Recent postmortem studies have documented degenerative changes in the cerebellum of ET cases, as manifested by increased numbers of torpedoes and loss of PCs (24
). These postmortem data complement a rich history of clinical, physiological and imaging studies that have long implicated cerebellar and cerebellothalamacortical circuitry abnormalities in ET (8
). While prior pathologic studies focused on degenerative changes in PCs, the relationship between ET and cerebellar interneurons has remained unexplored. We examined archival and prospectively acquired cerebellar tissue of ET cases and both diseased and non-diseased controls and found that the axonal plexuses of basket cells surrounding PCs in cerebellar ET brains are significantly “hairier,” as compared to controls.
Basket cells are GABA-ergic inhibitory interneurons that receive input from parallel fibers and to a limited extent from climbing and mossy fibers. In humans, up to 50 axon collaterals from neighboring basket cells descend from the molecular layer and combine to form a complex basket structure around the PC soma, to which the basket cells’ entire axonal output is devoted (44
). Reductions in cerebellar basket cell axonal plexuses have been reported in several disease states, including hypothyroidism (44
), ataxia telangiectasia (45
), and Menkes kinky hair disease (46
). Preservation of basket cell plexuses in the setting of PC death (i.e. “empty baskets”) has been reported in Creutzfeldt-Jakob disease (47
) and spinocerebellar ataxia type 1 (48
), but a dense and tangled appearance of the axonal plexus profile has not to our knowledge been documented in the ataxia literature. Curiously, an early 20th
century study, describing PC abnormalities in 16 brains of individuals diagnosed with “senile dementia”, noted increased basket cell fiber “tangles and masses” alongside PC soma showing signs of degeneration in 3 cases (31
). Another more recent study qualitatively examined basket cells in Creutzfeldt-Jakob disease and noted empty baskets with abnormally dense phosphorylated neurofilament positivity in a single case (31
). Here, we documented that individuals with a diagnosis of cerebellar ET were more than 10 times as likely as controls (both diseased and non-diseased) to have a hairy basket rating of 3 (the marked or severe end of the spectrum). Also, the median axonal plexus density in cerebellar ET was approximately 2 to 3 times higher than in controls. “Hairiness” of basket cell axonal plexuses in these independent semiquantitative and quantitative measures was positively associated with number of torpedoes and the degeneration index, and inversely with number of PCs, suggesting that hairy baskets and torpedoes may be concomitant features of cerebellar degeneration in ET.
The mechanism by which this increased “hairiness” occurs is unknown. One possible explanation is that the increased profiles observed in our study represent an accumulation of converging basket cell processes recruited from neighboring PCs that have been damaged or died. Although there has been little investigation of such a phenomenon in the human cerebellum, selective preservation and reorganization of basket cell axonal processes has been demonstrated in basket cells in the CA1 and CA3 region of the hippocampus (49
). These cells form baskets around hippocampal pyramidal cells and function as local circuit inhibitory GABA-ergic interneurons, analogous to the relationship between cerebellar basket cells and PCs. These disease-resistant hippocampal basket cells undergo extensive reorganization in the setting of pyramidal cell death (51
). In a few cases, basket cell plexuses displayed increased density adjacent to areas with severe pyramidal cell loss and no basket formations, suggesting that these basket cell processes might be converging on and reorganizing around remaining pyramidal cells (51
The molecular mechanisms that underlie the degenerative changes observed in the ET cerebellum are not clear. The array of cellular changes within the complex neuronal network in cerebellar cortex have yet to be fully catalogued in ET; thus, further work with human postmortem tissue is needed. In addition, genes have not been identified for ET and no transgenic mice currently exist that provide an animal model for ET. The most widely used animal model for ET is based on the administration of neurotoxins (e.g. harmaline and ibogaine) to rodents and other mammals. These toxins induce synchronous firing of inferior olivary neurons at an 8 to 12 Hz frequency and sustained glutamatergic stimulation of PCs via their climbing fiber afferents (52
), triggering a massive increase in intracellular calcium and an excitotoxic-mediated PC degeneration and cell death (55
). However, this toxin-based animal model produces an acute, reversible action tremor. Most animals shake for a matter of hours and there is more severe destruction of cerebellar cortex in readily identifiable linear bands. While these latter clinical and pathologic features clearly differ from those in ET, this model demonstrates the functional significance of the olivocerebellar system and a potential role for excitotoxic-mediated PC death in the generation of this type of tremor. It is intriguing that harmane (a tremor-producing, indole-alkaloid structurally similar to harmaline) has been found in the blood of some ET patients at levels up to 50% higher than age-matched controls (58
). While additional studies are needed to establish whether this exposure is of etiologic importance in ET, we postulate that the PC loss documented in ET (whether excitotoxic or not) leads to a reorganization of the cerebellar interneuron network, with sprouting and/or accumulation of neighboring basket cell processes on surviving PC bodies.
Ultrastructural analysis revealed the presence of multiple, closely spaced sites of synaptic contact between the dense, multi-layered basket cell processes and PC soma in ET. The neurophysiological abnormality in ET is not clear, but is thought to be the result of decreased cerebellar PC inhibitory output. Our observations raise the possibility that the increased density of basket cell processes identified in a significant number of ET cases contribute to this inhibitory effect on PC output. Recent studies have demonstrated that climbing fiber responses in PCs are modulated through cerebellar interneuron networks that include the basket cell (59
). Basket cell output is also inhibited by the axon collaterals of PCs and excited by both parallel fibers and collaterals of climbing fibers (60
). Cerebellar interneurons are also coupled by dendro-somatic, dendro-dendritic and somato-somatic gap junctions, which may lead to synchronization of firing in groups of organized neural networks (61
). Specialized axo-axonic septate junctions electrically couple basket fiber processes in the axon hillock region (62
). Thus, the disease process in ET may cause an imbalance in the interactions between cerebellar afferents, PCs and their axon collaterals and the cerebellar interneuron network that includes basket cells. Additional studies on other cerebellar neurons and their interactions in ET are clearly needed to understand the basis for tremor generation in this disorder.
Despite being the largest series of ET brains reported to date, the sample size was modest. Despite this limitation, we were able to detect significant case-control differences, indicating that the sample was sufficient. This study had several strengths. This was the first investigation of a relatively novel pathological finding in a large series of ET brains. We examined basket cell profiles both semiquantitatively and quantitatively and, furthermore, compared ET brains against both non-diseased control brains and a group of diseased controls with a variety of neurodegenerative pathologies.
In summary, the degenerative changes in PCs that characterize cerebellar ET are accompanied, in some cases, by a dense and tangled, or “hairy,” appearance of basket cell axonal plexus formations. This relatively novel finding was not observed to any degree in either diseased controls or normal control brains. This finding provides initial evidence that structural changes in ET are not restricted to the PC but also involve their functional network. The presence of hairy baskets is a heretofore uninvestigated phenomenon and further exploration is warranted in order to understand its relationship to PC degeneration and its role in the pathogenesis of ET.