PACSIN, a brain protein that is upregulated upon differentiation into neuronal cells

Inositol hexakisphosphate primes syndapin I/PACSIN 1 activation in endocytosis

Endocytosis is controlled by a well-orchestrated molecular machinery, where the individual players as well as their precise interactions are not fully understood. We now show that syndapin I/PACSIN 1 is expressed in pancreatic β cells and that its knockdown abrogates β cell endocytosis leading to disturbed plasma membrane protein homeostasis, as exemplified by an elevated density of L-type Ca²⁺ channels. Intriguingly, inositol hexakisphosphate (InsP₆) activates casein kinase 2 (CK2) that phosphorylates syndapin I/PACSIN 1, thereby promoting interactions between syndapin I/PACSIN 1 and neural Wiskott–Aldrich syndrome protein (N-WASP) and driving β cell endocytosis. Dominant-negative interference with endogenous syndapin I/PACSIN 1 protein complexes, by overexpression of the syndapin I/PACSIN 1 SH3 domain, decreases InsP₆-stimulated endocytosis. InsP₆ thus promotes syndapin I/PACSIN 1 priming by CK2-dependent phosphorylation, which endows the syndapin I/PACSIN 1 SH3 domain with the capability to interact with the endocytic machinery and thereby initiate endocytosis, as exemplified in β cells.

PACSIN proteins in vivo: Roles in development and physiology

Protein kinase C and casein kinase substrate in neurons (PACSINs), or syndapins (synaptic dynamin‐associated proteins), are a family of proteins involved in the regulation of cell cytoskeleton, intracellular trafficking and signalling. Over the last twenty years, PACSINs have been mostly studied in the in vitro and ex vivo settings, and only in the last decade reports on their function in vivo have emerged. We first summarize the identification, structure and cellular functions of PACSINs, and then focus on the relevance of PACSINs in vivo. During development in various model organisms, PACSINs participate in diverse processes, such as neural crest cell development, gastrulation, laterality development and neuromuscular junction formation. In mouse, PACSIN2 regulates angiogenesis during retinal development and in human, PACSIN2 associates with monosomy and embryonic implantation. In adulthood, PACSIN1 has been extensively studied in the brain and shown to regulate neuromorphogenesis, receptor trafficking and synaptic plasticity. Several genetic studies suggest a role for PACSIN1 in the development of schizophrenia, which is also supported by the phenotype of mice depleted of PACSIN1. PACSIN2 plays an essential role in the maintenance of intestinal homeostasis and participates in kidney repair processes after injury. PACSIN3 is abundant in muscle tissue and necessary for caveolar biogenesis to create membrane reservoirs, thus controlling muscle function, and has been linked to certain genetic muscular disorders. The above examples illustrate the importance of PACSINs in diverse physiological or tissue repair processes in various organs, and associations to diseases when their functions are disturbed.

Tau Protein Interaction Partners and Their Roles in Alzheimer's Disease and Other Tauopathies

Tau protein plays a critical role in the assembly, stabilization, and modulation of microtubules, which are important for the normal function of neurons and the brain. In diseased conditions, several pathological modifications of tau protein manifest. These changes lead to tau protein aggregation and the formation of paired helical filaments (PHF) and neurofibrillary tangles (NFT), which are common hallmarks of Alzheimer's disease and other tauopathies. The accumulation of PHFs and NFTs results in impairment of physiological functions, apoptosis, and neuronal loss, which is reflected as cognitive impairment, and in the late stages of the disease, leads to death. The causes of this pathological transformation of tau protein haven't been fully understood yet. In both physiological and pathological conditions, tau interacts with several proteins which maintain their proper function or can participate in their pathological modifications. Interaction partners of tau protein and associated molecular pathways can either initiate and drive the tau pathology or can act neuroprotective, by reducing pathological tau proteins or inflammation. In this review, we focus on the tau as a multifunctional protein and its known interacting partners active in regulations of different processes and the roles of these proteins in Alzheimer's disease and tauopathies.

Application of a C. trachomatis expression system to identify C. pneumoniae proteins translocated into host cells

Chlamydia pneumoniae is a Gram-negative, obligate intracellular pathogen that causes community-acquired respiratory infections. C. pneumoniae uses a cell contact-dependent type-III secretion (T3S) system to translocate pathogen effector proteins that manipulate host cellular functions. While several C. pneumoniae T3S effectors have been proposed, few have been experimentally confirmed in Chlamydia In this study, we expressed 382 C. pneumoniae genes in C. trachomatis as fusion proteins to an epitope tag derived from glycogen synthase kinase 3β (GSK3β) which is the target of phosphorylation by mammalian kinases. Based on the detection of the tagged C. pneumoniae protein with anti-phospho GSK3β antibodies, we identified 49 novel C. pneumoniae-specific proteins that are translocated by C. trachomatis into the host cytoplasm and thus likely play a role as modifiers of host cellular functions. In this manner, we identified and characterized a new C. pneumoniae effector CPj0678 that recruits the host cell protein PACSIN2 to the plasma membrane. These findings indicate that C. trachomatis provides a powerful screening system to detect candidate effector proteins encoded by other pathogenic and endosymbiotic Chlamydia species.Importance Chlamydia injects numerous effector proteins into host cells to manipulate cellular functions important for bacterial survival. Based on findings in C. trachomatis, it has been proposed that between 5-10% of the C. pneumoniae genome, a related respiratory pathogen, encodes secreted effectors. However only a few C. pneumoniae effectors have been identified and experimentally validated. With the development of methods for the stable genetic transformation of C. trachomatis, it is now possible to use C. trachomatis shuttle plasmids to express and explore the function of proteins from other Chlamydia in a more relevant bacterial system. In this study, we experimentally determined that 49 C. pneumoniae-specific proteins are translocated into the host cytoplasm by Chlamydia secretion systems, and identify a novel effector required to recruit the host factor PACSIN2 to the plasma membrane during infection.

The role of membrane-shaping BAR domain proteins in caveolar invagination: from mechanistic insights to pathophysiological consequences

The formation of caveolae, bulb-shaped plasma membrane invaginations, requires the coordinated action of distinct lipid-interacting and -shaping proteins. The interdependence of caveolar structure and function has evoked substantial scientific interest given the association of human diseases with caveolar dysfunction. Model systems deficient of core components of caveolae, caveolins or cavins, did not allow for an explicit attribution of observed functional defects to the requirement of caveolar invagination as they lack both invaginated caveolae and caveolin proteins. Knockdown studies in cultured cells and recent knockout studies in mice identified an additional family of membrane-shaping proteins crucial for caveolar formation, syndapins (PACSINs) — BAR domain superfamily proteins characterized by crescent-shaped membrane binding interfaces recognizing and inducing distinct curved membrane topologies. Importantly, syndapin loss-of-function resulted exclusively in impairment of caveolar invagination without a reduction in caveolin or cavin at the plasma membrane, thereby allowing the specific role of the caveolar invagination to be unveiled. Muscle cells of syndapin III KO mice showed severe reductions of caveolae reminiscent of human caveolinopathies and were more vulnerable to membrane damage upon changes in membrane tensions. Consistent with the lack of syndapin III-dependent invaginated caveolae providing mechanoprotection by releasing membrane reservoirs through caveolar flattening, physical exercise of syndapin III KO mice resulted in pathological defects reminiscent of the clinical symptoms of human myopathies associated with caveolin 3 mutation suggesting that the ability of muscular caveolae to respond to mechanical forces is a key physiological process.

PACSIN2-dependent apical endocytosis regulates the morphology of epithelial microvilli

Apical microvilli are critical for the homeostasis of transporting epithelia, yet mechanisms that control the assembly and morphology of these protrusions remain poorly understood. Previous studies in intestinal epithelial cell lines suggested a role for the F-BAR domain protein PACSIN2 in normal microvillar assembly. Here we report the phenotype of PACSIN2 KO mice and provide evidence that through its role in promoting apical endocytosis, this molecule plays a role in controlling microvillar morphology. PACSIN2 KO enterocytes exhibit reduced numbers of microvilli and defects in the microvillar ultrastructure, with membranes lifting away from rootlets of core bundles. Dynamin2, a PACSIN2 binding partner, and other endocytic factors were also lost from their normal localization near microvillar rootlets. To determine whether loss of endocytic machinery could explain defects in microvillar morphology, we examined the impact of PACSIN2 KD and endocytosis inhibition on live intestinal epithelial cells. These assays revealed that when endocytic vesicle scission fails, tubules are pulled into the cytoplasm and this, in turn, leads to a membrane-lifting phenomenon reminiscent of that observed at PACSIN2 KO brush borders. These findings lead to a new model where inward forces generated by endocytic machinery on the plasma membrane control the membrane wrapping of cell surface protrusions.

Clostridium difficile Toxin A Undergoes Clathrin-Independent, PACSIN2-Dependent Endocytosis

Clostridium difficile infection affects a significant number of hospitalized patients in the United States. Two homologous exotoxins, TcdA and TcdB, are the major virulence factors in C. difficile pathogenesis. The toxins are glucosyltransferases that inactivate Rho family-GTPases to disrupt host cellular function and cause fluid secretion, inflammation, and cell death. Toxicity depends on receptor binding and subsequent endocytosis. TcdB has been shown to enter cells by clathrin-dependent endocytosis, but the mechanism of TcdA uptake is still unclear. Here, we utilize a combination of RNAi-based knockdown, pharmacological inhibition, and cell imaging approaches to investigate the endocytic mechanism(s) that contribute to TcdA uptake and subsequent cytopathic and cytotoxic effects. We show that TcdA uptake and cellular intoxication is dynamin-dependent but does not involve clathrin- or caveolae-mediated endocytosis. Confocal microscopy using fluorescently labeled TcdA shows significant colocalization of the toxin with PACSIN2-positive structures in cells during entry. Disruption of PACSIN2 function by RNAi-based knockdown approaches inhibits TcdA uptake and toxin-induced downstream effects in cells indicating that TcdA entry is PACSIN2-dependent. We conclude that TcdA and TcdB utilize distinct endocytic mechanisms to intoxicate host cells.

Clostridium difficile is a bacterial pathogen that causes nearly half a million infections each year in the United States. It infects the human colon and causes diarrhea, colitis and, in some cases, death. C. difficile infection is mediated by the action of two large homologous toxins, TcdA and TcdB. Disruption of host cell function by these toxins requires entry into cells. There are multiple ways for pathogens and virulence factors such as viruses and toxins to enter host cells. The entry mechanism is often directed by a cell surface receptor and can impact the trafficking and virulence properties of the pathogenic factor. Investigating the internalization strategy can provide critical insight into the mechanism of action for specific pathogens and virulence factors. In our current study, we sought to determine the strategy utilized by TcdA to enter host cells. We show that TcdA uptake occurs by a clathrin- and caveolae-independent endocytic mechanism that is mediated by PACSIN2 and dynamin. We also show that TcdA and TcdB can utilize different routes of entry, which may have implications regarding their cytotoxic mechanisms. In summary, our results provide new insights into the mechanism of cellular intoxication by TcdA and the role of PACSIN2 in endocytosis.

Neuroproteomics and Systems Biology Approach to Identify Temporal Biomarker Changes Post Experimental Traumatic Brain Injury in Rats

Traumatic brain injury (TBI) represents a critical health problem of which diagnosis, management, and treatment remain challenging. TBI is a contributing factor in approximately one-third of all injury-related deaths in the United States. The Centers for Disease Control and Prevention estimate that 1.7 million people suffer a TBI in the United States annually. Efforts continue to focus on elucidating the complex molecular mechanisms underlying TBI pathophysiology and defining sensitive and specific biomarkers that can aid in improving patient management and care. Recently, the area of neuroproteomics–systems biology is proving to be a prominent tool in biomarker discovery for central nervous system injury and other neurological diseases. In this work, we employed the controlled cortical impact (CCI) model of experimental TBI in rat model to assess the temporal–global proteome changes after acute (1 day) and for the first time, subacute (7 days), post-injury time frame using the established cation–anion exchange chromatography-1D SDS gel electrophoresis LC–MS/MS platform for protein separation combined with discrete systems biology analyses to identify temporal biomarker changes related to this rat TBI model. Rather than focusing on any one individual molecular entity, we used in silico systems biology approach to understand the global dynamics that govern proteins that are differentially altered post-injury. In addition, gene ontology analysis of the proteomic data was conducted in order to categorize the proteins by molecular function, biological process, and cellular localization. Results show alterations in several proteins related to inflammatory responses and oxidative stress in both acute (1 day) and subacute (7 days) periods post-TBI. Moreover, results suggest a differential upregulation of neuroprotective proteins at 7 days post-CCI involved in cellular functions such as neurite growth, regeneration, and axonal guidance. Our study is among the first to assess temporal neuroproteome changes in the CCI model. Data presented here unveil potential neural biomarkers and therapeutic targets that could be used for diagnosis, for treatment and, most importantly, for temporal prognostic assessment following brain injury. Of interest, this work relies on in silico bioinformatics approach to draw its conclusion; further work is conducted for functional studies to validate and confirm the omics data obtained.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

Rho1- and Pkc1-dependent phosphorylation of the F-BAR protein Syp1 contributes to septin ring assembly

Septins often form filaments and rings at the neck of cellular appendages. Assembly of these structures must be coordinated with membrane remodeling. In budding yeast, the Rho1 GTPase and its effector, Pkc1, play a role in septin ring stabilization during budding at least partly through phosphorylation of the bud neck–associated F-BAR protein Syp1.

In many cell types, septins assemble into filaments and rings at the neck of cellular appendages and/or at the cleavage furrow to help compartmentalize the plasma membrane and support cytokinesis. How septin ring assembly is coordinated with membrane remodeling and controlled by mechanical stress at these sites is unclear. Through a genetic screen, we uncovered an unanticipated link between the conserved Rho1 GTPase and its effector protein kinase C (Pkc1) with septin ring stability in yeast. Both Rho1 and Pkc1 stabilize the septin ring, at least partly through phosphorylation of the membrane-associated F-BAR protein Syp1, which colocalizes asymmetrically with the septin ring at the bud neck. Syp1 is displaced from the bud neck upon Pkc1-dependent phosphorylation at two serines, thereby affecting the rigidity of the new-forming septin ring. We propose that Rho1 and Pkc1 coordinate septin ring assembly with membrane and cell wall remodeling partly by controlling Syp1 residence at the bud neck.

Identification of SH3 domain proteins interacting with the cytoplasmic tail of the a disintegrin and metalloprotease 10 (ADAM10)

The a disintegrin and metalloproteases (ADAMs) play a pivotal role in the control of development, adhesion, migration, inflammation and cancer. Although numerous substrates of ADAM10 have been identified, the regulation of its surface expression and proteolytic activity is still poorly defined. One current hypothesis is that both processes are in part modulated by protein-protein interactions mediated by the intracellular portion of the protease. For related proteases, especially proline-rich regions serving as docking sites for Src homology domain 3 (SH3) domain-containing proteins proved to be important for mediating regulatory interactions. In order to identify ADAM10-binding SH3 domain proteins, we screened the All SH3 Domain Phager library comprising 305 human SH3 domains using a GST fusion protein with the intracellular region of human ADAM10 as a bait for selection. Of a total of 291 analyzed phage clones, we found 38 SH3 domains that were precipitated with the ADAM10-derived fusion protein but not with GST. We verified the binding to the cytosolic portion of ADAM10 for several candidates by co-immunoprecipitation and/or pull down analyses. Intriguingly, several of the identified proteins have been implicated in regulating surface appearance and/or proteolytic activity of related ADAMs. Thus, it seems likely that they also play a role in ADAM10 biology.

Syndapin 3 modulates fusion pore expansion in mouse neuroendocrine chromaffin cells

Adrenal neuroendocrine chromaffin cells receive excitatory synaptic input from the sympathetic nervous system and secrete hormones into the peripheral circulation. Under basal sympathetic tone, modest amounts of freely soluble catecholamine are selectively released through a restricted fusion pore formed between the secretory granule and the plasma membrane. Upon activation of the sympathoadrenal stress reflex, elevated stimulation drives fusion pore expansion, resulting in increased catecholamine secretion and facilitating release of copackaged peptide hormones. Thus regulated expansion of the secretory fusion pore is a control point for differential hormone release of the sympathoadrenal stress response. Previous work has shown that syndapin 1 deletion alters transmitter release and that the dynamin 1-syndapin 1 interaction is necessary for coupled endocytosis in neurons. Dynamin has also been shown to be involved in regulation of fusion pore expansion in neuroendocrine chromaffin cells through an activity-dependent association with syndapin. However, it is not known which syndapin isoform(s) contributes to pore dynamics in neuroendocrine cells. Nor is it known at what stage of the secretion process dynamin and syndapin associate to modulate pore expansion. Here we investigate the expression and localization of syndapin isoforms and determine which are involved in mediating fusion pore expansion. We show that all syndapin isoforms are expressed in the adrenal medulla. Mutation of the SH3 dynamin-binding domain of all syndapin isoforms shows that fusion pore expansion and catecholamine release are limited specifically by mutation of syndapin 3. The mutation also disrupts targeting of syndapin 3 to the cell periphery. Syndapin 3 exists in a persistent colocalized state with dynamin 1.

GluN3A: an NMDA receptor subunit with exquisite properties and functions

N-methyl-D-aspartate receptors (NMDAR) are pivotal for synaptic plasticity and memory formation. Conventional NMDAR consist of heterotetrameric structures composed of GluN1 and GluN2 subunits. A third subunit, GluN3, can also assemble with NMDAR subunits giving a remarkable modification of their heteromeric structure, forming a "nonconventional" NMDAR. As a consequence, the stoichiometry and kinetic properties of the receptors are dramatically changed. Among the GluN3 family, the GluN3A subunit has been the focus of a large amount of studies during recent years. These studies reveal that GluN3A is transiently expressed during development and could play a role in the fine tuning of neuronal networks as well as associated diseases. Moreover, GluN3A distribution outside the postsynaptic densities, including perisynaptic astrocytes, places it at a strategic position to play an important role in the interactions between neurons and glial cells. This review highlights GluN3A properties and addresses its role in neurophysiology and associated pathologies.

Expression of cocaine-evoked synaptic plasticity by GluN3A-containing NMDA receptors

Drug-evoked synaptic plasticity in the mesolimbic dopamine (DA) system reorganizes neural circuits that may lead to addictive behavior. The first cocaine exposure potentiates AMPAR excitatory postsynaptic currents (EPSCs) onto DA neurons of the VTA but reduces the amplitude of NMDAR-EPSCs. While plasticity of AMPAR transmission is expressed by insertion of calcium (Ca(2+))-permeable GluA2-lacking receptors, little is known about the expression mechanism for altered NMDAR transmission. Combining ex vivo patch-clamp recordings, mouse genetics, and subcellular Ca(2+) imaging, we observe that cocaine drives the insertion of NMDARs that are quasi-Ca(2+)-impermeable and contain GluN3A and GluN2B subunits. These GluN3A-containing NMDARs appear necessary for the expression of cocaine-evoked plasticity of AMPARs. We identify an mGluR1-dependent mechanism to remove these noncanonical NMDARs that requires Homer/Shank interaction and protein synthesis. Our data provide insight into the early cocaine-driven reorganization of glutamatergic transmission onto DA neurons and offer GluN3A-containing NMDARs as new targets in drug addiction.

PICK1 interacts with PACSIN to regulate AMPA receptor internalization and cerebellar long-term depression

The dynamic trafficking of AMPA receptors (AMPARs) into and out of synapses is crucial for synaptic transmission, plasticity, learning, and memory. The protein interacting with C-kinase 1 (PICK1) directly interacts with GluA2/3 subunits of the AMPARs. Although the role of PICK1 in regulating AMPAR trafficking and multiple forms of synaptic plasticity is known, the exact molecular mechanisms underlying this process remain unclear. Here, we report a unique interaction between PICK1 and all three members of the protein kinase C and casein kinase II substrate in neurons (PACSIN) family and show that they form a complex with AMPARs. Our results reveal that knockdown of the neuronal-specific protein, PACSIN1, leads to a significant reduction in AMPAR internalization following the activation of NMDA receptors in hippocampal neurons. The interaction between PICK1 and PACSIN1 is regulated by PACSIN1 phosphorylation within the variable region and is required for AMPAR endocytosis. Similarly, the binding of PICK1 to the ubiquitously expressed PACSIN2 is also regulated by the homologous phosphorylation sites within the PACSIN2-variable region. Genetic deletion of PACSIN2, which is highly expressed in Purkinje cells, eliminates cerebellar long-term depression. This deficit can be fully rescued by overexpressing wild-type PACSIN2, but not by a PACSIN2 phosphomimetic mutant, which does not bind PICK1 efficiently. Taken together, our data demonstrate that the interaction of PICK1 and PACSIN is required for the activity-dependent internalization of AMPARs and for the expression of long-term depression in the cerebellum.

Polyglutamine domain flexibility mediates the proximity between flanking sequences in huntingtin

Huntington disease (HD) is a neurodegenerative disorder caused by a CAG expansion within the huntingtin gene that encodes a polymorphic glutamine tract at the amino terminus of the huntingtin protein. HD is one of nine polyglutamine expansion diseases. The clinical threshold of polyglutamine expansion for HD is near 37 repeats, but the mechanism of this pathogenic length is poorly understood. Using Förster resonance energy transfer, we describe an intramolecular proximity between the N17 domain and the downstream polyproline region that flanks the polyglutamine tract of huntingtin. Our data support the hypothesis that the polyglutamine tract can act as a flexible domain, allowing the flanking domains to come into close spatial proximity. This flexibility is impaired with expanded polyglutamine tracts, and we can detect changes in huntingtin conformation at the pathogenic threshold for HD. Altering the structure of N17, either via phosphomimicry or with small molecules, also affects the proximity between the flanking domains. The structural capacity of N17 to fold back toward distal regions within huntingtin requires an interacting protein, protein kinase C and casein kinase 2 substrate in neurons 1 (PACSIN1). This protein has the ability to bind both N17 and the polyproline region, stabilizing the interaction between these two domains. We also developed an antibody-based FRET assay that can detect conformational changes within endogenous huntingtin in wild-type versus HD fibroblasts. Therefore, we hypothesize that wild-type length polyglutamine tracts within huntingtin can form a flexible domain that is essential for proper functional intramolecular proximity, conformations, and dynamics.

Tip-to-tip interaction in the crystal packing of PACSIN 2 is important in regulating tubulation activity

The F-BAR domain containing proteins PACSINs are cytoplasmic phosphoproteins involved in various membrane deformations, such as actin reorganization, vesicle transport and microtubule movement. Our previous study shows that all PACSINs are composed of crescent shaped dimers with two wedge loops, and the wedge loop-mediated lateral interaction between neighboring dimers is important for protein packing and tubulation activity. Here, from the crystal packing of PACSIN 2, we observed a tight tip-to-tip interaction, in addition to the wedge loop-mediated lateral interaction. With this tip-to-tip interaction, the whole packing of PACSIN 2 shows a spiral-like assembly with a central hole from the top view. Elimination of this tip-to-tip connection inhibited the tubulation function of PACSIN 2, indicating that tip-to-tip interaction plays an important role in membrane deformation activity. Together with our previous study, we proposed a packing model for the assembly of PACSIN 2 on membrane, where the proteins are connected by tip-to-tip and wedge loop-mediated lateral interactions on the surface of membrane to generate various diameter tubules.

Syndapin--a membrane remodelling and endocytic F-BAR protein

Syndapin [also called PACSIN (protein kinase C and casein kinase II interacting protein)] is an Fes-CIP4 homology Bin-amphiphysin-Rvs161/167 (F-BAR) and Src-homology 3 domain-containing protein. Three genes give rise to three main isoforms in mammalian cells. They each function in different endocytic and vesicle trafficking pathways and provide critical links between the cytoskeletal network in different cellular processes, such as neuronal morphogenesis and cell migration. The membrane remodelling activity of syndapin via its F-BAR domain and its interaction partners, such as dynamin and neural Wiskott-Aldrich syndrome protein binding to its Src-homology 3 domain, are important with respect to its function. Its various partner proteins provide insights into its mechanism of action, as well as its differential roles in these cellular processes. Signalling pathways leading to the regulation of syndapin function by phosphorylation are now contributing to our understanding of the broader functions of this family of proteins.

Casein kinase 2 phosphorylation of protein kinase C and casein kinase 2 substrate in neurons (PACSIN) 1 protein regulates neuronal spine formation

The PACSIN (protein kinase C and casein kinase 2 substrate in neurons) adapter proteins couple components of the clathrin-mediated endocytosis machinery with regulators of actin polymerization and thereby regulate the surface expression of specific receptors. The brain-specific PACSIN 1 is enriched at synapses and has been proposed to affect neuromorphogenesis and the formation and maturation of dendritic spines. In studies of how phosphorylation of PACSIN 1 contributes to neuronal function, we identified serine 358 as a specific site used by casein kinase 2 (CK2) in vitro and in vivo. Phosphorylated PACSIN 1 was found in neuronal cytosol and membrane fractions. This localization could be modulated by trophic factors such as brain-derived neurotrophic factor (BDNF). We further show that expression of a phospho-negative PACSIN 1 mutant, S358A, or inhibition of CK2 drastically reduces spine formation in neurons. We identified a novel protein complex containing the spine regulator Rac1, its GTPase-activating protein neuron-associated developmentally regulated protein (NADRIN), and PACSIN 1. CK2 phosphorylation of PACSIN 1 leads to a dissociation of the complex upon BDNF treatment and induces Rac1-dependent spine formation in dendrites of hippocampal neurons. These findings suggest that upon BDNF signaling PACSIN 1 is phosphorylated by CK2 which is essential for spine formation.

The cytoplasmic protein Pacsin 2 in kidney development and injury repair

The protein kinase C and casein kinase 2 substrate in neurons (Pacsin) is a subfamily of membrane-binding proteins that participates in vesicle trafficking and cytoskeleton organization. Here, we studied Pacsin 2 in kidney development and repair following injury. In the postnatal developing kidneys, Pacsin 2 was found to be expressed in both ureteric bud- and mesenchyme-derived structures including proximal and distal tubules, Bowman's capsule, and the glomerular tuft. In the adult kidney, its expression was decreased in proximal tubules but increased in glomerular tuft when compared to that in the developing kidneys. Interestingly, Pacsin 2 expression was significantly upregulated during the repair phase after ischemia-reperfusion injury, especially on the apical brush border of proximal tubules that experienced massive damage. Pacsin 2 localized to the primary cilia of renal epithelial cells. Knockdown of Pacsin 2 by shRNA did not affect the cell cycle or cell polarity; however, it increased the length of primary cilia, and resulted in significant tubulogenic defects in three-dimensional cell culture. Thus, we propose that Pacsin 2 contributes to kidney development and repair in a nephron-specific manner.

PACSIN1, a Tau-interacting protein, regulates axonal elongation and branching by facilitating microtubule instability

Tau is a major member of the neuronal microtubule-associated proteins. It promotes tubulin assembly and stabilizes axonal microtubules. Previous studies have demonstrated that Tau forms cross-bridges between microtubules, with some particles located on cross-bridges, suggesting that some proteins interact with Tau and might be involved in regulating Tau-related microtubule dynamics. This study reports that PACSIN1 interacts with Tau in axon. PACSIN1 blockade results in impaired axonal elongation and a higher number of primary axonal branches in mouse dorsal root ganglia neurons, which is induced by increasing the binding ability of Tau to microtubules. In PACSIN1-blocked dorsal root ganglia neurons, a greater amount of Tau is inclined to accumulate in the central domain of growth cones, and it promotes the stability of the microtubule network. Taken together, these results suggest that PACSIN1 is an important Tau binding partner in regulating microtubule dynamics and forming axonal plasticity.

Rigidity of wedge loop in PACSIN 3 protein is a key factor in dictating diameters of tubules

BAR (Bin/amphiphysin/Rvs) domain-containing proteins participate in cellular membrane remodeling. The F-BAR proteins normally generate low curvature tubules. However, in the PACSIN subfamily, the F-BAR domain from PACSIN 1 and 2 can induce both high and low curvature tubules. We found that unlike PACSIN 1 and 2, PACSIN 3 could only induce low curvature tubules. To elucidate the key factors that dictate the tubule curvature, crystal structures of all three PACSIN F-BAR domains were determined. A novel type of lateral interaction mediated by a wedge loop is observed between the F-BAR neighboring dimers. Comparisons of the structures of PACSIN 3 with PACSIN 1 and 2 indicate that the wedge loop of PACSIN 3 is more rigid, which influences the lateral interactions between assembled dimers. We further identified the residues that affect the rigidity of the loop by mutagenesis and determined the structures of two PACSIN 3 wedge loop mutants. Our results suggest that the rigidity-mediated conformations of the wedge loop correlate well with the various crystal packing modes and membrane tubulations. Thus, the rigidity of the wedge loop is a key factor in dictating tubule diameters.

Barfly: sculpting membranes at the Drosophila neuromuscular junction

The ability of a cell to change the shape of its membranes is intrinsic to many cellular functions. Proteins that can alter or recognize curved membrane structures and those that can act to recruit other proteins which stabilize the membrane curvature are likely to be essential in cell functions. The BAR (Bin, amphiphysin, RVS167 homology) domain is a protein domain that can either induce lipidic membranes to curve or can sense curved membranes. BAR domains are found in several proteins at neuronal synapses. We will review BAR domain structure and the role that BAR domain containing proteins play in regulating the morphology and function of the Drosophila neuromuscular junction. In flies the BAR domain containing proteins, endophilin and syndapin affect synaptic vesicle endocytosis, whereas CIP4, dRich, nervous wreck and syndapin affect synaptic morphology. We will review the growing evidence implicating mutations in BAR domain containing proteins being the cause of human pathologies.

New insights into the brain protein metabolism of Gastrodia elata-treated rats by quantitative proteomics

Gastrodia elata (tianma) is a traditional Chinese herbal medicine (TCM) often used for the treatment of cerebrovascular diseases. In this study, we investigated the effects of tianma on the brain protein metabolism by quantitative proteomics to gain evidence for a direct relationship between tianma treatment and brain functions. One-year-old rats were treated with tianma (~2.5 g/kg/day) for 3months and the brain tissue proteome was analyzed by using the iTRAQ (isobaric tag for relative and absolute quantification) technology. According to our results, the long-term treatment with tianma could modulate the brain protein metabolism at the proteome level by down-regulating the expressions of various proteins, such as Gnao1 and Dctn2, which are related to neuronal growth cone control and synaptic activities. In addition, tianma treatment also induced the up-regulation of molecular chaperons and proteins related to the misfolded protein response, like Anxa5, and also other proteins involved in Huntington's disease (HD) (e.g. Pacsin1 and Arf3). Concluding, tianma could eventually contribute to activities related to synaptic plasticity and neuro-restorative processes and thus might be a novel candidate agent for the treatment of neurodegenerative diseases by regulating the brain proteome.

Identification of neuronal substrates implicates Pak5 in synaptic vesicle trafficking

Synaptic transmission is mediated by a complex set of molecular events that must be coordinated in time and space. While many proteins that function at the synapse have been identified, the signaling pathways regulating these molecules are poorly understood. Pak5 (p21-activated kinase 5) is a brain-specific isoform of the group II Pak kinases whose substrates and roles within the central nervous system are largely unknown. To gain insight into the physiological roles of Pak5, we engineered a Pak5 mutant to selectively radiolabel its substrates in murine brain extract. Using this approach, we identified two novel Pak5 substrates, Pacsin1 and Synaptojanin1, proteins that directly interact with one another to regulate synaptic vesicle endocytosis and recycling. Pacsin1 and Synaptojanin1 were phosphorylated by Pak5 and the other group II Paks in vitro, and Pak5 phosphorylation promoted Pacsin1-Synaptojanin1 binding both in vitro and in vivo. These results implicate Pak5 in Pacsin1- and Synaptojanin1-mediated synaptic vesicle trafficking and may partially account for the cognitive and behavioral deficits observed in group II Pak-deficient mice.

Phosphorylation of syndapin I F-BAR domain at two helix-capping motifs regulates membrane tubulation

Syndapin I (PACSIN 1) is a synaptically enriched membrane tubulating protein that plays important roles in activity-dependent bulk endocytosis and neuronal morphogenesis. While syndapin I is an in vitro phosphoprotein, it is not known to be phosphorylated in neurons. Here, we report the identification of two phosphorylation sites, S76 and T181, of syndapin I from nerve terminals. Both residues are located at the N-terminal helix-capping motifs (N-Cap) of different α-helices in the F-BAR domain, important for F-BAR homodimer curvature and dimer-dimer filament assembly, respectively. Phospho-mimetic mutations of these residues regulate lipid-binding and tubulation both in vitro and in cells. Neither phosphosite regulated syndapin I function in activity-dependent bulk endocytosis. Rather, T181 phosphorylation was developmentally regulated and inhibited syndapin I function in neuronal morphogenesis. This suggests a novel mechanism for phosphorylation control of an F-BAR function through the regulation of α-helix interactions and stability within the folded F-BAR domain.

Cloning, purification, crystallization and preliminary X-ray diffraction analysis of mouse PACSIN 3 protein

PACSIN-family proteins are cytoplasmic proteins that have vesicle-transport, membrane-dynamics, actin-reorganization and microtubule activities. Here, the N-terminal F-BAR domain of mouse PACSIN 3, which contains 341 amino acids, was successfully cloned, purified and crystallized. The crystal of PACSIN 3 (1-341) diffracted to 2.6 Å resolution and belonged to space group P2(1), with unit-cell parameters a = 46.9, b = 54.7, c = 193.7 Å, α = 90, β = 96.9, γ = 90°. These data should provide further information on PACSIN-family protein structures.

Pacsin 2 is recruited to caveolae and functions in caveolar biogenesis

The pacsin (also termed syndapin) protein family is well characterised structurally. They contain F-BAR domains associated with the generation or maintenance of membrane curvature. The cell biology of these proteins remains less understood. Here, we initially confirm that EHD2, a protein previously shown biochemically to be present in caveolar fractions and to bind to pacsins, is a caveolar protein. We go on to report that GFP-pacsin 2 can be recruited to caveolae, and that endogenous pacsin 2 partially colocalises with caveolin 1 at the plasma membrane. Analysis of the role of pacsin 2 in caveolar biogenesis using small interfering RNA (siRNA) reveals that loss of pacsin 2 function results in loss of morphologically defined caveolae and accumulation of caveolin proteins within the plasma membrane. Overexpression of the F-BAR domain of pacsin 2 (but not the related F-BAR domains of CIP4 and FBP17) disrupts caveolar morphogenesis or trafficking, implying that pacsin 2 interacts with components required for these processes. We propose that pacsin 2 has an important role in the formation of plasma membrane caveolae.

Proper synaptic vesicle formation and neuronal network activity critically rely on syndapin I

Synaptic transmission relies on effective and accurate compensatory endocytosis. F-BAR proteins may serve as membrane curvature sensors and/or inducers and thereby support membrane remodelling processes; yet, their in vivo functions urgently await disclosure. We demonstrate that the F-BAR protein syndapin I is crucial for proper brain function. Syndapin I knockout (KO) mice suffer from seizures, a phenotype consistent with excessive hippocampal network activity. Loss of syndapin I causes defects in presynaptic membrane trafficking processes, which are especially evident under high-capacity retrieval conditions, accumulation of endocytic intermediates, loss of synaptic vesicle (SV) size control, impaired activity-dependent SV retrieval and defective synaptic activity. Detailed molecular analyses demonstrate that syndapin I plays an important role in the recruitment of all dynamin isoforms, central players in vesicle fission reactions, to the membrane. Consistently, syndapin I KO mice share phenotypes with dynamin I KO mice, whereas their seizure phenotype is very reminiscent of fitful mice expressing a mutant dynamin. Thus, syndapin I acts as pivotal membrane anchoring factor for dynamins during regeneration of SVs.

The F-BAR domain protein PACSIN2 associates with Rac1 and regulates cell spreading and migration

The Rac1 GTPase controls cytoskeletal dynamics and is a key regulator of cell spreading and migration mediated by signaling through effector proteins, such as the PAK kinases and the Scar and WAVE proteins. We previously identified a series of regulatory proteins that associate with Rac1 through its hypervariable C-terminal domain, including the Rac1 activator β-Pix (also known as Rho guanine-nucleotide-exchange factor 7) and the membrane adapter caveolin-1. Here, we show that Rac1 associates, through its C-terminus, with the F-BAR domain protein PACSIN2, an inducer of membrane tubulation and a regulator of endocytosis. We show that Rac1 localizes with PACSIN2 at intracellular tubular structures and on early endosomes. Active Rac1 induces a loss of PACSIN2-positive tubular structures. By contrast, Rac1 inhibition results in an accumulation of PACSIN2-positive tubules. In addition, PACSIN2 appears to regulate Rac1 signaling; siRNA-mediated loss of PACSIN2 increases the levels of Rac1-GTP and promotes cell spreading and migration in a wound healing assay. Moreover, ectopic expression of PACSIN2 reduces Rac1-GTP levels in a fashion that is dependent on the PACSIN2-Rac1 interaction, on the membrane-tubulating capacity of PACSIN2 and on dynamin. These data identify the BAR-domain protein PACSIN2 as a Rac1 interactor that regulates Rac1-mediated cell spreading and migration.

Kinase inhibitors modulate huntingtin cell localization and toxicity

Two serine residues within the first 17 amino acid residues of huntingtin (N17) are crucial for modulation of mutant huntingtin toxicity in cell and mouse genetic models of Huntington's disease. Here we show that the stress-dependent phosphorylation of huntingtin at Ser13 and Ser16 affects N17 conformation and targets full-length huntingtin to chromatin-dependent subregions of the nucleus, the mitotic spindle and cleavage furrow during cell division. Polyglutamine-expanded mutant huntingtin is hypophosphorylated in N17 in both homozygous and heterozygous cell contexts. By high-content screening in live cells, we identified kinase inhibitors that modulated N17 phosphorylation and hence huntingtin subcellular localization. N17 phosphorylation was reduced by casein kinase-2 inhibitors. Paradoxically, IKKβ kinase inhibition increased N17 phosphorylation, affecting huntingtin nuclear and subnuclear localization. These data indicate that huntingtin phosphorylation at Ser13 and Ser16 can be modulated by small-molecule drugs, which may have therapeutic potential in Huntington's disease.

Essential role of PACSIN2/syndapin-II in caveolae membrane sculpting

Caveolae are flask-shaped invaginations of the plasma membrane that are associated with tumor formation, pathogen entry and muscular dystrophy, through the regulation of lipids, signal transduction and endocytosis. Caveolae are generated by the fusion of caveolin-1-containing vesicles with the plasma membrane, which then participate in endocytosis via dynamin. Proteins containing membrane-sculpting F-BAR (or EFC) domains organize the membrane in clathrin-mediated endocytosis. Here, we show that the F-BAR protein PACSIN2 sculpts the plasma membrane of the caveola. The PACSIN2 F-BAR domain interacts directly with caveolin-1 by unmasking autoinhibition of PACSIN2. Furthermore, the membrane invaginations induced by the PACSIN2 F-BAR domain contained caveolin-1. Knockdown of PACSIN2 resulted in abnormal morphology of caveolin-1-associated plasma membranes, presumably as a result of decreased recruitment of dynamin-2 to caveolin-1. These results indicate that PACSIN2 mediates membrane sculpting by caveolin-1 in caveola morphology and recruits dynamin-2 for caveola fission.

Dephosphorylation of F-BAR protein Cdc15 modulates its conformation and stimulates its scaffolding activity at the cell division site

Cytokinesis in Schizosaccharomyces pombe requires the function of Cdc15, the founding member of the pombe cdc15 homology (PCH) family of proteins. As an early, abundant contractile ring component with multiple binding partners, Cdc15 plays a key role in organizing the ring. We demonstrate that Cdc15 phosphorylation at many sites generates a closed conformation, inhibits Cdc15 assembly at the division site in interphase, and precludes interaction of Cdc15 with its binding partners. Cdc15 dephosphorylation induces an open conformation, oligomerization, and scaffolding activity during mitosis. Cdc15 mutants with reduced phosphorylation precociously appear at the division site in filament-like structures and display increased association with protein partners and the membrane. Our results indicate that Cdc15 phosphoregulation impels both assembly and disassembly of the contractile apparatus and suggest a regulatory strategy that PCH family and BAR superfamily members might broadly employ to achieve temporal specificity in their roles as linkers between membrane and cytoskeleton.

Structural requirements for PACSIN/Syndapin operation during zebrafish embryonic notochord development

PACSIN/Syndapin proteins are membrane-active scaffolds that participate in endocytosis. The structure of the Drosophila Syndapin N-terminal EFC domain reveals a crescent shaped antiparallel dimer with a high affinity for phosphoinositides and a unique membrane-inserting prong upon the concave surface. Combined structural, biochemical and reverse genetic approaches in zebrafish define an important role for Syndapin orthologue, Pacsin3, in the early formation of the notochord during embryonic development. In pacsin3-morphant embryos, midline convergence of notochord precursors is defective as axial mesodermal cells fail to polarize, migrate and differentiate properly. The pacsin3 morphant phenotype of a stunted body axis and contorted trunk is rescued by ectopic expression of Drosophila Syndapin, and depends critically on both the prong that protrudes from the surface of the bowed Syndapin EFC domain and the ability of the antiparallel dimer to bind tightly to phosphoinositides. Our data confirm linkage between directional migration, endocytosis and cell specification during embryonic morphogenesis and highlight a key role for Pacsin3 in this coupling in the notochord.

Mechanisms of membrane deformation by lipid-binding domains

Among an increasing number of lipid-binding domains, a group that not only binds to membrane lipids but also changes the shape of the membrane has been found. These domains are characterized by their strong ability to transform globular liposomes as well as flat plasma membranes into elongated membrane tubules both in vitro and in vivo. Biochemical studies on the structures of these proteins have revealed the importance of the amphipathic helix, which potentially intercalates into the lipid bilayer to induce and/or sense membrane curvature. Among such membrane-deforming domains, BAR and F-BAR/EFC domains form crescent-shaped dimers, suggesting a preference for a curved membrane, which is important for curvature sensing. Bioinformatics in combination with structural analyses has been identifying an increasing number of novel families of lipid-binding domains. This review attempts to summarize the evidence obtained by recent studies in order to gain general insights into the roles of membrane-deforming domains in a variety of biological events.

Syndapin is dispensable for synaptic vesicle endocytosis at the Drosophila larval neuromuscular junction

Syndapin is a conserved dynamin-binding protein, with predicted function in synaptic-vesicle endocytosis. Here, we combine genetic mutational analysis with in vivo cell biological assays to ask whether Drosophila syndapin (Synd) is an essential component of synaptic-vesicle recycling. The only isoform of Drosophila syndapin (synd) is broadly expressed and at high levels in the nervous system. synd mutants are late-larval lethals, but fertile adult "escapers" frequently emerge. Contrary to expectation, we report that the Synd protein is predominantly postsynaptic, undetectable at presynaptic varicosities at Drosophila third-instar larval neuromuscular junctions. Electrophysiological and synaptopHluorin imaging in control, synd-deficient or synd-overexpressing motor neurons reveals that synd is dispensable for synaptic-vesicle endocytosis. Our work in Drosophila leads to the suggestion that syndapin may not be a general or essential component in dynamin-dependent synaptic-vesicle endocytosis.

PACSIN proteins bind tubulin and promote microtubule assembly

PACSINs are intracellular adapter proteins involved in vesicle transport, membrane dynamics and actin reorganisation. In this study, we report a novel role for PACSIN proteins as components of the centrosome involved in microtubule dynamics. Glutathione S-transferase (GST)-tagged PACSIN proteins interacted with protein complexes containing alpha- and gamma-tubulin in brain homogenate. Analysis of cell lysates showed that all three endogenous PACSINs co-immunoprecipitated dynamin, alpha-tubulin and gamma-tubulin. Furthermore, PACSINs bound only to unpolymerised tubulin, not to microtubules purified from brain. In agreement, the cellular localisation of endogenous PACSIN 2 was not affected by the microtubule depolymerising reagent nocodazole. By light microscopy, endogenous PACSIN 2 localised next to gamma-tubulin at purified centrosomes from NIH 3T3 cells. Finally, reduction of PACSIN 2 protein levels with small-interfering RNA (siRNA) resulted in impaired microtubule nucleation from centrosomes, whereas microtubule centrosome splitting was not affected, suggesting a role for PACSIN 2 in the regulation of tubulin polymerisation. These findings suggest a novel function for PACSIN proteins in dynamic microtubuli nucleation.

Stimulus-specific modulation of the cation channel TRPV4 by PACSIN 3

TRPV4, a member of the vanilloid subfamily of the transient receptor potential (TRP) channels, is activated by a variety of stimuli, including cell swelling, moderate heat, and chemical compounds such as synthetic 4alpha-phorbol esters. TRPV4 displays a widespread expression in various cells and tissues and has been implicated in diverse physiological processes, including osmotic homeostasis, thermo- and mechanosensation, vasorelaxation, tuning of neuronal excitability, and bladder voiding. The mechanisms that regulate TRPV4 in these different physiological settings are currently poorly understood. We have recently shown that the relative amount of TRPV4 in the plasma membrane is enhanced by interaction with the SH3 domain of PACSIN 3, a member of the PACSIN family of proteins involved in synaptic vesicular membrane trafficking and endocytosis. Here we demonstrate that PACSIN 3 strongly inhibits the basal activity of TRPV4 and its activation by cell swelling and heat, while leaving channel gating induced by the synthetic ligand 4alpha-phorbol 12,13-didecanoate unaffected. A single proline mutation in the SH3 domain of PACSIN 3 abolishes its inhibitory effect on TRPV4, indicating that PACSIN 3 must bind to the channel to modulate its function. In line herewith, mutations at specific proline residues in the N terminus of TRPV4 abolish binding of PACSIN 3 and render the channel insensitive to PACSIN 3-induced inhibition. Taken together, these data suggest that PACSIN 3 acts as an auxiliary protein of TRPV4 channel that not only affects the channel's subcellular localization but also modulates its function in a stimulus-specific manner.

Roles of F-BAR/PCH proteins in the regulation of membrane dynamics and actin reorganization

The Pombe Cdc15 Homology (PCH) proteins have emerged in many species as important coordinators of signaling pathways that regulate actomyosin assembly and membrane dynamics. The hallmark of the PCH proteins is the presence of a Fes/CIP4 homology-Bin/Amphiphysin/Rvsp (F-BAR) domain; therefore they are commonly referred to as F-BAR proteins. The prototype F-BAR protein, Cdc15p of Schizosaccharomyces pombe, has a role in the formation of the contractile actomyosin ring during cytokinesis. Vertebrate F-BAR proteins have an established role in binding phospholipids and they participate in membrane deformations, for instance, during the internalization of transmembrane receptors. This way the F-BAR proteins will function as linkers between the actin polymerization apparatus and the machinery regulating membrane dynamics. Interestingly, some members of the F-BAR proteins are implicated in inflammatory or neurodegenerative disorders and the observations can be expected to have clinical implications for the treatment of the diseases.

Protective up-regulation of CK2 by mutant huntingtin in cells co-expressing NMDA receptors

Huntington's disease is caused by a polyglutamine expansion in the huntingtin (htt) protein, and previous data indicate that over-activation of NMDA receptors (NMDARs) may be involved in the selective degeneration of cells expressing NR1/NR2B NMDARs. We used Kinetworkstrade mark multi-immunoblotting screens to examine expression of 76 protein kinases, 18 protein phosphatases, 25 heat shock/stress proteins, and 27 apoptosis proteins in human embryonic kidney 293 cells transfected with NR1/NR2B and htt containing 15 (htt-15Q; wild-type) or 138 (htt-138Q; mutant) glutamine repeats. Follow-up experiments revealed several proteins involved in the heat-shock response pathway to be up-regulated in the soluble fraction from cells expressing htt-138Q, including protein phosphatase 5 and cyclin-dependent kinase 5. Increased expression in the soluble fraction of htt-138Q-expressing cells was also noted for the stress- and calcium-activated protein-serine/threonine kinase casein kinase 2, a change which was confirmed in striatal tissue of yeast artificial chromosome transgenic mice expressing full-length mutant htt. Inhibition of casein kinase 2 activity in cultured striatal neurons from these mice significantly exacerbated NMDAR-mediated toxicity, as assessed by labeling of apoptotic nuclei. Our findings are consistent with up-regulation of components of the stress response pathway in the presence of polyglutamine-expanded htt and NR1/NR2B which may reflect an attempt at the cellular level to ameliorate the detrimental effects of mutant htt expression.

New candidate targets of AMP-activated protein kinase in murine brain revealed by a novel multidimensional substrate-screen for protein kinases

AMP-activated protein kinase (AMPK) is a heterotrimeric serine/threonine kinase that is involved in the maintenance of energy homeostasis and recovery from metabolic stresses both at the cellular and whole body level. AMPK is found in all tissues examined so far, and a number of downstream targets have been identified. Recent work suggests that AMPK has specialized functions in the brain, such as involvement in appetite control. Nevertheless, brain-specific substrates of AMPK are unknown. Here, we performed a proteomic in vitro screen to identify new putative AMPK targets in brain. Prefractionation of murine brain lysates by liquid chromatography, utilizing four different, serially connected columns with different chemistries was found to be superior to a single column method. A pilot screen involving incubation of small volumes of individual fractions with radiolabeled ATP in the presence or absence of active AMPK, followed by one-dimensional SDS-PAGE and autoradiography, revealed the presence of potential AMPK substrates in a number of different fractions. On the basis of these results, several kinase assays were repeated with selected fractions on a preparative scale. Following separation of the radiolabeled proteins by two-dimensional electrophoresis and comparison of samples with or without added AMPK by differential autoradiography, 53 AMPK-specific phospho-spots were detected and excised. Thereof, 26 unique proteins were identified by mass spectrometry and were considered as new potential downstream targets of AMPK. Kinase assays with 14 highly purified candidate substrate proteins confirmed that at least 12 were direct targets of AMPK in vitro. Although the physiological consequences of these phosphorylation events remain to be established, hypotheses concerning the most intriguing potential targets of AMPK that have been identified by this search are discussed herein. Our data suggests that signaling by AMPK in brain is likely to be involved in the regulation of pathways that have not yet been linked to this kinase.

PACSIN 1 forms tetramers via its N-terminal F-BAR domain

The ability of protein kinase C and casein kinase 2 substrate in neurons (PACSIN)/syndapin proteins to self-polymerize is crucial for the simultaneous interactions with more than one Src homology 3 domain-binding partner or with lipid membranes. The assembly of this network has profound effects on the neural Wiskott-Aldrich syndrome protein-mediated attachment of the actin polymerization machinery to vesicle membranes as well as on the movement of the corresponding vesicles. Also, the sensing of vesicle membranes and/or the induction of membrane curvature are more easily facilitated in the presence of larger PACSIN complexes. The N-terminal Fes-CIP homology and Bin-Amphiphysin-Rvs (F-BAR) domains of several PACSIN-related proteins have been shown to mediate self-interactions, whereas studies using deletion mutants derived from closely related proteins led to the view that oligomerization depends on the formation of a trimeric complex via a coiled-coil region present in these molecules. To address whether the model of trimeric complex formation is applicable to PACSIN 1, the protein was recombinantly expressed and tested in four different assays for homologous interactions. The results showed that PACSIN 1 forms tetramers of about 240 kDa, with the self-interaction having a K(D) of 6.4 x 10(-8) M. Ultrastructural analysis of these oligomers after negative staining showed that laterally arranged PACSIN molecules bind to each other via a large globular domain and form a barrel-like structure. Together, these results demonstrate that the N-terminal F-BAR domain of PACSIN 1 forms the contact site for a tetrameric structure, which is able to simultaneously interact with multiple Src homology 3 binding partners.

EHD proteins are associated with tubular and vesicular compartments and interact with specific phospholipids

The four Eps15 homology (EH) domain-containing proteins, EHD1-EHD4, have recently been ascribed roles in the regulation of the recycling of distinct receptor molecules and are often found associated with tubular structures. Here, we report the analysis of all four EHD proteins with regard to tissue distribution, intracellular localization and lipid binding properties. Specific antibodies reveal distinct expression profiles for the individual proteins in tissues and at intracellular locations, where they potentially interact with specific phospholipids. Moreover, EHD proteins colocalize with vesicular and tubular structures, implying roles in transport processes and cytoskeletal dynamics. Protein variants carrying mutations in the N-terminal nucleotide-binding P-loop region are no longer associated with phospholipids or membrane compartments, while deletion of the C-terminal EH domain affects targeting to tubular structures. All EHD proteins are able to bind to phospholipids, but localizations differ for each protein.

The diaphanous-related formin DAAM1 collaborates with the Rho GTPases RhoA and Cdc42, CIP4 and Src in regulating cell morphogenesis and actin dynamics

Binding partners for the Cdc42 effector CIP4 were identified by the yeast two-hybrid system, as well as by testing potential CIP4-binding proteins in coimmunoprecipitation experiments. One of the CIP4-binding proteins, DAAM1, was characterised in more detail. DAAM1 is a ubiquitously expressed member of the mammalian diaphanous-related formins, which include proteins such as mDia1 and mDia2. DAAM1 was shown to bind to the SH3 domain of CIP4 in vivo. Ectopically expressed DAAM1 localised in dotted pattern at the dorsal side of transfected cells and the protein was accumulated in the proximity to the microtubule organising centre. Moreover, ectopic expression of DAAM1 induced a marked alteration of the cell morphology, seen as rounding up of the cells, the formation of branched protrusions as well as a reduction of stress-fibres in the transfected cells. Coimmunoprecipitation experiments demonstrated that DAAM1 bound to RhoA and Cdc42 in a GTP-dependent manner. Moreover, DAAM1 was found to interact and collaborate with the non-receptor tyrosine kinase Src in the formation of branched protrusions. Taken together, our data indicate that DAAM1 communicates with Rho GTPases, CIP4 and Src in the regulation of the signalling pathways that co-ordinate the dynamics of the actin filament system.

PACSINs bind to the TRPV4 cation channel. PACSIN 3 modulates the subcellular localization of TRPV4

TRPV4 is a cation channel that responds to a variety of stimuli including mechanical forces, temperature, and ligand binding. We set out to identify TRPV4-interacting proteins by performing yeast two-hybrid screens, and we isolated with the avian TRPV4 amino terminus the chicken orthologues of mammalian PACSINs 1 and 3. The PACSINs are a protein family consisting of three members that have been implicated in synaptic vesicular membrane trafficking and regulation of dynamin-mediated endocytotic processes. In biochemical interaction assays we found that all three murine PACSIN isoforms can bind to the amino terminus of rodent TRPV4. No member of the PACSIN protein family was able to biochemically interact with TRPV1 and TRPV2. Co-expression of PACSIN 3, but not PACSINs 1 and 2, shifted the ratio of plasma membrane-associated versus cytosolic TRPV4 toward an apparent increase of plasma membrane-associated TRPV4 protein. A similar shift was also observable when we blocked dynamin-mediated endocytotic processes, suggesting that PACSIN 3 specifically affects the endocytosis of TRPV4, thereby modulating the subcellular localization of the ion channel. Mutational analysis shows that the interaction of the two proteins requires both a TRPV4-specific proline-rich domain upstream of the ankyrin repeats of the channel and the carboxyl-terminal Src homology 3 domain of PACSIN 3. Such a functional interaction could be important in cell types that show distribution of both proteins to the same subcellular regions such as renal tubule cells where the proteins are associated with the luminal plasma membrane.

Endocytosis and synaptic removal of NR3A-containing NMDA receptors by PACSIN1/syndapin1

A key step in glutamatergic synapse maturation is the replacement of developmentally expressed N-methyl-D-aspartate receptors (NMDARs) with mature forms that differ in subunit composition, electrophysiological properties and propensity to elicit synaptic plasticity. However, the mechanisms underlying the removal and replacement of synaptic NMDARs are poorly understood. Here we demonstrate that NMDARs containing the developmentally regulated NR3A subunit undergo rapid endocytosis from the dendritic plasma membrane in cultured rat hippocampal neurons. This endocytic removal is regulated by PACSIN1/syndapin1, which directly and selectively binds the carboxy-terminal domain of NR3A through its NPF motifs and assembles a complex of proteins including dynamin and clathrin. Endocytosis of NR3A by PACSIN1 is activity dependent, and disruption of PACSIN1 function causes NR3A accumulation at synaptic sites. Our results reveal a new activity-dependent mechanism involved in the regulation of NMDAR expression at synapses during development, and identify a brain-specific endocytic adaptor that confers spatiotemporal and subunit specificity to NMDAR endocytosis.

Early changes in Huntington's disease patient brains involve alterations in cytoskeletal and synaptic elements

Huntington's disease (HD) is caused by a polyglutamine repeat expansion in the N-terminus of the huntingtin protein. Huntingtin is normally present in the cytoplasm where it may interact with structural and synaptic elements. The mechanism of HD pathogenesis remains unknown but studies indicate a toxic gain-of-function possibly through aberrant protein interactions. To investigate whether early degenerative changes in HD involve alterations of cytoskeletal and vesicular components, we examined early cellular changes in the frontal cortex of HD presymptomatic (PS), early pathological grade (grade 1) and late-stage (grade 3 and 4) patients as compared to age-matched controls. Morphologic analysis using silver impregnation revealed a progressive decrease in neuronal fiber density and organization in pyramidal cell layers beginning in presymptomatic HD cases. Immunocytochemical analyses for the cytoskeletal markers alpha -tubulin, microtubule-associated protein 2, and phosphorylated neurofilament demonstrated a concomitant loss of staining in early grade cases. Immunoblotting for synaptic proteins revealed a reduction in complexin 2, which was marked in some grade 1 HD cases and significantly reduced in all late stage cases. Interestingly, we demonstrate that two synaptic proteins, dynamin and PACSIN 1, which were unchanged by immunoblotting, showed a striking loss by immunocytochemistry beginning in early stage HD tissue suggesting abnormal distribution of these proteins. We propose that mutant huntingtin affects proteins involved in synaptic function and cytoskeletal integrity before symptoms develop which may influence early disease onset and/or progression.

The syndapin protein family: linking membrane trafficking with the cytoskeleton

Syndapins – also called PACSINs – are highly conserved Src-homology 3 (SH3)-domain-containing proteins that seem to exist in all multicellular eukaryotes. They interact with the large GTPase dynamin and several other proteins implicated in vesicle trafficking. Syndapin-dynamin complexes appear to play an important role in vesicle fission at different donor membranes, including the plasma membrane (endocytosis) and Golgi membranes. In addition, syndapins are implicated in later steps of vesicle cycling in neuronal and non-neuronal cells. Syndapins also interact with N-WASP, a potent activator of the Arp2/3 complex that forms a critical part of the actin polymerization machinery. Syndapin oligomers can thereby couple bursts of actin polymerization with the vesicle fission step involving dynamins. This allows newly formed vesicles to move away from the donor membrane driven by actin polymerization. Syndapins also engage in additional interactions with molecules involved in several signal transduction pathways, producing crosstalk at the interface between membrane trafficking and the cytoskeleton. Given the distinct expression patterns of the different syndapins and their splice forms, these proteins could have isoform-specific functions.

Slowly getting a clue on CD95 ligand biology

Since the ligand for the death factor CD95 (CD95L) was identified almost a decade ago, it has been established that this molecule (CD95L, FasL, Apo-1L, CD178, TNFSF6, APT1LG1) has multiple immunoregulatory and pathophysiologically relevant functions. CD95L does not only act as a death factor when externalized with secretory lysosomes on cytotoxic T and NK cells or when expressed on CD4(+) T cells in the course of activation-induced cell death, it is also a key molecule for the establishment of immune privilege or tumor cell survival and may serve as a costimulatory molecule during T cell activation. Moreover, alterations of expression or shedding of different forms of CD95L are associated with many diseases including various malignancies, HIV infection, autoimmune disorders (systemic lupus erythematodes, rheumatoid arthritis), acute myocardial infarction, traumatic injury and many others. In most cases, however, the physiological link between altered CD95L expression and pathophysiology is unknown. Given the potency of the molecule to regulate death and survival of many different cell types, the control of CD95L production, transport, storage, shedding and release is of tremendous biological and clinical interest. This commentary aims at briefly summarizing the current knowledge, hypotheses and controversies about CD95L as a multifunctional ligand and receptor. It touches upon the complex networks of intracellular dynamics of protein transport and trafficking and the potential bidirectional signal transduction capacity of CD95L with a focus on molecular interactions that have been worked out over the past years.