Calcium Distribution in the Human Eye Lens

Vrensen, Gijs F.J.M.; Wolf, Alexandra

doi:10.1159/000267960

Cited by 18 publications

(19 citation statements)

References 8 publications

(11 reference statements)

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“…Using electron tomographic reconstructions, we consistently found that the perimembranous precipitates along the fibres as described by Vrensen et al (1995Vrensen et al ( , 1996 were restricted to the intercellular space between the fibres since these precipitates were consistently observed to be smooth at the fibre cell membrane side irrespective of the orientation of the cross section through the reconstructed volume. The precipitates always protruded into the intercellular space, and in most cases the centre of the intercellular space was free of precipitate.…”

Section: Discussionsupporting

confidence: 76%

“…No reliable data on the intercellular concentration of calcium are available. Vrensen et al (1995Vrensen et al ( , 1996 used the oxalate-pyroantimonate (OPA) technique for EM localization of free or ionically bound calcium (VanReempts, Borgers and Offner, 1982) in normal rat and human lenses. It was observed in rat and human lenses that in the epithelium and superficial cortex calcium precipitates are scanty and if present at all they are restricted to mitochondria, to the cisterns of the endoplasmic reticulum and Golgi zone and to the nuclear envelope.…”

Section: Introductionmentioning

confidence: 99%

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Calcium and its Localization in Human Lens Fibres: An Electron Tomographic Study

VanMarle

Jonges

Vrensen

et al. 1997

Experimental Eye Research

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The ultrastructural distribution of calcium was studied in human lens fibres with the oxalate pyroantimonate technique. In the intermediate cortex precipitates were found perimembranous and occasionally in the fibre cytoplasm. The improved resolution of electron tomography revealed (a) that the perimembranous precipitates as described earlier by Vrensen et al. (1995) are restricted to the intercellular space ; no indications were found of an increased intracellular submembranous calcium level, and (b) the existence of an intracellular pool of small protein attached precipitates in the fibre cytoplasm not observed with conventional electron microscopy. It is concluded that in the intermediate and deep cortex, where in the mature fibres all cellular calcium pools have disappeared, two calcium pools exist : (a) a pool of free calcium ionically bound to the negatively charged phospholipids of the external face of the fibre membrane and (b) a cytoplasmic pool of protein associated calcium. Possible candidates for this cytoplasmic calcium binding are discussed.

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Section: Discussionsupporting

confidence: 76%

Section: Introductionmentioning

confidence: 99%

Calcium and its Localization in Human Lens Fibres: An Electron Tomographic Study

VanMarle

Jonges

Vrensen

et al. 1997

Experimental Eye Research

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“…Preceding organelle degradation, an increase in cytoplasmic calcium precipitates has been observed, before the Ca 2+ relocalises to the plasma membranes in organelle-free fibre cells [85,86]. It has been pointed out that the calcium measured in ultrastructural analyses of the lens only represents ionically bound Ca 2+ , since free Ca 2+ is lost during sample preparation [85], suggesting that the amount of Ca 2+ liberated during organelle breakdown is larger than that measured in this study. These data indicate that calcium signalling might be involved in the process of organelle loss in the lens.…”

Section: Intracellular Calciummentioning

confidence: 64%

“…As in most other cells, Ca 2+ in lens epithelial cells and early differentiating fibre cells has been shown to be sequestered in mitochondria, the Golgi apparatus, the endoplasmic reticulum and the nuclear envelope [85]. During fibre cell differentiation however, these organelles are broken down and Ca 2+ is liberated.…”

Section: Intracellular Calciummentioning

confidence: 91%

Lens Fibre Cell Differentiation – A Link with Apoptosis?

Dahm¹

1999

Ophthalmic Res

100

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During the process of terminal differentiation, the fibre cells in the eye lens undergo many changes that are reminiscent of apoptotic/necrotic changes. Mitochondria, for instance, undergo permeability transition and nuclear degradation is accompanied by chromatin condensation, disintegration of the nucleolus, dissolution of the nuclear lamina, nuclear pore clustering and fragmentation of the DNA into oligonucleosomal fragments. As during apoptosis, members of the caspase family of proteases were shown to be active during fibre cell differentiation and Bcl-2 overexpression was demonstrated to block normal differentiation of lens cells. In this review, the current knowledge of the sequence of events during cell death is summarised and contrasted with events during lens fibre cell differentiation. Due to the numerous similarities between these processes, lens fibre cell differentiation is suggested to represent an attenuated form of cell death.

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“…Thus there must exist calcium regulation systems in the lens, and it is of interest to identify what they are and how they change in health and in disease. Vrensen et al (3) have done an ultrastructural analysis of calcium distribution in the rat lens and have found calcium precipitates in the intermediate cortex fiber membranes, cytoplasm, and the nuclear envelope and very low levels of calcium in gap junctions, epithelial cells, and superficial fibers (3)(4)(5). The question of what the calcium-binding and -storing agents are in the lens is open; phospholipids and crystallins have been thought of as candidates.…”

mentioning

confidence: 99%

Calcium Binding Properties of γ-Crystallin

Rajini

Shridas

Sundari

et al. 2001

Journal of Biological Chemistry

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The ␤-and ␥-crystallins are closely related lens proteins that are members of the ␤␥-crystallin superfamily, which also include many non-lens members. Although ␤-crystallin is known to be a calcium-binding protein, this property has not been reported in ␥-crystallin. We have studied the calcium binding properties of ␥-crystallin, and we show that it binds 4 mol eq of calcium with a dissociation constant of 90 M. It also binds the calcium-mimic spectral probes, terbium and Stains-all. Calcium binding does not significantly influence protein secondary and tertiary structures. We present evidence that the Greek key crystallin fold is the site for calcium ion binding in ␥-crystallin. Peptides corresponding to Greek key motif of ␥-crystallin (42 residues) and their mutants were synthesized and studied for calcium binding. These peptides adopt ␤-sheet conformation and form aggregates producing ␤-sandwich. Our results with peptides show that, in Greek key motif, the amino acid adjacent to the conserved aromatic corner in the "a" strand and three amino acids of the "d" strand participate in calcium binding. We suggest that the ␤␥ superfamily represents a novel class of calcium-binding proteins with the Greek key ␤␥-crystallin fold as potential calcium-binding sites. These results are of significance in understanding the mechanism of calcium homeostasis in the lens.Calcium homeostasis plays an important role in lens transparency, opacification, and cataractogenesis. Cataracts can occur both under hypocalcemic and hypercalcemic conditions, so the actual amount of available calcium in the lens is an important parameter for the health of the lens (1, 2). The normal mammalian lens has around 0.2 mM total calcium, of which the amount of free Ca 2ϩ is only of the order of a few micromolars. Thus there must exist calcium regulation systems in the lens, and it is of interest to identify what they are and how they change in health and in disease. Vrensen et al. (3) have done an ultrastructural analysis of calcium distribution in the rat lens and have found calcium precipitates in the intermediate cortex fiber membranes, cytoplasm, and the nuclear envelope and very low levels of calcium in gap junctions, epithelial cells, and superficial fibers (3-5). The question of what the calcium-binding and -storing agents are in the lens is open; phospholipids and crystallins have been thought of as candidates. The major components of the lens are cytosolic proteins, crystallins, which account for about 40% of the wet weight of the lens. It is worth investigating whether any of the crystallins could act as a calcium sponge or storage depot in the tissue, particularly since the ultrastructural analysis shows calcium distribution in the cytoplasm. We have earlier shown that the ␤-and avian core protein ␦-crystallins show significant calcium-binding ability (6, 7). Thus, the possibility of crystallins acting as lenticular calcium-sequestering and -storing systems exists. However, the calcium binding properties of ␥-crystallin have not yet been...

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Calcium Distribution in the Human Eye Lens

Cited by 18 publications

References 8 publications

Calcium and its Localization in Human Lens Fibres: An Electron Tomographic Study

Calcium and its Localization in Human Lens Fibres: An Electron Tomographic Study

Lens Fibre Cell Differentiation – A Link with Apoptosis?

Calcium Binding Properties of γ-Crystallin

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