The space-time continuum in neurological disorders of the autophagosome-lysosome fusion machinery.

Autophagy is a highly conserved cellular pathway for the degradation and recycling of defective intracellular cargo and plays a vital role in the homeostasis of post-mitotic tissues, particularly the nervous system. Autophagosome-lysosome fusion represents the final critical step in macroautophagy with a tightly regulated process mediated by a complex molecular machinery of tethering vesicles for degradation. Since the first reports of human autophagy disorders, the scientific and clinical focus condensed on severe phenotypes with biallelic-truncating genotypes as monogenic models of near-complete autophagy perturbation. Recent reports suggest a much wider disease spectrum with defective autophagy, ranging from neurodevelopmental disorders to neurodegenerative phenotypes with later manifestation due to "milder" genotypes, including Alzheimer's disease (AD), Parkinson's disease (PD), and Amyotrophic Lateral Sclerosis-Frontotemporal Dementia (ALS-FTD). In addition, recent evidence identified molecular connections between physiological autophagy regulation during normal aging and pathophysiological hallmarks of aging-related disorders. These translational observations led to a more comprehensive understanding of autophagy at health and disease, in particular: 1) genetic location and allelism of pathogenic variants ("genomic space"); 2) protein-protein interaction in functional protein complexes ("proteomic space"); 3) metabolic autophagic flux with positive and negative regulators ("metabolomic space"); 4) age-related phenotypic progression over time. Here, we review the autophagosome-lysosome fusion machinery as a key structure both on the molecular level and with regards to the pathogenesis of the autophagy-related disease spectrum. We highlight the clinicopathological signature of disorders in the autophagosome-lysosome fusion machinery, in particular features warranting awareness from clinicians and geneticists to inform adequate diagnosis, surveillance, and patient guidance.