The SARS-CoV-2 Virus and the Cholinergic System: Spike Protein Interaction with Human Nicotinic Acetylcholine Receptors and the Nicotinic Agonist Varenicline.

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is responsible for the worldwide coronavirus disease 2019 (COVID-19) pandemic. Although the pathophysiology of SARS-CoV-2 infection is still being elucidated, the nicotinic cholinergic system may play a role. To evaluate the interaction of the SARS-CoV-2 virus with human nicotinic acetylcholine receptors (nAChRs), we assessed the in vitro interaction of the spike protein of the SARS-CoV-2 virus with various subunits of nAChRs. Electrophysiology recordings were conducted at α4ß2, α3ß4, α3α5ß4, α4α6ß2, and α7 neuronal nAChRs expressed in Xenopus oocytes. In cells expressing the α4ß2 or α4α6ß2 nAChRs, exposure to the 1 µg/mL Spike-RBD protein caused a marked reduction of the current amplitude; effects at the α3α5ß4 receptor were equivocal and effects at the α3ß4 and α7 receptors were absent. Overall, the spike protein of the SARS-CoV-2 virus can interact with select nAChRs, namely the α4ß2 and/or α4α6ß2 subtypes, likely at an allosteric binding site. The nAChR agonist varenicline has the potential to interact with Spike-RBD and form a complex that may interfere with spike function, although this effect appears to have been lost with the omicron mutation. These results help understand nAChR's involvement with acute and long-term sequelae associated with COVID-19, especially within the central nervous system.

Lumican is required for neutrophil extravasation following corneal injury and wound healing.

An important aspect of wound healing is the recruitment of neutrophils to the site of infection or tissue injury. Lumican, an extracellular matrix component belonging to the small leucine rich proteoglycan (SLRP) family, is one of the major keratan sulfate proteoglycans (KSPGs) within the corneal stroma. Increasing evidence indicates that lumican can serve as a regulatory molecule for several cellular processes, including cell proliferation and migration. In the present study, we addressed the role of lumican in the process of extravasation of polymorphonuclear leukocytes (PMNs) during the early inflammatory phase present in the healing of the corneal epithelium following debridement. We used Lum(-/-) mice and a novel transgenic mouse, Lum(-/-),Kera-Lum, which expresses lumican only in the corneal stroma, to assess the role of lumican in PMN extravasation into injured corneas. Our results showed that PMNs did not readily invade injured corneas of Lum(-/-) mice and this defect was rescued by the expression of lumican in the corneas of Lum(-/-),Kera-Lum mice. The presence of lumican in situ facilitates PMN infiltration into the peritoneal cavity in casein-induced inflammation. Our findings are consistent with the notion that in addition to regulating the collagen fibril architecture, lumican acts to aid neutrophil recruitment and invasion following corneal damage and inflammation.

Bone marrow chimeras and c-fms conditional ablation (Mafia) mice reveal an essential role for resident myeloid cells in lipopolysaccharide/TLR4-induced corneal inflammation.

The mammalian cornea contains an extensive network of resident macrophages and dendritic cells. To determine the role of these cells in LPS-induced corneal inflammation, TLR4(-/-) mice were sublethally irradiated and reconstituted with bone marrow cells from either enhanced GFP (eGFP)(+)/C57BL/6 or eGFP(+)/TLR4(-/-) mice. The corneal epithelium was abraded, LPS was added topically, and cellular infiltration to the corneal stroma and development of corneal haze were examined after 24 h. TLR4(-/-) mice reconstituted with C57BL/6, but not TLR4(-/-) bone marrow cells donor cells were found to cause infiltration of eGFP(+) cells to the cornea, including neutrophils, and also increased corneal haze compared with saline-treated corneas. In a second experimental approach, corneas of transgenic macrophage Fas induced apoptosis (Mafia) mice were stimulated with LPS. These mice express eGFP and a suicide gene under control of the c-fms promoter, and systemic treatment with the FK506 dimerizer (AP20187) causes Fas-mediated apoptosis of monocytic cells. AP20187-treated mice had significantly fewer eGFP(+) cells in the cornea than untreated mice. After stimulation with LPS neutrophil recruitment and development of corneal haze were impaired in AP20187-treated mice compared with untreated controls. Furthermore, LPS induced CXCL1/KC and IL-1alpha production within 4 h in corneas of untreated Mafia mice, which is before cellular infiltration; however, cytokine production was impaired after AP20187 treatment. Together, results from both experimental approaches demonstrate an essential role for resident corneal monocytic lineage cells (macrophages and dendritic cells) in development of corneal inflammation.

Conditional deletion of Cited2 results in defective corneal epithelial morphogenesis and maintenance.

Cited2 is an important transcriptional cofactor involved in multiple organ development. Gene profile analysis has identified Cited2 as one of the transcription factors expressed at high levels in adult mouse cornea. To address the function of Cited2 in corneal morphogenesis, we deleted Cited2 in surface ectoderm derived ocular structures including cornea by crossing Cited2-floxed mice with Le-Cre transgenic mice. Cited2(flox/flox);Le-Cre(+) eyes invariably displayed corneal opacity and developed spontaneous corneal neovascularization at older age. Fewer layers of corneal epithelial cells and the absence of cytokeratin 12 (K12) expression featured Cited2 deficient postnatal and adult eyes. Cited2 deficient cornea exhibited impaired healing in response to corneal epithelial debridement by manifesting abnormal histology, lack of K12 expression and corneal neovascularization. Moreover, mechanistic studies suggest that Cited2 may play a role in corneal morphogenesis in part through modulating the expression of Pax6 and Klf4. Collectively, these findings demonstrate a novel function of Cited2 in postnatal corneal morphogenesis and maintenance. Our study will help better understand the molecular mechanisms involved in corneal biology, and more importantly, it may provide a valuable animal model for testing therapeutics in the treatment of corneal disorders, especially blindness as a result of corneal epithelial cell deficiency.

Stem cell therapy restores transparency to defective murine corneas.

Corneal scarring from trauma and inflammation disrupts vision for millions worldwide, but corneal transplantation, the primary therapy for corneal blindness, is unavailable to many affected individuals. In this study, stem cells isolated from adult human corneal stroma were examined for the ability to correct stromal opacity in a murine model by direct injection of cells into the corneal stroma. In wild-type mice, injected human stem cells remained viable for months without fusing with host cells or eliciting an immune T-cell response. Human corneal-specific extracellular matrix, including the proteoglycans lumican and keratocan, accumulated in the treated corneas. Lumican-null mice have corneal opacity similar to that of scar tissue as a result of disruption of stromal collagen organization. After injection with human stromal stem cells, stromal thickness and collagen fibril defects in these mice were restored to that of normal mice. Corneal transparency in the treated mice was indistinguishable from that of wild-type mice. These results support the immune privilege of adult stem cells and the ability of stem cell therapy to regenerate tissue in a manner analogous to organogenesis and clearly different from that of normal wound healing. The results suggest that cell-based therapy can be an effective approach to treatment of human corneal blindness.

CD40 mediates retinal inflammation and neurovascular degeneration.

Retinopathies are major causes of visual impairment. We used a model of ischemic retinopathy to examine the role of CD40 in the pathogenesis of retinal injury. Retinal inflammation, loss of ganglion cells, and capillary degeneration were markedly attenuated in ischemic retinas of CD40(-/-) mice. Up-regulation of NOS2 and COX2 after retinal ischemia were blunted in CD40(-/-) mice. NOS2-COX-2 up-regulation in ischemic retinas from wild-type mice was at least in part explained by recruitment of NOS2(+)COX-2(+) leukocytes. Up-regulation of KC/CXCL1 and ICAM-1 also required CD40. Retinal endothelial and Muller cells expressed CD40. Stimulation of these cells through CD40 caused ICAM-1 up-regulation and KC/CXCL1 production. Bone marrow transplant experiments revealed that leukocyte infiltration, ganglion cell loss, and up-regulation of proinflammatory molecules after retinal ischemia were dependent on CD40 expression in the retina and not peripheral blood leukocytes. These studies identified CD40 as a regulator of retinal inflammation and neurovascular degeneration. They support a model in which CD40 stimulation of endothelial and Muller cells triggers adhesion molecule up-regulation and chemokine production, promoting the recruitment of leukocytes that express NOS2/COX-2, molecules linked to neurovascular degeneration.

Targeted expression of a lumican transgene rescues corneal deficiencies in lumican-null mice.

PURPOSE: To investigate whether targeted expression of lumican in the mouse cornea rescued the Lum(-/-) phenotype. METHODS: Lum(-/-)/Kera-Lum mice were generated by crossing Lum(-/-) mice with Kera-Lum transgenic mice that overexpressed lumican under the control of the keratocan promoter. Mouse eyes were analyzed in vivo by confocal microscopy through focusing (CMTF) to determine corneal sublayer thickness and haze. Subsequently, one cornea from each mouse was processed for SDS-PAGE/western blotting while the other was used for either electron microscopy (EM) or real-time polymerase chain reaction (RT-PCR). RESULTS: Overall, corneas of Lum(-/-)/Kera-Lum mice showed significant improvement over Lum(-/-) but were still deficient when compared to wildtype (WT) mice. Specifically, analysis of Lum(-/-)/Kera-Lum mouse eyes by CMTF showed a similar stromal but slightly increased epithelial thickness compared to matching Lum(-/-) mice. Analysis of the CMTF scans for light backscattering revealed a small yet significant reduction in corneal haze in Lum(-/-)/Kera-Lum mice as compared to Lum(-/-) mice. At the EM level, the pronounced disarray of the posterior fibrillar matrix seen in Lum(-/-) mice was not observed in Lum(-/-)/Kera-Lum mice. Moreover, analyses of collagen fibril diameter distributions showed a significant reduction in the number of large-diameter (>40 nm) fibrils in Lum(-/-)/Kera-Lum mice as compared to Lum(-/-) mice. No significant differences in keratocan expression were found at the mRNA level, but western blot analysis detected an approximately twofold increase in keratocan protein levels in Lum(-/-)/Kera-Lum over Lum(-/-) mice. CONCLUSIONS: Together these data suggest that despite the low keratocan promoter activity driving the transgene in Lum(-/-) cornea, transgenic lumican expression was sufficient to partially rescue corneal phenotypic deficiencies.

Visualization and characterization of inflammatory cell recruitment and migration through the corneal stroma in endotoxin-induced keratitis.

PURPOSE: The infiltration of inflammatory cells into the cornea is a major determinant in the outcome of keratitis. The purpose of this study was to use enhanced green fluorescence protein (EGFP) bone marrow chimeric mice to visualize and characterize the inflammatory cells that migrate to the corneal stroma during endotoxin-induced keratitis and to explore the mechanisms underlying this process. METHODS: Keratitis was induced by injecting endotoxin into the corneas of EGFP chimeric mice. In vivo fluorescence stereomicroscopy was used to visualize in real time the recruitment of EGFP-positive cells at different time points. Immunohistochemistry and three-dimensional (3D) confocal analysis of whole-mount corneas was used for histologic characterization. Macrophage inflammatory protein-2 (MIP-2) chemokine was neutralized in vivo to determine its contribution to this process. RESULTS: Recruitment of EGFP-positive inflammatory cells in the corneal stroma can be detected in vivo by 6 hours after the injection of endotoxin, and these were mainly neutrophils. Full-thickness whole corneal mount confocal image analysis showed a distinct pattern of migration of EGFP inflammatory cells through the anterior corneal stroma. Moreover, inflammatory cells did not colocalize with the injected lipopolysaccharide (LPS) deposits in the stroma but moved from all directions toward LPS, partially in response to the production of the chemokine MIP-2. CONCLUSIONS: EGFP chimeric mice and ex vivo 3D analysis of whole-mount corneas provides unique information on the interaction of infiltrating inflammatory cells in the cornea. These findings demonstrate that a chemotactic gradient triggered in part by MIP-2 is responsible for directing inflammatory cell migration through the corneal stroma.

In vivo gene delivery and visualization of corneal stromal cells using an adenoviral vector and keratocyte-specific promoter.

PURPOSE: This study was conducted to determine whether intrastromal injection of adenoviral construct could be used to transfect corneal stroma cells effectively in vivo and to determine whether a tissue-specific promoter could be used to express exogenous genes in keratocytes. METHODS: An adenoviral construct with a cytomegalovirus (pCMV)-driven enhanced green fluorescent protein (EGFP) reporter gene was injected into the stroma of murine corneas. In vivo expression was quantitated and samples were analyzed by in vivo stereomicroscopy, and ex vivo expression was determined by confocal three dimensional (3-D) reconstruction. The 3.2-kb keratocan promoter was used to drive tissue-specific reporter gene expression in vivo. RESULTS: EGFP expression was first detected in vivo 11 hours after injection of adeno-EGFP in the corneal stroma, with a duration of approximately 3 weeks. Ex vivo wholemount cornea confocal analysis with 3-D reconstruction allowed visualization of EGFP expression in corneal stroma cells, to accurately assess cellular architecture and distribution in the corneal stroma. Naked pCMV-EGFP plasmid DNA did not express the reporter gene to the levels of the adeno-EGFP. The 3.2-kb keratocan promoter was capable of driving EGFP tissue-specific expression in the cornea. CONCLUSIONS: Intrastromal injection of adenovirus packaged DNA constructs is a rapid and efficient way to deliver and express genes in the corneal stroma. Intrastromal injection is also capable of delivering tissue-specific promoter constructs to the corneal stroma for gene expression. Furthermore, 3-D reconstruction provides a powerful tool for enhanced visualization of the corneal stroma environment and cellular biology.

Altered KSPG expression by keratocytes following corneal injury.

PURPOSE: Keratocytes synthesize keratan-sulfate proteoglycans (KSPG), lumican and keratocan, to develop and maintain proper collagen interfibrillar spacing and fibril diameter characteristics of the transparent cornea. The purposes of this study are to compare the expression patterns of KSPGs and keratin 12 (K12) respectively by corneal keratocytes and epithelial cells after three different types of injuries; partial and total epithelial debridement and alkali burn. METHODS: Corneas of 8-12 week old C57Bl/6J or FVBN mice were wounded by partial epithelial (2 mm in diameter) and total epithelial debridement, and alkali burn (0.1 M NaOH, 30 s) and were allowed to heal for various periods of time, from 1 to 84 days. The corneas were then subjected to light microscopy, in situ and Northern hybridization and RT-PCR for examining the expression of K12 and KSPG in the corneal epithelium and stroma, respectively. Immunohistochemistry with anti-alpha-smooth muscle actin (alpha-SMA) was used to identify myofibroblasts in the stroma of injured cornea. RESULTS: In 2-3 days, partial epithelial denuded corneas were resurfaced by corneal epithelium positive for K12, and stromal edema caused by debridement disappeared. Total epithelial debridement wounded corneas were resurfaced by conjunctival epithelial cells in 2 weeks. Stromal edema in the total epithelial debridement corneas began to subside after 6 weeks. Corneal epithelial cells resurfaced alkali burned corneas within 3-5 days. In situ and Northern hybridization showed a decrease in keratocan and lumican expression at 6 weeks and increased at 12 weeks post-injury in all wound types. Alpha-SMA positive myofibroblasts in the cornea were detected via immunostaining at the time point when KSPG expression was lowest, 6 weeks post-injury. CONCLUSIONS: The results suggest keratocan and lumican are down-regulated during wound healing at 6 weeks and returned to higher levels at 12 weeks post-injury; implicating that the cells repopulating the injured corneal stroma regained the characteristic function of keratocytes independent of the wound types. However, complete epithelial removal results in irreversible loss of K12 expression.

Cis-regulatory elements of the mouse Krt1.12 gene.

PURPOSE: Keratin 12 is a cornea epithelial cell-specific intermediate filament component. To better understand the regulatory mechanism of its expression, the cis-regulatory elements located between the transcription start site and 600 bp upstream of the Krt1.12 gene were determined. METHODS: The promoter activity of reporter gene constructs containing 0.6, 0.4, and 0.2 kb of DNA 5' upstream of Krt1.12 coupled to the lac Z gene were determined in rabbit corneas using Gene Gun technology. DNA foot printing and EMSA (electrophoresis mobility shift assay) were employed to identify putative cis-regulatory elements of the Krt1.12 gene using bovine corneal epithelial cell nuclear extracts. RESULTS: Enzyme activity assays and histochemical analysis of beta-galactosidase from the 0.6, 0.4, and 0.2 kb K12 promoter constructs indicated that the DNA elements between -0.2 and -0.6 kb 5' of the Krt1.12 gene contain cis-regulatory elements for its corneal epithelial cell-specific expression. Foot printing and EMSA showed that the sequences between -181 to -111 and -256 to -193 upstream of the Krt1.12 gene reacted to nuclear proteins isolated from bovine corneal epithelial cells. A Genbank search revealed that these two regions were potential binding sites for many transcription factors such as AP1, c/EBP, and KLF6. Immunofluorescent staining indicated the presence of c-jun and c/EBP transcription factors in the nuclei of corneal epithelial cells. CONCLUSIONS: The data is consistent with the notion that the -182 to -111 and -256 to -193 fragments 5' of the Krt1.12 gene may serve as corneal epithelial cell-specific cis-regulatory elements, and the coordinated interactions of various transcription factors are required for cornea-specific expression of Krt1.12 gene.

Regulation of corneal inflammation by neutrophil-dependent cleavage of keratan sulfate proteoglycans as a model for breakdown of the chemokine gradient.

Keratocan and lumican are small, leucine-rich repeat KSPGs in the extracellular matrix (ECM) of the mammalian cornea, whose primary role is to maintain corneal transparency. In the current study, we examined the role of these proteoglycans in the breakdown of the chemokine gradient and resolution of corneal inflammation. LPS was injected into the corneal stroma of C57BL/6 mice, and corneal extracts were examined by immunoblot analysis. We found reduced expression of the 52-kD keratocan protein after 6 h and conversely, increased expression of 34/37 kD immunoreactive products. Further, appearance of the 34/37-kD proteins was dependent on neutrophil infiltration to the cornea, as the appearance of these products was coincident with neutrophil infiltration, and the 34/37-kD products were not detected in explanted corneas or in CXCR2(-/-) corneas with deficient neutrophil recruitment. Furthermore, the 34/37-kD products and CXCL1/KC were detected in the anterior chamber, into which the corneal stroma drains; and CXCL1/KC was elevated significantly in keratocan(-/-) and lumican(-/-) mice. Together, these findings indicate that the inflammatory response in the cornea is regulated by proteoglycan/CXCL1 complexes, and their diffusion into the anterior chamber is consistent with release of a chemokine gradient and resolution of inflammation.

Keratocan and lumican regulate neutrophil infiltration and corneal clarity in lipopolysaccharide-induced keratitis by direct interaction with CXCL1.

Keratocan and lumican are keratan-sulfate proteoglycans (KSPG), which have a critical role in maintaining corneal clarity. To determine whether these KSPGs have a role in corneal inflammation, we examined Kera(-/-) and Lum(-/-) mice in a model of lipopolysaccharide (LPS)-induced keratitis in which wild-type mice develop increased corneal thickness and haze due to neutrophil infiltration to the corneal stroma. Corneal thickness increases caused by LPS mice were significantly lower in Kera(-/-) and Lum(-/-) than wild-type mice. Further, LPS-injected Lum(-/-) mice had elevated corneal haze levels compared with that of Kera(-/-) and wild-type. At 24 h post-injection, total enhanced green fluorescent protein-positive bone marrow-derived inflammatory cells in chimeric mice was significantly lower in Kera(-/-) mice and Lum(-/-) mice compared with wild-type mice. Neutrophil infiltration was inhibited in Kera(-/-) and Lum(-/-) mice at 6 and 24 h post-stimulation, with Lum(-/-) corneas having the most profound defect in neutrophil migration. Reconstitution of keratocan and lumican expression in corneas of Kera(-/-) and Lum(-/-) mice using adeno-keratocan and adeno-lumican viral vectors, respectively, resulted in normal neutrophil infiltration in response to LPS. Immunoprecipitation/Western blot analysis showed that lumican and keratocan core proteins bind the CXC chemokine KC during a corneal inflammatory response, indicating that corneal KSPGs mediate neutrophil recruitment to the cornea by regulating chemokine gradient formation. Together, these data support a significant role for lumican and keratocan in a corneal inflammatory response with respect to edema, corneal clarity, and cellular infiltration.

Keratocan, a cornea-specific keratan sulfate proteoglycan, is regulated by lumican.

Lumican is an extracellular matrix glycoprotein widely distributed in mammalian connective tissues. Corneal lumican modified with keratan sulfate constitutes one of the major proteoglycans of the stroma. Lumican-null mice exhibit altered collagen fibril organization and loss of corneal transparency. A closely related protein, keratocan, carries the remaining keratan sulfate of the cornea, but keratocan-null mice exhibit a less severe corneal phenotype. In the current study, we examined the effect of lumican overexpression in corneas of wild type mice. These mice showed no alteration in collagen organization or transparency but had increased keratocan expression at both protein and mRNA levels. Corneas of lumican-null mice showed decreased keratocan. This coupling of keratocan expression with lumican also was observed after intrastromal injection of a lumican expression minigene into the corneal stroma of Lum-/- mice. Small interfering RNA knockdown of lumican in vitro reduced keratocan expression, whereas co-injection of a lumican-expressing minigene with a beta-galactosidase reporter driven by the keratocan promoter demonstrated an increase of keratocan transcriptional activity in response to lumican expression in Lum-/- corneas in vivo. These observations demonstrate that lumican has a novel regulatory role in keratocan expression at the transcriptional level. Such results help provide an explanation for the differences in severity of corneal manifestation found in Lum-/- and Kera-/- mice. The results also suggest a critical level of small proteoglycans to be essential for collagen organization but that overabundance is not detrimental to extracellular matrix morphogenesis.

Role of Cys41 in the N-terminal domain of lumican in ex vivo collagen fibrillogenesis by cultured corneal stromal cells.

The keratan sulphate proteoglycan lumican regulates collagen fibrillogenesis to maintain the integrity and function of connective tissues such as cornea. We examined the role of a highly conserved cysteine-containing domain proximal to the N-terminus of lumican in collagen fibrillogenesis using site-specific mutagenesis to prepare plasmid DNA encoding wild-type murine lumican (Cys(37)-Xaa(3)-Cys(41)-Xaa-Cys-Xaa(9)-Cys) and a Cys-->Ser (C/S) mutant (Cys(37)-Xaa(3)-Ser(41)-Xaa-Cys-Xaa(9)-Cys). cDNAs were cloned into the pSecTag2A vector, and cultures of MK/T-1 cells (an immortalized cell line from mouse keratocytes) were transfected with the cDNAs. Stable transformants were selected and cloned in the presence of Zeocin. All stable transformants maintained a dendritic morphology and growth rate similar to those of parental MK/T-1 cells. Western blot analysis with anti-lumican antibody detected a 42 kDa lumican protein secreted into the culture medium of both wild-type and C/S mutant lumican cell lines. Ultrastructural analyses by transmission electron microscopy showed both cell lines to form a multi-layered stroma ex vivo, but the matrix assembled by the two cell lines differed. Compared with the mutant cell line, the wild-type cells assembled a more organized matrix with regions containing orthogonal collagen fibrils. In addition, the fibrils in the extracellular matrix formed by the mutant cell line exhibited alterations in fibril packing and structure. Immunostaining analysed by confocal microscopy showed a further difference in this matrix, with the marked occurrence of lumican and collagen I co-localization in the lumican wild-type cells, but a lack thereof in the lumican C/S mutant cells. The results indicate that the cysteine-rich domain of lumican is important in collagen fibrillogenesis and stromal matrix assembly.