Specific deficiency in nuclear factor-kappaB activation in neurons of the central nervous system

NF-κB and JAK/STAT Signaling Pathways as Crucial Regulators of Neuroinflammation and Astrocyte Modulation in Spinal Cord Injury

Spinal cord injury (SCI) leads to significant functional impairments below the level of the injury, and astrocytes play a crucial role in the pathophysiology of SCI. Astrocytes undergo changes and form a glial scar after SCI, which has traditionally been viewed as a barrier to axonal regeneration and functional recovery. Astrocytes activate intracellular signaling pathways, including nuclear factor κB (NF-κB) and Janus kinase-signal transducers and activators of transcription (JAK/STAT), in response to external stimuli. NF-κB and STAT3 are transcription factors that play a pivotal role in initiating gene expression related to astrogliosis. The JAK/STAT signaling pathway is essential for managing secondary damage and facilitating recovery processes post-SCI: inflammation, glial scar formation, and astrocyte survival. NF-κB activation in astrocytes leads to the production of pro-inflammatory factors by astrocytes. NF-κB and STAT3 signaling pathways are interconnected: NF-κB activation in astrocytes leads to the release of interleukin-6 (IL-6), which interacts with the IL-6 receptor and initiates STAT3 activation. By modulating astrocyte responses, these pathways offer promising avenues for enhancing recovery outcomes, illustrating the crucial need for further investigation into their mechanisms and therapeutic applications in SCI treatment.

Neuroinflammation in schizophrenia: the role of nuclear factor kappa B

Neuroinflammation, particularly in the dorsolateral prefrontal cortex, is well-established in a subset of people with schizophrenia, with significant increases in inflammatory markers including several cytokines. Yet the cause(s) of cortical inflammation in schizophrenia remains unknown. Clues as to potential microenvironmental triggers and/or intracellular deficits in immunoregulation may be gleaned from looking further upstream of effector immune molecules to transcription factors that control inflammatory gene expression. Here, we focus on the 'master immune regulator' nuclear factor kappa B (NF-κB) and review evidence in support of NF-κB dysregulation causing or contributing to neuroinflammation in patients. We discuss the utility of 'immune biotyping' as a tool to analyse immune-related transcripts and proteins in patient tissue, and the insights into cortical NF-κB in schizophrenia revealed by immune biotyping compared to studies treating patients as a single, homogenous group. Though the ubiquitous nature of NF-κB presents several hurdles for drug development, targeting this key immunoregulator with novel or repurposed therapeutics in schizophrenia is a relatively underexplored area that could aid in reducing symptoms of patients with active neuroinflammation.

Reducing the Levels of Akt Activation by PDK1 Knock-in Mutation Protects Neuronal Cultures against Synthetic Amyloid-Beta Peptides

The Akt kinase has been widely assumed for years as a key downstream effector of the PI3K signaling pathway in promoting neuronal survival. This notion was however challenged by the finding that neuronal survival responses were still preserved in mice with reduced Akt activity. Moreover, here we show that the Akt signaling is elevated in the aged brain of two different mice models of Alzheimer Disease. We manipulate the rate of Akt stimulation by employing knock-in mice expressing a mutant form of PDK1 (phosphoinositide-dependent protein kinase 1) with reduced, but not abolished, ability to activate Akt. We found increased membrane localization and activity of the TACE/ADAM17 α-secretase in the brain of the PDK1 mutant mice with concomitant TNFR1 processing, which provided neurons with resistance against TNFα-induced neurotoxicity. Opposite to the Alzheimer Disease transgenic mice, the PDK1 knock-in mice exhibited an age-dependent attenuation of the unfolding protein response, which protected the mutant neurons against endoplasmic reticulum stressors. Moreover, these two mechanisms cooperatively provide the mutant neurons with resistance against amyloid-beta oligomers, and might singularly also contribute to protect these mice against amyloid-beta pathology.

Maternal IL-17A in autism

Although autism spectrum disorder (ASD) has a strong genetic basis, its etiology is complex, with several genetic factors likely to be involved as well as environmental factors. Immune dysregulation has gained significant attention as a causal mechanism in ASD pathogenesis. ASD has been associated with immune abnormalities in the brain and periphery, including inflammatory disorders and autoimmunity in not only the affected individuals but also their mothers. Prenatal exposure to maternal immune activation (MIA) has been implicated as an environmental risk factor for ASD. In support of this notion, animal models have shown that MIA results in offspring with behavioral, neurological, and immunological abnormalities similar to those observed in ASD. This raises the question of how MIA exposure can lead to ASD in susceptible individuals. Recent evidence points to a potential inflammation pathway linking MIA-associated ASD with the activity of T helper 17 (Th17) lymphocytes and their effector cytokine interleukin-17A (IL-17A). IL-17A has been implicated from human studies and elevated IL-17A levels in the blood have been found to correlate with phenotypic severity in a subset of ASD individuals. In MIA model mice, elevated IL-17A levels also have been observed. Additionally, antibody blockade to inhibit IL-17A signaling was found to prevent ASD-like behaviors in offspring exposed to MIA. Therefore, IL-17A dysregulation may play a causal role in the development of ASD. The source of increased IL-17A in the MIA mouse model was attributed to maternal Th17 cells because genetic removal of the transcription factor RORγt to selectively inhibit Th17 differentiation in pregnant mice was able to prevent ASD-like behaviors in the offspring. Similar to ASD individuals, the MIA-exposed offspring also displayed cortical dysplasia which could be prevented by inhibition of IL-17A signaling in pregnant mice. This finding reveals one possible cellular mechanism through which ASD-related cognitive and behavioral deficits may emerge following maternal inflammation. IL-17A can exert strong effects on cell survival and differentiation and the activity of signal transduction cascades, which can have important consequences during cortical development on neural function. This review examines IL-17A signaling pathways in the context of both immunity and neural function that may contribute to the development of ASD associated with MIA.

Gene regulation and genetics in neurochemistry, past to future

Ask any neuroscientist to name the most profound discoveries in the field in the past 60 years, and at or near the top of the list will be a phenomenon or technique related to genes and their expression. Indeed, our understanding of genetics and gene regulation has ushered in whole new systems of knowledge and new empirical approaches, many of which could not have even been imagined prior to the molecular biology boon of recent decades. Neurochemistry, in the classic sense, intersects with these concepts in the manifestation of neuropeptides, obviously dependent upon the central dogma (the established rules by which DNA sequence is eventually converted into protein primary structure) not only for their conformation but also for their levels and locales of expression. But, expanding these considerations to non-peptide neurotransmitters illustrates how gene regulatory events impact neurochemistry in a much broader sense, extending beyond the neurochemicals that translate electrical signals into chemical ones in the synapse, to also include every aspect of neural development, structure, function, and pathology. From the beginning, the mutability - yet relative stability - of genes and their expression patterns were recognized as potential substrates for some of the most intriguing phenomena in neurobiology - those instances of plasticity required for learning and memory. Near-heretical speculation was offered in the idea that perhaps the very sequence of the genome was altered to encode memories. A fascinating component of the intervening progress includes evidence that the central dogma is not nearly as rigid and consistent as we once thought. And this mutability extends to the potential to manipulate that code for both experimental and clinical purposes. Astonishing progress has been made in the molecular biology of neurochemistry during the 60 years since this journal debuted. Many of the gains in conceptual understanding have been driven by methodological progress, from automated high-throughput sequencing instruments to recombinant-DNA vectors that can convey color-coded genetic modifications in the chromosomes of live adult animals. This review covers the highlights of these advances, both theoretical and technological, along with a brief window into the promising science ahead. This article is part of the 60th Anniversary special issue.

NFκB-inducing kinase inhibits NFκB activity specifically in neurons of the CNS

The control of NFκB in CNS neurons appears to differ from that in other cell types. Studies have reported induction of NFκB in neuronal cultures and immunostaining in vivo, but others have consistently detected little or no transcriptional activation by NFκB in brain neurons. To test if neurons lack some component of the signal transduction system for NFκB activation, we transfected cortical neurons with several members of this signaling system along with a luciferase-based NFκB-reporter plasmid; RelA was cotransfected in some conditions. No component of the NFκB pathway was permissive for endogenous NFκB activity, and none stimulated the activity of exogenous RelA. Surprisingly, however, the latter was inhibited by cotransfection of NFκB-inducing kinase (NIK). Fluorescence imaging of RelA indicated that co-expression of NIK sequestered RelA in the cytoplasm, similar to the effect of IκBα. NIK-knockout mice showed elevated expression of an NFκB-reporter construct in neurons in vivo. Cortical neurons cultured from NIK-knockout mice showed elevated expression of an NFκB-reporter transgene. Consistent with data from other cell types, a C-terminal fragment of NIK suppressed RelA activity in astrocytes as well as neurons. Therefore, the inhibitory ability of the NIK C-terminus was unbiased with regard to cell type. However, inhibition of NFκB by full-length NIK is a novel outcome that appears to be specific to CNS neurons. This has implications for unique aspects of transcription in the CNS, perhaps relevant to aspects of development, neuroplasticity, and neuroinflammation. Full-length NIK was found to inhibit (down arrow) transcriptional activation of NFκB in neurons, while it elevated (up arrow) activity in astrocytes. Deletion constructs corresponding to the N-terminus or C-terminus also inhibited NFκB in neurons, while only the C-terminus did so in astrocytes. One possible explanation is that the inhibition in neurons occurs via two different mechanisms, including the potential for a neuron-specific protein (e.g., one of the 14-3-3 class) to create a novel complex in neurons, whereas the C-terminus may interact directly with NFκB. [Structure of NIK is based on Liu J., Sudom A., Min X., Cao Z., Gao X., Ayres M., Lee F., Cao P., Johnstone S., Plotnikova O., Walker N., Chen G., and Wang Z. (2012) Structure of the nuclear factor κB-inducing kinase (NIK) kinase domain reveals a constitutively active conformation. J Biol Chem. 287, 27326-27334); N-terminal lobe is oriented at top].

Minimal NF-κB activity in neurons

Nuclear factor-kappa B (NF-κB) is a ubiquitous transcription factor that regulates immune and cell-survival signaling pathways. NF-κB has been reported to be present in neurons wherein it reportedly responds to immune and toxic stimuli, glutamate, and synaptic activity. However, because the brain contains many cell types, assays specifically measuring neuronal NF-κB activity are difficult to perform and interpret. To address this, we compared NF-κB activity in cultures of primary neocortical neurons, mixed brain cells, and liver cells, employing Western blot of NF-κB subunits, electrophoretic mobility shift assay (EMSA) of nuclear κB DNA binding, reporter assay of κB DNA binding, immunofluorescence of the NF-κB subunit protein p65, quantitative real-time polymerase chain reaction (PCR) of NF-κB-regulated gene expression, and enzyme-linked immunosorbent assay (ELISA) of produced proteins. Assay of p65 showed its constitutive presence in cytoplasm and nucleus of neurons at levels significantly lower than in mixed brain or liver cells. EMSA and reporter assays showed that constitutive NF-κB activity was nearly absent in neurons. Induced activity was minimal--many fold lower than in other cell types, as measured by phosphorylation and degradation of the inhibitor IκBα, nuclear accumulation of p65, binding to κB DNA consensus sites, NF-κB reporting, or induction of NF-κB-responsive genes. The most efficacious activating stimuli for neurons were the pro-inflammatory cytokines tumor necrosis factor α (TNFα) and interleukin-beta (IL-β). Neuronal NF-κB was not responsive to glutamate in most assays, and it was also unresponsive to hydrogen peroxide, lipopolysaccharide, norepinephrine, ATP, phorbol ester, and nerve growth factor. The chemokine gene transcripts CCL2, CXCL1, and CXCL10 were strongly induced via NF-κB activation by TNFα in neurons, but many candidate responsive genes were not, including the neuroprotective genes SOD2 and Bcl-xL. Importantly, the level of induced neuronal NF-κB activity in response to TNFα or any other stimulus was lower than the level of constitutive activity in non-neuronal cells, calling into question the functional significance of neuronal NF-κB activity.

Unique aspects of transcriptional regulation in neurons--nuances in NFkappaB and Sp1-related factors

The unique physiology and function of neurons create differences in their cellular physiology, including their regulation of gene expression. We began several years ago exploring the relationships between the NFκB transcription factor, neuronal survival, and glutamate receptor activation in telencephalic neurons. These studies led us to conclude that this population of cells is nearly incapable of activating the NFκB that is nonetheless expressed at reasonable levels. A subset of the κB cis elements are instead bound by members of the Sp1 family in neurons. Also surprising was our discovery that Sp1 itself, typically described as ubiquitous, is severely restricted in expression within forebrain neurons; Sp4 seems to be substituted during neuronal differentiation. These findings and their implications for neuronal differentiation – as well as potential dedifferentiation during degenerative processes – are discussed here.

Altered levels of STAT1 and STAT3 influence the neuronal response to interferon gamma

As immune responses in the CNS are highly regulated, cell-specific differences in IFNgamma signaling may be integral in dictating the outcome of host cell responses. In comparing the response of IFNgamma-treated primary neurons to control MEF, we observed that neurons demonstrated lower basal expression of both STAT1 and STAT3, the primary signal transducers responsible for IFNgamma signaling. Following IFNgamma treatment of these cell populations, we noted muted and delayed STAT1 phosphorylation, no detectable STAT3 phosphorylation, and a 3-10-fold lower level of representative IFNgamma-responsive gene transcripts. Moreover, in response to a brief pulse of IFNgamma, a steady increase in STAT1 phosphorylation and IFNgamma gene expression over 48 h was observed in neurons, as compared to rapid attenuation in MEF. These distinct response kinetics in IFNgamma-stimulated neurons may reflect modifications in the IFNgamma negative feedback loop, which may provide a mechanism for the cell-specific heterogeneity of responses to IFNgamma.

TLR8: an innate immune receptor in brain, neurons and axons

Toll-like receptors (TLRs) play essential roles in generating innate immune responses, and are evolutionarily conserved across species. In mammals, TLRs specifically recognize the conserved microbial structural motifs referred to as pathogen-associated molecular patterns (PAMPs). Ligand recognition by TLRs activates signaling cascades that culminate in proinflammatory cytokine production and eventual elimination of invading pathogens. Although TLRs in mammals are expressed predominantly in the immune system, certain TLRs with poorly characterized function are also found in neurons. We recently profiled TLR8 expression during mouse brain development and established its localization in neurons and axons. We uncovered a novel role for TLR8 as a suppressor of neurite outgrowth as well as an inducer of neuronal apoptosis, and found that TLR8 functions in neurons through an NF-kappaB-independent mechanism. These findings add a new layer of complexity for TLR signaling, and expand the realm of mammalian TLR function to the CNS beyond the originally discovered immune context. Herein, we complement our earlier report with additional data, discuss their biological and mechanistic implications in central nervous system (CNS) developmental and pathological processes, and thus further our perspective on TLR signaling and potential physiological roles in mammals.

Differential transcriptional control of the superoxide dismutase-2 kappaB element in neurons and astrocytes

In addition to their conventional G-C/T target sequences, Sp1 family transcription factors (Sp-factors) can interact with a subset of the target sequences for NFkappaB. Due to the low level of bona fide NFkappaB activity in most resting cells, this interaction between Sp-factors and kappaB-sites could play important roles in cell function. Here we used mutagenesis of a canonical kappaB element from the immunoglobulin and HIV promoters to identify the GC-rich sequences at each end required for Sp-factor targeting. Through screening of multiple kappaB elements, a sequence element located in the second intron of superoxide dismutase-2 (SOD2) was identified as a good candidate for both NFkappaB and Sp-factor binding. In neurons, the prominent proteins interacting with this site were Sp3 and Sp4, whereas Sp1, Sp3, and NFkappaB were associated with this site in astroglia. The neuronal Sp-factors repressed transcriptional activity through this kappaB-site. In contrast, astroglial Sp-factors activated promoter activity through the same element. NFkappaB contributed to control of the SOD2 kappaB element only in astrocytes. These findings imply that cell-type specificity of transcription in the central nervous system, particularly with regard to kappaB elements, may include two unique aspects of neurons: 1) a recalcitrant NFkappaB and 2) the substitution of Sp4 for Sp1.

NFkappaB in neurons? The uncertainty principle in neurobiology

Nuclear factor kappaB (NFkappaB) is a dynamically modulated transcription factor with an extensive literature pertaining to widespread actions across species, cell types and developmental stages. Analysis of NFkappaB in a complex environment such as neural tissue suffers from a difficulty in simultaneously establishing both activity and location. Much of the available data indicate a profound recalcitrance of NFkappaB activation in neurons, as compared with most other cell types. Few studies to date have sought to distinguish between the various combinatorial dimers of NFkappaB family members. Recent research has illuminated the importance of these problems, as well as opportunities to move past them to the nuances manifest through variable activation pathways, subunit complexity and target sequence preferences.

S100B-induced microglial and neuronal IL-1 expression is mediated by cell type-specific transcription factors

Both the astrocytic cytokine S100B and the pro-inflammatory interleukin-1 (IL-1) are elevated in Alzheimer's disease, and each has been implicated in Alzheimer-related neuropathology. We examined the gene-regulatory events through which S100B induces IL-1beta expression. In primary microglia, S100B activated the transcription factors Sp1 and NFkappaB, followed by an increase in IL-1beta mRNA levels. The latter was blocked by a peptide inhibitor of NFkappaB or by a double-stranded oligonucleotide containing a NFkappaB-binding site to serve as "decoy" DNA and reduce available NFkappaB. But in primary cortical neurons, decoy and siRNA experiments indicated that the IL-1beta induction by S100B was mediated by Sp1 without evidence of a role for NFkappaB. Our results suggest that the elevation of S100B and IL-1 in Alzheimer brain and consequent neurodegenerative events are mediated through cell-type specific gene-regulatory events, providing mechanistic insight into connections between glial activation and neuronal dysfunction.

Combination gene therapy for glioblastoma involving herpes simplex virus vector-mediated codelivery of mutant IκBα and HSV thymidine kinase

To improve the effectiveness of herpes simplex virus (HSV) thymidine kinase/ganciclovir (HSV-tk/GCV) suicide gene therapy, the replication-defective HSV vector TOIkappaB expressing both HSV-TK and a mutant form of the NF-kappaB inhibitor IkappaBalpha (IkappaBalphaM) was developed. TOIkappaB was constructed by recombining the IkappaBalphaM gene into the U(L)41 locus of a replication-defective lacZ expression vector, TOZ.1. Expression of IkappaBalphaM was confirmed by Western blotting, and the ability of the mutant protein to inhibit NF-kappaB nuclear translocation was examined by electrophoretic mobility shift assay. In human glioblastoma U-87MG cells, the p50/p50 dimer of NF-kappaB was already translocated to the nucleus without receptor-dependent signaling by TNF-alpha. Following infection with TOIkappaB, nuclear translocation of NF-kappaB in U-87MG cells was significantly inhibited and caspase-3 activity increased compared with TOZ.1-infected cells. The cytotoxicity of TOIkappaB for U-87MG cells was investigated by colorimetric MTT assay. At an MOI of 3, TOIkappaB infection killed 85% of the cells compared to 20% killed by TOZ.1 infection. In the presence of GCV, these numbers increased to 95-100% for TOIkappaB and 80-85% for TOZ.1. TOIkappaB neurotoxicity measured on cultured murine neurons was relatively low and similar to that of TOZ.1. The survival of nude mice implanted into the brain with U-87MG tumor cells was markedly prolonged by intratumoral TOIkappaB injection and GCV administration. Survival of TOIkappaB+GCV group was significantly longer (P<.02, Wilcoxon test) than for the control groups (TOZ.1 or TOIkappaB only, PBS or PBS+GCV). These results suggest that IkappaBalphaM expression may be a safe enhancement of replication-defective HSV-based suicide gene therapy in vitro and in vivo.

Immune responses to RNA-virus infections of the CNS

A successful outcome for the host of virus infection of the central nervous system (CNS) requires the elimination of the virus without damage to essential non-renewable cells, such as neurons. As a result, inflammatory responses must be tightly controlled, and many unique mechanisms seem to contribute to this control. In addition to being important causes of human disease, RNA viruses that infect the CNS provide useful models in which to study immune responses in the CNS. Recent work has shown the importance of innate immune responses in the CNS in controlling virus infection. And advances have been made in assessing the relative roles of cytotoxic T cells, antibodies and cytokines in the clearance of viruses from neurons, glial cells and meningeal cells.

Interferon regulatory factor-1 is required for interferon-gamma-induced MHC class I genes in astrocytes

Recent studies have shown that the role of the transcription factor interferon regulatory factor-1 (IRF-1) in the expression of major histocompatibility complex (MHC) class I molecules is tissue-specific. Our previous studies indicated a role for IRF-1 in expression of MHC class I genes in cultured astrocytes in response to interferon-gamma (IFN-gamma). However, the requirement for IRF-1 in MHC class I expression has not been directly analyzed in neural tissue. To further ascertain the importance of IRF-1 in the induction of MHC class I genes in astrocytes in response to IFN-gamma, we analyzed astrocytes from mice with a targeted disruption of the IRF-1 gene (IRF-1(-/-) mice). As expected, astrocytes from wild type (IRF-1(+/+)) mice showed a coordinate increase in both IRF-1 and MHC class I gene expression in response to IFN-gamma. To the contrary, astrocytes from IRF-1(-/-) mice had greatly reduced MHC class I mRNA expression. MHC class I gene promoter activity in astrocytes was controlled entirely through a single enhancer, the MHC-IRF-E, to which IRF-1 bound in response to IFN-gamma in wild type but not in IRF-1(-/-) mouse astrocytes. In vivo, astrocytes in brains of wild type mice readily responded to IFN-gamma to express MHC class I molecules. This correlated with increased MHC class I mRNA in the brain. In contrast, brains of IRF-1(-/-) mice showed no MHC class I gene induction following exposure to IFN-gamma indicating that all cells in the central nervous system (CNS) including astrocytes with the potential to express MHC class I molecules were dependent on IRF-1. These studies conclusively demonstrate a major role for IRF-1/MHC-IRF-E interactions in controlling MHC class I gene expression in astrocytes in response to IFN-gamma.