Zhaolin Hua

Distinct Endocytic Pathways Control the Rate and Extent of Synaptic Vesicle Protein Recycling

Synaptic vesicles have been proposed to form through two mechanisms: one directly from the plasma membrane involving clathrin-dependent endocytosis and the adaptor protein AP2, and the other from an endosomal intermediate mediated by the adaptor AP3. However, the relative role of these two mechanisms in synaptic vesicle recycling has remained unclear. We now find that vesicular glutamate transporter VGLUT1 interacts directly with endophilin, a component of the clathrin-dependent endocytic machinery. In the absence of its interaction with endophilin, VGLUT1 recycles more slowly during prolonged, high-frequency stimulation. Inhibition of the AP3 pathway with brefeldin A rescues the rate of recycling, suggesting a competition between AP2 and -3 pathways, with endophilin recruiting VGLUT1 toward the faster AP2 pathway. After stimulation, however, inhibition of the AP3 pathway prevents the full recovery of VGLUT1 by endocytosis, implicating the AP3 pathway specifically in compensatory endocytosis.

v-SNARE Composition Distinguishes Synaptic Vesicle Pools

Synaptic vesicles belong to two distinct pools, a recycling pool responsible for the evoked release of neurotransmitter and a resting pool unresponsive to stimulation. The uniform appearance of synaptic vesicles has suggested that differences in location or cytoskeletal association account for these differences in function. We now find that the v-SNARE tetanus toxin-insensitive vesicle-associated membrane protein (VAMP7) differs from other synaptic vesicle proteins in its distribution to the two pools, providing evidence that they differ in molecular composition. We also find that both resting and recycling pools undergo spontaneous release, and when activated by deletion of the longin domain, VAMP7 influences the properties of release. Further, the endocytosis that follows evoked and spontaneous release differs in mechanism, and specific sequences confer targeting to the different vesicle pools. The results suggest that different endocytic mechanisms generate synaptic vesicles with different proteins that can endow the vesicles with distinct properties.

TLR signaling in B-cell development and activation

Expression of Toll-like receptors (TLRs) in B cells provides a cell-intrinsic mechanism for innate signals regulating adaptive immune responses. In combination with other signaling pathways in B cells, including through the B-cell receptor (BCR), TLR signaling plays multiple roles in B-cell differentiation and activation. The outcome of TLR signaling in B cells is largely context-dependent, which partly explains discrepancies among in vitro and in vivo studies, or studies using different immunogens. We focus on recent findings on how B-cell-intrinsic TLR signaling regulates antibody responses, including germinal center formation and autoantibody production in autoimmune disease models. In addition, TLR signaling also acts on the precursors of B cells, which could influence the immune response of animals by shaping the composition of the immune system. With TLR signaling modulating immune responses at these different levels, much more needs to be understood before we can depict the complete functions of innate signaling in host defense.

Secretion of L-glutamate from osteoclasts through transcytosis

Osteoclasts are involved in the catabolism of the bone matrix and eliminate the resulting degradation products through transcytosis, but the molecular mechanism and regulation of transcytosis remain poorly understood. Upon differentiation, osteoclasts express vesicular glutamate transporter 1 (VGLUT1), which is essential for vesicular storage and subsequent exocytosis of glutamate in neurons. VGLUT1 is localized in transcytotic vesicles and accumulates L‐glutamate. Osteoclasts secrete L‐glutamate and the bone degradation products upon stimulation with KCl or ATP in a Ca2+‐dependent manner. KCl‐ and ATP‐dependent secretion of L‐glutamate was absent in osteoclasts prepared from VGLUT1−/− knockout mice. Osteoclasts express mGluR8, a class III metabotropic glutamate receptor. Its stimulation by a specific agonist inhibits secretion of L‐glutamate and bone degradation products, whereas its suppression by a specific antagonist stimulates bone resorption. Finally, it was found that VGLUT1−/− mice develop osteoporosis. Thus, in bone‐resorbing osteoclasts, L‐glutamate and bone degradation products are secreted through transcytosis and the released L‐glutamate is involved in autoregulation of transcytosis. Glutamate signaling may play an important role in the bone homeostasis.

Requirement for MyD88 Signaling in B Cells and Dendritic Cells for Germinal Center Anti-Nuclear Antibody Production in Lyn-Deficient Mice

The intracellular tyrosine kinase Lyn mediates inhibitory receptor function in B cells and myeloid cells, and Lyn−/− mice spontaneously develop an autoimmune and inflammatory disease that closely resembles human systemic lupus erythematosus. TLR-signaling pathways have been implicated in the production of anti-nuclear Abs in systemic lupus erythematosus and mouse models of it. We used a conditional allele of Myd88 to determine whether the autoimmunity of Lyn−/− mice is dependent on TLR/MyD88 signaling in B cells and/or in dendritic cells (DCs). The production of IgG anti-nuclear Abs, as well as the deposition of these Abs in the glomeruli of the kidneys, leading to glomerulonephritis in Lyn−/− mice, were completely abolished by selective deletion of Myd88 in B cells, and autoantibody production and glomerulonephritis were delayed or decreased by deletion of Myd88 in DCs. The reduced autoantibody production in mice lacking MyD88 in B cells or DCs was accompanied by a dramatic decrease in the spontaneous germinal center (GC) response, suggesting that autoantibodies in Lyn−/− mice may depend on GC responses. Consistent with this view, IgG anti-nuclear Abs were absent if T cells were deleted (TCRβ−/− TCRδ−/− mice) or if T cells were unable to contribute to GC responses as the result of mutation of the adaptor molecule SAP. Thus, the autoimmunity of Lyn−/− mice was dependent on T cells and on TLR/MyD88 signaling in B cells and in DCs, supporting a model in which DC hyperactivity combines with defects in tolerance in B cells to lead to a T cell–dependent systemic autoimmunity in Lyn−/− mice.

AMP-activated kinase links serotonergic signaling to glutamate release for regulation of feeding behavior in C. elegans.

Serotonergic regulation of feeding behavior has been studied intensively, both for an understanding of the basic neurocircuitry of energy balance in various organisms and as a therapeutic target for human obesity. However, its underlying molecular mechanisms remain poorly understood. Here, we show that neural serotonin signaling in C. elegans modulates feeding behavior through inhibition of AMP-activated kinase (AMPK) in interneurons expressing the C. elegans counterpart of human SIM1, a transcription factor associated with obesity. In turn, glutamatergic signaling links these interneurons to pharyngeal neurons implicated in feeding behavior. We show that AMPK-mediated regulation of glutamatergic release is conserved in rat hippocampal neurons. These findings reveal cellular and molecular mediators of serotonergic signaling.

Vesicular Glutamate Transporter 1 Is Required for Photoreceptor Synaptic Signaling But Not For Intrinsic Visual Functions

Glutamatergic neurotransmission requires vesicular glutamate transporters (VGLUTs) to sequester glutamate into synaptic vesicles. Generally, VGLUT1 and VGLUT2 isoforms show complementary expression in the CNS and retina. However, little is known about whether isoform-specific expression serves distinct pathways and physiological functions. Here, by examining visual functions in VGLUT1-null mice, we demonstrate that visual signaling from photoreceptors to retinal output neurons requires VGLUT1. However, photoentrainment and pupillary light responses are preserved. We provide evidence that melanopsin-containing, intrinsically photosensitive retinal ganglion cells (RGCs), signaling via VGLUT2 pathways, support these non-image-forming functions. We conclude that VGLUT1 is essential for transmitting visual signals from photoreceptors to second- and third-order neurons, but VGLUT1 is not necessary for intrinsic visual functions. Furthermore, melanopsin and VGLUT2 expression in a subset of RGCs immediately after birth strongly supports the idea that intrinsic vision can function well before rod- and cone-mediated signaling has matured.

Characterization of T-Dependent and T-Independent B Cell Responses to a Virus-like Particle

Natural pathogens, such as viruses, often induce T-dependent and T-independent Ab responses. However, the activation and differentiation of Ag-specific B cells under these conditions had not been examined in detail. In this study, we used bacterial phage Qβ-derived virus-like particles (Qβ-VLPs) as an immunogen to examine the T-independent and T-dependent phases of the response in mice. Using Qβ-specific cell labeling and enrichment methods developed in this study, we were able to characterize the rare Ag-specific B cells in detail. Surprisingly, we found that Qβ-VLPs could induce Bcl-6 expression in pregerminal center B cells independently of T cell help. In addition, Qβ-VLP–induced T-independent responses could lead to isotype-switched and somatically mutated memory B cells. Finally, in contrast to what has been reported with several other Ags, long-lived IgG+ memory cells were induced by Qβ-VLPs, with IgM+ memory B cells being produced but only evident for a limited time, suggesting that different types of immunogens may preferentially generate or maintain IgM+ versus IgG+ memory B cells.

B Cell–Intrinsic MyD88 Signaling Promotes Initial Cell Proliferation and Differentiation To Enhance the Germinal Center Response to a Virus-like Particle

Although TLR signaling in B cells has been implicated in the germinal center (GC) responses during viral infections and autoimmune diseases, the underlying mechanism is unclear. Bacterial phage Qβ-derived virus-like particle (Qβ-VLP) contains TLR ligands, which can enhance Qβ-VLP-induced Ab response, including GC response, through TLR/MyD88 signaling in B cells. In this study, by examining Ag-specific B cell response to Qβ-VLP, we found that lack of B cell MyD88 from the beginning of the immune response led to a more severe defect in the GC scale than abolishing MyD88 at later time points of the immune response. Consistently, B cell–intrinsic MyD88 signaling significantly enhanced the initial proliferation of Ag-specific B cells, which was accompanied with a dramatic increase of plasma cell generation and induction of Bcl-6+ GC B cell precursors. In addition, B cell–intrinsic MyD88 signaling promoted strong T-bet expression independent of IFN-γ and led to the preferential isotype switching to IgG2a/c. Thus, by promoting the initial Ag-specific B cell proliferation and differentiation, B cell–intrinsic MyD88 signaling enhanced both T-independent and T-dependent Ab responses elicited by Qβ-VLP. This finding will provide additional insight into the role of TLR signaling in antiviral immunity, autoimmune diseases, and vaccine design.

B Cells Are the Dominant Antigen-Presenting Cells that Activate Naive CD4+ T Cells upon Immunization with a Virus-Derived Nanoparticle Antigen

Graphical Abstract Figure. No caption available. SUMMARY B cells can present antigens to CD4+ T cells, but it is thought that dendritic cells (DCs) are the primary initiators of naive CD4+ T cell responses. Nanoparticles, including virus‐like particles (VLPs), are attractive candidates as carriers for vaccines and drug delivery. Using RNA phage Q&bgr;‐derived VLP (Q&bgr;‐VLP) as a model antigen, we found that antigen‐specific B cells were the dominant antigen‐presenting cells that initiated naive CD4+ T cell activation. B cells were sufficient to induce T follicular helper cell development in the absence of DCs. Q&bgr;‐specific B cells promoted CD4+ T cell proliferation and differentiation via cognate interactions and through Toll‐like receptor signaling‐mediated cytokine production. Antigen‐specific B cells were also involved in initiating CD4+ T cell responses during immunization with inactivated influenza virus. These findings have implications for the rational design of nanoparticles as vaccine candidates, particularly for therapeutic vaccines that aim to break immune tolerance. HIGHLIGHTSB cells are the dominant APCs that initiate CD4+ T cell response to VLPsThe few antigen‐specific B cells in WT mice are sufficient to activate CD4+ T cellsInnate signaling in B cells is required for the VLP‐induced CD4+ T cell polarizationB cell‐mediated antigen‐presentation can break CD4+ T cell tolerance &NA; Initiation of CD4+ T cell response depends on antigen‐presenting cells. Contrary to the paradigm that dendritic cells are required for this process, Hong et al. show that antigen‐specific B cells are essential and sufficient to activate naive CD4+ T cells in response to virus‐like particles or influenza vaccination.