Cornea is the foremost part of the eye for transmitting and focusing extraocular light into the eye. On the ocular surface, excessive light exposure leads to a photo-oxidative reaction and potential photodamage, including apoptosis, suppression of motility, and further long-term effects on aging-related gene expression [[1], [2], [3]]. With different semiconductor materials, commercial light-emitting diodes (LEDs) exhibit various colors in a narrow band of wavelengths ranging from ultraviolet A (UVA) at 365 nm to infrared (IR) at 850 nm with their corresponding photoenergies.

LED lights are widely used for illuminations, medicine, agriculture, and clinical diagnostic and therapeutic devices [[4], [5], [6], [7], [8], [9]]. Photobiomodulation (PBM) is the intentional use of a low-power laser or LED in the visible and near-infrared light spectra as a medical treatment for living biological tissues [[9], [10], [11], [12], [13], [14]]. The biological effects of LED-induced PBM begin with the interaction of radiation photons with specific endogenous photoacceptors [15]. This subsequently increases ATP production in the mitochondria.

Cellular maturation and senescence are key drivers of disease pathogenesis and could be targeted as therapeutic strategies [16]. The TJ structure is a critical multiprotein complex between epithelial cells that build the cell barrier and maintain cell polarity [17,18]. The TJ framework is primarily composed of transmembrane proteins, including claudin-1 (CLDN1), claudin-4 (CLDN4), occludin (OCLN), junctional adhesion molecules, and angulins. Additionally, there are soluble peripheral cytoplasmic proteins represented by zonula occludens (ZOs; ZO-1, ZO-2, and ZO-3). Their expressions are regulated by intracellular regulatory proteins influenced by diverse phosphorylation pathways [19,20], and extracellular factors emanating from relevant cells [21]. The formation of TJ structures in epithelial cells requires the regulation of a series of intracellular signaling pathways, including those involving secondary messengers [[22], [23], [24], [25]] and TJ-associated proteins, by gene transcription, protein translation, and post-translational modification [26]. Among the required factors for TJ formation, increasing intracellular secondary messengers, such as adenosine 3′,5′-cyclic monophosphate (cAMP) [22,23] and calcium ions (Ca2+) [24,25], strengthens the TJ structure. Like secondary messengers, protein kinase A (PKA) is a downstream acceptor and is activated mainly by cAMP to regulate TJ-associated proteins and determine whether the TJ structure can be completely formed as a functional and integral barrier on epithelial and endothelial layers. Between PKA and TJ structure, vasodilator-stimulated phosphoprotein (VASP) phosphorylated by PKA, is also an important signal for membrane localization [27,28]. Generally, VASP plays a role in cell adhesion and motility through dynamic actin assembly at the leading front of migrating cells. In TJ structure, colocalization of the ZO-1 with VASP or phosphorylated VASP delineated the regulatory effect of VASP on ZO-1 [29,30]. These findings suggest that ZO-1 can be regulated by active VASP. However, dysfunction or reduction of any of the TJ modulating proteins, including cAMP, PKA, VASP, and ZOs, might directly interfere with physiological functions.

The commonly investigated themes on the impact of LED on daily irradiation include blue light on the retina [[31], [32], [33]] as well as UV in skin cells [34,35] and retinal cells [31,36]. Large epidemiologic studies demonstrate that the use of blue-blocking intraocular lenses does not reduce the risk or progression of age-related macular degeneration [37]. They have debunked the notion of the light hazard, refuting the idea that blue light is highly toxic to the retina [[31], [32], [33]]. In contrast, therapeutic PBM has been popularized in both the clinical setting and for use in home devices [9,10]. Currently, intense pulsed light (IPL), whose emission wavelength ranges from 500 nm to 1200 nm, is commonly used for treatment of meibomian gland dysfunction due to the high prevalence of dry eyes [[38], [39], [40], [41]]. The bright light applied during each pulse of IPL treatment traversed the corneal epithelium of dry eye patients. Whether IPL light also modulate corneal epithelial change in dry eye is not studied. Diabetes mellitus-related corneal epithelial dysfunction leads to delayed wound healing in corneal epithelial cells and impaired the tight-junction (TJ) formation [42]. Dry eye related hyperosmolar stress disrupted TJ proteins ZO-1 and occludin [13]. It is thus crucial to understand the cellular effects of different LEDs on ocular surface cells and to explore the potential of PBM for corneal treatment. Therefore, we evaluated the general characteristics of LED lights to demonstrate their cellular effects under non-lethal exposure conditions. Herein, we showed both the formation process and regulatory mechanism of TJ structures in cultivated human corneal epithelial cells (HCEs). We further investigated the critical factors involved in TJ formation in HCEs after non-lethal LED exposure.