Placental phenotype and resource allocation to fetal growth are modified by the timing and degree of hypoxia during mouse pregnancy

Key points

Hypoxia is a major cause of fetal growth restriction, particularly at high altitude, although little is known about its effects on placental phenotype and resource allocation to fetal growth.

In the present study, maternal hypoxia induced morphological and functional changes in the mouse placenta, which depended on the timing and severity of hypoxia, as well as the degree of maternal hypophagia.

Hypoxia at 13% inspired oxygen induced beneficial changes in placental morphology, nutrient transport and metabolic signalling pathways associated with little or no change in fetal growth, irrespective of gestational age.

Hypoxia at 10% inspired oxygen adversely affected placental phenotype and resulted in severe fetal growth restriction, which was due partly to maternal hypophagia.

There is a threshold between 13% and 10% inspired oxygen, corresponding to altitudes of ∼3700 m and 5800 m, respectively, at which the mouse placenta no longer adapts to support fetal resource allocation. This has implications for high altitude human pregnancies.

Abstract

The placenta adapts its transport capacity to nutritional cues developmentally, although relatively little is known about placental transport phenotype in response to hypoxia, a major cause of fetal growth restriction. The present study determined the effects of both moderate hypoxia (13% inspired O₂) between days (D)11 and D16 or D14 and D19 of pregnancy and severe hypoxia (10% inspired O₂) from D14 to D19 on placental morphology, transport capacity and fetal growth on D16 and D19 (term∼D20.5), relative to normoxic mice in 21% O₂. Placental morphology adapted beneficially to 13% O₂; fetal capillary volume increased at both ages, exchange area increased at D16 and exchange barrier thickness reduced at D19. Exposure to 13% O₂ had no effect on placental nutrient transport on D16 but increased placental uptake and clearance of ³H‐methyl‐d‐glucose at D19. By contrast, 10% O₂ impaired fetal vascularity, increased barrier thickness and reduced placental ¹⁴C‐methylaminoisobutyric acid clearance at D19. Consequently, fetal growth was only marginally affected in 13% O₂ (unchanged at D16 and −5% at D19) but was severely restricted in 10% O₂ (−21% at D19). The hypoxia‐induced changes in placental phenotype were accompanied by altered placental insulin‐like growth factor (IGF)‐2 expression and insulin/IGF signalling, as well as by maternal hypophagia depending on the timing and severity of the hypoxia. Overall, the present study shows that the mouse placenta can integrate signals of oxygen and nutrient availability, possibly through the insulin‐IGF pathway, to adapt its phenotype and optimize maternal resource allocation to fetal growth during late pregnancy. It also suggests that there is a threshold between 13% and 10% inspired O₂ at which these adaptations no longer occur.

Hypoxia is a major cause of fetal growth restriction, particularly at high altitude, although little is known about its effects on placental phenotype and resource allocation to fetal growth.

In the present study, maternal hypoxia induced morphological and functional changes in the mouse placenta, which depended on the timing and severity of hypoxia, as well as the degree of maternal hypophagia.

Hypoxia at 13% inspired oxygen induced beneficial changes in placental morphology, nutrient transport and metabolic signalling pathways associated with little or no change in fetal growth, irrespective of gestational age.

Hypoxia at 10% inspired oxygen adversely affected placental phenotype and resulted in severe fetal growth restriction, which was due partly to maternal hypophagia.

There is a threshold between 13% and 10% inspired oxygen, corresponding to altitudes of ∼3700 m and 5800 m, respectively, at which the mouse placenta no longer adapts to support fetal resource allocation. This has implications for high altitude human pregnancies.

Hypoxia, AMPK activation and uterine artery vasoreactivity

Genes near adenosine monophosphate-activated protein kinase-α1 (PRKAA1) have been implicated in the greater uterine artery (UtA) blood flow and relative protection from fetal growth restriction seen in altitude-adapted Andean populations. Adenosine monophosphate-activated protein kinase (AMPK) activation vasodilates multiple vessels but whether AMPK is present in UtA or placental tissue and influences UtA vasoreactivity during normal or hypoxic pregnancy remains unknown. We studied isolated UtA and placenta from near-term C57BL/6J mice housed in normoxia (n = 8) or hypoxia (10% oxygen, n = 7-9) from day 14 to day 19, and placentas from non-labouring sea level (n = 3) or 3100 m (n = 3) women. Hypoxia increased AMPK immunostaining in near-term murine UtA and placental tissue. RT-PCR products for AMPK-α1 and -α2 isoforms and liver kinase B1 (LKB1; the upstream kinase activating AMPK) were present in murine and human placenta, and hypoxia increased LKB1 and AMPK-α1 and -α2 expression in the high- compared with low-altitude human placentas. Pharmacological AMPK activation by A769662 caused phenylephrine pre-constricted UtA from normoxic or hypoxic pregnant mice to dilate and this dilatation was partially reversed by the NOS inhibitor l-NAME. Hypoxic pregnancy sufficient to restrict fetal growth markedly augmented the UtA vasodilator effect of AMPK activation in opposition to PE constriction as the result of both NO-dependent and NO-independent mechanisms. We conclude that AMPK is activated during hypoxic pregnancy and that AMPK activation vasodilates the UtA, especially in hypoxic pregnancy. AMPK activation may be playing an adaptive role by limiting cellular energy depletion and helping to maintain utero-placental blood flow in hypoxic pregnancy.

Corticosterone alters materno-fetal glucose partitioning and insulin signalling in pregnant mice

Glucocorticoids affect glucose metabolism in adults and fetuses, although their effects on materno-fetal glucose partitioning remain unknown. The present study measured maternal hepatic glucose handling and placental glucose transport together with insulin signalling in these tissues in mice drinking corticosterone either from day (D) 11 to D16 or D14 to D19 of pregnancy (term = D21). On the final day of administration, corticosterone-treated mice were hyperinsulinaemic (P < 0.05) but normoglycaemic compared to untreated controls. In maternal liver, there was no change in glycogen content or glucose 6-phosphatase activity but increased Slc2a2 glucose transporter expression in corticosterone-treated mice, on D16 only (P < 0.05). On D19, but not D16, transplacental (3) H-methyl-d-glucose clearance was reduced by 33% in corticosterone-treated dams (P < 0.05). However, when corticosterone-treated animals were pair-fed to control intake, aiming to prevent the corticosterone-induced increase in food consumption, (3) H-methyl-d-glucose clearance was similar to the controls. Depending upon gestational age, corticosterone treatment increased phosphorylation of the insulin-signalling proteins, protein kinase B (Akt) and glycogen synthase-kinase 3β, in maternal liver (P < 0.05) but not placenta (P > 0.05). Insulin receptor and insulin-like growth factor type I receptor abundance did not differ with treatment in either tissue. Corticosterone upregulated the stress-inducible mechanistic target of rapamycin (mTOR) suppressor, Redd1, in liver (D16 and D19) and placenta (D19), in ad libitum fed animals (P < 0.05). Concomitantly, hepatic protein content and placental weight were reduced on D19 (P < 0.05), in association with altered abundance and/or phosphorylation of signalling proteins downstream of mTOR. Taken together, the data indicate that maternal glucocorticoid excess reduces fetal growth partially by altering placental glucose transport and mTOR signalling.

Probes for investigating the effect of magnetic field, field orientation, temperature and strain on the critical current density of anisotropic high-temperature superconducting tapes in a split-pair 15 T horizontal magnet

We present the designs of probes for making critical current density (Jc) measurements on anisotropic high-temperature superconducting tapes as a function of field, field orientation, temperature and strain in our 40 mm bore, split-pair 15 T horizontal magnet. Emphasis is placed on the design of three components: the vapour-cooled current leads, the variable temperature enclosure, and the springboard-shaped bending beam sample holder. The vapour-cooled brass critical-current leads used superconducting tapes and in operation ran hot with a duty cycle (D) of ~0.2. This work provides formulae for optimising cryogenic consumption and calculating cryogenic boil-off, associated with current leads used to make J(c) measurements, made by uniformly ramping the current up to a maximum current (I(max)) and then reducing the current very quickly to zero. They include consideration of the effects of duty cycle, static helium boil-off from the magnet and Dewar (b'), and the maximum safe temperature for the critical-current leads (T(max)). Our optimized critical-current leads have a boil-off that is about 30% less than leads optimized for magnet operation at the same maximum current. Numerical calculations show that the optimum cross-sectional area (A) for each current lead can be parameterized by LI(max)/A = [1.46D(-0.18)L(0.4)(T(max) - 300)(0.25D(-0.09)) + 750(b'/I(max))D(10(-3)I(max)-2.87b') × 10⁶ A m⁻¹ where L is the current lead's length and the current lead is operated in liquid helium. An optimum A of 132 mm(2) is obtained when I(max) = 1000 A, T(max) = 400 K, D = 0.2, b' = 0.3 l h(-1) and L = 1.0 m. The optimized helium consumption was found to be 0.7 l h(-1). When the static boil-off is small, optimized leads have a boil-off that can be roughly parameterized by: b/I(max)  ≈ (1.35 × 10(-3))D(0.41) l h(‑1) A(-1). A split-current-lead design is employed to minimize the rotation of the probes during the high current measurements in our high-field horizontal magnet. The variable-temperature system is based on the use of an inverted insulating cup that operates above 4.2 K in liquid helium and above 77.4 K in liquid nitrogen, with a stability of ±80 mK to ±150 mK. Uniaxial strains of -1.4% to 1.0% can be applied to the sample, with a total uncertainty of better than ±0.02%, using a modified bending beam apparatus which includes a copper beryllium springboard-shaped sample holder.

On the difference in scattering behavior of cyclic and linear polymers in bulk

It has been suggested that, due to topological constraints, rings in the melt may assume a more compact shape than Gaussian chains. In this paper, we exploit the availability of narrow fractions of perdeuterated linear and cyclic polydimethylsiloxane (PDMS) and, through the analysis of the small angle neutron scattering (SANS) profiles, demonstrate the difference in scattering properties of linear and cyclic PDMS molecules. As expected for Gaussian chains, for the H/D linear PDMS samples, log-log plots of the scattered intensity versus scattering vector Q display a Q((-2)) dependence. However, for H/D cyclic blends, the scaling exponent is higher than 2, as predicted by computer simulations reported in the literature. We show that cyclic molecules in bulk display the characteristic maximum in plots of scattered intensity versus Q((-2)) that is expected on the basis of Monte Carlo calculations and from the Casassa equation [E. F. Casassa, J. Polym. Sci. A 3, 605 (1965)]. It is also shown that, for rings, the Debye equation [P. Debye, J. Appl. Phys. 15, 338 (1944)] is no longer appropriate to describe the SANS profiles of H/D cyclic blends, at least up to M(w) approximately 10 000. For these samples, the Casassa form factor gives a better representation of the SANS data and we show that this function which was developed for monodisperse cyclics is still adequate to describe our slightly polydisperse samples. Deviations from all above observations are noted for M(w)>11 000 and are attributed to partial contamination of cyclic samples with linear chains. The failure of both the Debye and the Casassa form factors could be due to contamination of the cyclic fractions by linear polymers or to a real conformational change.

Co-occurrence of superparamagnetism and anomalous hall effect in highly reduced cobalt-doped rutile TiO2-delta films

We report a detailed magnetic and structural analysis of highly reduced Co doped rutile TiO(2-delta) films displaying an anomalous Hall effect (AHE). The temperature and field dependence of magnetization, and transmission electron microscopy, clearly establish the presence of nanosized superparamagnetic cobalt clusters of approximately 8-10 nm size in the films at the interface. The co-occurrence of superparamagnetism and AHE raises questions regarding the use of the AHE as a test of the intrinsic nature of ferromagnetism in diluted magnetic semiconductors.

Tacticity effects on the barriers to rotation of the ester methyl group in poly (methyl methacrylate): A deuteron magnetic resonance study

In isotactic poly(methyl methacrylate) (PMMA), we investigate the dynamics of the ester methyl groups by means of deuteron magnetic resonance (DMR) in a deuterated sample. We find that the motion of the CD(3)-group affects the deuteron spin-lattice relaxation as well as the DMR line shape in a characteristic way. Quadrupolar order spin lattice relaxation measurements between T=291 K and T=70 K reveal a broad temperature dependent probability distribution of autocorrelation times tau(c) for the 2pi/3 reorientation. This broad distribution corresponds to a temperature independent Gaussian distribution of activation energies rho(E(a)) with variance sigma(E(a) )=13.8+/-0.5 meV (1.33 kJ/mol). The line shape transition between T=70 K and T=23 K is explained with the freezing in of the methyl group reorientation. By comparing our results in an 88% isotactic sample with results obtained from a 50% syndiotactic, 30% atactic, and 20% isotactic sample of a previous investigation, we demonstrate the higher local order of the 88% isotactic sample, which corresponds to a ratio of 1.6 in the relative width sigma(E(a) )/E(a) of the E(a) distribution. We show that different stereospecific forms of PMMA can be easily distinguished by the characteristics of their line shape transition between T=70 K and T=23 K.

Flow-induced enhancement of concentration fluctuations in polymer mixtures

Many mixtures of high-molecular-weight polymers are partially miscible, separating into two co-existing phases in some ranges of temperature and concentration. Generally these binary blends exhibit lower critical solution temperature (LCST) behaviour, becoming phase separated as the temperature is raised. We have been undertaking a systematic study of the effect of simple shear flow on the miscibility limits of a number of high-molecular-weight binary polymer blends exhibiting lower critical-solution temperatures. Using a parallel-plate shear cell and light-scattering detection, we have observed large shifts in the cloud-point curves as blends are heated while shear is applied, and we have been able to show, using ancillary measurements of glass-transition temperatures on quenched samples, that these effects are the results of changes in the miscibility of the blends and not artefacts of the scattering techniques. Both shear-induced 'mixing' and 'demixing' have been observed-the latter in temperature ranges well below the quiescent cloud points. We have investigated the kinetics of the appearance of the two-phase structures, and this is very reminiscent of the spinodal decomposition processes observed in zero shear from these blends. Small-angle neutron scattering is very sensitive to the rather short wavelength concentration fluctuations (a few nanometres) in binary systems, both in the stable one-phase region and in the early stages of instability in the two-phase region. Small-angle neutron-scattering results on blends sheared and quenched at temperatures far below the spinodal temperature show the effects of the shear in modifying the blend's structure factors.