Genetically Encoded Tools to Monitor and Interrogate Membrane Contact Sites

Chapter 10 - Advances in protein engineering and its application in synthetic biology

Synergistic advances in fluorescent protein engineering and live-cell imaging techniques in recent years have fueled the concurrent development and application of genetically encoded fluorescent reporters that are tailored for tracking signaling dynamics in living systems over multiple length and time scales. These biosensors are uniquely suited for this challenging task, owing to their specificity, sensitivity, and versatility, as well as to the noninvasive and nondestructive nature of fluorescence and the power of genetic encoding. Over the past 10 years, a growing number of fluorescent reporters have been developed for tracking a wide range of biological signals in living cells and animals, including second messenger and metabolite dynamics, enzyme activation and activity, and cell cycle progression and neuronal activity. Many of these biosensors are gaining wide use and are proving to be indispensable for unraveling the complex biological functions of individual signaling molecules in their native environment, the living cell, shedding new light on the structural and molecular underpinnings of cell signaling. In this review, we highlight recent advances in protein engineering that are likely to help expand and improve the design and application of these valuable tools. We then turn our focus to specific examples of live-cell imaging using genetically encoded fluorescent reporters as an important platform for advancing our understanding of G protein-coupled receptor signaling and neuronal activity.

ABSTRACTInterorganellar cross talk is often mediated by membrane contact sites (MCSs), which are zones where participating membranes come within 30 nm of one another. MCSs have been found in organelles, including the endoplasmic reticulum, Golgi bodies, endosomes, and mitochondria. Despite its seeming ubiquity, reports of MCS involving mitochondrion-related organelles (MROs) present in a few anaerobic parasitic protozoa remain lacking. Entamoeba histolytica, the etiological agent of amoebiasis, possesses an MRO called the mitosome. We previously discovered several Entamoeba-specific transmembrane mitosomal proteins (ETMPs) from in silico and cell-biological analyses. One of them, ETMP1 (EHI_175060), was predicted to have one transmembrane domain and two coiled-coil regions and was demonstrated to be mitosome membrane integrated based on carbonate fractionation and immunoelectron microscopy (IEM) data. Immunoprecipitation analysis detected a candidate interacting partner, EH domain-containing protein (EHD1; EHI_105270). We expressed hemagglutinin (HA)-tagged EHD1 in E. histolytica, and subsequent immunofluorescence and IEM data indicated an unprecedented MCS between the mitosome and the endosome. Live imaging of a green fluorescent protein (GFP)-EHD1-expressing strain demonstrated that EHD1 is involved in early endosome formation and is observed in MCS between endosomes of various sizes. In vitro assays using recombinant His-EHD1 demonstrated ATPase activity. MCSs are involved in lipid transfer, ion homeostasis, and organelle dynamics. The serendipitous discovery of the ETMP1-interacting partner EHD1 led to the observation of the mitosome-endosome contact site in E. histolytica. It opened a new view of how the relic mitochondria of Entamoeba may likewise be involved in organelle cross talk, a conserved feature of mitochondria and other organelles in general.

Organelle crosstalk in the kidney

Bispecific antibodies (BsAbs) are designed to recognize and bind to two different antigens or epitopes. In the last few decades, BsAbs have been developed within the context of cancer therapies and in particular for the treatment of hematologic B-cell malignancies. To date, more than one hundred different BsAb formats exist, including bispecific T-cell engagers (BiTEs), and new constructs are constantly emerging. Advances in protein engineering have enabled the creation of BsAbs with specific mechanisms of action and clinical applications. Moreover, a better understanding of resistance and evasion mechanisms, as well as advances in the protein engineering and in immunology, will help generating a greater variety of BsAbs to treat various cancer types. This review focuses on T-cell-engaging BsAbs and more precisely on the various BsAb formats currently being studied in the context of B-cell malignancies, on ongoing clinical trials and on the clinical concerns to be taken into account in the development of new BsAbs.

Polyarginine (poly-Arg) and arginine-rich peptides have been attracting enormous interest in chemical and cell biology as cell-penetrating peptides capable of direct intracellular penetration. Owing to advances in protein engineering, arginine-rich fragments are often incorporated into multifunctional bioorganic/inorganic core–shell nanoparticles, enabling them the novel unique ability to cross cells and deliver biopharmaceutical cargos. Therefore, understanding the molecular details of the adsorption, packing, and release of poly-Arg onto or from metal nanoparticles is one of the current challenges. In this work, we carry out atomistic molecular dynamics simulations to identify the most favorable location, orientation, and conformation of poly-Arg adsorbed onto a silver nanoparticle (AgNP). Herein, we utilize the constant protonation approach to identify the role of protonation of side chain arginine moieties in the adsorption of poly-Arg to AgNP as a function of pH. The strong adsorption of unprotonated poly-Arg30 onto the quasispherical surface of AgNP with an average diameter of 3.9 nm is primarily governed by multiple interactions of side chain guanidinium (Gdm) moieties, which get stacked and align flat onto the surface. The protonation of the arginine side chain enhances the protein–solvent interactions and promotes the weakening of the protein–nanoparticle binding. The formation of multiple H-bonds between the protonated Arg residues and water molecules favors exposing the charged Gdm+ moieties to the solvent. Protonated poly-Arg30 is found to be partially bound to AgNP due to some weak protein–nanoparticle contacts, maintained by binding of the amide oxygen atoms of the peptide bond. These results suggest that reversible acid–base switching between the arginine protonation states is able to drive the rearrangement of the polyarginine coating around AgNPs, which could be important for a rational design of “intelligent” multifunctional core–shell nanosystems.

Cell biology has long recognized that organelles can communicate with each other. Initially, such communication was thought to occur primarily via vesicular trafficking between biochemically distinct organelles. However, studies starting in the 1970s on lipid metabolism have unearthed another way how organelles can communicate and have spawned the field of membrane contact sites (MCS). While, initially, MCS had been recognized as fluid entities that mediate lipid and ion transport in an ad hoc manner, more recently MCS have been found to depend on protein-protein interactions that control themselves a variety of MCS functions. As a result, the cell biological definition of an intracellular organelle as an isolated membrane compartment is now being revised. Accordingly, the organelle definition now describes organelles as dynamic membrane compartments that function in a milieu of coordinated contacts with other organelles. Through these mercurial functions, MCS dictate the function of organelles to a large extent but also play important roles in a number of diseases, including type 2 diabetes, neurodegenerative diseases, infections, and cancer. This book assembles reviews that describe our quickly evolving knowledge about organellar communication on MCS and the significance of MCS for disease.

Understanding brain function requires experimental approaches that decipher the information coded by individual neurons. Calcium imaging with genetically encoded calcium indicators (GECIs) is a promising method that can visualize the spatiotemporal activity patterns of brain cells. Recent advances in protein engineering have greatly improved the properties of fluorescent GECIs, and they now have high flexibility for imaging defined cell populations over the course of months. The action spectra of single-wavelength GECIs have been extended by the development of color-shifted fluorescence probes, which increase the potential for multicolor imaging of different cell structures and, more importantly, can be integrated into optogenetics experiments with photoactivatable proteins. In particular, red-shifted GECIs are highly important for imaging deeper tissues because longer-wavelength light can reduce tissue scattering and background autofluorescence. This chapter mainly describes recent advances in the engineering of red GECIs, and highlights important neuroscience applications in optical monitoring and the manipulation of neuronal activity.

Amino acid transaminases (ATs) have garnered considerable attention in recent years as promising biocatalysts for the synthesis of high‐value chiral chemicals, including both natural and non‐canonical amino acids. These enzymes catalyze the transfer of amino groups from amino acids to keto acids, playing a pivotal role in various biological processes and industrial applications. Characterized by their high turnover rates, remarkable enantioselectivity, and broad substrate specificity, ATs exhibit exceptional versatility and potential. This review presents a comprehensive overview of the classification, reaction mechanisms, and activity assays of ATs. More crucially, we delve into the recent advancements in protein engineering of ATs through directed evolution and rational/semi‐rational design strategies, which have been instrumental in addressing limitations such as low catalytic efficiency and stability. Furthermore, we survey the recent synthetic applications of ATs in the production of aliphatic and aromatic amino acids, highlighting smart amino donors and coupling methods that effectively shift the equilibrium of transamination reactions, as well as enzyme cascades that further expand the scope of reactions. By bridging gaps in research on ATs, this review aims to provide valuable insights and guidance for future developments in the field of biocatalysis, ultimately fostering their continued utilization and advancement.

Recent advances in protein engineering and synthetic applications of amino acid oxidases.

Yeast surface display (YSD) is a powerful tool in biotechnology that links genotype to phenotype. In this review, the latest advancements in protein engineering and high-throughput screening based on YSD are covered. The focus is on innovative methods for overcoming challenges in YSD in the context of biotherapeutic drug discovery and diagnostics. Topics ranging from titrating avidity in YSD using transcriptional control to the development of serological diagnostic assays relying on serum biopanning and mitigation of unspecific binding are covered. Screening techniques against nontraditional cellular antigens, such as cell lysates, membrane proteins, and extracellular matrices are summarized and techniques are further delved into for expansion of the chemical repertoire, considering protein–small molecule hybrids and noncanonical amino acid incorporation. Additionally, in vivo gene diversification and continuous evolution in yeast is discussed. Collectively, these techniques enhance the diversity and functionality of engineered proteins isolated via YSD, broadening the scope of applications that can be addressed. The review concludes with future perspectives and potential impact of these advancements on protein engineering. The goal is to provide a focused summary of recent progress in the field.

Protein engineering has emerged as a powerful approach toward the development of novel therapeutics targeting the MYC/MAX/E-box network, an active driver of >70% of cancers. The MYC/MAX heterodimer regulates numerous genes in our cells by binding the Enhancer box (E-box) DNA site and activating the transcription of downstream genes. Traditional small molecules that inhibit MYC face significant limitations that include toxic effects, drug delivery challenges, and resistance. Recent advances in protein engineering offer promising alternatives by creating protein-based drugs that directly disrupt the MYC/MAX dimerization interface and/or MYC/MAX’s binding to specific DNA targets. Designed DNA binding proteins like Omomyc, DuoMyc, ME47, MEF, and Mad inhibit MYC activity through specific dimerization, sequestration, and DNA-binding mechanisms. Compared to small molecules, these engineered proteins can offer superior specificity and efficacy and provide a potential pathway for overcoming the limitations of traditional cancer therapies. The success of these protein therapeutics highlights the importance of protein engineering in developing cancer treatments.

Advances in protein engineering.

Precise control of gene expression is one of the fundamental goals of synthetic biology. Whether the objective is to modify endogenous cellular function or induce the expression of molecules for diagnostic and therapeutic purposes, gene regulation remains a key aspect of biological systems. Over time, advances in protein engineering and molecular biology have led to the creation of gene circuits capable of inducing the expression of specific proteins in response to external stimulus such as light. These optogenetic, or light-activated circuits hold significant potential for gene therapy as a tool for regulating the expression of therapeutic genes within cells. However, the applications of optogenetic systems can be limited by the lack of efficient ways for light delivery inside cells or tissue. Our approach to address this challenge is to harness the power of bioluminescence to produce light directly inside cells using a luminescent enzyme. Combined with a photosensitive transcription factor, we report the development of a fully genetically encoded optogenetic circuit for control of gene expression. Furthermore, we utilized a magneto sensitive protein to engineer a split protein version of this luminescent enzyme, where its reconstitution is driven by a 50mT magnetic stimulus. Thus, resulting in a first-of-its-kind gene circuit activated by a combination of light and magnetic stimulus. We expect this work to advance the implementation of light-controlled systems without the need of external light sources, as well as serve as a basis for the development of future magneto-sensitive tools.

Structure-driven protein engineering for production of valuable natural products.