Tau is a type of protein that is part of the structural function of nerves, necessary for nerves to transmit impulses efficiently. Certain diseases can change normal tau into forms that disrupt nerve impulses, and this group of diseases are called tauopathies. Examples include Alzheimer's Disease, Parkinson's Disease and corticobasal degeneration. Many of these diseases develop in the later stages of life, although some develop in middle age or earlier.
In people with healthy brains, tau protein forms part of the axons of nerves. Nerves are typically elongated cells with branch-like protrusions on one end called dendrites, and axons, which are more regularly shaped branch-like protrusions, on the opposite end. Nerves transmit information to each other through chemical signals. Dendrites receive the signals, whereas axons pass on signals to the next nerve.
Inside the nerve cell, also called a neuron, this information moves as an electrical impulse. Basically, the chemical signal is recognized by the dendrite end, which produces an electrical version, which gets to the axon end, and is turned back into a chemical signal. Tau protein, of which six individual types exist, is an essential component of the axon part of the nerve cell.
Healthy neurons use tiny structures called microtubules as scaffolding for the cell. Tau helps keep these microtubules in place and the cell in the correct shape. The presence of tau also helps essential cell nutrients like oxygen and glucose to travel around the cell. To do an efficient job, normal tau protein takes a specific shape.
When the tau present in the nerves is of a different shape, the function of the nerve cell can be affected. Tauopathies cover a wide range of nerve diseases, as "pathy" simply means disease, from the Greek word pathos. Any disease which affects the shape and function of tau proteins is therefore a tauopathy.
Normal tau folds in a certain way, but abnormal tau folds differently, into different shapes. Abnormal tau molecules contain extra phosphate groups, which affects the way the protein arranges itself. The different structure of the dangerous tau means that the protein acts differently inside the nerve cell. It tends to stick together in clumps on the dendrite end of the nerve cell, and block the transmission of electrical impulses.
This blocking of impulses is what causes the symptoms of tauopathies, which range from dementia to problems moving muscles. The variety of symptoms are due to the fact that tau is present in both the brain and in the nerves around the rest of the body. Abnormal tau can therefore affect the mental ability of the person, such as in Alzheimer's Disease, or instead damage the person's ability to control movements, such as in Parkinson's Disease.
Damaged nerve cells can also die off with time, making tauopathies worse with age, and causing them to fall into the group of medical conditions known as degenerative diseases. Certain gene mutations are implicated with some tauopathies, especially the ones that run in families, such as Younger Onset Alzheimer's Disease. Other tauopathies, like regular Alzheimer's Disease, have no known cause as of 2011, and research is ongoing.