Contribution of TRIP8b to the HCN channel function in TC neurons of different thalamic nuclei
A major function of TRIP8b in the mammalian brain is the regulation of HCN channel trafficking and surface localization (Lewis et al.
2009; Piskorowski et al.
2011; Santoro et al.
2009). In line with previous studies of the hippocampus (Lewis et al.
2011) and VB thalamus (Heuermann et al.
2016), we found a strong downregulation of
I
h
in TRIP8b
−/− TC neurons due to reduced HCN channel protein expression. Here, we extend this finding by demonstrating that TRIP8b upregulates
I
h
in several functionally different thalamic areas: dLGN, the primary relay for visual information; VB and PO, which form complementary somatosensory pathways with distinct inputs and targets (Kleinfeld and Deschênes
2011); and CM, involved in arousal and attention (Schiff et al.
2013). The fine-tuning of TC neuron properties within these different nuclei which is mandatory to suit their respective roles in the thalamocortical network is influenced by the variations in
I
h density, HCN subunit composition, and specific contribution(s) of TRIP8b. It is interesting to note that TRIP8b is not expressed in GABAergic thalamic neurons, namely nRT neurons and local circuit interneurons (Heuermann et al.
2016; present study, see Supplemental Figure 5 a–c). The preserved
I
h in these cells may partially explain the infrequent absence seizures (Heuermann et al.
2016) and short bursts of activity in our LFP recordings in TRIP8b
−/− mice, as compared to the severe epileptic phenotype of HCN2 knockouts (Ludwig et al.
2003).
HCN channels are partially open at rest and thus critically contribute to the RMP and
R
in in TC neurons. Knockout of HCN2 (Ludwig et al.
2003; Huang et al.
2009; Budde et al.
2012; Meuth et al.
2006), HCN4 (Budde et al.
2012), or pharmacological blockade of I
h (Leist et al.
2016) shifts the RMP to more hyperpolarized potentials. Similarly, we found that downregulating
I
h by knocking out TRIP8b produced a robust − 11 mV shift in RMP in TC neurons, accompanied by a 45% increase in
R
in. We further demonstrated that these changes in membrane properties have pronounced effects on the firing properties of TC neurons in TRIP8b
−/− mice, increasing the propensity for burst firing in response to depolarizing inputs but preventing rebound bursts following injection of hyperpolarizing currents. Due to the hyperpolarized RMP and lack of
I
h-dependent afterdepolarizing potential (see Fig.
5, right upper panel) that occurs in WT cells, TRIP8b
−/− neurons remain below the threshold for activating T-type Ca
2+ channels, which are responsible for the generation of the rebound burst. Notably, injection of hyperpolarizing current from a holding potential of − 60 mV (which is above the threshold for activating T-type Ca
2+ channels) resulted in the reappearance of rebound bursts in TRIP8b
−/− TC cells, suggesting that neuromodulatory influences could potentially restore this capability in vivo. Rebound bursting is important for different physiological and pathophysiological oscillatory activity within the thalamocortical network, particularly sleep spindles, delta oscillations and perhaps also spike-wave discharges (SWDs), mediated by reciprocal innervation between nRT and TC neurons (Steriade
2003).
In addition to alterations in burst firing, TRIP8b
−/− TC neurons showed a significant reduction in tonic AP firing upon injection of depolarizing current steps compared to WT animals. Extracellular application of 10 µM ZD7288 to WT TC neurons had a similar effect, as has been observed before in thalamic, hypothalamic and DRG neurons (Leist et al.
2016; Momin et al.
2008; Cai et al.
2012). This is in contrast to results in cortical and hippocampal pyramidal neurons, in which loss of
I
h
enhances AP firing (Huang et al.
2009; Lewis et al.
2011). This discrepancy may result from differences in HCN subunit expression and subcellular localization, causing the tonic depolarizing influence of
I
h to be more important in some neurons (e.g., TC neurons), whereas the effect on
R
in dominates in others (pyramidal neurons). Another notable finding is that the reduction in the number of APs induced by ZD7288 was less extensive compared to TRIP8b knockout animals, even after restoring the RMP to − 60 mV with DC current injection. These results indicate that there might be additional mechanisms underlying the reduction of AP firing in TC cells of TRIP8b
−/− mice. Possible mechanisms may be related to the decreased cAMP levels and/or alterations in K
+ channel function in TRIP8b
−/− mice. Since increases in cAMP enhance tonic firing in rodent TC neurons (Ehling et al.
2013) the lower basal cAMP level in TRIP8b
−/− mice brain compared to WT might contribute to the reduction of AP firing in TC cells in these animals. Another possible candidate is changes in Kv1.2 channels, which have been shown to increase their amplitude upon co-expression with TRIP8b (Santoro et al.
2009). Since Kv1.2 channels are expressed in TC neurons (Decher et al.
2010) and Kv1.2-deficient mice show decreased AP firing in neurons (Robinson et al.
2003), our results are in agreement with a reduction of current through Kv1.2 channels. Although no direct interaction was found between TRIP8b and channels underlying
I
A
in central neuron (Santoro et al.
2009), this fast transient K
+ current is a potential candidate since it is modulated by cAMP and known to shape bursting and tonic firing of TC neurons (Pape et al.
1994; Kanyshkova et al.
2011). The voltage-dependent properties of
I
A as described in the present study are well within the range of parameters found for activation (V
h: − 14 to − 37 mV), inactivation (V
h: − 65 to − 82 mV) and current kinetics (
τ
inact: 6 to 96 ms) of fast transient K
+ currents in TC neurons in different thalamic nuclei and species (Huguenard et al.
1991; Budde et al.
1992; Kanyshkova et al.
2011; McCormick
1991; Noh et al.
2011). In addition, a slow inactivating
I
A component generating a large window current was described (McCormick
1991). The firing properties of TC neurons are influenced by
I
A in several ways. The presence of a large K
+ current that generates a window current and is active at rest is expected to influence the RMP, bursting and tonic firing. Indeed computer modeling as well as pharmacological block by 4-amino pyridine (4-AP) revealed the hyperpolarizing influence of
I
A on the RMP (McCormick
1991; Amarillo et al.
2014). Furthermore, electrophysiological recordings and computer simulations have demonstrated that
I
A controls the generation, amplitude and duration of a LTS by functionally counteracting
I
T (Huguenard et al.
1991; Pape et al.
1994; Amarillo et al.
2014; Gutierrez et al.
2001; Noh et al.
2010). Since
I
A is active during repetitive bursting at the initial depolarizing phase of each cycle, the magnitude of the current influences the periodicity (Amarillo et al.
2014). While bursting requires hyperpolarized levels of the membrane potential,
I
A also affects tonic firing from more depolarized potentials when
I
T is inactivated. Since depolarizing inputs have to overcome the hyperpolarizing influence of the inactivating
I
A, the onset of firing is delayed thereby limiting the number of induced APs (McCormick
1991; Budde et al.
1992; Kanyshkova et al.
2011). Wash in of 4-AP abolishes the delayed onset of firing and increases the half width of single APs (Budde et al.
1992; Kanyshkova et al.
2011). While the latter decreases tonic firing frequencies, the total number of APs elicited by a given depolarizing pulse is increased, thus demonstrating the limitation of AP firing by
I
A. This view is corroborated by findings from different neuronal cell types where receptor stimulation-dependent activation and inhibition of I
A reduces and increases the rate of firing APs, respectively (Kloppenburg et al.
1999; Pitra and Stern
2017). In a similar way the knock down of Kv4.1 channels increased neuronal firing rates (Hermanstyne et al.
2017).
Since several members of different K
V channel subfamilies including K
V1.4, K
V3.3, K
V3.4, K
V4.1-4.3 reveal typical
I
A
properties, the molecular basis of fast transient K
+ currents is complex (Song
2002). This complexity is even increased by the formation of multi-protein ion channel complexes that underlie native currents. In case of
I
A
, the K
V channel-interacting proteins (KChIPs) and dipeptidyl peptidase-like proteins (DPLPs) interact with the pore-forming
α-subunits (Jerng and Pfaffinger
2014). Based on expression studies and analysis of native current, combinations of K
V4.2/K
V4.3, KChIP3/KChIP4, and DPP6/DPP10 may contribute to I
A in thalamic TC neurons (Serôdio and Rudy
1998; Rhodes et al.
2004; Xiong et al.
2004; Wang et al.
2014; Kanyshkova et al.
2011).
Based on the functional properties and expression profile of transient K
+ channels discussed above, a number of alterations found in TRIP8b
−/− mice may be attributed to the reduction in cAMP and consequent changes in
I
A. Channels based on the combination of K
V4.2 and KChiP3 have been shown to undergo a rightward shift in the activation curve and slowing of inactivation kinetics following cAMP-dependent modulation (Schrader et al.
2002). Therefore, the leftward shift in the
I
A activation curve and the faster inactivation kinetics are in line with the decreased cAMP levels found in TRIP8b
−/− mice. Additional interactions with DPP6 may result in the hyperpolarizing shift of the inactivation curve in TRIP8b
−/− mice (Jerng and Pfaffinger
2014). Together these changes enlarge the window current, increase the tonically active
I
A near the RMP and increase the voltage-activated
I
A close to the AP threshold (Kloppenburg et al.
1999). In combination with the reduction in
I
h, increased tonic
I
A hyperpolarizes the RMP (Amarillo et al.
2014) and reduces tonic firing by delaying the onset of firing and reducing the number of evoked APs. When an LTS is evoked in TC neurons of TRIP8b
−/− mice the enlarged
I
A is shaping the time course by contributing to reduced amplitude and duration. In addition, the combined changes in
I
A and
I
h may help to sustain oscillatory LTS generation at low frequency. While a decreased
I
h is expected to promote oscillations, conditions of imposed hyperpolarization may further facilitate oscillatory activity (Amarillo et al.
2014). With respect to thalamocortical oscillations, it is interesting to note that the block of I
A by 4-AP elicited spontaneous field potential activity in thalamocortical slices in vitro (D’Arcangelo et al.
2002). While sequences of fast (10–16 Hz) and slower (5–9 Hz) field potential bursts were recorded in WAG/Rij rats, a rodent absence epilepsy model (van Luijtelaar and Zobeiri
2014), only fast oscillations were seen in non-epileptic control rats. SWDs in the frequency range of 7–9 Hz are characteristic for a number of rodent absence epilepsy models. Therefore, the increased
I
A window current may hinder the transition from slow oscillations to faster epilepsy-related SWDs and wake-related rhythms.
TRIP8b regulates Ih in cortical pyramidal neurons
Previous studies demonstrated a high expression level of TRIP8b in layer V cortical pyramidal neurons with an expression gradient similar to HCN1 channels (Santoro et al.
2004). In these cells, TRIP8b is responsible for the trafficking of HCN1 subunits to the cell membrane. In TRIP8b
−/− mice, the lack of TRIP8b in cortical pyramidal neurons resulted in a reduction in the expression of HCN1/2 subunits in the somatosensory cortex whereas the mRNA levels were not affected. In accordance with these results, layer V and VI cortical pyramidal neurons of TRIP8b
−/− mice showed a significantly lower
I
h density compared to WT animals. The reduction of
I
h in cortex is responsible for an increased
R
in and a concomitant increase in dendritic excitability in these neurons. In fact, the downregulation of
I
h in cortical pyramidal neurons has been reported in several types of pathophysiological conditions including absence epilepsy (Huang et al.
2009; Phillips et al.
2014; Heuermann et al.
2016; Kole et al.
2007). However, in contrast to the previous study by Heuermann et al. (
2016), our in vivo LFP recordings from the somatosensory cortex of TRIP8b
−/− mice revealed only sporadic short (< 1 s) epileptiform-like activity in a small number of animals. No definite reason for the differences between the two studies can be named except a potential genetic drift, especially in a small founder colony (Sade et al.
2014). It has been noted before that differences in epilepsy severity between different colonies of GEARS rats that were derived from the same founder colony may be based on environmental conditions and/or genetic drift (Powell et al.
2014). Early life environmental experience can influence the frequency of SWD occurrence in adulthood (van Luijtelaar and Sitnikova
2006). In WAG/Rij rats, maternal deprivation and neonatal handling from postnatal day 1 to 21 reduced the number of SWDs in adulthood (4.5 months of age) with about 35%, while the mean duration was not affected (Schridde et al.
2006). In addition, introducing an enriched environment (for 2 months from 1 to 3 months of age) was found to have no effect on the number of animals with SWDs and no effect on the number of SWDs, only on the mean duration of SWDs (Schridde and van Luijtelaar
2004). In addition, cross-fostered WAG/Rij pups which were raised by Wistar mothers showed fewer SWDs compared to the condition in which they were cross-fostered within the WAG/Rij strain (Sitnikova et al.
2016). Altogether, these data suggest that environmental factors play a role in shaping the occurrence of SWDs in WAG/Rij rats, but not to a very large extent and especially not for the number of animals that show SWDs. Earlier manipulations (postnatal) seem to play a larger effect than post-weaning manipulations regarding the number and mean duration of SWDs. After comparing the environmental conditions in which the two groups of TRIP8b
−/− mice were raised, we found some minor differences (e.g., differences in light/dark cycle and hygienic procedures, but not general handling) that might to some degree affect the occurrence of SWDs in our TRIP8b
−/− mice colony.
The role of TRIP8b in regulation of thalamocortical oscillations and modulation of delta oscillations
In the present study, we demonstrated that the dysregulation of
I
h in the thalamocortical system of TRIP8b
−/− mice is associated with altered thalamocortical oscillations, revealing a significant increase in slow oscillations in the delta frequency range during episodes of active-wakefulness and reduced desynchronization of the EEG during transitions from slow-wave sleep to active-wakefulness. Considering that the behavior during active-wakefulness of TRIP8b
−/− mice did not appear qualitatively and quantitatively (as was established in the open field test (Lewis et al.
2011)) different from that of WT, it is safe to exclude that the differences in EEG between the mice strains is due to differences in overt behavior. More general, the appearance of prominent delta waves during wake is associated with both physiological and pathological conditions. In the cognitive domain, increased delta frequency oscillations are implicated in attention and salience detection and subliminal perception (Knyazev
2012). However, increased baseline delta power has also been associated with a range of neurological disorders including Alzheimer’s disease (Jeong
2004; Babiloni et al.
2009), and schizophrenia (Boutros et al.
2008). Interestingly, while TRIP8b
−/− mice were largely phenotypically normal during initial behavioral screening (Lewis et al.
2011), they did exhibit significantly impaired nest building, an established endophenotype for schizophrenia (Amann et al.
2010), raising the possibility that reduced thalamocortical HCN channel function may recapitulate some features of this disorder.
Several neurotransmitter systems contribute to promotion of sleep and wakefulness and regulation of cortical activation during different behavioral states. Both thalamus and neocortex receive a large number of cholinergic projections from pontine and midbrain reticular formation, as well as cholinergic (and non-cholinergic) inputs from nuclei in the basal forebrain (BF) activating system (Buzsaki et al.
1988). These cholinergic projections play an important role in the desynchronized EEG typical for wakefulness and REM sleep (Wikler
1952; Steriade et al.
1990). In addition, the serotonergic neurons of the dorsal raphe (DR) and noradrenergic neurons of the locus coeruleus (LC) contribute to both pathways and directly innervate cortical neurons (Brown et al.
2012). A number of observation point to a limited contribution of either TRIP8b or HCN channels to the function of these activating systems, thereby lowering the possibility that changes in the EEG of TRIP8b
−/− mice are related to alterations in these areas. In comparison to cortical and thalamic areas, the expression of TRIP8b was low in WT and expression of HCN2 was not altered following TRIP8b knock out in the ascending brainstem-activating system (see Supplemental Figures 10, 11). In addition, electrophysiological characterization of the cholinergic neurons of the BF and the locus coeruleus revealed only very little time-dependent anomalous rectification (Unal et al.
2012; Alreja and Aghajanian
1991; Hedrick and Waters
2010). Nevertheless, a role of activating systems in the aberrant waking EEG pattern in TRIP8b
−/− mice cannot be ruled out by the experiments presented here.
Several previous studies have established the importance of both thalamic and cortical I
h in generating network oscillations, particularly delta and spindle oscillations during slow-wave sleep (Steriade et al.
1993; Steriade and Deschenes
1984; Crunelli et al.
2015; Kanyshkova et al.
2009).
I
h in cortical pyramidal neurons helps to establish the subthreshold resonance frequency, typically in the theta band, which amplifies oscillations in this frequency range (Hu et al.
2002; Stark et al.
2013; Wahl-Schott and Biel
2009). Downregulation of
I
h shifts the resonant frequency from the theta to delta range both within individual pyramidal neurons (Karameh et al.
2006; Stadler et al.
2014) and at the network level (Schmidt et al.
2016). We, therefore, conclude that the significant reduction of
I
h in cortical pyramidal neurons of TRIP8b
−/− mice alters the preference of the cortical network in favor of slower oscillations.
I
h also contributes to delta rhythmicity at the level of the thalamus by promoting cycles of rebound bursts in reciprocally connected TC and nRT neurons (Crunelli et al.
2015; Steriade et al.
1993; Llinas and Steriade
2006; Steriade
2003; Timofeev and Bazhenov
2005; Kanyshkova et al.
2009; Meuth et al.
2003). Delta oscillations typical for slow-wave sleep occur due to hyperpolarization of TC neurons as sleep deepens, increasing the propensity for bursting (Steriade
2003; Llinas and Steriade
2006). Therefore, one might speculate that the hyperpolarized RMP of TC neurons in TRIP8b mice
−/−, coupled with a cortical network that resonates at delta frequencies, favors generation of thalamocortical delta oscillations during active-wakefulness. However, impaired rebound bursting in these cells would prevent reverberant, runaway delta activity in the form of SWDs. In line with this hypothesis, our LFP recordings of intrathalamic network activity showed slowed inter-burst frequency in TRIP8b
−/− mice and a reduction in overall oscillatory activity. Interestingly, a low concentration of ZD7288 (0.5 µM) generated time-dependent effects, with an increase in rhythmic burst activity during the first 20 min after application followed by a significant decrease in rhythmic burst activity towards the end of recording that mimics the TRIP8b
−/− condition. These findings suggest that the level of hyperpolarization and availability of
I
h in thalamic neurons is an important determinant of the behavior of the thalamic network. Slight hyperpolarization is able to increase the rhythmic burst activity in thalamus (as may be the case in the epileptic HCN2
−/− mouse), while further hyperpolarization (e.g., TRIP8b
−/− TC neurons, with downregulation of both HCN2 and HCN4) can abolish or significantly slow down the intrathalamic network oscillations (Yue and Huguenard
2001). Slowing of oscillatory activity was also obtained after introducing the conductance levels,
V
0.5 values and activation kinetics of
I
h obtained from TRIP8b
−/− TC cells to thalamocortical network models (Destexhe et al.
1996). Further analyses with similar models will be an excellent tool for probing the graded effects of
I
h within thalamocortical networks, and for guiding future experiments to understand the dynamic roles of HCN channels in modulating both physiological and pathophysiological brain states.