Distinct cellular expression pattern of annexins in Hydra vulgaris

A two-step process in the emergence of neurogenesis

Cnidarians belong to the first phylum differentiating a nervous system, thus providing suitable model systems to trace the origins of neurogenesis. Indeed corals, sea anemones, jellyfish and hydra contract, swim and catch their food thanks to sophisticated nervous systems that share with bilaterians common neurophysiological mechanisms. However, cnidarian neuroanatomies are quite diverse, and reconstructing the urcnidarian nervous system is ambiguous. At least a series of characters recognized in all classes appear plesiomorphic: (1) the three cell types that build cnidarian nervous systems (sensory-motor cells, ganglionic neurons and mechanosensory cells called nematocytes or cnidocytes); (2) an organization of nerve nets and nerve rings [those working as annular central nervous system (CNS)]; (3) a neuronal conduction via neurotransmitters; (4) a larval anterior sensory organ required for metamorphosis; (5) a persisting neurogenesis in adulthood. By contrast, the origin of the larval and adult neural stem cells differs between hydrozoans and other cnidarians; the sensory organs (ocelli, lens-eyes, statocysts) are present in medusae but absent in anthozoans; the electrical neuroid conduction is restricted to hydrozoans. Evo-devo approaches might help reconstruct the neurogenic status of the last common cnidarian ancestor. In fact, recent genomic analyses show that if most components of the postsynaptic density predate metazoan origin, the bilaterian neurogenic gene families originated later, in basal metazoans or as eumetazoan novelties. Striking examples are the ParaHox Gsx, Pax, Six, COUP-TF and Twist-type regulators, which seemingly exert neurogenic functions in cnidarians, including eye differentiation, and support the view of a two-step process in the emergence of neurogenesis.

Fluorescent nanocrystals reveal regulated portals of entry into and between the cells of Hydra

Initially viewed as innovative carriers for biomedical applications, with unique photophysical properties and great versatility to be decorated at their surface with suitable molecules, nanoparticles can also play active roles in mediating biological effects, suggesting the need to deeply investigate the mechanisms underlying cell-nanoparticle interaction and to identify the molecular players. Here we show that the cell uptake of fluorescent CdSe/CdS quantum rods (QRs) by Hydra vulgaris, a simple model organism at the base of metazoan evolution, can be tuned by modifying nanoparticle surface charge. At acidic pH, amino-PEG coated QRs, showing positive surface charge, are actively internalized by tentacle and body ectodermal cells, while negatively charged nanoparticles are not uptaken. In order to identify the molecular factors underlying QR uptake at acidic pH, we provide functional evidence of annexins involvement and explain the QR uptake as the combined result of QR positive charge and annexin membrane insertion. Moreover, tracking QR labelled cells during development and regeneration allowed us to uncover novel intercellular trafficking and cell dynamics underlying the remarkable plasticity of this ancient organism.

Hydra, a niche for cell and developmental plasticity

The silencing of genes whose expression is restricted to specific cell types and/or specific regeneration stages opens avenues to decipher the molecular control of the cellular plasticity underlying head regeneration in hydra. In this review, we highlight recent studies that identified genes involved in the immediate cytoprotective function played by gland cells after amputation; the early dedifferentiation of digestive cells into blastema-like cells during head regeneration, and the early late proliferation of neuronal progenitors required for head patterning. Hence, developmental plasticity in hydra relies on spatially restricted and timely orchestrated cellular modifications, where the functions played by stem cells remain to be characterized.

Hym-301, a novel peptide, regulates the number of tentacles formed in hydra

Hym-301 is a peptide that was discovered as part of a project aimed at isolating novel peptides from hydra. We have isolated and characterized the gene Hym-301, which encodes this peptide. In an adult, the gene is expressed in the ectoderm of the tentacle zone and hypostome, but not in the tentacles. It is also expressed in the developing head during bud formation and head regeneration. Treatment of regenerating heads with the peptide resulted in an increase in the number of tentacles formed, while treatment with Hym-301 dsRNA resulted in a reduction of tentacles formed as the head developed during bud formation or head regeneration. The expression patterns plus these manipulations indicate the gene has a role in tentacle formation. Furthermore, treatment of epithelial animals indicates the gene directly affects the epithelial cells that form the tentacles. Raising the head activation gradient, a morphogenetic gradient that controls axial patterning in hydra, throughout the body column results in extending the range of Hym-301 expression down the body column. This indicates the range of expression of the gene appears to be controlled by this gradient. Thus,Hym-301 is involved in axial patterning in hydra, and specifically in the regulation of the number of tentacles formed.

HyBMP5-8b, a BMP5-8 orthologue, acts during axial patterning and tentacle formation in hydra

Developmental gradients play a central role in axial patterning in hydra. As part of the effort towards elucidating the molecular basis of these gradients as well as investigating the evolution of the mechanisms underlying axial patterning, genes encoding signaling molecules are under investigation. We report the isolation and characterization of HyBMP5-8b, a BMP5-8 orthologue, from hydra. Processes governing axial patterning are continuously active in adult hydra. Expression patterns of HyBMP5-8b in normal animals and during bud formation, hydra's asexual form of reproduction, were examined. These patterns, coupled with changes in patterns of expression in manipulated tissues during head regeneration, foot regeneration as well as under conditions that alter the positional value gradient indicate that the gene is active in two different processes. The gene plays a role in tentacle formation and in patterning the lower end of the body axis.

Annexins: from structure to function

Annexins are Ca2+ and phospholipid binding proteins forming an evolutionary conserved multigene family with members of the family being expressed throughout animal and plant kingdoms. Structurally, annexins are characterized by a highly alpha-helical and tightly packed protein core domain considered to represent a Ca2+-regulated membrane binding module. Many of the annexin cores have been crystallized, and their molecular structures reveal interesting features that include the architecture of the annexin-type Ca2+ binding sites and a central hydrophilic pore proposed to function as a Ca2+ channel. In addition to the conserved core, all annexins contain a second principal domain. This domain, which NH2-terminally precedes the core, is unique for a given member of the family and most likely specifies individual annexin properties in vivo. Cellular and animal knock-out models as well as dominant-negative mutants have recently been established for a number of annexins, and the effects of such manipulations are strikingly different for different members of the family. At least for some annexins, it appears that they participate in the regulation of membrane organization and membrane traffic and the regulation of ion (Ca2+) currents across membranes or Ca2+ concentrations within cells. Although annexins lack signal sequences for secretion, some members of the family have also been identified extracellularly where they can act as receptors for serum proteases on the endothelium as well as inhibitors of neutrophil migration and blood coagulation. Finally, deregulations in annexin expression and activity have been correlated with human diseases, e.g., in acute promyelocytic leukemia and the antiphospholipid antibody syndrome, and the term annexinopathies has been coined.

Calcium-dependent binding of annexin 12 to phospholipid bilayers: stoichiometry and implications

Annexins (ANXs) are a superfamily of proteins whose functional hallmark is Ca2+-dependent binding to anionic phospholipids. Their core domains are usually composed of a 4-fold repeat of a conserved amino acid sequence, with each repeat containing a type II Ca2+ binding site that is generally thought to mediate Ca2+-dependent binding to the membrane. We now report that ANX12 binding to phospholipid vesicles is highly cooperative with respect to Ca2+ concentration (Hill constant approximately 7), thereby suggesting that more than the four well-characterized type II Ca2+ binding sites are involved in phospholipid binding. Two independent approaches, a novel 45Ca2+ copelleting assay and isothermal titration calorimetry, indicate a stoichiometry of approximately 12 mol of Ca2+/mol of ANX12 for binding to phospholipid vesicles. On the basis of the "low-affinity" Ca2+-binding sites in a number of ANX X-ray crystal structures, we propose a model for ANX12 bilayer binding that involves three types of Ca2+ sites in each of the four repeats. In this model, there is a complementarity between the spacing of the ANX12 Ca2+ binding sites and the spacing of the phospholipid headgroups in bilayers. We tested the implications of the model by manipulating the physical state of vesicles composed of phospholipids with saturated acyl chains with temperature and measuring its influence on ANX12 binding. ANX12 bound to vesicles in a Ca2+-dependent manner when the vesicles were in the liquid crystal phase but not when the phospholipid was in the gel phase. Furthermore, ANX12 bound initially to fluid bilayers remained bound when cooled to 4 degrees C, a temperature that should induce the gel phase transition. Overall, these studies suggest that ANX12 is well suited to being a Ca2+ sensor for rapid all-or-none intercellular membrane-related events.

HyAlx, an aristaless-related gene, is involved in tentacle formation in hydra

Developmental gradients are known to play important roles in axial patterning in hydra. Current efforts are directed toward elucidating the molecular basis of these gradients. We report the isolation and characterization of HyAlx, an aristaless-related gene in hydra. The expression patterns of the gene in adult hydra, as well as during bud formation, head regeneration and the formation of ectopic head structures along the body column, indicate the gene plays a role in the specification of tissue for tentacle formation. The use of RNAi provides more direct evidence for this conclusion. The different patterns of HyAlx expression during head regeneration and bud formation also provide support for a recent version of a reaction-diffusion model for axial patterning in hydra.

Annexin XII E105K crystal structure: identification of a pH-dependent switch for mutant hexamerization

Annexins are a family of calcium- and phospholipid-binding proteins involved with numerous cellular processes including membrane fusion, ion channel activity, and heterocomplex formation with other proteins. The annexin XII (ANXB12) crystal structure presented evidence that calcium mediates the formation of a hexamer through a novel intermolecular calcium-binding site [Luecke et al. (1995) Nature 378, 512-515]. In an attempt to disrupt hexamerization, we mutated a conserved key ligand in the intermolecular calcium-binding site, Glu105, to lysine. Despite its occurrence in a new spacegroup, the 1.93 A resolution structure reveals a hexamer with the Lys105 epsilon-amino group nearly superimposable with the original intermolecular calcium position. Our analysis shows that the mutation is directly involved in stabilizing the hexamer. The local residues are reoriented to retain affinity between the two trimers via a pH-dependent switch residue, Glu76, which is now protonated, allowing it to form tandem hydrogen bonds with the backbone carbonyl and nitrogen atoms of Thr103 located across the trimer interface. The loss of the intermolecular calcium-binding site is recuperated by extensive hydrogen bonding favoring hexamer stabilization. The presence of this mutant structure provides further evidence for hexameric annexin XII, and possible in vivo roles are discussed.

Annexin XII forms calcium-dependent multimers in solution and on phospholipid bilayers: a chemical cross-linking study

The annexins are a family of proteins that bind in a Ca2+-dependent manner to phospholipids that are preferentially located on the intracellular face of plasma membranes. Recent X-ray studies of hydra annexin XII showed that it crystallized as a homohexamer with an intermolecular Ca2+ binding site separate from the type II Ca2+-dependent phospholipid binding site. On the basis of this hexamer structure, a novel mechanism was proposed to explain how annexins interact with membranes. The first step toward evaluating this proposal is to determine whether the annexin XII hexamer exists when the protein is not in a crystalline form. We now report that annexin XII in solution can be cross-linked with dimethyl suberimidate into multimers with apparent Mr's corresponding to trimers and hexamers as determined by SDS--polyacrylamide gel electrophoresis--the trimer band may correspond to incompletely cross-linked hexamers. Multimer formation was dependent on Ca2+ and was enhanced when the protein first was bound to phospholipid vesicles. To evaluate the role of the intermolecular Ca2+ site in annexin XII hexamer formation, one of the residues used to coordinate Ca2+, glutamate 105, was replaced with lysine (E105K). In solution, the E105K mutation inhibited hexamer formation in the presence of moderate (3 mM) but not high (25 mM) Ca2+. No inhibition of E105K annexin XII hexamer formation was observed in the presence of phospholipid, thereby suggesting that either (i) other interactions are capable of stabilizing the hexamer when bound to bilayers or (ii) only trimers form on bilayers and the observed hexamer bands were due to cross-linking of closely packed trimers. In summary, this study shows for the first time that annexin XII can form hexamers in solution and implicates the intermolecular Ca2+ site in hexamer formation. This study also shows that multimers form on bilayers but does not clearly establish whether the multimers are trimers or hexamers.

Isolation of a marker for head-specific cell differentiation in hydra

A cDNA was isolated from Hydra vulgaris coding for a 360 amino acid long polypeptide with no significant similarity to any known protein. In situ hybridization showed that the corresponding transcript is expressed exclusively in endodermal cells of the hypostome, hence its name hyp 1. Hyp 1 therefore represents a highly specific marker for the head region of hydra. In accordance with this, we found that hyp 1 expression was strong in tissues regenerating a head. Hyp 1 reappeared early during head regeneration, namely 6-8 h after initiation of regeneration by cutting. These findings imply that hyp 1 is an excellent marker for monitoring early events in head-specific differentiation processes in hydra.

Evolutionary conservation of a cell fate specification gene: the Hydra achaete-scute homolog has proneural activity in Drosophila

Members of the Achaete-scute family of basic helix-loop-helix transcription factors are involved in cell fate specification in vertebrates and invertebrates. We have isolated and characterized a cnidarian achaete-scute homolog, CnASH, from Hydra vulgaris, a representative of an evolutionarily ancient branch of metazoans. There is a single achaete-scute gene in Hydra, and the bHLH domain of the predicted gene product shares a high degree of amino acid sequence similarity with those of vertebrate and Drosophila Achaete-scute proteins. In Hydra, CnASH is expressed in a subset of the interstitial cells as well as differentiation intermediates of the nematocyte pathways. In vitro translated CnASH protein can form heterodimers with the Drosophila bHLH protein Daughterless, and these dimers bind to consensus Achaete-scute DNA binding sites in a sequence-specific manner. Ectopic expression of CnASH in wild-type late third instar Drosophila larvae and early pupae leads to the formation of ectopic sensory organs, mimicking the effect of ectopic expression of the endogenous achaete-scute genes. Expression of CnASH in flies that are achaete and scute double mutants gives partial rescue of the mutant phenotype, comparable to the degree of rescue obtained by ectopic expression of the Drosophila genes. These results indicate that the achaete-scute type of bHLH genes for cell fate specification, as well as their mode of action, arose early and have been conserved during metazoan evolution.

A 25.7 × 10(3) M(r) hydra metalloproteinase (HMP1), a member of the astacin family, localizes to the extracellular matrix of Hydra vulgaris in a head-specific manner and has a developmental function

Hydra extracellular matrix (ECM) is composed of a number of components seen in vertebrate ECM such as laminin, type IV collagen, fibronectin, and heparan sulfate proteoglycan. A number of functional studies have shown that hydra ECM plays an important role in pattern formation and morphogenesis of this simple metazoan. The present study was designed to identify matrix degrading proteinases in hydra and determine their potential function in hydra morphogenesis. Using SDS-PAGE gelatin-zymography, five gelatinolytic bands were identified with relative molecular masses of 67 × 10(3), 51–58 × 10(3) (a triplet) and 25–29 × 10(3), respectively. Inhibition studies indicated that all of these gelatinases were metalloproteinases. Gelatin-zymography indicated that there was a differential distribution of these gelatinases along the longitudinal axis of hydra, with the 67 × 10(3) M(r) gelatinase being concentrated in the body column, while the 51–58 × 10(3) M(r) gelatinase triplet and the 25–29 × 10(3) M(r) gelatinase concentrated in the head region. Purification procedures were successfully developed for the 25–29 × 10(3) M(r) metalloproteinase which has been termed hydra metalloproteinase 1 (HMP1) and which appeared as a single band with a SDS-PAGE mobility of 25.7 × 10(3) M(r). The N-terminal sequence of purified HMP1 indicated that it has structural homology with metalloproteinases that belong to the astacin family. Subsequent cloning and sequencing of cDNA clones confirmed the identification of HMP1 as an astacin-like metalloproteinase. Immunocytochemical studies with antibodies generated against the purified enzyme and to a synthetic peptide indicated that HMP1 was localized to the ECM of tentacles. Functional studies were performed in which purified HMP1, anti-HMP1 IgG, or suspected substrates of HMP1 (e.g. growth factors such as TGF-beta 1) were introduced into the interepithelial compartment of hydra using a ‘DMSO loading’ procedure. These studies indicated that HMP1 has a functional role during a number of developmental processes such as head regeneration and cell differentiation/transdifferentiation of tentacle battery cells.

The expression of different annexins in the fish embryo is developmentally regulated

The expression of annexins, a family of Ca2+/phospholipid-binding proteins, was analyzed by biochemical and immunological criteria in the fish Misgurnus fossilis (loach), which is a good model system to study embryonic events. Five different annexins (loach annexins A to E) are present as a maternal pool in the unfertilized eggs. Only annexin E is newly synthesized in the early embryo. Its synthesis, already apparent at mid-blastula, decreases in later stages when two additional annexins (F and G) appear. They are present among the newly synthesized polypeptides of mid-gastrula and later embryonic stages and are also found in loach larvae. The developmentally controlled expression of several annexins indicates a specific role of these proteins at certain embryonic stages.

Differential tissue expression of Annexin VIII in human

The expression of Annexins V and VIII by human lung, liver, kidney, skin, heart, uterus, spleen and skeletal muscle was investigated by ELISA. All investigated tissues contained Annexin V. Its level varied with the tissue from around 5 microgram (skin) to approximately 120 micrograms (spleen) per g of wet tissue. Contradistinctionally Annexin VIII expression was less ubiquitous and less abundant. Only lung, skin, liver, and kidney expressed Annexin VIII. Its levels were approximately 100-fold less then the Annexin V levels. Immunohistochemical analysis of lung sections revealed Annexin VIII presence exclusively in the endothelia. Annexin V and VIII levels of cultured human umbilical vein endothelial cells, human arterial smooth muscle cells, human lung fibroblasts and HeLa cells were measured by ELISA. All cell types expressed Annexin V whereas only HeLa cells had detectable levels of Annexin VIII. The results indicate a tissue specific expression of Annexin VIII by lung endothelium, suggesting a highly specialised function.

The effects of PMA and TFP and alterations in intracellular pH and calcium concentration on the membrane associations of phospholipid-binding proteins fodrin, protein kinase C and annexin II in cultured MDCK cells

Annexin II, alpha-fodrin and protein kinase C (PKC) are associated with the cytoplasmic surface of the plasma membranes. When assayed with liposomes, they show affinity for acidic phospholipids and bind calcium ions. They also respond to or participate in cell signal transduction by altered membrane binding properties. In the present work we have studied the properties of these proteins in epithelial MDCK cells in response to elevated intracellular calcium ion concentration, lowered pH, treatment with tumor promoter phorbol myristoyl acetate (PMA) and calmodulin inhibitor trifluoperazine (TFP). In untreated polarized MDCK cells annexin II was seen both along the lateral walls and membranes of intracellular vesicles, fodrin was located along the lateral walls, whereas PKC was seen in the cytoplasm. There was no observable translocation of these proteins upon elevation of the intracellular calcium concentration using a calcium ionophore A23187. On the other hand, treatment with TFP led to a release of annexin II from the plasma membranes which was accompanied by a transient peak in the intracellular calcium. Treatment with PMA led to a loss of the cubic form of the cells, a slight elevation in the intracellular calcium concentration and a drop in the intracellular pH. Simultaneously fodrin was released from the lateral walls, but still remained insoluble in Triton X-100, PKC became associated with the intracellular membranes and fibers, whereas annexin II remained along the lateral walls. These changes could be prevented by clamping the intracellular pH neutral during PMA treatment. On the other hand, lowering of intracellular pH below 6.5 with the nigericin treatment led to a similar translocation of fodrin and PKC as PMA. This suggests that the protein redistribution is caused by cytoplasmic acidification and is due to an increased hydrophobicity and enhanced protonation of lipids and proteins. In contrast, no changes were seen in the annexin II distribution in response to altered pH. Hence, its release by TFP is presumably due to changes in the cationic properties of the inner phase of the plasma membrane. Thus, proteins which show similar binding properties with liposomes show different characteristics in their association with the intracellular membranes.

The subcellular distribution of early endosomes is affected by the annexin II2p11(2) complex

The tyrosine kinase substrate annexin II is a member of a multigene family of Ca2+ and lipid-binding proteins which have been implicated in a number of membrane-related events. We have analyzed the subcellular distribution of annexin II in relation to other cellular components in normal and specifically manipulated MDCK cells. In a polarized monolayer of MDCK cells annexin II and its cellular ligand p11 are restricted almost exclusively to the cortical regions of the cells which also contain peripheral early endosomes. Treatment of the polarized cells with low Ca2+ medium leads to a disintegration of the cortical cytoskeleton and a translocation of both, the annexin II2p11(2) complex and early endosomes, to the cytoplasm. A similar translocation which is however specific for the annexin II2p11(2) complex and early endosomes and does not affect other elements of the cell cortex is observed in cells expressing a trans-dominant annexin II-p11 mutant. This chimeric mutant protein causes the aggregation of endogenous annexin II and p11 and the simultaneous detachment of early endosomes from the cell periphery resulting in the binding of the early endosomes but no other components of the endocytotic or biosynthetic pathways to the annexin II/p11 aggregates. The specificity of this effect argues for the association of the annexin II2p11(2) complex with early endosomes and suggests that this association contributes to establish the peripheral localization of early endosomal structures.