Gobeaux Frédéric, Porcher Florence, Dattani Rajeev
LIONS-NIMBE CEA, CNRS, Université Paris-Saclay, CEA Saclay , 91191 Gif-sur-Yvette, France.
Laboratoire Léon Brillouin, CEA Saclay , 91191 Gif-sur-Yvette, France.
J Phys Chem B. 2017 Apr 13;121(14):3059-3069. doi: 10.1021/acs.jpcb.7b02448. Epub 2017 Apr 3.
Most amyloid assemblies are seen as irreversible and exhibit polymorphism because their assembly is kinetically controlled and different structures are trapped during the aggregation process. However, in the specific case of peptide hormones, formation of amyloid assemblies for storage purposes has been reported. This suggests a strict control of assembly and the ability to disassemble upon hormone secretion. In the present work, we have sought to test these assertions with a short peptide, the cholecystokinin (or gastrin) tetrapeptide (CCK-4), that has been found in both gastrointestinal tract and central nervous system, and whose sequence is shared by a large number of hormones. We have thus studied in vitro this peptide's self-assembling properties in dense phases at different pH levels, thus mimicking in vivo storage conditions. The solubility and morphology of the supramolecular assemblies have been shown to vary with the pH. At low pH, the tetrapeptide exhibits a low solubility and forms microcrystals. At higher pH levels, peptide solubility increases and above a high enough concentration, peptide monomers self-assemble into typical amyloid fibrils of 10-20 nm diameter. The physical network formed by these fibrils results in a birefringent hydrogel phase. Despite the different morphological features exhibited at different pH, structural analysis shows strong similarities. Both supramolecular assemblies-microcrystals and fibrils-are structured by β-sheets. We also show that all these morphologies are reversible and can be either dissolved or changed into one another by switching the pH. In addition, we demonstrate that a modification in the charge sequence of the peptide by amino acid mutation modifies its self-assembly properties. In conclusion, just as the CCK-4 sequence is the minimal sequence required for a complete biological activity at CCK receptors in the brain, it is also sufficient to form amyloid fibers whose properties can be related to hormone storage and release purposes in vivo.
大多数淀粉样蛋白聚集体被认为是不可逆的，并且表现出多态性，因为它们的组装受动力学控制，在聚集过程中会捕获不同的结构。然而，在肽类激素的特定情况下，已有报道称会形成用于储存目的的淀粉样蛋白聚集体。这表明对组装有严格的控制，并且在激素分泌时具有分解的能力。在本研究中，我们试图用一种短肽——胆囊收缩素（或胃泌素）四肽（CCK - 4）来验证这些说法，该四肽在胃肠道和中枢神经系统中均有发现，并且许多激素都共享其序列。因此，我们在体外研究了该肽在不同pH值下在浓相中自组装的特性，从而模拟体内储存条件。已表明超分子聚集体的溶解度和形态会随pH值变化。在低pH值下，四肽表现出低溶解度并形成微晶。在较高pH值时，肽的溶解度增加，并且在足够高的浓度以上，肽单体自组装成直径为10 - 20 nm的典型淀粉样纤维。由这些纤维形成的物理网络导致形成双折射水凝胶相。尽管在不同pH值下表现出不同的形态特征，但结构分析显示出很强的相似性。两种超分子聚集体——微晶和纤维——都是由β折叠片构成。我们还表明，所有这些形态都是可逆的，通过改变pH值可以使其溶解或相互转变。此外，我们证明通过氨基酸突变改变肽的电荷序列会改变其自组装特性。总之，正如CCK - 4序列是大脑中CCK受体完整生物活性所需的最小序列一样，它也足以形成淀粉样纤维，其特性可能与体内激素的储存和释放目的相关。
