An extracellular matrix, calmodulin-binding protein from Dictyostelium with EGF-like repeats that enhance cell motility

Cell polarity in the protist-to-animal transition

A signature feature of the animal kingdom is the presence of epithelia: sheets of polarized cells that both insulate the organism from its environment and mediate interactions with it. Epithelial cells display a marked apico-basal polarity, which is highly conserved across the animal kingdom, both in terms of morphology and of molecular regulators. How did this architecture first evolve? Although the last eukaryotic common ancestor almost certainly possessed a simple form of apico-basal polarity (marked by the presence of one or several flagella at a single cellular pole), comparative genomics and evolutionary cell biology reveal that the polarity regulators of animal epithelial cells have a surprisingly complex and stepwise evolutionary history. Here, we retrace their evolutionary assembly. We suggest that the "polarity network" that polarized animal epithelial cells evolved by integration of initially independent cellular modules that evolved at distinct steps of our evolutionary ancestry. The first module dates back to the last common ancestor of animals and amoebozoans and involved Par1, extracellular matrix proteins, and the integrin-mediated adhesion complex. Other regulators, such as Cdc42, Dlg, Par6 and cadherins evolved in ancient unicellular opisthokonts, and might have first been involved in F-actin remodeling and filopodial dynamics. Finally, the bulk of "polarity proteins" as well as specialized adhesion complexes evolved in the metazoan stem-line, in concert with the newly evolved intercellular junctional belts. Thus, the polarized architecture of epithelia can be understood as a palimpsest of components of distinct histories and ancestral functions, which have become tightly integrated in animal tissues.

A Proteomics Analysis of Calmodulin-Binding Proteins in Dictyostelium discoideum during the Transition from Unicellular Growth to Multicellular Development

Calmodulin (CaM) is an essential calcium-binding protein within eukaryotes. CaM binds to calmodulin-binding proteins (CaMBPs) and influences a variety of cellular and developmental processes. In this study, we used immunoprecipitation coupled with mass spectrometry (LC-MS/MS) to reveal over 500 putative CaM interactors in the model organism Dictyostelium discoideum. Our analysis revealed several known CaMBPs in Dictyostelium and mammalian cells (e.g., myosin, calcineurin), as well as many novel interactors (e.g., cathepsin D). Gene ontology (GO) term enrichment and Search Tool for the Retrieval of Interacting proteins (STRING) analyses linked the CaM interactors to several cellular and developmental processes in Dictyostelium including cytokinesis, gene expression, endocytosis, and metabolism. The primary localizations of the CaM interactors include the nucleus, ribosomes, vesicles, mitochondria, cytoskeleton, and extracellular space. These findings are not only consistent with previous work on CaM and CaMBPs in Dictyostelium, but they also provide new insight on their diverse cellular and developmental roles in this model organism. In total, this study provides the first in vivo catalogue of putative CaM interactors in Dictyostelium and sheds additional light on the essential roles of CaM and CaMBPs in eukaryotes.

Calmodulin-mediated events during the life cycle of the amoebozoan Dictyostelium discoideum

This review focusses on the functions of intracellular and extracellular calmodulin, its target proteins and their binding proteins during the asexual life cycle of Dictyostelium discoideum. Calmodulin is a primary regulatory protein of calcium signal transduction that functions throughout all stages. During growth, it mediates autophagy, the cell cycle, folic acid chemotaxis, phagocytosis, and other functions. During mitosis, specific calmodulin-binding proteins translocate to alternative locations. Translocation of at least one cell adhesion protein is calmodulin dependent. When starved, cells undergo calmodulin-dependent chemotaxis to cyclic AMP generating a multicellular pseudoplasmodium. Calmodulin-dependent signalling within the slug sets up a defined pattern and polarity that sets the stage for the final events of morphogenesis and cell differentiation. Transected slugs undergo calmodulin-dependent transdifferentiation to re-establish the disrupted pattern and polarity. Calmodulin function is critical for stalk cell differentiation but also functions in spore formation, events that begin in the pseudoplasmodium. The asexual life cycle restarts with the calmodulin-dependent germination of spores. Specific calmodulin-binding proteins as well as some of their binding partners have been linked to each of these events. The functions of extracellular calmodulin during growth and development are also discussed. This overview brings to the forefront the central role of calmodulin, working through its numerous binding proteins, as a primary downstream regulator of the critical calcium signalling pathways that have been well established in this model eukaryote. This is the first time the function of calmodulin and its target proteins have been documented through the complete life cycle of any eukaryote.

Calmodulin and Calmodulin Binding Proteins in Dictyostelium: A Primer

Dictyostelium discoideum is gaining increasing attention as a model organism for the study of calcium binding and calmodulin function in basic biological events as well as human diseases. After a short overview of calcium-binding proteins, the structure of Dictyostelium calmodulin and the conformational changes effected by calcium ion binding to its four EF hands are compared to its human counterpart, emphasizing the highly conserved nature of this central regulatory protein. The calcium-dependent and -independent motifs involved in calmodulin binding to target proteins are discussed with examples of the diversity of calmodulin binding proteins that have been studied in this amoebozoan. The methods used to identify and characterize calmodulin binding proteins is covered followed by the ways Dictyostelium is currently being used as a system to study several neurodegenerative diseases and how it could serve as a model for studying calmodulinopathies such as those associated with specific types of heart arrythmia. Because of its rapid developmental cycles, its genetic tractability, and a richly endowed stock center, Dictyostelium is in a position to become a leader in the field of calmodulin research.

Extracellular matrix dynamics and functions in the social amoeba Dictyostelium: A critical review

BACKGROUND

The extracellular matrix (ECM) is a dynamic complex of glycoproteins, proteoglycans, carbohydrates, and collagen that serves as an interface between mammalian cells and their extracellular environment. Essential for normal cellular homeostasis, physiology, and events that occur during development, it is also a key functionary in a number of human diseases including cancer. The social amoeba Dictyostelium discoideum secretes an ECM during multicellular development that regulates multicellularity, cell motility, cell differentiation, and morphogenesis, and provides structural support and protective layers to the resulting differentiated cell types. Proteolytic processing within the Dictyostelium ECM leads to specific bioactive factors that regulate cell motility and differentiation.

SCOPE OF REVIEW

Here we review the structure and functions of the Dictyostelium ECM and its role in regulating multicellular development. The questions and challenges that remain and how they can be answered are also discussed.

MAJOR CONCLUSIONS

The Dictyostelium ECM shares many of the features of mammalian and plant ECM, and thus presents an excellent system for studying the structure and function of the ECM.

GENERAL SIGNIFICANCE

As a genetically tractable model organism, Dictyostelium offers the potential to further elucidate ECM functions, and to possibly reveal previously unknown roles for the ECM.

Loss of Cln3 function in the social amoeba Dictyostelium discoideum causes pleiotropic effects that are rescued by human CLN3

The neuronal ceroid lipofuscinoses (NCL) are a group of inherited, severe neurodegenerative disorders also known as Batten disease. Juvenile NCL (JNCL) is caused by recessive loss-of-function mutations in CLN3, which encodes a transmembrane protein that regulates endocytic pathway trafficking, though its primary function is not yet known. The social amoeba Dictyostelium discoideum is increasingly utilized for neurological disease research and is particularly suited for investigation of protein function in trafficking. Therefore, here we establish new overexpression and knockout Dictyostelium cell lines for JNCL research. Dictyostelium Cln3 fused to GFP localized to the contractile vacuole system and to compartments of the endocytic pathway. cln3⁻ cells displayed increased rates of proliferation and an associated reduction in the extracellular levels and cleavage of the autocrine proliferation repressor, AprA. Mid- and late development of cln3⁻ cells was precocious and cln3⁻ slugs displayed increased migration. Expression of either Dictyostelium Cln3 or human CLN3 in cln3⁻ cells suppressed the precocious development and aberrant slug migration, which were also suppressed by calcium chelation. Taken together, our results show that Cln3 is a pleiotropic protein that negatively regulates proliferation and development in Dictyostelium. This new model system, which allows for the study of Cln3 function in both single cells and a multicellular organism, together with the observation that expression of human CLN3 restores abnormalities in Dictyostelium cln3⁻ cells, strongly supports the use of this new model for JNCL research.

Matricellular signal transduction involving calmodulin in the social amoebozoan dictyostelium

The social amoebozoan Dictyostelium discoideum undergoes a developmental sequence wherein an extracellular matrix (ECM) sheath surrounds a group of differentiating cells. This sheath is comprised of proteins and carbohydrates, like the ECM of mammalian tissues. One of the characterized ECM proteins is the cysteine-rich, EGF-like (EGFL) repeat-containing, calmodulin (CaM)-binding protein (CaMBP) CyrA. The first EGFL repeat of CyrA increases the rate of random cell motility and cyclic AMP-mediated chemotaxis. Processing of full-length CyrA (~63 kDa) releases two major EGFL repeat-containing fragments (~45 kDa and ~40 kDa) in an event that is developmentally regulated. Evidence for an EGFL repeat receptor also exists and downstream intracellular signaling pathways involving CaM, Ras, protein kinase A and vinculin B phosphorylation have been characterized. In total, these results identify CyrA as a true matricellular protein comparable in function to tenascin C and other matricellular proteins from mammalian cells. Insight into the regulation and processing of CyrA has also been revealed. CyrA is the first identified extracellular CaMBP in this eukaryotic microbe. In keeping with this, extracellular CaM (extCaM) has been shown to be present in the ECM sheath where it binds to CyrA and inhibits its cleavage to release the 45 kDa and 40 kDa EGFL repeat-containing fragments. The presence of extCaM and its role in regulating a matricellular protein during morphogenesis extends our understanding of CaM-mediated signal transduction in eukaryotes.

A matricellular protein and EGF-like repeat signalling in the social amoebozoan Dictyostelium discoideum

Matricellular proteins interact with the extracellular matrix (ECM) and modulate cellular processes by binding to cell surface receptors and initiating intracellular signal transduction. Their association with the ECM and the ability of some members of this protein family to regulate cell motility have opened up new avenues of research to investigate their functions in normal and diseased cells. In this review, we summarize the research on CyrA, an ECM calmodulin-binding protein in Dictyostelium. CyrA is proteolytically cleaved into smaller EGF-like (EGFL) repeat containing cleavage products during development. The first EGFL repeat of CyrA binds to the cell surface and activates a novel signalling pathway that modulates cell motility in this model organism. The similarity of CyrA to the most well-characterized matricellular proteins in mammals allows it to be designated as the first matricellular protein identified in Dictyostelium.

Cyclin-dependent kinase 5 is a calmodulin-binding protein that associates with puromycin-sensitive aminopeptidase in the nucleus of Dictyostelium

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase that has been implicated in a number of cellular processes. In Dictyostelium, Cdk5 localizes to the nucleus and cytoplasm, interacts with puromycin-sensitive aminopeptidase A (PsaA), and regulates endocytosis, secretion, growth, and multicellular development. Here we show that Cdk5 is a calmodulin (CaM)-binding protein (CaMBP) in Dictyostelium. Cdk5, PsaA, and CaM were all present in isolated nuclei and Cdk5 and PsaA co-immunoprecipitated with nuclear CaM. Although nuclear CaMBPs have previously been identified in Dictyostelium, the detection of CaM in purified nuclear fractions had not previously been shown. Putative CaM-binding domains (CaMBDs) were identified in Cdk5 and PsaA. Deletion of one of the two putative CaMBDs in Cdk5 ((132)LLINRKGELKLADFGLARAFGIP(154)) prevented CaM-binding indicating that this region encompasses a functional CaMBD. This deletion also increased the nuclear distribution of Cdk5 suggesting that CaM regulates the nucleocytoplasmic transport of Cdk5. A direct binding between CaM and PsaA could not be determined since deletion of the one putative CaMBD in PsaA prevented the nuclear localization of the deletion protein. Together, this study provides the first direct evidence for nuclear CaM in Dictyostelium and the first evidence in any system for Cdk5 being a CaMBP.

Extracellular calmodulin regulates growth and cAMP-mediated chemotaxis in Dictyostelium discoideum

The existence of extracellular calmodulin (CaM) has had a long and controversial history. CaM is a ubiquitous calcium-binding protein that has been found in every eukaryotic cell system. Calcium-free apo-CaM and Ca(2+)/CaM exert their effects by binding to and regulating the activity of CaM-binding proteins (CaMBPs). Most of the research done to date on CaM and its CaMBPs has focused on their intracellular functions. The presence of extracellular CaM is well established in a number of plants where it functions in proliferation, cell wall regeneration, gene regulation and germination. While CaM has been detected extracellularly in several animal species, including frog, rat, rabbit and human, its extracellular localization and functions are less well established. In contrast the study of extracellular CaM in eukaryotic microbes remains to be done. Here we show that CaM is constitutively expressed and secreted throughout asexual development in Dictyostelium where the presence of extracellular CaM dose-dependently inhibits cell proliferation but increases cAMP mediated chemotaxis. During development, extracellular CaM localizes within the slime sheath where it coexists with at least one CaMBP, the matricellular CaM-binding protein CyrA. Coupled with previous research, this work provides direct evidence for the existence of extracellular CaM in the Dictyostelium and provides insight into its functions in this model amoebozoan.

Ca2+ signaling regulates ecmB expression, cell differentiation and slug regeneration in Dictyostelium

Ca(2+) regulates cell differentiation and morphogenesis in a diversity of organisms and dysregulation of Ca(2+) signal transduction pathways leads to many cellular pathologies. In Dictyostelium Ca(2+) induces ecmB expression and stalk cell differentiation in vitro. Here we have analyzed the pattern of ecmB expression in intact and bisected slugs and the effect of agents that affect Ca(2+) levels or antagonize calmodulin (CaM) on this expression pattern. We have shown that Ca(2+) and CaM regulate ecmB expression and pstAB/pstB cell differentiation in vivo. Agents that increase intracellular Ca(2+) levels increased ecmB expression and/or pstAB and pstB cell differentiation, while agents that decrease intracellular Ca(2+) or antagonize CaM decreased it. In isolated slug tips agents that affect Ca(2+) levels and antagonize CaM had differential effect on ecmB expression and cell differentiation in the anterior versus posterior zones. Agents that increase intracellular Ca(2+) levels increased the number of ecmB expressing cells in the anterior region of slugs, while agents that decrease intracellular Ca(2+) levels or antagonize CaM activity increased the number of ecmB expressing cells in the posterior. We have also demonstrated that agents that affect Ca(2+) levels or antagonize CaM affect cells motility and regeneration of shape in isolated slug tips and backs and regeneration of tips in isolated slug backs. To our knowledge, this is the first study detailing the pattern of ecmB expression in regenerating slugs as well as the role of Ca(2+) and CaM in the regeneration process and ecmB expression.

Evolution of self-organisation in Dictyostelia by adaptation of a non-selective phosphodiesterase and a matrix component for regulated cAMP degradation

Dictyostelium discoideum amoebas coordinate aggregation and morphogenesis by secreting cyclic adenosine monophosphate (cAMP) pulses that propagate as waves through fields of cells and multicellular structures. To retrace how this mechanism for self-organisation evolved, we studied the origin of the cAMP phosphodiesterase PdsA and its inhibitor PdiA, which are essential for cAMP wave propagation. D. discoideum and other species that use cAMP to aggregate reside in group 4 of the four major groups of Dictyostelia. We found that groups 1-3 express a non-specific, low affinity orthologue of PdsA, which gained cAMP selectivity and increased 200-fold in affinity in group 4. A low affinity group 3 PdsA only partially restored aggregation of a D. discoideum pdsA-null mutant, but was more effective at restoring fruiting body morphogenesis. Deletion of a group 2 PdsA gene resulted in disruption of fruiting body morphogenesis, but left aggregation unaffected. Together, these results show that groups 1-3 use a low affinity PdsA for morphogenesis that is neither suited nor required for aggregation. PdiA belongs to a family of matrix proteins that are present in all Dictyostelia and consist mainly of cysteine-rich repeats. However, in its current form with several extensively modified repeats, PdiA is only present in group 4. PdiA is essential for initiating spiral cAMP waves, which, by organising large territories, generate the large fruiting structures that characterise group 4. We conclude that efficient cAMP-mediated aggregation in group 4 evolved by recruitment and adaptation of a non-selective phosphodiesterase and a matrix component into a system for regulated cAMP degradation.

EGF-like peptide of Dictyostelium discoideum is not a chemoattractant but it does restore folate-mediated chemotaxis in the presence of signal transduction inhibitors

A synthetic EGF-like (EGFL) peptide (DdEGFL1), equivalent to the first EGFL domain in the extracellular matrix protein CyrA, has previously been shown to enhance random cell motility and cAMP-mediated chemotaxis in Dictyostelium discoideum. However the role of DdEGFL1 as a potential chemoattractant had not been addressed. In this study, a micropipette assay and an under-agarose migration assay showed that DdEGFL1 is not a chemoattractant for Dictyostelium cells. A radial bioassay was used to show that DdEGFL1 does not significantly enhance folate-mediated chemotaxis in contrast to its chemokinetic effect during chemotaxis toward cAMP. However, DdEGFL1 was able to rescue chemotaxis toward folate when the pathway was inhibited by pharmacological agents that inhibit known components of the signaling cascade (e.g. phosphatidylinositol 3-kinase, phospholipase A2, tyrosine kinases, and calmodulin). These data suggest that DdEGFL1 may activate a novel motility pathway that when coupled with folic acid receptor activation, can maintain the normal migratory response to folic acid in vegetative cells. Together, this data provides new insight into the function of EGFL repeats during Dictyostelium chemotaxis and the existence of a novel motility pathway regulated by EGFL peptides and/or repeats in this model organism.

CyrA, a matricellular protein that modulates cell motility in Dictyostelium discoideum

CyrA, an extracellular matrix (slime sheath), calmodulin (CaM)-binding protein in Dictyostelium discoideum, possesses four tandem EGF-like repeats in its C-terminus and is proteolytically cleaved during asexual development. A previous study reported the expression and localization of CyrA cleavage products CyrA-C45 and CyrA-C40. In this study, an N-terminal antibody was produced that detected the full-length 63kDa protein (CyrA-C63). Western blot analyses showed that the intracellular expression of CyrA-C63 peaked between 12 and 16h of development, consistent with the time that cells are developing into a motile, multicellular slug. CyrA immunolocalization and CyrA-GFP showed that the protein localized to the endoplasmic reticulum, particularly its perinuclear component. CyrA-C63 secretion began shortly after the onset of starvation peaking between 8 and 16h of development. A pharmacological analysis showed that CyrA-C63 secretion was dependent on intracellular Ca(2+) release and active CaM, PI3K, and PLA2. CyrA-C63 bound to CaM both intra- and extracellularly and both proteins were detected in the slime sheath deposited by migrating slugs. In keeping with its purported function, CyrA-GFP over-expression enhanced cAMP-mediated chemotaxis and CyrA-C45 was detected in vinculin B (VinB)-GFP immunoprecipitates, thus providing a link between the increase in chemotaxis and a specific cytoskeletal component. Finally, DdEGFL1-FITC was detected on the membranes of cells capped with concanavalin A suggesting that a receptor exists for this peptide sequence. Together with previous studies, the data presented here suggests that CyrA is a bona fide matricellular protein in D. discoideum.

Nucleocytoplasmic transfer of cyclin dependent kinase 5 and its binding to puromycin-sensitive aminopeptidase in Dictyostelium discoideum

The Dictyostelium discoideum homolog of mammalian cyclin dependent kinase 5 (Cdk5) has previously been shown to be required for optimal growth and differentiation in this model organism, however, the subcellular localization of the protein has not previously been studied. In this study, immunolocalizations and a GFP fusion construct localized Cdk5 predominantly to the nucleus of vegetative cells. Western blots showed that Cdk5 was present in both nuclear and non-nuclear fractions, suggesting a functional role in both cellular locales. During the early stages of mitosis, Cdk5 gradually moved from a punctate nucleoplasmic distribution to localize adjacent to the inner nuclear envelope. During anaphase and telophase, Cdk5 localized to the cytoplasm and was not detected in the nucleoplasm. Cdk5 returned to the nucleus during cytokinesis. Proteolytic activity has been shown to be a critical regulator of the cell cycle. Immunoprecipitations coupled with immunolocalizations identified puromycin-sensitive aminopeptidase A (PsaA) as a potential Cdk5 binding partner in Dictyostelium. Immunoprecipitations also identified two phosphotyrosine proteins (35 and 18 kDa) that may interact with Cdk5 in vivo. Together, this work provides new insight into the localization of Cdk5, its function during cell division, and its binding to a proteolytic enzyme in Dictyostelium.