Fibrillins are a family of high sulphide containing glycoproteins that help create elasticity within connective tissue- any tissue that supports organs by forming a framework that binds different tissues together. Connective tissues can also store fat and transport different substances. It is made up of few calls but a large variety of proteins that create the extracellular matrix. Fibroblast cells secrete the extracellular matrix contents, which includes collagen, elastin and fibrillins.1 Fibrillins are made from amino acids joined together to form a peptide, as are all proteins. Amino acids interact with each other within the peptide chain to create a highly specific 3D shape. If the configuration of the protein is changed, the functionality of the protein may be affected, and it is likely that the protein will not be able to perform its role within the body.
I will be exploring the structure of fibrillins and how this allows fibrillins purpose within the extracellular matrix. Fibrillins form microfibrils 10-12 nm in length.2 There are two main sections in the fibrillin protein; 43 calcium-binding epidermal growth factor-like domains (cbEGF) and 8 transforming growth factor B binding protein-like cysteine domains (TB).3,4 cbEGF domains are present in many different proteins5, such as protein S, which works in the anticoagulant system6, and Notch-3, a receptor involved in gene expression.
7 In fibrillin-1, cbEGF domains have been shown to have antiparallel ? hairpin which contains three disulphide bridges. Calcium binds to cbEGF domains using ligands which arranges in a bipyramidal fashion. Six out of seven ligands are intradomain oxygen atoms, in the form of oxygen atoms on side groups or as part of a carbonyl. The seventh ligand has not yet been identified.8 Two cbEGF domains are involved in the stabilisation of the other cbEGF domains and do not directly bind to calcium. cbEGF domains are organised into a rod shape; calcium has been shown to control the rigidity of the rod shapeThe TB domain comprises six antiparallel ? strands and 2 ? helices. It contains four disulphide bridges which stabilise the structure. Six of the TB domains are covalently bonded to cbEGF domains and has been shown to increase cbEGF domain affinity to calcium atoms.
TB domains bind to transforming growth factor beta (TGF-?) proteins to store them within the matrix.6 Fibrillin-1 also contains a region that is high in proline and is thought to act as a hinge region.Fibrillin-1 is secreted from fibroblasts3 and has multiple functions within the extracellular matrix.
The microfibrils that are formed from fibrillin-1 form elastic fibres which also incorporate lysyl-oxidase, proteoglycans and elastin. Fibrillin-1 is also present in tissues that do not contain elastin. The full extent of fibrillin-1 protein functions and the processes involved within the extracellular matrix has not yet been identified, nonetheless some roles of fibrillin-1 are known. Fibrillin-1 provides a framework to deposit tropoelastin, which is an important protein that allows elastic fibres to stretch and recoil.10 The ? hairpin within cbEGF domains have been shown to be a key component within this process. Additionally, fibrillin-1 microfibrils are shown to be elastic, which can indicate their use in tissues, where elastin is not present, as an elastic fibre. The control of the rod shape in fibrillin-1 through calcium can lead to the protein being able to flatten the rod, elongating the protein, and can allow fibrillin-1 to behave in an elastic nature. Fibrillin-1 can provide support to non-elastic tissues, and anchors endothelial and epithelial tissues to elastic fibres.
11 Calcium, when bound to fibrillin-1, is shown to protect against proteolysis from proteases within the matrix, such as elastase and trypsin. This is important for fibrillin-1 to be able to function, as cbEGF domains that bind to calcium allow fibrillin to carry out its various functions without degradation. Calcium is also important in stabilising the fibrillin-1 structure; without calcium, microfibrils become distorted.8 Another function that fibrillin-1 has is to store TGF-?, which is inactive whilst bound to the fibrillin-1 within the microfibrils. TGF-? controls many cell processes, including cell proliferation, cell motility and apoptosis.12 The structure of the TB domains allows fibrillin-1 to bind to TGF-?, thus the structure of the TB domains allows fibrillin-1 to participate in the control of these mechanisms. The FBN1 gene codes for both fibrillin-1 and asprosin, which is used in glucose homeostasis.
13 It is located at 15q21.1, at base pairs 48,408,406 to 48,645,788 on chromosome 15.12 The size of transcript is around 10kb and the gene has 65 exons.11 Mutations in the FBN1 gene is known to cause Marfan Syndrome (MFS). MFS is an autosomal dominant disease. Around three quarters of cases are inherited, and a quarter occur from new mutations.14 There are over 3000 recorded mutations known to cause MFS,15 which shows that the FBN1 gene is highly susceptive to genetic changes.
Point mutations are the most common mutations that take place in the gene, with the occurrence at around 60%. Frameshifts are responsible for 13% of mutations, and splicing errors also account for 13%.7 The mutations can be split into two groups; one third lead to nonsense-mediated decay, where faulty mRNA is broken down and results in a lower amount of the protein being produced, and two thirds leads to improper folding of the fibrillin-1 protein.7 The mutations that bring about improper folding can affect disulphide bridges, amino acid residues involved in calcium bonding or other amino acids that affect the conformation of fibrillin-1. These mutations change the conformation of the fibrillin-1 protein and can lower the affinity for calcium, thus the protein is not protected from proteolysis and the stability of the protein is disrupted. Fibrillin-1 may not be able to bind to TGF-? if the 3D shape of fibrillin-1 changes, leading to fibrillin-1 being unable to store TGF-? within the extracellular matrix. MFS sufferers tend to have a delayed secretion of fibrillin-1, though some have normal secretion which supports that fibrillin-1 has compromised effectiveness in the extracellular environment. Some MFS sufferers have fibrillin-1 that is seen to have undergone glycosylation which suggests that their fibrillin-1 does not leave the cell and is retained in the endoplasmic reticulum.
3,16 MFS can be caused hundreds of different know mutations, so the nature of the mutation will affect the functionality and vesicular trafficking of fibrillin-1. MFS is the most common genetic disease affecting connective tissue. It has a frequency of 1 in 5000 people.14 MFS sufferers tend to be taller and slenderer than family members that are not affected and have very long fingers and toes (arachnodactyly). Over half of sufferers also develop scoliosis- abnormal curving of the spine. The sternum can protrude outwards or be sunken and fingers may be bent, as well as joint hypermobility.
Spinal abnormalities are due to TGF-? not being stored in the mutant fibrillin-1, leading to tissues overgrowing and instability within the tissues. This can also lead to cardiovascular problems, some of which can be very serious and life threatening. Aortic dilation (enlargement of the aorta) or aortic aneurysm (bulging of aorta walls) can occur.
This puts the MFS sufferer at risk of aortic dissection, where the aorta wall can rupture or tear. This is due to fragmentation and disorganisation in the elastic fibres, as tropoelastin deposition is disrupted. If the ascending aorta tears it is life threatening and requires immediate surgery. A tear in the descending aorta is not as dangerous, however still puts the vital organs at risk of reduced blood flow. Other cardiac problems include leakage of the mitral valve, causing an irregular heartbeat and chest pain. 14,17,18,19 MFS can cause problems with the eyes. Near sightedness is extremely common and often sufferers get ectopia lentis, where the lens detaches from the centre of the eyeball.
They can develop cataracts or glaucoma early in life, which leads to blindness if untreated. Some MFS sufferers develop pockets of air in the lung and spontaneous lung collapse. Other diseases can have very similar symptoms to MFS, however are due to mutations in genes for other very similar proteins, such as fibrillin-2. MFS has no cure as it is a genetic disease and treatments are directed towards specific symptoms of MFS. MFS has a high survival rate, though some more severely affected die in infancy