α-Synuclein in cutaneous autonomic nerves

Abstract

Objective: To develop a cutaneous biomarker for Parkinson disease (PD).

Methods: Twenty patients with PD and 14 age- and sex-matched control subjects underwent examinations, autonomic testing, and skin biopsies at the distal leg, distal thigh, and proximal thigh. α-Synuclein deposition and the density of intraepidermal, sudomotor, and pilomotor nerve fibers were measured. α-Synuclein deposition was normalized to nerve fiber density (the α-synuclein ratio). Results were compared with examination scores and autonomic function testing.

Results: Patients with PD had a distal sensory and autonomic neuropathy characterized by loss of intraepidermal and pilomotor fibers (p < 0.05 vs controls, all sites) and morphologic changes to sudomotor nerve fibers. Patients with PD had greater α-synuclein deposition and higher α-synuclein ratios compared with controls within pilomotor nerves and sudomotor nerves (p < 0.01, all sites) but not sensory nerves. Higher α-synuclein ratios correlated with Hoehn and Yahr scores (r = 0.58-0.71, p < 0.01), with sympathetic adrenergic function (r = -0.40 to -0.66, p < 0.01), and with parasympathetic function (r = -0.66 to -0.77, p > 0.01).

Conclusions: We conclude that α-synuclein deposition is increased in cutaneous sympathetic adrenergic and sympathetic cholinergic fibers but not sensory fibers of patients with PD. Higher α-synuclein deposition is associated with greater autonomic dysfunction and more advanced PD. These data suggest that measures of α-synuclein deposition in cutaneous autonomic nerves may be a useful biomarker in patients with PD.

Figure 1. Autonomic innervation and α-synuclein deposition

Sample images are shown for healthy control subjects (A, B, E, F) and subjects with PD (C, D, G, H). In (A) the pan-axonal marker PGP 9.5 in green reveals nerve fibers in a pilomotor muscle of a healthy subject. The fibers are linear with normal morphology. In (B), nerve fibers that contain α-synuclein (red) are shown for the corresponding pilomotor image (A). There is nonspecific α-synuclein staining in the hair follicle of the control subject and a small amount of α-synuclein deposition within the pilomotor nerve fibers. However, this small amount of α-synuclein deposition in a large number of fibers accumulates to a modest total amount of α-synuclein, but a low α-synuclein ratio. In (C), there is a reduction in the pilomotor nerve density in an individual with PD. Note the loss of linearity and thickened fibers. In contrast, (D), α-synuclein, shown in red, accumulates in pilomotor fibers of a patient with PD. The total amount of α-synuclein is not much greater than in the control subject (B), but the α-synuclein ratio is much higher. Sudomotor nerve fibers detected by the pan-axonal marker PGP 9.5 for a control subject (E) and a patient with PD (G). α-Synuclein is detected at low levels in the control with a low α-synuclein ratio (F), but is visible within the sudomotor fibers of a patient with PD and a high α-synuclein ratio (H). Scale bar = 100 μm. αSN = α-synuclein; PD = Parkinson disease; PGP = protein gene product 9.5.

Figure 2. α-Synuclein ratio

The pilomotor α-synuclein ratios (A) and sudomotor α-synuclein ratios (B) for healthy controls (black circles) and individuals with Parkinson disease (open circles) by biopsy site (distal leg, distal thigh, or proximal thigh). *p < 0.01 vs control subjects (paired t test with Bonferroni correction). Results shown are mean ± SEM.

Figure 3. α-Synuclein ratio by Hoehn and Yahr score

Pilomotor α-synuclein ratios, plotted against Hoehn and Yahr score (1 = mild PD, 4 = severe PD), at the distal leg (A) and proximal thigh (B). Sudomotor α-synuclein ratios, plotted against Hoehn and Yahr score, at the distal leg (C) and proximal thigh (D). *p < 0.05 vs control subjects (Kruskal-Wallis tests, with Mann-Whitney U tests for post hoc analysis using Bonferroni corrections for multiple comparisons). Results shown are mean ± SEM. PD = Parkinson disease.