in-vivo-dynamics-retinal-microglial-activation-during

<em>In Vivo </em>Dynamics of Retinal Microglial Activation During Neurodegeneration: Confocal Ophthalmoscopic Imaging and Cell Morphometry in Mouse Glaucoma

Microglia, which are CNS-resident neuroimmune cells, transform their morphology and size in response to CNS damage, switching to an activated state with ...

University of Utah

Developing amacrine cells in the vertebrate retina undergo naturally-occurring cell death which is accentuated by the early removal of retinal ganglion cells. We show that providing BDNF or decreasing endogenous BDNF via competitive binding with soluble TrkB receptors in a whole-retina culture assay modulates the frequency of dying cells in the amacrine cell layer. Ganglion cells synthesize BDNF, and amacrine cells express TrkB receptors, suggesting a likely signaling mechanism.

Cell death in the inner nuclear layer of the retina is modulated by BDNF.

During development of the retina, programmed cell death helps to establish the final size and distribution of various cell classes in distinct layers of the tissue. Here we show that dying cells in the developing ganglion and inner nuclear layers are clustered spatially and that gap junction inhibitors decrease the clustering of dying cells. To confirm the role of gap junctions in cell death, we induced targeted cell death via intracellular cytochrome c (Cc) and examined the induced cells and their neighbors for apoptotic morphology or caspase-3 cleavage. These studies indicate that bystander killing extends to coupled cells. Quantitative studies of bystander killing were performed by scrape-loading retinas with Cc in the presence of rhodamine dextran (RD; to identify Cc-loaded cells) and by counting pyknotic cells in cryosections. Although only 1.5% of control scrape-loaded cells (RD alone) showed apoptotic morphology, 97% of Cc scrape-loaded cells were pyknotic. Moreover, bystander killing extended to neighboring cells, not labeled with RD, and was reduced significantly by the gap junction inhibitors octanol and carbenoxolone. We hypothesize that dying cells in the retina generate a gap junction-permeant apoptotic signal that mediates bystander killing. This novel finding of naturally occurring bystander cell death may have important implications in the histogenesis and pathology of the nervous system.

Gap junctions mediate bystander cell death in developing retina.

In adult retina, aquaporin-4 (AQP4) and inwardly rectifying K(+) (Kir4.1) channels localize to astrocyte and Müller cell membranes facing vascular and vitreous compartments, optimizing clearance of extracellular K(+) and water from the synaptic layers. However, it is unknown whether these channels are expressed at early developmental stages, before gliogenesis or angiogenesis take place in the neural retina. This study was conducted to determine the presence of AQP4 and Kir4.1 proteins in the developing mouse retina.

A developmental switch in the expression of aquaporin-4 and Kir4.1 from horizontal to Müller cells in mouse retina.

Scaffolding of membrane proteins is a common strategy for forming complexes of proteins, including some connexins, within membrane microdomains. Here we describe studies indicating that Cx32 interacts with a PDZ-containing scaffolding protein, Dlgh1 (Discs Large homolog 1). Initial screens of liver lysates using antibody arrays indicated an interaction between Cx32 and Dlgh1 that was confirmed using coimmunoprecipitation studies. Yeast two-hybrid complementation determined that the Cx32 bound via interaction with the SH3/Hook domain of Dlgh1. Confocal microscopy of liver sections revealed that Cx32 and Dlgh1 could colocalize in hepatocyte membranes in wild type mice. Examination of levels and localization of Dlgh1 in livers from Cx32 null mice indicate that, in the absence of Cx32, Dlgh1 was decreased, and the remainder was translocated from the hepatocyte membrane to the nucleus with some remaining in cytoplasmic compartments. This translocation was confirmed by Western blots comparing Dlgh1 levels in nuclear extracts from wild type and Cx32 null murine livers. Using SKHep cells stably transfected with Cx32 under the control of a tet-off promoter, we found that acute removal of Cx32 led to a decrease of membrane-localized Dlgh1 and an increase in the nuclear localization of this tumor suppressor protein. Together, these results suggest that loss of Cx32 alters the levels, localization, and interactions of the tumor suppressor protein Dlgh1, events known in other systems to alter cell cycle and increase tumorigenicity.

The gap junction protein connexin32 interacts with the Src homology 3/hook domain of discs large homolog 1.

In the context of the retinal ganglion cell (RGC) axon degeneration in the optic nerve that occurs in glaucoma, microglia become activated, then phagocytic, and redistribute in the optic nerve head. The authors investigated the potential contribution of retinal microglia activation to glaucoma progression in the DBA/2J chronic mouse glaucoma model.

Reduced retina microglial activation and improved optic nerve integrity with minocycline treatment in the DBA/2J mouse model of glaucoma.

Changes in microglial cell activation and distribution are associated with neuronal decline in the central nervous system (CNS), particularly under pathological conditions. Activated microglia converge on the initial site of axonal degeneration in human glaucoma, yet their part in its pathophysiology remains unresolved. To begin with, it is unknown whether microglia activation precedes or is a late consequence of retinal ganglion cell (RGC) neurodegeneration. Here we address this critical element in DBA/2J (D2) mice, an established model of chronic inherited glaucoma, using as a control the congenic substrain DBA/2J Gpnmb(+/SjJ) (D2G), which is not affected by glaucoma. We analyzed the spatial distribution and timecourse of microglial changes in the retina, as well as within the proximal optic nerve prior to and throughout ages when neurodegeneration has been reported. Exclusively in D2 mice, we detected early microglia clustering in the inner central retina and unmyelinated optic nerve regions, with microglia activation peaking by 3 months of age. Between 5 and 8 months of age, activated microglia persisted and concentrated in the optic disc, but also localized to the retinal periphery. Collectively, our findings suggest microglia activation is an early alteration in the retina and optic nerve in D2 glaucoma, potentially contributing to disease onset or progression. Ultimately, detection of microglial activation may have value in early disease diagnosis, while modulation of microglial responses may alter disease progression.

Early microglia activation in a mouse model of chronic glaucoma.

Glaucoma is a neurodegenerative disease that results in the progressive decline and ultimate death of retinal ganglion cells (RGCs). While multiple risk factors are associated with glaucoma, the mechanisms leading to onset and progression of the disease remain unknown. Molecular analysis in various glaucoma models has revealed involvement of non-neuronal cell populations, including astrocytes, Mueller glia and microglia, at early stages of glaucoma. High-dose irradiation was reported to have a significant long-term protective effect in the DBA/2J (D2) mouse model of glaucoma, although the cellular and molecular basis for this effect remains unclear. In particular, the acute effects of irradiation on specific cell populations, including non-neuronal cells, in the D2 retina and nerve have not been assessed. Here we report that irradiation induces transient reduction in proliferating microglia within the optic nerve head and glial lamina within the first week post-irradiation. This was accompanied by reduced microglial activation, with no effect on astrocyte gliosis in those regions. At later stages we confirm that early high-dose irradiation of the mouse head results in improvement of axonal structural integrity and anterograde transport function, without reduction of intraocular pressure. Thus reduced microglial activation induced by irradiation at early stages is associated with reduced optic nerve and retinal neurodegeneration in the D2 mouse model of glaucoma.

Early reduction of microglia activation by irradiation in a model of chronic glaucoma.

Le glaucome est une maladie neurod&eacute;g&eacute;n&eacute;rative qui entra&icirc;ne le d&eacute;clin progressif et de la mort ultime des cellules ganglionnaires r&eacute;tiniennes (CGR). Alors que plusieurs facteurs de risque sont associ&eacute;s avec le glaucome, les m&eacute;canismes qui conduisent &agrave; l&#39;apparition et la progression de la maladie restent inconnues. Analyse mol&eacute;culaire dans divers mod&egrave;les de glaucome a r&eacute;v&eacute;l&eacute; l&#39;implication des populations de cellules non neuronales, y compris les astrocytes, cellules gliales Mueller et la microglie, aux premiers stades du glaucome. Irradiation de haut-dose a &eacute;t&eacute; signal&eacute;e pour avoir un effet protecteur &agrave; long terme significatif dans le mod&egrave;le de souris DBA/2J (D2) de glaucome, bien que les bases cellulaires et mol&eacute;culaires pour cet effet reste incertaine. En particulier, les effets aigus de l&#39;irradiation sur les populations cellule sp&eacute;cifique, y compris les cellules non neuronales, dans la r&eacute;tine de D2 et du syst&egrave;me nerveux n&#39;ont pas &eacute;t&eacute; &eacute;valu&eacute;es. Nous rapportons ici que l&#39;irradiation induit une r&eacute;duction transitoire dans la prolif&eacute;ration des microglies dans la t&ecirc;te du nerf optique et glial limbe dans la premi&egrave;re post-irradiation semaine. Cela s&#39;est accompagn&eacute; d&#39;activation microgliale r&eacute;duite, sans effet sur la gliose astrocytaire dans ces r&eacute;gions. &Agrave; un stade ult&eacute;rieur, nous confirmons que l&#39;irradiation fortes doses au d&eacute;but des r&eacute;sultats t&ecirc;te souris dans l&#39;am&eacute;lioration de l&#39;int&eacute;grit&eacute; structurale axonale et la fonction de transport ant&eacute;rograde, sans r&eacute;duction de la pression intraoculaire. Activation microgliale r&eacute;duite induite par l&#39;irradiation &agrave; des stades pr&eacute;coces est donc associ&eacute;e &agrave; la r&eacute;duction de nerf optique et neurod&eacute;g&eacute;n&eacute;rescence r&eacute;tinienne chez la souris D2 du glaucome.

R&eacute;duction rapide de l&#39;activation des microglies par irradiation dans un mod&egrave;le de glaucome chronique.

Il glaucoma &egrave; una malattia neurodegenerativa che provoca il progressivo declino e morte finale delle cellule gangliari della retina (RGCs). Mentre pi&ugrave; fattori di rischio sono associati a glaucoma, i meccanismi che portano all&#39;insorgenza e la progressione della malattia rimangano sconosciuti. Analisi molecolare in vari modelli di glaucoma ha rivelato il coinvolgimento delle popolazioni delle cellule non-neuronali, tra cui Mueller glia, astrociti e microglia, alle prime fasi di glaucoma. Irradiazione ad alte dosi &egrave; stato segnalato per avere un significativo effetto protettivo a lungo termine nel modello del mouse DBA/2J (D2) di glaucoma, anche se le basi cellulari e molecolari per questo effetto rimane poco chiara. In particolare, non sono stati valutati gli effetti acuti delle radiazioni sulle popolazioni cellulari specifici, tra cui le cellule non-neuronali, in D2 retina e nervo. Qui riportiamo che quello irradiazione induce riduzione transitoria in microglia proliferare all&#39;interno della testa del nervo ottico e gliale lamina entro il primo post-irradiation della settimana. Questo &egrave; stato accompagnato da microglia ridotta attivazione, senza alcun effetto sulla gliosi di astrociti in quelle regioni. Nelle fasi successive confermiamo che primo irradiazione ad alte dosi dei mouse testa risultati in miglioramento assonale integrit&agrave; strutturale e la funzione di trasporto anterograda, senza riduzione della pressione intraoculare. Cos&igrave; microgliali ridotta attivazione indotta dall&#39;irradiazione a stadi precoci &egrave; associato a ridotta nervo ottico e della retina neurodegenerazione nel modello del mouse D2 di glaucoma.

Precoce riduzione dell&#39;attivazione della microglia di irradiazione in un modello di glaucoma cronico.

Microglia serve key homeostatic roles, and respond to neuronal perturbation and decline with high spatiotemporal resolution. The course of all chronic CNS pathologies is thus paralleled by local microgliosis and microglia activation beginning at early stages. However, the possibility of using live monitoring of microglia during early disease progression to predict the severity of neurodegeneration has not been unexplored. Since the retina allows live tracking of fluorescent microglia in their intact niche, here we investigated their early changes in relation to later optic nerve neurodegeneration. Thus, we used the DBA/2J mouse model of inherited glaucoma, which develops progressive retinal ganglion cell degeneration of variable severity during aging, and thus represents a useful model to study pathogenic mechanisms of retinal ganglion cell decline similar to human glaucoma. We imaged CX3CR1(+/GFP) microglial cells in vivo at ages ranging from 1 to 5 months by confocal scanning laser ophthalmoscopy (cSLO) and quantified cell density and morphological activation. We detected early microgliosis at the optic nerve head (ONH), where axonopathy first manifests, and could track attenuation of this microgliosis induced by minocycline. We also observed heterogeneous and dynamic patterns of early microglia activation in the retina. When the same animals were aged and analyzed for the severity of optic nerve pathology at 10 months of age, we found a strong correlation with the levels of ONH microgliosis at 3 to 4 months. Our findings indicate that live imaging and monitoring the time course and levels of early retinal microgliosis and microglia activation in glaucoma could serve as indicators of future neurodegeneration severity.

Article	Year
Cell death in the inner nuclear layer of the retina is modulated by BDNF. Brain research. Developmental brain research\| PubMed ID: 12480149	2002
Gap junctions mediate bystander cell death in developing retina. The Journal of neuroscience : the official journal of the Society for Neuroscience\| PubMed ID: 12878681	2003
A developmental switch in the expression of aquaporin-4 and Kir4.1 from horizontal to Müller cells in mouse retina. Investigative ophthalmology & visual science\| PubMed ID: 16186376	2005
The gap junction protein connexin32 interacts with the Src homology 3/hook domain of discs large homolog 1. The Journal of biological chemistry\| PubMed ID: 17284442	2007
Reduced retina microglial activation and improved optic nerve integrity with minocycline treatment in the DBA/2J mouse model of glaucoma. Investigative ophthalmology & visual science\| PubMed ID: 18385061	2008
Early microglia activation in a mouse model of chronic glaucoma. The Journal of comparative neurology\| PubMed ID: 21246546	2011
Early reduction of microglia activation by irradiation in a model of chronic glaucoma. PloS one\| PubMed ID: 22952717	2012
Neurodegeneration severity is anticipated by early microglia alterations monitored in vivo in a mouse model of chronic glaucoma. Disease models & mechanisms\| PubMed ID: 25755083	2015

Alejandra Bosco

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