Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson’s disease. We created a LRRK2 transgenic mouse model that recapitulates cardinal features of the disease: an age-dependent and levodopa-responsive slowness of movement associated with diminished dopamine release and axonal pathology of nigrostriatal dopaminergic projection. These mice provide a valid model of Parkinson’s disease and are a resource for the investigation of pathogenesis and therapeutics.

There is increasing evidence that neuron death in neurodegenerative diseases, such as Parkinson's disease, is due to the activation of programmed cell death. However, the upstream mediators of cell death remain largely unknown. One approach to the identification of upstream mediators is to perform gene expression analysis in disease models. Such analyses, performed in tissue culture models induced by neurotoxins, have identified up-regulation of CHOP/ GADD153, a transcription factor implicated in apoptosis due to endoplasmic reticulum stress or oxidative injury. To evaluate the disease-related significance of these findings, we have examined the expression of CHOP/GADD153 in neurotoxin models of parkinsonism in living animals. Nuclear expression of CHOP protein is observed in developmental and adult models of dopamine neuron death induced by intrastriatal injection of 6-hydroxydopamine (6OHDA) and in models induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). CHOP is a mediator of neuron death in the adult 60HDA model because a null mutation results in a reduction in apoptosis. In the chronic MPTP model, however, while CHOP is robustly expressed, the null mutation does not protect from the loss of neurons. We conclude that the role of CHOP depends on the nature of the toxic stimulus. For 6OHDA, an oxidative metabolite of dopamine, it is a mediator of apoptotic death.

Objective-A prevailing concept in neuroscience has been that the adult mammalian central nervous system is incapable of restorative axon regeneration. Recent evidence, however, has suggested that reactivation of intrinsic cellular programs regulated by Akt/mTor signaling may restore this ability.Methods-To assess this possibility in the brain, we have examined the ability of adenoassociated virus mediated transduction of dopaminergic neurons of the substantia nigra with constitutively active forms of the kinase Akt and the GTPase Rheb to induce re-growth of axons after they have been destroyed by neurotoxin lesion.Results-Both constitutively active myristoylated Akt and hRheb(S16H) induce regrowth of axons from dopaminergic neurons to their target, the striatum. Histological analysis demonstrates that these new axons achieve morphologically accurate reinnervation. In addition, functional reintegration into target circuitry is achieved, as indicated by partial behavioral recovery.Interpretation-We conclude that regrowth of axons within the adult nigro-striatal projection, a system that is prominently affected in Parkinson's disease, can be achieved by activation of Akt/ mTor signaling in surviving endogenous mesencephalic dopaminergic neurons by viral vector transduction.

Axon degeneration is a hallmark of neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. Such degeneration is not a passive event but rather an active process mediated by mechanisms that are distinct from the canonical pathways of programmed cell death that mediate destruction of the cell soma. Little is known of the diverse mechanisms involved, particularly those of retrograde axon degeneration. We have previously observed in living animal models of degeneration in the nigrostriatal projection that a constitutively active form of the kinase, myristoylated Akt (Myr-Akt), demonstrates an ability to suppress programmed cell death and preserve the soma of dopamine neurons. Here, we show in both neurotoxin and physical injury (axotomy) models that Myr-Akt is also able to preserve dopaminergic axons due to suppression of acute retrograde axon degeneration. This cellular phenotype is associated with increased mammalian target of rapamycin (mTor) activity and can be recapitulated by a constitutively active form of the small GTPase Rheb, an upstream activator of mTor. Axon degeneration in these models is accompanied by the occurrence of macroautophagy, which is suppressed by Myr-Akt. Conditional deletion of the essential autophagy mediator Atg7 in adult mice also achieves striking axon protection in these acute models of retrograde degeneration. The protection afforded by both Myr-Akt and Atg7 deletion is robust and lasting, because it is still observed as protection of both axons and dopaminergic striatal innervation weeks after injury. We conclude that acute retrograde axon degeneration is regulated by Akt/Rheb/mTor signaling pathways.

Dopamine (DA) neurons of the substantia nigra undergo a developmental cell death event that is biphasic, with peaks just after birth and at postnatal day 14. As envisioned by neurotrophic theory, this cell death is likely to be regulated by target interactions because it is augmented by their disruption. However, the nature of the trophic molecules mediating this regulation are unknown. We showed in vitro that glial cell line-derived neurotrophic factor (GDNF) is able to suppress apoptotic death in DA neurons in postnatal primary culture. We now demonstrate in vivo that administration of GDNF into the striatal target is able to suppress apoptosis. Consistent with a possible physiologic role for endogenous striatal GDNF in regulating this event, two anti-GDNF neutralizing antibodies augment cell death. These antibodies augment cell death only during the first (immediately postnatal) phase of the biphasic death event. We conclude that GDNF is the leading candidate for a target-derived neurotrophic factor for the regulation of the early phase of natural cell death in DA neurons.

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.

The regulation of protein degradation is essential for maintaining the appropriate environment to coordinate complex cell signaling events and to promote cellular remodeling. The Autophagy linked FYVE protein (Alfy), previously identified as a molecular scaffold between the ubiquitinated cargo and the autophagic machinery, is highly expressed in the developing central nervous system, indicating that this pathway may have yet unexplored roles in neurodevelopment. To examine this possibility, we used mouse genetics to eliminate Alfy expression. We report that this evolutionarily conserved protein is required for the formation of axonal tracts throughout the brain and spinal cord, including the formation of the major forebrain commissures. Consistent with a phenotype reflecting a failure in axon guidance, the loss of Alfy in mice disrupts localization of glial guidepost cells, and attenuates axon outgrowth in response to Netrin-1. These findings further support the growing indication that macroautophagy plays a key role in the developing CNS.DOI: http://dx.doi.org/10.7554/eLife.14810.001

Programmed cell death has been proposed to play a role in the death of neurons in acute and chronic degenerative neurologic disease. There is now evidence that the caspases, a family of cysteine proteases, mediate programmed cell death in various cells. In neurons, caspase-3 (CPP32/Yama/apopain), in particular, has been proposed to play a role. We examined the expression of caspase-3 in three models of programmed cell death affecting neurons of the substantia nigra in the rat: natural developmental neuron death and induced developmental death following either striatal target injury with quinolinic acid or dopamine terminal lesion with intrastriatal injection of 6-hydroxydopamine. Using an antibody to the large (p17) subunit of activated caspase-3, we have found that activated enzyme is expressed in apoptotic profiles in all models. Increased p17 immunostaining correlated with increased enzyme activity. The subcellular distribution of activated caspase-3 differed among the models: In natural cell death and the target injury model, it was strictly nuclear, whereas in the toxin model, it was also cytoplasmic. We conclude that p17 immunostaining is a useful marker for programmed cell death in neurons of the substantia nigra. Key Words: Programmed cell death-Apoptosis-Caspase-Substantia nigra-Parkinson's disease.