Ausió Juan, González-Romero Rodrigo, Woodcock Christopher L
Department of Biochemistry and Microbiology, University of Victoria, Victoria, BC V8W 3P6, Canada.
Biology Department, University of Massachusetts Amherst, Amherst, MA 01003, USA.
J Struct Biol. 2014 Nov;188(2):142-55. doi: 10.1016/j.jsb.2014.09.004. Epub 2014 Sep 27.
A consistent feature of sperm nuclei is its exceptionally compact state in comparison with somatic nuclei. Here, we have examined the structural organization of sperm chromatin from representatives of three vertebrate lineages, bony fish (Danio rerio), birds (Gallus gallus domesticus) and mammals (Mus musculus) using light and transmission electron microscopy (TEM). Although the three sperm nuclei are all highly compact, they differ in morphology and in the complement of compaction-inducing proteins. Whereas zebrafish sperm retain somatic histones and a nucleosomal organization, in the rooster and mouse, histones are largely replaced by small, arginine-rich protamines. In contrast to the mouse, the rooster protamine contains no cysteine residues and lacks the potential stabilizing effects of S-S bonds. Protamine driven chromatin compaction results in a stable, highly condensed chromatin, markedly different from the somatic nucleosome-based beads-on-a-string architecture, but its structure remains poorly understood. When prepared gently for whole mount TEM, the rooster and mouse sperm chromatin reveal striking rod-like units 40-50 nm in width. Also present in the mouse, which has very flattened sperm nuclei, but not rooster, where nuclei take the form of elongated cylinders, are toroidal shaped structures, with an external diameter of about 90 nm. In contrast, similarly prepared zebrafish sperm exhibit nucleosomal chromatin. We also examined the early stages in the binding of salmine (the salmon protamine) to defined sequence DNA. These images suggest an initial side-by-side binding of linear DNA-protamine complexes leading to the nucleation of thin, flexible rods with the potential to bend, allowing the ends to come into contact and fuse to form toroidal structures. We discuss the relationship between these in vitro observations and the rods and toroids seen in nuclei, and suggest an explanation for the apparent absence of these structures in TEM images of fully condensed sperm nuclei.
与体细胞细胞核相比，精子细胞核的一个显著特征是其异常紧密的状态。在这里，我们使用光学显微镜和透射电子显微镜（TEM）研究了三种脊椎动物谱系代表——硬骨鱼（斑马鱼）、鸟类（家鸡）和哺乳动物（小鼠）精子染色质的结构组织。尽管这三种精子细胞核都高度紧密，但它们在形态和诱导紧密化的蛋白质组成上有所不同。斑马鱼精子保留了体细胞组蛋白和核小体组织，而在公鸡和小鼠中，组蛋白在很大程度上被小的、富含精氨酸的鱼精蛋白所取代。与小鼠不同，公鸡的鱼精蛋白不含半胱氨酸残基，也缺乏二硫键的潜在稳定作用。鱼精蛋白驱动的染色质紧密化导致形成一种稳定的、高度浓缩的染色质，这与基于体细胞核小体的串珠状结构明显不同，但其结构仍知之甚少。当轻柔制备用于整装TEM观察时，公鸡和小鼠的精子染色质呈现出宽度为40 - 50 nm的惊人杆状单元。外径约90 nm的环形结构也存在于精子细胞核非常扁平的小鼠中，但不存在于细胞核呈细长圆柱形的公鸡中。相比之下，同样制备的斑马鱼精子呈现出核小体染色质。我们还研究了鲑精蛋白（鲑鱼鱼精蛋白）与特定序列DNA结合的早期阶段。这些图像表明线性DNA - 鱼精蛋白复合物最初并排结合，导致形成细的、柔性的、有弯曲潜力的杆状物成核，使两端能够接触并融合形成环形结构。我们讨论了这些体外观察结果与细胞核中看到的杆状和环形结构之间的关系，并对完全浓缩的精子细胞核TEM图像中这些结构明显缺失的现象提出了解释。
