High levels of ERR2 and ERR3 are coexpressed by gamma motor neurons.

(A-F) Overview of transversal section of P21 Chat::tdTomato (ChatCre; Rosa26fxtdTomato) lumbar mouse spinal cord. (A) Expression of tdTomato, NeuN, ERR2, and ERR3. tdTomato (magenta) mostly labels motor neurons in lamina IX but also subsets of cholinergic interneurons in laminae III and X (see S4A Fig) (scale bar: 200 μm). High levels of ERR2 and ERR3 coexpression in tdTomato+ NeuNlow motor neurons (yellow-orange nuclei in boxed areas). Boxed areas around left and right lateral motor columns correspond to the higher magnification given in G-K and L-O, respectively. (B) Highest levels of ERR2 in motor neurons (boxed area), relatively lower but detectable levels in other motor neuron subtypes and subsets of interneurons throughout the spinal cord. (C) High levels of ERR3 in motor neurons (boxed area), lower but significant levels in other motor neuron subtypes, and occasional high levels in interneurons in the intermediate spinal laminae. (D) High levels of ERR2 and ERR3 coexpression in NeuNlow motor neurons (yellow-orange nuclei in boxed areas) (scale bar: 200 μm). (E, F) Separate channels depicting expression of NeuN only (E) or tdTomato only (F). (G-K) Higher magnification of left ventral horn (boxed area) in (A-F). Closed arrowheads: Highest levels of ERR2 (G) and ERR3 (H) in consistently NeuNlow (J) and tdTomato+ (K) small motor neurons (scale bar: 50 μm). Note that small NeuNlow motor neurons consistently exhibit lower tdTomato levels compared to large motor neurons (K, O). Open arrowheads: low but detectable ERR2 (G) and ERR3 (H) levels in tdTomato− interneurons (K). Triangles: low but detectable ERR2 (G) and ERR3 (H) levels in large tdTomato+ motor neurons (K) with intermediate to high NeuN levels (J). Note that NeuN levels vary considerably in large motor neuron subtypes (see Fig 2B and 2F). (L-O) Higher magnification of right ventral horn (boxed area) in (A-F). Closed arrowheads: Highest levels of ERR2 (L) and ERR3 (M) in consistently NeuNlow (N) and tdTomato+ (O) small motor neurons (scale bar: 50 μm). Open arrowheads: low but detectable ERR2 (L) and ERR3 (M) levels in tdTomato− interneurons (O). Triangles: low but detectable ERR2 (L) and ERR3 (M) levels in large tdTomato+ motor neurons (O) with high NeuN levels (N). (P–T) Consistent patterns of high ERR2 and ERR3 levels in small soma-size NeuNlow motor neurons at cervical-brachial levels (scale bar: 200 μm). (TIF)