Department of Bioengineering, University of Utah, Salt Lake City, Utah, USA.
FASEB J. 2019 Jan;33(1):572-583. doi: 10.1096/fj.201801029R. Epub 2018 Jul 9.
Aquatic caddisfly larvae (caddisworms) wet-spin fibers to construct composite cases of silk and stone. The silk emerges from labial ducts as a nanofibrous fluid gel, flowing over the stone substrate and making intimate interfacial adhesive contacts before being drawn into tough fibers that rapidly solidify underwater to span gaps in the construction. Divalent metal ions are responsible for the unique mechanical properties of naturally spun silk fibers; however, when and where divalent metal ions are incorporated into the metallofibers and other aspects of the fiber solidification mechanism are poorly understood. To investigate, the elemental composition and secondary structure of silk precursors stored in the silk gland lumen were compared with naturally spun fibers by inductively coupled plasma optical emission spectroscopy and attenuated total reflection Fourier transform infrared spectroscopy. Naturally spun fibers contained near equimolar ratios of Ca to P. In contrast, silk precursors stored in the silk gland lumen contained only traces of Ca and other multivalent metal ions. Ca was also undetectable in anterior lumenal silk using the histochemical Ca indicator, alizarin S red. Addition of Ca to isolated lumenal silk resulted in Ca complexation by H-fibroin phosphoserines (pSs) and a shift in secondary structure from random coils to β-structures, creating infrared spectra in the phosphate and amide I regions nearly equivalent to those found in naturally spun Ca-containing silk fibers. Light and electron microscopy within distinct regions of the silk gland suggested that posterior gland silk colloidal complexes transition into a nanofibrous morphology as they pass into the chitin-lined anterior lumen. Altogether, the results suggest that environmental Ca absorbed from natural water triggers silk fiber solidification postdraw by complexing H-fibroin pSs, creating Ca-stabilized crystalline β-nanodomains that cross-link and toughen the freshly drawn silk fibers.-Ashton, N. N., Stewart, R. J. Aquatic caddisworm silk is solidified by environmental metal ions during the natural fiber-spinning process.
水生石蛾幼虫（石蛾幼虫）湿纺纤维，构建丝和石的复合材料。丝从唇管中以纳米纤维状流体凝胶的形式出现，在流到石质基底上并在被拉入快速在水下凝固以弥合构建中的间隙的坚韧纤维之前，与基底形成紧密的界面粘合接触。二价金属离子是天然纺丝纤维独特机械性能的原因；然而，二价金属离子何时以及在何处掺入金属纤维以及纤维凝固机制的其他方面知之甚少。为了研究这一点，通过电感耦合等离子体发射光谱和衰减全反射傅里叶变换红外光谱比较了储存在丝腺管腔中的丝前体的元素组成和二级结构与天然纺丝纤维。天然纺丝纤维含有 Ca 和 P 的近等摩尔比。相比之下，储存在丝腺管腔中的丝前体仅含有痕量的 Ca 和其他多价金属离子。在前管腔丝中使用组织化学 Ca 指示剂茜素红 S 也检测不到 Ca。将 Ca 添加到分离的管腔丝中导致 H-丝氨酸磷酸丝氨酸（pSs）与 Ca 络合，并使二级结构从无规卷曲转变为β结构，在磷酸盐和酰胺 I 区域产生的红外光谱与天然纺丝 Ca 纤维几乎等效。丝腺不同区域的光镜和电子显微镜观察表明，后丝腺丝胶状复合物在进入富含几丁质的前管腔时转变为纳米纤维形态。总的来说，结果表明，从天然水中吸收的环境 Ca 通过与 H-丝氨酸 pSs 络合触发丝纤维凝固后拉伸，形成 Ca 稳定的结晶β-纳米区，交联并增强新拉伸的丝纤维。-Ashton，NN，Stewart，RJ 水生石蛾丝在自然纤维纺丝过程中通过环境金属离子固化。
