Nuclear pore composition regulates neural stem/progenitor cell differentiation in the mouse embryo

Abstract

Serving as the primary conduit for communication between the nucleus and the cytoplasm, nuclear pore complexes (NPCs) impact nearly every cellular process. The extent to which NPC composition varies and the functional significance of such variation in mammalian development has not been investigated. Here we report that a null allele of mouse nucleoporin Nup133, a structural subunit of the NPC, disrupts neural differentiation. We find that expression of Nup133 is cell type and developmental stage restricted, with prominent expression in dividing progenitors. Nup133-deficient epiblast and ES cells abnormally maintain features of pluripotency and differentiate inefficiently along the neural lineage. Neural progenitors achieve correct spatial patterning in mutant embryos; however, they are impaired in generating terminally differentiated neurons, as are Nup133 null ES cells. Our results reveal a role for structural nucleoporins in coordinating cell differentiation events in the developing embryo.

Figure 1. merm mutant phenotype and characterization of the merm allele

(A) Pax3 whole mount in situ hybridization revealed developmental delay and severely dysmorphic neural tube and somites in the merm e10.5 embryo compared to a wild-type embryo at e9.5. The arrow indicates the distended primitive streak/tail bud. (B) A comparison of e9.5 merm embryos to wild-type embryos at e8.5 and e9.5, hybridized to Snail - a marker of cephalic neural crest and limb mesenchyme - demonstrated the variability of the merm phenotype. (C) The merm mutation in intron 22 (red arrow) led to a C-terminal truncation of the Nup133 protein (red dashed line); the LacZ gene trap (RRK090; GT) inserted into intron 5 of Nup133. (D) The Nup133^GT allele failed to complement the merm allele. All embryos are at e10.5. (E) Extracts prepared from e9.5 embryos and ES cells of the indicated genotypes were analyzed by Western blot using a polyclonal serum raised against human Nup133. Note the lack of detectable Nup133 in the merm embryos and ES cells. A faint band in the GT/GT embryos was consistent with low level expression of the full length Nup133 transcript. The non-specific lower band (*) showed comparable loading.

Figure 2. NPC composition in merm neural tissue

Immunofluorescence using anti-Nup133, anti-Nup107, anti-Nup153, or MAb414 antibodies as indicated: (A) sections of e9.5 neural tube imaged by widefield (Nup133 and MAb414) and confocal microscopy (Nup107 and Nup153); (B) undifferentiated ES cells photographed under a widefield microscope. The portion of the nuclear envelope enclosed within the white rectangle is shown at higher magnification above the panel. Despite the absence of Nup133 in the merm mutant, merm and wild-type cells exhibited similar anti-Nup107, anti-Nup153, and MAb414 punctate staining of nuclear pores. C and D, Western blot analysis detected comparable levels of several nucleoporins in extracts of wild-type and merm mutant cells. (C) wild-type (+/+) and two independent merm (−/−) e9.5 embryos; (D) wild-type (+/+), heterozygous (+/−) and merm (−/−) ES cells. In D, MAb414 antibody was used to detect both Nup153 and p62. In C, β-actin antibody staining served as a loading control.

Figure 3. Nup133 expression in the developing embryo

Staining for β-Gal activity in GT/+ embryos at (A) the egg cylinder stage, (B–C) the headfold (0-somite) stage, and (D–F) the forelimb bud stage. D and D' show, respectively, dorsal and ventral views of the same embryo. Dashed lines in B, D and D' indicate the level of the section shown in C, E and F, respectively. e, epiblast; ee, extraembryonic ectoderm; eem, extraembryonic mesoderm; flb, forelimb bud; fp, floor plate; h, heart; hg, hindgut endoderm; lpm, lateral plate mesoderm; m, mesoderm; n, node; nc, notochord; ne, neuroectoderm; nt; neural tube; rp, roof plate; s, somite; se, surface ectoderm; tb, tail bud; ve, visceral endoderm.

Figure 4. merm ES cell contribution in chimeras

Chimeric embryos dissected at (A) e9.5; (B) e8.0- ventral view, and (C) e7.5. ES cell distribution detected by β-Gal activity (blue) in whole-mount preparations (left panels) and in sections (right panels). (A) Wild-type ES cells contributed uniformly to tissues of e9.5 chimeric embryos – far right panel, whereas merm ES cells were absent from the neural tube (nt) and somites (s) – middle panel. Arrowheads in the left panel indicate the level of the section in the middle panel. (B–C) In e8.0 and e7.5 chimeras, merm ES cells were detected in precursors to the neural tube - the neural plate (np) and anterior/distal epiblast (ep) – and in precursors to the somites – paraxial mesoderm (pxm). In the left most panels of (B) and (C) the arrowheads, labeled a, b, and c, indicate the level of the corresponding sections. mes, mesoderm.

Figure 5. merm ES cell differentiation in culture

(A) Upon LIF withdrawal and addition of retinoic acid (RA), merm colonies generated only small numbers of differentiated neurons marked by TuJ1 expression (green); these had short neurites compared to those in wild-type colonies. Gata4 (red) shows the presence of endodermal cells. (B) After growth in reduced levels of serum, merm ES cell colonies contained mostly alkaline phosphatase-positive (dark blue – b, higher magnification shown in lower panel), Oct4-positive (yellow, d), and Nanog-positive (magenta, f) cells. In contrast, wild-type ES cell colonies generated differentiated cells (unstained, a; green - phalloidin, c; and blue – DAPI, e).

Figure 6. Neural differentiation in merm embryos

(A) The e10.5 merm neural tube (b, d) contained fewer post-mitotic neurons (TuJ1- positive, a–b; Map2-positive, c–d), even compared to an e9.5 control (a, c). Wild-type and merm neural tubes expressed comparable levels of Sox2 (a–b), a marker of both epiblast and neural progenitor cells, and of Sox1 (e–f), a marker of neural progenitor cells. The persistence of Id1-positive cells throughout the merm neuroepithelium (h) indicated that the progenitors were more immature than those in the e9.5 wild-type neuroepithelium (g). (B) The patterning of neural progenitors was comparable between wild-type and merm neural tubes at e10.5 (a–h). (C) Markers of the G1 phase of the cell cycle were mostly absent in the merm neural tube (cyclin D1, a–b; cyclin D2, c–d; and p27Kip1, e–f), whereas expression of phosphorylated histone H3 (phospho-H3) indicated that merm and wild-type cells were undergoing mitosis at similar levels (g–h). Immunofluorescence with the indicated antibodies was performed on cryosections of neural tubes prepared from e9.5 or e10.5 merm and wild-type embryos and imaged by confocal or widefield microscopy.

Figure 7. Epiblast differentiation in merm embryos

(A) Patterning of merm e8.5 extraembryonic tissues was similar to that of an e7.5 wild-type embryo: Otx2, anterior visceral endoderm – arrowheads (a–b); Bmp4, extraembryonic ectoderm – arrowheads (e–f). Patterning of the merm epiblast was delayed by more than 24 hours: Otx2 (a–b); Fgf5 (c–d). Whole-mount in situ hybridization was performed on wild-type and merm embryos on the C3HeB/FeJ strain background; anterior is to the left. (B) Whole-mount in situ hybridization with an Oct4 probe to e8.5 wild-type (a, c) and e9.5 merm embryos (b, d) and anti-Oct4 immuno-labeling on e9.5 neural tube sections (e–f) revealed delayed extinction of Oct4 transcription in the differentiating merm epiblast/neural plate and the persistence of Oct4 nuclear localization in the merm neural tube. DAPI nuclear staining is shown in blue.