In spite of tremendous growth in recent years in our knowledge of the molecular basis of Parkinson's disease and the molecular pathways of cell injury and death, we remain without therapies that forestall disease progression. While there are many possible explanations for this lack of success, one is that experimental therapeutics to date have not adequately focused on an important component of the disease process, that of axon degeneration. It remains unknown what neuronal compartment, either the soma or the axon, is involved at disease onset, although some have proposed that it is the axons and their terminals that take the initial brunt of injury. Nevertheless, this concept has not been formally incorporated into many of the current theories of disease pathogenesis, and it has not achieved a wide consensus. More importantly, in view of growing evidence that the molecular mechanisms of axon degeneration are separate and distinct from the canonical pathways of programmed cell death that mediate soma destruction, the possibility of early involvement of axons in PD has not been adequately emphasized as a rationale to explore the neurobiology of axons for novel therapeutic targets. We propose that it is ongoing degeneration of axons, not cell bodies, that is the primary determinant of clinically apparent progression of disease, and that future experimental therapeutics intended to forestall disease progression will benefit from a new focus on the distinct mechanisms of axon degeneration.

mTOR is a regulator of cell growth and survival, protein synthesis-dependent synaptic plasticity, and autophagic degradation of cellular components. When triggered by mTOR inactivation, macroautophagy degrades long-lived proteins and organelles via sequestration into autophagic vacuoles. mTOR further regulates synaptic plasticity, and neurodegeneration occurs when macroautophagy is deficient. Nevertheless, whether macroautophagy is a modulator of presynaptic function was previously unknown. We find that the mTOR inhibitor, rapamycin, induces formation of autophagic vacuoles in prejunctional dopaminergic axons with associated decreased axonal profile volumes, synaptic vesicle numbers, and evoked dopamine release. Evoked dopamine secretion was enhanced and recovery accelerated in transgenic mice in which macroautophagy deficiency was restricted to dopaminergic neurons; rapamycin failed to decrease evoked dopamine release in the striatum of these mice. Macroautophagy that follows mTOR inhibition in presynaptic terminals, therefore, rapidly alters presynaptic structure and neurotransmission.

Activation of c‐jun N‐terminal kinase (JNK) by the mitogen‐activated protein kinase cascade has been shown to play an important role in the death of dopamine neurons of the substantia nigra, one of the principal neuronal populations affected in Parkinson’s disease. However, it has remained unknown whether the JNK2 and JNK3 isoforms, either singly or in combination, are essential for apoptotic death, and, if so, the mechanisms involved. In addition, it has been unclear whether they play a role in axonal degeneration of these neurons in disease models. To address these issues we have examined the effect of single and double jnk2 and jnk3 null mutations on apoptosis in a highly destructive neurotoxin model, that induced by intrastriatal 6‐hydroxydopamine. We find that homozygous jnk2/3 double null mutations result in a complete abrogation of apoptosis and a prolonged survival of the entire population of dopamine neurons. In spite of this complete protection at the cell soma level, there was no protection of axons. These studies provide a striking demonstration of the distinctiveness of the mechanisms that mediate cell soma and axon degeneration, and they illustrate the need to identify and target pathways of axon degeneration in the development of neuroprotective therapeutics.

Blockage of the p53 tumor suppressor has been found to impair nerve growth factor (NGF)-induced neurite outgrowth in PC-12 cells. We report herein that such impairment could be rescued by stimulation of the A 2A adenosine receptor (A 2A -R), a G protein-coupled receptor implicated in neuronal plasticity. The A 2A -R-mediated rescue occurred in the presence of protein kinase C (PKC) inhibitors or protein kinase A (PKA) inhibitors and in a PKA-deficient PC-12 variant. Thus, neither PKA nor PKC was involved. In contrast, expression of a truncated A 2A -R mutant harboring the seventh transmembrane domain and its C terminus reduced the rescue effect of A 2A -R. Using the cytoplasmic tail of the A 2A -R as bait, a novel-A 2A -R-interacting protein [translin-associated protein X (TRAX)] was identified in a yeast two-hybrid screen. The authenticity of this interaction was verified by pull-down experiments, coimmunoprecipitation, and colocalization of these two molecules in the brain. It is noteworthy that reduction of TRAX using an antisense construct suppressed the rescue effect of A 2A -R, whereas overexpression of TRAX alone caused the same rescue effect as did A 2A -R activation. Results of [ 3 H]thymidine and bromodeoxyuridine incorporation suggested that A 2A -R stimulation inhibited cell proliferation in a TRAX-dependent manner. Because the antimitotic activity is crucial for NGF function, the A 2A -R might exert its rescue effect through a TRAX-mediated antiproliferative signal. This antimitotic activity of the A 2A -R also enables a mitogenic factor (epidermal growth factor) to induce neurite outgrowth. We demonstrate that the A 2A -R modulates the differentiation ability of trophic factors through a novel interacting protein, TRAX.

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Following mitosis, specification and migration during embryogenesis, dopamine neurons of the mesencephalon undergo a postnatal naturally occurring cell death event that determines their final adult number, and a period of axonal growth that determines pattern and extent of target contacts. While a number of neurotrophic factors have been suggested to regulate these developmental events, little is known, especially in vivo, of the cell signaling pathways that mediate these effects. We have examined the possible role of Akt/Protein Kinase B by transduction of these neurons in vivo with adeno‐associated viral vectors to express either a constitutively active or a dominant negative form of Akt/protein kinase B. We find that Akt regulates multiple features of the postnatal development of these neurons, including the magnitude of the apoptotic developmental cell death event, neuron size, and the extent of target innervation of the striatum. Given the diversity and magnitude of its effects, the regulation of the development of these neurons by Akt may have implications for the many psychiatric and neurologic diseases in which these neurons may play a role.

For many neurodegenerative disorders, such as Parkinson's disease, there is evidence that the disease first affects axons and terminals of neurons that are selectively vulnerable. This would suggest that it may be possible to forestall progression by targeting the cellular mechanisms of axon degeneration. While it is now clear that these mechanisms are distinct from the pathways of programmed cell death, they are less well known. Compelling evidence of the distinctiveness of these mechanisms has derived from studies of the Wld S mutation, which confers resistance to axon degeneration. Little is known about how this mutation affects degeneration in dopaminergic axons, those that are affected in Parkinson's disease. We have characterized the Wld S phenotype in these axons in four models of injury: two that utilize the neurotoxin 6-hydroxydopamine or axotomy to induce anterograde degeneration, and two that use these methods to induce retrograde degeneration. For both 6-hydroxydopamine and axotomy, Wld S provides protection from anterograde, but not retrograde degeneration. This protection is observed as preserved immunostaining for tyrosine hydroxylase in axons and striatum, and by structural integrity visualized by GFP in tyrosine hydroxylase-GFP mice. Therefore, Wld S offers axon protection, but it reveals fundamentally different processes underlying antero-and retrograde degeneration in this system. Keywords: apoptosis, axon, axonopathy, Parkinson's disease, Wallerian. Relatively little is known about the Wld S phenotype in dopaminergic axons of the nigro-striatal projection, one of the major systems affected in PD. Sajadi and colleagues demonstrated that Wld S provides remarkable axon protection in a model of anterograde degeneration induced by injection of the neurotoxin 6-hydroxydopamine (6OHDA) into the medial forebrain bundle (MFB) (Sajadi et al. 2004). However, Wld S provided no protection in a model of retrograde degeneration of these axons induced by 6OHDA injection into the striatum. This result was unexpected because the MFB injection is generally considered to be more devastating. Sajadi et al. proposed that in the nigro-striatal projection, Wld S may provide protection from anterograde, but not retrograde, degeneration. This explanation, however, seems difficult to reconcile with observations that Wld S protects from retrograde degeneration in other models (Kerschensteiner et al. 2005;Mi et al. 2005). Additionally, in the nigrostriatal system, Wld S was subsequently found to protect from another toxin, MPTP, in the striatal terminals (Hasbani and O'Malley 2006). Therefore, to better delineate the Wld S phenotype in the nigro-striatal dopaminergic projection we have characterized its effect in four models of axon injury, two anterograde and two retrograde. In these investigations we monitor not only the expression of dopaminergic phenotype in axons by immunostaining for tyrosine hydroxylase (TH), but also their structural integrity by use of a novel optical technique that employs confocal opti...

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