Induction of STAT3-related genes in fast degenerating cone photoreceptors of cpfl1 mice

Nucleocytoplasmic transport at the crossroads of proteostasis, neurodegeneration and neuroprotection

Nucleocytoplasmic transport comprises the multistep assembly, transport, and disassembly of protein and RNA cargoes entering and exiting nuclear pores. Accruing evidence supports that impairments to nucleocytoplasmic transport are a hallmark of neurodegenerative diseases. These impairments cause dysregulations in nucleocytoplasmic partitioning and proteostasis of nuclear transport receptors and client substrates that promote intracellular deposits - another hallmark of neurodegeneration. Disturbances in liquid-liquid phase separation (LLPS) between dense and dilute phases of biomolecules implicated in nucleocytoplasmic transport promote micrometer-scale coacervates, leading to proteinaceous aggregates. This Review provides historical and emerging principles of LLPS at the interface of nucleocytoplasmic transport, proteostasis, aging and noxious insults, whose dysregulations promote intracellular aggregates. E3 SUMO-protein ligase Ranbp2 constitutes the cytoplasmic filaments of nuclear pores, where it acts as a molecular hub for rate-limiting steps of nucleocytoplasmic transport. A vignette is provided on the roles of Ranbp2 in nucleocytoplasmic transport and at the intersection of proteostasis in the survival of photoreceptor and motor neurons under homeostatic and pathophysiological environments. Current unmet clinical needs are highlighted, including therapeutics aiming to manipulate aggregation-dissolution models of purported neurotoxicity in neurodegeneration.

cGMP Signaling in Photoreceptor Degeneration

Photoreceptors in the retina are highly specialized neurons with photosensitive molecules in the outer segment that transform light into chemical and electrical signals, and these signals are ultimately relayed to the visual cortex in the brain to form vision. Photoreceptors are composed of rods and cones. Rods are responsible for dim light vision, whereas cones are responsible for bright light, color vision, and visual acuity. Photoreceptors undergo progressive degeneration over time in many hereditary and age-related retinal diseases. Despite the remarkable heterogeneity of disease-causing genes, environmental factors, and pathogenesis, the progressive death of rod and cone photoreceptors ultimately leads to loss of vision/blindness. There are currently no treatments available for retinal degeneration. Cyclic guanosine 3', 5'-monophosphate (cGMP) plays a pivotal role in phototransduction. cGMP governs the cyclic nucleotide-gated (CNG) channels on the plasma membrane of the photoreceptor outer segments, thereby regulating membrane potential and signal transmission. By gating the CNG channels, cGMP regulates cellular Ca2+ homeostasis and signal transduction. As a second messenger, cGMP activates the cGMP-dependent protein kinase G (PKG), which regulates numerous targets/cellular events. The dysregulation of cGMP signaling is observed in varieties of photoreceptor/retinal degenerative diseases. Abnormally elevated cGMP signaling interferes with various cellular events, which ultimately leads to photoreceptor degeneration. In line with this, strategies to reduce cellular cGMP signaling result in photoreceptor protection in mouse models of retinal degeneration. The potential mechanisms underlying cGMP signaling-induced photoreceptor degeneration involve the activation of PKG and impaired Ca2+ homeostasis/Ca2+ overload, resulting from overactivation of the CNG channels, as well as the subsequent activation of the downstream cellular stress/death pathways. Thus, targeting the cellular cGMP/PKG signaling and the Ca2+-regulating pathways represents a significant strategy for photoreceptor protection in retinal degenerative diseases.

Using Micro-Electrode-Array Recordings and Retinal Disease Models to Elucidate Visual Functions: Simultaneous Recording of Local Electroretinograms and Ganglion Cell Action Potentials Reveals the Origin of Retinal Oscillatory Potentials

BACKGROUND

The electroretinogram (ERG) is an essential diagnostic tool for visual function, both in clinical and research settings. Here, we establish an advanced in vitro approach to assess cell-type-specific ERG signal components.

METHODS

Retinal explant cultures, maintained under entirely controlled conditions, were derived from wild-type mice and rd10 rod- and cpfl1 cone-degeneration mouse models. Local micro-ERG (µERG) and simultaneous ganglion cell (GC) recordings were obtained from the retinal explants using multi-electrode arrays. Band-pass filtering was employed to distinguish photoreceptor, bipolar cell, amacrine cell (AC), and GC responses.

RESULTS

Scotopic and photopic stimulation discriminated between rod and cone responses in wild-type and mutant retina. The 25 kHz sampling rate allowed the visualization of oscillatory potentials (OPs) in extraordinary detail, revealing temporal correlations between OPs and GC responses. Pharmacological isolation of different retinal circuits found that OPs are generated by inner retinal AC electrical synapses. Importantly, this AC activity helped synchronise GC activity.

CONCLUSION

Our µERG protocol simultaneously records the light-dependent activities of the first-, second-, and third-order neurons within the native neuronal circuitry, providing unprecedented insights into retinal physiology and pathophysiology. This method now also enables complete in vitro retinal function testing of therapeutic interventions, providing critical guidance for later in vivo investigations.

Retinal Layer Separation (ReLayS) method enables the molecular analysis of photoreceptor segments and cell bodies, as well as the inner retina

Understanding the physiology of the retina, and especially of the highly polarized photoreceptors, is essential not only to broaden our knowledge of the processes required for normal vision, but also to develop effective therapies to prevent or slow retinal degenerative diseases. However, the molecular analysis of photoreceptors is a challenge due to the heterogeneity of the retinal tissue and the lack of easy and reliable methods for cell separation. Here we present the ReLayS method-a simple technique for the separation of photoreceptor segments (PS) containing both inner and outer segments, outer nuclear layer (ONL), and inner retina (InR) that contains the remaining retinal layers. The layer-specific material isolated from a mouse half-retina with the ReLayS method was sufficient for protein isolation and Western blotting or RNA isolation and real-time PCR studies. The separation of PS, ONL, and InR was successfully validated by Western blotting and real-time PCR using proteins and genes with known expression profiles within the retina. Furthermore, the separation of the PS from the ONL enabled the detection of light-driven translocation of transducin from the PS to the soma. ReLayS is a simple and useful method to address protein and possibly metabolites distribution in photoreceptor compartments in various situations including development, ageing, and degenerative diseases.

Protective effect of leukemia inhibitory factor on the retinal injury induced by acute ocular hypertension in rats

Glaucoma is one of the leading causes of irreversible blindness worldwide. As such, neuroprotective therapy is essential for the treatment of this disease. Leukemia inhibitory factor (LIF) is a member of the IL-6 cytokine family and the LIF signaling pathway is considered to be one of the major endogenous factors mediating neuroprotection in the retina. Therefore, the present study aimed to investigate the possible effects of LIF in acute ocular hypertension (AOH). The intraocular pressure in rat eyes was raised to 110 mmHg for 1 h by infusing the anterior chamber with normal saline to establish the AOH model. In the treatment group, LIF was then injected into the vitreous cavity after AOH was ceased. The retinal tissues were obtained after the termination of AOH, and H&E staining was conducted to assess the morphological damage. The number of retinal ganglion cells (RGCs) was counted using the Fluoro-Gold retrograde staining method. TUNEL staining was used to determine the extent of apoptosis among the retinal cells. In addition, the protein expression levels of cleaved caspase-3, poly (ADP-ribose) polymerase (PARP), STAT3 and components of the AKT/mTOR/70-kDa ribosomal protein S6 kinase (p70S6K) signaling pathway were examined by western blotting. The results showed that AOH induced tissue swelling and structural damage in the retina, which were reversed by LIF injection. In the LIF treatment group, RGC loss was significantly inhibited and the quantity of TUNEL-stained cells was also significantly reduced, whereas the expression of cleaved caspase-3 and PARP was decreased. Furthermore, increased phosphorylation of STAT3, AKT, mTOR and p70S6K was observed after LIF treatment. By contrast, pretreatment with the STAT3 inhibitor C188-9 or the PI3K/AKT/mTOR inhibitor LY3023414 reversed the LIF-induced inhibition of RGC loss. These results suggested that exogenous LIF treatment inhibited the retinal damage induced by AOH, which was associated with the activation of STAT3 and mTOR/p70S6K signaling. Therefore, LIF may serve a role in neuroprotection for glaucoma treatment.

In-Depth Functional Analysis of Rodents by Full-Field Electroretinography

Full-field electroretinography (ERG) belongs to the gold-standard of electrophysiological test systems in ophthalmology and reflects the sum response of the entire retina to light stimulation. The assessment of the retinal function is a fundamental diagnostic technique not only in the clinical ophthalmology it is also indispensable in the ophthalmic research, in particular, in therapeutic approaches where the in vivo follow up of the benefit after treatment is absolutely necessary. Several current therapeutic approaches have demonstrated long-lasting amelioration in respective disease models and show promise for a successful translation to human patients. In this chapter we provide electroretinography protocols of experimental data which may serve as informative features for upcoming gene therapeutic approaches and clinical trials.

Pyruvate Kinase M2 Increases Angiogenesis, Neurogenesis, and Functional Recovery Mediated by Upregulation of STAT3 and Focal Adhesion Kinase Activities After Ischemic Stroke in Adult Mice

Ischemic stroke remains a serious threat to human life. Generation of neuronal and vascular cells is an endogenous regenerative mechanism in the adult brain, which may contribute to tissue repair after stroke. However, the regenerative activity is typically insufficient for significant therapeutic effects after brain injuries. Pyruvate kinase isoform M2 (PKM2) is a key regulator for energy metabolism. PKM2 also has nonmetabolic roles involving regulations of gene expression, cell proliferation, and migration in cancer cells as well as noncancerous cells. In a focal ischemic stroke mouse model, recombinant PKM2 (rPKM2) administration (160 ng/kg, intranasal delivery) at 1 h after stroke showed the significant effect of a reduced infarct volume of more the 60%. Delayed treatment of rPKM2, however, lost the acute neuroprotective effect. We then tested a novel hypothesis that delayed treatment of PKM2 might show proregenerative effects for long-term functional recovery and this chronic action could be mediated by its downstream STAT3 signaling. rPKM2 (160 ng/kg) was delivered to the brain using noninvasive intranasal administration 24 h after the stroke and repeated every other day. Western blot analysis revealed that, 7 days after the stroke, the levels of PKM2 and phosphorylated STAT3 and the expression of angiogenic factors VEGF, Ang-1, and Tie-2 in the peri-infarct region were significantly increased in the rPKM2 treatment group compared with those of the stroke vehicle group. To label proliferating cells, 5-bromo-2'-deoxyuridine (BrdU, 50 mg/kg, i.p.) was injected every day starting 3 days after stroke. At 14 days after stroke, immunohistochemistry showed that rPKM2 increased cell homing of doublecortin (DCX)-positive neuroblasts to the ischemic cortex. In neural progenitor cell (NPC) cultures, rPKM2 (0.4-4 nM) increased the expression of integrin β1 and the activation/phosphorylation of focal adhesion kinase (FAK). A mediator role of FAK in PKM2-promoted cell migration was verified in FAK-knockout fibroblast cultures. In the peri-infarct region of the brain, increased numbers of Glut-1/BrdU and NeuN/BrdU double-positive cells indicated enhanced angiogenesis and neurogenesis, respectively, compared to stroke vehicle mice. Using Laser Doppler imaging, we observed better recovery of the local blood flow in the peri-infarct region of rPKM2-treated mice 14 days after stroke. Meanwhile, rPKM2 improved the sensorimotor functional recovery measured by the adhesive removal test. Inhibiting the STAT3 phosphorylation/activation by the STAT3 inhibitor, BP-1-102 (3 mg/kg/day, o.g.), abolished all beneficial effects of rPKM2 in the stroke mice. Taken together, this investigation provides the first evidence demonstrating that early treatment of rPKM2 shows an acute neuroprotective effect against ischemic brain damage, whereas delayed rPKM2 treatment promotes regenerative activities in the poststroke brain leading to better functional recovery. The underlying mechanism involves activation of the STAT3 and FAK signals in the poststroke brain.

HDAC inhibition in the cpfl1 mouse protects degenerating cone photoreceptors in vivo

Cone photoreceptor cell death as it occurs in certain hereditary retinal diseases is devastating, with the affected patients suffering from a loss of accurate and colour vision. Regrettably, these hereditary cone diseases are still untreatable to date. Thus, the identification of substances able to block or restrain cone cell death is of primary importance. We studied the neuroprotective effects of a histone deacetylase inhibitor, Trichostatin A (TSA), in a mouse model of inherited, primary cone degeneration (cpfl1). We show that HDAC inhibition protects cpfl1 cones in vitro, in retinal explant cultures. More importantly, in vivo, a single intravitreal TSA injection significantly increased cone survival for up to 16 days post-injection. In addition, the abnormal, incomplete cone migration pattern in the cpfl1 retina was significantly improved by HDAC inhibition. These findings suggest a crucial role for HDAC activity in primary cone degeneration and highlight a new avenue for future therapy developments for cone dystrophies and retinal diseases associated with impaired cone migration.

Inhibition of thyroid hormone receptor locally in the retina is a therapeutic strategy for retinal degeneration

Thyroid hormone (TH) signaling regulates cell proliferation, differentiation, and metabolism. Recent studies have implicated TH signaling in cone photoreceptor viability. Using mouse models of retinal degeneration, we demonstrated that antithyroid drug treatment and targeting iodothyronine deiodinases (DIOs) to suppress cellular tri-iodothyronine (T3) production or increase T3 degradation preserves cones. In this work, we investigated the effectiveness of inhibition of the TH receptor (TR). Two genes, THRA and THRB, encode TRs; THRB2 has been associated with cone viability. Using TR antagonists and Thrb2 deletion, we examined the effects of TR inhibition. Systemic and ocular treatment with the TR antagonists NH-3 and 1-850 increased cone density by 30-40% in the Rpe65^-/- mouse model of Leber congenital amaurosis and reduced the number of TUNEL⁺ cells. Cone survival was significantly improved in Rpe65^-/- and Cpfl1 (a model of achromatopsia with Pde6c defect) mice with Thrb2 deletion. Ventral cone density in Cpfl1/Thrb2^-/- and Rpe65^-/- /Thrb2^-/- mice was increased by 1- to 4-fold, compared with age-matched controls. Moreover, the expression levels of TR were significantly higher in the cone-degeneration retinas, suggesting locally elevated TR signaling. This work shows that the effects of antithyroid treatment or targeting DIOs were likely mediated by TRs and that suppressing TR protects cones. Our findings support the view that inhibition of TR locally in the retina is a therapeutic strategy for retinal degeneration management.-Ma, H., Yang, F., Butler, M. R., Belcher, J., Redmond, T. M., Placzek, A. T., Scanlan, T. S., Ding, X.-Q. Inhibition of thyroid hormone receptor locally in the retina is a therapeutic strategy for retinal degeneration.

Targeting iodothyronine deiodinases locally in the retina is a therapeutic strategy for retinal degeneration

Recent studies have implicated thyroid hormone (TH) signaling in cone photoreceptor viability. Using mouse models of retinal degeneration, we found that antithyroid treatment preserves cones. This work investigates the significance of targeting intracellular TH components locally in the retina. The cellular TH level is mainly regulated by deiodinase iodothyronine (DIO)-2 and -3. DIO2 converts thyroxine (T4) to triiodothyronine (T3), which binds to the TH receptor, whereas DIO3 degrades T3 and T4. We examined cone survival after overexpression of DIO3 and inhibition of DIO2 and demonstrated the benefits of these manipulations. Subretinal delivery of AAV5-IRBP/GNAT2-DIO3, which directs expression of human DIO3 specifically in cones, increased cone density by 30-40% in a Rpe65^-/- mouse model of Lebers congenital amaurosis (LCA) and in a Cpfl1 mouse with Pde6c defect model of achromatopsia, compared with their respective untreated controls. Intravitreal and topical delivery of the DIO2 inhibitor iopanoic acid also significantly improved cone survival in the LCA model mice. Moreover, the expression levels of DIO2 and Slc16a2 were significantly higher in the diseased retinas, suggesting locally elevated TH signaling. We show that targeting DIOs protects cones, and intracellular inhibition of TH components locally in the retina may represent a novel strategy for retinal degeneration management.-Yang, F., Ma, H., Belcher, J., Butler, M. R., Redmond, T. M., Boye, S. L., Hauswirth, W. W., Ding, X.-Q. Targeting iodothyronine deiodinases locally in the retina is a therapeutic strategy for retinal degeneration.

Calcium dynamics change in degenerating cone photoreceptors

Cone photoreceptors (cones) are essential for high-resolution daylight vision and colour perception. Loss of cones in hereditary retinal diseases has a dramatic impact on human vision. The mechanisms underlying cone death are poorly understood, and consequently, there are no treatments available. Previous studies suggest a central role for calcium (Ca²⁺) homeostasis deficits in photoreceptor degeneration; however, direct evidence for this is scarce and physiological measurements of Ca²⁺in degenerating mammalian cones are lacking.Here, we took advantage of the transgenic HR2.1:TN-XL mouse line that expresses a genetically encoded Ca²⁺biosensor exclusively in cones. We cross-bred this line with mouse models for primary ("cone photoreceptor function loss-1", cpfl1) and secondary ("retinal degeneration-1", rd1) cone degeneration, respectively, and assessed resting Ca²⁺levels and light-evoked Ca²⁺responses in cones using two-photon imaging. We found that Ca²⁺dynamics were altered in cpfl1 cones, showing higher noise and variable Ca²⁺levels, with significantly wider distribution than for wild-type and rd1 cones. Unexpectedly, up to 21% of cpfl1 cones still displayed light-evoked Ca²⁺responses, which were larger and slower than wild-type responses. In contrast, genetically intact rd1 cones were characterized by lower noise and complete lack of visual function.Our study demonstrates alterations in cone Ca²⁺dynamics in both primary and secondary cone degeneration. Our results are consistent with the view that higher (fluctuating) cone Ca²⁺levels are involved in photoreceptor cell death in primary (cpfl1) but not in secondary (rd1) cone degeneration. These findings may guide the future development of therapies targeting photoreceptor Ca²⁺homeostasis.

Proteomic Profiling Suggests Central Role Of STAT Signaling during Retinal Degeneration in the rd10 Mouse Model

The rd10 mouse is a model of retinitis pigmentosa characterized by the dysfunction of a rod-photoreceptor-specific phosphodiesterase. Compared to the rd1 mouse, retinal degeneration in the rd10 mouse begins later in age with a milder phenotype, making it ideal for investigating cell death and neuroprotective mechanisms. Alterations in the rd10 retina proteome at pre-, peak-, and postdegenerative time points were examined using a modified high-recovery filter-aided sample preparation (FASP) method in combination with label-free quantitative mass spectrometry, generating a proteomic data set on almost 3000 proteins. Our data confirmed a period of protein expression similar to age-matched wild-type mice predegeneration, with decreases in proteins associated with phototransduction and increases in signaling proteins at peak- and postdegenerative stages. A total of 57 proteins were differentially expressed in the rd10 retinae during peak-degeneration, compared to those in wild-type mice after stringent FDR correction (q < 0.05). Network analysis separated these proteins into one cluster of down-regulated photoreceptor proteins and one of up-regulated signaling proteins centered around GFAP, STAT3, and STAT1. This is the first study to identify alterations in STAT1 in the rd10 mouse, which were confirmed with gene expression and immunoblotting experiments, underpinning the efficacy of our approach. This unique proteomic data set on protein dynamics during retinal degeneration could serve as an information source for vision research in the future.

Targeting the cyclophilin domain of Ran-binding protein 2 (Ranbp2) with novel small molecules to control the proteostasis of STAT3, hnRNPA2B1 and M-opsin

Cyclophilins are peptidyl cis-trans prolyl isomerases (PPIases), whose activity is typically inhibited by cyclosporine A (CsA), a potent immunosuppressor. Cyclophilins are also chaperones. Emerging evidence supports that cyclophilins present nonoverlapping PPIase and chaperone activities. The proteostasis of the disease-relevant substrates, signal transducer and activator of transcription 3 and 5 (STAT3/STAT5), heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1), and M-opsin, is regulated by nonoverlapping chaperone and PPIase activities of the cyclophilin domain (CY) of Ranbp2, a multifunctional and modular scaffold that controls nucleocytoplasmic shuttling and proteostasis of selective substrates. Although highly homologous, CY and the archetypal cyclophilin A (CyPA) present distinct catalytic and CsA-binding activities owing to unique structural features between these cylophilins. We explored structural idiosyncrasies between CY and CyPA to screen in silico nearly 9 million small molecules (SM) against the CY PPIase pocket and identify SMs with selective bioactivity toward STAT3, hnRNPA2B1, or M-opsin proteostasis. We found three classes of SMs that enhance the cytokine-stimulated transcriptional activity of STAT3 without changing latent and activated STAT3 levels, down-regulate hnRNPA2B1 or M-opsin proteostasis, or a combination of these. Further, a SM that suppresses hnRNPA2B1 proteostasis also inhibits strongly and selectively the PPIase activity of CY. This study unravels chemical probes for multimodal regulation of CY of Ranbp2 and its substrates, and this regulation likely results in the allosterism stemming from the interconversion of conformational substates of cyclophilins. The results also demonstrate the feasibility of CY in drug discovery against disease-relevant substrates controlled by Ranbp2, and they open new opportunities for therapeutic interventions.

STAT3 promotes survival of mutant photoreceptors in inherited photoreceptor degeneration models

Inherited photoreceptor degenerations (IPDs), a group of incurable progressive blinding diseases, are caused by mutations in more than 200 genes, but little is known about the molecular pathogenesis of photoreceptor (PR) death. Increased retinal expression of STAT3 has been observed in response to many retinal insults, including IPDs, but the role of this increase in PR death is unknown. Here, we show that the expression of Stat3 is increased in PRs of the Tg(RHO P347S) and Prph2(rds) (/+) mouse models of IPD and is activated by tyrosine phosphorylation. PR-specific deletion of Stat3 substantially accelerated PR degeneration in both mutant strains. In contrast, increased PR-specific expression of ROSA26 (R26) alleles encoding either WT STAT3 (Stat3(wt)) or the gain-of-function variant STAT3(C) (Stat3(C)) improved PR survival in both models. Moreover, PR signaling in Tg(RHO P347S) mice carrying either a R26-Stat3(wt) or R26-Stat3(C) allele demonstrated increased a-wave amplitude of the scotopic electroretinogram. Phosphorylation of STAT3 at tyrosine 705 was required for the prosurvival effect because an R26-Stat3(Y705F) allele was not protective. The prosurvival role of enhanced Stat3 activity was validated using recombinant adenoassociated virus (rAAV) vector-mediated PR Stat3 expression in Tg(RHO P347S) mice. Our findings (i) establish that the increase in endogenous PR Stat3 expression is a protective response in IPDs, (ii) suggest that therapeutic augmentation of PR Stat3 expression has potential as a common neuroprotective therapy for these disorders, and (iii) indicate that prosurvival molecules whose expression is increased in mutant PRs may have promise as novel therapies for IPDs.

Suppressing thyroid hormone signaling preserves cone photoreceptors in mouse models of retinal degeneration

Cone phototransduction and survival of cones in the human macula is essential for color vision and for visual acuity. Progressive cone degeneration in age-related macular degeneration, Stargardt disease, and recessive cone dystrophies is a major cause of blindness. Thyroid hormone (TH) signaling, which regulates cell proliferation, differentiation, and apoptosis, plays a central role in cone opsin expression and patterning in the retina. Here, we investigated whether TH signaling affects cone viability in inherited retinal degeneration mouse models. Retinol isomerase RPE65-deficient mice [a model of Leber congenital amaurosis (LCA) with rapid cone loss] and cone photoreceptor function loss type 1 mice (severe recessive achromatopsia) were used to determine whether suppressing TH signaling with antithyroid treatment reduces cone death. Further, cone cyclic nucleotide-gated channel B subunit-deficient mice (moderate achromatopsia) and guanylate cyclase 2e-deficient mice (LCA with slower cone loss) were used to determine whether triiodothyronine (T3) treatment (stimulating TH signaling) causes deterioration of cones. We found that cone density in retinol isomerase RPE65-deficient and cone photoreceptor function loss type 1 mice increased about sixfold following antithyroid treatment. Cone density in cone cyclic nucleotide-gated channel B subunit-deficient and guanylate cyclase 2e-deficient mice decreased about 40% following T3 treatment. The effect of TH signaling on cone viability appears to be independent of its regulation on cone opsin expression. This work demonstrates that suppressing TH signaling in retina dystrophy mouse models is protective of cones, providing insights into cone preservation and therapeutic interventions.

Leukemia inhibitory factor signaling in degenerating retinas

Degeneration of cells in the retina is a hallmark of various inherited and acquired blinding diseases in humans. One of the most challenging problems to establish successful treatments for these diseases is to understand the molecular mechanisms that result in retinal degeneration and to identify endogenous rescue pathways which support cell survival. In many mouse models for retinal degeneration, expression of LIF in glial cells in response to a disease condition is crucial for the activation of an elaborate protective system. This mini review will summarize the findings that are related to LIF signaling and discuss the neuroprotective effects of LIF in different animal models.

p38 MAPK signaling acts upstream of LIF-dependent neuroprotection during photoreceptor degeneration

In many blinding diseases of the retina, loss of function and thus severe visual impairment results from apoptotic cell death of damaged photoreceptors. In an attempt to survive, injured photoreceptors generate survival signals to induce intercellular protective mechanisms that eventually may rescue photoreceptors from entering an apoptotic death pathway. One such endogenous survival pathway is controlled by leukemia inhibitory factor (LIF), which is produced by a subset of Muller glia cells in response to photoreceptor injury. In the absence of LIF, survival components are not activated and photoreceptor degeneration is accelerated. Although LIF is a crucial factor for photoreceptor survival, the detailed mechanism of its induction in the retina has not been elucidated. Here, we show that administration of tumor necrosis factor-alpha (TNF) was sufficient to fully upregulate Lif expression in Muller cells in vitro and the retina in vivo. Increased Lif expression depended on p38 mitogen-activated protein kinase (MAPK) since inhibition of its activity abolished Lif expression in vitro and in vivo. Inhibition of p38 MAPK activity reduced the Lif expression also in the model of light-induced retinal degeneration and resulted in increased cell death in the light-exposed retina. Thus, expression of Lif in the injured retina and activation of the endogenous survival pathway involve signaling through p38 MAPK.

Distinct and atypical intrinsic and extrinsic cell death pathways between photoreceptor cell types upon specific ablation of Ranbp2 in cone photoreceptors

Non-autonomous cell-death is a cardinal feature of the disintegration of neural networks in neurodegenerative diseases, but the molecular bases of this process are poorly understood. The neural retina comprises a mosaic of rod and cone photoreceptors. Cone and rod photoreceptors degenerate upon rod-specific expression of heterogeneous mutations in functionally distinct genes, whereas cone-specific mutations are thought to cause only cone demise. Here we show that conditional ablation in cone photoreceptors of Ran-binding protein-2 (Ranbp2), a cell context-dependent pleiotropic protein linked to neuroprotection, familial necrotic encephalopathies, acute transverse myelitis and tumor-suppression, promotes early electrophysiological deficits, subcellular erosive destruction and non-apoptotic death of cones, whereas rod photoreceptors undergo cone-dependent non-autonomous apoptosis. Cone-specific Ranbp2 ablation causes the temporal activation of a cone-intrinsic molecular cascade highlighted by the early activation of metalloproteinase 11/stromelysin-3 and up-regulation of Crx and CoREST, followed by the down-modulation of cone-specific phototransduction genes, transient up-regulation of regulatory/survival genes and activation of caspase-7 without apoptosis. Conversely, PARP1⁺-apoptotic rods develop upon sequential activation of caspase-9 and caspase-3 and loss of membrane permeability. Rod photoreceptor demise ceases upon cone degeneration. These findings reveal novel roles of Ranbp2 in the modulation of intrinsic and extrinsic cell death mechanisms and pathways. They also unveil a novel spatiotemporal paradigm of progression of neurodegeneration upon cell-specific genetic damage whereby a cone to rod non-autonomous death pathway with intrinsically distinct cell-type death manifestations is triggered by cell-specific loss of Ranbp2. Finally, this study casts new light onto cell-death mechanisms that may be shared by human dystrophies with distinct retinal spatial signatures as well as with other etiologically distinct neurodegenerative disorders.

The secondary demise of healthy neurons upon the degeneration of neurons harboring primary genetic defect(s) is hallmark to neurodegenerative diseases. However, the factors and mechanisms driving these cell-death processes are not understood, a severe limitation which has hampered the therapeutic development of neuroprotective approaches. The neuroretina is comprised of two main types of photoreceptor neurons, rods and cones. These undergo degeneration upon heterogeneous mutations or environmental stressors and the underlying diseases present conspicuous spatiotemporal pathological signatures whose molecular bases are not understood. We employed the multifunctional protein, Ran-binding protein-2 (Ranbp2), which is implicated in cell-type and stress-dependent clinical manifestations, to examine its role(s) in primary and secondary photoreceptor death mechanisms upon its specific loss in cones. Contrary to prior findings, we found that dying cones can trigger the loss of healthy rods. This process arises by the immediate activation of novel Ranbp2-responsive factors and downstream cascade events in cones that promote extrinsically the demise of rods. The mechanisms of rod and cone demise are molecularly distinct. Collectively, the data uncover distinct Ranbp2 roles in intrinsic and extrinsic cell-death and will likely contribute to our understanding of the spatiotemporal onset and progression of diseases affecting photoreceptor mosaics and other neural networks.

Loss of cone cyclic nucleotide-gated channel leads to alterations in light response modulating system and cellular stress response pathways: a gene expression profiling study

The cone photoreceptor cyclic nucleotide-gated (CNG) channel is essential for central and color vision and visual acuity. Mutations in the channel subunits CNGA3 and CNGB3 are associated with achromatopsia and cone dystrophy. We investigated the gene expression profiles in mouse retina with CNG channel deficiency using whole genome expression microarrays. As cones comprise only 2 to 3% of the total photoreceptor population in the wild-type mouse retina, the mouse lines with CNG channel deficiency on a cone-dominant background, i.e. Cnga3-/-/Nrl-/- and Cngb3-/-/Nrl-/- mice, were used in our study. Comparative data analysis revealed a total of 105 genes altered in Cnga3-/-/Nrl-/- and 92 in Cngb3-/-/Nrl-/- retinas, relative to Nrl-/- retinas, with 27 genes changed in both genotypes. The differentially expressed genes primarily encode proteins associated with cell signaling, cellular function maintenance and gene expression. Ingenuity pathway analysis (IPA) identified 26 and 9 canonical pathways in Cnga3-/-/Nrl-/- and Cngb3-/-/Nrl-/- retinas, respectively, with 6 pathways being shared. The shared pathways include phototransduction, cAMP/PKA-mediated signaling, endothelin signaling, and EIF2/endoplasmic reticulum (ER) stress, whereas the IL-1, CREB, and purine metabolism signaling were found to specifically associate with Cnga3 deficiency. Thus, CNG channel deficiency differentially regulates genes that affect cell processes such as phototransduction, cellular survival and gene expression, and such regulations play a crucial role(s) in the retinal adaptation to impaired cone phototransduction. Though lack of Cnga3 and Cngb3 shares many common pathways, deficiency of Cnga3 causes more significant alterations in gene expression. This work provides insights into how cones respond to impaired phototransduction at the gene expression levels.

Functional phenotyping of mouse models with ERG

In many situations it is important to be able to assess the degree of retinal function, e.g., for the characterization of mouse models with unknown retinal involvement, when studying degenerative processes, for the analysis of visual signal processing, and during the follow-up of therapeutic interventions. Full-field electroretinography (ERG), yielding a sum response of event-related transient electrical activity of the entire retina to light stimulation, is widely applied in human as well as experimental functional diagnostics. ERG examinations normally include initial dark-adapted (scotopic) measurements that enable rod-driven activity to be studied, followed by light-adapted (photopic) recordings to obtain information about cone system contributions. The results allow the correlation of acute or long-term disease-related changes or their alleviation by therapy with morphological data, in order to obtain a comprehensive understanding of the underlying processes and mechanisms.

Increased cone sensitivity to ABCA4 deficiency provides insight into macular vision loss in Stargardt's dystrophy

Autosomal recessive Stargardt macular dystrophy is caused by mutations in the photoreceptor disc rim protein ABCA4/ABCR. Key clinical features of Stargardt disease include relatively mild rod defects such as delayed dark adaptation, coupled with severe cone defects reflected in macular atrophy and central vision loss. In spite of this clinical divergence, there has been no biochemical study of the effects of ABCA4 deficiency on cones vs. rods. Here we utilize the cone-dominant Abca4(-/-)/Nrl(-/-) double knockout mouse to study this issue. We show that as early as post-natal day (P) 30, Abca4(-/-)/Nrl(-/-) retinas have significantly fewer rosettes than Abca4(+/+)/Nrl(-/-) retinas, a phenotype often associated with accelerated degeneration. Abca4-deficient mice in both the wild-type and cone-dominant background accumulate more of the toxic bisretinoid A2E than their ABCA4-competent counterparts, but Abca4(-/-)/Nrl(-/-) eyes generate significantly more A2E per mole of 11-cis-retinal (11-cisRAL) than Abca4(-/-) eyes. At P120, Abca4(-/-)/Nrl(-/-) produced 340 ± 121 pmoles A2E/nmol 11-cisRAL while Abca4(-/-) produced 50.4 ± 8.05 pmoles A2E/nmol 11-cisRAL. Nevertheless, the retinal pigment epithelium (RPE) of Abca4(-/-)/Nrl(-/-) eyes exhibits fewer lipofuscin granules than the RPE of Abca4(-/-) eyes; at P120: Abca4(-/-)/Nrl(-/-) exhibit 0.045 ± 0.013 lipofuscingranules/μm² of RPE vs. Abca4(-/-) 0.17 ± 0.030 lipofuscingranules/μm² of RPE. These data indicate that ABCA4-deficient cones simultaneously generate more A2E than rods and are less able to effectively clear it, and suggest that primary cone toxicity may contribute to Stargardt's-associated macular vision loss in addition to cone death secondary to RPE atrophy.