The cerebellum plays an integral role in learning motor skills. The cerebellar cortex forms an array of relatively simple neuronal networks where Purkinje cells (PCs) receive excitatory inputs from parallel fibre (PF) varicosities, as well as from climbing fibres (CFs). PC axons provide the single output system to the deep cerebellar nuclei (Fig. 1a). The PF input of granule cells is derived by the stimulation of mossy fibres (MFs), which originate from a variety of brainstem nuclei and the spinal cord (Nakanishi 2005). The CF input arises from the inferior olivary nuclei. The role of PCs in motor learning is clearly established (Ito 2001; Gao et al. 2012). Long-term depression (LTD) at the PF–PC synapse has been proposed to be the dominant type of plasticity for cerebellar learning (Ito 2001). However, recent studies suggested that various forms of synaptic plasticity work synergistically and can compensate each other when one is missing in cerebellar learning (Gao et al. 2012). Despite growing controversies over the role of cerebellar LTD in motor learning, a recent study found that the decrease in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors in PF-PC synapses and elimination of these synapses are in vivo engrams in short- and long-term motor learning, respectively (Wang et al. 2014), which seems to be consistent with the LTD hypothesis (Ito 2001). These previous studies indicate that an extensive capacity of the cerebellar cortex with dynamic reorganization of PF-PC synaptic connections is fundamentally important for the learning of motor skills (Wang et al. 2014). In spite of this, the molecular mechanisms that regulate the structural and functional reorganization of PF-PC synapses are not clear.
Synaptic plasticity-related changes in the structure and molecular composition of post-synaptic density (PSD) depends in part on the complex regulation of phosphorylation of specific proteins via different protein kinases, including the serine/threonine kinase, cycline-dependent kinase 5 (Cdk5) (Lai and Ip 2009). Cdk5 is activated upon association with its specific activating subunits, p35 or p39. Cdk5 is predominantly neural-specific kinase because of the restricted expression of p35 and p39 in the nervous system. The crucial roles of Cdk5 in neuronal migration, neuronal survival, differentiation, synapse development and synaptic plasticity in mature neurons are widely recognized (Lai and Ip 2009). Furthermore, dysregulation of Cdk5 has been linked to an array of neurodegenerative disorders (Lopes and Agostinho 2011; Cheung and Ip 2012). Considering the multifaceted role of Cdk5, it is not surprising that Cdk5 deficient mice exhibit perinatal lethality and defective positioning of several types of neurons (Kumazawa et al. 2013). In contrast, p35 deficient mice show a milder phenotype of positioning defects of neurons and survive to adulthood because of redundant and overlapping expression of p39 (Kumazawa et al. 2013). While previous studies revealed impaired hippocampus-dependent spatial learning and memory in p35 deficient mice (Ohshima et al. 2005), the role of Cdk5/p35 in cerebellar motor learning and synaptic plasticity has not been established until now.
In this issue, He et al. (2014), provide new insights into the role of Cdk5/p35 in motor coordination and cerebellar plasticity. The authors' initial investigations of previously developed p35 deficient (p35−/−) transgenic mice identified impaired ability to maintain balance without noticeable muscle weakness (He et al. 2014). However, these changes in motor coordination were difficult to interpret because of abnormalities in the laminar structure of the cerebellar cortex, which included the misalignment of Purkinje cells, sparse dendritic trees of Purkinje cells and the presence of granule cells in the molecular cell layer. Therefore, He and colleagues generated a Purkinje cell-specific p35 conditional knockout mouse model (L7-p35 cKO). Unlike in the p35−/− brains, there were no detectable structural abnormalities or gross histological changes in L7-p35 cKO brains. Immunohistochemical analysis confirmed the normal density and alignment of PCs and the elimination of p35 from PCs. Morphological analysis revealed no detectable differences in the density, lengths, area and branching of dendrites or axonal elongation of p35 lacking PCs. Carefully controlled behavioural tests revealed motor coordination deficiency and impaired ability to maintain balance or posture, but no gait disturbances in the L7-p35 cKO mice. There were no detectable differences in the relationship between PF stimulus intensity and excitatory post-synaptic current amplitudes (input-output relationship) or paired-pulse facilitation (form of short-term synaptic plasticity) or paired-pulse depression in L7-p35 cKO mice. These observations indicate that the selective loss of p35 activity from post-synaptic PCs produced no detectable functional deficits in pre-synaptic input at PF- and CF-PC synapses. A protocol that depressed PF-evoked excitatory post-synaptic currents failed to induce LTD in PCs of L7-p35 cKO mice. Moreover, a long-term potentiation (LTP) induction protocol evoked significantly reduced potentiation in L7-p35 cKO PCs. These results indicate that p35 is involved in both LTD and LTP at the PF-PC synapse (He et al. 2014).
In summary, He et al. (2014) demonstrated that Cdk5/p35 activity in PCs is required for proper motor coordination and for the formation of two forms of long-term synaptic plasticity of the cerebellum, LTD and LTP. These changes do not depend on anatomical or developmental defects. Because the elimination of p35 is restricted to PCs in L7-p35 cKO, this mouse model is a potentially useful tool for the investigation of Cdk5/p35-mediated post-synaptic signaling processes. While the molecular mechanisms underlying the post-synaptic involvement of Cdk5/p35 at PF-PC synapses remains to be determined, recent studies provide important clues on how this kinase may modulate the efficacy of synaptic transmission. For example, Cdk5/p35 may modulate the ionotropic glutamate receptor (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor and N-methyl-D-aspartate receptor) content of the PSD via the regulation of striatal-enriched tyrosine phosphatase, Ca2+/calmodulin-dependent protein kinase II, PSD-95 or direct phosphorylation of the GluN2A N-methyl-D-aspartate receptor subunit (Lai and Ip 2009; Barnett and Bibb 2011; Cheung and Ip 2012; He et al. 2014; Fig. 1b). It is also plausible that Cdk5/p35 modulate the plasticity of PF-PC synapses via the modification of intracellular Ca2+ levels in PCs through the phosphorylation of P/Q-type voltage-dependent Ca2+ channels (VDCCs), L-type VDCCs and/or inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) (He et al. 2014; Fig. 1b). Further studies are needed to identify the precise molecular mechanisms of Cdk5/p35-mediated changes in Purkinje cells that underlie learning and memory.