A neuronal‐specific surface‐exposed membrane proteasome complex modulates neuronal signaling through extracellular signaling peptides

Abstract

Proteasome‐dependent proteolysis is a tightly regulated process that is essential for life in all archaeal and eukaryotic cells. In the nervous system, proteasome function regulates a wide variety of cellular processes that are critical for proper neuronal function. Largely, the roles of proteasomes in the nervous system have been attributed to modulating the levels of proteins critical for particular cellular processes. However, certain rapidly‐occurring processes such as neuronal transmission, vesicle recycling, and calcium signaling are also affected by acute inhibition of proteasome function. It remains unclear how proteasomes can acutely regulate a cellular process within seconds to minutes, as this remains inconsistent with their canonical role in proteostasis. Here, we made the discovery of a nervous system‐specific membrane proteasome complex that directly and rapidly modulates neuronal function by degrading intracellular proteins into extracellular peptides that can stimulate neuronal signaling. This proteasome complex is tightly associated with the neuronal plasma membrane, exposed to the extracellular space, and is catalytically active. We have determined this complex largely to be made up of 20S proteasome subunits, with little to no 19S cap. Analysis of this membrane proteasome complex by mass spectrometry revealed nervous‐system specific auxiliary membrane glycoproteins which were sufficient for membrane proteasome expression. Selective inhibition of this membrane proteasome complex by a cell‐impermeable proteasome inhibitor blocked extracellular peptide production and attenuated neuronal activity‐induced calcium signaling within seconds. Moreover, membrane proteasome‐derived peptides are sufficient to robustly and rapidly induce neuronal calcium signaling, in part by acting through NMDA receptors. The nature of proteasomal membrane attachment, association with auxiliary transmembrane proteins, exposure to the extracellular space, and capability to signal across the membrane are characteristic of a transmembrane complex. Our discoveries challenge the prevailing notion that proteasomes primarily function to maintain protein homeostasis, and highlight the existence of a form of rapid neuronal communication through a novel transmembrane proteasome complex.Support or Funding InformationThis work was funded by institutional funding and the following grants to S.S.M. (R01 MH102364‐02). K.V.R. was supported by a training grant T32 GM007445 and NSF Graduate Research Fellowship DGE‐1232825.