Elastic fibres are designed to maintain elastic function for a lifetime. However, various enzymes matrix metalloproteinases and serine proteases are able to cleave elastic fibre molecules Kielty et al. Indeed, loss of elasticity due to degradative changes is a major contributing factor in ageing of connective tissues, in the development of aortic aneurysms and lung emphysema, and in degenerative changes in sun-damaged skin Watson et al.

The importance of elastic fibres is further highlighted by the severe heritable connective tissue diseases caused by mutations in components of elastic fibres for reviews, see Robinson and Godfrey, ; Milewicz et al. Fibrillin-1 mutations cause Marfan syndrome, which is associated with cardiovascular, ocular ectopia lentis and skeletal defects;fibrillin-2 mutations cause congenital contractural arachnodactyly CCA with overlapping skeletal and ocular symptoms; and elastin mutations cause Williams syndrome, supravalvular stenosis SVAS and cutis laxa Tassabehji et al.

Recently, pseudoxanthoma elasticum PXE , a heritable disease associated with elastic fibre calcification was linked to an ion channel protein Le Saux et al. The biology of elastic fibres is complex because of their multiple components, tightly regulated developmental pattern of deposition, multi-step hierarchical assembly, unique elastomeric properties and influence on cell phenotype. Below we discuss how the molecular complexity of the elastic fibre system is being unravelled by progress in identifying interactions between microfibrillar molecules and tropoelastin, detailed ultrastructural analyses and studies of mouse models.

Ultrastructural studies have provided insights into the complex organisation of fibrillin-rich microfibrils. Negative staining, rotary shadowing and atomic force microscopy AFM showed that isolated untensioned microfibrils have a 56 nm beaded periodicity Fig.

Calcium plays a key role in this microfibril organisation, since chelation of calcium induces a diffuse appearance and reduced beaded periodicity Kielty and Shuttleworth, ; Cardy and Handford, ; Wess et al. STEM analysis has defined microfibril mass and its axial distribution, predicted up to eight fibrillin molecules in cross-section, and shown that treatment of microfibrils with enzymes chondroitinase ABC lyase,matrix metalloproteinase can reduce mass and influence organisation Sherratt et al.

Automated electron tomography AET -generated 3D reconstructions at Localisation of antibody and gold-binding epitopes, and mapping of bead and interbead mass changes in untensioned and extended microfibrils have provided compelling evidence that fibrillin molecules, which form head-to-tail arrays in extended microfibrils Reinhardt et al.

D 3D height contour profile of the three beaded repeat units. In tissues, microfibrils form loosely packed parallel bundles. Mecham, personal communication. X-ray studies and mechanical testing of microfibril bundles showed that bound calcium influences load deformation but is not necessary for high extensibility and elasticity Wess et al.

Thus,microfibril elasticity is modified by, but not dependent on calcium-induced beaded periodic changes, which is consistent with the molecular folding model. Ultrastructural analysis has shown that the elastic fibre core is not really amorphous but instead laterally packed, thin ordered filaments Pasquali-Ronchetti and Baccarani-Contri, The architecture of mature elastic fibres is intricate and highly tissue specific, reflecting specific functions in different tissues. In the medial layer of the aorta and elastic arteries, elastic fibres form concentric fenestrated lamellae separated by smooth muscle cell SMC layers;this arrangement imparts elasticity and resilience to blood vessel walls.

In lung, elastic fibres are present in blood vessel walls and as thin highly branched elastic fibres throughout the respiratory tree that support alveolar expansion and recoil during breathing.

The reticular dermis of skin contains thick, horizontally arranged elastic fibres, whereas the papillary dermis contains thinner perpendicular elastic fibres elaunin fibres that merge with the microfibrillar cascade oxytalan fibres that intercalates into the dermal-epidermal junction.

This continuous elastic network imparts elasticity throughout skin from the reticular and papillary dermis to the epidermis. In auricular cartilage, a thin network of elastic fibres interspersed with collagen fibrils in the interterritorial zone contributes to tissue deformability.

Elastic fibres are abundant in flexible ligaments, but sparse in tendons. Tissue-specific arrangements are dictated by the mesenchymal cells that deposit and orientate the microfibril template, and by functional demands.

The biomechanical limitations in microfibrils that led to the evolution of elastin and the appearance of elastic fibres should be revealed once microfibril elastic properties are better understood. Although it has been recognised for many years that microfibrils form a template for elastin, precisely why microfibrils have this role is still a central question in elastic fibre biology.

It is also not clear whether elastin-associated microfibrils differ from those that do not associate with elastin. New insights into the cellular and extracellular basis of tissue-specific elastic fibre architecture are also needed. The inventory of known elastic-fibre-associated molecules has expanded dramatically in recent years following gene mapping of heritable elastic fibre defects, the development of mouse models and detailed immunohistochemical and biochemical studies of elastic tissues.

However, the biological roles of many of these molecular players are far from clear. They can be broadly categorised as molecules that colocalise or co-purify with microfibrils, molecules that occur at the elastin-microfibril interface or the elastic-fibre-cell interface, and molecules that are involved in the process of elastic fibre formation. Table 1 lists all the molecules so far identified in these categories, and below we give details of the major players and their potential roles.

Fibrillins are the principal structural components of elastic-fibre-associated microfibrils. Fibrillin-1 and fibrillin-2 are encoded by genes on chromosomes 15 and 5, respectively Pereira et al. Fibrillin-1 and fibrillin-2 have distinct but overlapping patterns of expression Zhang et al. Fibrillin-2 is generally expressed earlier in development than fibrillin-1 and may be particularly important in elastic fibre formation Pereira et al.

Fibrillin-3 was isolated from brain, and whether it is involved in elastic fibres is unknown Nagase et al. Apart from the fibrillins, microfibril-associated glycoprotein 1 MAGP-1 sometimes known as MFAP-2 is possibly the best candidate for an integral microfibril molecule important for structural integrity Gibson et al.

It is associated with virtually all microfibrils and widely expressed in mesenchymal and connective tissue cells throughout development. Human MAGP-1 is a residue molecule that has two distinctive domains: an acidic N-terminal half that is enriched in proline residues and has a clustering of glutamine residues, and a C-terminal portion that contains 13 cysteine residues and has a net positive charge.

MAGP-1 localises to microfibril beads sometimes two per bead Henderson et al. MAGP-2, the other member of this small microfibrillar protein family, is a residue protein structurally related to MAGP-1 mainly in a central region Gibson et al. MAGP-2 localises to elastin-associated and elastin-free microfibrils in a number of tissues Gibson et al. However, its restricted patterns of tissue localisation and developmental expression suggest that MAGP-2 has a function related to cell signalling during microfibril assembly and elastinogenesis.

LTBPs are smaller molecules than fibrillins but also comprise repeating cbEGF domains interspersed with TB modules, the latter being found only in the fibrillin superfamily Sinha et al.

LTBP-1 colocalises with microfibrils in skin and cell layers of cultured osteoblasts and in embryonic long bone but not cartilage Taipale et al.

LTBP-2 colocalises with fibrillin-rich microfibrils in elastic-fibre-rich tissues especially in the response to arterial injury, and in trabecular bone Gibson et al. Several other microfibril-associated proteins have been identified immunohistochemically, but little is known about whether they are essential microfibrillar components and how they might influence microfibril function.

Several proteoglycans PGs also engage in critically important interactions with microfibrils and contribute to their integration into the surrounding ECM. Early electron microscopy observations using polycationic dyes showed an association between PGs and elastic fibres Baccarani-Contri et al.

Two members of the small leucine-rich PG family, decorin and biglycan PG I and PG II were detected within elastic fibres in dermis;biglycan mapped to the elastin core and decorin mapped to microfibrils. More recent ultrastructural approaches have shown that chondroitin sulphate proteoglycans CSPGs are associated with microfibril beads, and a small CSPG co-immunoprecipitates with fibrillin from cultured smooth muscle cell medium Kielty et al. Small CSPGs may contribute to the beaded organisation of microfibrils, and versican may influence microfibril integration into the surrounding ECM.

Several molecules localise to the elastin-microfibril interface or to the cell-surface—elastic-fibre interface. These molecules could regulate tropoelastin deposition on microfibrils and link elastic fibres to cell surfaces. One such protein is emilin, a kDa glycoprotein, that localises to the elastin microfibril interface Bressan et al. Four family members have now been identified: emilin-1, emilin-2, emilin-3 and multimerin also known as emilin-4 , all of which possess a long coiled-coil central region Colombatti et al.

Apart from emilin-1, it remains to be determined which members of this family bind elastic fibres. Collagen VIII, a product of vascular smooth muscle cells and endothelial cells and a component of their pericellular basement membranes, localises to vascular elastic fibres and may link them to vascular cells Sadawa and Konomi, Members of the fibulin family of cbEGF-domain molecules are also present at elastic fibre interfaces. Three family members are strongly implicated in elastic fibre biology: fibulin-1, fibulin-2 and fibulin Fibulin-1 is located within the amorphous core of elastic fibres but not in fibrillin-rich microfibrils Kostka et al.

Fibulin-5 localises to the elastin-microfibril interface Nakamura et al. Fibulin-2 localises preferentially at the interface between microfibrils and the elastin core.

It colocalises with fibrillin-1 in skin except adjacent to the dermal-epithelial junction , perichondrium, elastic intima of blood vessels and the kidney glomerulus, although it does not appear to be present in ciliary zonules, tendon, and surrounding lung alveoli and kidney tubules Reinhardt et al. Fibulin-2 is probably not needed for microfibril biomechanical integrity,since labelling is not linearly periodic, and it is absent from tissues subject to strong tensional forces e.

It is strongly expressed by smooth muscle cells during cardiovascular development and may be important in elastic fibre deposition and cell migration Tsuda et al. Originally identified in bovine tissue extracts designed to solubilise microfibrils Gibson et al. No general localisation to elastic fibres was observed, but staining in most tissues closely resembles type VI collagen see Table 1.

Interactions between hydrophobic domains are important in assembly and essential for elasticity Bellingham et al. The formation of covalent lysyl-derived desmosine crosslinks by lysyl oxidase Csiszar, stabilises the polymerised insoluble product elastin.

All share homology in their catalytic C-terminal region, but the existence of distinct N-termini suggests different functions. Significant progress has thus been made in identifying molecular components of the elastic fibre system, and the challenge now is to determine their biological roles.

Of the long list of microfibril-associated molecules, it is highly unlikely that all are involved in assembly. Fibrillins form the backbone of microfibrils, but more sophisticated assembly assays are needed if we are to determine whether co-localising molecules are fundamental structural elements or associated components.

In few cases e. MAGP-1, versican has a direct link been made between co-immunolocalisation with fibrillin and localisation on or within beaded microfibrils, and no molecules other than fibrillins have yet been shown to be necessary for microfibril assembly. New analytical approaches are needed to define microfibril composition in different tissues and to clarify the roles in elastin deposition of molecules at microfibril interfaces with elastin and cells.

Much work will then be required to establish how these molecules modulate microfibril function and elastic fibre formation. Domain structures of fibrillin-1 and elastin, showing molecular interaction sites identified in vitro see Molecular Interactions. A proline-rich region is towards the N-terminus. N-glycosylation sites are indicated.

B Elastin contains alternating hydrophobic and crosslinking domains. The C-terminus has two cysteines and a negatively charged pocket. Refolded MAGP-1 produced in a bacterial system can bind to the fibrillin-1 N-terminus within exons in a calcium-dependent manner Jensen et al.

MAGP-1 and fibrillin-1 are both substrates for transglutaminase and, although only homotypic fibrillin-1 crosslinks have been identified to date, MAGP-1 might be crosslinked within microfibrils Brown-Augsberger et al. The fibrillininteracting sequence is within or adjacent to the proline-rich region, and the interaction is with the decorin core protein.

Decorin can interact with both fibrillin-1 and MAGP-1 individually,and together they form a ternary complex. Fibrillin-2 appears not to interact with MAGP-1 or decorin. Its inability to interact with MAGP-1 suggests either that fibrillin-2 does not support tropoelastin deposition or that MAGP-1 is not necessary for this process see below.

In these two studies, MAGP-1 was expressed in mammalian or bacterial systems,which could explain seemingly contradictory decorin-MAGP-1 interaction results. The versican C-terminal lectin domain binds N-terminal fibrillin-1 sequences Isogai et al. However, its non-periodic association with microfibrils indicates that versican is probably not an integral structural component. Instead, it may associate with microfibrils, and its negatively charged chondroitin sulphate chains may influence integration of microfibrils into the surrounding ECM.

MAGP-1 binds to tropoelastin as well as microfibrillar molecules and might be a critical elastic-fibre-linking molecule Brown-Augsberger et al. The tropoelastin-binding site in MAGP-1 is a tyrosine-rich sequence within its positively charged N-terminal half, which may interact with a negatively charged pocket near the tropoelastin C-terminus.

MAGP-1 may interact first with fibrillin-1 and decorin during microfibril assembly and then with tropoelastin during elastic fibre formation on the microfibrillar template. Sequences in fibrillin-1 and fibrillin-2 within exons interact with tropoelastin but only in solid-phase studies, which suggests that exposure of a cryptic site is needed Trask et al. Both decorin and biglycan can bind to tropoelastin. Biglycan binds more avidly than decorin, and the biglycan core protein binds more strongly than the intact PG Reinboth et al.

The ability of biglycan to form a ternary complex with tropoelastin and MAGP-1 suggests that it has a role in the elastinogenesis phase of elastic fibre formation. Fibulin-1 does not bind to fibrillin-1 but binds tropoelastin with low affinity Sasaki et al.

Fibulin-2 is not crosslinked within microfibrils but strongly binds a fibrillin-1 N-terminal sequence within residues ;exons in a calcium-dependent interaction Reinhardt et al. This sequence is also reported to contain an MAGP-1 binding site see above. Competition by two molecules for the same fibrillinbinding site could represent an important mechanism for regulating microfibril function.

Fibulin-2 has a particularly high affinity for tropoelastin and also binds to basement membrane molecules. Fibulin-1 and fibulin-2 interact with the versican C-terminal lectin domain, and fibulin-2 also binds to aggrecan and brevican Olin et al. Fibulin-5 is a critical determinant of elastic fibre formation see below.

It binds strongly to tropoelastin in a calcium-dependent manner, but not to fibrillin-1, and colocalises with tropoelastin Nakamura et al. A bewildering jigsaw of molecular interactions involving fibrillin, elastin and various microfibril-associated molecules has begun to emerge from in vitro binding studies, and it will be a major challenge to define the temporal hierarchy of interactions that drive microfibril and elastic fibre assembly in vivo.

The extracellular appearance of fibrillin and assembled microfibrils precedes elastin deposition, and so the timeframe of secretion may provide clues to roles for associated molecules in microfibril assembly or elastin deposition. Comparisons between invertebrate microfibrils and vertebrate microfibril-elastin composites could help identify molecules required for elastin deposition.

Cellular and in vitro assembly assays are needed to unravel the significance in assembly of homotypic fibrillin interactions and complexes of fibrillin-1, MAGP-1, decorin, biglycan and tropoelastin. Use of purified assembled microfibrils as ligands may be a useful approach to investigate interactions with associated molecules. The dogma that tropoelastin is deposited on a preformed microfibrillar template Fig.

However, it now needs updating in terms of the roles of newly identified component molecules and the control exerted by connective tissue cells, both intracellular and extracellular, over elastic fibre assembly and organisation. Microfibril assembly is, in part, a cell-regulated process that proceeds independently of tropoelastin. Fibrillin-1 may undergo limited initial assembly in the secretory pathway Ashworth et al. Such associations may influence intracellular fibrillin assembly.

Fibrillins have N- and C-terminal cleavage sites for furin convertase; extracellular deposition requires removal of the C-terminus Raghunath et al. Tiedemann et al. CSPGs may be needed for beaded microfibril formation see above. Sulphation is needed for microfibril assembly, since chlorate treatment ablates microfibril and elastic fibre formation Robb et al.

Different extracellular microfibril populations have been identified. The extracellular environment might thus play a major role in regulating microfibril fate. In human dermal fibroblast cultures, monoclonal antibody 11C1.

The time-dependent appearance of 11C1. In developing vascular tissues, 11C1. Kogake, S. Haworth, unpublished. Different microfibril-associated molecules may influence epitope availability and commit microfibrils to distinct extracellular fates.

Under appropriate in vitro conditions of temperature and ionic strength,elastin undergoes a process of ordered self-aggregation called coascervation caused by multiple and specific interactions of individual hydrophobic domains, which are usually induced by an increase in temperature Bellingham et al.

The elastin aggregates formed through coascervation appear as ordered fibrillar structures resembling the elastic fibre core, indicating that the protein has an intrinsic ability to organise into polymeric structures.

In vivo,tropoelastin probably binds microfibrils, and then coascervates and becomes crosslinked by lysyl oxidase Fig. The molecular form of fibrillin secreted from cells is controversial. As in the case of most major ECM polymers, fibrillins can undergo limited intracellular assembly to form dimers or trimers that could be intermediates during extracellular assembly. However, monomers that have been excluded from assembly are detected in cell culture medium.

Difficulties in expressing full-length fibrillin-1 have so far precluded detailed assessment of whether fibrillin can self assemble. However, microscopy studies indicate that assembly occurs in association with cell surfaces and predict a key role for receptors in this process.

Subsequent time-dependent microfibril maturation in the ECM could reflect association with other molecules or transglutaminase crosslink formation. Both fibrillin and elastin interact with chaperones in the secretory pathway, but more work is needed if we are to understand how cells coordinate the production of microfibrillar molecules and elastin during elastic fibre formation, and how they prevent uncontrolled or inappropriate intracellular interactions.

Mouse models are proving to be powerful tools for identifying elastic fibre components and unravelling their biological roles Dietz and Mecham, Below, we describe models that show clear elastic fibre defects.

Some mice show thinning of the proximal aortic wall, which suggests that they experience aneurysmal dilatation as in human Marfan syndrome. Substantially reduced extracellular fibrillin-1 staining but normal elastin staining suggests that organised elastic fibres could accumulate in the absence of normal fibrillinrich microfibrils.

Heterozygous mice appear normal at birth and throughout adult life. Homozygous mice gradually develop severe kyphosis and die of Marfan-like vascular complications at about 4 months. Newborn homozygous mice have normal vascular anatomy and architecture, including aortic medial elastic lamellae. However,fibrillin hypomorphism appears to trigger a secondary sequence of cell-mediated events, which begin with focal calcifications in the aortic elastic lamellae, progress to intimal hyperplasia, monocytic infiltration of the media, fragmentation of elastic lamellae and loss of elastin content, and finally result in aneurysmal dilatation of the aortic wall.

Tight skin Tsk mice are a naturally occurring strain that harbours a large in-frame insertion in the fibrillin-1 gene Kielty et al. Chaudry and co-workers recently described fibrillin-2 null and fibrillin-2 mutant mice Chaudry et al. Positional cloning demonstrated that the syndactyly phenotype is caused by loss-of-function mutations in the fibrillin-2 gene.

Mutations in the sy allelic series of mice include deletion of the fibrillin-2 gene,premature termination in which homozygotes have no detectable fibrillin-2 protein, and in-frame exon 38 deletion part of the fourth TB module , which may cause a molecular kink; the resulting syndactyly ranges in severity from hard tissue to soft tissue fusion.

A second fibrillinknockout mouse has now been described Arteaga-Solis et al. Further analysis identified a functional interaction between fibrillinrich microfibrils and bone morphogenetic protein 7 BMP-7 signalling which, when disrupted, may lead to syndactyly.

Studies of elastin-null mice have confirmed that elastin is an essential determinant of arterial morphogenesis Li et al. The mice die of obstructive arterial disease, which results from subendothelial cell proliferation and reorganisation of smooth muscle, but not endothelial damage, thrombosis or inflammation. Hemizyous mice have an increased number of lamellar units in the ascending and descending aorta consistent with early developmental compensatory alterations in vessel wall structure.

The hemizygous mice phenotype is similar to SVAS, which may be a disease of haploinsufficiency. Fibulinnull mice exhibit vascular wall weakness, which could involve elastic fibre defects Kostka et al. Homozygotes die days after birth owing to rupture of blood vessels and massive haemorrhages, and also display kidney glomerular malformation or podocyte disorganisation and lung pathology delayed alveolar development.

Unexpectedly, fibulinnull mice have no obvious phenotype. Fibulinnull mice exhibit a severely disorganised elastic fibre system throughout the body Nakamura et al. They survive to adulthood but have a tortuous aorta with loss of compliance, severe emphysema and loose skin. These tissues contain fragmented elastin but no increased elastase activity, which suggests they have defects in elastic fibre assembly rather than stability. Fibrillinnull mouse models have provided new insights into the Marfan phenotype.

Haploinsufficiency or expression of low levels of an allele product that has dominant-negative potential is associated with mild skeletal phenotypes, whereas abundant expression of a dominant-negative allele product leads to a more severe Marfan phenotype.

Fibrillinnull and -mutant mice exhibit recessive syndactyly, indicating a loss-of-function possibly associated with altered BMP-7 activity. Targeted deletion of elastin causes structural and cellular vessel wall abnormalities and altered haemodynamics, indicating a role for elastin in regulating smooth muscle cell proliferation and stabilising arterial structure. Defects in elastic fibre formation in fibulinnull mice suggest a key role for this pericellular matrix molecule in cell-matrix interactions.

Although these models have provided valuable insights into the physiological roles of several microfibril and elastic fibre molecules, the lack of elastic fibre phenotype in other knockout mice could either reflect compensatory mechanisms or the fact that these molecules are not critical to elastic fibre formation and function. Elastic fibres are large and complex, but still surprisingly poorly understood, ECM macromolecules.

They are important because they endow critical mechanical properties on elastic tissues and regulate cell fate in developing tissues such as blood vessels. The major challenges ahead are to establish how cells regulate microfibril and elastic fibre assembly, to define the temporal hierarchy and repertoire of molecular interactions in assembly and to resolve their molecular composition. The biomechanical properties of tissue microfibrils and microfibril-elastin composites, and their molecular basis,must be better understood.

At the whole organism level, virtually nothing is yet known about how elastic fibres influence cell behaviour, and so identification of cellular receptors, signalling responses and growth factor relationships is a priority.

Together, these approaches will provide a new level of understanding of elastic fibre biology that, in turn, should lead to new strategies for elastic fibre repair and regeneration in ageing and disease.

Studies from our laboratory that are described here were supported by the Medical Research Council, UK. Read more about our commitment to Open Access. Submit your essay by 1 December for a chance to be published in Journal of Cell Science. Journal of Cell Science is pleased to welcome submissions for consideration for an upcoming special issue, Cell Biology of Motors , which will be guest edited by Anne Straube University of Warwick, UK. Submission deadline: 15 July Our resident insectivore, Mole, continues his latest series — The Corona Files.

Sign In or Create an Account. Advanced Search. User Tools. Supplement Connective tissue is one of the major types of tissue s in higher animals, including humans. One of the features of connective tissues is the presence of fibers. There are three major types of fibers associated with connective tissues: 1 collagen fiber s, 2 elastic fiber s, and reticular fiber s.

An elastic fiber is the connective tissue fiber that is yellowish in colour, as opposed to the collagen fiber that is whitish.

It is relatively thick, with a diameter of about 0. In some ligaments, though, the elastic fiber could have larger diameter. While collagen fiber is made up of collagen , the elastic fiber is made up of, primarily, elastin.

Elastin is a glycoprotein comprised mainly of glycine and proline residues. Another elastin fiber component is the fibrous fibrillin. The fibrillin provide a scaffold for deposition of elastin. The elastic fiber is distinguished from other connective tissue fiber s for its great elasticity. It can stretch up to one and a half times their length then snap back to its original length when relaxed. The elastic fibers are produced by fibroblast s and smooth muscle cells in the arteries.

A connective tissue that consists mainly of elastic fibers is referred to as elastic tissue. Variant s :. Gregor Mendel, an Austrian monk, is most famous in this field for his study of the phenotype of pea plants, including.. A sensory system is a part of the nervous system consisting of sensory receptors that receive stimuli from the internal..