Katow Hideki, Katow Tomoko, Yoshida Hiromi, Kiyomoto Masato, Uemura Isao
Research Center for Marine Biology, Tohoku University, Asamushi, Aomori, Aomori 039-3501 Japan ; Center of Research Instruments, Institute of Development, Aging and Cancer, Tohoku University, Sendai, 980-8575 Japan.
Research Center for Marine Biology, Tohoku University, Asamushi, Aomori, Aomori 039-3501 Japan.
Front Zool. 2016 Jun 16;13:27. doi: 10.1186/s12983-016-0159-8. eCollection 2016.
The swimming activity of sea urchin larvae is dependent on the ciliary band (CB) on the larval surface and is regulated by several neurotransmitters, including serotonin (5HT), dopamine, and γ-aminobutyric acid (GABA). However, the CB signal transmission mechanism remains unknown. The present study investigated the structural relationship between the CB and external signal receptors by immunohistochemical and transmission electron microscopic analyses of sea urchin, Hemicentrotus pulcherrimus, larvae.
Glutamate decarboxylase (GAD; GABA synthetase) was detected in a strand of multiple cells along the circumoral CB in 6-arm plutei. The GAD-expressing strand was closely associated with the CB on the oral ectoderm side. The ciliary band-associated strand (CBAS) also expressed the 5HT receptor (5HThpr) and encephalopsin (ECPN) throughout the cytoplasm and comprised 1- to 2-μm diameter axon-like long stretched regions and sporadic 6- to 7-μm diameter bulbous nucleated regions (perikarya) that protruded into the oral ectoderm side. Besides the laterally polarized morphology of the CBAS cells, Epith-2, which is the epithelial lateral cell surface-specific protein of the sea urchin embryo and larva, was expressed exclusively by perikarya but not by the axon-like regions. The CBAS exposed its narrow apical surface on the larval epithelium between the CB and squamous cells and formed adherens junctions (AJs) on the apical side between them. Despite the presence of the CBAS axon-like regions, tubulins, such as α-, β-, and acetylated α-tubulins, were not detected. However, the neuroendocrine cell marker protein synaptophysin was detected in the axon-like regions and in bouton-like protrusions that contained numerous small ultrastructural vesicles.
The unique morphology of the CBAS in the sea urchin larva epithelium had not been reported. The CBAS expresses a remarkable number of receptors to environmental stimuli and proteins that are probably involved in signal transmission to the CB. The properties of the CBAS explain previous reports that larval swimming is triggered by environmental stimuli and suggest crosstalk among receptors and potential plural sensory functions of the CBAS.
海胆幼虫的游泳活动依赖于幼虫体表的纤毛带（CB），并受多种神经递质调节，包括血清素（5HT）、多巴胺和γ-氨基丁酸（GABA）。然而，CB信号传递机制仍不清楚。本研究通过对海胆（Hemicentrotus pulcherrimus）幼虫进行免疫组织化学和透射电子显微镜分析，研究了CB与外部信号受体之间的结构关系。
在六腕长腕幼虫沿口周CB的一串多细胞中检测到谷氨酸脱羧酶（GAD；GABA合成酶）。表达GAD的细胞串与口外胚层侧的CB紧密相连。纤毛带相关细胞串（CBAS）在整个细胞质中也表达5HT受体（5HThpr）和脑视蛋白（ECPN），并由直径1至2μm的轴突样长延伸区域和直径6至7μm的散在的球状有核区域（核周体）组成，这些区域向口外胚层侧突出。除了CBAS细胞的侧向极化形态外，Epith-2（海胆胚胎和幼虫上皮细胞侧面特异性蛋白）仅在核周体中表达，而不在轴突样区域表达。CBAS在幼虫上皮细胞上，在CB和鳞状细胞之间暴露其狭窄的顶端表面，并在它们之间的顶端形成黏着连接（AJs）。尽管存在CBAS轴突样区域，但未检测到微管蛋白，如α-、β-和乙酰化α-微管蛋白。然而，在轴突样区域和含有许多小超微结构囊泡的纽扣状突起中检测到神经内分泌细胞标记蛋白突触素。
海胆幼虫上皮细胞中CBAS的独特形态尚未见报道。CBAS表达大量对环境刺激的受体和可能参与向CB信号传递的蛋白质。CBAS的特性解释了先前关于幼虫游泳由环境刺激触发的报道，并提示受体之间的串扰以及CBAS潜在的多种感觉功能。
