Transcription factor Pax6 controls structure and function of the centrosome in cortical progenitors
by Marco Andreas Tylkowski
Date of Examination:2013-06-26
Date of issue:2013-07-15
Advisor:Prof. Dr. Anastassia Stoykova
Referee:Prof. Dr. Anastassia Stoykova
Referee:Prof. Dr. Sigrid Hoyer-Fender
Persistent Address:
http://dx.doi.org/10.53846/goediss-3936
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Abstract
English
The mammalian neocortex is a highly complex structure containing more than 100 billions neurons and ten times more glia cells that are generated during development by progenitor cells located at the apical surface of the forebrain ventricular zone (VZ). The cortical progenitors originate from neuroepithelial cells that transform into radial glia progenitor cells (RGPs) at the beginning of neurogenesis. Starting to divide in an asymmetric neurogenic mode each RGP produces a new radial glia progenitor for self-renewal and a neuron. Around midneurogenesis, the RGPs switch from this direct mode of neurogenesis to an indirect mode through which each RGP divides to self-renew and generates a new type of progenitor, the intermediate progenitor (IP), located in the subventricular zone (SVZ). Here, the IP undergoes a few symmetric proliferative divisions, thus amplifying the neuronal fate acquired at a particular developmental stage, before entering into a terminal symmetrical neurogenic division to produce two neurons. The transcription factor (TF) Pax6 is an intrinsic factor of RGPs that regulates multiple functions, e.g. cell morphology, cell cycle length, spindle orientation during mitosis, interkinetic nuclear migration and centrosome localisation. During the last few years some of the mechanistic backgrounds of defects in these processes in Pax6-deficiency could be discovered. However, how TF Pax6 controls a proper interkinetic nuclear migration during cell division that is most likely related to centrosome structure / function remains still unclear. Here, I show results that revealed a novel molecular mechanism, involved in Pax6-dependent control of centrosome structure and function. The observed findings could be summarized in the following: 1. Results from the electron microscopy analysis revealed a specific defect of the mother centrioles that were missing subdistal appendages in RGPs located at the cortical apical VZ in the mouse Pax6/Small eye mutant. Consequently, the RGPs showed a massive loss of primary cilia at the ventricular surface. Furthermore, analysis of the localisation of mother and daughter centrosomes in vivo revealed defect of centriole maturation in the Pax6-deficient cortex, evident by a dramatic loss of mother centrioles at VZ surface and a premature exit of RGPs from mitotic cycle.2. Mechanistically, the presented findings revealed that Pax6 transcriptionally regulates the expression of the appendage specific protein Odf2, thus controlling the maturation of the mother centriole (i.e. the assembly of the subdistal/distal appendages) in RGPs, more strongly during late neurogenesis. This process is of crucial importance for proper centrosome function that includes a correct assembly of primary cilia and the microtubules aster, which is most likely involved in correct interkinetic nuclear migration functioning. 3. In addition, results from Odf2 knock down assays in vivo indicated that a lack of Odf2, RGPs prematurely exit from mitotic cycle, suggesting an intrinsic relevance of Odf2 expression and centriole maturation for the RGP proliferative capacity and maintenance of cortical progenitor pool for late neurogenesis To sum up, the shown here direct dependence of appendage protein Odf2 expression by TF Pax6 represents a part of a complex molecular mechanism underlying the correct centrosome maturation and functioning during late cortical neurogenesis.
Keywords: Pax6; Centrosome; Cortex; Odf2; cortical progenitors