Unconventional Protein Secretion in the Central Nervous System: Mechanisms and Roles in Physiology and Disease.

Extracellular secretion of neurotransmitters, proteins, and peptides by cells of the central nervous system underpins neurological function and homeostasis. Decades of elegant research have illuminated the molecular mechanisms and machinery that support the release of neurotransmitters via synaptic vesicle exocytosis, as well as the secretion of signal-peptide bearing proteins through the endoplasmic reticulum (ER)-Golgi based secretory pathways. However, it is now increasingly appreciated that signal-peptide lacking "leaderless" proteins can also be secreted via ER-Golgi-independent mechanisms collectively termed unconventional protein secretion (UcPS). In this review, we highlight the physiological and pathological consequences of UcPS in the central nervous system. UcPS supports the secretion of aggregation-prone proteins such as α-synuclein and mutant huntingtin, pro-inflammatory mediators including interleukin-1β and high mobility group box protein 1, and neuroprotective or angiogenic factors such as fibroblast growth factor 2. Furthermore, several retroelement-derived proteins, encoded by ancient genomic elements with structural homology to retroviruses, are also secreted via unconventional pathways, and are thought to regulate essential CNS processes such as synaptic plasticity. These diverse cargoes underscore the functional range of UcPS in neuronal and glial biology. We summarize current understanding of the major UcPS pathways used by CNS cells. These mechanisms include plasma-membrane pore-mediated release facilitated by proteins such as gasdermin-D, as well as vesicular routes in which UcPS cargoes enter organelles of the autophagic and endolysosomal systems that subsequently fuse with the plasma membrane to enable extracellular release. Finally, we discuss key unresolved questionRecent evidence from HeLa cells suggests regarding the regulation of UcPS, including the molecular features that target select leaderless cargoes toward UcPS, how the balance between conventional secretion and UcPS shifts under cellular stress, and the current understanding of the diverse molecular machinery that mediates the vesicular form of UcPS.