanged. In chromatin immunoprecipitation analysis, endogenous necdin was associated with the cdc2 promoter containing an E2F-binding site in activity-deprived CGNs. After activity deprivation, CGNs underwent apoptosis, which was augmented in those prepared from mice defective in the paternal Ndn allele (Ndn(+m/-p)). The levels of cdc2 mRNA, protein, and kinase activity were significantly higher in Ndn(+m/-p) CGNs than in wild-type CGNs under activity-deprived conditions. Furthermore, the populations of Cdc2-immunoreactive and apoptotic cells were increased in the cerebellum in vivo of Ndn(+m/-p) mice. These results suggest that endogenous necdin attenuates neuronal apoptosis by suppressing the E2F1-Cdc2 system.

Epigenetics of autism spectrum disorders

The autism spectrum disorders (ASD) comprise a complex group of behaviorally related disorders that are primarily genetic in origin. Involvement of epigenetic regulatory mechanisms in the pathogenesis of ASD has been suggested by the occurrence of ASD in patients with disorders arising from epigenetic mutations (fragile X syndrome) or that involve key epigenetic regulatory factors (Rett syndrome). Moreover, the most common recurrent cytogenetic abnormalities in ASD involve maternally derived duplications of the imprinted domain on chromosome 15q11-13. Thus, parent of origin effects on sharing and linkage to imprinted regions on chromosomes 15q and 7q suggest that these regions warrant specific examination from an epigenetic perspective, particularly because epigenetic modifications do not change the primary genomic sequence, allowing risk epialleles to evade detection using standard screening strategies. This review examines the potential role of epigenetic factors in the etiology of ASD.

Expression of Drosophila MAGE gene encoding a necdin homologous protein in postembryonic neurogenesis

The MAGE (melanoma antigen) family is characterized by a large conserved domain termed MAGE homology domain. Originally identified MAGE genes encoding tumor rejection antigens are expressed only in cancers and male germ cells. Necdin, which contains the MAGE homology domain, is highly expressed in postmitotic cells such as neurons and skeletal muscle cells. The human necdin gene NDN is transcribed only from the paternal allele through genomic imprinting, and its deficiency is implicated in the pathogenesis of the neurodevelopmental disorder Prader-Willi syndrome. Although over 30 MAGE genes have been identified in humans, fruit fly (Drosophila melanogaster) has only a single MAGE gene that encodes a protein similar to necdin homologous MAGE proteins. In this study, we analyzed the spatiotemporal expression patterns of MAGE mRNA and the encoded protein during fly development. Whole-mount embryo in situ hybridization analysis revealed that MAGE mRNA was highly expressed at the syncytial blastoderm stage and in the ventral and procephalic neurogenic regions of the ectoderm during gastrulation. In contrast, MAGE expression was nearly undetectable in postmitotic neurons of the central nervous system at late embryonic stages. During postembryonic neurogenesis, MAGE was highly expressed in neural stem cells (neuroblasts) and their progeny (ganglion mother cells and postmitotic neurons) at larval and pupal stages. MAGE was also expressed in postmitotic neurons including mushroom body neurons and retinal photoreceptors in adulthood. These results indicate that MAGE expression lasts throughout the postembryonic neurogenesis in Drosophila.

Necdin promotes GABAergic neuron differentiation in cooperation with Dlx homeodomain proteins

Necdin, a member of the MAGE (melanoma antigen) protein family, is expressed predominantly in terminally differentiated neurons. The necdin gene NDN is maternally imprinted and expressed only from the paternal allele, the deficiency of which is implicated in the pathogenesis of the neurodevelopmental disorder Prader-Willi syndrome. Necdin binds to its homologous MAGE protein MAGE-D1 (also known as NRAGE or Dlxin-1), which interacts with Msx (msh homeobox) and Dlx (distal-less homeobox) family homeodomain transcription factors. Members of the Dlx homeobox gene family are involved in the differentiation and specification of forebrain GABAergic neurons. Here we demonstrate that necdin associates with Dlx homeodomain proteins via MAGE-D1 to promote the differentiation of GABAergic neurons in mouse embryonic forebrain. Immunohistochemical analysis revealed that necdin was coexpressed with Dlx2, Dlx5, or MAGE-D1 in a subpopulation of embryonic forebrain cells. Necdin bound to Dlx2 and Dlx5 via MAGE-D1 and enhanced Dlx2-dependent activation of the Wnt1 (wingless-type MMTV integration site family) promoter. Necdin significantly increased the populations of cells expressing the GABAergic neuron markers calbindin D-28k and glutamic acid decarboxylase when overexpressed by electroporation in cultured forebrain slices. In this assay, Dlx5N, a truncated Dlx5 mutant that competes with Dlx2 to bind MAGE-D1, diminished the effect of necdin on GABAergic neuron differentiation. Furthermore, mutant mice lacking the paternal necdin allele showed a significant reduction in the differentiation of forebrain GABAergic neurons in vivo and in vitro. These results suggest that paternally expressed necdin facilitates the differentiation and specification of GABAergic neurons in cooperation with Dlx homeodomain proteins.

Disruption of the paternal necdin gene diminishes TrkA signaling for sensory neuron survival

Necdin is a multifunctional signaling protein that stabilizes terminal differentiation of postmitotic neurons. The human necdin gene in chromosome 15q11-q12 is maternally imprinted, paternally transcribed, and not expressed in Prader-Willi syndrome, a human genomic imprinting-associated neurodevelopmental disorder. Although necdin-deficient mice display several abnormal phenotypes reminiscent of this syndrome, little is known about molecular mechanisms that lead to the neurodevelopmental defects. Here, we demonstrate that paternally expressed necdin is required for physiological development of nerve growth factor (NGF)-dependent sensory neurons. Mouse embryos defective in the paternal necdin allele displayed absent necdin expression in the dorsal root ganglia, in which the tropomyosin-related kinase A (TrkA) receptor tyrosine kinase and the p75 neurotrophin receptor were expressed in a normal manner. Necdin interacted with both TrkA and p75 to facilitate the association between these receptors. NGF-induced phosphorylation of TrkA and mitogen-activated protein kinase was significantly diminished in the necdin-null sensory ganglia. Furthermore, the mice lacking the paternal necdin allele displayed augmented apoptosis in the sensory ganglia in vivo and had a reduced population of substance P-containing neurons. These mutant mice showed significantly high tolerance to thermal pain, which is often seen in individuals with Prader-Willi syndrome. These results suggest that paternally expressed necdin facilitates TrkA signaling to promote the survival of NGF-dependent nociceptive neurons.

Differential gene expression profiling of human umbilical cord blood-derived mesenchymal stem cells by DNA microarray

Mesenchymal stem cells (MSCs) retain both self-renewal and multilineage differentiation capabilities. Despite wide therapeutic potential, many aspects of human MSCs, particularly the molecular parameters to define the stemness, remain largely unknown. Using high-density oligonucleotide micro-arrays, we obtained the differential gene expression profile between a fraction of mononuclear cells of human umbilical cord blood (UCB) and its MSC subpopulation. Of particular interest was a subset of 47 genes preferentially expressed at 50-fold or higher in MSCs, which could be regarded as a molecular foundation of human MSCs. This subset contains numerous genes encoding collagens, other extracellular matrix or related proteins, cytokines or growth factors, and cytoskeleton-associated proteins but very few genes for membrane and nuclear proteins. In addition, a direct comparison of this microarray-generated transcriptome with the published serial analysis of gene expression data suggests that a molecular context of UCB-derived MSCs is more or less similar to that of bone marrow-derived cells. Altogether, our results will provide a basis for studies on molecular mechanisms controlling core properties of human MSCs.

Biological Activities Encoded by the Murine Mesenchymal Stem Cell Transcriptome Provide a Basis for Their Developmental Potential and Broad Therapeutic Efficacy

We used serial analysis of gene expression to catalog the transcriptome of murine mesenchymal stem cells (MSCs) enriched from bone marrow by immunodepletion. Interrogation of this database, results of which are delineated in the appended databases, revealed that immunodepleted murine MSCs (IDmMSCs) highly express transcripts encoding connective tissue proteins and factors modulating T-cell proliferation, inflammation, and bone turnover. Categorizing the transcriptome based on gene ontologies revealed the cells also expressed mRNAs encoding proteins that regulate mesoderm development or that are characteristic of determined mesenchymal cell lineages, thereby reflecting both their stem cell nature and differentiation potential. Additionally, IDmMSCs also expressed transcripts encoding proteins regulating angiogenesis, cell motility and communication, hematopoiesis, immunity and defense as well as neural activities. Immunostaining and fluorescence-activated cell sorting analysis revealed that expression of various regulatory proteins was restricted to distinct subpopulations of IDmMSCs. Moreover, in some cases, these proteins were absent or expressed at reduced levels in other murine MSC preparations or cell lines. Lastly, by comparing their transcriptome to that of 17 other murine cell types, we also identified 43 IDmMSC-specific transcripts, the nature of which reflects their varied functions in bone and marrow. Collectively, these results demonstrate that IDmMSC express a diverse repertoire of regulatory proteins, which likely accounts for their demonstrated efficacy in treating a wide variety of diseases. The restricted expression pattern of these proteins within populations suggests that the cellular composition of marrow stroma and its associated functions are more complex than previously envisioned.

Chromatin modification of the human imprinted NDN (necdin) gene detected by in vivo footprinting

Allele-specific transcription is a characteristic feature of imprinted genes. Many imprinted genes are also transcribed in a tissue- or cell type-specific manner. Overlapping epigenetic signals must, therefore, modulate allele-specific and tissue-specific expression at imprinted loci. In addition, long-range interactions with an Imprinting Center (IC) may influence transcription, in an allele-specific or cell-type specific manner. The IC on human chromosome 15q11 controls parent-of-origin specific allelic identity of a set of genes located in cis configuration within 2 Mb. We have now examined the chromatin accessibility of the promoter region of one of the Imprinting Centre-controlled genes, NDN encoding necdin, using in vivo DNA footprinting to identify sites of DNA-protein interaction and altered chromatin configuration. We identified sites of modified chromatin that mark the parental alleles in NDN-expressing cells, and in cells in which NDN is not expressed. Our results suggest that long-lasting allele-specific marks and more labile tissue-specific marks layer epigenetic information that can be discriminated using DNA footprinting methodologies. Sites of modified chromatin mark the parental alleles in NDN-expressing cells, and in cells in which NDN is not expressed. Our results suggest that a layering of epigenetic information controls allele- and tissue-specific gene expression of this imprinted gene.

Functional domains of necdin for protein-protein interaction, nuclear matrix targeting, and cell growth suppression

Necdin is a growth suppressor expressed predominantly in postmitotic neurons. The necdin gene is involved in the etiology of the genomic imprinting-associated neurodevelopmental disorder Prader-Willi syndrome and belongs to the MAGE gene family. All the MAGE family proteins contain a large homology domain termed the MAGE homology domain (MHD). We here characterize the regions of necdin required for the protein-protein interaction, nuclear matrix targeting, and cell growth suppression. The region including entire MHD (amino acids 116-280) of necdin was required for its interaction with p53, while the regions amino acids 144-184 and 191-222 within the MHD were required for both the nuclear matrix targeting and the cell growth suppression of osteosarcoma SAOS-2 cells. The amino-terminal proline-rich acidic region (amino acids 60-100) was also necessary for cell growth suppression. Tetracycline-regulatable overexpression of necdin induced growth arrest of SAOS-2 cells in a reversible manner, and the necdin-overexpressing cells showed a large, flattened morphology with double nuclei. In contrast, a necdin mutant lacking amino acids 191-222 did not induce such changes. These findings suggest that different functions of necdin are mediated via its distinct domains.

Biological functions of melanoma-associated antigens

To date, dozens of melanoma-associated antigens (MAGEs) have been identified and classified into 2 subgroups, I and II. Subgroup I consists of antigens which expression is generally restricted to tumor or germ cells, also named as cancer/testis (CT) antigen. Proteins and peptides derived from some of these antigens have been utilized in promising clinical trials of immunotherapies for gastrointestinal carcinoma, esophageal carcinoma, pulmonary carcinoma and so on. Various MAGE family members play important physiological and pathological roles during embryogenesis, germ cell genesis, apoptosis, etc. However, little is known regarding the role of MAGE family members in cell activities. It is reasonable to speculate that the genes for subgroup I MAGEs, which play important roles during embryogenesis, could be later deactivated by a genetic mechanism such as methylation. In the case of tumor formation, these genes are reactivated and the resultant proteins may be recognized and attacked by the immune system. Thus, the subgroup I MAGEs may play important roles in the immune surveillance of certain tumor types. Here, we review the classifications of MAGE family genes and what is known of their biological functions.

Gene expression profiling in glomeruli from human kidneys with diabetic nephropathy

BACKGROUND

Diabetic nephropathy (DN) is a frequent complication in patients with diabetes mellitus. To find improved intervention strategies in this disease, it is necessary to investigate the molecular mechanisms involved. To obtain more insight into processes that lead to DN, messenger RNA expression profiles of diabetic glomeruli and glomeruli from healthy individuals were compared.

METHODS

Two morphologically normal kidneys and 2 kidneys from patients with DN were used for the study. Glomerular RNA was hybridized in duplicate on Human Genome U95Av2 Arrays (Affymetrix, Santa Clara, CA). Several transcripts were tested further in independent patient groups and at the protein level by immunohistochemistry.

RESULTS

Ninety-six genes were upregulated in diabetic glomeruli, whereas 519 genes were downregulated. The list of overexpressed genes in DN includes aquaporin 1, calpain 3, hyaluronoglucosidase, and platelet/endothelial cell adhesion molecule. The list of downregulated genes includes bone morphogenetic protein 2, vascular endothelial growth factor (VEGF), fibroblast growth factor 1, insulin-like growth factor binding protein 2, and nephrin. A decrease in VEGF and nephrin could be validated at the protein level and also at the RNA level in renal biopsy specimens from 5 additional patients with diabetes.

CONCLUSION

Results of oligonucleotide microarray analyses on control and diabetic glomeruli are presented and discussed in their relation to vascular damage, mesangial matrix expansion, proliferation, and proteinuria. Our findings suggest that progression of DN might result from diminished tissue repair capability.

Developmental stage-specific epigenetic control of human beta-globin gene expression is potentiated in hematopoietic progenitor cells prior to their transcriptional activation

To study epigenetic regulation of the human beta-globin locus during hematopoiesis, we investigated patterns of histone modification and chromatin accessibility along this locus in hematopoietic progenitor cells (HPCs) derived from both humans and transgenic mice. We demonstrate that the developmentally related activation of human beta-like globin genes in humans and transgenic mice HPCs is preceded by a wave of gene-specific histone H3 hyperacetylation and K4 dimethylation. In erythroid cells, expression of beta-like globin genes is associated with histone hyperacetylation along these genes and, surprisingly, with local deacetylation at active promoters. We also show that endogenous mouse beta major and human beta-like genes are subject to different epigenetic control mechanisms in HPCs. This difference is likely due to intrinsic properties of the human beta-globin locus since, in transgenic mice, this locus is epigenetically regulated in the same manner as in human HPCs. Our results suggest that a defined pattern of histone H3 acetylation/dimethylation is important for specific activation of human globin promoters during development in human and transgenic HPCs. We propose that this transient acetylation/dimethylation is involved in gene-specific potentiation in HPCs (ie, before extensive chromatin remodeling and transcription take place in erythroid cells).

Necdin-related MAGE proteins differentially interact with the E2F1 transcription factor and the p75 neurotrophin receptor

Necdin is a growth suppressor expressed predominantly in postmitotic neurons and implicated in their terminal differentiation. Necdin shows a moderate homology to the MAGE family proteins, the functional roles of which are largely unknown. Human genes encoding necdin, MAGEL2 (necdin-like 1), and MAGE-G1 (necdin-like 2) are located in proximal chromosome 15q, a region associated with neurodevelopmental disorders such as Prader-Willi syndrome, Angelman syndrome, and autistic disorder. The necdin and MAGEL2 genes are subjected to genomic imprinting and suggested to be involved in the etiology of Prader-Willi syndrome. In this study, we compared biochemical and functional characteristics of murine orthologs of these necdin-related MAGE proteins. The colony formation and bromodeoxyuridine incorporation analyses revealed that necdin and MAGE-G1, but not MAGEL2, induced growth arrest. Necdin and MAGE-G1 interacted with the transcription factor E2F1 via its transactivation domain, repressed E2F1-dependent transcription, and antagonized E2F1-induced apoptosis of N1E-115 neuroblastoma cells. In addition, necdin and MAGE-G1 interacted with the p75 neurotrophin receptor via its distinct intracellular domains. In contrast, MAGEL2 failed to bind to these necdin interactors, suggesting that MAGEL2 has no necdin-like function in developing brain. Overexpression of p75 translocated necdin and MAGE-G1 in the proximity of the plasma membrane and reduced their association with E2F1 to facilitate E2F1-induced death of neuroblastoma cells. These results suggest that necdin and MAGE-G1 target both E2F1 and p75 to regulate cell viability during brain development.

Epigenetic silencing of multiple interferon pathway genes after cellular immortalization

Abrogating cellular senescence is a necessary step in the formation of a cancer cell. Promoter hypermethylation is an epigenetic mechanism of gene regulation known to silence gene expression in carcinogenesis. Treatment of spontaneously immortal Li-Fraumeni fibroblasts with 5-aza-2'-deoxycytidine (5AZA-dC), an inhibitor of DNA methyltransferase (DNMT), induces a senescence-like state. We used microarrays containing 12 558 genes to determine the gene expression profile associated with cellular immortalization and also regulated by 5AZA-dC. Remarkably, among 85 genes with methylation-dependent downregulation (silencing) after immortalization, 39 (46%) are known to be regulated during interferon signaling, a known growth-suppressive pathway. This work indicates that gene silencing may be associated with an early event in carcinogenesis, cellular immortalization.

Molecular characterization of a metastatic neuroendocrine cell cancer arising in the prostates of transgenic mice

The features and functions of prostatic neuroendocrine (NE) cells remain ill-defined. Neuroendocrine differentiation (NED) in adenocarcinoma of the human prostate (CaP) is associated with more aggressive disease, but the underlying mediators are poorly understood. We examined these issues in transgenic mice that utilize regulatory elements from the cryptdin-2 gene (Defcr2) to express simian virus 40 large T antigen (TAg) in prostatic NE cells. CR2-TAg mice develop prostatic intraepithelial neoplasia at 8 weeks of age, 1 week after the onset of TAg expression. An invasive phase follows 2-4 weeks later, with lymph node, liver, lung, brain, and bone metastases appearing within 16 weeks. DNA microarray studies revealed 122 mRNAs that were increased >/=2-fold in duplicate assays of 16-week-old CR2-TAg versus normal prostates. Thirty two transcripts encode proteins associated with neurons and endocrine cells (e.g. basic helix loop helix, SRY-related high mobility group box and sine-oculis homeobox transcription factors, Hu RNA-binding proteins, neuronatin, Racgap1, collapsin response mediator protein-1, synaptotagmin-1, proprotein convertase, and secretogranins). Follow-up studies of candidate mediators and biomarkers of differentiation/growth in the microarray data set involved real time quantitative reverse transcriptase-PCR assays of laser capture microdissected NE cells from CR2-TAg prostates plus liver metastases, and immunohistochemical comparisons of transgenic mouse prostates and 35 human CaP samples. Our findings include (a) expression of the bHLH mouse achaete-scute homolog (mASH1) in normal and CR2-TAg NE cells and foci of NED in human CaP, (b) glutamic acid decarboxylase and its product (gamma-aminobutyric acid) in neoplastic NE cells juxtaposed next to cohorts of normal gamma-aminobutyric acid receptor expressing secretory cells (a potential route for paracrine interactions between these two epithelial lineages), and (c) aromatic l-amino-acid decarboxylase, but not its dopamine/serotonin products, in CR2-TAg NE cells and NED. These results underscore the value of CR2-TAg mice for characterizing normal NE cell biology and tumorigenesis.

Ectopic expression of necdin induces differentiation of mouse neuroblastoma cells

Necdin is expressed predominantly in postmitotic neurons, and ectopic expression of this protein strongly suppresses cell growth. Necdin has been implicated in the pathogenesis of Prader-Willi syndrome, a human neurodevelopmental disorder associated with genomic imprinting. Here we demonstrate that ectopic expression of necdin induces a neuronal phenotype in neuroblastoma cells. Necdin was undetectable in mouse neuroblastoma N1E-115 cells under undifferentiated and differentiated conditions. N1E-115 cells transfected with necdin cDNA showed morphological differentiation such as neurite outgrowth and expression of the synaptic marker proteins synaptotagmin and synaptophysin. In addition, Western blot analysis of the retinoblastoma protein (Rb) family members Rb, p130, and p107 revealed that necdin cDNA transfectants contained an increased level of p130 and a reduced level of p107, a pattern seen in differentiated G(0) cells. The transcription factors E2F1 and E2F4 physically interacted with necdin via their carboxyl-terminal transactivation domains, but only E2F1 abrogated necdin-induced growth arrest and neurite outgrowth of neuroblastoma cells. Overexpression of E2F1 in differentiated N1E-115 cells induced apoptosis, which was antagonized by co-expression of necdin. These results suggest that necdin promotes the differentiation and survival of neurons through its antagonistic interactions with E2F1.

Necdin is required for terminal differentiation and survival of primary dorsal root ganglion neurons

Necdin is expressed predominantly in postmitotic neurons and serves as a growth suppressor that is functionally similar to the retinoblastoma tumor suppressor protein. Using primary cultures of dorsal root ganglion (DRG) of mouse embryos, we investigated the involvement of necdin in the terminal differentiation of neurons. DRG cells were prepared from mouse embryos at 12.5 days of gestation and cultured in the presence of nerve growth factor (NGF). Immunocytochemistry revealed that necdin accumulated in the nucleus of differentiated neurons that showed neurite extension and expressed the neuronal markers microtubule-associated protein 2 and synaptophysin. Suppression of necdin expression in DRG cultures treated with antisense oligonucleotides led to a marked reduction in the number of terminally differentiated neurons. The antisense oligonucleotide-treated cells did not attempt to reenter the cell cycle, but underwent death with characteristics of apoptosis such as caspase-3 activation, nuclear condensation, and chromosomal DNA fragmentation. Furthermore, a caspase-3 inhibitor rescued antisense oligonucleotide-treated cells from apoptosis and significantly increased the population of terminally differentiated neurons. These results suggest that necdin mediates the terminal differentiation and survival of NGF-dependent DRG neurons and that necdin-deficient nascent neurons are destined to caspase-3-dependent apoptosis.

Cancer/testis antigens: structural and immunobiological properties

Characterization of tumor-associated antigens recognized by cytotoxic T lymphocytes which has evolved during recent years opens new possibilities for specific anti-cancer immunotherapy. Among different groups of tumor-associated antigens, cancer/testis (CT) antigens (expressed in many tumors and among normal tissues only in testes) represent the most perspective antigens for immunotherapy because of their broad tumor-specific expression. More than 50 CT antigens have been described so far and, for many of them, epitopes recognized by T lymphocytes have been identified. The most studied group of CT antigens is the MAGE proteins, which form the so-called MAGE superfamily, together with some MAGE-like proteins that have a different distribution than classical CT antigens. The MAGE superfamily includes five families: MAGE-A, MAGE-B, MAGE-C, MAGE-D, and necdin. Comparison of the structure of members of MAGE superfamily points to the existence of a domain organization of these proteins. The central, core domain (second domain) is highly conservative. The first domain is homologous among MAGE family members with a CT expression, but unique for each member of the MAGE-D and necdin families. In addition to the homology of the central domain, the third domain is also homologous among all members of MAGE superfamily, but to a much lesser extent. The MAGE-D proteins contain an additional, fourth domain, which in the case of MAGE-D3 coincides with trophinin, a separate molecule described previously as an adhesion molecule that participates in embryo implantation. The structural classification of the members of MAGE superfamily might help in the future to understand the biological function of MAGE proteins. One important property of the CT antigens is the up-regulation of their expression by DNA demethylating agents, indicating a possible mechanism for their re-expression in tumors. One of the implications of this particular property could be that a combination of immunotherapy targeting CT antigens with chemotherapy inducing up-regulation of CT antigens might result in more efficient tumor eradication.

Necdin acts as a transcriptional repressor that interacts with multiple guanosine clusters

Necdin is a growth suppressor expressed predominantly in postmitotic neurons, and ectopic expression of this protein suppresses cell growth. Here we report that Necdin directly binds to specific DNA sequences and serves as a transcriptional repressor. Polyhistidine-tagged Necdin was used for selection of random-sequence oligonucleotides by polymerase chain reaction-based amplification. Necdin recognized guanosine (G)-rich sequences that encompass multiple G clusters and intervening mono- or di-nucleotides of A, T and C. These sequences, termed GN boxes, resemble multiply aligned forms of the canonical GC box which is recognized by Sp family members. Necdin directly bound to a GN box consisting of contiguous two GC boxes with four G clusters, but not to a single GC box with two G clusters, whereas Sp1 bound to both. In a reporter system using Drosophila Schneider Line 2 cells, Necdin repressed Sp1-dependent activity of mouse c-myc P1 promoter that contains a typical GN box. Deletion of the GN box from the c-myc P1 promoter or its conversion to the single GC box abolished the Necdin-dependent repression. These results suggest that Necdin modulates gene transcription via the GN box that is potentially recognized by GC box-targeting Sp family members.

Systematic screening for mutations in the human necdin gene (NDN): identification of two naturally occurring polymorphisms and association analysis in body weight regulation

BACKGROUND

NDN, which codes for the human necdin protein, is a candidate gene for Prader-Willi syndrome (PWS). One feature of this neurogenetic disorder is hyperphagia resulting in extreme obesity observed later in development.

OBJECTIVE AND DESIGN

In this study we have used single-strand conformation polymorphism (SSCP) analysis to identify sequence variants at the human necdin gene. Furthermore we tested whether these variants were associated with obesity in extremely obese German children and adolescents.

RESULTS

Two gene variants could be identified: a g.1352T-->C polymorphism in the putative promotor region and a silent g.2311C-->T polymorphism in the coding region. Genotype and allele frequency distribution of both of the polymorphisms were not significantly different between lower and higher body mass index (BMI) subjects.

CONCLUSIONS

Hence, it is unlikely that these polymorphisms play a major role in the emergence of juvenile onset human obesity.

MAGE-F1, a novel ubiquitously expressed member of the MAGE superfamily

Most known members of the MAGE superfamily are expressed in tumors, testis and fetal tissues, which has been described as a cancer/testis or "CT" expression pattern. We have identified a novel member of this superfamily, MAGE-F1, which is expressed in all adult and fetal tissues tested. In addition to normal tissues, MAGE-F1 is expressed in many tumor types including ovarian, breast, cervical, melanoma and leukemia. MAGE-F1 is encoded on chromosome 3, identifying a sixth chromosomal location for a MAGE superfamily gene. The coding region of MAGE-F1 is contained within a single exon and includes a microsatellite repeat. Sequence analysis and expression profiles define a new class of ubiquitously expressed MAGE superfamily genes that includes MAGE-F1, MAGE-D1, MAGE-D2/JCL-1 and NDN. The finding that several MAGE genes are ubiquitously expressed suggests a role for MAGE encoded proteins in normal cell physiology. Furthermore, potential cross-reactivity to these ubiquitously expressed MAGE gene products should be considered in the design of MAGE-targeted immunotherapies for cancer.

The postmitotic growth suppressor necdin interacts with a calcium-binding protein (NEFA) in neuronal cytoplasm

Necdin, a growth suppressor expressed predominantly in postmitotic neurons, interacts with viral oncoproteins and cellular transcription factors E2F1 and p53. In search of other cellular targets of necdin, we screened cDNA libraries from neurally differentiated murine embryonal carcinoma P19 cells and adult rat brain by the yeast two-hybrid assay. We isolated cDNAs encoding partial sequences of mouse NEFA and rat nucleobindin (CALNUC), which are Ca(2+)-binding proteins possessing similar domain structures. Necdin interacted with NEFA via a domain encompassing two EF hand motifs, which had Ca(2+) binding activity as determined by (45)Ca(2+) overlay. NEFA was widely distributed in mouse organs, whereas necdin was expressed predominantly in the brain and skeletal muscle. In mouse brain in vivo, NEFA was localized in neuronal perikarya and dendrites. By immunoelectron microscopy, NEFA was localized to the cisternae of the endoplasmic reticulum and nuclear envelope in brain neurons. NEFA-green fluorescent protein (GFP) fusion protein expressed in neuroblastoma N1E-115 cells was retained in the cytoplasm and partly secreted into the culture medium. Necdin enhanced the cytoplasmic retention of NEFA-GFP and potentiated the effect of NEFA-GFP on caffeine-evoked elevation of cytosolic Ca(2+) levels. Thus, necdin and NEFA might be involved in Ca(2+) homeostasis in neuronal cytoplasm.

Cell cycle regulators in neural stem cells and postmitotic neurons

In the mammalian central nervous system, neurons withdraw from the cell cycle immediately after their differentiation from proliferative neuroepithelial cells. Even while postmitotic neurons remain in permanent mitotic quiescence, they express a number of cell cycle regulators required for cell cycle progression. This review focuses on the expression and functions of members of the retinoblastoma protein (Rb) family (Rb, p107, p130) and necdin, all of which are growth suppressors that interact with the viral oncoproteins and the E2F family proteins. These molecules are differentially expressed in proliferative neural progenitors and postmitotic neurons in the developing neuroepithelium in vivo and differentiating embryonal carcinoma cells in vitro. During neurogenesis, dysfunction of the Rb family proteins causes impaired neuronal differentiation accompanied by cell death (apoptosis). Thus, the Rb family proteins are essential for both terminal mitosis of neuronal progenitors and survival of nascent neurons. However, the Rb family proteins seem to be dispensable for the maintenance of the postmitotic state of terminally differentiated neurons. Necdin is expressed exclusively in postmitotic cells and may contribute to their permanent mitotic arrest. These cell cycle regulators coordinately act in the generation, survival and demise of postmitotic neurons.

Characterization and chromosomal mapping of a human Necdin pseudogene

The necdin gene is expressed predominantly in postmitotic neurons and encodes a growth suppressor that interacts with the transcription factors E2F1 and p53. Human necdin gene (NDN) is maternally imprinted and located in Prader-Willi syndrome deletion region 15q11.2-q12. We isolated an NDN homologous sequence from a human genomic DNA library. The homologous sequence is overall 83% identical with necdin cDNA sequence, and possesses a short poly(A) stretch at the 3' end and direct repeats at both ends. Expression of the homologous sequence, which lacks a 5' promoter sequence, was undetected in cultured human cell lines. We mapped this sequence to chromosome 12q14-q21.1 by fluorescence in situ hybridization. These characteristics of the NDN-homologous sequence are consistent with those of processed pseudogenes. The information about the necdin pseudogene in the human genome will be useful for genetic studies on NDN-associated neurogenic disorders.

Disruption of the mouse necdin gene results in early post-natal lethality

Prader-Willi syndrome (PWS) is a neurobehavioural disorder characterized by neonatal respiratory depression, hypotonia and failure to thrive in infancy, followed by hyperphagia and obesity among other symptoms. PWS is caused by the loss of one or more paternally expressed genes on chromosome 15q11-q13, which can be due to gene deletions, maternal uniparental disomy or mutations disrupting the imprinting mechanism. Imprinted genes mapped to this region include SNRPN (refs 3,4), ZNF127 (ref. 5), IPW (ref. 6) and NDN (which encodes the DNA-binding protein necdin; refs 7,8,9,10). The mouse homologues of these genes map to mouse chromosome 7 in a region syntenic with human chromosome 15q11-q13 (refs 7,11). Imprinting of the human genes is under the control of an imprinting center (IC), a long-range, cis-acting element located in the 5' region of SNRPN (ref. 12). A related control element was isolated in the mouse Snrpn genomic region which, when deleted on the paternally inherited chromosome, resulted in the loss of expression of all four genes and early post-natal lethality. To determine the possible contribution of Ndn to the PWS phenotype, we generated Ndn mutant mice. Heterozygous mice inheriting the mutated maternal allele were indistinguishable from their wild-type littermates. Mice carrying a paternally inherited Ndn deletion allele demonstrated early post-natal lethality. This is the first example of a single gene being responsible for phenotypes associated with PWS.

Prader Willi and Angelman syndromes: exemplars of genomic imprinting

The molecular phenomenon genomic imprinting provides an explanation for why two clinically distinct syndromes share genetic etiologies. Increased understanding of genomic imprinting is affecting diagnostics. Use of improved diagnostic tests can enable early, syndrome-specific, and anticipatory interventions and consequently, improved quality of life; however, these tests are of little use unless clinicians are able to identify at-risk patients. Nurses knowledgeable about Prader Willi and Angelman syndromes and their associated genetic mechanisms can play a significant role in early identification, referral, and intervention of patients with these conditions.

Physical and functional interactions of neuronal growth suppressor necdin with p53

Necdin is expressed in virtually all postmitotic neurons, and ectopic expression of this protein suppresses cell proliferation. Necdin, like the retinoblastoma protein, interacts with cell cycle promoting proteins such as simian virus 40 large T antigen, adenovirus E1A, and the transcription factor E2F1. Here we demonstrate that necdin interacts with the tumor suppressor protein p53 as well. The yeast two-hybrid and in vitro binding analyses revealed that necdin bound to a narrow region (amino acids 35-62) located between the MDM2-binding site and the proline-rich region in the amino-terminal domain of p53. The electrophoretic mobility shift assay showed that necdin supershifted a complex between p53 and its binding DNA, implying that the p53-necdin complex is competent for DNA binding. In p53-deficient osteosarcoma SAOS-2 cells, necdin markedly suppressed p53-dependent activation of the p21/WAF promoter. Necdin and p53 inhibited cell growth in an additive manner as assessed by the colony formation of SAOS-2 cells, suggesting that necdin does not affect p53-mediated growth suppression. On the other hand, necdin inhibited p53-induced apoptosis of osteosarcoma U2OS cells. Thus, necdin can be a growth suppressor that targets p53 and modulates its biological functions in postmitotic neurons.

Do we understand the evolution of genomic imprinting?

The conflict theory is the only hypothesis to have attracted any critical attention for the evolution of genomic imprinting. Although the earliest data appeared supportive, recent systematic analyses have not confirmed the model's predictions. The status of theory remains undecided, however, as post-hoc explanations can be provided as to why these predictions are not borne out.