Cellular localization of the FMRP in rat retina

Lack of FMRP in the retina: Evidence of a retinal specific transcriptomic profile

Fragile X Syndrome (FXS), the most common inherited form of human intellectual disability, is a monogenic neurodevelopmental disorder caused by a loss-of-function mutation of the FMR1 gene. FMR1 is encoding the Fragile X Messenger Ribonucleo Protein (FMRP) an RNA-binding protein that regulates the translation of synaptic proteins. The absence of FMRP expression has many important consequences on synaptic plasticity and function, leading to the FXS clinical phenotype. Over the last decade, a visual neurosensorial phenotype had been described in the FXS patients as well as in the murine model (Fmr1-/ymice), characterized by retinal deficits associated to retinal perception alterations. However, although the transcriptomic profile in the absence of FMRP has been studied in the cerebral part of the central nervous system (CNS), there are no actual data for the retina which is an extension of the CNS. Herein, we investigate the transcriptomic profile of mRNA from whole retinas of Fmr1-/ymice. Interestingly, we found a specific signature of Fmrp absence on retinal mRNA expression with few common genes compared to other brain studies. Gene Ontology on these retinal specific genes demonstrated an enrichment in retinal development genes as well as in synaptic genes. These alterations could be linked to the reported retinal phenotype of the FXS condition. In conclusion, we describe for the first time, retinal-specific transcriptomic changes in the absence of FMRP.

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.

FMRP-related retinal phenotypes: Evidence of glutamate-glutamine metabolic cycle impairment

FMRP, the fragile X mental retardation protein coded by the FMR1 gene, is an RNA-binding protein that assists transport, stabilization and translational regulation of specific synaptic mRNAs. Its expression has been found in multiple cell types of central nervous system (CNS) including glial cells where its involvement in glutamate neurotransmitter homeostasis have been shown. Indeed, glutamate homeostasis deficit has been observed in absence of FMRP in-vivo in cortex and hippocampus structures as well as in vitro on astroglial cell culture. Interestingly, the retina which is an extension of the CNS is presenting electrophysiological alterations in absence of FMRP in both human and murine models suggesting neurotransmitter impairments. Therefore, we investigate the consequences of Fmrp absence on Glutamate-Glutamine cycle in whole retinas and primary retinal Müller cells culture which are the main glial cells of the retina. Using the Fmr1^-/y mice, we have shown in vivo and in vitro that the absence of Fmrp in Müller cells is characterized by loss of Glutamate-Glutamine cycle homeostasis due to a lower Glutamine Synthetase protein expression and activity. The lack of Fmrp in the retina induces a reduced flow of glutamine synthesis. Our data established for the first time in literature a direct link between the lack of Fmrp and neurotransmitter homeostasis in the retina.

New Animal Models for Understanding FMRP Functions and FXS Pathology

Fragile X encompasses a range of genetic conditions, all of which result as a function of changes within the FMR1 gene and abnormal production and/or expression of the FMR1 gene products. Individuals with Fragile X syndrome (FXS), the most common heritable form of intellectual disability, have a full-mutation sequence (>200 CGG repeats) which brings about transcriptional silencing of FMR1 and loss of FMR protein (FMRP). Despite considerable progress in our understanding of FXS, safe, effective, and reliable treatments that either prevent or reduce the severity of the FXS phenotype have not been approved. While current FXS animal models contribute their own unique understanding to the molecular, cellular, physiological, and behavioral deficits associated with FXS, no single animal model is able to fully recreate the FXS phenotype. This review will describe the status and rationale in the development, validation, and utility of three emerging animal model systems for FXS, namely the nonhuman primate (NHP), Mongolian gerbil, and chicken. These developing animal models will provide a sophisticated resource in which the deficits in complex functions of perception, action, and cognition in the human disorder are accurately reflected and aid in the successful translation of novel therapeutics and interventions to the clinic setting.

Expression of SH3 and Multiple Ankyrin Repeat Domains Protein 3 in Mouse Retina

Synapse-associated gene mutations of SH3 and multiple ankyrin repeat domains protein 3 (SHANK3) may lead to autism spectrum disorder (ASD). In some ASD cases, patients may also have vision disorders. However, the effects of SHANK3 in the retina are barely mentioned in the literature. In this study, we used wild-type mice to systematically map the distribution of SHANK3 expression in entire mouse retinas. Using Western blot analysis and immunofluorescence double labeling, we identified a large number of prominent cells expressing high levels of SHANK3 in the inner retina, in particular, the ganglion cell layer (GCL) and inner nucleus layer. The inner plexiform layer and outer nucleus layer were also exhibited positive SHANK3 signals. In the inner layer, GABAergic amacrine cells (ACs) labeled by glutamate decarboxylase were colocalized with SHANK3-positive cells. Dopaminergic ACs (labeled by tyrosine hydroxylase) and cholinergic ACs (labeled by choline acetyltransferase) were also found to contain SHANK3-positive signals. Additionally, most GCs (labeled by Brn3a) were also found to be SHANK3 positive. In the outer retina, bipolar cells (labeled by homeobox protein ChX10) and horizontal cells (labeled by calbindin) were SHANK3 positive. In the outer nucleus layers, the somata of blue cones (labeled by S-opsin) were weekly co-labeled with SHANK3. The inner segments of blue cones and the outer segments of red/green cones (labeled by L/M-opsin) were partially colocalized with SHANK3 and the outer segments of rods (labeled by Rho4D2) were partially SHANK3 positive too. Moreover, SHANK3-positive labeling was also observed in Müller cells (labeled by cellular retinaldehyde-binding protein). These wide expression patterns indicate that SHANK3 may play an important role in the visual signaling pathway.

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.