Create and preserve: Proteostasis in development and aging is governed by Cdc48/p97/VCP

The AAA-ATPase Cdc48 (also called p97 or VCP) acts as a key regulator in proteolytic pathways, coordinating recruitment and targeting of substrate proteins to the 26S proteasome or lysosomal degradation. However, in contrast to the well-known function in ubiquitin-dependent cellular processes, the physiological relevance of Cdc48 in organismic development and maintenance of protein homeostasis is less understood. Therefore, studies on multicellular model organisms help to decipher how Cdc48-dependent proteolysis is regulated in time and space to meet developmental requirements. Given the importance of developmental regulation and tissue maintenance, defects in Cdc48 activity have been linked to several human pathologies including protein aggregation diseases. Thus, addressing the underlying disease mechanisms not only contributes to our understanding on the organism-wide function of Cdc48 but also facilitates the design of specific medical therapies. In this review, we will portray the role of Cdc48 in the context of multicellular organisms, pointing out its importance for developmental processes, tissue surveillance, and disease prevention. This article is part of a Special Issue entitled: Ubiquitin–Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.     

Lipid droplet proteins and metabolic diseases

Lipid droplets (LDs) are ubiquitous cellular organelles for lipid storage which are composed of a neutral lipid core bounded by a protein decorated phospholipid monolayer. Although lipid storage is their most obvious function, LDs are far from inert as they participate in maintaining lipid homeostasis through lipid synthesis, metabolism, and transportation. Furthermore, they are involved in cell signaling and other molecular events closely associated with human disease such as dyslipidemia, obesity, lipodystrophy, diabetes, fatty liver, atherosclerosis, and others. The last decade has seen a great increase in the attention paid to LD biology. Regardless, many fundamental features of LD biology remain obscure. In this review, we will discuss key aspects of LD biology including their biogenesis, growth and regression. We will also summarize the current knowledge about the role LDs play in human disease, especially from the perspective of the dynamics of the associated proteins. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.     

The oxidoreductase PYROXD1 uses NAD(P)+ as an antioxidant to sustain tRNA ligase activity in pre-tRNA splicing and unfolded protein response

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Quality Control of Membraneless Organelles

The formation of membraneless organelles (MLOs) by phase separation has emerged as a new way of organizing the cytoplasm and nucleoplasm of cells. Examples of MLOs forming via phase separation are nucleoli in the nucleus and stress granules in the cytoplasm. The main components of these MLOs are macromolecules such as RNAs and proteins. In order to assemble by phase separation, these proteins and RNAs have to undergo many cooperative interactions. These cooperative interactions are supported by specific molecular features within phase-separating proteins, such as multivalency and the presence of disordered domains that promote weak and transient interactions. However, these features also predispose phase-separating proteins to aberrant behavior. Indeed, evidence is emerging for a strong link between phase-separating proteins, MLOs, and age-related diseases. In this review, we discuss recent progress in understanding the formation, properties, and functions of MLOs. We pay special attention to the emerging link between MLOs and age-related diseases, and we explain how changes in the composition and physical properties of MLOs promote their conversion into an aberrant state. Furthermore, we discuss the key role of the protein quality control machinery in regulating the properties and functions of MLOs and thus in preventing age-related diseases.     

Identification and function of conformational dynamics in the multidomain GTPase dynamin

Vesicle release upon endocytosis requires membrane fission, catalyzed by the large GTPase dynamin. Dynamin contains five domains that together orchestrate its mechanochemical activity. Hydrogen–deuterium exchange coupled with mass spectrometry revealed global nucleotide‐ and membrane‐binding‐dependent conformational changes, as well as the existence of an allosteric relay element in the α2^S helix of the dynamin stalk domain. As predicted from structural studies, FRET analyses detect large movements of the pleckstrin homology domain (PHD) from a ‘closed’ conformation docked near the stalk to an ‘open’ conformation able to interact with membranes. We engineered dynamin constructs locked in either the closed or open state by chemical cross‐linking or deletion mutagenesis and showed that PHD movements function as a conformational switch to regulate dynamin self‐assembly, membrane binding, and fission. This PHD conformational switch is impaired by a centronuclear myopathy‐causing disease mutation, S619L, highlighting the physiological significance of its role in regulating dynamin function. Together, these data provide new insight into coordinated conformational changes that regulate dynamin function and couple membrane binding, oligomerization, and GTPase activity during dynamin‐catalyzed membrane fission.     

Autophagy activation in COL6 myopathic patients by a low-protein-diet pilot trial

A pilot clinical trial based on nutritional modulation was designed to assess the efficacy of a one-year low-protein diet in activating autophagy in skeletal muscle of patients affected by COL6/collagen VI-related myopathies. Ullrich congenital muscular dystrophy and Bethlem myopathy are rare inherited muscle disorders caused by mutations of COL6 genes and for which no cure is yet available. Studies in col6 null mice revealed that myofiber degeneration involves autophagy defects and that forced activation of autophagy results in the amelioration of muscle pathology. Seven adult patients affected by COL6 myopathies underwent a controlled low-protein diet for 12 mo and we evaluated the presence of autophagosomes and the mRNA and protein levels for BECN1/Beclin 1 and MAP1LC3B/LC3B in muscle biopsies and blood leukocytes. Safety measures were assessed, including muscle strength, motor and respiratory function, and metabolic parameters. After one y of low-protein diet, autophagic markers were increased in skeletal muscle and blood leukocytes of patients. The treatment was safe as shown by preservation of lean:fat percentage of body composition, muscle strength and function. Moreover, the decreased incidence of myofiber apoptosis indicated benefits in muscle homeostasis, and the metabolic changes pointed at improved mitochondrial function. These data provide evidence that a low-protein diet is able to activate autophagy and is safe and tolerable in patients with COL6 myopathies, pointing at autophagy activation as a potential target for therapeutic applications. In addition, our findings indicate that blood leukocytes are a promising noninvasive tool for monitoring autophagy activation in patients.     

A new mtDNA-tRNA(Glu) mutation (14728T>C) presenting a late-onset mitochondrial encephalomyopathy

We identified a new mutation in the mtDNA-encoded transfer RNA glutamate gene (tRNAGlu) in a patient presenting with late-onset myopathy. The mutation was nearly homoplasmic in muscle but hardly detectable in peripheral blood. Adding to the list of disease-related mtDNA variants, our findings propose to consider screening of tRNAGlu in cases of late-onset neuromuscular disorders.     

Primary desminopathies

Mutations of the human desmin gene on chromosome 2q35 cause a familial or sporadic form of skeletal myopathy frequently associated with cardiac abnormalities. Skeletal and cardiac muscle from patients with primary desminopathies characteristically display cytoplasmic accumulation of desmin-immunoreactive material and myofibrillar changes. However, desmin-positive protein aggregates in conjunction with myofibrillar abnormalities are also the morphological hallmark of the large group of secondary desminopathies (synonyms: myofibrillar myopathies, desmin-related myopathies), which comprise sporadic and familial neuromuscular conditions of considerable clinical and genetic heterogeneity. Here, we will give an overview on the functional role of desmin in striated muscle as well as the main clinical, myopathological, genetic and patho-physiological aspects of primary desminopathies. Furthermore, we will discuss recent genetic and biochemical advances in distinguishing primary from secondary desminopathies.