A microRNA feedback circuit in midbrain dopamine neurons. MicroRNAs (miRNAs) are evolutionarily conserved, 18-25 nucleotide non-protein coding transcripts that regulate gene expression post-transcriptionally. This study investigates the role of miRNAs in the terminal differentiation, function, and survival of mammalian midbrain dopaminergic neurons (DNs). The miRNA miR-133b is specifically expressed in midbrain DNs and is deficient in Parkinson's disease (PD) midbrain tissue. MiR-133b regulates the maturation and function of midbrain DNs within a negative feedback circuit that includes the paired-like homeodomain transcription factor Pitx3. MiR-133b is enriched in midbrain and is deficient in PD samples. The study shows that miR-133b is specifically expressed in midbrain DNs and is reduced in PD patient samples. MiR-133b is also reduced in dopamine neuron deficiency models, including Pitx3 mutant Aphakia mice and 6-hydroxydopamine (6-OHDA)-treated mice. The relative deficiency of miR-133b expression in Aphakia midbrain was surprising, as adult Aphakia mice maintain a population of midbrain dopamine neurons. This suggested that miR-133b is a direct target of Pitx3 transcription activation. The study shows that miR-133b suppresses dopamine neuron terminal differentiation and function. Overexpression of miR-133b in primary embryonic rat midbrain cultures leads to decreased expression of dopamine neuron mRNAs including TH and DAT, whereas Nurr1 and Pitx3 mRNAs are not altered. miR-133b overexpression reduces dopamine release in murine ES cultures derived dopamine neurons. Knockdown of miR-133b by modified oligonucleotides leads to increased expression of dopamine neuron mRNAs including TH and DAT. The study also shows that Pitx3 is a target of miR-133b activity. Overexpression of miR-133b in 293 cells decreases luciferase expression from pGL3-Pitx3. The study suggests that miR-133b and Pitx3 define a negative feedback loop in midbrain DN function and differentiation. This feedback loop is supported by the finding that downstream targets of Pitx3 activation, including TH and DAT, are modified in their transcription in the context of miR-133b mis-expression. The study also shows that miR-133b overexpression reduces Pitx3 protein expression in TH+ cells, whereas miR-133b knockdown leads to an increase in Pitx3 protein in TH+ cells. The study concludes that miR-133b functionsA microRNA feedback circuit in midbrain dopamine neurons. MicroRNAs (miRNAs) are evolutionarily conserved, 18-25 nucleotide non-protein coding transcripts that regulate gene expression post-transcriptionally. This study investigates the role of miRNAs in the terminal differentiation, function, and survival of mammalian midbrain dopaminergic neurons (DNs). The miRNA miR-133b is specifically expressed in midbrain DNs and is deficient in Parkinson's disease (PD) midbrain tissue. MiR-133b regulates the maturation and function of midbrain DNs within a negative feedback circuit that includes the paired-like homeodomain transcription factor Pitx3. MiR-133b is enriched in midbrain and is deficient in PD samples. The study shows that miR-133b is specifically expressed in midbrain DNs and is reduced in PD patient samples. MiR-133b is also reduced in dopamine neuron deficiency models, including Pitx3 mutant Aphakia mice and 6-hydroxydopamine (6-OHDA)-treated mice. The relative deficiency of miR-133b expression in Aphakia midbrain was surprising, as adult Aphakia mice maintain a population of midbrain dopamine neurons. This suggested that miR-133b is a direct target of Pitx3 transcription activation. The study shows that miR-133b suppresses dopamine neuron terminal differentiation and function. Overexpression of miR-133b in primary embryonic rat midbrain cultures leads to decreased expression of dopamine neuron mRNAs including TH and DAT, whereas Nurr1 and Pitx3 mRNAs are not altered. miR-133b overexpression reduces dopamine release in murine ES cultures derived dopamine neurons. Knockdown of miR-133b by modified oligonucleotides leads to increased expression of dopamine neuron mRNAs including TH and DAT. The study also shows that Pitx3 is a target of miR-133b activity. Overexpression of miR-133b in 293 cells decreases luciferase expression from pGL3-Pitx3. The study suggests that miR-133b and Pitx3 define a negative feedback loop in midbrain DN function and differentiation. This feedback loop is supported by the finding that downstream targets of Pitx3 activation, including TH and DAT, are modified in their transcription in the context of miR-133b mis-expression. The study also shows that miR-133b overexpression reduces Pitx3 protein expression in TH+ cells, whereas miR-133b knockdown leads to an increase in Pitx3 protein in TH+ cells. The study concludes that miR-133b functions