Neuronal Atrophy Early in Degenerative Ataxia Is a Compensatory Mechanism to Regulate Membrane Excitability.
Neuronal atrophy in neurodegenerative diseases is commonly viewed as an early event in a continuum that ultimately results in neuronal loss. In a mouse model of the polyglutamine disorder spinocerebellar ataxia type 1 (SCA1), we tested the hypothesis that cerebellar Purkinje neuron atrophy serves an adaptive role rather than being simply a nonspecific response to injury. In acute cerebellar slices from SCA1 mice, we find that Purkinje neuron pacemaker firing is initially normal but, with the onset of motor dysfunction, becomes disrupted, accompanied by abnormal depolarization. Remarkably, subsequent Purkinje cell atrophy is associated with a restoration of pacemaker firing. The early inability of Purkinje neurons to support repetitive spiking is due to unopposed calcium currents resulting from a reduction in large-conductance calcium-activated potassium (BK) and subthreshold-activated potassium channels. The subsequent restoration of SCA1 Purkinje neuron firing correlates with the recovery of the density of these potassium channels that accompanies cell atrophy. Supporting a critical role for BK channels, viral-mediated increases in BK channel expression in SCA1 Purkinje neurons improves motor dysfunction and partially restores Purkinje neuron morphology. Cerebellar perfusion of flufenamic acid, an agent that restores the depolarized membrane potential of SCA1 Purkinje neurons by activating potassium channels, prevents Purkinje neuron dendritic atrophy. These results suggest that Purkinje neuron dendritic remodeling in ataxia is an adaptive response to increases in intrinsic membrane excitability. Similar adaptive remodeling could apply to other vulnerable neuronal populations in neurodegenerative disease.


CONCLUSIONS
In neurodegenerative disease, neuronal atrophy has long been assumed to be an early nonspecific event preceding neuronal loss. However, in a mouse model of spinocerebellar ataxia type 1 (SCA1), we identify a previously unappreciated compensatory role for neuronal shrinkage. Purkinje neuron firing in these mice is initially normal, but is followed by abnormal membrane depolarization resulting from a reduction in potassium channels. Subsequently, these electrophysiological effects are counteracted by cell atrophy, which by restoring normal potassium channel membrane density, re-establishes pacemaker firing. Reversing the initial membrane depolarization improved motor function and Purkinje neuron morphology in the SCA1 mice. These results suggest that Purkinje neuron remodeling in ataxia is an active compensatory response that serves to normalize intrinsic membrane excitability.
Department of Neurology, University of Michigan, Ann Arbor, Michigan 48109.
James M
http://orcid.org/0000-0001-8565-7535
JM
Dell'Orco
Department of Neurology, University of Michigan, Ann Arbor, Michigan 48109.
Aaron H
AH
Wasserman
Department of Neurology, University of Michigan, Ann Arbor, Michigan 48109.
Ravi
R
Chopra
Department of Laboratory Medicine and Pathology, Institute for Translational Neuroscience, University of Minnesota, Minneapolis, Minnesota 55455.
Melissa A C
MA
Ingram
Ken and Ruth Davee Department of Neurology, Northwestern University, Chicago, Illinois 60611, and.
Yuan-Shih
http://orcid.org/0000-0002-8287-0186
YS
Hu
Department of Pharmacology, University of California, Davis, California 95616.
Vikrant
V
Singh
Department of Pharmacology, University of California, Davis, California 95616.
Heike
H
Wulff
Ken and Ruth Davee Department of Neurology, Northwestern University, Chicago, Illinois 60611, and.
Puneet
P
Opal
Department of Laboratory Medicine and Pathology, Institute for Translational Neuroscience, University of Minnesota, Minneapolis, Minnesota 55455.
Harry T
HT
Orr
Department of Neurology, University of Michigan, Ann Arbor, Michigan 48109, vikramsh@med.umich.edu.
Vikram G
VG
Shakkottai
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D000067696:Ataxin-1; D000067528:Ataxins; D001284:Atrophy / Q000473:pathology / Q000503:physiopathology; D002531:Cerebellum / Q000473:pathology* / Q000503:physiopathology; D004195:Disease Models, Animal; D008564:Membrane Potentials / Q000502:physiology*; D051379:Mice; D008822:Mice, Transgenic; D009419:Nerve Tissue Proteins / Q000235:genetics; D009687:Nuclear Proteins / Q000235:genetics; D011689:Purkinje Cells / Q000473:pathology* / Q000502:physiology; D020754:Spinocerebellar Ataxias / Q000473:pathology* / Q000503:physiopathology
D000067696:Ataxin-1; D000067528:Ataxins; C000593750:Atxn1 protein, mouse; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins
SCA1; ataxia; atrophy; cerebellum; channel; purkinje neuron