Amyloid-ß1-42 oligomers enhance mGlu5R-dependent synaptic weakening via NMDAR activation and complement C5aR1 signaling.

Synaptic weakening and loss are well-correlated with the pathology of Alzheimer's disease (AD). Oligomeric amyloid beta (oAß) is considered a major synaptotoxic trigger for AD. Recent studies have implicated hyperactivation of the complement cascade as the driving force for loss of synapses caused by oAß. However, the initial synaptic cues that trigger pathological complement activity remain elusive. Here, we examined a form of synaptic long-term depression (LTD) mediated by metabotropic glutamate receptors (mGluRs) that is disrupted in rodent models of AD. Exogenous application of oAß (1-42) to mouse hippocampal slices enhanced the magnitude of mGlu subtype 5 receptor (mGlu5R)-dependent LTD. We found that the enhanced synaptic weakening occurred via both N-methyl-D-aspartate receptors (NMDARs) and complement C5aR1 signaling. Our findings reveal a mechanistic interaction between mGlu5R, NMDARs, and the complement system in aberrant synaptic weakening induced by oAß, which could represent an early trigger of synaptic loss and degeneration in AD.

The anabolic effect of inorganic polyphosphate on chondrocytes is mediated by calcium signalling.

Inorganic polyphosphates (polyP) are polymers composed of phosphate residues linked by energy-rich phosphoanhydride bonds. As polyP can bind calcium, the hypothesis of this study is that polyP enters chondrocytes and exerts its anabolic effect by calcium influx through calcium channels. PolyP treatment of cartilage tissue formed in 3D culture by bovine chondrocytes showed an increase in proteoglycan accumulation but only when calcium was also present at a concentration of 1.5 mM. This anabolic effect could be prevented by treatment with either ethylene glycol-bis(ß-aminoethyl ether)-N,N,N',N'-tetraacetic acid or the calcium channel inhibitors gadolinium and nifedipine. Calcium and polyP cotreatment of chondrocytes in monolayer culture resulted in calcium oscillations that were polyP chain length specific and were inhibited by gadolinium and nifedipine. The calcium influx resulted in increased gene expression of sox9, collagen type II, and aggrecan which was prevented by treatment with either calphostin, an inhibitor of protein kinase C, and W7, an inhibitor of calmodulin; suggesting activation of the protein kinase C-calmodulin pathway. Tracing studies using  4',6-diamidino-2-phenylindole, Mitotracker Red, and/or Fura-AM staining showed that polyP was detected in the nucleus, mitochondria, and intracellular vacuoles suggesting that polyP may also enter the cell. PolyP colocalizes with calcium in mitochondria. This study demonstrates that polyP requires the influx of calcium to regulate chondrocyte matrix production, likely via activating calcium signaling. These findings identify the mechanism regulating the anabolic effect of polyP in chondrocytes which will help in its clinical translation into a therapeutic agent for cartilage repair.

Neuronal hypertrophy dampens neuronal intrinsic excitability and stress responsiveness during chronic stress.

KEY POINTS: The hypothalamic-pituitary-adrenal (HPA) axis habituates to repeated stress exposure. We studied hypothalamic corticotropin-releasing hormone (CRH) neurons that form the apex of the HPA axis in a mouse model of stress habituation using repeated restraint. The intrinsic excitability of CRH neurons decreased after repeated stress in a time course that coincided with the development of HPA axis habituation. This intrinsic excitability plasticity co-developed with an expansion of surface membrane area, which increased a passive electric load and dampened membrane depolarization in response to the influx of positive charge. We report a novel structure-function relationship for intrinsic excitability plasticity as a neural correlate for HPA axis habituation. ABSTRACT: Encountering a stressor immediately activates the hypothalamic-pituitary-adrenal (HPA) axis, but this stereotypic stress response also undergoes experience-dependent adaptation. Despite the biological and clinical importance, how the brain adjusts stress responsiveness in the long term remains poorly understood. We studied hypothalamic corticotropin-releasing hormone neurons that form the apex of the HPA axis in a mouse model of stress habituation using repeated restraint. Using patch-clamp electrophysiology in acute slices, we found that the intrinsic excitability of these neurons substantially decreased after daily repeated stress in a time course that coincided with their loss of stress responsiveness in vivo. This intrinsic excitability plasticity co-developed with an expansion of surface membrane area, which increased a passive electric load, and dampened membrane depolarization in response to the influx of positive charge. Multiphoton imaging and electron microscopy revealed that repeated stress augmented ruffling of the plasma membrane, suggesting an ultrastructural plasticity that may efficiently accommodate the membrane area expansion. Overall, we report a novel structure-function relationship for intrinsic excitability plasticity as a neural correlate for adaptation of the neuroendocrine stress response.

Autism-Misregulated eIF4G Microexons Control Synaptic Translation and Higher Order Cognitive Functions.

Microexons represent the most highly conserved class of alternative splicing, yet their functions are poorly understood. Here, we focus on closely related neuronal microexons overlapping prion-like domains in the translation initiation factors, eIF4G1 and eIF4G3, the splicing of which is activity dependent and frequently disrupted in autism. CRISPR-Cas9 deletion of these microexons selectively upregulates synaptic proteins that control neuronal activity and plasticity and further triggers a gene expression program mirroring that of activated neurons. Mice lacking the Eif4g1 microexon display social behavior, learning, and memory deficits, accompanied by altered hippocampal synaptic plasticity. We provide evidence that the eIF4G microexons function as a translational brake by causing ribosome stalling, through their propensity to promote the coalescence of cytoplasmic granule components associated with translation repression, including the fragile X mental retardation protein FMRP. The results thus reveal an autism-disrupted mechanism by which alternative splicing specializes neuronal translation to control higher order cognitive functioning.

Chronic stress dampens excitatory synaptic gain in the paraventricular nucleus of the hypothalamus.

KEY POINTS: Glutamatergic synaptic inputs to corticotrophin-releasing hormone (CRH) secreting neurons in the paraventricular nucleus of the hypothalamus (PVN) are required for stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis. These synapses also undergo stress-induced plasticity, thereby influencing HPA axis stress adaptation. By using patch clamp electrophysiology, we show that, in adult non-stressed mice, action potentials at these glutamatergic afferents elicit multiquantal transmission to the postsynaptic PVN-CRH neurons (i.e. synaptic multiplicity). Mechanistically, synaptic multiplicity results from multivesicular release at common synaptic sites, which is facilitated upon elevation of release probability, effectively increasing the upper limit of the dynamic range of synaptic transmission. Following chronic variable stress, functional PVN glutamate synapse number increases, although its synaptic multiplicity paradoxically decreases. These two contrasting synaptic changes can, respectively, increase the baseline excitatory drive while also limiting the capacity for potentiation, and may preferentially increase the baseline excitatory drive onto PVN-CRH neurons. ABSTRACT: The activation of the hypothalamic-pituitary-adrenal (HPA) axis relies on excitation of neuroendocrine neurons in the paraventricular nucleus of the hypothalamus (PVN) that secrete corticotrophin-releasing hormone (CRH). Afferent glutamate synapses onto these PVN-CRH neurons convey critical excitatory inputs during stress, and also undergo stress-induced plasticity, highlighting their roles in both stress activation and adaptation of the HPA axis. In the present study, using whole-cell patch clamp recordings from PVN-CRH neurons in brain slices from adult mice, we found that the amplitude of action potential-dependent spontaneous EPSCs (sEPSCs) was larger than that of action potential independent miniature EPSCs (mEPSCs), suggesting that action potentials at individual axons recruited multiquantal transmission onto the same postsynaptic neurons (i.e. synaptic multiplicity). The large, putative multiquantal sEPSCs had fast rise times similar to mEPSCs, and were abolished by replacing extracellular Ca2+ with Sr2+ , indicating Ca2+ -dependent synchronous release of multiple vesicles. Application of a low affinity, fast dissociating competitive AMPA receptor antagonist γ-d-glutamylglycine revealed that synaptic multiplicity resulted from multivesicular release targeting a common population of postsynaptic receptors. High-frequency afferent stimulation facilitated synaptic multiplicity, effectively increasing the upper limit of the dynamic range of synaptic transmission. Finally, we found that chronic variable stress (CVS), a stress model known to cause basal HPA axis hyperactivity, increased sEPSCs frequency but paradoxically decreased synaptic multiplicity. These results suggest that the CVS-induced synaptic changes may elevate the baseline excitatory drive at the same time as limiting the capacity for potentiation, and may contribute to the basal HPA axis hyperactivity.

Translational control of nociception via 4E-binding protein 1.

Activation of the mechanistic/mammalian target of rapamycin (mTOR) kinase in models of acute and chronic pain is strongly implicated in mediating enhanced translation and hyperalgesia. However, the molecular mechanisms by which mTOR regulates nociception remain unclear. Here we show that deletion of the eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), a major mTOR downstream effector, which represses eIF4E activity and cap-dependent translation, leads to mechanical, but not thermal pain hypersensitivity. Mice lacking 4E-BP1 exhibit enhanced spinal cord expression of neuroligin 1, a cell-adhesion postsynaptic protein regulating excitatory synapse function, and show increased excitatory synaptic input into spinal neurons, and a lowered threshold for induction of synaptic potentiation. Pharmacological inhibition of eIF4E or genetic reduction of neuroligin 1 levels normalizes the increased excitatory synaptic activity and reverses mechanical hypersensitivity. Thus, translational control by 4E-BP1 downstream of mTOR effects the expression of neuroligin 1 and excitatory synaptic transmission in the spinal cord, and thereby contributes to enhanced mechanical nociception.

Sustained increase in α5GABAA receptor function impairs memory after anesthesia.

Many patients who undergo general anesthesia and surgery experience cognitive dysfunction, particularly memory deficits that can persist for days to months. The mechanisms underlying this postoperative cognitive dysfunction in the adult brain remain poorly understood. Depression of brain function during anesthesia is attributed primarily to increased activity of γ-aminobutyric acid type A receptors (GABA(A)Rs), and it is assumed that once the anesthetic drug is eliminated, the activity of GABA(A)Rs rapidly returns to baseline and these receptors no longer impair memory. Here, using a murine model, we found that a single in vivo treatment with the injectable anesthetic etomidate increased a tonic inhibitory current generated by α5 subunit-containing GABA(A)Rs (α5GABA(A)Rs) and cell-surface expression of α5GABA(A)Rs for at least 1 week. The sustained increase in α5GABA(A)R activity impaired memory performance and synaptic plasticity in the hippocampus. Inhibition of α5GABA(A)Rs completely reversed the memory deficits after anesthesia. Similarly, the inhaled anesthetic isoflurane triggered a persistent increase in tonic current and cell-surface expression of α5GABA(A)Rs. Thus, α5GABA(A)R function does not return to baseline after the anesthetic is eliminated, suggesting a mechanism to account for persistent memory deficits after general anesthesia.