Novel substituted isoxazole FXR agonists with cyclopropyl, hydroxycyclobutyl and hydroxyazetidinyl linkers: Understanding and improving key determinants of pharmacological properties.

Several isoxazole-containing series of FXR agonists have been published over the last 15years, subsequent to the prototypical amphiphilic 'hammerhead'-type structure that was originally laid out by GW4064, the first potent synthetic FXR agonist. A set of novel compounds where the hammerhead is connected to the terminal carboxylic acid-bearing aryl or heteroaryl moiety by either a cyclopropyl, a hydroxycyclobutyl or a hydroxyazetidinyl linker was synthesized in order to improve upon the ADME properties of such isoxazoles. The resulting compounds all demonstrated high potencies at the target receptor FXR but with considerable differences in their physicochemical and in vivo profiles. The structure-activity relationships for key chemical features that have a major impact on the in vivo pharmacology of this series are discussed.

The nuclear bile acid receptor FXR controls the liver derived tumor suppressor histidine-rich glycoprotein.

The nuclear bile acid receptor Farnesoid X receptor (FXR) is strongly expressed in liver and intestine, controls bile acid and lipid homeostasis and exerts tumor-protective functions in liver and intestine. Histidine-rich glycoprotein (HRG) is an abundant plasma protein produced by the liver with the proposed function as a pattern recognition molecule involved in the clearance of immune complexes, necrotic cells and pathogens, the modulation of angiogenesis, the normalization of deranged endothelial vessel structure in tumors and tumor suppression. FXR recognition sequences were identified within a human HRG promoter fragment that mediated FXR/FXR-agonist dependent reporter gene activity in vitro. We show that HRG is a novel transcriptional target gene of FXR in human hepatoma cells, human upcyte® primary hepatocytes and 3D human liver microtissues in vitro and in mouse liver in vivo. Prolonged administration of the potent nonsteroidal FXR agonist PX20606 increases HRG levels in mouse plasma. Finally, daily oral administration of this FXR agonist for seven days resulted in a significant increase of HRG levels in the plasma of healthy human male volunteers during a clinical Phase I safety study. HRG might serve as a surrogate marker indicative of liver-specific FXR activation in future human clinical studies. Furthermore, potent FXR agonists might be beneficial in serious health conditions where HRG is reduced, for example, in hepatocellular carcinoma but also other solid cancers, liver failure, sepsis and pre-eclampsia.

Synthetic farnesoid X receptor agonists induce high-density lipoprotein-mediated transhepatic cholesterol efflux in mice and monkeys and prevent atherosclerosis in cholesteryl ester transfer protein transgenic low-density lipoprotein receptor (-/-) mice.

Farnesoid X receptor (FXR), a bile acid-activated nuclear hormone receptor, plays an important role in the regulation of cholesterol and more specifically high-density lipoprotein (HDL) homeostasis. Activation of FXR is reported to lead to both pro- and anti-atherosclerotic effects. In the present study we analyzed the impact of different FXR agonists on cholesterol homeostasis, plasma lipoprotein profiles, and transhepatic cholesterol efflux in C57BL/6J mice and cynomolgus monkeys and atherosclerosis development in cholesteryl ester transfer protein transgenic (CETPtg) low-density lipoprotein receptor (LDLR) (-/-) mice. In C57BL/6J mice on a high-fat diet the synthetic FXR agonists isopropyl 3-(3,4-difluorobenzoyl)-1,1-dimethyl-1,2,3,6-tetrahydroazepino[4,5-b]indole-5-carboxylate (FXR-450) and 4-[2-[2-chloro-4-[[5-cyclopropyl-3-(2,6-dichlorophenyl)-4-isoxazolyl]methoxy]phenyl]cyclopropyl]benzoic acid (PX20606) demonstrated potent plasma cholesterol-lowering activity that affected all lipoprotein species, whereas 3-[2-[2-chloro-4-[[3-(2,6-dichlorophenyl)-5-(1-methylethyl)-4-isoxazolyl]methoxy]phenyl]ethenyl]benzoic acid (GW4064) and 6-ethyl chenodeoxycholic acid (6-ECDCA) showed only limited effects. In FXR wild-type mice, but not FXR(-/-) mice, the more efficacious FXR agonists increased fecal cholesterol excretion and reduced intestinal cholesterol (re)uptake. In CETPtg-LDLR(-/-) mice PX20606 potently lowered total cholesterol and, despite the observed HDL cholesterol (HDLc) reduction, caused a highly significant decrease in atherosclerotic plaque size. In normolipidemic cynomolgus monkeys PX20606 and 6-ECDCA both reduced total cholesterol, and PX20606 specifically lowered HDL(2c) but not HDL(3c) or apolipoprotein A1. That pharmacological FXR activation specifically affects this cholesterol-rich HDL(2) subclass is a new and highly interesting finding and sheds new light on FXR-dependent HDLc lowering, which has been perceived as a major limitation for the clinical development of FXR agonists.

ATP distribution and localization of mitochondria in Suberites domuncula (Olivi 1792) tissue.

The metabolic energy state of sponge tissue in vivo is largely unknown. Quantitative bioluminescence-based imaging was used to analyze the ATP distribution of Suberites domuncula (Olivi 1792) tissue, in relation to differences between the cortex and the medulla. This method provides a quantitative picture of the ATP distribution closely reflecting the in vivo situation. The obtained data suggest that the highest ATP content occurs around channels in the sponge medulla. HPLC reverse-phase C-18, used for measurement of ATP content, established a value of 1.62 µmol ATP g⁻¹ dry mass in sponge medulla, as opposed to 0.04 µmol ATP g⁻¹ dry mass in the cortex, thus indicating a specific and defined energy distribution. These results correlate with the mitochondria localization, determined using primary antibodies against cytochrome oxidase c subunit 1 (COX1) (immunostaining), as well as with the distribution of arginine kinase (AK), essential for cellular energy metabolism (in situ hybridization with AK from S. domuncula; SDAK), in sponge sections. The highest energy consumption seemed to occur in choanocytes, the cells that drive the water through the channel system of the sponge body. Taken together, these results showed that the majority of energetic metabolism in S. domuncula occurs in the medulla, in the proximity of aqueous channels.

Synthesis and pharmacological validation of a novel series of non-steroidal FXR agonists.

To overcome the known liabilities of GW4064 a series of analogs were synthesized where the stilbene double bond is replaced by an oxymethylene or amino-methylene linker connecting a terminal benzoic acid with a substituted heteroaryl in the middle ring position. As a result we discovered compounds with increased potency in vitro that cause dose-dependent reduction of plasma triglycerides and cholesterol in db/db mice down to 2 x 1 mg/kg/day upon oral administration.

Regional and modular expression of morphogenetic factors in the demosponge Lubomirskia baicalensis.

Some sponges [phylum Porifera], e.g. the demosponges Lubomirskia baicalensis or Axinella polypoides, show an arborescent growth form. In the freshwater sponge L. baicalensis this morphotype is seen mostly in depths below 4m while in more shallow regions it grows as a crust. The different growth forms are determined in nature very likely by water current and/or light. The branches of this species are composed of modules, arranged along the apical-basal axis. The modules are delimited by a precise architecture of the spicule bundles; longitudinal bundles originate from the apex of the earlier module, while at the basis of each module these bundles are cross-linked by traverse bundles under formation of annuli. Genes encoding putative morphogenetic factors, myotrophin and epidermal growth factor (EGF)-like molecules, and one gene of an antagonist for the Wnt signaling pathway, the soluble frizzled molecule, have been identified and characterized. Their expression levels as well as those of silicatein, one major spicule-forming molecule, have been studied in the crusts and the modules. The data revealed that at the apices of each module higher level of expression of myotrophin and EGF can be detected, while the base of each module is characterized by a high steady-state expression level of soluble frizzled molecule. These results suggest that module formation in L. baicalensis is controlled by a tuned interaction of agonistic (e.g., myotrophin and EGF) as well as antagonistic morphogenetic factors (e.g., soluble frizzled molecule).

Cold stress defense in the freshwater sponge Lubomirskia baicalensis. Role of okadaic acid produced by symbiotic dinoflagellates.

The endemic freshwater sponge Lubomirskia baicalensis lives in Lake Baikal in winter (samples from March have been studied) under complete ice cover at near 0 degrees C, and in summer in open water at 17 degrees C (September). In March, specimens show high metabolic activity as reflected by the production of gametes. L. baicalensis lives in symbiosis with green dinoflagellates, which are related to Gymnodinium sanguineum. Here we show that these dinoflagellates produce the toxin okadaic acid (OA), which is present as a free molecule as well as in a protein-bound state. In metazoans OA inhibits both protein phosphatase-2A and protein phosphatase-1 (PP1). Only cDNA corresponding to PP1 could be identified in L. baicalensis and subsequently isolated from a L. baicalensis cDNA library. The deduced polypeptide has a molecular mass of 36 802 Da and shares the characteristic domains known from other protein phosphatases. As determined by western blot analysis, the relative amount of PP1 is almost the same in March (under ice) and September (summer). PP1 is not inhibited by low OA concentrations (100 nm); concentrations above 300 nm are required for inhibition. A sponge cell culture system (primmorphs) was used to show that at low temperatures (4 degrees C) expression of hsp70 is strongly induced and hsp70 synthesis is augmented after incubation with 100 nm OA to levels measured at 17 degrees C. In the enriched extract, PP1 activity at 4 degrees C is close to that measured at 17 degrees C. Immunoabsorption experiments revealed that hsp70 contributes to the high protein phosphatase activity at 4 degrees C. From these data we conclude that the toxin OA is required for the expression of hsp70 at low temperature, and therefore contributes to the cold resistance of the sponge.

Histochemical and electron microscopic analysis of spiculogenesis in the demosponge Suberites domuncula.

The skeleton of demosponges is built of spicules consisting of biosilica. Using the primmorph system from Suberites domuncula, we demonstrate that silicatein, the biosilica-synthesizing enzyme, and silicase, the catabolic enzyme, are colocalized at the surface of growing spicules as well as in the axial filament located in the axial canal. It is assumed that these two enzymes are responsible for the deposition of biosilica. In search of additional potential structural molecules that might guide the mineralization process during spiculogenesis to species-specific spicules, electron microscopic studies with antibodies against galectin and silicatein were performed. These studies showed that silicatein forms a complex with galectin; the strings/bundles of this complex are intimately associated with the surface of the spicules and arranged concentrically around them. Collagen fibers are near the silactein/galectin complexes. The strings/bundles formed from silicatein/galectin display a lower degree of orientation than the collagen fibers arranged in a highly ordered pattern around the spicules. These data indicate that species-specific formation of spicules involves a network of (diffusible) regulatory factor(s) controlling enzymatic silica deposition; this mineralization process proceeds on a galectin/collagen organic matrix.

Axial (apical-basal) expression of pro-apoptotic and pro-survival genes in the lake baikal demosponge Lubomirskia baicalensis.

Like in all other Metazoa, also in sponges (Porifera) proliferation, differentiation, and death of cells are controlled by apoptotic processes, thus allowing the establishment of a Bauplan (body plan). The demosponge Lubomirskia baicalensis from the Lake Baikal is especially suitable to assess the role of the apoptotic molecules, since its grade of construction is highly elaborated into an encrusting base and branches composed of modules lined up along the apical-basal axis. The four cDNAs, ALG-2, BAK, MA-3, and Bcl-2, were isolated from this sponge species. The expression levels of these genes follow characteristic gradients. While the proapoptotic genes are highly expressed at the base of the branches and comparably low at the top, the pro-survival gene follows an opposite gradient. Parallel with the tuned expression of these genes, the activities of the apoptosis-executing enzymes caspase-8 (IETDase activity) and caspase-3 (DEVDase activity) are lowest at the top of the branch and highest at their base. This characteristic expression/activity pattern of the genes/enzymes, which had been determined in a few specimens, collected from an unpolluted, natural site, appears reversed in specimens collected from an anthropogenically polluted site. These findings indicate the involvement of apoptotic proteins in the axis formation (branches) in L. baicalensis.

Silica transport in the demosponge Suberites domuncula: fluorescence emission analysis using the PDMPO probe and cloning of a potential transporter.

Silicon is, besides oxygen, the most abundant element on earth. Only two taxa use this element as a major constituent of their skeleton, namely sponges (phylum Porifera) and unicellular diatoms. Results from combined cytobiological and molecularbiological techniques suggest that, in the demosponge Suberites domuncula, silicic acid is taken up by a transporter. Incubation of cells with the fluorescent silica tracer PDMPO [2-(4-pyridyl)-5-[[4-(2-dimethylaminoethylaminocarbamoyl)methoxy]phenyl]-oxazole] showed a response to silicic acid by an increase in fluorescence; this process is temperature-dependent and can be blocked by DIDS (4,4-di-isothiocyanatostilbene-2,2-disulphonic acid). The putative NBC (Na+/HCO3-) transporter was identified, cloned and analysed. The deduced protein comprises all signatures characteristic of those molecules, and phylogenetic analysis also classifies it to the NBC transporter family. This cDNA was used to demonstrate that the expression of the gene is strongly up-regulated after treatment of cells with silicic acid. In situ hybridization demonstrated that the expression of the sponge transporter occurs in those cells that are located adjacent to the spicules (the skeletal element of the animal) or in areas in which spicule formation occurs. We conclude that this transporter is involved in silica uptake and have therefore termed it the NBCSA [Na+/HCO3-[Si(OH)4]] co-transporter.

Molecular markers for germ cell differentiation in the demosponge Suberites domuncula.

Sponges (phylum Porifera) are simple metazoans for which no molecular information on gametogenesis and larval development is available. To support the current study, it was confirmed by histology that oocytes and larvae were produced by the demosponge Suberites domuncula. Three genes/expressed products from S. domuncula whose expression correlated with sexual reproduction were identified and characterized (they are used here as marker genes): i) a receptor tyrosine kinase (RTK) with sequence similarity in the tyrosine kinase domain to fibroblast growth factor receptors; ii) the sex-determining protein FEM1 and iii) the sperm associated antigen (SAA) of triploblasts. Antibodies against the extracellular domain of the RTK specifically stained oocytes and larvae in S. domuncula tissue sections. Induction of these three genes was successful at elevated temperature, a factor which also promotes natural gametogenesis. In situ hybridization analyses revealed that FEM1 and SAA were expressed in those areas in which gametogenesis begins. Our results indicate that genes which play a role in sex determination may be present in Porifera.

Toll-like receptors are part of the innate immune defense system of sponges (demospongiae: Porifera).

During evolution and with the emergence of multicellular animals, the need arose to ward off foreign organisms that threaten the integrity of the animal body. Among many different receptors that participate in the recognition of microbial invaders, toll-like receptors (TLRs) play an essential role in mediating the innate immune response. After binding distinct microbial components, TLRs activate intracellular signaling cascades that result in an induced expression of diverse antimicrobial molecules. Because sponges (phylum Porifera) are filter feeders, they are abundantly exposed to microorganisms that represent a potential threat. Here, we describe the identification, cloning, and deduced protein sequence from 3 major elements of the poriferan innate response (to bacterial lipopeptides): the TLR, the IL-1 receptor-associated kinase-4-like protein (IRAK-4l), and a novel effector caspase from the demosponge Suberites domuncula. Each molecule shares significant sequence similarity with its homologues in higher Metazoa. Sequence homologies were found in particular within the family-specific domains toll/interleukin-1 receptor/resistance (TLR family), Ser/Thr/Tyr kinase domain (IRAK family), and CASc (caspase family). In addition, in situ hybridization and immunohistological analyses revealed an abundance of SDTLR (TLR) transcripts in epithelial layers of the sponge surface (exopinacoderm and endopinacoderm). Furthermore, it is shown that both SDTLR and SDIRAK-4 like (IRAK) are expressed constitutively, regardless of treatment with synthetic triacyl lipopeptide Pam(3)Cys-Ser-(Lys)(4). In contrast, SDCASL (caspase) expression is highly Pam(3)Cys-Ser-(Lys)(4) inducible. However, blocking of the lipopeptide with recombinant TLR prior to its application completely prevented the induced expression of this poriferan caspase. These results underscore that the phylogenetically oldest extant metazoan phylum is provided already with the signaling pathways of the antimicrobial host-defense system of Metazoa.

Damipipecolin and damituricin, novel bioactive bromopyrrole alkaloids from the Mediterranean sponge Axinella damicornis.

Two new bromopyrrole alkaloids, damipipecolin (1) and damituricin (2), have been isolated from the Mediterranean sponge Axinella damicornis, and their structures established through spectroscopic methods. Compounds 1 and 2 extend the structural variety of the so far known pyrrole alkaloids; in these compounds, the 4-bromopyrrole 2-carboxylic acid is directly condensed with a non-protein cyclic alpha-amino acid, the (2R, 4R)-trans-4-hydroxypipecolic acid and (2R, 4R)-cis-N,N'-dimethyl-4-hydroxyproline (D-turicine) in 1 and 2, respectively. Compounds 1 and 2 were found to display a modulating effect of the serotonin receptor activity in vitro.

Novel bioactive bromopyrrole alkaloids from the Mediterranean sponge Axinella verrucosa.

The Mediterranean sponge Axinella verrucosa has been investigated for its alkaloid composition and has been found to produce a complex mixture of bromopyrrole alkaloids. Along with the previously isolated compounds 5-18, four novel alkaloids of this class, compounds 1-4, have been isolated, and their structures established through spectroscopic methods. Compounds 1-4 were found to display neuroprotective activity against the agonists serotonin and glutamate in vitro.

Molecular control of serial module formation along the apical-basal axis in the sponge Lubomirskia baicalensis: silicateins, mannose-binding lectin and mago nashi.

The freshwater sponge Lubomirskia baicalensis (from Lake Baikal) is characterized by a body plan composed of serial modules which are arranged along an apical-basal axis. In shallow water, the sponge occurs only encrusting, while in deeper environment (>3 m), this species forms branches and grows in an arborescent manner. Each module is stabilized by bundles of spined oxeas (amphioxeae spicules). The spicules are surrounded by an organic matrix. cDNAs for structural proteins (silicatein and mannose-binding lectin (MBL)) as well as for one regulatory protein (mago nashi) were isolated from L. baicalensis. Surprisingly the silicatein alpha molecule exists in several, at least four, isoforms (a1 to a4). Expression studies revealed that the steady-state levels of transcripts for the silicateins, the mannose-binding lectin, and mago nashi are highest at the top of the branches, while only very low levels are found in cells at the base. Based on in situ hybridization studies, evidence is presented that the spicule formation (1) starts and is completed inside of the bundles, and (2) occurs together with the mannose-binding lectin from the surfaces of the bundles. The data suggest that the modules are sequentially formed. It is speculated that the expression of the silicateins and the mannose-binding lectin might be (partially) controlled by mago nashi.

Biosilica formation in spicules of the sponge Suberites domuncula: synchronous expression of a gene cluster.

The formation of spicules is a complicated morphogenetic process in sponges (phylum Porifera). The primmorph system was used to demonstrate that in the demosponge Suberites domuncula the synthesis of the siliceous spicules starts intracellularly and is dependent on the concentration of silicic acid. To understand spicule formation, a cluster of genes was isolated. In the center of this cluster is the silicatein gene, which codes for the enzyme that synthesizes spicules. This gene is flanked by an ankyrin repeat gene at one side and by a tumor necrosis factor receptor-associated factor and a protein kinase gene at the other side. All genes are strongly expressed in primmorphs and intact animals after exposure to silicic acid, and this expression is restricted to those areas where the spicule formation starts or where spicules are maintained in the animals. Our observations suggest that in S. domuncula a coordinated expression of physically linked genes is essential for the synthesis of the major skeletal elements.

Arginine kinase in the demosponge Suberites domuncula: regulation of its expression and catalytic activity by silicic acid.

In Demospongiae (phylum Porifera) the formation of the siliceous skeleton, composed of spicules, is an energetically expensive reaction. The present study demonstrates that primmorphs from the demosponge Suberites domuncula express the gene for arginine kinase after exposure to exogenous silicic acid. The deduced sponge arginine kinase sequence displays the two characteristic domains of the ATP:guanido phosphotransferases; it can be grouped to the 'usual' mono-domain 40 kDa guanidino kinases (arginine kinases). Phylogenetic studies indicate that the metazoan guanidino kinases evolved from this ancestral sponge enzyme; among them are also the 'unusual' two-domain 80 kDa guanidino kinases. The high expression level of the arginine kinase gene was already measurable 1 day after addition of silicic acid by northern blot, as well as by in situ hybridization analysis. Parallel determinations of enzyme activity confirmed that high levels of arginine kinase are present in primmorphs that had been exposed for 1-5 days to silicic acid. Finally, transmission electron-microscopical studies showed that primmorphs containing high levels of arginine kinase also produce siliceous spicules. These data highlight that silicic acid is an inorganic morphogenetic factor that induces the expression of the arginine kinase, which in turn probably catalyzes the reversible transfer of high-energy phosphoryl groups.

Innate immune defense of the sponge Suberites domuncula against bacteria involves a MyD88-dependent signaling pathway. Induction of a perforin-like molecule.

Sponges (phylum Porifera) are the phylogenetically oldest metazoa; as filter feeders, they are abundantly exposed to marine microorganisms. Here we present data indicating that the demosponge Suberites domuncula is provided with a recognition system for gram-negative bacteria. The lipopolysaccharide (LPS)-interacting protein was identified as a receptor on the sponge cell surface, which recognizes the bacterial endotoxin LPS. The cDNA was isolated, and the protein (Mr 49,937) was expressed. During binding to LPS, the protein dimerizes and interacts with MyD88, which was also identified and cloned. The sponge MyD88 (Mr 28,441) is composed of two protein interaction domains, a Toll/interleukin-1 receptor domain (found in MyD88 and in Toll-like receptors) and a death domain (present in MyD88 and interleukin-1 receptor-associated kinase). Northern blot experiments and in situ hybridization studies showed that after LPS treatment, the level of the LPS-interacting protein remains unchanged, whereas MyD88 is strongly up-regulated. A perforin-like molecule (Mr 74,171), the macrophage-expressed protein, was identified as an executing molecule of this pathway. This gene is highly expressed after LPS treatment, especially at the surfaces of the animals. The recombinant protein possesses biological activity and eliminates gram-negative bacteria; it is inactive against gram-positive bacteria. These data indicate that S. domuncula is provided with an innate immune system against gram-negative bacteria; the ligand LPS (a pathogen-associated molecular pattern) is recognized by the pattern recognition receptor (LPS-interacting protein), which interacts with MyD88. A signal transduction is established, which results in an elevated expression of MyD88 as well as of the macrophage-expressed protein as an executing protein.

Allograft rejection in the mixed cell reaction system of the demosponge Suberites domuncula is controlled by differential expression of apoptotic genes.

Until recently, the lack of molecular probes hampered the determination of the expression of pro-apoptotic and anti-apoptotic genes in sponge. In an approach to solve this problem, the present study describes a variety of cDNAs from the demosponge Suberites domuncula, coding for proteins that are characteristic for the initiation of apoptosis (caspase, MA3, ALG-2 protein), for the prevention of programmed cells death (2 Bcl-2 homology proteins, FAIM-related polypeptide, and DAD-1-related protein), and for morphogenetic processes (retinoid X receptor). They were used as probes to monitor the expression levels in vitro in the allogeneic mixed sponge cell reaction (MSCR) system. In the allogeneic MSCR, two-cell aggregates (primmorphs) from genetically different animals of the same species were positioned next to each other. After approximately 8 days in culture, one of the primmorphs underwent apoptotic death, while the second remained alive. The expression levels of the aforementioned genes were determined by Northern blotting and by in situ hybridization. These experiments revealed that in the apoptotic primmorph, the characteristic apoptotic genes were expressed, while in the non-apoptotic aggregates the cell-survival genes are highly upregulated. Interestingly, the transcript levels of retinoid X receptor were higher in apoptotic primmorphs than in the non-apoptotic aggregate in the assay. Our data show for the first time that in the in vitro MSCR system, allogeneic recognition led to apoptotic cell death in one partner, while the other one survived. We suggest that this process is controlled by a differential expression of the pro-apoptotic and pro-survival genes studied here.

Differentiation capacity of epithelial cells in the sponge Suberites domuncula.

Sponges (phylum Porifera) represent the oldest metazoans. Their characteristic metazoan adhesion molecules and transcription factors enable them to establish a complex "Bauplan"; three major differentiated cell types (epithelial cells, skeletal cells/sclerocytes, and contractile cells) can be distinguished. Since no molecular markers are as yet available to distinguish these somatic cells or the corresponding embryonic cells from which they originate, we have selected the following three genes for their characterization: noggin (a signaling molecule in development), a caspase that encodes an apoptotic molecule, and silicatein. Silicatein is an enzyme that is involved in the synthesis of siliceous spicules and can hence be considered as a marker for scleroblasts. We have used the demosponge Suberites domuncula as a model system. During the hatching of the gemmules (asexual reproduction bodies) of S. domuncula, the expression of both noggin and caspase increases, whereas no transcripts for silicatein can be detected, irrespective of the presence of silicate or ferric iron (Fe3+) in the medium. In contrast, in adult specimens, silicate/Fe3+ cause an increased expression of these genes. In situ analysis has revealed that the first cells that express noggin, caspase, and silicatein lie in the epithelial layer of the pinacoderm. In a later phase, the noggin- and silicatein-positive cells migrate into the mesohyl, where they are found in association with spicules. Thus, the pinacoderm of sponges contains cells that have a differentiating capacity and from which somatic cells, such as skeletal cells/sclerocytes, derive.