Inhibitors of lysosomal enzymes: accumulation of lipofuscin-like dense bodies in the brain

Synaptic plasticity in early aging

Studies of how aging affects brain plasticity have largely focused on old animals. However, deterioration of memory begins well in advance of old age in animals, including humans; the present review is concerned with the possibility that changes in synaptic plasticity, as found in the long-term potentiation (LTP) effect, are responsible for this. Recent results indicate that impairments to LTP are in fact present by early middle age in rats but only in certain dendritic domains. The search for the origins of these early aging effects necessarily involves ongoing analyses of how LTP is induced, expressed, and stabilized. Such work points to the conclusion that cellular mechanisms responsible for LTP are redundant and modulated both positively and negatively by factors released during induction of potentiation. Tests for causes of the localized failure of LTP during early aging suggest that the problem lies in excessive activity of a negative modulator. The view of LTP as having redundant and modulated substrates also suggests a number of approaches for reversing age-related losses. Particular attention will be given to the idea that induction of brain-derived neurotrophic factor, an extremely potent positive modulator, can be used to provide long periods of normal plasticity with very brief pharmacological interventions. The review concludes with a consideration of how the selective, regional deficits in LTP found in early middle age might be related to the global phenomenon of brain aging.

Tocopherol-mediated modulation of age-related changes in microglial cells: turnover of extracellular oxidized protein material

Proteins accumulate during aging and form insoluble protein aggregates. Microglia are responsible for their removal from the brain. During aging, changes within the microglia might play a crucial role in the malfunctioning of these cells. Therefore, we isolated primary microglial cells from adult rats and compared their activation status and their ability to degrade proteins to that of microglial cells isolated from newborn animals. The ability of adult microglial cells to degrade proteins is substantially decreased. However, the preincubation of microglial cells with vitamin E improves significantly the degradation of such modified proteins. The degradation of proteins from apoptotic vesicles is decreased in microglia isolated from adult rats. This might be the result of a suppression of the CD36 receptor due to vitamin E treatment. We concluded that microglial cells isolated from adult organisms have different metabolic properties and seem to be a more valuable model to study age-related diseases.

Catabolic Insufficiency and Aging

Cellular degradative processes, which include lysosomal (autophagic) and proteasomal degradation, as well as the activity of cytosolic and mitochondrial proteases, provide for a continuous turnover of damaged and obsolete biomolecules and organelles. Inherent insufficiency of these degradative processes results in progressive accumulation within long-lived postmitotic cells of biological "garbage" ("waste" material), such as indigestible protein aggregates, defective mitochondria, and lipofuscin (age pigment), an intralysosomal, polymeric, undegradable material. Intracellular "garbage" is neither completely catabolized, nor exocytosed to any considerable extent. Heavy lipofuscin loading of lysosomes, typical of old age, seems to pronouncedly decrease autophagic potential. As postulated in the mitochondrial-lysosomal axis theory of aging, this occurs on account of the transport of newly synthesized lysosomal enzymes to lipofuscin-loaded lysosomes rather than to active lysosomes/late endosomes, making the enzyme content of autophagolysosomes insufficient for proper degradation. Consequently, the turnover of mitochondria progressively declines, resulting in decreased ATP synthesis and enhanced formation of reactive oxygen species, inducing further mitochondrial damage and additional lipofuscin formation. With advancing age, lipofuscin-loaded lysosomes and defective mitochondria occupy increasingly larger parts of long-lived postmitotic cells, leaving less and less capability for normal turnover and ATP production, finally resulting in cell death.

You say lipofuscin, we say ceroid: defining autofluorescent storage material

Accumulation of intracellular autofluorescent material or "aging pigment" has been characterized as a normal aging event. Certain diseases also exhibit a similar accumulation of intracellular autofluorescent material. However, autofluorescent storage material associated with aging and disease has distinct characteristics. Lipofuscin is a common term for aging pigments, whereas ceroid is used to describe pathologically derived storage material, for example, in the neuronal ceroid lipofuscinoses (NCLs). NCLs are a family of neurodegenerative diseases that are characterized by an accumulation of autofluorescent storage material (ceroid) in the lysosome, which has been termed "lipofuscin-like". There have been many studies that describe this autofluorescent storage material, but what is it? Is this accumulation lipofuscin or ceroid? In this review we will try to answer the following questions: (1) What is lipofuscin and ceroid? (2) What contributes to the accumulation of this storage material in one or the other? (3) Does this material have an effect on cellular function? Studying parallels between the accumulation of lipofuscin and ceroid may provide insight into the biological relevance of these phenomena.

Involvement of a Novel Q-SNARE, D12, in Quality Control of the Endomembrane System

The cellular endomembrane system requires the proper kinetic balance of synthesis and degradation of its individual components, which is maintained in part by a specific membrane fusion apparatus. In this study, we describe the molecular properties of D12, which was identified from a mouse expression library. This C-terminal anchored membrane protein has sequence similarity to both a yeast soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE), Use1p/Slt1p, and a recently identified human syntaxin 18-binding protein, p31. D12 formed a tight complex with syntaxin 18 as well as Sec22b and bound to alpha-SNAP, indicating that D12 is a SNARE protein. Although the majority of D12 is located in the endoplasmic reticulum and endoplasmic reticulum-Golgi intermediate compartments at steady state, overexpression or knockdown of D12 had no obvious effects on membrane trafficking in the early secretory pathway. However, suppression of D12 expression caused rapid appearance of lipofuscin granules, accompanied by apoptotic cell death without the apparent activation of the unfolded protein response. The typical cause of lipofuscin formation is the impaired degradation of mitochondria by lysosomal degradative enzymes, and, consistent with this, we found that proper post-Golgi maturation of cathepsin D was impaired in D12-deficient cells. This unexpected observation was supported by evidence that D12 associates with VAMP7, a SNARE in the endosomal-lysosomal pathway. Hence, we suggest that D12 participates in the degradative function of lysosomes.

Cellular responses to protein accumulation involve autophagy and lysosomal enzyme activation

Protein oligomerization and aggregation are key events in age-related neurodegenerative disorders, causing neuronal disturbances including microtubule destabilization, transport failure and loss of synaptic integrity that precede cell death. The abnormal buildup of proteins can overload digestive systems and this, in turn, activates lysosomes in different disease states and stimulates the inducible class of lysosomal protein degradation, macroautophagy. These responses were studied in a hippocampal slice model well known for amyloidogenic species, tau aggregates, and ubiquitinated proteins in response to chloroquine-mediated disruption of degradative processes. Chloroquine was found to cause a pronounced appearance of prelysosomal autophagic vacuoles in pyramidal neurons. The vacuoles and dense bodies were concentrated in the basal pole of neurons and in dystrophic neurites. In hippocampal slice cultures treated with Abeta(142), ultrastructural changes were also induced. Autophagic responses may be an attempt to compensate for protein accumulation, however, they were not sufficient to prevent axonopathy indicated by swellings, transport deficits, and reduced expression of synaptic components. Additional chloroquine effects included activation of cathepsin D and other lysosomal hydrolases. Abeta(142) produced similar lysosomal activation, and the effects of Abeta(142) and chloroquine were not additive, suggesting a common mechanism. Activated levels of cathepsin D were enhanced with the lysosomal modulator Z-Phe-Ala-diazomethylketone (PADK). PADK-mediated lysosomal enhancement corresponded with the restoration of synaptic markers, in association with stabilization of microtubules and transport capability. To show that PADK can modulate the lysosomal system in vivo, IP injections were administered over a 5-day period, resulting in a dose-dependent increase in lysosomal hydrolases. The findings indicate that degradative responses can be modulated to promote synaptic maintenance.

Alpha-synuclein-positive structures induced in leupeptin-infused rats

Abnormal accumulation of alpha-synuclein is regarded as a key pathological step in a wide range of neurodegenerative processes, not only in Parkinson's disease (PD) and dementia with Lewy bodies (DLB) but also in multiple-system atrophy (MSA). Nevertheless, the mechanism of alpha-synuclein accumulation remains unclear. Leupeptin, a protease inhibitor, has been known to cause various neuropathological changes in vivo resembling those of aging or neurodegenerative processes in the human brain, including the accumulation of neuronal processes and neuronal cytoskeletal abnormalities leading to neurofibrillary tangle (NFT)-like formations. In the present study, we administered leupeptin into the rat ventricle and found that alpha-synuclein-positive structures appeared widely in the neuronal tissue, mainly in neuronal processes of the fimbria and alveus. Immunoelectron microscopic study revealed that alpha-synuclein immunoreactivity was located in the swollen axons of the fimbria and alveus, especially in the dilated presynaptic terminals. In addition colocalization of alpha-synuclein with ubiquitin was rarely observed in confocal laser-scan image. This is the first report of experimentally induced in vivo accumulation of alpha-synuclein in non-transgenic rodent brain injected with a well-characterized protease inhibitor by an infusion pump. The present finding suggests that the local accumulation of alpha-synuclein might be induced by the impaired metabolism of alpha-synuclein, which are likely related to lysosomal or ubiquitin-independent proteasomal systems.

Aging of the liver: facts and theories

Although most theories of aging assume that cellular functions decline with aging, many intracellular functions in the liver, such as enzyme activities, stay fairly stable in old age. This does not appear to be an antifact caused by in vitro experimental design, since in vivo pharmacokinetic data also demonstrate that most, if not all, biotransformation capacities of the liver remain stable during the aging process, if we take the decline in liver volume with age into account. Thus, many theories to explain the decline in cellular functions during aging appear to be based on erroneous assumptions. The stability of cellular function in old age does not necessarily mean, however, that all cellular functions are identical for young and old organisms. Once unfavorable conditions, such as malnutrition, infection, etc., are involved, the response of the liver is quite different for young and old subjects, demonstrating a more efficient and versatile response in young animal livers in comparison to old livers. Large differences in enzyme activities between young and old organisms appear during stress and especially during recovery from stress. Accordingly, any aging theory needs to explain a potential difference in liver functions (such as response capability) rather than the difference in basal functions. In contrast to rather stable intracellular functions, the uptake function of the hepatocyte surface membranes was found to be progressively decreased with age. This was shown for at least two different types of carrier systems in the surface membranes. Although the decrease of carrier unit number for these substances remains a possible causal factor, we suggest that the decline in hepatic uptake with age is at least partially the result of a gradual decrease in the mobility of surface membrane proteins, which can be shown by the fluorescence recovery after photobleaching (FRAP) technique. Theories of aging need to be elaborated on the basis of unbiased observations on the actual manifestations of cellular aging.

Aging and the liver: functional aspects

The drastic decline in the function of the hepatic microsomal cytochrome monooxygenase system, initially reported in male rat livers, was shown to be due to a feminization of male rat livers with aging. In female rat livers as well as in mouse livers, this system was found to stay unchanged with age. Phase II reactions which showed some decline with aging in male rat livers again stayed fairly stable with age in female rat and mouse livers. Glutathione S-transferase (GST) enzyme activities, which are very stable with age in female rat and mouse livers, demonstrated highly age-dependent changes when dietary conditions were manipulated, suggesting a potential age difference in the homeostatic regulation of this enzyme system. Using the fluorescence recovery after photobleaching (FRAP) technique, unique studies revealed an age-dependent decline in the lateral mobility of proteins in hepatocyte surface membranes. The protease inhibitor model of aging, initially proposed by Ivy for brain cells, has been validated in hepatocytes, demonstrating an accumulation of lipofuscin-like granules in young animals treated with i.p. infusion of leupeptin for only 2 weeks. Antioxidant enzyme activities such as superoxide dismutase (SOD) and catalase (CAT) in the liver were clearly demonstrated no to be reduced in general terms with aging. Rather, a clear increase in CAT enzyme activities with age was demonstrated in female rat livers, thus challenging the concept that intracellular enzyme activities generally decline with aging. In this paper, studies performed in Japan on aging and the liver over the past 30 years, with a focus on its functional aspects, are critically reviewed in terms of the clinical implications of these studies as well as on theories of aging in general.

Targeting senescence pathways to reverse drug resistance in cancer

Irreversible proliferation arrest (also called senescence) has emerged recently as a drug-responsive program able to influence the outcome of cancer chemotherapy. Since the drug amounts required for induction of proliferation arrest are much lower than those necessitated for induction of cell death, forcing cancer cells to undergo senescence may represent a less aggressive approach to control tumor progression. However, to achieve a long-standing control of proliferation, the ability of cancer cells to escape senescence and become drug resistant must be inhibited. Therefore, a clear understanding of the mechanisms that govern drug-induced senescence is critical and can lead to discovery of novel approaches to suppress drug resistance. The present review discusses the relevance of senescence in response to chemotherapy and the onset of drug resistance development. Particular emphasis is directed toward the utilization of findings from the field of research on aging, that can be applied to induction of senescence in cancer cells and reversal of their drug resistance phenotype. Proof of principle for this relationship is represented by the identification of inhibitors of aging associated proteases such as the proteasome and cathepsin L as novel and potent cancer drug resistance reversing agents.

Loss of the chloride channel ClC-7 leads to lysosomal storage disease and neurodegeneration

ClC-7 is a chloride channel of late endosomes and lysosomes. In osteoclasts, it may cooperate with H(+)-ATPases in acidifying the resorption lacuna. In mice and man, loss of ClC-7 or the H(+)-ATPase a3 subunit causes osteopetrosis, a disease characterized by defective bone resorption. We show that ClC-7 knockout mice additionally display neurodegeneration and severe lysosomal storage disease despite unchanged lysosomal pH in cultured neurons. Rescuing their bone phenotype by transgenic expression of ClC-7 in osteoclasts moderately increased their lifespan and revealed a further progression of the central nervous system pathology. Histological analysis demonstrated an accumulation of electron-dense material in neurons, autofluorescent structures, microglial activation and astrogliosis. Like in human neuronal ceroid lipofuscinosis, there was a strong accumulation of subunit c of the mitochondrial ATP synthase and increased amounts of lysosomal enzymes. Such alterations were minor or absent in ClC-3 knockout mice, despite a massive neurodegeneration. Osteopetrotic oc/oc mice, lacking a functional H(+)-ATPase a3 subunit, showed no comparable retinal or neuronal degeneration. There are important medical implications as defects in the H(+)-ATPase and ClC-7 can underlie human osteopetrosis.

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.

The role of macroautophagy in the ageing process, anti-ageing intervention and age-associated diseases

Macroautophagy is a degradation/recycling system ubiquitous in eukariotic cells, which generates nutrients during fasting under the control of amino acids and hormones, and contributes to the turnover and rejuvenation of cellular components (long-lived proteins, cytomembranes and organelles). Tight coupling between these two functions may be the weak point in cell housekeeping. Ageing denotes a post-maturational deterioration of tissues and organs with the passage of time, due to the progressive accumulation of the misfunctioning cell components because of oxidative damage and an age-dependent decline of turnover rate and housekeeping. Caloric restriction (CR) and lower insulin levels may slow down many age-dependent processes and extend lifespan. Recent evidence is reviewed showing that autophagy is involved in ageing and in the anti-ageing action of anti-ageing calorie restriction: function of autophagy declines during adulthood and is almost negligible at older age; CR prevents the age-dependent decline of autophagic proteolysis and improves the sensitivity of liver cells to stimulation of lysosomal degradation; protection of autophagic proteolysis from the age-related decline co-varies with the duration and level of anti-ageing food restriction like the effects of CR extending lifespan; the pharmacological stimulation of macroautophagy has anti-ageing effects. Besides the involvement in ageing, macroautophagy may have an essential role in the pathogenesis of many age-associated diseases. Higher protein turnover may not fully account for the anti-ageing effects of macroautophagy, and effects of macroautophagy on housekeeping of the cell organelles, antioxidant machinery of cell membranes and transmembrane cell signaling should also be considered.

Ultrastructure of the rostral ventral respiratory group neurons in the ventrolateral medulla of the rat

The neurons in the ventrolateral medulla that project to the spinal cord are called the rostral ventral respiratory group (rVRG) because they activate spinal respiratory motor neurons. We retrogradely labeled rVRG neurons with Fluoro-Gold (FG) injections into the fourth cervical spinal cord segment to determine their distribution. The rostral half of the rVRG was located in the area ventral to the semicompact formation of the nucleus ambiguus (AmS). A cluster of the neurons moved dorsally and intermingled with the palatopharyngeal motor neurons at the caudal end of the AmS. The caudal half of the rVRG was located in the area including the loose formation of the nucleus ambiguus caudal to the AmS. We also labeled the rVRG neurons retrogradely with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) to determine their ultrastructural characteristics. The neurons of the rVRG were medium to large (38.1 x 22.1 microm), oval or ellipsoid in shape, and had a dark cytoplasm containing numerous free ribosomes, rough endoplasmic reticulum (rER), mitochondria, Golgi apparatuses, lipofuscin granules and a round nucleus with an invaginated nuclear membrane. The average number of axosomatic terminals in a profile was 33.2. The number of axosomatic terminals containing round vesicles and making asymmetric synaptic contacts (Gray's type I) was almost equal to those containing pleomorphic vesicles and making symmetric synaptic contacts (Gray's type II). The axodendritic terminals were large (1.55 microm), and about 60% of them were Gray's type I. The rVRG neurons have ultrastructural characteristics, which are different from the palatopharyngeal motor neurons or the prorpiobulbar neurons.

Autolysosomes accumulate during in vitro CD8+ T-lymphocyte aging and may participate in induced death sensitization of senescent cells

As autophagic inclusions accumulate in senescent fibroblasts, we wondered whether an increase in cellular fragility during in vitro lymphocyte aging may be related to an autolysosome accumulation. We established that, during long-term cultures, repeatedly stimulated T-lymphocytes acquired characteristics of replicative senescence and became progressively intolerant to activation. Cell death following stimulations: (i) corresponded to apoptosis, associated with necrosis at the end of the culture; (ii) was not, for its main part, mediated through CD95/CD178 or TNFRII/TNF alpha interactions; and (iii) occurred in spite of bcl-2 increased expression. After 14 weeks of culture, the percentage of lymphocytes containing at least one autophagic inclusion (p<0.0001) and the lipofuscin autofluorescence in lymphocytes (p<0.0001) were significantly increased. The expression of several genes regulating autophagy did not significantly vary with the age of the culture. Forty-eight hours after each stimulation, the percentage of induced cell death rose while, in the remaining living cells, the percentage of lymphocytes with autophagic vacuoles (p<0.05), with beta-galactosidase activity and the lipofuscin autofluorescence (p<0.001) significantly decreased, suggesting the preferential death of cells with autophagy. Our data support the view that the accumulation of autolysosomes in senescent lymphocytes might aggravate cellular fragility, leading to apoptosis and necrosis mainly induced by lymphocyte activation.

The photoreactivity of ocular lipofuscin

Lipofuscin or "age pigment" is a lipid-protein complex which accumulates in a variety of postmitotic, metabolically active cells throughout the body. These complexes, which are thought to result from the incomplete degradation of oxidised substrate, have the potential for photoreactivity. This is particularly so in the retina in which the lipofuscin not only contains retinoid metabolites but is also exposed to high oxygen and fluxes of visible light all of which provide an ideal environment for the generation of reactive oxygen species (ROS). Lipofuscin is a potent photoinducible generator of ROS with the potential to damage proteins, lipids and DNA. Retinal cell dysfunction may be strongly associated with photoreactivity of lipofuscin and may contribute to age-related disease and vision loss.

Neuronal and microglial cathepsins in aging and age-related diseases

It has been long believed that cathepsins compensate for each other because of their overlapping substrate specificities. However, there is increasing evidence that disturbance of the normal balance of their enzymatic activities is the first insult in brain aging and age-related diseases. The imbalance of cathepsins may further cause age-related neuropathological changes such as accumulation of autophagic vacuoles and the formation of ceroid-lipofuscin leading to neuronal dysfunction and damage. Leakage of cathepsins due to the fragility of lysosomal membranes during aging also contributes to neurodegeneration. Furthermore, the deficiency of cathepsin D has been recently revealed to provoke a novel type of lysosomal storage disease associated with massive neurodegeneration. In these animals, microglia are activated to initiate inflammatory and cytotoxic responses by binding and phagocytosis of storage neurons. Activated microglia also release some members of cathepsins to induce neuronal death by degrading extracellular matrix proteins. Thus the microglial activation possibly through sensing neuronal storage may also be an important causative factor for neurodegeneration in lysosomal storage diseases and age-related diseases such as Alzheimer's disease. This review describes the pathological roles of neuronal and microglial cathepsins in brain aging and age-related diseases.

Aging, lipofuscin formation, and free radical-mediated inhibition of cellular proteolytic systems

Alterations in a wide array of physiological functions are a normal consequence of aging. Importantly, aged individuals exhibit an enhanced susceptibility to various degenerative diseases and appear less able than their young and adult counterparts to withstand (patho)physiological stress. Elucidation of mechanisms at play in the aging process would benefit the development of effective strategies for enhancing the quality of life for the elderly. It is likely that decrements in cellular and physiological function that occur during aging are the net result of numerous interacting factors. The current review focuses on the potential contribution(s) of free radical-mediated modifications to protein structure/function and alterations in the activities of two major proteolytic systems within cells, lysosomes and the proteasome, to the age-dependent accumulation of fluorescent intracellular granules, termed lipofuscin. Specifically, aging appears to influence the interplay between the occurrences of free radical-derived modifications to protein and the ability of cells to carry out critical proteolytic functions. We present immunochemical and ultrastructural evidence demonstrating the occurrence of a fluorescent protein cross-link derived from free radical-mediated reaction(s) within lipofuscin granules of rat cerebral cortex neurons. In addition, we provide evidence that a fluorophore-modified protein present in lipofuscin granules is the alpha subunit of F1F0-ATP synthase, a mitochondrial protein. It has previously been shown that protein(s) bearing this particular fluorescent cross-link are resistant to proteolysis and can inhibit the proteasome in a non-competitive fashion (J. Biol. Chem. 269 (1994a) 21639; FEBS Lett. 405 (1997) 21). Therefore, the current findings demonstrate that free radical-mediated modifications to protein(s) that lead to the production of inhibitor(s) of cellular proteolytic systems are present on specific protein components of lipofuscin. In addition, the mitochondrial origin of one of these proteins indicates specific intracellular pathways likely to be influenced by free radical events and participate in the formation of lipofuscin. The results of these studies are related to previous in vitro and in vivo observations in the field, thus shedding light on potential consequences to cellular function. In addition, future research directions suggested by the available evidence are discussed.

The analytical approach to the nature of lipofuscin (age pigment)

Analytical studies of three lipopigments show that much can be achieved. Lipopigment from ovine ceroid-lipofuscinosis is composed of discrete protein and lipid molecules in orderly arrays and lipid peroxidation is not involved in its formation. Subunit c of mitochondrial ATP synthase accounts for approximately 50% of accumulated material and is specific to the disease process in this and other forms of the disease. Lipofuscin from bovine heart was mostly soluble and also contained discrete proteins, lipids and metals. Equine thyroid lipofuscin was less soluble but also had a relatively high protein content, probably derived from thyroglobulin. Although sugar could not be measured quantitatively, staining reactions and elemental analyses suggested it could also be a significant component. Some may be present as derivatives in the form of advanced glycation products. It is proposed that protein, the dominant molecular species present, is the important constituent in lipofuscinogenesis rather than lipid peroxidation. Whereas this latter may play some part in the maturation of lipofuscin, this has not been shown experimentally and is not likely to be the initiating mechanism.

Quantifying dense bodies and lipofuscin during aging: a morphologist's perspective

Secondary lysosomes, residual or dense bodies containing lipofuscin or age pigment accumulate in post-mitotic and inter-mitotic cells during aging. The consensus is that the accumulation of this auto-fluorescent material is an index of cellular senescence. Biochemical and morphological studies have independently demonstrated marked age-related increases in the cell and tissue contents of lipofuscin. Most morphological studies on aging have been qualitative, have included only two or three age groups and have not yielded data that are easily correlated with biochemical analyses. One of the best documented age-related changes in hepatocytes and cardiac myocytes is the accumulation of dense bodies and lipofuscin inclusions. Independent stereologic studies reported two- to eightfold age-related increases in the dense body volume fraction of rat hepatocytes. Furthermore, we reported a fourfold increase in the dense body volume fraction of cardiac myocytes in rats between 6 and 30 months of age. These and other studies confirm the use of quantitative morphology to estimate the increases in dense body and lipofuscin inclusions as indices of age. Whether or not the accumulated lipofuscin compromises cell functions in senescent animals has not been adequately addressed. On the one hand, there is little evidence that several-fold increases in this subcellular compartment impair the functional capacities of either hepatocytes or cardiac myocytes. On the other hand, the age-related accumulation of immunoprecipitable, but catalytically inactive, lysosomal enzymes in both liver and heart muscle may be a reflection of increased lipofuscin deposits in the dense bodies.

Lysosomes and brain aging in mammals

Hypotheses about the factors controlling the rate of brain aging are usually derived from 1) correlates of maximum life span across mammals or 2) investigations into the causes of age-related neuropathologies in humans. With regard to the former, the strong correlation between metabolic rate and longevity prompted a variety of free radical hypotheses of aging. There is also evidence that brain size affects life span independently of body metabolism rates. The second approach has led to a diverse array of pathogenic mechanisms and, importantly for the development of general hypotheses, the discovery of animal analogues. The present paper discusses the possibility that age-associated lysosomal dysfunction constitutes a generalized mammalian phenomenon that accounts for specific features of the aged human brain. Immunocytochemical studies using rats and dogs have identified lysosomal changes that begin early in adulthood and are most pronounced in brain areas known to be particularly vulnerable to age-related pathogenesis in humans. Experimentally induced lysosomal dysfunction in cultured brain slices from rats and mutant mice triggers a wide array of changes associated with the aged human brain, including meganeurites and intraneuronal tangles. Finally, there is evidence that at least some forms of proteolysis decrease with increasing brain size across the mammals. The above observations lead to the suggestion that the expansion of neuronal arborizations that occurred in conjunction with increases in brain size secondarily slowed both neuronal metabolism and protein turnover. These events could have served to reduce the rate at which lysosomes (and other organelles) fail.

Inhibition of the ATP‐driven proton pump in RPE lysosomes by the major lipofuscin fluorophore A2‐E may contribute to the pathogenesis of age‐related macular degeneration

Lipofuscin accumulation in the retinal pigment epithelium (RPE) is associated with various blinding retinal diseases, including age-related macular degeneration (AMD). The major lipofuscin fluorophor A2-E is thought to play an important pathogenetic role. In previous studies A2-E was shown to severely impair lysosomal function of RPE cells. However, the underlying molecular mechanism remained obscure. Using purified lysosomes from RPE cells we now demonstrate that A2-E is a potent inhibitor of the ATP-driven proton pump located in the lysosomal membrane. Such inhibition of proton transport to the lysosomal lumen results in an increase of the lysosomal pH with subsequent inhibition of lysosomal hydrolases. An essential task of the lysosomal apparatus of postmitotic RPE for normal photoreceptor function is phagocytosis and degradation of membranous discs shed from photoreceptor outer segments (POS) and of biomolecules from autophagy. When the lysosomes of cultured RPE cells were experimentally loaded with A2-E, we observed intracellular accumulation of exogenously added POS with subsequent congestion of the phagocytic process. Moreover, the autophagic sequestration of cytoplasmic material was also markedly reduced after A2-E loading. These data support the hypothesis that A2-E-induced lysosomal dysfunction contributes to the pathogenesis of AMD and other retinal diseases associated with excessive lipofuscin accumulation.

Association of increased autophagic inclusions labeled for beta-galactosidase with fibroblastic aging

Replicative senescence appears after a finite number of cell divisions. After proliferation has ceased, senescent cells remain viable for long periods and metabolic modifications are observed such as lipofuscin accumulation. In order to understand this phenomenon, we examined the emergence of subcellular modifications corresponding to autophagy in MRC5 normal human fibroblasts. An increase of monodansylcadaverine fluorescence, a specific marker of autophagy, in aging compared to young fibroblasts was observed (p<0.0001). The increase of autophagic vacuoles in aging fibroblasts was confirmed by electron microscopy. We compared young versus senescent fibroblasts and showed that autophagic vacuoles, already present in young cells, became larger in senescent fibroblasts with a significant relative increase of inclusion area with respect to measured cell area (p=0.0041). However, autophagy-associated-gene expression remained stable in senescent compared to young fibroblasts, suggesting that the autophagy process per se is not enhanced. In parallel, transmission electron microscopy analysis showed that beta-galactosidase activity distribution was modified by aging: beta-galactosidase (an enzyme linked to lysosome) was scattered in young fibroblasts, but clustered at the level of autophagic vacuoles in senescent fibroblasts, suggesting a predominance of autolysosomes at this stage. These results support the hypothesis that, during fibroblast aging, the increase of autophagic vacuoles, as well as that of beta-galactosidase activity, may be associated to an increase of lysosomal mass and to an accumulation of degradative autolysosomes with lipofuscin. This phenomenon could be involved in the death of senescent fibroblasts.

Mitochondrial recycling and aging of cardiac myocytes: the role of autophagocytosis

The mechanisms of mitochondrial alterations in aged post-mitotic cells, including formation of so-called 'giant' mitochondria, are poorly understood. To test whether these large mitochondria might appear due to imperfect autophagic mitochondrial turnover, we inhibited autophagocytosis in cultured neonatal rat cardiac myocytes with 3-methyladenine. This resulted in abnormal accumulation of mitochondria within myocytes, loss of contractility, and reduced survival time in culture. Unlike normal aging, which is associated with slow accumulation of predominantly large defective mitochondria, pharmacological inhibition of autophagy caused only moderate accumulation of large (senescent-like) mitochondria but dramatically enhanced the numbers of small mitochondria, probably reflecting their normally more rapid turnover. Furthermore, the 3-methyladenine-induced accumulation of large mitochondria was irreversible, while small mitochondria gradually decreased in number after withdrawal of the drug. We, therefore, tentatively conclude that large mitochondria selectively accumulate in aging post-mitotic cells because they are poorly autophagocytosed. Mitochondrial enlargement may result from impaired fission, a possibility supported by depressed DNA synthesis in large mitochondria. Nevertheless, enlarged mitochondria retained immunoreactivity for cytochrome c oxidase subunit 1, implying that mitochondrial genes remain active in defective mitochondria. Our findings suggest that imperfect autophagic recycling of these critical organelles may underlie the progressive mitochondrial damage, which characterizes aging post-mitotic cells.

The anti-ageing effects of caloric restriction may involve stimulation of macroautophagy and lysosomal degradation, and can be intensified pharmacologically

Caloric restriction (CR) and a reduced growth hormone (GH)-insulin-like growth factor (IGF-1) axis are associated with an extension of lifespan across taxa. Evidence is reviewed showing that CR and reduced insulin of GH-IGF-1 axis may exhibit their effects at least partly by their common stimulatory action on autophagy, the cell repair mechanism responsible for the housekeeping of cell membranes and organelles including the free radical generators peroxisomes and mitochondria. It is shown that the life-long weekly administration of an anti-lipolytic drug may decrease glucose and insulin levels and stimulate autophagy and intensify anti-ageing effects of submaximal CR.

Lysosomal malfunction accompanies alpha-synuclein aggregation in a progressive mouse model of Parkinson's disease

We have detected granular and filamentous inclusions that are alpha-synuclein- and ubiquitin-immunoreactive in the cytoplasm of dopaminergic and cortical neurons of C57/black mice treated chronically with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and probenecid. The immunoreactive aggregates only become evident several weeks after large-scale dopaminergic cell death and a downregulation of alpha-synuclein gene expression. Numerous lipofuscin granules accumulate alpha-synuclein in the nigral and limbic cortical neurons of treated mice. These data provide evidence that insoluble proteins, such as alpha-synuclein, build up as granular and filamentous inclusions in dopaminergic neurons that survive the initial toxic MPTP insult. They further suggest that defective protein degradation rather than altered gene expression underlies deposition of alpha-synuclein and that abundant lysosomal compartments are present to seal off the potentially toxic material.

A novel monoclonal antibody recognizes lysosome-like structures and reflects regional and age-related differences in the rat dentate gyrus

The granule cells (GCs) of dentate gyrus exhibit regionally specific morphology, and continue to be born and to develop well into adult life. We used a novel monoclonal antibody, MAb2G7, elicited by immunization of a mouse with a microsome fraction of the hippocampus, to evaluate regional and age-related differences in GCs immunohistochemically. Weak cytoplasmic reactions were observed in many neurons, but intense MAb2G7-positive dots were observed only in GCs. Using electron microscopy, we observed that these dots were localized in the internal droplets of secondary lysosome-like structures in GCs. The MAb2G7-positive granules were quantitatively analyzed in young adult and middle-aged rats. Larger numbers of reactive granules were observed in the infrapyramidal blade (IPB) than in the suprapyramidal blade (SPB) and the numbers of positive granules were proportionally reduced in the two areas in middle-aged rats. The changes in the MAb2G7 immunoreactivity may reflect different activation or neurogeneration of GCs in the IPB versus the SPB, and in middle-aged versus young adult rats.

The effects of lysosomotropic agents on normal and INCL cells provide further evidence for the lysosomal nature of palmitoyl-protein thioesterase function

Fatty acylation of proteins on cysteine residues is a common post-translational modification that plays roles in protein-membrane and protein-protein interactions. Recently, we described a lysosomal palmitoyl-protein thioesterase that removes long-chain fatty acids from lipid-modified cysteine residues in proteins. Deficiency in palmitoyl-protein thioesterase results in a human lysosomal storage disorder, infantile neuronal ceroid lipofuscinosis (INCL), which primarily affects the central nervous system. The pathological hallmark of the disorder is the accumulation of granular osmiophilic deposits (GROD) that resemble lipofuscin, or aging pigment. In previous work, we have shown that [35S]cysteine-labeled lipid thioesters derived from fatty acylated proteins accumulate in cultured cells derived from palmitoyl-protein thioesterase-deficient patients. In the present work, we show that the lipid cysteine thioesters accumulate in the lysosomal fraction, and we further show that the appearance of these compounds in the organic phase is blocked by inhibitors of lysosomal proteolysis, demonstrating through biochemical means the lysosomal nature of the site of palmitoyl-protein thioesterase action. Furthermore, substrates for palmitoyl-protein thioesterase accumulate even in normal cells after leupeptin or chloroquine treatment. This was demonstrated by subjecting extracts of treated cells to exhaustive proteolysis to release protein-bound cysteine lipid for analysis. Cysteamine, a lysosomotropic drug recently proposed for the treatment of INCL, was found to have effects similar to leupeptin and chloroquine, suggesting that its mechanism of action may be more complex than previously understood.

Neuronal loss and brain atrophy in mice lacking cathepsins B and L

Cathepsins B and L are widely expressed cysteine proteases implicated in both intracellular proteolysis and extracellular matrix remodeling. However, specific roles remain to be validated in vivo. Here we show that combined deficiency of cathepsins B and L in mice is lethal during the second to fourth week of life. Cathepsin B(-/-)/L(-/-) mice reveal a degree of brain atrophy not previously seen in mice. This is because of massive apoptosis of select neurons in the cerebral cortex and the cerebellar Purkinje and granule cell layers. Neurodegeneration is accompanied by pronounced reactive astrocytosis and is preceded by an accumulation of ultrastructurally and biochemically unique lysosomal bodies in large cortical neurons and by axonal enlargements. Our data demonstrate a pivotal role for cathepsins B and L in maintenance of the central nervous system.

The origin of fluorescence in the neuronal ceroid lipofuscinoses (Batten disease) and neuron cultures from affected sheep for studies of neurodegeneration

Lipofuscin and ceroid are usually held responsible for impaired cellular performance, via oxidative damage and the irreversible accumulation of fluorescent products of lipid peroxidation. The neuronal ceroid lipofuscinoses (NCLs, Batten disease) are inherited neurodegenerative diseases characterized by intracellular accumulation of fluorescent lipofuscin-like bodies. However these bodies are lysosomes packed with a particular protein, subunit c of mitochondrial ATP synthase; not the result of oxidative damage. No individual storage body component was fluorescent nor were solutions of total storage bodies. UV-vis spectra confirmed the lack of a fluorophor. Crystals of non-fluorescent albumin and reconstituted storage bodies were fluorescent in glycerol suspensions. This fluorescence is probably caused by interference of light reflected from the protein array, as is often observed in protein crystals. Other lipofuscins may be secondary lysosomes with a high protein content and the source of fluorescence the same. The neurodegeneration associated with lipofuscin accumulation may be caused by that accumulation, or may be a separate manifestation of aging. Neuronal cell cultures offer a way to study these processes. Subunit c accumulation has been observed in cerebral bipolar neurons cultured from 90 day NCL affected sheep foetuses. Neurons from different parts of the brain behave differently. Normal 108 day cerebellar granule neurons migrated into clumps when cultured with tri-iodothyronine, but affected cerebellar neurons did not, nor did normal or affected cerebral neurons.

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.

The biological waste product formation in the light of the membrane hypothesis of aging

The membrane hypothesis of aging (MHA) explains the biological waste product (lipofuscin) formation as a disbalance between the rates of protein synthesis and damage, as well as of elimination of the damaged components. Although, this concept has not been refuted on the basis of any experimental evidence, it has neither been widely accepted. During the last decade the general interest has turned toward the molecular genetics so intensely, that research aimed at clarifying cell biological mechanisms became so to say hibernated. Nowadays it is being recognized more and more that after the complete description of the human genetic code, attention has to be dedicated again to the cellular mechanisms explaining the function of the gene products (proteins). In this context, our experimental findings described during the recent years may become again the subject of interest. We have shown that the in vivo inhibition of the lysosomal thiol-proteinase functions by sublethal doses of leupeptin in young, adult and old mice results in a considerable increase (about 30%) of the immobile fraction of membrane proteins in hepatocyte plasma membrane, meanwhile the lateral diffusion constant of the still mobile membrane proteins increased. These observations were interpreted as signs of a general slowing down of protein turnover in the plasma membrane, just by inhibiting the elimination mechanisms in the lysosomes. This paper will discuss the theoretical conclusions and significance of these findings for the biological waste product formation, as a basic cell biological function.

Oxidative damage causes formation of lipofuscin-like substances in the hippocampus of the senescence-accelerated mouse after kainate treatment

We have demonstrated that seizures induced by kainic acid (KA) are, at least in part, mediated via oxidative stress in rats [Life. Sci. 61 (1997) PL373; Brain Res. 853 (2000) 215; Brain Res. 874 (2000) 15; Neurosci. Lett. 281 (2000) 65]. In order to extend our findings, we employed the rodent aging model in this study. After KA treatments (once a day for 5 days; 20,20,20,20 and 40 mg/kg, i.p.), several parameters reflecting neurotoxic behaviors, oxidative stress [malondialdehyde (MDA) and protein carbonyl] and aging (lipofuscin-like substances) were compared between senile-prone (P8) and resistant (R1) strains of 9-month-old male senescence-accelerated mice (SAM). KA-induced neurotoxic signs as shown by mortality and seizure activity were more accentuated in the SAM-P8 than in the SAM-R1. Levels of MDA and carbonyl are consistently higher in the hippocampus of SAM-P8 than that of SAM-R1. Significant increases in the values of MDA and carbonyl were observed 4 h or 2 days after the final KA administration. This finding was more pronounced in the SAM-P8 than in the SAM-R1. Although a significant loss of hippocampal neurons was observed 7 days post-KA, at this time the MDA and carbonyl content had returned to near control levels. In contrast, fluorescent lipofuscin-like substances and lipofuscin granules were significantly increased 7 days after KA treatments. Therefore, our data suggests that mice in the senescence model are more susceptible to KA-induced seizures/oxidative damage, and that oxidative damage could be one of the casual factors in the accumulation of lipofuscin.

The protection of rat liver autophagic proteolysis from the age-related decline co-varies with the duration of anti-ageing food restriction

Restricting caloric intake (CR) well below that of ad libitum (AL) fed animals retards and/or delays many characteristics of ageing and the occurrence and progression of age-associated diseases, efficacy depending on duration. The hypothesis that the anti-ageing effect of CR might involve stimulation of the cell-repair mechanism autophagy was tested. The effects of ageing and duration of anti-ageing CR on liver autophagic proteolysis (AP) were explored in male AL Sprague-Dawley rats aged 2-, 6-, 12- and 24-months; and 24-month-old rats on a CR diet initiated at 2-, 6- and 12-month of age or initiated at age 2-months and interrupted at age 18 months. The age-related changes in the regulation of AP were studied by monitoring the rate of valine release in the incubation medium from isolated liver cells by an HPLC procedure. Results show that the maximum attainable rate and the regulation of AP decline with increasing age; that changes are prevented by anti-ageing CR initiated at young age, that the protective effects of CR change with the duration of diet. It is concluded that the data are compatible with the hypothesis that AP and improved membrane maintenance might be involved in the antiageing mechanism of CR.

Lipofuscin accumulation in proliferating fibroblasts in vitro: an indicator of oxidative stress

The amount of the ageing pigment, lipofuscin, found in replicating cells depends both on its rate of formation as well as its rate of dissolution by cell division. We present a model which allows the calculation of the lipofuscin accumulation rate from measurements of its amount and of the cell cycle duration. In two human fibroblast strains, the accumulation rate correlates well with differences in oxidative stress/antioxidative defence as measured by intracellular peroxide generation, protein carbonyl content, telomere shortening rate and replicative life span. The lipofuscin content increases with replicative age in both cultures. The rather steep increase in presenescent fibroblasts is not solely due to a slowing down of the cell turnover, but is partially caused by an increased rate of lipofuscin formation/ accumulation. This might indicate an increased level of oxidative stress in presenescent fibroblasts, or a decreased efficiency of proteolytic systems, or both. The results are in accordance with data demonstrating an adverse effect of lipofuscin accumulation on cellular protein turnover and suggest an active role for lipofuscin accumulation in cellular senescence.

Degradation of oxidized proteins by the 20S proteasome

Oxidatively modified proteins are continuously produced in cells by reactive oxygen and nitrogen species generated as a consequence of aerobic metabolism. During periods of oxidative stress, protein oxidation is significantly increased and may become a threat to cell survival. In eucaryotic cells the proteasome has been shown (by purification of enzymatic activity, by immunoprecipitation, and by antisense oligonucleotide studies) to selectively recognize and degrade mildly oxidized proteins in the cytosol, nucleus, and endoplasmic reticulum, thus minimizing their cytotoxicity. From in vitro studies it is evident that the 20S proteasome complex actively recognizes and degrades oxidized proteins, but the 26S proteasome, even in the presence of ATP and a reconstituted functional ubiquitinylating system, is not very effective. Furthermore, relatively mild oxidative stress rapidly (but reversibly) inactivates both the ubiquitin activating/conjugating system and 26S proteasome activity in intact cells, but does not affect 20S proteasome activity. Since mild oxidative stress actually increases proteasome-dependent proteolysis (of oxidized protein substrates) the 20S 'core' proteasome complex would appear to be responsible. Finally, new experiments indicate that conditional mutational inactivation of the E1 ubiquitin-activating enzyme does not affect the degradation of oxidized proteins, further strengthening the hypothesis that oxidatively modified proteins are degraded in an ATP-independent, and ubiquitin-independent, manner by the 20S proteasome. More severe oxidative stress causes extensive protein oxidation, directly generating protein fragments, and cross-linked and aggregated proteins, that become progressively resistant to proteolytic digestion. In fact these aggregated, cross-linked, oxidized proteins actually bind to the 20S proteasome and act as irreversible inhibitors. It is proposed that aging, and various degenerative diseases, involve increased oxidative stress (largely from damaged and electron 'leaky' mitochondria), and elevated levels of protein oxidation, cross-linking, and aggregation. Since these products of severe oxidative stress inhibit the 20S proteasome, they cause a vicious cycle of progressively worsening accumulation of cytotoxic protein oxidation products.

Cathepsin D deficiency induces lysosomal storage with ceroid lipofuscin in mouse CNS neurons

Cathepsin D-deficient (CD-/-) mice have been shown to manifest seizures and become blind near the terminal stage [approximately postnatal day (P) 26]. We therefore examined the morphological, immunocytochemical, and biochemical features of CNS tissues of these mice. By electron microscopy, autophagosome/autolysosome-like bodies containing part of the cytoplasm, granular osmiophilic deposits, and fingerprint profiles were demonstrated in the neuronal perikarya of CD-/- mouse brains after P20. Autophagosomes and granular osmiophilic deposits were detected in neurons at P0 but were few in number, whereas they increased in the neuronal perikarya within days after birth. Some large-sized neurons having autophagosome/autolysosome-like bodies in the perikarya appeared in the CNS tissues, especially in the thalamic region and the cerebral cortex, at P17. These lysosomal bodies occupied the perikarya of almost all neurons in CD-/- mouse brains obtained from P23 until the terminal stage. Because these neurons exhibited autofluorescence, it was considered that ceroid lipofuscin may accumulate in lysosomal structures of CD-/- neurons. Subunit c of mitochondrial ATP synthase was found to accumulate in the lysosomes of neurons, although the activity of tripeptidyl peptidase-I significantly increased in the brain. Moreover, neurons near the terminal stage were often shrunken and possessed irregular nuclei through which small dense chromatin masses were scattered. These results suggest that the CNS neurons in CD-/- mice show a new form of lysosomal accumulation disease with a phenotype resembling neuronal ceroid lipofuscinosis.

Proteasome inhibition by lipofuscin/ceroid during postmitotic aging of fibroblasts

We have studied the effects of hyperoxia and of cell loading with artificial lipofuscin or ceroid pigment on the postmitotic aging of human lung fibroblast cell cultures. Normobaric hyperoxia (40% oxygen) caused an irreversible senescence-like growth arrest after about 4 wk and shortened postmitotic life span from 1-1/2 years down to 3 months. During the first 8 wk of hyperoxia-induced 'aging', overall protein degradation (breakdown of [(35)S]methionine metabolically radiolabeled cell proteins) increased somewhat, but by 12 wk and thereafter overall proteolysis was significantly depressed. In contrast, protein synthesis rates were unaffected by 12 wk of hyperoxia. Lysosomal cathepsin-specific activity (using the fluorogenic substrate z-FR-MCA) and cytoplasmic proteasome-specific activity (measured with suc-LLVY-MCA) both declined by 80% or more over 12 wk. Hyperoxia also caused a remarkable increase in lipofuscin/ceroid formation and accumulation over 12 wk, as judged by both fluorescence measurements and FACscan methods. To test whether the association between lipofuscin/ceroid accumulation and decreased proteolysis might be causal, we next exposed cells to lipofuscin/ceroid loading under normoxic conditions. Lipofuscin/ceroid-loaded cells indeed exhibited a gradual decrease in overall protein degradation over 4 wk of treatment, whereas protein synthesis was unaffected. Proteasome specific activity decreased by 25% over this period, which is important since proteasome is normally responsible for degrading oxidized cell proteins. In contrast, an apparent increase in lysosomal cathepsin activity was actually caused by a large increase in the number of lysosomes per cell. To test whether lipofuscin/ceroid could in fact directly inhibit proteasome activity, thus causing oxidized proteins to accumulate, we incubated purified proteasome with lipofuscin/ceroid preparations in vitro. We found that proteasome is directly inhibited by lipofuscin/ceroid. Our results indicate that an accumulation of oxidized proteins (and lipids) such as lipofuscin/ceroid may actually cause further increases in damage accumulation during aging by inhibiting the proteasome.

Regionally selective changes in brain lysosomes occur in the transition from young adulthood to middle age in rats

The possibility that brain aging in rats exhibits regional variations of the type found in humans was studied using lysosomal chemistry as a marker. Age-related (two vs 12months; male Sprague-Dawley) differences in cathepsin D immunostaining were pronounced in the superficial layers of entorhinal cortex and in hippocampal field CA1, but not in neocortex and field CA3. Three changes were recorded: an increase in the intraneuronal area occupied by labeled lysosomes; clumping of immunopositive material within neurons; more intense cytoplasmic staining. Western blot analyses indicated that the increases involved the active forms of cathepsin D rather than their proenzyme. Shrinkage of cathepsin-D-positive neuronal cell bodies was observed in entorhinal cortex but not in neocortical sampling zones. Age-related lysosomal changes as seen with cathepsin B immunocytochemistry were considerably more subtle than those obtained with cathepsin D antibodies. In contrast, a set of glial and/or vascular elements located in a distal dendritic field of the middle-aged hippocampus was much more immunoreactive for cathepsin B than cathepsin D. The areas exhibiting sizeable changes in the present study are reported to be particularly vulnerable to aging in humans. The results thus suggest that aspects of brain aging common to mammals help shape neurosenescence patterns in humans.

Research on ageing in Germany

The present review on ageing research in Germany is biased towards experimental biogerontology, because this is the field which will define the future of ageing research as a whole. In absolute numbers of publications between 1995 and 1999, Germany is comparable to other large European countries. However, Germany ranks definitively last among 10 major developed countries if the numbers of scientific papers per year are seen in relation to the economic capability. This is true for the whole of biomedical research, but it is even more exaggerated for ageing research. There are potent groups in German ageing research capable of producing a good fraction of high-impact papers, however. There are many more researchers in areas highly relevant to gerontology which recently became attracted by gerontological problems. However, the importance of modern biogerontology has not made clear to decision-makers in Germany, so that structural and financial limitations will probably prevent any significant rise in the near future, which would be necessary to keep Germany along with other developed countries.

Telomere shortening is an in vivo marker of myocyte replication and aging

To determine whether adult cardiac myocytes are capable of multiple divisions and whether this form of growth is restricted to a subpopulation of cells that retain this capacity with age, telomere lengths were measured in myocyte nuclei isolated from the left ventricle of fetal and neonatal Fischer 344 rats and rats at 4, 12, and 27 months after birth. Two independent methodologies were used for this analysis: laser scanning cytometer and confocal microscopy. In each case, fluorescence intensity of a peptide nucleic acid probe specific for telomeric sequence was evaluated. The two techniques yielded comparable results. Telomeric shortening increased with age in a subgroup of myocytes that constituted 16% of the entire cell population. In the remaining nondividing cells, progressive accumulation of a senescent associated nuclear protein, p16(INK4), was evidenced. In conclusion, a significant fraction of myocytes divides repeatedly from birth to senescence, counteracting the continuous death of cells in the aging mammalian rat heart.

Age-related changes in ultrastructural features of cathepsin B- and D-containing neurons in rat cerebral cortex

The present study examines age-related changes in the subcellular localization of cathepsin B (cath B) and cathepsin D (cath D), as well as morphological features of the cathepsin-immunoreactive (ir) neurons in rat cerebral cortex. Sprague-Dawley rats were studied at 3 and 26 months. By immunoelectron microscopy cath B- or cath D-immunoreactivities were found in many, but not all, pyramidal neurons. In young rat cerebral cortical neurons, cath B was observed not only in lysosomal systems such as multivesicular bodies, dense bodies, and lipofuscin granules, but also in extralysosomal sites. By contrast, cath D was confined mainly to lysosomal systems in young rats. In aged rats, cath B showed a similar pattern in its subcellular localization compared to the young control, but some of the dense bodies containing cath B was closely apposed to the outer nuclear envelope. These cells exhibited a relatively normal appearance. Regardless of subcellular localization, approximately 10% of cath B-ir neurons displayed ultrastructural disturbances presumed to indicate an early stage of degeneration. The nucleus was indented, nuclear boundary was indistinct, nuclear pore structures appeared separately with high frequency, and the endoplasmic reticulum appeared to be affected. In addition to its presence in lysosomal structures, cath D-immunoreactivity in aged cerebral cortex was noted prominently in the cytosol as diffuse granules. About 37% of cath D-ir cells showed this age-related change. Among the neurons with the diffusely scattered form of cath D, approximately 70% of cells exhibited the degenerating features. These cells were characterized by large amounts of diffuse cath D, reduced cellular size, loss of the nuclear boundary, scattered nuclear pore structures, an often fragmentation of the nucleus, disturbances of endoplasmic reticular system, and in advanced stages, condensed nucleus and poor preservation of almost cytoplasmic organelles. Though some of these features were also found in cath B-ir neurons, findings of overt degeneration, such as fragmented and condensed nuclei and impaired almost cytoplasmic organelles, were generally not observed in cath B-ir neurons. In addition, lipofuscin aggregates containing cath D were observed frequently in the extracellular space close to sites of ruptured plasma membrane, whereas in the sections stained with anti-cath B antibodies, large-sized lipofuscin aggregates showed only very weak or no cath B-immunoreactivity at all. Taken together, the present results suggest that cath D and cath B may be regulated differently and play their specific roles in the aging of the brain, especially, the change in location of cath D from the lysosomal system to the cytosol in the aged brain may play an important role in age-related cell death.

Targeted disruption of the Cln3 gene provides a mouse model for Batten disease. The Batten Mouse Model Consortium [corrected]

Batten disease, a degenerative neurological disorder with juvenile onset, is the most common form of the neuronal ceroid lipofuscinoses. Mutations in the CLN3 gene cause Batten disease. To facilitate studies of Batten disease pathogenesis and treatment, a murine model was created by targeted disruption of the Cln3 gene. Mice homozygous for the disrupted Cln3 allele had a neuronal storage disorder resembling that seen in Batten disease patients: there was widespread and progressive intracellular accumulation of autofluorescent material that by EM displayed a multilamellar rectilinear/fingerprint appearance. Inclusions contained subunit c of mitochondrial ATP synthase. Mutant animals also showed neuropathological abnormalities with loss of certain cortical interneurons and hypertrophy of many interneuron populations in the hippocampus. Finally, as is true in Batten disease patients, there was increased activity in the brain of the lysosomal protease Cln2/TPP-1. Our findings are evidence that the Cln3-deficient mouse provides a valuable model for studying Batten disease.

Topographical localization of lipofuscin pigment in the brain of the aged fat-tailed dwarf lemur (Cheirogaleus medius) and grey lesser mouse lemur (Microcebus murinus): comparison to iron localization

The present study was undertaken to explore the distribution of lipofuscin in the brain of cheirogaleids by autofluorescence and compare it to other studies of iron distribution. Aged dwarf (Cheirogaleus medius) and mouse (Microcebus murinus) lemurs provide a reliable model for the study of normal and pathological cerebral aging. Accumulation of lipofuscin, an age pigment derived by lipid peroxidation, constitutes the most reliable cytological change correlated with neuronal aging. Brain sections of four aged (8-15 year old) and 3 young (2-3 year old) animals were examined. Lipofuscin accumulation was observed in the aged animals but not in the young ones. Affected regions include the hippocampus (granular and pyramidal cells), where no iron accumulation was observed, the olfactory nucleus and the olfactory bulb (mitral cells), the basal forebrain, the hypothalamus, the cerebellum (Purkinje cells), the neocortex (essentially in the pyramidal cells), and the brainstem. Even though iron is known to catalyse lipid oxidation, our data indicate that iron deposits and lipofuscin accumulation are not coincident. Different biochemical and morphological cellular compartments might be involved in iron and lipofuscin deposition. The nonuniform distribution of lipofuscin indicates that brain structures are not equally sensitive to the factors causing lipofuscin accumulation. The small size, the rapid maturity, and the relatively short life expectancy of the cheirogaleids make them a good model system in which to investigate the mechanisms of lipofuscinogenesis in primates.

Phenotypic reversal of the btn1 defects in yeast by chloroquine: a yeast model for Batten disease

BTN1 of Saccharomyces cerevisiae encodes an ortholog of CLN3, the human Batten disease gene. We have reported previously that deletion of BTN1, btn1-Delta, resulted in a pH-dependent resistance to D-(-)-threo-2-amino-1-[p-nitrophenyl]-1,3-propanediol (ANP). This phenotype was caused by btn1-Delta strains having an elevated ability to acidify growth medium through an elevated activity of the plasma membrane H(+)-ATPase, resulting from a decreased vacuolar pH during early growth. We have determined that growing btn1-Delta strains in the presence of chloroquine reverses the resistance to ANP, decreases the rate of medium acidification, decreases the activity of plasma membrane H(+)-ATPase, and elevates vacuolar pH. However, an additional effect of this phenotypic reversal is that activity of plasma membrane H(+)-ATPase is decreased further and vacuolar pH is increased further as btn1-Delta strains continue to grow. This phenotypic reversal of btn1-Delta can be considered for developing a therapy for Batten disease.

Increased glutathione levels in neurochemically identified fibre systems in the aged rat lumbar motor nuclei

The spinal cord motor nuclei have been the focus of a number of investigations exploring neurodegenerative mechanisms, e.g. excitotoxicity mediated by glutamate and oxidative stress. Here, high-resolution quantitative post-embedding immunocytochemistry with antibodies to oxidized and reduced glutathione (GSH), an ubiquitously expressed scavenger of free radicals, was used to examine if GSH synthesis is upregulated pre- and/or postsynaptically in the lumbar motor nuclei of aged (30 month old) rats. The purpose was, moreover, to resolve the extent of correlation between GSH expression, transmitter identity and degenerative changes. Tissue from young adult rats was co-processed for comparison. The quantitative immunogold analysis revealed an increase in GSH-immunoreactivity in both pre- and postsynaptic compartments in the lumbar motor nuclei of aged rats. Presynaptically, the enrichment of GSH-immunoreactivity was seen in axonal boutons of normal appearance, and was furthermore restricted to the extra-mitochondrial compartment. Postsynaptically, the aged rats disclosed, in comparison with young adults, higher values for GSH-immunoreactivity both over mitochondria (+49%) and cytoplasmic matrix (+130%). When analysing the transmitter identity of the bouton profiles, it turned out that close to 50% of all glutamate-immunoreactive boutons in the aged rats contained very high levels (> 40 gold particles/microm2) of GSH-immunoreactivity. Strong GSH-immunoreactivity was also a typical feature of a subset of axon terminal- and axon fibre-like profiles in the aged rat that showed signs of axon dystrophy and degeneration. When comparing with normally appearing axon fibre profiles located in close vicinity, the population of aberrant axons had higher average levels of glutamate-immunoreactivity (+93%), and lower average levels of glycine-immunoreactivity (-88%). No difference was seen regarding the levels of GABA. The results of this study lend support to the idea that aging in the spinal cord motor nuclei is associated with an increased oxidative stress and indicate that different transmitter systems are differentially affected by the degenerative process.

Lysosomal protease inhibitors induce meganeurites and tangle-like structures in entorhinohippocampal regions vulnerable to Alzheimer's disease

Lysosomal protease inhibitors induce signs of human brain aging in rat hippocampal slices. The present studies tested if they (1) also cause neurofibrillary tangles and (2) reproduce regional patterns of pathology found in Alzheimer's disease (AD). Slices of hippocampus plus retrohippocampal cortex were prepared from rats at postnatal days 6-7 and maintained for 2-5 weeks. In agreement with earlier studies, 6- to 12-day infusions of selective (ZPAD) or generalized (chloroquine) inhibitors of lysosomal proteases generated meganeurites of the type found in aged human cortex. Surveys and quantitative analyses established that the meganeurites developed almost exclusively in AD vulnerable regions. Antibodies against the phosphorylated tau protein in neurofibrillary tangles labeled thick filaments running through neurons in the superficial layers of entorhinal cortex in 6-day ZPAD-treated slices. The general appearance of the stained structures resembled that of early stage tangles. More mature tangle-like profiles were found at a number of sites after longer incubations; these were threefold more frequent in the superficial (AD vulnerable) than in the deep layers of the entorhinal cortex. Immunoblots indicated that essentially all phosphorylated tau labeling in the slices involved approximately 29-kDa fragments of the native isoforms. These findings establish that lysosomal dysfunction triggers the parallel formation of meganeurites and tangles with the regional distribution of both effects reflecting that for AD vulnerability.