Ageing Of The Taurine Transport System Implications For Disease

After release from the choriocapillaris, taurine must traverse Bruch's membrane by passive diffusional processes prior to uptake and delivery by active transporters in the RPE to primarily the photoreceptors of the retina. Ageing-related changes within these compartments are therefore likely to influence the trafficking of this amino acid, and the degree of modulation is expected to impact on both age-related diseases such as macular degeneration and other pathological alterations where the requirements for taurine are particularly exaggerated.

An analysis of the taurine transport system in the ageing human eye is complicated by uncontrollable post-mortem delays prior to analysis. For a transport system that relies on the energy status of the cell, such delays are expected to alter its functional characteristics. Since Bruch's membrane is an extracellular matrix, short post-mortem delays are expected to cause minimal damage and many studies have now confirmed the utility of such a preparation for transport studies (63-65). The stability of the high affinity taurine transport system in the retina/RPE of baboon and man has been investigated and results displayed in Fig. 3.

Baboon eyes were obtained fresh and stored on ice immediately after enucleation. Human donor eyes were enucleated at the earliest opportunity, but those within six hours were obtained from surgical enucleations for melanomas. Although some stability of transport was observed in baboon retinas, human retinal preparations showed a rapid fall in transport capability soon after death of donor. Kinetic analyses of human retinas showed a stable Km for the high affinity carrier of 47 ± 5 |M but the Vmax of the transport process diminished rapidly as a function of post-mortem time. As Fig. 3 demonstrates, the RPE was resistant to post-mortem delays and the tissue compartment was viable for the 48 hours of experimentation shown. Probing with a substrate concentration of 1 to 60 |M taurine,

Fig. 3. The effect of age on the diffusion of taurine across the isolated human Brach's-choroid complex. In these experiments, isolated samples of Bruch's-choroid were clamped in an Ussing chamber apparatus, and taurine, at a concentration of 10 mM, was added to one half-compartment. The opposing half-compartment was sampled at regular intervals and the amount of taurine traversing the preparation quantified by high-performance liquid chromatography. The respective diffusional fluxes were calculated and plotted as a function of age of donor. This showed a significant age-related decline in transport of taurine across Bruch's membrane (p < 0.05).

Fig. 3. The effect of age on the diffusion of taurine across the isolated human Brach's-choroid complex. In these experiments, isolated samples of Bruch's-choroid were clamped in an Ussing chamber apparatus, and taurine, at a concentration of 10 mM, was added to one half-compartment. The opposing half-compartment was sampled at regular intervals and the amount of taurine traversing the preparation quantified by high-performance liquid chromatography. The respective diffusional fluxes were calculated and plotted as a function of age of donor. This showed a significant age-related decline in transport of taurine across Bruch's membrane (p < 0.05).

a high-affinity carrier was identified in human RPE cells with kinetic characteristics of K = 50 ± 10 uM and V = 267 ±48 pmol taurine/10 mins/5 mm disc of tissue. Thus, the m r max r 7

status of taurine transport across Bruch's membrane and the RPE can be evaluated in relation to the ageing process.

Bruch's membrane undergoes age-related morphological and compositional alterations, with its thickness increasing two- to threefold over the life span of an individual (66,67). The increase in the thickness of Bruch's membrane alone is expected to decrease the driving concentration gradients across the membrane for diffusional transport of taurine. Compositional alterations include the deposition of lipids (68), proteins, and carbohydrate-associated moieties such as glycolipids, glycoproteins, and proteoglycans (69). The insoluble collagen content of Bruch's membrane was shown to increase linearly with age (70) and over a 10-decade period accounted for nearly 50% of membrane collagen. The insolubility is indicative of denaturation and subsequent chemical modification including greater cross-linking and formation of advanced glycation endproducts (AGEs). All these age-related changes compromise membrane porosity and their effects on the diffusion of taurine are shown in Fig. 4. Ageing was associated with a significant decline in diffusional transport of taurine (p < 0.05). This has ramifications not only for normal ageing, but more so for advanced changes associated with macular degeneration or in Sorsby's fundus dystrophy where Bruch's membrane is considerably thickened (71).

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