Protein synthesis inhibition promotes nitric oxide generation and activation of CGKII-dependent downstream signaling pathways in the retina

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Chemical signaling in the developing avian retina: Focus on cyclic AMP and AKT-dependent pathways

Communication between developing progenitor cells as well as differentiated neurons and glial cells in the nervous system is made through direct cell contacts and chemical signaling mediated by different molecules. Several of these substances are synthesized and released by developing cells and play roles since early stages of Central Nervous System development. The chicken retina is a very suitable model for neurochemical studies, including the study of regulation of signaling pathways during development. Among advantages of the model are its very well-known histogenesis, the presence of most neurotransmitter systems found in the brain and the possibility to make cultures of neurons and/or glial cells where many neurochemical functions develop in a similar way than in the intact embryonic tissue. In the chicken retina, some neurotransmitters or neuromodulators as dopamine, adenosine, and others are coupled to cyclic AMP production or adenylyl cyclase inhibition since early stages of development. Other substances as vitamin C and nitric oxide are linked to the major neurotransmitter glutamate and AKT metabolism. All these different systems regulate signaling pathways, including PKA, PKG, SRC, AKT and ERK, and the activation of the transcription factor CREB. Dopamine and adenosine stimulate cAMP accumulation in the chick embryo retina through activation of D1 and A2a receptors, respectively, but the onset of dopamine stimulation is much earlier than that of adenosine. However, adenosine can inhibit adenylyl cyclase and modulate dopamine-dependent cAMP increase since early developmental stages through A1 receptors. Dopamine stimulates different PKA as well as EPAC downstream pathways both in intact tissue and in culture as the CSK-SRC pathway modulating glutamate NMDA receptors as well as vitamin C release and CREB phosphorylation. By the other hand, glutamate modulates nitric oxide production and AKT activation in cultured retinal cells and this pathway controls neuronal survival in retina. Glutamate and adenosine stimulate the release of vitamin C and this vitamin regulates the transport of glutamate, activation of NMDA receptors and AKT phosphorylation in cultured retinal cells. In the present review we will focus on these reciprocal interactions between neurotransmitters or neuromodulators and different signaling pathways during retinal development.

Serine/Threonine Phosphatases in LTP: Two B or Not to Be the Protein Synthesis Blocker-Induced Impairment of Early Phase

Dephosphorylation of target proteins at serine/threonine residues is one of the most crucial mechanisms regulating their activity and, consequently, the cellular functions. The role of phosphatases in synaptic plasticity, especially in long-term depression or depotentiation, has been reported. We studied serine/threonine phosphatase activity during the protein synthesis blocker (PSB)-induced impairment of long-term potentiation (LTP). Established protein phosphatase 2B (PP2B, calcineurin) inhibitor cyclosporin A prevented the LTP early phase (E-LTP) decline produced by pretreatment of hippocampal slices with cycloheximide or anisomycin. For the first time, we directly measured serine/threonine phosphatase activity during E-LTP, and its significant increase in PSB-treated slices was demonstrated. Nitric oxide (NO) donor SNAP also heightened phosphatase activity in the same manner as PSB, and simultaneous application of anisomycin + SNAP had no synergistic effect. Direct measurement of the NO production in hippocampal slices by the NO-specific fluorescent probe DAF-FM revealed that PSBs strongly stimulate the NO concentration in all studied brain areas: CA1, CA3, and dentate gyrus (DG). Cyclosporin A fully abolished the PSB-induced NO production in the hippocampus, suggesting a close relationship between nNOS and PP2B activity. Surprisingly, cyclosporin A alone impaired short-term plasticity in CA1 by decreasing paired-pulse facilitation, which suggests bi-directionality of the influences of PP2B in the hippocampus. In conclusion, we proposed a minimal model of signaling events that occur during LTP induction in normal conditions and the PSB-treated slices.

mTORC1-mediated amino acid signaling is critical for cell fate determination under transplant-induced stress

Transplantation of in vitro-manipulated cells is widely used in hematology. While transplantation is well recognized to impose severe stress on transplanted cells, the nature of transplant-induced stress remains elusive. Here, we propose that the lack of amino acids in serum is the major cause of transplant-induced stress. Mechanistically, amino acid deficiency decreases protein synthesis and nutrient consummation. However, in cells with overactive AKT and ERK, mTORC1 is not inhibited and protein synthesis remains relatively high. This impaired signaling causes nutrient depletion, cell cycle block, and eventually autophagy and cell death, which can be inhibited by cycloheximide or mTORC1 inhibitors. Thus, mTORC1-mediated amino acid signaling is critical in cell fate determination under transplant-induced stress, and protein synthesis inhibition can improve transplantation efficiency.

Bicuculline regulated protein synthesis is dependent on Homer1 and promotes its interaction with eEF2K through mTORC1-dependent phosphorylation

The regulation of protein synthesis is a vital and finely tuned process in cellular physiology. In neurons, this process is very precisely regulated, as which mRNAs undergo translation is highly dependent on context. One of the most prominent regulators of protein synthesis is the enzyme eukaryotic elongation factor kinase 2 (eEF2K) that regulates the elongation stage of protein synthesis. This kinase and its substrate, eukaryotic elongation factor 2 (eEF2) are important in processes such as neuronal development and synaptic plasticity. eEF2K is regulated by multiple mechanisms including Ca²⁺ -ions and the mTORC1 signaling pathway, both of which play key roles in neurological processes such as learning and memory. In such settings, the localized control of protein synthesis is of crucial importance. In this work, we sought to investigate how the localization of eEF2K is controlled and the impact of this on protein synthesis in neuronal cells. In this study, we used both SH-SY5Y neuroblastoma cells and mouse cortical neurons, and pharmacologically and/or genetic approaches to modify eEF2K function. We show that eEF2K activity and localization can be regulated by its binding partner Homer1b/c, a scaffolding protein known for its participation in calcium-regulated signaling pathways. Furthermore, our results indicate that this interaction is regulated by the mTORC1 pathway, through a known phosphorylation site in eEF2K (S396), and that it affects rates of localized protein synthesis at synapses depending on the presence or absence of this scaffolding protein.