It is well established that long-term potentiation (LTP),

The majority of excitatory synaptic input in the brain is received by small bulbous actin-rich protrusions residing on the dendrites of glutamatergic neurons. These dendritic spines are the major sites of information processing in the brain. This conclusion is reinforced by the observation that many higher cognitive disorders, such as mental retardation, Rett syndrome, and autism, are associated with aberrant spine morphology. Mechanisms that regulate the maturation and plasticity of dendritic spines are therefore fundamental to understanding higher brain functions including learning and memory. It is well known that activity-driven changes in synaptic efficacy modulate spine morphology due to alterations in the underlying actin cytoskeleton. Recent studies have elucidated numerous molecular regulators that directly alter actin dynamics within dendritic spines. This review will emphasize activity-dependent changes in spine morphology and highlight likely roles of these actin-binding proteins.

Glutamatergic synapses in early postnatal development transiently express calcium-permeable AMPA receptors (CP-AMPARs). Although these GluA2-lacking receptors are essential and are elevated in response to brain-derived neurotrophic factor (BDNF), little is known regarding molecular mechanisms that govern their expression and synaptic insertion. Here we show that BDNF-induced GluA1 translation in rat primary hippocampal neurons requires the activation of mTOR via calcium calmodulin-dependent protein kinase kinase (CaMKK). Specifically, BDNF-mediated phosphorylation of T308 in AKT, a known substrate of CaMKK and an upstream activator of mTOR-dependent translation, was prevented by 1) pharmacological inhibition of CaMKK with STO-609, 2) overexpression of a dominant-negative CaMKK, or 3) short hairpin-mediated knockdown of CaMKK. GluA1 surface expression induced by BDNF, as assessed by immunocytochemistry using an extracellular N-terminal GluA1 antibody or by surface biotinylation, was impaired following knockdown of CaMKK or treatment with STO-609. Activation of CaMKK by BDNF requires TRPC channels as SKF-96365, but not the NMDA receptor antagonist D-APV, prevented BDNF-induced GluA1 surface expression as well as phosphorylation of CaMKI, AKTT308 and mTOR. Using siRNA we confirmed the involvement of TRPC5 and -6 subunits in BDNF-induced AKTT308 phosphorylation. The BDNF-induced increase in mEPSC was blocked by IEM-1460, a selected antagonist of CP-AMPARs, as well as by the specific repression of acute GluA1 translation via siRNA to GluA1 but not GluA2. Taken together these data support the conclusion that newly synthesized GluA1 subunits, induced by BDNF, are readily incorporated into synapses where they enhance the expression of CP-AMPARs and synaptic strength.

The glycosaminoglycan hyaluronan (HA), a component of the extracellular matrix, has been implicated in regulating neural differentiation, survival, proliferation, migration, and cell signaling in the mammalian central nervous system (CNS). HA is found throughout the CNS as a constituent of proteoglycans, especially within perineuronal nets that have been implicated in regulating neuronal activity. HA is also found in the white matter where it is diffusely distributed around astrocytes and oligodendrocytes. Insults to the CNS lead to long-term elevation of HA within damaged tissues, which is linked at least in part to increased transcription of HA synthases. HA accumulation is often accompanied by elevated expression of at least some transmembrane HA receptors including CD44. Hyaluronidases that digest high molecular weight HA into smaller fragments are also elevated following CNS insults and can generate HA digestion products that have unique biological activities. A number of studies, for example, suggest that both the removal of high molecular weight HA and the accumulation of hyaluronidase-generated HA digestion products can impact CNS injuries through mechanisms that include the regulation of progenitor cell differentiation and proliferation. These studies, reviewed here, suggest that targeting HA synthesis, catabolism, and signaling are all potential strategies to promote CNS repair.

ers by interactions of FoxO3 with the chromatin-remodeling factor Brg1 and the TF Olig2, which are involved in control of OPC differentiation (14-16). Hence, central myelination failure is regulated by a noncanonical TLR4/AKT/FoxO3 signaling pathway utilized by bHAf to induce a tolerance-like state that selectively constrains OPC maturation and myelination. ResultsNeonatal hypoxic-ischemic WMI promotes MDa HA depolymerization. To investigate the status of MDa HA in chronic neonatal WMI, we employed our preterm-equivalent rat hypoxia-ischemia (H-I) model, which generates myelination failure and replicates key features of human WMI ( Figure 1A) (17). MDa HA turnover in the ECM after H-I was visualized with a biotinylated HA-binding protein (HABP) and costained with glial fibrillary acidic protein (GFAP) as a marker of WMI. Unlike in age-matched uninpro-myelination signal (13). This tolerance-like action of bHAf was mediated through TLR4 but not via CD44 or TLR2. As in TLR4-mediated IT, bHAf 's influence on myelination was reversible when MDa HA depolymerization was attenuated or bHAf was removed. AKT desensitization was similarly reversible in vivo in neonatal WMI. Moreover, bHAf actions were mediated via a noncanonical TRIF-dependent pathway, also involved in IT, rather than the canonical MyD88 arm of TLR4 signaling. AKT desensitization resulted in maturation-dependent activation of the FoxO3 transcription factor (TF), which selectively constrained preOL maturation in a bHAf-dependent fashion. A role for activated FoxO3 in human myelination failure was supported by selective localization of nuclear FoxO3 to OPCs in human preterm WMI and multiple sclerosis (MS) plaques. bHAf-mediated OPC maturation arrest appears to be regulated at the FoxO3 and myelin basic protein (MBP) promot- (Contralateral). Only the lesion group had a significant reduction in HA recovery. (E) Incubation of MDa HA with the lesion lysate generated HAf below ~650 kDa. Lesion-lysate activity was sensitive to heat inactivation but insensitive to deferoxamine (50 μM). B and C: control n = 2; H-I n = 4 animals for each age group (P4 and P14). D: n = 6 (H-I), n = 5 (control), and n = 4 (hypoxia) animals (P7). E: n = 4 separate experiments on 4 different animals at P4 after H-I at P3; one representative experiment is shown. *P < 0.05 by ANOVA. Mean ± SD. Scale bars: 300 μm (B and C). The Journal of Clinical Investigation R E S E A R C H A R T I C L E2 0 2 7 jci.orgVolume 128 Number 5 May 2018 with MBP-labeled oligodendrocytes (Supplemental Figure 2A). To confirm de novo progressive myelin generation, we undertook ultrastructural studies that identified axons wrapped with multilamellar myelin sheaths ( Figure 2B). In contrast to vehicle-treated slices, which displayed robust myelination of the corpus callosum, slices incubated with MDa HA until 21 days in vitro (DIV21) displayed a pronounced reduction in myelinated axons ( Figure 2C). Myelination failure was not related to decreased OPC survival, since MDa HA treatment did not enhance OPC degeneratio...

ObjectiveAlthough the spectrum of white matter injury (WMI) in preterm infants is shifting from cystic necrotic lesions to milder forms, the factors that contribute to this changing spectrum are unclear. We hypothesized that recurrent hypoxia-ischemia (rHI) will exacerbate the spectrum of WMI defined by markers of inflammation and molecules related to the extracellular matrix (hyaluronan (HA) and the PH20 hyaluronidase) that regulate maturation of the oligodendrocyte (OL) lineage after WMI.MethodsWe employed a preterm fetal sheep model of in utero moderate hypoxemia and global severe but not complete cerebral ischemia that reproduces the spectrum of human WMI. The response to rHI was compared against corresponding early or later single episodes of HI. An ordinal rating scale of WMI was compared against an unbiased quantitative image analysis protocol that provided continuous histo-pathological outcome measures for astrogliosis and microglial activation. Late oligodendrocyte progenitors (preOLs) were quantified by stereology. Analysis of hyaluronan and the hyaluronidase PH20 defined the progressive response of the extracellular matrix to WMI.ResultsrHI resulted in a more severe spectrum of WMI with a greater burden of necrosis, but an expanded population of preOLs that displayed reduced susceptibility to cell death. WMI from single episodes of HI or rHI was accompanied by elevated HA levels and increased labeling for PH20. Expression of PH20 in fetal ovine WMI was confirmed by RT-PCR and RNA-sequencing.ConclusionsrHI is associated with an increased risk for more severe WMI with necrosis, but reduced risk for preOL degeneration compared to single episodes of HI. Expansion of the preOL pool may be linked to elevated hyaluronan and PH20.

Summary Ca2+/CaM-kinases (CaMKs) are essential for neuronal development and plasticity, processes requiring de novo protein synthesis. Roles for CaMKs in modulating gene transcription are well established, but their involvement in mRNA translation is evolving. Here we report that activity-dependent translational initiation in cultured rat hippocampal neurons is enhanced by CaMKI-mediated phosphorylation of Ser1156 in eukaryotic initiation factor eIF4GII (4GII). Treatment with bicuculline or gabazine to enhance neuronal activity promotes recruitment of wild-type 4GII, but not the 4GII S1156A mutant or 4GI, to the heterotrimeric eIF4F (4F) complex that assembles at the 5' cap structure (m7GTP) of mRNA to initiate ribosomal scanning. Recruitment of 4GII to 4F is suppressed by pharmacological inhibition (STO-609) of CaM-kinase kinase, the upstream activator of CaMKI. Post-hoc in vitro CaMKI phosphorylation assays confirm that activity promotes phosphorylation of S1156 in transfected 4GII in neurons. Changes in cap-dependent and cap-independent translation were assessed using a bi-cistronic luciferase reporter transfected into neurons. Activity upregulates cap-dependent translation, and RNAi knockdown of CaMKIβ and γ isoforms, but not α or δ, led to its attenuation as did blockade of NMDA receptors. Furthermore, RNAi knockdown of 4GII attenuates cap-dependent translation and reduces density of dendritic filopodia and spine formation without effect on dendritic arborization. Taken together, our results provide a mechanistic link between Ca2+ influx due to neuronal activity and regulation of cap-dependent RNA translation via CaMKI activation and selective recruitment of phosphorylated 4GII to the 4F complex that may function to regulate activity-dependent changes in spine density.

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