New Insight on the Factors Orienting the Axonal Outgrowth of Grafted Purkinje Cells in the pcd Cerebellum

Abstract

Despite Purkinje cell replacement, leading to the repair of the cortical circuit of the pcd mouse cerebellum grafted with E12 cerebellar primordium, the reestablishment of the corticonuclear projection only occurs for some Purkinje cells and in a small percentage of grafted mice. In order to assess the importance of: (1) competition between host and grafted deep nuclei, and (2) the distance between the implants and the host deep nuclei, new grafted experiments have been performed. In the latter, solid grafts were taken from E13 or E14 donor embryos after removal of the region containing the postmitotic deep nuclear neurons, and randomly positioned at various cerebellar depths. With cortical implants, the absence of donor nuclear neurons is not sufficient to allow the axons of the grafted Purkinje cells that have invaded the host molecular layer to escape the confinement of this layer. The molecular/granular layer interface appears as an almost impassable obstacle, and the granule cell layer as a nonpermissive milieu. With grafts located between the host deep nuclei and the 4th ventricle (deep grafts), the grafted Purkinje cells project massively to the host nuclei, but they are unable to leave the implant and, therefore, they are not integrated in the deficient cortical circuit. Finally, when the grafts positioned in the central white matter (intermediate grafts) disrupt the integrity of the host granule cell layer, some of the grafted Purkinje cells invade the host molecular layer, while most of them remain within the implant. Some axons of the cortically integrated Purkinje cells, using the nearby graft as a bridge, seem able to innervate the host deep nuclei. The latter, in addition, receive a massive projection from the nonintegrated Purkinje cells. These results emphasize the ability of grafted Purkinje cells to specifically innervate their target host neurons, when either there is proximity, or when a permissive microenvironment for their axonal outgrowth is created by embryonic grafted cortical cerebellar neurons, filling the gap between the molecular layer and the deep nuclei of the host.

Purkinje Cell Survival and Axonal Regeneration Are Age Dependent: AnIn VitroStudy

“…The ubiquitous invasion of the growing axons of immature Purkinje cells in the cocultures contrasts with previous results using transplants of embryonic cerebellar precursors to adult mutant cerebellum (Keep et al, 1992). Although in both situations the regenerative capacity appears similar, in the transplants the axons grow in the host molecular layer without entering the granular layer.…”

Cell Interactions Underlying Purkinje Cell Replacement by Neural Grafting in the pcd Mutant Cerebellum

“…Thus, although our results indicate that this prerequisite can occasionally be achieved, much more work is needed to provide to the growing axons of grafted PCs a permissive substratum for the re-establishment of a corticonuclear projection. 6 Without the projection, indicative of the complete synaptic integration of the grafted PCs, the restoration of an improved cerebellar function is inconceivable. …”

“…In order to determine whether or not grafted PCs are able to recapitulate their developmental history within the adult host cerebellum, short-term survivals have been analyzed in a timed sequence from 3 up to 21 days after grafting (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21), to compare at a given age the stage of maturation of the grafted PCs with that normally occurring in cerebellar ontogenesis.…”

Concise Review: Stem Cells for the Treatment of Cerebellar-Related Disorders

“…The fetal neural grafting in various ataxia murine models of the disease has been extensively studied over the last few decades (for reviews [15,16]) with the main focus on fate restriction and developmental potential of cerebellar progenitors as well as the role of specific regional brain environment in the determination of the fate of cerebellar progenitors. The most important studies were performed by different groups mostly using mouse cerebellar mutant model involving PC degeneration (pcd), first described by Mullen et al [17], and characterized by massive postnatal degradation of PCs or weaver mutant (wv:wv) [2,12,15,16,[18][19][20][21][22][23][24][25][26][27][28][29]. To re-establish the functional circuits in damaged pcd cerebellum, intracerebellar grafts derived from the wild type mouse embryo were transplanted into the mutant animals.…”

“…The results were improved by grafting fetal cerebellar progenitors directly to deep nuclei [22,27,29] resulting in robust PC axonal innervations of deep nuclei with the formation of synaptic contacts and the partial enhancement of behavioral response [28]. Successive studies have indicated that implantation of fetal progenitors can reverse a large number of symptoms (both motor and cognitive) caused by PC loss of various kinds [12,15,21,22,25,[27][28][29].…”