Small GTPases controlling autophagy-related membrane traffic in yeast and metazoans

Developmental program-independent secretory granule degradation in larval salivary gland cells of Drosophila

Both constitutive and regulated secretion require cell organelles that are able to store and release the secretory cargo. During development, the larval salivary gland of Drosophila initially produces high amount of glue-containing small immature secretory granules, which then fuse with each other and reach their normal 3-3.5 μm in size. Following the burst of secretion, obsolete glue granules directly fuse with late endosomes or lysosomes by a process called crinophagy, which leads to fast degradation and recycling of the secretory cargo. However, hindering of endosome-to-TGN retrograde transport in these cells causes abnormally small glue granules which are not able to fuse with each other. Here, we show that loss of function of the SNARE genes Syntaxin 16 (Syx16) and Synaptobrevin (Syb), the small GTPase Rab6 and the GARP tethering complex members Vps53 and Scattered (Vps54) all involved in retrograde transport cause intense early degradation of immature glue granules via crinophagy independently of the developmental program. Moreover, silencing of these genes also provokes secretory failure and accelerated crinophagy during larval development. Our results provide a better understanding of the relations among secretion, secretory granule maturation and degradation and paves the way for further investigation of these connections in other metazoans.

The entry of unclosed autophagosomes into vacuoles and its physiological relevance

It is widely stated in the literature that closed mature autophagosomes (APs) fuse with lysosomes/vacuoles during macroautophagy/autophagy. Previously, we showed that unclosed APs accumulated as clusters outside vacuoles in Vps21/Rab5 and ESCRT mutants after a short period of nitrogen starvation. However, the fate of such unclosed APs remains unclear. In this study, we used a combination of cellular and biochemical approaches to show that unclosed double-membrane APs entered vacuoles and formed unclosed single-membrane autophagic bodies after prolonged nitrogen starvation or rapamycin treatment. Vacuolar hydrolases, vacuolar transport chaperon (VTC) proteins, Ypt7, and Vam3 were all involved in the entry of unclosed double-membrane APs into vacuoles in Vps21-mutant cells. Overexpression of the vacuolar hydrolases, Pep4 or Prb1, or depletion of most VTC proteins promoted the entry of unclosed APs into vacuoles in Vps21-mutant cells, whereas depletion of Pep4 and/or Prb1 delayed the entry into vacuoles. In contrast to the complete infertility of diploid cells of typical autophagy mutants, diploid cells of Vps21 mutant progressed through meiosis to sporulation, benefiting from the entry of unclosed APs into vacuoles after prolonged nitrogen starvation. Overall, these data represent a new observation that unclosed double-membrane APs can enter vacuoles after prolonged autophagy induction, most likely as a survival strategy.

Molecular Mechanism and Regulation of Autophagy and Its Potential Role in Epilepsy

Autophagy is an evolutionally conserved degradation mechanism for maintaining cell homeostasis whereby cytoplasmic components are wrapped in autophagosomes and subsequently delivered to lysosomes for degradation. This process requires the concerted actions of multiple autophagy-related proteins and accessory regulators. In neurons, autophagy is dynamically regulated in different compartments including soma, axons, and dendrites. It determines the turnover of selected materials in a spatiotemporal control manner, which facilitates the formation of specialized neuronal functions. It is not surprising, therefore, that dysfunctional autophagy occurs in epilepsy, mainly caused by an imbalance between excitation and inhibition in the brain. In recent years, much attention has been focused on how autophagy may cause the development of epilepsy. In this article, we overview the historical landmarks and distinct types of autophagy, recent progress in the core machinery and regulation of autophagy, and biological roles of autophagy in homeostatic maintenance of neuronal structures and functions, with a particular focus on synaptic plasticity. We also discuss the relevance of autophagy mechanisms to the pathophysiology of epileptogenesis.

Autophagy reprogramming stem cell pluripotency and multiple-lineage differentiation

The cellular process responsible for the degradation of cytosolic proteins and subcellular organelles in lysosomes was termed "autophagy." This process occurs at a basal level in most tissues as part of tissue homeostasis that redounds to the regular turnover of components inside cytoplasm. The breakthrough in the autophagy field is the identification of key players in the autophagy pathway, compounded under the name "autophagy-related genes" (ATG) encoding for autophagy effector proteins. Generally, the function of autophagy can be classified into two divisions: intracellular clearance of defective macromolecules and organelles and generation of degradation products. Therapeutic strategies using stem cell-based approach come as a promising therapy and develop rapidly recently as stem cells have high self-renewability and differentiation capability as known as mesenchymal stem cells (MSCs). They are defined as adherent fibroblast-like population with the abilities to self-renew and multi-lineage differentiate into osteogenic, adipogenic, and chondrogenic lineage cells. To date, they are the most extensively applied adult stem cells in clinical trials. The properties of MSCs, such as immunomodulation, neuroprotection, and tissue repair pertaining to cell differentiation, processes to replace lost, or damaged cells, for aiding cell repair and revival. Autophagy has been viewed as a remarkable mechanism for maintaining homeostasis, ensuring the adequate function and survival of long-lived stem cells. In addition, authophagy also plays a remarkable role in protecting stem cells against cellular stress when the stem cell regenerative capacity is harmed in aging and cellular degeneration. Understanding the under-explored mechanisms of MSC actions and expanding the spectrum of their clinical applications may improve the utility of the MSC-based therapeutic approach in the future.

Role of Rab GTPases in Hepatocellular Carcinoma

The Rab GTPase family contains almost 70 genes in the human genome and acts as the key regulator of intracellular membrane trafficking in human cells. The dysregulation of Rab GTPase has been shown to be associated with multiple human diseases, ranging from neurodegeneration, and infection to cancer. Rab GTPases not only play important roles in genome replication, morphogenesis and the release of hepatitis B virus (HBV) or hepatitis C virus (HCV), but also contribute to hepatitis-related hepatocarcinogenesis and hepatocellular carcinoma (HCC) progression. The alteration of Rab GTPase expression in HCC plays an important role in tumour cell proliferation, invasion and migration. Notably, the expression of Rab genes can be regulated by some noncoding RNAs, such as miRNAs and circRNAs. Thus, Rab GTPases can serve as promising rational and therapeutic targets for HCC treatments. In this review, we summarized recent advancements in this field focusing on Rab GTPases in HCC.

Progranulin associates with Rab2 and is involved in autophagosome-lysosome fusion in Gaucher disease

Progranulin (PGRN) is a key regulator of lysosomes, and its deficiency has been linked to various lysosomal storage diseases (LSDs), including Gaucher disease (GD), one of the most common LSD. Here, we report that PGRN plays a previously unrecognized role in autophagy within the context of GD. PGRN deficiency is associated with the accumulation of LC3-II and p62 in autophagosomes of GD animal model and patient fibroblasts, resulting from the impaired fusion of autophagosomes and lysosomes. PGRN physically interacted with Rab2, a critical molecule in autophagosome-lysosome fusion. Additionally, a fragment of PGRN containing the Grn E domain was required and sufficient for binding to Rab2. Furthermore, this fragment significantly ameliorated PGRN deficiency-associated impairment of autophagosome-lysosome fusion and autophagic flux. These findings not only demonstrate that PGRN is a crucial mediator of autophagosome-lysosome fusion but also provide new evidence indicating PGRN's candidacy as a molecular target for modulating autophagy in GD and other LSDs in general. KEY MESSAGES : PGRN acts as a crucial factor involved in autophagosome-lysosome fusion in GD. PGRN physically interacts with Rab2, a molecule in autophagosome-lysosome fusion. A 15-kDa C-terminal fragment of PGRN is required and sufficient for binding to Rab2. This PGRN derivative ameliorates PGRN deficiency-associated impairment of autophagy. This study provides new insights into autophagy and may develop novel therapy for GD.

Pathogenic Single Nucleotide Polymorphisms on Autophagy-Related Genes

In recent years, the study of single nucleotide polymorphisms (SNPs) has gained increasing importance in biomedical research, as they can either be at the molecular origin of a determined disorder or directly affect the efficiency of a given treatment. In this regard, sequence variations in genes involved in pro-survival cellular pathways are commonly associated with pathologies, as the alteration of these routes compromises cellular homeostasis. This is the case of autophagy, an evolutionarily conserved pathway that counteracts extracellular and intracellular stressors by mediating the turnover of cytosolic components through lysosomal degradation. Accordingly, autophagy dysregulation has been extensively described in a wide range of human pathologies, including cancer, neurodegeneration, or inflammatory alterations. Thus, it is not surprising that pathogenic gene variants in genes encoding crucial effectors of the autophagosome/lysosome axis are increasingly being identified. In this review, we present a comprehensive list of clinically relevant SNPs in autophagy-related genes, highlighting the scope and relevance of autophagy alterations in human disease.

Signaling alterations caused by drugs and autophagy

Autophagy is an evolutionary conserved process that recycles cellular materials in times of nutrient restriction to maintain viability. In cancer therapeutics, the role of autophagy in response to multi-kinase inhibitors, alone or when combined with histone deacetylase (HDAC) inhibitors acts, generally, to facilitate the killing of tumor cells. Furthermore, the formation of autophagosomes and subsequent degradation of their contents can reduce the expression of HDAC proteins themselves as well as of other signaling regulatory molecules such as protein chaperones and mutated RAS proteins. Reduced levels of HDAC6 causes the acetylation and inactivation of heat shock protein 90, and, together with reduced expression of the chaperones HSP70 and GRP78, generates a strong endoplasmic reticulum (ER) stress response. Prolonged intense ER stress signaling causes tumor cell death. Reduced expression of HDACs 1, 2 and 3 causes the levels of programed death ligand 1 (PD-L1) to decline and the expression of Class I MHCA to increase which correlates with elevated immunogenicity of the tumor cells in vivo. This review will specifically focus on the downstream implications that result from autophagic-degradation of HDACs, RAS and protein chaperones.

RAB18 modulates autophagy in human stellate cells

BACKGROUND

Macroautophagy (or autophagy) is a conserved degradative pathway that breaks down sequestered cytoplasmic proteins and organelles in specialized double-membrane compartments called autophagosomes that fuse with lysosomes. Several proteins orchestrate this process, specifically Rab GTPases that are master regulators of molecular trafficking. RAB18 GTPase, a known mediator of stellate cell activation, is known to modulate autophagic flux in fibroblasts. However, its role in autophagy is unexplored in hepatic stellate cells.

OBJECTIVE

The aim of this study was to investigate the role of RAB18 in modulating autophagy in hepatic stellate cells.

METHODS

Role of RAB18 was determined by genetic depletion, pharmacologic inhibition, and overexpression studies to monitor autophagy flux and proteostasis in human LX2 stellate cell line.

RESULTS

RAB18 knockdown increases autophagy flux and regulates proteostasis. LX2 cells stimulated with transforming growth factor-beta robustly increases expression of profibrotic genes such as COL1A1 and ACTA2 along with RAB18 and its guanine nucleotide exchange factor, RAB3GAP1.

CONCLUSION

The study elucidates a role for RAB18 in autophagy and regulation of proteostasis in human stellate cells. Molecular insights into this process can provide therapeutic opportunities for intervention in liver fibrosis.

The where, what, and when of membrane protein degradation in neurons

Membrane protein turnover and degradation are required for the function and health of all cells. Neurons may live for the entire lifetime of an organism and are highly polarized cells with spatially segregated axonal and dendritic compartments. Both longevity and morphological complexity represent challenges for regulated membrane protein degradation. To investigate how neurons cope with these challenges, an increasing number of recent studies investigated local, cargo-specific protein sorting, and degradation at axon terminals and in dendritic processes. In this review, we explore the current answers to the ensuing questions of where, what, and when membrane proteins are degraded in neurons. © 2017 The Authors Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 78: 283-297, 2018.