Thorsten Naserke - Projects


Growth factors in mid-/hindbrain development


The mid-/hindbrain organizer is a structure that regulates development of the midbrain and hindbrain (Wurst and Bally-Cuif, 2001; Sato et al., 2004). Two secreted growth factors that mediate the function of the mid-/hindbrain organizer are Fgf8 in the hindbrain (Ye et al.,1998; Liu et al.,1999; Martinez et al.,1999; Chi et al.,2003) and Wnt1 in the midbrain (Prakash et al., 2006). The function of Fgf8 is mediated by binding to tyrosine kinase receptors of the Fgf receptor (Fgfr) family. Four Fgf receptors are known and have been numbered Fgfr1-4 accordingly. I am interested in the functions of the Fgf receptors and Wnt1 in the development of the midbrain and hindbrain.

  1. Analysis of Fgf receptor expression in the developing mid-/hindbrain region

    For some time it has been believed that Fgfr1 is the only Fgf receptor expressed close to the MHO. This led to the assumption that Fgfr1 would exclusively transmit the Fgf8 signal in the mid-/hindbrain region. However, conditional knock out of Fgfr1 in the mid-/hindbrain only led to a loss of dorsal tissues in the region, while tissues of the ventral mid-/hindbrain territory were still present (Trokovic et al., 2003). To find out whether other Fgf receptors are expressed in the developing mid-/hindbrain region, we mapped expression of Fgfr1-3 in the developing mouse mid-/hindbrain area between stages E8.5 and E12.5. Expression of Fgfr4 was not detected close to the MHO.

    Fgfr expression E8.5-12.5

    Our analysis revealed dynamic and distinct expression of Fgfr2 and Fgfr3 in the ventral mid-/hindbrain region, in close contact to the MHO. A summary of the expression domains of Fgfr1 (in yellow), Fgfr2 (in red) and Fgfr3 (in blue) in the mid-/hindbrain region is given to the left. These findings suggested a possible role for Fgfr2 and Fgfr3 in Fgf signaling in the developing ventral midbrain and hindbrain (Blak et al., 2005).

  2. Dopaminergic and serotonergic neurons in the ventral mid-/hindbrain region of conditional Fgfr1 mutants

    Caudal expansion of dopaminergic neurons Loss of anterior serotonergic neurons

    Ventral tissue is maintained after conditional inactivation of Fgfr1 in the mid-/hindbrain region. However, a detailed analysis of the ventral mid-/hindbrain region in En1Cre/+ Fgfr1lox/lox mutants revealed caudal expansion of midbrain dopaminergic neurons and a loss of anterior serotonergic neurons (Jukkola et al., 2006). Pitx3 and TH are genes that are expressed in midbrain dopaminergic neurons, whereas Pet1 and Sert are expressed in hindbrain serotonergic neurons. The pictures to the left show the expression of Pitx3, TH, Pet1 and Sert in the ventral mid-/hindbrain region of a wildtype and an En1Cre/+ Fgfr1lox/lox embryo in the ventral mid-/hindbrain region. Caudal expansion of midbrain dopaminergic neurons and loss of anterior serotonergic neurons in the mutant animals suggest a role for Fgfr1 in establishment of anterior hindbrain identity.

  3. A role for Wnt1 in the induction of midbrain dopaminergic neurons

    As we could demonstrate a role for Fgfr1 in the establishment of the anterior serotonergic neurons in the hindbrain, we wanted to find out whether Wnt1 has a similar function in the formation of dopaminergic neurons in the midbrain. We overexpressed Wnt1 in the mid-/hindbrain region and analyzed the mutants for the expression of midbrain dopaminergic marker genes.

    Induction of dopaminergic markers by ectopic Wnt1 expression

    Overexpression of Wnt1 in the mid-/hinbrain region led to ectopic development of midbrain dopaminergic neurons in the ventral hindbrain.The figure to the left shows the expression of Wnt1, Nurr1, Pitx3 and TH in wildtype and Wnt1 overexpressing mice. All of these genes are ectopically expressed in the ventral hindbrain of the mutants (Prakash et al., 2006). Therefore, Wnt1 is sufficient to induce midbrain dopaminergic neurons in the ventral hindbrain.

In summary, Fgf receptors 1-3 are expressed in the ventral midbrain and hindbrain. Besides a possible redundancy of the Fgf receptors in maintenance of ventral tissue, Fgfr1 alone is necessary for the generation of serotonergic neurons in the anterior hindbrain. Wnt1, another secreted factor expressed in the developing mid-/hindbrain region, is sufficient for the induction of midbrain dopaminergic neurons in the ventral midline of the hindbrain. Thus, the secreted factors Wnt1 and Fgf8 regulate development of neuronal cell populations in the ventral mid-/hindbrain region.


 
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