Dual Genetic Trigger for Parkinson's Neurodegeneration Identified in Fruit Fly Study
Parkinson's disease (PD), the second most common neurodegenerative disorder globally, affects over 10 million individuals. It is characterized by symptoms such as tremors, limb stiffness, and issues with gait and balance, which stem from the progressive death of specific brain cells. While some genetic risk factors are recognized, the exact reasons why certain individuals with these predispositions develop PD while others do not have been unclear.
Uncovering a Key Genetic Combination
A research team from Baylor College of Medicine, the Duncan Neurological Research Institute (Duncan NRI) at Texas Children's Hospital, and collaborating institutions conducted a pivotal study using fruit flies. Their findings, published in the journal Molecular Neurodegeneration, identified that two mutant genes are necessary to trigger neurodegeneration.
Flies possessing only one mutant copy of Gba1b, a known genetic risk factor for PD, did not exhibit neurological issues. However, neurodegeneration manifested when flies lacked one copy of Gba1b and one copy of anne, the fruit fly equivalent of the human gene ATP13A2. Significantly, the researchers also identified multiple human PD patients who carried variants in both ATP13A2 and GBA1.
The Search for a Second Factor
Dr. Hugo Bellen, a corresponding author on the study, highlighted a long-standing mystery:
"Human studies have indicated a five-fold increased risk of PD for individuals carrying one mutant copy of the GBA1 gene, but this does not always result in the condition's development, suggesting an additional contributing factor."
The research team focused their search for this second factor among genes linked to lysosomes, which are cellular structures responsible for breaking down and recycling cellular material. Many known PD risk genes, including GBA1, are associated with lysosome function.
A Progressive Decline in Cells
The study revealed that the presence of one mutant copy of Gba1b and one mutant copy of anne initiated a slow, progressive neurodegeneration in fruit flies. This led to movement difficulties, neuronal loss, and impaired communication between neurons and glial cells.
Further investigation into the neurodegenerative process indicated that Gba1b primarily affects glial cells, which provide support to neurons, while anne mainly impacts the neurons, which transmit electrical signals.
The earliest signs of damage were observed in glial cells. These support cells began to swell, detach from neighboring neurons, and show distress, which was linked to the accumulation of a fat molecule called glucosylceramide (GlcCer) within glial lysosomes.
In flies also carrying a mutant anne gene, lysosomes within neurons failed to maintain proper acidity. This resulted in neurons producing an excess of GlcCer, which was subsequently transferred to glial cells in quantities exceeding their processing capacity.
The buildup of waste materials caused swelling and structural damage in glial cells.
Without healthy glial support, neurons, particularly those involved in vision and movement, eventually deteriorated, leading to symptoms akin to early Parkinson's disease in the flies.
Promising Pathways for Intervention
The research also identified methods to potentially mitigate this damage. Treatment with ML SA1, a drug that improves lysosome function, helped restore healthier lysosome activity. Additionally, myriocin, a compound that reduces GlcCer production, lowered the toxic accumulation.
While these treatments do not represent an immediate cure for Parkinson's, they highlight promising biological pathways that could be investigated for future therapeutic interventions.