Calmodulin Promotes N-BAR Domain-Mediated Membrane Constriction and Endocytosis

Amphiphysin AoRvs167-Mediated Membrane Curvature Facilitates Trap Formation, Endocytosis, and Stress Resistance in Arthrobotrysoligospora

Bin1/Amphiphysin/Rvs (BAR) domain-containing proteins mediate fundamental cellular processes, including membrane remodeling and endocytosis. Nematode-trapping (NT) fungi can differentiate to form trapping structures through highly reorganized cell membranes and walls. In this study, we identified the NT fungus Arthrobotrys oligospora ortholog of yeast Rvs167 and documented its involvement in membrane bending and endocytosis. We further confirmed that the deletion of AoRvs167 makes the fungus more hypersensitive to osmotic salt (Nacl), higher temperatures (28 to 30 °C), and the cell wall perturbation agent Congo red. In addition, the disruption of AoRvs167 reduced the trap formation capacity. Hence, AoRvs167 may regulate fungal pathogenicity through the integrity of plasma membranes and cell walls.

Ca2+ Regulates Dimerization of the BAR Domain Protein PICK1 and Consequent Membrane Curvature

Bin-Amphiphysin-Rvs (BAR) domain proteins are critical regulators of membrane geometry. They induce and stabilize membrane curvature for processes, such as clathrin-coated pit formation and endosomal membrane tubulation. BAR domains form their characteristic crescent-shaped structure in the dimeric form, indicating that the formation of the dimer is critical to their function of inducing membrane curvature and suggesting that a dynamic monomer–dimer equilibrium regulated by cellular signaling would be a powerful mechanism for controlling BAR domain protein function. However, to the best of our knowledge, cellular mechanisms for regulating BAR domain dimerization remain unexplored. PICK1 is a Ca²⁺-binding BAR domain protein involved in the endocytosis and endosomal recycling of neuronal AMPA receptors and other transmembrane proteins. In this study, we demonstrated that PICK1 dimerization is regulated by a direct effect of Ca²⁺ ions via acidic regions in the BAR domain and at the N-terminus. While the cellular membrane tubulating activity of PICK1 is absent under basal conditions, Ca²⁺ influx causes the generation of membrane tubules that originate from the cell surface. Furthermore, in neurons, PICK1 dimerization increases transiently following NMDA receptor stimulation. We believe that this novel mechanism for regulating BAR domain dimerization and function represents a significant conceptual advance in our knowledge about the regulation of cellular membrane curvature.

Endophilin A1 drives acute structural plasticity of dendritic spines in response to Ca2+/calmodulin

Induction of long-term potentiation (LTP) in excitatory neurons triggers a large transient increase in the volume of dendritic spines followed by decays to sustained size expansion, a process termed structural LTP (sLTP) that contributes to the cellular basis of learning and memory. Although mechanisms regulating the early and sustained phases of sLTP have been studied intensively, how the acute spine enlargement immediately after LTP stimulation is achieved remains elusive. Here, we report that endophilin A1 orchestrates membrane dynamics with actin polymerization to initiate spine enlargement in NMDAR-mediated LTP. Upon LTP induction, Ca²⁺/calmodulin enhances binding of endophilin A1 to both membrane and p140Cap, a cytoskeletal regulator. Consequently, endophilin A1 rapidly localizes to the plasma membrane and recruits p140Cap to promote local actin polymerization, leading to spine head expansion. Moreover, its molecular functions in activity-induced rapid spine growth are required for LTP and long-term memory. Thus, endophilin A1 serves as a calmodulin effector to drive acute structural plasticity necessary for learning and memory.

Interplay between membrane curvature and the actin cytoskeleton

An intimate interplay of the plasma membrane with curvature-sensing and curvature-inducing proteins would allow for defining specific sites or nanodomains of action at the plasma membrane, for example, for protrusion, invagination, and polarization. In addition, such connections are predestined to ensure spatial and temporal order and sequences. The combined forces of membrane shapers and the cortical actin cytoskeleton might hereby in particular be required to overcome the strong resistance against membrane rearrangements in case of high plasma membrane tension or cellular turgor. Interestingly, also the opposite might be necessary, the inhibition of both membrane shapers and cytoskeletal reinforcement structures to relieve membrane tension to protect cells from membrane damage and rupturing during mechanical stress. In this review article, we discuss recent conceptual advances enlightening the interplay of plasma membrane curvature and the cortical actin cytoskeleton during endocytosis, modulations of membrane tensions, and the shaping of entire cells.

Yeast as a Model to Understand Actin-Mediated Cellular Functions in Mammals-Illustrated with Four Actin Cytoskeleton Proteins

The budding yeast Saccharomyces cerevisiae has an actin cytoskeleton that comprises a set of protein components analogous to those found in the actin cytoskeletons of higher eukaryotes. Furthermore, the actin cytoskeletons of S. cerevisiae and of higher eukaryotes have some similar physiological roles. The genetic tractability of budding yeast and the availability of a stable haploid cell type facilitates the application of molecular genetic approaches to assign functions to the various actin cytoskeleton components. This has provided information that is in general complementary to that provided by studies of the equivalent proteins of higher eukaryotes and hence has enabled a more complete view of the role of these proteins. Several human functional homologues of yeast actin effectors are implicated in diseases. A better understanding of the molecular mechanisms underpinning the functions of these proteins is critical to develop improved therapeutic strategies. In this article we chose as examples four evolutionarily conserved proteins that associate with the actin cytoskeleton: (1) yeast Hof1p/mammalian PSTPIP1, (2) yeast Rvs167p/mammalian BIN1, (3) yeast eEF1A/eEF1A1 and eEF1A2 and (4) yeast Yih1p/mammalian IMPACT. We compare the knowledge on the functions of these actin cytoskeleton-associated proteins that has arisen from studies of their homologues in yeast with information that has been obtained from in vivo studies using live animals or in vitro studies using cultured animal cell lines.

Yeast-model-based study identified myosin- and calcium-dependent calmodulin signalling as a potential target for drug intervention in chorea-acanthocytosis

Chorea-acanthocytosis (ChAc) is a rare neurodegenerative disease associated with mutations in the human VPS13A gene. The mechanism of ChAc pathogenesis is unclear. A simple yeast model was used to investigate the function of the single yeast VSP13 orthologue, Vps13. Vps13, like human VPS13A, is involved in vesicular protein transport, actin cytoskeleton organisation and phospholipid metabolism. A newly identified phenotype of the vps13Δ mutant, sodium dodecyl sulphate (SDS) hypersensitivity, was used to screen a yeast genomic library for multicopy suppressors. A fragment of the MYO3 gene, encoding Myo3-N (the N-terminal part of myosin, a protein involved in the actin cytoskeleton and in endocytosis), was isolated. Myo3-N protein contains a motor head domain and a linker. The linker contains IQ motifs that mediate the binding of calmodulin, a negative regulator of myosin function. Amino acid substitutions that disrupt the interaction of Myo3-N with calmodulin resulted in the loss of vps13Δ suppression. Production of Myo3-N downregulated the activity of calcineurin, a protein phosphatase regulated by calmodulin, and alleviated some defects in early endocytosis events. Importantly, ethylene glycol tetraacetic acid (EGTA), which sequesters calcium and thus downregulates calmodulin and calcineurin, was a potent suppressor of vps13Δ. We propose that Myo3-N acts by sequestering calmodulin, downregulating calcineurin and increasing activity of Myo3, which is involved in endocytosis and, together with Osh2/3 proteins, functions in endoplasmic reticulum-plasma membrane contact sites. These results show that defects associated with vps13Δ could be overcome, and point to a functional connection between Vps13 and calcium signalling as a possible target for chemical intervention in ChAc. Yeast ChAc models may uncover the underlying pathological mechanisms, and may also serve as a platform for drug testing.

This article has an associated First Person interview with the first author of the paper.

Summary: Using the vps13Δ strain, a yeast model of the neurodegenerative disorder chorea-acanthocytosis, we found that its defects can be overcome by reduction of calcineurin activity and/or type-I-myosin activation.

AtCaM4 interacts with a Sec14-like protein, PATL1, to regulate freezing tolerance in Arabidopsis in a CBF-independent manner

Calmodulin (CaM), a multifunctional Ca2+ sensor, mediates multiple reactions involved in regulation of plant growth and responses to environmental stress. In this study, we found that AtCaM4 plays a negative role in freezing tolerance in Arabidopsis. The deletion of AtCaM4 resulted in enhanced freezing tolerance in cam4 mutant plants. Although AtCaM4 and AtCaM1 were cold-induced isoforms, cam4/cam1Ri double-mutant and cam4 single-mutant plants exhibited similar improvements in freezing tolerance, indicating that AtCaM4 plays major role. Furthermore, we found that AtCaM4 may influence freezing tolerance in a C-repeat binding factor (CBF)-independent manner as cold-induced expression patterns of CBFs did not change in the cam4/cam1Ri mutant. In addition, among the cold-responsive (COR) genes detected, KIN1, COR15b, and COR8.6 exhibited clearly enhanced expression over the long term in cam4/cam1Ri mutant plants exposed to cold stress. Using immunoprecipitation and mass spectrometry, we identified multiple candidate AtCaM4-interacting proteins. Co-immunoprecipitation assays confirmed the interaction of AtCaM4 with PATL1 in vivo and a phenotype analysis showed that patl1 mutant plants exhibited enhanced freezing tolerance. Thus, we conclude that AtCaM4 negatively regulates freezing tolerance in Arabidopsis by interacting with the novel CaM-binding protein PATL1.

Balancing presynaptic release and endocytic membrane retrieval at hair cell ribbon synapses

The timely and reliable processing of auditory and vestibular information within the inner ear requires highly sophisticated sensory transduction pathways. On a cellular level, these demands are met by hair cells, which respond to sound waves - or alterations in body positioning - by releasing glutamate-filled synaptic vesicles (SVs) from their presynaptic active zones with unprecedented speed and exquisite temporal fidelity, thereby initiating the auditory and vestibular pathways. In order to achieve this, hair cells have developed anatomical and molecular specializations, such as the characteristic and name-giving 'synaptic ribbons' - presynaptically anchored dense bodies that tether SVs prior to release - as well as other unique or unconventional synaptic proteins. The tightly orchestrated interplay between these molecular components enables not only ultrafast exocytosis, but similarly rapid and efficient compensatory endocytosis. So far, the knowledge of how endocytosis operates at hair cell ribbon synapses is limited. In this Review, we summarize recent advances in our understanding of the SV cycle and molecular anatomy of hair cell ribbon synapses, with a focus on cochlear inner hair cells.

Endophilin A1 Promotes Actin Polymerization in Dendritic Spines Required for Synaptic Potentiation

Endophilin A1 is a member of the N-BAR domain-containing endophilin A protein family that is involved in membrane dynamics and trafficking. At the presynaptic terminal, endophilin As participate in synaptic vesicle recycling and autophagosome formation. By gene knockout studies, here we report that postsynaptic endophilin A1 functions in synaptic plasticity. Ablation of endophilin A1 in the hippocampal CA1 region of mature mouse brain impairs long-term spatial and contextual fear memory. Its loss in CA1 neurons postsynaptic of the Schaffer collateral pathway causes impairment in their AMPA-type glutamate receptor-mediated synaptic transmission and long-term potentiation. In KO neurons, defects in the structural and functional plasticity of dendritic spines can be rescued by overexpression of endophilin A1 but not A2 or A3. Further, endophilin A1 promotes actin polymerization in dendritic spines during synaptic potentiation. These findings reveal a physiological role of endophilin A1 distinct from that of other endophilin As at the postsynaptic site.

Heterologous mammalian Akt disrupts plasma membrane homeostasis by taking over TORC2 signaling in Saccharomyces cerevisiae

The Akt protein kinase is the main transducer of phosphatidylinositol-3,4,5-trisphosphate (PtdIns3,4,5P₃) signaling in higher eukaryotes, controlling cell growth, motility, proliferation and survival. By co-expression of mammalian class I phosphatidylinositol 3-kinase (PI3K) and Akt in the Saccharomyces cerevisiae heterologous model, we previously described an inhibitory effect on yeast growth that relied on Akt kinase activity. Here we report that PI3K-Akt expression in yeast triggers the formation of large plasma membrane (PM) invaginations that were marked by actin patches, enriched in PtdIns4,5P₂ and associated to abnormal intracellular cell wall deposits. These effects of Akt were mimicked by overproduction of the PtdIns4,5P₂ effector Slm1, an adaptor of the Ypk1 and Ypk2 kinases in the TORC2 pathway. Although Slm1 was phosphorylated in vivo by Akt, TORC2-dependent Ypk1 activation did not occur. However, PI3K-activated Akt suppressed the lethality derived from inactivation of either TORC2 or Ypk protein kinases. Thus, heterologous co-expression of PI3K and Akt in yeast short-circuits PtdIns4,5P₂- and TORC2-signaling at the level of the Slm-Ypk complex, overriding some of its functions. Our results underscore the importance of phosphoinositide-dependent kinases as key actors in the homeostasis and dynamics of the PM.

Sensing Exocytosis and Triggering Endocytosis at Synapses: Synaptic Vesicle Exocytosis-Endocytosis Coupling

The intact synaptic structure is critical for information processing in neural circuits. During synaptic transmission, rapid vesicle exocytosis increases the size of never terminals and endocytosis counteracts the increase. Accumulating evidence suggests that SV exocytosis and endocytosis are tightly connected in time and space during SV recycling, and this process is essential for synaptic function and structural stability. Research in the past has illustrated the molecular details of synaptic vesicle (SV) exocytosis and endocytosis; however, the mechanisms that timely connect these two fundamental events are poorly understood at central synapses. Here we discuss recent progress in SV recycling and summarize several emerging mechanisms by which synapses can “sense” the occurrence of exocytosis and timely initiate compensatory endocytosis. They include Ca²⁺ sensing, SV proteins sensing, and local membrane stress sensing. In addition, the spatial organization of endocytic zones adjacent to active zones provides a structural basis for efficient coupling between SV exocytosis and endocytosis. Through linking different endocytosis pathways with SV fusion, these mechanisms ensure necessary plasticity and robustness of nerve terminals to meet diverse physiological needs.

The anti-tumor drug 2-hydroxyoleic acid (Minerval) stimulates signaling and retrograde transport

2-hydroxyoleic acid (OHOA, Minerval^®) is an example of a substance used for membrane lipid therapy, where the cellular membranes rather than specific proteins constitute the therapeutical target. OHOA is thought to mediate its anti-tumor effect by affecting the biophysical properties of membranes, which leads to altered recruitment and activation of amphitropic proteins, altered cellular signaling, and eventual cell death. Little is known about the initial signaling events upon treatment with OHOA, and whether the altered membrane properties would have any impact on the dynamic intracellular transport system. In the present study we demonstrate that treatment with OHOA led to a rapid release of intracellular calcium and activation of multiple signaling pathways in HeLa cells, including the PI3K-AKT1-MTOR pathway and several MAP kinases, in a process independent of the EGFR. By lipidomics we confirmed that OHOA was incorporated into several lipid classes. Concomitantly, OHOA potently increased retrograde transport of the plant toxin ricin from endosomes to the Golgi and further to the endoplasmic reticulum. The OHOA-stimulated ricin transport seemed to require several amphitropic proteins, including Src, phospholipase C, protein kinase C, and also Ca²⁺/calmodulin. Interestingly, OHOA induced a slight increase in endosomal localization of the retromer component VPS35. Thus, our data show that addition of a lipid known to alter membrane properties not only affects signaling, but also intracellular transport.

Selective regulation of clathrin-mediated epidermal growth factor receptor signaling and endocytosis by phospholipase C and calcium

Clathrin-mediated endocytosis is a major regulator of cell-surface protein internalization. Clathrin and other proteins assemble into small invaginating structures at the plasma membrane termed clathrin-coated pits (CCPs) that mediate vesicle formation. In addition, epidermal growth factor receptor (EGFR) signaling is regulated by its accumulation within CCPs. Given the diversity of proteins regulated by clathrin-mediated endocytosis, how this process may distinctly regulate specific receptors is a key question. We examined the selective regulation of clathrin-dependent EGFR signaling and endocytosis. We find that perturbations of phospholipase Cγ1 (PLCγ1), Ca²⁺, or protein kinase C (PKC) impair clathrin-mediated endocytosis of EGFR, the formation of CCPs harboring EGFR, and EGFR signaling. Each of these manipulations was without effect on the clathrin-mediated endocytosis of transferrin receptor (TfR). EGFR and TfR were recruited to largely distinct clathrin structures. In addition to control of initiation and assembly of CCPs, EGF stimulation also elicited a Ca²⁺- and PKC-dependent reduction in synaptojanin1 recruitment to clathrin structures, indicating broad control of CCP assembly by Ca²⁺ signals. Hence EGFR elicits PLCγ1-calcium signals to facilitate formation of a subset of CCPs, thus modulating its own signaling and endocytosis. This provides evidence for the versatility of CCPs to control diverse cellular processes.

The SAGA complex, together with transcription factors and the endocytic protein Rvs167p, coordinates the reprofiling of gene expression in response to changes in sterol composition in Saccharomyces cerevisiae

The SAGA complex, together with transcription factors and Rvs167p, coordinates sterol-dependent transcription changes. In ergosterol mutants the SAGA complex increases its occupancy on ergosterol biosynthesis and anaerobic gene promoters, recruits the SWI/SNF complex, and binds to transcription factors and Rvs167p. Genes encoding stress proteins and basic amino acid synthesis are also affected even though promoter occupancy is not changed.

Changes in cellular sterol species and concentrations can have profound effects on the transcriptional profile. In yeast, mutants defective in sterol biosynthesis show a wide range of changes in transcription, including a coinduction of anaerobic genes and ergosterol biosynthesis genes, biosynthesis of basic amino acids, and several stress genes. However the mechanisms underlying these changes are unknown. We identified mutations in the SAGA complex, a coactivator of transcription, which abrogate the ability to carry out most of these sterol-dependent transcriptional changes. In the erg3 mutant, the SAGA complex increases its occupancy time on many of the induced ergosterol and anaerobic gene promoters, increases its association with several relevant transcription factors and the SWI/SNF chromatin remodeling complex, and surprisingly, associates with an endocytic protein, Rvs167p, suggesting a moonlighting function for this protein in the sterol-regulated induction of the heat shock protein, HSP42 and HSP102, mRNAs.

Molecular Mechanisms for the Coupling of Endocytosis to Exocytosis in Neurons

Neuronal communication and brain function mainly depend on the fundamental biological events of neurotransmission, including the exocytosis of presynaptic vesicles (SVs) for neurotransmitter release and the subsequent endocytosis for SV retrieval. Neurotransmitters are released through the Ca²⁺- and SNARE-dependent fusion of SVs with the presynaptic plasma membrane. Following exocytosis, endocytosis occurs immediately to retrieve SV membrane and fusion machinery for local recycling and thus maintain the homeostasis of synaptic structure and sustained neurotransmission. Apart from the general endocytic machinery, recent studies have also revealed the involvement of SNARE proteins (synaptobrevin, SNAP25 and syntaxin), synaptophysin, Ca²⁺/calmodulin, and members of the synaptotagmin protein family (Syt1, Syt4, Syt7 and Syt11) in the balance and tight coupling of exo-endocytosis in neurons. Here, we provide an overview of recent progress in understanding how these neuron-specific adaptors coordinate to ensure precise and efficient endocytosis during neurotransmission.