Ascidians belonging to Phylum Chordata are the most largest and diverse of the Sub-phylum Tunicata (Urochordata). Marine ascidians are one of the richest sources of bioactive peptides. These bioactive peptides from marine ascidians are confined to various types of structures such as cyclic peptides, acyclic peptides (depsipeptides), linear helical peptides with abundance of one amino acid (proline, trytophane, histidine), peptides forming hairpin like beta sheets or α-helical/β-sheet mixed structures stabilized by intra molecular disulfide bonding. Cyanobactins are fabricated through the proteolytic cleavage and cyclization of precursor peptides coupled with further posttranslational modifications such as hydroxylation, glycosylation, heterocyclization, oxidation, or prenylation of amino acids. Ascidians are known to be a rich source of bioactive alkaloids. β-carbolines form a large group of tryptophan derived antibiotics. Pyridoacridines from ascidians are tetra- or penta- cyclic aromatic alkaloids with broad range of bioactivities. Didemnidines derived from ascidian symbiotic microbes are inhibitors of phospholipase A2 and induce cell apoptosis. Meridianins are indulged in inhibiting various protein kinases such as, cyclindependent kinases, glycogen synthase kinase-3, cyclic nucleotide dependent kinases, casein kinase, and also implicate their activity of interfering with topoisomerase, altering the mitochondrial membrane potential and binding to the DNA minor groove to inhibit transcriptional activation. Most of these bioactive compounds from ascidians are already in different phases of the clinical and pre-clinical trials. They can be used for their nutraceutical values because of their antineoplastic, antihypertensive, antioxidant, antimicrobial, cytotoxic, antibacterial, antifungal, insecticidal, anti-HIV and anti-parasitic, anti-malarial, anti-trypanosomal, anti-cancer etc. This chapter mostly deals with antibacterial compounds from ascidian and their associate symbiotic cyanobacteria.
Ascidians commonly known as tunicates or sea squirts are soft bodied and sessile animals belonging to subphylum urochordates. Sac like sea squirt ascidians produce many toxic nitrogen bearing secondary metabolites that are implicated in their chemical defense . Ascidians belonging to family
Aplidine is a cyclodepsipeptide has sufficient activity against a variety of human cancer cell lines such as breast, melanoma and lung cancers . Aplidine has several functional activities such as Inhibition of protein synthesis, cell cycle arrest, induction of apoptosis on cancer cells and inhibition of vascular endothelial growth factor gene. Its actions on causing cytotoxicity, involves the inhibition of ornithine decarboxylase, an enzyme that is responsible for the tumor formation and tumor growth . Its approval on Phase-I clinical trial, induces on its minor toxicity tolerance limit with most of its side effects corresponding to asthenia, nausea, vomiting and transient transaminitis etc. .
Mollamides being a cyclodepsipeptide has suitable cytotoxic activity against a wide range of cancer cell lines such as human lung carcinoma and human colon carcinoma .
Trunkamide A, a cyclodepsipeptide with a thiazoline ring similar to mollamide, show antitumor activity under preclinical trials . This peptide contains the thiazoline-based proline on doubly prenylated cyclopeptides. Heterocyclic amino acids such as the tryptophan and histidine also forms the part of proline rich cyclic peptides structures such as wainunuamide, phakellistatin 15,17 and stylissatin B.
However, recently pharmaceutical industries are gaining more insights on antimicrobial peptides due to their increased efficacy, high specificity, low toxicity, decreased drug interaction and direct attacking properties.
Alkaloids are providing the majority of ascidian originating bioactive compounds. They represent a highly diverse group of compounds containing cyclic structures having a basic nitrogen atom incorporated within it. Ascidians on the other hand are produces of large quantity of alkaloids and modified peptides which exhibit a wide range of biological properties such as, Cytotoxicity, antibiotic, immunosuppressive activities, inhibition of topoisomerases (TOPO), cyclin kinase, display antimicrobial and anticancer activities by inhibiting kinase activity, including protein kinase B (PKB), Cyclin dependent kinases (CDKs), altering mitochondrial membrane potential and binding to the DNA minor groove to inhibit transcriptional activation .
Investigations on the biosynthesis of secondary metabolites provide evidence on the
A specific biosynthetic source of the alkaloids such as, granulatimide and isogranulatimides by specifically localizing these compounds lying inside ascidians. Granulatimide stored in
Didemnidines A and B are two indole spermidine alkaloids isolated from ascidian Didemnum species. Didemnidines A and B are both active as inhibitors of phospholipase A2, farnesyltransferase enzyme without cytotoxicity. It has moderate cytotoxicity towards malarial parasite, L6 cells and inhibition parasite proliferation. Antiparasitic activity of didemnidine B provides the opportunity to explore the didemnidines as antimalarial and antitrypanosomal agents .
Meridianins are brominated 3-(2-aminopyrimidine)-indoles isolated from the ascidian
Herdmanines represent a series of nucleoside derivatives isolated from the ascidian
This peptide belonging to tetrahydroisoquinoline alkaloid family exhibits potent antitumor activity. It binds with the major groove of DNA and leads to the sequence specific alterations in transcription, triggers DNA cleavage, causing double stranded break, interruption of the cell cycle, apoptosis of cancer cell and down regulation of some transcriptional  factors.
Eusynstyelaides, alkaloids isolated from ascidian
Sesbanimide A, peptide isolated from Agrobacterium. Sesbanimide C showed activity against the growth of mouse leukemia cells and inhibited the proliferation of mouth epidermal carcinoma (kb) cell . It was evaluated against various human cancer cell lines.
A cyclodepsipeptide isolated from the ascidian
3. Antimicrobial peptides from ascidians
Peptides are one of the major structural classes isolated from ascidians, including linear peptides, depsipeptides, and cyclic peptides, with residue numbers spanning from two to forty eight. Most of the active peptides from ascidians have complex cyclic of linear structures rarely found in terrestrial animals. These peptides are found to affect cell behavior with different mechanisms such as apoptosis, affecting the tubulin- microtubule environment and  inhibiting angiogenesis.
A bicyclic peptide isolated from ascidian
A group of macrocytic peptides isolated from the ascidians
3.3 Chondromodulin-1 (ChM-1)
Chondromodulin, a 25kD a glycoprotein isolated from fetal bovine cartilage. Recently, Chondromodulin isolated from the invertebrate ascidian
4. Polypeptide from ascidian associated microbes
Patellamides are cyclic peptides isolated from the cyanobacterium
Polyketides are the other important compounds in the screening of secondary metabolites. Polyketides are complex molecules built from simple carboxylic acids and synthesized by polyketide synthetase . Polyketides has been discovered as important lead compounds with various activities, such as blocking protein tyrosine phosphatase and inhibiting ATP synthetase complex . Highly cytotoxic patellazole A, thought to have a defensive role, is a polyketide peptide hybrid made by the alpha-proteobacterium Ca.
Mandelalides A-D are macrocyclic polyketides isolated from a new species
Mollecarbamates A-D po repeating O-carboxyphenethylamide units and a carbamate moiety. Molleures B-E contains tetra- and penta- repeating carboxyphenethylamide units and a urea bridge in different positions and molledihydroisoquinolone, a cyclic form of O-Carboxyphenethylamide. These metabolites were found to be the only compound known to contain Ortho-carboxyphenethylamide derivatives in their skeleton. None of these compounds produced any antibacterial or antiviral properties .
4.5 Palmirolide A
A macrocyclic polyketide isolated from the ascidian
5. Polyketides from ascidian associated microbes
Patellazoles A-C (Figure 3) are a family of compounds produced by the alpha-proteobacterium
5.2 Cyanobactins biosynthesis
Cyanobactin gene clusters are capable of encoding, two protease genes, A(N-terminal) and G(C-terminal) that are related to patA and patG genes from the patellamide biosynthetic pathway. A precursor peptide gene E being an homolog to patE which directly encodes cyanobactin structure that acts as a substrate for post translational modifications. Cyanobactin gene clusters may also encode homologs of PatD or PatF, denoted as D-protein and F-protein. Includes thiazoline/oxazoline dehydrogenases (responsible for the aromatization of the heterocycles to thiazoles and oxazoles, methyltransferases. This gene was proven to be essential for the synthesis of non-prenylated patellamides. Cyanobactins are classified into different groups based on a correspondence between genotypes and chemotypes.
5.2.1 Biosynthetic pathway
Cyanobactin biosynthesis begins with the precursor E-peptide, which is composed of an N-terminal conserved leader sequence that is recognized by some of the modifying and cleaving enzymes. Cyanobactin genetic cluster may also employ more than one precursor peptide. Genetic cluster may contain upto10 precursor peptide gene. E-peptide that contains the enzyme recognition sequences, 1,4 hypervariable core regions may be present and dictate amino acid backbone of cyanobactins.
Cyanobactins are ribosomally synthesized and post-translationally modifies peptides produced in the ribosome. Biosynthetic enzymes for cyclic peptide synthesis are encoded in the
In the presence of D-protein cyclodehydratase, heterocyclization of cysteins, serines or threonines will be directed by sequence recognition. A protease cleaves the precursor peptide RSII, leaving a free amine available for macrocyclization. G-protease splits the precursor peptide RSIII and causes the catalization of C-N macrocyclization. Other transformations may occur such as, prenylation of serine/threonines and tyrosines/tryptophans residues catalyzed by the PatF class of prenyltransferases. Oxidation of heterocycles to oxazoles and thiazoles when oxidized, domain is present within the G gene or separate and geranylation .
Heterocyclase accompanies heterocyclization of cysteins, serine and threonine residues to thiazolines or oxazolines and eliminates water. Cyanobactins heterocyclases D has been studied in partellamide and trunkamide pathways. Heterocyclases D in both pathways is ATP dependent. An adenylase mechanism has been proposed for TruD, from trunkamide.
An adenylase mechanism has been proposed for TruD, from trunkamide pathway, whose crystal structure presents as the three, domain protein. Enzyme progreesivity requires the presence of a lead protein to be attached to the core, indicating that heterocyclization occurs before cleavage and macrocyclization of the precursor peptide. The sequence element present in the lead sequence is responsible for heterocyclization. Cyanobactin pathway encoding a heterocyclase modifies a oxidase domain responsible for oxidation of thiazolines and oxazoles to thiazoles and oxazoles.
PatA protease from patellamide gene cluster catalyzes the N-terminal protease cleavage from the precursor peptide removing the leader sequence. This reaction catalyzed by the N-terminal protease A and C-terminal protease G, under subtilisin protein family encoded by cyanobactin gene cluster. A kind of protease called PatG isolated from Prochlorom was found to macrocyclize a wide range of synthetic substrates with non-proteinogenic and D-amino acids. Macrocyclase consists of PatG and PagG structural domains representing a catalytic triad. Macrocyclase crystal structure represents a domain of PatG, showing subtilisin folds containing two helices presented by the macrocyclization insert without any change in sequence length. This domain is insensitive to the identity of the residues within the core peptides, as PatG acts on RSIII residues and catalyzes the C-N macrocyclization. During this process, PatA protease removes the amino terminal linked to the core, producing a free amino terminal and PatG protease removes a catalytic terminal flanking the core. Cleaving site is protected by the PatG protease preventing access to water and continues hydrolysis until the transduction reactiobn is completed. PatG emphasizes macrocyclic peptide formation, by removing the C-terminal protease.
Prenylagarmide (pag), trunkamide (tru). Aesturamide (lyn) pathways, encodes the prenyltransferase gene, capable of synthesizing prenylated compounds. Prenyltransferase gene present in patellamide, generates non-prenylated patellamide A and C. Trunkamide contains O-prenylated threonine and serine (Figure 4(c)). Prenylagarmide contains O-prenylated tyrosine. Prenyltransferase from lyn (LynF) and tru (TruF) pathways. Prenyltransferase from lyn (LynF) and tru (TruF) pathways have been characterized biochemically. However, the reverse O-Prenylated tyrosine undergoes spontaneous claisen rearranging and yieding ortho-substituited phenol. LynF prenylates the oxygen atom of tyrosine residue by using dimethylallylpyrophosphate (DMAPP). TruF prenylates serine and threonine residues on the hydroxyl side chain. PatF from the patellamide pathway, it embraces the other prenyltransferase, classic TIM barrel fold. No enzymatic activity was detected, may be due to absence of prenylation in patellamides A and C. PatF essential for the production of patellamide in vivo and consequently responsible for another function in this pathway. Oxidase domain is conserved among PatG homologs and studies put up a prediction that FMN id dependent. Thiazoline oxidase has been related in sequence to the patellamide enzyme. However its action on microcin pathway was a matter of biochemical study. How this enzyme recognizes the substrate remains unclear, as the microns are linear and patellamides are macrocyclase. One homolog of the oxidase domain of PatG was capable of oxidizing both linear and macrocycle thiazoline containing compounds and another homolog has the ability to perform oxidation on a macrocyclic substrate.
PatA and PatG proteases contain a domain of unknown function sharing a sequence similarity of about 56%. DUF domains are found in PatA and PatG from the patellamide biosynthetic clusters. Epimerization follows heterocyclization and precedes oxidation. Epimerization is an important role of DUF domain, and its phenomenon is chemically spontaneous. Crystal structure of PatG-DUF is a novel fold dimer with two zinc ions. Practical importance of the dimer remains unclear since, the residues involved in Zn2+ binding, which is necessary for dimerization are not conserved among DUF domains. DUF domain does not bind to the macrocycle or the core peptide alone.
6. Sulfer containing metabolites
6.1 Polysulfides and alkylsulfides
Sulfur atom rarely found among the marine organism. Ascidians belonging to the genus
Lissoclinotoxins A and B. Toxins isolated from
A beta-carboline alkaloid derived from the ascidian
6.3 Ihenyamines A-B
A derivative from the ascidian,
6.4 Shishijimicins A-C
A class of beta-carboline alkaloids, isolated from the ascidian
An alkaloid Fascaplysin isolated from the sponge
6.5 Lamellarin D
This alkaloid represents an excellent cytotoxic effects towards tumor cells and proposed to be an anticancer agent for targeted topoisomerase –I cancer therapy . This series of alkaloids represents an ideal source for developing anti-cancer agents. A few aspects of the various mechanism imposed by the lamellarin analogue, made this peptide to be used in biotechnology and pharmaceutical industries  (Marco et al.,).
Schupp and his co-workers reported alkaloids of Staurosporine and their eight subderivative alkaloids analogues such as 3-hydroxystaurosporine, 4-N-demethylstaurosporine, 3-demethoxy-3-hydroxylstaurosporine, 3-hydroxy-3-demethoxy-3-hydroxystaurosporine, 11-hydroy-4-N-demethylstaurosporine, 11-hydroxystaurosporine, 4-N-methylstaurosporine, 3-hydroxystaurosporine. These alkaloids were isolated from
6.6 Somocystinamide A
A lipopeptide isolated from
6.7 Apratoxin A
A cyclodepsipeptide isolated from ascidian,
Cyanobacteria, known as blue-green algae, are ancient photosynthetic prokaryotes which inhabit a wide diversity of habitats including tropical reefs, fresh water ponds, streams and puddles and fresh water ponds. Luxuriant growth of cyanobacteria in these adverse environments conditions is based on their abilities of forming resistant spores, opportunistically colonizing micro-habitants and surviving under conditions of high UV-flux through production of UV-absorbing pigments, has made them one of the successful life forms on earth. Cyanobacteria associated with ascidians, their symbotic relationship was first found out in 1982 by Kott.
Cyanobacteria are phylum of bacteria that produce oxygen during photosynthesis. Host ascidians that exhibit symbiotic relationship with cyanobacteria, Prochloron, which belong to the Didemidae family and are therefore called as “Didemnin ascidians”. Cyanobacterial symbionts can both provide nutrients by means of Carbon fixation, nitrogen recycling and metabolite production and also exhibits defensive reaction for the host ascidian. Ascidian host are capable of producing some of the nitrogen containing nutrients that are needed for the cyanobacterial symbionts growth and also protection against the ultra violet radiation. Additional feature is that, a rich source of biologically active products, has assisted some of these organism to survive in predator-rich tropical reef ecosystems. Tropical marine Cyanobacteria particularly the filamentous forms such as
Cyanobateria have a rich complement of photosynthetic pigments, including chlorophyll a and b, as well as several accessory pigments (phycoerythrin, phycocyanin, and allophycocyanin). Phycoerythrin has found application in biotechnology as a conjugate to antibiotics that then allow visualization of cellular constituents and processes and chlorophyll is being explored for its cancer chemotherapeutic activity. Apratoxin A is a cyclic depsipeptide extracted from
Cyanobacteria inhibit Gram-negative and Gram-positive pathogenic bacterial species. Extracts of
CENA69 possibly caused cancer cell inhibition. Extracts from
Bisanthrantaquinones, isolated from blue green algae associated with the colonial ascidian
Cyanobacteria are considered to be an important source of bioactive metabolites, with various aspects of cytotoxic, antiviral, anticancer, antimitotic, antimicrobial, specific enzyme inhibitor and immunosuppressive activity. Cyanobacteria holds the presence of non-ribosomal peptide synthetase and polyketide synthetase genes, owing it to be the potential for finding novel natural drug products from these organism. Thus, cyanobacterium species are a rich strain enriched with the source of natural products with potential for pharmacological and biotechnological applications.
7.1 Tubulin binding proteins
Microtubules play many significant roles in cell biology. Formation of microtubules results from the polymerization of the subunit protein tubulin, first into heterodimers subsequently binds end to end with other heterodimer forming a protofilament, which in turn interacts to form sheets and eventually microtubules. Specifically, assembly and motility are crucially for the formation of the spindle apparatus during cell replication and mitosis where microtubule fibers direct the separation of sister chromatids into the resulting daughter cells. In case of rapidly dividing cancer cells, microtubule assembly has been an important target in the development of new chemotherapeutic agents. Various drugs have developed to disrupt the process of mitosis and cause catastrophic cell death by either stabilizing microtubule complexes, caused by taxol. Depolymerization of the tubulin protein complex caused by Vinblastin . Recently antimicrobial peptides targeting intracellular tubulin has developed from marine natural products.
Pharmacological properties of the marine mollusk, derivatives of
Dolastatin 10 binds to the ‘peptide groove’ lying within the r- subunit of tubulin. Molecular modeling suggested that the chiral centers of dolavaline, valine and dolaisoleucine binds in a manner that require the dolaphenine moiety to sterically block access to the vinca alkaloid and exchangeable nucleotide binding sites. Evidence of noncompetitive inhibition of Vinca alkaloid binding was realized by observation that tubulin polymerization and nucleotide binding are substantially diminished at sub-stoichiometric concentrations of dolastatin 10.
Dolastatin 10, on its Phase II clinical trial proved to be an antitumor agent, by evaluating its antitumor efficacy in patients with measurable recurrences of platinum –sensitive ovarian carcinoma in relation to the degree of toxicity .
7.2 Dolastatin 15
7.3 Actin binding proteins
Actin cytoskeleton is a dynamic network of filaments, which is associated with several proteins, plays an important role in cell shape, motility and signal transduction, this further switches on the other processes like embryonic development, tissue repair, immune response and tumor formation. However in cancer biology, actin cytoskeleton and actin associated proteins undergo modification in transformed tumor cell and impose ability to adhere and metastasize. It can be used for developing new chemotherapeutic agents. Actin targeting molecules could disrupt actin by destabilizing the filaments or induce hyperpolymerization .
This specific peptide was isolated from
8. Neurotoxic compounds
A lipopeptide isolated from
8.3 Cyanobacterial metabolites
Isolated from the lipid extract of
A specific bacterial species associated with
9. Symbiotic organisms
Symbiotic bacteria contribute secondary metabolites necessary for defense and the survival of ascidians. About 80 percent of the currently available secondary metabolites obtained from ascidians were made only by its symbiotic bacteria. These metabolites are essential for the interaction between the host and symbiont and the bacteria are phylogenetically diverse .
9.1 Callynormine A
This peptide represents a new class of heterodetic cyclic peptides possessing an –amido-aminoacrylamide cyclization functionality. Cyclic endiamino peptides composed of a Hyp part , which is likely to be present in its peptides such as callynormine A and callyaerin A-D.
9.2 Gombamide A
A cyclothiopeptide consisting of an unusual amino acid residues like pHSA and pyroGlu . It possess moderate inhibitory action towards the, Na+/K+-ATPase.
A specific new class of proline rich cycloheptapeptides derived from the photooxidation of tryptophan consisting of cytotoxic phakellistatin 3 and isophakellistatin 3. This peptides has an unusual amino acid residue such as “Hpi” respectively. These proline rich peptides act in a very divergent way, capable of causing stereospecific interaction with the membrane system. These interactions are caused by the intracellular targeting, compared to the general membrane disruption mode of action of the conventional antimicrobial peptides. It was found that proline rich antimicrobial peptides stereo specifically binds the intracellular foreign particles, such as the bacterial heat shock proteins DnaK. These peptides have a good water solubility, high potential for killing bacteria and lower cytotoxic activity at higher concentrations, these factors contribute to the development of the novel antimicrobial therapeutic agents in the field of medicine .
These peptides could easily enter the bacterial cell, binding and disrupting specific targets such as ribosome, thereby inhibiting protein synthesis. However all these factors, concludes that these peptides could subsequently be used as molecular hooks, for identifying intracellular or membrane proteins involved in this mechanism of action . And it could be used for specifically altering novel therapeutics for drug delivery.
Scleritodermin A causes inhibition of tubulin polymerization .
Immunosuppressive activity of cyclolinopeptide A results from the formation of complex with cyclophilin and causing inhibition of phosphatase activity of calcineurin, plays an important role in T-lymphocyte signaling .
Cemadotin, a water soluble synthetic component of linear peptide dolastatin 15, which is reported to act on microtubules and causing strong suppression of microtubule dynamics .
9.3 Didemnin B
A cyclic depsipeptide derived from the marine cyclopolypeptide undergo clinical trials because of its potential to target oncological patients. Its high toxicity, poor solubility and shorter life span led to the discontinuation of didemnin B in clinical trials .
Didemnin B, belonging to a class of heterodetic non-polar cyclic peptide associated with several Antiviral, antitumor, immunomodulating properties, potency inhibits protein and DNA synthesis by binding to eukaryotic translation elongation factor EF-1 in a GTP dependent manner. Formation of the  Didemnin B-GTP-EF-1 complex could be responsible for protein synthesis inhibition.
Inhibition of protein synthesis by didemnin B occurs by stabilization of aminoacyl-tRNA to the ribosomal A-site, preventing the translocation of phenylalanyl–tRNA from the A- to the P-site, preventing peptide bond formation.
Tamandarin A acts in a very same mechanism as didemnin B. Aplidine’s involves several mechanism of action such as cell cycle arrest and protein synthesis inhibition. It induces early oxidative stress and results in a rapid activation of JNK and p38 MAPK phosphorylation by activating both kinases occurring among before the activation of apoptosis .
Didemnin B causes the death of several transformed cells through apoptosis, DNA fragmentation within the cytosol and generation of DNA ladders .
A linear depsipeptide kahalalide F, has predominant antifungal and antitumor activity, and underway in clinical trials.
A cyclic depsipeptide Plitidepsin (dehydrodedemnin B or aplidine) is in its clinical trial for being developing it as a drug. In 2003, plitidepsin was given orphan drug status for treating acute lymphoblastic leukemia. In 2007, it underwent phase II clinical trials and in 2006 it is announced for small phase-I clinical trials for treating multiple myeloma .
Antimitotic dolastatins group, dolastatin 10 and 15 are undergoing phase-II clinical trials. A synthetic analogue of dolastatin 15, cemadotin is also in phase-II clinical trials for its promising cancer chemotherapeutic agent.
A specific alkaloid isolated from the tunicate
9.5 Helichondrin B
A complex polyether derived from the marine animals such as sponges, tunicates and their various predators. Compounds such as palytoxin, maitotoxin and halichondrins, because of their potential even very small quantities of these compounds could aid valuable commercial sense. Halichondrins was first isolated from the Japanese sponge
A blue green pigment-protein complex isolated from the marine cyanobacteria
However, a handful of antimicrobial peptides have found to be approved today for clinical use as anti-infectives. Cyclic peptides such as gramicidins and polymyxins are well characterized. Gramicidins are used in treating infections such as infection of the surface wounds as well as the infections of nasal, ocular and throat infections. On the other hand polymyxins are used for treating eye infections prior to local administration and for selective decontamination of the digestive tract and also for systemic infections caused by drug-resistant gram-negative pathogens. Daptomycin, a cyclic antimicrobial peptides in clinical practice to treat skin complications and skin-structure infections caused by Gram-positive bacteria mostly,