The intracellular Ig fold: a robust protein scaffold for the engineering of molecular recognition

Structural diversity in the atomic resolution 3D fingerprint of the titin M-band segment

In striated muscles, molecular filaments are largely composed of long protein chains with extensive arrays of identically folded domains, referred to as “beads-on-a-string”. It remains a largely unresolved question how these domains have developed a unique molecular profile such that each carries out a distinct function without false-positive readout. This study focuses on the M-band segment of the sarcomeric protein titin, which comprises ten identically folded immunoglobulin domains. Comparative analysis of high-resolution structures of six of these domains ‒ M1, M3, M4, M5, M7, and M10 ‒ reveals considerable structural diversity within three distinct loops and a non-conserved pattern of exposed cysteines. Our data allow to structurally interpreting distinct pathological readouts that result from titinopathy-associated variants. Our findings support general principles that could be used to identify individual structural/functional profiles of hundreds of identically folded protein domains within the sarcomere and other densely crowded cellular environments.

The ZT Biopolymer: A Self-Assembling Protein Scaffold for Stem Cell Applications

The development of cell culture systems for the naturalistic propagation, self-renewal and differentiation of cells ex vivo is a high goal of molecular engineering. Despite significant success in recent years, the high cost of up-scaling cultures, the need for xeno-free culture conditions, and the degree of mimicry of the natural extracellular matrix attainable in vitro using designer substrates continue to pose obstacles to the translation of cell-based technologies. In this regard, the ZT biopolymer is a protein-based, stable, scalable, and economical cell substrate of high promise. ZT is based on the naturally occurring assembly of two human proteins: titin-Z1Z2 and telethonin. These protein building blocks are robust scaffolds that can be conveniently functionalized with full-length proteins and bioactive peptidic motifs by genetic manipulation, prior to self-assembly. The polymer is, thereby, fully encodable. Functionalized versions of the ZT polymer have been shown to successfully sustain the long-term culturing of human embryonic stem cells (hESCs), human induced pluripotent stem cells (hiPSCs), and murine mesenchymal stromal cells (mMSCs). Pluripotency of hESCs and hiPSCs was retained for the longest period assayed (4 months). Results point to the large potential of the ZT system for the creation of a modular, pluri-functional biomaterial for cell-based applications.

Self-Assembling Proteins as High-Performance Substrates for Embryonic Stem Cell Self-Renewal

The development of extracellular matrix mimetics that imitate niche stem cell microenvironments and support cell growth for technological applications is intensely pursued. Specifically, mimetics are sought that can enact control over the self-renewal and directed differentiation of human pluripotent stem cells (hPSCs) for clinical use. Despite considerable progress in the field, a major impediment to the clinical translation of hPSCs is the difficulty and high cost of large-scale cell production under xeno-free culture conditions using current matrices. Here, a bioactive, recombinant, protein-based polymer, termed ZT^Fn , is presented that closely mimics human plasma fibronectin and serves as an economical, xeno-free, biodegradable, and functionally adaptable cell substrate. The ZT^Fn substrate supports with high performance the propagation and long-term self-renewal of human embryonic stem cells while preserving their pluripotency. The ZT^Fn polymer can, therefore, be proposed as an efficient and affordable replacement for fibronectin in clinical grade cell culturing. Further, it can be postulated that the ZT polymer has significant engineering potential for further orthogonal functionalization in complex cell applications.

Exploration of pathomechanisms triggered by a single-nucleotide polymorphism in titin's I-band: the cardiomyopathy-linked mutation T2580I

Missense single-nucleotide polymorphisms (mSNPs) in titin are emerging as a main causative factor of heart failure. However, distinguishing between benign and disease-causing mSNPs is a substantial challenge. Here, we research the question of whether a single mSNP in a generic domain of titin can affect heart function as a whole and, if so, how. For this, we studied the mSNP T2850I, seemingly linked to arrhythmogenic right ventricular cardiomyopathy (ARVC). We used structural biology, computational simulations and transgenic muscle in vivo methods to track the effect of the mutation from the molecular to the organismal level. The data show that the T2850I exchange is compatible with the domain three-dimensional fold, but that it strongly destabilizes it. Further, it induces a change in the conformational dynamics of the titin chain that alters its reactivity, causing the formation of aberrant interactions in the sarcomere. Echocardiography of knock-in mice indicated a mild diastolic dysfunction arising from increased myocardial stiffness. In conclusion, our data provide evidence that single mSNPs in titin's I-band can alter overall muscle behaviour. Our suggested mechanisms of disease are the development of non-native sarcomeric interactions and titin instability leading to a reduced I-band compliance. However, understanding the T2850I-induced ARVC pathology mechanistically remains a complex problem and will require a deeper understanding of the sarcomeric context of the titin region affected.

Structural advances on titin: towards an atomic understanding of multi-domain functions in myofilament mechanics and scaffolding

Titin is a gigantic filamentous protein of the muscle sarcomere that plays roles in myofibril mechanics and homoeostasis. 3D-structures of multi-domain fragments of titin are now available that start revealing the molecular mechanisms governing its mechanical and scaffolding functions. This knowledge is now being translated into the fabrication of self-assembling biopolymers. Here we review the structural advances on titin, the novel concepts derived from these and the emerging translational avenues.

Discovery of novel candidate therapeutics and diagnostics based on engineered human antibody domains

The smallest independently folded antibody fragments, the domains, are emerging as promising scaffolds for candidate therapeutics and diagnostics that bind specifically targets of interest. The discovery of such binders is based on several technologies including structure-based design and generation of libraries of mutants displayed on phage or yeast, next-generation sequencing for diversity analysis, panning and screening of the libraries, affinity maturation of selected binders, and their expression, purification, and characterization for specific binding, function, and aggregation propensity. In this review, we describe these technologies as applied for the generation of engineered antibody domains (eAds), especially those derived from the human immunoglobulin heavy chain variable region (VH) and the second domain of IgG1 heavy chain constant region (CH2) as potential candidate therapeutics and diagnostics, and discuss examples of eAds against HIV-1 and cancer-related proteins.