Thymosin Beta 4
Biochemically, Thymosin Beta 4 is a small peptide, i.e., it is composed of the chemically bonded amino acids via the peptide linkages and having amino group at one terminal while having carboxyl group on the other terminal. It is composed of 43 amino acids. The sequence of amino acids aa in the primary structure of Thymosin Beta 4 is schematically presented in Figure 1. Based on the knowledge of its primary structure, its secondary structure is also predicted via the method of Chou and Fasma. The details of this technique include: first, the residual amino acids are characterized as H, h, I, i, b, and B on the basis of their tendency to either form the alpha helix or beta pleated sheets, and represented in Figure 2 as the alpha parameters and beta parameters. Finally, the secondary structure of the Thymosin Beta 4 is predicted that is schematically shown in Figure 2, that is composed of only two alpha helices from the 4-12 and 32-40 residual amino acid. Normally, it is expressed in all the cell types except in the red blood cells while particularly in the thymocytes which are the cells of the thymus gland, that has a well-known role in the immunity. Apart from its biosynthesis, it is also synthesized in the laboratory conditions, and the complete process of its synthesis is well documented in the scientific literature.
Figure 1 The sequence of Amino Acids in the primary structure of the Thymosin Beta 4 peptide is given by the 3-letter representation of amino acid.
Figure 2 The prediction of the secondar structure for the Thymosin Beta 4 peptide, based on the presence of amino acids in its primary structure. H stands for strong former; h stands for former; I stands for weak former; i stands for indifferent; b stands for breaker, and B stands for strong breaker for the potential of the formation of alpha helix and for the formation of beta pleated sheet as well. Based on these observations, there is a high probability of the existence of two alpha helices and no beta pleated sheet, in the secondary structure of the Thymosin Beta 4.
Thymosin Beta 4 peptide has a healing role in the corneal wound/ injuries caused by any microbe, physical or chemical agent, especially the alkali injury; this role is schematically presented in Figure 3. Normally, Cornea is the transparent part of eye; it can be regarded as the biological lens as it is the very part of the eye that allows the light rays to enter the eye. Cornea is composed of three tissue layers namely epithelial, stroma and the endothelial. In the injury state, stroma becomes swellon and the fibroblasts in the stroma are activated thus fibers start forming. This process is accompanied by the presence of the infiltered inflammatory cell. All of these contribute to the decrease in the transparency of the cornea. Healing of cornea, immediately after the injury is central to the prevention of vision loss. In the corneal wound healing process, there are two major cellular responses that are always involved which are mediated by treating the corneal wound with the Thymosin Beta 4 peptide. First, the recruitment of the epithelial cells, keratinocytes and endothelial cells, at the wound site to cover the wound bed. The cell migration cell is supposed to the induced through the upregulated expression of the zyxin protein that regulates the actin cytoskeleton. Second, the proliferation via the mitosis, i.e., a type of cell division for the body cells, of all those recruited cells that must be followed by the cellular differentiation so that the lost epithelial can be restored.
Figure 3 The mechanism by which the Thymosin Beta 4 peptide promotes the wound healing in the cornea.
Apart from the healing of the corneal injuries, has also be observed to cause full wound healing of the skin injuries, in the animal study models namely normal, steroid-treated and diabetic ones.
In the corneal wound healing, to reduce the inflammation is as important as the cell migration/ proliferation. Because the prolonged inflammation caused the retardation of the re-epithelialization of the wound. The Thymosin Beta 4 peptide also downregulates the expression of Interleukin-1 beta and Tumor necrosis factor alpha (TNF-α); both of which are the main inflammatory cytokines that means that both of them promotes the inflammation process and negatively impact the corneal wound healing, while the latter one is the characteristic feature of the dry eye as observed in the murine model. And their downregulation directly contributes to the healing process. In addition, it also downregulates Macrophage inflammatory protein-1 alpha (MIP-1α) and Macrophage inflammatory protein-1 beta (MIP-1β); both of which are the chemokines. While, on the other hand, the Thymosin Beta 4 peptide upregulates the expression of transforming growth factor-beta 1 (TGFβ-1) and laminin-5.
Upregulated expression of Tumor necrosis factor alpha (TNF-α) promotes the transcription, that is the formation of mRNA transcripts, of Nuclear factor kappa B (NF-κB). Followed by the expression of Nuclear factor kappa B (NF-κB), it is held inactive as a dimer, in the cytoplasm of the cell by the inhibitory proteins. These inhibitory protein upon getting phosphorylated, are destroyed by the cells; thereby the Nuclear factor kappa B (NF-κB) molecules are released in free form, and thus are active. This active Nuclear factor kappa B (NF-κB) gets translocated in the cellular nucleus, where it binds to the kappa B element of a very wide range of genes. In this way, the Nuclear factor kappa B (NF-κB) can affect a vast number of inflammatory gene, thus the role of the Nuclear factor kappa B (NF-κB) is central to the inflammation process. As stated earlier, The Thymosin Beta 4 peptide also downregulates the expression of the Nuclear factor kappa B (NF-κB).
This is experimentally observed fact in the cultured corneal epithelial cells of human in which the expression of Tumor necrosis factor alpha (TNF-α) was upregulated. These cells were given the Thymosin Beta 4 peptide treatment. After this treatment, the level of the Nuclear factor kappa B (NF-κB) was reduced and thereby its activity was also reduced. In addition, its translocation to the nucleus was also hindered. Based on this experimental finding, it was confirmed that the Thymosin Beta 4 peptide reduces the inflammation via inhibiting the Nuclear factor kappa B (NF-κB) signaling pathway.
The Thymosin Beta 4 peptide prevents the cellular apoptosis, which is a type of a programmed cell death. It has been scientifically observed in the HeLa cell which were overexpressing the Thymosin Beta 4 peptide, showed a high growth rate as compared to the experimental control group. Moreover, these cells were also found to be more resistant to the cellular apoptosis mediated by the paclitaxel.
It is predicted by the researchers that Thymosin Beta 4 peptide is potent to be a therapeutic drug for the wound healing.