In vitro synthesis of thymosin beta 4 encoded by rat spleen mRNA

Thymosin β4 expression reveals intriguing similarities between fetal and cancer cells

Thymosin β4 (Tβ4) is highly expressed in saliva of human newborns but not in adults. Here preliminary immunohistochemical analyses on different human tissues are reported. Immunoreactivity for Tβ4 in human salivary glands show high quantities of Tβ4 before birth, followed by downregulation of expression in adulthood. In contrast, Tβ4 is detected in tumors of salivary glands, suggesting that tumor cells might utilize fetal programs, including Tβ4 synthesis. Immunohistochemical analyses in the gastrointestinal tract showed strong reactivity for Tβ4 in enterocytes during development, but weak immunostaining in mature enterocytes. In colorectal cancer, the association of a high expression of Tβ4 with epithelial-mesenchymal transition was observed. On the basis of these data, the process of epithelial-mesenchymal transition could represent the unifying process that explains the role of Tβ4 during fetal development and in cancer progression.

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.

The beta-thymosins: intracellular and extracellular activities of a versatile actin binding protein family

The beta-thymosins are N-terminally acetylated peptides of about 5 kDa molecular mass and composed of about 40-44 amino acid residues. The first member of the family, thymosin beta4, was initially isolated from thymosin fraction 5, prepared in five steps from calf thymus. Thymosin beta4 was supposed to be specifically produced and released by the thymic gland and to possess hormonal activities modulating the immune response. Various paracrine effects have indeed been reported for these peptides such as cardiac protection, angiogenesis, stimulation of wound healing, and hair growth. Besides these paracrine effects, it was noted that beta-thymosins occur in high concentration in the cytoplasm of many eukaryotic cells and bind to the cytoskeletal component actin. Subsequently it became apparent from in vitro experiments that they preferentially bind to monomeric (G-)actin and stabilize it in its monomeric form. Due to this ability the beta-thymosins are the main intracellular actin sequestering factor, i.e., they posses the ability to remove monomeric actin from the dynamic assembly and disassembly processes of the actin cytoskeleton that constantly occur in activated cells. In this review we will concentrate on the intracellular activity and localization of the beta-thymosins, i.e., their modulating effect on the actin cytoskeleton.

Alterations in actin-binding beta-thymosin expression accompany neuronal differentiation and migration in rat cerebellum

The beta 4- and beta 10-thymosins, recently identified as actin monomer-sequestering proteins, are developmentally regulated in brain. Using specific mRNA and protein probes, we have used in situ hybridization and immunohistochemical techniques to investigate the distribution of the beta-thymosin mRNAs and their proteins in developing rat cerebellum. Early in postnatal development, both beta-thymosin mRNAs were expressed at highest levels in the postmitotic, premigratory granule cells of the external granular layer; expression diminished as granule cells migrated to and differentiated within the developing internal granular layer. In addition, both beta-thymosin proteins were present in bundles of cerebellar afferent fibers in the white matter at this time. Throughout the maturation period, both proteins were present in elongating parallel fibers in the upper portion of the molecular layer. Later in cerebellar development, thymosin beta 4, but not thymosin beta 10, was expressed in Golgi epithelial cells and Bergmann processes. Thymosin beta 4 was expressed in a small population of cells with microglial morphology scattered throughout the gray and white matter. Thymosin beta 10 was detected in an even smaller population of glia. Expression of thymosin beta 4 and thymosin beta 10 in premigratory granule cells and in growing neuronal processes is consistent with the possibility that both beta-thymosins are involved in the dynamics of actin polymerization during migration and process extension of neurons.

Thymosins: both nuclear and cytoplasmic proteins

A simple procedure based on perchloric acid extraction has been developed for the preparation and purification of bovine prothymosin alpha and thymosins beta 4 and beta 9 in high yields. Spectroscopic observations show these proteins to be non-folding at neural pH. The cellular locations of human prothymosin alpha, rat parathymosin and calf thymosin beta 4, all so-called 'thymic hormones', have been studied by injection into the cytoplasm of Xenopus oocytes, followed by separate monitoring of nuclear and cytoplasmic concentrations. It is shown that human prothymosin alpha and rat parathymosin both migrate to the nucleus whilst thymosin beta 4 remains in the cytoplasm. The peptide (1-88) of calf prothymosin alpha is shown not to accumulate in the Xenopus nucleus, demonstrating that the C-terminal 21 residues, which include a KKQK sequence, are required for nuclear migration. The present data, in association with existing evidence of wide tissue distribution and the lack of signal peptides, indicate that these proteins do not behave as hormones in the usual sense of the word. It is suggested that thymosin beta 4 should be grouped separately from the pro- and parathymosins.

Developmental expression of mRNAs encoding thymosins beta 4 and beta 10 in rat brain and other tissues

In the course of screening a fetal rat cerebellum cDNA library for developmentally regulated sequences, we have identified a cDNA clone identical in sequence to that encoding a protein originally isolated from thymus, thymosin beta 10. Based on northern hybridization analyses with gene-specific oligonucleotide probes derived from the 3'-untranslated regions of thymosin beta 10 mRNA and the closely related beta 4 mRNA, we showed that both thymosin mRNAs were present at highest levels in fetal cortex and cerebellum but also were present at varying levels in all other fetal tissues examined (thymus, spleen, lung, kidney, adrenal, heart, and liver). Steady-state levels of thymosin beta 10 mRNA in cerebellum declined to negligible levels after day 14 of postnatal development. Its levels exhibited a similar pattern in developing cortex, although the adult cortex had slightly higher thymosin beta 10 mRNA levels. These results suggest that thymosin beta 10 mRNA is subject to strong developmental regulation in the rat central nervous system. Reduction of thymosin beta 10 mRNA levels also was seen during development of kidney, heart, and liver. Levels of both thymosin beta 10 and beta 4 mRNAs remained relatively constant during development of thymus, spleen, and lung. Thymosin beta 4 mRNA levels dropped much less sharply during brain development than did levels of the beta 10 mRNA. Testis and ovary contained the highest relative levels of thymosin beta 10 mRNA among adult tissues, but little thymosin beta 4 mRNA. A novel thymosin beta 10 mRNA species unique to adult testis was detected. These results indicate that both thymosins must function in the development of brain and many other organs, as well as in different subsets of organs in the adult.

Isolation and structural studies of porcine, ovine and murine thymosin beta 4 by high-performance liquid chromatography

Rapid high-performance liquid chromatographic (HPLC) procedures have been used to isolate and characterize thymosin beta 4 from different species. Crude extracts termed thymosin fraction 5A were prepared from porcine and ovine thymus glands as well as murine spleen. Each fraction 5A preparation was then fractionated by HPLC on a muBondapak C18 reversed-phase column. Porcine and ovine thymus fraction 5A, and murine spleen 5A, each yields a predominant peak at a retention time similar to that of bovine thymosin beta 4. Amino acid analysis as well as HPLC tryptic peptide mapping of these peaks indicate that they have homologous sequences to bovine thymosin beta 4. Chromatographic analysis of fresh murine thymus and spleen tissues also revealed protein peaks at the position of bovine beta 4, suggesting that thymosin beta 4 is the native protein present in the animal tissues.

Cloning and sequence analysis of cDNA for rat spleen thymosin beta 4

Molecular cloning of a cDNA has established the sequence of the translated portion of the mRNA for rat spleen thymosin beta 4. The presence of a methionyl initiator codon immediately preceding the codon for the first seryl residue of mature thymosin beta 4 is consistent with previous results indicating the absence of a signal peptide in the product translated in vitro from rat spleen mRNA. The cDNA sequence analysis also established the presence of two terminator codons immediately following the codon for the COOH-terminal seryl residue. Thymosin beta 4 is thus synthesized as a 5100-dalton peptide containing 44 amino acid residues. Removal of the initiator methionyl residue and acetylation of the NH2-terminal serine residue would yield mature thymosin beta 4 containing 43 amino acids. The absence of a signal peptide makes it unlikely that thymosin beta 4 is a secreted peptide.