Bidirectional regulation of fragile X mental retardation protein phosphorylation controls rhodopsin homoeostasis

FMR protein: Evidence of an emerging role in retinal aging?

Aging is a multifactorial process that affects the entire organism by cumulative alterations. Visual function impairments that go along with aging are commonly observed, causing lower visual acuity, lower contrast sensitivity, and impaired dark adaptation. Electroretinogram analysis revealed that the amplitudes of rod- and cone-mediated responses are reduced in aged mice and humans. Reports suggested that age-related changes observed in both rod and cone photoreceptor functionality were linked to oxidative stress regulation or free radical production homeostasis. Interestingly, several recent reports linked the fragile X mental retardation protein (FMRP) cellular activity with oxidative stress regulation in several tissue including brain tissue where FMRP participates to the response to stress via protein translation in neurite or is involved in free radical production and abnormal glutathione homeostasis. Based on these recent literatures, we raised the question about the effect of FMRP absence in the aging retina of Fmr1^-/y compared to their WT littermates. Indeed, up to now, only young or adult mice (<6 months) were investigated and have shown a specific retinal phenotype. Herein, we demonstrated that Fmr1^-/y mice do not present the aging effect on retinal function observed in WT littermates since ERG a- and b-waves amplitudes as well as oscillatory potentials amplitudes were not collapsed with age (12/18 months old). Absence of FMRP and its consequences seem to protect the retina against aging effect, rising a pivotal role of FMRP in retinal aging process.

Electroretinography and contrast sensitivity, complementary translational biomarkers of sensory deficits in the visual system of individuals with fragile X syndrome

BACKGROUND

Disturbances in sensory function are an important clinical feature of neurodevelopmental disorders such as fragile X syndrome (FXS). Evidence also directly connects sensory abnormalities with the clinical expression of behavioral impairments in individuals with FXS; thus, positioning sensory function as a potential clinical target for the development of new therapeutics. Using electroretinography (ERG) and contrast sensitivity (CS), we previously reported the presence of sensory deficits in the visual system of the Fmr1-/y genetic mouse model of FXS. The goals of the current study were two-folds: (1) to assess the feasibility of measuring ERG and CS as a biomarker of sensory deficits in individuals with FXS, and (2) to investigate whether the deficits revealed by ERG and CS in Fmr1-/y mice translate to humans with FXS.

METHODS

Both ERG and CS were measured in a cohort of male individuals with FXS (n = 20, 18-45 years) and age-matched healthy controls (n = 20, 18-45 years). Under light-adapted conditions, and using both single flash and flicker (repeated train of flashes) stimulation protocols, retinal function was recorded from individual subjects using a portable, handheld, full-field flash ERG device (RETeval®, LKC Technologies Inc., Gaithersburg, MD, USA). CS was assessed in each subject using the LEA SYMBOLS® low-contrast test (Good-Lite, Elgin, IL, USA).

RESULTS

Data recording was successfully completed for ERG and assessment of CS in most individuals from both cohorts demonstrating the feasibility of these methods for use in the FXS population. Similar to previously reported findings from the Fmr1-/y genetic mouse model, individuals with FXS were found to exhibit reduced b-wave and flicker amplitude in ERG and an impaired ability to discriminate contrasts compared to healthy controls.

CONCLUSIONS

This study demonstrates the feasibility of using ERG and CS for assessing visual deficits in FXS and establishes the translational validity of the Fmr1-/y mice phenotype to individuals with FXS. By including electrophysiological and functional readouts, the results of this study suggest the utility of both ERG and CS (ERG-CS) as complementary translational biomarkers for characterizing sensory abnormalities found in FXS, with potential applications to the clinical development of novel therapeutics that target sensory function abnormalities to treat core symptomatology in FXS.

TRIAL REGISTRATION

ID-RCB number 2019-A01015-52 registered on the 17 May 2019.

I-KCKT allows dissection-free RNA profiling of adult Drosophila intestinal progenitor cells

The adult Drosophila intestinal epithelium is a model system for stem cell biology, but its utility is limited by current biochemical methods that lack cell type resolution. Here, we describe a new proximity-based profiling method that relies upon a GAL4 driver, termed intestinal-kickout-GAL4 (I-KCKT-GAL4), that is exclusively expressed in intestinal progenitor cells. This method uses UV crosslinked whole animal frozen powder as its starting material to immunoprecipitate the RNA cargoes of transgenic epitope-tagged RNA binding proteins driven by I-KCKT-GAL4 When applied to the general mRNA-binder, poly(A)-binding protein, the RNA profile obtained by this method identifies 98.8% of transcripts found after progenitor cell sorting, and has low background noise despite being derived from whole animal lysate. We also mapped the targets of the more selective RNA binder, Fragile X mental retardation protein (FMRP), using enhanced crosslinking and immunoprecipitation (eCLIP), and report for the first time its binding motif in Drosophila cells. This method will therefore enable the RNA profiling of wild-type and mutant intestinal progenitor cells from intact flies exposed to normal and altered environments, as well as the identification of RNA-protein interactions crucial for stem cell function.

Post-translational modifications of the Fragile X Mental Retardation Protein in neuronal function and dysfunction

The Fragile X Mental Retardation Protein (FMRP) is an RNA-binding protein essential to the regulation of local translation at synapses. In the mammalian brain, synapses are constantly formed and eliminated throughout development to achieve functional neuronal networks. At the molecular level, thousands of proteins cooperate to accomplish efficient neuronal communication. Therefore, synaptic protein levels and their functional interactions need to be tightly regulated. FMRP generally acts as a translational repressor of its mRNA targets. FMRP is the target of several post-translational modifications (PTMs) that dynamically regulate its function. Here we provide an overview of the PTMs controlling the FMRP function and discuss how their spatiotemporal interplay contributes to the physiological regulation of FMRP. Importantly, FMRP loss-of-function leads to Fragile X syndrome (FXS), a rare genetic developmental condition causing a range of neurological alterations including intellectual disability (ID), learning and memory impairments, autistic-like features and seizures. Here, we also explore the possibility that recently reported missense mutations in the FMR1 gene disrupt the PTM homoeostasis of FMRP, thus participating in the aetiology of FXS. This suggests that the pharmacological targeting of PTMs may be a promising strategy to develop innovative therapies for patients carrying such missense mutations.

Cellular localization of the FMRP in rat retina

The fragile X mental retardation protein (FMRP) is a regulator of local translation through its mRNA targets in the neurons. Previous studies have demonstrated that FMRP may function in distinct ways during the development of different visual subcircuits. However, the localization of the FMRP in different types of retinal cells is unclear. In this work, the FMRP expression in rat retina was detected by Western blot and immunofluorescence double labeling. Results showed that the FMRP expression could be detected in rat retina and that the FMRP had a strong immunoreaction (IR) in the ganglion cell (GC) layer, inner nucleus layer (INL), and outer plexiform layer (OPL) of rat retina. In the outer retina, the bipolar cells (BCs) labeled by homeobox protein ChX10 (ChX10) and the horizontal cells (HCs) labeled by calbindin (CB) were FMRP-positive. In the inner retina, GABAergic amacrine cells (ACs) labeled by glutamate decarbonylase colocalized with the FMRP. The dopaminergic ACs (tyrosine hydroxylase marker) and cholinergic ACs (choline acetyltransferase (ChAT) marker) were co-labeled with the FMRP. In most GCs (labeled by Brn3a) and melanopsin-positive intrinsically photosensitive retinal GCs (ipRGCs) were also FMRP-positive. The FMRP expression was observed in the cellular retinal binding protein-positive Müller cells. These results suggest that the FMRP could be involved in the visual pathway transmission.

Visual Behavior Impairments as an Aberrant Sensory Processing in the Mouse Model of Fragile X Syndrome

Fragile X Syndrome (FXS), the most common inherited form of human intellectual disability (ID) associated with autistic-like behaviors, is characterized by dys-sensitivity to sensory stimuli, especially vision. In the absence of Fragile Mental Retardation Protein (FMRP), both retinal and cerebral structures of the visual pathway are impaired, suggesting that perception and integration of visual stimuli are altered. However, behavioral consequences of these defects remain unknown. In this study, we used male Fmr1^−/y mice to further define visual disturbances from a behavioral perspective by focusing on three traits characterizing visual modality: perception of depth, contrasts and movements. We performed specific tests (Optomotor Drum, Visual Cliff) to evaluate these visual modalities, their evolution from youth to adulthood, and to assess their involvement in a cognitive task. We show that Fmr1^−/y mice exhibit alteration in their visual skills, displaying impaired perspective perception, a drop in their ability to understand a moving contrasted pattern, and a defect in contrasts discrimination. Interestingly, Fmr1^−/y phenotypes remain stable over time from adolescence to late adulthood. Besides, we report that color and shape are meaningful for the achievement of a cognitive test involving object recognition. Altogether, these results underline the significance of visual behavior alterations in FXS conditions and relevance of assessing visual skills in neuropsychiatric models before performing behavioral tasks, such as cognitive assessments, that involve visual discrimination.

Metallophosphoesterase regulates light-induced rhodopsin endocytosis by promoting an association between arrestin and the adaptor protein AP2

Light-induced endocytosis of rhodopsin in the retina is critical for preventing photoreceptor hyperactivity and for the survival of photoreceptor cells. In Drosophila, this process is mediated by arrestin1 (Arr1). Because Arr1 lacks a clathrin-binding domain required for receptor internalization and the C-terminal sequence that interacts with the β-subunit of the clathrin adaptor protein AP2, the mechanism of how Arr1 mediates endocytosis of the major rhodopsin Rh1 is unclear. Here, using several approaches, including Arr binding and pulldown assays, immunofluorescence techniques, and EM imaging, we found that Drosophila metallophosphoesterase (dMPPE) is involved in light-induced rhodopsin endocytosis. We observed that the photoreceptor cells of a dmppe mutant exhibit impaired light-induced rhodopsin endocytosis and that this impairment is independent of dMPPE phosphoesterase activity. Furthermore, dMPPE directly interacted with Arr1 and promoted the association of Arr1 with AP2. Of note, genetic dmppe deletion largely prevented retinal degeneration in norpA (encoding phospholipase C) mutants, which were reported previously to contribute to retinal degeneration, by suppressing Rh1 endocytosis. Our findings demonstrate that Arr1 interacts with AP2 and that dMPPE functions as a critical regulator in Rh1 endocytosis and retinal degeneration.

A novel molecular probe for the detection of phosphorylated proteins

The compound ZKY-1 with a D-π-A structure was designed and synthesized in four steps. It undergoes complexation with a tetravalent zirconium ion and the complex can be used for the detection of phosphorylated proteins. The detection limit is estimated to be 100 nmin vitro. The successful detection of phosphorylated proteins in placental tissue samples indicates that this probe holds great potential for clinical diagnosis.

Early Retinal Defects in Fmr1-/y Mice: Toward a Critical Role of Visual Dys-Sensitivity in the Fragile X Syndrome Phenotype?

Fragile X Syndrome (FXS) is caused by a deficiency in Fragile X Mental Retardation Protein (FMRP) leading to global sensorial abnormalities, among which visual defects represent a critical part. These visual defects are associated with cerebral neuron immaturity especially in the primary visual cortex. However, we recently demonstrated that retinas of adult Fmr1^−/y mice, the FXS murine model, present molecular, cellular and functional alterations. However, no data are currently available on the evolution pattern of such defects. As retinal stimulation through Eye Opening (EO) is a crucial signal for the cerebral visual system maturation, we questioned the precocity of molecular and functional retinal phenotype. To answer this question, we studied the retinal molecular phenotype of Fmr1^−/y mice before EO until adult age and the consequences of the retinal loss of Fmrp on retinal function in young and adult mice. We showed that retinal molecular defects are present before EO and remain stable at adult age, leading to electrophysiological impairments without any underlying structural changes. We underlined that loss of Fmrp leads to a wide range of defects in the retina, settled even before EO. Our work demonstrates a critical role of the sensorial dysfunction in the Fmr1^−/y mice overall phenotype, and provides evidence that altered peripheral perception is a component of the sensory processing defect in FXS conditions.

Organic conjugated small molecule materials based optical probe for rapid, colorimetric and UV-vis spectral detection of phosphorylated protein in placental tissue

A novel organic small molecule with D-Pi-A structure was prepared, which was found to be a promising colorimetric and ratiometric UV-vis spetral probe for detection of phosphorylated proteins with the help of tetravalent zirconium ion. Such optical probe based on chromophore WYF-1 shows a rapid response (within 10s) and high selectivity and sensitivity for phosphorylated proteins, giving distinct colorimetric and ratiometric UV-vis changes at 720 and 560nm. The detection limit for phosphorylated proteins was estimated to be 100nM. In addition, detection of phosphorylated proteins in placental tissue samples with this probe was successfully applied, which indicates that this probe holds great potential for phosphorylated proteins detection.