New Research Suggests Kidney Function Linked to Parkinson’s Disease
A recent study in Nature Neuroscience has revealed that the onset of Parkinson’s disease and similar conditions might not begin in the brain, but rather in the kidneys. Researchers discovered that a significant protein, alpha-synuclein, which plays a role in these diseases, can accumulate in the kidneys and then travel to the brain through nerve pathways, particularly when kidney function is compromised. This finding implies that chronic kidney disease could increase the risk of developing Parkinson’s due to the accumulation and spreading of toxic proteins into the central nervous system.
Parkinson’s disease is a progressive neurological condition that impacts movement and coordination, along with various non-motor functions. Some of the most identifiable symptoms include tremors, muscle stiffness, slower movements, and balance issues. These arise mainly due to the loss of dopamine-producing neurons in the substantia nigra, a specific area of the brain. Dopamine, a key chemical messenger for motor control, is diminished, leading to the characteristic motor difficulties associated with the disease.
However, Parkinson’s is not solely about motor symptoms. Patients frequently experience mood disorders, cognitive decline, sleep problems, and digestive issues. Interestingly, many non-motor symptoms can appear years before the more recognized movement difficulties arise, suggesting that the disease process may initiate outside the brain. Researchers have increasingly studied alpha-synuclein, a protein typically found in neurons that can misfold and aggregate into toxic clumps. These aggregates form structures known as Lewy bodies, which are present in the brains of individuals with Parkinson’s and related disorders.
The concept that misfolded alpha-synuclein could move from peripheral organs to the brain has gained traction recently. For instance, research has shown that injecting these toxic aggregates into the guts of animals can lead to changes in the brain and eventual movement impairments. This study extends that idea to focus on the kidneys as a potentially overlooked starting point for the disease, especially in those with compromised kidney function.
The research team utilized both human tissue samples and animal testing. They evaluated kidney samples from individuals with Parkinson’s or related diseases, as well as from patients with end-stage kidney disease who weren’t diagnosed with any brain disorders. They also used genetically modified mice and normal mice to observe alpha-synuclein’s behavior under different circumstances. Surgical techniques and viral tracing were employed to map connections between the kidneys and the brain.
In their analysis, the researchers found misfolded and phosphorylated alpha-synuclein present in the kidneys of 10 out of 11 subjects with Parkinson’s or dementia linked to Lewy bodies. This abnormal protein was primarily found near nerve fibers close to small blood vessels. Remarkably, similar deposits were identified in the kidneys of 17 out of 20 patients who had chronic kidney disease but showed no signs of Parkinson’s or other neurological issues during their lives. In fact, some of these individuals exhibited early-stage alpha-synuclein pathology in the spinal cord and other brain areas associated with Parkinson’s, suggesting that kidney disease may quietly set the framework for future brain involvement.
In mice, the researchers demonstrated that the kidneys are actively involved in clearing alpha-synuclein from the bloodstream. When alpha-synuclein was injected into healthy mice, it initially accumulated in the kidneys before being cleared efficiently from the body. In contrast, when injected into mice with kidney failure, the protein lingered longer in the blood and accumulated within the kidneys. This impaired clearance was also noted in experiments with rabbits and tests using human kidney tissue. They found that enzymes in the kidneys, known as cathepsins, are essential for breaking down alpha-synuclein, and their effectiveness diminishes when kidney function is damaged.
The accumulation of alpha-synuclein in the kidneys produced hazardous outcomes. When the researchers injected toxic alpha-synuclein fibrils into the bloodstream of mice with kidney failure, they observed that the protein spread into the brain and spinal cord, resulting in Parkinson’s-like pathology in brain regions associated with movement and memory. These mice displayed loss of dopamine-producing neurons and various motor issues, such as poor balance. Notably, healthy mice that received the same protein showed none of these symptoms.
To further explore how the protein migrated from the kidneys to the brain, the team directly injected alpha-synuclein fibrils into the kidneys of genetically modified mice. Over several months, they tracked the protein moving along known nerve pathways into the spinal cord and various brain areas. By using viral tracers and surgical methods to disrupt these pathways, they confirmed that intact nerve connections are crucial for the kidney-to-brain spread. Mice who had their kidney nerves severed did not exhibit brain pathology, even after direct injections of toxic alpha-synuclein.
The researchers also examined whether blood cells contributed to the issue. Since alpha-synuclein is mainly found in red blood cells and kidney disease often leads to fragile or damaged red cells, they aimed to see if eliminating blood-derived alpha-synuclein could prevent disease progression. They performed bone marrow transplants, replacing the blood cells of genetically modified mice with cells from alpha-synuclein knockout mice.
The outcome was a notable decrease in alpha-synuclein levels in the blood and a significant reduction in brain pathology. Those mice also maintained more dopamine neurons and exhibited fewer motor symptoms. However, this protective effect only occurred when the mice were not exposed to external sources of toxic alpha-synuclein. Once the fibrils were reintroduced, the disease process resumed, even in mice without blood-derived alpha-synuclein.
The findings indicate that the kidneys could serve as a critical gateway organ in the development of Lewy body diseases, such as Parkinson’s. When kidneys function correctly, they clear alpha-synuclein from the bloodstream before it can cause harm. However, if kidney function is compromised, the protein can accumulate, settle in the kidneys, and traverse the brain through nerve pathways. This might explain why individuals with chronic kidney disease have an elevated risk of developing Parkinson’s.
Nonetheless, there are limitations to this study. While a strong correlation was established between kidney dysfunction and the spreading of alpha-synuclein, the role of this pathway in all Parkinson’s cases remains uncertain. Not everyone with kidney disease goes on to develop Parkinson’s, and not every case of Parkinson’s starts with peripheral involvement. Plus, the complexities of human biology mean that more research is needed to comprehend how the kidney-brain pathway operates over time in humans and to identify other potential routes for protein spread.
Future investigations might investigate whether manipulating peripheral alpha-synuclein could help prevent or delay Parkinson’s disease. For instance, medications that enhance the kidney’s ability to clear this protein or treatments that obstruct its nerve transmission might present new therapeutic options. Although methods like kidney denervation or bone marrow transplants are unlikely to be clinically practical, antibody-based therapies targeting circulating alpha-synuclein could be a more viable approach.
