Location of profilin at presynaptic sites in the cerebellar cortex; implication for the regulation of the actin-polymerization state during axonal elongation and synaptogenesis

A Drosophila model of ALS reveals a partial loss of function of causative human PFN1 mutants

Mutations in the profilin 1 (PFN1) gene are causative for familial amyotrophic lateral sclerosis (fALS). However, it is still not fully understood how these mutations lead to neurodegeneration. To address this question, we generated a novel Drosophila model expressing human wild-type and ALS-causative PFN1 mutants. We show that at larval neuromuscular junctions (NMJ), motor neuron expression of wild-type human PFN1 increases the number of ghost boutons, active zone density, F-actin content, and the formation of filopodia. In contrast, the expression of ALS-causative human PFN1 mutants causes a less pronounced phenotype, suggesting a loss of function of these mutants in promoting NMJ remodeling. Importantly, expression of human PFN1 in motor neurons results in progressive locomotion defects and shorter lifespan in adult flies, while ALS-causative PFN1 mutants display a less toxic effect. In summary, our study provides evidence that PFN1 is important in regulating NMJ morphology and influences survival and locomotion in Drosophila. Furthermore, our results suggest ALS-causative human PFN1 mutants display a partial loss of function relative to wild-type hPFN1 that may contribute to human disease pathogenesis.

The Proteome Profiles of the Cerebellum of Juvenile, Adult and Aged Rats--An Ontogenetic Study

In this study, we searched for proteins that change their expression in the cerebellum (Ce) of rats during ontogenesis. This study focuses on the question of whether specific proteins exist which are differentially expressed with regard to postnatal stages of development. A better characterization of the microenvironment and its development may result from these study findings. A differential two-dimensional polyacrylamide gel electrophoresis (2DE) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis of the samples revealed that the number of proteins of the functional classes differed depending on the developmental stages. Especially members of the functional classes of biosynthesis, regulatory proteins, chaperones and structural proteins show the highest differential expression within the analyzed stages of development. Therefore, members of these functional protein groups seem to be involved in the development and differentiation of the Ce within the analyzed development stages. In this study, changes in the expression of proteins in the Ce at different postnatal developmental stages (postnatal days (P) 7, 90, and 637) could be observed. At the same time, an identification of proteins which are involved in cell migration and differentiation was possible. Especially proteins involved in processes of the biosynthesis and regulation, the dynamic organization of the cytoskeleton as well as chaperones showed a high amount of differentially expressed proteins between the analyzed dates.

Targeting calpain in synaptic plasticity

INTRODUCTION

Calpains represent a family of neutral, calcium-dependent proteases, which modify the function of their target proteins by partial truncation. These proteases have been implicated in numerous cell functions, including cell division, proliferation, migration, and death. In the CNS, where µ-calpain and m-calpain are the main calpain isoforms, their activation has been linked to synaptic plasticity as well as to neurodegeneration. This review will focus on the role of calpains in synaptic plasticity and discuss the possibility of developing methods to manipulate calpain activity for therapeutic purposes.

AREAS COVERED

This review covers the literature showing how calpains are implicated in synaptic plasticity and in a number of conditions associated with learning impairment. The possibility of developing new drugs targeting these enzymes for treating these conditions is discussed.

EXPERT OPINION

As evidence accumulates that calpain activation participates in neurodegeneration and cancer, there is interest in developing therapeutic approaches using direct or indirect calpain inhibition. In particular, a peptide derived from the calpain truncation site of mGluR1α was shown to decrease neurodegeneration following neonatal hypoxia/ischemia. More selective approaches need to be developed to target calpain or some of its substrates for therapeutic indications associated with deregulation of synaptic plasticity.

Purkinje cell loss and motor coordination defects in profilin1 mutant mice

Profilin1 is an actin monomer-binding protein, essential for cytoskeletal dynamics. Based on its broad expression in the brain and the localization at excitatory synapses (hippocampal CA3-CA1 synapse, cerebellar parallel fiber (PF)-Purkinje cell (PC) synapse), an important role for profilin1 in brain development and synapse physiology has been postulated. We recently showed normal physiology of hippocampal CA3-CA1 synapses in the absence of profilin1, but impaired glial cell binding and radial migration of cerebellar granule neurons (CGNs). Consequently, brain-specific inactivation of profilin1 by exploiting conditional mutants and Nestin-mediated cre expression resulted in a cerebellar hypoplasia, aberrant organization of cerebellar cortex layers, and ectopic CGNs. Apart from these findings we noted a loss of PCs and an irregularly shaped PC layer in adult mutants. In this study, we show that PC migration and development are not affected in profilin1 mutants, suggesting cell type-specific functions for profilin1 in PCs and CGNs. PC loss begins during the second postnatal week and progresses until adulthood with no further impairment in aged mutants. In Nestin-cre profilin1 mutants, defects in cerebellar cortex cytoarchitecture are associated with impaired motor coordination. However, in L7-cre mutants, lacking profilin1 specifically in PCs, the cerebellar cortex cytoarchitecture is unchanged. Thereby, our results demonstrate that the loss of PCs is not caused by cell-autonomous defects, but presumably by impaired CGN migration. Finally, we show normal functionality of PF-PC synapses in the absence of profilin1. In summary, we conclude that profilin1 is crucially important for brain development, but dispensable for the physiology of excitatory synapses.

Accelerators, Brakes, and Gears of Actin Dynamics in Dendritic Spines

Dendritic spines are actin-rich structures that accommodate the postsynaptic sites of most excitatory synapses in the brain. Although dendritic spines form and mature as synaptic connections develop, they remain plastic even in the adult brain, where they can rapidly grow, change, or collapse in response to normal physiological changes in synaptic activity that underlie learning and memory. Pathological stimuli can adversely affect dendritic spine shape and number, and this is seen in neurodegenerative disorders and some forms of mental retardation and autism as well. Many of the molecular signals that control these changes in dendritic spines act through the regulation of filamentous actin (F-actin), some through direct interaction with actin, and others via downstream effectors. For example, cortactin, cofilin, and gelsolin are actin-binding proteins that directly regulate actin dynamics in dendritic spines. Activities of these proteins are precisely regulated by intracellular signaling events that control their phosphorylation state and localization. In this review, we discuss how actin-regulating proteins maintain the balance between F-actin assembly and disassembly that is needed to stabilize mature dendritic spines, and how changes in their activities may lead to rapid remodeling of dendritic spines.

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

Differentiating neurons extend membrane protrusions that develop into growing neurites. The driving force for neurite outgrowth is the dynamic actin cytoskeleton, which is regulated by actin-binding proteins. In this study, we describe for the first time, the role of profilin I and its ligand interactions in neuritogenesis of PC12 cells. High-level overexpression of wild-type profilin I had an inhibitory effect on neurite outgrowth. Low levels of profilin I did not disturb this process, but these cells developed many more filopodia along the neurite shafts. Low-level overexpression of mutant forms of profilin I changed one or more aspects of PC12 differentiation. Expression of a profilin I mutant that is defective in actin binding (profilin I(R74E)) decreased neurite length and strongly inhibited filopodia formation. Cells expressing mutants defective in binding proline-rich ligands (profilin I(W3A) and profilin I(R136D)) differentiated faster, developed more and longer neurites and more branches. The profilin I(R136D) mutant, which is also defective in phosphatidylinositol 4,5-bisphosphate binding, enhanced neurite outgrowth even in the absence of NGF. Parental PC12 cells treated with the ROCK inhibitor Y27632, differentiate faster and display longer neurites and more branches. Similar effects were seen in cells expressing profilin I(WT), profilin I(W3A) and profilin I(R74E). By contrast, the profilin I(R136D)-expressing cells were insensitive to the ROCK inhibitor, suggesting that regulation of profilin I by phosphatidylinositol 4,5-bisphosphate metabolism is crucial for proper neurite outgrowth. Taken together, our data show the importance of the interaction of profilin I with actin, proline-rich proteins and phosphatidylinositol 4,5-bisphosphate in neuronal differentiation of PC12 cells.

The actin-binding protein profilin I is localized at synaptic sites in an activity-regulated manner

Morphological changes at synaptic specializations have been implicated in regulating synaptic strength. Actin turnover at dendritic spines is regulated by neuronal activity and contributes to spine size, shape and motility. The reorganization of actin filaments requires profilins, which stimulate actin polymerization. Neurons express two independent gene products - profilin I and profilin II. A role for profilin II in activity-dependent mechanisms at spine synapses has recently been described. Although profilin I interacts with synaptic proteins, little is known about its cellular and subcellular localization in neurons. Here, we investigated the subcellular distribution of this protein in brain neurons as well as in hippocampal cultures. Our results indicate that the expression of profilin I varies in different brain regions. Thus, in cerebral cortex and hippocampus profilin I immunostaining was associated predominantly with dendrites and was present in a subset of dendritic spines. In contrast, profilin I in cerebellum was associated primarily with presynaptic structures. Profilin I immunoreactivity was partially colocalized with the synaptic molecules synaptophysin, PSD-95 and gephyrin in cultured hippocampal neurons, indicating that profilin I is present in only a subset of synapses. At dendritic spine structures, profilin I was found primarily in protrusions, which were in apposition to presynaptic terminal boutons. Remarkably, depolarization with KCl caused a moderate but significant increase in the number of synapses containing profilin I. These results show that profilin I can be present at both pre- and postsynaptic sites and suggest a role for this actin-binding protein in activity-dependent remodelling of synaptic structure.

Cytoskeletal dynamics and transport in growth cone motility and axon guidance

Recent studies indicate the actin and microtubule cytoskeletons are a final common target of many signaling cascades that influence the developing neuron. Regulation of polymer dynamics and transport are crucial for the proper growth cone motility. This review addresses how actin filaments, microtubules, and their associated proteins play crucial roles in growth cone motility, axon outgrowth, and guidance. We present a working model for cytoskeletal regulation of directed axon outgrowth. An important goal for the future will be to understand the coordinated response of the cytoskeleton to signaling cascades induced by guidance receptor activation.

Profilin II is alternatively spliced, resulting in profilin isoforms that are differentially expressed and have distinct biochemical properties

We deduced the structure of the mouse profilin II gene. It contains five exons that can generate four different transcripts by alternative splicing. Two transcripts encode different profilin II isoforms (designated IIa and IIb) that have similar affinities for actin but different affinities for polyphosphoinositides and proline-rich sequences. Profilins IIa and IIb are also present in humans, suggesting that all mammals have three profilin isoforms. Profilin I is the major form in all tissues, except in the brain, where profilin IIa is most abundant. Profilin IIb appears to be a minor form, and its expression is restricted to a limited number of tissues, indicating that the alternative splicing is tightly regulated. Western blotting and whole-mount in situ hybridization show that, in contrast to the expression of profilin I, the expression level of profilin IIa is developmentally regulated. In situ hybridization of adult brain sections reveals overlapping expression patterns of profilins I and IIa.

Actin-dependent regulation of neurotransmitter release at central synapses

Depolymerization of actin by latrunculin A transiently promotes neurotransmitter release. The mean rate of mEPSCs increases by a Ca2+-independent process, without a concomitant change in the mean amplitude. The readily releasable vesicle pool size and the rate of refilling of the readily releasable pool remain unaltered by latrunculin treatment. Evoked neurotransmitter release also increases in a manner consistent with an increase in vesicle release probability. The observed enhancement of neurotransmitter release is specific to actin depolymerization mediated by latrunculin A and is not caused by cytochalasin D. Our findings indicate that actin participates in a regulatory mechanism that restrains fusion of synaptic vesicles at the active zone.

Profilin is predominantly associated with monomeric actin in Acanthamoeba

We used biochemical fractionation, immunoassays and microscopy of live and fixed Acanthamoeba to determine how much profilin is bound to its known ligands: actin, membrane PIP(2), Arp2/3 complex and polyproline sequences. Virtually all profilin is soluble after gentle homogenization of cells. During gel filtration of extracts on Sephadex G75, approximately 60% of profilin chromatographs with monomeric actin, 40% is free and none voids with Arp2/3 complex or other large particles. Selective monoclonal antibodies confirm that most of the profilin is bound to actin: 65% in extract immunoadsorption assays and 74–89% by fluorescent antibody staining. Other than monomeric actin, no major profilin ligands are detected in crude extracts. Profilin-II labeled with rhodamine on cysteine at position 58 retains its affinity for actin, PIP(2) and poly-L-proline. When syringe-loaded into live cells, it distributes throughout the cytoplasm, is excluded from membrane-bounded organelles, and concentrates in lamellapodia and sites of endocytosis but not directly on the plasma membrane. Some profilin fluorescence appears punctate, but since no particulate profilin is detected biochemically, these spots may be soluble profilin between organelles that exclude profilin. The distribution of profilin in fixed human A431 cells is similar to that in amoebas. Our results show that the major pool of polymerizable actin monomers is complexed with profilin and spread throughout the cytoplasm.

Aczonin, a 550-kD putative scaffolding protein of presynaptic active zones, shares homology regions with Rim and Bassoon and binds profilin

Neurotransmitter exocytosis is restricted to the active zone, a specialized area of the presynaptic plasma membrane. We report the identification and initial characterization of aczonin, a neuron-specific 550-kD protein concentrated at the presynaptic active zone and associated with a detergent-resistant cytoskeletal subcellular fraction. Analysis of the amino acid sequences of chicken and mouse aczonin indicates an organization into multiple domains, including two pairs of Cys(4) zinc fingers, a polyproline tract, and a PDZ domain and two C2 domains near the COOH terminus. The second C2 domain is subject to differential splicing. Aczonin binds profilin, an actin-binding protein implicated in actin cytoskeletal dynamics. Large parts of aczonin, including the zinc finger, PDZ, and C2 domains, are homologous to Rim or to Bassoon, two other proteins concentrated in presynaptic active zones. We propose that aczonin is a scaffolding protein involved in the organization of the molecular architecture of synaptic active zones and in the orchestration of neurotransmitter vesicle trafficking.

Application of a peptide bank from porcine brain in isolation of regulatory peptides

Over the past years, the introduction of biological assay systems, random peptide sequencing and orphan receptor screening has led to the isolation and identification of new regulatory peptides with potential clinical impact. We have developed a method for separating peptides into about 300 fractions from large amounts of porcine brain tissue. The preparation of this peptide bank consists of three steps including ultrafiltration followed by cation-exchange separation and reversed-phase chromatography. These fractions represent the peptide bank with desalted and lyophilized peptides from brain tissue. Molecular masses of the peptides in the fractions are determined by matrix-assisted laser desorption ionization MS and a mass data bank is subsequently generated. For systematic analysis of the peptides, a subsequent two-step purification procedure is followed by Edman sequencing resulting in the identification of different peptides. A survival assay with a neuronal cell line revealing the stimulatory and inhibitory activities is applied as a model to test the 300 fractions. This primary screen indicates that the biological activities of the extracted peptides are easily characterized and, moreover, can be related to the biochemical entities. We conclude that the established peptide bank is an efficient and useful tool for the isolation of regulatory brain peptides applying different purification strategies.

Identification of a novel cortactin SH3 domain-binding protein and its localization to growth cones of cultured neurons

Cortactin is an actin-binding protein that contains several potential signaling motifs including a Src homology 3 (SH3) domain at the distal C terminus. Translocation of cortactin to specific cortical actin structures and hyperphosphorylation of cortactin on tyrosine have been associated with the cortical cytoskeleton reorganization induced by a variety of cellular stimuli. The function of cortactin in these processes is largely unknown in part due to the lack of information about cellular binding partners for cortactin. Here we report the identification of a novel cortactin-binding protein of approximately 180 kDa by yeast two-hybrid interaction screening. The interaction of cortactin with this 180-kDa protein was confirmed by both in vitro and in vivo methods, and the SH3 domain of cortactin was found to direct this interaction. Since this protein represents the first reported natural ligand for the cortactin SH3 domain, we designated it CortBP1 for cortactin-binding protein 1. CortBP1 contains two recognizable sequence motifs within its C-terminal region, including a consensus sequence for cortactin SH3 domain-binding peptides and a sterile alpha motif. Northern and Western blot analysis indicated that CortBP1 is expressed predominately in brain tissue. Immunofluorescence studies revealed colocalization of CortBP1 with cortactin and cortical actin filaments in lamellipodia and membrane ruffles in fibroblasts expressing CortBP1. Colocalization of endogenous CortBP1 and cortactin was also observed in growth cones of developing hippocampal neurons, implicating CortBP1 and cortactin in cytoskeleton reorganization during neurite outgrowth.

Motile areas of leech neurites are rich in microfilaments and two actin-binding proteins: gelsolin and profilin

Cell motility is produced by changes in the dynamics and organization of actin filaments. The aim of the experiments described here was to test whether growing neurites contain two actin-binding proteins, gelsolin and profilin, that regulate polymerization of actin and affect non-neuronal cell motility. The distribution of gelsolin, profilin and the microfilaments was compared by immunocytochemistry of leech neurons growing in culture. We observed that microfilaments are enriched in the peripheral motile areas of the neurites. Both gelsolin and profilin are also concentrated in these regions. Gelsolin is abundant in filopodia and is associated with single identifiable microfilament bundles in lamellipodia. Profilin is not prominent in filopodia and shows a diffuse staining pattern in lamellipodia. The colocalization of gelsolin and profilin in motile, microfilament-rich areas supports the hypothesis that they synergistically regulate the actin dynamics that underlie neurite growth.

Cytoskeletal reorganization underlying growth cone motility

Recent studies have implicated cytoskeletal dynamics as an important component in directing neuronal outgrowth. By using modern imaging techniques to observe the kinetics of individual cytoskeletal elements in living cells, these results have converged upon a common theme: functional coupling between the intracellular cytoskeleton and extracellular substrates, and regulation thereof, appears to be crucial in controlling neuronal migration.

Coordinated expression of five tropomyosin isoforms and beta-actin in astrocytes treated with dibutyryl cAMP and cytochalasin D

Cytochalasin D and dBcAMP cause cultured astrocytes to change from flat cells to retracted process-bearing cells. F-actin was present throughout cells stimulated with dBcAMP for 16 h, whereas cytochalasin D caused F-actin to form massive aggregates at the tips of the cell processes. The two drugs differently regulated the expression of both beta-actin and tropomyosin genes in astrocytes cultured in the presence or absence of serum: dBcAMP caused down-regulation and cytochalasin D caused up-regulation. Northern blot analyses indicated that: (1) serum deprivation halved the concentration of all tropomyosin transcripts (TM-1, TM-2, TM-4, TMBr-1, TMBr-2). Serum induced TM-4 via transcriptional activation, independent of protein synthesis, (2) dBcAMP induced down-regulation of beta-actin (-50%) and tropomyosin transcripts (-35 to 52%) even in the presence of serum. The concentration of profilin mRNA decreased in dBcAMP-reactive astrocytes (-46%). The decrease in beta-actin mRNA concentration was not blocked by cycloheximide, whereas down-regulation of tropomyosin transcripts was completely reversed when protein synthesis was inhibited, and (3) cytochalasin D induced an increase in the concentration of tropomyosin transcripts (+69 to 185%) which was cumulative with serum stimulation. Cytochalasin D induction of both beta-actin and TM-4 operated through transcriptional activation, independent of protein synthesis. The production of all tropomyosin transcripts examined here were strictly coordinated with beta-actin expression in serum-, dBcAMP- and cytochalasin D-treated astrocytes. This indicates that the differential expression of tropomyosin isoforms occurring during astrocyte maturation is due to more complex regulation than that involved in serum- or cAMP-stimulated astrocytes.