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Biology of Reproduction 63, 74-81 (2000)
© 2000 Society for the Study of Reproduction, Inc.

Regular article

Paternal Exposure to Cyclophosphamide Alters Cell-Cell Contacts and Activation of Embryonic Transcription in the Preimplantation Rat Embryo1

Wafa Harrouka, Bernard Robaireb, and Barbara F. Hales2,a

a Departments of Pharmacology and Therapeutics and b Obstetrics and Gynecology, McGill University, Montréal, Québec, Canada H3G 1Y6

ABSTRACT

Paternal exposure to chronic low doses of cyclophosphamide, an anticancer agent, results in aberrant embryonic development of the progeny. We hypothesized that paternal exposure to cyclophosphamide disturbs zygotic gene activity regulating proper progression through preimplantation development and that this disturbance results in improper cell-cell interactions. To test this hypothesis, we analyzed cell-cell interactions and the expression of cytoskeletal elements in preimplantation embryos sired by male rats gavaged with saline or 6 mg kg-1 day-1 cyclophosphamide for 5 wk. Embryos from control litters had 4–12 cells on Day 2 of gestation; cell-cell contacts were observed consistently. Embryos from litters sired by cyclophosphamide-treated males were frequently abnormal and had lower cell numbers and decreased cell-cell contacts. Steady state concentrations of the mRNAs for cell adhesion molecules (cadherins and connexin 43) and structural proteins (ß-actin, collagen, and vimentin) were low in two- and four-cell control embryos; expression increased dramatically by the eight-cell stage. In contrast, embryos sired by cyclophosphamide-treated males displayed the highest expression of most trancripts at the two-cell stage. In parallel with the mRNA profiles, E-cadherin immmunoreactivity was nearly absent in two-cell control embryos and was strong by the eight-cell stage; immunoreactivity in embryos sired by drug-treated fathers was strong at the two-cell stage but absent at later stages. Thus, drug exposure of the paternal genome led to dysregulated expression of structural elements and decreased cell interactions during preimplantation embryonic development.

conceptus, developmental biology, gametogenesis, implantation/early development, sperm, spermatid, spermatogenesis

INTRODUCTION

A key mechanism governing the behavior of cells during the early cleavage stages of mammalian development is a continuous series of cell-cell interactions. The number of cell-cell contacts determines successful embryonic development [1]. Although cell interactions in mammalian embryos are partly assured by the presence of the zona pellucida [2], the ability of zona-free mouse embryos to develop to term depends on the number of total points of contact between blastomeres; more contacts result in a higher number of inner cell mass cells and, subsequently, a higher number of live offspring [3].

A change in the cell surface of eight-cell embryos, associated with the process of compaction, represents the first differentiation event in the mammalian embryo [4]. Compaction consists of polarization, cell flattening, and junctional communication, all of which require cell-cell interactions [5]. Contact-mediated interactions in the compacting embryo require an intact cytoskeletal organization; treatment of compacting embryos with a cytoskeletal inhibitor prevents compaction [6]. Although the signal to initiate compaction does not occur until the eight-cell stage, two-cell embryos undergo morphological changes preceding the events of compaction. Ultrastructural analysis of human embryos reveals that blastomere surface modifications occur as early as the two-cell stage, as manifested by the loss of microvilli, the acquisition of endocytic activity, and the formation of cell junctions at cell-cell contact points [7]. In the mouse, the membrane proteins required for cell-cell interactions and cytoskeleton-membrane interactions are already synthesized and assembled at the two- to four-cell stage [8]. The capacity of the plasma membrane to undergo these changes precedes any detectable activity of the embryonic genome.

One common denominator for all three aspects of compaction (i.e., polarization, cell flattening, and junctional communication) is the requirement for expression of the Ca2+-dependent cadherin, E-cadherin, on adjacent cells. E-cadherin expression spans all stages of preimplantation development, starting at the one-cell stage [9], and is required for the initial cell-cell contacts at the two- and four-cell stages [10]. Deletion of the E-cadherin gene in mice results in embryos that undergo an initial compaction but subsequently decompact; blastomeres lose their polarity and fail to form a blastocyst [11, 12]. Among the proteins that interact with E-cadherin is the intermediate filament protein, vimentin; together, E-cadherin and vimentin form desmosomes in the mesenchymal epithelium [13, 14]. In the early embryo, a number of other proteins have been localized to cell-cell contacts, including filamentous actin [15] and members of the connexin family; the latter are involved in making up the intercellular membrane channels of gap junctions. Of the members of the connexin family, connexin 43 has the highest expression during preimplantation development [16].

Chronic treatment of adult male rats with a low dose of the alkylating agent cyclophosphamide affects male germ cells without an effect on the male reproductive system [17] or on the ability of a spermatozoon to fertilize an oocyte [18]. A 5-wk treatment with cyclophosphamide affects spermatozoa that are first exposed to the drug as late spermatids, thus encompassing spermiogenesis (maturation from spermatids to mature spermatozoa). Starting from the late spermatid stage, the chromatin of male germ cells is tightly packaged because of the replacement of somatic histones with male germ-cell-specific proteins, the protamines [19]. Little, if any, DNA repair or transcription takes place during this period of maturation [20]. It is not until after fertilization that the genome of the spermatozoon is capable of transcriptional activity [21]. As a result, any damage to the drug-treated sperm will only be repaired following fertilization. When treated males are mated to normal females, the progeny start dying at the preimplantation (15%) and early postimplantation stages (80%); some of the live pups (5%) have malformations [22]. Eight-cell embryos sired by cyclophosphamide-treated fathers have decreased cell numbers and a longer cell doubling time, coupled with a decrease in DNA synthesis capacity, when compared with litters sired by control males [23]. The presence of these abnormalities in eight-cell embryos, at the stage when the first events in differentiation requiring cell interactions take place, suggests that there is a disturbance in the gene expression program regulating major events. Thus, we hypothesized that the zygotic gene activity regulating cell division and the proper progression through preimplantation development is disturbed as a result of paternal exposure to cyclophosphamide and that this disturbance results in improper cell-cell interactions. The objectives of the current study were to elucidate the gene expression profiles of key regulators of the cytoskeletal architecture in preimplantation embryos sired by drug-treated males and to investigate the establishment of cell-cell contacts in these embryos.

MATERIALS AND METHODS

Animals

Adult male (300–325 g) and virgin female (225–250 g) Sprague-Dawley rats were obtained from Charles River Canada (St. Constant, PQ, Canada) and housed in the McIntyre Animal Centre, McGill University. Food and water were provided ad libitum, and animals were exposed to a 14L:10D cycle.

Treatment, Mating, and Embryo Collection

Male rats were randomly assigned to the control or cyclophosphamide groups, each consisting of six rats. Males were gavaged daily with saline or 6 mg kg-1 day-1 cyclophosphamide (Sigma, St. Louis, MO) for 5 wk. Starting on the fifth week of treatment, each male was mated overnight with two females in proestrus. On the night of mating, males were not treated to avoid the presence of the drug in the semen [24]. The following morning (considered Day 0 of gestation), females were checked for pregnancy as judged by the presence of spermatozoa in vaginal smears. Sperm-positive females were euthanized on Days 1, 1.5, or 2 of gestation. To ensure that embryos were collected at consistently similar stages of development, vaginal smears and embryo collection were performed always at the same time of day. Embryos were flushed from oviducts and were either snap frozen in liquid nitrogen and stored at -80°C until used for the antisense RNA (aRNA) profile analysis or fixed for immunofluorescence or embryo sectioning and light microscopy. For the aRNA analysis, six–eight litters were collected at the two-, four-, and eight-cell stages from both the control and cyclophosphamide-treated groups, and each litter was assayed separately. Each developmental stage was assessed under light microscopy prior to further processing.

Embryo Sectioning and Light Microscopy

Following collection, Day 2 embryos were transferred to a depression slide containing 2.5% glutaraldehyde in 0.1 M phosphate buffer, pH 7.4 (GA), and fixed for 45 min at room temperature, followed by two washes in phosphate buffer and one wash in Dulbecco phosphate buffer supplemented with 10% BSA (BSAD) (Gibco BRL, Burlington, ON, Canada). Embryos were then transferred to plastic capsules (MECA Lab Ltd., Montreal, PQ, Canada) containing 10 µl BSAD and allowed to settle to the bottom of the capsule for 30–40 min. Capsules were centrifuged horizontally for 15 min at 1800 x g at room temperature. Three drops of GA were placed on top of the BSAD, and capsules were centrifuged again for 60 min at 1800 x g. The capsules were then filled with GA and kept at 4°C overnight. The following day, the molds were removed and the bottoms of the capsules were cut away using a razor blade. Molds were then subjected to a postfixation step using osmium tetroxide (1% in phosphate buffer), dehydrated in serial alcohol washes, and infiltrated with propylene oxide alone followed by a mixture (50:50) of propylene oxide and Epon and finally Epon alone. Epon-embedded molds were left in an oven at 60°C overnight [25]. Thin sections (4 µm) were cut on a Reichert Ultramicrotome, stained with 0.5% toluidine blue O (Fisher Scientific, Nepean, ON, Canada) for 30 sec, dried, and stored until examination under a Leitz MPS60 phase microscope.

Amplified aRNA Procedure

The mRNA profiles were analyzed using the aRNA amplification approach. The protocol used for aRNA amplification in preimplantation embryos was as described previously with minor changes [26]. Embryos from individual litters (an average number of nine embryos/litter was collected and considered as one sample) were sonicated in the presence of lysis buffer containing 1 mg/ml digitonin, 5 mM dithiothreitol (DTT), and 1x in situ transcription buffer (50 mM Tris, 6 mM MgCl2, 0.12 mM KCl, pH 8.3). Embryo homogenates were reverse transcribed at 37°C for 2 h in the presence of an unlabeled oligo dT24 extended at the 5' end with the bacteriophage T7 RNA polymerase promoter (50 ng); recognition of the poly(A) tail of mRNA populations results in cDNA synthesis. Reverse transcription was performed using the avian myeloblastosis virus reverse transcriptase (50 U; NEB, Mississauga, ON, Canada), which binds to the primer and copies single-stranded mRNAs to cDNAs in the presence of deoxyribonucleotides (Boehringer Mannheim, Laval, PQ, Canada). Following a phenol/chloroform extraction, the first strand cDNAs were precipitated with ethanol and glycogen (4 mg). The pellet was resuspended in 10 µl of water, heated to 95°C to denature the DNA:RNA hybrid, and chilled on ice. Single-stranded cDNAs were allowed to self-prime (by hairpin loop formation) to form double cDNA strands (ds-cDNAs). Following excision of the self-primed hairpin loop by S1 nuclease (2 U) for 3 min at 37°C, ds-cDNAs were blunt ended and filled in with the addition of T4 DNA polymerase (5 U) and the Klenow fragment of Escherichia coli (5 U) in the presence of dATP, dCTP, dGTP, and dTTP nucleotides. The resulting ds-cDNAs, which contained the promoter region for the T7 RNA polymerase (NEB), were extracted with phenol/chloroform and amplified into aRNA in the following mixture: 40 mM Tris pH 7.5, 6 mM MgCl2, 10 mM DTT, 400 mM of ATP, GTP, and UTP and 4 mM CTP, 20 U RNasin, 100 U T7 RNA polymerase, and 32P-radiolabeled CTP (30 µl) in a final volume of 25 µl. This reaction was conducted for 4 h at 37°C.

Reverse Northern Blots and Genetic Expression Profiles

Equimolar concentrations of cDNA clones were calculated based on the approximate size of the insert relative to the vector size, which served to normalize the total amount of each of the cDNA clones immobilized on slot blots and hybridized with the aRNA probes. Radiolabeled amplified RNA populations obtained from embryos at the two-, four-, and eight-cell stages were utilized as probes to screen slot blots containing the following cloned cDNAs: E-cadherin [27], N-cadherin [28], P-cadherin [29], vimentin [30], collagen [31], connexin 43 [32], ß-actin [33], and retinoic acid binding protein 1 (RABP-1) [34]. Individual blots were prehybridized for 30 min in buffer (50% formamide, 0.12 M Na2HPO4, pH 7.2, 0.25 M NaCl, 7% w/v SDS) at 42°C. The heat-denatured aRNA probe from each embryo sample was applied to the blots and allowed to hybridize overnight at 42°C. The following day, blots were washed at decreasing stringency from 2x sodium chloride citrate (SSC) solution down to 0.1% SSC and 0.1% SDS at 42°C for 20 min in a shaking water bath. Blots were exposed to phosphorimager plates overnight. Individual band intensities were determined after exposure of the plates to a phosphorimager scanner (Molecular Dynamics, Sunnyvale, CA). The individual signals were corrected for nonspecific binding by subtracting the background values on each blot and normalizing to the internal standard. The use of an internal standard allows comparisons between different blots because the individual hybridization intensities of each cDNA on a blot are expressed as a ratio of expression to that of the internal standard. The three internal standards on each blot were RABP-1, DNA methyltransferase 1, and {gamma}-glutamylcysteinyl synthase 50; all three cDNAs showed similarly consistent results between stages and groups (data not shown). Subsequent data are expressed as a percentage of RABP-1 transcript concentrations; steady state concentrations of RABP-1 mRNA were consistent between groups (control and drug treated) and across blots, thus facilitating experiment-experiment and blot-blot quantification and comparison.

Because the cDNA population should reflect accurately the abundance of the original mRNA population, the intensity of the aRNA hybridization signal reflects the abundance of the original mRNA for any given gene. Six to eight expression profiles, each representing individual litters, were obtained for each stage and treatment group. A representative blot displaying the hybridization signal resulting from a litter at the two-cell stage is shown in Figure 1; bands representing the genes discussed here have been identified.



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  		FIG. 1. Blot illustrating the banding pattens resulting from hybridization of aRNA from a two-cell stage litter sired by a cyclophosphamide-treated rat to a slot blot membrane. Bands representing hybridization to the cDNAs are identified

Immunofluorescence and Laser-Scanning Confocal Microscopy

Embryos were fixed in a solution of 95% ethanol and glacial acetic acid (9:1 v/v) for 15 min; this step eliminated the need for removal of the zona pellucida, which made the embryos very sticky and caused disaggregation of blastomeres. Fixation was followed by rigorous washing in 1x PBS/polyvinylpyrolidone (1%). Embryos were permeabilized (0.05% Triton X-100, 1% BSA in PBS for 15 min, blocked for 1 h in 3% BSA, 0.1% Triton X-100 in PBS) and then incubated overnight at 4°C in a humidified chamber with the primary antibody for E-cadherin (a polyclonal anti-rabbit antibody; Signal Transduction Laboratories, Mississauga, ON, Canada) at a 1:50 dilution in blocking solution. For the negative controls, the primary antibody was omitted from the protocol. Embryos were washed for 3 x 20 min in blocking solution, incubated in a fluorescein-conjugated mouse anti-rabbit secondary antibody (Molecular Probes, Eugene, OR) for 1 h, washed 3 x 10 min, and incubated in propidium iodide (1 µg/ml) (Molecular Probes) in blocking solution to stain nuclei, followed by rigorous washing. To mount embryos, a drop of immunomount (Shandon, Pittsburgh, PA) was placed on a Superfrost microscope glass slide, and embryos were pipetted into the drop. Embryos were allowed to settle to the bottom of the drop to prevent them from floating and were covered with a glass cover slip. Slides were allowed to dry overnight in the dark before examination under a Zeiss LSM410 laser scanning microscope.

Statistical Analysis

Analysis of the developmental regulation of gene expression for each embryonic stage was conducted separately on litters sired by control and cyclophosphamide-treated males using a pairwise multicomparison procedure (ANOVA) followed by a Tukey post hoc analysis (P < 0.05). To reveal any effect of paternal treatment on gene expression in the progeny, a pairwise comparison consisting of a t-test followed by a Tukey post hoc test (P < 0.05) for each developmental stage was conducted between litters fathered by control and cyclophosphamide-treated males. Statistical analysis was done using a Sigmastat 2.3 (SPSS Inc., Chicago, IL) software package.

RESULTS

Morphological Assessment of Cell Numbers and Cell-Cell Interactions

Analysis of preimplantation embryos at the light microscope level revealed that control embryos on Day 1.5 of gestation were usually at the four-cell stage with the presence of cell-cell contacts (Fig. 2A). A significant delay in cell division starting from Day 1.5 postcoitum was seen in litters sired by cyclophosphamide-treated male rats when compared with controls (data not shown). By Day 2 of gestation, control litters had divided to reach up to the 16-cell stage (Fig. 2B). Even though progeny of cyclophosphamide-treated males had developed to the four-cell stage and beyond, a significant number of embryos contained dead blastomeres (Fig. 2, C and E) and some individual blastomeres looked slightly shrunken (Fig. 2D), thus contributing to a reduction in cell contacts in these embryos. Other abnormalities observed in litters sired by treated males included a higher percentage of abnormal/degenerated embryos (Fig. 2F).



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  		FIG. 2. Rat embryonic morphology examined under light microscopy on Days 1.5 (A) and 2 (B–F) of gestation. Embryos were scored for cell numbers, the presence of cell-cell contacts, and any abnormal structures within the zonae pellucidae. On Day 1.5, control embryos were at the four-cell stage (A). Day 2 control embryos had advanced to the 8- to 12-cell stages (B). Embryos sired by cyclophosphamide-treated fathers (C–F) displayed a number of abnormalities: loss of individual blastomeres (C and D), degenerate or abnormal morphology (F), and absence of cell-cell contacts (E)

A comparison of the cell numbers and cell contacts in approximately 100 embryos/group on Day 2 of gestation is shown in Figure 3. Analysis of control embryos revealed a range of cell numbers from 4–6 (39%) to 8 (42%) or 12–16 (15%) (Fig. 3A, solid bars). Analysis of progeny of cyclophosphamide-treated males revealed that these embryos were capable of progressing to the four- to six-cell stage (41%) and the eight-cell stage (21%). However, embryos in this group manifested a number of abnormalities as compared with those from control litters, including an increase in the percentage of embryos classified as abnormal or degenerating (26%) and an arrest of some embryos at the two-cell stage (9%); these findings explain the lower number of embryos that advanced to the 8-cell (21%) and the 12- to 16-cell (3%) stages when compared with control embryos (Fig. 3A, shaded bars).



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  		FIG. 3. Cell numbers (A) and cell contacts (B) in progeny sired by control and cyclophosphamide-treated rats on gestational Day 2. Embryos sired by control males (n = 95) (A, solid bars) ranged from the 4-cell to the 8-cell stages, with some reaching the 12-cell stage. Embryos sired by cyclophosphamide-treated males (n = 100) (A, shaded bars) had lower cell numbers and a higher percentage of abnormal, fragmented, or retarded embryos. The majority of embryos in control litters displayed cell contacts (B, solid bars), whereas a markedly reduced number of embryos with cell contacts were seen among the progeny of drug-exposed males (B, shaded bars)

Analysis of the percentage of embryos having cell-cell contacts on Day 2 of gestation revealed that most control embryos at all stages examined exhibited the presence of cell contacts (approximately 95%) (Fig. 3B, solid bars). In contrast, less than half of the embryos sired by cyclophosphamide-treated fathers displayed such cell-cell contacts at all of the stages examined with the exception of the 12- to 16-cell stage (Fig. 3B, shaded bars).

Temporal Expression of Cell Adhesion Molecules and Cytoskeletal Elements during Rat Preimplantation Development

Control embryos expressed low levels of transcripts at the two- and four-cell stages for all of the genes examined; only vimentin was expressed with a relatively moderate abundance. At the eight-cell stage, an increase in expression was seen for most of the genes examined; this was most marked for N-cadherin, E-cadherin, and vimentin (P < 0.01, 0.003, and 0.04, respectively, Fig. 4). Analysis of the gene expression profiles among the embryos sired by cyclophosphamide-treated males revealed a remarkable shift of maximal gene expression for many of the genes examined to the two-cell stage (P < 0.02–0.001). A decrease was evident by the four-cell stage, and a further dramatic decrease was observed at the eight-cell stage (Fig. 4). A comparison of the embryos in each treatment group was conducted separately for each developmental stage. An eightfold increase in expression for all genes except for ß-actin was seen at the two-cell stage in embryos sired by treated fathers (P < 0.001). By the four-cell stage, embryos sired by drug-treated rats still showed a dramatic increase in transcripts over controls, especially for N-cadherin, P-cadherin, and vimentin (P < 0.004, 0.006, and 0.025, respectively). At the eight-cell stage, except for connexin 43, the expression for all genes examined was remarkably reduced (P < 0.003–0.01) among the progeny of cyclophosphamide-treated fathers. These data suggest that a precocious increase in the steady state concentrations of cell structural gene transcripts among embryos sired by cyclophosphamide-treated fathers may be incompatible with normal embryonic development.



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  		FIG. 4. aRNA analysis of the expression profiles of cell adhesion and cytoskeletal genes in preimplantation rat embryos sired by control and cyclophosphamide-treated male rats. Results are expressed as the mean ± SEM (n = 8) of the intensity of expression of each probe as a percentage of the internal standard, RABP-1. Control embryos, shaded bars; embryos sired by cyclophosphamide-treated males, solid bars. At the two-cell stage, transcripts were low for all genes in control embryos; embryos sired by cyclophosphamide-treated males revealed a marked and significant increase in expression of most transcripts (P < 0.001–0.02). At the four-cell stage, expression remained low in control embryos; some expression persisted for E-cadherin, P-cadherin, and vimentin in embryos sired by cyclophosphamide-treated males, at a level significantly higher than that of controls (P < 0.004–0.026). By the eight-cell stage, control embryos showed an increase in expression of most genes examined, with a peak seen in transcripts for N-cadherin (P < 0.01), E-cadherin (P < 0.003), vimentin (P < 0.04), and collagen (P < 0.042); embryos sired by cyclophosphamide-treated males showed a consistently low expression at this stage. Asterisks indicate a significant difference from control (P <= 0.05) at that stage

Immunofluorescence of E-Cadherin

Immunostaining of E-cadherin in two-, four-, and eight-cell embryos sired by control and cyclophosphamide-treated fathers revealed a time-shift in maximal immunoreactivity (Fig. 5). In control embryos, E-cadherin immunoreactivity was low at the two- and four-cell stages and localized between blastomeres in the regions of cell-cell contacts. At the eight-cell stage, this expression increased to encompass cytoplasmic staining that was not seen in earlier stages (Fig. 5, A–C). The strong staining in the cytoplasm overwhelmed the staining at cell-cell contacts in the embryo shown in Figure 5. Embryos sired by cyclophosphamide-treated males showed a strong E-cadherin immunoreactivity at the two-cell stage in a pattern that was not restricted to cell-cell contacts; staining was evident throughout the cytoplasm and perinuclear areas. At the four- and eight-cell stages, embryos from the treated group showed cytoplasmic staining that was reduced substantially from that seen at the two-cell stage and in stage-matched control embryos (Fig. 5, D–F).



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  		FIG. 5. Immunolocalization of E-cadherin in two-, four-, and eight-cell rat embryos. Immunostaining of E-cadherin epitopes is depicted in green, and nuclear staining is seen in red. Embryos sired by control males (left) showed staining at cell-cell contacts at the two- and four-cell stages (A and B). This staining became more cytoplasmic by the eight-cell stage (C). Immunostaining of E-cadherin in embryos sired by cyclophosphamide-treated males (right) at the two-cell stage was not confined to cell contacts but spread to the cytoplasm and the perinuclear area (D). Four- and eight-cell stage embryos did not have a detectable signal for E-cadherin (E and F)

DISCUSSION

Chronic administration of the anti-cancer agent cyclophosphamide to male rats results in a dose- and time-dependent loss in the embryonic progeny of these rats starting from the preimplantation stages of development. In this study, we examined the spatial and temporal events that precede this embryonic loss. Using light microscopy, we traced the early morphological abnormalities to Day 2 of gestation, a critical time in embryonic development that corresponds to embryonic compaction and to the first signs of the differentiation program of the embryo. A diagrammatic representation of some key abnormalities found in progeny after paternal treatment with cyclophosphamide is shown in Figure 6; abnormalities include a delay in cell division, a higher proportion of embryos lacking cell-cell contacts, and a high number of degenerated and abnormal embryos.



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  		FIG. 6. Gene expression, cell numbers, and cell-cell contacts in rat preimplantation embryos. Gene expression patterns were examined for cytoskeletal elements at the two-, four-, and eight-cell stages. Although the expression of most genes peaked at the eight-cell stage in control embryos, embryos sired by cyclophosphamide-treated fathers displayed a precocious expression at the two-cell stage. Immunolocalization data for E-cadherin showed that this increase was not confined to cell interaction regions but expanded throughout the embryo. Cell numbers in the treated group lagged at the four-cell stage, when loss of individual blastomeres was often seen. By Day 2 of gestation, cell loss continued and abnormal and degenerate embryos became more frequent. Blastomere interaction was compromised in the treatment group, as manifested on Day 1.5 and Day 2 of gestation

The mechanisms underlying the loss of cell interactions have been studied in several other model systems. Disrupting cell-cell interactions in Xenopus embryos results in a differential response in mesodermal and ectodermal lineages; although expression of {alpha}-actin, a mesodermal gene, is inhibited when cell interactions are disrupted, expression of DG81, an ectodermal gene, is not affected [35]. In Caenorhabditis elegans embryos, cell-cell interactions are pivotal in the production of regions during embryogenesis [36]. In our model, a dysregulation in the gene expression program of cytoskeletal elements in the embryo, starting during preimplantation stages, leads to distinct abnormalities at various levels of cell lineage allocation in the developing embryo. This dysregulation is evident as early as the two-cell stage when steady state concentrations of all transcripts except collagen were high; a subsequent decrease was observed in all transcripts, except for connexin 43, at the eight-cell stage. The accumulation of transcripts in these embryos may be due to either an increase in transcription (induction) of the zygotic genome or a decrease in the rate of maternal mRNA degradation (reduced turnover). Our current experimental design does not distinguish between these two possibilities.

In accord with the aRNA data, embryos sired by cyclophosphamide-treated males showed a similar precocious increase of E-cadherin immunoreactivity at the two-cell stage that was not sustained in later stages. The immunoreactive E-cadherin was not targeted to the expected cellular compartment. In control embryos at the eight-cell stage, E-cadherin undergoes phosphorylation, which causes displacement to the cytoplasm [37] and might explain the presence of a strong cytoplasmic signal in control embryos.

Starting at the 16- to 32-cell transition during normal development, the extent to which outer blastomeres divide to contribute to either the inner cell mass (the nonpolar cells) or trophectoderm cells (the outer polar cells) is related to the extent of flattening of the outer blastomeres. When the flattening of these blastomeres is less extensive, their contribution to nonpolar cells is higher, a phenomenon referred to as differentiative division. The inside cells, by forcing outwards on polar cells, might delay or even inhibit division itself [38]. In our model, there was a general decrease in cell number (both inside and outside), thus the pressure exerted by the inner cells on the outer cells was not established. Analysis of embryonic morphology at the eight-cell stage revealed a high number of embryos missing inner cells (Fig. 6). As a result, it is tempting to speculate that the lower cell numbers may result in an initial delay of the differentiative division of eight-cell blastomeres and an eventual inhibition of division itself, thus potentiating the retardation and perhaps eventual failure of the embryos to proceed through implantation.

Normal cell associations are maintained in preimplantation embryos by the presence of a number of cytoskeletal and cell adhesion proteins (Xenopus laevis [35]; human [39]). In turn, cell interactions regulate the expression profile of a number of genes, including cingulin [40], syndecan [41], spectrin [42], and myosin [43]. In the present study, control embryos exhibited a dramatic increase in expression of cytoskeletal and cell adhesion molecules at the eight-cell stage. During normal development, a number of the transcripts important for cell proliferation, cavitation, and blastocyst formation accumulate in abundance in a second wave of gene activation, concomitant with compaction [44]. In contrast, embryos sired by cyclophosphamide-treated rats had a premature peak of expression at the two-cell stage. This peak was unexpected because the morphological data suggested a retardation of cell division and development. On the basis of differences in protein synthetic profiles from the one-cell to the four-cell stage [45], it is likely that the activation of embryonic transcription in rats occurs at the two-cell stage, as it does in mice and other rodents [46]. In the mouse, two-cell embryos acquire a repression of gene activation prior to any cell division, a property inherited from maternal factors [47]. Such an activity is lacking in male pronuclei and the cytoplasm of early one-cell embryos [48]. This suppression has been proposed to coordinate the zygotic clock of the embryo and to ensure a proper sequence of transcriptional and translational events in the embryo [49]. If this suppression is also an important feature controlling the timing of zygotic gene activation in the rat embryo, a disturbance in its regulation may be one of the consequences of exposure of the paternal genome to drugs.

We have shown previously that the nuclei of spermatozoa from male rats treated with cyclophosphamide decondense in vitro at a faster rate than do the nuclei of spermatozoa from control rats [50]. It is possible that the initial decondensation of cyclophosphamide-exposed spermatozoa is also faster in the oocyte cytoplasm. This more rapid decondensation might hasten the initial events of gene activation and transcription in these embryos, as observed in the present study. However, this initial gene activity was not sustained until later stages; transcript levels fell by the eight-cell stage, a period when transcripts are required in control litters for differentiation events. The embryo loss among litters sired by drug-treated males could be a result of this abnormal precocious gene activity.

An intact sperm nucleus is an essential requirement for normal embryonic development. A recent report illustrating the need for a normal sperm nucleus lends support to this hypothesis [51]. Exposure of males to cyclophosphamide resulted in an increased incidence of DNA breaks in their germ cells [52]. In addition, cross-links in spermatozoal chromatin have been reported following exposure to other alkylating agents [53]. The genome of spermatozoa obtained from cyclophosphamide-treated males had a more open conformation, as assessed by initial in vitro DNA template function, than did the genome of spermatozoa from control males [52]. Exposure of the paternal genome to cyclophosphamide may affect DNA synthesis in the first round of zygotic DNA replication. Early DNA replication in embryos sired by cyclophosphamide-treated males might relieve the transcriptionally repressive state that normally arrests maternal gene expression, thus leading to premature differentiation [54].

When two- and four-cell mouse embryos were incubated in the presence of aphidicolin, a reversible inhibitor of DNA polymerase {alpha}, during the G1 phase, only embryos incubated at the two-cell stage were affected adversely [55]. These effects were manifested as delayed DNA replication and the absence of cell division; there was no flattening or polarization, events that occur only later in eight-cell embryos. These data suggest that it is early inhibition of DNA replication that has a maximal impact on later events in the differentiation of the preimplantation embryo. Dysregulation of the pattern of gene expression in two-cell stage embryos sired by cyclophosphamide-treated males may result from inhibition of DNA synthesis, which is manifested later as a failure of these differentiative events.

The DDK syndrome is a genetic mouse model that shows an early embryonic lethal phenotype caused by an incompatibility between a maternal and a paternal factor of DDK origin [56]. Transmission of the gene through the maternal genome contributes to the defect [57]. One of the characteristics of the DDK syndrome is that embryos have a defect in cell interactions starting at the eight-cell stage [58].

It is tempting to speculate, based on the results obtained from the DDK model and from paternal exposure to cyclophosphamide, that there is a differential parent-of-origin effect that influences major events of the embryonic program such as the establishment of cell-cell interactions. Such events must be sensitive turning points that are susceptible to various insults and require components contributed by both the maternal and paternal genomes.

ACKNOWLEDGMENTS

We thank the following investigators for providing cDNA probes: G. Batist for connexin 43, M.J.G. Bussemakers for vimentin, A.C. Cuello for ß-actin, J. Douglass for retinoic-acid binding protein-1 (RABP-1, BC-1), N. Morris for the {alpha}1 (XI) chain of collagen, and M. Takeichi for E-, N-, and P-cadherins. We also thank R.H. Finnell for introducing us to the aRNA technique and M. Ballak and C. Huang for excellent technical assistance with embryo embedment and sectioning.

FOOTNOTES

First decision: 20 January 2000.

1 Supported by a grant from the Medical Research Council of Canada. Back

2 Correspondence: B.F. Hales, Department of Pharmacology and Therapeutics, McGill University, 3655 Drummond St., Rm 110, Montreal, PQ, Canada H3G 1Y6. FAX: 514 398 7120; bhales{at}pharma.mcgill.ca Back

Accepted: February 14, 2000.

Received: December 20, 1999.

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