New insights into cellular α-synuclein homeostasis in health and disease

α-Synuclein (αSyn) is a highly abundant neuronal protein whose exact structure and function are under debate. Misfolding and aggregation of this normally soluble, 140-residue polypeptide underlies a group of neurodegenerative disorders called synucleinopathies, including Parkinson's disease (PD) and dementia with Lewy bodies (DLB). The αSyn field has focused increasing attention on the hypotheses that certain aggregates of αSyn may be directly toxic to the neurons in which they arise and/or that aggregates can be released from some neurons and diffuse by undefined mechanisms to other neurons to seed αSyn in the recipient cells, thus propagating neuropathology by a non-cell autonomous process ('pathogenic spread'). While intense interest in these hypotheses has led to new approaches and tools to model aspects of the disorders, it is important to analyze which molecular events initiate αSyn aggregation inside neurons in the first place. Here, we review new insights into how neuronal αSyn homeostasis may be maintained under physiological conditions but perturbed by pathological factors.