title: Nucleolar release of Hand1 acts as a molecular switch to determine cell fate.
abstract: The bHLH transcription factor Hand1 is essential for placentation and cardiac morphogenesis in the developing embryo. Here we implicate Hand1 as a molecular switch that determines whether a trophoblast stem cell continues to proliferate or commits to differentiation. We identify a novel interaction of Hand1 with a protein that contains an I-mfa (inhibitor of myogenic factor) domain that anchors Hand1 in the nucleolus where it negatively regulates Hand1 activity. In the trophoblast stem-cell line Rcho-1, nucleolar sequestration of Hand1 accompanies sustained cell proliferation and renewal, whereas release of Hand1 into the nucleus leads to its activation, thus committing cells to a differentiated giant-cell fate. Site-specific phosphorylation is required for nucleolar release of Hand1, for its dimerization and biological function, and this is mediated by the non-canonical polo-like kinase Plk4 (Sak). Sak is co-expressed in Rcho-1 cells, localizes to the nucleolus during G2 and phosphorylates Hand1 as a requirement for trophoblast stem-cell commitment to a giant-cell fate. This study defines a novel cellular mechanism for regulating Hand1 that is a crucial step in the stem-cell differentiation pathway.
authors: Molecular Medicine Unit, UCL Institute of Child Health, London, WC1N 1EH, UK.
authors: David M J
authors: DM
authors: Martindill
authors: Catherine A
authors: CA
authors: Risebro
authors: Nicola
authors: N
authors: Smart
authors: Maria Del Mar
authors: Mdel M
authors: Franco-Viseras
authors: Carla O
authors: CO
authors: Rosario
authors: Carol J
authors: CJ
authors: Swallow
authors: James W
authors: JW
authors: Dennis
authors: Paul R
authors: PR
authors: Riley
mesh_terms: D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D015152:Blotting, Northern; D015153:Blotting, Western; D002454:Cell Differentiation; D002466:Cell Nucleolus / Q000378:metabolism*; D049109:Cell Proliferation; D018507:Gene Expression Regulation, Developmental; D015726:Giant Cells / Q000166:cytology / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D047468:Immunoprecipitation; D051379:Mice; D018006:Myogenic Regulatory Factors / Q000378:metabolism; D041681:NIH 3T3 Cells; D010766:Phosphorylation; D011485:Protein Binding; D054648:Protein Phosphatase 2; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013234:Stem Cells / Q000166:cytology / Q000378:metabolism*; D014327:Trophoblasts / Q000166:cytology / Q000378:metabolism; D020798:Two-Hybrid System Techniques
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D049452:Green Fluorescent Proteins; D018006:Myogenic Regulatory Factors; D054648:Protein Phosphatase 2; D017346:Protein Serine-Threonine Kinases; D011993:Recombinant Fusion Proteins; C095572:Hand1 protein, mouse; C491077:Mdfi protein, mouse; C088051:Plk4 protein, mouse; C475028:Ppp2r5d protein, mouse