Plasticity-driven gene expression in the rat retina

MEF2C and HDAC5 regulate Egr1 and Arc genes to increase dendritic spine density and complexity in early enriched environment

We investigated the effects of environmental enrichment during critical period of early postnatal life and how it interplays with the epigenome to affect experience-dependent visual cortical plasticity. Mice raised in an EE from birth to during CP have increased spine density and dendritic complexity in the visual cortex. EE upregulates synaptic plasticity genes, Arc and Egr1, and a transcription factor MEF2C. We also observed an increase in MEF2C binding to the promoters of Arc and Egr1. In addition, pups raised in EE show a reduction in HDAC5 and its binding to promoters of Mef2c, Arc and Egr1 genes. With an overexpression of Mef2c, neurite outgrowth increased in complexity. Our results suggest a possible underlying molecular mechanism of EE, acting through MEF2C and HDAC5, which drive Arc and Egr1. This could lead to the observed increased dendritic spine density and complexity induced by early EE.

Environmental enrichment protects the retina from early diabetic damage in adult rats

Diabetic retinopathy is a leading cause of reduced visual acuity and acquired blindness. Available treatments are not completely effective. We analyzed the effect of environmental enrichment on retinal damage induced by experimental diabetes in adult Wistar rats. Diabetes was induced by an intraperitoneal injection of streptozotocin. Three days after vehicle or streptozotocin injection, animals were housed in enriched environment or remained in a standard environment. Retinal function (electroretinogram, and oscillatory potentials), retinal morphology, blood-retinal barrier integrity, synaptophysin, astrocyte and Müller cell glial fibrillary acidic protein, vascular endothelial growth factor, tumor necrosis factor-α, and brain-derived neurotrophic factor levels, as well as lipid peroxidation were assessed in retina from diabetic animals housed in standard or enriched environment. Environmental enrichment preserved scotopic electroretinogram a-wave, b-wave and oscillatory potential amplitude, avoided albumin-Evan's blue leakage, prevented the decrease in retinal synaptophysin and astrocyte glial fibrillary acidic protein levels, the increase in Müller cell glial fibrillary acidic protein, vascular endothelial growth factor and tumor necrosis factor-α levels, as well as oxidative stress induced by diabetes. In addition, enriched environment prevented the decrease in retinal brain-derived neurotrophic factor levels induced by experimental diabetes. When environmental enrichment started 7 weeks after diabetes onset, retinal function was significantly preserved. These results indicate that enriched environment could attenuate the early diabetic damage in the retina from adult rats.

Environmental enrichment promotes neural plasticity and cognitive ability in fish

Different kinds of experience during early life can play a significant role in the development of an animal's behavioural phenotype. In natural contexts, this influences behaviours from anti-predator responses to navigation abilities. By contrast, for animals reared in captive environments, the homogeneous nature of their experience tends to reduce behavioural flexibility. Studies with cage-reared rodents indicate that captivity often compromises neural development and neural plasticity. Such neural and behavioural deficits can be problematic if captive-bred animals are being reared with the intention of releasing them as part of a conservation strategy. Over the last decade, there has been growing interest in the use of environmental enrichment to promote behavioural flexibility in animals that are bred for release. Here, we describe the positive effects of environmental enrichment on neural plasticity and cognition in juvenile Atlantic salmon (Salmo salar). Exposing fish to enriched conditions upregulated the forebrain expression of NeuroD1 mRNA and improved learning ability assessed in a spatial task. The addition of enrichment to the captive environment thus promotes neural and behavioural changes that are likely to promote behavioural flexibility and improve post-release survival.

Have studies of the developmental regulation of behavioral phenotypes revealed the mechanisms of gene-environment interactions?

This review addresses the recent convergence of our long-standing knowledge of the regulation of behavioral phenotypes by developmental experience with recent advances in our understanding of mechanisms regulating gene expression. This review supports a particular perspective on the developmental regulation of behavioral phenotypes: That the role of common developmental experiences (e.g. maternal interactions, peer interactions, exposure to a complex environment, etc.) is to fit individuals to the circumstances of their lives within bounds determined by long-standing (evolutionary) mechanisms that have shaped responses to critical and fundamental types of experience via those aspects of gene structure that regulate gene expression. The phenotype of a given species is not absolute for a given genotype but rather variable within bounds that is determined by mechanisms regulated by experience (e.g. epigenetic mechanisms). This phenotypic variation is not necessarily random, or evenly distributed along a continuum of description or measurement, but often highly disjointed, producing distinct, even opposing, phenotypes. The potentiality for these varying phenotypes is itself the product of evolution, the potential for alternative phenotypes itself conveying evolutionary advantage. Examples of such phenotypic variation, resulting from environmental or experiential influences, have a long history of study in neurobiology, and a number of these will be discussed in this review: neurodevelopmental experiences that produce phenotypic variation in visual perception, cognitive function, and emotional behavior. Although other examples will be discussed, particular emphasis will be made on the role of social behavior on neurodevelopment and phenotypic determination. It will be argued that an important purpose of some aspects of social behavior is regulation of neurobehavioral phenotypes by experience via genetic regulatory mechanisms.

The retina of the PCD/PCD mouse as a model of photoreceptor degeneration. A structural and functional study

In this work, we used the pcd (Purkinje cell degeneration) mutant mouse with a slow temporal progression of photoreceptor degeneration in order to analyze the structural and functional modifications in the neuronal populations of the outer and inner retina. Retinal immunocytochemistry and functional electroretinography were performed on the pcd/pcd mutant mice and control wild type animals of the C57/DBA strain at 45, 90, 180 and 270 post-natal days. Immunohistochemical studies were performed for a series of protein markers: calbindin, calretinin, PKCα, bassoon, synapsin, syntaxin and islet1. Full field electroretinography recordings were performed on control and dystrophic mice. Rod and mixed responses, and oscillatory potentials, were recorded in dark adapted conditions; cone and flicker responses were recorded under light adaptation. Our results show significant structural modifications in the photoreceptor populations and neurons of the inner retina. Changes in cell morphology affect mainly to the bipolar cells, which gradually lose their dendritic tufts. The electroretinography records reveal that in the pcd retinas the rod and cone systems show a reduction in the amplitude of the electrical signals. This decrease progresses slowly with the passage of time, although for the most advanced stage of photoreceptor degeneration considered, 270 post-natal days, it is still possible to record light induced responses. We conclude that pcd mice experience a loss of retinal function in correlation with the loss of photoreceptors with age, and significant changes in retinal synaptic processes.

EXPERIENCE-DEPENDENT IMMEDIATE EARLY GENE EXPRESSION IN THE ADULT CENTRAL NERVOUS SYSTEM: EVIDENCE FROM ENRICHED-ENVIRONMENT STUDIES

Here I discuss evidence from our group's work that implicates the immediate early genes NGFI-A and arc as possible regulators of neuronal plasticity. The enriched environment (EE) paradigm has been demonstrated to induce neural plasticity in both developing and mature mammals. Others and we have recently demonstrated that adult rats placed within an enriched environment underwent central nervous system-wide increases in the expression levels for the IEGs NGFI-A and arc. The relationships between the altered expression profiles for both genes in response to an EE exposure, and their putative role in orchestrating network restructuring in response to enhanced environmental complexity are discussed.

Synapsins are late activity-induced genes regulated by birdsong

The consolidation of long-lasting sensory memories requires the activation of gene expression programs in the brain. Despite considerable knowledge about the early components of this response, little is known about late components (i.e., genes regulated 2-6 h after stimulation) and the relationship between early and late genes. Birdsong represents one of the best natural behaviors to study sensory-induced gene expression in awake, freely behaving animals. Here we show that the expression of several isoforms of synapsins, a group of phosphoproteins thought to regulate the dynamics of synaptic vesicle storage and release, is induced by auditory stimulation with birdsong in the caudomedial nidopallium (NCM) of the zebra finch (Taeniopygia guttata) brain. This induction occurs mainly in excitatory (non-GABAergic) neurons and is modulated (suppressed) by early song-inducible proteins. We also show that ZENK, an early song-inducible transcription factor, interacts with the syn3 promoter in vivo, consistent with a direct regulatory effect and an emerging novel view of ZENK action. These results demonstrate that synapsins are a late component of the genomic response to neuronal activation and that their expression depends on a complex set of regulatory interactions between early and late regulated genes.

Profiling of experience-regulated proteins in the songbird auditory forebrain using quantitative proteomics

Auditory and perceptual processing of songs are required for a number of behaviors in songbirds such as vocal learning, territorial defense, mate selection and individual recognition. These neural processes are accompanied by increased expression of a few transcription factors, particularly in the caudomedial nidopallium (NCM), an auditory forebrain area believed to play a key role in auditory learning and song discrimination. However, these molecular changes are presumably part of a larger, yet uncharacterized, protein regulatory network. In order to gain further insight into this network, we performed two-dimensional differential in-gel expression (2D-DIGE) experiments, extensive protein quantification analyses, and tandem mass spectrometry in the NCM of adult songbirds hearing novel songs. A subset of proteins was selected for immunocytochemistry in NCM sections to confirm the 2D-DIGE findings and to provide additional quantitative and anatomical information. Using these methodologies, we found that stimulation of freely behaving birds with conspecific songs did not significantly impact the NCM proteome 5 min after stimulus onset. However, following 1 and 3 h of stimulation, a significant number of proteins were consistently regulated in NCM. These proteins spanned a range of functional categories that included metabolic enzymes, cytoskeletal molecules, and proteins involved in neurotransmitter secretion and calcium binding. Our findings suggest that auditory processing of vocal communication signals in freely behaving songbirds triggers a cascade of protein regulatory events that are dynamically regulated through activity-dependent changes in calcium levels.

Genetically induced retinal degeneration leads to changes in metabotropic glutamate receptor expression

In the retina, neurotransmission from photoreceptors to ON-cone and rod bipolar cells is sign reversing and mediated by the metabotropic glutamate receptor mGluR6, which converts the light-evoked hyperpolarization of the photoreceptors into depolarization of ON bipolar cells. The Royal College of Surgeons (RCS) rat retina undergoes progressive photoreceptor loss due to a genetic defect in the pigment epithelium cells. The consequences of photoreceptor loss and the concomitant loss of glutamatergic input to second-order retinal neurons on the expression of the metabotropic glutamate receptor was investigated in the RCS rat retina from early stages of photoreceptor degeneration (P17) up to several months after complete rod and cone degeneration (P120). The expression of the gene encoding mGluR6 was studied by in situ hybridization in the retina, using an [(35)S]dATP-labeled oligonucleotide probe. In congenic control and RCS retina, we found mRNA expression of mGluR6 receptor only in the outer half of the inner nuclear layer (INL) on emulsion-coated retinal sections. Quantitative analysis of the hybridization signal obtained from the autoradiographic films revealed decreased expression levels of the mGluR6 mRNA at early stages of photoreceptor degeneration (P17). On the contrary, increased expression levels were observed at late stages of degeneration (P60 and P120) in RCS compared to congenic control retina. In conclusion, our data demonstrate that the metabotropic glutamate receptor-6 mRNA levels are altered in the young and adult RCS rat retina and suggest that the genetically induced degeneration of photoreceptors affects the expression of this receptor by the INL retinal neurons.

Neural remodeling in retinal degeneration

Mammalian retinal degenerations initiated by gene defects in rods, cones or the retinal pigmented epithelium (RPE) often trigger loss of the sensory retina, effectively leaving the neural retina deafferented. The neural retina responds to this challenge by remodeling, first by subtle changes in neuronal structure and later by large-scale reorganization. Retinal degenerations in the mammalian retina generally progress through three phases. Phase 1 initiates with expression of a primary insult, followed by phase 2 photoreceptor death that ablates the sensory retina via initial photoreceptor stress, phenotype deconstruction, irreversible stress and cell death, including bystander effects or loss of trophic support. The loss of cones heralds phase 3: a protracted period of global remodeling of the remnant neural retina. Remodeling resembles the responses of many CNS assemblies to deafferentation or trauma, and includes neuronal cell death, neuronal and glial migration, elaboration of new neurites and synapses, rewiring of retinal circuits, glial hypertrophy and the evolution of a fibrotic glial seal that isolates the remnant neural retina from the surviving RPE and choroid. In early phase 2, stressed photoreceptors sprout anomalous neurites that often reach the inner plexiform and ganglion cell layers. As death of rods and cones progresses, bipolar and horizontal cells are deafferented and retract most of their dendrites. Horizontal cells develop anomalous axonal processes and dendritic stalks that enter the inner plexiform layer. Dendrite truncation in rod bipolar cells is accompanied by revision of their macromolecular phenotype, including the loss of functioning mGluR6 transduction. After ablation of the sensory retina, Müller cells increase intermediate filament synthesis, forming a dense fibrotic layer in the remnant subretinal space. This layer invests the remnant retina and seals it from access via the choroidal route. Evidence of bipolar cell death begins in phase 1 or 2 in some animal models, but depletion of all neuronal classes is evident in phase 3. As remodeling progresses over months and years, more neurons are lost and patches of the ganglion cell layer can become depleted. Some survivor neurons of all classes elaborate new neurites, many of which form fascicles that travel hundreds of microns through the retina, often beneath the distal glial seal. These and other processes form new synaptic microneuromas in the remnant inner nuclear layer as well as cryptic connections throughout the retina. Remodeling activity peaks at mid-phase 3, where neuronal somas actively migrate on glial surfaces. Some amacrine and bipolar cells move into the former ganglion cell layer while other amacrine cells are everted through the inner nuclear layer to the glial seal. Remodeled retinas engage in anomalous self-signaling via rewired circuits that might not support vision even if they could be driven anew by cellular or bionic agents. We propose that survivor neurons actively seek excitation as sources of homeostatic Ca(2+) fluxes. In late phase 3, neuron loss continues and the retina becomes increasingly glial in composition. Retinal remodeling is not plasticity, but represents the invocation of mechanisms resembling developmental and CNS plasticities. Together, neuronal remodeling and the formation of the glial seal may abrogate many cellular and bionic rescue strategies. However, survivor neurons appear to be stable, healthy, active cells and given the evidence of their reactivity to deafferentation, it may be possible to influence their emergent rewiring and migration habits.