Compartmental Differences in the Retinal Ganglion Cell Mitochondrial Proteome.

Among neurons, retinal ganglion cells (RGCs) are uniquely sensitive to mitochondrial dysfunction. The RGC is highly polarized, with a somatodendritic compartment in the inner retina and an axonal compartment projecting to targets in the brain. The drastically dissimilar functions of these compartments implies that mitochondria face different bioenergetic and other physiological demands. We hypothesized that compartmental differences in mitochondrial biology would be reflected by disparities in mitochondrial protein composition. Here, we describe a protocol to isolate intact mitochondria separately from mouse RGC somatodendritic and axonal compartments by immunoprecipitating labeled mitochondria from RGC MitoTag mice. Using mass spectrometry, 471 and 357 proteins were identified in RGC somatodendritic and axonal mitochondrial immunoprecipitates, respectively. We identified 10 mitochondrial proteins exclusively in the somatodendritic compartment and 19 enriched ≥2-fold there, while 3 proteins were exclusively identified and 18 enriched in the axonal compartment. Our observation of compartment-specific enrichment of mitochondrial proteins was validated through immunofluorescence analysis of the localization and relative abundance of superoxide dismutase ( SOD2 ), sideroflexin-3 ( SFXN3 ) and trifunctional enzyme subunit alpha ( HADHA ) in retina and optic nerve specimens. The identified compartmental differences in RGC mitochondrial composition may provide promising leads for uncovering physiologically relevant pathways amenable to therapeutic intervention for optic neuropathies.

Continuous Hypoxia Reduces Retinal Ganglion Cell Degeneration in a Mouse Model of Mitochondrial Optic Neuropathy.

Purpose: To test whether continuous hypoxia is neuroprotective to retinal ganglion cells (RGCs) in a mouse model of mitochondrial optic neuropathy. Methods: RGC degeneration was assessed in genetically modified mice in which the floxed gene for the complex I subunit NDUFS4 is deleted from RGCs using Vlgut2-driven Cre recombinase. Beginning at postnatal day 25 (P25), Vglut2-Cre;ndufs4loxP/loxP mice and control littermates were housed under hypoxia (11% oxygen) or kept under normoxia (21% oxygen). Survival of RGC somas and axons was assessed at P60 and P90 via histological analysis of retinal flatmounts and optic nerve cross-sections, respectively. Retinal tissue was also assessed for gliosis and neuroinflammation using western blot and immunofluorescence. Results: Consistent with our previous characterization of this model, at least one-third of RGCs had degenerated by P60 in Vglut2-Cre;ndufs4loxP/loxP mice remaining under normoxia. However, continuous hypoxia resulted in complete rescue of RGC somas and axons at this time point, with normal axonal myelination observed on electron microscopy. Though only partial, hypoxia-mediated rescue of complex I-deficient RGC somas and axons remained significant at P90. Hypoxia prevented reactive gliosis at P60, but the retinal accumulation of Iba1+ mononuclear phagocytic cells was not substantially reduced. Conclusions: Continuous hypoxia achieved dramatic rescue of early RGC degeneration in mice with severe mitochondrial dysfunction. Although complete rescue was not durable to P90, our observations suggest that investigating the mechanisms underlying hypoxia-mediated neuroprotection of RGCs may identify useful therapeutic strategies for optic neuropathies resulting from less profound mitochondrial impairment, such as Leber hereditary optic neuropathy.

Local Accumulation of Axonal Mitochondria in the Optic Nerve Glial Lamina Precedes Myelination.

Mitochondria are essential for neurons and must be optimally distributed along their axon to fulfill local functions. A high density of mitochondria has been observed in retinal ganglion cell (RGC) axons of an unmyelinated region of the optic nerve, called the glial lamina (GL) in mouse (lamina cribrosa in human). In glaucoma, the world's leading cause of irreversible blindness, the GL is the epicenter of RGC degeneration and is connected to mitochondrial dysfunction. It is generally accepted that the local accumulation of mitochondria in the GL is established due to the higher energy requirement of unmyelinated axons. Here we revisit the connection between mitochondrial positioning and myelin in RGC axons. We show that the high density of mitochondria in the GL is restricted to larger axons and is established before myelination. Thus, contrary to a longstanding belief in the field, the myelination pattern is not responsible for the establishment of the local accumulation of mitochondria in GL axons. Our findings open new research avenues likely critical to understanding the pathophysiology of glaucoma.

Initiation of HIV neutralizing B cell lineages with sequential envelope immunizations.

A strategy for HIV-1 vaccine development is to define envelope (Env) evolution of broadly neutralizing antibodies (bnAbs) in infection and to recreate those events by vaccination. Here, we report host tolerance mechanisms that limit the development of CD4-binding site (CD4bs), HCDR3-binder bnAbs via sequential HIV-1 Env vaccination. Vaccine-induced macaque CD4bs antibodies neutralize 7% of HIV-1 strains, recognize open Env trimers, and accumulate relatively modest somatic mutations. In naive CD4bs, unmutated common ancestor knock-in mice Env+B cell clones develop anergy and partial deletion at the transitional to mature B cell stage, but become Env- upon receptor editing. In comparison with repetitive Env immunizations, sequential Env administration rescue anergic Env+ (non-edited) precursor B cells. Thus, stepwise immunization initiates CD4bs-bnAb responses, but immune tolerance mechanisms restrict their development, suggesting that sequential immunogen-based vaccine regimens will likely need to incorporate strategies to expand bnAb precursor pools.

Method for single illumination source combined optical coherence tomography and fluorescence imaging of fluorescently labeled ocular structures in transgenic mice.

In vivo imaging permits longitudinal study of ocular disease processes in the same animal over time. Two different in vivo optical imaging modalities - optical coherence tomography (OCT) and fluorescence - provide important structural and cellular data respectively about disease processes. In this Methods in Eye Research article, we describe and demonstrate the combination of these two modalities producing a truly simultaneous OCT and fluorescence imaging system for imaging of fluorescently labeled animal models. This system uses only a single light source to illuminate both modalities, and both share the same field of view. This allows simultaneous acquisition of OCT and fluorescence images, and the benefits of both techniques are realized without incurring increased costs in variability, light exposure, time, and post-processing effort as would occur when the modalities are used separately. We then utilized this system to demonstrate multi-modal imaging in a progression of samples exhibiting both fluorescence and OCT scattering beginning with resolution targets, ex vivo thy1-YFP labeled neurons in mouse eyes, and finally an in vivo longitudinal time course of GFP labeled myeloid cells in a mouse model of ocular allergy.

SIRT3 regulates progression and development of diseases of aging.

The mitochondrial sirtuin SIRT3 is a protein deacylase that influences almost every major aspect of mitochondrial biology, including nutrient oxidation, ATP generation, reactive oxygen species (ROS) detoxification, mitochondrial dynamics, and the mitochondrial unfolded protein response (UPR). Interestingly, mice lacking SIRT3 (SIRT3KO), either spontaneously or when crossed with mouse models of disease, develop several diseases of aging at an accelerated pace, such as cancer, metabolic syndrome, cardiovascular disease, and neurodegenerative diseases, and, thus, might be a valuable model of accelerated aging. In this review, we discuss functions of SIRT3 in pathways involved in diseases of aging and how the lack of SIRT3 might accelerate the aging process. We also suggest that further studies on SIRT3 will help uncover important new pathways driving the aging process.

Visual impairment in an optineurin mouse model of primary open-angle glaucoma.

Primary open-angle glaucoma (POAG) is characterized by progressive neurodegeneration of retinal ganglion cells (RGCs). Why RGCs degenerate in low-pressure POAG remains poorly understood. To gain mechanistic insights, we developed a novel mouse model based on a mutation in human optineurin associated with hereditary, low-pressure POAG. This mouse improves the design and phenotype of currently available optineurin mice, which showed high global overexpression. Although both 18-month-old optineurin and nontransgenic control mice showed an age-related decrease in healthy axons and RGCs, the expression of mutant optineurin enhanced axonal degeneration and decreased RGC survival. Mouse visual function was determined using visual evoked potentials, which revealed specific visual impairment in contrast sensitivity. The E50K optineurin transgenic mouse described here exhibited clinical features of POAG and may be useful for mechanistic dissection of POAG and therapeutic development.

A role for repressive histone methylation in cocaine-induced vulnerability to stress.

Substance abuse increases an individual's vulnerability to stress-related illnesses, which is presumably mediated by drug-induced neural adaptations that alter subsequent responses to stress. Here, we identify repressive histone methylation in nucleus accumbens (NAc), an important brain reward region, as a key mechanism linking cocaine exposure to increased stress vulnerability. Repeated cocaine administration prior to subchronic social defeat stress potentiated depressive-like behaviors in mice through decreased levels of histone H3 lysine 9 dimethylation in NAc. Cre-mediated reduction of the histone methyltransferase, G9a, in NAc promoted increased susceptibility to social stress, similar to that observed with repeated cocaine. Conversely, G9a overexpression in NAc after repeated cocaine protected mice from the consequences of subsequent stress. This resilience was mediated, in part, through repression of BDNF-TrkB-CREB signaling, which was induced after repeated cocaine or stress. Identifying such common regulatory mechanisms may aid in the development of new therapies for addiction and depression.