Messenger RNA of Drosophila Amphiphysin is expressed widely during embryogenesis and has elevated expression in a number of sites including the foregut, hindgut and epidermis, but not in the central nervous system. Taken together, these data are consistent with a role for Drosophila Amphiphysin in endocytosis, but the details of this role may differ from that of vertebrate amphiphysins (Razzaq, 2000).
Vertebrate Amphiphysins 1 and 2 are soluble proteins that are associated in vivo with synaptic vesicles. Double staining for Amphiphysin and either a synaptic vesicle marker (Cysteine string protein, CSP) or a marker that is essentially postsynaptic using optical microscopy (Discs-large, Dlg) showed that Amphiphysin is detectable only postsynaptically at the larval neuromuscular junction (NMJ). Amphiphysin is found only around type I boutons, where its distribution overlaps with that of Dlg, a membrane-associated guanylate kinase (MAGUK) that is responsible for clustering ion channels and cell adhesion molecules. No Amphiphysin is detected around type II boutons, which lack Dlg protein postsynaptically. Although low levels of presynaptic Amphiphysin, or its presence on the presynaptic plasma membrane of type I boutons (indistinguishable from the postsynaptic compartment by optical microscopy) cannot be ruled out. Its absence from type II boutons and from the vesicle-containing cytosolic region of type I boutons suggests that it is unlikely to be required for synaptic vesicle recycling. No Amphiphysin staining is found postsynaptically at amphmut larval NMJs. However, postsynaptic structure, visualized by Dlg staining, is intact in amphmut larvae, showing that Amphiphysin is not required for the gross structural organization of the postsynaptic NMJ (Razzaq, 2001).
Amphiphysin expression is not limited to the neuromuscular junction and is also found throughout larval body wall and adult thoracic muscles, localized on a reticular network. A similar network in larval body wall muscles has been shown using antibodies against Dlg (Razzaq, 2001).
To examine the temporal and spatial distribution of Amph during development, rabbit polyclonal antibodies were generated to a full-length (602 aa) His6-tagged Amph protein. Western blots were performed on lysates from Drosophila at different stages of development. At all developmental stages examined, an 85-kDa band was observed and a cluster of bands at 80 kDa was observed that may represent additional splice variants or different phospho-isoforms of Amph and a 45-kDa band that may be the product of an as-of-yet unidentified transcript, or a proteolytic degradation product. However, all of the isoforms appear to be down-regulated in both first larval instars and pupae. Moreover, all 3 major isoforms are absent in null mutants. As with vertebrate amphiphysin I, Drosophila Amph has a reduced mobility on SDS-PAGE gels compared to the predicted size of the protein. The distribution of Amph in several tissues including body wall/muscle, central nervous system (CNS) and imaginal discs was examined. All of the isoforms can be detected in whole larvae and in the body wall, which is comprised primarily of muscle. In contrast, the 45 kDa isoform appears to be absent or strongly down-regulated in both the CNS and imaginal discs. Multiple isoforms of amphiphysin have also been observed in vertebrates, including brain-specific and ubiquitous forms. This suggests that in addition to a role in synaptic vesicle-mediated endocytosis, amphiphysins may also have additional cell biological functions within specific tissues (Leventis, 2001).
To further investigate the distribution of Amph, immunohistochemical and immunofluorescence staining of embryos and third instar larval CNS and imaginal discs were performed. Amph was widely expressed in embryos and larvae. Specifically, Amph is first detected in embryos at the onset of cellularization at the sites of developing membranes. At later stages, staining was observed in epithelial tissues, the foregut and hindgut. In contrast, no Amph expression was detected in either the embryonic nervous system or muscle. In third larval instars, Amph can be detected at high levels in both larval epithelial cells and muscles, with intense staining at the neuromuscular junction. In addition, and consistent with the Western blot analysis, ubiquitous expression of Amph was observed in both imaginal discs and the CNS (Leventis, 2001).
Since Amphiphysin has been widely implicated in synaptic vesicle endocytosis, the subcellular distribution of Amph at the larval neuromuscular junction (NMJ) was examined. Vertebrate amphiphysins are highly enriched at nerve terminals and associated with synaptic vesicles (SVs). To determine if Drosophila Amph is similarly found at sites of SV endocytosis, immunofluorescence techniques and confocal microscopy were used. Drosophila Amph was highly expressed at the NMJ. To determine if synaptic Amph is localized pre- or postsynaptically, co-immunolocalization studies were performed with the presynaptic protein, cysteine string protein (CSP), and with UAS-CD8::Sh-GFP that is driven postsynaptically by MHC-GAL4. CSP and CD8::Sh-GFP have discrete, nonoverlapping distributions. Surprisingly, double labeling of wildtype NMJs demonstrate that CSP and Amph do not colocalize. However, Amph and CD8::Sh-GFP show a high degree of colocalization at the NMJ, although Amph is more diffusely localized. Therefore, Amph appears not to be localized presynaptically, but is present at the synaptic surface of the muscle cell (Leventis, 2001).
In order to explain the locomoter defects of both larvae and adults, the larval neuromuscular junction (NMJ) was examined. Amph localization can be detected in the muscle and shows enrichment at the NMJ. To determine whether it is presynaptic or postsynaptic, colocalization experiments were done with a postsynaptic marker, Discs large (Dlg), and a presynaptic protein marker, Cysteine String Protein (Csp). Amph and the postsynaptic marker Dlg are perfectly colocalized, whereas Amph protein surrounds but does not colocalize with the presynaptic marker Csp. Thus, the majority of the Amph protein is postsynaptic, although the possibility that there are low levels of Amph at the presynaptic side of the NMJ cannot be excluded. In addition, Amph appears to be specifically localized to the type I Dlg+ glutamatergic synapses but absent from type II and III synapses, which are thought to contain other neurotransmitters and modulators (Zelhof, 2001).
The localization and function of Amph at the postsynaptic density of Type I synapses, which are exclusively characterized by an elaborate subsynaptic reticulum, suggests that Amph may play a role in regulating membrane organization or in selective protein targeting. To further investigate this possibility, a detailed examination of Amph localization and expression patterns in embryonic, larval, pupal and adult tissues was performed. Amph was detected in diverse cell types and in all Amp-expressing cells; Amph is restricted both spatially and temporally to a specific domain of the plasma membrane, typically a domain that is undergoing membrane remodeling or where a submembrane protein scaffold is being assembled (Zelhof, 2001).
During embryogenesis, Amph is first detected during cellularization, as membrane is being inserted between each nucleus of the syncitial blastoderm stage embryo. Initially, Amph is detected at the apical surface of the precellular embryo. As the membrane extends inward (basally) during cellularization, Amph decorates the leading edge; this differs from Bifocal, which consistently labels apical membrane domain. Upon completion of cellularization, Amph returns to the apical surface of the newly formed cells, where it is colocalized with many apical membrane markers. Amph is subsequently localized to the apical membrane domain in the ventral ectoderm and in the neuroblasts that delaminate from this ectoderm, as well as to the apical membrane of tubular internal tissues such as the esophagus, hindgut, trachea and salivary glands. In many of these tissues, Amph is also robustly detected in small punctate presumptive vesicles within the cytoplasm of the cell; the identity of these vesicles is unknown. Amph expression is not detected in embryonic postmitotic neurons or muscles, although it is detected in larval muscles. Surprisingly, analysis of Amph mutants and misexpression studies did not reveal any pronounced defects in the process of cellularization, trachea formation or asymmetric localization of cell fate determinants during embryonic neuroblast divisions: this is further indication that Amph function is either not necessary or redundant with other proteins (Zelhof, 2001).
Adult Drosophila photoreceptor cells are specialized neurons that have an apical microvillar stack of intricately folded membrane (the rhabdomere), which contains the light-sensing rhodopsin proteins. Twenty-four hours after puparium formation (APF) -- before the formation of the apical microvilli -- each photoreceptor shows F-actin accumulation at the apical membrane, while Amph is distributed more diffusely. Forty-eight hours APF, Amph has localized into a dense crescent at the apical cortex of each photoreceptor neuron because the first actin-filled apical microvilli can be detected. By 55 hours APF, F-actin and Amph are tightly colocalized at the apical membrane domain of each photoreceptor neuron, and it is this domain that will subsequently give rise to the rhabdomere. In the adult, Amph is no longer expressed in the photoreceptor cells, but can be detected in the lens-secreting cone cells (Zelhof, 2001).
In early Drosophila embryos, several mitotic cycles proceed with aborted cytokinesis before a modified cytokinesis, called cellularization, finally divides the syncytium into individual cells. This study finds that scission of endocytic vesicles from the plasma membrane (PM) provides a control point to regulate the furrowing events that accompany this development. At early mitotic cycles, local furrow-associated endocytosis is controlled by cell cycle progression, whereas at cellularization, which occurs in a prolonged interphase, it is controlled by expression of the zygotic gene nullo. nullo mutations impair cortical F-actin accumulation and scission of endocytic vesicles, such that membrane tubules remain tethered to the PM and deplete structural components from the furrows, precipitating furrow regression. Thus, Nullo regulates scission to restrain endocytosis of proteins essential for furrow stabilization at the onset of cellularization. It is proposed that developmentally regulated endocytosis can coordinate actin/PM remodeling to directly drive furrow dynamics during morphogenesis (Sokac, 2008).
Early morphogenetic events are accomplished by maternal cellular machinery that is developmentally controlled by expression of the zygotic genome. The zygotic gene product Nullo acts as a developmental switch at cycle 14 and targets the endocytic machinery to cellularize the embryo. This study assayed endocytic dynamics by following fluorophore-conjugated wheat-germ agglutinin (Alexa 488-WGA) internalization in living embryos and Amphiphysin (Amph) tubulation in fixed embryos. WGA is a general marker for endocytosis, whereas Amph tubules are more specifically associated with the initial ingression of PM furrows. Several findings support the hypothesis that these Amph tubules are endocytic intermediates. First, their structure is tubular rather than sheet like, consistent with a role in endocytosis. Second, perturbation of Dynamin, a catalyst of endocytic scission, increases the number of Amph tubules at cellularization furrows. Third, in living nulloX and Cyto-D-treated embryos, WGA is internalized in long, PM-tethered tubules that resemble Amph tubules. Fourth, DPATJ enters Amph tubules in nulloX embryos and accumulates at early endosomes. Thus, Amph tubules were used as quantifiable reporters of endocytosis at furrows (Sokac, 2008).
The membrane furrows that form during the early mitotic cycles regress, whereas those that form at cellularization stably ingress. It is proposed that endocytosis is differentially controlled to achieve these distinct morphogenetic events. During the mitotic cycles, metaphase furrows are transient, ingressing only ~5 μm before completely regressing. Restrained endocytosis, detected by both WGA internalization and Amph tubules, accompanies the initial furrow ingression that occurs at prophase/metaphase. This is followed by a fast wave of vigorous WGA endocytosis that traverses the embryo surface when metaphase furrows regress at anaphase/telophase. The endocytosis accompanying furrow regression is not associated with high levels of Amph tubules. Although this endocytosis may not recruit Amph, a model is favored whereby endocytic scission is more efficient at this time, precluding the capture of tubule intermediates by fixation. Thus, endocytosis at furrow regression may be mechanistically distinct from endocytosis at furrow ingression. It may also be functionally distinct and could even drive furrow regression, as endocytosis adjusts the surface area of both motile and dividing cells. Thus, throughout the early mitotic cycles, alternating and distinct endocytic dynamics are regulated by cell cycle progression and correlate with specific furrow events (Sokac, 2008).
At the onset of cellularization, furrows form in a way that resembles metaphase furrows, but then assemble furrow canals that stabilize the furrow and sustain ingression over ~40 μm. Endocytosis, marked by both WGA internalization and Amph tubules, also accompanies the initial ingression of cellularization furrows, but ceases by the time furrows reach 5 μm in length and furrow canals fully assemble. Since cellularization occurs during interphase, cell cycle progression cannot regulate this endocytosis. Instead, zygotic expression of Nullo aids endocytic scission, and this has the effect of limiting membrane dynamics at the tip of the incipient cellularization furrow, so that proteins, including Myosin-2, Septin, and DPATJ, that concentrate there are retained there. As a result, furrow canals assemble and stabilized furrows ingress to cellularize the embryo. Thus, Nullo regulation of endocytic dynamics could promote the developmental transition from transient furrowing that maintains the syncytium to stable furrowing that generates the primary epithelial cell sheet (Sokac, 2008).
Nullo activity facilitates endocytic scission, such that budding vesicles are rapidly released from the PM. When scission is impaired, some budding vesicles are distended into long Amph tubules that remain persistently tethered to the PM. This phenotype is mimicked when F-actin levels are reduced with Cyto-D, and Nullo regulates cortical F-actin. Thus, it is suggested that Nullo aids scission via its regulation of F-actin. How Nullo regulates actin remains elusive, since it is a small (213 amino acids), highly basic (pI 11.4), myristoylated protein with no readily identifiable globular domains to suggest interaction partners. Instead, sequence composition and hydropathy character suggest that Nullo is “natively unfolded,” containing 63% disorder-promoting amino acids (T, R, G, Q, S, N, P, D, E, and K) over its entire length and a disordered run of 50 consecutive amino acids. Other disordered proteins have been identified that control F-actin organization, such as MARCKs, MARCKs-related proteins, and GAP43, either by direct interaction with actin or by locally sequestering the actin-regulator PIP2 within the PM. Although sequence comparison and lack of characteristic acidic regions do not suggest that Nullo is MARCKs related, it was found that Nullo interacts with PIP2 in in vitro binding assays. Nullo may then concentrate PIP2 locally to regulate actin and/or to couple actin to components of the endocytic machinery that also interact with PIP2 at the PM (Sokac, 2008).
Nullo may aid endocytic scission via F-actin by either active or passive mechanisms. In yeast, F-actin actively drives endocytic scission by exerting polymerization and myosin-based forces to lengthen, and eventually break, the budding vesicle neck. In Drosophila hemocytes and mammalian cells, F-actin also contributes to a late step in endocytosis that just precedes vesicle release and that may be either bud invagination or scission. Additionally, cortical F-actin passively regulates endocytic dynamics by reinforcing the plasma membrane (PM) and thus antagonizing PM deformation. In the case of Bin-Amph-Rvs (BAR) domain activity, drug-mediated reduction of F-actin levels enhances PM tubulation. Reduced levels of cortical F-actin in nulloX mutants result in the appearance of more Amph tubules. However, fewer vesicles are released in living nulloX embryos, and Amph tubulation does not expand to regions beyond the furrow, arguing that there is not more endocytosis in mutants. Thus, impaired scission generates the appearance of more tubules, and it takes almost three times longer for budding vesicles to release from the PM in nulloX versus wild-type embryos. Consistently, dynamin defects enhance PM tubulation in cultured cells, and BAR-induced membrane tubulation is antagonized by coexpression of dynamin. Thus, Nullo may regulate a population of F-actin that either actively aids scission or stiffens the cortex and somehow contributes to endocytosis (i.e., if breaking the bud neck is aided by the PM being under cortically maintained tension) (Sokac, 2008).
This analysis strongly supports that membrane trafficking is differentially controlled at specific sites and times within embryos to achieve distinct morphogenetic events. This was previously suggested for fly embryos by the membrane-labeling analysis that demonstrates that exocytosis occurs at specific sites along cellularization furrows and thus helps establish apical/basal polarity in these cells. Two additional observations are relevant to the results described here. First, membrane labeling of the furrow canal is only possible at very early cellularization. After that time, the furrow canal persists as a stable membrane compartment in which no new membrane is either added or taken away. The current data also support the finding that membrane turnover at the furrow canal region is restricted to the very beginning of cellularization. In fact, endocytic dynamics are tightly controlled there to establish and/or maintain the concentration of proteins at the furrow canal. Second, it has been observed that the membrane label is cleared from the apical PM, and it has been suggested that clearing is mediated by endocytosis. At furrow lengths > 5 μm, WGA vesicles are seen moving away from the apical PM. But when perivitelline injections are done with higher lectin concentrations and at furrow lengths < 5 μm, they reveal WGA endocytosis from the tips of incipient furrows. In fixed embryos in which spatial resolution is better, Amph tubules clearly extend only from furrow tips. Thus, this analysis shows that, in addition to apical endocytosis, local endocytosis also occurs where furrows first ingress (Sokac, 2008).
During cytokinesis in some mammalian cells, membrane endocytosed at sites remote from the furrow is later delivered to the division plane via the endocytic pathway. At cellularization furrows in the fly embryo, the exocytosis of membrane derived from recycling endosomes suggests a similar pathway. In these cases, endocytosis from one site can provide a store of membrane to feed growth somewhere else. However, the observation that endocytosis occurs at the furrow itself is counterintuitive, since endocytosis would be expected to reduce surface area, whereas furrow ingression requires surface expansion. Nonetheless, there are now several reports that endocytic proteins including Clathrin, Clathrin adaptor-2, and Dynamin concentrate in cytokinesis furrows. In addition, endocytosis has been directly visualized at furrows in dividing zebrafish embryos, cultured cells, and fission yeast, although the function of this endocytosis remains unclear (Sokac, 2008).
Endocytosis occurs at the tips of both metaphase and cellularization furrows when the furrows are first ingressing, suggesting that it confers some temporally and spatially specific function. Both the PM and actin are significantly remodeled at these sites as furrows form. At the onset of cellularization in particular, the F-actin/Myosin-2 furrow canals are assembling at this place and time. Actin remodeling is intimately coupled to endocytosis in other cell types. Endocytic proteins control actin dynamics, and actin-binding proteins are required for endocytosis. Also, endocytic and actin-binding proteins are regulated by the same phosphoinositide pools at the PM. It follows that local endocytosis could influence local actin organization during furrow formation in fly embryos. This study shows that actin conversely provides developmental regulation of endocytic dynamics. This analysis leads to a speculation that the coupled regulation of actin and endocytosis can effectively coordinate actin/PM remodeling to drive furrow dynamics, and thus shape cells during morphogenesis (Sokac, 2008).
Jaguar (95F myosin) is required for spermatogenesis in Drosophila. Partial loss of function mutations in jaguar result in male sterility. During spermatogenesis the germ line precursor cells undergo mitosis and meiosis to form a bundle of 64 spermatids. The spermatids remain interconnected by cytoplasmic bridges until individualization. The process of individualization involves the formation of a complex of cytoskeletal proteins and membrane, the individualization complex (IC), around the spermatid nuclei. This complex traverses the length of each spermatid resolving the shared membrane into a single membrane enclosing each spermatid. Jaguar is a component of the IC whose function is essential for individualization. In wild-type testes, Jaguar localizes to the leading edge of the IC. Two independent mutations in jar reduce the amount of 95F myosin in only a subset of tissues, including the testes. This reduction of Jar causes male sterility as a result of defects in spermatid individualization. Germ line transformation with jar cDNA rescues the male sterility phenotype. IC movement is aberrant in these 95F myosin mutants, indicating a critical role for 95F myosin in IC movement. This report is the first identification of a component of the IC other than actin. It is proposed that 95F myosin is a motor that participates in membrane reorganization during individualization (Hicks, 1999).
Dynamin distribution was examined in individualizing spermatids; it localizes along the length of the actin cones in a distribution most similar to actin. As expected from the distribution of dynamin on actin cones, myosin VI concentrates at the front of dynamin-stained cones. Dynamin localization along the actin cones suggests that this region might either be an area of high endocytic membrane trafficking or a region where actin dynamics are regulated by dynamin. Furthermore, the close juxtaposition of myosin VI and dynamin suggests that they might participate in the same process during individualization (Rogat, 2002).
To gain a clearer understanding of dynamin's function in the actin cones, the distribution was examined of two proteins known to interact with dynamin in vertebrates, but which appear to function in different pathways in Drosophila. alpha-Adaptin is the alpha subunit of the AP-2 adaptor complex, which is known to bind clathrin and function in early endocytosis. alpha-Adaptin is also required for endocytosis in Drosophila. Amphiphysin, however, is not required for endocytosis in Drosophila. Amphiphysin can influence filamentous actin localization and has been implicated in membrane morphogenesis and organization in Drosophila (Rogat, 2002 and references therein).
alpha-Adaptin is neither concentrated on actin cones nor at the front of the cones, as indicated by anti-Drosophila alpha-adaptin antibodies. Instead, it localizes in a particulate fashion throughout the cystic bulge ahead of the actin cones. By contrast, Drosophila Amphiphysin antibodies localize to actin cones in a manner similar to dynamin. However, unlike dynamin, Amphiphysin also concentrates at the front of cones in a manner similar to Jaguar, Cortactin and the arp2/3 complex. Thus, Amphiphysin's distribution is intermediate between dynamin and cortactin/myosin VI. Since Amphiphysin localizes to the actin cones whereas alpha-Adaptin does not, it is concluded that dynamin on the actin cones participates in a non-endocytic function. It is hypothesized that Amphiphysin function is related to actin dynamics or organization on the basis of the Amphiphysin localization (Rogat, 2002).
De novo formation of cells in the Drosophila embryo is achieved when each nucleus is surrounded by a furrow of plasma membrane. Remodeling of the plasma membrane during cleavage furrow ingression involves the exocytic and endocytic pathways, including endocytic tubules that form at cleavage furrow tips (CFT-tubules). The tubules are marked by amphiphysin but are otherwise poorly understood. This study identified the septin family of GTPases as new tubule markers. Septins do not decorate CFT-tubules homogeneously: instead, novel septin complexes decorate different CFT-tubules or different domains of the same CFT-tubule. Using these new tubule markers, it was determined that all CFT-tubule formation requires the BAR domain of amphiphysin. In contrast, dynamin activity is preferentially required for the formation of the subset of CFT-tubules containing the septin Peanut. The absence of tubules in amphiphysin-null embryos correlates with faster cleavage furrow ingression rates. In contrast, upon inhibition of dynamin, longer tubules formed, which correlated with slower cleavage furrow ingression rates. These data suggest that regulating the recycling of membrane within the embryo is important in supporting timely furrow ingression (Su, 2013).
Cellularization in the Drosophila embryo involves de novo generation of 6000 columnar epithelial cells, which are generated by the ingression of plasma membrane furrows (cleavage furrows) that enclose each nucleus. At the tip of ingressing cleavage furrows, CFT-tubules form. This study demonstrated the existence of three populations of CFT-tubules, which can de defined by different septin family members. The different populations of CFT-tubules are differentially regulated, and their presence or absence correlates with changes in cleavage furrow ingression kinetics (Su, 2013).
Septins were identified as additional factors localizing to the CFT-tubules. Of interest, not all septins localize to the same CFT-tubules or the same domain within a single CFT-tubule. This suggests that although the CFT-tubules are formed by an endocytic pathway (Sokac, 2008), the tubules are not homogeneous. Instead, tubules can contain different domains that may have different functions. Three distinct types of tubules were identified: those that contain only amphiphysin and the septins Sep1 and Sep2, those that contain only the septins Peanut, Sep4, and Sep5, and those that possess heterogeneous subdomains each defined by a distinct composition of these various components. Of importance, localization studies suggest that distinct septin complexes localize to different structures. Because Peanut, Sep4, and Sep5 do not colocalize with Sep1 and Sep2 on CFT-tubules, it is predicted that Peanut, Sep4, and Sep5 form a novel septin complex. This new septin complex may resemble the previously isolated complex of Peanut, Sep1, and Sep2, as Sep2 is most closely related to Sep5 (72% identity) and Sep1 is most closely related to Sep4 (47% identity). It was not possible to isolate individual septin complexes by immunoprecipitation, as all septins coimmunoprecipitated. This finding is consistent with studies in mammalian cells and reflects either the heterogeneous nature of septin complexes within the entire embryo or that, in part, partial septin filaments were being immunoprecipitated. Unexpectedly, Peanut did not colocalize with Sep1 and Sep2 on CFT-tubules. This observation raises the possibility that Sep1 and Sep2 alone form a complex. Septin filaments in yeast and mammalian systems are generated from octamers containing two copies of four different septins arranged in an inverted repeat; however, this may not be true for all systems. In the case of Drosophila a hexamer of Peanut, Sep1, and Sep2 has been isolated, and in Caenorhabditis elegans there are only two septin genes (Su, 2013).
Septins have predominantly been implicated in modulating events at the plasma membrane in conjunction with the actin cytoskeleton. In mammalian cells, septins have also been linked to potential roles in membrane trafficking, especially in the exocytic pathway, possibly by regulating vesicle fusion. It seems unlikely that the septins on CFT-tubules are regulating exocytosis, as all evidence suggests that exocytosis occurs at distinct apical sites in the syncytial embryo). In contrast, one study suggests a role for septins in the endocytic pathway by regulating recruitment of the coat protein complex AP-3 to lysosomal membranes (Baust, 2008). The precise roles for septins in this process are unclear. In CFT-tubules, it is possible that septins exert an effect directly on the membrane. Septins can tubulate membranes containing phosphatidylinositol (4,5)-bisphosphate, a lipid that has a key role in cytokinesis. However, the current data demonstrate that CFT-tubule formation is dependent on amphiphysin. Septins have been proposed to stabilize membranes. Therefore septins could stabilize the CFT-tubules once formed. Indeed, reduced recruitment of septins to cleavage furrows destabilizes the entire cleavage cleavage furrow. Furthermore, embryos depleted of Peanut form unstable yolk channels at the end of cellularization, further supporting the model that septins can stabilize membrane structures to which they localize. These findings also suggest that mutations that deplete septins will not allow examination of the role of septins in CFT-tubule organization and function (Su, 2013).
This study found that CFT-tubule formation requires the BAR domain of amphiphysin. The N-BAR subfamily, to which amphiphysin belongs, can bind to membranes and promote their curvature. Amphiphysin is also involved in t-tubule formation in Drosophila indirect flight muscles and mouse heart muscle. These findings suggest a conserved role for amphiphysin in promoting tubule formation and organization (Su, 2013).
Loss of amphiphysin and the prevention of CFT-tubule formation did not inhibit furrow ingression, suggesting that amphiphysin is not required for remodeling of the membrane to drive furrow ingression. Instead, loss of amphiphysin increased the rate of furrow ingression. Because amphiphysin localizes to the tip of the furrow, it is possible that amphiphysin acts as a negative regulator of furrow ingression. Alternatively, by preventing CFT-tubule formation, amphiphysin may render more plasma membrane accessible for furrow ingression, and therefore the rate of furrow ingression increases. Consistent with this model, when CFT-tubules become longer upon disruption of dynamin, the rate of cleavage furrow ingression is reduced. One potential consequence of inhibiting endocytosis at the furrow tip would be to reduce the amount of membrane available for the expansion of the plasma membrane and the ingression of the furrow. In such a scenario membrane derived from endocytosis at the tip of the furrow would be recycled back to the plasma membrane through the exocytic pathway, thereby providing sufficient membrane for the expansion and ingression of the furrow. This reduced availability of membrane could manifest itself as a reduced rate of furrow ingression seen in shibirets embryos at the nonpermissive temperature, where CFT-tubules elongate due to a failure to pinch off. The additional membrane may be especially important for the rapid increase in furrow ingression that is seen once the furrow has ingressed ∼10 μm, a depth of ingression where CFT-tubules normally become shorter and disappear (Su, 2013).
Changes in tubule parameters correlate with changes in cleavage furrow ingression kinetics, especially in the fast phase of ingression; longer, more persistent tubules correlate with slower ingression kinetics, and the absence of tubules correlates with faster ingression kinetics. If the fast phase of cleavage furrow ingression were dependent upon new membrane being inserted into the plasma membrane, then restricting membrane insertion would suppress the fast phase. If membrane was recycled by endocytosis at the cleavage furrow tips through an endocytic compartment back to the plasma membrane, then changes in CFT-tubule parameters might be expected to affect cleavage furrow ingression kinetics (Su, 2013).
In the models outlined in this study CFT-tubules would function to buffer the amount of available membrane that is accessible for efficient cleavage furrow ingression. However, no comparable measurements have been made with respect to t-tubules in muscles. Therefore it remains unclear whether the tubules in these different systems have a common function, whether they are examples of specialized endocytosis, or whether the creation of extra membrane surface area facilitates specialized functions in these different systems (Su, 2013).
At the Drosophila larval neuromuscular junction, Amphiphysin is localized postsynaptically and Amphiphysin mutants have no major defects in neurotransmission; they are also viable, but flightless. Like mammalian amphiphysin 2 in muscles, Drosophila Amphiphysin does not bind clathrin, but can tubulate lipids and is localized on T-tubules. Amphiphysin mutants have a novel phenotype: a severely disorganized T-tubule/sarcoplasmic reticulum system. It is therefore proposed that muscle Amphiphysin is not involved in clathrin-mediated endocytosis, but in the structural organization of the membrane-bound compartments of the excitation-contraction coupling machinery of muscles (Razzaq, 2001).
The Amph gene contains 10 exons and occupies ~17.5 kb of DNA. A homozygous viable P insertion, EP(2)2175, lies 47 bp upstream of the Amphiphysin cDNA, LD19810. This insertion was mobilized to generate a number of imprecise excisions; one of these, amph26 (hereafter referred to as amphmut), had a deletion of the first exon including the beginning of the coding region, and part of the first intron of the Amphiphysin gene. A precise excision that left the Amphiphysin genomic region intact, amph+1 (hereafter referred to as amph+), was also recovered and used as a wild-type control in subsequent experiments. Western blots detected a number of related Amphiphysin proteins in amph+ flies, perhaps generated by alternative transcripts, or by physiological or artifactual protein degradation. All of these were absent in amphmut homozygotes, and it is therefore concluded that amphmut is a null allele of the Amphiphysin gene (Razzaq, 2001).
Unexpectedly, in light of the proposed central role for Amphiphysin in endocytosis, amphmut flies are viable and fertile both when homozygous, and when transheterozygous with a deficiency Df(2R)vg-C, which deletes the Amphiphysin genomic region. They also showed no obvious external defects in eye, bristle, or wing pattern that might result from the interference with endocytic mechanisms involved in wingless, EGF receptor, TGF-ß/decapentaplegic, or Notch signaling. Adult mutants are, however, flightless and generally sluggish when homozygous, or when transheterozygous with Df(2R)vg-C. EP(2)2175 homozygotes, amph+ homozygotes, and amph+/Df(2R)vg-C heterozygotes all showed no mutant phenotype (Razzaq, 2001).
In addition to a deletion of the 5' end of the Amph gene, amphmut also had a deletion of part of the first exon of the adjacent gene Sin3A, but left its coding region intact. No impaired function of Sin3A by amphmut could be detected: (1) loss of Sin3A is homozygous lethal, whereas amphmut is homozygous viable; (2) crosses between amphmut and four different Sin3A mutant alleles showed complementation for the flightless and sluggish phenotypes (Razzaq, 2001).
Mutations affecting endocytic proteins have pronounced effects on synaptic transmission in flies. For example, mutations in both lap (AP180) and stoned show reduced excitatory junction potential (EJP) amplitudes, increased variance in EJP size, and increased numbers of failures. In contrast, EJP amplitude is unaffected in amphmut larvae, and failures in synaptic transmission were never observed. High frequency stimulation results in a progressive rundown in EJPs in stoned and shibire mutants, owing to failure to replenish the vesicle pool by endocytosis. In all lines tested, there is an apparent initial rapid decrease in the EJP amplitude during high frequency stimulation, caused by a depletion of the synaptic vesicle pool during the first few stimulations, and by a failure of the muscle to return to its resting membrane potential during the interpulse period of 50 msec. However, shibirets1 (at the permissive temperature), amph+, and amphmut larvae show only a continuing slow decrease in EJP amplitude, in contrast to the much faster and more pronounced decrease observed in shibirets mutants at their restrictive temperature. Compared with amph+ and shibirets (at the permissive temperature), amphmut larvae have a larger initial drop in EJP amplitude, but the continued decline phase appears similar in all these genotypes. Although a mild defect in vesicle recycling cannot be ruled out, the subtle electrophysiological changes seen in amphmut larvae could also be explained by small changes in postsynaptic excitability (e.g., receptor or channel numbers or differences in desensitization). Nevertheless, because amphmut larvae clearly show a much milder phenotype than that of shibirets larvae at their restrictive temperature, Amphiphysin cannot be an absolute requirement for synaptic vesicle recycling at the NMJ (Razzaq, 2001).
The Amph mutants also show a small increase in amplitude of miniature excitatory junction potentials (mEJPs), but no significant effect on mEJP frequency. Given the localization of Amphiphysin at the postsynaptic NMJ of wild-type larvae, the increased mEJP amplitude in the mutant may reflect subtle changes in the postsynaptic physiology of the NMJ (Razzaq, 2001).
Amphiphysin distribution was analyzed in most detail in indirect flight muscles (IFMs), highly specialized muscles that can contract at up to 300 Hz. In IFMs, Amphiphysin is found on an extensive network of transverse and longitudinal projections that ramify around and between myofibrils. These transverse projections overlie sarcomeres periodically at positions midway between the M and Z lines, at which myosin and actin filaments, respectively, are anchored. The positions of transverse projections coincide with the locations of junctions formed by close apposition of the T-system to the junctional sarcoplasmic reticulum (jSR). These T-jSR junctions play a vital role in excitation-contraction coupling, in which depolarization of the sarcolemma activates a voltage sensor between the T-tubule and the underlying ryanodine receptors (RyRs) on the jSR surface, thus leading to calcium release from the jSR and to muscle contraction (Razzaq, 2001).
Ultrastructural studies of IFMs of the Dipteran Phormia regina have established that they possess mainly dyadic T-jSR junctions. Drosophila IFMs have dyadic junctions; these were generally localized alongside the sarcomere, midway between the M and Z lines. The jSR component of the dyad is electron-dense, and the T-tubule electron-lucent. In longitudinal sections along the peripheral surface of a myofibril, the lumina of the T-system were frequently seen to extend from dyads as transverse and longitudinal tubular processes that coursed between individual myofibrils and mitochondria. Classic triadic T-SR junctions consisting of two jSR cisternae flanking a single T-tubule are also seen occasionally. The only compartments seen to extend into longitudinal and transverse projections belonged to the T-system. In contrast, sarcoplasmic reticulum (SR) is only found as jSR at dyadic junctions. A network of free SR, which should be visible as an extensive, fenestrated tubular network extending longitudinally along myofibrils, is either very sparse or absent from these muscles. It is therefore concluded that the reticular Amphiphysin staining in IFMs seen using immunofluoresence is the extensive T-system observed at the ultrastructural level. This localization is consistent with the reported presence of vertebrate amphiphysin 2 on the surface of skeletal muscle T-tubules (Razzaq, 2001).
The flightless phenotype of Amph mutants, and localization of Amphiphysin at T-tubules, suggests a defect in muscle structure or function. Muscles of amphmut homozygotes have no detectable Amphiphysin. Despite this, mutant IFM structure is similar to that of wild type, although larval body wall muscles do display slightly looser packing of myofibrils (Razzaq, 2001).
In light of the similarity of the Dlg staining of larval body wall muscles to that of Amphiphysin, the localization of both proteins in these muscles and in IFMs was compared. There is substantial, but incomplete, overlap of the two proteins within the T-system, suggesting that Dlg is also localized on T-tubules. Double labeling with antibodies to RyR and Dlg show that the Dlg-containing T-tubular network runs between, but does not colocalize with RyR-containing jSR compartments that appear as regular puncta alongside myofibrils, approximately midway between the M and Z lines (Razzaq, 2001).
In amphmut muscles, Dlg staining shows that loss of Amphiphysin results in severe disorganization and reduction of the T-system. In amphmut IFMs, many transverse elements of the T-system are lost, and the remainder are predominantly longitudinal, and sometimes broader. Dyad junctions (visualized with antibody against RyR) are still apparent, but distributed irregularly, and occupy only 42% of the cell volume fraction they occupy in wild-type IFMs. They are often larger than in wild type, sometimes extending over half the length of a sarcomere. Only a sparse T-system is seen between expanded and mislocalized jSR compartments (Razzaq, 2001).
Electron microscopy of amphmut IFMs broadly supports the conclusions drawn from confocal analysis. Thin sections of IFMs confirm that the gross structure of mutant muscles is equivalent to wild type. Although dyads are still observed, they are no longer regularly distributed along the length of the sarcomere. The increased size of dyads seen using RyR staining is corroborated by the elongated T-jSR dyads often seen in thin sections. In agreement with the confocal observations, mutant T-tubules often have larger diameters. The reduction and mislocalization of the T/SR components suggests that amphmut flightlessness may be a consequence of a defect in excitation-contraction coupling (Razzaq, 2001).
Several lines of evidence indicate that the T-tubule defect is caused by loss of Amphiphysin: (1) the phenotype of transheterozygotes of amphmut over the deficiency Df(2R)vg-C, assessed by actin and RyR staining of IFMs, is identical to that of amphmut homozygotes; (2) RyR distribution in amphmut/Sin3A- IFMs is indistinguishable from wild type for all four Sin3A alleles tested, showing that the phenotypes observed in muscles are not caused by impairment of Sin3A function; (3) substantial rescue of the amphmut T-tubule phenotype was obtained by driving expression of the Amphiphysin cDNA, LD19810, under control of a heat-shock-inducible GAL4 gene. Here amphmut individuals that can express this cDNA are referred to as amphrescue individuals. In contrast to amphmut flies, which are flightless and possess a disrupted T-SR system, almost all amphrescue individuals can fly, and they exhibit varying degrees of recovery of the T-system. All show Amphiphysin staining in a reticular pattern around individual myofibrils, which colocalize mostly with that of Dlg. Although some flies have almost complete rescue of the T-SR defects, many individuals had only partial rescue, in which the degree of Amphiphysin branching and its ordered spacing midway between the M and Z lines is highly variable. Interestingly, even when rescue of T-system morphology is only partial, most individuals can still fly, suggesting that the precisely ordered spacing of the T-system is not absolutely essential for flight (Razzaq, 2001).
Comparison of the reticular localization of Amphiphysin seen using confocal microscopy, with electron microscopy of the T-SR system, suggests that Drosophila Amphiphysin is localized on muscle T-tubules. Since vertebrate amphiphysin 2 is also found on muscle T-tubules, this suggests a conserved function for Amphiphysin on these invaginations of the plasma membrane. Indeed, in flies that lack Amphiphysin, T-SR junctions and T-tubule projections are greatly reduced in number and generally mislocalized, and remaining T-SR junctions are often larger than normal. Therefore, Amphiphysin is essential for organization and normal morphology of the T-system (Razzaq, 2001).
A number of ways in which Amphiphysin might achieve this can be envisioned. (1) The N-terminal portion of Amphiphysin can tubulate lipids, and such an activity might contribute to T-tubule formation or stabilization. Although some T-tubules can still form even in the absence of Amphiphysin, the reduction in transverse elements of the T-system means that Amphiphysin might play a role in tubule branching, either alone or in cooperation with other proteins. (2) Amphiphysin might have a cytoskeletal role in anchoring transverse T-tubules close to the myofibril, approximately midway between the Z-line and the M-line. The yeast Amphiphysin homologs interact with a number of components of the actin cytoskeleton (Bon, 2000), and similar interactions of Amphiphysin could account for both the regular positioning of the transverse elements of the T-system in wild-type flies and the slightly looser arrangement of myofibrils in larval body wall muscles in the mutant. (3) The altered organization of T-tubules and T-SR junctions suggests that Amphiphysin plays a role in organizing the protein components of T-tubules. Vertebrate T-tubules contain several cell adhesion and associated cytoskeletal proteins and have subdomains that are defined by the presence of different membrane proteins. Surprisingly, Dlg protein is also detected on T-tubules in domains that partially overlap with those of Amphiphysin expression. Dlg and its vertebrate homologs, such as PSD-95, play an important role in localizing channels and cell adhesion proteins at synapses, and it could play an analogous role in T-tubules. These models of Amphiphysin function are not mutually exclusive. Testing them will involve addressing questions such as the interactions that localize T-tubules to the middle of the M-Z interval, the binding partners of the SH3 domain of Drosophila Amphiphysin if they do not include dynamin, and the contribution of other proteins to morphology of the T-system. One protein of potential relevance might have been caveolin, since the initial stages of vertebrate T-tubule formation may be analogous to caveola formation, and caveolin 3 is found on vertebrate T-tubules. However, no caveolin homolog has been detected in the Drosophila genome (Razzaq, 2001).
A predicted consequence of disrupted T-system would be altered spatial and temporal dynamics of calcium flux in the cytoplasm. Elevations in calcium concentration following muscle depolarization are first seen at discrete regions before the calcium gradients dissipate, and alterations in calcium dynamics might reduce the extent, speed, and synchronization of muscle contraction. Although this would not entirely block muscle contraction, it would probably interfere with the rapid coordinated cycles of contraction and relaxation of IFMs (Razzaq, 2001).
Given the localization of vertebrate amphiphysin 2 on T-tubules (Butler, 1997), mutations in human amphiphysin 2 might be implicated in myopathies that are associated with defects in excitation-contraction coupling. Experimentally induced congestive heart failure can lead to a disorganization and reduction in number of T-tubules in cardiac myocytes. Furthermore, the human conditions of malignant hyperthermia and central core disease have been linked to mutations affecting other components of excitation-contraction coupling, RyR or the L-type calcium channel. It is possible that myopathies that map to other loci might involve Amphiphysin. The phenotypes of mice deficient in amphiphysin 2 should help clarify this question (Razzaq, 2001).
Amp null mutants were created by imprecise excision of the EP(2)2175 transposon. Homozygous amph26 and amph54 flies were tested for protein expression using western blots and immunohistochemical staining of embryos, third instar neuromuscular junctions, and developing photoreceptor cells. Antibodies were used that recognized the Amph N terminus, as well as antibodies recognizing the Amph C terminus, but no protein expression could be detected. In addition, the lesions of amph26 and amph54 were mapped; both are deletions that remove the entire first exon and extend into the first intron. There is one predicted transcription start site in the 13 kb second intron (and no open reading frames), but it is thought not to be used because: (1) homozygous mutant embryos show no Amph protein; (2) using a primer representing the second putative start site, no RT-PCR products could be detected; and (3) all sequenced cDNAs contain the first exon and no alternative 5' end was identified. As such, it is concluded that both amph26 and amph54 are null alleles that produce no Amph protein product and do not affect the neighboring Sin3A gene (Zelhof, 2001).
Given the wide tissue distribution of Amp, and the existence of only one Drosophila amp gene, Amph mutants were expected to have severe pleiotropic defects and in particular endocytic defects. Surprisingly, Amph null mutant animals survive to adulthood, although larvae move sluggishly and adults do not fly (Zelhof, 2001).
Several approaches were tried to detect an endocytic defect in Amph mutants. In vertebrates, Amph is implicated in endocytosis via its interaction with Dynamin, but no interaction could be detected between the SH3 domain of Amph and Drosophila Dynamin, nor was protein colocalization with Dynamin observed in the embryo, neuromuscular junction or at any location that shows Amph expression. In addition, no genetic interactions could be detected between Amph and the endocytic mutant shibire (encoding Dynamin), although other endocytic mutants such as alpha-Adaptin show sensitive genetic interactions with shibire. Last, Amph mutants have normal presynaptic physiological properties, eliminating a role for Amph in synaptic vesicle recycling (Zelhof, 2001).
Given that Amph is localized to the NMJ, assays were performed for structural and functional defects at the neuromuscular junction (NMJ). Amph mutants show no defects in the localization of presynaptic marker proteins (e.g. Dynamin, Cysteine String Protein and HRP) and there are no defects in bouton structure or number. By contrast, several postsynaptic proteins show abnormal localization in Amph mutants. In wild-type larvae, type I glutamatergic synapses show postsynaptic enrichment of Dlg. In addition, two other proteins, Scribble (Scrib) and Lethal giant larvae (Lgl), are colocalized with Dlg at the NMJ. Scrib, Lgl and Dlg are all tumor suppressor proteins that are colocalized in epithelia. In Amph mutants, there is a clear increase in Dlg and Scrib protein delocalized throughout the muscle but no reduction at the synapse could be detected. Amph mutants show a more severe Lgl phenotype: no Lgl protein can be detected at the synapse, although the localization of Lgl to the muscle M band is unaffected (Zelhof, 2001).
Dlg is considered to be a scaffold molecule for organizing the postsynaptic density, whereas the role of Lgl and Scrib at the synapse is not known. Changes in the localization of these proteins could potentially alter postsynaptic responsiveness to transmitter release. To test whether Amph mutations alter synaptic physiology, the spontaneous release of transmitter was measured by detecting the miniature excitatory junctional potentials (mEJPs). The amplitude of mEJPs (quantal size) is considered to be a measurement of the sensitivity of the muscle glutamate receptors to the spontaneous release of transmitter from single synaptic vesicles. A small but statistically significant increase in mEJP amplitude was observed in Amph26 mutants. Although quantal size in Amph mutants is increased, elicited transmitter release measured by the amplitude of EJPs was found to be similar between amph26 mutant larvae and control larvae amph+1. The increase in quantal size suggests that the amount of transmitter per vesicle (i.e. quantum) is increased and/or that the postsynaptic receptors have become more sensitive to glutamate in the mutant. Since Amph is primarily a postsynaptic protein, it was reasoned that it is likely to play a direct role in regulation of the density or the ratio of glutamate receptor subtypes. This hypothesis was tested by overexpressing GluRIIB levels in the muscle and examining whether it could rescue the mEJP phenotype. Increased GluRIIB levels lead to a decrease in the amplitude of mEJPs in both controls and Amph mutants, but Amph mutants still have a larger mEJP than controls. Consistent with these physiological data, no significant change in the density of GlurRIIB receptors in Amph mutants was detected (Zelhof, 2001).
Unlike its vertebrate counterparts, Drosophila Amphiphysin is enriched postsynaptically at the larval neuromuscular junction. To determine the role of Drosophila Amphiphysin, null mutants were generated which are viable but give rise to larvae and adults with pronounced locomotory defects. Surprisingly, the locomotory defects cannot be accounted for by alterations in the morphology or physiology of the neuromuscular junction. Moreover, using stimulus protocols designed to test endocytosis under moderate and extreme vesicle cycling, no defect in the neuromuscular junction of the Amphiphysin mutant could be detected. Taken together, these findings suggest that Amphiphysin is not required for viability, nor is it absolutely required for clathrin-mediated endocytosis. However, Drosophila Amphiphysin function is required in both larvae and adults for normal locomotion (Leventis, 2001).
To characterize the function of Amph, a series of Amph mutants was created by imprecise excision of a transposable P-element, EP(2)2175, inserted 39 bp 5' to the Amph transcription start site. In total, four independent deletions were generated which removed various portions of the Amph gene and failed to complement each other. All of the Amph mutants are homozygous viable, although the larvae and adults appeared sluggish. Using a combination of Southern blot and PCR analyses, it was found that three lines had deletions of the first exon (amph2J6, amph3B1, amph6A2), whereas amph5E3 represented a deletion of the entire gene. Importantly, none of the deletions disrupted either of the flanking genes, Sin3A and Galpha49B, indicating that these mutations only affect Amph (Leventis, 2001).
To determine if any of the mutants produced Amph protein, Western blot analysis was performed on lysates from control and mutant third instar larvae. Not surprisingly, the genetic null mutant amph5E3 was also protein null. Additionally, the removal of exon 1 also led to a complete elimination of Amph protein expression. In contrast, Amph protein was detected from lysates generated from wildtype flies, the original EP insertion line and two precise excision lines (amph3B2, amph3E9). Consistent with these results, Amph is present at the NMJ of wildtype larvae and controls but not in mutants (Leventis, 2001).
All of the Amph mutants identified were viable but gave rise to larvae and adults that appeared sluggish and slower in their movements. For example, while adult flies are capable of hopping they are essentially flightless. To further quantify any potential locomotory defects, the total distance traveled by third instar larvae over 30 s was measured. The genetic null amph5E3 mutant larvae shows about a 70% reduction in movement (Leventis, 2001).
Locomotory defects such observed in Amph mutants could reflect defects in synaptic morphology or physiology. To determine if Amph mutants exhibit any alterations in NMJ morphology, anti-horseradish peroxidase (HRP)-FITC stained NMJs from mutant and control third instar larvae were compared and no difference was found in either the number of boutons or the amount of branching. Therefore, it is concluded that Amph is not required for the normal morphogenesis of the NMJ. Since no phenotypic difference was detected in any of the mutants and they were all protein null, all subsequent experiments were performed with a single Amph mutant line, amph5E3 (Leventis, 2001).
To determine whether the locomotory defects in Amph mutants were due to impaired synaptic physiology, synaptic currents were compared from neuromuscular junctions of third instar larvae. Because Amph is predominantly localized postsynaptically, spontaneously occurring quantal events from both the precise excision line and the mutant amph5E3 were analyzed to determine if there were any effects on glutamate receptors. While there were significant differences within the genotypes, there was no significant difference in mEJC amplitude between the mutant and controls. Thus it is concluded that the Amph mutant does not alter the mEJC amplitude (Leventis, 2001).
Although Amph is predominantly localized postsynaptically at the NMJ, it remains possible that low levels are present presynaptically but undetectable using immunofluorescence microscopy. These low levels could then be required for SV endocytosis and contribute to the locomotory behavior observed in both larvae and adults. If the mutations resulted in chronic suppression of vesicle endocytosis, it might be expected there would be a reduction in basal transmitter release. However, none was found (Leventis, 2001).
To determine if the Amph mutant affected SV endocytosis in a more dynamic manner, the synapses were challenged with long trains of stimuli. Again no differences were found. Also, recovery from synaptic fatigue was not altered in the Amph mutant. In these and other experiments, no evidence was found for either a chronic or acute impairment of SV endocytosis at the neuromuscular junction in null alleles for Drosophila Amph. Endogenous motor output from the CNS was examined by measuring the frequency and duration of synaptic activity from intact CNS-NMJ preparations. These experiments did not reveal a major difference in CNS-generated motor activity when Amph mutants were compared to controls (Leventis, 2001).
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date revised: 22 December 2017
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