Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Am Chem Soc. 2011 Jun 22;133(24):9545-55. doi: 10.1021/ja202406v. Epub 2011 May 26.
Asymmetric cell division is common in biology and plays critical roles in differentiation and development. Unicellular organisms are often used as model systems for understanding the origins and consequences of asymmetry during cell division. Although basic as compared to mammalian cells, these are already quite complex. We report complete budding and asymmetric fission of very simple nonliving model cells to produce daughter vesicles that are chemically distinct in both interior and membrane compositions. Our model cells are based on giant lipid vesicles (GVs, 10-30 μm) encapsulating a polyethylene glycol (PEG)/dextran aqueous two-phase system (ATPS) as a crowded and compartmentalized cytoplasm mimic. Ternary lipid compositions were used to provide coexisting micrometer-scale liquid disordered (L(d)) and liquid ordered (L(o)) domains in the membranes. ATPS-containing vesicles formed buds when sucrose was added externally to provide increased osmotic pressure, such that they became not only morphologically asymmetric but also asymmetric in both their interior and their membrane compositions. Further increases in osmolality drove formation of two chemically distinct daughter vesicles, which were in some cases connected by a lipid nanotube (complete budding), and in others were not (fission). In all cases, separation occurred at the aqueous-aqueous phase boundary, such that one daughter vesicle contained the PEG-rich aqueous phase and the other contained the dextran-rich aqueous phase. PEGylated lipids localized in the L(o) domain resulted in this membrane domain preferentially coating the PEG-rich bud prior to division, and subsequently the PEG-rich daughter vesicle. Varying the mole ratio of lipids resulted in excess surface area of L(o) or L(d) membrane domains such that, upon division, this excess portion was inherited by one of the daughter vesicles. In some cases, a second "generation" of aqueous phase separation and budding could be induced in these daughter vesicles. Asymmetric fission of a simple self-assembled model cell, with production of daughter vesicles that harbored different protein concentrations and lipid compositions, is an example of the seemingly complex behavior possible for simple molecular assemblies. These compartmentalized and asymmetrically dividing ATPS-containing GVs could serve as a test bed for investigating possible roles for spatial and organizational cues in asymmetric cell division and inheritance.
不对称细胞分裂在生物学中很常见，对细胞分化和发育起着至关重要的作用。单细胞生物通常被用作模型系统，以了解细胞分裂过程中不对称的起源和后果。尽管与哺乳动物细胞相比，这些细胞非常简单，但它们已经相当复杂了。我们报告了非常简单的非生命模型细胞的完全出芽和不对称分裂，产生了在内部和膜组成方面都具有化学差异的子囊泡。我们的模型细胞基于包裹聚乙二醇（PEG）/葡聚糖水相双相系统（ATPS）的巨大脂质囊泡（GVs，10-30μm），作为拥挤和分隔的细胞质模拟物。使用三元脂质组成在膜中提供共存的微米级无序（L(d)）和有序（L(o)）域。当外部添加蔗糖以提供增加的渗透压时，含有 ATPS 的囊泡形成芽，使得它们不仅在形态上不对称，而且在内部和膜组成上也不对称。渗透压的进一步增加导致形成两个化学上不同的子囊泡，在某些情况下，它们通过脂质纳米管连接（完全出芽），在其他情况下则没有（分裂）。在所有情况下，分离都发生在水-水相界面处，使得一个子囊泡包含富含 PEG 的水相，另一个包含富含葡聚糖的水相。PEG 化脂质定位于 L(o) 域，导致该膜域优先在分裂前包被富含 PEG 的芽，随后是富含 PEG 的子囊泡。改变脂质的摩尔比导致 L(o)或 L(d)膜域的表面积过多，使得在分裂时，这部分多余的部分被一个子囊泡继承。在某些情况下，这些子囊泡可以诱导第二次“代”水相分离和出芽。具有产生含有不同蛋白质浓度和脂质组成的子囊泡的简单自组装模型细胞的不对称分裂，是简单分子组装可能具有的复杂行为的一个例子。这些含有 ATPS 的分隔和不对称分裂的 GVs 可以作为一个测试平台，用于研究空间和组织线索在不对称细胞分裂和遗传中的可能作用。
