Site-specific modification approaches have been extensively employed in the development of protein-based technologies. In this field, stability and activity integrity are the envisioned features of chemically modified proteins. These methods are especially used in the design of antibody-drug conjugates (ADCs). Nevertheless, a biochemical feature of the target protein in these reactions is often overlooked, residue specificity. Usually, in the course of developing chemical probes to modify a protein of interest (POI), specific amino acids are selected due to their reactivity. It is not critical which residue is modified as long as its modification does not compromise the POI's activity. However, no attention is paid as to why certain residues are preferentially modified over others. Physicochemical and structural constraints are often involved in the reactivity of the residue and account for the preferential modification. We propose that site-specific protein modification approaches can be applied beyond the development of ADCs or protein-drug conjugates, and used as a tool to reveal functionally relevant residues. By preferentially modifying certain side chains in the POI, chemical probes can uncover new binding motifs to investigate. Here we describe methods for protein modification, and how some pitfalls in the field can be turned into tools to reveal and exploit druggable pockets. Thus, allowing the design of innovative inhibitors against disease-relevant POIs. We discuss methodologies for site-specific modification of lysine, tryptophan, cysteine, histidine and tyrosine and comment on instances where the modified residues were used as targets for functionalization or drug design.

Mesenchymal stem cell (MSC) transplantation has demonstrated its potential in repairing infarct heart tissue and recovering heart function after myocardial infarction (MI). However, its therapeutic effect is still limited due to poor MSC engraftment at the injury site whose tissue stiffness and local inflammation both dynamically and rapidly change after MI. Whether and how inflammatory cytokines could couple with stiffness change to affect MSC engraftment in the infarct zone still remain unclear. In this study, we characterized dynamic stiffness changes of and inflammatory cytokine expression in the infarct region of rat heart within a month after MI. We found that the tissue stiffness of the heart tissue gradually increased and peaked 21 days after MI along with the rapid upregulation of tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and interleukin-1 beta (IL-1 beta) in the first 3 days, followed by a sharp decline. We further demonstrated in vitro that immobilized inflammatory cytokine IL-6 performed better than the soluble form in enhancing MSC adhesion to stiffened substrate through IL-6/src homology 2 (SH2) domain-containing tyrosine phosphatase-2 (SHP2)/integrin signaling axis. We also confirmed such mechano-immune coupling of tissue stiffness and inflammatory cytokines in modulating MSC engraftment in the rat heart after MI in vivo. Our study provides new mechanistic insights of mechanical-inflammation coupling to improve MSC mechanosensing and adhesion, potentially benefiting MSC engraftment and its clinical therapy for MI.