Exocytosis, endocytosis, and development

Clathrin-dependent endocytosis plays a critical role in larval and pupal development, and female oocyte production in the red flour beetle (Tribolium castaneum)

BACKGROUND

Clathrin-dependent endocytosis is a vesicular transport process by which cells take macromolecules from the extracellular space to the intracellular space. It plays important roles in various cellular functions, but its biological significance in insect development and reproduction has not been well studied.

RESULTS

We characterized and functionally analyzed four major clathrin-dependent endocytic pathway genes (TcChc, TcAP50, TcVhaSFD, TcRab7) in Tribolium castaneum. RNA interference (RNAi) by injecting double-stranded RNA (dsRNA) targeting each gene at three doses (50, 100, or 200 ng per insect) in 20-day-old larvae led to 100% larval mortality. When the expressions of TcChc, TcVhaSFD, and TcRab7 were suppressed by injecting their respective dsRNAs at each dose in 1-day-old pupae, the adults that emerged from the dsRNA-injected pupae were deformed, with the absence of wing development. The deformed adults died within 2 days after eclosion. When the expression of TcAP50 was suppressed by injecting its dsRNA into 1-day-old pupae, although no apparent deformed adults were observed, all the adults died within 35 days after eclosion. In addition, when the expressions of TcChc and TcVhaSFD were suppressed by injecting their respective dsRNAs at a reduced dose (10 ng per insect) in 5-day-old pupae, the ovarian development and oocyte production in the resultant females were completely inhibited.

CONCLUSION

Our results indicate that clathrin-dependent endocytosis is essential for insect development and reproduction. The results from this study can help researchers identify potential molecular targets for developing novel strategies for insect pest management. © 2023 Society of Chemical Industry.

Syntaxin 1 is required for DCC/Netrin-1-dependent chemoattraction of migrating neurons from the lower rhombic lip

Directed cell migration and axonal guidance are essential steps in neural development that share many molecular mechanisms. The guidance of developing axons and migrating neurons is likely to depend on the precise control of plasmalemma turnover in selected regions of leading edges and growth cones, respectively. Previous results provided evidence of a signaling mechanism that couples chemotropic deleted in colorectal cancer (DCC)/Netrin-1 axonal guidance and exocytosis through Syntaxin1(Sytx1)/TI-VAMP SNARE proteins. Here we studied whether Netrin-1-dependent neuronal migration relies on a similar SNARE mechanism. We show that migrating neurons in the lower rhombic lip (LRL) express several SNARE proteins, and that DCC co-associates with Sytx1 and TI-VAMP in these cells. We also demonstrate that cleavage of Sytx1 by botulinum toxin C1 (BoNT/C1) abolishes Netrin-1-dependent chemoattraction of migrating neurons, and that interference of Sytx1 functions with shRNAs or Sytx1-dominant negatives disrupts Netrin-1-dependent chemoattraction of LRL neurons. These findings indicate that a Sytx1/DCC interaction is required for Netrin-1 guidance of migrating neurons, thereby highlighting a relationship between guidance signaling and SNARE proteins that regulate membrane turnover.

A signaling mechanism coupling netrin-1/deleted in colorectal cancer chemoattraction to SNARE-mediated exocytosis in axonal growth cones

Directed cell migration and axonal guidance are essential steps in neural development. Both processes are controlled by specific guidance cues that activate the signaling cascades that ultimately control cytoskeletal dynamics. Another essential step in migration and axonal guidance is the regulation of plasmalemma turnover and exocytosis in leading edges and growth cones. However, the cross talk mechanisms linking guidance receptors and membrane exocytosis are not understood. Netrin-1 is a chemoattractive cue required for the formation of commissural pathways. Here, we show that the Netrin-1 receptor deleted in colorectal cancer (DCC) forms a protein complex with the t-SNARE (target SNARE) protein Syntaxin-1 (Sytx1). This interaction is Netrin-1 dependent both in vitro and in vivo, and requires specific Sytx1 and DCC domains. Blockade of Sytx1 function by using botulinum toxins abolished Netrin-1-dependent chemoattraction of axons in mouse neuronal cultures. Similar loss-of-function experiments in the chicken spinal cord in vivo using dominant-negative Sytx1 constructs or RNAi led to defects in commissural axon pathfinding reminiscent to those described in Netrin-1 and DCC loss-of-function models. We also show that Netrin-1 elicits exocytosis at growth cones in a Sytx1-dependent manner. Moreover, we demonstrate that the Sytx1/DCC complex associates with the v-SNARE (vesicle SNARE) tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) and that knockdown of TI-VAMP in the commissural pathway in the spinal cord results in aberrant axonal guidance phenotypes. Our data provide evidence of a new signaling mechanism that couples chemotropic Netrin-1/DCC axonal guidance and Sytx1/TI-VAMP SNARE proteins regulating membrane turnover and exocytosis.

Ever-expanding network of dynamin-interacting proteins

Clathrin-mediated endocytosis is a major cellular pathway for internalization of proteins and lipids and for recycling of synaptic vesicles. The GTPase dynamin plays a key role in this process, and the proline-rich domain of dynamin participates in various protein-protein interactions to ensure a proper coordination of endocytic processes. Although dynamin is not directly associated with actin, several dynamin-binding proteins can interact with actin or with proteins that regulate actin assembly, thereby coordinately regulating actin assembly and trafficking events. This article summarizes dynamin interactions with various Src homology 3-containing proteins, many of which are actin-binding proteins. It also discusses the recently identified two new dynamin binding proteins, SH3 protein interacting with Nck, 90 kDa/Wiskott-Aldrich syndrome protein interacting with SH3 protein (SPIN90/WISH) and sorting nexin 9, and outlines their potential role as a link between endocytosis and actin dynamics.

A mutation in SNAP29, coding for a SNARE protein involved in intracellular trafficking, causes a novel neurocutaneous syndrome characterized by cerebral dysgenesis, neuropathy, ichthyosis, and palmoplantar keratoderma

Neurocutaneous syndromes represent a vast, largely heterogeneous group of disorders characterized by neurological and dermatological manifestations, reflecting the common embryonic origin of epidermal and neural tissues. In the present report, we describe a novel neurocutaneous syndrome characterized by cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma (CEDNIK syndrome). Using homozygosity mapping in two large families, we localized the disease gene to 22q11.2 and identified, in all patients, a 1-bp deletion in SNAP29, which codes for a SNARE protein involved in vesicle fusion. SNAP29 expression was decreased in the skin of the patients, resulting in abnormal maturation of lamellar granules and, as a consequence, in mislocation of epidermal lipids and proteases. These data underscore the importance of vesicle trafficking regulatory mechanisms for proper neuroectodermal differentiation.

Regulation of cortactin/dynamin interaction by actin polymerization during the fission of clathrin-coated pits

Separation of clathrin-coated pits from the plasma membrane, a key event during endocytosis, is thought to be driven by dynamin and the actin cytoskeleton. However, the mechanism for the actin-mediated endocytosis remains elusive. RNA interference-mediated suppression of cortactin, an F-actin binding protein that promotes Arp2/3 complex-mediated actin polymerization, effectively blocked transferrin uptake. Depletion of cortactin in brain cytosol inhibited formation of clathrin-coated vesicles by 70% as analyzed in a cell-free system. Interestingly, the interaction between cortactin and dynamin 2 in cells was dependent on actin polymerization and was attenuated upon cell exposure to cytochalasin D as analyzed by immunofluorescence and immunoprecipitation. Moreover, a cortactin mutant deficient in Arp2/3 binding colocalized less efficiently with dynamin 2 and inhibited the uptake of transferrin. The effect of actin polymerization on the interaction between cortactin and the dynamin proline-rich domain (PRD) was further evaluated under a condition for actin polymerization in vitro. Cortactin binds to the dynamin PRD with an equilibrium dissociation constant of 81 nM in the presence of the Arp2/3 complex and actin, and 617 nM in the absence of actin polymerization. Taken together, these data demonstrate that Arp2/3-mediated actin polymerization regulates the accessibility of cortactin to dynamin 2 and imply a novel mechanism by which cortactin and dynamin drive the fission of clathrin-coated pits in an actin polymerization dependent manner.

Control of vesicular trafficking by Rho GTPases

Although vesicular trafficking is essential for a large variety of cellular processes, the regulation of vesicular trafficking is still poorly understood. Members of the Rho family of small GTPases have recently emerged as important control elements of many stages of vesicular trafficking, providing new insight into the regulation of these events. We will discuss the diverse roles played by Rho proteins in membrane trafficking and focus on the biological implications of these functions.