Transgenic mice expressing F3/contactin from the TAG-1 promoter exhibit developmentally regulated changes in the differentiation of cerebellar neurons

Abstract

F3/contactin (CNTN1) and TAG-1 (CNTN2) are closely related axonal glycoproteins that are differentially regulated during development. In the cerebellar cortex TAG-1 is expressed first as granule cell progenitors differentiate in the premigratory zone of the external germinal layer. However, as these cells begin radial migration, TAG-1 is replaced by F3/contactin. To address the significance of this differential regulation, we have generated transgenic mice in which F3/contactin expression is driven byTAG-1 gene regulatory sequences, which results in premature expression of F3/contactin in granule cells. These animals (TAG/F3mice) display a developmentally regulated cerebellar phenotype in which the size of the cerebellum is markedly reduced during the first two postnatal weeks but subsequently recovers. This is due in part to a reduction in the number of granule cells, most evident in the external germinal layer at postnatal day 3 and in the inner granular layer between postnatal days 8 and 11. The reduction in granule cell number is accompanied by a decrease in precursor granule cell proliferation at postnatal day 3, followed by an increase in the number of cycling cells at postnatal day 8. In the same developmental window the size of the molecular layer is markedly reduced and Purkinje cell dendrites fail to elaborate normally. These data are consistent with a model in which deployment of F3/contactin on granule cells affects proliferation and differentiation of these neurons as well as the differentiation of their synaptic partners, the Purkinje cells. Together,these findings indicate that precise spatio-temporal regulation of TAG-1 and F3/contactin expression is critical for normal cerebellar morphogenesis.

F3/contactin and TAG1 play antagonistic roles in the regulation of sonic hedgehog-induced cerebellar granule neuron progenitor proliferation

“…Previously, since EGL thickness and TAG1 expression was largely unaffected, and because TAG1 was thought a post-mitotic marker, this was interpreted to mean that NrCAM and L1 promote survival of differentiated IGL GNs. As TAG1 is clearly expressed by cycling GNPs (this study) (Bizzoca et al, 2003), a simpler explanation is that few post-mitotic GNs are produced in these mice.…”

“…F3/contactin acts directly on cerebellar granule neuron precursors to suppress sonic hedgehoginduced proliferation Misexpression of F3/contactin (F3) in the developing cerebellum of TAGF3 mice delays the growth of the cerebellar cortex in the first two postnatal weeks (Bizzoca et al, 2003). However, it was unclear whether this was a direct effect of F3 on GNPs or due to F3 acting indirectly through other cell types.…”

“…Both misexpression of F3 in vivo (Bizzoca et al, 2003) and F3-fc treatment in vitro inhibit GNP proliferation and disrupt differentiation. As TAG1 and F3 expression is largely mutually exclusive (Fig.…”

“…In vivo, high level expression of F3 on GNPs is restricted to postmitotic cells of the iEGL and IGL, coincident with NeuN, whereas the related molecule TAG1 (Cntn2) appears more superficially in the EGL, largely non-overlapping with F3, but not on the Math1 + cells of the oEGL (Bizzoca et al, 2003;Virgintino et al, 1999) (Fig. 3A,B).…”

“…In addition, loss of F3 leads to postnatal lethality and severely disrupts parallel fibre development (Berglund et al, 1999). Premature expression of F3 in GNPs in transgenic TAGF3 mice, using the promoter of the related TAG1 gene (Cntn2) (which is normally expressed upon cycling as well as on postmitotic GNPs), suppresses GNP proliferation and leads to transient delay of cerebellar growth and reduced proliferative potential in vitro (Bizzoca et al, 2003). However, whether F3 affects SHHinduced GNP proliferation and whether its effects are direct or indirect has not been tested, although receptors for F3 are expressed on GNPs, including L1 and the related NrCAM, which are redundantly required for GNP survival (Brummendorf and Rathjen, 1996;Hu et al, 2003;Sakurai et al, 2001;Solecki et al, 2001).…”