Genetic analysis of dopaminergic neuron survival

Pathological changes in the dopaminergic system account for a
number of devastating illnesses including schizophrenia, psychosis,
depression, addiction, obsessive compulsive disorder or the most
well known Parkinson’s disease (PD). The nigrostriatal pathway is
an important component of the dopaminergic (DA) system mediating
voluntary movement and originates in the ventral midbrain from
where substantia nigra pars compacta (SN) neurons send their axons
to the dorsal striatum. Massive loss of SN neurons as seen in PD
leads to postural imbalance, rigidity, tremor and bradykinesia,
however, the precise mechanisms involved in the maintenance and the
demise of SN neurons are poorly understood. Endogenous neurotrophic
factors such as the Glial cell line-derived neurotrophic factor
(GDNF; signaling via the Ret receptor tyrosine kinase) and
Brain-derived neurotrophic factor (BDNF; signaling via the TrkB
receptor tyrosine kinase) were reported to have protective and
rescuing properties on DA neurons; however, their physiological
roles in SN neurons remained unknown. Inactivation of the oxidative
stress suppressor DJ-1 causes PD; remarkably, mice lacking DJ-1
function do not display overt SN degeneration, suggesting that
additional DJ-1 interactors compensate for loss of DJ-1 function.
To begin characterizing the cellular and molecular networks
mediating SN neuron survival, I used mouse genetics to investigate
the roles and the interaction between GDNF/BDNF-mediated trophic
signaling and the DJ-1-mediated stress response in SN neurons.
While mice lacking TrkB function specifically in SN neurons display
a normal complement of SN neurons up to 24-months, loss of Ret
function in DA neurons causes adult-onset and progressive SN
degeneration, suggesting that GDNF/Ret signaling is required for
long-term maintenance of SN neurons. I then generated and aged mice
lacking Ret and DJ-1 and found remarkably that they display an
enhanced SN degeneration relative to mice lacking Ret. Thus, DJ-1
promotes survival of Ret-deprived SN neurons. Interestingly, the
survival requirement for Ret and DJ-1 is restricted to those SN
neurons which express the ion channel GIRK2, project exclusively to
the striatum and specifically degenerate in PD. This is the first
in vivo evidence for a pro-survival role of DJ-1. To understand how
DJ-1 interacts molecularly with Ret signaling, I performed
epistasis analysis in Drosophila melanogaster. Although DJ-1
orthologs DJ-1A and DJ-1B are dispensable for fly development, the
developmental defects induced by targeting constitutively active
Ret to the retina were suppressed in a background of reduced
DJ-1A/B function. Moreover, DJ-1A/B interacted genetically with
Ras/ERK, but not PI3K/Akt signaling to regulate photoreceptor
neuron development. Flies with reduced ERK activity and lacking
DJ-1B function had more severe defects in photoreceptor neuron and
wing development than flies with reduced ERK function. These
observations establish, for the first time, a physiological role
for DJ-1B in the intact Drosophila. Our findings suggest that the
triple interaction between aging, trophic insufficiency and
cellular stress may cause Parkinsonism. Because Ret and DJ-1 show
convergence of their pro-survival activities, we predict that
striatal delivery of GDNF might be most effective in PD patients
carrying DJ-1 mutations. A better understanding of the molecular
connections between trophic signaling, cellular stress and aging
will accelerate the process of drug development in PD.