Structure and function of sialic acids at the plasma membrane

人类细胞的膜上覆盖着碳水化合物结构（聚糖）的异质混合物，这些结构充当与外部环境的界面，并具有重要的结构作用。这种结构通常被称为“糖萼”，由连接到蛋白质（糖蛋白）和脂质（糖脂）的聚糖组成。由于糖萼的异质性，我们对发生在膜上的特定变化的理解发展缓慢。在人类中，大多数细胞外聚糖以唾液酸（Sia）终止-使该残基成为与病原体，细胞和分泌蛋白相互作用的重要识别元件。* 本提案描述了一项研究计划，以促进我们对在质膜上加工含Sia聚糖的酶的理解，以及特定含Sia结构在细胞粘附和信号传导中的作用。在项目I中，我们将通过研究改变糖脂特异性神经氨酸酶NEU 3特异性的激活蛋白来建立我们在人类神经氨酸酶（hNEU）方面的专业知识。在人类细胞中发现了Sia的不寻常形式，这些包括在特定羟基上的乙酰化（OAc-Sia）和聚合唾液酸（poly-Sia）。在项目II中，我们将开发化学酶策略以更有效地合成OAc-Sia并修饰纯化的复合聚糖。在项目III中，我们将研究人类神经氨酸酶在一类称为半乳糖凝集素的可溶性人类蛋白质相互作用中的作用。半乳糖凝集素通过特异性聚糖结构结合并交联细胞表面受体，该聚糖结构不被神经氨酸酶掩蔽。在项目IV中，我们建议使用我们计划中开发的工具来研究聚Sia在整合素介导的细胞粘附中的作用。具体来说，我们将使用已知修饰poly-Sia的人神经氨酸酶同工酶，以及我们以前开发的这种酶的抑制剂。这些研究将揭示整合素糖基化对细胞粘附的影响。* 总之，该计划将促进我们对Sia聚糖在细胞过程中的理解，并为糖缀合物的开发提供新的工具，这些糖缀合物可能是未来疫苗，生物制剂或治疗剂的重要组成部分。

The membrane of human cells is coated with a heterogenous mixture of carbohydrate structures (glycans) that act as an interface to the external environment and have important structural roles. This structure is often referred to as the 'glycocalyx,' and consists of glycans attached to proteins (glycoproteins) and lipids (glycolipids). Due to the heterogeneous nature of the glycocalyx, our understanding of specific changes that occur at the membrane has been slow to develop. In humans, most extracellular glycans are terminated with sialic acids (Sia) - making this residue an important recognition element for interactions with pathogens, cells, and secreted proteins. ***This proposal describes a research program to advance our understanding of enzymes that process Sia-containing glycans at the plasma membrane, as well as the role of specific Sia-containing structures in cell adhesion and signaling. In Project I, We will build on our expertise in human neuraminidase enzymes (hNEU) by studying activator proteins that alter the specificity of the glycolipid-specific neuraminidase, NEU3. Unusual forms of Sia are found in human cells, these include acetylation at specific hydroxyl groups (OAc-Sia) and polymeric sialic acids (poly-Sia). In Project II, we will develop chemoenzymatic strategies to more efficiently synthesize OAc-Sia and to modify purified complex glycans. In Project III, we will examine the role of human neuraminidase enzymes in the interactions of a class of soluble human proteins known as Galectins. Galectins bind and crosslink cell surface receptors through specific glycan structures that are unmasked by neuraminidase enzymes. In Project IV, we propose to investigate the role of poly-Sia in integrin-mediated cell adhesion using tools developed within our program. Specifically, we will use an isoenzyme of human neuraminidase known to modify poly-Sia, as well as inhibitors of this enzyme that we have previously developed. These studies will reveal the influence of integrin glycosylation on cellular adhesion. ***Together, this program will advance our understanding of Sia glycans in cellular processes and provide new tools for the development of glycoconjugates that could be important components of future vaccines, biologics, or therapeutics.**