Hippocampal LTP triggers proteasome-mediated SPAR degradation in CA1 neurons

E3 Ubiquitin Ligase Uhrf2 Knockout Reveals a Critical Role in Social Behavior and Synaptic Plasticity in the Hippocampus

The hippocampal formation, particularly the CA2 subregion, is critical for social memory formation and memory processing, relying on synaptic plasticity-a fundamental mechanism by which synapses strengthen. Given the role of the ubiquitin-proteasome system (UPS) in various nervous system processes, including learning and memory, we were particularly interested in exploring the involvement of RING-type ubiquitin E3 ligases, such as UHRF2 (NIRF), in social behavior and synaptic plasticity. Our results revealed altered social behavior in mice with systemic Uhrf2 knockout, including changes in nest building, tube dominance, and the three-chamber social novelty test. In Uhrf2 knockout mice, the entorhinal cortex-CA2 circuit showed significant reductions in synaptic plasticity during paired-pulse facilitation and long-term potentiation, while the inability to evoke synaptic plasticity in the Schaffer-collateral CA2 synapses remained unaffected. These changes in synaptic plasticity correlated with significant changes in gene expression including genes related to vesicle trafficking and transcriptional regulation. The effects of Uhrf2 knockout on synaptic plasticity and the observed gene expression changes highlight UHRF2 as a regulator of learning and memory processes at both the cellular and systemic levels. Targeting E3 ubiquitin ligases, such as UHRF2, may hold therapeutic potential for memory-related disorders, warranting further investigation.

NRSF regulates age-dependently cognitive ability and its conditional knockout in APP/PS1 mice moderately alters AD-like pathology

NRSF/REST (neuron-restrictive silencer element, also known as repressor element 1-silencing transcription factor), plays a key role in neuronal homeostasis as a transcriptional repressor of neuronal genes. NRSF/REST relates to cognitive preservation and longevity of humans, but its specific functions in age-dependent and Alzheimer's disease (AD)-related memory deficits remain unclear. Here, we show that conditional NRSF/REST knockout either in the dorsal telencephalon or specially in neurons induced an age-dependently diminished retrieval performance in spatial or fear conditioning memory tasks and altered hippocampal synaptic transmission and activity-dependent synaptic plasticity. The NRSF/REST deficient mice were also characterized by an increase of activated glial cells, complement C3 protein and the transcription factor C/EBPβ in the cortex and hippocampus. Reduction of NRSF/REST by conditional depletion upregulated the activation of astrocytes in APP/PS1 mice, and increased the C3-positive glial cells, but did not alter the Aβ loads and memory retrieval performances of 6- and 12-month-old APP/PS1 mice. Simultaneously, overexpression of NRSF/REST improved cognitive abilities of aged wild type, but not in AD mice. These findings demonstrated that NRSF/REST is essential for the preservation of memory performance and activity-dependent synaptic plasticity during aging and takes potential roles in the onset of age-related memory impairments. However, while altering the glial activation, NRSF/REST deficiency does not interfere with the Aβ deposits and the electrophysiological and cognitive AD-like pathologies.

A Putative Role for Ubiquitin-Proteasome Signaling in Estrogenic Memory Regulation

Sex steroid hormones such as 17β-estradiol (E₂) are critical neuromodulators of hippocampal synaptic plasticity and hippocampus-dependent memory in both males and females. However, the mechanisms through which E₂ regulates memory formation in both sexes remain unclear. Research to date suggests that E₂ regulates hippocampus-dependent memory by activating numerous cell-signaling cascades to promote the synthesis of proteins that support structural changes at hippocampal synapses. However, this work has largely overlooked the equally important contributions of protein degradation mediated by the ubiquitin proteasome system (UPS) in remodeling the synapse. Despite being critically implicated in synaptic plasticity and successful formation of long-term memories, it remains unclear whether protein degradation mediated by the UPS is necessary for E₂ to exert its beneficial effects on hippocampal plasticity and memory formation. The present article provides an overview of the receptor and signaling mechanisms so far identified as critical for regulating hippocampal E₂ and UPS function in males and females, with a particular emphasis on the ways in which these mechanisms overlap to support structural integrity and protein composition of hippocampal synapses. We argue that the high degree of correspondence between E₂ and UPS activity warrants additional study to examine the contributions of ubiquitin-mediated protein degradation in regulating the effects of sex steroid hormones on cognition.

Cataloguing and Selection of mRNAs Localized to Dendrites in Neurons and Regulated by RNA-Binding Proteins in RNA Granules

Spatiotemporal translational regulation plays a key role in determining cell fate and function. Specifically, in neurons, local translation in dendrites is essential for synaptic plasticity and long-term memory formation. To achieve local translation, RNA-binding proteins in RNA granules regulate target mRNA stability, localization, and translation. To date, mRNAs localized to dendrites have been identified by comprehensive analyses. In addition, mRNAs associated with and regulated by RNA-binding proteins have been identified using various methods in many studies. However, the results obtained from these numerous studies have not been compiled together. In this review, we have catalogued mRNAs that are localized to dendrites and are associated with and regulated by the RNA-binding proteins fragile X mental retardation protein (FMRP), RNA granule protein 105 (RNG105, also known as Caprin1), Ras-GAP SH3 domain binding protein (G3BP), cytoplasmic polyadenylation element binding protein 1 (CPEB1), and staufen double-stranded RNA binding proteins 1 and 2 (Stau1 and Stau2) in RNA granules. This review provides comprehensive information on dendritic mRNAs, the neuronal functions of mRNA-encoded proteins, the association of dendritic mRNAs with RNA-binding proteins in RNA granules, and the effects of RNA-binding proteins on mRNA regulation. These findings provide insights into the mechanistic basis of protein-synthesis-dependent synaptic plasticity and memory formation and contribute to future efforts to understand the physiological implications of local regulation of dendritic mRNAs in neurons.

Translation of BDNF-gene transcripts with short 3' UTR in hippocampal CA1 neurons improves memory formation and enhances synaptic plasticity-relevant signaling pathways

While the brain-derived neurotrophic factor (BDNF) gene and its multiple transcripts have been recognized as a key factor for learning, but the specific involvement of BDNF translated from BDNF transcripts with short-3' untranslated region (short 3' UTR) in learning and memory requires further analysis. In this paper, we present data to show that the transduction of hippocampal CA1 neurons with AAV9-5' UTR-BDNF (short 3' UTR)-IRES-ZsGreen and the subsequent expression of BDNF enhanced the phosphorylation of synaptic plasticity relevant proteins and improved passive avoidance and object location, but not object recognition memory. In addition, BDNF improved the relearning of object location. At higher BDNF overexpression levels, the fear behavior was accompanied with a decline in the passive avoidance memory 24h post training, and with an enhanced fear conditioning performance. In addition, these animals developed spontaneous seizures. Thus, the expression of BDNF in the hippocampal CA1 region has the potential to improve fear and object location memory in wild type mouse strains when the region and expression levels of BDNF are well controlled.

Sipa1l3/SPAR3 is targeted to postsynaptic specializations and interacts with the Fezzin ProSAPiP1/Lzts3

Rap GTPase-activating proteins (RapGAPs) are essential for synaptic function as they tightly regulate synaptic Rap signaling. Among the most abundant synaptic RapGAPs in brain are the Spine-associated RapGAPs (SPARs) Sipa1l1/SPAR and Sipa1l2/SPAR2, whereas nothing has been reported on Sipa1l3/SPAR3. In this study, we show that Sipa1l3/SPAR3 is conserved across species, has a distinct expression pattern in the developing rat brain and is localized at excitatory postsynapses. We further demonstrate that the Sipa1l3/SPAR3 C-terminus is required for postsynaptic targeting and represents an interaction module for Fezzins such as ProSAPiP1/Lzts3, a binding partner of the postsynaptic scaffold protein Shank3. Taken together, our data imply that Sipa1l3/SPAR3 is a hitherto unknown synaptic RapGAP, which is targeted to postsynaptic specializations and interacts with Fezzins. Spine-associated RapGAPs (SPARs) are essential modulators of synaptic signaling. Our study is the first to characterize the SPAR family member Sipa1l3/SPAR3 in neuronal tissue. We show that Sipa1l3/SPAR3 is conserved across species, has a distinct expression pattern in brain and is localized to excitatory postsynapses via its C-terminus, which represents an interaction module for other postsynaptic proteins including the Fezzin ProSAPiP1/Lzts3.

Involvement of protein kinase ζ in the maintenance of hippocampal long-term potentiation in rats with chronic visceral hypersensitivity

The hippocampal long-term potentiation (LTP) was implicated in the formation of visceral hypersensitivity in rats with irritable bowel syndrome in our previous study. Recent studies have shown that protein kinase M ζ (PKMζ) may be responsible for the maintenance of LTP in memory formation. However, it remains unclear whether PKMζ is involved in the visceral hypersensitivity. In this study, a rat model of visceral hypersensitivity was generated by neonatal maternal separation (NMS). The visceral hypersensitivity was assessed by recording responses of the external oblique abdominal muscle to colorectal distension. Our results demonstrated that hippocampal LTP and visceral hypersensitivity were enhanced significantly in rats of NMS. ζ-Pseudosubstrate inhibitory peptide (ZIP) could dose dependently inhibit the maintenance of Cornu Ammonis area 1 LTP in rats of NMS. Furthermore, Western blot data showed that the expression of hippocampal phosphorylated PKMζ (p-PKMζ) significantly increased in rats of NMS. In addition, bilateral intrahippocampal injections of ZIP attenuated the visceral hypersensitivity dose dependently in rats of NMS. The maximal inhibition was observed at 30 min, and significant inhibition lasted for 1.5–2 h after ZIP application. Besides, data from the open-field test and Morris water maze showed that ZIP did not influence the movement and spatial procedural memory in rats of NMS. In conclusion, p-PKMζ might be a critical protein in the maintenance of hippocampal LTP, which could result in visceral hypersensitivity.

Regulation of hippocampal synaptic plasticity by BDNF

The neurotrophin brain-derived neurotrophic factor (BDNF) has emerged as a major regulator of activity-dependent plasticity at excitatory synapses in the mammalian central nervous system. In particular, much attention has been given to the role of the neurotrophin in the regulation of hippocampal long-term potentiation (LTP), a sustained enhancement of excitatory synaptic strength believed to underlie learning and memory processes. In this review we summarize the evidence pointing to a role for BDNF in generating functional and structural changes at synapses required for both early- and late phases of LTP in the hippocampus. The available information regarding the pre- and/or postsynaptic release of BDNF and action of the neurotrophin during LTP will be also reviewed. Finally, we discuss the effects of BDNF on the synaptic proteome, either by acting on the protein synthesis machinery and/or by regulating protein degradation by calpains and possibly by the ubiquitin-proteasome system (UPS). This fine-tuned control of the synaptic proteome rather than a simple upregulation of the protein synthesis may play a key role in BDNF-mediated synaptic potentiation. This article is part of a Special Issue entitled SI: Brain and Memory.

Ubiquitin proteasome system-mediated degradation of synaptic proteins: An update from the postsynaptic side

The ubiquitin proteasome system is one of the principle mechanisms for the regulation of protein homeostasis in mammalian cells. In dynamic cellular structures such as neuronal synapses, ubiquitin proteasome system and protein translation provide an efficient way for cells to respond promptly to local stimulation and regulate neuroplasticity. The majority of research related to long-term plasticity has been focused on the postsynapses and has shown that ubiquitination and subsequent degradation of specific proteins are involved in various activity-dependent plasticity events. This review summarizes recent achievements in understanding ubiquitination of postsynaptic proteins and its impact on synapse plasticity and discusses the direction for advancing future research in the field.

Parkin overexpression ameliorates hippocampal long-term potentiation and β-amyloid load in an Alzheimer's disease mouse model

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by a severe decline of memory performance. A widely studied AD mouse model is the APPswe/PSEN1ΔE9 (APP/PS1) strain, as mice exhibit amyloid plaques as well as impaired memory capacities. To test whether restoring synaptic plasticity and decreasing β-amyloid load by Parkin could represent a potential therapeutic target for AD, we crossed APP/PS1 transgenic mice with transgenic mice overexpressing the ubiquitin ligase Parkin and analyzed offspring properties. Overexpression of Parkin in APP/PS1 transgenic mice restored activity-dependent synaptic plasticity and rescued behavioral abnormalities. Moreover, overexpression of Parkin was associated with down-regulation of APP protein expression, decreased β-amyloid load and reduced inflammation. Our data suggest that Parkin could be a promising target for AD therapy.

SCRAPPER regulates the thresholds of long-term potentiation/depression, the bidirectional synaptic plasticity in hippocampal CA3-CA1 synapses

SCRAPPER, which is an F-box protein encoded by FBXL20, regulates the frequency of the miniature excitatory synaptic current through the ubiquitination of Rab3-interacting molecule 1. Here, we recorded the induction of long-term potentiation/depression (LTP/LTD) in CA3-CA1 synapses in E3 ubiquitin ligase SCRAPPER-deficient hippocampal slices. Compared to wild-type mice, Scrapper-knockout mice exhibited LTDs with smaller magnitudes after induction with low-frequency stimulation and LTPs with larger magnitudes after induction with tetanus stimulation. These findings suggest that SCRAPPER regulates the threshold of bidirectional synaptic plasticity and, therefore, metaplasticity.

MPTP modulates hippocampal synaptic transmission and activity-dependent synaptic plasticity via dopamine receptors

Parkinson's disease (PD)-like symptoms and cognitive deficits are inducible by 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP). Since cognitive abilities, including memory formations rely also on hippocampus, we set out to clarify the effects of MPTP on hippocampal physiology. We show that bath-application of MPTP (25 μM) to acute hippocampal slices enhanced AMPA receptor-mediated field excitatory postsynaptic potentials (AMPAr-fEPSPs) transiently, whereas N-methyl-D-aspartate (NMDA) receptor-mediated fEPSPs (NMDAr-fEPSPs) were facilitated persistently. The MPTP-mediated transient AMPAr-fEPSP facilitation was antagonized by the dopamine D2-like receptor antagonists, eticlopride (1 μM) and sulpiride (1 and 40 μM). In contrast, the persistent enhancement of NMDAr-fEPSPs was prevented by the dopamine D1-like receptor antagonist SCH23390 (10 μM). In addition, we show that MPTP decreased paired-pulse facilitation of fEPSPs and mEPSCs frequency. Regarding activity-dependent synaptic plasticity, 25 μM MPTP transformed short-term potentiation (STP) into a long-term potentiation (LTP) and caused a slow onset potentiation of a non-tetanized synaptic input after induction of LTP in a second synaptic input. This heterosynaptic slow onset potentiation required activation of dopamine D1-like and NMDA-receptors. We conclude that acute MPTP application affects basal synaptic transmission by modulation of presynaptic vesicle release and facilitates NMDAr-fEPSPs as well as activity-dependent homo- and heterosynaptic plasticity under participation of dopamine receptors.