title: Nuclear envelope localization of LEMD2 is developmentally dynamic and lamin A/C dependent yet insufficient for heterochromatin tethering.
abstract: Peripheral heterochromatin in mammalian nuclei is tethered to the nuclear envelope by at least two mechanisms here referred to as the A- and B-tethers. The A-tether includes lamins A/C and additional unknown components presumably INM protein(s) interacting with both lamins A/C and chromatin. The B-tether includes the inner nuclear membrane (INM) protein Lamin B-receptor, which binds B-type lamins and chromatin. Generally, at least one of the tethers is always present in the nuclear envelope of mammalian cells. Deletion of both causes the loss of peripheral heterochromatin and consequently inversion of the entire nuclear architecture, with this occurring naturally in rod photoreceptors of nocturnal mammals. The tethers are differentially utilized during development, regulate gene expression in opposite manners, and play an important role during cell differentiation. Here we aimed to identify the unknown chromatin binding component(s) of the A-tether. We analyzed 10 mouse tissues by immunostaining with antibodies against 7 INM proteins and found that every cell type has specific, although differentially and developmentally regulated, sets of these proteins. In particular, we found that INM protein LEMD2 is concomitantly expressed with A-type lamins in various cell types but is lacking in inverted nuclei of rod cells. Truncation or deletion of Lmna resulted in the downregulation and mislocalization of LEMD2, suggesting that the two proteins interact and pointing at LEMD2 as a potential chromatin binding mediator of the A-tether. Using nuclei of mouse rods as an experimental model lacking peripheral heterochromatin, we expressed a LEMD2 transgene alone or in combination with lamin C in these cells and observed no restoration of peripheral heterochromatin in either case. We conclude that in contrary to the B-tether, the A-tether has a more intricate composition and consists of multiple components that presumably vary, at differing degrees of redundancy, between cell types and differentiation stages.
authors.affiliation: Department of Biology II, Ludwig-Maximilians-University Munich, Grosshadernerstrasse 2, 82152 Planegg-Martinsried, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Thanisch
authors.affiliation: Department of Biology II, Ludwig-Maximilians-University Munich, Grosshadernerstrasse 2, 82152 Planegg-Martinsried, Germany.
authors.forename: Congdi
authors.initials: C
authors.lastname: Song
authors.affiliation: Transgenic Service Facility, BTE, Franz-Penzoldt-Centre, Friedrich-Alexander-University of Erlangen-Nürnberg, Erwin-Rommel-Str.3, D-91058 Erlangen, Germany.
authors.forename: Dieter
authors.initials: D
authors.lastname: Engelkamp
authors.affiliation: Department of Biology II, Ludwig-Maximilians-University Munich, Grosshadernerstrasse 2, 82152 Planegg-Martinsried, Germany.
authors.forename: Jeannette
authors.initials: J
authors.lastname: Koch
authors.affiliation: Institute of Medical Biology, 8A Biomedical Grove and Dept of Biological Sciences, NUS, 138648, Singapore.
authors.forename: Audrey
authors.initials: A
authors.lastname: Wang
authors.affiliation: Department of Neurochemistry, Stockholm University, Se-106 91 Stockholm, Sweden.
authors.forename: Einar
authors.initials: E
authors.lastname: Hallberg
authors.affiliation: Max F. Perutz Laboratories, Medical University of Vienna, Vienna Biocenter (VBC), Dr. Bohr-Gasse 9, A-1030 Vienna, Austria.
authors.forename: Roland
authors.initials: R
authors.lastname: Foisner
authors.affiliation: Department of Biology II, Ludwig-Maximilians-University Munich, Grosshadernerstrasse 2, 82152 Planegg-Martinsried, Germany.
authors.forename: Heinrich
authors.initials: H
authors.lastname: Leonhardt
authors.affiliation: Institute of Medical Biology, 8A Biomedical Grove and Dept of Biological Sciences, NUS, 138648, Singapore. Electronic address: colin.stewart@imb.a-star.edu.sg.
authors.forename: Colin L
authors.initials: CL
authors.lastname: Stewart
authors.affiliation: Department of Biology II, Ludwig-Maximilians-University Munich, Grosshadernerstrasse 2, 82152 Planegg-Martinsried, Germany.
authors.forename: Boris
authors.initials: B
authors.lastname: Joffe
authors.affiliation: Department of Biology II, Ludwig-Maximilians-University Munich, Grosshadernerstrasse 2, 82152 Planegg-Martinsried, Germany. Electronic address: Irina.Solovei@lrz.uni-muenchen.de.
authors.forename: Irina
authors.initials: I
authors.lastname: Solovei
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000235:genetics; D002467:Cell Nucleus / Q000235:genetics* / Q000378:metabolism; D006570:Heterochromatin / Q000235:genetics / Q000378:metabolism; D034904:Lamin Type A / Q000235:genetics* / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics* / Q000378:metabolism; D051379:Mice; D009685:Nuclear Envelope / Q000235:genetics* / Q000378:metabolism; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism; D019076:Transgenes
chemical_list: D006570:Heterochromatin; D034904:Lamin Type A; C000603518:Lem2 protein, mouse; D008565:Membrane Proteins; D009687:Nuclear Proteins
keywords: LEMD proteins; LEMD2; Nuclear architecture; Nuclear envelope; Samp1; Transgenic rod cells