Traditional and Modern Biomedical Prospecting: Part II-the Benefits: Approaches for a Sustainable Exploitation of Biodiversity (Secondary Metabolites and Biomaterials from Sponges)

Abstract

The progress in molecular and cell biology has enabled a rational exploitation of the natural resources of the secondary metabolites and biomaterials from sponges (phylum Porifera). It could be established that these natural substances are superior for biomedical application to those obtained by the traditional combinatorial chemical approach. It is now established that the basic structural and functional elements are highly conserved from sponges to the crown taxa within the Protostomia (Drosophila melanogaster and Caenorhabditis elegans) and Deuterostomia (human); therefore, it is obvious that the molecular etiology of diseases within the metazoan animals have a common basis. Hence, the major challenge for scientists studying natural product chemistry is to elucidate the target(s) of a given secondary metabolite, which is per se highly active and selective. After this step, the potential clinical application can be approached. The potential value of some selected secondary metabolites, all obtained from sponges and their associated microorganisms, is highlighted. Examples of compounds that are already in medical use (inhibition of tumor/virus growth [arabinofuranosyl cytosine and arabinofuranosyl adenine]), or are being considered as lead structures (acting as cytostatic and anti-inflammatory secondary metabolites [avarol/avarone], causing induction of apoptosis [sorbicillactone]) or as prototypes for the interference with metabolic pathways common in organisms ranging from sponges to humans (modulation of pathways activated by fungal components [aeroplysinin], inhibition of angiogenesis [2-methylthio-1,4-napthoquinone], immune modulating activity [FK506]) are discussed in this study. In addition, bioactive proteins from sponges are listed (antibacterial activity [pore-forming protein and tachylectin]). Finally, it is outlined that the skeletal elements-the spicules-serve as blueprints for new biomaterials, especially those based on biosilica, which might be applied in biomedicine. These compounds and biomaterials have been isolated/studied by members of the German Center of Excellence BIOTECmarin. The goal for the future is to successfully introduce some of these compounds in the treatment of human diseases in order to raise the public awareness on the richness and diversity of natural products, which should be sustainably exploited for human benefit.

Figure 1

High value of (natural) secondary metabolites in comparison with compounds obtained by combinatorial chemistry. Left: The number of chemicals synthesized solely in a combinatorial chemical manner, which have potential biomedical value, decreases drastically during the preclinical and clinical screening/testing phases. If at all, very few compounds reach the stage of registration (9). Right: Natural secondary metabolites are per se biologically active. If the biological target of a given compound could be identified, (ideally) all of them can reach the clinical phases.

Figure 2

Strategies of sponges, together with their symbiotic microorganisms, bacteria and fungi, for protection against attackers. Sponges are provided with two forms of direct protections; (i) single protection and (ii) dual protection. In single protection, bioactive molecules are produced which are directed only toward the invading pro- and eukaryotic organisms. In the dual form of protection, the secondary metabolites are directed against the non-self organisms and also positively modulate the host metabolism. (iii) The sponges produce defense proteins that neutralize/kill foreign prokaryotic pathogenic microorganisms and eukaryotic predators (immune protection). (iv) The sponges facultatively comprise surface bacteria that produce antifouling secondary metabolites (indirect protection). The red arrows indicate bacterial origin, and the blue ones indicate that the compounds are produced by sponges. Examples of secondary metabolites and proteins involved in these strategies of protection are provided.

Figure 3

Research area around Rovinj (Croatia). Left: A map of Ortelius (year 1580 [Histriae tabula a Petro Coppo]) depicting the Limski Canal (arrow) with the connected river Lemo flumen. It has its source in the 20 km distant mountains. Right: A map from 1900 (67), showing the 11 km long Limski Canal. Similarly, another river (Arsa flumen) (arrow head), which had been connected with the lake Vrana in the year 1580, disappeared later. In the Limski Canal, endemic sponge species, e.g., Tethya limski, Geodia rovinjensis and Thoosa istriaca have been described. The scale bar is provided close to the location of Rovinj.

Figure 4

Arabinofuranosyl cytosine. (A) Sponges of the genus Tethya (Demospongiae)—views from the past and the present. (Aa) a sketch of this sponge by Gesner (1558); (Ab) a picture of Tetia sphaerica (Donati 1753); (Ac) Tethya lyncurium or Tethya aurantium with its protruding buds [×3]; (Ad) Tethya limski, an endemic species from the ‘Limski Canal’ (Rovinj; Croatia) [×0.5]. (B) Evolutionary shaping (evochemistry) of a normal metabolite (adenosine) into a bioactive secondary metabolite (ara-A) in the sponge Cryptotethya crypta during over 500 million years of biochemical selection for the highest potency in action and selectivity in function. (C) Mode of action of ara-A. This nucleoside analog in its triphosphorylated form competitively inhibits the herpes simplex virus (HSV) coded DNA polymerase. The ratio of the Michaelis constant (K_m) and the inhibitor constant (K_i) reflects the efficacy of this inhibitor on HSV polymerase.

Figure 5

Aeroplysinin. The Mediterranean sponge Verongia aerophoba (left) produces the secondary metabolite aeroplysinin (right). This tyrosine kinase inhibitor modulates the recognition system of sponges for fungi. The fungal model compound curdlan, composed of (1→3)-b-D-glucan, interacts with its receptor, the (1→3)-β-D-glucan binding protein (GLUBP), in the S. domuncula system. Controlled by a tyrosine kinase, which can be inhibited by the aeroplysinin, this recognition results in an expression of three genes: one coding for the glucan binding protein precursor (GLUBPp), the second for the fibrinogen-like protein (FIBG) and the third for the epidermal growth factor (EGF)-precursor. While fibrinogen is assumed to be involved in the recognition of fungi, EGF might be involved in the control of proliferation of sponge cells.

Figure 6

Sorbicillactone A. (A) This secondary metabolite is synthesized by the sponge-associated fungus, Biconiosporella corniculata Schaumann (Ascomycota). (B) Structure of sorbicillactone A. (C) Inhibitory activity of 0.4 μg/ml of sorbicillactone A on murine leukemic lymphoblasts L5178y (leukaemia), human cervix HeLa S3 cells (sarcoma) and rat adrenal phaeochromocytoma cells PC-12 (adenoma). The inhibitory activity of the compound on the respective tumor cell lines is given in percentage. In parallel, the inhibition of sorbicillactone A was tested against S. domuncula cells, which remained almost non-affected. (D) Crystals of sorbicillactone A, after having upscaled the production of this natural product to amounts in grams (courtesy Dr G Bringmann, Würzburg).

Figure 7

Sponge-bacteria relationship. (A) Formation of diphenols from monophenols mediated by the sponge enzyme tyrosinase. The reaction product(s) is taken up by surface-associated bacteria and used as carbon source. (B) Schematic model of the domains encoding the polyketide synthase (KS) cloned from a bacterium that had been isolated from the sponge S. domuncula (the scales indicate the stretches on the DNA [given in kb]). Lower scheme: the modular polyketide synthase is composed of at least the following enzymic activities: β-keto acylthioester synthase (KS), acyltransferase (AT), ketoacyl-reductase (KR), dehydratase (DH), enoyl reductase (ER) and thioesterase domain (TE). Multiple copies of active site units (in this case, KS-1 to KS-4) are usually present. The photograph at the center of the figure shows a semi-thin section through the mesohyl compartment adjacent to the lacunae, which are surrounded by an epithelium (formed by endopinacocytes). One bacteriocyte (arrow head) is shown, which is embedded in the epithelium that surrounds a water canal (C); it is almost completely filled with bacteria (light micrograph; [× 250]). Upper scheme: upstream of the polyketide synthase cluster of this sponge-associated bacterium, genes encoding for enzymes of the protocatechuate/β-ketoadipate pathway have been identified (abbreviated as pcaB-pcaQ). This pathway uses benzoate or p-hydroxybenzoate and forms succinyl-CoA and acetyl-CoA in the cascade, metabolites that also enter the polyketide synthase cycle. This pathway is used by the sponge-associated bacteria to utilize the carbon source.

Figure 8

Spicules from siliceous sponges. (A) (Aa) Differentiation of embryonic ‘Schwärmsporen’ into differentiated spicule-forming sclerocytes (in Spongilla fluviatilis) (59). (Ab) The intracellular formation of the spicules was also precisely observed by DeLage (60). (B) Scanning electron microscopic analysis of basal spicules (Ba: cross section; Bb: side view) from Hyalonema sieboldi showing the concentric silica layers that constitute them. (C) Use of one basal spicule from H. sieboldi as a light conductor. A laser beam is directed at one end of a spicule through which the light is transmitted to the other end.