Postsynaptic complexin controls AMPA receptor exocytosis during LTP

Long-term potentiation (LTP) is a compelling synaptic correlate of learning and memory. LTP induction requires NMDA receptor (NMDAR) activation, which triggers SNARE-dependent exocytosis of AMPA receptors (AMPARs). However, the molecular mechanisms mediating AMPAR exocytosis induced by NMDAR activation remain largely unknown. Here, we show that complexin, a protein that regulates neurotransmitter release via binding to SNARE complexes, is essential for AMPAR exocytosis during LTP but not for the constitutive AMPAR exocytosis that maintains basal synaptic strength. The regulated postsynaptic AMPAR exocytosis during LTP requires binding of complexin to SNARE complexes. In hippocampal neurons, presynaptic complexin acts together with synaptotagmin-1 to mediate neurotransmitter release. However, postsynaptic synaptotagmin-1 is not required for complexin-dependent AMPAR exocytosis during LTP. These results suggest?a complexin-dependent molecular mechanism for regulating AMPAR delivery to synapses, a mechanism that is surprisingly similar to presynaptic exocytosis but controlled by regulators other than synaptotagmin-1.     

Cortical development and asymmetric cell divisions

The development of the mammalian neocortex involves rounds of symmetric and asymmetric cell division of neural progenitors to fulfill needs of both self-renewal of progenitors and production of differe...     

Drosophila Anillin is unequally required during asymmetric cell divisions of the PNS

During Drosophila embryogenesis, timely and orderly asymmetric cell divisions ensure the correct number of each cell type that make up the sensory organs of the larval PNS. We report a role of scraps, Drosophila Anillin, during these divisions. Anillin, a constitutive member of the contractile ring is essential for cytokinesis in Drosophila and vertebrates. During embryogenesis we find that zygotically transcribed scraps is required specifically for the unequal cell divisions, those in which cytokinesis occurs in an “off-centred” manner, of the pIIb and pIIIb neuronal precursor cells, but not the equal cell divisions of the lineage related precursor cells. Complementation and genetic rescue studies demonstrate this effect results from zygotic scraps and leads to polyploidy, ectopic mitosis, and loss of the neuronal precursor daughter cells. The net result of which is the formation of incomplete sense organs and embryonic lethality.     

BASL and EPF2 act independently to regulate asymmetric divisions during stomatal development

The initiation of stomatal development in the developing Arabidopsis epidermis is characterized by an asymmetric ‘entry’ division in which a small cell, known as a meristemoid, and a larger daughter cell is formed. The meristemoid may undergo further asymmetric divisions, regenerating a meristemoid each time, before differentiating into a guard mother cell which divides symmetrically to form a pair of guard cells surrounding a stomatal pore. Recently EPF2 and BASL have emerged as regulators of these asymmetric divisions and here we present results indicating that these two factors operate independently to control stomatal developmentKey words: stomata, development, meristemoids, asymmetric cell division, leaf epidermis, cell polarity, peptide signal     

中枢神经系统发育的不对称细胞分裂机制

无论在无脊椎动物还是脊椎动物中,组成中枢神经系统（CNS）的大多数细胞都是由极性神经祖细胞不对称分裂而来。通过简要综述果蝇（Drosophila melanogaste）成神经母细胞（NB）不对称分裂机制,并与近年来在脊椎动物不对称细胞分裂上取得的研究成果相比较,尝试找出两个系统的相似性和相异性。     

A complexin/synaptotagmin 1 switch controls fast synaptic vesicle exocytosis

Ca(2+) binding to synaptotagmin 1 triggers fast exocytosis of synaptic vesicles that have been primed for release by SNARE-complex assembly. Besides synaptotagmin 1, fast Ca(2+)-triggered exocytosis requires complexins. Synaptotagmin 1 and complexins both bind to assembled SNARE complexes, but it is unclear how their functions are coupled. Here we propose that complexin binding activates SNARE complexes into a metastable state and that Ca(2+) binding to synaptotagmin 1 triggers fast exocytosis by displacing complexin from metastable SNARE complexes. Specifically, we demonstrate that, biochemically, synaptotagmin 1 competes with complexin for SNARE-complex binding, thereby dislodging complexin from SNARE complexes in a Ca(2+)-dependent manner. Physiologically, increasing the local concentration of complexin selectively impairs fast Ca(2+)-triggered exocytosis but retains other forms of SNARE-dependent fusion. The hypothesis that Ca(2+)-induced displacement of complexins from SNARE complexes triggers fast exocytosis accounts for the loss-of-function and gain-of-function phenotypes of complexins and provides a molecular explanation for the high speed and synchronicity of fast Ca(2+)-triggered neurotransmitter release.     

Palmitoylated Ras proteins traffic through recycling endosomes to the plasma membrane during exocytosis

Ras proteins regulate cell growth, death, and differentiation, and it is well established that this functional versatility is accomplished through their different subcellular localizations. Palmitoylated H- and N-Ras are believed to localize at the perinuclear Golgi and plasma membrane (PM). Notably, however, recycling endosomes (REs) also localize to a perinuclear region, which is often indistinguishable from the Golgi. In this study, we show that active palmitoylated Ras proteins mainly localize intracellularly at REs and that REs act as a way station along the post-Golgi exocytic pathway to the PM. H-Ras requires two palmitoyl groups for RE targeting. The lack of either or both palmitoyl groups leads to the mislocalization of the mutant proteins to the endoplasmic reticulum, Golgi apparatus, or the PM. Therefore, we demonstrate that palmitoylation directs Ras proteins to the correct intracellular organelles for trafficking and activity.     

Molecular mechanisms of asymmetric divisions in mammary stem cells

Stem cells have the remarkable ability to undergo proliferative symmetric divisions and self‐renewing asymmetric divisions. Balancing of the two modes of division sustains tissue morphogenesis and homeostasis. Asymmetric divisions of Drosophila neuroblasts (NBs) and sensory organ precursor (SOP) cells served as prototypes to learn what we consider now principles of asymmetric mitoses. They also provide initial evidence supporting the notion that aberrant symmetric divisions of stem cells could correlate with malignancy. However, transferring the molecular knowledge of circuits underlying asymmetry from flies to mammals has proven more challenging than expected. Several experimental approaches have been used to define asymmetry in mammalian systems, based on daughter cell fate, unequal partitioning of determinants and niche contacts, or proliferative potential. In this review, we aim to provide a critical evaluation of the assays used to establish the stem cell mode of division, with a particular focus on the mammary gland system. In this context, we will discuss the genetic alterations that impinge on the modality of stem cell division and their role in breast cancer development.     

The HNF-1 target collectrin controls insulin exocytosis by SNARE complex formation

Defective glucose-stimulated insulin secretion is the main cause of hyperglycemia in type 2 diabetes mellitus. Mutations in HNF-1 cause a monogenic form of type 2 diabetes, maturity-onset diabetes of the young (MODY), characterized by impaired insulin secretion. Here we report that collectrin, a recently cloned kidney-specific gene of unknown function, is a target of HNF-1 in pancreatic β cells. Expression of collectrin was decreased in the islets of HNF-1 (−/−) mice, but was increased in obese hyperglycemic mice. Overexpression of collectrin in rat insulinoma INS-1 cells or in the β cells of transgenic mice enhanced glucose-stimulated insulin exocytosis, without affecting Ca²⁺ influx. Conversely, suppression of collectrin attenuated insulin secretion. Collectrin bound to SNARE complexes by interacting with snapin, a SNAP-25 binding protein, and facilitated SNARE complex formation. Therefore, collectrin is a regulator of SNARE complex function, which thereby controls insulin exocytosis.     

Osteogenic differentiation of hypertrophic chondrocytes involves asymmetric cell divisions and apoptosis

We have investigated the early cellular events that take place during the change in lineage commitment from hypertrophic chondrocytes to osteoblast-like cells. We have induced this osteogenic differentiation by cutting through the hypertrophic cartilage of embryonic chick femurs and culturing the explants. Immunocytochemical characterization, [3H]thymidine pulse-chase labeling, in situ nick translation or end labeling of DNA breaks were combined with ultrastructural studies to characterize the changing pattern of differentiation. The first responses to the cutting, seen after 2 d, were upregulation of alkaline phosphatase activity, synthesis of type I collagen and single-stranded DNA breaks, probably indicating a metastable state. Associated with the change from chondrogenic to osteogenic commitment was an asymmetric cell division with diverging fates of the two daughter cells, where one daughter cell remained viable and the other one died. The available evidence suggests that the viable daughter cell then divided and generated osteogenic cells, while the other daughter cell died by apoptosis. The results suggest a new concept of how changes in lineage commitment of differentiated cells may occur. The concepts also reconcile previously opposing views of the fate of the hypertrophic chondrocyte.     

Drosophila neuroblast asymmetric divisions: cell cycle regulators, asymmetric protein localization, and tumorigenesis

Over the past decade, many of the key components of the genetic machinery that regulate the asymmetric division of Drosophila melanogaster neural progenitors, neuroblasts, have been identified and their functions elucidated. Studies over the past two years have shown that many of these identified components act to regulate the self-renewal versus differentiation decision and appear to function as tumor suppressors during larval nervous system development. In this paper, we highlight the growing number of molecules that are normally considered to be key regulators of cell cycle events/progression that have recently been shown to impinge on the neuroblast asymmetric division machinery to control asymmetric protein localization and/or the decision to self-renew or differentiate.     

C. elegans HAM-1 positions the cleavage plane and regulates apoptosis in asymmetric neuroblast divisions

Asymmetric cell division occurs when a mother cell divides to generate two distinct daughter cells, a process that promotes the generation of cellular diversity in metazoans. During Caenorhabditis elegans development, the asymmetric divisions of neural progenitors generate neurons, neural support cells and apoptotic cells. C. elegans HAM-1 is an asymmetrically distributed cortical protein that regulates several of these asymmetric neuroblast divisions. Here, we show that HAM-1 is a novel protein and define residues important for HAM-1 function and distribution to the cell cortex. Our phenotypic analysis of ham-1 mutant embryos suggests that HAM-1 controls only neuroblast divisions that produce apoptotic cells. Moreover, ham-1 mutant embryos contain many unusually large cell-death corpses. An investigation of this corpse phenotype revealed that it results from a reversal of neuroblast polarity. A misplacement of the neuroblast cleavage plane generates daughter cells of abnormal size, with the apoptotic daughters larger than normal. Thus, HAM-1 regulates the position of the cleavage plane, apoptosis and mitotic potential in C. elegans asymmetric cell divisions.     

Membrane recycling occurs during asymmetric tip growth and cell plate formation inFucus distichus zygotes

Summary Zygotes of the brown algaFucus distichus undergo a series of intracellular changes resulting in the establishment of a polar growth axis prior to the first embryonic cell division. In order to examine the dynamics of membrane recycling which occur in the zygote during polar growth of the rhizoid, we probed living Fucus zygotes with the vital stain FM4-64, N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylammo)phenyl)hexatrienyl)pyridinium dibromide. In newly fertilized, spherical zygotes, FM4-64 staining is symmetric and predominantly in the perinuclear region which is rich in endoplasmic reticulum, Golgi, and vacuolar membranes. As rhizoid or tip growth is initiated, this population of stained membranes becomes asymmetrically redistributed, concentrating at the rhizoid tip and extending centrally to the perinuclear region. This asymmetric localization is maintained in the zygote throughout polar growth of the rhizoid and during karyokinesis. Subsequently, FM4-64 staining also begins to accumulate in a central location between the daughter nuclei. As cytokinesis proceeds, this region of stain expands laterally from this central location, perpendicular to the plane of polar rhizoid outgrowth. The staining pattern thus delineates the formation of a cell plate, similar spatially to the accumulation of nascent plate membranes of higher plants. Treatment of Fucus zygotes with brefeldin-A inhibits both asymmetric growth of the rhizoid and formation of a new cell plate. These data suggest that inF. distichus FM4-64 is labeling a Golgi-derived membrane fraction that appears to be recycling between the site of tip growth, perinuclear region, and new cell plate.Abbreviations AF after fertilization - ASW artificial seawater - BFA brefeldin A - ER endoplasmic reticulum - FM4-64 N-(3-triethylam-moniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide     

The GC kinase Fray and Mo25 regulate Drosophila asymmetric divisions

Drosophila neuroblasts provide an excellent model for asymmetric cell divisions, where cell-fate determinants such as Miranda localize at the basal cortex and segregate to one daughter cell. Mechanisms underlying this process, however, remain elusive. We found that Mo25 and the GC kinase Fray act in this regulation. mo25 and fray mutants show an indistinguishable defect in Miranda localization. On the other hand, Drosophila Mo25 interacts with the tumor suppressor kinase Lkb1 in vivo, as have shown in mammals. Overexpression of Lkb1, which accumulates in the cell cortex, drastically relocalizes both Mo25 and Fray from the cytoplasm to the cortex, causing the same phenotype as mo25-mutant neuroblasts. Recovery from this defect caused by Lkb1 overexpression requires simultaneous overexpression of Mo25 and Fray. We suggest from those results that Mo25 and Fray operate together or in the same pathway in Drosophila asymmetric processes, and that their function counterbalances Lkb1.     

Endocytosis, recycling, and regulated exocytosis of glucose transporter 4

Glucose transporter 4 (GLUT4) is responsible for the uptake of glucose into muscle and adipose tissues. Under resting conditions, GLUT4 is dynamically retained through idle cycling among selective intracellular compartments, from whence it undergoes slow recycling to the plasma membrane (PM). This dynamic retention can be released by command from intracellular signals elicited by insulin and other stimuli, which result in 2-10-fold increases in the surface level of GLUT4. Insulin-derived signals promote translocation of GLUT4 to the PM from a specialized compartment termed GLUT4 storage vesicles (GSV). Much effort has been devoted to the characterization of the intracellular compartments and dynamics of GLUT4 cycling and to the signals by which GLUT4 is sorted into, and recruited from, GSV. This review summarizes our understanding of intracellular GLUT4 traffic during its internalization from the membrane, its slow, constitutive recycling, and its regulated exocytosis in response to insulin. In spite of specific differences in GLUT4 dynamic behavior in adipose and muscle cells, the generalities of its endocytic and exocytic itineraries are consistent and an array of regulatory proteins that regulate each vesicular traffic event emerges from these cell systems.     

The Epac-Rap1 signaling pathway controls cAMP-mediated exocytosis of Weibel-Palade bodies in endothelial cells

Endothelial cells contain specialized storage organelles called Weibel-Palade bodies (WPBs) that release their content into the vascular lumen in response to specific agonists that raise intracellular Ca(2+) or cAMP. We have previously shown that cAMP-mediated WPB release is dependent on protein kinase A (PKA) and involves activation of the small GTPase RalA. Here, we have investigated a possible role for another PKA-independent cAMP-mediated signaling pathway in the regulation of WPB exocytosis, namely the guanine nucleotide exchange factor Epac1 and its substrate, the small GTPase Rap1. Epinephrine stimulation of endothelial cells leads to Rap1 activation in a PKA-independent fashion. siRNA-mediated knockdown of Epac1 abolished epinephrine-induced activation of Rap1 and resulted in decreased epinephrine-induced WPB exocytosis. Down-regulation of Rap1 expression and prevention of Rap1 activation through overexpression of Rap1GAP effectively reduced epinephrine- but not thrombin-induced WPB exocytosis. Taken together, these data uncover a new Epac-Rap1-dependent pathway by which endothelial cells can regulate WPB exocytosis in response to agonists that signal through cAMP.