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. Author manuscript; available in PMC: 2013 Jan 30.
Published in final edited form as: Biol Reprod. 2010 Oct 27;84(3):537–545. doi: 10.1095/biolreprod.110.085951

Survival and size are differentially regulated by placental and fetal PKBα/Akt1

Vicki Plaks 1,*, Elina Berkovitz 2,*, Katrien Vandoorne 1, Tamara Berkutzki 2, Golda M Damari 2, Rebecca Haffner 2, Nava Dekel 1, Brian A Hemmings 3, Michal Neeman 1, Alon Harmelin 2
PMCID: PMC3558738  EMSID: EMS51211  PMID: 20980686

Abstract

The importance of the placental circulation is exemplified by the correlation of placental size and blood flow with fetal weight and survival during normal and compromised human pregnancies in conditions as preeclampsia and intrauterine growth restriction (IUGR). Utilizing non-invasive magnetic resonance imaging (MRI) we evaluated the role of PKBα/Akt1, a major mediator of angiogenesis, on placental vascular function. PKBα/Akt1 deficiency reduced maternal blood volume fraction without affecting the integrity of the feto-maternal blood barrier. In addition to angiogenesis, PKBα/Akt1 regulates additional processes related to survival and growth. In accordance to reports in adult mice, we demonstrated a role for PKBα/Akt1 in regulating chondrocyte organization in fetal long bones. Using tetraploid complementation experiments, with PKBα/Akt1-expressing placentas, we found that while placental PKBα/Akt1 restored fetal survival, fetal PKBα/Akt1 regulated fetal size, as tetraploid complementation did not prevent intrauterine growth retardation. Histological examination of rescued fetuses showed reduced liver blood vessel and renal glomeruli capillary density in PKBα/Akt1 null fetuses, both of which were restored by tetraploid complementation. However, bone development was still impaired in tetraploid-rescued PKBα/Akt1 null fetuses. Although PKBα/Akt1-expressing placentas restored chondrocyte cell number in the hypertrophic layer of humeri, fetal PKBα/Akt1 was found to be necessary for chondrocyte columnar organization. Remarkably, a dose-dependent phenotype was exhibited for PKBα/Akt1, when examining PKBα/Akt1 heterozygous fetuses as well as those complimented by tetraploid placentas. The differential role of PKBα/Akt1 on mouse fetal survival and growth may shed light on its roles in human IUGR.

Keywords: PKBα, placenta, fetus, tetraploid, MRI

Introduction

In the mammalian placenta, respiratory gases, nutrients, and wastes are exchanged between the maternal and fetal vasculature. From embryonic day (E) 10.5, mouse fetal growth depends on the umbilical flow directing blood via the placenta, which includes three parts: placental labyrinth, spongiotrophoblast layer and decidua basalis. Placental circulation is a critical determinant for fetal and placental size[1]. Reduced maternal placental blood flow is associated with early embryonic mortality, fetal growth retardation[2] and impaired neonatal survival and growth[3]. Etiologies of preeclampsia and intrauterine growth restriction (IUGR) are frequently associated with abnormalities in placental growth, structure, and function, eventually giving rise to decelerated fetal growth and subsequent infant mortality and morbidity[4].

Angiogenesis refers to the formation of a new vascular bed, and is a critical process for normal tissue growth and development[5]. Placental angiogenesis is a major determinant in the increase of fetal placental blood flow throughout gestation[1]. Vascular endothelial growth factors (VEGFs), representing a major class of placental angiogenic factors, stimulate angiogenic processes[6]. Trophoblasts are a rich source of angiogenic growth factors, such as VEGF-A, which directs the growth of maternal blood vessels towards the embryonic implantation site[7]. The maternal circulation in the placenta involves vascular mimicry by fetal trophoblast cells, which respond as well to the angiogenic signals.

PKB/Akt1 acts downstream of VEGF-A via the VEGF-receptor-2/PI3K/PKB signaling cascade, known to mediate the formation of new blood vessels[8]. In addition to angiogenesis, the three isoforms of PKB/Akt (1, 2 and 3) regulate many other cellular and physiological processes such as glucose metabolism, transcription, cell cycle regulation and survival. PI3K and PKB/Akt are expressed and functional from the 1-cell stage of the mouse preimplantation embryo and specifically, PKB can be detected in the inner cell mass (ICM) and the trophoblast cells[9, 10]. PKBα/Akt1 (also known as Akt1; one of three known PKB isoforms) was found to be present in all types of trophoblast cells and vascular endothelial cells [11].

Placentas of PKBα/Akt1 null (−/−) fetuses were shown to display decreased vascularization and significant hypotrophy with marked reduction of the decidua basalis[11]. PKBα/Akt1 −/− mice were found to be smaller with increased perinatal mortality and disordered fetal vasculature[12, 13]. In human pregnancies, placentas of IUGR exhibit signs of oxidative stress, with reduced Akt signaling[14] and therefore, the PKBα/Akt1 −/− mice were suggested as a model for the human IUGR. The persistence of the reduced size in adult PKBα/Akt1 −/− mice was further suggested to be associated with bone mineralization defects characterized by decreased length and mass of the long bones[15, 16]. We recently reported that impaired endochondral bone growth in these mice was associated with decreased bone vascularization, which was significant also for mice lacking a single copy of PKBα/Akt1 [17]. From the evidence so far, it is clear that the absence of PKBα/Akt1 from the placenta and embryo results in substantial embryonic defects mainly characterized by fetal mortality and reduced size, the latter persist after birth, possibly due to bone developmental defects. However, it is still unclear whether the embryonic defects are secondary to placental hypovascularity caused by lack of PKBα/Akt1 or due to its absence from the embryo proper.

The study reported here aimed at differentiating between the role of PKBα-associated placental deficiency and the role of fetal PKBα/Akt1 in fetal survival and development. We utilized non-invasive macromolecular dynamic contrast enhanced (DCE) magnetic resonance imaging (MRI) to evaluate placental vascular functionality in PKBα/Akt1 −/− fetuses. In contrast to histology, which allows quantification of vessel number and morphology, this MRI methodology allows the assessment of vessel functionality, as shown in our previous work on deciduas at embryo implantation sites [18]. Macromolecular DCE MRI was also applied to assess the maternal circulation in normal placental vasculature and in tetraploid placental complementation [19]. Tetraploid complementation/rescue is typically used to rescue embryonic lethality caused by defects in extraembryonic tissues like placenta. By doubling their ploidity at the 2-cell stage, tetraploid embryos are able to contribute to the extraembryonic tissue and complement diploid embryos upon aggregation with their ICM[20]. Tetraploid-aggregated embryos will give rise to the placenta while the ICM will give rise to the embryo proper. Tetraploid rescue was used here to complement PKBα/Akt1 −/− embryos with PKBα-expressing placentas, allowing us to differentiate between the roles of placental versus fetal PKBα/Akt1 in determining fetal survival and size.

Materials and Methods

Embryo transfer and tetraploid complementation/rescue

Embryo collection and culture

All animal experiments were approved by the Weizmann Institutional Animal Care and Use Committee. Mice were maintained on a 12h light (from 6h00-18h00), 12h dark cycle. For superovulation, 3-week old ICR females were superovulated by intraperitoneal injection (i.p.) of 5 international units (IU) of pregnant mare serum gonadotropin (PMSG) at 13h00 and 5IU of human chorionic gonadotropin (hCG) 46h later. Females were then mated with B5/EGFP males[21], examined for vaginal plugs the following morning (defined as E0.5; 0.5 days post coitum, d.p.c), and sacrificed at 46-48h after hCG (1.5 d.p.c) to collect late 2-cell embryos, by oviduct flushing. These were later used to produce tetraploid embryos by electrofusion. Five-to 6-week old B6(C57Bl/6J)/PKBα/Akt1 heterozygote (+/−) females were superovulated as described above, mated with B6/PKBα/Akt1 knockout (−/−) males and sacrificed on 2.5 d.p.c to isolate 8-cell embryos for aggregation with the tetraploid embryos.

Immunosurgery

To minimize the contribution of the B6 origin (of the PKBα/Akt1 −/− embryos) to the tetraploid placenta, the trophectoderm (TE) layer of the blastocysts was lysed and removed by immunosurgery, as previously described[22], thus isolating the ICM. Briefly, blastocysts were exposed to rabbit anti-mouse lymphocyte serum (1:20 in DMEM) for 30min at 37°C, washed with media and transferred to guinea pig serum (1:2, as complement) for 10min. Gentle pipettation allowed the final separation of the ICM from the lysed TE.

Production of tetraploid embryos

Electrofusion was done using a CF-150 pulse generator equipped with a 250μm electrode chamber (BLS Ltd, Hungary). Electrodes were covered by a drop of 3M manitol (Sigma). Two-cell embryos were placed perpendicular to and between the electrodes and two pulses of 30V were delivered over 40ms (AC field between 1.3-1.5V). Following the pulse, embryos were transferred to the incubator for the fusion to take place within 30-40min (with more than 90% efficiency). Fused embryos were separated and cultured overnight in KSO media [23] and after 24h, compacting 4-cell stage embryos were used for aggregation.

Aggregation of tetraploid embryos and isolated ICM

A single isolated (diploid) ICM was sandwiched between two tetraploid embryos (after removal of the zona pellucida with acidic Tyrode’s solution, Sigma) in an aggregation plate. After 24h incubation, blastocysts were transferred to the uteri of 2.5 d.p.c pseudopregnant ICR females, 8-10 blastocysts per uterine horn. As a control, embryo transfer only with no tetraploid complementation was used: E3.5 blastocysts derived from PKBα/Akt1 +/− females mated with either +/− or −/− males (without mixing of blastocysts from the different matings prior to transfer) were transferred to 2.5 d.p.c pseudopregnant ICR females, 8-10 blastocysts per uterine horn [23].

In vivo contrast enhanced MRI studies

Female ICR mice (12 weeks old) carrying transferred embryos (ICR; B6/PKBα/Akt1 wild type (+/+); +/− or −/− with native or tetraploid placentas), were analyzed by MRI on E18.5 of pregnancy. Images were taken before and sequentially till 13.5 min after intravenous (i.v.) tail vein injection of the macromolecular contrast agent, biotin-BSA-GdDTPA [18, 19, 24, 25] (See supplementary material for full details).

Histology, histochemistry and fluorescence microscopy

BSA labeled with rhodamine (BSA-ROX) was i.v. injected 3-5 min prior to animal sacrifice for histological detection of functional blood vessels as reported previously [24]. Placentas and fetuses were retrieved for further analysis. Macro images of ex-vivo fetuses and placentas were achieved using regular and fluorescent light (Olympus SZX12 microscope equipped with DP50 camera) and then samples were fixed (fetuses in 4% PFA and placentas in Carnoy mixture), embedded in paraffin, sectioned serially at 4μm thickness and stained with eosin and hematoxilin (H&E) and for biotin-BSA-GdDTPA (using fluorescein labeled avidin, avidin-FITC, Molecular probes, San Francisco, CA, USA) as previously described[26]. See details for the morphometric analysis in the supplementary material.

Statistical Analysis

Statistical analysis accounted for the mixed genotypes of the litters (resulting from matings of PKBα/Akt1 +/− females mated with either +/− or −/− males ). A 2 sample 2 tail t-test ± standard error (s.e.m.) was applied for the analysis of significance of the MRI, histological and ex-vivo data. The data was considered significant for p< 0.05.

Results

PKBα/Akt1 deficient placentas exhibited impaired vascular function with maintained integrity of the fetal/maternal blood barrier

DCE MRI, using biotin-BSA-GdDTPA as a contrast agent (i.v.), was use for visualizing the maternal circulation of E18.5 placentas in utero (Fig. 1a). The distribution of the albumin based contrast media was validated using fluorescently labeled albumin, BSA-ROX (Fig. 1b). Both biotin-BSA-GdDTPA and BSA-ROX did not extravasate to the fetal side, and thus the MRI contrast enhancement exclusively revealed the maternal circulatory bed in the placenta.

Figure 1.

Figure 1

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PKBα/Akt1 deficient placentas exhibited impaired vascular function with maintained integrity of the fetal/maternal blood barrier. (a) Representative MR image (7.5min after intravenous administration of biotin-BSA-GdDTPA) shows signal enhancement in E18.5 placentas (lighter) versus the adjacent non-enhanced embryos (darker). This is the result of the accumulation of the contrast material, biotin-BSA-GdDTPA that was injected into the maternal circulation and was excluded from the fetal vasculature. The enhanced maternal-placental circulation can also be exhibited in the enlarged MR images of placentas (p1=cross section; p2=longitudinal section; VC= vena cava). (b) The exclusion of the BSA-based contrast material from the fetal vasculature was verified using BSA-ROX. BSA-ROX fluorescence is shown in macro views of placentas from the side of the umbilical cord connection (upper panels) and the side of the decidual connection (lower panels), both in bright field (left) and fluorescence (right). (arrow=embryo paw; uc=umbilical cord, lab= maternal vessels with BSA-ROX in the placental labyrinth). (c) Early injected biotin-BSA-GdDTPA (stained with avidin-FITC, green; localized more to the edges of the labyrinth or vessel rim-arrowheads) and late injected BSA-ROX (red, localized within vessels-arrows) are both confined to the maternal circulation in the placental labyrinth as exhibited by histological analysis. Asterisks in the inserts mark the fetal blood spaces empty of both early and late-injected contrast materials (Inserts magnify micrographs by 400%). Co-localization of the early and late injected contrast materials appears in yellow. Hoechst (blue) stained nuclei (lab=labyrinth layer). (d) Representative sections of placental mid-transverse histological sections showing the circulatory bed of the placental labyrinths (example for circulatory bed vessels is pointed by black arrows in inserts; Inserts magnify micrographs by 200%). (e) Placental initial enhancement of PKBα/Akt1 −/− and +/− calculated using the MRI data were significantly reduced versus enhancement of PKBα/Akt1 +/+ placentas (PKBα/Akt1 +/+: n=11 in 4 dams; PKBα/Akt1 +/−: n=5 in 3 dams, *p=0.007; PKBα/Akt1 −/−: n=11 in 4 dams, *p=0.02). (f and g) A morphometric analysis of the placental circulatory bed (n=3 placentas each, 3 sections per each placenta) verified the significant reduction in circulatory bed area (*p=0.005) and length (*p=9.6−10-6) of PKBα/Akt1 −/− placentas. PKBα/Akt1 +/− circulatory bed area was similar to PKBα/Akt1 −/− (*p=0.01) while circulatory bed length was intermediate (*0.009>p>0.015).

PKBα/Akt1, as a central downstream target of VEGF-receptor-2, plays an important role in mediating VEGF-A-induced vascular permeability [27]. In the placenta, vascular permeability could be suppressed by heterodimers of VEGF-A and placental growth factor (PlGF) [28]. Previous studies reported that vascular permeability was elevated for PKBα/Akt1 null mice upon induction of angiogenesis[28]. Therefore, we examined the impact of PKBα/Akt1 deficiency on the integrity of the feto-maternal blood barrier. Histological analysis of early injected biotin-BSA-GdDTPA (stained with avidin-FITC, green) and late injected BSA-ROX (red), revealed that they are both confined to the maternal circulatory bed of the placental labyrinth in PKBα/Akt1 +/+, +/− and −/− placentas and did not extravasate to the feto-placental vasculature (Fig. 1c; asterisks within inserts illustrate the feto-placental spaces void of either contrast materials injected). Therefore, PKBα/Akt1 −/− placentas, as well as +/− maintain the integrity of the feto-placental barrier (with no interstitial accumulation of the contrast material), similar to wildtype, PKBα/Akt expressing placentas [19]. Although the early and late injected contrast materials mostly co-localize (yellow), early injected biotin-BSA-GdDTPA is localized more at the edges of the placental labyrinth or vessel rim (arrowheads), while the late injected BSA-ROX filled most of the blood vessels within the labyrinth itself (white arrows). This spatial mismatch visualized by the dual labeling approach, revealed slow dynamics of inflow and clearance of maternal blood from the placental blood pool at E18.5, as well as accumulation of albumin towards the end of pregnancy with possible uptake by various trophoblast cells [19].

The contribution of PKBα/Akt1 to the vascular functionality of the placenta was evaluated by MRI (Fig. 1e) and by histological morphometric analysis (Fig. 1d and f-g). The initial enhancement measured from the MRI data (analogous to the late injected BSA-ROX; Fig. 1c) corresponds to the blood volume entering the maternal-placental blood spaces (See supplementary Fig. 1). The ability of the placenta to deliver sufficient blood supply is a prerequisite to its proper functionality. The initial enhancement of PKBα/Akt1 −/− and interestingly also, PKBα/Akt1 +/− placentas were significantly reduced compared to the enhancement of PKBα/Akt1 +/+ placentas (Fig. 1e). Morphometric analysis of the placental circulatory bed (i.e. placental labyrinth, includes both maternal and fetal blood spaces) also revealed a significant reduction in the circulatory bed area and length of blood vessels in PKBα/Akt1 −/− versus normal placentas (Fig. 1 f,g). PKBα/Akt1 +/− revealed an overall intermediate phenotype: the circulatory bed area was similar to PKBα/Akt1 −/− (corresponding to the initial enhancement measured by MRI) while the circulatory bed length was intermediate between PKBα/Akt1 +/+ and −/−.

Osteopenia in PKBα/Akt1 deficient mice originates prenatally and persists after birth

Determining the functional placental insufficiency caused by the absence of two copies or even a single copy of the PKBα/Akt1 gene led us to examine the effect that the absence of PKBα/Akt1 has on the fetal bone phenotype. The bone defects reported for adult PKBα/Akt1 −/− mice [15, 16], were attempted to be traced down to E18.5 fetuses. Long bones were characterized in fetuses and neonates of various ages (E18.5-P40; Fig. 2). Morphological abnormalities in the growth plate cartilage were observed in PKBα/Akt1 −/− and +/− mice, which were most profound at E18.5 and their severity diminished with age. The growth plate cartilage was narrowed due to a reduction in the width and disturbed organization of the proliferating and hypertrophic zones (Fig. 2a- between lines). A gradation in severity was observed between the −/− and +/− mice, where +/− appeared intermediate between −/− and +/+ mice (Fig 2 a-d). Furthermore, the columnar organization of the hypertrophic chondrocytes was less evident and there was significant heterogeneity in their size (anisocytosis) (Fig. 2c- box). Endochondral bone production was reduced in the −/− and to a lesser extent in +/− mice (Fig. 2d- asterisk). Therefore, it seemed that the bone defects found in the adult mice were more severe in the PKBα/Akt1 −/− E18.5 fetuses with defects found in PKBα/Akt1 +/− fetuses, although milder.

Figure 2.

Figure 2

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Osteopenia in PKBα/Akt1 deficient mice originates prenatally and persists after birth Histological sections of humeri of (a) E18.5 fetuses as well as (b) day 7 (c) day 22 and (d) day 40 neonates showing morphological abnormalities in the growth plate cartilage. The growth plate cartilage was narrowed due to disturbed organization of the proliferating and hypertrophic zones (Fig. 2a – between lines). Heterozygous PKBα/Akt1+/− mice appeared intermediate between −/− and +/+ mice (Fig 2 a-d). Furthermore, the columnar organization of the hypertrophic chondrocytes was less evident and there was significant heterogeneity in their size (anisocytosis) (Fig 2c – box). Endochondral bone production was reduced in the PKBα/Akt1−/− and to a lesser extent in PKBα/Akt1+/− mice. (Fig. 2d- asterisk) (e) Magnification of the hypertrophic layer of postnatal day 7 (arrowheads=hypertrophic chondrocytes; arrows=columnar organization of chondrocytes).

Vascular function in the placentas of PKBα/Akt1 deficient fetuses can be restored by tetraploid rescue

Previously reported perinatatal mortality[12, 13] together with our conclusions concerning the placental functional insufficiency and the fetal bone defects caused by PKBα/Akt1 gene deficiency, emphasized the need to distinguish between the role of placental and fetal PKBα/Akt1 in determining fetal survival and size. Moreover, the greater severity of the bone phenotype in fetuses versus older mice deficient of PKBα/Akt1 also justified checking a putative role for the placental functional insufficiency in this phenotype. Therefore, we decided to study survival and size in tetraploid-rescued fetuses, in which tetraploid placentas originated from GFP expressing (PKBα/Akt1 +/+) transgenic mice. We first wanted to examine the vascular functionality of tetraploid placentas serving either PKBα−/− and +/− fetuses versus normal, PKBα-expressing C57Bl/6J +/+ placentas. ICM of PKBα−/− and +/− (C57Bl/6J background) blastocysts were aggregated with the GFP expressing tetraploid embryos and transplanted into 2.5 d.p.c. ICR pseudopregnant mice. Embryos complemented with tetraploid placentas were identified by placental GFP fluorescence with no fluorescence in the embryo (Fig. 3a, b; control, non-aggregated diploid GFP embryos show fluorescence of both the placenta and embryo, Fig. 3b insert). The tetraploid placental vasculature was then visualized and studied by MRI (Fig. 3c, d). Surface projections of the enhanced areas in the 3 dimensional MRI data illustrates how this method resolved the functional blood vessels of the tetraploid placenta along with blood vessels in other maternal internal organs (Fig. 3e, f). The tetraploid placentas serving either −/− or +/− PKBα/Akt1 fetuses were not significantly different from non-tetraploid C57Bl/6J +/+ placentas in both their initial enhancement (Fig. 3g) and size (Fig. 3h). Thus, tetraploid complementation restored normal placental vascular function.

Figure 3.

Figure 3

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Vascular function in the placentas of PKBα/Akt1 deficient fetuses can be restored by tetraploid rescue. (a and c) E18.5 PKBα/Akt1 +/− fetus (non fluorescent) with tetraploid GFP placenta versus (b and d) PKBα/Akt1 −/− fetus (non fluorescent) with tetraploid GFP placenta transplanted into 2.5 d.p.c. ICR pseudopregnant mice. (a and b) Fluorescence microscopy depicting the GFP placentas (green), BSA-ROX, as a vascular marker (red), in contrast to the non-fluorescent aggregated embryos. Non-aggregated diploid GFP embryos (insert in b; GFP embryo and placenta). Chromatic light was also applied to show the non fluorescent fetuses. (c and d) three dimensional (3D) gradient echo maximal intensity projection (MIP) of PKBα/Akt1 +/− fetus (c) and PKBα/Akt1 −/− fetus (d) both with GFP tetraploid placentas. Insert-enlarged image of the materno-placental vascular bed with arrow pointing at ub Oa&v (see below). Note the functional blood vessels of the tetraploid placenta in a and c, also in frontal (e) and dorsal (f) 3 dimentional surface projections of enhanced biotin-BSA-GdDTPA blood vessels (p=placenta; ov=ovary; k=kidney; vc=vena cava; ub Oa&v=uterine branch of ovarian artery and vein). (g) Placental initial enhancement (arbitrary units) and (h) placental size (ml) of the tetraploid placentas serving either hetero (tetra PKBα/Akt1 +/−) or null (tetra PKBα/Akt1 −/−) fetuses was not different from C57Bl/6J PKBα/Akt1 +/+ placentas.

Teraploid placentas rescue PKBα/Akt1 null fetuses from death in utero but do not prevent fetal growth retardation

Normally-functioning tetraploid placentas were used in an attempt to rescue the PKBα/Akt1 −/− phenotype of reduced survival and size. The weight of PKBα/Akt1 −/− placentas was found to be significantly reduced as compared to that of +/−, +/+ and tetraploid placentas (Fig. 4a). Despite the improved placental vascular function, the weight of PKBα/Akt1 −/− fetuses complemented with tetraploid placentas remained significantly lower than +/− and +/+ fetuses and was not different from non-rescued PKBα/Akt1 −/− fetuses (p=0.8; Fig. 4b). Gene dosage was found important, as PKBα/Akt1 +/− fetuses were also significantly smaller than PKBα/Akt1 +/+ fetuses. The intermediate phenotype of PKBα/Akt1 +/− placental vascularity (Fig. 1) was less pronounced by placental weight since, although slightly reduced, no significant difference in placental weight was found between PKBα/Akt1 +/− and PKBα/Akt1 +/+ placentas.

Figure 4.

Figure 4

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Teraploid placentas rescue PKBα/Akt1 null fetuses from death in utero but do not prevent fetal growth retardation. (a) The weight of PKBα/Akt1 −/− placentas is significantly lower than that of +/− (*p=0.001), +/+ (*p=0.003) and tetraploid placentas (*p=0.016; serving PKBα/Akt1 null fetuses, i.e. rescued −/−; −/−: n=6 in 4 dams, +/−: n= 22 in 14 dams, +/+: n= 13 in 4 dams tetraploid (rescued −/−): n= 5 in 4 dams). (b) Fetal weight of PKBα/Akt1 −/− fetuses is significantly lower than that of +/− (*p=0.046) and +/+ (*p=5.6×10-5) fetuses but not different from tetraploid rescued PKBα/Akt1 −/− fetuses (p=0.8). Also, PKBα/Akt1 +/− fetuses are significantly smaller than PKBα/Akt1 +/+ fetuses (*p=5.6×10-5). (c) Tetraploid placentas rescued PKBα/Akt1 −/− fetuses from death in utero, as the fraction of −/− non-rescued fetuses was significantly lower than that of rescued −/− fetuses from total pups (−/− and +/− littermates) examined. Gray line indicates Mendelian frequency pups in natural heterozygous over homozygous mating (*p= 0.007). (d) Out of 296 mice genotyped at age 3 weeks, 93 were +/+ (31% instead of the expected 25%), 169 were +/− (57% but lower than the expected 186 relative to the +/+ offspring) and only and 34 were −/− (12%; significantly fewer than the expected 93).

Normally-functioning tetraploid placentas were able to rescue PKBα/Akt1 −/− embryos from fetal death in utero (Fig. 4c). The average number of total tetraploid-rescued fetuses per dam was 2 times lower than the average number of non-rescued fetuses that were subjected to embryo transfer only, due to the general low yield of surviving embryos post tetraploid rescue. However, all but one fetuses that were recovered from the uterus at E18.5 (after tetraploid rescue or embryo transfer only) were viable. Two E18.5 pups (−/− and +/−) were given to a foster mother and developed normally and were fertile.

The frequency of +/− and −/− versus the +/+ mice derived from routine +/− mating provided further evidence to the mortality associated with PKBα/Akt1 deficiency (Fig. 4d). Moreover, since the −/− fetuses suffer from death in utero, we had to use +/− and −/− matings for tetraploid rescue to increase the finite number of −/− embryos for tetraploid aggregation. Therefore, the survival of +/− mice, which served as our control in this experiment, had to be evaluated. Of 296 mice genotyped at age 3 weeks, 93 were +/+ (31% instead of the expected 25%), 169 were +/− (57% but lower than the expected 186 relative to the +/+ offspring) and only 34 were −/− (12%; significantly fewer than the expected 93). These results complement the study, since the intermediate phenotype of PKBα/Akt1 +/−placentas correlated with the mild reduction in live +/− pups. In PKBα/Akt1 deficient embryos, a significant reduction in placental vascularity and weight correlated with a severe reduction in live −/− pups (with no rescue). These results are consistent with the rescue results, since a healthy placenta could rescue −/− pups from death in utero but did not prevent their growth retardation.

Phenotypic changes in various organs in PKBα/Akt1 null fetuses versus rescued PKBα/Akt1 null fetuses

In order to determine the reason for the yet smaller size of the rescued PKBα/Akt1 −/− E18.5 fetuses vital internal organs, from these fetuses, such as liver and kidney, were histopathologically examined and subjected to morphometric analysis. As major size determinants, long bones (humeri) were examined as well. Blood vessels volume fraction in the liver (Fig. 5a) was significantly reduced in both +/− and −/− livers, but normal in rescued −/− and +/− livers (Fig. 5b). The observation that the livers of −/− and +/− fetuses had reduced percentage of blood vessels is consistent with the observation made for these genotypes in the placenta, and both were rescued by the tetraploid placentas. However, examination of the hematopoietic islets volume fraction in the liver revealed that −/− fetuses had a significantly reduced percentage of hematopoietic islets that were not rescued by the tetraploid placentas. The +/− livers showed an intermediate phenotype (Fig. 5c).

Figure 5.

Figure 5

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Tetraploid normally-functioning placentas rescue the vascular-features of the liver and kidney of PKBα/Akt1 – deficient fetuses. (a) H&E staining of liver from E18.5 PKBα/Akt1 +/+ (n=2 dams), non-rescued PKBα/Akt1 −/+ (n=3 dams, 1 fetus each), non-rescued PKBα/Akt1 −/− (n=2 dams) and rescued PKBα/Akt1 +/− (n=2 dams) and rescued PKBα/Akt1 −/− (n=3 dams; for all: 1 fetus of each dam). Green arrowhead in insert (200% magnification)-blood vessel; yellow arrowhead-hematopoietic islet. (b) Morphometric analysis of blood vessels volume fraction. *0.0067>p>5.1×10-8. (c) morphometric analysis of hematopoietic islets volume fraction. *0.05>p>0.003. (d) H&E staining of kidney from E18.5 PKBα/Akt1 +/+, non-rescued PKBα/Akt1 +/−, non-rescued PKBα/Akt1 −/− and rescued PKBα/Akt1 +/− and PKBα/Akt1 −/−. (The number of fetuses in each category is the same as in a). red arrowheads in insert (200% magnification)-glumeruli. e) Morphometric analysis of glomeruli density. *0.05>p>3.9×10-5 f) morphometric analysis of glomeruli size *1.3×10-5>p>4.3×10-8.

The renal glomeruli (capillaries that perform the first step of blood filtration to form urine) were also examined. Similar to the liver blood vessel phenotype, gluomerular density was significantly reduced in both PKBα/Akt1 −/− and +/− fetuses (Fig. 5d) that were both rescued by the tetraploid placentas (Fig. 5e). Surprisingly, the size of the glomeruli in rescued −/− was reduced as compared to +/+, +/−, rescued +/− and −/− fetuses (Fig. 5f). The latter is probably a secondary effect, since total kidney size was still small in the rescued −/−, restricting the ability of the glomeruli to regain normal size, as their density returned to normal.

The yet reduced size of the rescued PKBα/Akt1 −/− fetuses can probably be mostly attributed to the phenotype exhibited in the bone. The hypertrophic layer of the growth plate demonstrated significantly reduced size in −/− fetuses (Fig. 6a,c). In the rescued fetuses, the hypertrophic layer was rescued in size and even significantly enlarged than its +/+ equivalents. The rescue effect in the rescued +/− was more subtle in accordance to the initial, more subtle effect in the non-rescued +/−. In the rescued −/− and +/−, the columnar organization of the chondrocytes in the proliferative and hypertrophic zones remained disorganized (also see Fig. 2). Interestingly, examination of the mineralized bone revealed impaired bone mineralization in +/− and −/− fetuses and even further impaired bone mineralization in rescued −/− (Fig. 6a,d). The over-shortened mineralized bone in the rescued −/− directly depends on the normal development (proliferation and differentiation) of the chondrocytes. In the −/− fetuses, the over-grown hypertrophic layer with highly disorganized chondrocytes may explain the over-shortened mineralized bone phenotype. In +/− mice, the effect on the hypertrophic layer is more subtle and so is the effect on the mineralized bone. Overall, while normally-functioning (PKBα-expressing) placenta governed chondrocyte cell number, fetal PKBα/Akt1 controlled chondrocyte columnar organization (all the phenotypical changes are summarized in supplementary Table S1).

Figure 6.

Figure 6

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Tetraploid normally-functioning placentas rescue fetal PKBα/Akt1 deficiency-related impaired proliferation but not columnar organization of chondrocytes. (a) Alcian blue staining of humerus from E18.5 PKBα/Akt1 +/+, non-rescued PKBα/Akt1 −/+, non-rescued PKBα/Akt1 −/− and rescued PKBα/Akt1 +/− and −/−. (b) Calcium staining of humerus from E18.5 PKBα/Akt1 +/+, non-rescued PKBα/Akt1 −/+, non-rescued PKBα/Akt1 −/− and rescued PKBα/Akt1 +/− and −/−. (The number of fetuses in each category in a and b is the same as in Fig. 5a) (c) Morphometric analysis of growth plate length. *0.016>p>4.3×10-7 (d) Morphometric analysis of bone mineralization length. *0.05>p>2×10-9.

Discussion

Tetraploid complementation allowed us to differentiate between the roles of placental versus fetal expression of PKBα/Akt1, suggesting differential contributions of PKBα/Akt1 to in utero embryonic development according to its physiological compartment. Specifically, we demonstrated that a normally-functioning, PKBα/Akt1-expressing placenta is sufficient for regulating fetal survival in utero, while fetal expression of PKBα/Akt1 regulates fetal size, in a gene-dosage-dependent manner. Despite these differences in the effects of placental versus fetal expression of PKBα/Akt1, it is clear that extensive fetal-placental interactive processes contribute to the outcome of any specific alteration of gene expression in either the placental or the fetus. Thus these results represent the integrated effects of direct as well as indirect consequences of the manipulation of placental expression of PKBα/Akt1 by tetraploid complementation.

Utilizing DCE MRI, we were able to detect a significant contribution of placental PKBα/Akt1 not only to the vascular content but also to the functionality of the maternal blood flow in a PKBα/Akt1 dose-dependent manner. Although the integrity of the feto-maternal blood barrier was not compromised by the PKBα/Akt1 deficiency, PKBα/Akt1 −/− and +/− placentas were characterized by a marked reduction in the placental initial enhancement. Besides playing a role in compromised fetal survival, placental hypovascularity in the absence of maternal PKBα/Akt1 was previously suggested to influence fetal phenotype, as placentas and embryos from homozygous mating were found to be smaller than those from heterozygous mating[11]. On the other hand, growth retardation of PKBα/Akt1 −/− mice was attributed to a direct effect on bone, which was recently reported in adult mice[15-17], and was also addressed here with morphological abnormalities in the growth plate cartilage in PKBα/Akt1 −/− and +/− E18.5 fetuses. Therefore, segregating between the role of placental and fetal PKBα/Akt1 deficiency in determining the phenotype was important. DCE MRI showed that tetraploid placentas exhibited functional vascular characteristics similar to normal, healthy placentas, enabling us to monitor the impact of tetraploid rescue of PKBα/Akt1 −/− embryos from placental insufficiency. Tetraploid placentas serving PKBα/Akt1 −/− fetuses, were sufficient to significantly reduce PKBα/Akt1 −/− fetal mortality. However, the tetraploid placentas did not prevent PKBα-associated IUGR. Thus, expression of PKBα/Akt1 by the fetus is critical to normal fetal size, suggesting that growth retardation related to PKBα/Akt1 deficiency may be a direct effect and is not secondary to placenta hypovascularity. Transplantation of the tetraploid aggregates into pseudopregnant WT dams also allowed to isolate the feto-placental and embryo proper components from the materno-placental component. Rescuing PKBα/Akt1 −/− embryos from death in utero by tetraploid complementation suggests that defective placental formation causing the phenotype of impaired survival is a primary trophoblast defect and not due to defective allantoic blood vessels (the latter is ICM-derived). Disruption of genes involved in extraembryonic allantoic-vitelline vasculature development frequently present a similar phenotype, but cannot be rescued by tetraploid complementation [29]. Moreover, the yolk sac vasculature in the case of PKBα/Akt1 deficiency is apparently normal since in our study, the yolk sac which is both ICM and trophectoderm-derived, was occasionally GFP positive after tetraploid rescue but the umbilical cord and yolk sac vessels were never GFP positive.

The placental complementation experiments elucidated a role for placental PKBα/Akt1 in fetal hepatic blood vessel development and renal glomerular capillary density, in a PKBα/Akt1 dose-dependent manner. One hypothesis could be that by restoration of the impaired blood pressure and/ or growth factors transport caused by the under-developed placental circulatory bed in the absence of PKBα/Akt1 affected the development of the vascular components in those major organs. Liver is also a major site for hematopoiesis in embryos and it was previously shown that knockdown of PKB/Akt activity significantly inhibits fetal liver–derived erythroid-cell colony formation and gene expression[30]. The allantoic mesoderm in the placenta, which is a putative source for hematopoietic stem cells in the liver[31], does not originate from the PKBα-expressing tetraploid trophectoderm, but rather from PKBα/Akt1 −/−ICM[23]. This may explain why hematopoiesis in the liver was not rescued. The most prominent feature underlying the yet reduced size of the rescued PKBα/Akt1 fetuses is the impaired development of long bones. Interestingly, while normally-functioning (PKBα-expressing) placentas governed chondrocyte cell number in fetal humeri, fetal PKBα/Akt1 controlled chondrocyte columnar organization, a prerequisite for chondrocyte differentiation and ossification. The rescue of chondrocyte cell number could have resulted from an adequate transport of growth factors by the tetraploid placenta, such as parathyroid hormone-related protein (PTHrP). PTHrP has profound actions on the growth and differentiation of bones [32]. Within the placenta, PTHrP is expressed by placental syncytial trophoblasts and will be transported to the fetus[33]. In the absence of PTHrP, chondrocytes do not proliferate normally, giving rise to shortened bone [32]. Such a mechanism could possibly explain the rescue of chondrocyte cell number in PKBα/Akt1 null rescued fetuses. Terminal chondrocyte differentiation featuring adequate columnar organization of chondrocyes that was not rescued by the tetraploid placenta, was previously shown to be directly regulated by Akt, both in embryonic and adult chondrogenesis. In transgenic embryos that expressed constitutively active or dominant-negative Akt in chondrocytes, Akt was found to positively regulate all processes of chondrocyte maturation[34]. Among the Akt isoforms, PKBα/Akt1 was the most highly expressed in chondrocytes [35] and a delayed secondary ossification in the long bones of PKBα/Akt1 null mice was previously observed at 1 week of age [15, 16]. Surprisingly, gene dosage of PKBα/Akt1 was also apparent when comparing the rescued PKBα/Akt1 +/− to the effects exhibited in PKBα/Akt1 −/−, overall exhibiting an intermediate, milder but significant phenotype looking at the parameters mentioned above for liver, kidney and bone.

In summary, in utero death of PKBα/Akt1 −/− fetuses was attributed to insufficient placental function of PKBα/Akt1 −/− placentas and could be prevented by tetraploid complementation. Fetal growth retardation on the other hand, was associated with the absence of PKBα/Akt1 in the embryo proper. Since Akt deficiency is relevant to IUGR in humans[14], this study provides substantial insights into the roles that PKBα/Akt1 may play in this pathology of human pregnancies.

Supplementary Material

Supplementary data

Acknowledgement

We would like to thank Dr. Sylvain Provot for helpful discussions on bone developmental biology. We would also like to thank Itzhak Ino from the Animal Facility for technical assistance as well as Idan Aharon and Naama Cirkin for genotyping. Michal Neeman is incumbent of the Helen and Morris Mauerberger Chair in Biological Sciences.

Grant Support: This work was supported by grants from the Israel Science Foundation (391-02), the Minerva Foundation and by the 7th Framework European Research Council Advanced grant 232640-IMAGO (to MN).

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