High levels of correlated ERR2/3 expression by gamma motor neurons.

(A-D) Transversal section of P21 ChatCre; Rosa26floxtdTomato mouse lumbar spinal cord ventral horn: motor neurons genetically labeled by tdTomato (scale bar: 50 μm). Arrowheads: high ERR2 and ERR3 levels in small NeuNlow, tdTomato+ motor neuron nuclei. Open arrowheads: relatively moderate-to-high levels ERR2/3 levels in NeuNhigh tdTomato− interneurons. Triangles: consistently lower but detectable levels in some large motor neurons with moderate-to-high NeuN levels. (E-H). Transversal section of P21 (wild-type) mouse lumbar spinal cord ventral horn: motor neurons labeled by retrograde tracer Fluoro-Gold (FG) (scale bar: 50 μm). Arrowheads: high ERR2 and ERR3 levels in small NeuNlow that retain high levels of FGhigh. Open arrowheads: relatively moderate-to-high levels ERR2/3 levels in NeuNhigh FG− interneurons. Triangles: consistently lower but detectable levels in some large motor neurons with moderate-to-high NeuN levels. (I) Quantitative analysis: high ERR2 (red) and ERR3 (green) levels in motor neurons with small somas. (I’) L Linear regression: strong positive correlation of motor neuron Err2 and Err3 levels. (F) High Err2 (red) and NeuN (black) levels in nonoverlapping motor neuron populations (n = 93, N = 3, Pearson’s correlation coefficient r = 0.86). (J) High ERR2 (red) and low NeuN (blue) levels in motor neurons with small somas. (J’) Linear regression: lack of positive correlation between Err2 and NeuN levels in motor neurons (n = 93, N = 3, Pearson’s correlation coefficient r = −0.32). (K) Loss of high ERR2 (red) and ERR3 (green) levels by Egr3-deficient small motor neurons (n = 184, N = 3). (L-N) Imaris 3D reconstruction: VGLUT1+ synaptic varicosities associated with ERR2low NeuNhigh motor neuron, but not with adjacent ERR2high NeuNlow motor neuron ERR2low motor neuron (motor neurons retrogradely labeled by FG) (scale bar: 20 μm). (N) Quantification of VGLUT1+ synaptic varicosities associated with ERR2high or ERR2low motor neurons. N = # of mice and n = # of neurons. Statistically significant differences between motor neurons are indicated as: *p p p t test). Data for Fig 2I, 2J, 2K, and 2N can be found in S2 Data.