Can non-muscle myosin II be targeted for therapeutic intervention?

Why is there interest in targeting NMII in disease?

NMIIs represent a potential untapped target for therapeutic development because of their presence in a wide array of diseases (see Figure 1), and combined with their exclusive skillset, make it a highly attractive target. Diseases where NMII has been implicated appear to display dysfunction in a variety of NMII-dependent cellular processes and NMII isoforms⁶.

For example, over the past several years, there has been a growing body of work implicating myosin heavy chain 9 (MYH9), which is part of the NMIIA isoform, as a critical regulator in tumor progression⁹. NMIIA has also been linked to chemoradiotherapy resistance, stemness, and metastasis in cancer studies⁷. One recent example identified NMIIA as a binding partner of HER3, and was shown to be upregulated in HER2+ breast cancers, where it promoted tumorigenesis¹⁰. In another study, NMIIA was shown to bind the hepatitis B X-interacting protein, and this interaction blocked NMIIA-dependent migration and metastasis of breast cancer¹¹.

A recent study highlights the importance of NMII in immune cell function in the setting of cancer, where they identified an NMII-NKX3-2-ADGRB3 pathway, which was important for T cell traction force and potentiated T cell cytotoxicity¹². In animal models, suppressing this pathway in T cells depressed tumor growth and improved immunotherapy.

Other components of NMII, such as the myosin light chain family members, have also been implicated in tumor progression¹³.Additionally, upstream signaling pathways, such as the Rho Kinase (ROCK) pathway, have been shown to regulate tumor progression through an NMII-dependent process^7-9. Curiously, NMIIA may also function as a tumor suppressor in specific cancer types, which adds a layer of complexity to consider from a therapeutic target perspective⁹.

NMII dysfunction is also strongly implicated in neurobiological diseases, including schizophrenia, autism, neurodegeneration, and intellectual disability¹⁴. But it also has important roles in neurobiology, for example, a recent study using a phenotypic screen on human motor neurons identified blebbistatin, an NMII inhibitor, as the most effective neurite outgrowth promoter of the molecules tested¹⁵, implicating it as an important regulator of axon regeneration.

Similarly, blebbistatin was shown to have beneficial effects in substance use disorder, through disrupting methamphetamine-associated memory, implicating NMII as an important protein in memories associated with drug use¹⁶.NMII’s role in other neurologic-dependent processes, vascular diseases, lung disease, and disorders of the blood have also been reported and are reviewed here^2,6,8, thus providing additional evidence of its role in diseases that may ultimately benefit from NMII-targeted therapeutic intervention.

Two other NMIIB inhibitors, MT-110 and MT-228, which specifically inhibit NMIIB, were shown to have promising effects at disrupting methamphetamine seeking²⁰. In preclinical models of methamphetamine use disorder, a single treatment of MT-110 or MT-228 produced long-lasting effects to suppress methamphetamine seeking.

In a third example, Heo et al. showed that a blebbistatin analog, NMIIi2, from this same cohort of small molecules, enhanced regeneration of human motor neurons when applied to the neuron directly¹⁵. Furthermore, local administration of the NMII inhibitor following sciatic nerve crush injury in mice led to motor neuron regeneration.

While these studies show great promise for these NMII-specific inhibitors as viable therapeutic targets, other approaches are also under investigation. For example, ROCK-targeting inhibitors were used to suppress NMII-induced survival in melanoma models²². Other potential NMII targeting agents include 4-hydroxyacetophenone (activates NMIIB and NMIIC to induce tumor cell stiffness)²³,covalent inhibitors of S1004A (disrupt S1004A-NMII interactions to reduce cancer cell migration)²⁴, and a treatment combination that included the lidamycin antibiotic and MYH9 antibody²⁵.

Why is there hesitancy in targeting NMII?

It is fair to wonder whether a ubiquitous protein like NMII, which performs essential functions in the majority of cells in vertebrates, can be a viable target. This presumption has likely contributed to the hesitancy in developing NMII-targeting therapies.

In fact, the group that developed these novel NMII inhibitors pondered this very question and hypothesized why their MT-125 drug was well tolerated in their GBM studies⁸, even though NMII is widely expressed and deletion in mice was lethal in utero²⁶.

They suggested that the tolerance of MT-125 can be attributed to two factors, including the “addiction” or dependency that glioblastoma cells have for NMII and the relatively short half-life of MT-125. Additional studies will reveal whether this hypothesis holds true; nevertheless, it is worthwhile to note that these NMII-specific inhibitors were also well tolerated in the axon regeneration and methamphetamine studies as well.

Still, it is important to note that NMII contributions in physiology and pathology continue to be identified. For instance, a recent study on herpes simplex virus 1 (HSV-1) identified TMEFF1 as a critical neuronal-specific protein that can restrict HSV-1 viral entry into neurons in an NMII-dependent manner²⁷. NMIIA and NMIIB are known to be important in virus-cell fusion, and loss of TMEFF1 in mice led to elevated HSV-1 viral load specifically in neurons within their brains²⁷. Thus, it will be interesting to see whether these NMII inhibitors have a deleterious effect on neuron-specific viral infection.

Are there other interesting targets for NMII therapies?

The recent identification of these highly specific NMII inhibitors, which are well tolerated and highly efficacious, suggest than NMII may eventually be a viable therapeutic target.

As therapeutics targeting NMII continue to be developed, it will be important to consider balancing efficacy and toxicity, which can be achieved through the following: understanding how dependent specific pathological cells are on NMII, developing high-precision small molecule inhibitors, and gaining a more robust understanding of NMII’s role in physiology and pathology.

Several recent studies on NMII describe its role in blood-brain barrier disruption²⁸, crypt cell adhesion in intestinal epithelial cells (Crohn’s disease model)²⁹, and motor neuron bouton remodeling³⁰. Some of these studies suggest that NMII inhibitors may provide benefit in these disease models as well. However, should side effects to these drugs arise, it may be necessary to evaluate these NMII inhibitors against these mechanisms and others to pinpoint the cause of the toxicity.

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