ホヤやフジツボ、イガイといった海産付着動物の多くは成長の過程で変態し、体構造を劇的に変化させることにより生活様式を変える。しかしながら環境からのキューとなる刺激を、体内の変態シグナルへと変換する分子メカニズムは不明である。カタユウレイボヤの変態を機械刺激により人工的に誘導する系を開発した。変態誘導時に付着器で2度のCa2+濃度上昇が生じること、体幹部の表皮細胞全体でCa2+波が伝播したのち表皮細胞が後方に移動することを発見した（Wakai et al., 2021 Proc.R.Soc.B）。しかし、これ以降変態シグナルは明らかでなはい。今回、2度のCa2+濃度上昇前後のリアルタイムイメージングにより、間充織細胞が表皮を越えて体外被嚢層に移動する（溢出する）ことを見出した。溢出のタイミングは表皮細胞のCa2+伝播後、後方移動と同時だった。
溢出細胞の役割を調べるために、尾部退縮後も観察を続けた。変態前、遊泳幼生の被嚢の周囲はテスト細胞で覆われていた。テスト細胞は未受精卵から存在し、母体由来の卵にとっては非自己の付属細胞である。尾部退縮完了後、体幹部前方からテスト細胞は脱落した。一方で尾部退縮前に溢出した自己由来の間充織細胞は被嚢内に留まることから、変態を境に、体外細胞が親由来の非自己テスト細胞から自己由来の被嚢細胞へと置き換わることが明らかになった(投稿準備中、Totsuka et al., 2022 11th International Tunicate Meeting)。溢出と表皮伝播の関係を調べるため、ニポウディスク顕微鏡を用いて表皮におけるGCaMP6sの3Dライブイメージングを行い表皮伝播動態を解析した。眼点直上の神経細胞(aATEN)付近の表皮からCa2+伝播が開始されることが示唆された。付着器神経から投射を受けるこの細胞は、変態を駆動する特異な細胞であると考えられる。また変態の駆動にはcAMP が関与し、付着器の表皮細胞でCa2+濃度上昇に続いてcAMP 濃度は上昇した。本研究のような組織レベルの解析は、今後ホヤおよびその他の海産付着生物の変態メカニズムの解明に役立つ。
Many marine sessile organisms, such as ascidians, sea urchins, and barnacles, undergo metamorphosis during their growth, dramatically altering their body structure and lifestyle. However, the molecular mechanisms that convert environmental cues into metamorphic signals within these organisms are largely unknown. Recently, a system was developed to induce metamorphosis in the ascidian Ciona intestinalis using mechanical stimulation. During metamorphosis induction, it was discovered that there were two increases in Ca2+ concentration in the palp, and that Ca2+ waves propagated throughout the entire epidermis of the trunk before the trunk epidermal cells moved posteriorly (Wakai et al., 2021 Proc.R.Soc.B). However, the metamorphic signal after this event is still unclear. In this study, real-time imaging before and after the two increases in Ca2+ concentration revealed that mesenchymal cells move out of the epidermis and into the extracellular matrix layer. This cell migration occurred simultaneously with the posterior movement of epidermal cells, following the Ca2+ wave propagation. To investigate the role of these migrating cells, observations were continued after the tail regression. Prior to metamorphosis, the perimeter of the ascidian larva's tunic was covered with test cells. Test cells are non-self accessory cells derived from unfertilized eggs. After tail regression, the test cells were shed from the anterior trunk. Conversely, self-derived mesenchymal cells that spilled out prior to tail regression remained within the tunic, revealing that external cells are replaced by self-derived tunic cells after metamorphosis. To investigate the relationship between cell migration and Ca2+ propagation, 3D live imaging of GCaMP6s in the epidermis was performed using a Nikon Disk microscope to analyze the dynamics of Ca2+ propagation. It was suggested that Ca2+ propagation was initiated from the epidermis near the aATEN neuron directly above the ocellis. This cell, which receives projections from the palp neurons, is considered to be a unique cell that drives metamorphosis. Additionally, cAMP was found to be involved in metamorphosis, as the Ca2+ concentration in the palp epidermal cells was followed by an increase in cAMP concentration. Tissue-level analysis like this study will shed light on the mechanism of metamorphosis in sessile organisms including Ciona.
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