Restoring Compromised Cl- in D2 Neurons of a HD Mouse Model Rescues Motor Disability.

Huntington's disease (HD) is a progressive neurodegenerative disorder with no cure, characterized by significant neurodegeneration of striatal GABAergic medium spiny neurons (MSNs). Early stages of the disease are characterized by the loss of dopamine 2 receptor-expressing MSNs (D2 MSNs) followed by degeneration of dopamine 1 receptor-expressing MSNs (D1 MSNs), leading to aberrant basal ganglia signaling. While the early degeneration of D2 MSNs and impaired GABAergic transmission are well-documented, potassium chloride cotransporter 2 (KCC2), a key regulator of intracellular chloride (Cl^-), and therefore GABAergic signaling, has not been characterized in D1 and D2 MSNs in HD. We aimed to investigate whether Cl^- regulation was differentially altered in D1 and D2 MSNs and may contribute to the early degeneration of D2 MSNs in male and female symptomatic R6/2 mice. We used electrophysiology to record the reversal potential for GABA_A receptors (E_GABA), a read-out for the efficacy of Cl^- regulation, in striatal D1 and D2 MSNs and their corresponding output structures. During the early symptomatic phase (P55-P65)_, Cl^- impairments were observed in D2 MSNs in R6/2 mice, with no change in D1 MSNs. Cl^- regulation was also dysfunctional in the globus pallidus externa, resulting in GABA-mediated excitation. When we overexpressed KCC2 in D2 MSNs using AAV-mediated delivery, we delayed the onset of motor impairments in R6/2 mice. We demonstrate that Cl^- homeostasis is differentially altered in D1 and D2 MSNs and may contribute to the enhanced susceptibility of D2 MSNs during HD progression.Significance Statement Huntington's Disease is an inherited neurodegenerative disease caused by a repeat expansion in the Huntingtin gene and characterized by the sequential loss of dopamine 2 and dopamine 1 receptor-expressing medium spiny neurons (D2 and D1 MSNs) of the striatum. MSNs release GABA, which depends on proper Cl^- regulation for inhibition. We asked whether Cl^- homeostasis is differentially altered in D1 and D2 MSNs and their output structures, and whether this altered expression contributes to the pattern of degeneration between these two principal striatal cell types. Using electrophysiology, biochemistry, and fluorescence imaging, we determined that Cl^- regulation was impaired in D2 MSNs in R6/2 mice, with no change in D1 MSNs. Cl^- was also dysregulated in the globus pallidus externa resulting in excitatory GABA.