Figure 2.

Egr3 has no role in proprioceptive neuron-dependent muscle spindle morphogenesis. A, Left, Historical Egr3 expression is detected in intraganglionic nerve fibers (arrow) and neurons (arrowheads) in newborn Egr3^+/Cre; R26R^+/flx DRG using lacZ histochemistry. Right, However, no lacZ⁺ proprioceptive neurons, labeled by Pv immunofluorescence, were identified. B, Similarly, using Egr3^+/Cre; Ai14^+/flx mice, which express tandem dimer tomato (T; left, red) in Egr3-lineage-traced cells, no Pv⁺ neurons (middle, green) were found to coexpress T (historical Egr3 expression) (n = 3 mice, > 10 DRG per mouse analyzed). C, Adult skeletal muscle spindles from Pv^Cre/Cre; Egr3^flx/flx (Prop-cKO) mice remain innervated by Pv⁺ proprioceptive axons (top, green), which continue to express Egr3 (bottom, red) in intrafusal muscle fiber nuclei. D, Loss of Egr3 in proprioceptive neurons does not affect the number of muscle spindles in medial gastrocnemius, plantaris, or soleus hindlimb muscles of 8-week-old mice (n = 3 mice per muscle and genotype). E, Prop-cKO mice do not exhibit proprioceptive abnormalities as demonstrated by rotarod performance (n = 8 mice for each genotype) at 4 weeks of age. Scale bars: A, B, 50 μm; C, 10 μm. *p < 0.05, Student's t test.