The immune molecule CD3zeta and its downstream effectors ZAP-70/Syk mediate ephrin signaling in neurons to regulate early neuritogenesis

A Function of Amyloid-β in Mediating Activity-Dependent Axon/Synapse Competition May Unify Its Roles in Brain Physiology and Pathology

Amyloid-β protein precursor (AβPP) gives rise to amyloid-β (Aβ), a peptide at the center of Alzheimer's disease (AD). AβPP, however, is also an ancient molecule dating back in evolution to some of the earliest forms of metazoans. This suggests a possible ancestral function that may have been obscured by those that evolve later. Based on literature from the functions of Aβ/AβPP in nervous system development, plasticity, and disease, to those of anti-microbial peptides (AMPs) in bacterial competition as well as mechanisms of cell competition uncovered first by Drosophila genetics, I propose that Aβ/AβPP may be part of an ancient mechanism employed in cell competition, which is subsequently co-opted during evolution for the regulation of activity-dependent neural circuit development and plasticity. This hypothesis is supported by foremost the high similarities of Aβ to AMPs, both of which possess unique, opposite (i.e., trophic versus toxic) activities as monomers and oligomers. A large body of data further suggests that the different Aβ oligomeric isoforms may serve as the protective and punishment signals long predicted to mediate activity-dependent axonal/synaptic competition in the developing nervous system and that the imbalance in their opposite regulation of innate immune and glial cells in the brain may ultimately underpin AD pathogenesis. This hypothesis can not only explain the diverse roles observed of Aβ and AβPP family molecules, but also provide a conceptual framework that can unify current hypotheses on AD. Furthermore, it may explain major clinical observations not accounted for and identify approaches for overcoming shortfalls in AD animal modeling.

SYK Inhibition Potentiates the Effect of Chemotherapeutic Drugs on Neuroblastoma Cells in Vitro

Neuroblastoma is a malignancy arising from the developing sympathetic nervous system and the most common and deadly cancer of infancy. New therapies are needed to improve the prognosis for high-risk patients and to reduce toxicity and late effects. Spleen tyrosine kinase (SYK) has previously been identified as a promising drug target in various inflammatory diseases and cancers but has so far not been extensively studied as a potential therapeutic target in neuroblastoma. In this study, we observed elevated SYK gene expression in neuroblastoma compared to neural crest and benign neurofibroma. While SYK protein was detected in the majority of examined neuroblastoma tissues it was less frequently observed in neuroblastoma cell lines. Depletion of SYK by siRNA and the use of small molecule SYK inhibitors significantly reduced the cell viability of neuroblastoma cell lines expressing SYK protein. Moreover, SYK inhibition decreased ERK1/2 and Akt phosphorylation. The SYK inhibitor BAY 61-3606 enhanced the effect of different chemotherapeutic drugs. Transient expression of a constitutive active SYK variant increased the viability of neuroblastoma cells independent of endogenous SYK levels. Collectively, our findings suggest that targeting SYK in combination with conventional chemotherapy should be further evaluated as a treatment option in neuroblastoma.

MHC Class I Molecules and PirB Shape Neuronal Morphology by Affecting the Dendritic Arborization of Cortical Neurons

Neuronal MHC class I proteins have been previously reported to regulate synaptic plasticity. Several reports indicate MHC class I proteins are expressed early during development of the nervous system, suggesting they may also play a role in neuronal development. Using cultured cortical neurons, we show MHC class I proteins aggregate at specific sites in neuronal cell bodies, which overlap with the actin cytoskeleton. Knockout of MHC class I in cultured neurons increases total dendritic length and the number of branch points. These effects are abolished by reintroducing MHC class I expression. Similarly, blocking of MHC class I proteins or PirB by an MHCI antibody or a soluble PirB ectodomain respectively, mimics the knock out phenotype of increased dendritic branching. This effect is correlated with decreased phosphorylation of both LIMK and cofilin, suggesting it may be mediated by an induction of cofilin activity. Finally, layer II and III cortical neurons in the sensorimotor region of an MHC class I deficiency mouse model show increased dendritic growth and branching. Altogether, our results suggest MHC class I plays a role in inhibiting or limiting the degree of dendrite arborization during the development of cortical neurons.

Gestational Hypothyroxinemia Imprints a Switch in the Capacity of Astrocytes and Microglial Cells of the Offspring to React in Inflammation

Hypothyroxinemia (Hpx) is a highly frequent condition characterized by low thyroxine (T₄) and normal 3,3',5'-triiodothyronine (T₃) and thyroid stimulating hormone (TSH) levels in the blood. Gestational Hpx is closely related to cognitive impairment in the human offspring. In animal models gestational Hpx causes impairment at glutamatergic synapsis, spatial learning, and the susceptibility to suffer strong autoimmune diseases like experimental autoimmune encephalomyelitis (EAE). However, the mechanisms underlying these phenotypes are unknown. On the other hand, it has been shown that astrocytes and microglia affect the outcome of EAE. In fact, the activation of astrocytes and microglia in the central nervous system (CNS) contributes to EAE progression. Thus, in this work, the reactivity of astrocytes and microglia from rats gestated in Hpx was evaluated aiming to understand whether these cells are targets of gestational Hpx. Interestingly, microglia derived from the offspring gestated in Hpx were less reactive compared to microglia derived from offspring gestated in euthyroidism. Instead, astrocytes derived from the offspring gestated in Hpx were significantly more reactive than the astrocytes from the offspring gestated in euthyroidism. This work contributes with novel information regarding the effects of gestational Hpx over astrocytes and microglia in the offspring. It suggests that astrocyte could react strongly to an inflammatory insult inducing neuronal death in the CNS.

Syk kinases are required for spinal commissural axon repulsion at the midline via the ephrin/Eph pathway

In the hematopoietic system, Syk family tyrosine kinases are essential components of immunoreceptor ITAM-based signaling. While there is increasing data indicating the involvement of immunoreceptors in neural functions, the contribution of Syk kinases remains obscure. Previously, we identified phosphorylated forms of Syk kinases in specialized populations of migrating neurons or projecting axons. Moreover, we identified ephrin/Eph as guidance molecules utilizing the ITAM-bearing CD3zeta (Cd247) and associated Syk kinases for the growth cone collapse response induced in vitro Here, we show that in the developing spinal cord, Syk is phosphorylated in navigating commissural axons. By analyzing axon trajectories in open-book preparations of Syk(-/-); Zap70(-/-) mouse embryos, we show that Syk kinases are dispensable for attraction towards the midline but confer growth cone responsiveness to repulsive signals that expel commissural axons from the midline. Known to serve a repulsive function at the midline, ephrin B3/EphB2 are obvious candidates for driving the Syk-dependent repulsive response. Indeed, Syk kinases were found to be required for ephrin B3-induced growth cone collapse in cultured commissural neurons. In fragments of commissural neuron-enriched tissues, Syk is in a constitutively phosphorylated state and ephrin B3 decreased its level of phosphorylation. Direct pharmacological inhibition of Syk kinase activity was sufficient to induce growth cone collapse. In conclusion, Syk kinases act as a molecular switch of growth cone adhesive and repulsive responses.

Gestational Hypothyroxinemia Affects Glutamatergic Synaptic Protein Distribution and Neuronal Plasticity Through Neuron-Astrocyte Interplay

Gestational hypothyroxinemia, characterized by low levels of maternal thyroxine (T₄) during gestation, is closely associated with cognitive impairment in offspring. Studies in animal models have shown that this condition alters neuronal glutamatergic synapses in the hippocampus. Given that astrocytes critically contribute to the establishment and functioning of synapses, the aim of this study was to determine the effects of gestational hypothyroxinemia on the capacity of astrocytes to regulate glutamatergic synapses. In an in vitro co-culture model of astrocytes and hippocampal neurons, gestational hypothyroxinemia profoundly affected the synaptic patterns of GluN1 and CD3ζ in an astrocyte-dependent manner. These effects were associated with impaired plasticity that was dependent on both neuronal and astrocyte contributions. These results highlight the importance of neuron-astrocyte interplay in the deleterious effects of gestational hypothyroxinemia and the timely diagnosis and treatment of this condition during gestation to ensure proper central nervous system development in offspring.

Regulating Rac in the nervous system: molecular function and disease implication of Rac GEFs and GAPs

Rho family GTPases, including RhoA, Rac1, and Cdc42 as the most studied members, are master regulators of actin cytoskeletal organization. Rho GTPases control various aspects of the nervous system and are associated with a number of neuropsychiatric and neurodegenerative diseases. The activity of Rho GTPases is controlled by two families of regulators, guanine nucleotide exchange factors (GEFs) as the activators and GTPase-activating proteins (GAPs) as the inhibitors. Through coordinated regulation by GEFs and GAPs, Rho GTPases act as converging signaling molecules that convey different upstream signals in the nervous system. So far, more than 70 members of either GEFs or GAPs of Rho GTPases have been identified in mammals, but only a small subset of them have well-known functions. Thus, characterization of important GEFs and GAPs in the nervous system is crucial for the understanding of spatiotemporal dynamics of Rho GTPase activity in different neuronal functions. In this review, we summarize the current understanding of GEFs and GAPs for Rac1, with emphasis on the molecular function and disease implication of these regulators in the nervous system.

Impaired spatial memory in mice lacking CD3ζ is associated with altered NMDA and AMPA receptors signaling independent of T-cell deficiency

The immunoreceptor-associated protein CD3ζ is known for its role in immunity and has also been implicated in neuronal development and synaptic plasticity. However, the mechanism by which CD3ζ regulates synaptic transmission remains unclear. In this study, we showed that mice lacking CD3ζ exhibited defects in spatial learning and memory as examined by the Barnes maze and object location memory tasks. Given that peripheral T cells have been shown to support cognitive functions and neural plasticity, we generated CD3ζ(-/-) mice in which the peripheral T cells were repopulated to a normal level by syngeneic bone marrow transplantation. Using this approach, we showed that T-cell replenishment in CD3ζ(-/-) mice did not restore spatial memory defects, suggesting that the cognitive deficits in CD3ζ(-/-) mice were most likely mediated through a T-cell-independent mechanism. In support of this idea, we showed that CD3ζ proteins were localized to glutamatergic postsynaptic sites, where they interacted with the NMDAR subunit GluN2A. Loss of CD3ζ in brain decreased GluN2A-PSD95 association and GluN2A synaptic localization. This effect was accompanied by a reduced interaction of GluN2A with the key NMDAR downstream signaling protein calcium/calmodulin-dependent protein kinase II (CaMKII). Using the glycine-induced, NMDA-dependent form of chemical long-term potentiation (LTP) in cultured cortical neurons, we showed that CD3ζ was required for activity-dependent CaMKII autophosphorylation and for the synaptic recruitment of the AMPAR subunit GluA1. Together, these results support the model that the procognitive function of CD3ζ may be mediated through its involvement in the NMDAR downstream signaling pathway leading to CaMKII-dependent LTP induction.

Epigenetic biomarkers of T-cells in human glioma

Immune factors are thought to influence glioma risk and outcomes, but immune profiling studies to further our understanding of the immune response are limited by current immunodiagnostic methods. We developed a new assay to capture glioma immune biology based on quantitative methylation specific PCR (qMSP) of two T-cell genes (CD3Z: T-cells, and FOXP3: Tregs). Flow cytometry of T-cells correlated well with the CD3Z demethylation assay (r = 0.93; p < 2.2 × 10 (-16) ), demonstrating the validity of the assay. Furthermore, there was a high correlation between qMSP and immunohistochemistry (IHC) in quantifying tumor infiltrating T-cells (r = 0.85; p = 3.4 × 10 (-11) ). Applying our qMSP methods to archival whole blood from 65 glioblastoma multiforme (GBM) cases and 94 non-diseased controls, GBM cases had highly statistically significantly lower T-cells (p = 1.7 × 10 (-9) ) as well as Tregs (p = 5.2 × 10 (-11) ) and a modestly lower ratio of Tregs/T-cells (p = 0.024). Applying the methods to 120 excised glioma tumors, we observed that tumor infiltrating CD3+ T-cells were positively correlated with glioma tumor grade (p = 5.7 × 10 (-7) ), and that Tregs were enriched in tumors compared with peripheral blood indicating active chemoattraction of suppressive Tregs into the tumor compartment. Poorer patient survival was correlated with higher levels of tumor infiltrating T-cells (p = 0.01) and Tregs (p = 0.04). DNA methylation based immunodiagnostics represent a new generation of powerful laboratory tools offering many advantages over conventional methods that will facilitate large clinical epidemiologic studies and capitalize on stored archival blood and tissue banks.

NFκB signaling regulates embryonic and adult neurogenesis

Both embryonic and adult neurogenesis involves the self-renewal/proliferation, survival, migration and lineage differentiation of neural stem/progenitor cells. Such dynamic process is tightly regulated by intrinsic and extrinsic factors and complex signaling pathways. Misregulated neurogenesis contributes much to a large range of neurodevelopmental defects and neurodegenerative diseases. The signaling of NFκB regulates many genes important in inflammation, immunity, cell survival and neural plasticity. During neurogenesis, NFκB signaling mediates the effect of numerous niche factors such as cytokines, chemokines, growth factors, extracellular matrix molecules, but also crosstalks with other signaling pathways such as Notch, Shh, Wnt/β-catenin. This review summarizes current progress on the NFκB signaling in all aspects of neurogenesis, focusing on the novel role of NFκB signaling in initiating early neural differentiation of neural stem cells and embryonic stem cells.