Secretion of functional α1-antitrypsin is cell type dependent: Implications for intramuscular delivery for gene therapy

From Rat Tails to Glycoproteostasis: Motivated by Biology, Enabled by Biophysics, and Lucky

In this article I tell the story of my career path and how I have come to focus my research on protein folding in the cell. My early fascination with protein folding began during my undergraduate research. My graduate work exploited reductionist approaches to explore structural features in proteins by using cyclic peptide models ofβ-turns. My career trajectory from these early days to present, described in the first section of this article, illustrates the importance of pursuing the scientific questions that one finds most exciting and seizing professional opportunities that enable these questions to be tackled productively. In addition, this trajectory shows how serendipity can shape a career path. The second section describes the extraordinary scientific discoveries I have witnessed in protein folding during my career. Here I explain how I was drawn into the world of protein folding in thecell. This turning point allowed me to participate in the explosion of research on molecular chaperones in the early 90's and to help elucidate the nature of chaperone-substrate recognition, a problem I continue to focus on. Examples of our research contributions are presented in the third section, with a perspective on major challenges for the future offered in the last section. Throughout my career I have engaged in many collaborations;each has opened new scientific doors. Importantly, I seek to instill in my trainees the same excitement about research that I feel and to foster their growth as scientists and their discovery of their own passions and talents.

N-glycan-dependent protein maturation and quality control in the ER

The vast majority of proteins that traverse the mammalian secretory pathway become N-glycosylated in the endoplasmic reticulum (ER). The bulky glycan protein modifications, which are conserved in fungi and humans, act as maturation and quality-control tags. In this Review, we discuss findings published in the past decade that have rapidly expanded our understanding of the transfer and processing of N-glycans, as well as their role in protein maturation, quality control and trafficking in the ER, facilitated by structural insights into the addition of N-glycans by the oligosaccharyltransferases A and B (OST-A and OST-B). These findings suggest that N-glycans serve as reporters of the folding status of secretory proteins as they traverse the ER, enabling the lectin chaperones to guide their maturation. We also explore how the emergence of co-translational glycosylation and the expansion of the glycoproteostasis network in metazoans has expanded the role of N-glycans in early protein-maturation events and quality control.

Monitoring the Secretion and Activity of Alpha-1 Antitrypsin in Various Mammalian Cell Types

Overexpression of recombinant protein in mammalian cells is widely used for producing biologics, as protein maturation and post-translational modifications are similar to human cells. Some therapeutics, such as mRNA vaccines, target nonnative cells that may contain inefficient secretory machinery. For example, gene replacement therapies for alpha-1 antitrypsin (AAT), a glycoprotein normally produced in hepatocytes, are often targeted to muscle cells due to ease of delivery. In this chapter, we define methods for expressing AAT in representative cell types such as Huh-7; hepatocytes; Chinese hamster ovarian cells (CHO), a common host to produce biologics; and C2C12, a muscle progenitor cell line. Methods for metabolically labeling AAT to monitor secretion in these cell lines are described along with the use of proteostasis activators to increase the amount of AAT secreted in both C2C12 myoblasts and differentiated myotubes. Assays to assess the activity and glycan composition of overexpressed AAT are also presented. The usage of the proteostasis activator SAHA provided a 40% improvement in expression of active AAT in muscle-like cells and may be an advantageous adjuvant for recombinant production of proteins delivered by mRNA vaccines.

ER chaperones use a protein folding and quality control glyco-code

N-glycans act as quality control tags by recruiting lectin chaperones to assist protein maturation in the endoplasmic reticulum. The location and composition of N-glycans (glyco-code) are key to the chaperone-selection process. Serpins, a class of serine protease inhibitors, fold non-sequentially to achieve metastable active states. Here, the role of the glyco-code in assuring successful maturation and quality control of two human serpins, alpha-1 antitrypsin (AAT) and antithrombin III (ATIII), is described. We find that AAT, which has glycans near its N terminus, is assisted by early lectin chaperone binding. In contrast, ATIII, which has more C-terminal glycans, is initially helped by BiP and then later by lectin chaperones mediated by UGGT reglucosylation. UGGT action is increased for misfolding-prone disease variants, and these clients are preferentially glucosylated on their most C-terminal glycan. Our study illustrates how serpins utilize N-glycan presence, position, and composition to direct their proper folding, quality control, and trafficking.