"Hairy baskets" associated with degenerative Purkinje cell changes in essential tremor

Abstract

Essential tremor (ET) is one of the most common neurologic diseases. Increased numbers of torpedoes and Purkinje cell (PC) loss have been documented in the brains of patients with ET. We recently observed a dense and tangled appearance ("hairiness") of the basket cell axonal plexuses that surround PC soma in Bielschowsky preparations of cerebellar cortex in ET brains. Here, we assessed basket cell "hairiness" in 37 ET (32 cerebellar ET; 5 Lewy body variant ET), 21 nondisease control, and 48 disease control brains using a semiquantitative scale. In 8 cerebellar ET cases (25%), there were high basket scores (rating = 3), whereas no Lewy body variant ET, 1 nondisease control (4.8%), and 2 diseased controls (4.2%) had high basket scores (p = 0.001). The hairy basket scores correlated with numbers of torpedoes (p < 0.001) and inversely with numbers of PCs (p = 0.06). Axonal plexus density obtained by image analysis of basket cell processes traced from digitized images was higher in ET than in nondiseased control cases (p = 0.016). Closely spaced sites of synaptic contact between basket cell processes and PCs were identified by electron microscopy in ET cases. These data indicate that structural changes are not restricted to PCs in ET, and that other neurons within their functional network may be involved in its pathogenesis.

Figure 1

Semiquantitative rating of basket cell processes. (A–D) A rating scale (from 0 to 3) of the appearance of the basket cell plexus surrounding extant Purkinje cell bodies was determined in a blinded analysis of Bielschowsky preparations of neocerebellum sections. (A) Score = 0 (few, or no discernible processes); (B) Score = 1 (sparse number of processes); (C) score = 2 (moderate number of processes); (D) score = 3 (dense tangle of processes, i.e. hairy basket).

Figure 2

Immunohistochemical analysis of basket cell processes. (A, B) Vibratome sections (100-μm-thick) immunostained with monoclonal antibody SMI-31 demonstrate prominence of phosphorylated neurofilaments in basket cell processes (long arrows). (A) A dense and tangled, or “hairy,” appearance of basket cell axonal plexus formation =in a section from a cerebellar essential tremor case with a hairy basket rating of 3. (B) A control case with a rating of 0. Torpedoes (short arrows) are also seen.

Figure 3

Quantification of basket cell axonal plexus density. (A–D) Examples of tracings scored according to a semiquantitative scale are shown: (A) 0 (few, or no discernible processes); (B) 1 (sparse processes); (C) 2 (moderate numbers of processes); and (D) 3 (dense tangle of processes). (E) Basket cell plexus densities were higher in cerebellar essential tremor (ET) cases (closed circles) (median value = 15.10) than in controls (open squares) (median value = 6.98) (p = 0.016).

Figure 4

Ultrastructure of neurofilament-rich basket cell processes in cerebellar essential tremor (ET) and control cases. (A) The dense plexus of basket cell processes (arrows) adjacent to the Purkinje cell soma (P) in a cerebellar ET case contains neurofilaments and mitochondria. (B) Only 1 to 2 layer(s) of basket cell processes (arrows) are seen adjacent to a Purkinje cell soma (P) in a control case. (C) Higher magnification of basket cell axonal plexus in (A) shows many layers of the basket cell formation (arrows). (D) A dense nest of basket cell processes (arrows) surrounds another Purkinje cell in an ET case. (E) Intertwining of basket cell processes around one another in an ET case. Some processes are longitudinal to the plane of section (long arrows); others have been cut transversely (short arrows). Magnifications: A, B, 10,000×; C, 20,000×; D, 25,000×; E, 30,000×.

Figure 5

Ultrastructural morphology of basket cell-Purkinje cell (P) contacts in essential tremor cases. (A–C) Multiple, closely spaced sites of synaptic contact (arrows) between a neurofilament-rich basket fiber and a Purkinje cell soma. Post-synaptic densities are apparent in (A) and (B) (arrows). The basket fiber in (C) shows several vesicle-lined contacts with the Purkinje cell (arrows). (D) Numerous processes converging on the initial segment (axon hillock) of a Purkinje cell (P-axh); at least one of these is a neurofilament-rich basket cell axon (large arrow). (d′) In a higher magnification of (D), the Purkinje cell contact shows 2 distinct clusters of vesicles (arrows). Magnifications: A, B, d’, 40,000×; C, 50,000×; D, 20,000×.

Figure 6

Correlations of hairy basket scores in cerebellar essential tremor (ET) and non-diseased control cases. (A–C) Hairy basket score correlated with number of torpedoes (A) (Spearman’s r = 0.49, p < 0.001), inversely with the number of Purkinje cells (B) (Spearman’s r = −0.27, p = 0.06), and degeneration index (C) (Spearman’s r = 0.48, p = 0.001).