In the course of our project, we discovered a striking and altogether unexpected interaction of aSyn with a certain cellular organelle in a few selected cell types. These aSyn-lipid droplet (LD) interactions were strongly dependent on cellular identity, individual metabolic cell states and vastly exacerbated under cellular stress. Upon further investigating aSyn-LD interactions, we realised that binding to this organelle was prominently observed in all the cell types that we were investigating under appropriate metabolic conditions, including in iPCS-derived, human dopaminergic neurons. Eventually, we identified the molecular determinant that drove aSyn-LD binding and delineated that aSyn acted as a cholesteryl-ester (CE) sensor on LDs. We further showed that enhanced cholesterol uptake in neuronal and non-neuronal cells led to the accumulation of CE-rich LDs, which were efficiently bound by cellular aSyn.
Moreover, LD interactions led to the formation of membrane-induced aSyn oligomers, although without the characteristics of mature amyloid fibrils and clearly reversible in their abilities to disassemble and reassemble. Given the striking uniqueness of this behaviour and its implicative relevance for Parkinson's disease as a possible scenario that may 'wear out' over time and turn irreversible and toxic, we focused on aSyn-LD binding as a possible route to cellular toxicity in PD. This decision was further supported by cumulating evidence in the wider PD community that dysfunctional lipid homeostasis forms the basis of many aspects of PD pathology and that cellular aSyn lipid and membrane interactions may hold the key to its true biological function, which is still unknown. Tantalised by the prospects of being able to add a possible 'loss-of-physiological-function' angle to the widespread 'gain-of-toxic-function' aspect of aSyn's role in PD development, we embarked on deciphering the structural and functional details of aSyn-LD interactions in various cell types.
aSyn-LD interactions proved invaluably rich in providing numerous insights into the possible biological function(s) of aSyn (too many to summarise here) and they also formed the basis for new structural approaches at the interface of protein and membrane biology. Given the vastly different dynamic properties of soluble versus membrane-bound proteins, we faced the challenge to experimentally bridge this gap through the integrated use of different tools and methods. While we are making good progress in delineating the first experimentally derived, atomic-resolution model of aSyn bound to a truly native, cellular membrane compartment, we face persisting challenges to resolve the oligomeric state(s) of aSyn on LDs, both in terms of compositions of assemblies and their resulting architectures. Because these difficulties are technical in nature, we hope to resolve them within the near future.
Overall, NeuroInCellNMR provided us with a wealth of new information about physiological and pathological aspects about aSyn biology, with clear implications for strategies of novel therapeutic interventions. Given that the bulk of this work is still unpublished and, hence, not disseminated amongst the wider PD community, we are eagerly awaiting the general response to our findings and what colleagues will draw from our conclusions. Having worked in this field for many years, I am confident that our results will stir considerable excitement and offer an altogether new angle on a broadly relevant and utterly debilitating human disease.
