Behavioral deficits in neurodegenerative diseases are often attributed to the selective dysfunction of vulnerable neurons via cell-autonomous mechanisms. Although vulnerable neurons are embedded in neuronal circuits, the contributions of their synaptic partners to disease process are largely unknown. Here we show that, in a mouse model of spinal muscular atrophy (SMA), a reduction in proprioceptive synaptic drive leads to motor neuron dysfunction and motor behavior impairments. In SMA mice or after the blockade of proprioceptive synaptic transmission, we observed a decrease in the motor neuron firing that could be explained by the reduction in the expression of the potassium channel Kv2.1 at the surface of motor neurons. Chronically increasing neuronal activity pharmacologically in vivo led to a normalization of Kv2.1 expression and an improvement in motor function. Our results demonstrate a key role of excitatory synaptic drive in shaping the function of motor neurons during development and the contribution of its disruption to a neurodegenerative disease.
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We thank M. Goulding (Salk Institute) for generously providing the R26floxstop-TeNT mice. We also thank S. Morton and T. Jessell (Columbia University) for help with VGluT1 antibody production and a kind gift of GAD65 and GAD67 antibodies. We would like to thank C. Kellendonk (Columbia University) for his kind gift of the custom-made Cre antibody. We are also grateful to J. Trimmer (University of California, Davis) for his kind gift of Kv2.1 knockout mouse spinal cord tissue. We would like to thank T. Jessell, L. Pellizzoni, F.J. Alvarez, S. Przedborski, D. Logothetis, S. Schorge and F. Polleux for critical comments on the manuscript. G.Z.M. was supported by the NINDS, NIH (RO1-NS078375, R21-NS079981, R21-NS084185), the Department of Defense (GR.10235006), The SMA Foundation, Cure SMA, SMA-Europe and Target-ALS.
The authors declare no competing financial interests.
Integrated supplementary information
Supplementary Figure 1 Iliopsoas and quadratus lumborum motor neurons in the L2 spinal segment.
(A) IL and QL motor neurons were labeled after CTb-488 (green) muscle injections at birth. At P4, the L2 motor neurons were retrogradely filled with Texas Red Dextran (red) using the ex vivo spinal cord preparation to identify the rostral and caudal borders (dotted line) of the L2 spinal segment. IL and QL motor neurons (CTb-488, green) were quantified as a percentage of ChAT (blue) immunoreactive motor neurons. (B) Transverse sections of the L2 motor neuron nucleus containing IL/QL motor neurons (CTb-488, green) and ChAT immunoreactivity (red). Merged image is shown at the bottom. R: rostral, C: caudal, D: dorsal, V: ventral, M: medial, L: lateral.
Supplementary Figure 2 Resistant (L5) SMA motor neurons are not hyperexcitable, whereas vulnerable (L2) SMA motor neurons exhibit the first signs of dysfunction at P2.
(A) Schematic of experimental protocol. Extracellular electrodes in the L2 ventral root (1) and L2 dorsal root (2) were used for stimulation. Responses from individual motor neurons were recorded intracellularly (3) (whole-cell patch clamp) using the intact ex vivo spinal cord preparation. Traces on the right show: an antidromically evoked action potential following ventral root stimulation (1), a synaptic response from a motor neuron following dorsal root stimulation (2) and current-to-voltage relationship in whole-cell configuration (3).
(B) Input resistance-time constant relationship for L2 WT and SMA motor neurons at P2. The SMA-affected motor neurons exhibit high input resistance and high values of time constant (marked by red dotted lines). The SMA-unaffected motor neurons exhibit similar values of input resistance and time constant with WT motor neurons (marked by blue dotted lines).
(C) Membrane responses following current injections in a WT and an SMA L5 motor neuron at P4.
(D) Current/voltage relationships for the two motor neurons shown in (C).
(E) The average input resistance (Rin), membrane time constant (τ) and rheobase current (IRh) for WT (n=4) and SMA (n=5) L5 motor neurons at P4.
(F) Intracellular responses to supramaximal stimulation of the L5 dorsal root in a WT and a SMA motor neuron at P4. The peak amplitude of monosynaptic EPSP is indicated by arrows. The arrowheads show the stimulus artifact.
(G) The average peak EPSP amplitude for WT (n=4) and SMA (n=5) L5 motor neurons.
(H) Relationship between transverse area of L2 motor neurons and the number of VGluT1 synapses in WT and SMA motor neurons at P2.
(I) The average transverse area of WT and SMA motor neurons at P2.
Supplementary Figure 3 Parvalbumin expression in the spinal cord and brain during first postnatal week.
(A) Transverse sections from WT and SMA spinal cords at P0, P4 and P11 showing parvalbumin (green) and ChAT (blue) immunoreactivity. Arrows indicate parvalbumin+ interneurons at P11. DRG: dorsal root ganglion.
(B) Parvalbumin (green) and NeuN (red) expression in the cerebellum, hippocampus and cortex in P5 WT mice.
(C) Sagittal section of the brain following transection at a high medulla level in a P3 WT mouse. Arrow indicates the level of complete transection.
(D) Average righting times before and 3 hours after transection. NS: no significant difference (p=0.20, Mann-Whitney test).
Supplementary Figure 4 Absence of parvalbumin from the quadratus lumborum muscle during the first 2 postnatal weeks.
Sections from QL and EDL muscles immunoreacted against parvalbumin at P5 (A) and P12 (B). Nuclear stain DAPI in blue.
Supplementary Figure 5 Expression of the fluorescent reporter TdTomato in PvCre mice.
Expression of TdTomato (red) and DAPI (blue) in cerebellum (A), spinal cord and DRG in the L2 segment (B) and its absence in cortex and hippocampus (C).(D) shows examples from EDL and QL muscles. TdTomato was only observed in EDL muscle.
Supplementary Figure 6 Validation of selective upregulation of SMN protein in proprioceptive and motor neurons
(A) Confocal images from L2 spinal segment at P11 showing motor neurons expressing TdTomato (red) driven by ChAT::Cre, ChAT immunoreactivity (green) and retrogradely filled motor neurons with Cascade Blue Dextran (blue) from the ventral root and their merged image. Graph shows the percentage expression of TdTomato+ to ChAT+ motor neurons.
(B) Confocal images from an L2 dorsal root ganglion at P11 showing TdTomato (red) expression driven by PvCre, Parvalbumin (Pv) immunoreactivity (green) and their merged image. Graph shows the percentage expression of TdTomato+ to Paralbumin+ neurons.
(C) Confocal images from L2 ChAT+ motor neurons (red) and Cre immunoreactivity (green) in ChATCre mice at P11 and their merged image. Graph indicates the percentage of ChAT+ motor neurons expressing Cre.
(D) Confocal images from an L2 dorsal root ganglion showing Parvalbumin (white) and Cre (red) immunoreactivity in PvCre mice at P11. Yellow arrows indicate co-localization and blue arrow indicates no co-localization. Graph indicates the percentage of Parvalbumin+ neurons expressing Cre.
(E) Expression of Gems (arrows) revealed by SMN immunoreactivity (yellow) in nuclei (nuclear stain DAPI in blue) of proprioceptive neurons labeled by parvalbumin (red) in L2 dorsal root ganglia in WT, SMA and SMA+PvCre mice at P4.
(F) Similar to (A) but for motor neurons labeled by ChAT immunoreactivity (red) in the L2 segment of the spinal cord in WT, SMA and SMA+ChATCre mice at P4.
(G) The average percentage expression of Gems in proprioceptive neurons in WT, SMA, SMA+PvCre and SMA+ChATCre mice at P4.
(H) The average percentage expression of Gems in motor neurons in WT, SMA, SMA+PvCre and SMA+ChATCre mice at P4.
Supplementary Figure 7 Changes in motor neuron input resistance do not depend on motor neuron soma size.
(A) Location of intracellularly recorded L2 WT and SMA motor neurons within the motor neuron nucleus at P4. The grey line indicates the approximate border of the motor neuron nucleus. The blue dotted line indicates the approximate grey-white matter border. The solid blue line indicates the edge of the spinal cord. The location of the medial motor nucleus (MMC) is noted in a few instances. Scale bars: 50 μm. D: dorsal, V: ventral, M: medial, L: lateral.
(B) Confocal images (single optical plane) of a WT and an SMA P4 L2 motor neuron filled with Neurobiotin following intracellular recording. Scale bar applies to both images.
(C) Graph shows the relationship between the input resistance and the corresponding soma size for 5 WT and 4 SMA motor neurons.
(D) Average soma size (left graph) and input resistance (right graph) for the WT and SMA motor neurons shown in (C). * P= 0.02, t-test.
All data are represented as mean ± s.e.m. For details, see online methods checklist.
Supplementary Figure 8 Maintenance of rescued VGluT1 synapses at P11.
(A) Z-stack projection of confocal images from retrogradely labeled L2 motor neurons (blue) and VGluT1 synaptic boutons (green) from WT, SMA, SMA+PvCre, SMA+ChATCre and SMA+(Pv+ChAT)Cre mice at P11. The total distance in the z-axis for all images was 7 μm (20 optical planes at 0.35 μm intervals).
(B) The average number of VGluT1 boutons on somata of L2 WT, SMA, SMA+PvCre, SMA+ChATCre and SMA+(Pv+ChAT)Cre motor neurons at P11. # P<0.05, ## P<0.01, ### P<0.001, one-way ANOVA, Tukey's post hoc analysis [SMA v SMA+PvCre, SMA v SMA+ChATCre and SMA v SMA+(Pv+ChAT)Cre]. * P<0.05, ** P<0.01, *** P<0.001, one-way ANOVA, Tukey's post hoc analysis [WT v SMA, WT v SMA+PvCre, SMA v SMA+ChATCre and WT v SMA+(Pv+ChAT)Cre]. All data are represented as mean ± s.e.m. For details, see online methods checklist.
(C) VGluT1 synaptic density on 50 μm dendritic compartments from the soma, for the same groups shown in (B). # P<0.05, ## P<0.01, ### P<0.001, one-way ANOVA, Tukey's post hoc analysis [SMA v SMA+PvCre, SMA v SMA+ChATCre and SMA v SMA+(Pv+ChAT)Cre]. *** P<0.001, one-way ANOVA, Tukey's post hoc analysis [WT v SMA, SMA v SMA+ChATCre]. All data are represented as mean ± s.e.m. For details, see online methods checklist.
Supplementary Figure 9 Variability in rheobase, required for motor neuron repetitive firing at P2, and absence of changes in firing frequencies for resistant SMA motor neurons at P4.
(A) Frequency to current plots for seven WT motor neurons at P2.
(B) Frequency to current plots for six SMA-unaffected motor neurons at P2.
(C) Frequency to current plots for four SMA-affected motor neurons at P2.
(D) Repetitive firing following 50 pA current injection above the minimum current required for continuous spiking, in a WT and an SMA L5 motor neuron at P4.
(E) Frequency-to-current relationships for the two groups shown in (D).
Supplementary Figure 10 NMJ functional assays and lifespan of SMA mice with selective restoration of SMN in either proprioceptive neurons, motor neurons or both neuronal classes.
(A) Schematic showing the set-up used to record the compound muscle action potential (CMAP) from the QL muscle following stimulation of the L2 ventral root at P4.
(B) CMAP responses at 20 Hz stimulation frequency.
(C) CMAP response before (black) and after 30 μM pancuronium (red). Arrow indicates the stimulus artifact.
(D) Average lifespan in SMA+PvCre+ compared to SMA+PvCre- mice. ** P=0.013, unpaired t-test.
(E) Average lifespan in SMA+ChATCre+ and SMA+(Pv+ChAT)Cre+ compared to SMA+ChATCre- mice. * P<0.05, one-way ANOVA, Tukey's post hoc analysis SMA+ChATCre- v SMA+ChATCre+; *** P<0.001, one-way ANOVA, Tukey's post hoc analysis, SMA+ChATCre- v SMA+(Pv+ChAT)Cre+.
Supplementary Figure 11 Input resistance, motor neuron numbers, NMJ innervation and function in PvTeNT mice.
(A) Average number of L2 motor neurons in WT and PvTeNT mice.
(B) Confocal images of NMJs from the QL muscle labeled by the presynaptic markers synaptophysin (green), neurofilament (blue) and the postsynaptic marker bungarotoxin (red) in PvTeNT mouse at P4.
(C) Percentage of innervation of the QL muscle in PvTeNT mouse. No denervation was observed.
(D) A compound muscle action potential (CMAP) recorded from the QL muscle following stimulation of the L2 ventral root in a PvTeNT mouse at P4.
(E) The average peak CMAP amplitude in WT and PvTeNT mice at P4.
(F) Current-to-voltage relationship in an L5 WT and PvTeNT motor neurons.
(G) I-V plots for the motor neurons shown in (F).
(H) Average of input resistance for L5 WT (black) and PvTeNT (purple) motor neurons. ** p<0.01, t-test.
Supplementary Figure 12 Overexpression of BDNF in SMA proprioceptive neurons does not affect the input resistance or firing frequency of SMA motor neurons.
(A) Images from L2 DRG (at P5) with parvalbumin (red) and GFP (green) immunoreactivity, following ICV injection with AAV9-GFP-BDNF at P0.
(B) Percentage of parvalbumin+ neurons transduced by AAV9-GFP-BDNF.
(C) Images of ChAT+ motor neurons (red) and GFP (green) following injection with AAV9-GFP-BDNF.
(D) Percentage of motor neurons transduced by AAV9-GFP-BDNF.
(E) Immunoreactivity of GAD65 (red) in GABApre terminals contacting VGluT1 (green) proprioceptive afferent terminals in SMA (top row) and SMA+AAV9-BDNF (bottom row) P5 mice. Yellow arrows indicate GABApre terminals.
(F) Synaptic marker intensity measurements in GAD65+ GABApre terminals at P5. (GAD65: 39 terminals, 3 mice/experimental group). *** P<0.001, Mann-Whitney test.
(G) Immunoreactivity of GAD67 (red) in GABApre terminals contacting VGluT1 (green) proprioceptive afferent terminals in SMA (top row) and SMA+AAV9-BDNF (bottom row) P5 mice. Yellow arrows indicate GABApre terminals.
(H) Synaptic marker intensity measurements (arbitrary units) in GAD67+ GABApre terminals at P5. (GAD67: 39 terminals, 3 mice/experimental group). NS: no significance (P=0.72, Mann-Whitney test).
(I) Intracellular recorded and subsequently filled motor neuron (red) combined with GFP (green) and ChAT (white). Arrow indicates that the recorded motor neuron was not transduced by the virus. Asterisk indicates a nearby transduced motor neuron.
(J) Current to voltage relationship for motor neuron shown in (I).
(K) The average input resistance for 3 non-transduced SMA motor neurons (yellow; SMA+AAV9-BDNFproprio) compared with SMA motor neurons (red; SMA-uninjected).
(L) The firing frequency in 3 non-transduced SMA motor neurons was similar to SMA motor neurons.
Supplementary Figure 13 Afterhyperpolarization properties, effects of GxTx-1E on input resistance and trough voltage analysis in WT and SMA motor neurons.
(A) Afterhyperpolarization (AHP) in a WT and SMA motor neuron at P4. The AHP was measured from the first action potential evoked following current injection. The AHP duration is indicated between the two grey vertical lines. The AHP amplitude was calculated between the red dotted line (membrane voltage prior to spike initiation) to the peak negative potential.
(B) The average AHP duration (left) and amplitude (right) in WT and SMA motor neurons at P4.
(C) Superimposed membrane responses (top) to current injections (bottom) before and after GxTx-1E exposure.
(D) Current-to-voltage relationship before (black) and after GxTx-1E (red) exposure.
(E) Average values of input resistance before and after GxTx-1E in WT and SMA motor neurons. Lines indicate the change in input resistance for the individual motor neurons.
(F) Superimposed recordings from repetitive firing at a low frequency (black) and twice that frequency (blue) in a WT (top) and an SMA (bottom) motor neuron. On the right, traces at higher voltage to indicate the change in trough voltage (dotted red lines).
(G) The average trough voltage change in WT (n=8) and SMA (n=6) motor neurons. ** P=0.0023, Mann-Whitney test.
Supplementary Figure 14 Methodology of measuring Kv2.1 surface expression; Kv2.1 in L5 motor neurons in WT, SMA and PvTeNT motor neurons; Kv2.2 in WT and SMA motor neurons; Kv4.3 expression in WT and SMA mice
(A) Kv2.1 (red), VGluT1 (green), ChAT (blue) immunoreactivity. Yellow arrow indicates the association of a VGluT1+ synapse with post-synaptic Kv2.1 expression.
(B) Analysis of Kv2.1 expression (yellow) was performed from single optical plane images. Only motor neuron somata (inset; ChAT in blue) were analyzed. To calculate the coverage by Kv2.1, a line (red) was drawn along the soma perimeter to acquire the fluorescence intensity. Baseline fluorescence intensity measurement was achieved by drawing a straight line within the cytoplasm (white).
(C) Fluorescence intensity measurement along the perimeter of the motor neuron soma shown in (A). The signal higher than 3 Standard Deviations (black dotted line) of the baseline intensity (blue dotted line) was considered as Kv2.1 expression, while the signal below was considered as background.
(D) Single optical level confocal images of L5 motor neurons (ChAT in blue) expressing Kv2.1 immunoreactivity (yellow) in WT, SMA and PvTeNT mice at P4.
(E) Percentage somatic coverage of Kv2.1 in L5 WT, SMA and PvTeNT motor neurons at P4. *** P=0.001, One-Way ANOVA (WT vs PvTeNT motor neurons).
(F) Single optical plane confocal images of L2 motor neurons (ChAT in blue) expressing Kv2.2 immunoreactivity (yellow) in WT and SMA mice at P4. Graph shows the average percentage somatic coverage of Kv2.2 in L2 WT and SMA motor neurons at P4.
(G) Single optical plane confocal images from an area near the motor neuron nucleus (ChAT in blue) in the L2 spinal segment and Kv4.3 immunoreactivity (yellow) in WT and SMA mice at P4. Kv4.3 was prominently expressed in spinal interneurons (arrows) but not in motor neurons. D: dorsal, V: ventral, L: lateral, M: medial.
Supplementary Figure 15 Reduced sensory–motor synaptic excitation impairs motor neuron output through the potassium channel Kv2.1
Dysfunction of excitatory proprioceptive synapses caused by disease (SMA) or blocked neurotransmission at sensory-motor synapses, results in reduced excitatory postsynaptic potentials (EPSPs) and subsequently reduced membrane surface expression of Kv2.1 channels on the somata and proximal dendrites of motor neurons. This reduction leads to a significant decrease in the firing frequency of motor neurons. The inability of motor neurons to fire at high frequencies (rate coding) may be a key determinant in muscle contraction. These events can be significantly ameliorated by increasing excitatory synaptic drive (SMA+PvCre experiments or treatment of SMA mice with kainate).
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Fletcher, E., Simon, C., Pagiazitis, J. et al. Reduced sensory synaptic excitation impairs motor neuron function via Kv2.1 in spinal muscular atrophy. Nat Neurosci 20, 905–916 (2017). https://doi.org/10.1038/nn.4561
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