Modulation of the cAMP signaling pathway after traumatic brain injury
Introduction
Traumatic brain injury (TBI) is a prevalent, debilitating health problem, occurring in 1.4 million people each year and disabling 5 million people in the United States (Langlois et al., 2004). The subsequent progressive injury after brain trauma develops from hours to days after the initiating insult, providing an accessible time window for pharmacological therapies. Despite intense efforts, research in TBI has not yielded a therapy that has passed Phase III clinical trials (Doppenberg et al., 2004).
Brain trauma results in contusion formation, neuronal apoptosis, and axonal tract damage. These pathologies are worsened by the inflammatory cascade set into motion by the initial injury (Morganti-Kossmann et al., 2002, Dietrich et al., 2004). Two pro-inflammatory cytokines released after TBI are tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). Numerous studies have documented rapid increases in TNF-α and IL-1β levels after TBI (Taupin et al., 1993, Shohami et al., 1994, Fan et al., 1996, Kinoshita et al., 2002, Vitarbo et al., 2004).
IL-1β synergistically acts with TNF-α to induce cell death after TBI. These pro-inflammatory cytokines stimulate inflammatory cells to release damaging reactive oxygen and nitrogen species, raise glutamate levels to excitotoxic levels, impair the ability of glia cells to buffer extracellular potassium, compromise the blood–brain barrier, and attract more inflammatory cells into the brain (Tanaka et al., 1994, Meda et al., 1995, Soares et al., 1995, Hu et al., 1997, Keeling et al., 2000). Once initiated, the inflammatory cascade becomes a toxic positive-feedback loop, further exacerbating brain pathology.
In other models of CNS injury, several studies have demonstrated that restoration of cyclic AMP (cAMP) levels improves outcome. In spinal cord injury, application of rolipram to inhibit the degradation of cAMP promotes axon sparing and results in locomotor improvements (Nikulina et al., 2004, Pearse et al., 2004). Similarly, in transient global ischemia rolipram improves neuronal survival in the hippocampus and hippocampal-dependent learning (Kato et al., 1995, Block et al., 1997, Imanishi et al., 1997, Block et al., 2001).
The effects of cAMP are short-lived because phosphodiesterases (PDEs) rapidly degrade cAMP (Manganiello et al., 1995). Of the 10 classes of PDEs, two isoforms are highly selective for degrading cAMP, PDE IV and VII. Rolipram, a selective inhibitor of PDE IV, reduces inflammation in a number of diseases including asthma, multiple sclerosis, septic shock, rheumatoid arthritis, and inflammatory bowel disease (Dal Piaz and Giovannoni, 2000, Castro et al., 2005). Consequently, PDE IV inhibitors are widely-utilized by the pharmaceutical industry as anti-inflammatory drugs.
A primary action of cAMP is activation of protein kinase A (PKA). PKA phosphorylates transcription factors, including cAMP-responsive element binding (CREB) protein and nuclear factor-κB (NF-κB) p50 (Montminy and Bilezikjian, 1987, Hou et al., 2003). Phosphorylation of CREB stimulates transcription of cell survival genes (Mayr and Montminy, 2001). Phosphorylation of NF-κB p50 subunit suppresses transcription of genes with IκB elements in their promoters; this includes the pro-inflammatory cytokines TNF-α and IL-1β (Cogswell et al., 1994, Verghese et al., 1995, Hou et al., 2003). Thus, we hypothesized that rolipram treatment may improve TBI outcome by decreasing pro-inflammatory cytokine production.
Section snippets
Traumatic brain injury
All experimental procedures were in compliance with the NIH Guide for the Care and Use of Laboratory Animals and approved by the University of Miami Animal Care and Use Committee. Male Sprague–Dawley rats (270–320 g; Charles River Laboratories, Raleigh, NC, USA) were anesthetized with 3% halothane, 70% N2O, and 30% O2, then intubated endotracheally and mechanically ventilated (Harvard Apparatus, Holliston, MA, USA) with 1.5% halothane, 70% N2O, and 30% O2. To immobilize the animals and
Results
To ascertain if the cAMP–PKA pathway is a potential therapeutic target after TBI, we first determined if the cAMP–PKA pathway is modulated after TBI. At various times after sham or FPI surgery, the ipsilateral parietal cortex, hippocampus, and thalamus were assayed by ELISA for cAMP. Absolute levels of cAMP from cortices of sham animals were similar to levels previously reported in the literature (parietal cortex cAMP levels 184.1 ± 5.6 pmol/ml, n = 6) (Pearse et al., 2004). We found that cAMP
Discussion
The parasagittal FPI model leads to reproducible histopathology in the brain, similar to the pathology typically seen in TBI patients (Dietrich et al., 1994, Gennarelli, 1994, Keane et al., 2001, Thompson et al., 2005). Accordingly, there are consistent, quantifiable focal and diffuse histopathologies that are all potential therapeutic targets (Dietrich et al., 1994, Bramlett et al., 1997, Ciallella et al., 2002, Grady et al., 2003, Suzuki et al., 2003, Suzuki et al., 2004, Witgen et al., 2005
Acknowledgments
This work was supported by NIH grants NS30291 and NS42133 (W.D.D.). We thank the Dietrich and Pearse labs for helpful discussions and Beata Frydel and Jarret Weinrich for technical assistance.
References (107)
Role of impaired cAMP and calcium-sensitive K+ channel function in altered cerebral hemodynamics following brain injury
Brain Res.
(1997)- et al.
Ca(2+)/CREB/CBP-dependent gene regulation: a shared mechanism critical in long-term synaptic plasticity and neuronal survival
Cell Calcium
(2003) - et al.
Inflammation contributes to the postponed ischemic neuronal damage following treatment with a glutamate antagonist in rats
Neurosci. Lett.
(2001) - et al.
Type VIII adenylyl cyclase. A Ca2+/calmodulin-stimulated enzyme expressed in discrete regions of rat brain
J. Biol. Chem.
(1994) - et al.
Phosphodiesterase 4 inhibitors, structurally unrelated to rolipram, as promising agents for the treatment of asthma and other pathologies
Eur. J. Med. Chem.
(2000) - et al.
Beneficial effects of rolipram in a quinolinic acid model of striatal excitotoxicity
Neurobiol. Dis.
(2007) - et al.
Expression of the neurotrophin receptor trkB is regulated by the cAMP/CREB pathway in neurons
Mol. Cell. Neurosci.
(2004) - et al.
Experimental brain injury induces differential expression of tumor necrosis factor-α mRNA in the CNS
Brain Res. Mol. Brain Res.
(1996) - et al.
Relationships between seizure activity and cyclic nucleotide levels in brain
Brain Res.
(1980) - et al.
Concussive brain injury is associated with a prolonged accumulation of calcium: A 45Ca autoradiographic study
Brain Res.
(1993)