Lipofuscin in retinal pigment epithelium of rhesus monkey: lack of diminution with centrophenoxine treatment

Multispectral LEDs Eliminate Lipofuscin-Associated Autofluorescence for Immunohistochemistry and CD44 Variant Detection by in Situ Hybridization in Aging Human, non-Human Primate, and Murine Brain

A major limitation of mechanistic studies in aging brains is the lack of routine methods to robustly visualize and discriminate the cellular distribution of tissue antigens using fluorescent immunohistochemical multi-labeling techniques. Although such approaches are routine in non-aging brains, they are not consistently feasible in the aging brain due to the progressive accumulation of autofluorescent pigments, particularly lipofuscin, which strongly excite and emit over a broad spectral range. Consequently, aging research has relied upon colorimetric antibody techniques, where discrimination of tissue antigens is often challenging. We report the application of a simple, reproducible, and affordable protocol using multispectral light-emitting diodes (mLEDs) exposure for the reduction/elimination of lipofuscin autofluorescence (LAF) in aging brain tissue from humans, non-human primates, and mice. The mLEDs lamp has a broad spectral range that spans from the UV to infrared range and includes spectra in the violet/blue and orange/red. After photo quenching, the LAF level was markedly reduced when the tissue background fluorescence before and after mLEDs exposure was compared (p < 0.0001) across the spectral range. LAF elimination was estimated at 95 ± 1%. This approach permitted robust specific fluorescent immunohistochemical co-visualization of commonly studied antigens in aging brains. We also successfully applied this method to specifically visualize CD44 variant expression in aging human cerebral white matter using RNAscope fluorescent in-situ hybridization. Photo quenching provides an attractive means to accelerate progress in aging research by increasing the number of molecules that can be topologically discriminated by fluorescence detection in brain tissue from normative or pathological aging.

Lipofuscin and Aging: A Matter of Toxic Waste

Lipofuscin is membrane-bound cellular waste that can be neither degraded nor ejected from the cell but can only be diluted through cell division and subsequent growth. The fate of postmitotic cells is to accumulate lipofuscin, which as an "aging pigment" has been considered a reliable biomarker for the age of cells such as neurons and, by extension, their hosts. In the aging human brain, deposits of lipofuscin are not uniformly distributed but are concentrated in specific regions of functional interest. The prevailing thought is that the major source of lipofuscin is incomplete lysosomal degradation of damaged mitochondria. Accumulating evidence suggests that lipofuscin is not benign but can impair the functioning of seemingly unrelated cellular systems, including the ubiquitin/proteasome pathway. A damaging feedback loop of lysosomal and proteasomal inhibition may occur as lipofuscin accumulates, leading to what has been appropriately named a "garbage catastrophe." Reversing this catastrophe presents a formidable challenge.

Potential reversibility of lipofuscin accumulation

It is well established that the lipofuscin content of many post-mitotic cell types increases progressively during normal senescence. This age-related accumulation of lipofuscin may occur either: (1) because lipofuscin, once formed, is never degraded or eliminated from cells; or (2) because, despite turnover of this pigment, the rate of lipofuscin formation exceeds the rate at which it is eliminated. Little research has been performed to distinguish between these possibilities. Several studies suggest that lipofuscin may be turned over. However, definitive demonstrations that such turnover does occur under normal circumstances are lacking. It is possible to specifically halt new lipofuscin formation in the retinal pigment epithelium (RPE) of animals at an age when significant amounts of lipofuscin have accumulated in these cells. By monitoring the RPE for possible decreases in lipofuscin content after new pigment formation has been halted, it should be possible to determine whether previously formed lipofuscin can be eliminated from the RPE.

Advances in age pigment research

Although it is presently accepted that lipofuscin (age-pigment) is the end product of the physiological decay of the cells' own constituents, the intimate mechanisms involved in its formation are largely unknown. The advances in the field of lipofuscinogenesis have been relatively slow, mainly due to the persistent confusion between the naturally occurring normal lipofuscin and the pathologically formed ceroid pigments. Therefore, attempts have been made in this presentation to review first the differential features between these pigments and second, to provide a general overview on the physicochemical properties of lipofuscin. The two prevailing theories on lipofuscinogenesis, the peroxidative theory and the proteolytic decline theory, are critically discussed, and future lines of research are suggested for the resolution of present uncertainties on lipofuscinogenesis. Since lipofuscin is properly considered the hallmark of cellular aging, it is expected that the unraveling of the mechanisms involved in lipofuscin formation will provide important clues to the still unknown underlying causes of cellular aging.

Effects of centrophenoxine on cholinephosphotransferase activity in maternal and fetal guinea pig lung

Cholinephosphotransferase activities of guinea pig lung mitochondria and microsomes are inhibited by centrophenoxine and one of its metabolites, p-chlorophenoxyacetate. 2-Dimethylaminoethanol, the second metabolite of centrophenoxine, has no inhibitory effect on the enzyme activity. The inhibition of enzyme activity by centrophenoxine is noncompetitive. Intravenous injection of centrophenoxine and p-chlorophenoxyacetate to pregnant animals causes inhibition of cholinephosphotransferase activity in adult lung but not in fetal lung. However, direct administration of centrophenoxine to the fetus after laparotomy causes inhibition of both subcellular enzyme activity in fetal lung. It is suggested that the drug injected to the pregnant animals does not reach the fetal lung or is metabolized. Furthermore, while centrophenoxine injection does not change the total phosphatidylcholine content of adult lung, the acyl group composition of phosphatidylcholine was modulated.