Fibroblast Growth Factor Signalling in the Development of the Midbrain and Anterior Hindbrain
Year: 2012
Type: dissertation
Abstract: The embryonic midbrain and hindbrain give rise to brain stem structures and the cerebellum. The ventral midbrain and anterior hindbrain include highly important brain nuclei such as the dopaminergic substantia nigra and the ventral tegmental area, as well as serotonergic dorsal raphe neurons. These specific brain structures are affected in several disorders such as Parkinson s disease, depression, schizophrenia and drug addiction. Between the developing midbrain and hindbrain is a signalling centre called the Isthmic Organizer. This Isthmic Organizer secretes signalling molecules, such as Wnts and Fibroblast growth factors (Fgfs). Fgf8 is able to induce midbrain and anterior hindbrain characteristics in ectopic locations, and thus Fgf8 can act as an organizer molecule. Fgf signals are mediated by Fgf receptors (Fgfr). Of the four Fgfrs, Fgfr1-Fgfr3 are expressed in the nervous system. Fgfr1 is required to maintain coherence of a slowly dividing midbrain-hindbrain boundary cell population. However, the role of Fgfr2 and Fgfr3 in the development of midbrain and anterior hindbrain is poorly understood as well as cell adhesion molecules related to the maintenance of the coherent isthmic constriction. In this study, we elucidated the role of Fgfr2 and Fgfr3 during the development of the mid-brain and hindbrain. We showed that loss of either Fgfr2 or Fgfr3 alone or even both together did not result in any structural abnormalities. Thus, Fgfr1 is the major Fgf receptor in the midbrain and anterior hindbrain region. However, when Fgfr1 and Fgfr2, or all three Fgfr1, Fgfr2 and Fgfr3 were simultaneously inactivated, the defects in the midbrain-hindbrain development were much more severe than in the Fgfr1 mutants alone. Dorsal midbrain structures and the cerebellum were lost. Although some dopaminergic precursors appeared in the ventral midbrain, all dopaminergic neurons and several other ventral neuronal populations were lost by birth. We showed that Fgfr cooperatively regulate cell survival, antero-posterior patterning, and the maintenance of neural progenitor properties. Loss of Fgf signalling in the ventral midbrain resulted in a thinner ventricular zone and premature neurogenesis. This was not caused by shortened cell cycle length or abnormalities in cellular polarity, cellular architecture or the orientation of mitotic spindles. Instead, loss of Fgf signalling lead to a downregulation of neural stem cell transcription factors, which allowed upregulation of proneural genes. Thus, these gene expression changes drove neural progenitors to exit the cell cycle. In addition, we showed that Fgf8 is localized in the basal membrane. Thus, Fgf signalling may maintain proliferative identity of the midbrain neural progenitors, and the cells likely receive these guiding Fgf signals through their basal processes. Finally, we showed that an Fgf-regulated adhesion molecule Cadherin22 (Cdh22) is not essential for the maintenance of the coherent compartment boundary between the midbrain and the hindbrain. Possibly, Cdh22 acts redundantly with other type II cadherins. In addition, specific expression patterns in distinct brain nuclei suggest roles for Cdh22 in the segregation of neuronal populations cooperatively with other cadherins. In summary, these results demonstrate that Fgf signalling, and especially cooperation of the Fgf receptors, is required for proliferation, cell survival, and patterning of the neural progenitors in the midbrain and anterior hindbrain. A good understanding of developmental processes such as detailed mechanisms of signalling pathways and their regulation elucidates possibilities for therapeutic use.
Source: Työväentutkimus Vuosikirja
Author: Jonna Saarimäki‐Vire
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
Funder: Helsingin Yliopisto
OpenAlex ID: W2105854986