The maternal transcript for truncated voltage-dependent Ca2+ channels in the ascidian embryo: A potential suppressive role in Ca2+ channel expression

Ca2+ entry during electrical activity plays several critical roles in devel opment. However, the mechanisms that regulate Ca2+ influx-during early embr yogenesis remain unknown. In ascidians, a primitive chordate, development i s rapid and blastomeres of the muscle and neuronal lineages are easily iden tified, providing a simple model for studying the expression of voltage-dep endent Ca2+ channels (VDCCs) in cell differentiation. Here we isolate an as cidian cDNA, TuCa1, a homologue of the alpha (1)-subunit of L-type class Ca 2+ channels. We unexpectedly found another form of Ca2+ channel cDNA (3-dom ain-type) potentially encoding a truncated type which lacked the first doma in and a part of the second domain. An analysis of genomic sequence suggest ed that 3-domain-type RNA and the full-length type have alternative transcr iptional start sites. The temporal pattern of the amount of 3-domain-type R NA was the reverse of that of the full-length type; the 3-domain type was p rovided maternally and persisted during early embryogenesis, whereas the fu ll-length type was expressed zygotically in neuronal and muscular lineage c ells. Switching of the two forms occurred at a critical stage when VDCC cur rents appeared in neuronal or muscular blastomeres. To examine the function al roles of the 3-domain type, it was coexpressed with the full-length type in Xenopus oocyte. The 3-domain type did not produce a functional VDCC cur rent, whereas it had a remarkable inhibitory effect on the functional expre ssion of the full-length form. In addition, overexpression of the 3-domain type under the control of the muscle-specific actin promoter in ascidian mu scle blastomeres led to a significant decrease in endogenous VDCC currents. These findings raise the possibility that the 3-domain type has some regul atory role in tuning current amplitudes of VDCCs during early development. (C) 2001 Academic Press.