Could Lichens Cure Alzheimer s Disease?

Review Article

J Bacteriol Mycol. 2021; 8(7): 1191.

Could Lichens Cure Alzheimer’s Disease?

Khadhri A*

Faculty of Sciences, Plant, Soil and Environment Interactions Laboratory, University of El-Manar II, Campus Academia, Tunis, Tunisia

*Corresponding author: Ayda Khadhri, Faculty of Sciences, Plant, Soil and Environment Interactions Laboratory, University of El-Manar II, Campus Academia, 2092 Tunis, Tunisia

Received: October 29, 2021; Accepted: December 11, 2021; Published: December 18, 2021


Alzheimer’s disease is a neurodegenerative illness marked by a gradual memory impairment and certain intellectual (neurocognitive) functions leading to repercussions in the activities of daily living. Until now, there is no drug to treat neurodegenerative disorders; for this, it is preferable to seek to delay the progression of this disease. Lichens show vital therapeutic activity in several neurological diseases, including Alzheimer’s disease. Several isolated lichenic compounds have been tested for anti-acetylcholinesterase potency and may play a key role in the prevention of this dementia. This review deals with previous work on the therapeutic activity of some lichens and their bioactive components for them neurodegenerative diseases. Thus, compounds isolated from lichens can be considered favorable and promising for the prevention of neurodegenerative diseases.

Keywords: Lichens; Alzheimer; Anti-acetylcholinesterase; Neurodegenerative; Usnic acid


Oxidative stress is an imbalance between the excessive amount of free radicals and antioxidants.

Free radicals are molecules containing oxygen and are the origin of the natural process of oxidation in cells. Too much in the body, they can be harmful to the body and attack fatty tissue, proteins, DNA and all parts of the body. During an antioxidant/free radical imbalance, the body’s immune response is weak and therefore the body’s coping strategies are damaged.

This anomaly is the root cause of chronic disorders like diabetes, tumor, inflammatory diseases, Alzheimer’s disease and Parkinson’s. However, it should be remembered that if the balance of antioxidants and free radicals is present, the latter are used by certain white blood cells and contribute to the destruction of bacteria and the regulation of dead cells.

Free radicals are therefore very unstable and chemically reactive molecules, which are at the origin of oxidative stress and can be neutralized by antioxidants [1].

In a normal situation, the antioxidant/prooxidant balance is balanced. However, the body can be faced with over-exposure to oxidizing compounds when the endogenous production of Reactive Oxygen Species (ROS) becomes excessive or following exposure to an exogenous toxic phenomenon. When an imbalance occurs (by overproduction of pro-oxidant compounds or by a deficit in antioxidant substances), we speak of oxidative stress [2]. One of the main causes of Alzheimer’s disease is oxidative stress, which is caused by the excess production of free radicals. The main contributing elements are increased formation of Reactive Oxygen Species (ROS) and reactive nitrogen species (RNS). Certain environmental factors (i.e. Contaminants, pesticides, environmental pollutants, and Ultraviolet rays) can lead to the production of free radicals. Lipid peroxidation, which is the major cause of the decrease in membrane phospholipids in Alzheimer’s disease, is caused by free radicals reacting with enzymes, transporters, and proteins [3]. Indeed, the antioxidant activity would limit the oxidative damage linked to the neurodegenerative disorders associated with Parkinson’s and Alzheimer’s diseases [4].

Alzheimer’s disease (AD) is the most well-known and widespread neurodegenerative disease that impairs older people’s memory and behavior. The clinical manifestation of this neurological disease is the progressive degradation of brain tissue, which is driven by Acetylcholine (ACh) insufficiency [5]. Acetylcholinesterase (AChE) is a key neurotransmission enzyme (Figure 1). By hydrolyzing the cationic neurotransmitter ACh, it allows cholinergic neurons to return to their resting state by hydrolyzing acetylcholine (ACh) (Figure 2). AChE transforms acetylcholine (ACh) into choline (Ch) and acetate [6]. Reduced ACh levels in the hippocampus and cortex have been linked to significant biochemical alterations in Alzheimer’s patients [6]. AChE inhibitors (AChEI) are natural compounds that have been tested in clinical trials, primarily for the treatment of Alzheimer’s disease. Secondary metabolites have also been found as AChEIs, indicating that they could be used to treat Alzheimer’s disease [7]. Acetylcholine levels are particularly low on those suffering from Alzheimer’s disease, which explains the cognitive impairment observed. The solution to increasing the level of acetylcholine at the synaptic level is therefore to decrease its degradation by inhibiting the action of acetylcholinesterase [8]. Based on the hypothesis of inhibiting the action of AChE to better treat AD, several inhibitors of this enzyme have appeared on the market [9], such as galantamine, a natural alkaloid from Galanthus nivalis, in 2000. Although most of the known AChE enzyme inhibitors are alkaloids, various investigations have lately been conducted to uncover alternative naturally occurring compounds with strong anti-AChE activity. Several substances, other than alkaloids, exhibit a high ability to inhibit the AChE enzyme, according to Houghton et al. [8], including terpenoids, phenolics, flavonoids, and isocoumarins. In addition to secondary metabolites extracted from plants, natural products from lichens have aroused enormous interest from researchers around the world in the search for new drugs due to their positive effects on bioactivity. With the same objective, this review article focuses on the search for natural alternatives based on lichens that have antioxidant substances that indicate that they can slow the progression of Alzheimer’s disease [10]. This review is an attempt to compile information on various ethnomedicinal uses of lichens to fight Alzheimer’s disease.