TTBK2 and primary cilia are essential for the connectivity and survival of cerebellar Purkinje neurons

Abstract

Primary cilia are vital signaling organelles that extend from most types of cells, including neurons and glia. These structures are essential for development of many tissues and organs; however, their function in adult tissues, particularly neurons in the brain, remains largely unknown. Tau tubulin kinase 2 (TTBK2) is a critical regulator of ciliogenesis, and is also mutated in a hereditary neurodegenerative disorder, spinocerebellar ataxia type 11 (SCA11). Here, we show that conditional knockout of Ttbk2 in adult mice results in degenerative cerebellar phenotypes that recapitulate aspects of SCA11 including motor coordination deficits and defects to Purkinje cell (PC) integrity. We also find that the Ttbk2 conditional mutant mice quickly lose cilia throughout the brain. We show that conditional knockout of the key ciliary trafficking gene Ift88 in adult mice results in nearly identical cerebellar phenotypes to those of the Ttbk2 knockout, indicating that disruption of ciliary signaling is a key driver of these phenotypes. Our data suggest that primary cilia play an integral role in maintaining the function of PCs in the adult cerebellum and reveal novel insights into mechanisms involved in neurodegeneration.

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

“…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). These data are comparable, for example, to cilia loss in the cerebellum, in which IFT88 removal is associated with a greater loss of ADCY3‐positive cilia compared to Arl13b‐positive cilia (Bowie & Goetz, 2020). In Gad2:Ift88 KO brains, some Arl13b cilia are still present, although this may reflect cilia not on GAD2 cells.…”

“…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).…”

“…Most neurons have a single primary cilium that projects from the soma, although some, such as gonadotropin‐releasing hormone neurons, can have 2–3 (Berbari et al., 2007; Bishop et al., 2007; Kirschen & Xiong, 2017; Koemeter‐Cox et al., 2014). Neuronal cilia range in size from 3 to 15 µm in length, with some regional variation in average lengths (Bowie & Goetz, 2020; Guadiana et al., 2016). Following maturation, average length of neuronal cilia is relatively stable over the life span in rodent brains (Chakravarthy et al., 2012; Guadiana et al., 2016), but they can be dynamic and change lengths in response to extracellular cues (Miyoshi et al., 2014; Shiwaku et al., 2017).…”

“…Diseases caused by mutations in cilia‐specific genes, known collectively as ciliopathies, are often accompanied by deficits in cognition and motivated behavior (Berbari et al., 2014; Brinckman et al., 2013; Poretti & Gerner, 2016; Sherafat‐Kazemzadeh et al., 2013). Neuronal cilia signaling influences the formation of dendritic arbors, synapse maintenance, and axon growth (Bowie & Goetz, 2020; Guadiana et al., 2013; Guo et al., 2017, 2019; Higginbotham et al., 2012). For example, disruption of cilia signaling during development reduces dendritic complexity of cortical neurons and striatal interneurons (Guadiana et al., 2013; Guo et al., 2017) as well as synaptic connectivity (Bowie & Goetz, 2020; Guo et al., 2017).…”

“…Neuronal cilia signaling influences the formation of dendritic arbors, synapse maintenance, and axon growth (Bowie & Goetz, 2020; Guadiana et al., 2013; Guo et al., 2017, 2019; Higginbotham et al., 2012). For example, disruption of cilia signaling during development reduces dendritic complexity of cortical neurons and striatal interneurons (Guadiana et al., 2013; Guo et al., 2017) as well as synaptic connectivity (Bowie & Goetz, 2020; Guo et al., 2017). Across organ systems, cilia play a variety of signaling roles, largely related to sensing extracellular cues.…”