Profilin-I-ligand interactions influence various aspects of neuronal differentiation

Biochemical characterization of actin assembly mechanisms with ALS-associated profilin variants

Eight separate mutations in the actin-binding protein profilin-1 have been identified as a rare cause of amyotrophic lateral sclerosis (ALS). Profilin is essential for many neuronal cell processes through its regulation of lipids, nuclear signals, and cytoskeletal dynamics, including actin filament assembly. Direct interactions between profilin and actin monomers inhibit actin filament polymerization. In contrast, profilin can also stimulate polymerization by simultaneously binding actin monomers and proline-rich tracts found in other proteins. Whether the ALS-associated mutations in profilin compromise these actin assembly functions is unclear. We performed a quantitative biochemical comparison of the direct and formin mediated impact for the eight ALS-associated profilin variants on actin assembly using classic protein-binding and single-filament microscopy assays. We determined that the binding constant of each profilin for actin monomers generally correlates with the actin nucleation strength associated with each ALS-related profilin. In the presence of formin, the A20T, R136W, Q139L, and C71G variants failed to activate the elongation phase of actin assembly. This diverse range of formin-activities is not fully explained through profilin-poly-L-proline (PLP) interactions, as all ALS-associated variants bind a formin-derived PLP peptide with similar affinities. However, chemical denaturation experiments suggest that the folding stability of these profilins impact some of these effects on actin assembly. Thus, changes in profilin protein stability and alterations in actin filament polymerization may both contribute to the profilin-mediated actin disruptions in ALS.

Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities

Intellectual disability (ID) is a pathological condition characterized by limited intellectual functioning and adaptive behaviors. It affects 1–3% of the worldwide population, and no pharmacological therapies are currently available. More than 1000 genes have been found mutated in ID patients pointing out that, despite the common phenotype, the genetic bases are highly heterogeneous and apparently unrelated. Bibliomic analysis reveals that ID genes converge onto a few biological modules, including cytoskeleton dynamics, whose regulation depends on Rho GTPases transduction. Genetic variants exert their effects at different levels in a hierarchical arrangement, starting from the molecular level and moving toward higher levels of organization, i.e., cell compartment and functions, circuits, cognition, and behavior. Thus, cytoskeleton alterations that have an impact on cell processes such as neuronal migration, neuritogenesis, and synaptic plasticity rebound on the overall establishment of an effective network and consequently on the cognitive phenotype. Systems biology (SB) approaches are more focused on the overall interconnected network rather than on individual genes, thus encouraging the design of therapies that aim to correct common dysregulated biological processes. This review summarizes current knowledge about cytoskeleton control in neurons and its relevance for the ID pathogenesis, exploiting in silico modeling and translating the implications of those findings into biomedical research.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

Proteomic analysis of rat serum revealed the effects of chronic sleep deprivation on metabolic, cardiovascular and nervous system

Sleep is an essential and fundamental physiological process that plays crucial roles in the balance of psychological and physical health. Sleep disorder may lead to adverse health outcomes. The effects of sleep deprivation were extensively studied, but its mechanism is still not fully understood. The present study aimed to identify the alterations of serum proteins associated with chronic sleep deprivation, and to seek for potential biomarkers of sleep disorder mediated diseases. A label-free quantitative proteomics technology was used to survey the global changes of serum proteins between normal rats and chronic sleep deprivation rats. A total of 309 proteins were detected in the serum samples and among them, 117 proteins showed more than 1.8-folds abundance alterations between the two groups. Functional enrichment and network analyses of the differential proteins revealed a close relationship between chronic sleep deprivation and several biological processes including energy metabolism, cardiovascular function and nervous function. And four proteins including pyruvate kinase M1, clusterin, kininogen1 and profilin-1were identified as potential biomarkers for chronic sleep deprivation. The four candidates were validated via parallel reaction monitoring (PRM) based targeted proteomics. In addition, protein expression alteration of the four proteins was confirmed in myocardium and brain of rat model. In summary, the comprehensive proteomic study revealed the biological impacts of chronic sleep deprivation and discovered several potential biomarkers. This study provides further insight into the pathological and molecular mechanisms underlying sleep disorders at protein level.

Structure-based virtual screening identifies a small-molecule inhibitor of the profilin 1-actin interaction

Profilin 1 (Pfn1) is an important regulator of the actin cytoskeleton and plays a vital role in many actin-based cellular processes. Therefore, identification of a small-molecule intervention strategy targeted against the Pfn1-actin interaction could have broad utility in cytoskeletal research and further our understanding of the role of Pfn1 in actin-mediated biological processes. Based on an already resolved Pfn1-actin complex crystal structure, we performed structure-based virtual screening of small-molecule libraries to seek inhibitors of the Pfn1-actin interaction. We identified compounds that match the pharmacophore of the key actin residues of Pfn1-actin interaction and therefore have the potential to act as competitive inhibitors of this interaction. Subsequent biochemical assays identified two candidate compounds with nearly identical structures that can mitigate the effect of Pfn1 on actin polymerization in vitro As a further proof-of-concept test for cellular effects of these compounds, we performed proximity ligation assays in endothelial cells (ECs) to demonstrate compound-induced inhibition of Pfn1-actin interaction. Consistent with the important role of Pfn1 in regulating actin polymerization and various fundamental actin-based cellular activities (migration and proliferation), treatment of these compounds reduced the overall level of cellular filamentous (F) actin, slowed EC migration and proliferation, and inhibited the angiogenic ability of ECs both in vitro and ex vivo In summary, this study provides the first proof of principle of small-molecule-mediated interference with the Pfn1-actin interaction. Our findings may have potential general utility for perturbing actin-mediated cellular activities and biological processes.

Characterization of the Molecular Mechanisms Underlying the Chronic Phase of Stroke in a Cynomolgus Monkey Model of Induced Cerebral Ischemia

Stroke is one of the main causes of mortality and long-term disability worldwide. The pathophysiological mechanisms underlying this disease are not well understood, particularly in the chronic phase after the initial ischemic episode. In this study, a Macaca fascicularis stroke model consisting of two sample groups, as determined by MRI-quantified infarct volumes as a measure of the stroke severity 28 days after the ischemic episode, was evaluated using qualitative and quantitative proteomics analyses. By using multiple online multidimensional liquid chromatography platforms, 8790 nonredundant proteins were identified that condensed to 5223 protein groups at 1% global false discovery rate (FDR). After the application of a conservative criterion (5% local FDR), 4906 protein groups were identified from the analysis of cerebral cortex. Of the 2068 quantified proteins, differential proteomic analyses revealed that 31 and 23 were dysregulated in the elevated- and low-infarct-volume groups, respectively. Neurogenesis, synaptogenesis, and inflammation featured prominently as the cellular processes associated with these dysregulated proteins. Protein interaction network analysis revealed that the dysregulated proteins for inflammation and neurogenesis were highly connected, suggesting potential cross-talk between these processes in modulating the cytoskeletal structure and dynamics in the chronic phase poststroke. Elucidating the long-term consequences of brain tissue injuries from a cellular prospective, as well as the molecular mechanisms that are involved, would provide a basis for the development of new potentially neurorestorative therapies.

Profilin1 biology and its mutation, actin(g) in disease

Profilins were discovered in the 1970s and were extensively studied for their significant physiological roles. Profilin1 is the most prominent isoform and has drawn special attention due to its role in the cytoskeleton, cell signaling, and its link to conditions such as cancer and vascular hypertrophy. Recently, multiple mutations in the profilin1 gene were linked to amyotrophic lateral sclerosis (ALS). In this review, we will discuss the physiological and pathological roles of profilin1. We will further highlight the cytoskeletal function and dysfunction caused by profilin1 dysregulation. Finally, we will discuss the implications of mutant profilin1 in various diseases with an emphasis on its contribution to the pathogenesis of ALS.

Combination of fasudil and celecoxib promotes the recovery of injured spinal cord in rats better than celecoxib or fasudil alone

Resistance mechanisms of rho-associated kinase (ROCK) inhibitors are associated with the enhanced expression of cyclooxygenase-2 (COX-2). The therapeutic effects of ROCK on nervous system diseases might be enhanced by COX-2 inhibitors. This study investigated the synergistic effect of the combined use of the ROCK inhibitor fasudil and a COX-2 inhibitor celecoxib on spinal cord injury in a rat model established by transecting the right half of the spinal cord at T₁₁. Rat models were orally administrated with celecoxib (20 mg/kg) and/or intramuscularly with fasudil (10 mg/kg) for 2 weeks. Results demonstrated that the combined use of celecoxib and fasudil significantly decreased COX-2 and Rho kinase II expression surrounding the lesion site in rats with spinal cord injury, improved the pathomorphology of the injured spinal cord, and promoted the recovery of motor function. Moreover, the effects of the drug combination were better than celecoxib or fasudil alone. This study demonstrated that the combined use of fasudil and celecoxib synergistically enhanced the functional recovery of injured spinal cord in rats.

Molecular insights on context-specific role of profilin-1 in cell migration

Profilin-1 (Pfn1) is a ubiquitously expressed actin-monomer binding protein that has been linked to many cellular activities ranging from control of actin polymerization to gene transcription. Traditionally, Pfn1 has been considered to be an essential control element for actin polymerization and cell migration. Seemingly contrasting this view, a few recent studies have shown evidence of an inhibitory action of Pfn1 on motility of certain types of carcinoma cells. In this review, we summarize biochemistry and functional aspects of Pfn1 in normal cells and bring in newly emerged action of Pfn1 in cancer cells that may explain its context-specific role in cell migration.

Cochlin, intraocular pressure regulation and mechanosensing

Fluid shear modulates many biological properties. How shear mechanosensing occurs in the extracellular matrix (ECM) and is transduced into cytoskeletal change remains unknown. Cochlin is an ECM protein of unknown function. Our investigation using a comprehensive spectrum of cutting-edge techniques has resulted in following major findings: (1) over-expression and down-regulation of cochlin increase and decrease intraocular pressure (IOP), respectively. The overexpression was achieved in DBA/2J-Gpnmb⁺/SjJ using lentiviral vectors, down-regulation was achieved in glaucomatous DBA/2J mice using targeted disruption (cochlin-null mice) and also using lentiviral vector mediated shRNA against cochlin coding region; (2) reintroduction of cochlin in cochlin-null mice increases IOP; (3) injection of exogenous cochlin also increased IOP; (4) increasing perfusion rates increased cochlin multimerization, which reduced the rate of cochlin proteolysis by trypsin and proteinase K; The cochlin multimerization in response to shear stress suggests its potential mechanosensing. Taken together with previous studies, we show cochlin is involved in regulation of intraocular pressure in DBA/2J potentially through mechanosensing of the shear stress.

Neuronal profilin isoforms are addressed by different signalling pathways

Profilins are prominent regulators of actin dynamics. While most mammalian cells express only one profilin, two isoforms, PFN1 and PFN2a are present in the CNS. To challenge the hypothesis that the expression of two profilin isoforms is linked to the complex shape of neurons and to the activity-dependent structural plasticity, we analysed how PFN1 and PFN2a respond to changes of neuronal activity. Simultaneous labelling of rodent embryonic neurons with isoform-specific monoclonal antibodies revealed both isoforms in the same synapse. Immunoelectron microscopy on brain sections demonstrated both profilins in synapses of the mature rodent cortex, hippocampus and cerebellum. Both isoforms were significantly more abundant in postsynaptic than in presynaptic structures. Immunofluorescence showed PFN2a associated with gephyrin clusters of the postsynaptic active zone in inhibitory synapses of embryonic neurons. When cultures were stimulated in order to change their activity level, active synapses that were identified by the uptake of synaptotagmin antibodies, displayed significantly higher amounts of both isoforms than non-stimulated controls. Specific inhibition of NMDA receptors by the antagonist APV in cultured rat hippocampal neurons resulted in a decrease of PFN2a but left PFN1 unaffected. Stimulation by the brain derived neurotrophic factor (BDNF), on the other hand, led to a significant increase in both synaptic PFN1 and PFN2a. Analogous results were obtained for neuronal nuclei: both isoforms were localized in the same nucleus, and their levels rose significantly in response to KCl stimulation, whereas BDNF caused here a higher increase in PFN1 than in PFN2a. Our results strongly support the notion of an isoform specific role for profilins as regulators of actin dynamics in different signalling pathways, in excitatory as well as in inhibitory synapses. Furthermore, they suggest a functional role for both profilins in neuronal nuclei.

Synaptotagmin 1 causes phosphatidyl inositol lipid-dependent actin remodeling in cultured non-neuronal and neuronal cells

Here we demonstrate that a dramatic actin polymerizing activity caused by ectopic expression of the synaptic vesicle protein synaptotagmin 1 that results in extensive filopodia formation is due to the presence of a lysine rich sequence motif immediately at the cytoplasmic side of the transmembrane domain of the protein. This polybasic sequence interacts with anionic phospholipids in vitro, and, consequently, the actin remodeling caused by this sequence is interfered with by expression of a phosphatidyl inositol (4,5)-bisphosphate (PIP2)-targeted phosphatase, suggesting that it intervenes with the function of PIP2-binding actin control proteins. The activity drastically alters the behavior of a range of cultured cells including the neuroblastoma cell line SH-SY5Y and primary cortical mouse neurons, and, since the sequence is conserved also in synaptotagmin 2, it may reflect an important fine-tuning role for these two proteins during synaptic vesicle fusion and neurotransmitter release.

Cochlin induced TREK-1 co-expression and annexin A2 secretion: role in trabecular meshwork cell elongation and motility

Fluid flow through large interstitial spaces is sensed at the cellular level, and mechanistic responses to flow changes enables expansion or contraction of the cells modulating the surrounding area and brings about changes in fluid flow. In the anterior eye chamber, aqueous humor, a clear fluid, flows through trabecular meshwork (TM), a filter like region. Cochlin, a secreted protein in the extracellular matrix, was identified in the TM of glaucomatous patients but not controls by mass spectrometry. Cochlin undergoes shear induced multimerization and plays a role in mechanosensing of fluid shear. Cytoskeletal changes in response to mechanosensing in the ECM by cochlin will necessitate transduction of mechanosensing. TREK-1, a stretch activated outward rectifying potassium channel protein known to act as mechanotransducer was found to be expressed in TM. Cochlin expression results in co-expression of TREK-1 and filopodia formation. Prolonged cochlin expression results in expression and subsequent secretion of annexin A2, a protein known to play a role in cytoskeletal remodeling. Cochlin interacts with TREK-1 and annexin A2. Cochlin-TREK-1 interaction has functional consequences and results in changes in cell shape and motility. Annexin A2 expression and secretion follows cochlin-TREK-1 syn-expression and correlates with cell elongation. Thus cytoskeleton changes in response to fluid shear sensed by cochlin are further mediated by TREK-1 and annexin A2.

Fine-tuning of neuronal architecture requires two profilin isoforms

Two profilin isoforms (PFN1 and PFN2a) are expressed in the mammalian brain. Although profilins are essential for regulating actin dynamics in general, the specific role of these isoforms in neurons has remained elusive. We show that knockdown of the neuron-specific PFN2a results in a significant reduction in dendrite complexity and spine numbers of hippocampal neurons. Overexpression of PFN1 in PFN2a-deficient neurons prevents the loss of spines but does not restore dendritic complexity. Furthermore, we show that profilins are involved in differentially regulating actin dynamics downstream of the pan-neurotrophin receptor (p75(NTR)), a receptor engaged in modulating neuronal morphology. Overexpression of PFN2a restores the morphological changes in dendrites caused by p75(NTR) overexpression, whereas PFN1 restores the normal spine density. Our data assign specific functions to the two PFN isoforms, possibly attributable to different affinities for potent effectors also involved in actin dynamics, and suggest that they are important for the signal-dependent fine-tuning of neuronal architecture.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Both actin and polyproline interactions of profilin-1 are required for migration, invasion and capillary morphogenesis of vascular endothelial cells

The objective of the present study was to evaluate how different ligand interactions of profilin-1 (Pfn1), an actin-binding protein that is upregulated during capillary morphogenesis of vascular endothelial cells (VEC), contribute to migration and capillary forming ability of VEC. We adopted a knockdown-knockin experimental system to stably express either fully functional form or mutants of Pfn1 that are impaired in binding to two of its major ligands, actin (H119E mutant) and proteins containing polyproline domains (H133S mutant), in a human dermal microvascular cell line (HmVEC) against near-null endogenous Pfn1 background. We found that silencing endogenous Pfn1 expression in HmVEC leads to slower random migration, reduced velocity of membrane protrusion and a significant impairment in matrigel-induced cord formation. Only re-expression of fully functional but not any of the two ligand-binding deficient mutants of Pfn1 rescues the above defects. We further show that loss of Pfn1 expression in VEC inhibits three-dimensional capillary morphogenesis, MMP2 secretion and ECM invasion. VEC invasion through ECM is also inhibited when actin and polyproline interactions of Pfn1 are disrupted. Together, these experimental data demonstrate that Pfn1 regulates VEC migration, invasion and capillary morphogenesis through its interaction with both actin and proline-rich ligands.

The interaction of proline-rich ligands with profilin probed with an enzyme-linked immunosorbent assay

To detect interactions of different proline-rich ligands with profilins, the authors developed a simple analytical antibody-based screening method. Profilin I or profilin IIa was coated in microplates, and ligand binding was monitored via antibody detection. Using purified components, the authors show that the assay is very sensitive as nanomolar concentrations of recombinant profilin ligands can be used. They further apply this technique to detect interaction of profilin with various proline-rich partners, either endogenously present or ectopically expressed as tagged fusions, using lysates. With this assay, the authors identify Shootin1 as a novel profilin IIa partner. In addition, they demonstrate that this assay can be used for studying competition or ternary complex formation. In conclusion, they developed a sensitive, easy-to-use, and versatile method for the study of the interaction between profilin and different ligands.

Role of the Rho-ROCK (Rho-associated kinase) signaling pathway in the regulation of pancreatic beta-cell function

Extracellular matrix has a beneficial impact on beta-cell spreading and function, but the underlying signaling pathways have yet to be fully elucidated. In other cell types, Rho, a well-characterized member of the family of Rho GTPases, and its effector Rho-associated kinase (ROCK), play an important role as downstream mediators of outside in signaling from extracellular matrix. Therefore, a possible role of the Rho-ROCK pathway in beta-cell spreading, actin cytoskeleton dynamics, and function was investigated. Rho was inhibited using a new cell-permeable version of C3 transferase, whereas the activity of ROCK was repressed using the specific ROCK inhibitors H-1152 and Y-27632. Inhibition of Rho and of ROCK increased spreading and improved both short-term and prolonged glucose-stimulated insulin secretion but had no impact on basal secretion. Inhibition of this pathway led to a depolymerization of the actin cytoskeleton. Furthermore, the impact of the inhibition of ROCK on stimulated insulin secretion was acute and reversible, suggesting that rapid signaling such as phosphorylation is involved. Finally, quantification of the activity of RhoA indicated that the extracellular matrix represses RhoA activity. Overall these results show for the first time that the Rho-ROCK signaling pathway contributes to the stabilization of the actin cytoskeleton and inhibits glucose-stimulated insulin secretion in primary pancreatic beta-cells. Furthermore, they indicate that inhibition of this pathway might be one of the mechanisms by which the extracellular matrix exerts its beneficial effects on pancreatic beta-cell function.

Profilin 1 is required for abscission during late cytokinesis of chondrocytes

Profilins are key factors for dynamic rearrangements of the actin cytoskeleton. However, the functions of profilins in differentiated mammalian cells are uncertain because profilin deficiency is early embryonic lethal for higher eukaryotes. To examine profilin function in chondrocytes, we disrupted the profilin 1 gene in cartilage (Col2pfn1). Homozygous Col2pfn1 mice develop progressive chondrodysplasia caused by disorganization of the growth plate and defective chondrocyte cytokinesis, indicated by the appearance of binucleated cells. Surprisingly, Col2pfn1 chondrocytes assemble and contract actomyosin rings normally during cell division; however, they display defects during late cytokinesis as they frequently fail to complete abscission due to their inability to develop strong traction forces. This reduced force generation results from an impaired formation of lamellipodia, focal adhesions and stress fibres, which in part could be linked to an impaired mDia1-mediated actin filament elongation. Neither an actin nor a poly-proline binding-deficient profilin 1 is able to rescue the defects. Taken together, our results demonstrate that profilin 1 is not required for actomyosin ring formation in dividing chondrocytes but necessary to generate sufficient force for abscission during late cytokinesis.

In birds, profilin-2a is ubiquitously expressed and contributes to actin-based motility

Profilins are small actin-binding proteins expressed in all eukaryotes. They are involved in the regulation of actin filament dynamics and various signalling pathways. The identification of a variety of profilin isoforms led to the assumption that there may be isoform-specific functions. In mammals, profilin-1 (PFN1) is ubiquitously expressed and engaged in the regulation of various motility processes in all cell types. By contrast, profilin-2a (PFN2a) is mainly restricted to neuronal cells and there is evidence that it is involved in neuronal plasticity and membrane trafficking. However, the PFN2a sequence is much better conserved than PFN1 throughout different phyla, indicating that its restricted expression and specialized function in mammals might be unique. Using isoform-specific antibodies, we show that the situation is different in birds. PFN2a is ubiquitously expressed in embryonic and adult chicken tissues at equal and frequently higher amounts than in mammals. Together with PFN1, it is present in cultivated chicken fibroblasts, but differentially localized. Knockdown experiments with miRNA reveal that PFN2a is involved in cell adhesion, spreading and locomotion, and silencing this isoform has pronounced consequences on these processes. Our results indicate profilin isoform expression is differentially regulated among vertebrates.

Profilin, a multi-modal regulator of neuronal plasticity

Thirty years after its initial characterization and more than 1000 publications listed in PubMed describing its properties, the small (ca 15 kDa) protein profilin continues to surprise us with new, recently discovered functions. Originally described as an actin-binding protein, profilin has now been shown to interact with more than a dozen proteins in mammalian cells. Some of the more recently described and intriguing interactions are within neurons involving a neuronal profilin family member. Profilin is now regarded as a regulator of various cellular processes such as cytoskeletal dynamics, membrane trafficking and nuclear transport. Profilin is a necessary element in key steps of neuronal differentiation and synaptic plasticity, and embodies properties postulated for a synaptic tag. These findings identify profilin as an important factor linking cellular and behavioural plasticity in neural circuits.

Expression analysis and characteristics of profilin gene from silkworm, Bombyx mori

A recombinant Bombyx mori profilin protein (rBmPFN) was overexpressed in Escherichia coli BL21. Purified rBmPFN was used to generate anti-BmPFN polyclonal antibody, which were used to determine the subcellular localization of BmPFN. Immunostaining indicated that profilin can be found in both the nucleus and cytoplasm but is primarily located in the cytoplasm. Real-time RT-PCR and Western blot analyses indicated that, during the larvae stage, profilin expression levels are highest in the silk gland, followed by the gonad, and are lowest in the fatty body. Additionally, BmPFN expression begins during the egg stage, increases during the larvae stage, reaches a peak during the pupa stage, and decreases significantly in the moth. Therefore, we propose that BmPFN may play an important role during larva stage development, especially in the silk gland.

Anaplasma phagocytophilum-induced gene expression in both human neutrophils and HL-60 cells

Anaplasma phagocytophilum (Ap), the etiologic agent of the tick-borne disease human granulocytic anaplasmosis, is an obligate intracellular pathogen unique in its ability to target and replicate within neutrophils. We define and compare the spectra of host gene expression in response to Ap infection of human neutrophils and of HL-60 cells using long (70-mer)-oligonucleotide array technology. In addition to apoptosis-related genes, genes involved in signaling pathways, transcriptional regulation, immune response, host defense, cell adhesion, and cytoskeleton were modulated in neutrophils infected with Ap. Ap infection affected the same pathways in HL-60 cells but transcriptional changes occurred more slowly and in a reduced spectrum of genes. Gene expression changes detected by microarray were confirmed for randomly selected genes by QRT-PCR and Western blot studies. These studies demonstrate for the first time that the ERK pathway is activated in Ap-infected neutrophils and also define multiple pathways that are activated during intracellular Ap infection, which together serve to prolong the cell survival that is needed to allow bacterial replication and survival in neutrophils, which otherwise would rapidly apoptose.

Profilin induces lamellipodia by growth factor-independent mechanism

Profilin has been implicated in cell motility and in a variety of cellular processes, such as membrane extension, endocytosis, and formation of focal complexes. In vivo, profilin replenish the pool of ATP-actin monomers by increasing the rate of nucleotide exchange of ADP-actin for ATP-actin, promoting the incorporation of new actin monomers at the barbed end of actin filaments. For this report, we generated a membrane-permeable version of profilin I (PTD4-PfnI) for the alteration of intracellular profilin levels taking advantage of the protein transduction technique. We show that profilin I induces lamellipodia formation independently of growth factor presence in primary bovine trabecular meshwork (BTM) cells. The effects are time- and concentration-dependent and specific to the profilin I isoform. Profilin II, the neuronal isoform, failed to extend lamellipodia in the same degree as profilin I. H133S, a mutation in the polyproline binding domain, showed a reduced ability to induce lamellipodia. H199E, mutation in the actin binding domain failed to induce membrane spreading and inhibit fetal bovine serum (FBS) -induced lamellipodia extension. Incubation with a synthetic polyproline domain peptide (GP5)3, fused to a transduction domain, abolished lamellipodia induction by profilin or FBS. Time-lapse microscopy confirmed the effects of profilin on lamellipodia extension with a higher spreading velocity than FBS. PTD4-Pfn I was found in the inner lamellipodia domain, at the membrane leading edge where it colocalizes with endogenous profilin. While FBS-induced lamellipodia formation activates Rac1, PTD4-Pfn I stimulation did not induce Rac1 activation. We propose a role of profilin I favoring lamellipodia formation by a mechanism downstream of growth factor.

Profilin-1 is a negative regulator of mammary carcinoma aggressiveness

Expression of profilin-1 (Pfn1) is downregulated in breast cancer cells, the functional significance of which is yet to be understood. To address this question, in this study we evaluated how perturbing Pfn1 affects motility and invasion of breast cancer cells. We show that loss of Pfn1 expression leads to enhanced motility and matrigel invasiveness of MDA-MB-231 breast cancer cells. Interestingly, silencing Pfn1 expression is associated with downregulation of both cell–cell and cell–matrix adhesions with concomitant increase in motility and dramatic scattering of normal human mammary epithelial cells. Thus, these data for the first time suggest that loss of Pfn1 expression may have significance in breast cancer progression. Consistent with these findings, even a moderate overexpression of Pfn1 induces actin stress-fibres, upregulates focal adhesion, and dramatically inhibits motility and matrigel invasiveness of MDA-MB-231 cells. Using mutants of Pfn1 that are defective in binding to either actin or proline-rich ligands, we further show that overexpressed Pfn1 must have a functional actin-binding site to suppress cell motility. Finally, animal experiments reveal that overexpression of Pfn1 suppresses orthotopic tumorigenicity and micro-metastasis of MDA-MB-231 cells in nude mice. These data imply that perturbing Pfn1 could be a good molecular strategy to limit the aggressiveness of breast cancer cells.

Scaffold mediated regulation of MAPK signaling and cytoskeletal dynamics: a perspective

Cell migration is critical for many physiological processes and is often misregulated in developmental disorders and pathological conditions including cancer and neurodegeneration. MAPK signaling and the Rho family of proteins are known regulators of cell migration that exert their influence on cellular cytoskeleton during cell adhesion and migration. Here we review data supporting the view that localized ERK signaling mediated through recently identified scaffold proteins may regulate cell migration.

The Down syndrome critical region protein TTC3 inhibits neuronal differentiation via RhoA and Citron kinase

The Down syndrome critical region (DSCR) on Chromosome 21 contains many genes whose duplication may lead to the major phenotypic features of Down syndrome and especially the associated mental retardation. However, the functions of DSCR genes are mostly unknown and their possible involvement in key brain developmental events still largely unexplored. In this report we show that the protein TTC3, encoded by one of the main DSCR candidate genes, physically interacts with Citron kinase (CIT-K) and Citron N (CIT-N), two effectors of the RhoA small GTPase that have previously been involved in neuronal proliferation and differentiation. More importantly, we found that TTC3 levels can strongly affect the NGF-induced differentiation of PC12 cells, by a CIT-K-dependent mechanism. Indeed, TTC3 overexpression leads to strong inhibition of neurite extension, which can be reverted by CIT-K RNAi. Conversely, TTC3 knockdown stimulates neurite extension in the same cells. Finally, we find that Rho, but not Rho kinase, is required for TTC3 differentiation-inhibiting activity. Our results suggest that the TTC3-RhoA-CIT-K pathway could be a crucial determinant of in vivo neuronal development, whose hyperactivity may result in detrimental effects on the normal differentiation program.

Profilin isoforms in Dictyostelium discoideum

Eukaryotic cells contain a large number of actin binding proteins of different functions, locations and concentrations. They bind either to monomeric actin (G-actin) or to actin filaments (F-actin) and thus regulate the dynamic rearrangement of the actin cytoskeleton. The Dictyostelium discoideum genome harbors representatives of all G-actin binding proteins including actobindin, twinfilin, and profilin. A phylogenetic analysis of all profilins suggests that two distinguishable groups emerged very early in evolution and comprise either vertebrate and viral profilins or profilins from all other organisms. The newly discovered profilin III isoform in D. discoideum shows all functions that are typical for a profilin. However, the concentration of the third isoform in wild type cells reaches only about 0.5% of total profilin. In a yeast-2-hybrid assay profilin III was found to bind specifically to the proline-rich region of the cytoskeleton-associated vasodilator-stimulated phosphoprotein (VASP). Immunolocalization studies showed similar to VASP the profilin III isoform in filopodia and an enrichment at their tips. Cells lacking the profilin III isoform show defects in cell motility during chemotaxis. The low abundance and the specific interaction with VASP argue against a significant actin sequestering function of the profilin III isoform.

The profile of profilins

Profilins are small proteins involved in actin dynamics. In accordance with this function, they are found in all eukaryotes and are structurally highly conserved. However, their precise role in regulating actin-related functions is just beginning to emerge. This article recapitulates the wealth of information on structure, expression and functions accumulated on profilins from many different organisms in the 30 years after their discovery as actin-binding proteins. Emphasis is given to their interaction with a plethora of many different ligands in the cytoplasm as well as in the nucleus, which is considered the basis for their various activities and the significance of the tissue-specific expression of profilin isoforms.