Immunoproteasome remodeling in senescing human macrophages reveals the loss of PA28αβ capping as a hallmark of immunosenescence.

Abstract

Aging negatively impacts proteasome activity and/or content, and this impairment contributes to disrupted protein homeostasis and cellular dysfunction. However, little is known about proteasome complex dynamics during aging, particularly in the context of immunosenescence. Indeed, only limited data are available on the immunoproteasome, a specialized variant expressed in immune cells. We establish an in vitro model of monocyte-derived human macrophages that develop a senescence-like phenotype upon long-term culture. Our data demonstrate that immunoproteasome complexes undergo deep structural and functional alterations, with the downregulation of immunosubunit expression at the mRNA and protein level, uncapping of the 20S catalytic particle by the PA28αβ regulator, and loss of activity. Immunosubunits are partly replaced by their constitutive counterparts with a shift toward the building of 19S-capped 20S complexes to maintain proteostasis. Similar proteasome dynamics are found in the lymph nodes of aged C57BL/6 and BTBR mice, the latter of which have a naturally activated immune system. Overall, these findings propose long-term cultures of human monocyte-derived macrophages as a model to study macrophage senescence. They also provide a molecular rationale for immunoproteasome dysfunction with remodeling of the proteasome, indicating that the loss of the PA28αβ regulator is a critical event and a hallmark of immunosenescence.