My laboratory has focused on the development of potential pharmacologic treatments for strabismus, with an emphasis on treatments that increase muscle strength. My lab was the first to demonstrate that treatment with insulin-like growth factor-II (IGF-II) increased extraocular muscle (EOM) force and size.1 Further, we demonstrated that sustained treatment of the EOM with a variety of neurotrophic and growth factors alters muscle force and size in predetermined ways.2,3 In addition, my laboratory has contributed novel insights into mechanisms that explain why strabismus treatments fail and suggested potential signaling pathways that might explain why strabismus develops. Our most recent work demonstrates that sustained IGF-1 treatment of infant primate extraocular muscles can result in the development of strabismus.4 This study is very exciting and paradigm-shifting, as it demonstrates that in an otherwise normal oculomotor system, disrupting the IGF-1 signaling pathway in EOM can prevent the normal development of binocularity. This strongly suggested that aberrant IGF-1 signaling in development can cause strabismus.
EOM Cell Biological Complexity and EOM Adaptability
My laboratory made the fundamental discovery that unlike limb skeletal muscle, the EOM continuously and significantly remodel individual muscle fibers throughout life, even in aging human eye muscles.5,6 This, in part, explains the tremendous resilience of the EOM to injury, as well as their significant capacity for adaptation after strabismus surgery. We have shown that almost immediately after either recession, putting an EOM on slack, or resection, increasing EOM load, results in activation of muscle stem cells. Most importantly, there is coordinated adaptation of the unoperated antagonist and yoked muscle pairs – suggesting that there is significant plasticity mediated through central nervous system mechanisms.7 Not only are the EOM capable of rapid adaptation to injury, the EOM express large number of molecules not normally expressed in adult limb skeletal muscle. In an attempt to understand how the effector organ for oculomotor control, the EOM and its innervation, ultimately controls eye movements, we examined hundreds of single EOM myofibers and showed a continuum of shortening velocities and a continuum in their expression of different myosin heavy chain isoforms.8 Recent studies of receptor expression for various neurotrophic factors suggest a potential mechanism for local control over these complex single fiber characteristics. Collectively, these studies suggest that there is great potential for manipulation of EOM as a potential approach to treat strabismus1-4, nystagmus, or modulate the EOM after strabismus surgery in order to provide more physiological approaches for improved treatment.7
Sparing of the EOM in Degenerative Muscle Diseases
The EOM are spared in various forms of muscular dystrophy and in amyotrophic lateral sclerosis (ALS), but the reason is unknown. Our working hypothesis is that the EOM contain an enriched population of muscle stem cells that allows for continuous remodeling and repair in EOM in muscular dystrophy.9 Subsequent studies showed these muscle stem cells to be Pitx2-positive. We used gamma irradiation to deplete these muscle stem cells in the EOM and leg of the mdx mouse, a model of Duchenne muscular dystrophy (DMD). We found this created dystrophic pathology in the EOM. Surprisingly, the dystrophic phenotype was short lived, and normal EOM morphology had returned by 2 months after stem cell depletion. This result supports the role of Pitx2 positive EOM stem cell in the sparing of the EOM in DMD. Just as importantly, this finding demonstrates that there is a radiation-resistant stem cell in the EOM that is able to allow complete recovery of normalcy in the irradiated EOM. ALS is another degenerative disease where the EOM are spared. Based on published literature we set out to determine if the Wnt family of signaling molecules might play a role in EOM sparing. Wnts are a conserved family of secreted signaling molecules that play a critical role in neuromuscular junction formation. We showed significant differences in Wnt expression in EOM and limb skeletal muscles from ALS patients. This supports their potential role in preserving normal function and structure of the EOM and their innervating motor neurons from the devastating neurodegeneration seen in limb skeletal muscles in ALS.11 This study was the first to link Wnt signaling to sparing of the EOM in ALS, and suggests a potential therapeutic target for this fatal neurodegenerative disease.
Early studies in my laboratory generated critical normative data on the structure of the orbicularis oculi muscles, which are critical for eye blink and forceful eyelid closure. We then went on to develop two different pharmacologic approaches to the treatment of blepharospasm, a focal dystonia that causes functional blindness, as well as related focal dystonias such as hemifacial spasm and cervical dystonia. In the first series of studies, we demonstrated that a drug called doxorubicin could effectively and permanently reduce these forceful contractions.13 While this treatment was extremely effective, it was hard to convince Ophthalmologists to try it. We thus moved to a different approach. The most common method for treating focal dystonias in general, and blepharospasm and hemifacial spasm in particular, is with botulinum toxin A. However, its effects are temporary, and overtime patients find they need more frequent injections for relief. Part of the mechanism is significantly increased neuromuscular density on the affected myofibers with concomitant nerve sprouting.14 Building on work in my laboratory which showed that local injection of corticotropin releasing factor (CRF) reduced hyperalgesia by limiting neurite sprouting after local injury,15,16 we tested the ability of a combination drug composed of botulinum toxin A and CRF to reduce neuromuscular junction density increases as well as nerve sprouting. As a result of the efficacy of this highly novel combination treatment, we applied for and were awarded as U.S. patent for this drug (US 8,791,072 B2, Patent Granted, July 29, 2014.) We hope to move this drug treatment to the clinic in the future.
My most recent studies focus on another intractable eye movement disorder common in children, infantile nystagmus syndrome (INS). This is characterized for involuntary oscillatory movements of the eyes, and results in significant loss of visual acuity in most of the affected children. We have done extensive analysis of surgical waste specimens from patients with INS, and shown that there are significant abnormalities compared to normal age-matched control EOM.17 Further studies, which we will be presenting as a platform talk at ARVO, suggest that there is an innervation deficit in the EOM from these patients, with a concomitant change in neuromuscular junction morphology – reduced size and increased expression of the immature subunit of the acetylcholine receptor. Albino mice have been shown to have nystagmus-type eye movements. We believe that retrogradely transported neurotrophic factors, after sustained treatment at the periphery, should be able to modulate and potentially stabilize the abnormal eye oscillations. For example, our recent study showed that treatment with brain-derived neurotrophic factor increases the number of slow contracting myofibers,18 and theoretically could modulate the functional properties of muscle contraction and shortening velocity. As there is no effective treatment for this motor impairment, it is exciting to have a potential approach to test for its ability to reduce the uncontrolled eye movements of INS.
Rudell J, Stager DR, Felius J, McLoon LK. Morphological differences in inferior oblique muscles from subjects with over-elevation in adduction. Invest Ophthalmol Vis Sci. 2020;61: in press.
Torres Jimenez N, Lines J, Kueppers RB, Rankila A,Wei H, Kofuji P, Coyle J, Miller RF, McLoon LK. Electroretinographic abnormalities and sex differences detected with mesopic adaptation in a mouse model of schizophrenia: A and B wave analysis. PMCID: 32053730. Invest. Ophthalmol. Vis. Sci. 2020 Feb 7;61(2):16.
Moghimi P*, Torres Jimenez N*, McLoon LK, Netoff TI, Lee MS, MacDonald III, A, Miller RG. Electroretinographic evidence of altered retinal function in schizophrenia. PMID: 31615740. Schizophr. Res. 2019 Oct 12. pii: S0920-9964(19)30414-1 *: co-first authors
Olson RM, Mokhtarzadeh A, McLoon LK, Harrison AR. Effects of repeated eyelid injections with botulinum toxin A on innervation of treated muscles in patients with blepharospasm. PMC6397080. Current Eye Res. 2019;44:257-263.
Fleuriet J, McLoon LK. Visualizing neuronal adaptation over time after treatment of strabismus. PMC6188464. Invest. Ophthalmol. Vis. Sci. 2018;59:5022-5024.
Verma M, Asakura Y, Murakonda BSR, Pengo T, Latroche C, Chazaud B, McLoon LK. Asakura A. Muscle satellite cell cross-talk with a vascular niche maintains quiescence via VEGF and notch aignaling. PMC6178221. Cell Stem Cell. 2018;23(4):530-543.
Fitzpatrick KR, Cucak A, McLoon LK. Changing muscle function with sustained glial derived neurotrophic factor treatment of rabbit extraocular muscle. PMC6108505. PLoS One. 2018 Aug 24;13(8):e0202861.
McLoon LK, Redish AD. Demystifying graduate school: Navigating a PhD in neuroscience and beyond. PMC6153002. J. Undergrad. Neurosci. Educ. 2018;16:A203-A209.
McLoon LK, Vincente A, Fitzpatrick KR, Lindström M, Pedrosa-Domellöf F. Composition, architecture, and functional implications of the connective tissue network of the extraocular muscles. PMC5773232. Invest. Ophthalmol. Vis. Sci. 2018;59:322-329.
Hebert SL*, Fitzpatrick KR*, McConnell SA, Cucak A, Yuan C, McLoon LK. Effects of retinoic acid signaling on extraocular muscle myogenic precursor cells in vitro. PMC6546114. Exp. Cell Res. 2017;361:101-111. *co-first authors.
Verma M, Fitzpatrick KR, McLoon LK. Extraocular muscle repair and regeneration. PMC5669281. Current Ophthalmology Reports. Invited review. 2017;5:207-215.
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