Phosphorylation of the menin tumor suppressor protein on serine 543 and serine 583

GLP-1 signaling suppresses menin's transcriptional block by phosphorylation in β cells

Both menin and glucagon-like peptide 1 (GLP-1) pathways play central yet opposing role in regulating β cell function, with menin suppressing, and GLP-1 promoting, β cell function. However, little is known as to whether or how GLP-1 pathway represses menin function. Here, we show that GLP-1 signaling-activated protein kinase A (PKA) directly phosphorylates menin at the serine 487 residue, relieving menin-mediated suppression of insulin expression and cell proliferation. Mechanistically, Ser487-phosphorylated menin gains increased binding affinity to nuclear actin/myosin IIa proteins and gets sequestrated from the Ins1 promoter. This event leads to reduced binding of repressive epigenetic histone modifiers suppressor variegation 3-9 homologue protein 1 (SUV39H1) and histone deacetylases 1 (HDAC1) at the locus and subsequently increased Ins1 gene transcription. Ser487 phosphorylation of menin also increases expression of proproliferative cyclin D2 and β cell proliferation. Our results have uncovered a previously unappreciated physiological link in which GLP-1 signaling suppresses menin function through phosphorylation-triggered and actin/myosin cytoskeletal protein-mediated derepression of gene transcription.

Molecular pathogenesis of parathyroid tumours

Parathyroid tumors represent an elusive endocrine neoplasia, which lead to primary hyperparathyroidism, pHPT, a common endocrine calcium disorder characterized by hypercalcemia and normal-high parathormone secretion. Parathyroid tumours are benign adenomas or multiple glands hyperplasia in the vast majority (>99% of cases), while malignant neoplasms are rare (less than 1%). Despite pHPT is a common disorder, our knowledge about the genetic predisposition and molecular pathophysiology is limited to the familial syndromic forms of parathyroid tumour, that, however, represent not more than the 10% of all the cases; instead, the pathophysiology of sporadic forms remains an open field, although data about epigenetic mechanisms or private genes have been supposed. Here we present an overview of more recent acquisitions about the genetic causes along with their molecular mechanisms of benign, but also, malignant parathyroid tumours either in sporadic and familial presentation.

Pathophysiology of Gastric NETs: Role of Gastrin and Menin

PURPOSE OF REVIEW

Neuroendocrine tumors (NETs) were initially identified as a separate entity in the early 1900s as a unique malignancy that secretes bioactive amines. GI-NETs are the most frequent type and represent a unique subset of NETs, because at least 75% of these tumors represent gastrin stimulation of the enterochromaffin-like cell located in the body of the stomach. The purpose of this review is to understand the specific role of gastrin in the generation of Gastric NETs (G-NETs).

RECENT FINDINGS

We review here the origin of enterochromaffin cells gut and the role of hypergastrinemia in gastric enteroendocrine tumorigenesis. We describe generation of the first genetically engineered mouse model of gastrin-driven G-NETs that mimics the human phenotype. The common mechanism observed in both the hypergastrinemic mouse model and human carcinoids is translocation of the cyclin-dependent inhibitor p27^kip to the cytoplasm and its subsequent degradation by the proteasome. Therapies that block degradation of p27^kip, the CCKBR2 gastrin receptor, or gastrin peptide are likely to facilitate treatment.

Epigenetic regulation by the menin pathway

There is a trend of increasing prevalence of neuroendocrine tumors (NETs), and the inherited multiple endocrine neoplasia type 1 (MEN1) syndrome serves as a genetic model to investigate how NETs develop and the underlying mechanisms. Menin, encoded by the MEN1 gene, at least partly acts as a scaffold protein by interacting with multiple partners to regulate cellular homeostasis of various endocrine organs. Menin has multiple functions including regulation of several important signaling pathways by controlling gene transcription. Here, we focus on reviewing the recent progress in elucidating the key biochemical role of menin in epigenetic regulation of gene transcription and cell signaling, as well as posttranslational regulation of menin itself. In particular, we will review the progress in studying structural and functional interactions of menin with various histone modifiers and transcription factors such as MLL, PRMT5, SUV39H1 and other transcription factors including c-Myb and JunD. Moreover, the role of menin in regulating cell signaling pathways such as TGF-beta, Wnt and Hedgehog, as well as miRNA biogenesis and processing will be described. Further, the regulation of the MEN1 gene transcription, posttranslational modifications and stability of menin protein will be reviewed. These various modes of regulation by menin as well as regulation of menin by various biological factors broaden the view regarding how menin controls various biological processes in neuroendocrine organ homeostasis.

Tumor suppressor menin is required for subunit-specific nAChR α5 transcription and nAChR-dependent presynaptic facilitation in cultured mouse hippocampal neurons

In the central nervous system (CNS), cholinergic transmission induces synaptic plasticity that is required for learning and memory. However, our understanding of the development and maintenance of cholinergic circuits is limited, as the factors regulating the expression and clustering of neuronal nicotinic acetylcholine receptors (nAChRs) remain poorly defined. Recent studies from our group have implicated calpain-dependent proteolytic fragments of menin, the product of the MEN1 tumor suppressor gene, in coordinating the transcription and synaptic clustering of nAChRs in invertebrate central neurons. Here, we sought to determine whether an analogous cholinergic mechanism underlies menin's synaptogenic function in the vertebrate CNS. Our data from mouse primary hippocampal cultures demonstrate that menin and its calpain-dependent C-terminal fragment (C-menin) regulate the subunit-specific transcription and synaptic clustering of neuronal nAChRs, respectively. MEN1 knockdown decreased nAChR α5 subunit expression, the clustering of α7 subunit-containing nAChRs at glutamatergic presynaptic terminals, and nicotine-induced presynaptic facilitation. Moreover, the number and function of glutamatergic synapses was unaffected by MEN1 knockdown, indicating that the synaptogenic actions of menin are specific to cholinergic regulation. Taken together, our results suggest that the influence of menin on synapse formation and synaptic plasticity occur via modulation of nAChR channel subunit composition and functional clustering.

Two proteolytic fragments of menin coordinate the nuclear transcription and postsynaptic clustering of neurotransmitter receptors during synaptogenesis between Lymnaea neurons

Synapse formation and plasticity depend on nuclear transcription and site-specific protein targeting, but the molecular mechanisms that coordinate these steps have not been well defined. The MEN1 tumor suppressor gene, which encodes the protein menin, is known to induce synapse formation and plasticity in the CNS. This synaptogenic function has been conserved across evolution, however the underlying molecular mechanisms remain unidentified. Here, using central neurons from the invertebrate Lymnaea stagnalis, we demonstrate that menin coordinates subunit-specific transcriptional regulation and synaptic clustering of nicotinic acetylcholine receptors (nAChR) during neurotrophic factor (NTF)-dependent excitatory synaptogenesis, via two proteolytic fragments generated by calpain cleavage. Whereas menin is largely regarded as a nuclear protein, our data demonstrate a novel cytoplasmic function at central synapses. Furthermore, this study identifies a novel synaptogenic mechanism in which a single gene product coordinates the nuclear transcription and postsynaptic targeting of neurotransmitter receptors through distinct molecular functions of differentially localized proteolytic fragments.

Challenges and opportunities in targeting the menin-MLL interaction

Menin is an essential co-factor of oncogenic MLL fusion proteins and the menin-MLL interaction is critical for development of acute leukemia in vivo. Targeting the menin-MLL interaction with small molecules represents an attractive strategy to develop new anticancer agents. Recent developments, including determination of menin crystal structure and development of potent small molecule and peptidomimetic inhibitors, demonstrate the feasibility of targeting the menin-MLL interaction. On the other hand, biochemical and structural studies revealed that MLL binds to menin in a complex bivalent mode engaging two MLL motifs, and therefore inhibition of this protein-protein interaction represents a challenge. This review summarizes the most recent achievements in targeting the menin-MLL interaction as well as discusses potential benefits of blocking menin in cancer.

Remodeling of the enhancer landscape during macrophage activation is coupled to enhancer transcription

Recent studies suggest a hierarchical model in which lineage-determining factors act in a collaborative manner to select and prime cell-specific enhancers, thereby enabling signal-dependent transcription factors to bind and function in a cell-type-specific manner. Consistent with this model, TLR4 signaling primarily regulates macrophage gene expression through a pre-existing enhancer landscape. However, TLR4 signaling also induces priming of ∼3,000 enhancer-like regions de novo, enabling visualization of intermediates in enhancer selection and activation. Unexpectedly, we find that enhancer transcription precedes local mono- and dimethylation of histone H3 lysine 4 (H3K4me1/2). H3K4 methylation at de novo enhancers is primarily dependent on the histone methyltransferases Mll1, Mll2/4, and Mll3 and is significantly reduced by inhibition of RNA polymerase II elongation. Collectively, these findings suggest an essential role of enhancer transcription in H3K4me1/2 deposition at de novo enhancers that is independent of potential functions of the resulting eRNA transcripts.

Menin: a scaffold protein that controls gene expression and cell signaling

The protein menin is encoded by the MEN1 gene, which is mutated in patients with multiple endocrine neoplasia type 1 (MEN1) syndrome. Although menin acts as a tumor suppressor in endocrine organs, it is required for leukemic transformation in mouse models. Menin possesses these dichotomous functions probably because it can both positively and negatively regulate gene expression, as well as interact with a multitude of proteins with diverse functions. Here, we review the recent progress in understanding the molecular mechanisms by which menin functions. The crystal structures of menin with different binding partners reveal that menin is a key scaffold protein that functionally crosstalks with various partners to regulate gene transcription and interplay with multiple signaling pathways.

Crystal structure of menin reveals binding site for mixed lineage leukemia (MLL) protein

Menin is a tumor suppressor protein that is encoded by the MEN1 (multiple endocrine neoplasia 1) gene and controls cell growth in endocrine tissues. Importantly, menin also serves as a critical oncogenic cofactor of MLL (mixed lineage leukemia) fusion proteins in acute leukemias. Direct association of menin with MLL fusion proteins is required for MLL fusion protein-mediated leukemogenesis in vivo, and this interaction has been validated as a new potential therapeutic target for development of novel anti-leukemia agents. Here, we report the first crystal structure of menin homolog from Nematostella vectensis. Due to a very high sequence similarity, the Nematostella menin is a close homolog of human menin, and these two proteins likely have very similar structures. Menin is predominantly an α-helical protein with the protein core comprising three tetratricopeptide motifs that are flanked by two α-helical bundles and covered by a β-sheet motif. A very interesting feature of menin structure is the presence of a large central cavity that is highly conserved between Nematostella and human menin. By employing site-directed mutagenesis, we have demonstrated that this cavity constitutes the binding site for MLL. Our data provide a structural basis for understanding the role of menin as a tumor suppressor protein and as an oncogenic co-factor of MLL fusion proteins. It also provides essential structural information for development of inhibitors targeting the menin-MLL interaction as a novel therapeutic strategy in MLL-related leukemias.

The menin tumor suppressor protein is phosphorylated in response to DNA damage

BACKGROUND

Multiple endocrine neoplasia type 1 (MEN1) is a heritable cancer syndrome characterized by tumors of the pituitary, pancreas and parathyroid. Menin, the product of the MEN1 gene, is a tumor suppressor protein that functions in part through the regulation of transcription mediated by interactions with chromatin modifying enzymes.

PRINCIPAL FINDINGS

Here we show menin association with the 5' regions of DNA damage response genes increases after DNA damage and is correlated with RNA polymerase II association but not with changes in histone methylation. Furthermore, we were able to detect significant levels of menin at the 3' regions of CDKN1A and GADD45A under conditions of enhanced transcription following DNA damage. We also demonstrate that menin is specifically phosphorylated at Ser394 in response to several forms of DNA damage, Ser487 is dynamically phosphorylated and Ser543 is constitutively phosphorylated. Phosphorylation at these sites however does not influence the ability to interact with histone methyltransferase activity. In contrast, the interaction between menin and RNA polymerase II is influenced by phosphorylation, whereby a phospho-deficient mutant had a higher affinity for the elongating form of RNA polymerase compared to wild type. Additionally, a subset of MEN1-associated missense point mutants, fail to undergo DNA damage dependent phosphorylation.

CONCLUSION

Together, our findings suggest that the menin tumor suppressor protein undergoes DNA damage induced phosphorylation and participates in the DNA damage transcriptional response.

Menin expression modulates mesenchymal cell commitment to the myogenic and osteogenic lineages

Menin plays an established role in the differentiation of mesenchymal cells to the osteogenic lineage. Conversely, whether Menin influences the commitment of mesenschymal cells to the myogenic lineage, despite expression in the developing somite was previously unclear. We observed that Menin is down-regulated in C2C12 and C3H10T1/2 mesenchymal cells when muscle differentiation is induced. Moreover, maintenance of Menin expression by constitutive ectopic expression inhibited muscle cell differentiation. Reduction of Menin expression by siRNA technology results in precocious muscle differentiation and concomitantly attenuates BMP-2 induced osteogenesis. Reduced Menin expression antagonizes BMP-2 and TGF-beta1 mediated inhibition of myogenesis. Furthermore, Menin was found to directly interact with and potentiate the transactivation properties of Smad3 in response to TGF-beta1. Finally in concert with these observations, tissue-specific inactivation of Men1 in Pax3-expressing somite precursor cells leads to a patterning defect of rib formation and increased muscle mass in the intercostal region. These data invoke a pivotal role for Menin in the competence of mesenchymal cells to respond to TGF-beta1 and BMP-2 signals. Thus, by modulating cytokine responsiveness Menin functions to alter the balance of multipotent mesenchymal cell commitment to the osteogenic or myogenic lineages.

Menin: the protein behind the MEN1 syndrome

The cloning of the MEN1 gene in 1997 led to the characterization of menin, the protein behind the multiple endocrine neoplasia Type 1 syndrome. Menin, a novel nuclear protein with no homology to other gene products, is expressed ubiquitously. MEN1 missense mutations are dispersed along the coding region of the gene but are more common in the most conserved regions. Likewise, domains of protein interaction often correspond to the more conserved segments of menin. These protein interactions are generally facilitated by multiple domains or encompass a large portion of menin. The exception to this rule is a small stretch of amino acids mediating the interaction of menin with the mSin3A corepressor and histone deacetylase complexes. The C-terminal region of menin harbors several nuclear localization signals that play redundant functions in the localization of menin to the nuclear compartment. The nuclear localization signals are also important for the interaction of menin with the nuclear matrix. Menin is the target of several kinases and a candidate substrate of the ATM/ATR kinases, implying a role for this tumor suppressor in the DNA damage response. Menin is highly conserved from Drosophila to human but is absent in the nematode and in yeast.