Pathophysiology

Background

Dementia

Dementia is not a specific disease but a syndrome associated with an ongoing decline of brain function (Duong et al., 2017). It is a general term that encompasses difficulties in memory, language, and behaviour that lead to impairments in activities of daily living (Robinson et al., 2015). Its symptoms can vary widely from person to person and are usually mild and get worse very gradually (Duong et al., 2017). While some mild changes in cognition are considered a normal part of the ageing process, dementia is not (Harada et al., 2013). There are many different causes and types of dementia, the most common being Alzheimer’s Disease (Cunningham et al., 2015).

Alzheimer's disease

AD is both a cause and type of dementia. It is a progressive condition with symptoms developing gradually over many years (Duong et al., 2017). The first sign of AD is usually minor memory problems. With the condition developing, memory problems become more severe and further symptoms can develop such as confusion and disorientation, difficulty planning and making decisions, problems with speech and language, problems with moving around and performing self-care tasks, personality changes, hallucinations and low mood or anxiety (Duong et al., 2017).

Most cases of AD are sporadic, with late onset, in individuals above 65 years, and unclear aetiology (Bekris et al., 2011). Early-onset AD is rare, representing 5%-10% of all AD patients. This is commonly connected to an inherited change in genes and patients develop symptoms before the age of 65 (Ayodele et al., 2021). The risk of AD increases with age, it is estimated to affect 1 in 14 people over the age of 65 and 1 in every 6 people over the age of 80. There is no known way to prevent the condition but there are possible ways to reduce the risk of developing AD or delay its onset such as stopping smoking, cutting down on alcohol, eating a healthy, balanced diet and maintaining a healthy weight and staying physically as well as mentally active (Livingston et al., 2020).

The two pathologic hallmarks of Alzheimer disease are extracellular beta-amyloid deposits (in senile plaques) and intracellular neurofibrillary tangles (paired helical filaments) (Murphy & LeVine, 2010). A sustained immune response and inflammation have also been observed in the brain of patients with AD and some experts have proposed that inflammation is the 3rd core pathologic feature of AD (Kinney et al., 2018).

Pathophysiology

Brain Atrophy

AD is a highly complex and progressive neurodegenerative disease with the affected brain exhibiting astrogliosis, nerve cell atrophy and neuronal loss (Kametani & Hasegawa, 2018). At first, AD typically destroys neurons and their connections in parts of the brain involved in the memory. It later affects areas in the cerebral cortex responsible for language, reasoning, and social behaviour. Eventually, many other areas of the brain are damaged. When neurons are destroyed and tissue is lost throughout the brain, it dramatically exacerbates the normal brain atrophy that comes with ageing, affecting nearly all of its functions (Alzheimer's Associations, 2020). This appears as exacerbated brain volume loss, cortical thinning, sulcal widening, and ventricular enlargement. The hippocampus is one of the earliest cortical substructures to undergo detectable atrophy in Alzheimer’s disease and changes in this area can be detected as early as 10 years prior to the onset of symptoms (Blinkouskaya and Weickenmeier, 2021).

Amyloid Beta and Tau Tangles

Reported histopathological characteristics of AD are extracellular aggregates of amyloid beta plaques and intracellular aggregations of neurofibrillary tangles, composed of hyperphosphorylated microtubule-associated tau. These disrupt axonal transport causing loss of signal transmission and axon death (Blinkouskaya and Weickenmeier, 2021). Amyloid beta plaques form as a result of improper cleavage of the amyloid precursor protein (APP) creating amyloid beta monomers that aggregate forming oligomeric amyloid beta and eventually aggregating into amyloid beta fibrils and plaques (Kinney et al., 2018). Amyloid beta plaques develop initially in basal, temporal and orbitofrontal neocortex regions of the brain and in later stages progress throughout the neocortex, hippocampus, amygdala, diencephalon and basal ganglia. In critical cases, amyloid beta is found throughout the mesencephalon, lower brain stem, and cerebellar cortex as well. This increased concentration of amyloid beta in addition to inflammation triggers tau-tangle formation, worsening the progression (Tiwari et al., 2019).

Neurofibrillary tangles arise from hyperphosphorylation of tau, a protein used in stabilisation of tubules. This results in removal of tau from microtubules and collapse of the microtubule structures leading to disruption of cellular processes. Additionally, the hyperphosphorylated tau aggregates into paired helical fragments that eventually form neurofibrillary tangles (Kinney et al., 2018). Tau tangles are found in the locus coeruleus and transentorhinal and entorhinal areas of the brain and in the critical stage, they spread to the hippocampus and neocortex. Increasing age plays a role in slowing down natural processes to mediate this progression, but AD greatly exacerbates this (Tiwari et al., 2019).

Aggregations Are Not Enough to Cause AD

Study by Pascoal et al. (2021) has shown that many older adults have amyloid plaques in their brains but never progress to developing AD suggesting that amyloid accumulation on its own is not enough to cause dementia. Amyloid beta plaques can accumulate up to 10 years before any observable AD symptoms. Additionally, mice models have shown that amyloid beta does not induce tau accumulation (Kametani & Hasegawa, 2018). Results of the study by Pascoal et al. (2021) suggest that it is an interaction between neuroinflammation (lead by activation of microglia) and amyloid pathology that is needed to unleash tau propagation that eventually leads to widespread brain damage and cognitive impairment. The overall tau tangle load is correlated with cognitive decline in AD, however the appearance of tau tangles occurs before the clinical appearance of the disease (Kinney et al., 2018). Hence, neuroinflammation is a promising target for AD medication (Pascoal et al., 2021).

Changes in Character of Inflammation

Some research studies postulate that CNS inflammation, pathology, starts at very early stages of AD, most likely before amyloid plaques are formed. It then evolves in its intensity and character as the disease progresses. The inflammatory process is likely to start decades before the clinical manifestation of AD, and it culminates in a mixed overt inflammatory and immune response (Cuello, 2017).

Neuro-inflammation in the early (preclinical) stages has been shown to be amenable by anti-inflammatory therapy. In later stages, tissue resolution and innate/adaptive immune reactions predominate which are not amenable to anti-inflammatory therapy (Hampel et al., 2020).

Stages of AD

There are different ways of classifying AD patients depending on whether we look at their symptoms or the development of the underlying pathology. It is important to note that as of now, AD is diagnosed based on clinical symptoms (Figure 1). As mentioned, AD can start to develop decades before symptoms start to appear. By then, the pathology is already quite developed and a very early clinical stage would need to be targeted to significantly delay progression. It should be noted that the team has not came across in literature nor outreach where there is an exact connection between clinical and pathological stages, but correlation can be found, with many variations between patients.

NLRP3/caspase-1 Pathway

In brief, the NLRP3/caspase-1 pathway promotes the assembly of the NLRP3 inflammasome complex, resulting in caspase-1 activation and production of pro-inflammatory cytokines including IL-1β and IL-18 and induces pyroptosis. NLRP3 inflammasome is an intracellular protein complex consisting of NLRP3, the adaptor protein apoptosis associated speck-like protein containing a caspase recruitment domain (ASC) and procaspase-1.4. The NLRP3/caspase-1 pathway plays an important role in inflammatory response, but when exacerbated in the brain, critically contributes to the pathology of AD. This pathway is still being studied but is shown to be a viable target for our potential therapeutic.

The upregulation of this pathway is heavily impacted by the transcription factor NfkappaB. If NLRP3 is primed by this factor and excessively activated it promotes the onset of AD by increasing the expression of the NLRP3 inflammasome. This leads to the activation of caspase-1, which turns pro-IL-1β to the secreted IL-1β and gasdermin-D (GSDMD) to its active form. Significant effects of IL-1β include how it induces neurotoxicity that causes degeneration of neurons and facilitates a response that leads to an infiltration of immune cells. GSDMD triggers pyroptosis, a pro-inflammatory type of cell death. It releases more inflammatory factors and causes nerve cell damage. Furthermore, activation of the NLRP3 inflammasome can reduce the phagocytosis of microglia on Aβ, thus promoting the occurrence and development of AD lesions. On top of that, Aβ also induces the activation of the NLRP3 inflammasome and perpetuates a feedback loop (Liang et al., 2022).

Pterostilbene has been shown to inhibit neuroinflammation by inactivating the NLRP3/caspase-1 pathway in preclinical research (Li Q et al., 2018), (Wang et al., 2020) and mice studies as well. It attenuated Aβ cytotoxicity while also inhibiting Aβ-induced nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) expression, attenuating enhancement of IL-6, IL-1β, and TNF-α by Aβ.

Pterostilbene

What is it?

Pterostilbene is a phenolic compound of the stilbenoid class. Stilbenoids are compounds that are hydroxylated versions of stilbenes and have a C2C6C6 structure. Chemically related to resveratrol, pterostilbene is a similar compound that is currently used as a supplement and has been a compound of interest in therapeutic studies (McCormack & McFadden, 2013). It is naturally found in various plants including grapes, peanuts, and blueberries. Like many other phenolic compounds, they are currently of interest in research due to their numerous health benefits, particularly as protection from non-communicable diseases. Among them are cancer, cardiovascular diseases, and neurodegenerative diseases. For neuroprotection specifically, it is known to exert anti-inflammatory and antioxidant effects.

Therapeutic Potential

Research is still being done on the clinical use of pterostilbene on AD and other neurodegenerative diseases, but there is promise as to their potential to become a treatment through preclinical and mice studies. For example, mice study with SAMP8 mice has shown that when administered pterostilbene, functional deficits and AD pathology were improved (Chang et al., 2012).

As mentioned, neuroinflammation is a hallmark in the pathophysiology of AD. With the targeting of the NLRP3/caspase-1 pathway, progression would be delayed as Aβ-induced neuroinflammation is attenuated. Pterostilbene likely directly binds to NLRP3-ASC and NLRP3-NEK7 and targets their ATPase activity, which is significant as it is needed for the assembly and activation of the NLRP3 inflammasome (Chen et al., 2021).

Furthermore, pterostilbene displays high bioavailability compared to other stilbenoids and phenolic compounds. It is lipophilic and has 80% bioavailability, which is significantly higher compared to the 20% of resveratrol (Kapetanovic et al., 2011). This means that less intake of pterostilbene is needed to achieve the desired neuroprotective effect. Higher permeability, especially across the blood brain barrier, and higher cellular uptake can be achieved with pterostilbene.

Derivatives of pterostilbene have been studied in direct relation to the NLRP3/caspase-1 pathway. A compound has been found to have high efficacy for both preventing the maturation of caspase-1 and pyroptosis (Chen et al., 2021). It also showed low cytotoxicity. This shows further promise for the use of pterostilbene and our project hopefully contributes a starting point to this.

References