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Abstract
Pituitary homeobox 3 (Pitx3) is required for the terminal differentiation of nigrostriatal dopaminergic neurons during neuronal development. However, whether Pitx3 contributes to the normal physiological function and cell-type identity of adult neurons remains unknown. To explore the role of Pitx3 in maintaining mature neurons, we selectively deleted Pitx3 in the mesodiencephalic dopaminergic (mdDA) neurons of Pitx3fl/fl/DATCreERT2 bigenic mice using a tamoxifen inducible CreERT2/loxp gene-targeting system. Pitx3fl/fl/DATCreERT2 mice developed age-dependent progressive motor deficits, concomitant with a rapid reduction of striatal dopamine (DA) content and a profound loss of mdDA neurons in the substantia nigra pars compacta (SNc) but not in the adjacent ventral tegmental area (VTA), recapitulating the canonical neuropathological features of Parkinson’s disease (PD). Mechanistic studies showed that Pitx3-deficiency significantly increased the number of cleaved caspase-3+ cells in SNc, which likely underwent neurodegeneration. Meanwhile, the vulnerability of SNc mdDA neurons was increased in Pitx3fl/fl/DATCreERT2 mice, as indicated by an early decline in glial cell line-derived neurotrophic factor (GDNF) and aldehyde dehydrogenase 1a1 (Aldh1a1) levels. Noticeably, somatic accumulation of α-synuclein (α-syn) was also significantly increased in the Pitx3-deficient neurons. Together, our data demonstrate that the loss of Pitx3 in fully differentiated mdDA neurons results in progressive neurodegeneration, indicating the importance of the Pitx3 gene in adult neuronal survival. Our findings also suggest that distinct Pitx3-dependent pathways exist in SNc and VTA mdDA neurons, correlating with the differential vulnerability of SNc and VTA mdDA neurons in the absence of Pitx3.
Highlights
The main pathological characteristic of Parkinson’s disease (PD) is a profound loss of mesodiencephalic dopaminergic (mdDA) neurons in substantia nigra pars compacta (SNc) [1, 2]
Selective deletion of Pituitary homeobox 3 (Pitx3) in mature mdDA neurons To investigate the role of Pitx3 in mature mdDA neurons, we first established a mouse model with selective Pitx3 deletion in mdDA neurons using a TAM-inducible CreERT2/loxp gene-targeting system (Fig. 1a)
Tissues were first collected at 2 months after TAM administration (4 months of age), and the Pitx3 expression profile in mdDA neurons was detected by immunofluorescence staining (IFC) staining (Fig. 1c)
Summary
The main pathological characteristic of PD is a profound loss of mdDA neurons in SNc [1, 2]. A cascade of developmental transcription factors has long been known to define the mdDA neuronal fate specification in the neuroepithelium, including Pitx, Nurr, Engrailed, and Lmx1b [7, 8] These molecules are involved in the early neuronal events but are continuously expressed during neuronal maturation and even throughout adulthood [9]. Conventional knockout mice of Nurr die shortly after birth [10], whereas recent studies have shown that the haploinsufficient ones can survive until the late stages, given exhibiting mdDA neuron loss and locomotor deficits [11] This Nurr1-related neuronal loss might be largely attributed to the intrinsic dysregulation of Bcl-2 and P53 genes, both critical for cell survival [12, 13]. The conditional deletion of Nurr in mature neurons results in a rapid reduction of striatal DA, a loss of mdDA neuronal markers, and neuronal degeneration [14, 15]
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