Müller cells and microglia of the mouse eye react throughout the entire retina in response to the procedure of an intravitreal injection

Gliosis-dependent expression of complement factor H truncated variants attenuates retinal neurodegeneration following ischemic injury

Inherited, age-related, and acute retinal diseases are often exacerbated by an aberrant or excessive activity of the complement system. Consequently, cells not directly affected by an acute event or genetic variants may degenerate, resulting in enhanced visual impairment. The therapeutic potential of supplementation of complement factor H (FH), a key regulator of the complement cascade, is therefore particularly promising in the context of retinal diseases caused by complement activation. In this study, we engineered adeno-associated viruses (AAVs) containing sequences of two truncated human FH variants. The expression of these variants was regulated by the glial fibrillary acidic protein (GFAP) promoter, which is selectively active in gliotic Müller cells. Both FH variants consisted of FH domains 19-20, which were connected to domains 1-4 and 1-7, respectively, by a polyglycine linker. These AAVs were intravitreally injected following ischemic injury of C57BL/6J mouse retinas. We observed transgene expression in gliotic Müller cells and to some extent in astrocytes. The expression correlated directly with damage severity. Interventions resulted in decreased complement activation, accelerated normalization of microglia activity and morphological improvements. Reduced levels of C3 transcripts and C3d protein in conjunction with higher transcript levels of inhibitory regulators like Cfi and Cfh, hinted at attenuated complement activity. This study demonstrates the great potential of complement regulatory gene addition therapy. With further in vivo testing it could be applied to treat a wide range of retinal diseases where no causative therapies are available.

SARM1 acts downstream of neuroinflammatory and necroptotic signaling to induce axon degeneration

Neuroinflammation and necroptosis are major contributors to neurodegenerative disease, and axon dysfunction and degeneration is often an initiating event. SARM1 is the central executioner of pathological axon degeneration. Here, we demonstrate functional and mechanistic links among these three pro-degenerative processes. In a neuroinflammatory model of glaucoma, TNF-α induces SARM1-dependent axon degeneration, oligodendrocyte loss, and subsequent retinal ganglion cell death. TNF-α also triggers SARM1-dependent axon degeneration in sensory neurons via a noncanonical necroptotic signaling mechanism. MLKL is the final executioner of canonical necroptosis; however, in axonal necroptosis, MLKL does not directly trigger degeneration. Instead, MLKL induces loss of the axon survival factors NMNAT2 and STMN2 to activate SARM1 NADase activity, which leads to calcium influx and axon degeneration. Hence, these findings define a specialized form of axonal necroptosis. The demonstration that neuroinflammatory signals and necroptosis can act locally in the axon to stimulate SARM1-dependent axon degeneration identifies a therapeutically targetable mechanism by which neuroinflammation can stimulate axon loss in neurodegenerative disease.

Interaction of complement system and microglia activation in retina and optic nerve in a NMDA damage model

It is known that intravitreally injected N-methyl-d-aspartate (NMDA) leads to fast retina and optic nerve degeneration and can directly activate microglia. Here, we analyzed the relevance for microglia related degenerating factors, the proteins of the complement system, at a late stage in the NMDA damage model. Therefore, different doses of NMDA (0 (PBS), 20, 40, 80 nmol) were intravitreally injected in rat eyes. Proliferative and activated microglia/macrophages (MG/Mϕ) were found in retina and optic nerve 2 weeks after NMDA injection. All three complement pathway proteins were activated in retinas after 40 and 80 nmol NMDA treatment. 80 nmol NMDA injection also lead to more numerous depositions of complement factors C3 and membrane attack complex (MAC) in retina and MAC in optic nerve. Additionally, more MAC⁺ depositions were detected in optic nerves of the 40 nmol NMDA group. In this NMDA model, the retina is first affected followed by optic nerve damage. However, we found initiating complement processes in the retina, while more deposits of the terminal complex were present 2 weeks after NMDA injection in the optic nerve. The complement system can be activated in waves and possibly a second wave is still on-going in the retina, while the first activation wave is in the final phase in the optic nerve. Only the damaged tissues showed microglia activation as well as proliferation and an increase of complement proteins. Interestingly, the microglia/macrophages (MG/Mϕ) in this model were closely connected with the inductors of the classical and lectin pathway, but not with the alternative pathway. However, all three initiating complement pathways were upregulated in the retina. The alternative pathway seems to be triggered by other mechanisms in this NMDA model. Our study showed an ongoing interaction of microglia and complement proteins in a late stage of a degenerative process.

Concentration-Dependent Inner Retina Layer Damage and Optic Nerve Degeneration in a NMDA Model

The intravitreal injection of N-methyl-D-aspartate (NMDA), a glutamate analogue, is an established model for fast retinal ganglion cell (RGC) degeneration. Yet, NMDA does not cause specific RGC damage. Now, the effects on the whole retina were analyzed. Additionally, the related effects for the structure and apoptotic levels of the optic nerve were investigated. Therefore, different NMDA concentrations were intravitreally injected in rats (20, 40, or 80 nmol NMDA or PBS). At days 3 and 14, Brn-3a⁺ RGCs were degenerated. A damage of calretinin⁺ amacrine cells was also recognized at day 14. Only a slight damage was observed in regard to PKCα⁺ bipolar cells, while rhodopsin⁺ photoreceptors remained intact. A long-lasting retinal microglia response was observed from day 3 up to day 14. Furthermore, a partial degeneration of the optic nerve was noted. At day 3, the SMI-32⁺ neurofilaments were just slightly affected, whereas the neurofilament structure was further degenerated at day 14. However, the luxol fast blue (LFB)-stained myelin structure remained intact from day 3 up to day 14. Interestingly, apoptotic mechanisms, like FasL and Fas co-localization as well as caspase 3 activation, were restricted to the optic nerve of the highest NMDA group at this late stage of degeneration. The degeneration of the optic nerve is probably only a side effect of neuronal degeneration of the inner retinal layers. The intact myelin structure might form a barrier against the direct influence of NMDA. In conclusion, this model is very suitable to test therapeutic agents, but it is important to analyze all inner retina layers and the optic nerve to determine their efficacy in this model more precisely.

Neonatal disease environment limits the efficacy of retinal transplantation in the LCA8 mouse model

Background

Mutations of Crb1 gene cause irreversible and incurable visual impairment in humans. This study aims to use an LCA8-like mouse model to identify host-mediated responses that might interfere with survival, retinal integration and differentiation of grafted cells during neonatal cell therapy.

Methods

Mixed retinal donor cells (1 ~ 2 × 10⁴) isolated from neural retinas of neonatal eGFP transgenic mice were injected into the subretinal space of LCA8-like model neonatal mice. Markers of specific cell types were used to analyze microglial attraction, CSPG induction and retinal cell differentiation. The positions of host retinal cells were traced according to their laminar location during disease progression to look for host cell rearrangements that might inhibit retinal integration of the transplanted cells.

Results

Transplanted retinal cells showed poor survival and attracted microglial cells, but CSPG was not greatly induced. Retinas of the LCA8 model hosts underwent significant cellular rearrangement, including rosette formation and apical displacement of inner retinal cells.

Conclusions

Local disease environment, particularly host immune responses to injected cells and formation of a physical barrier caused by apical migration of host retinal cells upon disruption of outer limiting membrane, may impose two major barriers in LCAs cell transplantation therapy.

Tamoxifen-Containing Eye Drops Successfully Trigger Cre-Mediated Recombination in the Entire Eye

Embryonic lethality in mice with targeted gene deletion is a major issue that can be circumvented by using Cre-loxP-based animal models. Various inducible Cre systems are available, e.g. such that are activated following tamoxifen treatment, and allow deletion of a specific target gene at any desired time point during the life span of the animal. In this study, we describe the efficiency of topical tamoxifen administration by eye drops using a Cre- reporter mouse strain (R26R). We report that tamoxifen-responsive CAGGCre-ER (TM) mice show a robust Cre- mediated recombination throughout the entire eye.

The external limiting membrane in early-onset Stargardt disease

PURPOSE

To describe pathologic changes of the external limiting membrane (ELM) in young patients with early-onset Stargardt (STGD1) disease.

METHODS

Twenty-six STGD1 patients aged younger than 20 years with confirmed disease-causing adenosine triphosphate-binding cassette, subfamily A, member 4 (ABCA4) alleles and 30 age-matched unaffected individuals were studied. Spectral-domain optical coherence tomography (SD-OCT), fundus autofluorescence (AF), and color fundus photography (CFP) images, as well as full-field electroretinograms were obtained and analyzed for one to four visits in each patient.

RESULTS

The ELM in all patients exhibited a distinct thickening that was not observed in unaffected individuals. In addition, accumulations of reflective deposits were noted in the outer nuclear layer in every patient. Four patients exhibited a concave protuberance or bulging of a thickened and hyperreflective ELM band within the fovea containing preserved photoreceptors. Longitudinal SD-OCT data in several patients revealed the persistence of this ELM abnormality over a period of time (1-4 years). Furthermore, the edges of the inner segment ellipsoid band appeared to recede earlier than the ELM band in active lesions.

CONCLUSIONS

Structural changes seen in the ELM of this cohort may reflect a gliotic response to cellular stress at the photoreceptor level in early-onset STGD1.