Peter Proksch

Drugs from the seas – current status and microbiological implications

Abstract. The oceans are the source of a large group of structurally unique natural products that are mainly accumulated in invertebrates such as sponges, tunicates, bryozoans, and molluscs. Several of these compounds (especially the tunicate metabolite ET-743) show pronounced pharmacological activities and are interesting candidates for new drugs primarily in the area of cancer treatment. Other compounds are currently being developed as an analgesic (ziconotide from the mollusc Conus magus) or to treat inflammation. Numerous natural products from marine invertebrates show striking structural similarities to known metabolites of microbial origin, suggesting that microorganisms (bacteria, microalgae) are at least involved in their biosynthesis or are in fact the true sources of these respective metabolites. This assumption is corroborated by several studies on natural products from sponges that proved these compounds to be localized in symbiotic bacteria or cyanobacteria. Recently, molecular methods have successfully been applied to study the microbial diversity in marine sponges and to gain evidence for an involvement of bacteria in the biosynthesis of the bryostatins in the bryozoan Bugula neritina.

Fungal endophytes from higher plants: a prolific source of phytochemicals and other bioactive natural products

Bioactive natural products from endophytic fungi, isolated from higher plants, are attracting considerable attention from natural product chemists and biologists alike as indicated by the steady increase of publications devoted to this topic during recent years (113 research articles on secondary metabolites from endophytic fungi in the period of 2008–2009, 69 in 2006–2007, 36 in 2004–2005, 14 in 2002–2003, and 18 in 2000–2001). This overview will highlight the chemical potential of endophytic fungi with focus on the detection of pharmaceutically valuable plant constituents, e.g. paclitaxel, camptothecin and podophyllotoxin, as products of fungal biosynthesis. In addition, it will cover new bioactive metabolites reported in recent years (2008–2009) from fungal endophytes of terrestrial and mangrove plants. The presented compounds are selected based on their antimicrobial, antiparasitic, cytotoxic as well as neuroprotective activities. Furthermore, possible factors influencing natural product production in endophytes cultivated in vitro and hence the success of bioprospecting from endophytes are likewise discussed in this review.

Fungal endophytes: unique plant inhabitants with great promises

Fungal endophytes residing in the internal tissues of living plants occur in almost every plant on earth from the arctic to the tropics. The endophyte–host relationship is described as a balanced symbiotic continuum ranging from mutualism through commensalism to parasitism. This overview will highlight selected aspects of endophyte diversity, host specificity, endophyte–host interaction and communication as well as regulation of secondary metabolite production with emphasis on advanced genomic methods and their role in improving our current knowledge of endophytic associations. Furthermore, the chemical potential of endophytic fungi for drug discovery will be discussed with focus on the detection of pharmaceutically valuable plant constituents as products of fungal biosynthesis. In addition, selected examples of bioactive metabolites reported in recent years (2008–2010) from fungal endophytes residing in terrestrial plants are presented grouped according to their reported biological activities.

Bioactive Compounds from Marine Bacteria and Fungi

Marine bacteria and fungi are of considerable importance as new promising sources of a huge number of biologically active products. Some of these marine species live in a stressful habitat, under cold, lightless and high pressure conditions. Surprisingly, a large number of species with high diversity survive under such conditions and produce fascinating and structurally complex natural products. Up till now, only a small number of microorganisms have been investigated for bioactive metabolites, yet a huge number of active substances with some of them featuring unique structural skeletons have been isolated. This review covers new biologically active natural products published recently (2007–09) and highlights the chemical potential of marine microorganisms, with focus on bioactive products as well as on their mechanisms of action.

New pathway to polyketides in plants

The repertoire of secondary metabolism (involving the production of compounds not essential for growth) in the plant kingdom is enormous, but the genetic and functional basis for this diversity is hard to analyse as many of the biosynthetic enzymes are unknown. We have now identified a key enzyme in the ornamental plant Gerbera hybrida (Asteraceae) that participates in the biosynthesis of compounds that contribute to insect and pathogen resistance. Plants transformed with an antisense construct of gchs2, a complementary DNA encoding a previously unknown function,, completely lack the pyrone derivatives gerberin and parasorboside. The recombinant plant protein catalyses the principal reaction in the biosynthesis of these derivatives: GCHS2 is a polyketide synthase that uses acetyl-CoA and two condensation reactions with malonyl-CoA to form the pyrone backbone of thenatural products. The enzyme also accepts benzoyl-CoA to synthesize the backbone of substances that have become of interest as inhibitors of the HIV-1 protease. GCHS2 is related to chalcone synthase (CHS) and its properties define a new class of function in the protein superfamily. It appears that CHS-related enzymes are involved in the biosynthesis of a much larger range of plant products than was previously realized.

Defensive roles for secondary metabolites from marine sponges and sponge-feeding nudibranchs.

In the marine environment sponges (Porifera) constitute one of the most interesting sources of bioactive natural products. The high frequency of bioactive components in these primitive filter-feeders is interpreted as chemical defence of sponges against environmental stress factors such as predation, overgrowth by fouling organisms or competition for space. The highest incidence of toxic or deterrent sponge metabolites is consequently found in habitats such as coral reefs that are characterized by intense competition and feeding pressure due, for example, to carnivorous fish. Further support for the adaptive significance of sponge constituents is derived from the observation that sponges which are growing exposed are usually more toxic than those growing unexposed. Whereas the chemical defence of sponges seems to be highly effective against most species of fish, a group of shell-less gastropods, the nudibranchs, has specialized on sponges. While feeding on sponges the nudibranchs sequester the effective chemical armoury of their prey, which is subsequently employed for their own protection. Some nudibranchs, however, have become independent of this interspecific flow of natural products and are able to accumulate defensive compounds through de novo synthesis.

Cytotoxic metabolites from the fungal endophyte Alternaria sp. and their subsequent detection in its host plant Polygonum senegalense

From the Egyptian medicinal plant Polygonum senegalense the fungal endophyte Alternaria sp. was isolated. Extracts of the fungus grown either in liquid culture or on solid rice media exhibited cytotoxic activity when tested in vitro against L5178Y cells. Chromatographic separation of the extracts yielded 15 natural products, out of which seven were new compounds, with both fungal extracts differing considerably with regard to their secondary metabolites. Compounds 1, 2, 3, 6, and 7 showed cytotoxic activity with EC 50 values ranging from 1.7 to 7.8 microg/mL. When analyzed in vitro for their inhibitory potential against 24 different protein kinases, compounds 1- 3, 5- 8, and 15 inhibited several of these enzymes (IC 50 values 0.22-9.8 microg/mL). Interestingly, compounds 1, 3, and 6 were also identified as constituents of an extract derived from healthy leaves of the host plant P. senegalense, thereby indicating that the production of natural products by the endophyte proceeds also under in situ conditions within the plant host.

Chromenes and benzofurans of the asteraceae, their chemistry and biological significance

Abstract Chromenes (benzopyrans) and benzofurans are characteristic natural products of certain tribes of the Asteraceae. This review highlights recent findings

Co-cultivation--a powerful emerging tool for enhancing the chemical diversity of microorganisms.

Marine-derived bacteria and fungi are promising sources of novel bioactive compounds that are important for drug discovery programs. However, as encountered in terrestrial microorganisms there is a high rate of redundancy that results in the frequent re-discovery of known compounds. Apparently only a part of the biosynthetic genes that are harbored by fungi and bacteria are transcribed under routine laboratory conditions which involve cultivation of axenic microbial strains. Many biosynthetic genes remain silent and are not expressed in vitro thereby seriously limiting the chemical diversity of microbial compounds that can be obtained through fermentation. In contrast to this, co-cultivation (also called mixed fermentation) of two or more different microorganisms tries to mimic the ecological situation where microorganisms always co-exist within complex microbial communities. The competition or antagonism experienced during co-cultivation is shown to lead to a significantly enhanced production of constitutively present compounds and/or to an accumulation of cryptic compounds that are not detected in axenic cultures of the producing strain. This review highlights the power of co-cultivation for increasing the chemical diversity of bacteria and fungi drawing on published studies from the marine and from the terrestrial habitat alike.

From anti-fouling to biofilm inhibition: new cytotoxic secondary metabolites from two Indonesian Agelas sponges

Chemical investigation of Indonesian marine sponges Agelas linnaei and A. nakamurai afforded 24 alkaloid derivatives representing either bromopyrrole or diterpene alkaloids. A. linnaei yielded 16 bromopyrrole alkaloids including 11 new natural products with the latter exhibiting unusual functionalities. The new compounds include the first iodinated tyramine-unit bearing pyrrole alkaloids, agelanesins A-D. These compounds exhibited cytotoxic activity against L5178Y mouse lymphoma cells with IC(50) values between 9.25 and 16.76 muM. Further new compounds include taurine acid substituted bromopyrrole alkaloids and a new dibromophakellin derivative. A. nakamurai yielded eight alkaloids among them are three new natural products. The latter include the diterpene alkaloids (-)-agelasine D and its oxime derivative and the new bromopyrrole alkaloid longamide C. (-)-Agelasine D and its oxime derivative exhibited cytotoxicity against L5178Y mouse lymphoma cells (IC(50) 4.03 and 12.5 microM, respectively). Furthermore, both agelasine derivatives inhibited settling of larvae of Balanus improvisus in an anti-fouling bioassay and proved to be toxic to the larvae. (-)-Agelasine D inhibited the growth of planktonic forms of biofilm forming bacteria S. epidermidis (MIC<0.0877 microM) but did not inhibit biofilm formation whereas the oxime derivative showed the opposite activity profile and inhibited only biofilm formation but not bacterial growth. The structures of the isolated secondary metabolites were elucidated based on extensive spectroscopic analysis involving one- and two-dimensional NMR as well as mass spectrometry and comparison with literature data.