Neurogranin expression regulates mitochondrial function and redox balance in endothelial cells

Endothelial dysfunction and endothelial activation are common early events in vascular diseases and can arise from mitochondrial dysfunction. Neurogranin (Ng) is a 17kD protein well known to regulate intracellular Ca2+-calmodulin (CaM) complex signaling, and its dysfunction is significantly implicated in brain aging and neurodegenerative diseases. We found that Ng is also expressed in human aortic endothelial cells (HAECs), and depleting Ng promotes Ca2+-CaM complex-dependent endothelial activation and redox imbalances. Endothelial-specific Ng knockout (Cre-CDH5-Ngf/f) mice demonstrate a significant delay in the flow-mediated dilation (FMD) response. Therefore, it is critical to characterize how endothelial Ng expression regulates reactive oxygen species (ROS) generation and affects cardiovascular disease. Label-free quantification proteomics identified that mitochondrial dysfunction and the oxidative phosphorylation pathway are significantly changed in the aorta of Cre-CDH5-Ngf/f mice. We found that a significant amount of Ng is expressed in the mitochondrial fraction of HAECs using western blotting and colocalized with the mitochondrial marker, COX IV, using immunofluorescence staining. Seahorse assay demonstrated that a lack of Ng decreases mitochondrial respiration. Treatment with MitoEbselen significantly restores the oxygen consumption rate in Ng knockdown cells. With the RoGFP-Orp1 approach, we identified that Ng knockdown increases mitochondrial-specific hydrogen peroxide (H2O2) production, and MitoEbselen treatment significantly reduced mitochondrial ROS (mtROS) levels in Ng knockdown cells. These results suggest that Ng plays a significant role in mtROS production. We discovered that MitoEbselen treatment also rescues decreased eNOS expression and nitric oxide (NO) levels in Ng knockdown cells, which implicates the critical role of Ng in mtROS-NO balance in the endothelial cells.

Time Course of Homeostatic Structural Plasticity in Response to Optogenetic Stimulation in Mouse Anterior Cingulate Cortex

Plasticity is the mechanistic basis of development, aging, learning, and memory, both in healthy and pathological brains. Structural plasticity is rarely accounted for in computational network models due to a lack of insight into the underlying neuronal mechanisms and processes. Little is known about how the rewiring of networks is dynamically regulated. To inform such models, we characterized the time course of neural activity, the expression of synaptic proteins, and neural morphology employing an in vivo optogenetic mouse model. We stimulated pyramidal neurons in the anterior cingulate cortex of mice and harvested their brains at 1.5 h, 24 h, and $48\,\mathrm{h}$ after stimulation. Stimulus-induced cortical hyperactivity persisted up to 1.5 h and decayed to baseline after $24\,\mathrm{h}$ indicated by c-Fos expression. The synaptic proteins VGLUT1 and PSD-95, in contrast, were upregulated at $24\,\mathrm{h}$ and downregulated at $48\,\mathrm{h}$, respectively. Spine density and spine head volume were also increased at $24\,\mathrm{h}$ and decreased at $48\,\mathrm{h}$. This specific sequence of events reflects a continuous joint evolution of activity and connectivity that is characteristic of the model of homeostatic structural plasticity. Our computer simulations thus corroborate the observed empirical evidence from our animal experiments.

Neurogranin Expression Is Regulated by Synaptic Activity and Promotes Synaptogenesis in Cultured Hippocampal Neurons

Neurogranin (Ng) is a calmodulin (CaM)-binding protein that is phosphorylated by protein kinase C (PKC) and is highly enriched in the dendrites and spines of telencephalic neurons. It is proposed to be involved in regulating CaM availability in the post-synaptic environment to modulate the efficiency of excitatory synaptic transmission. There is a close relationship between Ng and cognitive performance; its expression peaks in the forebrain coinciding with maximum synaptogenic activity, and it is reduced in several conditions of impaired cognition. We studied the expression of Ng in cultured hippocampal neurons and found that both protein and mRNA levels were about 10% of that found in the adult hippocampus. Long-term blockade of NMDA receptors substantially decreased Ng expression. On the other hand, treatments that enhanced synaptic activity such as long-term bicuculline treatment or co-culture with glial cells or cholesterol increased Ng expression. Chemical long-term potentiation (cLTP) induced an initial drop of Ng, with a minimum after 15 min followed by a slow recovery during the next 2-4 h. This effect was most evident in the synaptosome-enriched fraction, thus suggesting local synthesis in dendrites. Lentiviral expression of Ng led to increased density of both excitatory and inhibitory synapses in the second and third weeks of culture. These results indicate that Ng expression is regulated by synaptic activity and that Ng promotes the synaptogenesis process. Given its relationship with cognitive function, we propose targeting of Ng expression as a promising strategy to prevent or alleviate the cognitive deficits associated with aging and neuropathological conditions.

Brugada syndrome trafficking-defective Nav1.5 channels can trap cardiac Kir2.1/2.2 channels

Cardiac Nav1.5 and Kir2.1-2.3 channels generate Na (INa) and inward rectifier K (IK1) currents, respectively. The functional INa and IK1 interplay is reinforced by the positive and reciprocal modulation between Nav15 and Kir2.1/2.2 channels to strengthen the control of ventricular excitability. Loss-of-function mutations in the SCN5A gene, which encodes Nav1.5 channels, underlie several inherited arrhythmogenic syndromes, including Brugada syndrome (BrS). We investigated whether the presence of BrS-associated mutations alters IK1 density concomitantly with INa density. Results obtained using mouse models of SCN5A haploinsufficiency, and the overexpression of native and mutated Nav1.5 channels in expression systems - rat ventricular cardiomyocytes and human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) - demonstrated that endoplasmic reticulum (ER) trafficking-defective Nav1.5 channels significantly decreased IK1, since they did not positively modulate Kir2.1/2.2 channels. Moreover, Golgi trafficking-defective Nav1.5 mutants produced a dominant negative effect on Kir2.1/2.2 and thus an additional IK1 reduction. Moreover, ER trafficking-defective Nav1.5 channels can be partially rescued by Kir2.1/2.2 channels through an unconventional secretory route that involves Golgi reassembly stacking proteins (GRASPs). Therefore, cardiac excitability would be greatly affected in subjects harboring Nav1.5 mutations with Golgi trafficking defects, since these mutants can concomitantly trap Kir2.1/2.2 channels, thus unexpectedly decreasing IK1 in addition to INa.

Alzheimer-associated cerebrospinal fluid fragments of neurogranin are generated by Calpain-1 and prolyl endopeptidase

Background

Neurogranin (Ng) is a small 7.6 kDa postsynaptic protein that has been detected at elevated concentrations in cerebrospinal fluid (CSF) of patients with Alzheimer’s disease (AD), both as a full-length molecule and as fragments from its C-terminal half. Ng is involved in postsynaptic calcium (Ca) signal transduction and memory formation via binding to calmodulin in a Ca-dependent manner. The mechanism of Ng secretion from neurons to CSF is currently unknown, but enzymatic cleavage of Ng may be of relevance. Therefore, the aim of the study was to identify the enzymes responsible for the cleavage of Ng, yielding the Ng fragment pattern of C-terminal fragments detectable and increased in CSF of AD patients.

Methods

Fluorigenic quenched FRET probes containing sequences of Ng were utilized to identify Ng cleaving activities among enzymes known to have increased activity in AD and in chromatographically fractionated mouse brain extracts.

Results

Human Calpain-1 and prolyl endopeptidase were identified as the candidate enzymes involved in the formation of endogenous Ng peptides present in CSF, cleaving mainly in the central region of Ng, and between amino acids 75_76 in the Ng sequence, respectively. The cleavage by Calpain-1 affects the IQ domain of Ng, which may deactivate or change the function of Ng in Ca²⁺/calmodulin -dependent signaling for synaptic plasticity. While shorter Ng fragments were readily cleaved in vitro by prolyl endopeptidase, the efficiency of cleavage on larger Ng fragments was much lower.

Conclusions

Calpain-1 and prolyl endopeptidase cleave Ng in the IQ domain and near the C-terminus, respectively, yielding specific fragments of Ng in CSF. These fragments may give clues to the roles of increased activities of these enzymes in the pathophysiology of AD, and provide possible targets for pharmacologic intervention.

Electronic supplementary material

The online version of this article (10.1186/s13024-018-0279-z) contains supplementary material, which is available to authorized users.

Characterization of a nuclear localization signal in the foot-and-mouth disease virus polymerase

We have experimentally tested whether the MRKTKLAPT sequence in FMDV 3D protein (residues 16 to 24) can act as a nuclear localization signal (NLS). Mutants with substitutions in two basic residues within this sequence, K18E and K20E, were generated. A decreased nuclear localization was observed in transiently expressed 3D and its precursor 3CD, suggesting a role of K18 and K20 in nuclear targeting. Fusion of MRKTKLAPT to the green fluorescence protein (GFP) increased the nuclear localization of GFP, which was not observed when GFP was fused to the 3D mutated sequences. These results indicate that the sequence MRKTKLAPT can be functionally considered as a NLS. When introduced in a FMDV full length RNA replacements K18E and K20E led to production of revertant viruses that replaced the acidic residues introduced (E) by K, suggesting that the presence of lysins at positions 18 and 20 of 3D is essential for virus multiplication.

Stimulation-mediated translocation of calmodulin and neurogranin from soma to dendrites of mouse hippocampal CA1 pyramidal neurons

Calmodulin (CaM) and neurogranin (Ng) are two abundant neuronal proteins in the forebrain whose interactions are implicated in the enhancement of synaptic plasticity. To gain further insight into the actions of these two proteins we investigated whether they co-localize in principle neurons and whether they respond to high frequency stimulation in a coordinated fashion. Immunohistochemical staining of CaM and Ng in mouse hippocampal slices revealed that CaM was highly concentrated in the nucleus of CA1 pyramidal neurons, whereas Ng was more broadly localized throughout the soma and dendrites. The asymmetrical localization of CaM in the nucleus of pyramidal neurons was in sharp contrast to the distribution observed in pyramidal cells of the neighboring subiculum, where CaM was uniformly localized throughout the soma and dendrites. The somatic concentrations of CaM and Ng in CA1 pyramidal neurons were approximately 10- and two-fold greater than observed in the dendrites, respectively. High frequency stimulation (HFS) of hippocampal slices promoted mobilization of CaM and Ng from soma to dendrites. These responses were spatially restricted to the area close to the site of stimulation and were inhibited by the N-methyl-D-asparate receptor antagonist 2-amino-5-phosphonopentanoic acid. Furthermore, HFS failed to promote translocation of CaM from soma to dendrites of slices from Ng knockout mice, which also exhibited deficits in HFS-induced long-term potentiation. Translocated CaM and Ng exhibited distinct puncta decorating the apical dendrites of pyramidal neurons and appeared to be concentrated in dendritic spines. These findings suggest that mobilization of CaM and Ng to stimulated dendritic spines may enhance synaptic efficacy by increasing and prolonging the Ca2+ transients and activation of Ca2+/CaM-dependent enzymes.

Immunonucleochemistry: a new method for in situ detection of antigens in the nucleus of cells in culture

The advancement of immunocytochemistry (ICC) allows one to observe detailed spatial distribution of cellular antigens, but, with some limitations. Using conventional ICC, it is difficult to distinguish the nuclear localization from cytoplasm, as two large subcellular compartments overlap on the z-axis. In this study, we have investigated whether in situ immunostaining of 'naked' nuclei could provide an unambiguous method for detection of nuclear antigens. We have designed a protocol that efficiently lyses plasmalemma, while keeping the nuclear envelope intact. The optimal condition for lysing the plasmalemma was 0.5% Nonidet P-40 for 5 min in both neuronal and non-neuronal cultured cells. Using this protocol, we could unambiguously isolate nuclear from cytoplasmic ICC signals. Since the present protocol has been designed for immunostaining of 'naked' nuclei from cultured or isolated cells, we have coined a new term to refer to this procedure as 'immunonucleochemistry' ('INC' for abbreviation).