Post by Amanda McFarlan
What's the science?
The Scn2a gene, encoding the protein for the Nav1.2 voltage-gated sodium channel, has been identified as one of the most commonly affected genes in Autism Spectrum Disorder. In the early stages of brain development, the Nav1.2 channel is the only sodium channel isoform that is expressed in the axons of cortical pyramidal neurons and is therefore critical for the initiation and propagation of action potentials. In later stages of development, however, the Nav1.2 channels in the axon and distal initial axon segment are replaced with the Nav1.6 channel (another voltage-gated sodium channel encoded by the Scn8a gene). This switch in ion channel expression in cortical pyramidal cells leaves the Nav1.2 channel restricted to the proximal initial axon segment where it is thought to be important for the backpropagation of action potentials. This week in the Neuron, Spratt and colleagues investigated the consequences of reduced function of the Nav1.2 channel on cortical neuron excitability and plasticity throughout development.
How did they do it?
The authors assessed the role of Scn2a haploinsufficiency (reduced function of Nav1.2 channel) on neuron excitability during development by targeting layer 5b pyramidal neurons in the medial prefrontal cortex for whole-cell recording. To do this, they used acute slices from Scn2a+/- mice (reduced function of Nav1.2 channel) and wildtype mice (normal function of Nav1.2 channel) between postnatal day 4 and 64. Next, the authors investigated the impact of reduced neuronal excitability on cell signaling by altering the localization and density of Nav1.2 and Nav1.6 channels in the soma, axon and dendrites in a computational model of cortical pyramidal cells. Then, they explored the effect of Scn2a haploinsufficiency on dendritic excitability by imaging calcium transients from backpropagating action potentials along the apical dendrite of layer 5 pyramidal neurons in Scn2a+/- mice. They repeated this experiment in wild-type mice while acutely blocking sodium channels with a low, sub-saturating dose of tetrodotoxin, to determine whether changes in dendritic excitability were caused by an acute loss of Nav1.2 rather than a loss of Nav1.2 during development. Next, the authors recorded miniature excitatory postsynaptic currents and miniature inhibitory postsynaptic currents in Scn2a+/- mice and wildtype mice at postnatal days 6 and 27 (important developmental stages for axon and dendritic development, respectively) to assess whether Scn2a haploinsufficiency affects the development of functional synapses. They investigated this further by performing plasticity experiments in Scn2a+/- mice, wild-type mice and wildtype mice treated with a low dose of tetrodotoxin. For these experiments, they targeted layer 5 pyramidal cells for whole-cell recording and induced long-term potentiation by pairing extracellular stimulation of layer 1 dendritic processes with action potentials. Finally, the authors used behavioural tests to assess different behavioural traits, including locomotion, anxiety, repetitive behaviour, sociability, and learning, in both male and female Scn2a+/- mice.
What did they find?
The authors found that the action potential threshold was more depolarized in pyramidal neurons in Scn2a+/- mice compared to wildtype mice during the first postnatal week, but these differences did not persist afterward. They also determined that pyramidal neurons in Scn2a+/- mice had a reduction in action potential velocity compared to wildtype mice that became more prominent as the neurons matured, suggesting that Scn2a haploinsufficiency impairs neuronal excitability throughout development and into adulthood.
Next, the authors revealed that their computational model best fit their real-world observations when Nav1.2 and Nav1.6 channels were equally expressed in the somatodendritic region. They showed that removing half the Nav1.2 channels resulted in a reduction of the action potential velocity (similar to what was observed in Scn2a+/- mice) as well as an attenuation in backpropagating action potentials, suggesting that the mechanisms involved in the backpropagation of action potentials may be impaired by Scn2a haploinsufficiency. In support of this finding, the authors determined that calcium transients in the apical dendrites of pyramidal neurons (indicative of backpropagating action potentials) rapidly decreased in amplitude in a distance-dependent manner from the soma in Scn2a+/- mice and wildtype mice treated with tetrodotoxin but were reliably observed throughout the entire apical dendrite in wildtype mice. These findings suggest that Scn2a haploinsufficiency results in impairments to dendritic excitability that are likely due to an acute loss of Nav1.2 channels in the dendrite rather than a loss of Nav1.2 during development.
Lastly, the authors found that there was a reduction in the frequency, but not amplitude, of miniature excitatory postsynaptic currents at postnatal day 27 (associated with dendritic development) in Scn2a+/- compared to wildtype mice. Further, they determined that long-term potentiation was abolished in Scn2a+/- mice and wildtype mice treated with tetrodotoxin, suggesting that Scn2a haploinsufficiency disrupts the formation of mature, functional synapses and impairs synaptic plasticity. Finally, the authors revealed that Scn2a+/- mice display trends towards impairments in learning and social behaviour that are sex-specific.
What's the impact?
This is the first study to show that Nav1.2 channels play an important role in controlling dendritic excitability and synaptic plasticity in excitatory pyramidal neurons. They show that haploinsufficiency of Scn2a in mice causes impairments in synapse formation and synaptic plasticity that are likely due to the acute loss of the Nav1.2 channels in the dendrites. Together, these findings provide valuable insight into the functional role of the Scn2a gene and how it may be implicated in Autism Spectrum Disorder.
Spratt et al. The Autism-Associated Gene Scn2a Contributes to Dendritic Excitability and Synaptic Function in the Prefrontal Cortex. Neuron (2019). Access the original scientific publication here.