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observing less dramatic effects involved the use of confocal microscopy of whole-
mount brains to detect TH-positive neurons. Thus, one possible explanation for
these conflicting results is that the different methods employed to identify TH-
positive neurons may differ in their sensitivity of detection. Auluck
directly
addressed this possibility and demonstrated that the subpopulation of DA neu-
rons reported to degenerate in
et al.
-synuclein-expressing flies could still be detected
by confocal analysis of whole-mount brains from flies expressing
-synuclein.
Furthermore, they demonstrated that this same cell population generally stains
less intensely than those in clusters reportedly unaffected by
-synuclein expres-
sion (Auluck
, 2005, supplementary data). These findings indicate that the
loss of TH staining previously reported in
et al.
-synuclein transgenic flies reflects
reduced TH levels rather than overt cell loss. While the reduced TH expression
conferred by
-synuclein expression suggests that these cells are dysfunctional,
whether this phenotype reflects an early stage leading to the death of these
neurons, as proposed by Auluck
(2005), remains an open question. Irre-
spective of the final outcomes between individual laboratory environments,
these findings underscore the paramount requirement for scientific rigor and
the absolute necessity to conduct DA neurons analysis in a blinded manner.
The standardization of reliable and efficient staining protocols, which has re-
cently been attempted (Drobysheva
et al.
, 2008), would also help. If these
standards are upheld, one can be confident to interpret the results.
et al.
1. Mechanistic insights from the
-synuclein models
The coincidence of neuronal loss and the formation of
-synuclein-containing
Lewy body aggregates in PD prompted speculation of the possible involvement of
protein aggregation in promoting DA neuron loss. This hypothesis was addressed
directly using the Drosophila
-synuclein models. Overexpression of the chaper-
one, HSP70, abrogated the
-synuclein-induced loss of TH-positive neurons
without detectably influencing the appearance of Lewy body-like aggregates
(Auluck
-synuclein transgenic flies the
chaperone-inducing compound geldanamycin was found to phenocopy the pro-
tective effect of HSP70 expression (Auluck and Bonini, 2002). These findings,
together with other work from the field, suggest that the formation of large
et al.
, 2002). Furthermore, feeding
-
synuclein-containing aggregates is not the primary toxic species and that HSP70
induction is protective from the harmful effects of
-synuclein. This model is
supported by observations that upregulation of the K48-linked ubiquitin-
proteasome-dependent degradation can also ameliorate the toxic effects of
-
synuclein expression (Lee
, 2009), again arguing that the toxic species are
mono- or oligomeric rather than aggregates. This is further supported by analysis
of synthetically mutated
et al.
-synuclein with an increased propensity to exist as
soluble oligomers that demonstrate a higher neurotoxicity (Karpinar
et al.
, 2009).
