The dorsal cochlear nucleus of the adult lurcher mouse is specifically invaded by embryonic grafted Purkinje cells.

The fate of embryonic Purkinje cells grafted over the brainstem surface of the adult Lurcher mouse was analyzed using anti-calbindin (CaBP) immunocytochemistry. Purkinje cells are able to migrate specifically into the molecular layer of the host dorsal cochlear nucleus (DCoN) and develop dendritic trees that are practically isoplanar, suggesting synaptic interactions with the parallel fibres of the DCoN. These results provide a new argument in favour of the homology between the cerebellum and the DCoN.

Partial reconstruction of the adult Lurcher cerebellar circuitry by neural grafting.

Solid cerebellar grafts, taken from normal mouse embryos (gestational day 12-14), were transplanted into the cerebellum of adult Lurcher mice. The degree of Purkinje cell replacement was analysed one to three months after transplantation by means of immunocytochemistry (antibodies against calbindin, cGMP-dependent protein kinase and neurofilament proteins) and electron microscopy. Grafted Purkinje cells succeed in moving out of the graft and migrate into the host cerebellar cortex. They are present next to the graft in the granule cell and molecular layers, and far from the graft remnant, only in the molecular layer, indicating that, although both layers subserve Purkinje cell migration, the molecular layer is the ultimate target. In the host molecular layer, axons of transplanted Purkinje cells form thick bundles running in the frontal plane over long distances. Most of them terminate in the upper granule cell layer by enlarged bulbs resembling collapsed growth cones. Axons reaching their normal targets (the neurons of the deep cerebellar nuclei) are observed only in cases where the granule cell layer is disrupted and/or grafted Purkinje cells remain in the white matter. The projection is massive only from grafts lying in the close vicinity of the target neurons. Electron-microscopic analysis of grafted Purkinje cells populating the host cerebellar cortex reveals that their synaptic investment is abnormal. In the molecular layer, where the normal inputs are reduced, the compartmentation in proximal and distal dendritic segments is severely affected, climbing fibre synapses only form on a minority of grafted cells and "pinceau" formations are absent. In the granule cell layer, the synaptic investment is similar to that of Purkinje cells in agranular cerebellum, and even heterelogous synapses with mossy fibres have been observed. These results, compared to those previously obtained with grafting experiments in Purkinje cell degeneration mutant mouse, allow us to conclude that: (i) the Purkinje cell-deficient molecular layer of the host, despite its severe atrophy and reactive gliosis, still exerts a positive neurotropism specific for grafted Purkinje cells; (ii) the unlesioned host granule cell layer underlying the molecular layer containing grafted Purkinje cells, even if almost depleted of granule cells, remains an obstacle for the re-establishment of a corticonuclear projection; and (iii) the degree of synaptic integration of grafted Purkinje cells is directly related to the nearby presence of available host axon terminals. Hence, owing to the atrophy of the Lurcher cerebellum, the postgrafting restoration of the cerebellar cortical circuit is much less complete in this mutant.

Early development of the Lurcher cerebellum: Purkinje cell alterations and impairment of synaptogenesis.

The postnatal development of the heterozygous Lurcher (Lc/+) mouse cerebellum is characterized by Purkinje cell death with a concomitant reduction in granule cell number. In order to evaluate possible relationships between these two events, this study investigates early morphological abnormalities of the Purkinje cells and possible defects in the formation of their synaptic investment. Cerebella of Lurcher and control age-matched (from P8 to P16) mice were analysed by calbindin immunostaining, silver impregnation and quantitative electron microscopy. Direct signs of Purkinje cell anomaly are obvious from P8, four days before the onset of the necrotic process. These signs include the presence of axonal swellings and perinuclear clumps of chromatin, and a general delayed process of maturation, evidenced in cell bodies (incomplete development of the basal polysomal mass) and in dendritic trees (hyperspinous dendrites, delayed formation of proximal and distal compartments). Also from P8, the external granular layer is reduced in thickness. Despite these abnormalities, the onset of the synaptogenesis between Purkinje cells and their specific inputs (parallel fibres, climbing fibres and basket cell axons) takes place on schedule and, at P8, no defect has been noticed. On and after P10, the rate of parallel fibre synaptogenesis is decreased. Very few climbing fibres translocate from their perisomatic to their peridendritic locations, and basket cell axons fail to develop 'pinceau' formations. All these results suggest that before the death of the Purkinje cell by P12, there is an impaired maturation of these neurons provoked by the Lurcher gene action. The hypoplasia of the external granular layer and the altered synaptic investment of the Purkinje cell after P10 are considered to be consequences of the early Purkinje cell defect.