29 March 2013

Can Salmonella cure Alzheimer's disease?

A promising drug against Alzheimer's disease –
a component of the cell wall of the salmonella bacterium

Anastasia Gorelova, "Elements"Alzheimer's disease is associated with the deposition of beta-amyloid protein aggregates in the human brain.

One of the possible ways to combat these deposits is to search for substances that stimulate the phagocytic activity of microglia cells – the main component of the immune defense of the brain and spinal cord – that is, their ability to absorb foreign particles, including amyloid protein. A group of scientists from Canada, Belgium and the USA proposed as such a substance monophosphoryl-lipid A (MPL) – a modified lipopolysaccharide of the cell wall of the bacterium Salmonella minnesota. MPL solution reduces the number and area of amyloid plaques in the brains of transgenic mice and improves their memory ability.

More than 35 million people around the world are living with a diagnosis of Alzheimer's disease. And although the main risk factor is old age, this disease is also diagnosed in people aged 30-40 years. Due to the progressive deterioration of memory and developing dementia, such patients are confined to hospitals and nursing homes, living the rest of their lives without hope of recovery.

The pathogenesis of Alzheimer's disease is associated with the accumulation of insoluble plaques of beta-amyloid peptides in brain tissues, which occurs due to spontaneous imbalance of transformations of the precursor of beta-amyloid - a protein embedded in the cell membranes of neurons.

The amyloid precursor protein (Amyloid precursor protein, APP) is found in many tissues of the body, where it is a normal transmembrane protein, but its functions have not yet been definitively established. Depending on the availability of specific sites of the protein molecule for peculiar proteins-"scissors" – secretase enzymes (see Secretase), APP can enter one of two possible conversion pathways – amyloidogenic or non-amyloidogenic (see the review article by Querfurth & LaFerla, 2010. Alzheimer's Disease). Both pathways are characteristic of normal cells (Fig. 1).

Fig. 1. Amyloid precursor protein (APP) conversion pathways.
Figure from the review
of Alzheimer's Disease (Querfurth & LaFerla, 2010).

We are interested in amyloidogenic protein transformation, since the non-amyloidogenic pathway associated with protein cleavage by the enzyme α-secretase cannot lead to the formation of the amyloid protein A-beta42, which is dangerous for cells, since already at the first stage cleavage occurs within the A-beta sequence.

The amyloidogenic transformation of the precursor begins with the cleavage of its extracellular fragment under the action of the beta-secretase enzyme. The remaining fragment undergoes further cleavage by gamma secretase to form beta-amyloid. Beta-amyloid peptides consisting of 36-43 amino acids are natural products of metabolism; at the same time, normally monomeric peptides with a length of 40 amino acids are formed significantly more than damaging longer peptides prone to aggregation (usually their size is 42 amino acids: A-beta42). The imbalance between the two pathways of APP conversion, the formation, destruction and aggregation (adhesion) of peptides, leads to the accumulation of A-beta42, which can become a trigger factor in the development of the disease.

Scientists from the Research Clinical Center of Laval University (Quebec, Canada), together with colleagues from Belgium and the USA, suggested that a promising direction of searching for a possible cure for Alzheimer's disease are substances that stimulate the phagocytic activity of microglia cells - the main component of active immune protection of the brain and spinal cord.

Phagocytic activity characteristic of many types of immune cells, that is, the ability of the cell to capture and process solid particles (including bacteria, viruses, aggregated proteins), is the first line of defense against many diseases. It can be stimulated with the help of various components of viruses and bacteria. In microglial cells, as in other cells of the immune system, there are special sensors of molecules characteristic of pathogens – the so-called Toll-like receptors (TLRs). When a pathogen enters the body, the cells of the immune system are mobilized to protect the body by activating these receptors. An increase in the phagocytic activity of microglia is also important for protection against amyloid protein aggregates formed in Alzheimer's disease.

Lipopolysaccharide (Lipopolysaccharide), a component of the cell wall of gram–negative bacteria, is one of such pathogen-specific molecules by which the body can recognize an infection and mobilize the immune system. The problem is that the lipopolysaccharide of pathogenic bacteria can be toxic to the human body. Therefore, the authors of the article decided to use a harmless chemically modified lipopolysaccharide of the cell wall of the Salmonella minnesota bacterium – monophosphoryl lipid A (MPL) to stimulate microglial cells (Fig. 2).

Fig. 2. Chemical structure of the cell wall derivative
Salmonella minnesota bacteria are monophosphoryl-lipid A.
Figure from the article under discussion in PNASAs a model for studying the mechanism of A-beta deposition, in vivo experiments used a line of transgenic mice with chimeric protein, the precursor of amyloid.

By 9 months of life, a large number of beta-amyloid plaques are found in the brains of such animals, resembling those formed in the brains of patients suffering from Alzheimer's disease. The ability to remember in such mice decreases with age and worsens compared to mice who do not have plaques in the brain.

The results obtained after incubation of microglial cells of this mouse line with MPL solution confirmed the stimulating effect of monophosphoryl-lipid A on phagocytic activity (Fig. 3).

3. MPL stimulates phagocytosis in mouse microglial cells. A – both MPL and unmodified lipopolysaccharide (LPS) stimulate phagocytosis of fluorescent beads of Escherichia coli. B – intracellular localization of these granules (green) is confirmed by confocal microscopy data. The length of the scale ruler is 10 microns. A drawing from the article under discussion in PNAS.To evaluate the effectiveness of MPL in vivo, transgenic mice from three experimental groups were injected with solutions of the studied substances for a long time.

The control group was injected with biologically neutral sodium-phosphate buffer (Phosphate buffered saline, PBS), the experimental groups were injected with solutions of MPL or unmodified lipopolysaccharide. It turned out that compared with the control group, which was injected with a sodium-phosphate buffer solution, the number and total area of beta-amyloid plaques in the cerebral cortex of mice injected with MPL solution were significantly reduced (Fig. 4). In addition, unlike the original extremely toxic salmonella lipopolysaccharide, MPL injection did not it led to the development of life-threatening inflammation.

4. The number (D) and total area (E) of beta-amyloid plaques in the cerebral cortex of mice that were injected with MPL solution, nitrium phosphate buffer (PBS) or unmodified lipopolysaccharide (LPS). The figure shows that the number of plaques decreases after MPL injection. Interestingly, in the case of unmodified lipopolysaccharide, a very large variation in values is observed. Figure from the original article in PNAS.The results of behavioral tests also showed significant improvements in the animals' learning and memory indicators compared to the control.

An aquatic T-shaped maze was used to determine the ability of mice to memorize. In such a maze, the animal must choose between two arms, one of which contains an invisible platform on which the animal can climb (the mouse, once in the water, tends to be back on land). The authors first "taught" the mouse to systematically choose the left sleeve, in which the platform was located all the time. After that, it was estimated how many attempts were needed for the mouse to start selecting the right sleeve into which the platform was moved. The criterion of training was considered 5 correct choices in a row. It turned out that for transgenic mice that were used in the work, it takes an average of about 17 attempts to remember that the platform is in the right sleeve. At the same time, 10 attempts were enough for mice injected with MPL.

The results of this work present monophosphoryl-lipid A as a promising drug for the safe and effective treatment of Alzheimer's disease.

Source: Michauda et al., Toll-like receptor 4 stimulation with the detoxified ligand monophosphoryl lipid A improves Alzheimer's disease-related pathology http://www.pnas.org/content/110/5/1941.short // PNAS, January 29, 2013.

Portal "Eternal youth" http://vechnayamolodost.ru29.03.2013

Found a typo? Select it and press ctrl + enter Print version