Proteins of the Bcl-2 family are intracellular membrane-associated proteins that regulate programmed cell death (apoptosis) either positively or negatively by as yet unknown mechanisms. Bax, a pro-apoptotic member of the Bcl-2 family, was shown to form channels in lipid membranes. Bax triggered the release of liposome-encapsulated carboxyfluorescein at both neutral and acidic pH. At physiological pH, release could be blocked by Bcl-2. Bcl-2, in contrast, triggered carboxyfluorescein release at acidic pH only. In planar lipid bilayers, Bax formed pH- and voltage-dependent ion-conducting channels. Thus, the pro-apoptotic effects of Bax may be elicited through an intrinsic pore-forming activity that can be antagonized by Bcl-2.

Approximately half of the neurons produced during embryogenesis normally die before adulthood. Although target-derived neurotrophic factors are known to be major determinants of programmed cell death--apoptosis--the molecular mechanisms by which trophic factors interfere with cell death regulation are largely unknown. Overexpression of the bcl-2 proto-oncogene in cultured sympathetic neurons has now been shown to prevent apoptosis normally induced by deprivation of nerve growth factor. This finding, together with the previous demonstration of bcl-2 expression in the nervous system, suggests that the Bcl-2 protein may be a major mediator of the effects of neurotrophic factors on neuronal survival.

MKP-1 (also known as CL100, 3CH134, Erp, and hVH-1) exemplifies a class of dual-specificity phosphatase able to reverse the activation of mitogen-activated protein (MAP) kinase family members by dephosphorylating critical tyrosine and threonine residues. We now report the cloning of MKP-3, a novel protein phosphatase that also suppresses MAP kinase activation state. The deduced amino acid sequence of MKP-3 is 36% identical to MKP-1 and contains the characteristic extended activesite sequence motif VXVHCXXGXSRSXTXXXAYLM (where X is any amino acid) as well as two N-terminal CH2 domains displaying homology to the cell cycle regulator Cdc25 phosphatase. When expressed in COS-7 cells, MKP-3 blocks both the phosphorylation and enzymatic activation of ERK2 by mitogens. Northern analysis reveals a single mRNA species of 2.7 kilobases with an expression pattern distinct from other dual-specificity phosphatases. MKP-3 is expressed in lung, heart, brain, and kidney, but not significantly in skeletal muscle or testis. In situ hybridization studies of MKP-3 in brain reveal enrichment within the CA1, CA3, and CA4 layers of the hippocampus. Metrazole-stimulated seizure activity triggers rapid (<1 h) but transient up-regulation of MKP-3 mRNA in the cortex, piriform cortex, and some amygdala nuclei. Metrazole stimulated similar regional up-regulation of MKP-1, although this was additionally induced within the thalamus. MKP-3 mRNA also undergoes powerful induction in PC12 cells after 3 h of nerve growth factor treatment. This response appears specific insofar as epidermal growth factor and dibutyryl cyclic AMP fail to induce significant MKP-3 expression. Subcellular localization of epitope-tagged MKP-3 in sympathetic neurons reveals expression in the cytosol with exclusion from the nucleus. Together, these observations indicate that MKP-3 is a novel dual-specificity phosphatase that displays a distinct tissue distribution, subcellular localization, and regulated expression, suggesting a unique function in controlling MAP kinase family members. Identification of a second partial cDNA clone (MKP-X) encoding the C-terminal 280 amino acids of an additional phosphatase that is 76% identical to MKP-3 suggests the existence of a distinct structurally homologous subfamily of MAP kinase phosphatases.A wide range of cell-surface stimuli, including growth and differentiation factors and cytokines as well as ultraviolet radiation and osmotic shock, trigger rapid and powerful activation of mitogen-activated protein (MAP) 1 kinase family members (1-5). Currently, three major subclasses of MAP kinase can be identified, and these comprise the ERK, SAPK/JNK, and p38/HOG1 families (2, 3, 6). Full activation of MAP kinase requires phosphorylation on critical tyrosine and threonine residues, and several upstream dual-specificity kinases catalyzing this modification have now been identified (1-3, 6). Once activated, MAP kinases phosphorylate and regulate several cellular proteins, including additional protein kinases, cytoskeletal elements, stathmin,...

A number of DNA viruses carry apoptosis-inhibiting genes which enable the virus to escape from the host response. The adenovirus E1B 19K protein can inhibit apoptosis induced by E1A, tumour-necrosis factor-alpha, FAS antigen and nerve growth factor deprivation. The molecular basis of this inhibition remains poorly understood, but the fact that protection is seen in the absence of other viral proteins suggests that E1B 19K targets cellular proteins. We report here the identification of three cellular proteins that bind E1B 19K. One of these is a new member of the bcl-2 family, which we have called bak (for bcl-2 homologous antagonist/killer). This protein, which is expressed in a wide variety of cell types, binds to E1B 19K and to the Bcl-2 homologue Bcl-XL (ref. 17) in yeast. In addition, overexpression of bak in sympathetic neurons deprived of nerve growth factor accelerates apoptosis and blocks the protective effect of co-injected E1B 19K.

During apoptosis induced by various stimuli, cytochrome c is released from mitochondria into the cytosol where it participates in caspase activation. This process has been proposed to be an irreversible consequence of mitochondrial permeability transition pore opening, which leads to mitochondrial swelling and rupture of the outer mitochondrial membrane. Here we present data demonstrating that NGF-deprived sympathetic neurons protected from apoptosis by caspase inhibitors possess mitochondria which, though depleted of cytochrome c and reduced in size, remained structurally intact as viewed by electron microscopy. After re-exposure of neurons to NGF, mitochondria recovered their normal size and their cytochrome c content, by a process requiring de novo protein synthesis. Altogether, these data suggest that depletion of cytochrome c from mitochondria is a controlled process compatible with function recovery. The ability of sympathetic neurons to recover fully from trophic factor deprivation provided irreversible caspase inhibitors have been present during the insult period, has therapeutical implications for a number of acute neuropathologies.