Dysfunction of mitochondrial complex I is associated with a wide spectrum of neurodegenerative disorders, including Parkinson's disease (PD). In rodents, inhibition of complex I leads to degeneration of dopaminergic neurons of the substantia nigra pars compacta (SNpc), as seen in PD, through activation of mitochondriadependent apoptotic molecular pathways. In this scenario, complex I blockade increases the soluble pool of cytochrome c in the mitochondrial intermembrane space through oxidative mechanisms, whereas activation of pro-cell death protein Bax is actually necessary to trigger neuronal death by permeabilizing the outer mitochondrial membrane and releasing cytochrome c into the cytosol. Activation of Bax after complex I inhibition relies on its transcriptional induction and translocation to the mitochondria. apoptosis ͉ Bax ͉ Bim ͉ complex I ͉ 1-methyl-4-pheny-1,2,3,6-tetrahydropyridine C omplex I deficiency impairs mitochondrial respiration and is associated with a wide spectrum of neurodegenerative disorders, including Parkinson's disease (PD). Reduced complex I activity is found in both autopsy brain tissues and platelets of patients affected with sporadic PD (1-3). Furthermore, complex I inhibitors, such as 1-methyl-4-pheny-1,2,3,6-tetrahydropyridine (MPTP), reproduce some of the clinical and neuropathological hallmarks of PD in monkeys and humans, including degeneration of dopaminergic (DA) neurons of the substantia nigra pars compacta (SNpc) (4). Studies in rodents and human postmortem PD samples indicate that SNpc DA neurodegeneration linked to complex I deficiency occurs, at least in part, through activation of mitochondria-dependent apoptotic molecular pathways (5, 6). Complex I blockade, however, is not the actual executioner but rather sensitizes neurons to mitochondria-dependent apoptosis through oxidative damage and activation of the proapoptotic Bcl-2 family member Bax (6).Activation of Bax relies, in most instances, not only on its transcriptional induction but also on its posttranslational modification. The latter results in Bax translocation and insertion into the mitochondrial outer membrane, thereby eliciting cytochrome c release and activation of the caspase cascade, which ultimately causes cell death (7). Both transcriptional and posttranslational activation of Bax have been observed in the SNpc of MPTPintoxicated mice (6,8) and PD patients (9, 10). Furthermore, genetic ablation of Bax in mutant mice prevents mitochondriadependent apoptotic SNpc DA cell death caused by complex I inhibition with MPTP (6, 8). In contrast, both Bid and Bak, which cooperate with Bax to initiate mitochondria-dependent apoptosis in response to activation of cell-surface death receptors, are probably dispensable for MPTP-induced neuronal death (11, 12). Bax thus governs SNpc DA cell death linked to PD-related complex I deficiency. However, the molecular mechanisms of Bax activation after complex I blockade remain unknown.Although Bax transcriptional induction associated with complex I blockade might be media...

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces clinical, biochemical, and neuropathological changes reminiscent of those occurring in idiopathic Parkinson's disease (PD).Here we show that a peptide caspase inhibitor, N-benzyloxycarbonyl-val-ala-asp-fluoromethyl ketone, or adenoviral gene transfer (AdV) of a protein caspase inhibitor, X-chromosome-linked inhibitor of apoptosis (XIAP), prevent cell death of dopaminergic substantia nigra pars compacta (SNpc) neurons induced by MPTP or its active metabolite 1-methyl-4-phenylpyridinium in vitro and in vivo. Because the MPTP-induced decrease in striatal concentrations of dopamine and its metabolites does not differ between AdV-XIAP-and control vector-treated mice, this protection is not associated with a preservation of nigrostriatal terminals. In contrast, the combination of adenoviral gene transfer of XIAP and of the glial cell line-derived neurotrophic factor to the striatum provides synergistic effects, rescuing dopaminergic SNpc neurons from cell death and maintaining their nigrostriatal terminals. These data suggest that a combination of a caspase inhibitor, which blocks death, and a neurotrophic factor, which promotes the specific function of the rescued neurons, may be a promising strategy for the treatment of PD.

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by the loss of ␣-motoneurons in the spinal cord followed by atrophy of skeletal muscles. SMA-determining candidate genes, SMN1 and SMN2, have been identified on human chromosome 5q. The corresponding SMN protein is expressed ubiquitously. It is coded by seven exons and contains conspicuous proline-rich motifs in its COOH-terminal third (exons 4, 5, and 6). Such motifs are known to bind to profilins (PFNs), small proteins engaged in the control of actin dynamics. We tested whether profilins interact with SMN via its polyproline stretches. Using the yeast two-hybrid system we show that profilins bind to SMN and that this binding depends on its proline-rich motifs. These results were confirmed by coimmunoprecipitation and by in vitro binding studies. Two PFN isoforms, I and II, are known, of which II is characteristic for central nervous system tissue. We show by in situ hybridization that both PFNs are highly expressed in mouse spinal cord and that PFN II is expressed predominantly in neurons. In motoneurons, the primary target of neurodegeneration in SMA, profilins are highly concentrated and colocalize with SMN in the cytoplasm of the cell body and in nuclear gems. Likewise, SMN and PFN I colocalize in gems of HeLa cells. Although SMN interacts with both profilin isoforms, binding of PFN II was stronger than of PFN I in all assays employed. Because the SMN genes are expressed ubiquitously, our findings suggest that the interaction of PFN II with SMN may be involved in neuron-specific effects of SMN mutations.Spinal muscular atrophies (SMAs) 1 types I, II, and III are autosomal hereditary diseases of graded severity in which loss of motoneurons leads to paralysis and subsequent atrophy of skeletal muscles, and in the most severe Werdnig-Hoffmann type I form, to death in early infancy. The corresponding SMA disease genes have been mapped to human chromosome 5q (1). There are two genes in close vicinity, the telomeric SMN1 (or SMN T ) and the centromeric SMN2 (or SMN C ; 2). Although they have identical coding sequences for a 294-amino acid SMN polypeptide, pathogenic mutations were found solely in SMN1. Its gene product, the 40-kDa protein "survival motoneuron," SMN, is expressed ubiquitously, and its concentration is reduced drastically in the spinal cord of SMA patients (3, 4). There is evidence, mostly from a yeast two-hybrid screen and from a Xenopus oocyte model system, that the SMN protein is engaged in the assembly of spliceosomal U snRNPs in the cytoplasm (5-7). Recently, the function of SMN has been demonstrated by a dominant-negative mutant of SMN which inhibits pre-mRNA splicing by blocking the formation of a mature spliceosome (8).The severity of SMA has been correlated with a deficient oligomerization of mutated SMN proteins, and it has been hypothesized that the critical level of functional SMN oligomers in normal motoneurons may be controlled by SMNЈs binding to a motoneuron-specific factor (9). There are seven coding exo...

Duplications and overexpression of the proteolipid protein (PLP) gene are known to cause the dysmyelinating disorder Pelizaeus-Merzbacher disease (PMD). To understand the cellular response to overexpressed PLP in PMD, we have overexpressed PLP in BHK cells and primary cultures of oligodendrocytes with the Semliki Forest virus expression system. Overexpressed PLP was routed to late endosomes/lysosomes and caused a sequestration of cholesterol in these compartments. Similar results were seen in transgenic mice overexpressing PLP. With time, the endosomal/lysosomal accumulation of cholesterol and PLP led to an increase in the amount of detergent-insoluble cellular cholesterol and PLP. In addition, two fluorescent sphingolipids, BODIPY–lactosylceramide and –galactosylceramide, which under normal conditions are sorted to the Golgi apparatus, were missorted to perinuclear structures. This was also the case for the lipid raft marker glucosylphosphatidylinositol–yellow fluorescence protein, which under normal steady-state conditions is localized on the plasma membrane and to the Golgi complex. Taken together, we show that overexpression of PLP leads to the formation of endosomal/lysosomal accumulations of cholesterol and PLP, accompanied by the mistrafficking of raft components. We propose that these accumulations perturb the process of myelination and impair the viability of oligodendrocytes.