A novel function for Rab1 and Rab11 during secretory granule maturation

Vitamin E Attenuates Red-Light-Mediated Vasodilation: The Benefits of a Mild Oxidative Stress

Red light (670 nm) energy controls vasodilation via the formation of a transferable endothelium-derived nitric oxide (NO)-precursor-containing substance, its intracellular traffic, and exocytosis. Here we investigated the underlying mechanistic effect of oxidative stress on light-mediated vasodilation by using pressure myography on dissected murine arteries and immunofluorescence on endothelial cells. Treatment with antioxidants Trolox and catalase decreased vessel dilation. In the presence of catalase, a lower number of exosomes were detected in the vessel bath. Light exposure resulted in increased cellular free radical levels. Mitochondrial reactive oxygen species were also more abundant but did not alter cellular ATP production. Red light enhanced the co-localization of late exosome marker CD63 and cellular S-nitrosoprotein to a greater extent than high glucose, suggesting that a mild oxidative stress favors the localization of NO precursor in late exosomes. Exocytosis regulating protein Rab11 was more abundant after irradiation. Our findings conclude that red-light-induced gentle oxidative stress facilitates the dilation of blood vessels, most likely through empowering the traffic of vasodilatory substances. Application of antioxidants disfavors this mechanism.

The Ykt6-Snap29-Syx13 SNARE complex promotes crinophagy via secretory granule fusion with Lamp1 carrier vesicles

In the Drosophila larval salivary gland, developmentally programmed fusions between lysosomes and secretory granules (SGs) and their subsequent acidification promote the maturation of SGs that are secreted shortly before puparium formation. Subsequently, ongoing fusions between non-secreted SGs and lysosomes give rise to degradative crinosomes, where the superfluous secretory material is degraded. Lysosomal fusions control both the quality and quantity of SGs, however, its molecular mechanism is incompletely characterized. Here we identify the R-SNARE Ykt6 as a novel regulator of crinosome formation, but not the acidification of maturing SGs. We show that Ykt6 localizes to Lamp1+ carrier vesicles, and forms a SNARE complex with Syntaxin 13 and Snap29 to mediate fusion with SGs. These Lamp1 carriers represent a distinct vesicle population that are functionally different from canonical Arl8+, Cathepsin L+ lysosomes, which also fuse with maturing SGs but are controlled by another SNARE complex composed of Syntaxin 13, Snap29 and Vamp7. Ykt6- and Vamp7-mediated vesicle fusions also determine the fate of SGs, as loss of either of these SNAREs prevents crinosomes from acquiring endosomal PI3P. Our results highlight that fusion events between SGs and different lysosome-related vesicle populations are critical for fine regulation of the maturation and crinophagic degradation of SGs.

Advancements in the study of synaptic plasticity and mitochondrial autophagy relationship

Synapses serve as the points of communication between neurons, consisting primarily of three components: the presynaptic membrane, synaptic cleft, and postsynaptic membrane. They transmit signals through the release and reception of neurotransmitters. Synaptic plasticity, the ability of synapses to undergo structural and functional changes, is influenced by proteins such as growth-associated proteins, synaptic vesicle proteins, postsynaptic density proteins, and neurotrophic growth factors. Furthermore, maintaining synaptic plasticity consumes more than half of the brain's energy, with a significant portion of this energy originating from ATP generated through mitochondrial energy metabolism. Consequently, the quantity, distribution, transport, and function of mitochondria impact the stability of brain energy metabolism, thereby participating in the regulation of fundamental processes in synaptic plasticity, including neuronal differentiation, neurite outgrowth, synapse formation, and neurotransmitter release. This article provides a comprehensive overview of the proteins associated with presynaptic plasticity, postsynaptic plasticity, and common factors between the two, as well as the relationship between mitochondrial energy metabolism and synaptic plasticity.

A Rab6 to Rab11 transition is required for dense-core granule and exosome biogenesis in Drosophila secondary cells

Secretory cells in glands and the nervous system frequently package and store proteins destined for regulated secretion in dense-core granules (DCGs), which disperse when released from the cell surface. Despite the relevance of this dynamic process to diseases such as diabetes and human neurodegenerative disorders, our mechanistic understanding is relatively limited, because of the lack of good cell models to follow the nanoscale events involved. Here, we employ the prostate-like secondary cells (SCs) of the Drosophila male accessory gland to dissect the cell biology and genetics of DCG biogenesis. These cells contain unusually enlarged DCGs, which are assembled in compartments that also form secreted nanovesicles called exosomes. We demonstrate that known conserved regulators of DCG biogenesis, including the small G-protein Arf1 and the coatomer complex AP-1, play key roles in making SC DCGs. Using real-time imaging, we find that the aggregation events driving DCG biogenesis are accompanied by a change in the membrane-associated small Rab GTPases which are major regulators of membrane and protein trafficking in the secretory and endosomal systems. Indeed, a transition from trans-Golgi Rab6 to recycling endosomal protein Rab11, which requires conserved DCG regulators like AP-1, is essential for DCG and exosome biogenesis. Our data allow us to develop a model for DCG biogenesis that brings together several previously disparate observations concerning this process and highlights the importance of communication between the secretory and endosomal systems in controlling regulated secretion.

Interaction of the sorting nexin 25 homologue Snazarus with Rab11 balances endocytic and secretory transport and maintains the ultrafiltration diaphragm in nephrocytes

Proper balance of exocytosis and endocytosis is important for the maintenance of plasma membrane lipid and protein homeostasis. This is especially critical in human podocytes and the podocyte-like Drosophila nephrocytes that both use a delicate diaphragm system with evolutionarily conserved components for ultrafiltration. Here, we show that the sorting nexin 25 homologue Snazarus (Snz) binds to Rab11 and localizes to Rab11-positive recycling endosomes in Drosophila nephrocytes, unlike in fat cells where it is present in plasma membrane/lipid droplet/endoplasmic reticulum contact sites. Loss of Snz leads to redistribution of Rab11 vesicles from the cell periphery and increases endocytic activity in nephrocytes. These changes are accompanied by defects in diaphragm protein distribution that resemble those seen in Rab11 gain-of-function cells. Of note, co-overexpression of Snz rescues diaphragm defects in Rab11 overexpressing cells, whereas snz knockdown in Rab11 overexpressing nephrocytes or simultaneous knockdown of snz and tbc1d8b encoding a Rab11 GTPase-activating protein (GAP) leads to massive expansion of the lacunar system that contains mislocalized diaphragm components: Sns and Pyd/ZO-1. We find that loss of Snz enhances while its overexpression impairs secretion, which, together with genetic epistasis analyses, suggest that Snz counteracts Rab11 to maintain the diaphragm via setting the proper balance of exocytosis and endocytosis.

The secretory ability of newly formed secretory granules is regulated by pro-cathepsin B and amylase in parotid glands

Parotid glands are exocrine glands that release saliva into the oral cavity. Acinar cells of parotid glands produce many secretory granules (SGs) that contain the digestion enzyme amylase. After the generation of SGs in the Golgi apparatus, they mature by enlarging and membrane remodeling. VAMP2, which is involved in exocytosis, accumulates in the membrane of mature SGs. The remodeling of SG membranes is regarded as a preparation process for exocytosis but its detailed mechanism remains unknown. To address that subject, we investigated the secretory ability of newly formed SGs. Although amylase is a useful indicator of secretion, the cell leakage of amylase might affect the measurement of secretion. Thus, in this study, we focused on cathepsin B (CTSB), a lysosomal protease, as an indicator of secretion. It has been reported that some procathepsin B (pro-CTSB), which is a precursor of CTSB, is initially sorted to SGs after which it is transported to lysosomes by clathrin-coated vesicles. Because pro-CTSB is processed to mature CTSB after its arrival in lysosomes, we can distinguish between the secretion of SGs and cell leakage by measuring the secretion of pro-CTSB and mature CTSB, respectively. When acinar cells isolated from parotid glands were stimulated with isoproterenol (Iso), a β-adrenergic agonist, the secretion of pro-CTSB was increased. In contrast, mature CTSB was not detected in the medium although it was abundant in the cell lysates. To prepare parotid glands rich in newly formed SGs, the depletion of per-existing SGs was induced by an intraperitoneal injection of Iso into rats. At 5 h after that injection, newly formed SGs were observed in parotid acinar cells and the secretion of pro-CTSB was also detected. We confirmed that the purified newly formed SGs contained pro-CTSB, but not mature CTSB. At 2 h after Iso injection, few SGs were observed in the parotid glands and the secretion of pro-CTSB was not detected, which proved that the Iso injection depleted pre-existing SGs and the SGs observed at 5 h were newly formed after the Iso injection. These results suggest that newly formed SGs have a secretory ability prior to membrane remodeling.

Extracellular HSPA5 is autocrinally involved in the regulation of neuronal process elongation

The molecular mechanisms by which neuronal processes grow are extremely complicated, involving fine-tuned regulation of extracellular and intracellular signals. It remains to be elucidated which molecules are contained in the regulation. Herein, we report for the first time that heat shock protein family A member 5 (HSPA5, also called immunoglobulin heavy chain binding endoplasmic reticulum [ER] protein [BiP]) is secreted from mouse primary dorsal neuronal ganglion (DRG) cells or neuronal cell line N1E-115, a frequently used neuronal differentiation model. Supporting these results, HSPA5 protein was co-localized not only with ER antigen KDEL but also with intracellular vesicles such as Rab11-positive secretory vesicles. Unexpectedly, addition of HSPA5 inhibited elongation of neuronal processes, whereas neutralization of extracellular HSPA5 with the antibodies elongated processes, characterizing extracellular HSPA5 as a negative regulator of neuronal differentiation. Treatment of cells with neutralizing antibodies for low-density lipoprotein receptor (LDLR) did not have significant effects on process elongation, whereas LDLR-related protein 1 (LRP1) antibodies promoted differentiation, implying that LRP1 may act as a receptor candidate for HSPA5. Interestingly, extracellular HSPA5 was greatly decreased following treatment with tunicamycin, an ER stress inducer, illustrating that the ability to form neuronal processes could be preserved, even under stress. These results suggest that neuronal HSPA5 itself is secreted to contribute to inhibitory effects on neuronal cell morphological differentiation and can be included on the list of extracellular signaling molecules negatively controlling differentiation.

Rab26 controls secretory granule maturation and breakdown in Drosophila

At the onset of Drosophila metamorphosis, plenty of secretory glue granules are released from salivary gland cells and the glue is deposited on the ventral side of the forming (pre)pupa to attach it to a dry surface. Prior to this, a poorly understood maturation process takes place during which secretory granules gradually grow via homotypic fusions, and their contents are reorganized. Here we show that the small GTPase Rab26 localizes to immature (smaller, non-acidic) glue granules and its presence prevents vesicle acidification. Rab26 mutation accelerates the maturation, acidification and release of these secretory vesicles as well as the lysosomal breakdown (crinophagy) of residual, non-released glue granules. Strikingly, loss of Mon1, an activator of the late endosomal and lysosomal fusion factor Rab7, results in Rab26 remaining associated even with the large glue granules and a concomitant defect in glue release, similar to the effects of Rab26 overexpression. Our data thus identify Rab26 as a key regulator of secretory vesicle maturation that promotes early steps (vesicle growth) and inhibits later steps (lysosomal transport, acidification, content reorganization, release, and breakdown), which is counteracted by Mon1.

Immunofluorescence as a Method to Study Golgi Organization in Larval Salivary Glands of Drosophila melanogaster

Immunofluorescence is an important research tool in cell biology that reveals structural organization of subcellular organelles by detecting their associated constituents. Here, we describe an antibody staining method to detect Golgi-associated proteins in Drosophila larval salivary glands, using the cis-Golgi protein Lava lamp and the clathrin adaptor AP-1 as a suitable example. Golgi bodies immunostained using this protocol can be visualized using confocal or structured illumination microscopy.

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.

Convergence of secretory, endosomal, and autophagic routes in trans-Golgi-associated lysosomes

At the trans-Golgi, complex traffic connections exist to the endolysosomal system additional to the main Golgi-to-plasma membrane secretory route. Here, we investigated three hits in a Drosophila screen displaying secretory cargo accumulation in autophagic vesicles: ESCRT-III component Vps20, SNARE-binding Rop, and lysosomal pump subunit VhaPPA1-1. We found that Vps20, Rop, and lysosomal markers localize near the trans-Golgi. Furthermore, we document that the vicinity of the trans-Golgi is the main cellular location for lysosomes and that early, late, and recycling endosomes associate as well with a trans-Golgi-associated degradative compartment where basal microautophagy of secretory cargo and other materials occurs. Disruption of this compartment causes cargo accumulation in our hits, including Munc18 homolog Rop, required with Syx1 and Syx4 for Rab11-mediated endosomal recycling. Finally, besides basal microautophagy, we show that the trans-Golgi-associated degradative compartment contributes to the growth of autophagic vesicles in developmental and starvation-induced macroautophagy. Our results argue that the fly trans-Golgi is the gravitational center of the whole endomembrane system.

Focus on the Small GTPase Rab1: A Key Player in the Pathogenesis of Parkinson's Disease

Parkinson’s disease (PD) is the second most frequent neurodegenerative disease. It is characterized by the loss of dopaminergic neurons in the substantia nigra and the formation of large aggregates in the survival neurons called Lewy bodies, which mainly contain α-synuclein (α-syn). The cause of cell death is not known but could be due to mitochondrial dysfunction, protein homeostasis failure, and alterations in the secretory/endolysosomal/autophagic pathways. Survival nigral neurons overexpress the small GTPase Rab1. This protein is considered a housekeeping Rab that is necessary to support the secretory pathway, the maintenance of the Golgi complex structure, and the regulation of macroautophagy from yeast to humans. It is also involved in signaling, carcinogenesis, and infection for some pathogens. It has been shown that it is directly linked to the pathogenesis of PD and other neurodegenerative diseases. It has a protective effect against α–σψν toxicity and has recently been shown to be a substrate of LRRK2, which is the most common cause of familial PD and the risk of sporadic disease. In this review, we analyze the key aspects of Rab1 function in dopamine neurons and its implications in PD neurodegeneration/restauration. The results of the current and former research support the notion that this GTPase is a good candidate for therapeutic strategies.