Lewis, Kate; Hilinski, William (2016)
      The correct specification of neurotransmitter phenotypes is crucial for properly functioning neuronal circuitry. Neurons specify their neurotransmitter phenotypes via transcription factors that they express as they differentiate. Often, transcription factors that specify neurotransmitter phenotypes are expressed in multiple populations with the same neurotransmitter phenotype. Since, the transcription factors that specify this characteristic are not yet known for all spinal cord glutamatergic populations, we identified additional transcription factors expressed at relatively higher levels in glutamatergic neurons compared to inhibitory neurons. We have functionally tested three of these (Lmx1bb, Skor1a and Skor1b) to determine if they are required for correct spinal, glutamatergic phenotypes. We demonstrate that Lmx1b likely maintains a subset of glutamatergicphenotypes in the spinal cord. In lmx1bb mutant embryos, the number of cells that initially express glutamatergic markers are unchanged but become reduced at 36 h and to a greater degree at 48 h. In contrast, we observe no changes in the total number of dI5 or V0v neurons, which express lmx1bb,nor do we detect elevated levels of apoptosis between 36 h and 48 h in lmx1bb mutants. Lastly, we show that at least some of the cells that lose their glutamatergic neurotransmitter phenotype are likely to be V0v cells.Additionally, we demonstrate that skor1a and skor1b are expressed predominantly by glutamatergic spinal interneurons, many of which are V0v neurons. When skor1a and skor1b are knocked-down, we observe a significant reduction in the number of glutamatergic neurons and no change in the number of inhibitory neurons suggesting that these genesmay be required to specify the glutamatergic neurotransmitter phenotype of a subset of spinal neurons. In parallel studies, it was shown that evx1 and evx2, genes expressed exclusively by V0v spinal neurons, are required to specify the V0v glutamatergic phenotype. Interestingly, we show that lmx1bb, skor1a and skor1b require evx1 and evx2 for their expression. In summary, these results suggest that skor1, skor1b and lmx1bb may function downstream of evx1 and evx2 to specify and/ormaintain the glutamatergic neurotransmitter phenotype ofat least a subset of V0v neurons.