A research group headed by HHMI investigator Richard D. Palmiter at the University of Washington has shown conclusively that catecholamines are essential in mouse development, and most likely in human development, too.
Catecholamines are neurotransmitters that have long been known to play vital roles in many adult physiological processes, from heart stimulation to motor control. Nothing has been known, however, of the impact this battery of biochemicals that includes dopamine, noradrenaline, and adrenaline has on embryonic development.
Now, in a pair of letters to Nature published April 13, 1995, a research group headed by HHMI investigator Richard D. Palmiter at the University of Washington has shown conclusively that catecholamines are essential in mouse development, and most likely in human development, too.
The whole system is a gold mine to look at ways animals can compensate for the lack of noradrenaline and other catecholamines.
Richard D. Palmiter
Using gene targeting, the Seattle group created a mouse that lacked the gene for tyrosine hydroxylase (TH), the first enzyme in the catecholamine biosynthetic pathway, crippling the mouse's ability to synthesize any of the catecholamines. Other knockout mice had inactivated dopamine beta-hydroxylase (DBH) genes, which prevented them from making noradrenaline from dopamine.
Palmiter, who has spent more than a decade experimenting with transgenic animals to study gene function and regulation, bred heterozygous mice carrying one mutant copy and one normal copy for TH or DBH. About 25 percent of the embryos derived from breeding the TH or DBH heterozygotes would be expected to be homozygotes with two copies of the mutant genes—so-called knockouts. However, less than five percent of the mice born were DBH or TH knockouts. About 80 percent of DBH or TH knockout mouse embryos died between days 11.5 and 13.5 of fetal development.
"The first major surprise was the death of the fetuses, implying that catecholamines are needed for embryonic development," said Palmiter. "No one had realized that they were necessary at this critical stage of development. Because the embryos likely died of heart failure, it's possible that catecholamines are necessary to prepare the heart and vessels for the rapid growth that occurs during this period of development."
The TH-deficient mice could be sustained through birth by providing the mothers with L-DOPA in their drinking water. L-DOPA is normally produced by TH activity. The L-DOPA passed into the fetal circulation and was taken up by cells that metabolize it to essential catecholamines. Thus, all of the embryos were saved, but, after birth, when deprived of L-DOPA, none of them survived for more than three weeks.
"This is strong evidence that the major defect is in catecholamine synthesis--not some mutation in embryonic cells or some other metabolic effect of TH," Palmiter said. "It's also proof that dopamine is needed soon after birth."
Similarly, DBH-deficient mice were rescued by giving the mothers dihydroxyphenlyserine (L-DOPS), which can be converted directly into noradrenaline by the second enzyme in the catecholamine pathway, AADC. This confirmed the requirement of noradrenaline for embryo survival. Palmiter hypothesizes that noradrenaline interacts during the critical embryonic period between 10 and 13 days with surface proteins called adrenergic receptors, which spark a biochemical cascade that eventually activates protein kinase A, a vital enzyme for cell function.
The findings have immediate implications for humans. Steve Thomas of Palmiter's laboratory observed a condition called ptosis— characterized by drooping eyelids, and caused by flawed nervous systems—in the DBH-deficient mice. This correlates with six human cases of DBH deficiency and ptosis reported in the late 1980s. Also, the relationship between noradrenaline and adrenergic receptors cautions against using adrenergic blockers for pregnant women suffering from hypertension. Such drugs might mimic DBH deficiency in the fetus.
Palmiter and his colleagues now have generated many catecholamine-deficient mice to study a variety of physiological and behavioral questions. "The whole system is a gold mine to look at ways animals can compensate for the lack of noradrenaline and other catecholamines," Palmiter said.