Journal: eLife
Article Title: Spinal V1 inhibitory interneuron clades differ in birthdate, projections to motoneurons, and heterogeneity
doi: 10.7554/eLife.95172
Figure Lengend Snippet: ( A ) Motor column identification from lower thoracic to upper sacral spinal cord in P20 mice following labeling with choline acetyltransferase (ChAT) antibodies: PGC = preganglionic cell column; MMC = medial motor column; HMC = hypaxial motor column; LMCd/v=lateral motor column (dorsal/ventral). ( B ) Schematic representation of the rostro-caudal span of each motor column in the spinal segments studied. ( C ) Synapse quantification. Axons of Foxp2 and non-Foxp2 V1 interneurons were respectively labeled with EGFP and tdT in En1 Cre , Foxp2 flpo , R26 CE:dualGFP/Ai9-lsl-tdT mice. In en1 Cre , Ai9 R26 lsl-tdT mice we identified V1-Renshaw cell axons using calbindin antibodies. Synaptic locations were labeled with VGAT antibodies and the postsynaptic motoneurons with ChAT antibodies. Synapse densities were analyzed in a ribbon of membrane at mid-cell body level (7 optical planes, 1 µm z-step). C1, Single optical plane of an L4/5 LMCv motoneuron surrounded by genetically labeled Foxp2-V1 and non-Foxp2-V1 axons. Inhibitory synapses on ChAT-IR motoneurons are VGAT+. C2, Single optical image of an L4/5 LMCv ChAT-IR motoneuron receiving synapses from V1 Renshaw cells (genetically labeled V1 axons with calbindin-IR and VGAT). C3–4, Method for estimating synapse densities on motoneuron cell bodies using C2 as example. C3, V1-VGAT (red arrowheads) and V1-CB-VGAT synapses (yellow arrowhead) are marked (VGAT-IR is not shown for clarity), and the cell body contour annotated with regions corresponding to dendrite exits. This process was repeated in seven consecutive mid-cell body optical planes (cross-sections with well-defined nucleus and nucleolus). C4, A membrane surface slab is reconstructed in 3D (two different rotations shown). The surface area corresponding to dendrite exits is subtracted from the total surface area of the slab to calculate the available surface area on the motoneuron cell body. V1-VGAT synapses (red), V1-CB-VGAT synapses (yellow), and CB-VGAT synapses (green) are marked. A similar process was followed for calculating Foxp2-V1 synapse density. ( D ) Quantification of total V1-VGAT synapse densities on motoneuron cell bodies in different motor columns (n=21–30 motoneurons per motor column, n=5 animals with 4–9 motoneurons per animal per motor column). Each data point is one motoneuron color-coded by mouse origin. Average synaptic densities ± SD indicated to the right of scatter plots. A nested ANOVA found significant differences among motor column/segments (p<0.0001) with no inter-animal variability (p=0.4768). The table summarizes all post hoc pairwise comparisons for average V1 synaptic densities of each motor column and segment (Bonferroni-corrected t-tests) (further statistical details are found in ). Colors indicate increased (>1, red) or decreased ratios (<1, blue) of column motoneurons vs row motoneurons. PGC neurons receive significantly fewer V1 synapses than MMC or LMC motoneurons. The LMC (ventral and dorsal) in lower lumbar (L4/L5) had significantly more V1 contacts than MMC motoneurons or L6 dorsal LMC. ( E ) Comparison of synaptic densities from Foxp2-V1 and non-Foxp2-V1 neurons (top) or Renshaw cells (bottom). All motoneurons sampled in 2–3 animals for each comparison were pooled together. Densities of V1-VGAT synapses from Foxp2-V1s, non-Foxp2 V1s, or calbindin (CB)+ V1s (Renshaw cells) (n=6–17 motoneurons sampled per motor column/segments, average = 12.1 ± 2.9 SD) were compared using a two-way ANOVA for axon type vs motor column and segment. Foxp2-V1 vs non-Foxp2-V1 synapses: significant differences in density were found for type of synapse (p=0.001), motor column location (p<0.0001), and their interaction (p<0.0001). Significant differences after post hoc Bonferroni tests are indicated (*p<0.05; ****p<0.0001). In general, synapses from Foxp2-V1 axons have higher density than non-Foxp2-V1 axons on HMC and LMC columns at all spinal segments except for L1/L2 LMC. MMC motoneurons receive similar synaptic densities from both types of V1 axons, except at the sacral level in which non-Foxp2 V1 synapses predominate. PGC neurons receive very low densities of V1 axons and there are no significant differences between either type in any region. Foxp2-V1 vs CB+ V1 synapses: significant density differences were found for type of synapse (p<0.0001), motor column location (p<0.0001), and their interaction (p<0.0001). Significant differences between Foxp2-V1 and CB+ V1 synapses after post hoc Bonferroni tests are indicated (*p<0.05; ****p<0.0001). Synapses from Foxp2-V1 axons have higher density than CB+ V1 axons in HMC and LMC columns at all spinal segments except for L1/L2 LMC. MMC motoneurons receive similar synaptic densities from both types of V1 axons in upper lumbar regions, but Foxp2-V1 synapse predominate in lower lumbar. In S1 the density of CB+/V1 synapses is significantly higher. The low synaptic densities estimated in PGC neurons for Foxp2-V1s and CB+ V1s are not significantly different. Further details of statistical comparisons are in . ( F ) Comparing the numbers of Foxp2 and CB+ (Renshaw) V1 synapses to the total number of V1 synapses, we estimated their respective percentages. From these estimates we calculated that the remainder belongs to non-Foxp2 and non-CB+ Renshaw cells. The large majority of V1 synapses on the cell bodies of LMC, HMC, and MMC motoneurons are either from Renshaw cells or Foxp2-V1s. ( G ) Summary diagram of major V1 clade connectivity to motoneuron cell bodies. Foxp2-V1s and Renshaw cells form the majority of inhibitory V1 contacts on LMC and HMC motoneurons, with slightly higher density from Foxp2-V1s. The MMC receives roughly equal portions of V1 contacts from Foxp2-V1s and Renshaw cells. V1s provide only sparse inhibition on preganglionic sympathetic neurons and most originate in V1 clades other than Renshaw cells and Foxp2-V1s.
Article Snippet: Calbindin , Recombinant protein (calbindin D-28k from rat) , Rabbit, polyclonal , Swant , CB-38a , RRID: AB_10000340 , 1:1000.
Techniques: Labeling, Membrane, Comparison, Inhibition