Neuromelanin, neurotransmitter status and brainstem location determine the differential vulnerability of catecholaminergic neurons to mitochondrial DNA deletions

Background: Deletions of the mitochondrial DNA (mtDNA) accumulate to high levels in dopaminergic neurons of the substantia nigra pars compacta (SNc) in normal aging and in patients with Parkinson's disease (PD). Human nigral neurons characteristically contain the pigment neuromelanin (NM), which is believed to alter the cellular redox-status. The impact of neuronal pigmentation, neurotransmitter status and brainstem location on the susceptibility to mtDNA damage remains unclear. We quantified mtDNA deletions (Delta mtDNA) in single pigmented and non-pigmented catecholaminergic, as well as non-catecholaminergic neurons of the human SNc, the ventral tegmental area (VTA) and the locus coeruleus (LC), using laser capture microdissection and single-cell real-time PCR. Results: In healthy aged individuals, Delta mtDNA levels were highest in pigmented catecholaminergic neurons (25.2 +/- 14.9%), followed by non-pigmented catecholamergic (18.0 +/- 11.2%) and non-catecholaminergic neurons (12.3 +/- 12.3%; p < 0.001). Within the catecholaminergic population, Delta mtDNA levels were highest in dopaminergic neurons of the SNc (33.9 +/- 21.6%) followed by dopaminergic neurons of the VTA (21.9 +/- 12.3%) and noradrenergic neurons of the LC (11.1 +/- 11.4%; p < 0.001). In PD patients, there was a trend to an elevated mutation load in surviving non-pigmented nigral neurons (27.13 +/- 16.73) compared to age-matched controls (19.15 +/- 11.06; p = 0.052), but levels where similar in pigmented nigral neurons of PD patients (41.62 +/- 19.61) and controls (41.80 +/- 22.62). Conclusions: Catecholaminergic brainstem neurons are differentially susceptible to mtDNA damage. Pigmented dopaminergic neurons of the SNc show the highest Delta mtDNA levels, possibly explaining the exceptional vulnerability of the nigro-striatal system in PD and aging. Although loss of pigmented noradrenergic LC neurons also is an early feature of PD pathology, mtDNA levels are not elevated in this nucleus in healthy controls. Thus, Delta mtDNA are neither an inevitable consequence of catecholamine metabolism nor a universal explanation for the regional vulnerability seen in PD.