Functional and anatomical aspects of leg innervation in the giant cockroach Blabera craniifer as compared with that in the american cockroach Periplaneta americana

Inversion of allosteric effect of arginine on N-acetylglutamate synthase, a molecular marker for evolution of tetrapods

Background

The efficient conversion of ammonia, a potent neurotoxin, into non-toxic metabolites was an essential adaptation that allowed animals to move from the aquatic to terrestrial biosphere. The urea cycle converts ammonia into urea in mammals, amphibians, turtles, snails, worms and many aquatic animals and requires N-acetylglutamate (NAG), an essential allosteric activator of carbamylphosphate synthetase I (CPSI) in mammals and amphibians, and carbamylphosphate synthetase III (CPSIII) in fish and invertebrates. NAG-dependent CPSI and CPSIII catalyze the formation of carbamylphosphate in the first and rate limiting step of ureagenesis. NAG is produced enzymatically by N-acetylglutamate synthase (NAGS), which is also found in bacteria and plants as the first enzyme of arginine biosynthesis. Arginine is an allosteric inhibitor of microbial and plant NAGS, and allosteric activator of mammalian NAGS.

Results

Information from mutagenesis studies of E. coli and P. aeruginosa NAGS was combined with structural information from the related bacterial N-acetylglutamate kinases to identify four residues in mammalian NAGS that interact with arginine. Substitutions of these four residues were engineered in mouse NAGS and into the vertebrate-like N-acetylglutamate synthase-kinase (NAGS-K) of Xanthomonas campestris, which is inhibited by arginine. All mutations resulted in arginine losing the ability to activate mouse NAGS, and inhibit X. campestris NAGS-K. To examine at what point in evolution inversion of arginine effect on NAGS occur, we cloned NAGS from fish and frogs and examined the arginine response of their corresponding proteins. Fish NAGS were partially inhibited by arginine and frog NAGS were activated by arginine.

Conclusion

Difference in arginine effect on bacterial and mammalian NAGS most likely stems from the difference in the type of conformational change triggered by arginine binding to these proteins. The change from arginine inhibition of NAGS to activation was gradual, from complete inhibition of bacterial NAGS, to partial inhibition of fish NAGS, to activation of frog and mammalian NAGS. This change also coincided with the conquest of land by amphibians and mammals.

Evolution of urea synthesis in vertebrates: the piscine connection

Elasmobranch fishes, the coelacanth, estivating lungfish, amphibians, and mammals synthesize urea by the ornithine-urea cycle; by comparison, urea synthetic activity is generally insignificant in teleostean fishes. It is reported here that isolated liver cells of two teleost toadfishes, Opsanus beta and Opsansus tau, synthesize urea by the ornithine-urea cycle at substantial rates. Because toadfish excrete ammonia, do not use urea as an osmolyte, and have substantial levels of urease in their digestive systems, urea may serve as a transient nitrogen store, forming the basis of a nitrogen conservation shuttle system between liver and gut as in ruminants and hibernators. Toadfish synthesize urea using enzymes and subcellular distributions similar to those of elasmobranchs: glutamine-dependent carbamoyl phosphate synthethase (CPS III) and mitochondrial arginase. In contrast, mammals have CPS I (ammonia-dependent) and cytosolic arginase. Data on CPS and arginases in other fishes, including lungfishes and the coelacanth, support the hypothesis that the ornithine-urea cycle, a monophyletic trait in the vertebrates, underwent two key changes before the evolution of the extant lungfishes: a switch from CPS III to CPS I and replacement of mitochondrial arginase by a cytosolic equivalent.

Glutamine- and N-acetylglutamate-dependent carbamoyl phosphate synthetase in elasmobranchs

High levels of glutamine- and N-acetyl-L-glutamate-dependent carbamoyl phosphate synthetase activity are present in liver extracts of marine species of fish that retain high levels of urea in their tissues for the purpose of osmoregulation. The function of the synthetase in these species appears to be related to urea synthesis.

Organization of motoneurones in the prothoracic ganglion of the cockroach Periplaneta americana (L.)

The location within the prothoracic ganglion of neurone somata with axons in identified peripheral nerves is examined by the cobalt iontophoresis technique. Axons are filled with cobalt by diffusion through their cut ends and the cobalt is then precipitated as the black sulphide inside the neurone. It is assumed that neurones with axons in peripheral nerves and somata in central ganglia are either motor or neuro-secretory. Fifteen nerves are examined and maps of the location of somata with axons in each nerve are presented. The axon distribution in peripheral nerves of three common inhibitory neurones is described. Dendritic morphology of one common inhibitory neurone and two coxal depressor motoneurones is illustrated. It is proposed that some individual neurones can be reliably identified from their soma dimensions and location within the ganglion. The number of motoneurones with somata in the prothoracic ganglion and their homology with cells in the other thoracic ganglia are discussed.