Unfolded protein response regulates yeast small GTPase Arl1p activation at late Golgi via phosphorylation of Arf GEF Syt1p

The SNARE-associated protein Sft2 functions in Imh1-mediated SNARE recycling transport upon ER stress

Vesicular trafficking involving SNARE proteins play a crucial role in the delivery of cargo to the target membrane. Arf-like protein 1 (Arl1) is an important regulator of the endosomal trans-Golgi network (TGN) and secretory trafficking. In yeast, ER stress-enhances Arl1 activation and Golgin Imh1 recruitment to the late-Golgi. Although Arl1 and Imh1 are critical for GARP-mediated endosomal SNARE-recycling transport in response to ER stress, their downstream effectors are unknown. Here, we report that the SNARE-associated protein Sft2 acts downstream of the Arl1-Imh1 axis to regulate SNARE recycling upon ER stress. We first demonstrated that Sft2 is required for Tlg1/Snc1 SNARE-recycling transport under tunicamycin-induced ER stress. Interestingly, we found that Imh1 regulates Tlg2 retrograde transport to the late-Golgi under ER stress, which in turn is required for Sft2 targeting to the late-Golgi. We further showed that Sft2 with 40 amino acids deleted from the N-terminus exhibits defective mediation of SNARE recycling and decreased association with Tlg1 under ER stress. Finally, we demonstrated that Sft2 is required for GARP-dependent endosome-to-Golgi transport in the absence of Rab protein Ypt6. This study highlights Sft2 as a critical downstream effector of the Arl1-Imh1 axis, mediating the endosome-to-Golgi transport of SNAREs.

Involvement of Sec71 and Ubp2 in tunicamycin-induced ER stress response in the fission yeast

BACKGROUND

Accumulation of unfolded or misfolded proteins in the cellular environment result in ER stress and activates the unfolded protein response (UPR). The UPR alleviates ER stress and restores homeostasis, but it triggers cell death under prolonged stress. Here, we aimed to investigate the involvement of Sec71, an Arf-GEF involved in vesicular transport, in the tunicamycin-induced ER stress response. Since deubiquitinases and ER stress are known to be closely linked, we investigated this response by evaluating the potential role of Ubp2, a deubiquitinase, in the ER stress response in fission yeast.

METHODS AND RESULTS

Tunicamycin-induced ER stress responses were assessed by analyzing cell viability, apoptosis, intracellular oxidation levels, and proteasomal activities in sec71 and ubp2-deficient cells. The cell viability of Δsec71 and Δubp2 decreased after exposure to 0.5 µg/mL tunicamycin. Deleting either ubp2 or sec71 genes significantly decreased proteasomal activity and sensitized cells to ER stress, resulting in increased apoptosis compared with wild-type cells after tunicamycin treatment. DCFDA (2,7-dichlorodihydrofluorescein diacetate) reduction increased in correlation with apoptosis observed in the mutant cells, indicating higher levels of reactive oxygen species.

CONCLUSIONS

The results highlight the involvement of S. pombe Ubp2 in the known role of the ubiquitin-proteasome system in the ER stress response. We hypothesise that Sec71 is associated with ER homeostasis, and our findings on Sec71 provide new insight into the regulation of cell death mechanisms arising from the ER stress.

Golgin Imh1 and GARP complex cooperate to restore the impaired SNARE recycling transport induced by ER stress

The accumulation of misfolded proteins in the endoplasmic reticulum (ER) induces the unfolded protein response (UPR), which acts through various mechanisms to reduce ER stress. While the UPR has been well studied for its effects on the ER, its impact on the Golgi is less understood. The Golgi complex receives transport vesicles from the endosome through two types of tethering factors: long coiled-coil golgin and the multisubunit Golgi-associated retrograde protein (GARP) complex. Here, we report that ER stress increases the phosphorylation of golgin Imh1 to maintain the GARP-mediated recycling of the SNAREs Snc1 and Tlg1. We also identify a specific function of the Golgi affected by ER stress and elucidate a homeostatic response to restore this function, which involves both an Ire1-dependent and a MAP kinase Slt2/ERK2-dependent mechanism. Furthermore, our findings advance a general understanding of how two different types of tethers act cooperatively to mediate a transport pathway.

Tunicamycin Protects against LPS-Induced Lung Injury

The pulmonary endothelium is a dynamic semipermeable barrier that orchestrates tissue-fluid homeostasis; regulating physiological and immunological responses. Endothelial abnormalities are caused by inflammatory stimuli interacting with intracellular messengers to remodel cytoskeletal junctions and adhesion proteins. Those phenomena are associated with sepsis, acute lung injury, and acute respiratory distress syndrome. The molecular processes beyond those responses are the main interest of our group. Unfolded protein response (UPR) is a highly conserved molecular pathway resolving protein-folding defects to counteract cellular threats. An emerging body of evidence suggests that UPR is a promising target against lung and cardiovascular disease. In the present study, we reveal that Tunicamycin (TM) (UPR inducer) protects against lipopolysaccharide (LPS)-induced injury. The barrier function of the inflamed endothelium was evaluated in vitro (transendothelial and paracellular permeability); as well as in mice exposed to TM after LPS. Our study demonstrates that TM supports vascular barrier function by modulating actomyosin remodeling. Moreover, it reduces the internalization of vascular endothelial cadherin (VE-cadherin), enhancing endothelial integrity. We suggest that UPR activation may deliver novel therapeutic opportunities in diseases related to endothelial dysregulation.

A Quantitative Analysis of Cellular Lipid Compositions During Acute Proteotoxic ER Stress Reveals Specificity in the Production of Asymmetric Lipids

The unfolded protein response (UPR) is central to endoplasmic reticulum (ER) homeostasis by controlling its size and protein folding capacity. When activated by unfolded proteins in the ER-lumen or aberrant lipid compositions, the UPR adjusts the expression of hundreds of target genes to counteract ER stress. The proteotoxic drugs dithiothreitol (DTT) and tunicamycin (TM) are commonly used to induce misfolding of proteins in the ER and to study the UPR. However, their potential impact on the cellular lipid composition has never been systematically addressed. Here, we report the quantitative, cellular lipid composition of Saccharomyces cerevisiae during acute, proteotoxic stress in both rich and synthetic media. We show that DTT causes rapid remodeling of the lipidome when used in rich medium at growth-inhibitory concentrations, while TM has only a marginal impact on the lipidome under our conditions of cultivation. We formulate recommendations on how to study UPR activation by proteotoxic stress without interferences from a perturbed lipid metabolism. Furthermore, our data suggest an intricate connection between the cellular growth rate, the abundance of the ER, and the metabolism of fatty acids. We show that Saccharomyces cerevisiae can produce asymmetric lipids with two saturated fatty acyl chains differing substantially in length. These observations indicate that the pairing of saturated fatty acyl chains is tightly controlled and suggest an evolutionary conservation of asymmetric lipids and their biosynthetic machineries.

System-Wide Characterization of MoArf GTPase Family Proteins and Adaptor Protein MoGga1 Involved in the Development and Pathogenicity of Magnaporthe oryzae

ADP ribosylation factor (Arf) small GTPase family members are involved in vesicle trafficking and organelle maintenance in organisms ranging from Saccharomyces cerevisiae to humans. A previous study identified Magnaporthe oryzae Arf6 (MoArf6) as one of the Arf proteins that regulates growth and conidiation in the rice blast fungus M. oryzae, but the remaining family proteins remain unknown. Here, we identified six additional Arf proteins, including MoArf1, MoArl1, MoArl3, MoArl8, MoCin4, and MoSar1, as well as their sole adaptor protein, MoGga1, and determined their shared and specific functions. We showed that the majority of these proteins exhibit positive regulatory functions, most notably, in growth. Importantly, MoArl1, MoCin4, and MoGga1 are involved in pathogenicity through the regulation of host penetration and invasive hyphal growth. MoArl1 and MoCin4 also regulate normal vesicle trafficking, and MoCin4 further controls the formation of the biotrophic interfacial complex (BIC). Moreover, we showed that Golgi-cytoplasm cycling of MoArl1 is required for its function. Finally, we demonstrated that interactions between MoArf1 and MoArl1 with MoGga1 are important for Golgi localization and pathogenicity. Collectively, our findings revealed the shared and specific functions of Arf family members in M. oryzae and shed light on how these proteins function through conserved mechanisms to govern growth, transport, and virulence of the blast fungus.IMPORTANCEMagnaporthe oryzae is the causal agent of rice blast, representing the most devastating diseases of rice worldwide, which results in losses of amounts of rice that could feed more than 60 million people each year. Arf (ADP ribosylation factor) small GTPase family proteins are involved in vesicle trafficking and organelle maintenance in eukaryotic cells. To investigate the function of Arf family proteins in M. oryzae, we systematically characterized all seven Arf proteins and found that they have shared and specific functions in governing the growth, development, and pathogenicity of the blast fungus. We have also identified the pathogenicity-related protein MoGga1 as the common adaptor of MoArf1 and MoArl1. Our findings are important because they provide the first comprehensive characterization of the Arf GTPase family proteins and their adaptor protein MoGga1 functioning in a plant-pathogenic fungus, which could help to reveal new fungicide targets to control this devastating disease.

Type 2 deiodinase polymorphism causes ER stress and hypothyroidism in the brain

Levothyroxine (LT4) is a form of thyroid hormone used to treat hypothyroidism. In the brain, T4 is converted to the active form T3 by type 2 deiodinase (D2). Thus, it is intriguing that carriers of the Thr92Ala polymorphism in the D2 gene (DIO2) exhibit clinical improvement when liothyronine (LT3) is added to LT4 therapy. Here, we report that D2 is a cargo protein in ER Golgi intermediary compartment (ERGIC) vesicles, recycling between ER and Golgi. The Thr92-to-Ala substitution (Ala92-D2) caused ER stress and activated the unfolded protein response (UPR). Ala92-D2 accumulated in the trans-Golgi and generated less T3, which was restored by eliminating ER stress with the chemical chaperone 4-phenyl butyric acid (4-PBA). An Ala92-Dio2 polymorphism-carrying mouse exhibited UPR and hypothyroidism in distinct brain areas. The mouse refrained from physical activity, slept more, and required additional time to memorize objects. Enhancing T3 signaling in the brain with LT3 improved cognition, whereas restoring proteostasis with 4-PBA eliminated the Ala92-Dio2 phenotype. In contrast, primary hypothyroidism intensified the Ala92-Dio2 phenotype, with only partial response to LT4 therapy. Disruption of cellular proteostasis and reduced Ala92-D2 activity may explain the failure of LT4 therapy in carriers of Thr92Ala-DIO2.

Multiple activities of Arl1 GTPase in the trans-Golgi network

ADP-ribosylation factors (Arfs) and ADP-ribosylation factor-like proteins (Arls) are highly conserved small GTPases that function as main regulators of vesicular trafficking and cytoskeletal reorganization. Arl1, the first identified member of the large Arl family, is an important regulator of Golgi complex structure and function in organisms ranging from yeast to mammals. Together with its effectors, Arl1 has been shown to be involved in several cellular processes, including endosomal trans-Golgi network and secretory trafficking, lipid droplet and salivary granule formation, innate immunity and neuronal development, stress tolerance, as well as the response of the unfolded protein. In this Commentary, we provide a comprehensive summary of the Arl1-dependent cellular functions and a detailed characterization of several Arl1 effectors. We propose that involvement of Arl1 in these diverse cellular functions reflects the fact that Arl1 is activated at several late-Golgi sites, corresponding to specific molecular complexes that respond to and integrate multiple signals. We also provide insight into how the GTP-GDP cycle of Arl1 is regulated, and highlight a newly discovered mechanism that controls the sophisticated regulation of Arl1 activity at the Golgi complex.

Role of Arf GTPases in fungal morphogenesis and virulence

Virulence of the human fungal pathogen Candida albicans depends on the switch from budding to filamentous growth, which requires sustained membrane traffic and polarized growth. In many organisms, small GTPases of the Arf (ADP-ribosylation factor) family regulate membrane/protein trafficking, yet little is known about their role in fungal filamentous growth. To investigate these GTPases in C. albicans, we generated loss of function mutants in all 3 Arf proteins, Arf1-Arf3, and 2 Arf-like proteins, Arl1 and Arl3. Our results indicate that of these proteins, Arf2 is required for viability and sensitivity to antifungal drugs. Repressible ARF2 expression results in defects in filamentous growth, cell wall integrity and virulence, likely due to alteration of the Golgi. Arl1 is also required for invasive filamentous growth and, although arl1/arl1 cells can initiate hyphal growth, hyphae are substantially shorter than that of the wild-type, due to the inability of this mutant to maintain hyphal growth at a single site. We show that this defect does not result from an alteration of phospholipid distribution and is unlikely to result from the sole Golgin Imh1 mislocalization, as Imh1 is not required for invasive filamentous growth. Rather, our results suggest that the arl1/arl1 hyphal growth defect results from increased secretion in this mutant. Strikingly, the arl1/arl1 mutant is drastically reduced in virulence during oropharyngeal candidiasis. Together, our results highlight the importance of Arl1 and Arf2 as key regulators of hyphal growth and virulence in C. albicans and identify a unique function of Arl1 in secretion.

Small GTPase proteins in macroautophagy

Macroautophagy, a highly conserved process in eukaryotic cells, is initiated in response to stress, especially nutrient starvation. Macroautophagy helps cells survive by engulfing proteins and organelles into an unusual double-membraned structure called the autophagosome, which then fuses with the lysosome. Upon degradation of the engulfed contents, the building blocks are recycled for synthesis of new macromolecules. Recent work has demonstrated that construction of the autophagosome requires a variety of small GTPases in variations of their normal roles in membrane traffic. In this Commentary, we review our own recent findings with respect to 2 different GTPases, Arl1, a member of the Arf/Arl/Sar family, and Ypt6, a member of the Rab family, in the yeast S. cerevisiae in light of other information from the literature and discuss future directions for further discerning the roles of small GTPases in autophagy.