Pleiotropic and Isoform-Specific Functions of PITX2 in Brain Development.

Abstract

Neuronal diversification in the developing brain is a coordinated process requiring complex genetic regulation. Transcriptional control of gene expression is known to regulate proliferation, migration, differentiation, and survival of developing neurons, although the precise mechanisms underlying these processes are not fully understood. One transcription factor, pituitary homeobox 2 (Pitx2), is expressed in the developing and adult mammalian brain, eye, and pituitary, and in asymmetric organs such as the heart, lungs, and gut. In humans, mutations in PITX2 result in the autosomal dominant disorder Rieger syndrome which is characterized by defects in the eyes, umbilicus, and teeth along with variable abnormalities in the heart, pituitary, and brain. Pitx2 encodes a paired-like homeodomain transcription factor expressed in highly restricted domains of the mouse brain beginning at E9.25 and continuing through adulthood. Alternative splicing of Pitx2 in the mouse results in three isoforms (Pitx2a, Pitx2b, and Pitx2c) which are expressed symmetrically in the brain throughout development. The contribution of each isoform to brain development is not known. We have identified three Pitx2-positive neuronal populations in the midbrain and at least two populations in rhombomere 1 (r1). In the dorsal midbrain and r1, Pitx2-positive neurons are GABAergic, whereas in the ventral midbrain Pitx2-positive neurons are glutamatergic, consistent with the idea that Pitx2- viii positive cell fate depends on anatomic context and axial level. Loss of Pitx2 in the dorsal midbrain results in failure of cells to fully migrate and undergo GABAergic differentiation. Chimera analyses suggest that dorsal midbrain Pitx2 regulates migration through cell autonomous mechanisms. Isoform-specific knockout studies show unique dosage-specific requirements for Pitx2 isoforms in midbrain neuronal migration, GABAergic differentiation, and mammillothalamic tract formation. Loss of Pitx2 in the ventral midbrain and r1 does not affect neuronal migration per se, indicating region-specific requirements for Pitx2. Collectively, these studies expand our knowledge about region- and isoform-specific requirements for transcription factors in the developing brain, which may prove critical for developing future stem cell and other therapies in neurological disorders.