A ‘tache noirâtre’ (in English, blackish spot): this is the expression that the French scientist Fèlix Vicq d'Azyr (1748-1794) coined to describe a set of black-colored neurons located deep in the human brain, which he drew in two of the thirty-five anatomical plates that make up his Treatise on Anatomy and Physiology, with Colored Plates Representing the Various Organs of Man in Their Natural State, published in 1786. The anatomy books that followed in the years, and then centuries, would rename the blackish spot drawn by Vicq d'Azyr as substantia nigra, from the Latin, black substance.

Still today, we call the substantia nigra the region of the brain whose neurons release the neurotransmitter dopamine (so-called, dopaminergic neurons) within the circuits that regulate motor control. These dopaminergic neurons are the first to degenerate in Parkinson's disease, a neurodegenerative pathology that affects at least 8.5 million people in the world and for which, unfortunately, there are still no neuroprotective treatments. “Today we still do not fully understand why it is specifically the dopaminergic neurons of the substantia nigra that die in Parkinson's disease. Over the years, this pathology has revealed itself to be a complex condition, which manifests not only through the alterations in movement such as tremors, slowness, and muscle rigidity, but also through anosmia (loss of the sense of smell), sleep disorders and disorders of the sensory sphere and cognition”, Dr. Vania Broccoli tells us, Director of Research at the Institute of Neuroscience of the CNR and Head of the Stem Cells and Neurogenesis Unit at IRCCS Ospedale San Raffaele, whom we met on the occasion of April 11, World Parkinson's Disease Day.

 

Parkinson's Disease and Alpha-Synuclein Accumulation

A brain affected by Parkinson's disease is characterized by widespread accumulations of the protein alpha-synuclein, which cause the selective death of dopaminergic neurons in the substantia nigra and, consequently, the onset of motor symptoms. In later stages of the disease, these alpha-synuclein aggregates impair the function of other types of neurons in other regions of the brain as well, resulting in cognitive and behavioral symptoms, such as depression and apathy.

Alpha-synuclein is normally present in the synapses. Throughout the individual’s entire existence, multiple alpha-synuclein molecules can aggregate together, forming insoluble and toxic accumulations within neurons. “We have defined the molecular structure of these aggregates and how they become toxic, but it remains unclear why dopamine neurons in the substantia nigra are the most vulnerable to their accumulation,” comments Dr. Broccoli.

Neurons try to get rid of these accumulations of alpha-synuclein, first through degradation mechanisms happening within the cell itself. However, at a certain point these mechanisms are no longer sufficient to remove these protein aggregates. “The neurons then begin to release them into  the extracellular environment, still in the attempt to get rid of them,” adds Dr. Broccoli. “Once outside, however, these alpha-synuclein aggregates are internalized by neighboring neurons, glial cells, and immune microglia.”

 

Parkinson’s disease and inflammatory response

Microglia cells in the brain, activated by the toxic alpha-synuclein aggregates, trigger an inflammatory response and  engulf these protein deposits to protect neurons.

“We have demonstrated that the continuous accumulation of alpha-synuclein, however, exhausts the capacity of microglial cells to engulf neurotoxic aggregates. These functionally exhausted microglia recall T-cells, which enter the brain from the blood, triggering a response which in turn might amplify and accelerate neuronal degeneration and death”, explains Dr. Broccoli.

 

Parkinson’s disease and inflammatory response: the gut-brain axis

Among the most relevant discoveries of the last few years, it has been observed that alpha-synuclein aggregates can deposit both in the brain and in peripheral organs, such as the heart and the gut. “There are several epidemiological studies indicating that patients who have suffered from clinical inflammatory bowel disease are at increased risk of developing Parkinson’s disease in later years,” comments Dr. Broccoli.

A new classification of the disease is based on the initial site of alpha-synuclein deposition and inflammation, thus distinguishing 2 specific forms:

1. The “brain-first” form of Parkinson’s, in which the accumulation of alpha-synuclein and the consequent inflammatory response take place first and directly in the brain. Subsequently, the inflammation would spread to other areas in the body.
2. The “body-first” form of Parkinson’s, in which inflammation of the gut (and other peripheral organs) would precede inflammation in the brain and would favor the formation of alpha-synuclein aggregates at the intestinal level. Through peripheral nerves, these intestinal protein deposits would spread to the brain, where they can trigger the inflammatory response and increase the likelihood of neuron degeneration.

 “Specific diets, which are for example rich in fiber, vitamins and polyphenols, are being tested to alleviate intestinal inflammation and all the complications associated with it, and to test whether they can have an impact in preventing or slowing down the progression of Parkinson’s disease,” explains Dr. Broccoli. “However, these measures alone do not resolve the disease, especially once it has already manifested.”

 

Parkinson’s disease and new therapeutic strategies

Today, there are no neuroprotective treatments for Parkinson’s disease; there are only drugs as well as surgical or rehabilitation therapies that can alleviate motor and cognitive symptoms without solving the pathological mechanisms underlying the disease.

A possible therapeutical strategy, currently under investigation in several clinical trials, consists of transplanting new dopaminergic neurons differentiated from stem cells into the brains of patients. These transplanted neurons, once integrated into the brain circuits, would replace those lost in the disease, supplying the brain with the dopamine needed to regulate movement. “These are important and innovative experiments, which however do not resolve the mechanisms underlying the pathology. In fact, the new neurons are transplanted into an environment still characterized by chronic accumulation of alpha-synuclein and inflammation. Therefore, since these cell transplant therapies do not resolve the accumulation of alpha-synuclein and inflammation, the transplanted neurons are also destined to die in the long term,” explains Dr. Broccoli.

Recent discoveries on the inflammatory mechanisms underlying Parkinson's disease open new research perspectives for therapies based on immunomodulation – that is, the modulation of the immune response both at the systemic and brain level. "For example, a recent clinical study is testing the systemic use of immunosuppressive drugs, such as those approved for autoimmune diseases like rheumatoid arthritis, also in the treatment of Parkinson's disease," continues Dr. Broccoli. However, these immunomodulatory drugs that are administered systemically have a reduced ability to act directly on the brain and may have greater side effects, due to the need to be taken chronically by patients.

For this reason, Dr. Broccoli’s research group is testing gene therapy strategies in preclinical models of the disease. In this case, gene therapy uses a “shuttle” to transport the genes encoding for the synthesis of anti-inflammatory and neuroprotective molecules directly into the microglial cells of the substantia nigra. These molecules aim at restoring, in a long-term perspective, the capacity of the microglial cells to phagocytize alpha-synuclein aggregates. In this context, last year Dr. Broccoli’s research group showed how interleukin 10 restores the capacity of microglial cells to engulf alpha-synuclein deposits in pre-clinical models of Parkinson’s disease. “We are now testing other combinations of anti-inflammatory and neuroprotective molecules to be expressed directly in the brain of preclinical models” adds Dr. Broccoli.

 

Parkinson's disease and monoclonal antibodies

The search for new therapies for Parkinson's disease is also moving in the field of monoclonal antibodies, drugs that act by removing the neurotoxic protein aggregates characteristic of various neurodegenerative diseases. Lecanemab, for example, a drug approved by EMA for marketing authorization during the last November 2024 for the treatment of Alzheimer's disease at an early stage, another neurodegenerative disease, characterized by the accumulation of the beta amyloid protein. "There are numerous active clinical studies to test the efficacy of various monoclonal antibodies also in the treatment of Parkinson's disease. In this case, the antibodies are directed against alpha-synuclein proteins and are intended to block their propagation and promote their removal", concludes Doctor Broccoli.