During cerebral ischemia NF B is a pri mary regulator of the inflammatory response to ischemic injury, affecting cell death and dilution calculator survival. Microglia, the resident immune cells in the brain, are activated follow ing ischemia and play a controversial role in this decision. Microglia respond to injury in part by releasing both cytoprotective and cytotoxic signaling molecules to sur rounding cells, many of which are regulated by NF B. As the dynamics of NF B activation control gene expression, characterizing the dynamics of NF B activation in microglia is of great interest. Members of the NF B family of transcription factors are found in their inactive state as dimers bound to their IkB inhibitor proteins. Upon stimulation by a diverse set of stimuli, NF B is freed from its inhibitor to coordinate gene expression in a highly specific and tightly regulated manner.

The I Ba inhibitor and p65,p50 NF B heterodimer are the most extensively studied members of their respective families, and their response to extracellu lar stimuli illustrates the canonical pathway of NF B activation. In the canonical pathway, binding of extracellular TNFa trimers to TNFR1 receptors at the cell membrane initiates NF B activation. The ligand receptor complex interacts with several adapter proteins, including TNF receptor associated factor 2 and receptor inter acting protein 1, which are essential for recruit ment and activation of the I B kinase complex. The IKK complex involved in canonical NF B activation is composed primarily of the regulatory subunit IKKg and two catalytic subunits, IKKa IKK1 and IKKb IKK2.

Upstream signals activate IKK by phosphor ylation of the kinase domain of IKKb, which in turn phosphorylates I Ba on serines 32 and 36. Phos phorylated I Ba is recognized by the bTrCP containing Skp1 Culin Roc1 RBx1 Hrt 1 F box E3 ubiquitin ligase complex, which facilitates K48 linked dation by the 26S proteasome. NF B is released following proteasomal degradation of I Ba and translocates to the nucleus, where it activates gene expression. Of the hundreds of genes tar geted by NF B, two in particular are ikba and a20. The expression of these genes is rapidly induced by NF B and triggers the synthesis of de novo I Ba and A20 proteins. Newly synthesized I Ba sequesters NF B from the nucleus to inhibit further transcriptional activ ity, forming a strong negative feedback regulatory mechanism.

The synthesis of A20 proteins creates a sec ond negative feedback loop by regulating the ubiquitina tion of adapter proteins responsible for activating the IKK complex, thus inhibiting further NF B activation. Many characteristics that define Batimastat TNFa induced NF B activation also underlie cellular responses to many other stimuli, necessitating a thorough under standing of this pathway.