Myopia is the leading cause of visual impairment worldwide. Its alarmingly rapid increase in prevalence has raised significant concerns regarding its impact on public health. Two of its signature changes include excessive optical axis elongation, concurrent scleral thinning accompanying remodeling of its extracellular matrix. These alterations result in declines in the visual acuity of distant images.
Causes of myopia
Scleral hypoxia is the main mechanism of myopia, which affects scleral extracellular matrix remodeling and the development of myopia by activating the signaling pathway of hypoxia inducible factor (HIF), which is a transcriptionally active protein and only be expressed stably under hypoxic conditions [1].
Specifically, myopia-related visual signals can specifically lower choroidal blood flow, which results in insufficient oxygen and nutrition reaching the sclera. Scleral fibroblasts detect the extracellular microenvironment for the first time and react swiftly by accumulating HIF-1α, increasing eIF2 and mTOR phosphorylation levels, and changing their phenotype from fibroblasts to myofibroblasts while decreasing the production of type I collagen. The sclera weakens and thins as a result. Consequently, axial length increases, and myopia ensues [1].
Fig.1 Paradigm for myofibroblast transdifferentiation involved in the pathogenesis of myopia [1].
HIF-1α has become a new target of myopia research in recent years. Anti hypoxia therapy by inhibiting HIF-1α can suppress the development of myopia. It’s reported that anti hypoxia drug salidroside, a drug with high biological activity [4], can downregulate HIF-1α expression, thus slow down the progression of myopia without affecting normal eye growth [1].
Fig.2 Salidroside has high biological activity[4].
Elevated intraocular pressure (IOP) can also cause myopia. During childhood eye growth or the progression of high myopia, elevated intraocular pressure may lead to scleral dilation, especially at the posterior pole of the sclera. After prolonged stretching, the elasticity of the sclera may decrease, affecting the growth and expansion speed of the sclera, ultimately leading to axial elongation [2]. Besides, the mechanical stress generated by eye pressure on scleral fibroblasts can trigger matrix metalloproteinases (MMPs), especially MMP2, a downstream protein of HIF-1α [5]. It can induce the degradation of type I collagen and other extracellular matrix remodeling, leading to scleral thinning during the progression of myopia.
Fig. 3. Decreased scleral elasticity caused by increased intraocular pressure [3]
Fig. 4. HIF-1α signal pathway [5]
Carrier
Because therapeutic contact lenses can boost the bioavailability of medications by over 50% through continuous residence time and close contact with the cornea, we chose them as our carrier of choice. They have a ten-fold longer retention period than conventional eye drops.
Fig. 5. Therapeutic contact lenses have better bioavailability than eye drops [7]
We initially intended to isolate the engineered bacterium from the eye by fixing it to the contact
lens and isolating it with a layer of nanofiltration membrane. Only salidroside can flow through the
membrane, nevertheless, because the pore size of the nanofiltration membrane cannot restrict small
molecule nutrients or bacterial metabolites inside the membrane. As a result, we developed the
concept of constructing salidroside transporters on the phospholipid bilayer, which requires
encasing the bacterial bodies in vesicles and integrating them into contact lenses. We ultimately
settled on this option because there have been reports of encapsulating Escherichia coli with
phospholipid bilayer and numerous application cases of contact lenses using liposomes as carriers.
Considering that the vesicle size is typically around 25 µm, we chose Escherichia coli as the
chassis cell.
Fig. 6. encapsulating E.coli with vesicles
Fig. 7. contact lenses loaded with vesicles
Detection signal
We chose intraocular pressure as the detection signal, which was inspired by iGEM NCKU Tainan 2020, because there are no readily observable indications during the development of myopia, such as no notable changes in the makeup of the eye surface and tear fluid. High intraocular pressure has already been demonstrated to put pressure on contact lenses, causing them to distort and leak water, which in turn raises the concentration of inducers.
Fig. 8. Schematic diagram of increased inducer concentration caused by elevated intraocular pressure.
Therefore, this project hopes that E. coli can synthesize salidroside so that it can downregulate HIF-1α when detecting elevated intraocular pressure, thus slow down the development of myopia.