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Latrunculins are a family of natural products and toxins produced by certain sponges, including genus Latrunculia and Negombata, whence the name is derived. Latrunculins bind actin monomers near the nucleotide binding cleft with 1:1 stoichiometry and prevent them from polymerizing. Administered in vivo, this effect results in disruption of the actin filaments of the cytoskeleton, and allows visualization of the corresponding changes made to the cellular processes. This property is similar to that of cytochalasin, but has a narrow effective concentration range. Latrunculins have been used to great effect in the discovery of cadherin distribution regulation and have potential medical applications. The latrunculin A congener was found to make reversible morphological changes to mammalian cells by disrupting the actin network.

Latunculin A: Target and functions

Gelsolin - Latrunculin A causes end- blocking; this protein binds to the barbed sides of the actin filaments which accelerates nucleation. This calcium- regulated protein also plays a role in assembly and disassembly of cilia which plays a crucial role in handedness.

History
Latrunculins are toxins produced by sponges. The red-coloured Latrunculia Magnifica Keller is an abundant sponge in the gulf of Eilat and the gulf of Suez in the red sea, where it lives at a depth of 6–30 meters. The toxin was discovered around 1970. Researchers observed that the red-coloured sponges, Latrunculia Magnifica Keller, were never damaged or eaten by fish, while others were. Furthermore, when researchers squeezed the sponges in the sea, they observed that a red fluid was released. Fish nearby immediately fled the surrounding area when the sponge secreted the fluid. These were the first indications that these sponges produced a toxin. Later, researchers confirmed this hypothesis by squeezing these sponges into an aquarium with fish. The fish showed a loss of balance and severe bleeding, dying within only 4–6 minutes. Similar effects were observed when the toxin was injected into mice.

Latrunculin makes up to 0.35% of the dry weight of the sponge. There are two main congeners of the toxin, A and B. Latrunculin A is only present in sponges which live in the Gulf of Suez while latrunculin B only exists in sponges in the Gulf of Eilat. Why this is the case is still under investigation.

Structure and reactivity
Several latrunculin congeners have been isolated, including A, B, C, D, G, H, M, S and T. Structural features common to latrunculin congeners include the presence of a macrolactone and a 2-thiazolidinone moiety, which is biologically rare. Structure-activity relationships have revealed that the OH side group and the NH group on the thiazolidinone moiety are important for potent inhibition of actin. These structural features are highlighted in Figure 2.

Latrunculins A, B, and C were the first latrunculins to be isolated. The A and B congeners are the most well-studied, with molecular formulas of C22H31NO5S and C20H29NO5S, respectively. Shape and size of the macrocyclic ring in latrunculins A and B have been shown to affect binding affinity to actin. The 16-membered macrocyclic ring of latrunculin A interacts with actin via hydrophobic interactions, stabilizing the thiazolidinone ring. Latrunculin B, on the other hand, is a 14-membered ring and binds with lower affinity to actin, which is likely due to decreased interactions with a glutamine residue of actin in comparison to latrunculin A in complex with actin.

Besides these naturally-occurring forms, synthetic forms have been produced with different toxic strengths. Figure 3 shows some of these forms with their relative abilities to disrupt microfilament activity. Semisynthetic forms that contained N-alkylated derivates were inactive.

Mechanism of action
Latrunculins A and B disrupt the polymerization of actin, a process involved in many important cellular activities, including cell motility, cell division, and phagocytosis. Actin polymerization involves the binding of two G-actin monomers to another monomer in a head-to-tail manner to create F-actin filaments. Monomeric G-actin binds ATP, and ATP is hydrolyzed to ADP after F-actin assembly. Latrunculins form a 1:1 stoichiometric complex with G-actin monomers, sequestering G-actin and disrupting microfilament assembly. However, these toxins do not affect microtubular structure. Latrunculin A has also been shown to bind to the nucleotide binding cleft on actin and to prevent nucleotide exchange. In comparison to latrunculin B, latrunculin A is the more potent toxin.

Toxicity
As latrunculin inhibits actin polymerization and actomyosin contractile ability, exposure to latrunculin may result in cellular relaxation, expansion of drainage tissues and decreased outflow resistance in, for example, the trabecular meshwork.

Animal
Squeezing Latrunculia magnifica into aquariums with fish causes their almost immediate agitation, followed by hemorrhage, loss of balance and death in 4–6 minutes.

Prevention of actin filament polymerization causes reversible changes in the morphology of mammalian cells. Lantrunculin interferes with the structure of the cytoskeleton in rats.

After latrunculin B exposure, mouse fibroblasts grow bigger and PtK2 kidney cells from a potoroo stem produced long, branched extensions. The extensions seem to be an accumulation of actin monomers.

Latrunculin A has been used as acrosome reaction inhibitor of guinea pigs in laboratory conditions.

Human
Lat-A-induces reduction of actomyosin contractility. This is associated with trabecular meshwork porous expansion without evidence of reduced structural extracellular matrix protein expression or cellular viability. In high doses, latrunculin can induce acute cell injury and programmed cell death through activating the caspase-3/7 pathway.

In human cells, Shiga toxins were shown to more easily translocate across the intestinal epithelial monolayer when actin was inhibited by latrunculin B.

Plant
Latrunculin B causes marked and dose-dependent reductions in pollen germination frequency and pollen tube growth rate.

Adding latrunculin B to solutions of pollen F-actin produced a rapid decrease in the total amount of polymer, the extent of depolymerization increasing with the concentrations of the toxin. The concentration of latrunculin B required for half-maximal inhibition of pollen germination is 40 to 50 nM, whereas pollen tube extension is much more sensitive, requiring only 5 to 7 nM LATB for half-maximal inhibition. The disruption of germination and pollen tube growth by latrunculin B is partially reversible at low concentrations. (<30 nM).

Lethal doses
TDLO - Lowest Published Toxic Dose

LD50 – median Lethal Dose

Applications
In nature, latrunculins are used by the sponges themselves as a defense mechanism, and for the same purpose are also sequestered by certain nudibranchs.

Latrunculins are produced for fundamental research and have potential medical applications, as latrunculins and their derivatives show antiangionic, antiproliferative, antimcrobial and antimetastatic activities.

Defense mechanism
Like many other sessile organisms, sponges are rich of secondary metabolites with toxic properties and most of them, including Latrunculin, have a defense role against predators, competitors and epibionts.

The sponges themselves are not damaged by latrunculins. As a measure against self-toxination, they keep latrunculin toxins in membrane-bound vacuoles, that also function as secretory and storage vesicles. These vacuoles are free of actin and prevent the latrunculin toxins from entering the cytosol where actin would be damaged. After production in the choanocytes, the latrunculin is transferred via the archeocytes to the vulnerable areas of the sponges where defense is needed, such as injured or regenerating sites.

Sequestering by nudibranchs
Sea slugs of the genus Chromodoris sequester different toxics from the sponges that they eat as defensive metabolites, including latrunculins. They selectively transfer and store latrunculins in the sites of the mantle that are most exposed to potential predators. It is thought that the digestive system of the nudibranchs plays an important role in the detoxification.

In 2015, the discovery that five closely related sea slugs of the genus Chromodoris all use latrunculins as a defense mechanism indicated that the toxic might be used via Müllerian mimicry.

Research
Latrunculins are used for fundamental research like cytoskeletal studies. Many functions of actin have been determined by using latrunculins to block actin polymerization followed by examining the effects on the cell. Using this method, the importance of actin for the polarized localization of proteins, polarized exocytosis and the maintenance of cell polarity have been shown. Additionally, the role of actin in regulating voltage-gated ion channels in different nerve cells has been examined, showing that with latrunculin treatment the electrical activity of nerve cells can be altered. Latrunculin shows a dose-dependent inhibition of K+ currents and acute application can induce multiple action potential firing, which could underlie a mechanism of defense via nociceptors.

No research has been done to figure out how the biotransformation of latrunculin works in eukaryotic cells. However, research suggests that it is the unaltered form of latrunculin that causes toxic effects.

Studies in yeast
Yeast cells in absence of the proteins osh3 or osh5 demonstrated hypersensitivity to latrunculin B. The osh proteins are homologous to OSBP generated enzymes that appear in mammals, indicating that these might play a role in the toxicokinetics of latrunculins.

Yeast mutants that are resistant to latrunculin show a mutation, D157E, that initiates a hydrogen bond with latrunculin. Other yeast mutants adjust the binding site, thus making it resistant to latrunculin.

Medical applications
Latrunculins A and B and derivatives have potential roles as novel chemotherapeutic agents. The potential use of latrunculins as growth inhibitors of tumor cells has already been investigated for certain forms of gastric cancer, metastatic breast cancer and prostate tumors. In lower doses, latrunculin can be used to decrease disaggregation and cell migration, thereby preventing invasive activities of tumor cells. In higher doses, latrunculin can induce acute cell injury and programmed cell death through activating the caspase-3/7 pathway, and thus be used to kill tumor cells.

Latrunculin also is a potential therapeutic for ocular hypertension and glaucoma. Latrunculins A and B are shown to disrupt the actin cytoskeleton of the trabecular meshwork that is important for regulating humor outflow resistance and thereby intraocular pressure. By cellular relaxation and loosened cell-cell junctions, latrunculin can increase humor outflow facility. The first human trial of lantruculin B as treatment of ocular hypertension and glaucoma showed significantly lower intraocular pressure in patients.

Popular Science
Memories associated with previous drug use can lead to substance abuse relapse. Researchers at the Scripps Research Institute in Florida attempted to disrupt drug-related memories in rats to better understand the underlying mechanisms associated with these memories and to eventually develop therapeutics. In this experimental study, latrunculin A was used to inhibit actin polymerization, which plays a role in the formation and maintenance of drug-associated memories. Their results suggest that latrunculin A can disrupt these memories.

The methodology in this study involved moving rats between chambers with different sensory cues, such as distinct tactile characteristics. In one chamber, the rats were injected with methamphetamine (METH), while in another chamber they were injected with saline as a control. Rats that were injected with latrunculin A after METH training did not show a preference for the chamber in which they were injected with METH, while those that were not treated with latrunculin A showed a strong preference for this chamber. However, latrunculin A did not disrupt memories associated with other types of training, such as training based on food reward. The disruption in drug-related memories of the rats treated with latrunculinA also lasted for multiple days. Because latrunculin A disrupts actin, this molecule was attributed to affecting the rats’ drug-related memories. Although these results are intriguing, many other biological processes rely on actin polymerization; therefore, latrunculin A is not a viable therapeutic.