Biology subjects

Van Hook, M. J.

Publications and source records attributed to Van Hook, M. J..

7 recordsLinked to original sources

bioRxiv2025

Compensatory responses to glaucoma pathology in the dorsolateral geniculate nucleus

Glaucoma disrupts the conveyance of retinal signals to visual regions of the brain such as the dorsolateral geniculate nucleus (dLGN) due to degeneration of retinal ganglion cells (RGCs) and their axons. Although plasticity during development allows altered visual experience to modulate dLGN synapses and excitability, evidence for experience-dependent dLGN plasticity in adults is limited. However, glaucoma might trigger compensatory plasticity in adult dLGN, thereby compensating for diminished RGC synaptic drive. Here, we tested this using aged (11-15 month-old) DBA/2J mice, which develop high intraocular pressure and glaucoma. In brain slice recordings, we found that diminished RGC inputs could drive robust action potential firing in dLGN relay neurons that was comparable to controls. This was accompanied by increased intrinsic excitability and decreased magnitude of sustained inhibitory currents from GABA spillover to extrasynaptic receptors. These results implicate multiple cellular and synaptic mechanisms that support signaling despite the diminished RGC inputs in glaucoma. HIGHLIGHTSO_LIRetinogeniculate synapse strength in the dLGN is diminished in the DBA/2J mouse model of glaucoma C_LIO_LIDespite a loss of synaptic strength, retinogeniculate synapses drive robust action potential firing in dLGN relay neurons, suggestive of homeostatic compensation C_LIO_LIdLGN relay neurons from DBA/2J mice have an increase in intrinsic excitability, supporting action potential generation C_LIO_LIReduced extrasynaptic sustained inhibition in DBA/2J dLGN relay neurons can also support synaptically driven action potential firing C_LI

neuroscience↗

bioRxiv2024

Microglia respond to elevated intraocular pressure and synapse loss in the visual thalamus in a mouse model of glaucoma.

Microglia are resident immune cells of the central nervous system and mediate a broad array of adaptations and responses during disease, injury, and development. Typically, microglia morphology is understood to provide a window into their functional state. However, it is apparent that they have the capacity to adopt a broad spectrum of functional phenotypes characterized by numerous morphological profiles and associated gene expression profiles. Glaucoma, which leads to blindness from retinal ganglion cell (RGC) degeneration, is commonly associated with elevated intraocular pressure and has been shown to trigger microglia responses within the retinal layers, at the optic nerve head, and in retinal projection targets in the brain. The goal of this study was to determine the relationship of microglia morphology to intraocular pressure and the loss of retinal ganglion cell output synapses in the dorsolateral geniculate nucleus (dLGN), a RGC projection target in the thalamus that conveys information to the primary visual cortex. We accomplished this by analyzing dLGN microglia morphologies in histological sections from DBA/2J mice, which develop a form of inherited glaucoma. Microglia morphology was analyzed using skeletonized Iba1-fluorescence images and fractal analyses of individually reconstructed microglia cells. We found that microglia adopted more simplified morphologies, characterized by fewer endpoints and less total process length per microglia cell. There was an age-dependent shift in microglia morphology in tissue from control mice (DBA/2JGpnmb+) that was accelerated in DBA/2J mice. Microglia morphological measurements correlated with cumulative intraocular pressure, immunofluorescence labeling for the complement protein C1q, and density of vGlut2-labeled RGC axon terminals. Additionally, fractal analysis revealed a clear distinction between control and glaucoma dLGN, with microglia from ocular hypertensive DBA/2J dLGN tissue showing an elongated rod-like morphology. RNA-sequencing of dLGN tissue samples showed an upregulation of immune system-related gene expression and several specific genes associated with microglia activation and potential neuroprotective functions. These results suggest that microglia in the dLGN alter their physiology to respond to RGC degeneration in glaucoma, potentially contributing to CNS adaptations to neurodegenerative vision loss.

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bioRxiv2024

Amyloid Beta Pathology Accelerates Alterations in the Visual Pathway of the 5xFAD Mouse

Alzheimers disease (AD) is a neurodegenerative disorder characterized by the formation of amyloid beta plaques and neurofibrillary tangles that leads to decreased quality of life due to behavioral, motor, and cognitive impairments. Due to the widespread pathological nature of AD, many brain regions are affected by amyloid beta plaques including regions important for vision such as the lateral geniculate nucleus (LGN) of the thalamus which is critical for relaying signals from the retina to the primary visual cortex. Using a wide range of techniques including electrophysiological approaches, in vivo and ex vivo imaging methods, and immunohistochemistry in a mouse model with progressing amyloidosis (5xFAD), the goal of this study was to determine whether AD-like pathology disrupts neuronal and synaptic structure and function in the visual system. In vivo electroretinogram recordings revealed photoreceptor dysfunction in the 6- and 9-month-old 5xFAD mice, while optical coherence tomography indicated no changes in retinal thickness. In the dorsolateral geniculate nucleus (dLGN), the rodent homolog of the primate LGN, we identified decreased densities of retinal ganglion cell axon terminals and fewer thalamocortical (TC) neuron cell bodies. No detectable deficits in excitatory synaptic function or TC neuron dendritic structure were seen in the dLGN, and reflexive visual behavior was also found to be normal in the 5xFAD mice. These results indicate relatively modest amyloid-triggered dysfunction in these stages of the visual system suggesting that amyloid beta plaque formation may play only a small role in the visual system dysfunction seen in AD patients. These results may also point to potential compensatory mechanisms that preserve function of visual pathways in the 5xFAD visual system.

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bioRxiv2024

Nonuniform scaling of synaptic inhibition in the dorsolateral geniculate nucleus in a mouse model of glaucoma.

Elevated intraocular pressure (IOP) triggers glaucoma by damaging the output neurons of the retina called retinal ganglion cells (RGCs). This leads to the loss of RGC signaling to visual centers of the brain such as the dorsolateral geniculate nucleus (dLGN), which is critical for processing and relaying information to the cortex for conscious vision. In response to altered levels of activity or synaptic input, neurons can homeostatically modulate postsynaptic neurotransmitter receptor numbers, allowing them to scale their synaptic responses to stabilize spike output. While prior work has indicated unaltered glutamate receptor properties in the glaucomatous dLGN, it is unknown whether glaucoma impacts dLGN inhibition. Here, using DBA/2J mice, which develop elevated IOP beginning at 6-7 months of age, we tested whether the strength of inhibitory synapses on dLGN thalamocortical relay neurons is altered in response to the disease state. We found an enhancement of feed-forward disynaptic inhibition arising from local interneurons along with increased amplitude of quantal inhibitory synaptic currents. A combination of immunofluorescence staining for the GABAA-1 receptor subunit, peak-scaled nonstationary fluctuation analysis, and measures of homeostatic synaptic scaling indicated this was the result of an approximately 1.4-fold increase in GABA receptor number at post-synaptic inhibitory synapses, although several pieces of evidence strongly indicate a non-uniform scaling across inhibitory synapses within individual relay neurons. Together, these results indicate an increase in inhibitory synaptic strength in the glaucomatous dLGN, potentially pointing toward homeostatic compensation for disruptions in network and neuronal function triggered by increased IOP. Significance StatementElevated eye pressure in glaucoma leads to loss of retinal outputs to the dorsolateral geniculate nucleus (dLGN), which is critical for relaying information to the cortex for conscious vision. Alterations in neuronal activity, as could arise from excitatory synapse loss, can trigger homeostatic adaptations to synaptic function that attempt to maintain activity within a meaningful dynamic range, although whether this occurs uniformly at all synapses within a given neuron or is a non-uniform process is debated. Here, using a mouse model of glaucoma, we show that dLGN inhibitory synapses undergo non-uniform upregulation due to addition of post-synaptic GABA receptors. This is likely to be a neuronal adaptation to glaucomatous pathology in an important sub-cortical visual center.

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bioRxiv2022

Loss of retinogeniculate synaptic function in the DBA/2J mouse model of glaucoma.

BackgroundRetinal ganglion cell (RGC) axons comprise the optic nerve and carry information to the dorsolateral geniculate nucleus (dLGN) that is relayed to the cortex for conscious vision. Glaucoma is a blinding neurodegenerative disease that commonly results from intraocular pressure (IOP)-associated injury leading to RGC axonal pathology, disruption of RGC outputs to the brain, and eventual apoptotic loss of RGC somata. The consequences of elevated IOP and glaucomatous pathology on RGC signaling to the dLGN are largely unknown and likely to be important contributors to visual system dysfunction in glaucoma. Thus, the goal of this study was to determine how glaucoma affects RGC outputs to the dLGN. MethodsWe used a combination of anatomical and physiological approaches to study the structure and function of retinogeniculate synapses in male and female DBA/2J mice at multiple ages before and after IOP elevation. These included measures of anterograde axonal transport, immunofluorescence staining of RGC axon terminals, patch-clamp recording retinogeniculate (RG) synapses in living brain slices, Sholl analysis of thalamocortical relay neuron dendrites, measurements of RGC somatic density, and treatment with a topical ophthalmic alpha-2 adrenergic agonist (brimonidine). ResultsDBA/2J mice showed progressive loss of anterograde optic tract transport to the dLGN and vGlut2 labeling of RGC axon terminals. Patch-clamp measurements of RG synaptic function showed that the strength of synaptic transmission was lower in 9 and 12-month DBA/2J mice and that this was the result of loss of individual RGC axon contributions. TC neuron dendrites showed a reduction in complexity at 12 months, suggestive of a delayed reorganization following reduced synaptic input. There was no detectable change in RGC soma density in 11-12m DBA/2J retinas indicating that observed effects occurred prior to RGC somatic loss. Finally, treatment with brimonidine eye drops prevented the loss of vGlut2-labeled RGC terminals in the dLGN. ConclusionsThese findings identify glaucoma- and IOP-associated functional deficits in an important subcortical RGC projection target. This sheds light on the processes linking IOP to vision loss and will be critical for informing future diagnostic approaches and vision-restoration therapies.

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bioRxiv2020

Neuronal and synaptic plasticity in the visual thalamus in mouse models of glaucoma.

Homeostatic plasticity plays important roles in regulating synaptic and intrinsic neuronal function to stabilize output following perturbations to circuit activity. In glaucoma, a neurodegenerative disease of the visual system commonly associated with elevated intraocular pressure (IOP), early disease is associated with altered synaptic inputs to retinal ganglion cells (RGCs), changes in RGC intrinsic excitability, and deficits in optic nerve transport and energy metabolism. These early functional changes can precede RGC degeneration and are likely to alter RGC outputs to their target structures in the brain and thereby trigger homeostatic changes in synaptic and neuronal properties in those brain regions. In this study, we sought to determine whether and how neuronal and synaptic function is altered in the dorsal lateral geniculate nucleus (dLGN), an important RGC projection target in the thalamus, and how functional changes relate to IOP. We accomplished this using patch-clamp recordings from thalamocortical (TC) relay neurons in the dLGN in two established mouse models of glaucoma - the DBA/2J (D2) genetic mouse model and an inducible glaucoma model with intracameral microbead injections to elevate IOP. We found that the intrinsic excitability of TC neurons was enhanced in D2 mice and these functional changes were mirrored in recordings of TC neurons from microbead-injected mice. Notably, many neuronal properties were correlated with IOP in older D2 mice, but not younger D2 mice or microbead-injected mice. The frequency of miniature excitatory synaptic currents (mEPSCs) was reduced in both ages of D2 mice, and vGlut2 staining of RGC synaptic terminals was reduced in an IOP-dependent manner in older D2 mice. Among D2 mice, functional changes observed in younger mice without elevated IOP were distinct from those observed in older mice with elevated IOP and RGC degeneration, suggesting that glaucoma-associated changes to neurons in the dLGN might represent a combination of stabilizing/homeostatic plasticity at earlier stages and pathological dysfunction at later stages.

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bioRxiv2019

Early-Stage Ocular Hypertension Alters Retinal Ganglion Cell Synaptic Transmission in the Visual Thalamus

Axonopathy is a hallmark of many neurodegenerative diseases including glaucoma, where elevated intraocular pressure (ocular hypertension, OHT) stresses retinal ganglion cell (RGC) axons as they exit the eye and form the optic nerve. OHT causes early changes in the optic nerve such as axon atrophy, transport inhibition, and gliosis. Importantly, many of these changes appear to occur prior to irreversible neuronal loss, making them promising points for early diagnosis of glaucoma. It is unknown whether OHT has similarly early effects on the function of RGC output to the brain. To test this possibility, we elevated eye pressure in mice by anterior chamber injection of polystyrene microbeads. 5 weeks post-injection, bead-injected eyes showed a modest RGC loss in the peripheral retina, as evidenced by RBPMS antibody staining. Additionally, we observed reduced dendritic complexity and lower spontaneous spike rate of On-RGCs, targeted for patch clamp recording and dye filling using a Opn4-cre reporter mouse line. To determine the influence of OHT on retinal projections to the brain, we expressed Channelrhodopsin-2 (ChR2) in melanopsin-expressing retinal ganglion cells by crossing the Opn4-cre mouse line with a ChR2-reporter mouse line and recorded post-synaptic responses in thalamocortical relay neurons in the dorsal lateral geniculate nucleus (dLGN) of the thalamus evoked by stimulation with 460 nm light. The use of a Opn4-cre reporter system allowed for expression of ChR2 in a narrow subset of RGCs responsible for image-forming vision in mice. Five weeks following OHT induction, paired pulse and high-frequency stimulus train experiments revealed that presynaptic vesicle release probability at retinogeniculate synapses was elevated. Additionally, miniature synaptic current frequency was slightly reduced in brain slices from OHT mice and proximal dendrites of post-synaptic dLGN relay neurons, assessed using a Sholl analysis, showed a reduced complexity. Strikingly, these changes occurred prior to major loss of RGCs labeled with the Opn4-Cre mouse, as indicated by immunofluorescence staining of ChR2-expressing retinal neurons. Thus, OHT leads to pre- and post-synaptic functional and structural changes at retinogeniculate synapses. Along with RGC dendritic remodeling and optic nerve transport changes, these retinogeniculate synaptic changes are among the earliest signs of glaucoma.

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