The Skaggs Institute
for Chemical Biology

Scientific Report 2007

Cytokine Signaling in Hypothalamic and Hippocampal Neurons in Thermoregulation and Seizures

T. Bartfai, B. Conti, I. Tabarean, A. Vezzani,* O. Osborn, M. Sanchez-Alavez, I. Klein

* Mario Negri Institute for Pharmacological Research, Milan, Italy

Warm-sensitive neurons in the anterior hypothalamic and preoptic areas are key regulators of energy metabolism, core body temperature, and fever. These specialized neurons express receptors for pyrogens such prostaglandin E₂ and the proinflammatory cytokine IL-1. Using pharmacologic approaches based on chemical tools designed and synthesized by J. Rebek, the Skaggs Institute, we delineated a novel, transcription-independent, fast reaction pathway for IL-1 signaling in hypothalamic neurons. This pathway is important in early temperature and endocrine responses in inflammation and infection.

The signaling mediated by IL-1 in hippocampal neurons has long been suspected to be the link between neuroinflammation in neurotrauma and stroke and the altered seizure threshold. Using the tools from Dr. Rebek's laboratory, we established that IL-1 is a proconvulsant that acts through the same pathway we established in hypothalamic neurons. These findings open up novel targets for development of antiepileptic drugs.

IL-1β Signaling in Warm-Sensitive Neurons

Warm-sensitive neurons are detected by electrophysiologic recording of individual neurons. We showed that warm-sensitive neurons express the IL-1 type 1 receptor (IL-1R1) and respond to IL-1 by hyperpolarization. This response involves interaction between the adapter protein myeloid differentiation primary response protein 88 (MyD88) and the complex formed by IL-1R1 and the IL-1 receptor accessory protein (IL-1RAcP) and occupied by IL-1β. We found that IL-1β–mediated activation of N-sphingomyelinase and subsequent ceramide production are blocked by MyD88 mimics such as AS-1 and EM 163, which were supplied by Dr. Rebek. These low molecular weight compounds inhibited IL-1β–mediated hyperpolarization in cultured neurons in vitro and IL-1β–mediated early fever response in mice in vivo and thus indicate interactions between MyD88, IL-1R1, and IL-1RAcP as targets in the control of neuroinflammation in stroke and traumatic brain injury, both of which are followed by fever and seizures.

Proconsulvant Effects of IL-1β

In brain trauma, microglia and neurons rapidly synthesize and secrete IL-1β, and the secreted proinflammatory cytokine acting at the IL-1R1–IL-1RAcP complex in hippocampal neurons can lower the seizure threshold and thus enhance generation of seizures, a common sequela of trauma and stroke. The molecular mechanism of the proconvulsant effects of IL-1β has not been known, and the classical antiepileptic drugs that block sodium and calcium channels or glutamate release or enhance GABAergic transmission do not suppress these effects.

Using the results from our studies of the hypothalamic, nontranscriptional, neuronal IL- 1β signaling via MYD88–N-sphingomyelinase–ceramide pathway, we tested MyD88 mimics and N-sphingomyelinase inhibitors in a seizure model in which kainate was injected into the hippocampus in mice. In collaboration with A. Vezzani, Mario Negri Institute for Pharmacological Research, Milan, Italy, we found that both the MyD88 mimic and the N-sphingomyelinase inhibitor can attenuate the seizure-enhancing effects of IL-1β. Furthermore, ceramide can mimic the proconvulsant effects of IL-1β and thus can be regarded as a second messenger for this cytokine in hippocampal neurons.

These results define the interactions between MyD88, IL-1R1, and IL-1RAcP as drug targets for controlling seizures in neurotrauma and stroke. The results also indicate that AS-1 and EM-163 are prototype compounds that can interrupt this interaction. These findings have major consequences for the development of MyD88 mimics that can penetrate the blood-brain barrier as putative anticonvulsant drugs.

Publications

Bartfai, T., Sanchez-Alavez, M., Andell-Jonsson, S., Schultzberg, M., Vezzani, A., Danielsson, E., Conti, B. Interleukin-1 system in CNS stress: seizures, fever and neurotrauma. Ann. N. Y. Acad. Sci. 1113:173, 2007.