Nerve cells throughout the brain and body are covered in a white, waxy coating known as the myelin sheath. Its function is to allow conduction of nerve impulses between cells, working in much the same way as the insulating coating on a wire does. According to the Myelin Project, myelin is composed of a number of packed proteins that are sandwiched between two layers of lipids or fats. The most abundant lipid found in myelin is cholesterol.
Cholesterol
Cholesterol is a type of lipid or fat that is a requirement of life. It is a major component of cell membranes and myelin, and also acts as a building block from which our bodies produce hormones, bile acids and vitamin D. While cholesterol may be obtained from the diet, it is also produced within the body, primarily in the liver and intestines. Cholesterol molecules used in myelin production are normally synthesized exclusively in the glial cells of the brain, according to Dr. G. Saher of the Max Planck Institute.
Myelin Disorders
Interruptions or gaps within the myelin sheath may cause disruption of nerve impulses, similar to a short in a wire. Gaps may be caused by injury or disease, and will result in a wide variety of neurological symptoms. According to the Myelin Project, diseases that damage myelin, known as demyelinating disorders, can be either hereditary or acquired. Childhood hereditary disorders collectively known as the leukodystrophies are caused by an enzyme deficiency, which allows toxins to build up and destroy myelin, causing paralysis, the loss of all senses and death. The most common acquired demyelinating disorder is multiple sclerosis (MS).
Myelin Repair
Myelin that has been damaged by injuries or MS can repair spontaneously. According to the Myelin Project, specialized stem cells known as OPCs migrate to the damage site and activate a cascade of reactions, one of which is cholesterol formation in glial cells. Indeed, one of the areas under current investigation in MS research is to determine why some MS patients are able to repair myelin while others cannot, and what role cholesterol plays.
Cholesterol and Disease
Cholesterol molecules must be coated in a protein layer in order to travel throughout the body in the blood; the resulting cholesterol-protein complex is called a lipoprotein. According to Iowa State University, there are five classes of lipoproteins, and two of them--HDL and LDL--are regularly measured as indicators of coronary heart disease. Deficiencies of cholesterol are also known to exist, such as Smith-Lemli-Opitz Syndrome (SLOS). Characteristics of SLOS include lack of muscle tone and cognitive deficits. One current theory is these symptoms are the result of improper myelin formation due to a lack of cholesterol.
Statins
Statins are a class of cholesterol-lowering drugs. They are used to treat individuals at risk of coronary heart disease due to high amounts of LDL in their blood. V. Miron of the Montreal Neurological Institute and colleagues published a study in 2009 examining the effect of statins in MS patients with recent myelin damage. Statins were found to inhibit myelin repair at the sites of injury, presumably by lowering the amount of available cholesterol required to form new myelin. The authors of the study recommended finding alternative drug therapies to treat MS patients with high cholesterol.
References
- The Myelin Project: What is Myelin?
- Max Planck Institute for Experimental Medicine: High Cholesterol is Essential for Myelin Membrane Growth
- Iowa State University: Cholesterol in Your Body
- "The American Journal of Pathology"; Statin Therapy Inhibits Remyelination in the Central Nervous System; V. Miron et. al., May 2009
