Experimental traumatic brain injury (TBI) and spinal cord injury (SCI) result in a rapid and significant necrosis of neuronal tissue at the site of injury. In the ensuing hours and days, secondary injury exacerbates the primary damage, resulting in significant neurologic dysfunction. It is believed that alterations in excitatory amino acids (EAA), increased reactive oxygen species (ROS), and the disruption of Ca(2+) homeostasis are major factors contributing to the ensuing neuropathology. Mitochondria serve as the powerhouse of the cell by maintaining ratios of ATP:ADP that thermodynamically favor the hydrolysis of ATP to ADP + P(i), yet a byproduct of this process is the generation of ROS. Proton-pumping by components of the electron transport system (ETS) generates a membrane potential (DeltaPsi) that can then be used to phosphorylate ADP or sequester Ca(2+) out of the cytosol into the mitochondrial matrix. This allows mitochondria to act as cellular Ca(2+) sinks and to be in phase with changes in cytosolic Ca(2+) levels. Under extreme loads of Ca(2+), however, opening of the mitochondrial permeability transition pore (mPTP) results in the extrusion of mitochondrial Ca(2+) and other high- and low-molecular weight components. This catastrophic event discharges DeltaPsi and uncouples the ETS from ATP production. Cyclosporin A (CsA), a potent immunosuppressive drug, inhibits mitochondrial permeability transition (mPT) by binding to matrix cyclophilin D and blocking its binding to the adenine nucleotide translocator. Peripherally administered CsA attenuates mitochondrial dysfunction and neuronal damage in an experimental rodent model of TBI, in a dose-dependent manner. The underlying mechanism of neuroprotection afforded by CsA is most likely via interaction with the mPTP because the immunosuppressant FK506, which has no effect on the mPT, was not neuroprotective. When CsA was administrated after experimental SCI at the same dosage and regimen used TBI paradigms, however, it had no beneficial neuroprotective effects. This review takes a comprehensive and critical look at the evidence supporting the role for mPT in central nervous system (CNS) trauma and highlights the differential responses of CNS mitochondria to mPT induction and the implications this has for therapeutically targeting the mPT in TBI and SCI.

Ras is a universal eukaryotic intracellular protein integrating extracellular signals from multiple receptor types. To investigate its role in the adult central nervous system, constitutively activated V12-Ha-Ras was expressed selectively in neurons of transgenic mice via a synapsin promoter. Ras-transgene protein expression increased postnatally, reaching a four- to fivefold elevation at day 40 and persisting at this level, thereafter. Neuronal Ras was constitutively active and a corresponding activating phosphorylation of mitogen-activated kinase was observed, but there were no changes in the activity of phosphoinositide 3-kinase, the phosphorylation of its target kinase Akt/PKB, or expression of the anti-apoptotic proteins Bcl-2 or Bcl-XL. Neuronal Ras activation did not alter the total number of neurons, but induced cell soma hypertrophy, which resulted in a 14.5% increase of total brain volume. Choline acetyltransferase and tyrosine hydroxylase activities were increased, as well as neuropeptide Y expression. Degeneration of motorneurons was completely prevented after facial nerve lesion in Ras-transgenic mice. Furthermore, neurotoxin-induced degeneration of dopaminergic substantia nigra neurons and their striatal projections was greatly attenuated. Thus, the Ras signaling pathway mimics neurotrophic effects and triggers neuroprotective mechanisms in adult mice. Neuronal Ras activation might become a tool to stabilize donor neurons for neural transplantation and to protect neuronal populations in neurodegenerative diseases.

Cyclosporin A (CsA) shows cytoprotective properties in many cellular and in vivo models that may depend on interference of the interaction of cyclophilin A with calcineurin or of cyclophilin D with the mitochondrial permeability transition (PT) pore. The nonimmunosuppressive cyclosporin derivative N-methyl-4-valine-cyclosporin (PKF220-384) inhibits the mitochondrial permeability transition (MPT) like CsA but without calcineurin inactivation. PKF220-384 has been used to discriminate between PT pore-and calcineurin mediated effects but is no longer available. Here, we evaluated the effects of another nonimmunosuppressive cyclosporin derivative, N-methyl-4-isoleucinecyclosporin (NIM811) on the MPT. Using two newly developed microtiter plate assays, one measuring mitochondrial swelling from absorbance and the other measuring mitochondrial membrane potential from changes in safranin fluorescence, we show that NIM811 blocks the MPT induced by calcium and inorganic phosphate, alone or in combination with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, the complex I inhibitor rotenone, and the prooxidant tbutylhydroperoxide. NIM811 was equipotent to CsA and half as potent as PKF220-384. Additionally, we show that NIM811 blocks cell killing and prevents in situ mitochondrial inner membrane permeabilization and depolarization during tumor necrosis factor-␣-induced apoptosis to cultured rat hepatocytes. NIM811 inhibition of apoptosis was equipotent with CsA except at higher concentrations: CsA lost efficacy but NIM 811 did not. We conclude that NIM811 is a useful alternative to PKF220-384 to investigate the role of the mitochondrial permeability transition in apoptotic and necrotic cell death.Permeabilization of mitochondrial membranes triggered by proapoptotic molecules and damage pathways is implicated as an important event in the control of cell death and survival and may be involved in both apoptosis and necrosis (for a recent review, see Kroemer and Reed, 2000). Inner membrane permeabilization causes dissipation of the mitochondrial membrane potential, uncoupling of oxidative phosphorylation, ATP depletion, and equilibration of small solutes and ions between the cytosol and the mitochondrial matrix. Solute influx into the matrix can cause swelling followed by rupture of the outer membrane and consequent efflux of proteins from the intermembrane space, including cytochrome c, procaspase 9, apoptosis-inducing factor, and endonuclease G. However, outer membrane permeabilization is not always related to inner membrane permeabilization but seems also to occur independently.Several models of mitochondrial inner membrane permeabilization, specifically the mitochondrial permeability transition (MPT), are currently under discussion. Onset of the MPT is mediated by opening of a proteinaceous permeability transition (PT) pore that probably forms at contact sites between the inner and outer membranes. The PT pore is postulated to form through combination of the adenine nucleotide translocator of ...

The mitochondrial permeability transition (MPT) plays an important role in damage-induced cell death, and agents inhibiting the MPT may have a therapeutic potential for treating human conditions such as ischemia/reperfusion injury, trauma, and neurodegenerative diseases. The mitochondrial matrix protein, cyclophilin D (CYP D), a member of a family of highly homologous peptidylprolyl cis-trans isomerases (PPIases), plays a decisive role in MPT, being an integral constituent of the MPT pore. Other putative MPT pore proteins include the adenine nucleotide translocator (ANT) and the voltage-dependent anion channel (VDAC). In an alternative model, the MPT pore is formed by clusters of misfolded membrane proteins outlining aqueous channels that are regulated by CYP D and other chaperone-like proteins. Like cyclophilin A (CYP A) and other cyclophilin family members, CYP D is targeted by the immunosuppressant cyclosporin A (CsA). CsA is cytoprotective in many cellular and animal models, but protection may result from either inhibition of the MPT through an interaction with CYP D or inhibition of calcineurin-mediated dephosphorylation of BAD through an interaction with CYP A. The relevance of MPT inhibition by CsA for its cytoprotective effects is well documented in many cellular models. Mechanisms of action in vivo are more difficult to define, and accordingly the evidence is as yet less compelling in in vivo animal models of ischemia/reperfusion injury, trauma and neurodegenerative diseases. Notwithstanding, CYP D is a drug target of high interest. Structural considerations suggest feasibility of designing CYP D ligands without immunosuppressant properties. This is highly desirable, since they have the potential of being useful therapeutic agents in a variety of disease states. It might be a tougher challenge to obtain compounds specific for CYP D vs. other cyclophilins, and/or of small molecular weight, allowing brain penetration to make them suitable for treating neurodegenerative diseases.

gamma-Hydroxybutyrate (GHB), a metabolite of gamma-aminobutyric acid (GABA), is proposed to function as a neurotransmitter or neuromodulator. gamma-Hydroxybutyrate and its prodrug, gamma-butyrolactone (GBL), recently received increased public attention as they emerged as popular drugs of abuse. The actions of GHB/GBL are believed to be mediated by GABAB and/or specific GHB receptors, the latter corresponding to high-affinity [3H]GHB-binding sites coupled to G-proteins. To investigate the contribution of GABAB receptors to GHB actions we studied the effects of GHB in GABAB(1)-/- mice, which lack functional GABAB receptors. Autoradiography reveals a similar spatial distribution of [3H]GHB-binding sites in brains of GABAB(1)-/- and wild-type mice. The maximal number of binding sites and the KD values for the putative GHB antagonist [3H]6,7,8,9-tetrahydro-5-hydroxy-5H-benzocyclohept-6-ylidene acetic acid (NCS-382) appear unchanged in GABAB(1)-/- compared with wild-type mice, demonstrating that GHB- are distinct from GABAB-binding sites. In the presence of the GABAB receptor positive modulator 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol GHB induced functional GTPgamma[35S] responses in brain membrane preparations from wild-type but not GABAB(1)-/- mice. The GTPgamma[35S] responses in wild-type mice were blocked by the GABAB antagonist [3-[[1-(S)-(3,4dichlorophenyl)ethyl]amino]-2-(S)-hydroxy-propyl]-cyclohexylmethyl phosphinic acid hydrochloride (CGP54626) but not by NCS-382. Altogether, these findings suggest that the GHB-induced GTPgamma[35S] responses are mediated by GABAB receptors. Following GHB or GBL application, GABAB(1)-/- mice showed neither the hypolocomotion, hypothermia, increase in striatal dopamine synthesis nor electroencephalogram delta-wave induction seen in wild-type mice. It, therefore, appears that all studied GHB effects are GABAB receptor dependent. The molecular nature and the signalling properties of the specific [3H]GHB-binding sites remain elusive.