Marsh-Armstrong, N.*; Brown, D.D.: Transgenic studies of metamorphic changes in the nervous system of Xenopus laevis

During Xenopus laevis metamorphosis, herbivorous filter feeding tadpoles that swim by flicking their tails are transformed into carnivorous frogs that use their arms to shovel in food and their legs to swim. The nervous system changes during metamorphosis to accommodate these new behaviors. Until recently, it has been difficult to dissociate nervous system changes from the other changes that occur during metamorphosis. However, the recent development of transgenic technology in Xenopus laevis now allows us to directly study the metamorphosis of the nervous system. During metamorphosis, the spinal cord develops the circuitry to control arm and leg movement. The interference of thyroid hormone action in the spinal cord by means of nervous system specific transgenes leads to the development of limbs that look normal but that in severe cases lack any motion and in less severe cases have uncoordinated movement. Examination of these paralyzed limbs show abnormal development of the neuromuscular junction. Also during metamorphosis, the eyes shift their position and for the first time develop a region of visual overlap. Concurrent with these eye position changes, the retinas alter their proliferation pattern from symmetric to asymmetric and for the first time develop an axonal projection to the same side of the brain. Transgenic interference of the action of the action of thyroid hormone in the retina leads to inhibition of both these changes. Further, transgenic studies also demonstrate that the localized expression of a thyroid hormone degrading enzyme in the dorsal margin of the retina is sufficient to account for the asymmetry that develops in the retina at metamorphosis.