Instability of the cellular lipidome with age

Abstract

The human lens nucleus is formed in utero, and from birth onwards, there appears to be no significant turnover of intracellular proteins or membrane components. Since, in adults, this region also lacks active enzymes, it offers the opportunity to examine the intrinsic stability of macromolecules under physiological conditions. Fifty seven human lenses, ranging in age from 12 to 82 years, were dissected into nucleus and cortex, and the nuclear lipids analyzed by electrospray ionization tandem mass spectrometry. In the first four decades of life, glycerophospholipids (with the exception of lysophosphatidylethanolamines) declined rapidly, such that by age 40, their content became negligible. In contrast the level of ceramides and dihydroceramides, which were undetectable prior to age 30, increased approximately 100-fold. The concentration of sphingomyelins and dihydrosphingomyelins remained unchanged over the whole life span. As a consequence of this marked alteration in composition, the properties of fiber cell membranes in the centre of young lenses are likely to be very different from those in older lenses. Interestingly, the identification of age 40 years as a time of transition in the lipid composition of the nucleus coincides with previously reported macroscopic changes in lens properties (e.g., a massive age-related increase in lens stiffness) and related pathologies such as presbyopia. The underlying reasons for the dramatic change in the lipid profile of the human lens with age are not known, but are most likely linked to the stability of some membrane lipids in a physiological environment.

Fig. 1

The main glycerophospholipid classes present in the human lens. Lysophosphatidylethanolamines have the same structure as phosphatidylethanolamine except for the absence of a fatty acyl chain on the sn-2 position of the glycerol backbone. 1-O-Alkyl phosphatidylethanolamines have a 1-O-alkyl ether linked fatty acid at the sn-1 position instead of the diester linkage commonly found on glycerophospholipids. 1-O-Alkyl ether linked species are also abundant in phosphatidylserine and lysophosphatidylethanolamine in the human lens. R₁, R₂ hydrocarbon chain that corresponds to the substituent at the sn-1 and sn-2 positions, respectively

Fig. 2

The main sphingolipid classes present in the human lens. Dihydrosphingomyelin and sphingomyelin differ based on the presence of a double bond at the trans-4 position of the sphingoid backbone. Dihydroceramide and ceramide differ in an analogous manner. Sphingomyelins have a phosphocholine head group that differentiates them from their related ceramides. R hydrocarbon chains of various length and unsaturation levels

Fig. 3

Total phosphatidylcholine present in individual human lens nuclei of different ages. All values are expressed as nmol g⁻¹ tissue wet weight. The solid line is a generalized linear model fit; the dashed lines give a 95% confidence band

Fig. 4

Total phosphatidylserine present in individual human lens nuclei of different ages. All values are expressed as nmol g⁻¹ tissue wet weight. The solid line is a generalized linear model fit; the dashed lines give a 95% confidence band

Fig. 5

Total phosphatidylethanolamine present in individual human lens nuclei of different ages. All values are expressed as nmol g⁻¹ tissue wet weight. The solid line is a generalized linear model fit; the dashed lines give a 95% confidence band

Fig. 6

Total lysophosphatidylethanolamine present in individual human lens nuclei of different ages. All values are expressed as nmol g⁻¹ tissue wet weight. The solid line is a generalized linear model fit; the dashed lines give a 95% confidence band

Fig. 7

Total a ceramide and b dihydroceramide present in individual human lens nuclei of different ages. All values are expressed as nmol g⁻¹ tissue wet weight. The solid line is a generalized linear model fit; the dashed lines give a 95% confidence band

Fig. 8

Total a sphingomyelin and b dihydrosphingomyelin present in individual human lens nuclei of different ages. All values are expressed as nmol g⁻¹ tissue wet weight. In both cases, a horizontal line fits into the 95% confidence band of the generalized additive model fit; hence, the null hypothesis of no change with age cannot be rejected

Fig. 9

Precursor ion scan (m/z 184.1) for the phosphocholine head group, displaying dihydrosphingomyelin, sphingomyelin and phosphatidylcholine molecules in a 19-week-old fetus, 12-year-old, 36-year-old and 80-year-old human lens nucleus. Internal standards are present at m/z 649.6 and 818.6 (SM (d18:0/12:0) and PC (19:0/19:0), respectively)