Long-term Potentiation In Knockout Mice Research Articles

Hippocampal Homer1b/c is necessary for contextual fear conditioning and group I metabotropic glutamate receptor mediated long-term depression.

Coiled-coil forms of Homer1, including Homer1b and c (Homer1b/c) have been shown to play a role in hippocampal learning and memory and synaptic plasticity. We have previously found that overexpression of hippocampal Homer1c is sufficient to rescue learning and memory ability in aged learning impaired rats and in Homer1 knockout (KO) mice, and to rescue group I metabotropic glutamate receptor (mGluR1/5) mediated long-term potentiation in KO mice. Here, to determine if Homer1b/c is necessary for successful learning and memory we have utilized a rAAV5 vector expressing a Homer1b/c-targeting short hairpin RNA to knock down the expression of hippocampal Homer1b/c in adult 4-6-month old male Sprague Dawley rats. We have found that reduced hippocampal Homer1b/c expression elicits significant learning deficits in contextual fear conditioning, but not in the Morris water maze or novel object recognition tasks. Furthermore, we demonstrate that reduced hippocampal Homer1b/c is sufficient to completely block mGluR1/5 mediated long-term depression in the Schaffer collateral pathway. These results support a significant role for Homer1b/c in learning and synaptic plasticity; however, the exact role of each of these two protein isoforms in learning and memory remains elusive.

βCaMKII regulates bidirectional long-term plasticity in cerebellar Purkinje cells by a CaMKII/PP2B switch mechanism

It is thought that modifications in the strength of synaptic connections onto cerebellar Purkinje cells (PCs) such as long-term depression (LTD) and long-term potentiation (LTP) contribute to learning. Repeated paired stimulations of parallel fibres (PFs) and climbing fibre (CF) evoke high-amplitude calcium signals that lead to LTD in PCs, while LTP is induced by low-amplitude calcium pulses as a result of PF input alone. Although the events following these stimulation protocols are becoming clearer, the precise interactions of the molecular machinery underlying LTD and LTP induction remain something of an enigma. Calcium-calmodulin dependent protein kinase II (CaMKII) is an important component of the signalling network that is responsible for cerebellar plasticity in PCs. The CaMKII holoenzyme contains isoforms such as αCaMKII and βCaMKII. Experiments with Camk2b knockout mice that lack the β isoform of CaMKII have demonstrated that βCaMKII regulates the direction of plasticity at PF-PC synapses [1]. Stimulation protocols that induce LTD in wild-type mice, which contain both α and βCaMKII, lead to LTP in knockout mice without βCaMKII, and vice versa. In previous work, we have developed a kinetic simulation of the phosphorylation and dephosphorylation of AMPA receptors by CaMKII and protein phosphatase 2B (PP2B) to investigate how βCaMKII controls bidirectional plasticity in cerebellar PCs [2]. This bidirectional plasticity model was based on our earlier model of CaMKII activation [3,4]. Because the βCaMKII, but not αCaMKII, isoforms can bind to filamentous actin (F-actin), we have included the binding of F-actin to CaMKII to simulate plasticity induction in wild-type mice, whereas the F-actin binding was omitted in the knockout mice that lack the β isoform. Our previous simulation results have demonstrated that the binding of F-actin to βCaMKII can contribute to the regulation of bidirectional plasticity at PF-PC synapses, as suggested by Van Woerden and collaborators [1]. However, we have investigated the induction of bidirectional plasticity only in the presence of calcium signals, i.e. during the early phase of LTD and LTP. But the experimental observations in [1] have demonstrated that the bidirectional plasticity is also observed after the offset of paired PF and CF inputs and PF input alone. To analyse the persistent AMPA receptor phosphorylation after calcium stimulation protocols that last 5 min, we modified our previous model [2] because it did not simulate the CaMKII dephosphorylation, and PP2B was rapidly inactivated with the interruption of calcium signals. Here, we allow the CaMKII subunits in the autonomous form, which are not bound to calcium-calmodulin but still retain some activity levels, to be dephosphorylated and switch to the inactive state of CaMKII. The kinase is therefore gradually inactivated after calcium stimulation stops. Moreover, calcium-calmodulin slowly dissociates from PP2B, leading to a gradual deactivation of PP2B. Results of computer simulations of our mathematical model suggest that the balance of CaMKII-mediated phosphorylation and PP2B-mediated dephosphorylation of AMPA receptors determines whether LTD or LTP occurs at the PF-PC synapse. PF activity alone evokes PP2B concentration levels higher than CaMKII, leading to LTP induction. Instead, LTD occurs when the CaMKII concentration levels surpass the PP2B concentration in response to the paired activation of PF and CF. Moreover, our recent simulation results replicate the experimental observations by Van Woerden et al. [1] not only in the early phase of LTD and LTP as shown in [2], but also in their intermediate and late phases after the termination of PF and CF stimulation and PF input alone. The mechanisms observed in our previous simulations with 5 min duration are also observed in simulations that last 100 min. The loss of F-actin binding in the knockout mice enhances the availability of active CaMKII when compared to the wild-type mice at low calcium concentrations. This favours the induction of LTD rather than LTP. In contrast, the reduction in the CaMKII concentration in the knockout mice leads to AMPA receptor dephosphorylation by PP2B for high calcium concentrations, leading to the induction of LTP instead of LTD. We again demonstrate that the binding of F-actin to βCaMKII can indeed contribute to the control of bidirectional plasticity at PF-PC synapses, as suggested in [1,2].

Aquaporin-4 Deficiency Impairs Synaptic Plasticity and Associative Fear Memory in the Lateral Amygdala: Involvement of Downregulation of Glutamate Transporter-1 Expression

Astrocytes are implicated in information processing, signal transmission, and regulation of synaptic plasticity. Aquaporin-4 (AQP4) is the major water channel in adult brain and is primarily expressed in astrocytes. A growing body of evidence indicates that AQP4 is a potential molecular target for the regulation of astrocytic function. However, little is known about the role of AQP4 in synaptic plasticity in the amygdala. Therefore, we evaluated long-term potentiation (LTP) in the lateral amygdala (LA) and associative fear memory of AQP4 knockout (KO) and wild-type mice. We found that AQP4 deficiency impaired LTP in the thalamo-LA pathway and associative fear memory. Furthermore, AQP4 deficiency significantly downregulated glutamate transporter-1 (GLT-1) expression and selectively increased NMDA receptor (NMDAR)-mediated EPSCs in the LA. However, low concentration of NMDAR antagonist reversed the impairment of LTP in KO mice. Upregulating GLT-1 expression by chronic treatment with ceftriaxone also reversed the impairment of LTP and fear memory in KO mice. These findings imply a role for AQP4 in synaptic plasticity and associative fear memory in the amygdala by regulating GLT-1 expression.

Impairment of Select Forms of Spatial Memory and Neurotrophin-Dependent Synaptic Plasticity by Deletion of Glial Aquaporin-4

Aquaporin-4 (AQP4) is the major water channel in the CNS and is primarily expressed in astrocytes. Little is known about the potential for AQP4 to influence synaptic plasticity, although many studies have shown that it regulates the response of the CNS to injury. Therefore, we evaluated long-term potentiation (LTP) and long-term depression (LTD) in AQP4 knock-out (KO) and wild-type mice. KO mice exhibited a selective defect in LTP and LTD without a change in basal transmission or short-term plasticity. Interestingly, the impairment in LTP in KO mice was specific for the type of LTP that depends on the neurotrophin BDNF, which is induced by stimulation at theta rhythm [theta-burst stimulation (TBS)-LTP], but there was no impairment in a form of LTP that is BDNF independent, induced by high-frequency stimulation. LTD was also impaired in KO mice, which was rescued by a scavenger of BDNF or blockade of Trk receptors. TrkB receptors, which mediate effects of BDNF on TBS-LTP, were not altered in KO mice, but p75NTR, the receptor that binds all neurotrophins and has been implicated in some types of LTD, was decreased. The KO mice also exhibited a cognitive defect, which suggests a new role for AQP4 and astrocytes in normal cognitive function. This defect was evident using a test for location-specific object memory but not Morris water maze or contextual fear conditioning. The results suggest that AQP4 channels in astrocytes play an unanticipated role in neurotrophin-dependent plasticity and influence behavior.