Targeted Depletion of Primary Cilia in Dopaminoceptive Neurons in a Preclinical Mouse Model of Huntington’s Disease

Abstract

Multiple pathomechanisms triggered by mutant Huntingtin (mHTT) underlie progressive degeneration of dopaminoceptive striatal neurons in Huntington's disease (HD). The primary cilium is a membrane compartment that functions as a hub for various pathways that are dysregulated in HD, for example, dopamine (DA) receptor transmission and the mechanistic target of rapamycin (mTOR) pathway. The roles of primary cilia (PC) for the maintenance of striatal neurons and in HD progression remain unknown. Here, we investigated PC defects in vulnerable striatal neurons in a progressive model of HD, the mHTT-expressing knock-in zQ175 mice. We found that PC length is affected in striatal but not in cortical neurons, in association with the accumulation of mHTT. To explore the role of PC, we generated conditional mutant mice lacking IFT88, a component of the anterograde intraflagellar transport-B complex lacking PC in dopaminoceptive neurons. This mutation preserved the expression of the dopamine 1 receptor (D1R), and the survival of striatal neurons, but resulted in a mild increase of DA metabolites in the striatum, suggesting an imbalance of ciliary DA receptor transmission. Conditional loss of PC in zQ175 mice did not trigger astrogliosis, however, mTOR signaling was more active and resulted in a more pronounced accumulation of nuclear inclusions containing mHTT. Further studies will be required of aged mice to determine the role of aberrant ciliary function in more advanced stages of HD.

Neuron‐specific cilia loss differentially alters locomotor responses to amphetamine in mice

“…Second, Cre‐dependent removal of IFT88 in the olfactory epithelium, either in horizontal basal stem cells or in olfactory sensory neurons, leads to cilia loss as determined by immunostaining for acetylated α‐tubulin, as well as ADCY3 and Arl13b (Green et al., 2018; Joiner et al., 2015). Third, Cre‐dependent IFT88 deletion consistently leads to loss of ADCY3 staining from multiple types of neurons across various brain regions (Berbari et al., 2014; Bowie & Goetz, 2020; Mustafa et al., 2019). The present results show that IFT88 deletion produces loss of ADCY3 in both GAD2‐ and DAT‐expressing cell types, as well as a decrease in Arl13b positive cilia in Gad2:Ift88 mice (Figure S2).…”

Targeted Ablation of Primary Cilia in Differentiated Dopaminergic Neurons Reduces Striatal Dopamine and Responsiveness to Metabolic Stress

“…For PC frequency and length quantification, we used a Leica SP8 confocal microscope with a 63x oil immersion objective and maximal intensity projection z-stacked images (1 µm interval), as previously described [ 16 , 17 ]. To determine the number of DA neurons showing primary cilia, on average, 240 TH positive neurons per mouse were analyzed in five non-consecutive 30 μm sections (one every four sections).…”

“…The graphical and statistical analysis was performed using GraphPad Prism 7.0 (GraphPad Software, San Diego, CA, USA). In general, 80 cells per mouse were analyzed after tracing the ACIII signal in the ImageJ software in line with established protocols [ 16 , 17 , 34 ].…”

“…D1-type dopamine receptors (D1Rs) are G-protein-coupled receptors localized to the primary cilium of different cell types, suggesting a ciliary role in dopamine signaling transmission [ 9 , 10 ]. Increased PC length has been reported in various models of Huntington’s disease (HD) and HD patients, characterized by progressive degeneration of GABAergic striatal medium spiny neurons (MSNs) that receive glutamatergic and DA synaptic inputs [ 11 , 12 , 13 , 14 , 15 , 16 ]. The DA input originates from DA midbrain neurons.…”

“…As these mutant mice were perinatally lethal, the role of PC in postnatal survival and during aging could not be investigated. More recently, by the ablation of Ift88 in dopaminoceptive neurons, we have shown that PC are dispensable for maintenance of D1R expression in striatal neurons; however, striatal content of dopamine metabolites increases in aged mutant mice, supporting a PC-dependent functional crosstalk between dopaminoceptive and DA neurons [ 16 ].…”