Microglial Function Is Distinct in Different Anatomical Locations during Retinal Homeostasis and Degeneration.

Microglia from different nervous system regions are molecularly and anatomically distinct, but whether they also have different functions is unknown. We combined lineage tracing, single-cell transcriptomics, and electrophysiology of the mouse retina and showed that adult retinal microglia shared a common developmental lineage and were long-lived but resided in two distinct niches. Microglia in these niches differed in their interleukin-34 dependency and functional contribution to visual-information processing. During certain retinal-degeneration models, microglia from both pools relocated to the subretinal space, an inducible disease-associated niche that was poorly accessible to monocyte-derived cells. This microglial transition involved transcriptional reprogramming of microglia, characterized by reduced expression of homeostatic checkpoint genes and upregulation of injury-responsive genes. This transition was associated with protection of the retinal pigmented epithelium from damage caused by disease. Together, our data demonstrate that microglial function varies by retinal niche, thereby shedding light on the significance of microglia heterogeneity.

Microglia Drive Pockets of Neuroinflammation in Middle Age.

During aging, microglia produce inflammatory factors, show reduced tissue surveillance, altered interactions with synapses, and prolonged responses to CNS insults, positioning these cells to have profound impact on the function of nearby neurons. We and others recently showed that microglial attributes differ significantly across brain regions in young adult mice. However, the degree to which microglial properties vary during aging is largely unexplored. Here, we analyze and manipulate microglial aging within the basal ganglia, brain circuits that exhibit prominent regional microglial heterogeneity and where neurons are vulnerable to functional decline and neurodegenerative disease. In male and female mice, we demonstrate that VTA and SNc microglia exhibit unique and premature responses to aging, compared with cortex and NAc microglia. This is associated with localized VTA/SNc neuroinflammation that may compromise synaptic function as early as middle age. Surprisingly, systemic inflammation, local neuron death, and astrocyte aging do not appear to underlie these early aging responses of VTA and SNc microglia. Instead, we found that microglial lysosome status was tightly linked to early aging of VTA microglia. Microglial ablation/repopulation normalized VTA microglial lysosome swelling and suppressed increases in VTA microglial density during aging. In contrast, CX3CR1 receptor KO exacerbated VTA microglial lysosome rearrangements and VTA microglial proliferation during aging. Our findings reveal a previously unappreciated regional variation in onset and magnitude of microglial proliferation and inflammatory factor production during aging and highlight critical links between microglial lysosome status and local microglial responses to aging.SIGNIFICANCE STATEMENT Microglia are CNS cells that are equipped to regulate neuronal health and function throughout the lifespan. We reveal that microglia in select brain regions begin to proliferate and produce inflammatory factors in late middle age, months before microglia in other brain regions. These findings demonstrate that CNS neuroinflammation during aging is not uniform. Moreover, they raise the possibility that local microglial responses to aging play a critical role in determining which populations of neurons are most vulnerable to functional decline and neurodegenerative disease.

PRCD is essential for high-fidelity photoreceptor disc formation.

Progressive rod-cone degeneration (PRCD) is a small protein residing in the light-sensitive disc membranes of the photoreceptor outer segment. Until now, the function of PRCD has remained enigmatic despite multiple demonstrations that its mutations cause blindness in humans and dogs. Here, we generated a PRCD knockout mouse and observed a striking defect in disc morphogenesis, whereby newly forming discs do not properly flatten. This leads to the budding of disc-derived vesicles, specifically at the site of disc morphogenesis, which accumulate in the interphotoreceptor matrix. The defect in nascent disc flattening only minimally alters the photoreceptor outer segment architecture beyond the site of new disc formation and does not affect the abundance of outer segment proteins and the photoreceptor's ability to generate responses to light. Interestingly, the retinal pigment epithelium, responsible for normal phagocytosis of shed outer segment material, lacks the capacity to clear the disc-derived vesicles. This deficiency is partially compensated by a unique pattern of microglial migration to the site of disc formation where they actively phagocytize vesicles. However, the microglial response is insufficient to prevent vesicular accumulation and photoreceptors of PRCD knockout mice undergo slow, progressive degeneration. Taken together, these data show that the function of PRCD is to keep evaginating membranes of new discs tightly apposed to each other, which is essential for the high fidelity of photoreceptor disc morphogenesis and photoreceptor survival.

Retinal laser burn-induced neuropathy leads to substance P-dependent loss of ocular immune privilege.

Inflammation in the eye is tightly regulated by multiple mechanisms that together contribute to ocular immune privilege. Many studies have shown that it is very difficult to abrogate the immune privileged mechanism called anterior chamber-associated immune deviation (ACAID). Previously, we showed that retinal laser burn (RLB) to one eye abrogated immune privilege (ACAID) bilaterally for an extended period of time. In an effort to explain the inflammation in the nonburned eye, we postulated that neuronal signals initiated inflammation in the contralateral eye. In this study, we test the role of substance P, a neuroinflamatory peptide, in RLB-induced loss of ACAID. Histological examination of the retina with and without RLB revealed an increase of the substance P-inducible neurokinin 1 receptor (NK1-R) in the retina of first, the burned eye, and then the contralateral eye. Specific antagonists for NK1-R, given locally with Ag within 24 h, but not 3, 5, or 7 d post-RLB treatment, prevented the bilateral loss of ACAID. Substance P knockout (KO) mice retained their ability to develop ACAID post-RLB. These data support the postulate that substance P transmits early inflammatory signals from the RLB eye to the contralateral eye to induce changes to ocular immune privilege and has a central role in the bilateral loss of ACAID. The possibility is raised that blocking of the substance P pathway with NK1-R antagonists postocular trauma may prevent unwanted and perhaps extended consequences of trauma-induced inflammation in the eye.

Microglia at sites of atrophy restrict the progression of retinal degeneration via galectin-3 and Trem2.

Outer retinal degenerations, including age-related macular degeneration (AMD), are characterized by photoreceptor and retinal pigment epithelium (RPE) atrophy. In these blinding diseases, macrophages accumulate at atrophic sites, but their ontogeny and niche specialization remain poorly understood, especially in humans. We uncovered a unique profile of microglia, marked by galectin-3 upregulation, at atrophic sites in mouse models of retinal degeneration and human AMD. In disease models, conditional deletion of galectin-3 in microglia led to phagocytosis defects and consequent augmented photoreceptor death, RPE damage, and vision loss, indicating protective roles. Mechanistically, Trem2 signaling orchestrated microglial migration to atrophic sites and induced galectin-3 expression. Moreover, pharmacologic Trem2 agonization led to heightened protection but in a galectin-3-dependent manner. In elderly human subjects, we identified this highly conserved microglial population that expressed galectin-3 and Trem2. This population was significantly enriched in the macular RPE-choroid of AMD subjects. Collectively, our findings reveal a neuroprotective population of microglia and a potential therapeutic target for mitigating retinal degeneration.

Intrinsic disorder mediates the diverse regulatory functions of the Cdk inhibitor p21.

Traditionally, well-defined three-dimensional structure has been thought to be essential for protein function. However, myriad biological functions are performed by highly dynamic, intrinsically disordered proteins (IDPs). IDPs often fold upon binding their biological targets and frequently show 'binding diversity' by targeting multiple ligands. We sought to understand the physical basis of IDP binding diversity and report here that the cyclin-dependent kinase (Cdk) inhibitor p21(Cip1) adaptively binds to and inhibits the various Cdk-cyclin complexes that regulate eukaryotic cell division. Using results from NMR spectroscopy and biochemical and cellular assays, we show that structural adaptability of a helical subdomain within p21, termed LH, enables two other subdomains, D1 and D2, to specifically bind conserved surface features of the cyclin and Cdk subunits, respectively, within otherwise structurally distinct Cdk-cyclin complexes. Adaptive folding upon binding is likely to mediate the diverse biological functions of the thousands of IDPs present in eukaryotes.

Microglia at Sites of Atrophy Restrict the Progression of Retinal Degeneration via Galectin-3 and Trem2 Interactions.

Degenerative diseases of the outer retina, including age-related macular degeneration (AMD), are characterized by atrophy of photoreceptors and retinal pigment epithelium (RPE). In these blinding diseases, macrophages are known to accumulate ectopically at sites of atrophy, but their ontogeny and functional specialization within this atrophic niche remain poorly understood, especially in the human context. Here, we uncovered a transcriptionally unique profile of microglia, marked by galectin-3 upregulation, at atrophic sites in mouse models of retinal degeneration and in human AMD. Using disease models, we found that conditional deletion of galectin-3 in microglia led to defects in phagocytosis and consequent augmented photoreceptor death, RPE damage and vision loss, suggestive of a protective role. Mechanistically, Trem2 signaling orchestrated the migration of microglial cells to sites of atrophy, and there, induced galectin-3 expression. Moreover, pharmacologic Trem2 agonization led to heightened protection, but only in a galectin-3-dependent manner, further signifying the functional interdependence of these two molecules. Likewise in elderly human subjects, we identified a highly conserved population of microglia at the transcriptomic, protein and spatial levels, and this population was enriched in the macular region of postmortem AMD subjects. Collectively, our findings reveal an atrophy-associated specialization of microglia that restricts the progression of retinal degeneration in mice and further suggest that these protective microglia are conserved in AMD.

Meibomian gland dysfunction is suppressed via selective inhibition of immune responses by topical LFA-1/ICAM antagonism with lifitegrast in the allergic eye disease (AED) model.

PURPOSE: The etiology of meibomian gland dysfunction (MGD) is incompletely understood, despite being a common ophthalmic condition and an area of unmet medical need. It is characterized by an insufficiency in glandular provision of specialized lipids (meibum) to the tear film and is a major cause of dry eye. Work in the allergic eye disease (AED) mouse model has revealed an immunopathogenic role in MGD causation, now raising interest in the applicability of immunomodulatory therapies. As such, we herein ask whether inhibition of lymphocyte function associated antigen (LFA)-1/intracellular adhesion molecules (ICAM)-1 signaling via topical lifitegrast administration has a therapeutic effect on MGD in AED mice. METHODS: Mice were induced with AED by i.p. injection of ovalbumin (OVA) mixed with alum and pertussis toxin, followed 2 weeks later by once daily topical OVA challenges for 7 days. Mice were treated topically with 5% lifitegrast ophthalmic solution or vehicle (PBS) 30 min prior to challenge. We developed a clinical ranking method to assess MGD severity, and also scored clinical allergy. Conjunctivae and draining lymph nodes were collected for flow cytometry. RESULTS: Topical lifitegrast significantly inhibited clinical MGD severity, which was associated with diminished pathogenic TH17 cell and neutrophil numbers in the conjunctiva. No significant change in conjunctival TH2 cells or eosinophils, and only marginal differences in ocular allergy were observed. CONCLUSIONS: In AED mice, lifitegrast inhibited MGD severity marked by a reduction in select immune populations in the conjunctiva. Our findings warrant future examination of lifitegrast in the treatment of patients with forms of MGD.

Mucosal-associated invariant T cell responses differ by sex in COVID-19.

BACKGROUND: Sexual dimorphisms in immune responses contribute to coronavirus disease 2019 (COVID-19) outcomes, but the mechanisms governing this disparity remain incompletely understood. METHODS: We carried out sex-balanced sampling of peripheral blood mononuclear cells from hospitalized and non-hospitalized individuals with confirmed COVID-19, uninfected close contacts, and healthy control individuals for 36-color flow cytometry and single-cell RNA sequencing. FINDINGS: Our results revealed a pronounced reduction of circulating mucosal-associated invariant T (MAIT) cells in infected females. Integration of published COVID-19 airway tissue datasets suggests that this reduction represented a major wave of MAIT cell extravasation during early infection in females. Moreover, MAIT cells from females possessed an immunologically active gene signature, whereas cells from males were pro-apoptotic. CONCLUSIONS: Our findings uncover a female-specific protective MAIT cell profile, potentially shedding light on reduced COVID-19 susceptibility in females. FUNDING: This work was supported by NIH/NIAID (U01AI066569 and UM1AI104681), the Defense Advanced Projects Agency (DARPA; N66001-09-C-2082 and HR0011-17-2-0069), the Veterans Affairs Health System, and Virology Quality Assurance (VQA; 75N93019C00015). The content is solely the responsibility of the authors and does not necessarily represent the official view of the National Institutes of Health. COVID-19 samples were processed under Biosafety level 2 (BSL-2) with aerosol management enhancement or BSL-3 in the Duke Regional Biocontainment Laboratory, which received partial support for construction from NIH/NIAID (UC6AI058607).

A unique E1-E2 interaction required for optimal conjugation of the ubiquitin-like protein NEDD8.

Ubiquitin-like proteins (UBLs) such as NEDD8 are transferred to their targets by distinct, parallel, multienzyme cascades that involve the sequential action of E1, E2 and E3 enzymes. How do enzymes within a particular UBL conjugation cascade interact with each other? We report here that the unique N-terminal sequence of NEDD8's E2, Ubc12, selectively recruits NEDD8's E1 to promote thioester formation between Ubc12 and NEDD8. A peptide corresponding to Ubc12's N terminus (Ubc12N26) specifically binds and inhibits NEDD8's E1, the heterodimeric APPBP1-UBA3 complex. The structure of APPBP1-UBA3- Ubc12N26 reveals conserved Ubc12 residues docking in a groove generated by loops conserved in UBA3s but not other E1s. These data explain why the Ubc12-UBA3 interaction is unique to the NEDD8 pathway. These studies define a novel mechanism for E1-E2 interaction and show how enzymes within a particular UBL conjugation cascade can be tethered together by unique protein-protein interactions emanating from their common structural scaffolds.

Fate Mapping In Vivo to Distinguish Bona Fide Microglia Versus Recruited Monocyte-Derived Macrophages in Retinal Disease.

With the new understanding that adult microglia in mice have embryonic origins and are maintained in situ throughout life, it has become pertinent to now understand how these unique cells differ from monocyte-derived macrophages. The latter are recruited into the neural retina (and elsewhere in CNS) in certain diseased states, such as in various forms of retinal degeneration. However, phenotypic markers expressed by microglia and monocyte-derived macrophages largely overlap, thereby making it technically challenging to distinguish the two cell types in disease. To address this problem in mice, we have established an in vivo fate mapping system that enables distinguishing these two cell types in retinal disease models. Our approach leverages the seminal work that originally developed Cx3cr1-CreER mice and is based on commercially available mouse strains. Here, we detail our protocol and how to apply this fate mapping method paired with flow cytometry (or immunohistochemistry) to faithfully distinguish and examine microglia vs. monocyte-derived macrophages in a mutually exclusive manner. This approach will henceforth empower new efforts to identify functional specializations of these two populations in the pathobiology of retinal degenerative diseases and possibly other conditions of the retina where monocyte recruitment is observed, such as in glaucoma, diabetic retinopathy, ischemia reperfusion, retinal detachment, and so on.

Resolvin D1 treatment on goblet cell mucin and immune responses in the chronic allergic eye disease (AED) model.

Severe, chronic eye allergy is an understudied, vision-threatening condition. Treatments remain limited. We used a mouse model of severe allergic eye disease (AED) to determine whether topical application of the pro-resolution mediator Resolvin D1 (RvD1) terminates the response. AED was induced by injection of ovalbumin (OVA) followed by topical challenge of OVA daily. RvD1 was applied topically prior to OVA. Clinical symptoms were scored. Eye washes were assayed for MUC5AC. After 7 days, eyes were removed and the number of goblet cells, T helper cell responses and presence of immune cells in draining lymph nodes and conjunctiva determined. Topical RvD1 treatment significantly reduced symptoms of AED. RvD1 did not alter the systemic type 2 immune response in the lymph nodes. AED increased the total amount of goblet cell mucin secretion, but not the number of goblet cells. RvD1 prevented this increase, but did not alter goblet cell number. Absolute numbers of CD4 + T cells, total CD11b + myeloid cells, eosinophils, neutrophils, and monocytes, but not macrophages increased in AED versus RvD1-treated mice. We conclude that topical application of RvD1 reduced the ocular allergic response by local actions in conjunctival immune response and a decrease in goblet cell mucin secretion.

Complement and CD4+ T cells drive context-specific corneal sensory neuropathy.

Whether complement dysregulation directly contributes to the pathogenesis of peripheral nervous system diseases, including sensory neuropathies, is unclear. We addressed this important question in a mouse model of ocular HSV-1 infection, where sensory nerve damage is a common clinical problem. Through genetic and pharmacologic targeting, we uncovered a central role for C3 in sensory nerve damage at the morphological and functional levels. Interestingly, CD4 T cells were central in facilitating this complement-mediated damage. This same C3/CD4 T cell axis triggered corneal sensory nerve damage in a mouse model of ocular graft-versus-host disease (GVHD). However, this was not the case in a T-dependent allergic eye disease (AED) model, suggesting that this inflammatory neuroimmune pathology is specific to certain disease etiologies. Collectively, these findings uncover a central role for complement in CD4 T cell-dependent corneal nerve damage in multiple disease settings and indicate the possibility for complement-targeted therapeutics to mitigate sensory neuropathies.

Induction and Characterization of the Allergic Eye Disease Mouse Model.

Ocular IgE-associated allergy ranges from mild disease (seasonal and perennial allergic conjunctivitis) to more chronic/severe and vision-threatening forms (atopic and vernal keratoconjunctivitis). Whereas mild forms of disease have been studied extensively, less is known about the more chronic forms. Our lab has helped to address this knowledge gap by developing and characterizing an allergen-induced, chronic/severe, IgE-associated model of ocular allergy referred to as the severe allergic eye disease (AED) model. It is distinct from previously described models that mimic the more mild forms, referred to in the literature as the allergic conjunctivitis (AC) model. The purpose of this method article is to detail the protocol to induce and characterize the AED model and directly compare these mice to the mild AC model. Troubleshooting and implications are also discussed.

Supercritical fluid chromatography-photodiode array detection-electrospray ionization mass spectrometry as a framework for impurity fate mapping in the development and manufacture of drug substances.

Impurity fate and purge studies are critical in order to establish an effective impurity control strategy for approval of the commercial filing application of new medicines. Reversed phase liquid chromatography-diode array-mass spectrometry (RPLC-DAD-MS) has traditionally been the preferred tool for impurity fate mapping. However, separation of some reaction mixtures by LC can be very problematic requiring combination LC-UV for area % analysis and a different LC-MS method for peak identification. In addition, some synthetic intermediates might be chemically susceptible to the aqueous conditions used in RPLC separations. In this study, the use of supercritical fluid chromatography-photodiode array-electrospray ionization mass spectrometry (SFC-PDA-ESIMS) for fate and purge of two specified impurities in the 1-uridine starting material from the synthesis of a bis-piv 2'keto-uridine, an intermediate in the synthesis of uprifosbuvir, a treatment under investigation for chronic hepatitis C infection. Readily available SFC instrumentation with a Chiralpak IC column (4.6 × 150 mm, 3 µm) and ethanol: carbon dioxide based mobile phase eluent enabled the separation of closely related components from complex reaction mixtures where RLPC failed to deliver optimal chromatographic performance. These results illustrate how SFC combined with PDA and ESI-MS detection can become a powerful tool for direct impurity fate mapping across multiple reaction steps.

Neutrophils cause obstruction of eyelid sebaceous glands in inflammatory eye disease in mice.

Meibomian glands (MGs) are sebaceous glands of the eyelid margin that secrete lipids needed to avert tear evaporation and to help maintain ocular surface homeostasis. Obstruction of MGs or other forms of MG dysfunction can promote chronic diseases of the ocular surface. Although chronic eyelid inflammation, such as allergic eye disease, is an associated risk factor for obstructive MG dysfunction, it is not clear whether inflammatory processes contribute to the pathophysiology of MG obstruction. We show that polymorphonuclear neutrophils (PMNs) promoted MG obstruction in a chronic inflammatory model of allergic eye disease in mice. Analysis of leukocytes in tears of patients with MG dysfunction showed an increase in PMN numbers compared to healthy subjects. Moreover, PMN numbers in tears positively correlated with clinical severity of MG dysfunction. Our findings point to a role for PMNs in the pathogenesis and progression of MG dysfunction.

Commensal microflora-induced T cell responses mediate progressive neurodegeneration in glaucoma.

Glaucoma is the most prevalent neurodegenerative disease and a leading cause of blindness worldwide. The mechanisms causing glaucomatous neurodegeneration are not fully understood. Here we show, using mice deficient in T and/or B cells and adoptive cell transfer, that transient elevation of intraocular pressure (IOP) is sufficient to induce T-cell infiltration into the retina. This T-cell infiltration leads to a prolonged phase of retinal ganglion cell degeneration that persists after IOP returns to a normal level. Heat shock proteins (HSP) are identified as target antigens of T-cell responses in glaucomatous mice and human glaucoma patients. Furthermore, retina-infiltrating T cells cross-react with human and bacterial HSPs; mice raised in the absence of commensal microflora do not develop glaucomatous T-cell responses or the associated neurodegeneration. These results provide compelling evidence that glaucomatous neurodegeneration is mediated in part by T cells that are pre-sensitized by exposure to commensal microflora.

Author Correction: Commensal microflora-induced T cell responses mediate progressive neurodegeneration in glaucoma.

The originally published version of this Article contained an error in Figure 4. The bar chart in panel f was inadvertently replaced with a duplicate of the bar chart in panel e. This error has now corrected in both the PDF and HTML versions of the Article.

A multifunctional catalyst that stereoselectively assembles prodrugs.

The catalytic stereoselective synthesis of compounds with chiral phosphorus centers remains an unsolved problem. State-of-the-art methods rely on resolution or stoichiometric chiral auxiliaries. Phosphoramidate prodrugs are a critical component of pronucleotide (ProTide) therapies used in the treatment of viral disease and cancer. Here we describe the development of a catalytic stereoselective method for the installation of phosphorus-stereogenic phosphoramidates to nucleosides through a dynamic stereoselective process. Detailed mechanistic studies and computational modeling led to the rational design of a multifunctional catalyst that enables stereoselectivity as high as 99:1.

Paraneoplastic cerebellar degeneration: Yo-expressing tumor revealed after a 5-year follow-up with FDG-PET.

We report a patient with anti-Yo associated paraneoplastic cerebellar degeneration (PCD) whose tumor was demonstrated 5 years after developing PCD and had strong expression of Yo (cdr2) antigen. Review of this case along with clinical series and studies of tumor growth rates question the effectiveness of the anti-tumor immune response. These studies and similar cases suggest that the tumor may trigger the anti-Yo immune response at microscopic stages of development. An overwhelming majority of anti-Yo positive patients eventually develop a detectable malignancy, which argues in favor of a poorly effective or non-sustained anti-tumor immune response.