Neuroimmunomodulatory and Neuroprotective Effects of the Flavonoid Apigenin in vitro Models of Neuroinflammation Associated With Alzheimer’s Disease

In the study Neuroimmunomodulatory and Neuroprotective Effects of the Flavonoid Apigenin in In Vitro Models of Neuroinflammation Associated with Alzheimer’s Disease, Dourado et al. explore the potential of apigenin, a plant-based flavonoid, to reduce inflammation and protect neurons in the context of Alzheimer's Disease (AD). The authors focus on neuroinflammation, a key factor in AD progression, and investigate how apigenin can modulate immune responses in the brain, particularly by preventing the overactivation of microglia and astrocytes. Using in vitro models that simulate Alzheimer's conditions, the study reveals that apigenin not only protects neurons from inflammatory damage but also decreases the production of harmful inflammatory molecules, offering a promising avenue for future treatments of neurodegenerative diseases.

Alzheimer's Disease (AD) is a neurodegenerative disorder that affects millions of people worldwide. It leads to memory loss, cognitive decline, and, eventually, the inability to perform basic tasks. One of the key features of AD is chronic inflammation in the brain, which worsens the condition over time. This type of inflammation is primarily driven by overactive brain immune cells, like microglia and astrocytes, which release harmful chemicals that damage neurons.

Apigenin, a plant-based flavonoid found in fruits and vegetables like parsley, celery, and chamomile, has shown promise in fighting inflammation and protecting brain cells. This study dives into the potential of apigenin to protect neurons from the damaging effects of neuroinflammation, which is a hallmark of Alzheimer's and similar neurodegenerative diseases. According to the research, "apigenin acts as a potent modulator of microglial activation, protecting neurons and reducing the harmful effects of neuroinflammatory stimuli."

To assess the effects of apigenin, the researchers used co-cultures of neurons and glial cells. These cells were extracted from the brains of neonatal and embryonic Wistar rats. The neurons and glial cells were grown together in a controlled environment to mimic the brain's natural interactions. Glial cells, which include astrocytes and microglia, are crucial for supporting neurons and responding to brain injury. In this study, the researchers exposed these co-cultures to different inflammatory triggers to create conditions similar to Alzheimer's Disease.

The inflammatory stimuli used were:

1. Lipopolysaccharide (LPS): A toxin from bacteria that is widely known to cause inflammation.
2. Interleukin-1 beta (IL-1β): A protein produced by immune cells that contributes to inflammation, especially in the brain.
3. Beta-amyloid oligomers (Aβ): Protein fragments that form clumps in the brains of Alzheimer's patients, leading to neuron death.

After applying these inflammatory triggers, the cells were treated with apigenin (at a concentration of 1 µM) for 24 hours. Several techniques were used to measure the impact of apigenin on the brain cells. For instance, immunocytochemistry was used to look at changes in cell structure, and RT-qPCR (a method of measuring gene expression) helped track how apigenin influenced the levels of key inflammatory and neuroprotective molecules.

The study produced several important results that demonstrate apigenin's potential to protect brain cells and reduce inflammation. The findings provide new insights into how natural compounds like apigenin could be used to treat neurodegenerative diseases.

Neurons exposed to LPS, IL-1β, and Aβ oligomers showed significant signs of damage, including the loss of their long, branch-like structures (neurites) and an increase in apoptosis (cell death). However, when apigenin was applied, the neurons retained their structure, and fewer cells showed signs of apoptosis. The researchers observed that "apigenin preserved neurons and astrocytes integrity, determined by staining methods that highlight their structural markers."

Specifically, apigenin decreased the activation of caspase-3, a key enzyme involved in cell death, which was elevated after the inflammatory triggers. The Fluoro-Jade-B assay confirmed that apigenin protected neurons from degeneration, showing fewer degenerating cells in apigenin-treated cultures compared to untreated ones.

Microglia, the brain's immune cells, usually take on an active role during inflammation, which can be harmful when this activation goes on for too long. In this study, inflammatory triggers caused the microglia to switch to an activated ameboid shape, which is a sign of ongoing inflammation. After apigenin treatment, microglia returned to their resting state, taking on a branched shape that indicates lower activity. "Apigenin reduced microglial activation, as shown by the decrease in Iba-1+ and CD68+ microglia markers," noted the researchers.

Additionally, apigenin decreased the expression of CD68, a marker associated with microglia's harmful M1 inflammatory state. The reduction of CD68-positive cells highlights apigenin's ability to switch the microglia from a pro-inflammatory state to a more protective, anti-inflammatory form.

One of the significant issues in Alzheimer's is the overproduction of inflammatory molecules, such as IL-6 and IL-1β, which worsen neuron damage. After the cells were exposed to IL-1β, the levels of these pro-inflammatory cytokines rose dramatically. However, apigenin treatment reduced their expression significantly. The study also showed that apigenin increased the expression of brain-derived neurotrophic factor (BDNF), which is crucial for neuron survival and growth. According to the research, "apigenin increased BDNF mRNA levels, helping neurons survive the inflammatory conditions caused by IL-1β."

The treatment also decreased the expression of other inflammatory molecules like OX42 and gp130, which are involved in triggering and maintaining inflammation in the brain. This suggests that apigenin works not only to protect neurons but also to tone down the overall inflammatory environment.

The results from this research indicate that apigenin has strong potential as a natural treatment for Alzheimer's Disease and possibly other neurodegenerative diseases characterized by inflammation. Here's why:

1. Neuroprotection: Apigenin can prevent neurons from dying and maintain their structural integrity, even under conditions of intense inflammation. This means that it could slow down the loss of brain cells in Alzheimer's patients, potentially leading to better brain function over time.
2. Inflammation Control: The fact that apigenin reduces the harmful activation of microglia and lowers the levels of inflammatory molecules like IL-6 and IL-1β suggests that it could help prevent the chronic inflammation seen in neurodegenerative diseases.
3. Natural Source: Apigenin is found naturally in everyday foods like parsley, chamomile tea, and celery. This makes it an accessible and low-risk option for reducing inflammation in the brain, compared to synthetic drugs that may have more side effects.

The study provides a strong foundation for further research into how apigenin could be used in humans. Since current treatments for Alzheimer's mainly focus on managing symptoms, apigenin might offer a way to slow down the disease's progression by targeting the underlying inflammation.

This study highlights apigenin's promising neuroprotective and anti-inflammatory properties. It shows that it can protect neurons and reduce brain inflammation, which are critical in conditions like Alzheimer's Disease. By lowering levels of harmful cytokines and preventing the overactivation of microglia, apigenin demonstrates potential as a natural compound that could help slow the progression of neurodegenerative diseases.

While these findings are based on in vitro models, they provide valuable insights into how apigenin might work in the human brain. Further research, including clinical trials, will be needed to fully explore how apigenin can be used in Alzheimer's treatment. However, this study suggests that increasing apigenin intake through foods like parsley, celery, and chamomile tea could be beneficial for brain health and help protect against neuroinflammation.