Researchers have identified a Parkinson’s disease-linked mutation that impacts the endocytic protein Endophilin A1, which is involved in autophagy.
In a recent study, published in Neuron, researchers investigated the process of autophagy in Drosophila and showed that the endocytic protein Endophilin-A modulates autophagy at synapses and thus mediates neuronal survival.
Autophagy is a crucial cellular process by which cells degrade and “recycle” waste components. This process is particularly important in neurons. In neurodegenerative diseases such as Parkinson’s, toxic debris builds up and causes neurons to die. Adekunle Bademosi, first author of the study, explains, “Autophagy is a waste disposal and recycling machinery present in every neuron of the brain. As neurons of the brain function, their components get worn out and need to be degraded and also replaced, to avoid the build-up of aggregates – debris. Autophagy is the cells way of recognising the damaged parts, then engulfing them in a double-membraned organelle, which later fuses with the highly acidic structure called lysosomes that helps to destroy the debris.”
Most research and current Parkinson’s disease treatments focus on clearing out the debris, and replacing the dopamine loss that occurs when too many neurons die. The research team from The Queensland Brain Institute and Flanders Institute of Biotechnology decided to instead focus on investigating how autophagy becomes disrupted in Parkinson’s, to elucidate the underlying mechanisms that lead to the build-up of toxic debris in the brain.
Investigating the role of Endophilin A
Bademosi sheds some light on why they decided to focus on the Endophilin A protein in their study, telling us, “We had previously shown that the phosphorylation of EndophilinA was involved in the onset of autophagy especially upon amino acid deprivation. What we also observed but didn’t understand at the time was that increase in neuronal transmission also induced autophagy. This study set out to investigate how neuronal communication (specifically Ca2+ influx during neuronal transmission) is linked with autophagy, and whether EndoA was the link.”
“We knew that in its traditional known role, EndoA functions at the synapse in endocytosis, by inducing the curvature of the plasma membrane. So, we wondered whether: 1.) Autophagy which requires highly curved membranes (of structures called autophagosomes) occurred within synapses, and 2.) Whether EndoA was involved in inducing the curvature of membranes needed to form the autophagosomes in the first place.”
The findings from the study reveal that the calcium (Ca2+) influx induced by neuronal stimulation is crucial in the formation of autophagosomes at pre-synaptic terminals, and it is essential for neuronal survival. “Electrical signals travel from the cell body down the axons, to the synapses,” Bademosi explained, saying “When the electrical signals arrive at the synapse, it triggers certain gates (sensitive to electrical signals) to open and they allow the influx of Ca2+ ions into the synapse. The Ca2+ ions within the synapse induce a cascade of evens that ultimately make vesicles (containing chemical signals) to fuse to the plasma membrane and release their content, neurotransmitters, into the small space between the synapse and the next neuron.”
ENDOA1 mediates this process by controlling nanoscale organization at Drosophila synapses in a calcium-dependent manner. When the neurons are in their resting state, ENDOA1 is in the periphery, promoting synaptic vesicle endocytosis. However, during stimulation and Ca2+ influx, it relocalizes to the synapse lumen to facilitate autophagosome formation. The Parkinson’s disease risk variant G276V in the disordered loop region of the human ENDOA1 protein blocks this process.
“The risk mutation in SH3 GL2 (the human EndoA gene) had been identified in previous GWAS studies [researching Parkinson’s Disease]” said Bademosi, elaborating: “How this variant induced Parkinson’s disease was unknown. We first established a link between Ca2+ influx during neurotransmission and EndoA in autophagy. We showed that mutations in a negatively charged region of EndoA (amino acid 264 in humans and 265 in Drosophila), made it insensitive to Ca2+ and depending on the mutation, it made EndoA behave in a way as though Ca2+ was absent or present within the synapse.”
“What is interesting is that the Parkinson risk variant (at amino acid 276) on EndoA is close to this Ca2+ ‘responding’ region and it was similarly unresponsive to Ca2+ and led to a dampening of autophagy as we saw in one of the EndoA ‘Ca2+’ mutants. Dampened autophagy would ultimately led to build up debris.”
Shifting the focus of Parkinson’s Disease research
This research could shift the focus of Parkinson’s disease treatment from primarily managing symptoms to targeting the underlying cause of the disease, namely the build-up of toxic debris in the brain. By investigating how autophagy becomes disrupted in Parkinson’s and how the endocytic protein Endophilin A1 modulates autophagy at synapses, the researchers hope to develop new therapies that can clear out or prevent the build-up of debris in the brain, rather than simply replacing dopamine.
“The sole treatment for Parkinson’s at the moment uses L-DOPA”, said Bademosi, explaining, “Ultimately in Parkinson’s a specific type of neuron is destroyed by the toxicity of the debris. They are called dopaminergic neurons, because they release dopamine. So, the available treatment helps to supplement the brains supply of dopamine (L-DOPA). Here in lies the reason for our study. L-DOPA does not do anything to the debris that causes the neurons to die. L-DOPA simply replaces the decreased dopamine due to the loss of those neurons. So, it is more of a management drug, rather that curative therapy. We need to focus on clearing the debris or even preventing the build-up of the debris, rather than simply replacing dopamine.”
Future research is required in other animal models of Parkinson’s to further investigate the role of the endocytic protein Endophilin A1 in autophagy and its potential as a target for developing new therapies that can clear out or prevent the build-up of toxic debris in the brain. Bademosi elaborates on this, saying, “Ultimately we need to invest in researching more on compounds that modulate autophagy. There are several of these already available. We need to investigate them more in animal models of Parkinson’s and see how they can precisely help to clear/prevent Parkinson’s associated debris – Lewy Bodies, or aggregates of the protein – alpha-synuclein.”
Moreover, this research is applicable not just to Parkinson’s, but all neurodegenerative diseases. As Bademosi explains: “The build-up of debris is a common theme in almost all neurodegenerative diseases. So, providing clear answers in one disease, will provide the right platform for the whole field.”