Monoclonal antibodies against components of the myoplasm of eggs of the ascidian Ciona intestinalis partially block the development of muscle-specific acetylcholinesterase

Cytosolic subunits of ATP synthase are localized to the cortical endoplasmic reticulum-rich domain of the ascidian egg myoplasm

Previously, we revealed that p58, one of the ascidian maternal factors, is identical to the alpha-subunit of F1-ATP synthase (ATPα), a protein complex of the inner mitochondrial membrane. In the current study, we used immunological probes for ascidian mitochondria components to show that the ascidian ATPα is ectopically localized to the cytosol. Virtually all mitochondrial components were localized to the mitochondria-rich myoplasm. However, in detail, ATP synthase subunits and the matrix proteins showed different localization patterns. At least at the crescent stage, transmission electron microscopy (TEM) distinguished the mitochondria-less, endoplasmic reticulum (ER)-rich cortical region and the mitochondria-rich internal region. ATPα was enriched in the cortical region and MnSOD was limited to the internal region. Using subcellular fractionation, although all of the mitochondria components were highly enriched in the mitochondria-enriched fraction, a considerable amount of ATPα and F1-ATP synthase beta-subunit (ATPβ) were recovered in the insoluble cytoplasmic fraction. Even under these conditions, F1-ATP synthase gamma-subunit (ATPγ) and F0-ATP synthase subunit b (ATPb) were not recovered in the insoluble cytoplasmic fraction. This result strongly supports the exomitochondrial localization of both ATPα and ATPβ. In addition, the detergent extraction of eggs supports the idea that these cytosolic ATP synthase subunits are associated with the egg cytoskeleton. These results suggest that the subunits of ATP synthase might play dual roles at different subcellular compartments during early development.

Purification of mitochondrial proteins HSP60 and ATP synthase from ascidian eggs: implications for antibody specificity

Use of antibodies is a cornerstone of biological studies and it is important to identify the recognized protein with certainty. Generally an antibody is considered specific if it labels a single band of the expected size in the tissue of interest, or has a strong affinity for the antigen produced in a heterologous system. The identity of the antibody target protein is rarely confirmed by purification and sequencing, however in many cases this may be necessary. In this study we sought to characterize the myoplasm, a mitochondria-rich domain present in eggs and segregated into tadpole muscle cells of ascidians (urochordates). The targeted proteins of two antibodies that label the myoplasm were purified using both classic immunoaffinity methods and a novel protein purification scheme based on sequential ion exchange chromatography followed by two-dimensional gel electrophoresis. Surprisingly, mass spectrometry sequencing revealed that in both cases the proteins recognized are unrelated to the original antigens. NN18, a monoclonal antibody which was raised against porcine spinal cord and recognizes the NF-M neurofilament subunit in vertebrates, in fact labels mitochondrial ATP synthase in the ascidian embryo. PMF-C13, an antibody we raised to and purified against PmMRF, which is the MyoD homolog of the ascidian Phallusia mammillata, in fact recognizes mitochondrial HSP60. High resolution immunolabeling on whole embryos and isolated cortices demonstrates localization to the inner mitochondrial membrane for both ATP synthase and HSP60. We discuss the general implications of our results for antibody specificity and the verification methods which can be used to determine unequivocally an antibody's target.

From oocyte to 16-cell stage: Cytoplasmic and cortical reorganizations that pattern the ascidian embryo

The dorsoventral and anteroposterior axes of the ascidian embryo are defined before first cleavage by means of a series of reorganizations that reposition cytoplasmic and cortical domains established during oogenesis. These domains situated in the periphery of the oocyte contain developmental determinants and a population of maternal postplasmic/PEM RNAs. One of these RNAs (macho-1) is a determinant for the muscle cells of the tadpole embryo. Oocytes acquire a primary animal-vegetal (a-v) axis during meiotic maturation, when a subcortical mitochondria-rich domain (myoplasm) and a domain rich in cortical endoplasmic reticulum (cER) and maternal postplasmic/PEM RNAs (cER-mRNA domain) become polarized and asymmetrically enriched in the vegetal hemisphere. Fertilization at metaphase of meiosis I initiates a series of dramatic cytoplasmic and cortical reorganizations of the zygote, which occur in two major phases. The first major phase depends on sperm entry which triggers a calcium wave leading in turn to an actomyosin-driven contraction wave. The contraction concentrates the cER-mRNA domain and myoplasm in and around a vegetal/contraction pole. The precise localization of the vegetal/contraction pole depends on both the a-v axis and the location of sperm entry and prefigures the future site of gastrulation and dorsal side of the embryo. The second major phase of reorganization occurs between meiosis completion and first cleavage. Sperm aster microtubules and then cortical microfilaments cause the cER-mRNA domain and myoplasm to reposition toward the posterior of the zygote. The location of the posterior pole depends on the localization of the sperm centrosome/aster attained during the first major phase of reorganization. Both cER-mRNA and myoplasm domains localized in the posterior region are partitioned equally between the first two blastomeres and then asymmetrically over the next two cleavages. At the eight-cell stage the cER-mRNA domain compacts and gives rise to a macroscopic cortical structure called the Centrosome Attracting Body (CAB). The CAB is responsible for a series of unequal divisions in posterior-vegetal blastomeres, and the postplasmic/PEM RNAs it contains are involved in patterning the posterior region of the embryo. In this review, we discuss these multiple events and phases of reorganizations in detail and their relationship to physiological, cell cycle, and cytoskeletal events. We also examine the role of the reorganizations in localizing determinants, postplasmic/PEM RNAs, and PAR polarity proteins in the cortex. Finally, we summarize some of the remaining questions concerning polarization of the ascidian embryo and provide comparisons to a few other species. A large collection of films illustrating the reorganizations can be consulted by clicking on "Film archive: ascidian eggs and embryos" at http://biodev.obs-vlfr.fr/recherche/biomarcell/.

Cell fate specification by localized cytoplasmic determinants and cell interactions in ascidian embryos

Tadpole larvae of ascidians show the basic body plan of chordates. An ascidian larva consists of only a few types of cells and has a relatively small number of cells. Cell lineages are invariant among individuals and have been described in detail. These advantages facilitate the analysis of how the fate of each blastomere becomes specified during development. Over a century of research on ascidian embryogenesis has uncovered many interesting features concerning cellular mechanisms responsible for the fate specification. During embryogenesis, the developmental fate of a blastomere is specified by one of three different mechanisms: localized maternal cytoplasmic determinants, inductive interactions, or lateral inhibition in an equivalence cell group.

An ankryin-like protein in ascidian eggs and its role in the evolution of direct development

An erythrocyte anti-ankryin antibody was used to investigate the presence and distribution of ankryin in eggs and embryos of ascidian species with different modes of development. In eggs of the indirect developer Ascidia ceratodes, anti-ankryin reacted with a 210 kDa polypeptide which has an electrophoretic mobility similar to the vertebrate ankryins. Immunofluorescence microscopy showed that the ankryin-like protein is co-distributed with the myoplasm throughout development. It is restricted to a thin layer under the plasma membrane in unfertilised eggs, undergoes ooplasmic segregation to the posterior pole of the zygote after fertilisation, and is distributed to the tail muscle cells during cleavage and embryogenesis. After gastrulation and neurulation, lower levels of the ankryin-like protein, presumably of zygotic origin, were observed in brain cells and in the apical margin of epidermal cells. The ankryin-like protein was also localised in the myoplasm in eggs and embryos of another indirect developing species, Halocynthia roretzi. The ankryin-like protein may link the cytoskeleton with the plasma membrane in ascidian eggs, as it does in vertebrate erythrocytes. In contrast to A. ceratodes and H. roretzi, which are members of the families Ascidiidae and Pyuridae respectively, the pattern of ankryin-like protein expression was changed in five species in the family Molgulidae. These molgulid ascidians exhibit either indirect or direct development, and eggs of the direct developing species have lost or modified the myoplasm. The ankryin-like protein was present in young oocytes but failed to persist during oogenesis and disappeared in mature eggs and embryos of these molgulid species. The change in ankryin-like protein expression may be a preadaptation for loss of the myoplasm and the evolution of direct development.

GABA-mediated synaptic interaction between the visual and vestibular pathways of Hermissenda

The synaptic convergence of the eyes and the vestibular hair cells in the nudibranch mollusc Hermissenda has been shown previously to mediate the learning of simple visual-vestibular associations. The neurotransmitter mediating this interaction between the visual and vestibular organs was characterized. HPLC chromatography, confirmed by mass spectroscopic analysis, demonstrated endogenous GABA in the statocysts, in a concentration approximately 150 times greater than in the whole CNS. Additional confirmation was provided by immunocytochemical localization of GABA in hair cell axons and branches that converge with photoreceptor terminal branches. Depolarization of the hair cells in the caudal region of the statocyst in response to positive current injection or vibratory stimulation caused a hyperpolarization and a cessation of the type B photoreceptor impulse activity. The inhibition of the B cell was unaffected by addition to the artificial sea water bath of the adrenergic antagonist yohimbine (250 microM), the cholinergic antagonist atropine (250 microM), and the serotonergic antagonist imipramine (50 microM). In contrast, the GABAA antagonist bicuculline (250 microM) significantly reduced the inhibitory interaction. Moreover, the GABA reuptake inhibitor guvisine (250 microM) increased the hyperpolarization. Pressure microapplication of GABA (12.5 or 25 microM) onto the terminal branches of the B cell resulted in a concentration-dependent hyperpolarization and cessation of spikes in the B cell. Depolarization of the caudal hair cell, or direct GABA application, decreased input resistance across the B cell soma membrane. Moreover, removal of chloride from the extracellular solution reduced inhibition of the B cell induced by GABA application or hair cell stimulation. Furthermore, application of the GABAB agonist baclofen hyperpolarized the type B cell and reduced or eliminated spontaneous impulse activity at the resting membrane potential. The reversal potentials for inhibition induced in all three procedures ranged from -70 to -80 mV and were consistent with mixed Cl- and K+ conductances. These results implicate GABA as the endogenous neurotransmitter mediating visual-vestibular interactions in this animal, and suggest a possible role of GABA in visual-vestibular associative learning.

The repetitive calcium waves in the fertilized ascidian egg are initiated near the vegetal pole by a cortical pacemaker

Ascidian eggs respond to fertilization with a series of repetitive calcium waves that originate mostly from the vegetal/contraction pole region (J. E. Speksnijder, C. Sardet, and L. F. Jaffe, 1990, Dev. Biol. 142, 246-249), where the myoplasm is concentrated during the first phase of ooplasmic segregation. This suggests that the myoplasm may be involved in initiating these calcium waves. To test this possibility, the starting position of the calcium waves was determined in eggs that had the subcortical, mitochondria-rich part of the myoplasm displaced by centrifugation. Such centrifuged eggs display four cytoplasmic layers: a large centrifugal yolk zone, a narrow clear zone, a mitochondria-rich layer, and a small clear zone at the centripetal pole. Imaging of the cytosolic calcium in centrifuged eggs that were injected with the calcium-specific photoprotein aequorin reveals a series of repetitive calcium waves after fertilization. About 70% of these waves start in the vegetal/contraction pole area, which is similar to the number of waves previously found to start in this area in uncentrifuged eggs. In contrast, only about 25% of the waves start close to the displaced mitochondria-rich layer. From this result it is concluded that the main wave initiation site is not displaced by the centrifugal forces that displace the subcortical, mitochondria-rich part of the myoplasm. Moreover, the observation that the animal-vegetal polarity of cortical components such as actin filaments and the endoplasmic reticulum has been retained after centrifugation further suggests that a cortical component located in the vegetal hemisphere--most likely the endoplasmic reticulum network in the cortical region of the myoplasm--is involved in initiating the repetitive calcium waves in the fertilized ascidian egg.