A molecular switch for neuroprotective astrocyte reactivity.

The intrinsic mechanisms that regulate neurotoxic versus neuroprotective astrocyte phenotypes and their effects on central nervous system degeneration and repair remain poorly understood. Here we show that injured white matter astrocytes differentiate into two distinct C3-positive and C3-negative reactive populations, previously simplified as neurotoxic (A1) and neuroprotective (A2)1,2, which can be further subdivided into unique subpopulations defined by proliferation and differential gene expression signatures. We find the balance of neurotoxic versus neuroprotective astrocytes is regulated by discrete pools of compartmented cyclic adenosine monophosphate derived from soluble adenylyl cyclase and show that proliferating neuroprotective astrocytes inhibit microglial activation and downstream neurotoxic astrocyte differentiation to promote retinal ganglion cell survival. Finally, we report a new, therapeutically tractable viral vector to specifically target optic nerve head astrocytes and show that raising nuclear or depleting cytoplasmic cyclic AMP in reactive astrocytes inhibits deleterious microglial or macrophage cell activation and promotes retinal ganglion cell survival after optic nerve injury. Thus, soluble adenylyl cyclase and compartmented, nuclear- and cytoplasmic-localized cyclic adenosine monophosphate in reactive astrocytes act as a molecular switch for neuroprotective astrocyte reactivity that can be targeted to inhibit microglial activation and neurotoxic astrocyte differentiation to therapeutic effect. These data expand on and define new reactive astrocyte subtypes and represent a step towards the development of gliotherapeutics for the treatment of glaucoma and other optic neuropathies.

Quantitative BONCAT Allows Identification of Newly Synthesized Proteins after Optic Nerve Injury.

Retinal ganglion cells (RGCs) die after optic nerve trauma or in degenerative disease. However, acute changes in protein expression that may regulate RGC response to injury are not fully understood, and detailed methods to quantify new protein synthesis have not been tested. Here, we develop and apply a new in vivo quantitative measure of newly synthesized proteins to examine changes occurring in the retina after optic nerve injury. Azidohomoalanine, a noncanonical amino acid, was injected intravitreally into the eyes of rodents of either sex with or without optic nerve injury. Isotope variants of biotin-alkyne were used for quantitative BONCAT (QBONCAT) mass spectrometry, allowing identification of protein synthesis and transport rate changes in more than 1000 proteins at 1 or 5 d after optic nerve injury. In vitro screening showed several newly synthesized proteins regulate axon outgrowth in primary neurons in vitro This novel approach to targeted quantification of newly synthesized proteins after injury uncovers a dynamic translational response within broader proteostasis regulation and enhances our understanding of the cellular response to injury.SIGNIFICANCE STATEMENT Optic nerve injury results in death and degeneration of retinal ganglion cells and their axons. The specific cellular response to injury, including changes in new protein synthesis, is obscured by existing proteins and protein degradation. In this study, we introduce QBONCAT to isolate and quantify acute protein synthesis and subsequent transport between cellular compartments. We identify novel candidate protein effectors of the regenerative response and uncover their regulation of axon growth in vitro, validating the utility of QBONCAT for the discovery of novel regulatory and therapeutic candidates after optic nerve injury.

Mouse γ-Synuclein Promoter-Mediated Gene Expression and Editing in Mammalian Retinal Ganglion Cells.

Optic neuropathies are a group of optic nerve (ON) diseases caused by various insults including glaucoma, inflammation, ischemia, trauma, and genetic deficits, which are characterized by retinal ganglion cell (RGC) death and ON degeneration. An increasing number of genes involved in RGC intrinsic signaling have been found to be promising neural repair targets that can potentially be modulated directly by gene therapy, if we can achieve RGC specific gene targeting. To address this challenge, we first used adeno-associated virus (AAV)-mediated gene transfer to perform a low-throughput in vivo screening in both male and female mouse eyes and identified the mouse γ-synuclein (mSncg) promoter, which specifically and potently sustained transgene expression in mouse RGCs and also works in human RGCs. We further demonstrated that gene therapy that combines AAV-mSncg promoter with clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing can knock down pro-degenerative genes in RGCs and provide effective neuroprotection in optic neuropathies.SIGNIFICANCE STATEMENT Here, we present an RGC-specific promoter, mouse γ-synuclein (mSncg) promoter, and perform extensive characterization and proof-of-concept studies of mSncg promoter-mediated gene expression and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing in RGCs in vivo To our knowledge, this is the first report demonstrating in vivo neuroprotection of injured RGCs and optic nerve (ON) by AAV-mediated CRISPR/Cas9 inhibition of genes that are critical for neurodegeneration. It represents a powerful tool to achieve RGC-specific gene modulation, and also opens up a promising gene therapy strategy for optic neuropathies, the most common form of eye diseases that cause irreversible blindness.

Regulation of Neuronal Survival and Axon Growth by a Perinuclear cAMP Compartment.

cAMP signaling is known to be critical in neuronal survival and axon growth. Increasingly the subcellular compartmentation of cAMP signaling has been appreciated, but outside of dendritic synaptic regulation, few cAMP compartments have been defined in terms of molecular composition or function in neurons. Specificity in cAMP signaling is conferred in large part by A-kinase anchoring proteins (AKAPs) that localize protein kinase A and other signaling enzymes to discrete intracellular compartments. We now reveal that cAMP signaling within a perinuclear neuronal compartment organized by the large multivalent scaffold protein mAKAPα promotes neuronal survival and axon growth. mAKAPα signalosome function is explored using new molecular tools designed to specifically alter local cAMP levels as studied by live-cell FRET imaging. In addition, enhancement of mAKAPα-associated cAMP signaling by isoform-specific displacement of bound phosphodiesterase is demonstrated to increase retinal ganglion cell survival in vivo in mice of both sexes following optic nerve crush injury. These findings define a novel neuronal compartment that confers cAMP regulation of neuroprotection and axon growth and that may be therapeutically targeted in disease.SIGNIFICANCE STATEMENT cAMP is a second messenger responsible for the regulation of diverse cellular processes including neuronal neurite extension and survival following injury. Signal transduction by cAMP is highly compartmentalized in large part because of the formation of discrete, localized multimolecular signaling complexes by A-kinase anchoring proteins. Although the concept of cAMP compartmentation is well established, the function and identity of these compartments remain poorly understood in neurons. In this study, we provide evidence for a neuronal perinuclear cAMP compartment organized by the scaffold protein mAKAPα that is necessary and sufficient for the induction of neurite outgrowth in vitro and for the survival of retinal ganglion cells in vivo following optic nerve injury.

Sox2 regulates astrocytic and vascular development in the retina.

Sox2 is a transcriptional regulator that is highly expressed in retinal astrocytes, yet its function in these cells has not previously been examined. To understand its role, we conditionally deleted Sox2 from the population of astrocytes and examined the consequences on retinal development. We found that Sox2 deletion does not alter the migration of astrocytes, but it impairs their maturation, evidenced by the delayed upregulation of glial fibrillary acidic protein (GFAP) across the retina. The centro-peripheral gradient of angiogenesis is also delayed in Sox2-CKO retinas. In the mature retina, we observed lasting abnormalities in the astrocytic population evidenced by the sporadic loss of GFAP immunoreactivity in the peripheral retina as well as by the aberrant extension of processes into the inner retina. Blood vessels in the adult retina are also under-developed and show a decrease in the frequency of branch points and in total vessel length. The developmental relationship between maturing astrocytes and angiogenesis suggests a causal relationship between the astrocytic loss of Sox2 and the vascular architecture in maturity. We suggest that the delay in astrocytic maturation and vascular invasion may render the retina hypoxic, thereby causing the abnormalities we observe in adulthood. These studies uncover a novel role for Sox2 in the development of retinal astrocytes and indicate that its removal can lead to lasting changes to retinal homeostasis.

Directly induced human retinal ganglion cells mimic fetal RGCs and are neuroprotective after transplantation in vivo.

Retinal ganglion cell (RGC) replacement therapy could restore vision in glaucoma and other optic neuropathies. We developed a rapid protocol for directly induced RGC (iRGC) differentiation from human stem cells, leveraging overexpression of NGN2. Neuronal morphology and neurite growth were observed within 1 week of induction; characteristic RGC-specific gene expression confirmed identity. Calcium imaging demonstrated γ-aminobutyric acid (GABA)-induced excitation characteristic of immature RGCs. Single-cell RNA sequencing showed more similarities between iRGCs and early-stage fetal human RGCs than retinal organoid-derived RGCs. Intravitreally transplanted iRGCs survived and migrated into host retinas independent of prior optic nerve trauma, but iRGCs protected host RGCs from neurodegeneration. These data demonstrate rapid iRGC generation in vitro into an immature cell with high similarity to human fetal RGCs and capacity for retinal integration after transplantation and neuroprotective function after optic nerve injury. The simplicity of this system may benefit translational studies on human RGCs.

Fusogenic liposome-enhanced cytosolic delivery of magnetic nanoparticles.

Magnetic nanoparticles (MNPs) are widely used in cell sorting, organelle selection, drug delivery, cell delivery, and cell tracking applications. However, organelle manipulation in living cells has been limited due to the endocytic uptake and sequestration of MNPs. Here, we introduce a method for modifying MNPs with fusogenic liposomes that facilitate MNP passage directly into the cytosol. MNPs were enclosed in fusogenic liposomes that exhibit a core-shell structure under a transmission electron microscope (TEM). The lipid-to-MNP ratio was optimized for one layer of liposome coating around each MNP, so that MNPs were delivered to the cytosol without endosomal or liposomal coatings. After incubation with the retinal pigment epithelial cell line ARPE-19, single-layer liposome-coated MNPs exhibited the highest MNP delivery efficiency. Although uncoated MNPs are taken up through endocytosis, less than 15% of the fusogenic liposome-coated MNPs co-localized with early endosomes. MNPs delivered by fusogenic liposomes showed cytosolic localization early on and increased lysosomal localization at later time points. The movement of intracellular MNPs could be manipulated with an external magnet to estimate cytosolic viscosity. Bypassing endocytosis in this way allowed efficient delivery of MNPs to the cytosol, potentially allowing for the targeting of specific organelles and controlling their motion in living cells.

Posttranslational Modification of Sox11 Regulates RGC Survival and Axon Regeneration.

The failure of adult CNS neurons to survive and regenerate their axons after injury or in neurodegenerative disease remains a major target for basic and clinical neuroscience. Recent data demonstrated in the adult mouse that exogenous expression of Sry-related high-mobility-box 11 (Sox11) promotes optic nerve regeneration after optic nerve injury but exacerbates the death of a subset of retinal ganglion cells (RGCs), α-RGCs. During development, Sox11 is required for RGC differentiation from retinal progenitor cells (RPCs), and we found that mutation of a single residue to prevent SUMOylation at lysine 91 (K91) increased Sox11 nuclear localization and RGC differentiation in vitro Here, we explored whether this Sox11 manipulation similarly has stronger effects on RGC survival and optic nerve regeneration. In vitro, we found that non-SUMOylatable Sox11K91A leads to RGC death and suppresses axon outgrowth in primary neurons. We furthermore found that Sox11K91A more strongly promotes axon regeneration but also increases RGC death after optic nerve injury in vivo in the adult mouse. RNA sequence (RNA-seq) data showed that Sox11 and Sox11K91A increase the expression of key signaling pathway genes associated with axon growth and regeneration but downregulated Spp1 and Opn4 expression in RGC cultures, consistent with negatively regulating the survival of α-RGCs and ipRGCs. Thus, Sox11 and its SUMOylation site at K91 regulate gene expression, survival and axon growth in RGCs, and may be explored further as potential regenerative therapies for optic neuropathy.

Physiologic maturation is both extrinsically and intrinsically regulated in progenitor-derived neurons.

During development, newly-differentiated neurons undergo several morphological and physiological changes to become functional, mature neurons. Physiologic maturation of neuronal cells derived from isolated stem or progenitor cells may provide insight into maturation in vivo but is not well studied. As a step towards understanding how neuronal maturation is regulated, we studied the developmental switch of response to the neurotransmitter GABA, from excitatory depolarization to inhibitory hyperpolarization. We compared acutely isolated retinal ganglion cells (RGCs) at various developmental stages and RGCs differentiated in vitro from embryonic retinal progenitors for the effects of aging and, independently, of retinal environment age on their GABAA receptor (GABAAR) responses, elicited by muscimol. We found that neurons generated in vitro from progenitors exhibited depolarizing, immature GABA responses, like those of early postnatal RGCs. As progenitor-derived neurons aged from 1 to 3 weeks, their GABA responses matured. Interestingly, signals secreted by the early postnatal retina suppressed acquisition of mature GABA responses. This suppression was not associated with changes in expression of GABAAR or of the chloride co-transporter KCC2, but rather with inhibition of KCC2 dimerization in differentiating neurons. Taken together, these data indicate GABA response maturation depends on release of inhibition by developmentally regulated diffusible signals from the retina.

Opposing Effects of Growth and Differentiation Factors in Cell-Fate Specification.

Following ocular trauma or in diseases such as glaucoma, irreversible vision loss is due to the death of retinal ganglion cell (RGC) neurons. Although strategies to replace these lost cells include stem cell replacement therapy, few differentiated stem cells turn into RGC-like neurons. Understanding the regulatory mechanisms of RGC differentiation in vivo may improve outcomes of cell transplantation by directing the fate of undifferentiated cells toward mature RGCs. Here, we report a new mechanism by which growth and differentiation factor-15 (GDF-15), a ligand in the transforming growth factor-beta (TGF-ß) superfamily, strongly promotes RGC differentiation in the developing retina in vivo in rodent retinal progenitor cells (RPCs) and in human embryonic stem cells (hESCs). This effect is in direct contrast to the closely related ligand GDF-11, which suppresses RGC-fate specification. We find these opposing effects are due in part to GDF-15's ability to specifically suppress Smad-2, but not Smad-1, signaling induced by GDF-11, which can be recapitulated by pharmacologic or genetic blockade of Smad-2 in vivo to increase RGC specification. No other retinal cell types were affected by GDF-11 knockout, but a slight reduction in photoreceptor cells was observed by GDF-15 knockout in the developing retina in vivo. These data define a novel regulatory mechanism of GDFs' opposing effects and their relevance in RGC differentiation and suggest a potential approach for advancing ESC-to-RGC cell-based replacement therapies.

Magnetic Human Corneal Endothelial Cell Transplant: Delivery, Retention, and Short-Term Efficacy.

Purpose: Corneal endothelial dysfunction leads to corneal edema, pain, and vision loss. Adequate animal models are needed to study the safety and efficacy of novel cell therapies as an alternative to corneal transplantation. Methods: Primary human corneal endothelial cells (HCECs) were isolated from cadaveric donor corneas, expanded in vitro, transduced to express green fluorescent protein (GFP), loaded with superparamagnetic nanoparticles, and injected into the anterior chamber of adult rabbits immediately after endothelial cell or Descemet's membrane stripping. The same volume of balanced salt solution plus (BSS+) was injected in control eyes. We compared different models for inducing corneal edema in rabbits, and examined the ability of transplanted HCECs to reduce corneal edema over time by measuring central corneal thickness and tracking corneal clarity. GFP-positive donor cells were tracked in vivo using optical coherence tomography (OCT) fluorescence angiography module, and the transplanted cells were confirmed by human nuclei immunostaining. Results: Magnetic HCECs integrated onto the recipient corneas with intact Descemet's membrane, and donor identity was confirmed by GFP expression and immunostaining for human nuclei marker. Donor HCECs formed a monolayer on the posterior corneal surface and expressed HCEC functional markers of tight junction formation. No GFP-positive cells were observed in the trabecular meshwork or on the iris, and intraocular pressure remained stable through the length of the study. Conclusions: Our results demonstrate magnetic cell-based therapy efficiently delivers HCECs to restore corneal transparency without detectable toxicity or adverse effect on intraocular pressure. Magnetic delivery of HCECs may enhance corneal function and should be explored further for human therapies.

MTP18 is a Novel Regulator of Mitochondrial Fission in CNS Neuron Development, Axonal Growth, and Injury Responses.

The process of mitochondrial fission-fusion has been implicated in diverse neuronal roles including neuronal survival, axon degeneration, and axon regeneration. However, whether increased fission or fusion is beneficial for neuronal health and/or axonal growth is not entirely clear, and is likely situational and cell type-dependent. In searching for mitochondrial fission-fusion regulating proteins for improving axonal growth within the visual system, we uncover that mitochondrial fission process 1,18 kDa (MTP18/MTFP1), a pro-fission protein within the CNS, is critical to maintaining mitochondrial size and volume under normal and injury conditions, in retinal ganglion cells (RGCs). We demonstrate that MTP18's expression is regulated by transcription factors involved in axonal growth, Kruppel-like factor (KLF) transcription factors-7 and -9, and that knockdown of MTP18 promotes axon growth. This investigation exposes MTP18's previously unexplored role in regulating mitochondrial fission, implicates MTP18 as a downstream component of axon regenerative signaling, and ultimately lays the groundwork for investigations on the therapeutic efficacy of MTP18 expression suppression during CNS axon degenerative events.

Induced Pluripotent Stem Cells Promote Retinal Ganglion Cell Survival After Transplant.

Purpose: The purpose of this study was to characterize whether induced pluripotent stem cells (iPSCs) affect survival of grafted retinal ganglion cells (RGCs) after transplantation. Methods: For in vitro studies, human iPSCs were either directly cocultured with mouse RGCs or plated in hanging inserts in RGC cultures for 1 week. For ex vivo studies, RGCs and iPSCs were seeded onto the inner surface of an adult rat retina explant and cultured for 1 week. For in vivo studies, RGCs and iPSCs were intravitreally coinjected into an adult rat eye 1 week before examining retinas by explant and immunostaining. Results: A dose-dependent increase in RGC survival was observed in RGC-iPSC direct cocultures, and RGC-iPSC indirect cocultures showed a similar RGC protective effect, but to a lesser extent than in direct coculture. Enhanced RGC survival was also identified in RGC-iPSC cotransplantations to adult retinas ex vivo and in vivo. In addition, RGCs with iPSC cotransplantation extended significantly longer neurites than RGC-only transplants. Conclusions: Human iPSCs promote transplanted RGC survival and neurite extension. This effect may be mediated at least partially through secretion of diffusible neuroprotective factors.