Proteomic Analysis of the Bovine and Human Ciliary Zonule

Abstract

PurposeThe zonule of Zinn (ciliary zonule) is a system of fibers that centers the crystalline lens on the optical axis of the eye. Mutations in zonule components underlie syndromic conditions associated with a broad range of ocular pathologies, including microspherophakia and ectopia lentis. Here, we used HPLC–mass spectrometry to determine the molecular composition of the zonule.MethodsTryptic digests of human and bovine zonular samples were analyzed by HPLC–mass spectrometry. The distribution of selected components was confirmed by immunofluorescence confocal microscopy. In bovine samples, the composition of the equatorial zonule was compared to that of the hyaloid zonule and vitreous humor.ResultsThe 52 proteins common to the zonules of both species accounted for >95% of the zonular protein. Glycoproteins constituted the main structural components, with two proteins, FBN1 and LTBP2, constituting 70%–80% of the protein. Other abundant components were MFAP2, EMILIN-1, and ADAMTSL-6. Lysyl oxidase-like 1, a crosslinking enzyme implicated in collagen and elastin biogenesis, was detected at significant levels. The equatorial and hyaloid zonular samples were compositionally similar to each other, although the hyaloid sample was relatively enriched in the proteoglycan opticin and the fibrillar collagens COL2A1, COL11A1, COL5A2, and COL5A3.ConclusionsThe zonular proteome was surprisingly complex. In addition to structural components, it contained signaling proteins, protease inhibitors, and crosslinking enzymes. The equatorial and hyaloid zonules were similar in composition, but the latter may form part of a composite structure, the hyaloid membrane, that stabilizes the vitreous face.

Fell‐Muir Lecture: Fibrillin microfibrils: structural tensometers of elastic tissues?

“…This result confirmed our earlier proteomic analysis of microfibrils purified from zonules, aorta and skin, which had detected only the fibrillin-1 isoform in human zonules, alongside low levels of the microfibril-associated glycoprotein MAGP-1 (see 'Microfibril-associated molecules') (Cain et al 2006). A recent proteomic analysis (De Maria et al 2017) confirmed that fibrillin-1 and MAGP-1, with LTBP-2, are the most abundant components of human zonules.…”

“…A recent proteomic analysis (De Maria et al . ) confirmed that fibrillin‐1 and MAGP‐1, with LTBP‐2, are the most abundant components of human zonules.…”

“…It was also not detected in a proteomic analysis of zonules, despite the fact that ectopia lentis is often a feature of WMS (De Maria et al . ), although it did localize to ciliary zonules as shown by immunogold labelling (Kutz et al . ).…”

The lens growth process

“…The zonule serves to suspend the lens in the eye (see Figure 7A). Recent proteomic analysis of the human and bovine zonule has shown that Fibrillin 1 and LTBP2 (genes implicated in WMS-2, WMS-3, MSPKA, and GPHYSD-2) are the two most abundant components (de Maria et al, 2017). Neither is strongly expressed in the lens, however (Shi et al, 2013).…”

“…Neither is strongly expressed in the lens, however (Shi et al, 2013). Similarly, ADAMTSL2 has been detected in the zonule and to a lesser degree in the vitreous humor (de Maria et al, 2017). These clinical observations raise the possibility that the zonule, which constitutes the mechanical linkage between the lens and the wall of the eye, may actively modulate lens growth.…”