How are diseases of the plasma membrane caused and what are their effects?
One of the important roles of the plasma membrane is to control what goes in and out of a cell to maintain homeostasis. To do so, functional integral protein channels are needed. Diseases of the plasma membrane, such as Cystic Fibrous, impair the body’s ability to keep internal conditions constant. To keep optimal conditions, the flow and balance of ions must be regulated across a membrane, faulty genes prevent this from happening. Membrane phospholipids additionally are essential for membrane functioning, degradation of phospholipids result in diseases of the membrane such as Alzheimer’s.
What is Cystic Fibrosis?
Cystic fibrosis is an inherited genetic condition that affects the ability of cells to control the movement of salt and water between one another, due to the impaired plasma membrane. The faulty movements cause a build-up of sticky mucus on the lungs and digestive system resulting in problems with breathing and vulnerability to infection. The disease of the membrane results in an accumulation of ions within the cell. The autosomal recessive genetic disease is caused by various mutations in the CFTR gene (cystic fibrosis transmembrane conductance regulator) which is located on chromosome 7. The CFTR gene provides synthesis instructions to make the CFTR protein. Normally the protein will transport chloride ions out of the cell by active transport. When in mutated form, the tertiary structure is altered and CFTR is unable to carry out its normal function in maintaining a thin water layer above cells in the lung epithelium thus mucus builds up. The absent water layer means mucus cannot be cleared and therefore it becomes infected by bacteria and oxygen cannot get into the blood efficiently.
CFTR consists of two membrane domains each with six transmembrane domain segments, two nucleotide binding domains (NBD 1 and NBD 2) and a cytoplasmic domain. The CFTR gene (known as an ABC gene) is responsible in the creation of chloride ion channels allowing sweat, digestive juices and mucus to be made. Movement of chloride ions requires phosphorylation of the regulatory domain. Two of the protein domains are ATP-hydrolysing, and thus use ATP as an energy source. Binding and hydrolysis of ATP are required for the opening and closing of the CFTR channel (known as gating.) Another two domains are made up of six alpha helices each, these allow the crossing of proteins across the plasma membrane (membrane trafficking.)
What causes Cystic Fibrosis?
There are a variety of mutations which cause cystic fibrosis, for example the most common mutation F508 as a result of a three nucleotide deletion (loss of phenylalanine). This leads to incorrect folding of the protein and is then degraded. Transmembrane topology is not altered by mutations blocking one pathway, but instead the mutations disrupt intracellular trafficking. Abnormal trafficking consists of CFTR protein being retained in the endoplasmic reticulum and the absence of it in the plasma membrane. Vesicles from the rough endoplasmic reticulum are responsible for transporting CFTR to the cell surface membrane. Dependent on what type of mutation occurs will determine how much CFTR is found on the plasma membrane.