Alzheimer's disease (AD), Parkinson's disease (PD), and Amyotrophic Lateral Sclerosis (ALS) are neurodegenerative diseases characterized by dysfunction of the endosomal-lysosomal system (ELS). Four shared neurodegenerative mechanisms across ALS, PD and AD are regulated by the ELS, namely proteostasis and related protein misfolding, mitochondrial function, neurotransmission and neuroinflammation. These mechanisms are interconnected, contributing to neurodegeneration in a "snowball" manner. The retromer, a multimeric, evolutionarily conserved protein complex involved in intracellular protein trafficking, is at the crossroad of these neurodegenerative processes. This narrative review focuses on exploring the retromer structure, function as a master regulator of the ELS, and how this impacts proteostasis, mitochondrial biogenesis and homeostasis, neurotransmission and neuroinflammation across neurodegenerative diseases. We explore how alterations in retromer function can play an important role in neurodegeneration and discuss the impact of genetic and pharmacological manipulations of VPS35, one of the main retromer subunits. In vitro and in vivo studies have identified that the retromer enhances the activity of protein degradation pathways via the ELS, namely macroautophagy, chaperone-mediated autophagy, and the ELS itself, with concomitant reduction in misfolded protein levels. Also, in some model systems, when the retromer role is enhanced or restored, mitochondrial function is rescued, dysfunctional neurotransmission is restored, and the damaging effects of neuroinflammation are dampened. Lastly, we highlight the role of a novel pharmacological class of agents that enhance retromer function as a strategy for potentially slowing the progression of these neurodegenerative diseases in in vivo models of PD and ALS. We discuss the challenges in targeting the retromer, current limitations and potential off-target effects of retromer enhancement. Overall, the retromer regulates shared mechanisms across neurodegenerative diseases and retromer enhancers could represent a novel disease-modifying strategy in AD, PD and ALS.
