While stem cell and tissue engineering approaches may ultimately deliver on their clinical promise, small molecules that control innate regeneration mechanisms offer a less complex and more economical path to realizing regenerative medicine’s transformative potential.
The U.S. Department of Health and Human Services has called regenerative medicine the “vanguard of 21st century healthcare”1. Regenerative medicine R&D efforts are focused largely on developing stem cell and tissue engineering therapies as a means to regenerate, replace or repair damaged tissues and organs. Despite over 15 years of extensive research, these approaches remain challenged by problems with efficacy, by their complexity and expense, and by regulatory hurdles7,10,11,17. In addition, the hyperbole that has been generated around stem cells has contributed significantly to scientific fraud and the growth of unregulated stem cell clinics and spas offering unproven and costly treatments12,16.
Is there another approach? What if we could take a drug that would stimulate the regeneration of lost and damaged tissues and organs? Regenerative medicine small molecule therapies offer many advantages over other therapeutic approaches including reduced complexity, ease of administration, ready reversibility, fewer regulatory hurdles and the absence of ethical concerns like those that have challenged the stem cell field. Small molecules would also likely be less costly to administer and thus be available to a larger patient population.
“What if we could take a drug that would stimulate the regeneration of lost and damaged tissues and organs?”
Small molecule drugs are discovered utilizing target- or phenotype-based approaches. Target-based strategies typically employ in vitro high-throughput screening methods to identify small molecules that alter the activity of an identified candidate protein implicated in a disease process. Phenotypic approaches screen for the effects of small molecules on an observable characteristic of an animal, tissue or cell model, typically without a priori knowledge of the target.
Development of small molecules for regenerative medicine applications faces three challenges. The first is an emphasis within the regenerative medicine R&D community on developing non-drug based therapies like stem cell transplants. This emphasis overshadows alternative approaches and translates into limited funding resources for regenerative medicine drug discovery and development. Second is the lack of understanding of the detailed molecular mechanisms of regeneration and thus the lack of identified proteins that can be targeted with small molecules.
The third challenge is one that is pervasive in biomedical R&D. Non-mammalian animals like zebrafish, fruit flies, nematode worms and even seemingly more exotic creatures like salamanders and Mexican blind cavefish provide powerful experimental models for understanding fundamental biology and disease mechanisms. Unfortunately, this is not widely appreciated and mammalian models such as mice are generally thought to be the best systems for informing our understanding human physiology and pathophysiology.
Arguably, the most economical and efficient strategy for development of small molecules with regenerative medicine applications is to use non-mammalian animal models for both target- and phenotype-based drug discovery. Countless invertebrate and lower vertebrate animals, including fruit flies, salamanders and zebrafish, exhibit remarkable regenerative capabilities2,14. The zebrafish in particular is able to fully regenerate many lost or damaged body parts5,6. In contrast, humans and most other mammals have limited capacity for regenerating damaged tissues even though they possess the genetic instructions needed for building tissues and organs de novo during embryogenesis.
“In only a short period of time and with modest resources, the MDI Bioloigcal Laboratory’s approach has generated promising results”
Regeneration is a predictable biological process. The source of this predictability is the regulatory information encoded in gene and signaling networks. Research at the MDI Biological Laboratory is narrowly focused on using invertebrate and lower vertebrate animals to define these networks and identify regenerative medicine drug targets. To facilitate target identification, a novel NIH-supported bioinformatics resource, the Comparative Models of Regeneration Database, is being developed. The focus of the database is to integrate gene function data across multiple animal, tissue and cell models in order to validate and inform hypotheses needed to discover and develop drug therapies for regenerative medicine applications.
In only a short period of time and with modest resources, the MDI Bioloigcal Laboratory’s narrowly focused approach has generated promising results. For example, Rieger and co-workers used the zebrafish to model peripheral neuropathy induced by the cancer chemotherapeutic agent paclitaxel9. Wild type zebrafish readily regenerate damaged peripheral neurons, but paclitaxel treatment causes permanent neuronal damage. Paclitaxel-induced neurotoxicity is associated with increased expression of matrix-metalloproteinase 13 (MMP-13). Pharmacological inhibition of MMP-13 in zebrafish rescues the neurotoxic effects of paclitaxel suggesting a therapeutic strategy for treating peripheral neuropathy in chemotherapy patients.
In addition to target-based approaches, scientists at the MDI Biological Laboratory are also using animals like the zebrafish as drug screening platforms for phenotype-based regenerative medicine drug discovery. Yin and co-workers recently carried out a small molecule screen to identify compounds capable of stimulating zebrafish caudal fin regeneration13. The caudal fin is a composite tissue comprising bone, nerve, vasculature, connective and skin tissues. It fully regenerates within 10-14 days following amputation and the rate of fin regeneration is readily quantified by simple visual assay 5. The screen carried out by Yin and co-workers identified the protein tyrosine phosphatase 1B (PTP1B) inhibitor MSI-1436. This small molecule dramatically stimulates caudal fin regeneration without overgrowth or malformation and has no effect on sensitive developmental processes. Knockout of PTP1B in mice also has no effect on development3,20 suggesting that inhibition of this protein impacts tissue homeostasis predominantly in the context of injury.
“At least three other groups have identified drug candidates capable of stimulating mammalian tissue regeneration”
The magnitude of the stimulation induced by MSI-1436 is noteworthy. A recent study identifying a combination of two drugs that stimulate healing of cutaneous wounds by 25-30%8 was hailed as having the “potential to change clinical practice”15. By contrast, MSI-1436 stimulates by ~300% the coordinated regeneration of bone, skin, connective, vascular and nerve tissues that comprise the caudal fin13.
MSI-1436 also stimulates zebrafish heart regeneration by 2- to 3-fold. In adult mice, MSI-1436 stimulates stem celI activation in injured skeletal muscle ~2-fold and increases survival, improves heart function ~2-fold, reduces infarct size by 53% and stimulates cardiomyocyte proliferation ~5-fold 4 weeks after induction of myocardial infarction by permanent ligation of the left anterior descending coronary artery13. To the best of our knowledge, this is the first small molecule that has been shown to induce heart regeneration in an adult mammal. Importantly, MSI-1436 has been tested in Phase 1 and 1b clinical trials as a potential obesity and type 2 diabetes treatment and was shown to be well-tolerated by patients. The doses effective at stimulating tissue regeneration in zebrafish and mice are 50-times lower than the maximum well-tolerated human dose.
Our small molecule discoveries are not the only ones in the regenerative medicine field. At least three other groups have identified drug candidates capable of stimulating mammalian tissue regeneration4,18,19.
Stem cell and tissue engineering strategies may eventually deliver on their clinical promise and should continue to be a component of regenerative medicine R&D efforts. However, given the challenges these approaches face, it is time to begin devoting significant attention and resources to the development of small molecule regenerative medicine therapies. Patients with devastating diseases and injuries deserve it and there are no sound scientific, medical or business reasons not to do so.
- U.S. Department of Health and Human Services. 2020: A New Vision–A Future for Regenerative Medicine. http://www.hhs.gov/reference/FutureofRegenerativeMedicine.pdf (2003)
- Birnbaum KD, Sanchez AA. Slicing across kingdoms: regeneration in plants and animals. Cell. 132:697-710, (2008)
- Elchebly M, Payette P, Michaliszyn E et al. Increased insulin sensitivity and obesity resistance in mice lacking the protein tyrosine phosphatase-1B gene. Science. 283:1544-48 (1999)
- Fan F. Pharmacological targeting of kinases MST1 and MST2 augments tissue repair and regeneration. Sci Transl Med. 8:352ra108 (2016)
- Gemberling M, Bailey TJ, Hyde DR, Poss KD. The zebrafish as a model for complex tissue regeneration. Trends Genet. 29:611-620 (2013)
- Goessling W, North TE. Repairing quite swimmingly: advances in regenerative medicine using zebrafish. Dis Model Mech. 7:769-776 (2014)
- Golpanian S, Wolf A, Hatzistergos KE, Hare JM. Rebuilding the damaged heart: mesenchymal stem cells, cell-based therapy, and engineered heart tissue. Physiol Rev. 96:1127-1168 (2016)
- Lin Q, Wesson RN, Maeda H et al. Pharmacological mobilization of endogenous stem cells significantly promotes skin regeneration after full-thickness excision: the synergistic activity of AMD3100 and tacrolimus. J Invest Dermatol. 134:2458-2468 (2014)
- Lisse TS, Middleton LJ, Pellegrini AD et al. Paclitaxel-induced epithelial damage and ectopic MMP-13 expression promotes neurotoxicity in zebrafish. Proc Natl Acad Sci, U S A 113:E2189-E2198 (2016)
- Mari C, Winyard P. Concise review: understanding the renal progenitor cell niche in vivo to recapitulate nephrogenesis in vitro. Stem Cells Transl Med. 4:1463-1471 (2015)
- Nguyen PK, Rhee JW, Wu JC. Adult stem cell therapy and heart failure, 2000 to 2016: a systematic review. JAMA Cardiol. 1:831-841 (2016)
- Sipp D. The unregulated commercialization of stem cell treatments: a global perspective. Front Med. 5:348-355 (2011)
- Smith A, Nguyen K, Rando TA, Zasloff M, Strange K, Yin VP. The protein tyrosine phosphatase 1B inhibitor MSI-1436 stimulates regeneration of heart and multiple other tissues. Npj Regenerative Medicine. In press (2017)
- Tanaka EM, Reddien PW. The cellular basis for animal regeneration. Dev Cell. 21:172-185 (2011)
- Tolar J, McGrath JA. Augmentation of cutaneous wound healing by pharmacologic mobilization of endogenous bone marrow stem cells. J Invest Dermatol. 134:2312-2314 (2014)
- van der Heyden MA, van der Ven T, Opthof T. Fraud and misconduct in science: the stem cell seduction: Implications for the peer-review process. Neth Heart. J 17:25-29 (2009)
- Yoshihara M, Hayashizaki Y, Murakawa Y. Genomic instability of iPSCs: challenges towards their clinical applications. Stem Cell Rev. doi: 10.1007/s12015-016-9680-6 (2016)
- Zhang Y, Desai A, Yang SY et al. Inhibition of the prostaglandin-degrading enzyme 15-PGDH potentiates tissue regeneration. Science. 348:aaa2340 (2015)
- Zhang Y, Strehin I, Bedelbaeva K et al. Drug-induced regeneration in adult mice. Sci Transl Med. 7:290ra92, (2015)
- Zhang ZY. Protein tyrosine phosphatases: prospects for therapeutics. Curr Opin Chem Biol. 5:416-423 (2001)