Proteins can become modified in a wide range of ways via addition or removal of various motifs. This is a necessary part of our biochemistry, but some modifications are harmful rather than useful. The pattern of protein modifications present in cells changes with age, and some pathological modifications begin to appear more often. The underlying reasons for these changes are usually poorly understood, at least once stepping beyond the immediate causal chemical reactions, as cellular biochemistry is very complex. As researchers here demonstrate, given a problematic modified protein that exists outside cells, it is possible to target it for removal and thereby produce benefits.
At a molecular level, aging is thought to be underpinned by progressive biomolecular damage caused by degenerative protein modifications (DPMs), including oxidation, deamidation, glycation, and a range of other non-enzymatic structural changes. We now recognize that aging is a consequence of deleterious chemical processes that damage biomolecules and impair the homeostatic functions programmed by our genomes. The functional impact of DPMs depends on the mode of modification and the target molecule involved. For example, deamidation leads to the accumulation of isoaspartate residues that progressively disrupt protein integrity and alter biological activity. However, "gain of function" structural changes caused by DPMs may play equally important roles in human pathology. DPMs greatly increase the diversity of biomolecules present in body tissues, with a high probability of generating proteoforms capable of interacting with or binding to key biomolecules in novel ways.
Indeed, we recently reported that deamidation of the amino acid sequence NGR (Asn-Gly-Arg) in extracellular matrix (ECM) proteins results in "gain-of-function" conformational switching to isoDGR (isoAsp-Gly-Arg) motifs that can bind to integrins and promote immune cell activation. Unlike isoaspartate-modified proteins within cells that can be repaired by the Pcmt1 enzyme, long-lived ECM proteins cannot be repaired by intracellular mechanisms and are thus susceptible to progressive damage over time. Accordingly, age-linked isoDGR modifications have previously been detected in several ECM proteins derived from human carotid plaque tissues, suggesting that these molecules may be capable of enhancing leukocyte binding to the atherosclerotic matrix, thereby accelerating progression of atherosclerosis.
We now report that anti-isoDGR immunotherapy mitigates lifespan reduction of Pcmt1-/- mouse. We observed extensive accumulation of isoDGR and inflammatory cytokine expression in multiple tissues from Pcmt1-/- and naturally aged wild type (WT) animals, which could also be induced via injection of isoDGR-modified plasma proteins or synthetic peptides into young WT animals. However, weekly injection of anti-isoDGR monoclonal antibody was sufficient to significantly reduce isoDGR-protein levels in body tissues, decreased pro-inflammatory cytokine concentrations in blood plasma, improved cognition/coordination metrics, and extended the average lifespan of Pcmt1-/- mice. Mechanistically, isoDGR-mAb mediated immune clearance of damaged isoDGR-proteins via antibody-dependent cellular phagocytosis. These results indicate that immunotherapy targeting age-linked protein damage may represent an effective intervention strategy in a range of human degenerative disorders.