Homology models for domains 21-23 of human tropoelastin shed light on lysine crosslinking

MFAP4-Mediated Effects in Elastic Fiber Homeostasis, Integrin Signaling and Cancer, and Its Role in Teleost Fish

Microfibrillar-associated protein 4 (MFAP4) is an extracellular matrix (ECM) protein belonging to the fibrinogen-related domain superfamily. MFAP4 is highly expressed in elastin-rich tissues such as lung, blood vessels and skin. MFAP4 is involved in organization of the ECM, regulating proper elastic fiber assembly. On the other hand, during pathology MFAP4 actively contributes to disease development and progression due to its interactions with RGD-dependent integrin receptors. Both tissue expression and circulating MFAP4 levels are associated with various disorders, including liver fibrosis and cancer. In other experimental models, such as teleost fish, MFAP4 appears to participate in host defense as a macrophage-specific innate immune molecule. The aim of this review is to summarize the accumulating evidence that indicates the importance of MFAP4 in homeostasis as well as pathological conditions, discuss its known biological functions with special focus on elastic fiber assembly, integrin signaling and cancer, as well as describe the reported functions of non-mammalian MFAP4 in fish. Overall, our work provides a comprehensive overview on the role of MFAP4 in health and disease.

Tropoelastin and Elastin Assembly

Elastic fibers are an important component of the extracellular matrix, providing stretch, resilience, and cell interactivity to a broad range of elastic tissues. Elastin makes up the majority of elastic fibers and is formed by the hierarchical assembly of its monomer, tropoelastin. Our understanding of key aspects of the assembly process have been unclear due to the intrinsic properties of elastin and tropoelastin that render them difficult to study. This review focuses on recent developments that have shaped our current knowledge of elastin assembly through understanding the relationship between tropoelastin’s structure and function.

Allysine modifications perturb tropoelastin structure and mobility on a local and global scale

Elastin provides elastic tissues with resilience through stretch and recoil cycles, and is primarily made of its extensively cross-linked monomer, tropoelastin. Here, we leverage the recently published full atomistic model of tropoelastin to assess how allysine modifications, which are essential to cross-linking, contribute to the dynamics and structural changes that occur in tropoelastin in the context of elastin assembly. We used replica exchange molecular dynamics to generate structural ensembles of allysine containing tropoelastin. We conducted principal component analysis on these ensembles and found that the molecule departs from the canonical structural ensemble. Furthermore, we showed that, while the canonical scissors-twist movement was retained, new movements emerged that deviated from those of the wild type protein, providing evidence for the involvement of a variety of molecular motions in elastin assembly. Additionally, we highlighted secondary structural changes and linked these perturbations to the longevity of specific salt bridges. We propose a model where allysines in tropoelastin contribute to hierarchical elastin assembly through global and local perturbations to molecular structure and dynamics.

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converting lysine to allysine by lysyl oxidases is needed to generate crosslinks between tropoelastin molecules in order to make elastin

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structural changes in the intact tropoelastin molecule ensue where modified tropoelastin molecules structurally depart from the canonical ensemble

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new molecular motions deviate from those of unmodified tropoelastin

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persistence times of specific salt bridges contribute to these perturbations

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allysines in tropoelastin contribute to hierarchical elastin assembly through global and local perturbations to molecular structure and dynamics

Targeted Modulation of Tropoelastin Structure and Assembly

Tropoelastin, as the monomer unit of elastin, assembles into elastic fibers that impart strength and resilience to elastic tissues. Tropoelastin is also widely used to manufacture versatile materials with specific mechanical and biological properties. The assembly of tropoelastin into elastic fibers or biomaterials is crucially influenced by key submolecular regions and specific residues within these domains. In this work, we identify the functional contributions of two rarely occurring negatively charged residues, glutamate 345 in domain 19 and glutamate 414 in domain 21, in jointly maintaining the native conformation of the tropoelastin hinge, bridge and foot regions. Alanine substitution of E345 and/or E414 variably alters the positioning and interactive accessibility of these regions, as illustrated by nanostructural studies and detected by antibody and cell probes. These structural changes are associated with a lower propensity for monomer coacervation, cross-linking into morphologically and functionally atypical hydrogels, and markedly impaired and abnormal elastic fiber formation. Our work indicates the crucial significance of both E345 and E414 residues in modulating specific local structure and higher-order assembly of human tropoelastin.

Elastin-based biomaterials and mesenchymal stem cells

Elastin is the dominant mammalian elastic protein found in soft tissue. Elastin-based biomaterials have the potential to repair elastic tissues by improving local elasticity and providing appropriate cellular interactions and signaling. Studies that combine these biomaterials with mesenchymal stem cells have demonstrated their capacity to also regenerate non-elastic tissue. Mesenchymal stem cell differentiation can be controlled by their immediate environment, and their sensitivity to elasticity makes them an ideal candidate for combining with elastin-based biomaterials. With the growing accessibility of the elastin precursor, tropoelastin, and elastin-derived materials, the amount of research interest in combining these two fields has increased and, subsequently, is leading to the realization of a potentially new strategy for regenerative medicine.

Tropoelastin: a versatile, bioactive assembly module

Elastin provides structural integrity, biological cues and persistent elasticity to a range of important tissues, including the vasculature and lungs. Its critical importance to normal physiology makes it a desirable component of biomaterials that seek to repair or replace these tissues. The recent availability of large quantities of the highly purified elastin monomer, tropoelastin, has allowed for a thorough characterization of the mechanical and biological mechanisms underpinning the benefits of mature elastin. While tropoelastin is a flexible molecule, a combination of optical and structural analyses has defined key regions of the molecule that directly contribute to the elastomeric properties and control the cell interactions of the protein. Insights into the structure and behavior of tropoelastin have translated into increasingly sophisticated elastin-like biomaterials, evolving from classically manufactured hydrogels and fibers to new forms, stabilized in the absence of incorporated cross-linkers. Tropoelastin is also compatible with synthetic and natural co-polymers, expanding the applications of its potential use beyond traditional elastin-rich tissues and facilitating finer control of biomaterial properties and the design of next-generation tailored bioactive materials.

Evaluating the conformation and binding interface of cap-binding proteins and complexes via ultraviolet photodissociation mass spectrometry

We report the structural analysis of cap-binding proteins using a chemical probe/ultraviolet photodissociation (UVPD) mass spectrometry strategy for evaluating solvent accessibility of proteins. Our methodology utilized a chromogenic probe (NN) to probe the exposed amine residues of wheat eukaryotic translation initiation factor 4E (eIF4E), eIF4E in complex with a fragment of eIF4G ("mini-eIF4F"), eIF4E in complex with full length eIF4G, and the plant specific cap-binding protein, eIFiso4E. Structural changes of eIF4E in the absence and presence of excess dithiothreitol and in complex with a fragment of eIF4G or full-length eIF4G are mapped. The results indicate that there are particular lysine residues whose environment changes in the presence of dithiothreitol or eIF4G, suggesting that changes in the structure of eIF4E are occurring. On the basis of the crystal structure of wheat eIF4E and a constructed homology model of the structure for eIFiso4E, the reactivities of lysines in each protein are rationalized. Our results suggest that chemical probe/UVPD mass spectrometry can successfully predict dynamic structural changes in solution that are consistent with known crystal structures. Our findings reveal that the binding of m(7)GTP to eIF4E and eIFiso4E appears to be dependent on the redox state of a pair of cysteines near the m(7)GTP binding site. In addition, tertiary structural changes of eIF4E initiated by the formation of a complex containing a fragment of eIF4G and eIF4E were observed.

Elastic fibres in health and disease

Elastic fibres are insoluble components of the extracellular matrix of dynamic connective tissues such as skin, arteries, lungs and ligaments. They are laid down during development, and comprise a cross-linked elastin core within a template of fibrillin-based microfibrils. Their function is to endow tissues with the property of elastic recoil, and they also regulate the bioavailability of transforming growth factor β. Severe heritable elastic fibre diseases are caused by mutations in elastic fibre components; for example, mutations in elastin cause supravalvular aortic stenosis and autosomal dominant cutis laxa, mutations in fibrillin-1 cause Marfan syndrome and Weill–Marchesani syndrome, and mutations in fibulins-4 and -5 cause autosomal recessive cutis laxa. Acquired elastic fibre defects include dermal elastosis, whereas inflammatory damage to fibres contributes to pathologies such as pulmonary emphysema and vascular disease. This review outlines the latest understanding of the composition and assembly of elastic fibres, and describes elastic fibre diseases and current therapeutic approaches.

Engineered zinc-finger proteins can compensate genetic haploinsufficiency by transcriptional activation of the wild-type allele: application to Willams-Beuren syndrome and supravalvular aortic stenosis

Williams-Beuren syndrome (WBS) and supravalvular aortic stenosis (SVAS) are genetic syndromes marked by the propensity to develop severe vascular stenoses. Vascular lesions in both syndromes are caused by haploinsufficiency of the elastin gene. We used these distinct genetic syndromes as models to evaluate the feasibility of using engineered zinc-finger protein transcription factors (ZFPs) to achieve compensatory expression of haploinsufficient genes by inducing augmented expression from the remaining wild-type allele. For complex genes with multiple splice variants, this approach could have distinct advantages over cDNA-based gene replacement strategies. Targeting the elastin gene, we show that transcriptional activation by engineered ZFPs can induce compensatory expression from the wild-type allele in the setting of classic WBS and SVAS genetic mutations, increase elastin expression in wild-type cells, induce expression of the major elastin splice variants, and recapitulate their natural stoichiometry. Further, we establish that transcriptional activation of the mutant allele in SVAS does not overcome nonsense-mediated decay, and thus ZFP-mediated transcriptional activation is not likely to induce production of a mutant protein, a crucial consideration. Finally, we show in bioengineered blood vessels that ZFP-mediated induction of elastin expression is capable of stimulating functional elastogenesis. Haploinsufficiency is a common mechanism of genetic disease. These findings have significant implications for WBS and SVAS, and establish that haploinsufficiency can be overcome by targeted transcriptional activation without inducing protein expression from the mutant allele.