Proteomics analysis of the expression of neurogranin in murine neuroblastoma (Neuro-2a) cells reveals its involvement for cell differentiation

Expression and secretion of synaptic proteins during stem cell differentiation to cortical neurons

Synaptic function and neurotransmitter release are regulated by specific proteins. Cortical neuronal differentiation of human induced pluripotent stem cells (hiPSC) provides an experimental model to obtain more information about synaptic development and physiology in vitro. In this study, expression and secretion of the synaptic proteins, neurogranin (NRGN), growth-associated protein-43 (GAP-43), synaptosomal-associated protein-25 (SNAP-25) and synaptotagmin-1 (SYT-1) were analyzed during cortical neuronal differentiation. Protein levels were measured in cells, modeling fetal cortical development and in cell-conditioned media which was used as a model of cerebrospinal fluid (CSF), respectively. Human iPSC-derived cortical neurons were maintained over a period of at least 150 days, which encompasses the different stages of neuronal development. The differentiation was divided into the following stages: hiPSC, neuro-progenitors, immature and mature cortical neurons. We show that NRGN was first expressed and secreted by neuro-progenitors while the maximum was reached in mature cortical neurons. GAP-43 was expressed and secreted first by neuro-progenitors and its expression increased markedly in immature cortical neurons. SYT-1 was expressed and secreted already by hiPSC but its expression and secretion peaked in mature neurons. SNAP-25 was first detected in neuro-progenitors and the expression and secretion increased gradually during neuronal stages reaching a maximum in mature neurons. The sensitive analytical techniques used to monitor the secretion of these synaptic proteins during cortical development make these data unique, since the secretion of these synaptic proteins has not been investigated before in such experimental models. The secretory profile of synaptic proteins, together with low release of intracellular content, implies that mature neurons actively secrete these synaptic proteins that previously have been associated with neurodegenerative disorders, including Alzheimer's disease. These data support further studies of human neuronal and synaptic development in vitro, and would potentially shed light on the mechanisms underlying altered concentrations of the proteins in bio-fluids in neurodegenerative diseases.

Intrahippocampal injection of a lentiviral vector expressing neurogranin enhances cognitive function in 5XFAD mice

Progressive cognitive declines are the main clinical symptoms of Alzheimer's disease (AD). Cognitive impairment in AD is directly correlated with amyloid beta (Aβ)-mediated synaptic deficits. It is known that upregulation of neurogranin (Ng), a postsynaptic protein, contributes to the enhancement of synaptic plasticity and cognitive function. By contrast, downregulation of Ng expression results in learning and memory impairments. Interestingly, Ng expression is significantly reduced in the parenchyma of brains with AD. However, the pathological role that downregulated Ng plays in the cognitive dysfunctions observed in AD remains unclear. Therefore, the present study examined whether enhancing Ng expression affected cognitive functions in 5XFAD mice, an animal model of AD. We found that the Ng reductions and cognitive decline observed in 5XFAD mice were restored in mice that were intrahippocampally injected with an Ng-expressing lentiviral vector. Furthermore, overexpression of Ng upregulated expression of postsynaptic density protein-95 in the hippocampus of 5XFAD mice. These results suggest that the cause of cognitive decline in AD may be at least partially associated with reduced Ng levels, and thus, supplementation of Ng may be an appropriate therapeutic strategy for individuals with AD.

Neuronal NOS Induces Neuronal Differentiation Through a PKCα-Dependent GSK3β Inactivation Pathway in Hippocampal Neural Progenitor Cells

We investigated the role of neuronal nitric oxide synthase (nNOS) in the neuronal differentiation of neural progenitor cells (NPCs) from hippocampi of E16.5 rat embryos. The production of nitric oxide (NO) and nNOS expression increased markedly during neuronal differentiation as did the expression of neurotrophin-3 (NT3), neurotrophin-4/5 (NT 4/5), and synapsin I. nNOS siRNA or the nNOS inhibitor, 7-nitroindazole (7-NI), decreased expression of the neurotrophins and synapsin I, and suppressed neurite outgrowth. These results suggest that nNOS plays a critical role in neuronal differentiation of hippocampal NPCs. nNOS-mediated neuronal differentiation is controlled by calcineurin since cyclosporin A (CsA), a calcineurin inhibitor, decreased nNOS activation and NO production, and inhibited neurite outgrowth. We found that inactivation of glycogen synthase kinase-3 beta (GSK3β) resulting from activation of protein kinase C alpha (PKCα) is involved in the nNOS-mediated neuronal differentiation. Moreover, lithium chloride (LiCl), a GSK3β inhibitor, increased neuronal differentiation by inhibiting the proliferation of NPCs. Taken together, these results suggest that neuronal differentiation is dependent on calcineurin-mediated activation of nNOS; this induces PKCα-dependent inactivation of GSK3β, which leads to inhibition of the proliferation of hippocampal NPCs.

Zinc regulation of transcriptional activity during retinoic acid-induced neuronal differentiation

Zinc deficiency impairs the proliferation and differentiation of stem cells in the central nervous system that participate in neurogenesis. To examine the molecular mechanisms responsible for the role of this essential nutrient in neuronal precursor cells and neuronal differentiation, we identified zinc-dependent changes in the DNA-binding activity of zinc finger proteins and other transcription factors in proliferating human Ntera-2 neuronal precursor cells undergoing retinoic acid-stimulated differentiation into a neuronal phenotype. We found that zinc deficiency altered binding activity of 28 transcription factors including retinoid X receptor (RXR) known to participate in neuronal differentiation. Alterations in zinc finger transcription factor activity were not simply the result of removal of zinc from these proteins during zinc deficiency, as the activity of other zinc-binding transcription factors such as the glucocorticoid receptor was increased by as much as twofold over zinc-adequate conditions, and nonzinc-binding transcription factors such as nuclear factor-1 and heat shock transcription factor-1 were increased by as much as fourfold over control. Western analysis did not detect significant decreases in total RXR protein abundance in neuronal precursors, suggesting that the decrease in DNA-binding activity was not simply the result of a reduction in RXR levels in neuronal precursor cells. Rather, use of a reporter gene construct containing retinoic acid response elements upstream from a luciferase coding sequence revealed that zinc deficiency results in decreased transcriptional activity of RXR and reductions in retinoic acid-mediated gene transcription during neuronal differentiation. These results show that zinc deficiency has implications for both developmental and adult neurogenesis.

Structural basis for the interaction of unstructured neuron specific substrates neuromodulin and neurogranin with Calmodulin

Neuromodulin (Nm) and neurogranin (Ng) are neuron-specific substrates of protein kinase C (PKC). Their interactions with Calmodulin (CaM) are crucial for learning and memory formation in neurons. Here, we report the structure of IQ peptides (24aa) of Nm/Ng complexed with CaM and their functional studies with full-length proteins. Nm/Ng and their respective IQ peptides are intrinsically unstructured; however, upon binding with CaM, IQ motifs adopt a helical conformation. Ser41 (Ser36) of Nm (Ng) is located in a negatively charged pocket in the apo CaM and, when phosphorylated, it will repel Nm/Ng from CaM. These observations explain the mechanism by which PKC-induced Ser phosphorylation blocks the association of Nm/Ng with CaM and interrupts several learning- and memory-associated functions. Moreover, the present study identified Arg as a key CaM interacting residue from Nm/Ng. This residue is crucial for CaM-mediated function, as evidenced by the inability of the Ng mutant (Arg-to-Ala) to potentiate synaptic transmission in CA1 hippocampal neurons.