Development of the dopaminergic system and the reticular thalamic nucleus in polysialic acid-deficient mice

The neural cell adhesion molecule NCAM and its post-translational modification with polysialic acid (polySia) are broadly implicated in neural development. Mice lacking the polysialyltransferases ST8SiaII and ST8SiaIV (II-/-IV-/-) are devoid of polySia and are characterized by postnatal growth retardation, precocious death, high incidence of hydrocephalus, and severe defects of major brain axon tracts. In the present work, the impact of polySia on (i) the development of the dopaminergic system and (ii) thalamocortical projections as well as the reticular thalamic nucleus was analyzed. In the first part of my thesis, I studied the role of polySia during embryonic development of the midbrain dopamine system in mice. PolySia immunoreactivity was detected on radial glia processes and on cell somata in the pial zone of the developing midbrain. Real-time RT-PCR analyses revealed that the mRNA profiles of polysialyltransferases and NCAM matched the time course of dopaminergic marker gene expression. Despite these parallels, tyrosine hydroxylase immunohistochemistry at embryonic day 14.5 and postnatal days 1 and 30 demonstrated that the loss of polySia caused no defects in the neuroarchitecture of the midbrain dopaminergic system. Furthermore, polySia-deficiency in vivo as well as enzymatic removal of polySia from cultured midbrain dopaminergic neurons in vitro had no effect on the expression of dopaminergic marker genes. Together, these data indicate that polySia is dispensable for the development of the midbrain dopamine system. In the second part of my thesis, I analyzed thalamocortical projections in relation to the previously described hypoplasia of the internal capsule in II-/-IV-/- mice. Furthermore, I asked for a role of the reticular thalamic nucleus (Rt) in the aberrant development of this fiber tract. Immunohistochemistry and tract tracing established that early misguidance of thalamocortical axons and subsequent deficits of corticofugal projections contribute to the internal capsule defect in II-/-IV-/- mice. As the Rt has been proposed to serve as a guidepost during early thalamocortical pathfinding, its malformation could be the reason for the observed phenotype. Indeed, numbers of parvalbumin-positive cells were drastically reduced in four week old II-/-IV-/- mice. However, initial formation of the Rt was largely uncompromised and together with increased apoptotic cell death at postnatal day 5, Rt neurons disappeared postnatally. Thus, neuronal damage was preceded by deficits of thalamocortical and corticothalamic axons, the collaterals of which provide the major excitatory input into the Rt. These findings suggest that defective innervation causes anterograde degeneration of the Rt in II-/-IV-/- mice.