Amyloidogenic lysozymes accumulate in the endoplasmic reticulum accompanied by the augmentation of ER stress signals

Binding site prediction between lysozyme and glucose-regulated protein 78, a hope to fight amyloidosis

Amyloidosis is an extraordinarily vigorous and heterogeneous group of disorders that causes numerous organ failures due to the precipitation of misfolded proteins. Many of these damaged proteins are discarded before causing any fatal diseases due to the contribution of the protein quality control (PQC) system and its chaperons, including glucose-regulated protein (GRP78). One of the most important enzymatic proteins inside the body is lysozyme, which is reported to have many mutated variants that may cause amyloid fibrils. This study used structural bioinformatics and molecular dynamics simulations to test and suggest binding sites for the human lysozyme protein with GRP78. Multiple sequence alignment (MSA) shows that part of the lysozyme envelope protein (C65-C81 cyclic region) has high similarities (30.77% identity) with the cyclic Pep42. Additionally, the binding between the lysozyme cyclic region (C65-C81) and GRP78 substrate binding domain (SBD) is found favorable. The number and types of interactions vary between each of the mutant isoforms of lysozyme. The more significant the conformational changes in the mutation, the greater its probability of aggregation and the formation of amyloid fibrils. Each mutation leads to different interactions and binding patterns with GRP78. The present computational study suggests a lysozyme-GRP78 binding site, thus paving the way for drug designers to construct suitable carriers that can collect misfolded lysozyme proteins and eliminate them from the body, preventing their aggregation and amyloidogenesis.Communicated by Ramaswamy H. Sarma.

Promotion of degradative autophagy by 6-bromoindirubin-3'-oxime attenuates neuropathy

Damage to the central or peripheral nervous system causes neuropathic pain. Endoplasmic reticulum (ER) stress plays a role in peripheral neuropathy. Increase in ER stress is seen in diabetic neuropathy. Inducers of ER stress also give rise to peripheral neuropathy. ER stress leads to the formation of autophagosome but as their degradation is also stalled during ER stress accumulation of autophagosomes is seen. Accumulation of autophagosomes has deleterious effects on cells. In the present study, we show that treatment with tunicamycin (TM) (ER stress inducer) in mice leads to peripheral neuropathy as assessed by Von Frey and Hot plate method. Administration of a promoter of autophagy viz. 6-bromoindirubin-3'-oxime (6-BIO) subsequent to ER stress induced by TM exhibits a decrease in peripheral neuropathy. 6-BIO was also effective in reducing diabetic peripheral neuropathy. To understand the type of autophagy activated, SH-SY5Y cells were treated with 6-BIO after TM treatment. Levels of cathepsin D (CTSD), a marker for degradative autophagy was higher in cells treated with 6-BIO after TM treatment compared to only TM-treated SH-SY5Y cells while levels of Rab8A,-a marker for secretory autophagy was reduced. Furthermore, in parallel during ER stress secretory, we noted increased levels of lysozyme in autophagosomes destined for secretion. Cells treated with 6-BIO showed reduction of lysozyme in secretory autophagosomes. This shows that 6-BIO increased degradative autophagy and reduced the secretory autophagy. 6-BIO also reduced the caspase-3 activity in 6-BIO-treated cells. Thus, 6-BIO reduced neuropathy in animals by activating degradative autophagy and reducing the secretory autophagy.

Stress-responsive regulation of extracellular proteostasis

Genetic, environmental, and aging-related insults can promote the misfolding and subsequent aggregation of secreted proteins implicated in the pathogenesis of numerous diseases. This has led to considerable interest in understanding the molecular mechanisms responsible for regulating proteostasis in extracellular environments such as the blood and cerebrospinal fluid (CSF). Extracellular proteostasis is largely dictated by biological pathways comprising chaperones, folding enzymes, and degradation factors localized to the ER and extracellular space. These pathways limit the accumulation of nonnative, potentially aggregation-prone proteins in extracellular environments. Many reviews discuss the molecular mechanisms by which these pathways impact the conformational integrity of the secreted proteome. Here, we instead focus on describing the stress-responsive mechanisms responsible for adapting ER and extracellular proteostasis pathways to protect the secreted proteome from pathologic insults that challenge these environments. Further, we highlight new strategies to identify stress-responsive pathways involved in regulating extracellular proteostasis and describe the pathologic and therapeutic implications for these pathways in human disease.

Serum Amyloid P Component Ameliorates Neurological Damage Caused by Expressing a Lysozyme Variant in the Central Nervous System of Drosophila melanogaster

Lysozyme amyloidosis is a hereditary disease in which mutations in the gene coding for lysozyme leads to misfolding and consequently accumulation of amyloid material. To improve understanding of the processes involved we expressed human wild type (WT) lysozyme and the disease-associated variant F57I in the central nervous system (CNS) of a Drosophila melanogaster model of lysozyme amyloidosis, with and without co-expression of serum amyloid p component (SAP). SAP is known to be a universal constituent of amyloid deposits and to associate with lysozyme fibrils. There are clear indications that SAP may play an important role in lysozyme amyloidosis, which requires further elucidation. We found that flies expressing the amyloidogenic variant F57I in the CNS have a shorter lifespan than flies expressing WT lysozyme. We also identified apoptotic cells in the brains of F57I flies demonstrating that the flies' neurological functions are impaired when F57I is expressed in the nerve cells. However, co-expression of SAP in the CNS prevented cell death and restored the F57I flies' lifespan. Thus, SAP has the apparent ability to protect nerve cells from damage caused by F57I. Furthermore, it was found that co-expression of SAP prevented accumulation of insoluble forms of lysozyme in both WT- and F57I-expressing flies. Our findings suggest that the F57I mutation affects the aggregation process of lysozyme resulting in the formation of cytotoxic species and that SAP is able to prevent cell death in the F57I flies by preventing accumulation of toxic F57I structures.

Lysozyme Mutants Accumulate in Cells while Associated at their N-terminal Alpha-domain with the Endoplasmic Reticulum Chaperone GRP78/BiP

Amyloidogenic human lysozyme variants deposit in cells and cause systemic amyloidosis. We recently observed that such lysozymes accumulate in the endoplasmic reticulum (ER) with the ER chaperone GRP78/BiP, accompanying the ER stress response. Here we investigated the region of lysozyme that is critical to its association with GRP78/BiP. In addition to the above-mentioned variants of lysozyme, we constructed lysozyme truncation or substitution mutants. These were co-expressed with GRP78/BiP (tagged with FLAG) in cultured human embryonic kidney cells, which were analyzed by western blotting and immunocytochemistry using anti-lysozyme and anti-FLAG antibodies. The amyloidogenic variants were confirmed to be strongly associated with GRP78/BiP as revealed by the co-immunoprecipitation assay, whereas N-terminal mutants pruned of 1-41 or 1-51 residues were found not to be associated with the chaperone. Single amino acid substitutions for the leucine array along the α-helices in the N-terminal region resulted in wild-type lysozyme remaining attached to GRP78/BiP. These mutations also tended to show lowered secretion ability. We conclude that the N-terminal α-helices region of the lysozyme is pivotal for its strong adhesion to GRP78/BiP. We suspect that wild-type lysozyme interacts with the GRP at this region as a step in the proper folding monitored by the ER chaperone.

Endoplasmic reticulum quality control and systemic amyloid disease: Impacting protein stability from the inside out

The endoplasmic reticulum (ER) is responsible for regulating proteome integrity throughout the secretory pathway. The ER protects downstream secretory environments such as the extracellular space by partitioning proteins between ER protein folding, trafficking, and degradation pathways in a process called ER quality control. In this process, ER quality control factors identify misfolded, aggregation-prone protein conformations and direct them toward ER protein folding or degradation, reducing their secretion to the extracellular space where they could further misfold or aggregate into proteotoxic conformations. Despite the general efficiency of ER quality control, many human diseases, such as the systemic amyloidoses, involve aggregation of destabilized, aggregation-prone proteins in the extracellular space. A common feature for all systemic amyloid diseases is the ability for amyloidogenic proteins to evade ER quality control and be efficiently secreted. The efficient secretion of these amyloidogenic proteins increases their serum concentrations available for the distal proteotoxic aggregation characteristic of these diseases. This indicates that ER quality control, and the regulation thereof, is a critical determinant in defining the onset and pathology of systemic amyloid diseases. Here, we discuss the pathologic and potential therapeutic relationship between ER quality control, protein secretion, and distal deposition of amyloidogenic proteins involved in systemic amyloid diseases. Furthermore, we present evidence that the unfolded protein response, the stress-responsive signaling pathway that regulates ER quality control, is involved in the pathogenesis of systemic amyloid diseases and represents a promising emerging therapeutic target to intervene in this class of human disease.