Rods contribute to the light-induced phase shift of the retinal clock in mammals.

While rods, cones, and intrinsically photosensitive melanopsin-containing ganglion cells (ipRGCs) all drive light entrainment of the master circadian pacemaker of the suprachiasmatic nucleus, recent studies have proposed that entrainment of the mouse retinal clock is exclusively mediated by a UV-sensitive photopigment, neuropsin (OPN5). Here, we report that the retinal circadian clock can be phase shifted by short duration and relatively low-irradiance monochromatic light in the visible part of the spectrum, up to 520 nm. Phase shifts exhibit a classical photon dose-response curve. Comparing the response of mouse models that specifically lack middle-wavelength (MW) cones, melanopsin, and/or rods, we found that only the absence of rods prevented light-induced phase shifts of the retinal clock, whereas light-induced phase shifts of locomotor activity are normal. In a "rod-only" mouse model, phase shifting response of the retinal clock to light is conserved. At shorter UV wavelengths, our results also reveal additional recruitment of short-wavelength (SW) cones and/or OPN5. These findings suggest a primary role of rod photoreceptors in the light response of the retinal clock in mammals.

Tau-Driven Neuronal and Neurotrophic Dysfunction in a Mouse Model of Early Tauopathy.

Tauopathies are neurodegenerative diseases characterized by intraneuronal inclusions of hyperphosphorylated tau protein and abnormal expression of brain-derived neurotrophic factor (BDNF), a key modulator of neuronal survival and function. The severity of both these pathological hallmarks correlate with the degree of cognitive impairment in patients. However, how tau pathology specifically modifies BDNF signaling and affects neuronal function during early prodromal stages of tauopathy remains unclear. Here, we report that the mild tauopathy developing in retinal ganglion cells (RGCs) of the P301S tau transgenic (P301S) mouse induces functional retinal changes by disrupting BDNF signaling via the TrkB receptor. In adult P301S mice, the physiological visual response of RGCs to pattern light stimuli and retinal acuity decline significantly. As a consequence, the activity-dependent secretion of BDNF in the vitreous is impaired in P301S mice. Further, in P301S retinas, TrkB receptors are selectively upregulated, but uncoupled from downstream extracellular signal-regulated kinase (ERK) 1/2 signaling. We also show that the impairment of TrkB signaling is triggered by tau pathology and mediates the tau-induced dysfunction of visual response. Overall our results identify a neurotrophin-mediated mechanism by which tau induces neuronal dysfunction during prodromal stages of tauopathy and define tau-driven pathophysiological changes of potential value to support early diagnosis and informed therapeutic decisions. SIGNIFICANCE STATEMENT: This work highlights the potential molecular mechanisms by which initial tauopathy induces neuronal dysfunction. Combining clinically used electrophysiological techniques (i.e., electroretinography) and molecular analyses, this work shows that in a relevant model of early tauopathy, the retina of the P301S mutant human tau transgenic mouse, mild tau pathology results in functional changes of neuronal activity, likely due to selective impairment of brain-derived neurotrophic factor signaling via its receptor, TrkB. These findings may have important translational implications for early diagnosis in a subset of Alzheimer's disease patients with early visual symptoms and emphasize the need to clarify the pathophysiological changes associated with distinct tauopathy stages to support informed therapeutic decisions and guide drug discovery.

Lamin B1 overexpression increases nuclear rigidity in autosomal dominant leukodystrophy fibroblasts.

The architecture and structural mechanics of the cell nucleus are defined by the nuclear lamina, which is formed by A- and B-type lamins. Recently, gene duplication and protein overexpression of lamin B1 (LB1) have been reported in pedigrees with autosomal dominant leukodystrophy (ADLD). However, how the overexpression of LB1 affects nuclear mechanics and function and how it may result in pathology remain unexplored. Here, we report that in primary human skin fibroblasts derived from ADLD patients, LB1, but not other lamins, is overexpressed at the nuclear lamina and specifically enhances nuclear stiffness. Transient transfection of LB1 in HEK293 and neuronal N2a cells mimics the mechanical phenotype of ADLD nuclei. Notably, in ADLD fibroblasts, reducing LB1 protein levels by shRNA knockdown restores elasticity values to those indistinguishable from control fibroblasts. Moreover, isolated nuclei from ADLD fibroblasts display a reduced nuclear ion channel open probability on voltage-step application, suggesting that biophysical changes induced by LB1 overexpression may alter nuclear signaling cascades in somatic cells. Overall, the overexpression of LB1 in ADLD cells alters nuclear mechanics and is linked to changes in nuclear signaling, which could help explain the pathogenesis of this disease.

Enhanced Robustness of the Mouse Retinal Circadian Clock Upon Inherited Retina Degeneration.

Daily biological rhythms are fundamental to retinal physiology and visual function. They are generated by a local circadian clock composed of a network of cell type/layer-specific, coupled oscillators. Animal models of retinal degeneration have been instrumental in characterizing the anatomical organization of the retinal clock. However, it is still unclear, among the multiple cell-types composing the retina, which ones are essential for proper circadian function. In this study, we used a previously well-characterized mouse model for autosomal dominant retinitis pigmentosa to examine the relationship between rod degeneration and the retinal circadian clock. This model carries the P23H mutation in rhodopsin, which induces mild rod degeneration in heterozygous and rapid loss of photoreceptors in homozygous genotypes. By measuring PER2::LUC bioluminescence rhythms, we show that the retinal clock in P23H/+ heterozygous mice displays circadian rhythms with significantly increased robustness and amplitude. By treating retinal explants with L-α aminoadipic acid, we further provide evidence that this enhanced rhythmicity might involve activation of Müller glial cells.

Rev-Erbα and Photoreceptor Outer Segments modulate the Circadian Clock in Retinal Pigment Epithelial Cells.

Retinal photoreceptor outer segments (POS) are renewed daily through phagocytosis by the adjacent retinal pigment epithelial (RPE) monolayer. Phagocytosis is mainly driven by the RPE circadian clock but the underlying molecular mechanisms remain elusive. Using ARPE-19 (human RPE cell-line) dispersed and monolayer cell cultures, we investigated the influence of cellular organization on the RPE clock and phagocytosis genes. PCR analysis revealed rhythmic expression of clock and phagocytosis genes in all ARPE-19 cultures. Monolayers had a tendency for higher amplitudes of clock gene oscillations. In all conditions ARNTL, CRY1, PER1-2, REV-ERBα, ITGB5, LAMP1 and PROS1 were rhythmically expressed with REV-ERBα being among the clock genes whose expression showed most robust rhythms in ARPE-19 cells. Using RPE-choroid explant preparations of the mPer2Luc knock-in mice we found that Rev-Erbα deficiency induced significantly longer periods and earlier phases of PER2-bioluminescence oscillations. Furthermore, early phagocytosis factors ß5-Integrin and FAK and the lysosomal marker LAMP1 protein levels are rhythmic. Finally, POS incubation affects clock and clock-controlled phagocytosis gene expression in RPE monolayers in a time-dependent manner suggesting that POS can reset the RPE clock. These results shed some light on the complex interplay between POS, the RPE clock and clock-controlled phagocytosis machinery which is modulated by Rev-Erbα.

Resting microglia react to Aß42 fibrils but do not detect oligomers or oligomer-induced neuronal damage.

In Alzheimer's disease (AD), amyloid-ß (Aß) deposits accumulate in the brain parenchyma and contain fibrils of aggregated heterogeneous Aß peptides. In addition to fibrils, Aß aggregates into stable soluble species (termed Aß oligomers), which are increasingly viewed as the key drivers of early neurodegenerative events in AD. Aß aggregates stimulate microglia recruitment and activation. In the AD brain, microglia surround Aß deposits, activate, and abnormally produce inflammatory mediators, contributing to AD pathogenesis. However, it remains unclear to which of the conformationally diverse Aß species microglia specifically react. Here, we explore the "sensor" capability of murine microglia. We examine whether they can detect and discriminate the toxic Aß oligomers, Aß fibrils, and Aß-induced neuronal damage and investigate whether they are activated by diverse human Aß species cell autonomously or through neuron-derived factors. We find that, on aggregation in vitro, Aß42 peptides form stable oligomers and fibrils, which are neurotoxic and trigger dendritic spine loss in mature primary mouse hippocampal neurons. Further, in resting primary murine microglia, Aß42 fibrils induce a pattern of expression of inflammatory genes typical of the classical inflammatory response induced by infectious agents (e.g., the bacterial toxin lipopolysaccharide). Conversely, Aß42 oligomers never elicit a microglia inflammatory response, whether applied alone, in combination with neuron-derived secreted factors, or in contact with neurons. Thus, microglia strongly react to Aß42 fibrils, but do not sense Aß oligomers or oligomer-induced neuronal damage. This suggests that early neurotoxic species can escape detection by microglia, leading to the chronic unfolding of amyloid pathology in AD.