Loss of HCN2 in Dorsal Hippocampus of Young Adult Mice Induces Specific Apoptosis of the CA1 Pyramidal Neuron Layer

Abstract

Neurons inevitably rely on a proper repertoire and distribution of membrane-bound ion‑conducting channels. Among these proteins, the family of hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels possesses unique properties giving rise to the corresponding Ih-current that contributes to various aspects of neural signaling. In mammals, four genes (hcn1-4) encode subunits of HCN channels. These subunits can assemble as hetero- or homotetrameric ion-conducting channels. In order to elaborate on the specific role of the HCN2 subunit in shaping electrical properties of neurons, we applied an Adeno-associated virus (AAV)-mediated, RNAi-based knock-down strategy of hcn2 gene expression both in vitro and in vivo. Electrophysiological measurements showed that HCN2 subunit knock-down resulted in specific yet anticipated changes in Ih-current properties in primary hippocampal neurons and, in addition, corroborated that the HCN2 subunit participates in postsynaptic signal integration. To further address the role of the HCN2 subunit in vivo, we injected recombinant (r)AAVs into the dorsal hippocampus of young adult male mice. Behavioral and biochemical analyses were conducted to assess the contribution of HCN2-containing channels in shaping hippocampal network properties. Surprisingly, knock-down of hcn2 expression resulted in a severe degeneration of the CA1 pyramidal cell layer, which did not occur in mice injected with control rAAV constructs. This finding might pinpoint to a vital and yet unknown contribution of HCN2 channels in establishing or maintaining the proper function of CA1 pyramidal neurons of the dorsal hippocampus.

HCN2 Channels in the Ventral Hippocampal CA1 Regulate Nociceptive Hypersensitivity in Mice

“…Our data, therefore, support the hypothesis that reduced HCN2 expression led to decreased neuronal activities in the vCA1 after CFA injection. These findings align with previous studies that have reported severe degeneration of the CA1 pyramidal cell layer following the knockdown of HCN2 expression [ 34 ]. Moreover, HCN2-knockin neurons in the thalamic produce bursts of action potentials and reveal a larger voltage–sag ratio [ 44 ].…”

“…Both HCN1 and HCN2 channels in the peripheral nervous system participate in nociceptive processing, thereby influencing neuropathic and inflammatory pain [ 32 , 33 ]. In the hippocampus, HCN2 knockdown results in severe degeneration in CA1 pyramidal cell layer [ 34 ], whereas knockdown of HCN1 increases the cellular excitability of the hippocampus [ 35 ], indicating distinct roles of HCN1 and HCN2 in regulating hippocampal physiology. However, whether HCN channels participate in hippocampal modulation of pain remains unclear.…”

Localization of hyperpolarization-activated cyclic nucleotide-gated channels in the vertebrate retinas across species and their physiological roles

“…Second, HCN channels are essential for oscillations in both individual cells and cellular networks. A prime example of this can be observed in thalamocortical neurons, where two distinct depolarization modes exist: the transmission mode and the burst mode ( Benarroch, 2013 ; Deutsch et al, 2021 ). Sensory inputs depolarize thalamocortical neurons, generating single spikes that appear as delta waves in electroencephalography (EEG) during wakefulness and non-rapid eye movement (non-REM) sleep ( Benarroch, 2013 ; Maki-Marttunen and Maki-Marttunen, 2022 ).…”