Signaling, polyubiquitination, trafficking, and inclusions: sequestosome 1/p62's role in neurodegenerative disease

Glial fibrillary acidic protein is elevated in the lysosomal storage disease classical late-infantile neuronal ceroid lipofuscinosis, but is not a component of the storage material

Classical late-infantile neuronal ceroid lipofuscinosis (LINCL) is a fatal neurodegenerative disease of children caused by mutations in TPP1, the gene encoding the lysosomal protease tripeptidyl peptidase 1. LINCL is characterized by lysosomal accumulation of storage material of which only a single protein component, subunit c of mitochondrial ATP synthase, has been well established to date. Identification of other protein constituents of the storage material could provide useful insights into the pathophysiology of disease and the natural substrates for TPP1. We have therefore initiated a proteomic analysis of storage material in brain from a LINCL mouse model. One protein, GFAP (glial fibrillary acidic protein), was found to be elevated in the LINCL mice compared with normal controls in both isolated storage bodies and a lysosome-enriched subcellular fraction that contains storage material. To determine whether GFAP accumulates within the lysosome in LINCL, we examined its intracellular distribution using subcellular fractionation and morphological methods. These experiments demonstrate that GFAP is not a component of the storage material in LINCL, suggesting that reports of GFAP storage in other NCLs may need to be re-examined. A number of other proteins were elevated in the storage material and/or lysosome-enriched fraction from the LINCL mice, but it remains unclear whether these proteins are true constituents of the storage material or, like GFAP, whether they associate with this material upon purification.

A small molecule inhibitor of inducible heat shock protein 70

The multifunctional, stress-inducible molecular chaperone HSP70 has important roles in aiding protein folding and maintaining protein homeostasis. HSP70 expression is elevated in many cancers, contributing to tumor cell survival and resistance to therapy. We have determined that a small molecule called 2-phenylethynesulfonamide (PES) interacts selectively with HSP70 and leads to a disruption of the association between HSP70 and several of its cochaperones and substrate proteins. Treatment of cultured tumor cells with PES promotes cell death that is associated with protein aggregation, impaired autophagy, and inhibition of lysosomal function. Moreover, this small molecule is able to suppress tumor development and enhance survival in a mouse model of Myc-induced lymphomagenesis. The data demonstrate that PES disrupts actions of HSP70 in multiple cell signaling pathways, offering an opportunity to better understand the diverse functions of this molecular chaperone and also to aid in the development of new cancer therapies.

Defining an Embedded Code for Protein Ubiquitination

It has been more than 30 years since the initial report of the discovery of ubiquitin as an 8.5 kDa protein of unknown function expressed universally in living cells. And still, protein modification by covalent conjugation of the ubiquitin molecule is one of the most dynamic posttranslational modifications studied in terms of biochemistry and cell physiology. Ubiquitination plays a central regulatory role in number of eukaryotic cellular processes such as receptor endocytosis, growth-factor signaling, cell-cycle control, transcription, DNA repair, gene silencing, and stress response. Ubiquitin conjugation is a three step concerted action of the E1-E2-E3 enzymes that produces a modified protein. In this review we investigate studies undertaken to identify both ubiquitin and SUMO (small ubiquitin-related modifier) substrates with the goal of understanding how lysine selectivity is achieved. The SUMOylation pathway though distinct from that of ubiquitination, draws many parallels. Based upon the recent findings, we present a model to explain how an individual ubiquitin ligase may target specific lysine residue(s) with the co-operation from a scaffold protein.

Inclusion body myositis: a degenerative muscle disease associated with intra-muscle fiber multi-protein aggregates, proteasome inhibition, endoplasmic reticulum stress and decreased lysosomal degradation

Sporadic inclusion body myositis (s-IBM), the most common muscle disease of older persons, is of unknown cause, and there is no enduring treatment. Abnormal accumulation of intracellular multi-protein inclusions is a characteristic feature of the s-IBM phenotype, and as such s-IBM can be considered a "conformational disorder," caused by protein unfolding/misfolding combined with the formation of inclusion bodies. Abnormal intracellular accumulation of unfolded proteins may lead to their aggregation and inclusion body formation. The present article is focusing on the multiple proteins that are accumulated in the form of aggregates within s-IBM muscle fibers, and it explores the most recent research advances directed toward a better understanding of mechanisms causing their impaired degradation and abnormal aggregation. We illustrate that, among other factors, abnormal misfolding, accumulation and aggregation of proteins are associated with their inadequate disposal-and these factors are combined with, and perhaps provoked by, an aging intracellular milieu. Other concurrent and possibly provocative phenomena known within s-IBM muscle fibers are: endoplasmic reticulum stress and unfolded protein response, mitochondrial abnormalities, proteasome inhibition, lysosome abnormality and endodissolution. Together, these appear to lead to the s-IBM-specific vacuolar degeneration, and muscle fiber atrophy, concluding with muscle fiber death.

Parkin deficiency increases the resistance of midbrain neurons and glia to mild proteasome inhibition: the role of autophagy and glutathione homeostasis

Parkin mutations in humans produce parkinsonism whose pathogenesis is related to impaired protein degradation, increased free radicals and abnormal neurotransmitter release. In this study, we have investigated whether partial proteasomal inhibition by epoxomicin, an ubiquitin proteasomal system (UPS) irreversible inhibitor, further aggravates the cellular effects of parkin suppression in midbrain neurons and glia. We observed that parkin null (PK-KO) midbrain neuronal cultures are resistant to epoxomicin-induced cell death. This resistance is due to increased GSH and DJ-1 protein levels in PK-KO mice. The treatment with epoxomicin increases, in wild type (WT) cultures, the pro-apoptotic Bax/Bcl-2 ratio, the phosphorylation of tau, and the levels of chaperones heat-shock protein 70 and C-terminal Hsc-interacting protein, but none of these effects took place in epoxomicin-treated PK-KO cultures. Poly-ubiquitinated proteins increased more in WT than in PK-KO-treated neuronal cultures. Parkin accumulated in WT neuronal cultures treated with epoxomicin. Markers of autophagy, such as LC3II/I, were increased in naïve PK-KO cultures, and further increased after treatment with epoxomicin, implying that the blockade of the proteasome in PK-KO neurons triggers the enhancement of autophagy. The treatment with l-buthionine-S,R-sulfoximine and the inhibition of autophagy, however, reverted the increase resistance to epoxomicin of the PK-KO cultures. We also found that PK-KO glial cells, stressed by growth in defined medium and depleted of GSH, were more susceptible to epoxomicin induced cell death than WT glia treated similarly. This susceptibility was linked to reduced GSH levels and less heat-shock protein 70 response, and to activation of p-serine/threonine kinase protein signaling pathway as well as to increased poly-ubiquitinated proteins. These data suggest that mild UPS inhibition is compensated by other mechanisms in PK-KO midbrain neurons. However the depletion of GSH, as happens in stressed glia, suppresses the protection against UPS inhibition-induced cell death. Furthermore, GSH inhibition regulated differentially UPS activity and in old PK-KO mice, which have depletion of GSH, UPS activity is decreased in comparison with that of old-WT.

Oxidative damage to the promoter region of SQSTM1/p62 is common to neurodegenerative disease

Recently we reported that declined SQSTM1/p62 expression in Alzheimer disease brain was age-correlated with oxidative damage to the p62 promoter. The objective of this study was to examine whether oxidative damage to the p62 promoter is common to DNA recovered from brain of individuals with neurodegenerative disease. Increased 8-OHdG staining was observed in brain sections from Alzheimer's disease (AD), Parkinson disease (PD), Huntington disease (HD), Frontotemporal dementia (FTD), and Pick's disease compared to control subjects. In parallel, the p62 promoter exhibited elevated oxidative damage in samples from various diseases compared to normal brain, and damage was negatively correlated with p62 expression in FTD samples. Oxidative damage to the p62 promoter induced by H2O2 treatment decreased its transcriptional activity. In keeping with this observation, the transcriptional activity of a Sp-1 element deletion mutant displayed reduced stimulus-induced activity. These findings reveal that oxidative damage to the p62 promoter decreased its transcriptional activity and might therefore account for decreased expression of p62. Altogether these results suggest that pharmacological means to increase p62 expression may be beneficial in delaying the onset of neurodegeneration.

Neuropathology of the Mcoln1(-/-) knockout mouse model of mucolipidosis type IV

The recently developed Mcoln1(-/-) knockout mouse provides a novel model for analyzing mucolipin 1 function and mucolipidosis type IV disease. Here we characterize the neuropathology of Mcoln1(-/-) mouse at the end stage. Evidence of ganglioside accumulation, including increases in GM2, GM3, and GD3 and redistribution of GM1, was found throughout the central nervous system (CNS) independent of significant cholesterol accumulation. Unexpectedly, colocalization studies using immunofluorescence confocal microscopy revealed that GM1 and GM2 were present in separate vesicles within individual neurons. While GM2 was significantly colocalized with LAMP2, consistent with late-endosomal/lysosomal processing, some GM2-immunoreactivity occurred in LAMP2-negative sites, suggesting involvement of other vesicular systems. P62/Sequestosome 1 (P62/SQSTM1) inclusions were also identified in the CNS of the Mcoln1(-/-) mouse, suggesting deficiencies in protein degradation. Glial cell activation was increased in brain, and there was evidence of reduced myelination in cerebral and cerebellar white matter tracts. Autofluorescent material accumulated throughout the brains of the knockout mice. Finally, axonal spheroids were prevalent in white matter tracts and Purkinje cell axons. This neuropathological characterization of the Mcoln1(-/-) mouse provides an important step in understanding how mucolipin 1 loss of function affects the CNS and contributes to mucolipidosis type IV disease.

Autophagy inhibition compromises degradation of ubiquitin-proteasome pathway substrates

The two main routes that cells use for degrading intracellular proteins are the ubiquitin-proteasome and autophagy-lysosome pathways, which have been thought to have largely distinct clients. Here, we show that autophagy inhibition increases levels of proteasome substrates. This is largely due to p62 (also called A170/SQSTM1) accumulation after autophagy inhibition. Excess p62 inhibits the clearance of ubiquitinated proteins destined for proteasomal degradation by delaying their delivery to the proteasome's proteases. Our data show that autophagy inhibition, which was previously believed to only affect long-lived proteins, will also compromise the ubiquitin-proteasome system. This will lead to increased levels of short-lived regulatory proteins, like p53, as well as the accumulation of aggregation-prone proteins, with predicted deleterious consequences.

The sequestosome 1/p62 attenuates cytokine gene expression in activated macrophages by inhibiting IFN regulatory factor 8 and TNF receptor-associated factor 6/NF-kappaB activity

Sequestosome 1/p62 (p62) is a scaffold/adaptor protein with multiple functions implicated for neuronal and bone diseases. It carries a ubiquitin binding domain through which it mediates proteasome-dependent proteolysis. In addition, p62 is reported to regulate NF-kappaB activity in some cells. To date, however, the role of p62 in innate immunity has not been fully elucidated. In this study, we report that IFN-gamma plus TLR signaling stimulates late expression of p62 in murine macrophages. Overexpression of p62 inhibited expression of multiple cytokines, IL-12p40, TNF-alpha, IL-1beta, IL-6, and IFN-beta, whereas p62 underexpression by small hairpin RNA markedly elevated their expression, indicating that p62 is a broad negative regulator of cytokine expression in stimulated macrophages. We show that p62 interacts with IFN regulatory factor 8 and Ro52, the transcription factor and ubiquitin E3 ligase that are important for IL-12p40 expression. This interaction, detectable at a late stage in stimulated macrophages, led to increased polyubiquitination and destabilization of IFN regulatory factor 8. We also show that upon macrophage stimulation, p62 binds to TNFR-associated factor 6, another E3 ligase important for NF-kappaB activation, but later this interaction was replaced by the recruitment of the deubiquitinating enzyme, cylindromatosis, an inhibitor of NF-kappaB activity. Recruitment of cylindromatosis coincided with reduced TNFR-associated factor 6 autoubiquitination and lower NF-kappaB activation. Our results indicate that p62 orchestrates orderly regulation of ubiquitin modification processes in macrophages to ensure attenuation of cytokine transcription postactivation. Together, p62 may provide a mechanism by which to control excessive inflammatory responses after macrophage activation.

Age-associated oxidative damage to the p62 promoter: implications for Alzheimer disease

The absence of the p62 gene in mouse brain leads to biochemical and cognitive deficits that resemble Alzheimer disease (AD). In this context, the objective of this study was to examine the relationship between age-induced oxidative damage to the p62 promoter and AD. Increased 8-OHdG staining, a marker of oxidative stress, was observed in brain sections from mice deficient in the p62 gene compared to control. Treatment of MEF cells deficient in p62 with H(2)O(2) resulted in decreased cell survival and an absence of Nrf2 nuclear translocation. The mouse p62 promoter exhibited elevated oxidative damage with increasing age, and the degree of p62 promoter damage was also age-correlated in human brain samples. In human subjects, the expression of p62 was decreased in AD brain relative to age-matched controls, and likewise decreased p62 expression correlated with oxidative damage to the promoter. Treatment of HEK cells with H(2)O(2) resulted in decreased p62 expression concomitant with increased promoter damage. Consistent with these findings, a transgenic AD mouse model also exhibited increased p62 promoter damage and reduced p62 levels in brain. Altogether, our results reveal that oxidative damage to the p62 promoter correlates with decreased expression of p62 and may contribute to age-associated neurodegenerative disease such as AD and others.

Physiological functions of autophagy

The field of autophagy research has advanced rapidly in recent years, with important discoveries made in relation to both molecular mechanisms and physiological functions. Initially, autophagy was thought to be primarily a response to starvation. Although this might be true in lower eukaryotes, this catabolic process exerts various physiological functions in higher eukaryotes. This review summarizes the physiological roles of autophagy in amino acid pool maintenance, intracellular quality control, development, cell death, tumor suppression and anti-aging.

Hypoxia-activated autophagy accelerates degradation of SQSTM1/p62

Sequestosome 1 (SQSTM1/p62) is a multifunctional protein involved in signal transduction, protein degradation and cell transformation. Hypoxia is a common feature of solid tumours that promotes cancer progression. Here, we report that p62 is downregulated in hypoxia in carcinoma cells and that the expression is rapidly restored in response to reoxygenation. The hypoxic p62 downregulation did not occur at the mRNA level and was independent of the hypoxic signal mediators hypoxia-inducible factor (HIF) and von Hippel-Lindau tumour suppressor protein as well as the activity of HIF-prolyl hydroxylases and was not mediated by proteosomal destruction. Autophagy was activated in hypoxia and was required for p62 degradation. The hypoxic degradation of p62 was blocked by autophagy inhibitors as well as by the attenuation of Atg8/LC3 expression. Downregulation of p62 was required for hypoxic extracellular regulated kinase (ERK)-1/2 phosphorylation. Attenuation of p62 in normoxia activated and forced expression of p62 in hypoxia blocked the activation of ERK-1/2. The results demonstrate that hypoxic activation of autophagy induces clearance of p62 protein and implies a role for p62 in the regulation of hypoxic cancer cell survival responses.

White matter lesions in the brain with frontotemporal lobar degeneration with motor neuron disease: TDP-43-immunopositive inclusions co-localize with p62, but not ubiquitin

Recently, TDP-43 was established as a major component of the ubiquitinated inclusions found in both amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with motor neuron disease (FTLD-MND). However, differences in the underlying pathogenesis between ALS and FTLD-MND remain yet to be elucidated. Originally, TDP-43-immunopositive inclusions were found in neuronal cells and reported to be ubiquitinated. This study shows that TDP-43-positive inclusions were distributed throughout the subcortical white matter except for the occipital lobe in the FTLD-MND brain, but not in the ALS brain. TDP-43-positive inclusions were also prominent features of pathologically proven FTLD-MND cases (p-FTLD-MND) without history of apparent clinical cognitive decline. A substantial fraction of these inclusions was also p62-immunoreactive, and another noteworthy feature was that those inclusions did not stain positively for ubiquitin. Significant correlations between immunoreactivity for TDP-43 and p62 were observed, particularly in p-FTLD-MND (Pearson correlation coefficient, 0.976). Furthermore, TDP-43 extracted from white matter appeared to be uncleaved. These results indicate that pathological changes might take place within the white matter also in the brain with FTLD-MND, but in a different manner than within the gray matter.

Neuropathologic features of frontotemporal lobar degeneration with ubiquitin-positive inclusions visualized with ubiquitin-binding protein p62 immunohistochemistry

Genetic, clinical, and neuropathologic heterogeneity have been observed in frontotemporal lobar degeneration with ubiquitin (Ubq)-positive inclusions (FTLD-U) and FTLD-U with motor neuron disease. Here, the distribution and morphologic features of neuronal and glial inclusions in the brains of 20 FTLD-U and 2 FTLD-U/motor neuron disease cases were assessed using immunohistochemistry for Ubq-binding protein p62. Eighteen cases displayed TAR DNA-binding protein 43-immunoreactive lesions and were classified as Types 3 (neuronal cytoplasmic inclusions and neurites; 72%), 2 (primarily neuronal cytoplasmic inclusions; 17%), or 1 (primarily neurites; 11%) FTLD-U. The distribution of p62-immunoreactivity varied considerably in each type. Of 4 unclassifiable cases, 2 displayed p62-immunoreactive lesions suggestive of FTLD-U with a mutation in the charged multivesicular body protein 2B gene; 1 suggested basophilic inclusion body disease, and 1 was of a type not previously described. By immunohistochemistry for Ubq-binding protein p62, the distribution of abnormalities was wider than expected; in approximately half of the cases, there were p62-positive but TAR DNA-binding protein 43-negative inclusions in the cerebellum, a region not previously considered to be affected. In other regions, TAR DNA-binding protein 43-, Ubq-, and Ubq-binding protein p62 labeling of inclusions was variable. Whether variations in inclusion morphologies, immunoreactivity, and topographic distribution are due to methodologic factors, different stages of inclusion and disease evolution, different disease entities or biologic modifications of the same disease are presently unclear.

Ref(2)P, the Drosophila melanogaster homologue of mammalian p62, is required for the formation of protein aggregates in adult brain

p62 has been proposed to mark ubiquitinated protein bodies for autophagic degradation. We report that the Drosophila melanogaster p62 orthologue, Ref(2)P, is a regulator of protein aggregation in the adult brain. We demonstrate that Ref(2)P localizes to age-induced protein aggregates as well as to aggregates caused by reduced autophagic or proteasomal activity. A similar localization to protein aggregates is also observed in D. melanogaster models of human neurodegenerative diseases. Although atg8a autophagy mutant flies show accumulation of ubiquitin- and Ref(2)P-positive protein aggregates, this is abrogated in atg8a/ref(2)P double mutants. Both the multimerization and ubiquitin binding domains of Ref(2)P are required for aggregate formation in vivo. Our findings reveal a major role for Ref(2)P in the formation of ubiquitin-positive protein aggregates both under physiological conditions and when normal protein turnover is inhibited.

Autophagy-physiology and pathophysiology

"Autophagy" is a highly conserved pathway for degradation, by which wasted intracellular macromolecules are delivered to lysosomes, where they are degraded into biologically active monomers such as amino acids that are subsequently re-used to maintain cellular metabolic turnover and homeostasis. Recent genetic studies have shown that mice lacking an autophagy-related gene (Atg5 or Atg7) cannot survive longer than 12 h after birth because of nutrient shortage. Moreover, tissue-specific impairment of autophagy in central nervous system tissue causes massive loss of neurons, resulting in neurodegeneration, while impaired autophagy in liver tissue causes accumulation of wasted organelles, leading to hepatomegaly. Although autophagy generally prevents cell death, our recent study using conditional Atg7-deficient mice in CNS tissue has demonstrated the presence of autophagic neuron death in the hippocampus after neonatal hypoxic/ischemic brain injury. Thus, recent genetic studies have shown that autophagy is involved in various cellular functions. In this review, we introduce physiological and pathophysiological roles of autophagy.

Homeostatic levels of p62 control cytoplasmic inclusion body formation in autophagy-deficient mice

Inactivation of constitutive autophagy results in formation of cytoplasmic protein inclusions and leads to liver injury and neurodegeneration, but the details of abnormalities related to impaired autophagy are largely unknown. Here we used mouse genetic analyses to define the roles of autophagy in the aforementioned events. We report that the ubiquitin- and LC3-binding protein "p62" regulates the formation of protein aggregates and is removed by autophagy. Thus, genetic ablation of p62 suppressed the appearance of ubiquitin-positive protein aggregates in hepatocytes and neurons, indicating that p62 plays an important role in inclusion body formation. Moreover, loss of p62 markedly attenuated liver injury caused by autophagy deficiency, whereas it had little effect on neuronal degeneration. Our findings highlight the unexpected role of homeostatic level of p62, which is regulated by autophagy, in controlling intracellular inclusion body formation, and indicate that the pathologic process associated with autophagic deficiency is cell-type specific.

Autophagy: process and function

Autophagy is an intracellular degradation system that delivers cytoplasmic constituents to the lysosome. Despite its simplicity, recent progress has demonstrated that autophagy plays a wide variety of physiological and pathophysiological roles, which are sometimes complex. Autophagy consists of several sequential steps--sequestration, transport to lysosomes, degradation, and utilization of degradation products--and each step may exert different function. In this review, the process of autophagy is summarized, and the role of autophagy is discussed in a process-based manner.

Use of p62/SQSTM1 antibodies for neuropathological diagnosis

The demonstration of proteinaceous inclusions in the brain is the key step in the pathological diagnosis of degenerative dementias. The diversity of these diseases has necessitated the use of a panel of (immuno)stains to visualize all suspect pathologies, elevating diagnostic costs. Immunodetection of p62 (sequestosome 1), an abundant constituent in diverse pathological inclusions, holds the potential for a broad-specificity, high-contrast inclusion label. In the brain, pathological p62-positive aggregates comprise both cytoplasmic and nuclear types in neurones and glia, with abnormal tau, alpha-synuclein, TAR DNA-binding protein 43 or polyglutamine proteins as primary components. We therefore set out to evaluate the performance of p62 antibodies for diagnostic immunohistochemistry. We optimized the application conditions and compared the staining profiles of eight commercial p62 antibodies with each other and with reference immunostains, using 2-mm tissue multiarrays representing the major tauo- and synucleinopathies and frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U). The lesions were best visualized using monoclonal antibodies, displaying most types of hallmark inclusions with excellent contrast. Expanding the list of p62-containing aggregates, we demonstrated p62 in tufted astrocytes, coiled bodies, astrocytic plaques, and variform neocortical inclusions and pathological processes in FTLD-U. Polyclonal antibodies exhibited lower sensitivities with variable background levels. We also noted more subtle p62-immunoreactive features lacking overt disease associations. Emphasizing the importance of proper antibody and epitope unmasking methods for maximum sensitivity, we recommend p62 immunodetection as a screening stain for diagnostic practice.

How a cell deals with abnormal proteins. Pathogenetic mechanisms in protein aggregation diseases

Defective protein folding is responsible for many diseases. Although these diseases seem to be quite diverse at the first glance, there is evidence for common pathogenetic principles. The basis of the pathological changes is the cell's inability to prevent protein misfolding, to revert misfolded proteins to normal or to eliminate misfolded proteins by degradation. This could result in deposition of potentially cytotoxic protein aggregates (protein aggregation diseases). Chronic degenerative diseases of the central nervous system (e.g. Alzheimer's and Parkinson's disease), the amyloidoses, but also chronic liver diseases, for example alcoholic and nonalcoholic steatohepatitis, belong to this category of disorders. This review highlights general pathogenic principles of protein aggregation diseases based on immunohistochemical and biochemical studies as well as observations in a mouse model for protein aggregation in the context of alcoholic and nonalcoholic steatohepatitis. The cellular defense mechanisms involved in protein quality control as well as the pathogenesis of protein aggregation diseases will be discussed.

Oxidative stress in desminopathies and myotilinopathies: a link between oxidative damage and abnormal protein aggregation

Myotilinopathies and desminopathies are subgroups of myofibrillar myopathies (MFM) caused by mutations in myotilin and desmin genes, respectively. They are characterized by the presence of protein aggregates in muscle cells. As oxidation of proteins facilitates their aggregation and makes them more resistant to proteolysis, the present study was geared to analyze oxidative stress in MFM. For this purpose, markers of glycoxidation, lipoxidation and nitration were examined with gel electrophoresis and Western blotting, single immunohistochemistry, and double- and triple-labeling immunofluorescence and confocal microscopy in muscle biopsies from patients suffering from myotilinopathy and desminopathy. Increased levels of glycation-end products (AGEs), N-carboxymethyl-lysine (CML) and N-carboxyethyl-lysine (CEL), malondialdehyde-lysine (MDAL), 4-hydroxynonenal (HNE) and nitrotyrosine (N-tyr) were found in MFM. Furthermore, aberrant expression of AGE, CML, CEL, MDAL and HNE, as well as of neuronal, inducible and endothelial nitric oxide synthases (nNOS, iNOS, eNOS), and superoxide dismutase 2 (SOD2), was found in muscle fibers containing protein aggregates in myotilinopathies and desminopathies. AGE, ubiquitin and p62 co-localized in several muscle fibers in MFM. As oxidized proteins are vulnerable to misfolding and are resistant to degradation by the UPS, the present observations support a link between oxidative stress, protein aggregation and abnormal protein deposition in MFMs.

In vitro production of Mallory bodies and intracellular hyaline bodies: the central role of sequestosome 1/p62

Mallory bodies (MBs) and intracellular hyaline bodies (IHBs) are characteristic hepatocellular inclusions. MBs are hallmarks of steatohepatitis, whereas IHBs have first been detected in hepatocellular carcinoma. MBs and IHBs contain ubiquitin and sequestosome 1/p62 (p62), a stress-inducible adapter protein with affinity to polyubiquitinated proteins. MBs differ from IHBs by their keratin content and morphology. In vitro transfections were undertaken to study under defined conditions MB and IHB formation, their pathogenesis, and relationship. CHO-K1, TIB73, and HeLa cells were transfected with keratin 8, keratin 18, ubiquitin, p62, and p62 lacking the ubiquitin binding domain (p62DeltaUBA) and analyzed by immunofluorescence, immunoelectron microscopy, and immunoblotting. Transfection of p62 complementary deoxyribonucleic acid (cDNA) alone led to cytoplasmic aggregates consisting of filaments mostly arranged in parallel arrays resembling amyloid and type 1 MBs. Transfection of p62 and ubiquitin resulted in globular cytoplasmic aggregates with indistinct fibrillar ultrastructure resembling IHBs. Cotransfection of p62, keratin 8, and ubiquitin was necessary to produce in vitro type 2 MBs-like aggregates consisting of randomly oriented 10- to 15-nm filaments. A similar result was obtained when keratin 8 was replaced by keratin 18. After cotransfection of p62DeltaUBA, keratin 8, and ubiquitin, keratin formed irregular aggregates with electron-dense granular-amorphous ultrastructure (resembling type 3 MBs), whereas p62DeltaUBA remained in diffuse cytoplasmic distribution.

CONCLUSION

Our studies show that in vitro development of classical type 2 MBs requires overexpression of keratin 8 (or keratin 18), ubiquitin, and p62 containing the ubiquitin binding domain, whereas IHBs result from overexpression of p62 together with ubiquitin without keratin involvement.