C-terminal truncation of thymosin beta10 by an intracellular protease and its influence on the interaction with G-actin studied by ultrafiltration

Top down proteomic analysis of gingival crevicular fluid in deciduous, exfoliating and permanent teeth in children

Gingival Crevicular Fluid (GCF), a plasma-derived exudate present in the gingival crevice was collected from deciduous, exfoliating and permanent teeth from 20 children (60 samples) with the aim to characterize and quantify by a mass spectrometry based top-down proteomic approach, the peptide/proteins in the fluid and verify possible variations occurring during the exfoliating process. The results obtained confirmed the presence in GCF of α-Defensins 1-4, Thymosin β4 and Thymosin β10, as described in previous works and revealed the presence of other interesting peptides never described before in GCF such as specific fragments of α-1-antitrypsin, α-1-antichymotrypsin; fragments of Thymosin β4 and Thymosin β10; Fibrinopeptide A and its fragments and Fibrinopeptide B; S100A8 and S100A9, LVV Hemorphin-7 (hemoglobin chain β fragment), as well as some other peptides deriving from α and β subunits of hemoglobin. Statistical analysis evidenced different levels in 5 proteins/peptides in the three groups. Our study demonstrate that an in-depth analysis of a biological fluid like GCF, present in small amount, can provide useful information for the understanding of different biological processes like teeth eruption. Data are available via ProteomeXchange with identifier PXD016010 and PXD016049. SIGNIFICANCE: GCF due to his site-specific nature has a great potential in containing factors that are specific for action at a given site and might have diagnostic value to detect qualitative and quantitative variations of proteins/peptides composition linked to physiological or pathological conditions.

High-resolution mass spectrometry for thymosins detection and characterization

OBJECTIVES

The aim of this study was to characterize β and α thymosins and their proteoforms in various tissues and bodily fluids by mass spectrometry and to look at their association with a wide variety of pathologies.

METHODS

A top-down proteomic platform based on high-performance liquid chromatography (HPLC) coupled to high-resolution LTQ-Orbitrap mass spectrometry (MS) was applied to the characterization of naturally occurring peptides.

RESULTS

In addition to thymosin β4 (Tβ4) and β10 (Tβ10), several post-translational modifications of both these peptides were identified not only in bodily fluids but also in normal and pathological tissues of different origins. The analysis of tissue specimens allowed the characterization of different C-terminal truncated forms of Tβ4 and Tβ10 together with other proteolytic fragments. The sulfoxide derivative of both Tβ4 and Tβ10 and the acetylated derivatives at lysine residues of Tβ4 were also characterized. Different proteoforms of prothymosin α, parathymosin α, thymosin α1 and thymosin α11 together with diverse proteolytic fragments were identified too.

CONCLUSION

The clinical and prognostic significance and the origin of these proteoforms have to be deeply investigated.

Thymosin beta4 and its posttranslational modifications

Thymosin beta(4) as well as the other members of the beta-thymosin family are important G-actin sequestering peptides. The chemical properties, the biosynthesis, and posttranslational modifications (PTMs) of these peptides are discussed. During biosynthesis of thymosin beta(4) the initiator methionine is removed and the N-terminus is acetylated. Research on proteomics revealed several acetylated lysine residues and two phosphorylated threonine residues. The enormous number of phosphorylable and acetylable sites in the human proteome raises the question about the biological significance of these PTMs in the context of beta-thymosins. Presently, this question cannot be answered because neither the concentration of these modified beta-thymosins in cells is known nor the consequences of the modifications on the biological function(s) of beta-thymosins have been studied yet. Thymosin beta(4) is also posttranslationally modified by transglutaminase forming covalent bonds with other molecules. Prolyl oligopeptidase generates ac-SDKP from thymosin beta(4). The concentration of C-terminal peptide fragments of thymosin beta(4) is elevated in the blood of patients with rheumatoid arthritis.

Functional and profiling studies prove that prostate cancer upregulated neuroblastoma thymosin β is the true human homologue of rat thymosin β15

A peptide with a sequence identical to rat thymosin beta(Tb)15 was reported to be upregulated in human prostate cancer. However, in this report we provide evidence that TbNB, initially identified in human neuroblastoma, is the only Tb isoform upregulated in human prostate cancer and that the Tb15 sequence is not present herein. In addition, we demonstrate that human TbNB has a higher affinity for actin in comparison to Tb4 and promotes cell migration. In combination, this experimentally validates TbNB as functional homologue of rat Tb15 in the human organism and clarifies the current composition of the human Tb family.

Influence of the N terminus and the actin-binding motif of thymosin beta4 on its interaction with G-actin

Thymosin beta(4) binds G-actin in a 1:1 ratio and prevents its aggregation to F-actin by sequestration. Substitution or modification of single amino acid residues within the N-terminal sequence 1 to 22 of thymosin beta(4) alters its interaction with G-actin. We generated thymosin beta(4) variants with amino acid substitutions within the N-terminal alpha-helix and the putative actin-binding motif. None of the E. coli-generated thymosin beta(4) variants was modified or acetylated at its N terminus. The stability of the complex of G-actin with nonacetylated thymosin beta(4) or beta(4)(A7V) is higher than the one with naturally occurring thymosin beta(4), which is always acetylated. The complex of G-actin with nonacetylated thymosin beta(4)(A7V,K18,19A) and beta(4)(K14,16,18,19A) is 15 times less stable compared to the complex with thymosin beta(4). The G-actin sequestering activities of all thymosin beta(4) variants correspond to their complex stabilities with G-actin, except for nonacetylated thymosin beta(4)(A7V), where it is attenuated. Thymosin beta(4)(Delta17-23) missing the putative actin-binding motif shows no interaction with G-actin.

beta-Thymosins

The development of thymosin beta(4) from a thymic hormone to an actin-sequestering peptide and back to a cytokine supporting wound healing will be outlined. Thymosin fraction 5 consists of a mixture of polypeptides and improves immune response. Starting with fraction 5, several main peptides (thymosin alpha(1), polypeptide beta(1), and thymosin beta(4)) were isolated and tested for biological activity. However, none of the isolated peptides were really thymic hormones. They are all biological important peptides with diverse functions. Polypeptide beta(1) is identical to ubiquitin truncated by two C-terminal glycine residues. Several peptides related to thymosin beta(4) were isolated and sequenced from various species. Large amounts of thymosin beta(4) were found in many cells. It was postulated that the beta-thymosins might have a general function. The identification of a biological function of thymosin beta(4) was tedious. In 1990, Dan Safer and his colleagues recognized that thymosin beta(4) sequesters G-actin. The dissociation constant of the complex in the micromolar range allows for fast binding and release of G-actin. beta-Thymosins are the main intracellular G-actin-sequestering peptides in most vertebrate cells. Thymosin beta(4) is unstructured but folds into a stable conformation on binding to G-actin. It is present in the nucleus as well as the cytoplasm and might be responsible for sequestering nuclear actin. Several biological effects are attributed to thymosin beta(4), oxidized thymosin beta(4), or to ac-SDKP possibly generated from thymosin beta(4). However, very little is known about molecular mechanisms mediating the effects attributed to extracellular beta-thymosins.

Thymosin beta4 Is Not Always the Main beta-Thymosin in Mammalian Platelets

beta-thymosins constitute a family of highly conserved 5-kDa polypeptides. Thymosin beta(4), the most abundant member of this family, is expressed in most mammalian cell types and is regarded as the main intracellular G-actin sequestering peptide. In addition to this important intracellular function several other activities have been attributed to this peptide. Thymosin beta(4) is released from human platelets and cross-linked to fibrin after activation of platelets with thrombin. While in most mammalian tissues thymosin beta(4) is accompanied by a second member of this peptide family, in human platelets only thymosin beta(4) is present. To elucidate if it is common to mammalian platelets that only one beta-thymosin is present, we analyzed platelets from several mammals for their beta-thymosin content. In human platelets only thymosin beta(4) could be detected, whereas in bovine platelets thymosin beta(9), which is normally the minor beta-thymosin in bovine tissues, was identified as the main beta-thymosin. In rabbit platelets, thymosin beta(4) is not simply replaced by the most homologous thymosin beta(4)(Ala), as might be expected from sequence homology. Thymosin beta(4)(Ala) and thymosin beta(10) were found, but thymosin beta(10) is present in about 2.5-fold higher amounts. Because thymosin beta(4)(Ala) possesses about threefold higher affinity to G-actin, compared to thymosin beta(4), beta(10), and beta(9), we suggest that expression of beta-thymosins is triggered by functional requirements and not sequence homology.

The Identification of Apoptosis-related Residues in Human Thymosin β-10 by Mutational Analysis and Computational Modeling

Thymosin beta-10 (TB10) is an actin monomer-sequestering peptide that consists of 43 amino acid residues and that tends to form alpha-helical structures. Previously, we showed that the overexpression of TB10 dramatically increases the frequency of apoptosis in human ovarian cancer cells. To identify the critical residues responsible for TB10-mediated apoptosis, we used a series of computational methods. First, a three-dimensional structure of human TB10 was constructed using the homology modeling method with the calf thymosin beta-9 NMR structure as a template. Although the sequences of both of these structures are almost identical, 200-ps molecular dynamics simulation results showed that their secondary structures differ. Analyses of molecular dynamics snapshot structures suggested that the TB10 structure is conformationally more complicated than the TB9 structure. The conserved 17LKKTET(22 motif region of TB10 was tested by Ala and Ser scanning mutagenesis using computational and biochemical methods, and 12 mutants were transfected into cancer cell lines and tested for their effects on growth arrest. Of the 12 mutants examined, only the Thr20 to Ser20 mutation showed reduced growth arrest. These results strongly suggest that Thr20 is specifically required for actin sequestration by TB10 in ovarian cancer cells. These results may provide useful information for the development of a new ovarian cancer therapy.

The thymosins. Prothymosin alpha, parathymosin, and beta-thymosins: structure and function

The studies on thymosins were initiated in 1965, when the group of A. White searched for thymic factors responsible for the physiological functions of thymus. To restore thymic functions in thymic-deprived or immunodeprived animals, as well as in humans with primary immuno-deficiency diseases and in immunosuppressed patients, a standardized extract from bovine thymus gland called thymosin fraction 5 was prepared. Thymosin fraction 5 indeed improved immune response. It turned out that thymosin fraction 5 consists of a mixture of small polypeptides. Later on, several of these peptides (polypeptide beta 1, thymosin alpha 1, prothymosin alpha, parathymosin, and thymosin beta 4) were isolated and tested for their biological activity. The research of many groups has indicated that none of the isolated peptides is really a thymic hormone; nevertheless, they are biologically important peptides with diverse intracellular and extracellular functions. Studies on these functions are still in progress. The current status of knowledge of structure and functions of the thymosins is discussed in this review.

Formation and implications of a ternary complex of profilin, thymosin beta 4, and actin

Data from affinity chromatography, analytical ultracentrifugation, covalent cross-linking, and fluorescence anisotropy show that profilin, thymosin beta(4), and actin form a ternary complex. In contrast, steady-state assays measuring F-actin concentration are insensitive to the formation of such a complex. Experiments using a peptide that corresponds to the N terminus of thymosin beta(4) (residues 6-22) confirm the presence of an extensive binding surface between actin and thymosin beta(4), and explain why thymosin beta(4) and profilin can bind simultaneously to actin. Surprisingly, despite much lower affinity, the N-terminal thymosin beta(4) peptide has a very slow dissociation rate constant relative to the intact protein, consistent with a catalytic effect of the C terminus on conformational change occurring at the N terminus of thymosin beta(4). Intracellular concentrations of thymosin beta(4) and profilin may greatly exceed the equilibrium dissociation constant of the ternary complex, inconsistent with models showing sequential formation of complexes of profilin-actin or thymosin beta(4)-actin during dynamic remodeling of the actin cytoskeleton. The formation of a ternary complex results in a very large amplification mechanism by which profilin and thymosin beta(4) can sequester much more actin than is possible for either protein acting alone, providing an explanation for significant sequestration even if molecular crowding results in a very low critical concentration of actin in vivo.

beta-Thymosins, small acidic peptides with multiple functions

The beta-thymosins are a family of highly conserved polar 5 kDa peptides originally thought to be thymic hormones. About 10 years ago, thymosin beta(4) as well as other members of this ubiquitous peptide family were identified as the main intracellular G-actin sequestering peptides, being present in high concentrations in almost every cell. beta-Thymosins bind monomeric actin in a 1:1 complex and act as actin buffers, preventing polymerization into actin filaments but supplying a pool of actin monomers when the cell needs filaments. Changes in the expression of beta-thymosins appear to be related to the differentiation of cells. Increased expression of beta-thymosins or even the synthesis of a beta-thymosin normally not expressed might promote metastasis possibly by increasing mobility of the cells. Thymosin beta(4) is detected outside of cells in blood plasma or in wound fluid. Several biological effects are attributed to thymosin beta(4), oxidized thymosin beta(4), or to the fragment, acSDKP, possibly generated from thymosin beta(4). Among the effects are induction of metallo-proteinases, chemotaxis, angiogenesis and inhibition of inflammation as well as the inhibition of bone marrow stem cell proliferation. However, nothing is known about the molecular mechanisms mediating the effects attributed to extracellular beta-thymosins.

Single-cell MALDI: a new tool for direct peptide profiling

Matrix-assisted laser desorption-ionization (MALDI) mass spectrometry (MS) is a rapid and sensitive analytical approach that is well suited for obtaining molecular weights of peptides and proteins from complex samples. MALDI-MS can profile the peptides and proteins from single-cell and small tissue samples without the need for extensive sample preparation, except for the cell isolation and matrix application. Strategies for peptide identification and characterization of post-translational modifications are presented. Furthermore, several recent enhancements in MALDI-MS technology, including in situ peptide sequencing as well as the direct spatial mapping of peptides in cells and tissues are discussed.

C-terminal variations in beta-thymosin family members specify functional differences in actin-binding properties

Mammalian cells express several isoforms of beta-thymosin, a major actin monomer sequestering factor, including thymosins beta4, beta10, and beta15. Differences in actin-binding properties of different beta-thymosin family members have not been investigated. We find that thymosin beta15 binds actin with a 2.4-fold higher affinity than does thymosin beta4. Mutational analysis was performed to determine the amino acid differences in thymosin beta15 that specify its increased actin-affinity. Previous work with thymosin beta4 identified an alpha-helical domain, as well as a conserved central motif, as crucial for actin binding. Mutational analysis confirms that these domains are also vital for actin binding in thymosin beta15, but that differences in these domains are not responsible for the variation in actin-binding properties between thymosins beta4 and beta15. Truncation of the unique C-terminal residues in thymosin beta15 inhibits actin binding, suggesting that this domain also has an important role in mediating actin-binding affinity. Replacement of the 10 C-terminal amino acids of thymosin beta15 with those of thymosin beta4 did, however, reduce the actin-binding affinity of the hybrid relative to thymosin beta15. Similarly, replacement of the thymosin beta4 C-terminal amino acids with those of thymosin beta15 led to increased actin binding. We conclude that functional differences between closely related beta-thymosin family members are, in part, specified by the C-terminal variability between these isoforms. Such differences may have consequences for situations where beta-thymosins are differentially expressed as in embryonic development and in cancer.

Direct profiling of proteins in biological tissue sections by MALDI mass spectrometry

The direct profiling of proteins present in tissue sections for several organs of the mouse has been accomplished using matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS). Fresh tissue was sectioned and blotted on a conductive polyethylene membrane. The dried membrane blot was coated with matrix, typically sinapinic acid, and directly analyzed in the mass spectrometer. Generally, well over 100 peptide/protein signals in the 2000-30,000 Da range were observed, with 30-50 having relatively high signal intensities. Analysis of different areas of the same tissue gave remarkably similar mass spectra with greater than 90% homology. However, different parts of a segmented tissue, such as the proximal, intermediate, and distal colon, gave some unique protein signals. After treatment of the tissue blot with protease and subsequent MALDI MS analysis using postsource decay methods for peptide sequencing, some of the proteins were identified. The unique protein profiles measured from these tissue blots also showed differences from strain to strain of the mouse, with genetically similar strains having very similar patterns.

The dipyridyls paraquat and diquat attenuate the interaction of G-actin with thymosin beta4

Beta-thymosins sequester G-actin and preserve a pool of monomers of actin which constitute an important prerequisite for cellular function of the microfilament system. To study the influence of paraquat binding to G-actin on the interaction of G-actin with thymosin beta4 we determined the apparent dissociation constant of the G-actin-thymosin beta4 complex in the absence or presence of paraquat using an ultrafiltration assay. Paraquat (1,1'-dimethyl-4,4'-dipyridylium dichloride) attenuates this interaction in a concentration- and time-dependent manner. When exposed to 10 mM paraquat, the apparent dissociation constant increased 10-85-fold within 15 min to 24 h. After incubation for 24 h even a paraquat concentration as low as 100 microM increased the dissociation constant of the G-actin-thymosin beta4 complex from 0.66 microM to 0.82 microM (P < 0.05). Diquat (1,1'-ethylene-2,2'-dipyridylium dibromide) similarly weakens the interaction of G-actin and beta-thymosins. In none of the experiments was oxidation of the methionine residue or any other modification of thymosin beta4 detected. Therefore we conclude that the dipyridyls paraquat and diquat directly interact with G-actin and thereby impede the interaction between G-actin and thymosin beta4.