Localized products of futile cycle/lrmp promote centrosome-nucleus attachment in the zebrafish zygote

Abstract

Background: The centrosome has a well-established role as a microtubule organizer during mitosis and cytokinesis. In addition, it facilitates the union of parental haploid genomes following fertilization by nucleating a microtubule aster along which the female pronucleus migrates toward the male pronucleus. Stable associations between the sperm aster and the pronuclei are essential during this directed movement.

Results: Our studies reveal that the zebrafish gene futile cycle (fue) is required in the zygote for male pronucleus-centrosome attachment and female pronuclear migration. We show that fue encodes a novel, maternally-provided long form of lymphoid-restricted membrane protein (lrmp), a vertebrate-specific gene of unknown function. Both maternal lrmp messenger RNA (mRNA) and protein are highly localized in the zygote, in a largely overlapping pattern at nuclear membranes, centrosomes, and spindles. Truncated Lrmp-EGFP fusion proteins identified subcellular targeting signals in the C terminus of Lrmp; however, endogenous mRNA localization is likely important to ensure strict spatial expression of the protein. Localization of both Lrmp protein and lrmp RNA is defective in fue mutant embryos, indicating that correct targeting of lrmp gene products is dependent on Lrmp function.

Conclusions: Lrmp is a conserved vertebrate gene whose maternally inherited products are essential for nucleus-centrosome attachment and pronuclear congression during fertilization. Precise subcellular localization of lrmp products also suggests a requirement for strict spatiotemporal regulation of their function in the early embryo.

Figure 1. Centrosomes fail to attach to pronuclear envelopes in fue embryos

In vitro fertilized embryos from wild-type and fue females fixed and labeled for centrosomes (γ-tubulin antibody, red) and DNA (DAPI, blue) at 10 (A,C) and 15 mpf (B,D). Asterisks indicate polar bodies, and male and female pronuclei are indicated with symbols. Scale bar represents 20 μm and applies to all panels. Images are projections from confocal z-stacks. (E,F) Distance between centrosomal γ-tubulin labeling and male or female pronuclear envelopes quantified at 10 and 15 mpf. Error bars indicate +/- 1 standard error. See also Figure S1, and Movies S1 and S2.

Figure 2. The fue molecular lesion results in an amino acid substitution at a conserved residue of lymphoid-restricted membrane protein

(A) Positional cloning indicated linkage of fue to chromosome 4 between markers z17278 and z7918. On the physical map, the mutation lies between markers in cancer susceptibility candidate 1 (casc1) and insulin-like growth factor 1 (igf1). Fractions below markers indicate the number of recombinant meiotic events at each position out of 2075 examined meioses. (B) A point mutation was discovered in the fourth exon of lrmp, predicted to cause a valine-to-glutamate amino acid substitution at a conserved residue. (C) Alternative splicing involving exon 36 results in proteins of 1447 (Lrmp+EX36, NCBI accession #CAI20727) and 1418 (Lrmp−EX36) residues. The N-terminal region of zebrafish Lrmp is homologous to hypothetical proteins in humans, chick, and several other species. The C-terminal 500 residues in zebrafish Lrmp correspond to Lrmp proteins described in mouse and human cells, and predicted in chick. Predicted domains and fue mutation site (orange marker at amino acid 246) are indicated. See also Figure S2 and Table S1.

Figure 3. lrmp transcripts show dynamic localization patterns lost in fue mutants

(A) Chromogenic in situ hybridization with lrmp antisense probes in wild-type (left) and fue mutants (center), and negative control sense probes (right). (B) Wild-type embryos fixed at 5-minute intervals and labeled with γ–tubulin antibody (red) and DAPI (blue), in combination with fluorescent in situ detection of lrmp mRNA (green). Scale bar represents 20 μm and applies to all panels in (B). See also Figure S3.

Figure 4. Lrmp protein localizes to nuclear membranes and subregions of the mitotic apparatus

Wild-type embryos fixed at 5-minute intervals and labeled with γ–tubulin antibody (red), anti-LrmpMD antiserum (green) and DAPI (blue). Scale bars represent 20 μm. See also Figure S4.

Figure 5. C-terminal domains of Lrmp facilitate subcellular targeting

(A) Diagram of EFGP-fusion constructs. (B) Fusion construct RNAs encoding EGFP::LrmpCC-TM-L (top row), EGFP::LrmpTM-L (second row), and EGFP::LrmpCC construct (bottom two rows), were injected into one-cell wild-type embryos. Embryos expressing EGFP were fixed and processed for DAPI and anti-γ–tubulin immunostaining between 2.5-3.5 hpf. White boxes indicate fields shown at higher magnification (right). Scale bar represents 20 μm in all lower magnification panels. (C) Summary of results from EGFP::Lrmp C-terminal protein expression. Low viability (far right column) manifested as cell division defects and failure to undergo gastrulation, which lead to embryo lysis. See also Figure S5.

Figure 6. Mutant oocyte injection and rescue with wild-type lrmp mRNA

Stage IV oocytes from fue mutant females were isolated and injected with wild-type lrmp mRNA. Following maturation and in vitro fertilization, embryos were fixed at 1 hpf and labeled for DNA, γ–tubulin, and Lrmp. Examples of weakly rescued embryos (top row), moderately rescued embryos (second row), and strongly rescued embryos (third and fourth rows) are shown. White box in the third row indicates the region shown at higher magnification in the fourth row. Embryos from uninjected fue oocytes, derived from the same set of mothers and treated in parallel with injected oocytes, showed the typical mutant phenotype (bottom row). Scale bars represent 20 μm. See also Table S2.

Figure 7. Model for Lrmp function in early zebrafish development

(A, B) At fertilization, Lrmp protein localizes to pronuclear membranes and lrmp mRNA accumulates at centrosomes. Lrmp facilitates the association of pronuclei to the sperm aster (black arrows indicate direction of hypothesized minus-oriented motor movement), promoting pronuclear congression and fusion (C). (D) Spindle-associated lrmp mRNA may provide a localized source of newly synthesized Lrmp protein to the reforming nuclear membranes in late mitosis.