Marfan syndrome: fibrillin expression and microfibrillar abnormalities in a family with predominant ocular defects

Case report: Biochemical and clinical phenotypes caused by cysteine substitutions in the epidermal growth factor-like domains of fibrillin-1

Marfan syndrome, an autosomal dominant disorder of connective tissue, is primarily caused by mutations in the fibrillin-1 (FBN1) gene, which encodes the protein fibrillin-1. The protein is composed of epidermal growth factor-like (EGF-like) domains, transforming growth factor beta-binding protein-like (TB) domains, and hybrid (Hyb) domains and is an important component of elastin-related microfibrils in elastic fiber tissue. In this study, we report a cysteine to tyrosine substitution in two different domains of fibrillin-1, both of which cause Marfan syndrome with ocular abnormalities, in two families. Using protease degradation and liquid chromatography-tandem mass spectrometry analyses, we explored the different effects of substitution of cysteine by tyrosine in an EGF-like and a calcium-binding (cb) EGF-like domain on protein stability. The results showed that cysteine mutations in the EGF domain are more likely to result in altered proteolytic sensitivity and thermostability than those in the cbEGF domain. Furthermore, cysteine mutations can lead to new enzymatic sites exposure or hidden canonical cleavage sites. These results indicate the differential clinical phenotypes and molecular pathogenesis of Marfan syndrome caused by cysteine mutations in different fibrillin-1 domains. These results strongly suggest that failure to form disulfide bonds and abnormal proteolysis of fibrillin-1 caused by cysteine mutations may be an important factor underlying the pathogenesis of diseases caused by fibrillin-1 mutations, such as Marfan syndrome.

Multiscale Imaging Reveals the Hierarchical Organization of Fibrillin Microfibrils

Fibrillin microfibrils are evolutionarily ancient, structurally complex extracellular polymers found in mammalian elastic tissues where they endow elastic properties, sequester growth factors and mediate cell signalling; thus, knowledge of their structure and organization is essential for a more complete understanding of cell function and tissue morphogenesis. By combining multiple imaging techniques, we visualize three levels of hierarchical organization of fibrillin structure ranging from micro-scale fiber bundles in the ciliary zonule to nano-scale individual microfibrils. Serial block-face scanning electron microscopy imaging suggests that bundles of zonule fibers are bound together by circumferential wrapping fibers, which is mirrored on a shorter-length scale where individual zonule fibers are interwoven by smaller fibers. Electron tomography shows that microfibril directionality varies from highly aligned and parallel, connecting to the basement membrane, to a meshwork at the zonule fiber periphery, and microfibrils within the zonule are connected by short cross-bridges, potentially formed by fibrillin-binding proteins. Three-dimensional reconstructions of negative-stain electron microscopy images of purified microfibrils confirm that fibrillin microfibrils have hollow tubular structures with defined bead and interbead regions, similar to tissue microfibrils imaged in our tomograms. These microfibrils are highly symmetrical, with an outer ring and interwoven core in the bead and four linear prongs, each accommodating a fibrillin dimer, in the interbead region. Together these data show how a single molecular building block is organized into different levels of hierarchy from microfibrils to tissue structures spanning nano- to macro-length scales. Furthermore, the application of these combined imaging approaches has wide applicability to other tissue systems.

•

Extracellular matrix fibrillin microfibrils assemble to form ocular ligaments.

•

Individual beaded fibrillin microfibrils are highly symmetric biological polymers.

•

Zonule fibers are composed of aligned, organized arrays of fibrillin microfibrils.

•

Bundles of zonule fibers are wrapped by large fibers providing structural support.

•

Fibrillin organization shows how a single building block constructs an elastic tissue.

Structural and compositional diversity of fibrillin microfibrils in human tissues

Elastic fibers comprising fibrillin microfibrils and elastin are present in many tissues, including the skin, lungs, and arteries, where they confer elasticity and resilience. Although fibrillin microfibrils play distinct and tissue-specific functional roles, it is unclear whether their ultrastructure and composition differ between elastin-rich (skin) and elastin-poor (ciliary body and zonule) organs or after in vitro synthesis by cultured cells. Here, we used atomic force microscopy, which revealed that the bead morphology of fibrillin microfibrils isolated from the human eye differs from those isolated from the skin. Using newly developed pre-MS preparation methods and LC-MS/MS, we detected tissue-specific regions of the fibrillin-1 primary structure that were differentially susceptible to proteolytic extraction. Comparing tissue- and culture-derived microfibrils, we found that dermis- and dermal fibroblast–derived fibrillin microfibrils differ in both bead morphology and periodicity and also exhibit regional differences in fibrillin-1 proteolytic susceptibility. In contrast, collagen VI microfibrils from the same dermal or fibroblast samples were invariant in ultrastructure (periodicity) and protease susceptibility. Finally, we observed that skin- and eye-derived microfibril suspensions were enriched in elastic fiber– and basement membrane–associated proteins, respectively. LC-MS/MS also identified proteins (such as calreticulin and protein-disulfide isomerase) that are potentially fundamental to fibrillin microfibril biology, regardless of their tissue source. Fibrillin microfibrils synthesized in cell culture lacked some of these key proteins (MFAP2 and -4 and fibrillin-2). These results showcase the structural diversity of these key extracellular matrix assemblies, which may relate to their distinct roles in the tissues where they reside.

Vesicoureteral reflux and the extracellular matrix connection

Primary vesicoureteral reflux (VUR) is a common pediatric condition due to a developmental defect in the ureterovesical junction. The prevalence of VUR among individuals with connective tissue disorders, as well as the importance of the ureter and bladder wall musculature for the anti-reflux mechanism, suggest that defects in the extracellular matrix (ECM) within the ureterovesical junction may result in VUR. This review will discuss the function of the smooth muscle and its supporting ECM microenvironment with respect to VUR, and explore the association of VUR with mutations in ECM-related genes.

A potential role for endogenous proteins as sacrificial sunscreens and antioxidants in human tissues

Excessive ultraviolet radiation (UVR) exposure of the skin is associated with adverse clinical outcomes. Although both exogenous sunscreens and endogenous tissue components (including melanins and tryptophan-derived compounds) reduce UVR penetration, the role of endogenous proteins in absorbing environmental UV wavelengths is poorly defined. Having previously demonstrated that proteins which are rich in UVR-absorbing amino acid residues are readily degraded by broadband UVB-radiation (containing UVA, UVB and UVC wavelengths) here we hypothesised that UV chromophore (Cys, Trp and Tyr) content can predict the susceptibility of structural proteins in skin and the eye to damage by physiologically relevant doses (up to 15.4 J/cm²) of solar UVR (95% UVA, 5% UVB). We show that: i) purified suspensions of UV-chromophore-rich fibronectin dimers, fibrillin microfibrils and β- and γ-lens crystallins undergo solar simulated radiation (SSR)-induced aggregation and/or decomposition and ii) exposure to identical doses of SSR has minimal effect on the size or ultrastructure of UV chromophore-poor tropoelastin, collagen I, collagen VI microfibrils and α-crystallin. If UV chromophore content is a factor in determining protein stability in vivo, we would expect that the tissue distribution of Cys, Trp and Tyr-rich proteins would correlate with regional UVR exposure. From bioinformatic analysis of 244 key structural proteins we identified several biochemically distinct, yet UV chromophore-rich, protein families. The majority of these putative UV-absorbing proteins (including the late cornified envelope proteins, keratin associated proteins, elastic fibre-associated components and β- and γ-crystallins) are localised and/or particularly abundant in tissues that are exposed to the highest doses of environmental UVR, specifically the stratum corneum, hair, papillary dermis and lens. We therefore propose that UV chromophore-rich proteins are localised in regions of high UVR exposure as a consequence of an evolutionary pressure to express sacrificial protein sunscreens which reduce UVR penetration and hence mitigate tissue damage.

Major structural proteins such as collagen I and tropoelastin are UVA-resistant.

In contrast, proteins which are rich in Cys, Trp and Tyr residues are UV-susceptible.

These proteins are concentrated in UV exposed tissues.

UV-chromophore (Cys, Trp, Tyr)-rich proteins may act as endogenous sunscreens.

Intraocular lens subluxation in marfan syndrome

PURPOSE

Ectopia lentis (EL) is a major criteria for the diagnosis of Marfan syndrome, it may vary from an asymptomatic mild displacement to a significant subluxation that places the equator of the lens in the pupillary axis. The purpose of this work is to present the case of a patient with Marfan syndrome who received treatment for subluxation at our institution.

CASE REPORT

A 51-year-old female diagnosed with Marfan syndrome presented to the emergency department with bilateral eye redness, foreign body sensation and crusting around the eyes on awakening. She had the following history of cardiac and ophthalmologic complications, including: 1. Lens subluxation 2. High myopia 3. Aortic root dilation, 4. Mitral valve prolapse and 5. Tricuspid insufficiency.

CONCLUSION

The ophthalmological management of Marfan patients is challenging and periodical follow-up is needed. Surgical versus conservative management is controversial, each case needs to be evaluated individually to analyze the risks and benefits of the procedures.

Damage to skin extracellular matrix induced by UV exposure

SIGNIFICANCE

Chronic exposure to environmental ultraviolet radiation (UVR) plays a key role in both photocarcinogenesis and induction of accelerated skin aging. Although the spatiotemporal consequences of UVR exposure for the composition and architecture of the dermal extracellular matrix (ECM) are well characterized, the pathogenesis of photoaging remains poorly defined. Given the compelling evidence for the role of reactive oxygen species (ROS) as mediators of photoaging, UVR-exposed human skin may be an accessible model system in which to characterize the role of oxidative damage in both internal and external tissues.

RECENT ADVANCES

Although the cell-mediated degradation of dermal components via UVR-induced expression of ECM proteases has long been identified as an integral part of the photoaging pathway, the relative importance and identity of cellular and extracellular photosensitizers (direct hit and bystanders models, respectively) in initiating this enzymatic activity is unclear. Recently, both age-related protein glycation and relative amino-acid composition have been identified as potential risk factors for photo-ionization and/or photo-sensitization. Here, we propose a selective multi-hit model of photoaging.

CRITICAL ISSUES

Bioinformatic analyses can be employed to identify candidate UVR targets/photosensitizers, but the action of UVR on protein structure and/or ROS production should be verified experimentally. Crucially, in the case of biochemically active ECM components such as fibronectin and fibrillin, the downstream effects of photo-degradation on tissue homeostasis remain to be confirmed.

FUTURE DIRECTIONS

Both topical antioxidants and inhibitors of detrimental cell signaling may be effective in abrogating the effects of specific UVR-mediated protein degradation in the dermis.

Molecular pathogenesis and management strategies of ectopia lentis

Ectopia lentis (EL) is a condition that can either herald underlying systemic conditions, or be isolated. The recent expansion in the genetics of these conditions has furthered the understanding of the underlying molecular aetiology. It is becoming apparent that novel genes, and in particular the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family, are important in ocular development. The common link in these genes seems to be EL. The clinical management of EL is challenging. In particular, the options for addressing surgically induced aphakia in the context of an ectopic capsule are varied. Little evidence exists to direct management of these issues. This review summarises the molecular pathogenesis of EL and conditions associated with it, using the genetic aetiology as a framework. Furthermore, it summarises some of the issues involved in its clinical management.

Fibrillin-1 and fibrillin-2 are essential for formation of thick oxytalan fibers in human nonpigmented ciliary epithelial cells in vitro

The ciliary zonule, also known as Zinn's zonule, is composed of oxytalan fibers. However, the mechanism by which epithelial cells in the ciliary body form these fibers in not fully understood. We examined human nonpigmented ciliary epithelial cells to determine the appearance and amount of oxytalan fibers in terms of positivity for their major components, fibrillin-1 and fibrillin-2. Examination of fibrillin-1 and fibrillin-2 expression by immunofluorescence revealed that thin fibers positive for fibrillin-1 on Day 2 changed to thick fibers by Day 8. The fibers positive for fibrillin-2 appeared on the thick fibrillin-1-positive fibers after Day 4. Northern blot analysis revealed that the level of fibrillin-1 did not change markedly, while induction of fibrillin-2 gene was evident on Day 5. Western blot analysis showed that fibrillin-1 deposition increased gradually, while that of fibrillin-2 increased markedly from Day 5 to Day 8. Fibrillin-1 suppression did not lead to the formation of fibrillin-2-positive thick fibers, whereas fibrillin-2 suppression led to the formation of fibrillin-1-positive thin fibers, but not thick fibers. These results suggest that both fibrillin-1 and fibrillin-2 are essential for the formation of thick oxytalan fibers in the ciliary zonule and are informative for clarifying the mechanism of homeostasis of the ocular matrix.

Okuläre Manifestationen und chirurgische Ergebnisse beim Marfan-Syndrom

AIM

This retrospective study should examine and judge the surgical indications and the therapeutic possibilities as well as their complications in patients with ocular manifestations of Marfan syndrome (MFS) diagnosed according to the criteria of the Ghent nosology.

PATIENTS AND METHODS

The study included 17 patients. Operative indications were increasing subluxation of the lens, retinal detachments and secondary glaucoma. The operative procedure depended on patient age and findings. Eleven MFS patients were operated in both eyes and six MFS patients in one eye.

RESULTS

Stabilization or functional improvement of visual acuity could be achieved in all patients in whom no disorders limiting visual acuity or amblyopia were present preoperatively. In six eyes of five patients, lens insertion was accomplished via a pars plana approach. Lens removal without implantation of an intraocular lens was performed in 16 eyes of 10 patients. Pars plana vitrectomy was accomplished in 12 eyes. Complications were well controlled by pars plana vitrectomy.

CONCLUSIONS

Difficult preoperative situations and postoperative complications are not rare in MFS patients. However, they can be controlled well by means of modern vitreous surgery.

The molecular genetics of Marfan syndrome and related disorders

Marfan syndrome (MFS), a relatively common autosomal dominant hereditary disorder of connective tissue with prominent manifestations in the skeletal, ocular, and cardiovascular systems, is caused by mutations in the gene for fibrillin-1 (FBN1). The leading cause of premature death in untreated individuals with MFS is acute aortic dissection, which often follows a period of progressive dilatation of the ascending aorta. Recent research on the molecular physiology of fibrillin and the pathophysiology of MFS and related disorders has changed our understanding of this disorder by demonstrating changes in growth factor signalling and in matrix-cell interactions. The purpose of this review is to provide a comprehensive overview of recent advances in the molecular biology of fibrillin and fibrillin-rich microfibrils. Mutations in FBN1 and other genes found in MFS and related disorders will be discussed, and novel concepts concerning the complex and multiple mechanisms of the pathogenesis of MFS will be explained.

Early Implantation Events in the Baboon (Papio anubis) with Special Reference to the Establishment of Anchoring Villi

The development of the baboon anchoring villus has been studied from day 14 to day 48 of gestation, using light and electron microscopy. At day 14, cords of trophoblast could be seen streaming into the endometrium, invading maternal vessels and forming blood-filled lacunae; by 20 days gestation some of these had differentiated into distinctive anchoring villi, with an outer covering of syncytiotrophoblast and inner cytotrophoblast cells which differed from those of floating villi in that a subpopulation detached from the syncytium to form an interconnecting network of cells within the centre of the villus. Subsequent migration of cytotrophoblast into the endometrium formed the cytotrophoblastic shell while fibrillin-like extracellular matrix biosynthesis within the body of the villus provided a firm mechanical support. At the trophoblast-decidual interface, a zone of necrosis and phagocytosis initially developed, which became less extensive with time, so that by 40 days a stable interface was evident with only residual pockets of necrosis. During this period, there was differentiation of decidual cells which by 28 days developed characteristic pedunculated cell processes, and later became surrounded by a basal lamina. The factors that control detachment of cytotrophoblast from the syncytium and the biosynthesis of the specialized, fibrillar extracellular matrix, features that are not apparent in other placental villi, require further investigation, possibly by carefully controlled in vitro experimentation.

Structure of anchoring villi and the trophoblastic shell in the human, baboon and macaque placenta

Anchoring villi of first trimester placentae of the macaque, baboon and human were examined by light and electron microscopy. The anchoring villi of the baboon and macaque are similar in having more elongated cell columns than those of the human and in having more extracellular matrix between cytotrophoblast cells. These species also have a thicker and more uniform trophoblastic shell. The generative region of cytotrophoblast cells adjacent to the villous mesenchyme is similar in all three species, with the aspect of the core abutting this area being lined by a thickened basal lamina. Similarly, migratory cytotrophoblast cells form extracellular matrix in all three species, but matrix-rich regions of the anchoring villi and shell are more extensive in the non-human primates. The extracellular matrix and especially the material resembling fibrillin may serve to strengthen the villi, particularly the elongated villi of the non-human primate, and also may prevent maternal cells migrating into the trophoblastic shell. The baboon and macaque cytotrophoblast cells that form this matrix tend to be linked by gap and desmosomal junctions and are in contiguous arrays, whereas those in the human that are blocked from reaching normal decidua form abundant extracellular matrix but have no gap junctions. Whether the lack of extensive invasion of the endometrium by baboon and macaque cytotrophoblast cells is related to the increased amount of extracellular matrix, their greater distance from the mesenchymal core, or their intercellular linkages is not known. The investigation of isolated villi from the macaque or baboon, as has been extensively carried out in the human, might help to determine whether the cytotrophoblast cells are intrinsically different or are responding to different environmental cues.

Fibrillin-rich microfibrils of the extracellular matrix: ultrastructure and assembly

Fibrillin-rich microfibrils are a unique class of extensible connective tissue macromolecules. Their critical contribution to the establishment and maintenance of diverse extracellular matrices was underlined by the linkage of their principal structural component fibrillin to Marfan syndrome, a heritable connective tissue disorder with pleiotropic manifestations. Microscopy and preparative techniques have contributed substantially to the understanding of microfibril structure and function. The supramolecular organisation of microfibrillar assemblies in tissues has been examined by tissue sectioning and X-ray diffraction methods. Published findings are discussed and new information reported on the organisation of microfibrils in the ciliary zonular fibrils by environmental scanning electron microscopy. This review summarises microscopy and X-ray diffraction studies that are informing current understanding of the ultrastructure of fibrillin-rich microfibrils.

Frequency of His1069Gln and Gly1267Lys mutations in Polish Wilson's disease population

Wilson's disease is an autosomal recessive disorder. More than 60 mutations of the Wilson's disease gene have been described so far. We have analysed 148 Polish Wilson's disease patients from 95 families for His1069Gln and Gly1267Lys mutations and correlated this finding with age and clinical form of the disease at presentation. To identify these mutations, single strand conformation polymorphism analysis was performed. In our group there were 94 patients with neurological presentation, 28 with hepatic presentation, whilst 26 were in a pre-clinical stage of the disease. His1069Gln mutation was present on 171 (57%) of the 296 studied chromosomes, and Gly1267Lys mutation was present on 27 chromosomes (9.1%). Most of our patients were homozygous or heterozygous for His1069Gln mutation (39.9% and 30.4%, respectively); 4% of the patients were homozygous for Gly1267Lys mutation and 5.4% had both of these described mutations on their chromosomes. His1069Gln and Gly1267Lys mutations occurred often in our Wilson's disease patient population but we did not find any relationship between investigated mutations and the clinical form of Wilson's disease or age of first symptoms.

The molecular genetics of Marfan syndrome and related microfibrillopathies

Mutations in the gene for fibrillin-1 (FBN1) have been shown to cause Marfan syndrome, an autosomal dominant disorder of connective tissue characterised by pleiotropic manifestations involving primarily the ocular, skeletal, and cardiovascular systems. Fibrillin-1 is a major component of the 10-12 nm microfibrils, which are thought to play a role in tropoelastin deposition and elastic fibre formation in addition to possessing an anchoring function in some tissues. Fibrillin-1 mutations have also been found in patients who do not fulfil clinical criteria for the diagnosis of Marfan syndrome, but have related disorders of connective tissue, such as isolated ectopia lentis, familial aortic aneurysm, and Marfan-like skeletal abnormalities, so that Marfan syndrome may be regarded as one of a range of type 1 fibrillinopathies. There appear to be no particular hot spots since mutations are found throughout the entire fibrillin-1 gene. However, a clustering of mutations associated with the most severe form of Marfan syndrome, neonatal Marfan syndrome, has been noted in a region encompassing exons 24 to 32. The gene for fibrillin-2 (FBN2) is highly homologous to FBN1, and mutations in FBN2 have been shown to cause a phenotypically related disorder termed congenital contractural arachnodactyly. Since mutations in the fibrillin genes are likely to affect the global function of the microfibrils, the term microfibrillopathy may be the most appropriate to designate the spectrum of disease associated with dysfunction of these molecules. The understanding of the global and the molecular functions of the fibrillin containing microfibrils is still incomplete and, correspondingly, no comprehensive theory of the pathogenesis of Marfan syndrome has emerged to date. Many, but not all, fibrillin-1 gene mutations are expected to exert a dominant negative effect, whereby mutant fibrillin monomers impair the global function of the microfibrils. In this paper we review the molecular physiology and pathophysiology of Marfan syndrome and related microfibrillopathies.

Calcium determines the supramolecular organization of fibrillin-rich microfibrils

Microfibrils are ubiquitous fibrillin-rich polymers that are thought to provide long-range elasticity to extracellular matrices, including the zonular filaments of mammalian eyes. X-ray diffraction of hydrated bovine zonular filaments demonstrated meridional diffraction peaks indexing on a fundamental axial periodicity (D) of approximately 56 nm. A Ca2+-induced reversible change in the intensities of the meridional Bragg peaks indicated that supramolecular rearrangements occurred in response to altered concentrations of free Ca2+. In the presence of Ca2+, the dominant diffracting subspecies were microfibrils aligned in an axial 0.33-D stagger. The removal of Ca2+ caused an enhanced regularity in molecular spacing of individual microfibrils, and the contribution from microfibrils not involved in staggered arrays became more dominant. Scanning transmission electron microscopy of isolated microfibrils revealed that Ca2+ removal or addition caused significant, reversible changes in microfibril mass distribution and periodicity. These results were consistent with evidence from x-ray diffraction. Simulated meridional x-ray diffraction profiles and analyses of isolated Ca2+-containing, staggered microfibrillar arrays were used to interpret the effects of Ca2+. These observations highlight the importance of Ca2+ to microfibrils and microfibrillar arrays in vivo.