Maternal serotonin is crucial for murine embryonic development

Abstract

The early appearance of serotonin and its receptors during prenatal development, together with the many effects serotonin exerts during CNS morphogenesis, strongly suggest that serotonin influences the development and maturation of the mammalian brain before it becomes a neuromodulator/neurotransmitter. Sites of early serotonin biosynthesis, however, have not been detected in mouse embryos or extraembryonic structures, suggesting that the main source of serotonin could be of maternal origin. This hypothesis was tested by using knockout mice lacking the tph1 gene, which is responsible for the synthesis of peripheral serotonin. Genetic crosses were performed to compare the phenotype of pups born from homozygous and heterozygous mothers. Observations provide the first clear evidence that (i) maternal serotonin is involved in the control of morphogenesis during developmental stages that precede the appearance of serotonergic neurons and (ii) serotonin is critical for normal murine development. Most strikingly, the phenotype of tph1-/- embryos depends more on the maternal genotype than on that of the concepti. Consideration of the maternal genotype may thus help to clarify the influence of other genes in complex diseases, such as mental illness.

Fig. 1.

Phenotypic comparison of E12.5 embryos from wild-type (A, embryo 1), heterozygous (A, embryo 2), and homozygous (B, embryos 3 and 4) tph1 mothers. Embryos displayed no obvious macroscopic anomalies except for an overall size reduction in embryos obtained from tph1−/− mothers (embryos 3 and 4). Size reduction was evaluated by measuring the CRL of the embryos. An overall 15–30% reduction was observed in the mutant embryos obtained from tph1-null mothers. CRL values are 7.8 mm (embryo 1), 7.7 mm (embryo 2), 6.8 mm (embryo 3), and 7.0 mm (embryo 4). (Scale bars: 0.25 cm.)

Fig. 2.

Longitudinal section of whole tph1 embryos 2 and 3 of Fig. 1. (A and B) Embryos 2 and 3 were obtained, respectively, from a cross between a tph1+/− mother and a tph1+/− father and a cross between a tph1−/− mother and a wild-type father. Microscopic investigations revealed flattening of the head region at the level of the IV ventricle in the embryos obtained from a null mother. The corresponding CNS area in an embryo obtained from a heterozygous mother showed a normal histology. (C and D) A 2-h BrdU pulse revealed a reduced number (30% less) of labeled cells in the ventricular zone of the heterozygous embryo obtained from a null mother as compared with a null embryo obtained from a heterozygous mother. The analysis also revealed a 24% reduction in BrdU labeling in the roof of neopallial cortex of heterozygous embryos from null mothers (compare F with E). No major difference in BrdU labeling was observed at the level of the spinal cord (G and H). (Scale bars: 0.25 cm in A and B and 100 μm in C–H.)

Fig. 3.

mRNA expression pattern of TPH1 in mouse embryos by ISH with a specific 3′ UTR tph1 probe. The expression of the nonneuronal enzyme TPH1 is restricted to the pineal gland and enterochromaffin cells of the gut starting at E14.5 in the pineal gland (A) and at E15.5 in the gut (B).

Fig. 4.

mRNA expression pattern of TPH2 in mouse embryos by ISH with a specific 3′ UTR tph2 probe. The neuronal enzyme TPH2 is expressed in the developing nervous system at early embryonic stages (E10.5) (A) and in neurons of the myenteric plexus at E12.5 (B).