However, TH+ mDA were not observed emanating from most regions of the Otx2+ hindbrain FP. Since forced expression of Otx2 alone in the hindbrain FP is sufficient to generate mDA neurons, our expectation was that in the Shh::cre Ctnnb1 lox(ex3) embryos, induction of Otx2 would be sufficient to drive mDA production. Analysis of the hindbrain FP in such embryos, revealed the induction of Lmx1a, Otx2, Msx1, and Ngn2, and suppression of Shh in the hindbrain. To determine whether Wnt signaling is sufficient to induce mDA production, we generated Shh::cre Ctnnb1 lox(ex3) embryos in which beta–catenin was stabilized in the FP. In accordance with the notion that Wnt signaling is critical for mDA production, a recent study has used canonical Wnt-promoting regimens to greatly improve mDA derived from human embryonic stem cells. These studies suggest that canonical Wnt signaling is necessary for mDA specification and neurogenesis, although it is possible that some of the effects seen in these studies are due to loss of integrity of adherens junctions. In these mutants, the midbrain FP, at least partially loses expression of Otx2, Msx1, Ngn2 and Lmx1a, , maintains the expression of Shh, and thereby molecularly resembles the hindbrain FP. Next, both FP-specific, and temporally controlled conditional ablation of beta–catenin results in the reduction of mDA neurogenesis. First, Wnt1 is expressed in the ventral midbrain but not ventral hindbrain or spinal cord – and the Wnt1 null mutant has severely reduced numbers of mDA. The stark difference in neurogenic capacity is at least in part, due to canonical Wnt signaling. In contrast, at midbrain levels the FP is remarkably neurogenic, generating among others, midbrain dopamine neurons (mDA – ). Recent studies have confirmed that the FP does not generate large numbers of neurons in the spinal cord and hindbrain regions. The floor plate (FP) has long been recognized as a structure at the ventral midline of the central nervous system (CNS), conspicuously negative for neuronal elements. Taken together, these data elucidate both patterning and neurogenic functions of Wnt/beta catenin signaling in the FP, and thereby add to our understanding of the molecular logic of mDA specification and neurogenesis. This is, at least in part, due to depressed Lmx1b levels by Wnt/beta catenin signaling indeed, when Lmx1b levels are restored in this mutant, mDA are observed not only in rostral r1, but also at more caudal axial levels in the hindbrain, but not in the spinal cord. Moreover, in this mutant, mDA progenitor markers are induced throughout the rostrocaudal axis of the hindbrain FP, although TH+ mDA neurons are produced only in the rostral aspect of rhombomere (r)1.
We reveal that a neurogenic hindbrain FP results in the altered settling pattern of neighboring precerebellar neuronal clusters.
Here, we unambiguously show by fate mapping FP cells in this mutant, that the hindbrain and spinal cord FP are rendered highly neurogenic, producing large numbers of neurons. To further develop the hypothesis that canonical Wnt signaling promotes mDA specification and FP neurogenesis, we have generated a model wherein beta–catenin is conditionally stabilized throughout the FP. Wnt1, and other Wnts are expressed in the ventral midbrain, and Wnt/beta catenin signaling can at least in part account for the difference in neurogenic capacity of the FP between midbrain and hindbrain levels. The FP is largely non-neurogenic at the hindbrain and spinal cord levels, but generates large numbers of dopamine (mDA) neurons at the midbrain levels. The most ventral structure of the developing neural tube, the floor plate (FP), differs in neurogenic capacity along the neuraxis.
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