**7. Cyanobacteria**

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 *Lyngbya* species or *Symploca* species have been a source of novel natural products with therapeutic and biotechnological potential. Marine cyanobacteria are considered to be an important source of structurally diverse and biologically active natural products. Different peptides isolated from a wide variety of marine cyanobacteria, induces

#### *Antimicrobial Peptides Derived from Ascidians and Associated Cyanobacteria DOI: http://dx.doi.org/10.5772/intechopen.99183*

anticancer effects on various human cell lines. Most studied cytotoxic cyanobacteria on human tumor cell lines inducing minimal inhibitory effects [53].

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 *Lyngbya majuscule*. Exhibited cytotoxic effects on Human HeLa cervical carcinoma cells by cell cycle inhibition [54]. Similar mechanism was also reported on cyclic depsipeptide Coibamide A, isolated from *Leptolyngbya* species on Human lung cancer cell line and Lyngbyabellin B isolated from *Lyngbya majuscule* on Human Burkitt [55] lymphoma cells. Dolastatin 10 and Symplostatin 1, isolated from *Symploca* species, showed cytotoxic effect on human lung cancer cell line and Human breast carcinoma cell line by both Bcl-2 phosphorylation and Caspase-3 protein activation. Anticancer peptides such as *Lyngbya sp* and *Nostoc sp*, shows activity against cancer on different cell lines through microfilament disruption, secretory pathway inhibition.

Cyanobacteria inhibit Gram-negative and Gram-positive pathogenic bacterial species. Extracts of *Cylindrospermopsis raciborskii*, CYP011K and *Nostoc* species,

CENA69 possibly caused cancer cell inhibition. Extracts from *Fischerella* species, CENA213 showed inhibition of 3LL lung cancer cells. NPLJ-4 extracts isolated from *M.aeruginosa* reported to have inhibition against CT26 colon cancer cells. All of these extracts are prone to have low inhibitory activity towards human peripheral blood lymphocytes.

*Aphanazomenon flos-aquae*, freshwater cyanobacteria reported to contain immune stimulating properties, Other cyanobacteria Spirulina, a rich source of digestable proteins with a complete complement of essential amino acids [56].

*Lyngbya majuscule* from Curacao, yielded metabolites with broad biological properties, including those with toxicity to arthropods, those toxic to fish and those toxic to gastropods.

Bisanthrantaquinones, isolated from blue green algae associated with the colonial ascidian *Ecteinascidia turbinate*. These are the antimicrobial metabolites from ascidian-associated cyanobacteria (**Figure 7**), available to date. It has greater antibacterial effectiveness, and has resistance towards multi-drug resistant bacteria and vancomycin-resistant *Enterococcus faecalis* with an MIC of 0.6 and 12uM [57].

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

**Figure 7.** *Image showing marine cyanobacteria.*

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 [58]. Recently antimicrobial peptides targeting intracellular tubulin has developed from marine natural products.

Pharmacological properties of the marine mollusk, derivatives of *Aplysiidae* commonly know as the 'sea hare' or 'nudibrunch' has been reported for their toxic secretions. Biological activity of the cyanobacteria present in sea hare *Dolabella auricularia*, its structure consists of an active constituent, dolastatin 10. Dolastatin 10 displayed exceptional activity against the P388 lymphocytic leukemia cell line with ED50 value of about 4.6\*10-5 g/ml. It is also reported to have potent antineoplastic activity. Dolastatin 10 isolated from field collections of the marine cyanobacterim *Symploca* species, clarifying the concept that the true biosynthetic source is the cyanobacteria and not the sea hare. Dolastatin 10, a unique linear pentapeptide is composed of four novel amino acid residues such as dolavaline, dolaisoleucine, dolaproline, dolaphenine and valine. High resolution mass spectroscopic analysis such as 1H (COSY, 2D-J resolved) and 13C NMR methodologies revealed the structure of this linear pentapeptide. This peptide sequence was assigned on the basis of several low resolution mass spectral fragmentation techniques. This peptide was found to be involved in the binding of Dolastatin 10, whereas other antimicrobial peptides near the exchangeable nucleotide and vinca alkaloid sites on microtubules [59]. Dolastatin 10 inhibits microtubule assembly in vitro and subsequently blocks cytokinesis. Also this peptide was noted for its non-competitive inhibition of radiolabeled vinblastine and vincristine to tubulin as well as tubulin dependent hydrolysis of GTP.

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 [60]. *Antimicrobial Peptides Derived from Ascidians and Associated Cyanobacteria DOI: http://dx.doi.org/10.5772/intechopen.99183*
