Routes of excretion of neuronal lysosomal dense bodies after ventricular infusion of leupeptin in the rat: a study using ubiquitin and PGP 9.5 immunocytochemistry

Species-Specific Ultrastructure of Neuronal Lipofuscin in Hippocampus and Neocortex of Subhuman Mammals and Humans

Lipofuscin represents an integral part of neurons and glial cells in mammals and in submammalian species. It is a special lysosomal organelle, takes part of cellular metabolism, and is a structural expression of catabolic pathways. Species-specific differences of lipofuscin indicate metabolic differences of the relevant neurons. The authors have studied the ultrastructure of neuronal lipofuscin in the hippocampus and cerebral neocortex of dogs, horses, cows, elephants, rats, mice, apes, and humans to answer the question of species-specific differences of this organelle. Paraffin sections of formalin-fixed material were investigated by hematoxylin-eosin and PAS staining, by fluorescence microscopy for autofluorescence, with a laser scanning confocal microscope and by electron microscopy. In the animals studied and in humans the lipofuscin displayed, in addition to the general trilaminar substructure, species-specific appearances. No differences were found in the lipofuscin structure between neocortical and hippocampal neurons of the separate animal species. In contrast, in humans, neurons of the hippocampus showed a particular lipofuscin structure, not only different from the neocortical one, but also with differences between CA1 and CA3/4 sectors. Interestingly, in apes a transitional situation was found with slight differences between neocortical and hippocampal lipofuscin, especially in the rhesus monkey. This peculiarity was corroborated by the distribution of special pentilaminar linear structures in the lipofuscin pigment in all animals, only sparsely in the rhesus monkey and not in humans. The results indicate that lipofuscin ultrastructure of neocortical and hippocampal neurons is species specific and that lipofuscin in the human hippocampal neurons displays structures characteristic of man differing from the neocortical neuronal lipofuscin. The neuronal lipofuscin of apes, especially of the rhesus monkey displays structures in between humans and lower mammals. Nothing is known about the functional significance of these findings. They may indicate metabolic and/or functional characteristics of the relevant neurons.

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.

Reversal of a hallmark of brain ageing: lipofuscin accumulation

The prospect of removing cellular deposits of lipofuscin is of considerable interest because they may contribute to age related functional decline and disease. Here, we use a decapod crustacean model to circumvent a number of problems inherent in previous studies on lipofuscin loss. We employ (a) validated lipofuscin quantification methods, (b) an in vivo context, (c) essentially natural environmental conditions and (d) a situation without accelerated production of residual material or (e) application of pharmacological compounds. We use a novel CNS biopsy technique that produces both an anti-ageing effect and also permits longitudinal sampling of individuals, thus (f) avoiding conventional purely cross-sectional population data that may suffer from selective mortality biases. We quantitatively demonstrate that lipofuscin, accrued through normal ageing, can be lost from neural tissue. The mechanism of loss probably involves exocytosis and possibly blood transport. If non-disruptive ways to accelerate lipofuscin removal can be found, our results suggest that therapeutic reversal of this most universal manifestation of cellular ageing may be possible.

Intracellular deposition, microtubule destabilization, and transport failure: an "early" pathogenic cascade leading to synaptic decline

Protein deposition is a common event in age-related neurological diseases that are characterized by neuronal dysfunction and eventual cell death. Here, cultured hippocampal slices were infused with the lysosomal disrupter chloroquine to examine the link between abnormal protein processing/deposition and early synaptopathogenesis. Tau species of 55 to 69 kDa increased over several days of treatment with chloroquine, while the protein and message levels of synaptic markers were selectively reduced. Neurons of subfields CA1, CA3, and dentate gyrus accumulated protein deposits recognized by antibodies against paired helical filaments and ubiquitin, and this was accompanied by tubulin fragmentation and deacetylation. The deposition filled the basal pole of pyramidal neurons, encompassing the area of the axon hillock and initial dendritic branching but without causing overt neuronal atrophy. Neurons containing the polar aggregates exhibited severely impaired transport along basal dendrites. Transport capability was also lost along apical dendrites, the opposite direction of deposited material in the basal pole; thus, perpetuating the problem beyond physical blockage must be the associated loss of microtubule integrity. These data indicate that transport failure forms a link between tau deposition and synaptic decline, thus shedding light on how protein aggregation events disrupt synaptic and cognitive functions before the ensuing cellular destruction.

Globus pallidus glial pigment and its changes with age and chronic illness in childhood

Glial lipopigment appears in the globus pallidus without accumulating in neurons (except for late adolescence) in multiple chronic childhood diseases. In this observational study, we compared the age-related development of glial pigmentation in children with the chronic illness (cystic fibrosis) and children dying acutely. A secondary goal was to search for pallidal neuronal lipopigment in childhood. We recorded pigmentation in the brains of 37 consecutive cystic fibrosis children ranging in age from 0-23 yr and in 17 controls ranging in age from 0-18 yr. We characterized the lipofuscin histochemically and used several regression models to describe the mode of deposition. We observed that in the controls, intraglial pallidal pigment accumulated in 2 forms (relatively large globules and, separately, as clusters of fine granules) at a slow rate during childhood. In cystic fibrosis, both forms of pallidal glial pigment started accumulating at a younger age and were deposited far more rapidly. There was a further increase in the rate of accumulation between 8 and 10 yr of age. We did not encounter pallidal neuronal lipofuscin at any age. These observations are consistent with 2 propositions: 1) that globus pallidus glial cells are unique in their ability to accumulate lipofuscin before it accumulates in nearby neurons; and 2) that they are particularly susceptible to some systemic effect of this chronic illness.

Expression of A53T mutant but not wild-type alpha-synuclein in PC12 cells induces alterations of the ubiquitin-dependent degradation system, loss of dopamine release, and autophagic cell death

Alpha-synuclein mutations have been identified in certain families with Parkinson's disease (PD), and alpha-synuclein is a major component of Lewy bodies. Other genetic data indicate that the ubiquitin-dependent proteolytic system is involved in PD pathogenesis. We have generated stable PC12 cell lines expressing wild-type or A53T mutant human alpha-synuclein. Lines expressing mutant but not wild-type alpha-synuclein show: (1) disruption of the ubiquitin-dependent proteolytic system, manifested by small cytoplasmic ubiquitinated aggregates and by an increase in polyubiquitinated proteins; (2) enhanced baseline nonapoptotic death; (3) marked accumulation of autophagic-vesicular structures; (4) impairment of lysosomal hydrolysis and proteasomal function; and (5) loss of catecholamine-secreting dense core granules and an absence of depolarization-induced dopamine release. Such findings raise the possibility that the primary abnormality in these cells may involve one or more deficits in the lysosomal and/or proteasomal degradation pathways, which in turn lead to loss of dopaminergic capacity and, ultimately, to death. These cells may serve as a model to study the effects of aberrant alpha-synuclein on dopaminergic cell function and survival.

Proteasomal inhibition leads to formation of ubiquitin/alpha-synuclein-immunoreactive inclusions in PC12 cells

Proteasomal dysfunction has been recently implicated in the pathogenesis of several neurodegenerative diseases, including Parkinson's disease and diffuse Lewy body disease. We have developed an in vitro model of proteasomal dysfunction by applying pharmacological inhibitors of the proteasome, lactacystin or ZIE[O-tBu]-A-leucinal (PSI), to dopaminergic PC12 cells. Proteasomal inhibition caused a dose-dependent increase in death of both naive and neuronally differentiated PC12 cells, which could be prevented by caspase inhibition or CPT-cAMP. A percentage of the surviving cells contained discrete cytoplasmic ubiquitinated inclusions, some of which also contained synuclein-1, the rat homologue of human alpha-synuclein. However the total level of synuclein-1 was not altered by proteasomal inhibition. The ubiquitinated inclusions were present only within surviving cells, and their number was increased if cell death was prevented. We have thus replicated, in this model system, the two cardinal pathological features of Lewy body diseases, neuronal death and the formation of cytoplasmic ubiquitinated inclusions. Our findings suggest that inclusion body formation and cell death may be dissociated from one another.

Corpora-amylacea and the family of polyglucosan diseases

The history, characters, composition and topography of corpora amylacea (CA) in man and the analogous polyglucosan bodies (PGB) in other species are documented, noting particularly the wide variation in the numbers found with age and in neurological disease. Their origins from both neurons and glia and their probable migrations and ultimate fate are discussed. Their presence is also noted in other organs, particularly in the heart. The occurrence in isolated cases of occasional 'massive' usually focal accumulations of similar polyglucosan bodies in association with certain chronic neurological diseases is noted and the specific conditions Adult Polyglucosan body disease and type IV glycogenosis where they are found throughout the nervous system in great excess is discussed. The distinctive differences of CA from the PGB of Lafora body disease and Bielschowsky body disease are emphasised. When considering their functional roles, a parallel is briefly drawn on the one hand between normal CA and the bodies in the polyglucosan disorders and on the other with the lysosomal system and its associated storage diseases. It is suggested that these two systems are complementary ways by which large, metabolically active cells such as neurons, astrocytes, cardiac myocytes and probably many other cell types, dispose of the products of stressful metabolic events throughout life and the continuing underlying process of aging and degradation of long lived cellular proteins. Each debris disposal system must be regulated in its own way and must inevitably, a priori, be heir to metabolic defects that give rise in each to its own set of metabolic disorders.

Lipofuscin: mechanisms of formation and increase with age

Lipofuscin (age pigment) is a brown-yellow, electron-dense, autofluorescent material that accumulates progressively over time in lysosomes of postmitotic cells, such as neurons and cardiac myocytes. The exact mechanisms behind this accumulation are still unclear. This review outlines the present knowledge of age pigment formation, and considers possible mechanisms responsible for the increase of lipofuscin with age. Numerous studies indicate that the formation of lipofuscin is due to the oxidative alteration of macromolecules by oxygen-derived free radicals generated in reactions catalyzed by redox-active iron of low molecular weight. Two principal explanations for the increase of lipofuscin with age have been suggested. The first one is based on the notion that lipofuscin is not totally eliminated (either by degradation or exocytosis) even at young age, and, thus, accumulates in postmitotic cells as a function of time. Since oxidative reactions are obligatory for life, they would act as age-independent enhancers of lipofuscin accumulation, as well as of many other manifestations of senescence. The second explanation is that the increase of lipofuscin is an effect of aging, caused by an age-related enhancement of autophagocytosis, a decline in intralysosomal degradation, and/or a decrease in exocytosis.

The effects of the tremorgenic mycotoxin penitrem A on the rat cerebellum

Within 10 minutes of intraperitoneal injection of penitrem A (3 mg/kg), rats develop severe generalized tremors and ataxia that persist for up to 48 hours. These are accompanied by a three- to fourfold increase in cerebellar cortical blood flow. Mitochondrial swelling occurs in cerebellar stellate and basket cells within 30 minutes of dosing and persists for more than 12 hours without leading to cell death. From 2 hours, Purkinje cell dendrites show early cytoplasmic condensation accompanied by fine vacuolation of smooth endoplasmic reticulum and enlargement of perikaryal mitochondria. From 6 hours, many Purkinje cells develop intense cytoplasmic condensation with eosinophilia that resembles "ischemic cell change," and from 12 hours, many other Purkinje cells show marked watery swelling. Astrocytes begin to swell from 0.5 hours after injection and show hypertrophy of organelles from 6 hours. Also from 6 hours onward, discrete foci of necrosis appear in the granule cell layer, while permeability of overlying meningeal vessels to horseradish peroxidase becomes evident at 8 hours. All changes are more severe in vermis and paravermis. Despite widespread loss of Purkinje cells, the animals' behavior becomes almost normal within a week. While tremor occurs with doses of 1.5 and 0.5 mg/kg, cellular damage is minimal. The tremor mechanism differs from that of harmaline since destruction of inferior olivary nuclei abolishes neither the tremor response to penitrem A nor the cellular damage. No morphological changes are found in other brain regions. The affinities of penitrem A for high-conductance calcium-dependent potassium channels and for gamma-aminobutyric acid receptors with the probability of resultant excitotoxity are considered to be important underlying factors for these changes.

Suppression of cathepsins B and L causes a proliferation of lysosomes and the formation of meganeurites in hippocampus

Cultured hippocampal slices exhibited prominent ultrastructural features of brain aging after exposure to an inhibitor of cathepsins B and L. Six days of treatment with N-CBZ-L-phenylalanyl-L-alanine-diazomethylketone (ZPAD) resulted in a dramatic increase in the number of lysosomes in the perikarya of neurons and glial cells throughout the slices. Furthermore, lysosomes in CA1 and CA3 pyramidal cells were not restricted to the soma but instead were located throughout dendritic processes. Clusters of lysosomes were commonly found within bulging segments of proximal dendrites that were notable for an absence of microtubules and neurofilaments. Although pyknotic nuclei were sometimes encountered, most of the cells in slices exposed to ZPAD for 6 d appeared relatively normal. Slices given 7 d of recovery contained several unique features, compared with those processed immediately after incubation with the inhibitor. Cell bodies of CA1 neurons were largely cleared of the excess lysosomes but had gained fusiform, somatic extensions that were filled with fused lysosomes and related complex, dense bodies. These appendages, similar in form and content to structures previously referred to as "meganeurites," were not observed in CA3 neurons or granule cells. Because meganeurites were often interposed between cell body and axon, they have the potential to interfere with processes requiring axonal transport. It is suggested that inactivation of cathepsins B and L results in a proliferation of lysosomes and that meganeurite generation provides a means of storing residual catabolic organelles. The accumulated material could be eliminated by pinching off the meganeurite but, at least in some cases, this action would result in axotomy. Reduced cathepsin L activity, increased numbers of lysosomes, and the formation of meganeurites are all reported to occur during brain aging; thus, it is possible that the infusion of ZPAD into cultured slices sets in motion a greatly accelerated gerontological sequence.

Immunolocalization of ubiquitin and related enzymes in human retina and retinal pigment epithelium

PURPOSE

To examine the localization of ubiquitin (Ub) and related enzymes in human retina with emphasis on the retinal pigment epithelium (RPE)-Bruch's membrane complex.

METHODS

Thirty human eyes enucleated for various disease processes were examined. Immunohistochemistry was performed on paraffin sections using antibodies against Ub, Ub-conjugating enzyme (E2), Ub carboxyl-terminal hydrolase (PGP 9.5), and, for comparison, arrestin (Arr). Immunoreactivity (IR) was tested using the avidin-biotin method.

RESULTS

Ub was present throughout retina but was particularly prominent in ganglion cells and RPE. Most intriguing was the presence of Ub IR in age-related, sub-RPE deposits such as drusen and basal laminar deposits (BLD). RPE immunolabeling was more intense in older tissue, but otherwise no pattern of Ub IR could be linked to specific diseases. E2 IR colocalized with Ub, with one exception; E2 IR was not found in drusen or BLD. PGP 9.5 IR was intense in nerve fibers, ganglion cells, and the inner nuclear and plexiform layers. RPE staining was faint and patchy; sub-RPE deposits were not labeled. Arr IR was present in photoreceptors but not within or beneath RPE cells.

CONCLUSION

The ubiquitination process is important in human retina and particularly in ganglion cells. Ub-related processes are also active in RPE and may be involved in the degradation and disposal of proteins from these cells. The presence of Ub in sub-RPE deposits--without related Ub-processing enzymes--raises the possibility that certain proteins become ubiquitinated within RPE but that further degradation of the Ub-protein complexes does not occur.

Endothelial lipopigment as an indicator of alpha-tocopherol deficiency in two equine neurodegenerative diseases

Two spontaneous neurodegenerative diseases of the horse, equine motor neuron disease (EMND) and equine degenerative myeloencephalopathy (EDM), have been associated with alpha-tocopherol deficiency, and both were characterized by prominent accumulations of endothelial lipopigment in the small vessels of the spinal cord. These endothelial pigment deposits appear to be reversible. In EMND horses pasture-supplemented for 9 months or more after the progression of weakness and wasting had arrested, there was very little endothelial lipopigment. The origin and the potential effects of these endothelial lipopigment accumulations are discussed.

Fate of lysosomes transported to the dendrites by a colchicine-induced mechanism

Lysosomes in hypoglossal motoneurons were retrogradely labeled with fluorescent latex microspheres and their distribution as well as that of acid phosphatase was examined after a single 50 microgram intracerebroventricular (i.c.v.) injection of colchicine. In saline injected controls the fluorescent label was distributed mainly in cell bodies. Twenty-four hours after the colchicine injection we observed a re-distribution of fluorescent label from the cell body of neurons to the dendrites. Seventy-two hours after the colchicine injection the fluorescent label had returned from the dendrites to the cell body. A similar pattern was obtained by following the effect of colchicine on the distribution of acid phosphatase reaction product. We conclude that the reappearance of fluorescent label in cell bodies following colchicine treatment is the result of the retrograde transport of dendritic lysosomes.

Cytoskeletal changes in rat cortical neurons induced by long-term intraventricular infusion of leupeptin

Neurofibrillary tangles (NFTs), which are composed of paired helical filament (PHF)-like filaments, were induced by the long-term intraventricular infusion of leupeptin, a potent protease inhibitor. The fibrils composing the NFTs were 20 nm in maximal width and had periodic constrictions at 40-nm intervals. They were identical to the PHF that had been found in aged rat neurons. Dystrophic axons filled with mainly tubular structures were also abundantly found in the parietal and temporal isocortices, which were not affected in the acute or subacute phases of leupeptin treatment. An immunohistochemical study using antibodies related to the neuronal cytoskeleton showed that neuronal cytoskeletal changes accompanying ubiquitination occurred in dystrophic axons distributed widely in the isocortex as well as the hippocampal formation. The present findings suggest that long-term administration of leupeptin accelerates the neuronal ageing process in rats and causes other neuronal changes: NFT formation, such as seen in the aged brain or in neurodegenerative diseases including Alzheimer's disease, in addition to accumulation of lipofuscin granules and degeneration of neuronal processes. In other words, some disturbance of the balance between proteases and their inhibitors may play an important role in the neuronal ageing process, and some regulatory intervention in the intraneuronal protease activity may provide a new therapeutic strategy for the neurodegenerative diseases.

beta-Amyloid precursor protein fragments and lysosomal dense bodies are found in rat brain neurons after ventricular infusion of leupeptin

Infusion of the serine and thiol protease inhibitor, leupeptin, is known to cause a reduction of fast axoplasmic transport, and accumulation of lysosomal dense bodies in neuronal perikarya. We have found these dense bodies in hippocampal and cerebellar neurons contain ubiquitin conjugated proteins. We now demonstrate that these accumulated neuronal lysosomes are labeled by antisera to the cytoplasmic, transmembrane and extracellular domains of beta-amyloid precursor protein (APP) and also that lysosomal APP is fragmented. This in vivo model confirms that neurons can process APP via a lysosomal pathway and that neuronal lysosomes in vivo contain both N-terminal and potentially amyloidogenic C-terminal fragments of APP. We also show that increased APP immunoreactivity after leupeptin treatment is seen first in neurons and later in astrocytes. On recovery from infusion, APP N-terminal immunoreactivity diminishes whilst C-terminal reactivity remains in neurons. These findings are consistent with production in whole brain of potentially amyloidogenic fragments of APP within neuronal lysosomes in perikarya and dendrites implying that neurons may play a role in forming the beta-amyloid of plaques.

Ubiquitin, PGP 9.5 and dense body formation in trimethyltin intoxication: differential neuronal responses to chemically induced cell damage

Ubiquitin in normal cells may be important in degrading or transferring short-lived or aberrant proteins to lysosomal dense bodies. To examine its role in degrading proteins produced by a chemical insult, changes in the distribution of ubiquitin and the carboxy-terminal hydrolase, PGP 9.5, have been studied in rat hippocampal neurons and cerebellar Purkinje cells in trimethyltin intoxication. Here tubulovesicular dense bodies (TVBs) form from 12h onwards associated with vacuolation of the Golgi apparatus. Striking accumulations of lysosomal dense bodies follow in hippocampal pyramidal cells but not in cerebellar Purkinje cells; many of the hippocampal neurons later die, while the Purkinje cells generally survive. Ubiquitin immunoreactivity was diffusely increased in hippocampal pyramidal and Purkinje cells 6 h after dosing. By 12 h both diffuse and granular ubiquitin immunoreactivity was present that intensified over 24 and 48 h. Both by light and electron microscopy TVBs showed ubiquitin immunoreactivity, but dense bodies in hippocampal perikarya did not stain with an anti-ubiquitin antibody. PGP 9.5 immunoreactivity was not altered in hippocampal cells at any time, while Purkinje and Golgi cell dendrites and perikarya showed intensified labelling at 3 h that reached a peak of 12 h. At 48 h Western blot analysis of hippocampal homogenates showed significant increases in high molecular weight (HMW) ubiquitin conjugates, while cerebellar homogenates showed an increase in ubiquitin-histone conjugates. Northern blot analyses showed no change in ubiquitin or PGP9.5 gene expression in hippocampus or cerebellum. These findings suggest that the material in the TVBs in hippocampal cells is not being degraded by the ubiquitin system but passes ubiquitinated into the lysosomal system, while material in Purkinje cell TVBs is degraded by the ubiquitin system, suggesting it may have a different composition in each type of neuron.