Synapse elimination in the developing cerebellum

Kouichi Hashimoto 0 1 Masanobu Kano 0 1 0 K. Hashimoto PReSTO, Japan Science and Technology Agency , Saitama 332-0012, Japan 1 K. Hashimoto Department of Neurophysiology, Graduate School of Biomedical Sciences, Hiroshima University , 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan 2 ) Department of Neurophysiology, Graduate School of Medicine, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Neural circuits in neonatal animals contain numerous redundant synapses that are functionally immature. During the postnatal period, unnecessary synapses are eliminated while functionally important synapses become stronger and mature. The climbing fiber (CF) to the Purkinje cell (PC) synapse is a representative model for the analysis of postnatal refinement of neuronal circuits in the central nervous system. PCs are initially innervated by multiple CFs with similar strengths around postnatal day 3 (P3). Only a single CF is selectively strengthened during P3-P7 (functional differentiation), and the strengthened CF undergoes translocation from soma to dendrites of PCs from P9 on (dendritic translocation). Following the functional differentiation, supernumerary CF synapses on the soma are eliminated, which proceeds in two distinct phases: the early phase from P7 to around P11 and the late phase from around P12 to P17. Here, we review our current understanding of cellular and molecular mechanisms of CF synapse elimination in the developing cerebellum. - Formation of precise neuronal connections during development is a prerequisite for proper functions of the nervous system. At birth, neuronal connections are redundant, but they are refined and become functionally mature through activity-dependent competition among redundant synaptic inputs to each postsynaptic neuron. During the postnatal period, functionally important synapses are selectively strengthened and stabilized, whereas unnecessary surplus connections are weakened and eventually eliminated. Many studies indicate that these processes are dependent on neural activity particularly during the limited postnatal period known as the critical period or sensitive period [15]. The climbing fiber (CF) to the Purkinje cell (PC) synapse in the cerebellar cortex is regarded as a representative model system for analyzing the mechanisms for developmental establishment of the functional connections in the central nervous system. In the adult cerebellum, each PC is innervated by a single CF (mono innervation) originating from the inferior olive of the contralateral medulla oblongata. each CF forms hundreds of synaptic contacts on the proximal part of PC dendrites [6, 7]. Therefore, activation of a single CF causes strong depolarization that triggers a Ca2+ transient due to activation of voltage-dependent Ca2+ channels (vDCCs) in PC dendrites [8]. In early postnatal days, however, all PCs are innervated by multiple CFs with weak synaptic responses (multiple innervation) [911]. Surplus CFs are eventually eliminated during postnatal development, and mono innervation is attained by the end of the third postnatal week in mice [1216]. In this review article, we will integrate our current knowledge and provide an overview of the mechanisms of CF synapse elimination in the developing cerebellum. Synaptogenesis of CFs to immature PCs The cerebellar cortex is known to be subdivided into longitudinally elongated parasagittal bands [1719]. PCs in each subdivision are innervated by neurons in distinct subnuclei of the inferior olive [2022]. These parasagittal bands are further subdivided into smaller units called microzones, in each of which PCs display high synchronity of complex spike activity [23, 24] and resultant Ca2+ transients [25, 26]. In adult rats, each olivocerebellar axon ramifies several times in the cerebellum and gives rise to 6.6 CFs on average [27]. These CFs terminate in one lobule or multiple continuous or discontinuous lobules, but all of them are aligned within a single rostrocaudally oriented area [28]. Olivo-cerebellar axons reach the primitive cerebellum around e18 [29, 30]. Their projections are largely topographic at birth, and are roughly aligned within rostrocaudally oriented areas [31, 32]. However, the typical climbing fiber morphology is not observed at this postnatal age. Immature olivo-cerebellar axons extensively ramify in the white matter and granule cell (GC) layer, and give rise to many thick and thin collaterals around PCs. This stage is called the creeper stage [33]. At this stage, PCs still have bipolar shapes (called simple- and complex-fusiform cells [34]), have just completed their migration, and are organized in a multilayer. Initially, each olivo-cerebellar axon forms about 100 creeper fibers [32]. Morphological analyses have demonstrated that the synapse formation of olivary axons on PCs starts from their arrival to the cerebellar cortex [29, 33, 35, 36]. electrophysiological analyzes indicate that functional olivo-cerebellar synapses are formed on immature PCs around P3. In juvenile rats and mice in vivo, stimulation in the inferior olive after P3 elicits excitatory responses in PCs [37, 38]. However, the responses of juvenile PCs are graded in parallel with the increase in the stimulus strength [10], which indicates that PCs are innervated by multiple olivo-cerebellar axons. while molecules related to cell identity are differentially expressed in parasagittal bands in developing cerebellum and inferior olivery neurons [39], it remains unclear how such parasagittal organization is formed during development. Several molecules have been proposed but none of them has been proven to provide spatial cues that may direct CF targeting to appropriate cerebellar zones. Future studies should elucidate a molecular logic for constructing the topographic olivo-cerebellar projection during cerebellar development. Postnatal refinement of CF to PC synapses while the microzonal projection is largely established at early postnatal days, each PC is innervated by multiple CFs. PCs are devoid of large primary dendrites and CFs mainly form terminals on the fine processes emerging from the PC somata. Adult-like, mono innervation is gradually established during postnatal development. Studies on the mechanisms underlying postnatal development of the CF to PC synapse were initiated from the analyses of spontaneously occurring mutant animals. Several mutant mice that have malformation of cerebellar cortex (reeler [40]), degeneration of cerebellar GCs (weaver [41, 42]), or impairment of PC morphogenesis and parallel fiber (PF)-PC synaptogenesis (staggerer [43, 44]) show persistent multiple CF innervation of PCs in adulthood. Defects in CF synapse elimination are also observed in mice or rats whose GCs are artificially destroyed by methylazoxy methanol acetate [13], virus infection [45], or X-ray irradiation [46, 47]. These pioneering analyses suggest that proper formation of GCs and PFs is crucial for (...truncated)