Primary Sequence and Developmental Expression Pattern of mRNAs and Protein for an α₁Subunit of the Sodium Pump Cloned from the Neural Plate ofXenopus laevis

Abstract

Expression of a catalytic α subunit of the sodium pump was followed in earlyXenopusembryos for correlation with physiological experiments showing that the sodium pump controls cavity expansion and the differentiation of neurones from the neural plate. Two cDNAs (one full length, one partial) for α₁subunit isoforms were cloned from a neural plate stageXenopuslibrary and sequenced. Other isoforms were not detected. Temporal and spatial expression patterns for α₁subunit transcripts and protein revealed extensive developmental regulation. At all stages, cells involved in cavity generation (outer ectoderm and cells lining the archenteron) expressed α₁transcripts with protein confined to the lateral and basal membranes. Before gastrulation, transcript levels were low and predominantly in animal cells. During gastrulation, α₁mRNAs rose significantly. Transcripts and protein were down-regulated in future outer neural plate cells as the mesoderm invaginated. Protein appeared at the blastopore on apical surfaces of lip cells and apposing surfaces of invaginating cells, suggesting that the Na pump opposes entry of fluid. In early neurulae, α₁mRNAs rose sharply. Transcript expression remained low in outer neural plate cells and increased in the endoderm, and protein appeared in the notochord. In midneurulae, transcripts returned in outer neural plate cells. Protein expression appeared on basal surfaces of deep neural plate cells and the floor plate, matching physiological observations. After neural tube closure, transcripts were detected in all dorsal structures. Protein was retained in the notochord and floor plate, was eliminated from the outer layer of the neural tube, and appeared on ependymal cells. The results are discussed in relation to previous physiological observations.