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.

Identification of imprinted genes in the skeletal muscle of newborn piglets by high-throughput sequencing

Imprinted genes - exhibiting parent-specific transcription - play essential roles in the process of mammalian development and growth. Skeletal muscle growth is crucial for meat production. To further understand the role of imprinted genes during the porcine skeletal muscle growth, DNA-seq and RNA-seq were used to explore the characteristics of imprinted genes from porcine reciprocal crosses. A total of 584 545 single-nucleotide variations were discovered in the DNA-seq data of F0 parents, heterozygous in two pig breeds (Yorkshire and Min pigs) but homozygous in each breed. These single-nucleotide variations were used to determine the allelic-specific expression in F1 individuals. Finally, eight paternal expression sites and three maternal expression sites were detected, whereas two paternally expressed imprinted genes (NDN and IGF2) and one maternally expressed imprinted gene (H1-3) were validated by Sanger sequencing. DNA methylation regulates the expression of imprinted genes, and all of the identified imprinted genes in this study were predicted to possess CpG islands. PBX1 and YY1 binding motifs were discovered in the promoter regions of all three imprinted genes, which were candidate elements regulating the transcription of imprinted genes. For these identified imprinted genes, IGF2 and NDN promoted muscle growth whereas H1-3 inhibited cell proliferation, corroborating the 'parental conflict' theory that paternally expressed imprinted genes assisted descendants' growth whereas maternally expressed imprinted genes inhibited it. This study discovered porcine imprinted genes in skeletal muscle and was the first to reveal that H1-3 was expressed by the maternal allele to our knowledge. Our findings provided valuable resources for the potential utilization of imprinted genes in pig breeding.

The necdin interactome: evaluating the effects of amino acid substitutions and cell stress using proximity-dependent biotinylation (BioID) and mass spectrometry

Prader-Willi syndrome (PWS) is a neurodevelopmental disorder caused by the loss of function of a set of imprinted genes on chromosome 15q11-15q13. One of these genes, NDN, encodes necdin, a protein that is important for neuronal differentiation and survival. Loss of Ndn in mice causes defects in the formation and function of the nervous system. Necdin is a member of the melanoma-associated antigen gene (MAGE) protein family. The functions of MAGE proteins depend highly on their interactions with other proteins, and in particular MAGE proteins interact with E3 ubiquitin ligases and deubiquitinases to form MAGE-RING E3 ligase-deubiquitinase complexes. Here, we used proximity-dependent biotin identification (BioID) and mass spectrometry (MS) to determine the network of protein-protein interactions (interactome) of the necdin protein. This process yielded novel as well as known necdin-proximate proteins that cluster into a protein network. Next, we used BioID-MS to define the interactomes of necdin proteins carrying coding variants. Variant necdin proteins had interactomes that were distinct from wildtype necdin. BioID-MS is not only a useful tool to identify protein-protein interactions, but also to analyze the effects of variants of unknown significance on the interactomes of proteins involved in genetic disease.

Preclinical Testing in Translational Animal Models of Prader-Willi Syndrome: Overview and Gap Analysis

Prader-Willi syndrome (PWS) is a rare neurodevelopmental disorder causing endocrine, musculoskeletal, and neurological dysfunction. PWS is caused by the inactivation of contiguous genes, complicating the development of targeted therapeutics. Clinical trials are now underway in PWS, with more trials to be implemented in the next few years. PWS-like endophenotypes are recapitulated in gene-targeted mice in which the function of one or more PWS genes is disrupted. These animal models can guide priorities for clinical trials or provide information about efficacy of a compound within the context of the specific disease. We now review the current status of preclinical studies that measure the effect of therapeutics on PWS-like endophenotypes. Seven categories of therapeutics (oxytocin and related compounds, K⁺-ATP channel agonists, melanocortin 4 receptor agonists, incretin mimetics and/or GLP-1 receptor agonists, cannabinoids, ghrelin agents, and Caralluma fimbriata [cactus] extract) have been tested for their effect on endophenotypes in both PWS animal models and clinical trials. Many other therapeutics have been tested in clinical trials, but not preclinical models of PWS or vice versa. Fostering dialogs among investigators performing preclinical validation of animal models and those implementing clinical studies will accelerate the discovery and translation of therapies into clinical practice in PWS.

The Prader-Willi syndrome proteins MAGEL2 and necdin regulate leptin receptor cell surface abundance through ubiquitination pathways

In Prader-Willi syndrome (PWS), obesity is caused by the disruption of appetite-controlling pathways in the brain. Two PWS candidate genes encode MAGEL2 and necdin, related melanoma antigen proteins that assemble into ubiquitination complexes. Mice lacking Magel2 are obese and lack leptin sensitivity in hypothalamic pro-opiomelanocortin neurons, suggesting dysregulation of leptin receptor (LepR) activity. Hypothalamus from Magel2-null mice had less LepR and altered levels of ubiquitin pathway proteins that regulate LepR processing (Rnf41, Usp8, and Stam1). MAGEL2 increased the cell surface abundance of LepR and decreased their degradation. LepR interacts with necdin, which interacts with MAGEL2, which complexes with RNF41 and USP8. Mutations in the MAGE homology domain of MAGEL2 suppress RNF41 stabilization and prevent the MAGEL2-mediated increase of cell surface LepR. Thus, MAGEL2 and necdin together control LepR sorting and degradation through a dynamic ubiquitin-dependent pathway. Loss of MAGEL2 and necdin may uncouple LepR from ubiquitination pathways, providing a cellular mechanism for obesity in PWS.

Necdin shapes serotonergic development and SERT activity modulating breathing in a mouse model for Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a genetic neurodevelopmental disorder that presents with hypotonia and respiratory distress in neonates. The Necdin-deficient mouse is the only model that reproduces the respiratory phenotype of PWS (central apnea and blunted response to respiratory challenges). Here, we report that Necdin deletion disturbs the migration of serotonin (5-HT) neuronal precursors, leading to altered global serotonergic neuroarchitecture and increased spontaneous firing of 5-HT neurons. We show an increased expression and activity of 5-HT Transporter (SERT/Slc6a4) in 5-HT neurons leading to an increase of 5-HT uptake. In Necdin-KO pups, the genetic deletion of Slc6a4 or treatment with Fluoxetine, a 5-HT reuptake inhibitor, restored normal breathing. Unexpectedly, Fluoxetine administration was associated with respiratory side effects in wild-type animals. Overall, our results demonstrate that an increase of SERT activity is sufficient to cause the apneas in Necdin-KO pups, and that fluoxetine may offer therapeutic benefits to PWS patients with respiratory complications.

Prader-Willi syndrome results from the disruption of a cluster of neighboring genes, including one called Necdin. Symptoms begin in early infancy and worsen with age. Affected children tend to develop an insatiable appetite, which often leads to obesity. They also experience serious problems with their breathing. Chest infections, high altitude and intense physical activity can be dangerous for children with Prader-Willi syndrome. This is because a slight shortage of oxygen may trigger breathing difficulties that could prove fatal.

The brain cells that produce a chemical messenger called serotonin help to control breathing. Several lines of evidence suggest that loss of Necdin may trigger breathing difficulties in Prader-Willi syndrome via effects on the serotonin system. First, serotonin neurons produce the Necdin protein. Second, laboratory mice that lack the gene for Necdin have abnormally shaped serotonin neurons. Third, these mice show breathing difficulties like those of individuals with Prader-Willi syndrome. But while this implies a connection between serotonin, Necdin and breathing difficulties, it falls short of establishing a causal link.

Matarazzo et al. now reveal an increase in the quantity and activity of a protein called the serotonin transporter in mutant mice that lacked the gene for Necdin compared to normal mice. Serotonin transporter proteins mop up the serotonin that neurons release when they signal to one another. Neurons in the mutant mice take up more serotonin than their counterparts in normal mice; this means they have less serotonin available for signaling. This may make it harder for the mutant mice to regulate their breathing.

Drugs called selective serotonin-reuptake inhibitors (or SSRIs for short) can block the serotonin transporter. These drugs, which include Fluoxetine (also called Prozac), are antidepressants. Matarazzo et al. show that SSRIs temporarily restore normal breathing in young mice that lack the gene for Necdin. However, these drugs have harmful long-term effects on breathing in non-mutant mice. Further studies should test whether short-term use of SSRIs could offer immediate relief for breathing difficulties in infants and children with Prader-Willi syndrome.

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.

Brain-derived neurotrophic factor interacts with astrocytes and neurons to control respiration

Respiratory rhythm is generated and modulated in the brainstem. Neuronal involvement in respiratory control and rhythmogenesis is now clearly established. However, glial cells have also been shown to modulate the activity of brainstem respiratory groups. Although the potential involvement of other glial cell type(s) cannot be excluded, astrocytes are clearly involved in this modulation. In parallel, brain-derived neurotrophic factor (BDNF) also modulates respiratory rhythm. The currently available data on the respective roles of astrocytes and BDNF in respiratory control and rhythmogenesis lead us to hypothesize that there is BDNF-mediated control of the communication between neurons and astrocytes in the maintenance of a proper neuronal network capable of generating a stable respiratory rhythm. According to this hypothesis, progression of Rett syndrome, an autism spectrum disease with disordered breathing, can be stabilized in mouse models by re-expressing the normal gene pattern in astrocytes or microglia, as well as by stimulating the BDNF signaling pathway. These results illustrate how the signaling mechanisms by which glia exerts its effects in brainstem respiratory groups is of great interest for pathologies associated with neurological respiratory disorders.

Recommendations for the investigation of animal models of Prader-Willi syndrome

Prader-Willi syndrome (PWS) occurs in about 1 in 15,000 individuals and is a contiguous gene disorder causing developmental disability, hyperphagia usually with obesity, and behavioral problems, including an increased incidence of psychiatric illness. The genomic imprinting that regulates allele-specific expression of PWS candidate genes, the fact that multiple genes are typically inactivated, and the presence of many genes that produce functional RNAs rather than proteins has complicated the identification of the underlying genetic pathophysiology of PWS. Over 30 genetically modified mouse strains that have been developed and characterized have been instrumental in elucidating the genetic and epigenetic mechanisms for the regulation of PWS genes and in discovering their physiological functions. In 2011, a PWS Animal Models Working Group (AMWG) was established to generate discussions and facilitate exchange of ideas regarding the best use of PWS animal models. Here, we summarize the goals of the AMWG, describe current animal models of PWS, and make recommendations for strategies to maximize the utility of animal models and for the development and use of new animal models of PWS.

Molecular characterization of porcine NECD, SNRPN and UBE3A genes and imprinting status in the skeletal muscle of neonate pigs

Imprinted genes are expressed monoallelically depending on their parental origin, and play important roles in embryo survival and postnatal growth regulation. In this study, we characterized the porcine NECD (necdin), SNRPN (small nuclear ribonucleoprotein polypeptide N) and UBE3A (UBE3A ubiquitin protein ligase E3A) genes, analyzed their expression in nine tissues including liver, lung, small intestine, skeletal muscle, heart, kidney, spleen, inguinal lymph nodes and fat, and also examined their imprinting status in the skeletal muscle of neonate pigs. Results indicated that these three genes were highly homologous between pigs and cattle, being 95.02 % in nucleotide and 99.17 % in amino acid with the cattle SNRPN gene, and 96.46 % in nucleotide and 98.63 % in amino acid with the cattle UBE3A gene, respectively. The three genes were expressed in all the tissues investigated. Three single nucleotide polymorphisms (SNPs) in the coding region of these genes, i.e. g.263G>C, g.402T>C and g.340A>G for porcine NECD, SNRPN and UBE3A genes, respectively, were revealed; and imprinting analysis with which indicated that, in the skeletal muscle of neonate pigs, both NECD and SNRPN were maternally imprinted, while UBE3A was not imprinted.

Loss of the Prader-Willi obesity syndrome protein necdin promotes adipogenesis

We investigated the role of necdin during adipogenic differentiation. Necdin is one of several genes inactivated in children with Prader-Willi syndrome, who are predisposed to increased adiposity at the expense of lean mass. Necdin promotes neuronal and muscle differentiation and survival through interactions with a variety of proteins, including cell surface receptors, modifiers of protein stability, and transcription factors. In pre-adipocytes, necdin over-expression inhibits adipogenesis, while reducing necdin levels enhances adipogenic differentiation in tissue culture cells. We now directly demonstrate a role for necdin in inhibiting adipogenesis using cells derived from necdin deficient mice.

Necdin protects embryonic motoneurons from programmed cell death

NECDIN belongs to the type II Melanoma Associated Antigen Gene Expression gene family and is located in the Prader-Willi Syndrome (PWS) critical region. Necdin-deficient mice develop symptoms of PWS, including a sensory and motor deficit. However, the mechanisms underlying the motor deficit remain elusive. Here, we show that the genetic ablation of Necdin, whose expression is restricted to post-mitotic neurons in the spinal cord during development, leads to a loss of 31% of specified motoneurons. The increased neuronal loss occurs during the period of naturally-occurring cell death and is not confined to specific pools of motoneurons. To better understand the role of Necdin during the period of programmed cell death of motoneurons we used embryonic spinal cord explants and primary motoneuron cultures from Necdin-deficient mice. Interestingly, while Necdin-deficient motoneurons present the same survival response to neurotrophic factors, we demonstrate that deletion of Necdin leads to an increased susceptibility of motoneurons to neurotrophic factor deprivation. We show that by neutralizing TNFα this increased susceptibility of Necdin-deficient motoneurons to trophic factor deprivation can be reduced to the normal level. We propose that Necdin is implicated through the TNF-receptor 1 pathway in the developmental death of motoneurons.