Tumor necrosis factor (TNF) is an important factor in various acute and chronic neurodegenerative disorders. In retinal ischemia, we show early, transient upregulation of TNF, TNF receptor 1 (TNF-R1), and TNF-R2 6 hr after reperfusion preceding neuronal cell loss. To assess the specific role of TNF and its receptors, we compared ischemia-reperfusion-induced retinal damage in mice deficient for TNF-R1, TNF-R2, or TNF by quantifying neuronal cell loss 8 d after the insult. Surprisingly, TNF deficiency did not affect overall cell loss, yet absence of TNF-R1 led to a strong reduction of neurodegeneration and lack of TNF-R2 led to an enhancement of neurodegeneration, indicative of TNF-independent and TNF-dependent processes in the retina, with TNF-R1 augmenting neuronal death and TNF-R2 promoting neuroprotection. Western blot analyses of retinas revealed that reduction of neuronal cell loss in TNF-R1/ animals correlated with the presence of activated Akt/protein kinase B (PKB). Inhibition of the phosphatidylinositol 3-kinase signaling pathway reverted neuroprotection in TNF-R1-deficient mice, indicating an instrumental role of Akt/PKB in neuroprotection and TNF-R2 dependence of this pathway. Selective inhibition of TNF-R1 function may represent a new approach to reduce ischemia-induced neuronal damage, being potentially superior to strategies aimed at suppression of TNF activity in general.

We have previously shown that two tumor necrosis factor (TNF) receptors (TNFR) exhibit antagonistic functions during neurodegenerative processes in vivo with TNFR1 aggravating and TNFR2 reducing neuronal cell loss, respectively. To elucidate the neuroprotective signaling pathways of TNFR2, we investigated glutamate-induced excitotoxicity in primary cortical neurons. TNF-expressing neurons from TNF-transgenic mice were found to be strongly protected from glutamate-induced apoptosis. Neurons from wild type and TNFR1^-/- mice prestimulated with TNF or agonistic TNFR2-specific antibodies were also resistant to excitotoxicity, whereas TNFR2^-/- neurons died upon glutamate and/or TNF exposures. Both protein kinase B/Akt and nuclear factor-κB (NF-κB) activation were apparent upon TNF treatment. Both TNFR1 and TNFR2 induced the NF-κB pathway, yet with distinguishable kinetics and upstream activating components, TNFR1 only induced transient NF-κB activation, whereas TNFR2 facilitated long term phosphatidylinositol 3-kinase-dependent NF-κB activation strictly. Glutamate-induced triggering of the ionotropic N-methyl-d-aspartate receptor was required for the enhanced and persistent phosphatidylinositol 3-kinase-dependent NF-κB activation by TNFR2, indicating a positive cooperation of TNF and neurotransmitter-induced signal pathways. TNFR2-induced persistent NF-κB activity was essential for neuronal survival. Thus, the duration of NF-κB activation is a critical determinant for sensitivity toward excitotoxic stress and is dependent on a differential upstream signal pathway usage of the two TNFRs.

Tetanus toxin is a potent neurotoxin that inhibits the release of neurotransmitters from presynaptic nerve endings. The mature toxin is composed of a heavy and a light chain that are linked via a disulfide bridge. After entry of tetanus toxin into the cytoplasm, the released light chain causes block of neurotransmitter release. Recent evidence suggests that the L-chain may act as a metalloendoprotease. Here we demonstrate that blockade of neurotransmission by tetanus toxin in isolated nerve terminals is associated with a selective proteolysis of synaptobrevin, an integral membrane protein of synaptic vesicles. No other proteins appear to be affected by tetanus toxin. In addition, recombinant light chain selectively cleaves synaptobrevin when incubated with purified synaptic vesicles. Our data suggest that cleavage of synaptobrevin is the molecular mechanism of tetanus toxin action.

Extensive research has been performed to unravel the mechanistic signaling pathways mediated by tumor necrosis factor receptor 1 (TNFR1), by contrast there is limited knowledge on cellular signaling upon activation of TNFR2. Recently published data have revealed that these two receptors not only function independently, but also can influence each other via cross‐talk between the different signaling pathways initiated by TNFR1 and TNFR2 stimulation. Furthermore, the complexity of this cross‐talk is also dependent on the different signaling kinetics between TNFR1 and TNFR2, by which a delicate balance between cell survival and apoptosis can be maintained. Some known signaling factors and the kinetics that are involved in the receptor cross‐talk between TNFR1 and TNFR2 are the topic of this review.

Alzheimer's disease (AD) is associated with an altered immune response, resulting in chronic increased inflammatory cytokine production with a prominent role of TNF‐α. TNF‐α signals are mediated by two receptors: TNF receptor 1 (TNFR1) and TNF receptor 2 (TNFR2). Signaling through TNFR2 is associated with neuroprotection, whereas signaling through TNFR1 is generally proinflammatory and proapoptotic. Here, we have identified a TNF‐α‐induced proinflammatory agent, lipocalin 2 (Lcn2) via gene array in murine primary cortical neurons. Further investigation showed that Lcn2 protein production and secretion were activated solely upon TNFR1 stimulation when primary murine neurons, astrocytes, and microglia were treated with TNFR1 and TNFR2 agonistic antibodies. Lcn2 was found to be significantly decreased in CSF of human patients with mild cognitive impairment and AD and increased in brain regions associated with AD pathology in human postmortem brain tissue. Mechanistic studies in cultures of primary cortical neurons showed that Lcn2 sensitizes nerve cells to β‐amyloid toxicity. Moreover, Lcn2 silences a TNFR2‐mediated protective neuronal signaling cascade in neurons, pivotal for TNF‐a‐mediated neuroprotection. The present study introduces Lcn2 as a molecular actor in neuroinflammation in early clinical stages of AD.—Naudé, P.J. W., Nyakas, C., Eiden, L. E., Ait‐Ali, D., van der Heide, R., Engelborghs, S., Luiten, P. G. M., De Deyn, P. P., den Boer, J. A., Eisel, U. L. M. Lipocalin 2: Novel component of proinflammatory signaling in Alzheimer's disease. FASEB J. 26, 2811–2823 (2012). www.fasebj.org