Under physiological conditions, mitochondrial morphology dynamically shifts between a punctuate appearance and tubular networks. However, little is known about upstream signal transduction pathways that regulate mitochondrial morphology. We show that mitochondrial fission is a very early and kinetically invariant event during neuronal cell death, which causally contributes to cytochrome c release and neuronal apoptosis. Using a small molecule CDK5 inhibitor, as well as a dominantnegative CDK5 mutant and RNAi knockdown experiments, we identified CDK5 as an upstream signalling kinase that regulates mitochondrial fission during apoptosis of neurons. Vice versa, our study shows that mitochondrial fission is a modulator contributing to CDK5-mediated neurotoxicity. Thereby, we provide a link that allows integration of CDK5 into established neuronal apoptosis pathways. Mitochondria can acquire a broad range of morphologies, from a punctuate shape to a tubular appearance or even extended networks. Several proteins, most of them belonging to the family of large GTPases, have been identified that energy-dependently regulate mitochondrial morphology and subcellular distribution by promoting either fission (Fis1, dynamin-related protein 1 (Drp1), MTP18) or fusion of mitochondria (Mfn1/2 and Opa1). 1-3 As a potential upstream regulatory factor, the actin cytoskeleton has been shown to be a prerequisite for mitochondrial fission, possibly by its influence on Drp1 recruitment to mitochondria. 4 Apoptotic release of the mitochondrial cytochrome c and other proapoptotic mitochondrial proteins results in formation of the so-called apoptosome, which in turn activates downstream effector caspases with death executing function. 5 This intrinsic, mitochondrial death pathway is relevant for most forms of neuronal apoptosis, in contrast to extrinsic, for example death receptor mediated, activation of programmed cell death. 6 However, despite the contribution of mitochondrial dysfunction specifically to the demise of neurons, there is only scarce knowledge about mitochondrial fission during neuronal apoptosis, nor about its functional contribution to neuronal cell death. Furthermore, little is known about upstream regulatory pathways for mitochondrial morphology in general.Cyclin-dependent kinase 5 (CDK5) is a member of the CDK family, but, distinct from other CDKs, it does not participate in cell cycle regulation. 7 Instead, CDK5 is physiologically implicated in cytoskeletal functions, as well as regulation of membrane turnover and morphology, for example cell migration or endocytosis. Moreover, it is involved in neuronspecific functions including, for example, synaptic plasticity, axonal outgrowth or transmitter release. [8][9][10][11] In neurological diseases, deregulated CDK5 turned out to be an apical instigator of neuronal cell death cascades. Amyloid-toxicity was shown to induce calpain-mediated cleavage of the CDK5 activators p35 or p39 to p25 and p29. This leads to redistribution and overactivation of CDK5 after its association...

We have recently shown that the hematopoietic GranulocyteColony Stimulating Factor (G-CSF) is neuroprotective in rodent stroke models, and that this action appears to be mediated via a neuronal G-CSF receptor. Here, we report that the G-CSF receptor is expressed in rodent dopaminergic substantia nigra neurons, suggesting that G-CSF might be neuroprotective for dopaminergic neurons and a candidate molecule for the treatment of Parkinson's disease. Thus, we investigated protective effects of G-CSF in 1-methyl-4-phenylpyridinium (MPP + )-challenged PC12 cells and primary neuronal midbrain cultures, as well as in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease. Substantial protection was found against MPP + -induced dopaminergic cell death in vitro. Moreover, subcutaneous application of G-CSF at a dose of 40 lg/Kg body weight daily over 13 days rescued dopaminergic substantia nigra neurons from MPTP-induced death in aged mice, as shown by quantification of tyrosine hydroxylase-positive substantia nigra cells. Using HPLC, a corresponding reduction in striatal dopamine depletion after MPTP application was observed in G-CSF-treated mice. Thus our data suggest that G-CSF is a novel therapeutic opportunity for the treatment of Parkinson's disease, because it is well-tolerated and already approved for the treatment of neutropenic conditions in humans.

No abstract