Prenatal mTORC1 Inhibition causes Fetal Growth Restriction and Impairs Heart Development

Objectives: Intrauterine growth restriction (IUGR) due to maternal undernutrition or placental insufficiency Results in cardiac hypoplasia at birth. The mechanistic target of rapamycin complex 1 (mTORC1) integrates nutrient and growth factor availability with cell growth, thereby regulating organ size. Although multiple studies have characterized mTORC1 function in cardiac hypertrophy in adulthood, much less is known about its role before birth. This study aimed at elucidating a possible involvement of mTORC1 in IUGR and fetal heart growth.

Methods: We inhibited mTORC1 in fetal and neonatal mice by rapamycin treatment of pregnant dams during the final quarter of gestation.

Results: Prenatal rapamycin treatment sufficiently reduces mTORC1 activity in various organs including the heart at birth, evident as reduced phosphorylation of known downstream targets. Rapamycin treated neonates showed overall growth restriction resulting in a 16% reduction in body weight (BW) compared with vehicle treated controls (p< 0.001). Although kidney weight was reduced to the same extend (19%), heart weight (HW) decreased by 35% resulting in a significantly reduced HW/BW ratio in rapamycin versus vehicle treated neonates (4.87 ± 0.02 vs. 6.23 ± 0.03 mg/g, p< 0.001). Although histological examination confirmed reduced left ventricular (LV) mass and wall thickness after prenatal mTORC1 inhibition, echocardiography revealed normal LV contractility at birth. While proliferation rates in neonatal rapamycin treated hearts were unaffected, the reduced heart size was attributed to decreased cardiomyocyte size (42.8 ± 0.44 vs. 64.7 ± 2.45 µm² cross-sectional area (CSA), p< 0.001) and increased apoptosis (0.2 ± 0.01 vs. 0.07 ± 0.02% apoptotic cells, p< 0.001) compared with vehicle treated neonates. Body and heart size were largely normalized during postnatal life until the age of 11 weeks. This was achieved by increased cardiomyocyte size in adult hearts after prenatal rapamycin versus vehicle treatment (427.5 ± 89.05 vs. 280.03 ± 14.66 µm² CSA, p< 0.01). Considering normal heart weight, the latter suggests a reduced cardiomyocyte number after prenatal mTORC1 inhibition. Nevertheless, no signs of maladaptive LV remodelling were observed, resulting in normal cardiac function as revealed by echocardiography.

Conclusion: Prenatal rapamycin treatment of pregnant dams represents a new mouse model of IUGR and identifies an important role of mTORC1 in perinatal cardiac growth and organ size control.