MEF2 transcription factors differentially contribute to retinal ganglion cell loss after optic nerve injury

Perinuclear compartment controls calcineurin/MEF2 signaling for axonal outgrowth of hippocampal neurons

Central to the process of axon elongation is the concept of compartmentalized signaling, which involves the A-kinase anchoring protein (AKAP)-dependent organization of signaling pathways within distinct subcellular domains. This spatial organization is also critical for translating electrical activity into biochemical events. Despite intensive research, the detailed mechanisms by which the spatial separation of signaling pathways governs axonal outgrowth and pathfinding remain unresolved. In this study, we demonstrate that mAKAPα (AKAP6), located in the perinuclear space of primary hippocampal neurons, scaffolds calcineurin, NFAT, and MEF2 transcription factors for activity-dependent axon elongation. By employing anchoring disruptors, we show that the mAKAPα/calcineurin/MEF2 signaling pathway, but not NFAT, drives the process of axonal outgrowth. Furthermore, mAKAPα-controlled axonal elongation is linked to the changes in the expression of genes involved in Ca2+/cAMP signaling. These findings reveal a novel regulatory mechanism of axon growth that could be targeted therapeutically for neuroprotection and regeneration.

NFATc4 Knockout Promotes Neuroprotection and Retinal Ganglion Cell Regeneration After Optic Nerve Injury

Retinal ganglion cells (RGCs), neurons transmitting visual information via the optic nerve, fail to regenerate their axons after injury. The progressive loss of RGC function underlies the pathophysiology of glaucoma and other optic neuropathies, often leading to irreversible blindness. Therefore, there is an urgent need to identify the regulators of RGC survival and the regenerative program. In this study, we investigated the role of the family of transcription factors known as nuclear factor of activated T cells (NFAT), which are expressed in the retina; however, their role in RGC survival after injury is unknown. Using the optic nerve crush (ONC) model, widely employed to study optic neuropathies and central nervous system axon injury, we found that NFATc4 is specifically but transiently up-regulated in response to mechanical injury. In the injured retina, NFATc4 immunolocalized primarily to the ganglionic cell layer. Utilizing NFATc4-/- and NFATc3-/- mice, we demonstrated that NFATc4, but not NFATc3, knockout increased RGC survival, improved retina function, and delayed axonal degeneration. Microarray screening data, along with decreased immunostaining of cleaved caspase-3, revealed that NFATc4 knockout was protective against ONC-induced degeneration by suppressing pro-apoptotic signaling. Finally, we used lentiviral-mediated NFATc4 delivery to the retina of NFATc4-/- mice and reversed the pro-survival effect of NFATc4 knockout, conclusively linking the enhanced survival of injured RGCs to NFATc4-dependent mechanisms. In summary, this study is the first to demonstrate that NFATc4 knockout may confer transient RGC neuroprotection and decelerate axonal degeneration after injury, providing a potent therapeutic strategy for optic neuropathies.

Reversal of injury-associated retinal ganglion cell gene expression by a phosphodiesterase anchoring disruptor peptide

Loss of retinal ganglion cells (RGCs) is central to the pathogenesis of optic neuropathies such as glaucoma. Increased RGC cAMP signaling is neuroprotective. We have shown that displacement of the cAMP-specific phosphodiesterase PDE4D3 from an RGC perinuclear compartment by expression of the modified PDE4D3 N-terminal peptide 4D3(E) increases perinuclear cAMP and protein kinase A activity in cultured neurons and in vivo RGC survival after optic nerve crush (ONC) injury. To explore mechanisms by which PDE4D3 displacement promotes neuroprotection, in this study mice intravitreally injected with an adeno-associated virus to express an mCherry-tagged 4D3(E) peptide were subjected to ONC injury and analyzed by single cell RNA-sequencing (scRNA-seq). 4D3(E)-mCherry expression was associated with an attenuation of injury-induced changes in gene expression, thereby supporting the hypothesis that enhanced perinuclear PKA signaling promotes neuroprotective RGC gene expression.

Atlas of phosphoinositide signatures in the retina identifies heterogeneity between cell types

Phosphoinositides (PIPs) are a family of minor acidic phospholipids in the cell membrane. Phosphoinositide (PI) kinases and phosphatases can rapidly convert one PIP product into another resulting in the generation of seven distinct PIPs. The retina is a heterogeneous tissue composed of several cell types. In the mammalian genome, around 50 genes encode PI kinases and PI phosphatases; however, there are no studies describing the distribution of these enzymes in the various retinal cell types. Using translating ribosome affinity purification, we have identified the in vivo distribution of PI-converting enzymes from the rod, cone, retinal pigment epithelium (RPE), Müller glia, and retinal ganglion cells, generating a physiological atlas for PI-converting enzyme expression in the retina. The retinal neurons, rods, cones, and RGCs, are characterized by the enrichment of PI-converting enzymes, whereas the Müller glia and RPE are characterized by the depletion of these enzymes. We also found distinct differences between the expression of PI kinases and PI phosphatases in each retinal cell type. Since mutations in PI-converting enzymes are linked to human diseases including retinal diseases, the results of this study will provide a guide for what cell types are likely to be affected by retinal degenerative diseases brought on by changes in PI metabolism.

Long Non-Coding RNAs in Retinal Ganglion Cell Apoptosis

Traumatic optic neuropathy or other neurodegenerative diseases, including optic nerve transection, glaucoma, and diabetic retinopathy, can lead to progressive and irreversible visual damage. Long non-coding RNAs (lncRNAs), which belong to the family of non-protein-coding transcripts, have been linked to the pathogenesis, progression, and prognosis of these lesions. Retinal ganglion cells (RGCs) are critical for the transmission of visual information to the brain, damage to which results in visual loss. Apoptosis has been identified as one of the most essential modes of RGC death. Emerging evidence suggests that lncRNAs can regulate RGC degeneration by directly or indirectly modulating apoptosis-associated signaling pathways. This review presents a comprehensive overview of the role of lncRNAs in RGC apoptosis at transcriptional, post-transcriptional, translational, and post-translational levels, emphasizing on the potential mechanisms of action. The current limitations and future perspectives of exploring the connection between lncRNAs and RGC apoptosis have been summarized. Understanding the intricate molecular interaction network of lncRNAs and RGC apoptosis will open new avenues for the identification of novel diagnostic biomarkers, therapeutic targets, and molecules for prognostic evaluation of diseases related to RGC injury.

The Role of MEF2 Transcription Factor Family in Neuronal Survival and Degeneration

The family of myocyte enhancer factor 2 (MEF2) transcription factors comprises four highly conserved members that play an important role in the nervous system. They appear in precisely defined time frames in the developing brain to turn on and turn off genes affecting growth, pruning and survival of neurons. MEF2s are known to dictate neuronal development, synaptic plasticity and restrict the number of synapses in the hippocampus, thus affecting learning and memory formation. In primary neurons, negative regulation of MEF2 activity by external stimuli or stress conditions is known to induce apoptosis, albeit the pro or antiapoptotic action of MEF2 depends on the neuronal maturation stage. By contrast, enhancement of MEF2 transcriptional activity protects neurons from apoptotic death both in vitro and in preclinical models of neurodegenerative diseases. A growing body of evidence places this transcription factor in the center of many neuropathologies associated with age-dependent neuronal dysfunctions or gradual but irreversible neuron loss. In this work, we discuss how the altered function of MEF2s during development and in adulthood affecting neuronal survival may be linked to neuropsychiatric disorders.

Myocyte Enhancer Factor 2A Plays a Central Role in the Regulatory Networks of Cellular Physiopathology

Cell regulatory networks are the determinants of cellular homeostasis. Any alteration to these networks results in the disturbance of cellular homeostasis and induces cells towards different fates. Myocyte enhancer factor 2A (MEF2A) is one of four members of the MEF2 family of transcription factors (MEF2A-D). MEF2A is highly expressed in all tissues and is involved in many cell regulatory networks including growth, differentiation, survival and death. It is also necessary for heart development, myogenesis, neuronal development and differentiation. In addition, many other important functions of MEF2A have been reported. Recent studies have shown that MEF2A can regulate different, and sometimes even mutually exclusive cellular events. How MEF2A regulates opposing cellular life processes is an interesting topic and worthy of further exploration. Here, we reviewed almost all MEF2A research papers published in English and summarized them into three main sections: 1) the association of genetic variants in MEF2A with cardiovascular disease, 2) the physiopathological functions of MEF2A, and 3) the regulation of MEF2A activity and its regulatory targets. In summary, multiple regulatory patterns for MEF2A activity and a variety of co-factors cause its transcriptional activity to switch to different target genes, thereby regulating opposing cell life processes. The association of MEF2A with numerous signaling molecules establishes a central role for MEF2A in the regulatory network of cellular physiopathology.

Construction and analysis of mRNA, lncRNA, and transcription factor regulatory networks after retinal ganglion cell injury

Retinal ganglion cell (RGC) injury is a critical pathological feature of several optic neurodegenerative diseases. The regulatory mechanisms underlying RGC injury remain poorly understood. Recent evidence has highlighted the important roles of long noncoding RNAs (lncRNAs) in degenerative neuropathy but few studies have focused on lncRNAs associated with RGC injury. In this study, we analyzed dysregulated lncRNAs associated with RGC injury, their potential regulatory functions, and the molecular mechanisms underlying the regulation of lncRNAs and transcription factors (TFs). We analyzed lncRNA and mRNA profiles in the GSE142881 dataset associated with RGC injury and identified 1049 differentially expressed genes (DEGs), with 18 differentially expressed (DE) TFs among 883 DE mRNAs and 312 DE lncRNAs. The predicted DE lncRNAs and DE mRNAs were used to construct a lncRNA-mRNA co-expression network. Functional enrichment analysis was performed to explore the functions of the lncRNAs and mRNAs. The co-expression network between DE lncRNAs and DE mRNAs was highly enriched in inflammatory and immune-related pathways, indicating that they play role in the process of RGC injury. Among the DE mRNAs, we screened 18 DE TFs, including activating transcription factor 3 (ATF3), associated with RGC injury. Co-expression analysis predicted that 13 lncRNAs were potential binding targets of ATF3. The screening of the potential targets of these 13 lncRNAs showed that they were also significantly enriched in functional pathways associated with inflammation and apoptosis. After analysis, we constructed the mRNA-ATF3-lncRNA regulatory network after RGCs injury. In summary, we identified the gene module associated with immune and inflammatory responses after optic nerve injury and constructed a regulatory network of lncRNA-TF-mRNA. The results indicate that lncRNAs, by binding to TFs, can regulate downstream genes and function during RGC injury. The results provide a foundation for further studies of the mechanism of RGC injury and provide insight into the clinical diagnosis and investigation direction of neurodegenerative diseases such as traumatic optic neuropathy and glaucoma.

Genome-wide chromatin accessibility analyses provide a map for enhancing optic nerve regeneration

Retinal Ganglion Cells (RGCs) lose their ability to grow axons during development. Adult RGCs thus fail to regenerate their axons after injury, leading to vision loss. To uncover mechanisms that promote regeneration of RGC axons, we identified transcription factors (TF) and open chromatin regions that are enriched in rat embryonic RGCs (high axon growth capacity) compared to postnatal RGCs (low axon growth capacity). We found that developmental stage-specific gene expression changes correlated with changes in promoter chromatin accessibility. Binding motifs for TFs such as CREB, CTCF, JUN and YY1 were enriched in the regions of the chromatin that were more accessible in embryonic RGCs. Proteomic analysis of purified rat RGC nuclei confirmed the expression of TFs with potential role in axon growth such as CREB, CTCF, YY1, and JUND. The CREB/ATF binding motif was widespread at the open chromatin region of known pro-regenerative TFs, supporting a role of CREB in regulating axon regeneration. Consistently, overexpression of CREB fused to the VP64 transactivation domain in mouse RGCs promoted axon regeneration after optic nerve injury. Our study provides a map of the chromatin accessibility during RGC development and highlights that TF associated with developmental axon growth can stimulate axon regeneration in mature RGC.