Lewy body pathology refers to the formation of abnormal protein deposits, known as Lewy bodies, within specific nerve cells. First described by neurologist Friederich Lewy in 1912, these deposits are a feature of several progressive neurodegenerative disorders. The accumulation of these protein clumps inside neurons unfolds over many years, disrupting brain function. This process causes cells to malfunction and die, leading to a gradual decline in abilities.
The Role of Alpha-Synuclein Protein
At the heart of Lewy body pathology is a protein called alpha-synuclein. In a healthy brain, alpha-synuclein is abundant at the presynaptic terminals of neurons. Its job involves regulating synaptic vesicles, which release the neurotransmitters that allow brain cells to communicate. For alpha-synuclein to perform correctly, it must be folded into a specific three-dimensional shape.
The problem arises when this protein misfolds. This can be triggered by genetic mutations or the aging process, which can weaken a cell’s quality control systems. Once a protein misfolds, it becomes sticky and prone to clumping with other misfolded alpha-synuclein proteins. This process is like bent paperclips that hook onto each other, forming a tangled, useless mass.
This clumping occurs in stages. Initially, misfolded proteins form small clusters called oligomers, which then assemble into longer, thread-like fibrils. Over time, these fibrils accumulate and combine with other cellular proteins, forming the dense, spherical deposits identified as Lewy bodies. This aggregation is a toxic gain-of-function, meaning the clumped protein acquires harmful properties that disrupt the neuron’s activities.
Distribution Within the Brain
The specific symptoms a person experiences are determined by where in the brain Lewy bodies first form and accumulate. The pathology does not appear everywhere at once but follows distinct patterns of distribution that correspond to different clinical presentations.
In many cases, the pathology begins in the brainstem, which houses centers that control automatic functions and movement. When Lewy bodies accumulate in brainstem nuclei like the substantia nigra, they disrupt the production of dopamine. This neurotransmitter facilitates smooth, coordinated muscle movements, so this pattern leads to the early onset of motor symptoms.
In other instances, Lewy bodies are more widely distributed from the outset, appearing in the cerebral cortex. When the pathology is concentrated in cortical areas, it interferes with cognitive processes, perception, and behavior. This can result in problems with thinking, memory, and attention. The pathology can spread from one brain region to another over time, explaining why symptoms evolve and worsen.
Associated Neurodegenerative Disorders
Lewy body pathology is the underlying cause of a spectrum of neurodegenerative diseases, most notably Parkinson’s disease (PD), Parkinson’s disease dementia (PDD), and dementia with Lewy bodies (DLB). Though they are separate diagnoses, they are considered part of a continuum of Lewy body disorders because they share the same core protein abnormality.
Parkinson’s disease is defined by its motor symptoms, such as tremor, stiffness, and slowness of movement. These symptoms arise because the initial Lewy body formation is concentrated in the brainstem, impacting dopamine-producing neurons. Many individuals with Parkinson’s eventually develop cognitive problems and dementia as the pathology spreads to the cortex, a condition known as Parkinson’s disease dementia.
Dementia with Lewy bodies is diagnosed when cognitive decline is the presenting symptom or appears within one year of motor problems. In DLB, Lewy bodies are more diffusely spread throughout the cerebral cortex early in the disease, leading to issues with attention and executive function. The “one-year rule” is a clinical guideline used to differentiate DLB from PDD. If dementia begins more than a year after motor symptoms are established, the diagnosis is PDD.
How Lewy Bodies Disrupt Cell Function
The presence of Lewy bodies inside neurons disrupts cellular machinery, leading to cell death. One issue is overwhelming the cell’s protein disposal systems, like the ubiquitin-proteasome system, which identifies and breaks down damaged proteins. The volume of aggregated alpha-synuclein can clog this system, preventing the removal of other cellular waste and leading to a toxic buildup.
Another target of Lewy body pathology is the mitochondria, which generate energy for the cell. Alpha-synuclein aggregates interfere with mitochondrial function, particularly with complex I of the electron transport chain. This impairment starves the neuron of energy, generates harmful reactive oxygen species, and can trigger programmed cell death. The mitochondria are often drawn into the developing Lewy body, further compromising their integrity.
Furthermore, these protein clumps disrupt the neuron’s internal transport system. Neurons have long extensions called axons that require a constant supply of molecules transported from the cell body. Lewy bodies can physically obstruct this transport by damaging the microtubule tracks. This interference isolates distant parts of the neuron from vital supplies, leading to a breakdown in communication between brain cells.