Developmental underpinnings of substantia nigra vulnerability

Abstract: DA neuron degeneration, resulting in deficient DA signaling, underpins the debilitating motor
symptoms of Parkinson’s disease (PD). Among DA neurons, those located in ventral tier of the substantia nigra
pars compacta (SNc), are particularly vulnerable, compared to those in the dorsal tier of the SNc or ventral
tegmental area (VTA). A mechanistic explanation of selective DA neuron vulnerability remains an important
goal, that has been hampered in part by a lack of understanding of the intrinsic differences between DA neurons.
We hypothesize that even within a single neuroanatomical cluster like the SNc, there exist DA
subtypes with distinct developmental histories and intrinsic properties that may influence their vulnerability.
Single cell expression profiling based DA neuron classification from our lab revealed the presence of a key
SNc population defined by Sox6 and Aldh1a1 - this subtype was located in the ventral tier of the SNc, and was
preferentially vulnerable in a toxin model of PD. Our preliminary data indicate that this population exists in
human SNc, and is also selectively vulnerable in post-mortem PD samples. To interrogate the basis for
selectively vulnerability in the SNc in depth, we next developed a set of intersectional genetic strategies, which
strikingly defined a fault-line in the SNc defined by Sox6 expression, with Sox6+ cells being located ventrally
and Sox6- cells forming the dorsal tier. Building on these studies, several key questions remain unanswered.
Are dorsal and ventral SNc subtypes developmentally distinct? Do these neurons have different anatomical
features and DA release characteristics? Is the size of arborizations of these neurons, a property linked to
vulnerability, different? Are calcium fluxes and mitochondrial bioenergetic properties distinct?
To answer these questions, in Aim 1, we will determine the origin of SNc neuron subtypes. We will use
intersectional, subtractive, and inducible genetic approaches, as well as a novel progenitor anchored fate
mapping approach (PRISM) to test the hypothesis that the dorsal and ventral tier DA neurons are
developmentally distinguished by Sox6. We will then test the potential of Sox6+ medial vs Sox6- lateral floor
plate progenitors to give rise to SNc neurons when transplanted into a PD model. In Aim 2, we will examine the
arborization of genetically defined SNc neurons, since this feature has been linked to their vulnerability. Using
a new sparse labeling tool, we will plot the projections of genetically defined SNc DA neuron subtypes, and
determine the size of their arbors, and explore the molecular determinants of extensive arborization. We will
also study DA release from axons and dendrites to determine if dorsal and ventral tier neurons are
distinguishable by these criteria, also linked to vulnerability. In Aim 3, we will test the hypothesis that the SNc
DA neuron subtypes have distinctive physiological properties and that these differences translate into
differences in mitochondrial oxidant stress in somatodendritic and axonal compartments. Overall, our studies
redefining the SNc based on lineage will provide important insights into selective vulnerability.

Grant Number: 5R01NS119690-05
NIH Institute/Center: NIH
Principal Investigator: Rajeshwar Awatramani