Conformation and self-association of human recombinant transforming growth factor-beta3 in aqueous solutions

Convergent evolution of a parasite-encoded complement control protein-scaffold to mimic binding of mammalian TGF-β to its receptors, TβRI and TβRII

The mouse intestinal helminth Heligmosomoides polygyrus modulates host immune responses by secreting a transforming growth factor (TGF)-β mimic (TGM), to expand the population of Foxp3⁺ T_regs. TGM comprises five complement control protein (CCP)-like domains, designated D1-D5. Though lacking homology to TGF-β, TGM binds directly to the TGF-β receptors TβRI and TβRII and stimulates the differentiation of naïve T-cells into T_regs. However, the molecular determinants of binding are unclear. Here, we used surface plasmon resonance, isothermal calorimetry, NMR spectroscopy, and mutagenesis to investigate how TGM binds the TGF-β receptors. We demonstrate that binding is modular, with D1-D2 binding to TβRI and D3 binding to TβRII. D1-D2 and D3 were further shown to compete with TGF-β(TβRII)₂ and TGF-β for binding to TβRI and TβRII, respectively. The solution structure of TGM-D3 revealed that TGM adopts a CCP-like fold but is also modified to allow the C-terminal strand to diverge, leading to an expansion of the domain and opening potential interaction surfaces. TGM-D3 also incorporates a long structurally ordered hypervariable loop, adding further potential interaction sites. Through NMR shift perturbations and binding studies of TGM-D3 and TβRII variants, TGM-D3 was shown to occupy the same site of TβRII as bound by TGF-β using both a novel interaction surface and the hypervariable loop. These results, together with the identification of other secreted CCP-like proteins with immunomodulatory activity in H. polygyrus, suggest that TGM is part of a larger family of evolutionarily plastic parasite effector molecules that mediate novel interactions with their host.

Aspartic protease-pepstatin A interactions: Structural insights on the thermal inactivation mechanism

Aspartic proteases are the targets for structure-based drug design for their role in physiological processes and pharmaceutical applications. Structural insights into the thermal inactivation mechanism of an aspartic protease in presence and absence of bound pepstatin A have been obtained by kinetics of thermal inactivation, CD, fluorescence spectroscopy and molecular dynamic simulations. The irreversible thermal inactivation of the aspartic protease comprised of loss of tertiary and secondary structures succeeded by the loss of activity, autolysis and aggregation The enthalpy and entropy of thermal inactivation of the enzyme in presence of pepstatin A increased from 81.2 to 148.5 kcal mol^-1, and from 179 to 359 kcal mol^-1 K^-1 respectively. Pepstatin A shifted the mid-point of thermal inactivation of the protease from 58 °C to 77 °C. The association constant (K) for pepstatin A with aspartic protease was 2.5 ± 0.3 × 10 ⁵ M^-1 and ΔG^o value was -8.3 kcal mol^-1. Molecular dynamic simulation studies were able to delineate the role of pepstatin A in stabilizing backbone conformation and side chain interactions. In the Cα-backbone, the short helical segments and the conserved glycines were part of the most unstable segments of the protein. Understanding the mechanism of thermal inactivation has the potential to develop re-engineered thermostable proteases.

TGF-β3-loaded graphene oxide - self-assembling peptide hybrid hydrogels as functional 3D scaffolds for the regeneration of the nucleus pulposus

Intervertebral disc (IVD) degeneration is a process that starts in the central nucleus pulposus (NP) and leads to inflammation, extracellular matrix (ECM) degradation, and progressive loss of disc height. Early treatment of IVD degeneration is critical to the reduction of low back pain and related disability. As such, minimally invasive therapeutic approaches that can halt and reverse NP degeneration at the early stages of the disease are needed. Recently, we developed an injectable graphene oxide (GO) - self-assembling peptide FEFKFEFK (F: phenylalanine; K: lysine; E: glutamic acid) hybrid hydrogels as potential delivery platform for cells and/or drugs in the NP. In this current study, we explored the possibility of using the GO present in these hybrid hydrogels as a vehicle for the sequestration and controlled delivery of transforming growth factor beta-3 (TGF-β3), an anabolic growth factor (GF) known to direct NP cell fate and function. For this purpose, we first investigated the potential of GO to bind and sequestrate TGF-β3. We then cultured bovine NP cells in the new functional scaffolds and investigated their response to the presence of GO and TGF-β3. Our results clearly showed that GO flakes can sequestrate TGF-β3 through strong binding interactions resulting in a slow and prolonged release, with the GF remaining active even when bound to the GO flakes. The adsorption of the GF on the GO flakes to create TGF-β3-loaded GO flakes and their subsequent incorporation in the hydrogels through mixing, [(GO/TGF-β3_Ads)-F8] hydrogel, led to the upregulation of NP-specific genes, accompanied by the production and deposition of an NP-like ECM, rich in aggrecan and collagen II. NP cells actively interacted with TGF-β3-loaded GO flakes and remodeled the scaffolds through endocytosis. This work highlights the potential of using GO as a nanocarrier for the design of functional hybrid peptide-based hydrogels. STATEMENT OF SIGNIFICANCE: Intervertebral disc (IVD) degeneration is a process that starts in the central nucleus pulposus (NP) and leads to inflammation, extracellular matrix (ECM) degradation, and progressive loss of disc height. As such, minimally invasive therapeutic approaches that can halt and reverse NP degeneration at the early stages of the disease are needed. In this current study, we explored the possibility of using peptide - GO hybrid hydrogels as a vehicle for the sequestration and controlled delivery of transforming growth factor beta-3 (TGF-β3), an anabolic growth factor (GF) known to direct NP cell fate and function.

The folding propensity of α/sulfono-γ-AA peptidic foldamers with both left- and right-handedness

The discovery and application of new types of helical peptidic foldamers have been an attractive endeavor to enable the development of new materials, catalysts and biological molecules. To maximize their application potential through structure-based design, it is imperative to control their helical handedness based on their molecular scaffold. Herein we first demonstrate the generalizability of the solid-state right-handed helical propensity of the 4₁₃-helix of L-α/L-sulfono-γ-AA peptides that as short as 11-mer, using the high-resolution X-ray single crystallography. The atomic level folding conformation of the foldamers was also elucidated by 2D NMR and circular dichroism under various conditions. Subsequently, we show that the helical handedness of this class of foldamer is controlled by the chirality of their chiral side chains, as demonstrated by the left-handed 4₁₃-helix comprising 1:1 D-α/D-sulfono-γ-AA peptide. In addition, a heterochiral coiled-coil-like structure was also revealed for the first time, unambiguously supporting the impact of chirality on their helical handedness. Our findings enable the structure-based design of unique folding biopolymers and materials with the exclusive handedness or the racemic form of the foldamers in the future.

Mycobacterium tuberculosis Rv0991c Is a Redox-Regulated Molecular Chaperone

M. tuberculosis infections are responsible for more than 1 million deaths per year. Developing effective strategies to combat this disease requires a greater understanding of M. tuberculosis biology. As in all cells, protein quality control is essential for the viability of M. tuberculosis, which likely faces proteotoxic stress within a host. Here, we identify an M. tuberculosis protein, Ruc, that gains chaperone activity upon oxidation. Ruc represents a previously unrecognized family of redox-regulated chaperones found throughout the bacterial superkingdom. Additionally, we found that oxidized Ruc promotes the protein-folding activity of the essential M. tuberculosis Hsp70 chaperone system. This work contributes to a growing body of evidence that oxidative stress provides a particular strain on cellular protein stability.

The bacterial pathogen Mycobacterium tuberculosis is the leading cause of death by an infectious disease among humans. Here, we describe a previously uncharacterized M. tuberculosis protein, Rv0991c, as a molecular chaperone that is activated by oxidation. Rv0991c has homologs in most bacterial lineages and appears to function analogously to the well-characterized Escherichia coli redox-regulated chaperone Hsp33, despite a dissimilar protein sequence. Rv0991c is transcriptionally coregulated with hsp60 and hsp70 chaperone genes in M. tuberculosis, suggesting that Rv0991c functions with these chaperones in maintaining protein quality control. Supporting this hypothesis, we found that, like oxidized Hsp33, oxidized Rv0991c prevents the aggregation of a model unfolded protein in vitro and promotes its refolding by the M. tuberculosis Hsp70 chaperone system. Furthermore, Rv0991c interacts with DnaK and can associate with many other M. tuberculosis proteins. We therefore propose that Rv0991c, which we named “Ruc” (redox-regulated protein with unstructured C terminus), represents a founding member of a new chaperone family that protects M. tuberculosis and other species from proteotoxicity during oxidative stress.

Trodusquemine displaces protein misfolded oligomers from cell membranes and abrogates their cytotoxicity through a generic mechanism

The onset and progression of numerous protein misfolding diseases are associated with the presence of oligomers formed during the aberrant aggregation of several different proteins, including amyloid-β (Aβ) in Alzheimer’s disease and α-synuclein (αS) in Parkinson’s disease. These small, soluble aggregates are currently major targets for drug discovery. In this study, we show that trodusquemine, a naturally-occurring aminosterol, markedly reduces the cytotoxicity of αS, Aβ and HypF-N oligomers to human neuroblastoma cells by displacing the oligomers from cell membranes in the absence of any substantial morphological and structural changes to the oligomers. These results indicate that the reduced toxicity results from a mechanism that is common to oligomers from different proteins, shed light on the origin of the toxicity of the most deleterious species associated with protein aggregation and suggest that aminosterols have the therapeutically-relevant potential to protect cells from the oligomer-induced cytotoxicity associated with numerous protein misfolding diseases.

Limbocker et al. show that trodusquemine, an aminosterol, reduces the cytotoxicity of protein misfolded oligomers by displacing them from cell membranes in the absence of any overt structural/ morphological changes in them. This mechanism appears to be general, as they test it for oligomers of αS, Aβ and the model protein HypF-N to human neuroblastoma cells.

Interaction of TGFβ3 ligand with its receptors type II (TβRII) and type I (TβRI): A unique mechanism of protein-protein association

The proper orchestration of transforming growth factor beta (TGFβ) mediated signal transduction depends upon a delicate set of interactions between specific ligands and their receptors. Here we present an in-depth profiling of the binding mechanism of TGFβ3 ligand with its type II and type I receptors (TβRII and TβRI) using isothermal titration calorimetry (ITC). Studies were carried out in acidic pH as it has great physiological relevance for TGFβ3 activity. Our findings reveal an unusual positive enthalpy (∆H) compensated by a large favourable entropy (∆S) during TGFβ3-TβRII interaction. In addition to the hydrophobic effect, we propose that a distinct conformational switch from "closed" to "open" form as experienced by TGFβ3 on binding to TβRII is contributing significantly to the increase in overall entropy of the system. Binding studies of TGFβ3 and TβRII were carried out at different pH values and salt concentrations to gain further insight into the thermodynamics of the interaction. Furthermore, the importance of hydrophobic interactions on the binding affinity of TβRII with TGFβ3 was confirmed by two TβRII variants (interfacial). Finally, a distinct shift from entropy to enthalpy dominated interaction was observed upon recruitment of TβRI to the binary complex forming the ternary complex.

Chondrogenic Differentiation from Induced Pluripotent Stem Cells Using Non-Viral Minicircle Vectors

Human degenerative cartilage has low regenerative potential. Chondrocyte transplantation offers a promising strategy for cartilage treatment and regeneration. Currently, chondrogenesis using human pluripotent stem cells (hiPSCs) is accomplished using human recombinant growth factors. Here, we differentiate hiPSCs into chondrogenic pellets using minicircle vectors. Minicircles are a non-viral gene delivery system that can produce growth factors without integration into the host genome. We generated minicircle vectors containing bone morphogenetic protein 2 (BMP2) and transforming growth factor beta 3 (TGFβ3) and delivered them to mesenchymal stem cell-like, hiPSC-derived outgrowth (OG) cells. Cell pellets generated using minicircle-transfected OG cells successfully differentiated into the chondrogenic lineage. The implanted minicircle-based chondrogenic pellets recovered the osteochondral defects in rat models. This work is a proof-of-concept study that describes the potential application of minicircle vectors in cartilage regeneration using hiPSCs.

Probing the Nongeneralized Amyloid Inhibitory Mechanism of Hydrophobic Chaperone

Increasing prevalence of protein misfolding disorders urges the search for effective therapies. Although several antiaggregation molecules have been identified, their molecular process of aggregation and clinical trials are underway. The present study is focused on the mechanism through which phenyl butyrate (PB), a chemical chaperone, triggers inhibition of human serum albumin (HSA) fibrillation. Turbidity and Rayleigh light scattering (RLS) measurements reveal the marked presence of aggregates in HSA that were confirmed as amyloid fibrils by thioflavin T (ThT) and Congo red (CR) and were subsequently inhibited by PB in a dose dependent manner. ThT fluorescence kinetics reveals a decrease in the apparent rate constant, K_app, in the presence of PB without triggering a lag phase in HSA suggesting PB's interference with the elongation phase. Dynamic light scattering (DLS) results display a reduction in the aggregate size in the presence of PB. Isothermal titration calorimetry (ITC) data reveals strong binding of PB at site II both at 25 °C (K_b ≈ 1.94 × 10⁵ M^-1) and 65 °C (K_b ≈ 2.90 × 10⁴ M^-1), mediated by hydrogen bonding. Overall, our finding establishes that PB stabilizes partially unfolded HSA molecules through hydrogen bonding, thereby preventing establishment of hydrogen bonds between them and hindering their progression into amyloid fibrils. This is in contrast to its chaperone effect manifested with other proteins.

Selective Regulation of Neurons, Glial Cells, and Neural Stem/Precursor Cells by Poly(allylguanidine)-Coated Surfaces

Poly(allylguanidine) (PAG) was synthesized and characterized as a polycationic coating material for culturing neurons, glial cells, and neural stem/precursor cells (NSPCs) to apply PAG for neural tissue engineering. For comparison, poly-d-lysine (PDL), the golden benchmark of the neuron cell culture system, was also used in this study. When PAG was subjected to a mixed culture of neurons and glial cells, cell adhesion and neurite extension of neuronal cells were clearly observed but only few glial cells could be found alongside the neurons. Compared to PDL, the significantly lower density of the glial fibrillary acidic protein-positive cells implied that PAG suppressed the glial cell development. Likewise, PAG was demonstrated to dominate the differentiation of NSPCs principally into neurons. To investigate whether the different effects of PAG and PDL on neuron and glial cell behaviors resulted from the difference of guanidinium cations and ammonium cations, poly-l-arginine (PLA) was included and compared in this study. Similar to PDL, PLA supported high neuron and glial cell viability simultaneously. Consequently, glial cell growth and viability restrained on PAG was not only affected by the side-chain guanidino groups but also by the backbone structure property. The absence of the peptide structure in the backbone of PAG and the conformation of coated PAG on tissue culture polystyrene possibly determined the polycationic biomaterial to limit the growth of glial cells.

Graphene oxide: A growth factor delivery carrier to enhance chondrogenic differentiation of human mesenchymal stem cells in 3D hydrogels

Cartilage engineering with stem cells in 3D scaffolds is a promising future therapy to treat cartilage defects. One challenge in the field is to design carriers to efficaciously deliver biological factors in 3D scaffolds containing stem cells to appropriately guide differentiation of these cells in same scaffolds and promote specific tissue synthesis. Graphene-based 2D nanomaterials have recently attracted extensive interest for their biomedical applications as they can adsorb a plethora of biological molecules, thus offering high potential as delivery carriers. This study utilized graphene oxide (GO) flakes to adsorb transforming growth factor β3 (TGF-β3), which were then incorporated into a collagen hydrogel. Human mesenchymal stem cells (hMSCs) were encapsulated in the same gel and chondrogenic differentiation assessed. The study showed GO flakes adsorbed > 99% TGF-β3 with <1.7% release. Adsorbed TGF-β3 retained a similar conformation to its dissolved counterpart (free protein) but importantly demonstrated greater conformational stability. Smad2 phosphorylation was promoted, and higher chondrogenic gene expression and cartilage-specific extracellular matrix deposition were achieved compared to exogenously delivering TGF-β3 in culture media. Effects were sustained in long-term 28-day culture. The results demonstrate GO flakes as highly-efficient for delivering GFs in 3D to guide cells in the same scaffold and induce tissue formation. The ability of GO flakes to provide sustained local delivery makes this material attractive for tissue engineering strategies, in particular for regionally-specific MSC differentiation (e.g. osteochondral tissue engineering). STATEMENT OF SIGNIFICANCE: Cartilage engineering involving stem cells in 3D scaffolds is a promising future therapy to treat cartilage defects which can lead to debilitating conditions such as osteoarthritis. However, this field faces the challenge to design delivery carriers to efficaciously deliver biological factors inside these 3D cell-containing scaffolds for appropriately-guided cell differentiation. Graphene-based 2D nanomaterials offer high potential as delivery carriers, but to date studies using them to deliver biological factors have been restricted to 2D substrates, non-scaffold cell masses, or acellular 3D scaffolds. Our study for the first time demonstrated simultaneously incorporating both human mesenchymal stem cells (hMSCs) and GO (graphene oxide)-adsorbed growth factor TGFβ3 into a 3D scaffold, where GO-adsorbed TGFβ3 enhanced chondrogenic differentiation of hMSCs and cartilage-tissue synthesis throughout the scaffold without needing to repeatedly supply TGFβ3 exogenously.

TGF-β2 uses the concave surface of its extended finger region to bind betaglycan's ZP domain via three residues specific to TGF-β and inhibin-α

Betaglycan (BG) is a membrane-bound co-receptor of the TGF-β family that selectively binds transforming growth factor-β (TGF-β) isoforms and inhibin A (InhA) to enable temporal-spatial patterns of signaling essential for their functions in vivo Here, using NMR titrations of methyl-labeled TGF-β2 with BG's C-terminal binding domain, BGZP-C, and surface plasmon resonance binding measurements with TGF-β2 variants, we found that the BGZP-C-binding site on TGF-β2 is located on the inner surface of its extended finger region. Included in this binding site are Ile-92, Lys-97, and Glu-99, which are entirely or mostly specific to the TGF-β isoforms and the InhA α-subunit, but they are unconserved in other TGF-β family growth factors (GFs). In accord with the proposed specificity-determining role of these residues, BG bound bone morphogenetic protein 2 (BMP-2) weakly or not at all, and TGF-β2 variants with the corresponding residues from BMP-2 bound BGZP-C more weakly than corresponding alanine variants. The BGZP-C-binding site on InhA previously was reported to be located on the outside of the extended finger region, yet at the same time to include Ser-112 and Lys-119, homologous to TGF-β2 Ile-92 and Lys-97, on the inside of the fingers. Therefore, it is likely that both TGF-β2 and InhA bind BGZP-C through a site on the inside of their extended finger regions. Overall, these results identify the BGZP-C-binding site on TGF-β2 and shed light on the specificity of BG for select TGF-β-type GFs and the mechanisms by which BG influences their signaling.

Challenges in the local delivery of peptides and proteins for oral mucositis management

Oral mucositis, a common inflammatory side effect of oncological treatments, is a disorder of the oral mucosa that can cause painful ulcerations, local motor disabilities, and an increased risk of infections. Due to the discomfort it produces and the associated health risks, it can lead to cancer treatment restrains, such as the need for dose reduction, cycle delays or abandonment. Current mucositis management has low efficiency in prevention and treatment. A topical drug application for a local action can be a more effective approach than systemic routes when addressing oral cavity pathologies. Local delivery of growth factors, antibodies, and anti-inflammatory cytokines have shown promising results. However, due to the peptide and protein nature of these novel agents, and the several anatomic, physiological and environmental challenges of the oral cavity, their local action might be limited when using traditional delivering systems. This review is an awareness of the issues and strategies in the local delivery of macromolecules for the management of oral mucositis.

An engineered transforming growth factor β (TGF-β) monomer that functions as a dominant negative to block TGF-β signaling

The transforming growth factor β isoforms, TGF-β1, -β2, and -β3, are small secreted homodimeric signaling proteins with essential roles in regulating the adaptive immune system and maintaining the extracellular matrix. However, dysregulation of the TGF-β pathway is responsible for promoting the progression of several human diseases, including cancer and fibrosis. Despite the known importance of TGF-βs in promoting disease progression, no inhibitors have been approved for use in humans. Herein, we describe an engineered TGF-β monomer, lacking the heel helix, a structural motif essential for binding the TGF-β type I receptor (TβRI) but dispensable for binding the other receptor required for TGF-β signaling, the TGF-β type II receptor (TβRII), as an alternative therapeutic modality for blocking TGF-β signaling in humans. As shown through binding studies and crystallography, the engineered monomer retained the same overall structure of native TGF-β monomers and bound TβRII in an identical manner. Cell-based luciferase assays showed that the engineered monomer functioned as a dominant negative to inhibit TGF-β signaling with a Ki of 20-70 nm Investigation of the mechanism showed that the high affinity of the engineered monomer for TβRII, coupled with its reduced ability to non-covalently dimerize and its inability to bind and recruit TβRI, enabled it to bind endogenous TβRII but prevented it from binding and recruiting TβRI to form a signaling complex. Such engineered monomers provide a new avenue to probe and manipulate TGF-β signaling and may inform similar modifications of other TGF-β family members.

Structural basis for potency differences between GDF8 and GDF11

Background

Growth/differentiation factor 8 (GDF8) and GDF11 are two highly similar members of the transforming growth factor β (TGFβ) family. While GDF8 has been recognized as a negative regulator of muscle growth and differentiation, there are conflicting studies on the function of GDF11 and whether GDF11 has beneficial effects on age-related dysfunction. To address whether GDF8 and GDF11 are functionally identical, we compared their signaling and structural properties.

Results

Here we show that, despite their high similarity, GDF11 is a more potent activator of SMAD2/3 and signals more effectively through the type I activin-like receptor kinase receptors ALK4/5/7 than GDF8. Resolution of the GDF11:FS288 complex, apo-GDF8, and apo-GDF11 crystal structures reveals unique properties of both ligands, specifically in the type I receptor binding site. Lastly, substitution of GDF11 residues into GDF8 confers enhanced activity to GDF8.

Conclusions

These studies identify distinctive structural features of GDF11 that enhance its potency, relative to GDF8; however, the biological consequences of these differences remain to be determined.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0350-1) contains supplementary material, which is available to authorized users.

Binding Properties of the Transforming Growth Factor-β Coreceptor Betaglycan: Proposed Mechanism for Potentiation of Receptor Complex Assembly and Signaling

Transforming growth factor (TGF) β1, β2, and β3 (TGF-β1–TGF-β3, respectively) are small secreted signaling proteins that each signal through the TGF-β type I and type II receptors (TβRI and TβRII, respectively). However, TGF-β2, which is well-known to bind TβRII several hundred-fold more weakly than TGF-β1 and TGF-β3, has an additional requirement for betaglycan, a membrane-anchored nonsignaling receptor. Betaglycan has two domains that bind TGF-β2 at independent sites, but how it binds TGF-β2 to potentiate TβRII binding and how the complex with TGF-β, TβRII, and betaglycan undergoes the transition to the signaling complex with TGF-β, TβRII, and TβRI are not understood. To investigate the mechanism, the binding of the TGF-βs to the betaglycan extracellular domain, as well as its two independent binding domains, either directly or in combination with the TβRI and TβRII ectodomains, was studied using surface plasmon resonance, isothermal titration calorimetry, and size-exclusion chromatography. These studies show that betaglycan binds TGF-β homodimers with a 1:1 stoichiometry in a manner that allows one molecule of TβRII to bind. These studies further show that betaglycan modestly potentiates the binding of TβRII and must be displaced to allow TβRI to bind. These findings suggest that betaglycan functions to bind and concentrate TGF-β2 on the cell surface and thus promote the binding of TβRII by both membrane-localization effects and allostery. These studies further suggest that the transition to the signaling complex is mediated by the recruitment of TβRI, which simultaneously displaces betaglycan and stabilizes the bound TβRII by direct receptor–receptor contact.

New strategy for high-level expression and purification of biologically active monomeric TGF-β1/C77S in Escherichia coli

Mature transforming growth factor beta1 (TGF-β1) is a homodimeric protein with a single disulfide bridge between Cys77 on the respective monomers. The synthetic DNA sequence encoding the mature human TGF-β1/C77S (further termed TGF-β1m) was cloned into plasmid pET-32a downstream to the gene of fusion partner thioredoxin (Trx) immediately after the DNA sequence encoding enteropeptidase recognition site. High-level expression (~1.5 g l(-1)) of Trx/TGF-β1m fusion was achieved in Escherichia coli BL21(DE3) strain mainly in insoluble form. The fusion was solubilized and refolded in glutathione redox system in the presence of zwitterionic detergent CHAPS. After refolding, Trx/TGF-β1m fusion was cleaved by enteropeptidase, and the carrier protein of TGF-β1m was separated from thioredoxin on Ni-NTA agarose. Separation of monomeric molecules from the noncovalently bounded oligomers was done using cation-exchange chromatography. The structure of purified TGF-β1m was confirmed by circular dichroism analysis. The developed technology allowed purifying biologically active tag-free monomeric TGF-β1m from bacteria with a yield of about 2.8 mg from 100 ml cell culture. The low-cost and easy purification steps allow considering that our proposed preparation of recombinant monomeric TGF-β1 could be employed for in vitro and in vivo experiments as well as for therapeutic intervention.

Biological activity differences between TGF-β1 and TGF-β3 correlate with differences in the rigidity and arrangement of their component monomers

TGF-β1, -β2, and -β3 are small, secreted signaling proteins. They share 71-80% sequence identity and signal through the same receptors, yet the isoform-specific null mice have distinctive phenotypes and are inviable. The replacement of the coding sequence of TGF-β1 with TGF-β3 and TGF-β3 with TGF-β1 led to only partial rescue of the mutant phenotypes, suggesting that intrinsic differences between them contribute to the requirement of each in vivo. Here, we investigated whether the previously reported differences in the flexibility of the interfacial helix and arrangement of monomers was responsible for the differences in activity by generating two chimeric proteins in which residues 54-75 in the homodimer interface were swapped. Structural analysis of these using NMR and functional analysis using a dermal fibroblast migration assay showed that swapping the interfacial region swapped both the conformational preferences and activity. Conformational and activity differences were also observed between TGF-β3 and a variant with four helix-stabilizing residues from TGF-β1, suggesting that the observed changes were due to increased helical stability and the altered conformation, as proposed. Surface plasmon resonance analysis showed that TGF-β1, TGF-β3, and variants bound the type II signaling receptor, TβRII, nearly identically, but had small differences in the dissociation rate constant for recruitment of the type I signaling receptor, TβRI. However, the latter did not correlate with conformational preference or activity. Hence, the difference in activity arises from differences in their conformations, not their manner of receptor binding, suggesting that a matrix protein that differentially binds them might determine their distinct activities.

Transforming growth factor-β3 (TGF-β3) knock-in ameliorates inflammation due to TGF-β1 deficiency while promoting glucose tolerance

Three homologues of TGF-β exist in mammals as follows: TGF-β1, TGF-β2, and TGF-β3. All three proteins share high homology in their amino acid sequence, yet each TGF-β isoform has unique heterologous motifs that are highly conserved during evolution. Although these TGF-β proteins share similar properties in vitro, isoform-specific properties have been suggested through in vivo studies and by the unique phenotypes for each TGF-β knock-out mouse. To test our hypothesis that each of these homologues has nonredundant functions, and to identify such isoform-specific roles, we genetically exchanged the coding sequence of the mature TGF-β1 ligand with a sequence from TGF-β3 using targeted recombination to create chimeric TGF-β1/3 knock-in mice (TGF-β1(Lβ3/Lβ3)). In the TGF-β1(Lβ3/Lβ3) mouse, localization and activation still occur through the TGF-β1 latent associated peptide, but cell signaling is triggered through the TGF-β3 ligand that binds to TGF-β receptors. Unlike TGF-β1(-/-) mice, the TGF-β1(Lβ3/Lβ3) mice show neither embryonic lethality nor signs of multifocal inflammation, demonstrating that knock-in of the TGF-β3 ligand can prevent the vasculogenesis defects and autoimmunity associated with TGF-β1 deficiency. However, the TGF-β1(Lβ3/Lβ3) mice have a shortened life span and display tooth and bone defects, indicating that the TGF-β homologues are not completely interchangeable. Remarkably, the TGF-β1(Lβ3/Lβ3) mice display an improved metabolic phenotype with reduced body weight gain and enhanced glucose tolerance by induction of beneficial changes to the white adipose tissue compartment. These findings reveal both redundant and unique nonoverlapping functional diversity in TGF-β isoform signaling that has relevance to the design of therapeutics aimed at targeting the TGF-β pathway in human disease.

BMP-2 and TGF-β3 do not prevent spontaneous degeneration in rabbit disc explants but induce ossification of the annulus fibrosus

INTRODUCTION

Different approaches for disc regeneration are currently under investigation. Beside gene therapy and tissue engineering techniques, the application of growth and differentiation factors own promising potential. Studies using reduced intervertebral disc models, such as cell or tissue fragment cultures, have limited validity and show controversial results depending on the employed experimental model. Therefore, the goal of the current study was to investigate the effect of BMP-2 and TGF-β3 on intervertebral disc degeneration using an in vitro full-organ disc/endplate culture system.

MATERIALS AND METHODS

Intervertebral rabbit disc explants were cultured in the presence of 1 μg/ml BMP-2 or TGF-β3 for 21 days in DMEM/F12 media. Nucleus and annulus were analyzed for gene expression of collagen type I and II (Col I/II), aggrecan, collagenases (MMP-1/MMP-13) with RT-qPCR, histological changes with bone and proteoglycan-specific staining (von Kossa, toluidine blue) and differences in cellularity (DNA) and proteoglycan content (alcian blue binding assay).

RESULTS

The results demonstrate that disc proteoglycan concentration decreased with time in the TGF-β3 and BMP-2 groups. In the annulus fibrosus (AF), TGF-β3 and BMP-2 resulted in an up-regulation of Col I and type II, and of aggrecan gene expression. In contrast, MMP genes were inhibited. In the nucleus, the growth factors decreased gene expression of aggrecan and spontaneous Col I up-regulation was inhibited by TGF-β3, whereas expression of Col II was decreased with BMP-2. There was no effect on expression of MMP-1 and MMP-13 for most sampling points. However, TGF-β3 and BMP-2 induced ossification of the AF was demonstrated by histology.

CONCLUSION

It can be concluded that both growth factors, at the tested concentrations, may not be suitable to regenerate the whole intervertebral disc organ but they are interesting candidates for being injected alone or in combination into a painful intervertebral disc to induce osseous fusion (spondylodesis).

Physical instability, aggregation and conformational changes of recombinant human bone morphogenetic protein-2 (rhBMP-2)

The influence of two different pH values on the physical stability of recombinant human bone morphogenetic protein-2 (rhBMP-2) in aqueous solution was evaluated in the present work. RhBMP-2 in solution at pH 4.5 or 6.5 was characterized by intrinsic and extrinsic (Nile Red and 1,8-ANS) fluorescence spectroscopy, 90 degrees light-scattering and transmission electron microscopy (TEM). Compared to the pH 4.5 solution, rhBMP-2 at pH 6.5 had (i) a stronger intrinsic fluorescence intensity, (ii) a longer fluorescence lifetime, (iii) a stronger 90 degrees light-scattering intensity, (iv) a stronger Nile Red fluorescence intensity, (v) a higher Nile Red fluorescence anisotropy, (vi) a lower 1,8-ANS fluorescence intensity, (vii) a higher 1,8-ANS fluorescence anisotropy and (viii) a longer 1,8-ANS fluorescence lifetime. Electron microscopy showed that rhBMP-2 at pH 4.5 contained aggregates of about 100 nm in diameter. More and larger protein aggregates (0.1-2 microm) were observed in solution at pH 6.5. Taken together, these results indicate conformational changes and increased aggregation of rhBMP-2 at pH 6.5 compared to pH 4.5, demonstrating a strong influence of pH on rhBMP-2 physical stability. These observations must be considered when developing a delivery system for rhBMP-2.

TbetaR-II discriminates the high- and low-affinity TGF-beta isoforms via two hydrogen-bonded ion pairs

The TGF-beta isoforms, TGF-beta1, -beta2, and -beta3, share greater than 70% sequence identity and are almost structurally identical. TGF-beta2 differs from the others, however, in that it binds the TGF-beta type II receptor (TbetaR-II) with much lower affinity than either TGF-beta1 or -beta3. It has been previously shown that three conserved interfacial residues, Arg25, Val92, Arg94, in TGF-beta1 and -beta3 are responsible for their high-affinity interaction with TbetaR-II. In this study, the role of each of these residues was examined by creating single, double, and triple substitutions resulting in both TGF-beta3 loss-of-function and TGF-beta2 gain-of-function variants. One-dimensional 1H NMR spectra of the variants confirmed a lack of large structural perturbations. Affinities, kinetics, and thermodynamics for TbetaR-II binding were determined by surface plasmon resonance biosensor analysis. Double substitutions revealed that nearly all of the high-affinity binding is contributed by Arg25 and Arg94. Single site substitutions showed that Arg94 makes the greatest contribution. Substitution of Arg25 and Arg94 with alanine verified the requirement of the arginine guanidinium functional groups for the highly specific hydrogen-bonded ion pairs formed between Arg25 and Arg94 of TGF-beta1 and -beta3, and Glu119 and Asp32 of TbetaR-II. Further kinetic and thermodynamic analyses confirmed that Arg25 and Arg94 are primarily responsible for high-affinity binding and also revealed that noninterfacial longer range effects emanating from the TGF-beta structural framework contribute slightly to TbetaR-II binding. Growth inhibition assays showed that binding changes generally correlate directly with changes in function; however, a role Val92 in this cellular context was uncovered.

Ion-specific modulation of protein interactions: anion-induced, reversible oligomerization of a fusion protein

Ions can significantly modulate the solution interactions of proteins. We aim to demonstrate that the salt-dependent reversible heptamerization of a fusion protein called peptibody A or PbA is governed by anion-specific interactions with key arginyl and lysyl residues on its peptide arms. Peptibody A, an E. coli expressed, basic (pI = 8.8), homodimer (65.2 kDa), consisted of an IgG1-Fc with two, C-terminal peptide arms linked via penta-glycine linkers. Each peptide arm was composed of two, tandem, active sequences (SEYQGLPPQGWK) separated by a spacer (GSGSATGGSGGGASSGSGSATG). PbA was monomeric in 10 mM acetate, pH 5.0 but exhibited reversible self-association upon salt addition. The sedimentation coefficient (s(w)) and hydrodynamic diameter (D(H)) versus PbA concentration isotherms in the presence of 140 mM NaCl (A5N) displayed sharp increases in s(w) and D(H), reaching plateau values of 9 s and 16 nm by 10 mg/mL PbA. The D(H) and sedimentation equilibrium data in the plateau region (>12 mg/mL) indicated the oligomeric ensemble to be monodisperse (PdI = 0.05) with a z-average molecular weight (M(z)) of 433 kDa (stoichiometry = 7). There was no evidence of reversible self-association for an IgG1-Fc molecule in A5N by itself or in a mixture containing fluorescently labeled IgG1-Fc and PbA, indicative of PbA self-assembly being mediated through its peptide arms. Self-association increased with pH, NaCl concentration, and anion size (I(-) > Br(-) > Cl(-) > F(-)) but could be inhibited using soluble Trp-, Phe-, and Leu-amide salts (Trp > Phe > Leu). We propose that in the presence of salt (i) anion binding renders PbA self-association competent by neutralizing the peptidyl arginyl and lysyl amines, (ii) self-association occurs via aromatic and hydrophobic interactions between the ..xxCTRWPWMC..xxxCTRWPWMCxx.. motifs, and (iii) at >10 mg/mL, PbA predominantly exists as heptameric clusters.

A cytokine-neutralizing antibody as a structural mimetic of 2 receptor interactions

TGF-beta isoforms are key modulators of a broad range of biological pathways and increasingly are exploited as therapeutic targets. Here, we describe the crystal structures of a pan-TGF-beta neutralizing antibody, GC-1008, alone and in complex with TGF-beta3. The antibody is currently in clinical evaluation for idiopathic pulmonary fibrosis, melanoma, and renal cell cancer. GC-1008 recognizes an asymmetric binding interface across the TGF-beta homodimer with high affinity. Whereas both cognate receptors, TGF-beta-receptor types I and II, are required to recognize all 3 TGF-beta isoforms, GC-1008 has been engineered to bind with high affinity to TGF-beta1, 2, and 3 via a single interaction surface. Comparison with existing structures and models of TGF-beta interaction with its receptors suggests that the antibody binds to a similar epitope to the 2 receptors together and is therefore a structurally different but functionally identical mimic of the binding mode of both receptors.

High-throughput formulation screening of therapeutic proteins

High-throughput screening (HTS) is used extensively in drug discovery to identify active compounds. Automated preparation and sample analysis in multiwell plates using a combination of liquid and/or powder handling stations, robotics and sensitive detection devices provide powerful tools. At present, protein formulation remains a slow process and will benefit from a fast formulation screening approach. The use of multiwell plates enables the simultaneous screening of many excipients and experimental conditions, such as buffers, salts, surfactants, sugars, storage temperature and mechanical stress. This article reviews the application of the HTS methodology for the development of different protein formulations, such as stable liquids, lyophilisates and slow release forms.:

High throughput screening of protein formulation stability: practical considerations

The formulation of protein drugs is a difficult and time-consuming process, mainly due to the complexity of protein structure and the very specific physical and chemical properties involved. Understanding protein degradation pathways is essential for the success of a biopharmaceutical drug. The present review concerns the application of high throughput screening techniques in protein formulation development. A protein high throughput formulation (HTF) platform is based on the use of microplates. Basically, the HTF platform consists of two parts: (i) sample preparation and (ii) sample analysis. Sample preparation involves automated systems for dispensing the drug and the formulation ingredients in both liquid and powder form. The sample analysis involves specific methods developed for each protein to investigate physical and chemical properties of the formulations in microplates. Examples are presented of the use of protein intrinsic fluorescence for the analysis of protein aqueous properties (e.g., conformation and aggregation). Different techniques suitable for HTF analysis are discussed and some of the issues concerning implementation are presented with reference to the use of microplates.

Mechanistically identified suitable biomarkers of exposure, effect, and susceptibility for silicosis and coal-worker's pneumoconiosis: a comprehensive review

Clinical detection of silicosis is currently dependent on radiological and lung function abnormalities, both late manifestations of disease. Markers of prediction and early detection of pneumoconiosis are imperative for the implementation of timely intervention strategies. Understanding the underlying mechanisms of the etiology of coal workers pneumoconiosis (CWP) and silicosis was essential in proposing numerous biomarkers that have been evaluated to assess effects following exposure to crystalline silica and/or coal mine dust. Human validation studies have substantiated some of these proposed biomarkers and argued in favor of their use as biomarkers for crystalline silica- and CWP-induced pneumoconiosis. A number of "ideal" biological markers of effect were identified, namely, Clara cell protein-16 (CC16) (serum), tumor necrosis factor-alpha (TNF-alpha) (monocyte release), interleukin-8 (IL-8) (monocyte release), reactive oxygen species (ROS) measurement by chemiluminescence (neutrophil release), 8-isoprostanes (serum), total antioxidant levels measured by total equivalent antioxidant capacity (TEAC), glutathione, glutathione peroxidase activity, glutathione S-transferase activity, and platelet-derived growth factor (PDGF) (serum). TNF-alpha polymorphism (blood cellular DNA) was identified as a biomarker of susceptibility. Further studies are planned to test the validity and feasibility of these biomarkers to detect either high exposure to crystalline silica and early silicosis or susceptibility to silicosis in gold miners in South Africa.

Assembly of TbetaRI:TbetaRII:TGFbeta ternary complex in vitro with receptor extracellular domains is cooperative and isoform-dependent

Transforming growth factor-beta (TGFbeta) isoforms initiate signaling by assembling a heterotetrameric complex of paired type I (TbetaRI) and type II (TbetaRII) receptors on the cell surface. Because two of the ligand isoforms (TGFbetas 1, 3) must first bind TbetaRII to recruit TbetaRI into the complex, and a third (TGFbeta2) requires a co-receptor, assembly is known to be sequential, cooperative and isoform-dependent. However the source of the cooperativity leading to recruitment of TbetaRI and the universality of the assembly mechanism with respect to isoforms remain unclear. Here, we show that the extracellular domain of TbetaRI (TbetaRI-ED) binds in vitro with high affinity to complexes of the extracellular domain of TbetaRII (TbetaRII-ED) and TGFbetas 1 or 3, but not to either ligand or receptor alone. Thus, recruitment of TbetaRI requires combined interactions with TbetaRII-ED and ligand, but not membrane attachment of the receptors. Cell-based assays show that TbetaRI-ED, like TbetaRII-ED, acts as an antagonist of TGFbeta signaling, indicating that receptor-receptor interaction is sufficient to compete against endogenous, membrane-localized receptors. On the other hand, neither TbetaRII-ED, nor TbetaRII-ED and TbetaRI-ED combined, form a complex with TGFbeta2, showing that receptor-receptor interaction is insufficient to compensate for weak ligand-receptor interaction. However, TbetaRII-ED does bind with high affinity to TGFbeta2-TM, a TGFbeta2 variant substituted at three positions to mimic TGFbetas 1 and 3 at the TbetaRII binding interface. This proves both necessary and sufficient for recruitment of TbetaRI-ED, suggesting that the three different TGFbeta isoforms induce assembly of the heterotetrameric receptor complex in the same general manner.

Purified human SUV3p exhibits multiple-substrate unwinding activity upon conformational change

Suv3 of Saccharomyces cerevisiae has been classified as a mitochondrial RNA helicase. However, the helicase domain in both yeast and human SUV3 varies considerably from the typical RNA helicase motifs. To investigate its enzymatic activities, a homogeneously purified preparation of SUV3 is required. Expression of a processed form of human SUV3 carrying an N-terminal deletion of 46 amino acids (SUV3DeltaN46) in a yeast suv3 null mutant, which otherwise fails to grow in a nonfermentable carbon source and forms petite colonies in glucose medium, rescues the null phenotype. Through a five-step chromatographic procedure, an 83 kDa SUV3DeltaN46 protein (SUV3-83) and a partially degraded 70 kDa product (SUV3-70) containing amino acids 68-685 were purified to homogeneity. Single- or double-stranded DNA and RNA stimulated ATPase activity of both proteins. SUV3-70, which retains core catalytic domains, can bind and unwind multiple duplex substrates of RNA and DNA with a 5'-3' directionality over a wide range of pH, while SUV3-83 has helicase activity at only acidic pH. ATP, but not nonhydrolyzable ATP, is essential for the unwinding activity, suggesting the requirement of the energy derived from ATP hydrolysis. Consistent with this notion, suv3 mutants containing alanine (A) or arginine (R) substitutions at the conserved lysine residue in the ATP binding site (K213) lost ATPase activity and also failed to unwind the substrates. Importantly, circular dichroism (CD) spectral analysis showed that SUV3-83, at pH 5.0, adopts a conformation similar to that of SUV3-70, suggesting a conformational change in SUV3-83 is required for its helicase activity. The physiological relevance of the multiple-substrate helicase activity of human SUV3 is discussed.

Solution structure and backbone dynamics of the TGFbeta type II receptor extracellular domain

Isoforms of transforming growth factor beta (TGFbeta) are 25 kDa homodimeric polypeptides that signal by binding and bringing together two related, functionally distinct cell surface receptors designated as TbetaR1 and TbetaR2. Here, we report the solution structure of the 13.8 kDa extracellular domain of human TbetaR2 (ecTbetaR2) as calculated from N(N)-H(N), C(alpha)-H(alpha), and C(alpha)-C(O) residual dipolar coupling restraints in conjunction with NOE distance, dihedral angle, and scalar coupling restraints. Comparison of the free ecTbetaR2 solution structure with the TGFbeta3-bound ecTbetaR2 crystal structure reveals backbone conformations that superimpose with RMSDs of 1.0 A over the regions of regular secondary structure and 1.4 A overall. The differences in structure fall mainly in loop regions that are either poorly defined by the available NMR data or are involved in crystal contacts. The noted similarities between the NMR structure of the free form and the crystal structure of the TGFbeta-bound form are also consistent with the close correspondence, 0.16 A RMSD for regions of secondary structure and 0.51 A RMSD overall, for the crystal structure of free ecTbetaR2 as compared to the crystal structure of TGFbeta3-bound ecTbetaR2. Despite the apparent similarities between the free and the bound forms, there appears to be small but significant differences in structure involving the interfacial contact region of the receptor. Measurements of backbone (15)N relaxation times and interpretation of these by the model-free formalism with axial diffusional anisotropy further reveal significant ms to micros time scale motions centered about two of the conserved disulfide bonds and in several residues that comprise the TGFbeta binding surface. Together, these observations indicate that binding likely occurs through a mechanism with a small component of induced fit character, whereby flexibility within the receptor facilitates the transition to the TGFbeta-bound state.

Dynamics-modulated biological activity of transforming growth factor beta3

Transforming growth factor beta3 (TGF-beta3) is an important mediator of growth, maintenance, and repair processes in human cells. Internal dynamic properties have been derived from (15)N NMR relaxation data and mapped onto the spatial structure of TGF-beta3. The pattern of internal dynamics in the structure identifies potential "hot spots" of binding free energy and reveals the importance of conformational entropy in the interaction of TGF-beta3 with the receptors. The observed internal dynamics set TGF-beta3 apart from other TGF-beta isoforms, with which it shares the same fold. These findings may explain functional differences among the various TGF-beta isoforms and thus prove essential in the search for related therapeutic agents.

Transforming growth factor beta-3 crystals as reservoirs for slow release of active TGF-beta3

Transforming growth factor betas (TGF-betas) play critical roles in many diseased states and injury repair processes. Exogenous delivery of TGF-beta may thus have therapeutic applications. Here, crystals of TGF-beta3 (TGF-beta3) are being evaluated as protected reservoirs for sustained local release. A sensitive Mv1Lu cell growth inhibition assay established that in vitro, active TGF-beta3 can be delivered from physically stable crystals. Non-sink release experiments revealed that crystal solubility at pH 7.4 was higher in cell culture medium (2.7+/-0.1 microg/ml) than in saline buffers (approximately 1-1.5 microg/ml, P<0.05). Addition of serum induced a five-fold delay in equilibration of soluble-crystal TGF-beta3. Semi-sink experiments cumulated in higher TGF-beta3 release than under non-sink conditions; the observed steady states correlated with crystal solubility and the frequency of buffer exchange. Release of TGF-beta3 from crystals was also strongly dependent on solubility changes as affected by pH. At neutral pH the solubilities were the lowest, and increased with both higher and lower pH. The results indicate that TGF-beta3 crystals may have promising features for local pH-triggered sustained-release applications.