Novel alterations in corneal neuroimmune phenotypes in mice with central nervous system tauopathy

Interconnections between diabetic corneal neuropathy and diabetic retinopathy: diagnostic and therapeutic implications

Diabetic corneal neuropathy and diabetic retinopathy are ocular complications occurring in the context of diabetes mellitus. Diabetic corneal neuropathy refers to the progressive damage of corneal nerves. Diabetic retinopathy has traditionally been considered as damage to the retinal microvasculature. However, growing evidence suggests that diabetic retinopathy is a complex neurovascular disorder resulting from dysfunction of the neurovascular unit, which includes both the retinal vascular structures and neural tissues. Diabetic retinopathy is one of the leading causes of blindness and is frequently screened for as part of diabetic ocular screening. However, diabetic corneal neuropathy is commonly overlooked and underdiagnosed, leading to severe ocular surface impairment. Several studies have found that these two conditions tend to occur together, and they share similarities in their pathogenesis pathways, being triggered by a status of chronic hyperglycemia. This review aims to discuss the interconnection between diabetic corneal neuropathy and diabetic retinopathy, whether diabetic corneal neuropathy precedes diabetic retinopathy, as well as the relation between the stage of diabetic retinopathy and the severity of corneal neuropathy. We also endeavor to explore the relevance of a corneal screening in diabetic eyes and the possibility of using corneal nerve measurements to monitor the progression of diabetic retinopathy.

Redefining the human corneal immune compartment using dynamic intravital imaging

The healthy human cornea is a uniquely transparent sensory tissue where immune responses are tightly controlled to preserve vision. The cornea contains immune cells that are widely presumed to be intraepithelial dendritic cells (DCs). Corneal immune cells have diverse cellular morphologies and morphological alterations are used as a marker of inflammation and injury. Based on our imaging of corneal T cells in mice, we hypothesized that many human corneal immune cells commonly defined as DCs are intraepithelial lymphocytes (IELs). To investigate this, we developed functional in vivo confocal microscopy (Fun-IVCM) to investigate cell dynamics in the human corneal epithelium and stroma. We show that many immune cells resident in the healthy human cornea are T cells. These corneal IELs are characterized by rapid, persistent motility and interact with corneal DCs and sensory nerves. Imaging deeper into the corneal stroma, we show that crawling macrophages and rare motile T cells patrol the tissue. Furthermore, we identify altered immune cell behaviors in response to short-term contact lens wear (acute inflammatory stimulus), as well as in individuals with allergy (chronic inflammatory stimulus) that was modulated by therapeutic intervention. These findings redefine current understanding of immune cell subsets in the human cornea and reveal how resident corneal immune cells respond and adapt to chronic and acute stimuli.

Neuroimmune crosstalk in the cornea: The role of immune cells in corneal nerve maintenance during homeostasis and inflammation

In the cornea, resident immune cells are in close proximity to sensory nerves, consistent with their important roles in the maintenance of nerves in both homeostasis and inflammation. Using in vivo confocal microscopy in humans, and ex vivo immunostaining and fluorescent reporter mice to visualize corneal sensory nerves and immune cells, remarkable progress has been made to advance our understanding of the physical and functional interactions between corneal nerves and immune cells. In this review, we summarize and discuss recent studies relating to corneal immune cells and sensory nerves, and their interactions in health and disease. In particular, we consider how disrupted corneal nerve axons can induce immune cell activity, including in dendritic cells, macrophages and other infiltrating cells, directly and/or indirectly by releasing neuropeptides such as substance P and calcitonin gene-related peptide. We summarize growing evidence that the role of corneal intraepithelial immune cells is likely different in corneal wound healing versus other inflammatory-dominated conditions. The role of different types of macrophages is also discussed, including how stromal macrophages with anti-inflammatory phenotypes communicate with corneal nerves to provide neuroprotection, while macrophages with pro-inflammatory phenotypes, along with other infiltrating cells including neutrophils and CD4⁺ T cells, can be inhibitory to corneal re-innervation. Finally, this review considers the bidirectional interactions between corneal immune cells and corneal nerves, and how leveraging this interaction could represent a potential therapeutic approach for corneal neuropathy.

The Efficient Regeneration of Corneal Nerves via Tunable Transmembrane Signaling Channels Using a Transparent Graphene-Based Corneal Stimulation Electrode

The efficient regeneration of corneal nerves is of limited success in the field of ophthalmology. This work reports the use of a non-invasive electrical stimulation technique that uses a transparent graphene-based corneal stimulation electrode and that can achieve efficient regeneration of corneal nerves. The corneal stimulation electrode is prepared using electroactive nitrogen-containing conducting polymers such as polyaniline functionalized graphene (PAG). This composite can carry a high capacitive current. It can be used to tune transmembrane signaling pathways including calcium channels and the MAPK signaling pathway. Tuning can lead to the efficient regeneration of corneal damaged nerves after the surgery of laser in-situ keratomileusis (LASIK). The composite and its application reported have the potential to provide a new way to treat nerve-related injuries.

Tauopathy induces degeneration and impairs regeneration of sensory nerves in the cornea

The cornea is transparent and innervated by a dense collection of sensory nerves originating from the ocular branch of the trigeminal nerve. This study was designed to comprehensively analyze alterations of corneal sub-basal nerve plexus in a mouse model of tauopathy (P301L transgenic mice) to test the possibility of using corneal nerves as a biomarker for tauopathy. Corneal sensitivity, thickness and epithelial wound healing were measured non-invasively by aeshesiometer, optical coherence tomography and fluorescein staining, respectively. Tau, corneal nerves and immune cells were examined by immunohistochemistry or Western blot. At the early stage of tauopathy, although corneal sensitivity, thickness and nerve fiber density were not greatly altered, corneal nerve abnormalities were observed in the peripheral region of young P301L mice. With aging, the density of abnormal nerves increased, while corneal sensitivity, epithelial thickness, nerve fiber density and length decreased in middle-aged P301L mice compared with WT mice. After corneal epithelial injury in young mice, no difference in reepithelialization was observed between two groups of mice, however, the regeneration of corneal nerves in P301L mice lagged behind WT mice, which was reflected by delayed recovery of corneal sensitivity, decreased corneal nerve density and length and density of CD45⁺ dendriform cells in P301L mice. In conclusion, our data provide compelling evidence that corneal nerves were changed in a mouse model of tauopathy in an age-dependent manner. Moreover, tau overexpression impairs corneal nerve regeneration. These results suggest that cornea may serve as a promising ocular site for the early diagnosis of tauopathy.

Corneal immune cell morphometry as an indicator of local and systemic pathology: A review

The corneal epithelium contains a population of resident immune cells commonly referred to as dendritic cells (DCs), or Langerhans cells. A unique advantage of the transparent cornea being situated at the surface of the eye is that these cells can be readily visualised using in vivo confocal microscopy. Over the past decade, interest in the involvement of corneal DCs in a range of ocular and systemic diseases has surged. For most studies, the number of corneal DCs has been the main outcome of interest. However, more recently attention has shifted towards understanding how DC morphology may provide insights into the inflammatory status of the cornea, and in some cases, the health of the peripheral nervous system. In this review, we provide examples of recent methodologies that have been used to classify and measure corneal DC morphology and discuss how this relates to local and systemic inflammatory conditions in humans and rodents.

Altered Corneal Epithelial Dendritic Cell Morphology and Phenotype Following Acute Exposure to Hyperosmolar Saline

Purpose

The purpose of this study was to assess the morphological and phenotypic responses of corneal epithelial dendritic cells (DCs) to acute topical hyperosmolar stress, given a pathogenic role for tear hyperosmolarity in dry eye disease (DED).

Methods

C57BL/6J mice were anesthetized and received 350 mOsm/L (physiological; n = 5 mice), 450 mOsm/L (n = 6), or 600 mOsm/L (n = 6) saline on a randomly assigned eye. Corneas were harvested 2 hours later. Immunofluorescent staining was performed using CD45, CD86, and CD68 antibodies to investigate DC morphology (density, viability, field area, circularity, and dendritic complexity) and immunological phenotype. Flow cytometry was used to confirm CD86 and CD68 expression in CD11c⁺ DCs, using C57BL/6J mice that received topical applications of 350 mOsm/L, 450 mOsm/L, or 600 mOsm/L (n = 5 per group) bilaterally for 2 hours.

Results

Following exposure to 450 mOsm/L topical saline for 2 hours, DCs in the central and peripheral cornea were larger (field area: P_central = 0.005, P_peripheral = 0.037; circularity: P_central = 0.026, and P_peripheral = 0.013) and had higher expression of CD86 compared with 350 mOsm/L controls (immunofluorescence: P < 0.0001; flow cytometry: P = 0.0058). After application of 600 mOsm/L saline, DC morphology was unchanged, although the percentage of fragmented DCs, and phenotypic expression of CD86 (immunofluorescence: P < 0.0001; and flow cytometry: P = 0.003) and CD68 (immunofluorescence: P = 0.024) were higher compared to 350 mOsm/L controls.

Conclusions

Short-term exposure to mild hyperosmolar saline (450 mOsm/L) induced morphological and phenotypic maturation in corneal epithelial DCs. More severe hyperosmolar insult (600 mOsm/L) for 2 hours appeared toxic to these cells. These data suggest that hyperosmolar conditions activate corneal DCs, which may have implications for understanding DC activation in DED.

Tau associated peripheral and central neurodegeneration: Identification of an early imaging marker for tauopathy

Pathological hyperphosphorylated tau is a key feature of Alzheimer's disease (AD) and Frontotemporal dementia (FTD). Using transgenic mice overexpressing human non-mutated tau (htau mice), we assessed the contribution of tau to peripheral and central neurodegeneration. Indices of peripheral small and large fiber neuropathy and learning and memory performances were assessed at 3 and 6 months of age. Overexpression of human tau is associated with peripheral neuropathy at 6 months of age. Our study also provides evidence that non-mutated tau hyperphosphorylation plays a critical role in memory deficits. In addition, htau mice had reduced stromal corneal nerve length with preservation of sub-basal corneal nerves, consistent with a somatofugal degeneration. Corneal nerve degeneration occurred prior to any cognitive deficits and peripheral neuropathy. Stromal corneal nerve loss was observed in patients with FTD but not AD. Corneal confocal microscopy may be used to identify early neurodegeneration and differentiate FTD from AD.

Morphometric Changes to Corneal Dendritic Cells in Individuals With Mild Cognitive Impairment

Purpose

There has been increasing interest in identifying non-invasive, imaging biomarkers for neurodegenerative disorders of the central nervous system (CNS). The aim of this proof-of-concept study was to investigate whether corneal sensory nerve and dendritic cell (DC) parameters, captured using in vivo confocal microscopy (IVCM), are altered in individuals with mild cognitive impairment (MCI) and Alzheimer’s disease (AD).

Methods

Fifteen participants were recruited from the Australian Imaging Biomarkers and Lifestyle (AIBL) study in Melbourne, VIC, Australia. The cohort consisted of cognitively normal (CN) individuals (n = 5), and those with MCI (n = 5) and AD (n = 5). Participants underwent a slit lamp examination of the anterior segment, followed by corneal imaging using laser-scanning in vivo confocal microscopy (IVCM) of the central and inferior whorl regions. Corneal DC density, field area, perimeter, circularity index, aspect ratio, and roundness were quantified using Image J. Quantitative data were derived for corneal nerve parameters, including nerve fiber length (CNFL), fiber density (CNFD), branch density (CNBD), and diameter.

Results

Corneal DC field area and perimeter were greater in individuals with MCI, relative to CN controls, in both the central and inferior whorl regions (p < 0.05 for all comparisons). In addition, corneal DCs in the whorl region of MCI eyes had lower circularity and roundness indices and a higher aspect ratio relative to CNs (p < 0.05 for all comparisons). DC density was similar across participant groups in both corneal regions. There was a trend toward lower quantitative parameters for corneal nerve architecture in the AD and MCI groups compared with CN participants, however, the inter-group differences did not reach statistical significance. Central corneal nerve diameters were similar between groups.

Conclusion

This study is the first to report morphological differences in corneal DCs in humans with MCI. These differences were evident in both the central and mid-peripheral cornea, and in the absence of significant nerve abnormalities or a difference in DC density. These findings justify future large-scale studies to assess the utility of corneal IVCM and DC analysis for identifying early stage pathology in neurodegenerative disorders of the CNS.

Role of Peripheral Immune Cells-Mediated Inflammation on the Process of Neurodegenerative Diseases

Neurodegenerative diseases are characterized by progressive loss of selectively vulnerable neuronal populations, which contrasts with selectively static loss of neurons due to toxic or metabolic disorders. The mechanisms underlying their progressive nature remain unknown. To date, a timely and well-controlled peripheral inflammatory reaction is verified to be essential for neurodegenerative diseases remission. The influence of peripheral inflammation on the central nervous system is closely related to immune cells activation in peripheral blood. The immune cells activation participated in the uncontrolled and prolonged inflammation that drives the chronic progression of neurodegenerative diseases. Thus, the dynamic modulation of this peripheral inflammatory reaction by interrupting the vicious cycle might become a disease-modifying therapeutic strategy for neurodegenerative diseases. This review focused on the role of peripheral immune cells on the pathological progression of neurodegenerative diseases.