Ultrastructural properties of ciliary zonule microfibrils

Davis, Elaine C.; Roth, Robyn; Heuser, John E.; Mecham, Robert P.

doi:10.1016/s1047-8477(02)00559-2

Cited by 32 publications

(18 citation statements)

References 43 publications

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“…Beads and interbead filaments in zonular microfibrils, viewed after quick-freeze deep-etch techniques, were also not apparent (38). In the latter study, intact native microfibrils were continuous and uniform, with a twisting beaded appearance.…”

Section: Discussionmentioning

confidence: 60%

Effects of Fibrillin-1 Degradation on Microfibril Ultrastructure

Kuo

Isogai

Keene

et al. 2007

Journal of Biological Chemistry

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Current models of the elastic properties and structural organization of fibrillin-containing microfibrils are based primarily on microscopic analyses of microfibrils liberated from connective tissues after digestion with crude collagenase. Results presented here demonstrate that this digestion resulted in the cleavage of fibrillin-1 and loss of specific immunoreactive epitopes. The proline-rich region and regions near the second 8-cysteine domain in fibrillin-1 were easily cleaved by crude collagenase. Other sites that may also be cleaved during microfibril digestion and extraction were identified. In contrast to collagenase-digested microfibrils, guanidine-extracted microfibrils contained all fibrillin-1 epitopes recognized by available antibodies. The ultrastructure of guanidine-extracted microfibrils differed markedly from that of collagenase-digested microfibrils. Fibrillin-1 filaments splayed out, extending beyond the width of the periodic globular beads. Both guanidine-extracted and collagenase-digested microfibrils were subjected to extensive digestion by crude collagenase. Collagenase digestion of guanidine-extracted microfibrils removed the outer filaments, revealing a core structure. In contrast to microfibrils extracted from tissues, cell culture microfibrils could be digested into short units containing just a few beads. These data suggest that additional cross-links stabilize the long beaded microfibrils in tissues. Based on the microfibril morphologies observed after these experiments, on the crude collagenase cleavage sites identified in fibrillin-1, and on known antibody binding sites in fibrillin-1, a model is proposed in which fibrillin-1 molecules are staggered in microfibrils. This model further suggests that the N-terminal half of fibrillin-1 is asymmetrically exposed in the outer filaments, whereas the C-terminal half of fibrillin-1 is present in the interior of the microfibril.Microfibrils have been extracted from a variety of tissues and visualized by electron microscopy as distinctive beaded string structures (1, 2). The molecular composition of these structures is assumed to be complex. However, based upon immunolocalization of fibrillin to these structures (2, 3) and the shape of fibrillin monomers (4, 5), fibrillins are thought to be the major backbone components of these extracted microfibrils. Fibrillincontaining microfibrils are ubiquitous in the connective tissue space (6), providing architectural support as well as information essential for appropriate signaling during morphogenesis (7-9). In human disorders associated with fibrillins, the structural integrity of fibrillin microfibrils is required for the proper function of certain connective tissues (10). Therefore, determination of the organization of fibrillin molecules within the microfibril is important basic information.The organization of fibrillin molecules within microfibrils has been controversial. Both parallel (head-to-tail) (3, 4) and antiparallel (11) arrangements of fibrillin molecules within microfibrils have been...

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Section: Discussionmentioning

confidence: 60%

Effects of Fibrillin-1 Degradation on Microfibril Ultrastructure

Kuo

Isogai

Keene

et al. 2007

Journal of Biological Chemistry

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“…Our data suggests that LTBP1 elongates in an end-to-end manner while the N-terminus may act as a branching point. Small filaments seen to bridge fibrillin microfibrils in the ciliary zonules of the eye 38 , could represent short LTBP filaments connecting adjacent microfibrils, indeed LTBP2 is required for correct ciliary zonule structure and microfibril bundling 4 .…”

Section: Discussionmentioning

confidence: 99%

Independent multimerization of Latent TGFβ Binding Protein-1 stabilized by cross-linking and enhanced by heparan sulfate

Troilo

Steer

Collins

et al. 2016

Sci Rep

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TGFβ plays key roles in fibrosis and cancer progression, and latency is conferred by covalent linkage to latent TGFβ binding proteins (LTBPs). LTBP1 is essential for TGFβ folding, secretion, matrix localization and activation but little is known about its structure due to its inherent size and flexibility. Here we show that LTBP1 adopts an extended conformation with stable matrix-binding N-terminus, extended central array of 11 calcium-binding EGF domains and flexible TGFβ-binding C-terminus. Moreover we demonstrate that LTBP1 forms short filament-like structures independent of other matrix components. The termini bind to each other to facilitate linear extension of the filament, while the N-terminal region can serve as a branch-point. Multimerization is enhanced in the presence of heparin and stabilized by the matrix cross-linking enzyme transglutaminase-2. These assemblies will extend the span of LTBP1 to potentially allow simultaneous N-terminal matrix and C-terminal fibrillin interactions providing tethering for TGFβ activation by mechanical force.

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“…Among their proposed independent roles are conferring structural integrity to tissues, cell–anchorage through Arg–Gly–Asp (RGD) and heparin-binding sites present in each fibrillin, and growth factor regulation via binding to the large latent complexes of transforming growth factor-β (TGFβ) or bone morphogenetic proteins (BMPs) [4–9]. An unusual structure, the ocular zonule, is a cell free, macroscopic, microfibril-based rigging that spans two basement membranes, the lens capsule and the internal limiting membrane of the ciliary body, providing a matrix–matrix anchorage [10–12]. The zonule centers the lens in the optic path and mediates accommodation by transmitting ciliary sphincter contraction and relaxation to the lens.…”

Section: Introductionmentioning

confidence: 99%

ADAMTS proteins as modulators of microfibril formation and function

2015

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The ADAMTS (a disintegrin-like and metalloproteinase domain with thrombospondin-type 1 motifs) protein superfamily includes 19 secreted metalloproteases and 7 secreted ADAMTS-like (ADAMTSL) glycoproteins. The possibility of functional linkage between ADAMTS proteins and fibrillin microfibrils was first revealed by a human genetic consilience, in which mutations in ADAMTS10, ADAMTS17, ADAMTSL2 and ADAMTSL4 were found to phenocopy rare genetic disorders caused by mutations affecting fibrillin-1 (FBN1), the major microfibril component in adults. The manifestations of these ADAMTS gene disorders in humans and animals suggested that they participated in the structural and regulatory roles of microfibrils. Whereas two such disorders, Weill–Marchesani syndrome 1 and Weill–Marchesani-like syndrome involve proteases (ADAMTS10 and ADAMTS17, respectively), geleophysic dysplasia and isolated ectopia lentis in humans involve ADAMTSL2 and ADAMTSL4, respectively, which are not proteases. In addition to broadly similar dysmorphology, individuals affected by Weill–Marchesani syndrome 1, Weill–Marchesani-like syndrome or geleophysic dysplasia each show characteristic anomalies suggesting molecule-, tissue-, or context-specific functions for the respective ADAMTS proteins. Ectopia lentis occurs in each of these conditions except geleophysic dysplasia, and is due to a defect in the ciliary zonule, which is predominantly composed of FBN1 microfibrils. Together, this strongly suggests that ADAMTS proteins are involved either in microfibril assembly, stability, and anchorage, or the formation of function-specific supramolecular networks having microfibrils as their foundation. Here, the genetics and molecular biology of this subset of ADAMTS proteins is discussed from the perspective of how they might contribute to fully functional or function-specific microfibrils.

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Ultrastructural properties of ciliary zonule microfibrils

Cited by 32 publications

References 43 publications

Effects of Fibrillin-1 Degradation on Microfibril Ultrastructure

Effects of Fibrillin-1 Degradation on Microfibril Ultrastructure

Independent multimerization of Latent TGFβ Binding Protein-1 stabilized by cross-linking and enhanced by heparan sulfate

ADAMTS proteins as modulators of microfibril formation and function

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