Search

Abstract There are no available therapies targeting the underlying molecular mechanisms of neurodegenerative diseases. Although chaperone therapies that alleviate endoplasmic reticulum (ER) stress recently showed promise in the treatment of neurodegenerative diseases, the detailed mechanisms remain unclear. We previously reported that mice with central nervous system-specific deletion of Derlin-1, which encodes an essential component for ER quality control, are useful as models of neurodegenerative diseases such as spinocerebellar degeneration. Cholesterol biosynthesis is essential for brain development, and its disruption inhibits neurite outgrowth, causing brain atrophy. In this study, we report a novel mechanism by which chemical chaperones ameliorate brain atrophy and motor dysfunction. ER stress was induced in the cerebella of Derlin-1 deficiency mice, whereas the administration of a chemical chaperone did not alleviate ER stress. However, chemical chaperone treatment ameliorated cholesterol biosynthesis impairment through SREBP-2 activation and simultaneously relieved brain atrophy and motor dysfunction. Altogether, these findings demonstrate that ER stress may not be the target of action of chaperone therapies and that chemical chaperone-mediated improvement of brain cholesterol biosynthesis is a promising novel therapeutic strategy for neurodegenerative diseases.

Summary: Derlin family members (Derlins) are primarily known as components of the endoplasmic reticulum-associated degradation pathway that eliminates misfolded proteins. Here we report a function of Derlins in the brain development. Deletion of Derlin-1 or Derlin-2 in the central nervous system of mice impaired postnatal brain development, particularly of the cerebellum and striatum, and induced motor control deficits. Derlin-1 or Derlin-2 deficiency reduced neurite outgrowth in vitro and in vivo and surprisingly also inhibited sterol regulatory element binding protein 2 (SREBP-2)-mediated brain cholesterol biosynthesis. In addition, reduced neurite outgrowth due to Derlin-1 deficiency was rescued by SREBP-2 pathway activation. Overall, our findings demonstrate that Derlins sustain brain cholesterol biosynthesis, which is essential for appropriate postnatal brain development and function.

Abstract The maintenance of endoplasmic reticulum (ER) homeostasis is essential for cell function. ER stress-induced pre-emptive quality control (ERpQC) helps alleviate the burden to a stressed ER by limiting further protein loading. We have previously reported the mechanisms of ERpQC, which includes a rerouting step and a degradation step. Under ER stress conditions, Derlin family proteins (Derlins), which are components of ER-associated degradation, reroute specific ER-targeting proteins to the cytosol. Newly synthesized rerouted polypeptides are degraded via the cytosolic chaperone Bag6 and the AAA-ATPase p97 in the ubiquitin-proteasome system. However, the mechanisms by which ER-targeting proteins are rerouted from the ER translocation pathway to the cytosolic degradation pathway and how the E3 ligase ubiquitinates ERpQC substrates remain unclear. Here, we show that ERpQC substrates are captured by the carboxyl-terminus region of Derlin-1 and ubiquitinated by the HRD1 E3 ubiquitin ligase prior to degradation. Moreover, HRD1 forms a large ERpQC-related complex composed of Sec61α and Derlin-1 during ER stress. These findings indicate that the association of the degradation factor HRD1 with the translocon and the rerouting factor Derlin-1 may be necessary for the smooth and effective clearance of ERpQC substrates.

Cells possess ER quality control systems to adapt to ER stress and maintain their function. ER-stress-induced pre-emptive quality control (ER pQC) selectively degrades ER proteins via translocational attenuation during ER stress. However, the molecular mechanism underlying this process remains unclear. Here, we find that most newly synthesized endogenous transthyretin proteins are rerouted to the cytosol without cleavage of the signal peptide, resulting in proteasomal degradation in hepatocytes during ER stress. Derlin family proteins (Derlins), which are ER-associated degradation components, reroute specific ER proteins, but not ER chaperones, from the translocon to the proteasome through interactions with the signal recognition particle (SRP). Moreover, the cytosolic chaperone Bag6 and the AAA-ATPase p97 contribute to the degradation of ER pQC substrates. These findings demonstrate that Derlins-mediated substrate-specific rerouting and Bag6- and p97-mediated effective degradation contribute to the maintenance of ER homeostasis without the need for translocation.

Mutations in the Cu, Zn superoxide dismutase (SOD1) gene are one of the causative agents of amyotrophic lateral sclerosis (ALS). Although more than 100 different mutations in SOD1 have been identified, it is unclear whether all the mutations are pathogenic or just single nucleotide polymorphisms (SNPs) unrelated to the disease. Our previous systematic analysis found that all pathogenic SOD1 mutants (SOD1mut) have a common property, namely, an association with Derlin-1, a component of the endoplasmic reticulum-associated degradation machinery. For the proposed mechanism, we found that most pathogenic SOD1mut have a constitutively exposed Derlin-1-binding region (DBR), which is concealed in wild-type SOD1 (SOD1WT). Moreover, we generated MS785, a monoclonal antibody against DBR. MS785 distinguished most ALS-causative SOD1mut from both SOD1WT and non-toxic SOD1mut. However, MS785 could not recognize SOD1mut that has mutations in the MS785 epitope region. Here, we developed a new diagnostic antibody, which could compensate for this shortcoming of MS785. We hypothesized that in ALS-causative SOD1mut, the DBR-neighboring region [SOD1(30–40)] may also be exposed. We then generated MS27, a monoclonal antibody against SOD1(30–40). We found that MS27 could distinguish SOD1WT from the pathogenic SOD1mut, which has mutations in the MS785 epitope region. Moreover, all pathogenic SOD1mut, without exception, were immunoprecipitated with a combination of MS785 and MS27. The MS785–MS27 combination could be developed as a novel mechanism-based biomarker for the diagnosis of ALS.

The unfolded protein response (UPR) is an intracellular homeostatic signalling pathway that is induced by accumulated misfolded/unfolded proteins in the endoplasmic reticulum (ER). The UPR is closely associated with the development of disease in several tissues, including the central nervous system (CNS), in response to ER stress. More recently, the unique features and importance of the UPR have been revealed in neural stem cells (NSCs) and differentiated CNS cells [neurons and glial cells (astrocytes and oligodendrocytes)]. Although several UPR signalling pathways dynamically change in each CNS cell during brain development, the role of UPR signalling in CNS cells (especially NSCs and glial cells) under pathological or physiological conditions is poorly understood. Here, we discuss and summarize the recent progress in understanding how the UPR regulates the proliferation, differentiation, maturation and viability of CNS cells. [ABSTRACT FROM AUTHOR]

The endoplasmic reticulum (ER) is a dynamic organelle that is essential for multiple cellular functions. During cellular stress conditions, including nutrient deprivation and dysregulation of protein synthesis, unfolded/misfolded proteins accumulate in the ER lumen, resulting in activation of the unfolded protein response (UPR). The UPR also contributes to the regulation of various intracellular signaling pathways such as calcium signaling and lipid signaling. More recently, the mitochondria-associated ER membrane (MAM), which is a site of close contact between the ER and mitochondria, has been shown to function as a platform for various intracellular stress responses including apoptotic signaling, inflammatory signaling, the autophagic response, and the UPR. Interestingly, in cancer, these signaling pathways from the ER are often dysregulated, contributing to cancer cell metabolism. Thus, the signaling pathway from the ER may be a novel therapeutic target for various cancers. In this review, we discuss recent research on the roles of stress responses from the ER, including the MAM. [ABSTRACT FROM AUTHOR]

The endoplasmic reticulum (ER) is an organelle in which newly synthesized secretory and transmembrane proteins are assembled and folded into their correct tertiary structures. However, many of these ER proteins are misfolded as a result of various stimuli and gene mutations. The accumulation of misfolded proteins disrupts the function of the ER and induces ER stress. Eukaryotic cells possess a highly conserved signaling pathway, termed the unfolded protein response (UPR), to adapt and respond to ER stress conditions, thereby promoting cell survival. However, in the case of prolonged ER stress or UPR malfunction, apoptosis signaling is activated. Dysfunction of the UPR causes numerous conformational diseases, including neurodegenerative disease, metabolic disease, inflammatory disease, diabetes mellitus, cancer, and cardiovascular disease. Thus, ER stress-induced signaling pathways may serve as potent therapeutic targets of ER stress-related diseases. In this review, we will discuss the molecular mechanisms of the UPR and ER stress-induced apoptosis, as well as the possible roles of ER stress in several diseases. [ABSTRACT FROM AUTHOR]

Background: Fn14 is a highly inducible member of the TNF receptor family. Results: Large-magnitude mechanical stress induced Fn14 expression via JNK in osteoblasts. Conclusion: Expression of Fn14 regulates mechanical stress-induced apoptosis in osteoblasts. Significance: This is the first elucidation of themechanismof excessivemechanical stress-induced apoptosismediated by Fn14. [ABSTRACT FROM AUTHOR]