Rac1 Modulates Excitatory Synaptic Transmission in Mouse Retinal Ganglion Cells

Ras-related C3 botulinum toxin substrate 1(Racl),a member of the Rho GTPase family which plays important roles in dendritic spine morphology and plasticity,is a key regulator of cytoskeletal reorganization in dendrites and spines.Here,we investigated whether and how Racl modulates synaptic transmission in mouse retinal ganglion cells(RGCs)using selective conditional knockout of Racl(Racl-cKO).Racl-cKO significantly reduced the frequency of AMPA receptor-mediated miniature excitatory postsynaptic currents,while glycine/GABA_A receptor-mediated miniature inhibitory postsynaptic currents were not affected.Although the total GluA1 protein level was increased in Racl-cKO mice,its expression in the membrane component was unchanged.RaclcKO did not affect spine-like branch density in single dendrites,but significantly reduced the dendritic complexity,which resulted in a decrease in the total number of dendritic spine-like branches.These results suggest that Racl selectively affects excitatory synaptic transmission in RGCs by modulating dendritic complexity.

Dopamine D2 Receptor-Mediated Modulation of Rat Retinal Ganglion Cell Excitability

Ganglion cells(RGCs) are the sole output neurons of the retinal circuity. Here, we investigated whether and how dopamine D2 receptors modulate the excitability of dissociated rat RGCs. Application of the selective D2 receptor agonist quinpirole inhibited outward K^+ currents, which were mainly mediated by glybenclamide-and 4-aminopyridine-sensitive channels, but not the tetraethylammonium-sensitive channel. In addition,quinpirole selectively enhanced Nav1.6 voltage-gated Na^+ currents. The intracellular c AMP/protein kinase A,Ca^2+/calmodulin-dependent protein kinase Ⅱ, and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling pathways were responsible for the effects of quinpirole on K^+ and Na^+ currents, while phospholipase C/protein kinase C signaling was not involved. Under current-clamp conditions, the number of action potentials evoked by positive current injection was increased by quinpirole. Our results suggest that D2 receptor activation increases RGC excitability by suppressing outward K+currents and enhancing Nav1.6 currents, which may affect retinal visual information processing.

P2X7/P2X4 Receptors Mediate Proliferation and Migration of Retinal Microglia in Experimental Glaucoma in Mice

Microglia are involved in the inflammatory response and retinal ganglion cell damage in glaucoma.Here,we investigated how microglia proliferate and migrate in a mouse model of chronic ocular hypertension(COH).In COH retinas,the microglial proliferation that occurred was inhibited by the P2X7 receptor(P2X7R)blocker BBG or P2X7R knockout,but not by the P2X4R blocker 5-BDBD.Treatment of primary cultured microglia with BzATP,a P2X7R agonist,mimicked the effects of cell proliferation and migration in COH retinas through the intracellular MEK/ERK signaling pathway.Transwell migration assays showed that the P2X4R agonist CTP induced microglial migration,which was completely blocked by 5-BDBD.In vivo and in vitro experiments demonstrated that ATP,released from activated Müller cells through connexin43 hemichannels,acted on P2X7R to induce microglial proliferation,and acted on P2X4R/P2X7R(mainly P2X4R)to induce microglial migration.Our results suggest that inhibiting the interaction of Müller cells and microglia may attenuate microglial proliferation and migration in glaucoma.

Mdm2 links genotoxic stress and metabolism to p53

Mouse double minute 2(Mdm2)gene was isolated from a cDNA library derived from transformed mouse 3T3 cells,and was classified as an oncogene as it confers 3T3 and Rat2 cells tumorigenicity when overexpressed.It encodes a nucleocytoplasmic shuttling ubiquitin E3 ligase,with its main target being tumor suppressor p53,which is mutated in more than 50%of human primary tumors.Mdm2’s oncogenic activity is mainly mediated by p53,which is activated by various stresses,especially genotoxic stress,via Atm(ataxia telangiectasia mutated)and Atr(Atm and Rad3-related).Activated p53 inhibits cell proliferation,induces apoptosis or senescence,and maintains genome integrity.Mdm2 is also a target gene of p53 transcription factor.Thus,Mdm2 and p53 form a feedback regulatory loop.External and internal cues,through multiple signaling pathways,can act on Mdm2 to regulate p53 levels and cell proliferation,death,and senescence.This review will focus on how Mdm2 is regulated under genotoxic stress,and by the Akt1-mTOR-S6K1 pathway that is activated by insulin,growth factors,amino acids,or energy status.