Conservation and variation of structure and function in a newly identified GCM homolog from chicken

Nuclear factor erythroid-derived 2 (Nfe2) regulates JunD DNA-binding activity via acetylation: a novel mechanism regulating trophoblast differentiation

We recently demonstrated that the bZip transcription factor nuclear factor erythroid-derived 2 (Nfe2) represses protein acetylation and expression of the transcription factor glial cell missing 1 (Gcm1) in trophoblast cells, preventing excess syncytiotrophoblast formation and permitting normal placental vascularization and embryonic growth. However, the Gcm1 promoter lacks a Nfe2-binding site and hence the mechanisms linking Nfe2 and Gcm1 expression remained unknown. Here we show that Nfe2 represses JunD DNA-binding activity to the Gcm1 promoter during syncytiotrophoblast differentiation. Interventional studies using knockdown and knockin approaches show that enhanced JunD DNA-binding activity is required for increased expression of Gcm1 and syncytiotrophoblast formation as well as impaired placental vascularization and reduced growth of Nfe2(-/-) embryos. Induction of Gcm1 expression requires binding of JunD to the -1441 site within the Gcm1 promoter, which is distinct from the -1314 site previously shown to induce Gcm1 expression by other bZip transcription factors. Nfe2 modulates JunD binding to the Gcm1 promoter via acetylation, as reducing JunD acetylation using the histone acetyltransferase inhibitor curcumin reverses the increased JunD DNA-binding activity observed in the absence of Nfe2. This identifies a novel mechanism through which bZip transcription factors interact. Within the placenta this interaction regulates Gcm1 expression, syncytiotrophoblast formation, placental vascularization, and embryonic growth.

Retrotransposon silencing by DNA methylation contributed to the evolution of placentation and genomic imprinting in mammals

The evolution of two mammalian-specific traits, viviparous reproduction with a placenta and genomic imprinting, have been addressed by multiple studies of two retrotransposon derived, mammalian-specific genes. These gene targeting experiments in mice, together with recent comparative genomic analyses among three mammalian groups, suggest that extremely rare events; namely exaptations from retrotransposons, made crucial contributions to the establishment and diversification of mammals via placental formation. We propose that nearly neutral evolution, as well as Darwinian evolution (natural selection), plays an important role in the exaptation process. Comparative genomic analysis of various imprinted regions has also revealed that an imprinting control element essential for parent-of-origin specific monoallelic expression of imprinted genes emerged in each of the imprinted regions, possibly by the insertion of exogenous DNAs, such as retrotransposons. In both cases, DNA methylation in germ cells must have been of critical importance to repress the exogenous DNAs inserted into the genomes of mammalian ancestors. We propose that the ability of germ line DNA methylation enabled the emergence of certain mammalian-specific features during the course of evolution.

Neurogenic role of Gcm transcription factors is conserved in chicken spinal cord

Although glial cells missing (gcm) genes are known as glial determinants in the fly embryo, the role of vertebrate orthologs in the central nervous system is still under debate. Here we show for the first time that the chicken ortholog of fly gcm (herein referred to as c-Gcm1), is expressed in early neuronal lineages of the developing spinal cord and is required for neural progenitors to differentiate as neurons. Moreover, c-Gcm1 overexpression is sufficient to trigger cell cycle exit and neuronal differentiation in neural progenitors. Thus, c-Gcm1 expression constitutes a crucial step in the developmental cascade that prompts progenitors to generate neurons: c-Gcm1 acts downstream of proneural (neurogenin) and progenitor (Sox1-3) factors and upstream of NeuroM neuronal differentiation factor. Strikingly, this neurogenic role is not specific to the vertebrate gene, as fly gcmand gcm2 are also sufficient to induce the expression of neuronal markers. Interestingly, the neurogenic role is restricted to post-embryonic stages and we identify two novel brain neuronal lineages expressing and requiring gcm genes. Finally, we show that fly gcm and the chick and mouse orthologs induce expression of neural markers in HeLa cells. These data, which demonstrate a conserved neurogenic role for Gcm transcription factors, call for a re-evaluation of the mode of action of these genes during evolution.

Casein kinase 2-dependent serine phosphorylation of MuSK regulates acetylcholine receptor aggregation at the neuromuscular junction

The release of Agrin by motoneurons activates the muscle-specific receptor tyrosine kinase (MuSK) as the main organizer of subsynaptic specializations at the neuromuscular junction. MuSK downstream signaling is largely undefined. Here we show that protein kinase CK2 interacts and colocalizes with MuSK at post-synaptic specializations. We observed CK2-mediated phosphorylation of serine residues within the kinase insert (KI) of MuSK. Inhibition or knockdown of CK2, or exchange of phosphorylatable serines by alanines within the KI of MuSK, impaired acetylcholine receptor (AChR) clustering, whereas their substitution by residues that imitate constitutive phosphorylation led to aggregation of AChRs even in the presence of CK2 inhibitors. Impairment of AChR cluster formation after replacement of MuSK KI with KIs of other receptor tyrosine kinases correlates with potential CK2-dependent serine phosphorylation within KIs. MuSK activity was unchanged but AChR stability decreased in the presence of CK2 inhibitors. Muscle-specific CK2beta knockout mice develop a myasthenic phenotype due to impaired muscle endplate structure and function. This is the first description of a regulatory cross-talk between MuSK and CK2 and of a role for the KI of the receptor tyrosine kinase MuSK for the development of subsynaptic specializations.

Huckebein-mediated autoregulation of Glide/Gcm triggers glia specification

Cell specification in the nervous system requires patterning genes dictating spatio-temporal coordinates as well as fate determinants. In the case of neurons, which are controlled by the family of proneural transcription factors, binding specificity and patterned expression trigger both differentiation and specification. In contrast, a single gene, glide cell deficient/glial cell missing (glide/gcm), is sufficient for all fly lateral glial differentiation. How can different types of cells develop in the presence of a single fate determinant, that is, how do differentiation and specification pathways integrate and produce distinct glial populations is not known. By following an identified lineage, we here show that glia specification is triggered by high glide/gcm expression levels, mediated by cell-specific protein-protein interactions. Huckebein (Hkb), a lineage-specific factor, provides a molecular link between glide/gcm and positional cues. Importantly, Hkb does not activate transcription; rather, it physically interacts with Glide/Gcm thereby triggering its autoregulation. These data emphasize the importance of fate determinant cell-specific quantitative regulation in the establishment of cell diversity.

Stimulation of GCMa transcriptional activity by cyclic AMP/protein kinase A signaling is attributed to CBP-mediated acetylation of GCMa

Human GCMa is a zinc-containing transcription factor primarily expressed in placenta. GCMa regulates expression of syncytin gene, which encodes for a placenta-specific membrane protein that mediates trophoblastic fusion and the formation of syncytiotrophoblast layer required for efficient fetal-maternal exchange of nutrients and oxygen. The adenylate cyclase activator, forskolin, stimulates syncytin gene expression and cell fusion in cultured placental cells. Here we present evidence that cyclic AMP (cAMP) signaling pathway activates the syncytin gene expression by regulating GCMa activity. We found that forskolin and protein kinase A (PKA) enhances GCMa-mediated transcriptional activation. Furthermore, PKA treatment stimulates the association of GCMa with CBP and increases GCMa acetylation. CBP primarily acetylates GCMa at lysine367, lysine406, and lysine409 in the transactivation domain (TAD). We found that acetylation of these residues is required to protect GCMa from ubiquitination and increases the TAD stability with a concomitant increase in transcriptional activity, supporting the importance of acetylation in PKA-dependent GCMa activation. Our results reveal a novel regulation of GCMa activity by cAMP-dependent protein acetylation and provide a molecular mechanism by which cAMP signaling regulates trophoblastic fusion.

Interaction, cooperative promoter modulation, and renal colocalization of GCMa and Pitx2

The transcription factor GCMa is a member of a new small family of transcription factors with a conserved zinc-containing DNA-binding domain. All members of this transcription factor family play crucial roles as master regulators during development. GCMa is restricted to placenta during development and to kidney and thymus at postnatal stages. It is essential for the formation of the placental labyrinth and as a consequence for survival of the embryo from mid-embryogenesis onwards. Here, we identify Pitx transcription factors as GCMa-interacting proteins. We show that Pitx proteins interact via their conserved homeodomain with the DNA-binding domain of GCMa. As a consequence, Pitx proteins and GCMa exhibit cooperative DNA binding. Furthermore, Pitx proteins influence GCMa-dependent promoter activation in a cell-specific manner. One of the three Pitx paralogues in mice, Pitx2, is the predominant Pitx member present in the placenta and colocalizes on the cellular level with GCMa in the kidney. This is the first description of a regulatory cross-talk between a transcription factor of the GCM family and a homeodomain protein.

Impacts of a new transcription factor family: mammalian GCM proteins in health and disease

GCM proteins constitute a small transcription factor family with a DNA-binding domain exhibiting a novel fold composed of two subdomains rigidly held together by coordination of one of two structural zinc cations. In all known cases, GCM proteins exert the role of master regulators: the prototypical family member determines gliogenesis in Drosophila melanogaster, whereas mammalian GCM proteins orchestrate divergent aspects of development and physiology in placenta, kidney, thymus, and parathyroid gland. Recent data point to an involvement of GCM proteins in different pathological contexts, such as preeclampsia, hyper- or hypoparathyroidism, and parathyroid gland tumors.