Motor Cortex Layer V Pyramidal Neurons Exhibit Dendritic Regression, Spine Loss, and Increased Synaptic Excitation in the Presymptomatic hSOD1^G93AMouse Model of Amyotrophic Lateral Sclerosis

Abstract

Motor cortex layer V pyramidal neurons (LVPNs) regulate voluntary control of motor output and selectively degenerate (along with lower motor neurons) in amyotrophic lateral sclerosis. Using dye-filling and whole-cell patch clamping in brain slices, together with highresolution spinning disk confocal z-stack mosaics, we characterized the earliest presymptomatic cortical LVPN morphologic and electrophysiological perturbations in hSOD1 G93A (SOD1) mice to date. Apical dendritic regression occurred from postnatal day (P) 28, dendritic spine loss from P21, and increased EPSC frequency from P21 in SOD1 LVPNs. These findings demonstrate extensive early changes in motor cortex of the SOD1 mouse model, which thus recapitulates clinically relevant cortical pathophysiology more faithfully than previously thought.

Glycinergic Neurotransmission: A Potent Regulator of Embryonic Motor Neuron Dendritic Morphology and Synaptic Plasticity

“…By contrast, higher levels of respiratory MN activity (XII and phrenic) decreased MN survival and muscle innervation (Banks et al, 2005). Similar alterations of MN survival and axonal branching in mice lacking GABA (Fogarty et al, 2013b) and combined GABAergic and glycinergic neurotransmission loss (Fogarty et al, 2015b) further support the hypothesis that central synaptic inputs to MNs play a vital role in regulating MN survival and muscle innervation.…”

“…For quantifying spine density, a high-powered (63ϫ oil, NA 1.4) objective was used with a 2.5ϫ zoom to collect z-series with a 0.33 m z-step. Dendritic processes were classified as spines only if they were Ͻ3 m long and Ͻ0.8 m in cross-sectional diameter, in accordance with past studies (Harris, 1999;Fogarty et al, 2015a). Proximal and distal dendrites were classified according to centrifugal branch orders where branches extending from the cell soma are considered first order, branch ramifications from this dendritic segment second order, and ramifications from these segments being classified as third order.…”

“…Morphological properties of NB-filled XII MNs were analyzed using Neurolucida (MBF Bioscience) and manual soma (volume and surface area), dendrite and spine tracing as for previous studies (Fogarty et al, 2015a). Low-powered (20ϫ objective) z-series (with a 1.0 m z-step) from a Zeiss LSM 510 META scanning confocal microscope (Carl Zeiss) using 555 nm laser excitation/ detection filters were used to create images of the MN and its entire dendritic arbor.…”

“…A total MN dendrite length of 54,384 m was traced for morphometric analysis in 19 neurons. XII motor nucleus volume was measured using a Cavalieri volume estimate obtained on the slices recovered for XII MN dendritic morphology using previously established methods (Fogarty et al, 2013b(Fogarty et al, , 2015b and an average of 4 sequential transverse slices containing XII motor nucleus per animal (Kanjhan and Bellingham, 2013).…”

“…When the central activation of MNs by other neurons has been perturbed in mice, similar alterations in MN survival and muscle innervation have been shown to occur (Oppenheim et al, 2003;Banks et al, 2005;Fogarty et al, 2013bFogarty et al, , 2015b and are thought to depend on the level of MN activity passed onto muscle via intact neuromuscular synapses (Banks et al, 2003(Banks et al, , 2005Oppenheim et al, 2003). This has been demonstrated for central synaptic inputs releasing the amino acid neurotransmitters GABA and glycine, which provide an important source (along with glutamate) of excitation during MN development (Nishimaru et al, 1996;Singer and Berger, 2000;Ben-Ari, 2002).…”