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PLoS Genet
2008 Oct 01;410:e1000240. doi: 10.1371/journal.pgen.1000240.
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Mutation of the zebrafish nucleoporin elys sensitizes tissue progenitors to replication stress.
Davuluri G
,
Gong W
,
Yusuff S
,
Lorent K
,
Muthumani M
,
Dolan AC
,
Pack M
.
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The recessive lethal mutation flotte lotte (flo) disrupts development of the zebrafish digestive system and other tissues. We show that flo encodes the ortholog of Mel-28/Elys, a highly conserved gene that has been shown to be required for nuclear integrity in worms and nuclear pore complex (NPC) assembly in amphibian and mammalian cells. Maternal elys expression sustains zebrafish flo mutants to larval stages when cells in proliferative tissues that lack nuclear pores undergo cell cycle arrest and apoptosis. p53 mutation rescues apoptosis in the flo retina and optic tectum, but not in the intestine, where the checkpoint kinase Chk2 is activated. Chk2 inhibition and replication stress induced by DNA synthesis inhibitors were lethal to flo larvae. By contrast, flo mutants were not sensitized to agents that cause DNA double strand breaks, thus showing that loss of Elys disrupts responses to selected replication inhibitors. Elys binds Mcm2-7 complexes derived from Xenopus egg extracts. Mutation of elys reduced chromatin binding of Mcm2, but not binding of Mcm3 or Mcm4 in the flo intestine. These in vivo data indicate a role for Elys in Mcm2-chromatin interactions. Furthermore, they support a recently proposed model in which replication origins licensed by excess Mcm2-7 are required for the survival of human cells exposed to replication stress.
Figure 1. flo intestinal and retinal defects.(A,B) Lateral view of live 5 dpf wild type (wt) and flo larvae. The flo intestine lacks folds (arrow) and the lumen contains detached epithelial cells (arrowheads). (C,D) Dorsal view showing reduced size of the 5 dpf flo eye. (E,F) Histological cross section showing cells with condensed nuclei typical of apoptotic cells in the 60 hpf flo retina, and disorganization of the flo photoreceptor (*) and outer plexiform (arrow) layers. (G,H) Histological cross section showing marked disorganization of the 4 dpf flo retina and cells with condensed nuclei typical of apoptotic cells (white arrow). (I,J) Acridine orange staining showing apoptotic cells in the 48 hpf flo retina but not sibling wild types. (K,L) TUNEL staining showing apoptotic cells in the 75 hpf flo intestine (arrowheads) but not sibling wild types (anteriorâleft, posteriorâright). (M,N) Dorsal view showing mild reduction in the number of flo retinal ganglion cells (arrowheads) and optic nerve diameter (arrow) identified with the Zn5 antibody. (O,P) Confocal projection of immunostained wt and flo larvae showing reduced rod cells in the flo retina including the large ventral cluster of cells and in the periphery of the mid retina (arrow). onl, outer nuclear layer; inl, inner nuclear layer; ipl, inner plexiform layer; rgc, retinal ganglion cell layer; on, optic nerve; le, lens; y, yolk.
Figure 2. The flo locus encodes zebrafish elys.(A) Schematic representation of the genomic region surrounding the flo locus. The names of the polymorphic markers with the corresponding number of recombinants are listed. (B) DNA sequence analysis showing the cytosine to thymidine transition encoding the premature stop codon in the elysti262c allele. (C) Schematic representation of the functional domains of the human (hs) and zebrafish (dr) Elys protein and the protein encoded by the elysti262c allele (flo-ELYS). (DâI) Acridine orange staining showing apoptotic cells in the retina and growth plate of the optic tectum of 48 hpf flo (E,H) and elys-morpholino injected (F,I) larvae but not wt (D,G). (JâR) Confocal projections through the posterior intestine of 96 hpf larvae showing wheat-germ agglutinin positive goblet cells in the epithelium of the posterior intestine of wt (J) but not flo (K) or elys-morpholino injected (L) larvae; secretory cells in wt (M) but not flo (N) or elys-morpholino (O) injected larvae; enterocytes in wt (P) but not flo (Q) or elys-morpholino (R) injected larvae. (SâT) Carboxy-peptidase A positive cells are abundant in the 5 dpf wt (S) but not in elys-morpholino injected (T) exocrine pancreas. (U) Histological cross section through the retina of a 4 dpf elys morpholino injected larva showing retinal disorganization that is comparable to the 4 dpf flo retina (Figure 1H).
Figure 3. elys expression in developing zebrafish.Images (AâH) are whole mount RNA in situ hybridizations. (A) Maternal elys expression at 3 hpf. (B) Strong elys expression is evident in the midbrain (black arrow) and eye (white arrow) at 24 hpf. (CâF) Expression at 48 hpf in the retina (C,E), growth plate of the optic tectum (F), and digestive organs [(D): pancreas (arrow), anterior intestine (white arrowhead), posterior intestine (black arrowheads), liver (*)]. (GâH) Weak elys expression in the 5 dpf anterior (G) and posterior (H) intestine. (I) Graph showing elys expression in whole embryos (4 hpfâ120 hpf) as determined via real-time quantitative PCR.
Figure 4. Nuclear pore disruption in flo mutants.(AâC) Confocal projections through the posterior intestine of 75 hpf wild type (A), flo (B), and elys morpholino injected larvae (C), following anti-FG nucleoporin immunostainings with mAb414 (green; DAPIâblue). There is a dramatic reduction of nuclear pores in the flo and morpholino injected larvae. Inset shows higher magnification of localized regions of the DAPI-stained image. (DâF) Identical findings are evident in the retina of these larvae. Note apparent cytoplasmic accumulation of the immunoreactive FG-nucleoporins in flo and morpholino injected larvae. (G,H) Normal nuclear distribution of FG nucleoporins in wild type and flo skeletal muscle. (I) Western blot showing levels of FG nucleoporin proteins relative to beta-actin in nuclear (nucl) and cytoplasmic (cyto) extracts derived from the intestine of 75 hpf flo and sibling wild type larvae.
Figure 5. Nuclear ultrastructure in flo mutants.Transmission electron micrographs of nuclei from representative 5 dpf wild type and flo intestinal epithelial cells. (A,B) Intact nuclear envelope in wild type (A) and flo (B). (CâF) Tangential sections through the nuclear envelope showing abundant nuclear pores (arrows) in the wild type larva (C,E) but few if any well defined pores in the flo larva (D,F). (E) and (F) are higher magnification views of (C) and (D), respectively.
Figure 6. The flo mutation activates the DNA damage response.(AâC) Acridine orange staining showing apoptosis in the 50 hpf flo retina that is rescued by the tp53 morpholino (mo) knockdown. (D) Histological cross section showing rescue of flo retinal architecture defects by tp53 knockdown [compare (F) with Figure 1G and 1H]. (E) Intestinal defects persist in flo/tp53 double mutants. Arrow, thin intestinal wall; Arrowhead, apoptotic cells in the intestinal lumen. (F) Western blot showing elevated levels of phospho-Chk2 (Serine 33) in the intestine of flo larvae, compared with sibling wild type larvae, but not slim jim larvae (I). (G) Western blot showing comparable levels of phospho-Chk1 (Ser 345) in flo and sibling wild type larvae, before and after γ-radiation (30 Gy) and treatment with hydroxyurea (HU). (H) Western blot showing enhanced phospho-Chk2 activation in the intestine of flo and wild type larvae following γ-radiation (30 Gy). (I) γH2AX is not detected in the flo or wild type intestine (75 hpf), but is detected at this stage following γ-irradiation (30 Gy) or hydroxyurea treatment (HU).
Figure 7. Reduced chromatin bound Mcm2 in the flo intestine.(A) Western analysis showing reduced levels of chromatin bound Mcm2 in 75 hpf and 96 hpf flo intestine compared with wild type siblings. By contrast, levels of chromatin bound Mcm3 (B) and Mcm4 (C) in the flo intestine are comparable to wild type. Multiple bands (*) corresponding to phospho-Mcm3 and phospho-Mcm4 are recognized by the antibodies directed against the native proteins in wt and flo samples.
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