Local Circuits in the Cerebellum Interact with Biochemical Events

In a recent study published in Signal Transduction and Targeted Therapy, Liu et al. [1] studied a particular circuit from lobules IV/V in the cerebellar vermis to neurons within the fastigial nucleus (FN), which regulates sensorimotor coordination. They also identified biorientation defective 1 (BOD1) as part of the molecular mechanism of the circuit, in which a deficit in Purkinje cells (PCs) in lobules IV/V triggers hyperactivation in FN CaMKIIα⁺-neurons, resulting in behavioral signs of ataxia (Fig. 1).

Fig. 1.

Schematic of the mechanism by which BOD1 regulates the cerebellar IV/V lobe^PCs→FN^CaMKIIα+ circuit associated with motor coordination

Ataxia is a neurological disorder resulting from cerebellar lesions. In light of the limited evidence on how ataxia arises from dysfunctions of cerebellar structure and its afferent and efferent connections, we currently lack effective interventions to treat this disease. Therefore, clarifying the precise regulatory mechanism that connects different cerebellar lobes and deep cerebellar nuclei (DCN) will help us to find pharmacological and non-pharmacological treatment options for patients with ataxia. The mammalian cerebellar cortex has ten lobules that are separated from one another by a series of fissures; these lobules are segregated into the anterior (lobules I–V) and posterior lobes (lobules VI–X), which are associated with motor and non-motor functions, respectively. However, recent findings have indicated that lobules IV/V of the anterior lobe subserve non-motor behaviors, including social behavior, temporal memory [2], and working memory [3]. This raises a critical question: Can behavioral differences arise specifically from dysfunction of the projections between lobules IV/V and the DCN? The DCN comprise at least three subdivisions: the FN, interposed nucleus, and lateral nucleus, which differ in their axonal projections and potential gene expression. The FN has widespread anatomical connections to brain regions involved in motor and non-motor functions [4]. For example, the FN plays a role in cognitive processes [5]. Moreover, inactivation of the FN alters social behavior in rats [6], and activation of this nucleus is a potent control measure for temporal lobe seizures [7]. Therefore, it would be of great significance to characterize the anatomical projections and the motor and non-motor functions of the FN and to investigate related diseases. Using anatomical tracing, Liu et al. and colleagues established comprehensive structural and functional networks of the specific cerebellar lobule IV/V–FN circuit associated with motor coordination.

Extensive examination by Liu et al. revealed the specific anatomical segregation of an inhibitory circuit between PCs in lobules IV/V and CAMKIIα⁺ neurons in the FN. As the sole output of the cerebellar cortex, PCs send GABAergic projections to innervate the FN and shape its neuronal activity. FN neurons include at least five distinct populations (large glutamatergic neurons, large glycinergic neurons, medium GABAergic neurons, small GABA/glycine interneurons, and non-GABAergic interneurons). The activation of histamine H2 receptors or 5-HT_2A receptors by histamine or serotonin microinjection into the bilateral FN enhances rat motor performance and balance [8]. Another study demonstrated that the loss of GABAergic inhibition in the FN is responsible for myoclonus [9]. Therefore, it is necessary to further characterize the FN neuronal activity that may positively modulate motor behaviors. By combining optogenetic and chemogenetic manipulation, Liu et al. found that the PCs in lobules IV/V project to CaMKIIα⁺ neurons in the FN. Likewise, the authors used optogenetic manipulations based on the rotarod test and found that the optogenetic inhibition of PCs increased c-fos expression in CaMKIIα⁺ neurons but not in PV⁺ neurons in the FN. These results imply that different types of neurons in the FN mediate different functions and behaviors in various pathophysiological contexts.

In addition to abnormal electrophysiological activity, it is also necessary to pay attention to whether multilevel biological events are involved in and mediate the pathological process of cerebellar ataxia. Through transcriptomic profiling analysis, the authors made and validated the discovery that BOD1 plays an important role in the circuit from lobules IV/V to the FN. Liu et al. demonstrated that BOD1 deficiency in PCs of lobules IV/V attenuated the excitability of PCs; this change was accompanied by behavioral signs of ataxia. In addition, the authors found that BOD1 protein is involved in regulating the excitability of CAMKIIα⁺ neurons and that overexpression of BOD1 in PCs of lobules IV/V ameliorated the ataxic behavior in L7-Cre; BOD1^f/f mice. Recognizing that neuronal firing contributes to the intrinsic electrical properties of neurons as well as to excitatory and inhibitory synaptic inputs [10], Liu et al. next investigated how BOD1 deficiency in lobules IV/V induced PC activity. The authors found that the frequency of miniature inhibitory postsynaptic current was significantly decreased, and this change was accompanied by dysfunction of excitability/inhibitory balance, which contributed to the decrease in PC activity. Furthermore, they showed that BOD1 deletion selectively decreases the number of mature dendritic spines. Hence, ataxia may ultimately be caused by an imbalance between neurobiochemical signals and electrical signal oscillations.

In conclusion, this work highlights the functional validation of the cerebellar lobule IV/V^PC→FN^CaMKIIα+ circuit and provides novel mechanistic insights into ataxia. Some of the most notable findings by Liu et al. were that BOD1 deficiency decreased the number of GABAergic projections from PCs to CaMKIIα⁺ neurons in the deep cerebellar FN, resulting in CaMKIIα⁺ neuronal hyperactivation in the FN and ataxic behavior in mice. These encouraging findings suggest that BOD1 is a potential key molecule in the pathological process underlying cerebellar ataxia, meaning that pharmacological regulation strategies based on increasing BOD1 expression or enhancing its downstream signaling targets may have important clinical implications. Going forward, research priorities should include collecting data on the changes in neurochemical and electrophysiological indicators after clinical intervention; these clinical data could have implications for the basic understanding of cerebellar ataxia, helping investigators to identify the key BOD1-associated signaling targets that lead to the development of cerebellar ataxia.