Reconstitution defines the roles of p62, NBR1 and TAX1BP1 in ubiquitin condensate formation and autophagy initiation

The autophagic degradation of misfolded and ubiquitinated proteins is important for cellular homeostasis. In this process, which is governed by cargo receptors, ubiquitinated proteins are condensed into larger structures and subsequently become targets for the autophagy machinery. Here we employ in vitro reconstitution and cell biology to define the roles of the human cargo receptors p62/SQSTM1, NBR1 and TAX1BP1 in the selective autophagy of ubiquitinated substrates. We show that p62 is the major driver of ubiquitin condensate formation. NBR1 promotes condensate formation by equipping the p62-NBR1 heterooligomeric complex with a high-affinity UBA domain. Additionally, NBR1 recruits TAX1BP1 to the ubiquitin condensates formed by p62. While all three receptors interact with FIP200, TAX1BP1 is the main driver of FIP200 recruitment and thus the autophagic degradation of p62–ubiquitin condensates. In summary, our study defines the roles of all three receptors in the selective autophagy of ubiquitin condensates.

Misfolded proteins are ubquitinated and subsequently condensed by cargo receptors for selective autophagy. Here, the authors use in vitro reconstitution to elegantly dissect how the receptors p62/SQSTM1, NBR1 and TAX1BP1 contribute to p62-ubiquitin condensate formation and degradation by autophagy.

A cross-kingdom conserved ER-phagy receptor maintains endoplasmic reticulum homeostasis during stress

Eukaryotes have evolved various quality control mechanisms to promote proteostasis in the endoplasmic reticulum (ER). Selective removal of certain ER domains via autophagy (termed as ER-phagy) has emerged as a major quality control mechanism. However, the degree to which ER-phagy is employed by other branches of ER-quality control remains largely elusive. Here, we identify a cytosolic protein, C53, that is specifically recruited to autophagosomes during ER-stress, in both plant and mammalian cells. C53 interacts with ATG8 via a distinct binding epitope, featuring a shuffled ATG8 interacting motif (sAIM). C53 senses proteotoxic stress in the ER lumen by forming a tripartite receptor complex with the ER-associated ufmylation ligase UFL1 and its membrane adaptor DDRGK1. The C53/UFL1/DDRGK1 receptor complex is activated by stalled ribosomes and induces the degradation of internal or passenger proteins in the ER. Consistently, the C53 receptor complex and ufmylation mutants are highly susceptible to ER stress. Thus, C53 forms an ancient quality control pathway that bridges selective autophagy with ribosome-associated quality control in the ER.

Phasing out the bad-How SQSTM1/p62 sequesters ubiquitinated proteins for degradation by autophagy

The degradation of misfolded, ubiquitinated proteins is essential for cellular homeostasis. These proteins are primarily degraded by the ubiquitin-proteasome system (UPS) and macroautophagy/autophagy serves as a backup mechanism when the UPS is overloaded. How autophagy and the UPS are coordinated is not fully understood. During the autophagy of misfolded, ubiquitinated proteins, referred to as aggrephagy, substrate proteins are clustered into larger structures in a SQSTM1/p62-dependent manner before they are sequestered by phagophores, the precursors to autophagosomes. We have recently shown that SQSTM1/p62 and ubiquitinated proteins spontaneously phase separate into micrometer-sized clusters in vitro. This enabled us to characterize the properties of the ubiquitin-positive substrates that are necessary for the SQSTM1/p62-mediated cluster formation. Our results suggest that aggrephagy is triggered by the accumulation of substrates with multiple ubiquitin chains and that the process can be inhibited by active proteasomes.

p62 filaments capture and present ubiquitinated cargos for autophagy

The removal of misfolded, ubiquitinated proteins is an essential part of the protein quality control. The ubiquitin-proteasome system (UPS) and autophagy are two interconnected pathways that mediate the degradation of such proteins. During autophagy, ubiquitinated proteins are clustered in a p62-dependent manner and are subsequently engulfed by autophagosomes. However, the nature of the protein substrates targeted for autophagy is unclear. Here, we developed a reconstituted system using purified components and show that p62 and ubiquitinated proteins spontaneously coalesce into larger clusters. Efficient cluster formation requires substrates modified with at least two ubiquitin chains longer than three moieties and is based on p62 filaments cross-linked by the substrates. The reaction is inhibited by free ubiquitin, K48-, and K63-linked ubiquitin chains, as well as by the autophagosomal marker LC3B, suggesting a tight cross talk with general proteostasis and autophagosome formation. Our study provides mechanistic insights on how substrates are channeled into autophagy.

Regional differences in diabetic patients' pharmacotherapy in Bulgaria

BACKGROUND

The regional analyses play an important role in understanding a state of diabetes mellitus management and to support informed policy options. They need to be explored in more details in order to ensure an equal patients' access to health care services of the same value and quality.

AIM

The aim of this study is to analyze regional differences in a cost of diabetes therapy for insulin users in Bulgaria.

MATERIALS AND METHODS

It is a combined prospective and retrospective observational study with duration of 6 months. Diabetic patients on insulin therapy were recruited by 35 endocrinologists. Information about the health care resources used was collected within 3-prospective and 3 retrospective months in 2010 and 2011. The regional cost of illness analysis was performed by calculating the average cost attributable to each individual patient despite the fact that some might not use a particular health care resource. Subgroup analysis was performed for hospitalized patients.

RESULTS

A detailed analysis revealed cost differences in the regions, especially with more vulnerable population like Burgas and Pleven regions. Another reason for the cost differences is the type of insulin or type of therapy. Our study confirms the fact that the hospitalizations are the major cost driver. Rising diabetes prevalence and associated costs, including hospitalizations and management of diabetes complications, are a growing concern. The last possible reason for regional differences is the patients' characteristics and therapy differences. We add evidence demonstrating that the patients on insulin and OAD consume more resources including hospitalizations and suffer from more complications of diabetes.

CONCLUSIONS

Reasons for regional differences might have different origin as there are various population characteristics, type of therapy, socio economic status and others.