Johnson Christopher J, Razy-Krajka Florian, Stolfi Alberto
Georgia Institute of Technology, Atlanta, GA USA.
Evodevo. 2020 Aug 3;11:15. doi: 10.1186/s13227-020-00162-x. eCollection 2020.
The evolution of vertebrate smooth muscles is obscured by lack of identifiable smooth muscle-like cells in tunicates, the invertebrates most closely related to vertebrates. A recent evolutionary model was proposed in which smooth muscles arose before the last bilaterian common ancestor, and were later diversified, secondarily lost or modified in the branches leading to extant animal taxa. However, there is currently no data from tunicates to support this scenario.
Here, we show that the axial columnar cells, a unique cell type in the adhesive larval papillae of the tunicate are enriched for orthologs of vertebrate smooth/non-muscle-specific effectors of contractility, in addition to developing from progenitors that express conserved cardiomyocyte regulatory factors. We show that these cells contract during the retraction of the papillae during larval settlement and metamorphosis.
We propose that the axial columnar cells of are a myoepithelial cell type required for transducing external stimuli into mechanical forces that aid in the attachment of the motile larva to its final substrate. Furthermore, they share developmental and functional features with vertebrate myoepithelial cells, vascular smooth muscle cells, and cardiomyocytes. We discuss these findings in the context of the proposed models of vertebrate smooth muscle and cardiomyocyte evolution.
由于在与脊椎动物关系最为密切的无脊椎动物——被囊动物中缺乏可识别的平滑肌样细胞，脊椎动物平滑肌的进化情况尚不明确。最近提出了一种进化模型，其中平滑肌在最后一个两侧对称动物共同祖先之前就已出现，随后在导致现存动物类群的分支中发生多样化、次生丢失或改变。然而，目前尚无来自被囊动物的数据支持这一设想。
在此，我们表明，被囊动物附着性幼虫乳头中的独特细胞类型——轴向柱状细胞，除了由表达保守心肌细胞调节因子的祖细胞发育而来外，还富含脊椎动物平滑肌/非肌肉特异性收缩效应器的直系同源物。我们发现这些细胞在幼虫定居和变态过程中乳头回缩时会收缩。
我们提出，被囊动物的轴向柱状细胞是一种肌上皮细胞类型，它将外部刺激转化为机械力，有助于活动的幼虫附着到其最终底物上。此外，它们与脊椎动物的肌上皮细胞、血管平滑肌细胞和心肌细胞具有共同的发育和功能特征。我们在脊椎动物平滑肌和心肌细胞进化的提出模型的背景下讨论了这些发现。
