Blebbistatin is a

myosin Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...

inhibitor mostly specific for myosin II. It is widely used in research to inhibit heart muscle myosin, non-muscle myosin II, and

skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

myosin. Blebbistatin has been especially useful in optical mapping of the heart, and its recent use in cardiac muscle cell cultures has improved cell survival time. However, its adverse characteristics e.g. its cytotoxicity and blue-light instability or low solubility in water often make its application challenging. Recently its applicability was improved by chemical design and its derivatives overcome the limitations of blebbistatin. E.g. para-nitroblebbistatin and para-aminoblebbistatin are photostable, and they are neither cytotoxic nor fluorescent.

Mode of action and biological effects

Blebbistatin inhibits myosin ATPase activity and this way acto-myosin based

motility Motility is the ability of an organism to move independently using metabolism, metabolic energy. This biological concept encompasses movement at various levels, from whole organisms to cells and subcellular components. Motility is observed in ...

. It binds halfway between the nucleotide binding pocket and the actin binding cleft of myosin, predominantly in an actin detached conformation. This type of inhibition relaxes the acto-myosin

myofilament Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act togeth ...

s and leads to several biological effects. Blebbistatin inhibits the formation of blebs in melanoma cell culture, hence its name. At a cellular level, blebbistatin also inhibits

cytokinesis Cytokinesis () is the part of the cell division process and part of mitosis during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division ...

and may also disrupt mitotic spindle formation. Migration of cells can be either enhanced or inhibited depending on other conditions. In neurons, blebbistatin was found to promote neurite outgrowth. At the organ level blebbistatin stops the contraction of

or heart muscle. Blebbistatin has also been found to stabilize the super relaxed state in the myofilaments, where myosin heads are in a helical order and interact with each other but not with actin.

Adverse characteristics

A number of physicochemical deficiencies hamper the use of blebbistatin as a chemical tool in particular applications.

Photo-instability

Upon blue light illumination, blebbistatin becomes inactive and phototoxic due to changes in the structure of the compound accompanied by the generation of ROS

Fluorescence

Blebbistatin is a relatively strong

fluorophore A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with se ...

. When dissolved in water, it absorbs at 420 nm and emits at 490 nm however in DMSO or when perfused through cardiac tissue, it absorbs around 430 nm and emits around 560 nm, therefore at high concentrations its fluorescence interferes with GFP imaging or

FRET A fret is any of the thin strips of material, usually metal wire, inserted laterally at specific positions along the neck or fretboard of a stringed instrument. Frets usually extend across the full width of the neck. On some historical inst ...

experiments. Reduction of the concentration of blebbistatin to 6.25 uM allows for

imaging in isolated adult mouse cardiac muscle cells.

Cytotoxicity

Long-term incubation with blebbistatin results in cell damage and cytotoxicity, which are independent of the myosin inhibitory effect. This photo-instability, phototoxicity and fluorescence makes in-vivo imaging of blebbistatin-treated samples impossible.

Myosin specificity

Blebbistatin is a potent inhibitor of nonmuscle myosin IIA and IIB, cardiac myosin, skeletal myosin and smooth muscle but does not inhibit myosin I, V and X. The table below summarizes IC50 data of blebbistatin on different myosin isoforms.

Derivatives

The main aims of the structure-activity relationship work on the blebbistatin scaffold are the improvement of the physicochemical properties and the ATPase inhibitory potency, for use as chemical or pharmacological tools. Several analogs with superior properties have been developed, and guidelines for their optimal use have been described.

''para''-Nitroblebbistatin

A non-fluorescent, non-phototoxic, non-cytotoxic derivative developed in 2014. Its myosin inhibitory properties are similar to those of blebbistatin (for rabbit skeletal muscle myosin S1 IC50=0.4 μM, for Dictyostelium discoideum myosin II motor domain IC50=2.3 μM, for human β-cardiac myosin subfragment 1 IC50=13 μM, for heavy meromyosin fragment of chicken skeletal muscle myosin IC50=0.4 μM). It has been successfully used in fluorescent imaging experiments involving myosin IIA-GFP expressing live dendritic cells

''para''-Aminoblebbistatin

A water-soluble blebbistatin derivative developed in 2016, its high water solubility (~400 uM) enables in vivo research applications. Para-aminoblebbistatin is a slightly weaker myosin inhibitor than blebbistatin (for rabbit skeletal muscle myosin S1 IC50=1.3 μM, for Dictyostelium discoideum myosin II motor domain IC50=6.6 μM with only 90% maximal inhibition), it is non-fluorescent, photostable, neither cytotoxic nor phototoxic.

Azidoblebbistatin

A photoreactive myosin inhibitor developed in 2012. A permanent inhibition of myosin may be achieved by covalently crosslinking the inhibitor azidoblebbistatin to its target by photoaffinity labeling (PAL). Briefly, upon UV illumination, the aryl-azide moiety in azidoblebbistatin forms a reactive nitrene. This reaction is utilized to form covalent link between the inhibitor and myosin. Azidoblebbistatin is also sensitive to two-photon irradiation, i.e. the covalent crosslink may also be generated by two-photon excitation microscope, therefore azidoblebbistatin is suitable for molecular tattooing.

(''S'')-Nitroblebbistatin

This derivative was developed in 2005 to increase the photostability and decrease the fluorescence of blebbistatin. (S)-nitro-blebbistatin is indeed stable to prolonged irradiation at 450-490 nm and has been successfully used in fluorescent live cell imaging. However its affinity to myosin significantly decreased with the nitro-substitution (for nonmuscle myosin IIA, the IC₅₀ = 27 μM). In many cases due to the low solubility, it is not possible to achieve full inhibition of myosin with (S)-nitro-blebbistatin. It is effective for FRET imaging of isolated adult mouse cardiac muscle cells.

(+)-Blebbistatin

(+)-Blebbistatin (or (''R'')-blebbistatin) is the inactive enantiomer of blebbistatin which inhibits the ATPase activity by maximum 10%. In research, it is useful compound for control treatment, to check the non-myosin related toxic effects of blebbistatin.

Other derivatives

The blebbistatin scaffold has been modified in several ways to optimize myosin isoform specificity or to improve the inhibitory properties and to map the structure-activity relationship. Major steps in the optimization include the work of Lucas-Lopez et al. from 2008 and the works of Verhasselt et al. from 2017. The latter studies also include modifications of the A- and C-rings of the scaffold.

''para''-Chloroblebbistatin

A photostable, non-fluorescent, phototoxic derivative. Its fluorescence is less than 1% of that of blebbistatin myosin inhibitory properties are similar to those of blebbistatin. It is even more phototoxic than blebbistatin.

References

{{Reflist Pyrroloquinolines Tertiary alcohols Enones Cardiac myosin inhibitors