Although the peripheral nervous system has an innate repair capacity, there are various cases, in which this process is impaired, especially in organisms with restricted regenerative potential, such as humans. Given the limitations of current treatment options, there is a critical need to develop new strategies to enhance nerve regeneration. One approach is to examine mechanisms in lower vertebrates, in which peripheral nerve repair is quite efficient. The anthelminic drug ivermectin was recently shown to enhance innervation in regenerative stage frogs. Previous experiments performed in
X. laevis tadpoles demonstrated a role for ivermectin in promoting increased nerve growth from ectopic eye tissue.
(17) Fluorescently labeled donor eye primordia or unlabeled host tadpoles were treated with or without ivermectin before ectopic engraftment on the host animal flank. Hyperinnervation of the engrafted eye was only seen in those tadpoles, in which the donor tadpole received ivermectin pretreatment (and not vice versa).
(17) These experiments revealed that ivermectin treatment of non-neuronal stromal tissue can be exploited to induce the expansion of neurons from the adjacent engrafted neuronal tissue. Although these results were striking, it is important to understand whether this phenomenon was specific to this amphibian model or could be recapitulated in mammalian systems.
In this work,
we provide a role for ivermectin in promoting peripheral nerve growth in mammals. We discovered that co-culturing hiNSCs with ivermectin-treated fibroblasts resulted in increased hiNSC proliferation and migration. In addition, we found that ivermectin causes fibroblasts to adopt a glial-like phenotype; increasing uptake of extracellular glutamate, expressing neurotrophic factor GDNF, and displaying characteristics of Schwann cells, including elongated morphology and GFAP expression.
These transformed glial-like cells allow for the expansion of resident neurons, providing a supportive environment for nerve regeneration. Furthermore, we demonstrate a physiologically relevant in vivo role for ivermectin in promoting nerve regeneration using a murine model of wound healing. Importantly, our results are in accordance with current findings implicating a critical role for peripheral glia during mammalian tissue regeneration. It was recently shown that dermal injury activates peripheral glia in an in vivo model of full-thickness skin repair. Further, it was demonstrated that depletion of these activated glia functionally impairs the wound healing process.
(24) Similarly, transplantation of Schwann cell precursors promoted digit tip regeneration via localized secretion of paracrine factors in a murine amputation model
(25)
Given that FDA-approved Ivermectin is already currently used to treat a variety of infestations, including scabies, lice, and onchocerciasis,
(26) its use could be further adapted for clinical applications in peripheral nerve repair. It is known that ivermectin eradicates parasitic invertebrates by binding and activating glutamate-gated chloride channels present only in neurons and muscle cells of these organisms, ultimately leading to muscle paralysis and death.
(26) In mammals, these types of glutamate-gated chloride channels were only thought to be expressed in the brain, and were thereby protected by the blood–brain barrier:
rationale that has lead to its deemed safety for human use. Indeed, at low levels comparable to what is used in both clinical and veterinary medicines, there is essentially no discernible effect on these types of mammalian brain-specific glutamate-gated chloride channels. It has been shown, however, that ivermectin at higher concentrations (i.e., micromolar range) can act as an allosteric modulator of multiple channels, including the human glycine receptor;
(19) γ-aminobutyric acid A (GABAA) receptors from chicken,
(27) rodents,
(28) and humans;
(29) chicken and
human α7 nicotinic receptors;
(30) as well as human purinergic receptors P2X4
(20) and P2X7.
(31) Many of these receptors are found in multiple cell types in mammals and more specifically humans, suggesting that the effects of ivermectin may be more widespread than initially realized.
Within the microenvironment of a healing wound, there are multiple cell types involved in the healing process, many of which are known to express a number of relevant ion channels. For example, human fibroblasts have been shown to express many of the aforementioned receptors, such as
glycine,(32) GABA,(33) purinergic,(34) and nicotinic.
(35) This endogenous expression combined with the relatively nonspecific binding and functioning of ivermectin on a variety of different ion channels and receptors makes it somewhat challenging to identify which specific receptor or receptors ivermectin is acting upon in our in vitro and in vivo systems. This complexity of potential interactions of ivermectin with multiple channels and receptors limits insight into mode of action. However, the findings here provide compelling evidence for a broader impact of ivermectin, in both downstream efficacy and potential clinical utility.
Although the effects of ivermectin on neuronal and glial growth in vivo were quite striking, it is important to acknowledge that the drug’s effect on wound closure was not as profound. It is accepted that wound healing in healthy mice is particularly difficult to improve experimentally,
(36) and more specifically, the BALB/c wildtype mice used in this study are known to heal relatively quickly.
(37) It will be important to assess whether this drug can also play a role in improving nerve regeneration in other models, in which the nerve defect is more profound and/or impaired. For example, similar experiments could be repeated in various in vivo models of neuropathy, which is often associated with other co-morbidities, such as diabetes, autoimmune disorders, and chemotherapy treatment.
(38) Ivermectin could also be explored for its potential use in promoting repair of larger nerve defect models, such as sciatic nerve resection or spinal cord injury. Furthermore, because we demonstrate that ivermectin causes fibroblasts to secrete GDNF, this technique can also be adapted as a method to induce endogenous localized delivery of GDNF as a potential analgesic
(39) to promote innervation in ischemic tissue
(40) or perhaps modified further to develop strategies of GDNF delivery for treating Parkinson’s disease.
(41)
