title: The maternal transcript for truncated voltage-dependent Ca2+ channels in the ascidian embryo: a potential suppressive role in Ca2+ channel expression.
abstract: Ca2+ entry during electrical activity plays several critical roles in development. However, the mechanisms that regulate Ca2+ influx during early embryogenesis remain unknown. In ascidians, a primitive chordate, development is rapid and blastomeres of the muscle and neuronal lineages are easily identified, providing a simple model for studying the expression of voltage-dependent Ca2) channels (VDCCs) in cell differentiation. Here we isolate an ascidian cDNA, TuCa1, a homologue of the alpha(1)-subunit of L-type class Ca2+ channels. We unexpectedly found another form of Ca2+ channel cDNA (3-domain-type) potentially encoding a truncated type which lacked the first domain and a part of the second domain. An analysis of genomic sequence suggested that 3-domain-type RNA and the full-length type have alternative transcriptional start sites. The temporal pattern of the amount of 3-domain-type RNA was the reverse of that of the full-length type; the 3-domain type was provided maternally and persisted during early embryogenesis, whereas the full-length type was expressed zygotically in neuronal and muscular lineage cells. Switching of the two forms occurred at a critical stage when VDCC currents appeared in neuronal or muscular blastomeres. To examine the functional 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 current, whereas it had a remarkable inhibitory effect on the functional expression of the full-length form. In addition, overexpression of the 3-domain type under the control of the muscle-specific actin promoter in ascidian muscle blastomeres led to a significant decrease in endogenous VDCC currents. These findings raise the possibility that the 3-domain type has some regulatory role in tuning current amplitudes of VDCCs during early development.
authors.affiliation: Ion Channel Group, National Institute of Bioscience and Human Technology, Ibaraki, 305-8566, Japan.
authors.forename: R
authors.initials: R
authors.lastname: Okagaki
authors.forename: H
authors.initials: H
authors.lastname: Izumi
authors.forename: T
authors.initials: T
authors.lastname: Okada
authors.forename: H
authors.initials: H
authors.lastname: Nagahora
authors.forename: K
authors.initials: K
authors.lastname: Nakajo
authors.forename: Y
authors.initials: Y
authors.lastname: Okamura
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D015220:Calcium Channels / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D020746:Calcium Channels, L-Type / Q000737:chemistry / Q000235:genetics; D003001:Cloning, Molecular; D018076:DNA, Complementary; D004625:Embryo, Nonmammalian / Q000502:physiology*; D005260:Female; D018392:Genomic Imprinting; D008969:Molecular Sequence Data; D009024:Morphogenesis; D009132:Muscles / Q000196:embryology; D009865:Oocytes / Q000502:physiology; D017433:Protein Structure, Secondary; D012326:RNA Splicing; D012333:RNA, Messenger / Q000032:analysis; D011817:Rabbits; D011994:Recombinant Proteins / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012689:Sequence Homology, Nucleic Acid; D014158:Transcription, Genetic; D014561:Urochordata / Q000196:embryology* / Q000235:genetics*; D014982:Xenopus laevis
chemical_list: D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D018076:DNA, Complementary; D012333:RNA, Messenger; D011994:Recombinant Proteins