Authors :
Eliza Schnitzer, _ – Stanford University
Shreya Malhotra, MPH – Stanford University
Annie Goettemoeller, PhD – Stanford University
Presenting Author: Peter Klein, PhD – Stanford University
Robert Lupoiu, BS – Stanford University
Fraser Sparks, PhD – Columbia University
Attila Losonczy, PhD – Columbia University
Ivan Soltesz, PhD – Stanford University
Rationale:
As we seek to improve the therapeutic options available to people with temporal lobe epilepsy (TLE), better understanding the nature of the underlying neuronal circuits that generate seizures will potentially provide crucial insights. Within the range of interneuron populations that regulate signaling among hippocampal neurons, axo-axonic cells (AACs) are uniquely positioned as the only cell type that provides inhibitory inputs to the axon initial segment, likely providing AACs with an outsized ability to regulate excitatory signaling propagation. We therefore aimed to determine the degree to which AACs remain integrated in hippocampal circuits in TLE and how well targeted regulation of AAC activity may be leveraged to provide seizure control.
Methods:
TLE was induced in adult Unc5b-CreERT2 AAC reporter mice through unilateral intrahippocampal kainic acid (IHKA) injections and epileptogenesis was later confirmed in all animals using video-EEG recordings. Hippocampal AACs were specifically targeted using stereotaxic injections of AAVs to allow expression of Cre-dependent genetic constructs. TLE-induced impacts on AAC properties were assessed using immunohistochemistry, slice electrophysiology and in vivo 2-photon Ca2+ imaging during behavior in head-fixed animals.
Results:
We observe that AACs survive in similar numbers between hippocampi ipsilateral and contralateral to the site of IHKA injection following TLE induction. The cell intrinsic electrophysiological properties of hippocampal AACs are also largely unchanged between hemispheres, displaying comparable excitability. As we have previously observed in control animals, in vivo 2-photon Ca2+ imaging reveals that CA1 AACs in TLE mice remain tightly correlated to the activity of nearby pyramidal cells during locomotor activity, as well as concurrent to electrographic seizure activity. However, we observe indications that the typically highly coordinated synchronous activity of AACs is potentially impacted by TLE. As we have previously observed that selective activation of hippocampal interneuron populations using closed-loop optogenetics can effectively control seizures in TLE, we are currently investigating the degree to which AACs can provide a similar therapeutic benefit.
Conclusions:
Our study demonstrates that AACs largely survive during TLE and remain actively integrated within hippocampal neuronal networks. As AAC synapse onto the AIS of excitatory neurons, we believe they may be ideally positioned to provide targeted regulation of hippocampal excitability. Determining the specific hippocampal neuronal populations that can best exert fine-tuned control over circuit excitability during seizures will provide opportunities to improve therapeutic options for people with TLE.
Funding:
National Institutes of Health (NIH) under award R01NS121106 (I.S. and A.L.), Stanford Bio-X Undergraduate Research Fellowship (E.S.) and AES Predoctoral Research Fellowship (S.M.)