Vinblastine-induced autophagocytosis in cultured fibroblasts

Alzheimer's disease-associated ubiquilin-1 regulates presenilin-1 accumulation and aggresome formation

The Alzheimer's disease (AD)-associated ubiquilin-1 regulates proteasomal degradation of proteins, including presenilin (PS). PS-dependent γ-secretase generates β-amyloid (Aβ) peptides, which excessively accumulate in AD brain. Here, we have characterized the effects of naturally occurring ubiquilin-1 transcript variants (TVs) on the levels and subcellular localization of PS1 and other γ-secretase complex components and subsequent γ-secretase function in human embryonic kidney 293, human neuroblastoma SH-SY5Y and mouse primary cortical cells. Full-length ubiquilin-1 TV1 and TV3 that lacks the proteasome-interaction domain increased full-length PS1 levels as well as induced accumulation of high-molecular-weight PS1 and aggresome formation. Accumulated PS1 colocalized with TV1 or TV3 in the aggresomes. Electron microscopy indicated that aggresomes containing TV1 or TV3 were targeted to autophagosomes. TV1- and TV3-expressing cells did not accumulate other unrelated proteasome substrates, suggesting that the increase in PS1 levels was not because of a general impairment of the ubiquitin-proteasome system. Furthermore, PS1 accumulation and aggresome formation coincided with alterations in Aβ levels, particularly in cells overexpressing TV3. These effects were not related to altered γ-secretase activity or PS1 binding to TV3. Collectively, our results indicate that specific ubiquilin-1 TVs can cause PS1 accumulation and aggresome formation, which may impact AD pathogenesis or susceptibility.

Redox activity within the lysosomal compartment: implications for aging and apoptosis

The lysosome is a redox-active compartment containing low-mass iron and copper liberated by autophagic degradation of metalloproteins. The acidic milieu and high concentration of thiols within lysosomes will keep iron in a reduced (ferrous) state, which can react with endogenous or exogenous hydrogen peroxide. Consequent intralysosomal Fenton reactions may give rise to the formation of lipofuscin or "age pigment" that accumulates in long-lived postmitotic cells that cannot dilute it by division. Extensive accumulation of lipofuscin seems to hinder normal autophagy and may be an important factor behind aging and age-related pathologies. Enhanced oxidative stress causes lysosomal membrane permeabilization, with ensuing relocation to the cytosol of iron and lysosomal hydrolytic enzymes, with resulting apoptosis or necrosis. Lysosomal copper is normally not redox active because it will form non-redox-active complexes with various thiols. However, if cells are exposed to lysosomotropic chelators that do not bind all the copper coordinates, highly redox-active complexes may form, with ensuing extensive lysosomal Fenton-type reactions and loss of lysosomal stability. Because many malignancies seem to have increased amounts of copper-containing macromolecules that are turned over by autophagy, it is conceivable that lysosomotropic copper chelators may be used in the future in ROS-based anticancer therapies.

Mitochondrial turnover and aging of long-lived postmitotic cells: the mitochondrial-lysosomal axis theory of aging

It is now generally accepted that aging and eventual death of multicellular organisms is to a large extent related to macromolecular damage by mitochondrially produced reactive oxygen species, mostly affecting long-lived postmitotic cells, such as neurons and cardiac myocytes. These cells are rarely or not at all replaced during life and can be as old as the whole organism. The inherent inability of autophagy and other cellular-degradation mechanisms to remove damaged structures completely results in the progressive accumulation of garbage, including cytosolic protein aggregates, defective mitochondria, and lipofuscin, an intralysosomal indigestible material. In this review, we stress the importance of crosstalk between mitochondria and lysosomes in aging. The slow accumulation of lipofuscin within lysosomes seems to depress autophagy, resulting in reduced turnover of effective mitochondria. The latter not only are functionally deficient but also produce increased amounts of reactive oxygen species, prompting lipofuscinogenesis. Moreover, defective and enlarged mitochondria are poorly autophagocytosed and constitute a growing population of badly functioning organelles that do not fuse and exchange their contents with normal mitochondria. The progress of these changes seems to result in enhanced oxidative stress, decreased ATP production, and collapse of the cellular catabolic machinery, which eventually is incompatible with survival.

New insights into the mechanisms of macroautophagy in mammalian cells

Macroautophagy is a self-digesting pathway responsible for the removal of long-lived proteins and organelles by the lysosomal compartment. Parts of the cytoplasm are first segregated in double-membrane-bound autophagosomes, which then undergo a multistep maturation process including fusion with endosomes and lysosomes. The segregated cytoplasm is then degraded by the lysosomal hydrolases. The discovery of ATG genes has greatly enhanced our understanding of the mechanisms of this pathway. Two novel ubiquitin-like protein conjugation systems were shown to function during autophagosome formation. Autophagy has been shown to play a role in a wide variety of physiological processes including energy metabolism, organelle turnover, growth regulation, and aging. Impaired autophagy can lead to diseases such as cardiomyopathy and cancer. This review summarizes current knowledge about the formation and maturation of autophagosomes, the role of macroautophagy in various physiological and pathological conditions, and the signaling pathways that regulate this process in mammalian cells.

Amyloid-β 1–42 induced endocytosis and clusterin/apoJ protein accumulation in cultured human astrocytes

Recent studies indicate that astrocytes may be the primary target of secreted amyloid-beta 1-42 peptides, with the neurotoxicity representing a secondary response to astrocytic stress. Our purpose was to clarify the astrocytic stress response induced by amyloid-beta peptides in human and rat astrocytes. Human amyloid-beta 1-42 peptides and fibrils induced the appearance of cytoplasmic vacuoles in normal human astrocytes (NHA) and CCFsttg1 astrocytoma cells. Vacuoles appeared 9-12h after the amyloid-beta exposure and remained present for several days. Rat primary neonatal astrocytes showed similar but less prominent vacuolar response. Human amyloid-beta peptides 1-16, 1-28, 10-20, 17-21 and 25-35 did not cause vacuole formation. Electron microscopic observations revealed large endocytic vacuoles containing fibrillar amyloid material. Stress marker analysis did not show any increase in protein levels of HSP70, HSP90, GRP78 and GRP94. However, the protein level of clusterin/apoJ, a secreted chaperone, was strongly increased both in NHA and CCFsttg1 astrocytes. Endocytic response associated with the accumulation of clusterin/apoJ protein suggests that clusterin/apoJ has a role in the clearance of amyloid-beta peptides.

Inhibition of autophagy with 3-methyladenine results in impaired turnover of lysosomes and accumulation of lipofuscin-like material

Autophagy (which includes macro-, micro-, and chaperone-mediated autophagy) is an important biological mechanism for degradation of damaged/obsolete macromolecules and organelles. Ageing non-dividing cells, however, progressively accumulate oxidised proteins, defective organelles and intralysosomal lipofuscin inclusions, suggesting inherent insufficiency of autophagy. To learn more about the role of macroautophagy in the turnover of organelles and lipofuscin formation, we inhibited autophagic sequestration with 3-methyladenine (3 MA) in growth-arrested human fibroblasts, a classical model of cellular ageing. Such treatment resulted in a dramatic accumulation of altered lysosomes, displaying lipofuscin-like autofluorescence, as well as in a moderate increase of mitochondria with lowered membrane potential. The size of the late endosomal compartment appeared not to be significantly altered following 3 MA exposure. The accumulation of lipofuscin-like material was enhanced when 3 MA administration was combined with hyperoxia. The findings suggest that macroautophagy is essential for normal turnover of lysosomes. This notion is supported by reports in the literature of lysosomal membrane proteins inside lysosomes and/or late endosomes, as well as lysosomes with active hydrolases within autophagosomes following vinblastine-induced block of fusion between lysosomes and autophagosomes. The data also suggest that specific components of lysosomes, such as membranes and proteins, may be direct sources of lipofuscin.