Impaired presynaptic function and elimination of synapses at premature stages during postnatal development of the cerebellum in the absence of CALEB (CSPG5/neuroglycan C)

Chicken acidic leucine‐rich EGF‐like domain‐containing brain protein (CALEB), also known as chondroitin sulfate proteoglycan (CSPG)5 or neuroglycan C, is a neural chondroitin sulfate‐containing and epidermal growth factor (EGF)‐domain‐containing transmembrane protein that is implicated in synaptic maturation. Here, we studied the role of CALEB within the developing cerebellum. Adult CALEB‐deficient mice displayed impaired motor coordination in Rota‐Rod experiments. Analysis of the neuronal connectivity of Purkinje cells by patch‐clamp recordings demonstrated impairments of presynaptic maturation of inhibitory synapses. GABAergic synapses on Purkinje cells revealed decreased evoked amplitudes, altered paired‐pulse facilitation and reduced depression after repetitive stimulation at early postnatal but not at mature stages. Furthermore, the elimination of supernumerary climbing fiber synapses on Purkinje cells was found to occur at earlier developmental stages in the absence of CALEB. For example, at postnatal day 8 in wild‐type mice, 54% of Purkinje cells had three or more climbing fiber synapses in contrast to mutants where this number was decreased to less than 25%. The basic properties of the climbing fiber Purkinje cell synapse remained unaffected. Using Sholl analysis of dye‐injected Purkinje cells we revealed that the branching pattern of the dendritic tree of Purkinje cells was not impaired in CALEB‐deficient mice. The alterations observed by patch‐clamp recordings correlated with a specific pattern and timing of expression of CALEB in Purkinje cells, i.e. it is dynamically regulated during development from a high chondroitin sulfate‐containing form to a non‐chondroitin sulfate‐containing form. Thus, our results demonstrated an involvement of CALEB in the presynaptic differentiation of cerebellar GABAergic synapses and revealed a new role for CALEB in synapse elimination in Purkinje cells.