两篇Nature文章吸引眼球

　　

生物通报道：3月26日公布的Nature杂志有两项相关的研究成果，一项是由中国科学家完成的硬骨鱼类的起源提的研究，一项是由美国国立卫生研究院完成的骨再吸收研究。

在一篇文章中，中国科学院古脊椎动物与古人类研究所的朱敏研究员所带领的早期脊椎动物课题组新发现的古鱼将有颌脊椎动物几大类群的特征汇于一身，大大填充了它们之间的形态学鸿沟，第一次近乎完整地呈现了有颌脊椎动物祖先可能具有的特征组合。

这一新发现的硬骨鱼形态距今超过4.18亿年，是已知最早的、保存完好的硬骨鱼，是“肉鳍鱼”这个分支中位于演化树根部的一个成员，这个分支今天还包括肺鱼、腔棘鱼和所有陆地脊椎动物。在该化石中，衍生特征与原始特征如人们所料混合在一起。这意味着，“条鳍”硬骨鱼和“肉鳍”硬骨鱼的分化一定发生在距今至少4.19亿年前，说明有颚类脊椎动物有一个久远的历史。

同期配发的美国芝加哥大学迈克－科兹（Michael Coates）教授的评述文章称：朱敏等为解开围绕有颌类分化和硬骨鱼类起源的重重谜团“提供了一条活灵活现的古鱼”。“至关紧要的是，这条鱼代表着人类遥远祖先的一个分支，它不但罕见地被完整保存下来，而且出人意料的古老。它为脊椎动物进化的一个重大分歧事件（辐鳍鱼类与肉鳍鱼类的分化）提供了一个新的确凿无疑的最近时间校正点，将会掀起新一轮的在志留纪地层中的野外考察热潮。”

过去十年，有颌类的早期分化、硬骨鱼类的起源与早期分化逐渐成为国际演化生物学界关注的重点，相关研究成果在《自然》《科学》等杂志上屡有报道。但由于早期化石保存的不完整，多数早泥盆世鱼类都是由零散材料拼合到一起进行研究，特征的解释存在很多不确定因素，这会直接影响到对演化格局的重建。尤其是当恢复出来的古鱼完全不同于过去任何见过的鱼类，学术界就会有一定的疑虑和保留。要彻底澄清这些疑虑，最好的解决方案就是在4亿多年前的地层中寻找出一条完整的古鱼。

为此，朱敏等在云南曲靖、昭通等地组织了多次发掘，2007年春，课题组成员赵文金博士带队测制曲靖志留系剖面过程中，发现了几处具有鱼类大化石的地点。随即，朱敏带队对这些地点及周边做了更细致的考察，在重点层位组织了定点发掘，发现了大量的盾皮鱼类化石，同时发现了一件完整的硬骨鱼下颌标本。

第二篇文章中，研究人员利用一种激素缺陷型骨质疏松症小鼠模型，发现血液中的脂质介质“鞘氨醇-1-磷酸盐”是骨头去矿质作用的一个关键调控因子。

骨头是一种动态组织，不断生长、重塑和退化。在这些过程中起中心作用的是破骨细胞，即能够再吸收骨的多核巨型细胞，它们是从单核巨噬细胞/单核细胞系造血细胞前体分化出来的。正常情况下，骨再吸收会被形成骨头的成骨细胞的活性所平衡，但在骨质疏松症等骨头被破坏的疾病中，破骨细胞活性超过成骨细胞活性。现在，研究人员利用一种荷尔蒙匮乏型骨质疏松症小鼠模型，发现血液中的脂质介质“鞘氨醇-1-磷酸盐”是骨头去矿质作用的一个关键调控因子。它控制破骨细胞前体的迁移行为，从而调控骨头的体内平衡。作为破骨细胞生成中的一个至关重要的控制点，“鞘氨醇-1-磷酸盐”有可能成为骨再吸收疾病的一个潜在治疗目标。

原文摘要：

Sphingosine-1-phosphate mobilizes osteoclast precursors and regulates bone homeostasis

Osteoclasts are the only somatic cells with bone-resorbing capacity and, as such, they have a critical role not only in normal bone homeostasis (called 'bone remodelling') but also in the pathogenesis of bone destructive disorders such as rheumatoid arthritis and osteoporosis1. A major focus of research in the field has been on gene regulation by osteoclastogenic cytokines such as receptor activator of NF-B-ligand (RANKL, also known as TNFSF11) and TNF-, both of which have been well documented to contribute to osteoclast terminal differentiation2, 3. A crucial process that has been less well studied is the trafficking of osteoclast precursors to and from the bone surface, where they undergo cell fusion to form the fully differentiated multinucleated cells that mediate bone resorption. Here we report that sphingosine-1-phosphate (S1P), a lipid mediator enriched in blood4, 5, induces chemotaxis and regulates the migration of osteoclast precursors not only in culture but also in vivo, contributing to the dynamic control of bone mineral homeostasis. Cells with the properties of osteoclast precursors express functional S1P1 receptors and exhibit positive chemotaxis along an S1P gradient in vitro. Intravital two-photon imaging of bone tissues showed that a potent S1P1 agonist, SEW2871, stimulated motility of osteoclast precursor-containing monocytoid populations in vivo. Osteoclast/monocyte (CD11b, also known as ITGAM) lineage-specific conditional S1P1 knockout mice showed osteoporotic changes due to increased osteoclast attachment to the bone surface. Furthermore, treatment with the S1P1 agonist FTY720 relieved ovariectomy-induced osteoporosis in mice by reducing the number of mature osteoclasts attached to the bone surface. Together, these data provide evidence that S1P controls the migratory behaviour of osteoclast precursors, dynamically regulating bone mineral homeostasis, and identifies a critical control point in osteoclastogenesis that may have potential as a therapeutic target.

The oldest articulated osteichthyan reveals mosaic gnathostome characters

The evolutionary history of osteichthyans (bony fishes plus tetrapods) extends back to the Ludlow epoch of the Silurian period. However, these Silurian forms have been documented exclusively by fragmentary fossils. Here we report the discovery of an exceptionally preserved primitive fish from the Ludlow of Yunnan, China, that represents the oldest near-complete gnathostome (jawed vertebrate). The postcranial skeleton of this fish includes a primitive pectoral girdle and median fin spine as in non-osteichthyan gnathostomes, but a derived macromeric squamation as in crown osteichthyans, and substantiates the unexpected mix of postcranial features in basal sarcopterygians, previously restored from the disarticulated remains of Psarolepis. As the oldest articulated sarcopterygian, the new taxon offers insights into the origin and early divergence of osteichthyans, and indicates that the minimum date for the actinopterygian–sarcopterygian split was no later than 419 million years ago.