Maintaining Golgi Homeostasis: A Balancing Act of Two Proteolytic Pathways

Unremodeled GPI-anchored proteins at the plasma membrane trigger aberrant endocytosis

The plasma membrane has a complex organization that includes the polarized distribution of membrane proteins and lipids. Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are ubiquitously expressed in eukaryotes and represent a functionally diverse, extensively remodeled, ER-derived group of proteins critical for the organization and function of the plasma membrane. Little is known about how the transport of incompletely remodeled GPI-APs to the plasma membrane affects cell function. Here, we investigated how failure to remodel mannose 2 (Man2) of the GPI moiety impacted endocytic activity on the plasma membrane. We find that Man2 unremodeled GPI-APs increased membrane disorder and generated a stress response that triggered abnormal ubiquitin- and clathrin-dependent endocytosis. The resulting stress-induced endocytosis disrupted the trafficking repertoire of a subset of plasma membrane proteins, which were redirected, via the multivesicular body, to numerous small vacuoles for degradation. Our findings highlight the critical importance of GPI-AP Man2 remodeling for maintaining the integrity and homeostasis of the plasma membrane.

Organelle-level toxicity of nanometals relevant to titanium implants. Original research and comprehensive literature overview

OBJECTIVE

This study analyzed organelle toxicities of nanometals applied as free formulations or titanium rod-coating materials in rats.

METHODS

All materials were injected intraperitoneally, including the physiological saline applied to the control group. The first experimental group was implanted with nanosilver-coated titanium rods, and the second, third, and fourth groups received free nanosilver at rising levels. The fifth group was implanted with nanosilver, nanocopper, and nanozinc-coated titanium rods, and the sixth group received the same nanometals as free formulations. Light and electron microscopy and ICP-Mass Spectrometry were utilized to determine the neural, hepatic, and renal toxicities and tissue metal levels.

RESULTS

In brains, neuropil, myelin, and cellular damages occurred, especially in groups receiving high-dose nanosilver or nanometal combinations. Histiocyte accumulation and dark mitochondria within hepatocytes were discernible in the liver. Kidneys were the organs that were most severely affected by nanometal toxicity. The nephrotoxicity was apparent with the perturbations of the membrane infoldings and mitochondrial damage in the proximal and distal convoluted epithelia. Large angular peroxisomes developed inside the mesangial cells, and Golgi bodies increased in epithelial cells. Systemic metal levels increased on the thirtieth and prominently dropped on the sixtieth day.

CONCLUSION

These results provide insights into the extent of injury and organelle targets of nanometals and will guide optimizing the nanomaterials and implants used in the surgical practice.

The Dsc ubiquitin ligase complex identifies transmembrane degrons to degrade orphaned proteins at the Golgi

The Golgi apparatus is essential for protein sorting, yet its quality control mechanisms are poorly understood. Here we show that the Dsc ubiquitin ligase complex uses its rhomboid pseudo-protease subunit, Dsc2, to assess the hydrophobic length of α-helical transmembrane domains (TMDs) at the Golgi. Thereby the Dsc complex likely interacts with orphaned ER and Golgi proteins that have shorter TMDs and ubiquitinates them for targeted degradation. Some Dsc substrates will be extracted by Cdc48 for endosome and Golgi associated proteasomal degradation (EGAD), while others will undergo ESCRT dependent vacuolar degradation. Some substrates are degraded by both, EGAD- or ESCRT pathways. The accumulation of Dsc substrates entails a specific increase in glycerophospholipids with shorter and asymmetric fatty acyl chains. Hence, the Dsc complex mediates the selective degradation of orphaned proteins at the sorting center of cells, which prevents their spreading across other organelles and thereby preserves cellular membrane protein and lipid composition.

Selective delivery of imaging probes and therapeutics to the endoplasmic reticulum or Golgi apparatus: Current strategies and beyond

To maximize therapeutic effects and minimize unwanted effects, the interest in drug targeting to the endoplasmic reticulum (ER) or Golgi apparatus (GA) has been recently growing because two organelles are distributing hubs of cellular building/signaling components (e.g., proteins, lipids, Ca2+) to other organelles and the plasma membrane. Their structural or functional damages induce organelle stress (i.e., ER or GA stress), and their aggravation is strongly related to diseases (e.g., cancers, liver diseases, brain diseases). Many efforts have been developed to image (patho)physiological functions (e.g., oxidative stress, protein/lipid-related processing) and characteristics (e.g., pH, temperature, biothiols, reactive oxygen species) in the target organelles and to deliver drugs for organelle disruption using organelle-targeting moieties. Therefore, this review will overview the structure, (patho)physiological functions/characteristics, and related diseases of the organelles of interest. Future direction on ER or GA targeting will be discussed by understanding current strategies and investigations on targeting, imaging/sensing, and therapeutic systems.

Arf6 GTPase deficiency leads to porcine oocyte quality decline during aging

Arf6 is a member of ADP-ribosylation factor (Arf) family, which is widely implicated in the regulation of multiple physiological processes including endocytic recycling, cytoskeletal organization, and membrane trafficking during mitosis. In this study, we investigated the potential relationship between Arf6 and aging-related oocyte quality, and its roles on organelle rearrangement and cytoskeleton dynamics in porcine oocytes. Arf6 expressed in porcine oocytes throughout meiotic maturation, and it decreased in aged oocytes. Disruption of Arf6 led to the failure of cumulus expansion and polar body extrusion. Further analysis indicated that Arf6 modulated ac-tubulin for meiotic spindle organization and microtubule stability. Besides, Arf6 regulated cofilin phosphorylation and fascin for actin assembly, which further affected spindle migration, indicating the roles of Arf6 on cytoskeleton dynamics. Moreover, the lack of Arf6 activity caused the dysfunction of Golgi and ER for protein synthesis and signal transduction. Mitochondrial dysfunction was also observed in Arf6-deficient porcine oocytes, which was supported by the increased ROS level and abnormal membrane potential. In conclusion, our results reported that insufficient Arf6 was related to aging-induced oocyte quality decline through spindle organization, actin assembly, and organelle rearrangement in porcine oocytes.

Loss of GABARAP mediates resistance to immunogenic chemotherapy in multiple myeloma

ABSTRACT

Immunogenic cell death (ICD) is a form of cell death by which cancer treatments can induce a clinically relevant antitumor immune response in a broad range of cancers. In multiple myeloma (MM), the proteasome inhibitor bortezomib is an ICD inducer and creates durable therapeutic responses in patients. However, eventual relapse and resistance to bortezomib appear inevitable. Here, by integrating patient transcriptomic data with an analysis of calreticulin (CRT) protein interactors, we found that GABA type A receptor-associated protein (GABARAP) is a key player whose loss prevented tumor cell death from being perceived as immunogenic after bortezomib treatment. GABARAP is located on chromosome 17p, which is commonly deleted in patients with high risk MM. GABARAP deletion impaired the exposure of the eat-me signal CRT on the surface of dying MM cells in vitro and in vivo, thus reducing tumor cell phagocytosis by dendritic cells and the subsequent antitumor T-cell response. Low GABARAP was independently associated with shorter survival in patients with MM and reduced tumor immune infiltration. Mechanistically, we found that GABARAP deletion blocked ICD signaling by decreasing autophagy and altering Golgi apparatus morphology, with consequent defects in the downstream vesicular transport of CRT. Conversely, upregulating autophagy using rapamycin restored Golgi morphology, CRT exposure, and ICD signaling in GABARAPKO cells undergoing bortezomib treatment. Therefore, coupling an ICD inducer, such as bortezomib, with an autophagy inducer, such as rapamycin, may improve patient outcomes in MM, in which low GABARAP in the form of del(17p) is common and leads to worse outcomes.

The helicase-like transcription factor redirects the autophagic flux and restricts human T cell leukemia virus type 1 infection

Retroviruses and their host have coevolved in a delicate balance between viral replication and survival of the infected cell. In this equilibrium, restriction factors expressed by infected cells control different steps of retroviral replication such as entry, uncoating, nuclear import, expression, or budding. Here, we describe a mechanism of restriction against human T cell leukemia virus type 1 (HTLV-1) by the helicase-like transcription factor (HLTF). We show that RNA and protein levels of HLTF are reduced in primary T cells of HTLV-1-infected subjects, suggesting a clinical relevance. We further demonstrate that the viral oncogene Tax represses HLTF transcription via the Enhancer of zeste homolog 2 methyltransferase of the Polycomb repressive complex 2. The Tax protein also directly interacts with HLTF and induces its proteasomal degradation. RNA interference and gene transduction in HTLV-1-infected T cells derived from patients indicate that HLTF is a restriction factor. Restoring the normal levels of HLTF expression induces the dispersal of the Golgi apparatus and overproduction of secretory granules. By synergizing with Tax-mediated NF-κB activation, physiologically relevant levels of HLTF intensify the autophagic flux. Increased vesicular trafficking leads to an enlargement of the lysosomes and the production of large vacuoles containing viral particles. HLTF induction in HTLV-1-infected cells significantly increases the percentage of defective virions. In conclusion, HLTF-mediated activation of the autophagic flux blunts the infectious replication cycle of HTLV-1, revealing an original mode of viral restriction.

Ubiquitin-mediated degradation at the Golgi apparatus

The Golgi apparatus is an essential organelle of the secretory pathway in eukaryotic cells. It processes secretory and transmembrane proteins and orchestrates their transport to other endomembrane compartments or the plasma membrane. The Golgi apparatus thereby shapes the cell surface, controlling cell polarity, cell-cell communication, and immune signaling. The cytosolic face of the Golgi hosts and regulates signaling cascades, impacting most notably the DNA damage response and mitosis. These essential functions strongly depend on Golgi protein homeostasis and Golgi integrity. Golgi fragmentation and consequent malfunction is associated with neurodegenerative diseases and certain cancer types. Recent studies provide first insight into the critical role of ubiquitin signaling in maintaining Golgi integrity and in Golgi protein quality control. Similar to well described pathways at the endoplasmic reticulum, ubiquitin-dependent degradation of non-native proteins prevents the accumulation of toxic protein aggregates at the Golgi. Moreover, ubiquitination regulates Golgi structural rearrangements in response to cellular stress. Advances in elucidating ubiquitination and degradation events at the Golgi are starting to paint a picture of the molecular machinery underlying Golgi (protein) homeostasis.

Membrane trafficking in breast cancer progression: protein kinase D comes into play

Protein kinase D (PKD) is a serine/threonine kinase family that controls important cellular functions, most notably playing a key role in the secretory pathway at the trans-Golgi network. Aberrant expression of PKD isoforms has been found mainly in breast cancer, where it promotes various cellular processes such as growth, invasion, survival and stem cell maintenance. In this review, we discuss the isoform-specific functions of PKD in breast cancer progression, with a particular focus on how the PKD controlled cellular processes might be linked to deregulated membrane trafficking and secretion. We further highlight the challenges of a therapeutic approach targeting PKD to prevent breast cancer progression.

Volleying plasma membrane proteins from birth to death: Role of J-domain proteins

The function, stability, and turnover of plasma membrane (PM) proteins are crucial for cellular homeostasis. Compared to soluble proteins, quality control of plasma membrane proteins is extremely challenging. Failure to meet the high quality control standards is detrimental to cellular and organismal health. J-domain proteins (JDPs) are among the most diverse group of chaperones that collaborate with other chaperones and protein degradation machinery to oversee cellular protein quality control (PQC). Although fragmented, the available literature from different models, including yeast, mammals, and plants, suggests that JDPs assist PM proteins with their synthesis, folding, and trafficking to their destination as well as their degradation, either through endocytic or proteasomal degradation pathways. Moreover, some JDPs interact directly with the membrane to regulate the stability and/or functionality of proteins at the PM. The deconvoluted picture emerging is that PM proteins are relayed from one JDP to another throughout their life cycle, further underscoring the versatility of the Hsp70:JDP machinery in the cell.

Autophagy in glaucoma pathogenesis: Therapeutic potential and future perspectives

Glaucoma is a common blinding eye disease characterized by progressive loss of retinal ganglion cells (RGCs) and their axons, progressive loss of visual field, and optic nerve atrophy. Autophagy plays a pivotal role in the pathophysiology of glaucoma and is closely related to its pathogenesis. Targeting autophagy and blocking the apoptosis of RGCs provides emerging guidance for the treatment of glaucoma. Here, we provide a systematic review of the mechanisms and targets of interventions related to autophagy in glaucoma and discuss the outlook of emerging ideas, techniques, and multidisciplinary combinations to provide a new basis for further research and the prevention of glaucomatous visual impairment.