Seizures exacerbate excitatory: inhibitory imbalance in Alzheimer's disease and 5XFAD mice

Approximately 22% of Alzheimer's disease (AD) patients suffer from seizures, and the co-occurrence of seizures and epileptiform activity exacerbates AD pathology and related cognitive deficits, suggesting that seizures may be a targetable component of AD progression. Given that alterations in neuronal excitatory:inhibitory (E:I) balance occur in epilepsy, we hypothesized that decreased markers of inhibition relative to those of excitation would be present in AD patients. We similarly hypothesized that in five times familial AD (5XFAD) mice, the E:I imbalance would progress from an early stage (prodromal) to later symptomatic stages, and be further exacerbated by pentylenetetrazol (PTZ) kindling. Post-mortem AD temporal cortical tissue from patients with or without seizure history were examined for changes in several markers of E:I balance, including levels of the inhibitory GABAA receptor, the sodium potassium chloride cotransporter 1 (NKCC1) and potassium chloride cotransporter 2 (KCC2), and the excitatory NMDA and AMPA type glutamate receptors. We performed patch clamp electrophysiological recordings from CA1 neurons in hippocampal slices and examined the same markers of E:I balance in prodromal 5XFAD mice. We next examined 5XFAD mice at chronic stages, after PTZ or control protocols, and in response to chronic mTORC1 inhibitor rapamycin, administered following kindled seizures, for markers of E:I balance. We found that AD patients with comorbid seizures had worsened cognitive and functional scores and had decreased GABAA receptor subunit expression, and increased NKCC1/KCC2 ratios, indicative of depolarizing GABA responses. Patch clamp recordings of prodromal 5XFAD CA1 neurons showed increased intrinsic excitability, along with decreased GABAergic inhibitory transmission and altered glutamatergic neurotransmission, indicating that E:I imbalance may occur in early disease stages. Furthermore, seizure induction in prodromal 5XFAD mice led to later dysregulation of NKCC1/KCC2 and a reduction in GluA2 AMPA glutamate receptor subunit expression, indicative of depolarizing GABA receptors and calcium permeable AMPA receptors. Finally, we found that chronic treatment with the mTORC1 inhibitor, rapamycin, at doses we have previously shown to attenuate seizure-induced β-amyloid pathology and cognitive deficits, could also reverse elevations to NKCC1/KCC2 ratio in these mice. Our data demonstrate novel mechanisms of interaction between AD and epilepsy and indicate that targeting E:I balance, potentially with FDA-approved mTOR inhibitors, hold therapeutic promise for AD patients with a seizure history.

Reversible synaptic adaptations in a subpopulation of murine hippocampal neurons following early-life seizures

Early-life seizures (ELSs) can cause permanent cognitive deficits and network hyperexcitability, but it is unclear whether ELSs induce persistent changes in specific neuronal populations and whether these changes can be targeted to mitigate network dysfunction. We used the targeted recombination of activated populations (TRAP) approach to genetically label neurons activated by kainate-induced ELSs in immature mice. The ELS-TRAPed neurons were mainly found in hippocampal CA1, remained uniquely susceptible to reactivation by later-life seizures, and displayed sustained enhancement in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated (AMPAR-mediated) excitatory synaptic transmission and inward rectification. ELS-TRAPed neurons, but not non-TRAPed surrounding neurons, exhibited enduring decreases in Gria2 mRNA, responsible for encoding the GluA2 subunit of the AMPARs. This was paralleled by decreased synaptic GluA2 protein expression and heightened phosphorylated GluA2 at Ser880 in dendrites, indicative of GluA2 internalization. Consistent with increased GluA2-lacking AMPARs, ELS-TRAPed neurons showed premature silent synapse depletion, impaired long-term potentiation, and impaired long-term depression. In vivo postseizure treatment with IEM-1460, an inhibitor of GluA2-lacking AMPARs, markedly mitigated ELS-induced changes in TRAPed neurons. These findings show that enduring modifications of AMPARs occur in a subpopulation of ELS-activated neurons, contributing to synaptic dysplasticity and network hyperexcitability, but are reversible with early IEM-1460 intervention.

The role of mTORC1 activation in seizure-induced exacerbation of Alzheimer's disease

The risk of seizures is 10-fold higher in patients with Alzheimer's disease than the general population, yet the mechanisms underlying this susceptibility and the effects of these seizures are poorly understood. To elucidate the proposed bidirectional relationship between Alzheimer's disease and seizures, we studied human brain samples (n = 34) from patients with Alzheimer's disease and found that those with a history of seizures (n = 14) had increased amyloid-β and tau pathology, with upregulation of the mechanistic target of rapamycin (mTOR) pathway, compared with patients without a known history of seizures (n = 20). To establish whether seizures accelerate the progression of Alzheimer's disease, we induced chronic hyperexcitability in the five times familial Alzheimer's disease mouse model by kindling with the chemoconvulsant pentylenetetrazol and observed that the mouse model exhibited more severe seizures than the wild-type. Furthermore, kindled seizures exacerbated later cognitive impairment, Alzheimer's disease neuropathology and mTOR complex 1 activation. Finally, we demonstrated that the administration of the mTOR inhibitor rapamycin following kindled seizures rescued enhanced remote and long-term memory deficits associated with earlier kindling and prevented seizure-induced increases in Alzheimer's disease neuropathology. These data demonstrated an important link between chronic hyperexcitability and progressive Alzheimer's disease pathology and suggest a mechanism whereby rapamycin may serve as an adjunct therapy to attenuate progression of the disease.

Dysregulation of GABAA Receptor-Mediated Neurotransmission during the Auditory Cortex Critical Period in the Fragile X Syndrome Mouse Model

Fragile X syndrome (FXS) is the leading monogenic form of intellectual disability and autism, with patients exhibiting numerous auditory-related phenotypes during their developmental period, including communication, language development, and auditory processing deficits. Despite FXS studies describing excitatory-inhibitory (E-I) imbalance in the auditory circuit and an impaired auditory critical period, evaluation of E-I circuitry maturation in the auditory cortex of FXS models remains limited. Here, we examined GABAA receptor (GABAAR)-mediated inhibitory synaptic transmission within the auditory cortex, characterizing normal intracortical circuit development patterns in wild-type (WT) mice and examining their dysregulation in developing Fmr1 knock-out (KO) mice. Electrophysiological recordings revealed gradual developmental shifts in WT L4-L2/3 connectivity, where circuit excitability significantly increased after critical period onset. KO mice exhibited accelerated developmental shifts related to aberrant GABAergic signaling. Specifically, Fmr1 KO L2/3 pyramidal neurons have enhanced developmental sensitivity to pharmacological GABAAR modulators, altered maturation of GABAAR voltage-dependent conductance, with additional presynaptic GABA release alterations. These differences are further accompanied by alterations in developmental long-term potentiation. Together, our results suggest that altered GABAergic signaling within developing Fmr1 KOs impairs the normal patterning of E-I circuit and synaptic plasticity maturation to contribute to the impaired auditory cortex critical period and functional auditory deficits in FXS.

HIV-1 Tat and Morphine Differentially Disrupt Pyramidal Cell Structure and Function and Spatial Learning in Hippocampal Area CA1: Continuous versus Interrupted Morphine Exposure

About half the people infected with human immunodeficiency virus (HIV) have neurocognitive deficits that often include memory impairment and hippocampal deficits, which can be exacerbated by opioid abuse. To explore the effects of opioids and HIV on hippocampal CA1 pyramidal neuron structure and function, we induced HIV-1 transactivator of transcription (Tat) expression in transgenic mice for 14 d and co-administered time-release morphine or vehicle subcutaneous implants during the final 5 d (days 9-14) to establish steady-state morphine levels. Morphine was withheld from some ex vivo slices during recordings to begin to assess the initial pharmacokinetic consequences of opioid withdrawal. Tat expression reduced hippocampal CA1 pyramidal neuronal excitability at lower stimulating currents. Pyramidal cell firing rates were unaffected by continuous morphine exposure. Behaviorally, exposure to Tat or high dosages of morphine impaired spatial memory Exposure to Tat and steady-state levels of morphine appeared to have largely independent effects on pyramidal neuron structure and function, a response that is distinct from other vulnerable brain regions such as the striatum. By contrast, acutely withholding morphine (from morphine-tolerant ex vivo slices) revealed unique and selective neuroadaptive shifts in CA1 pyramidal neuronal excitability and dendritic plasticity, including some interactions with Tat. Collectively, the results show that opioid-HIV interactions in hippocampal area CA1 are more nuanced than previously assumed, and appear to vary depending on the outcome assessed and on the pharmacokinetics of morphine exposure.

Restoration of KCC2 Membrane Localization in Striatal Dopamine D2 Receptor-Expressing Medium Spiny Neurons Rescues Locomotor Deficits in HIV Tat-Transgenic Mice

People infected with HIV (PWH) are highly susceptible to striatal and hippocampal damage. Motor and memory impairments are common among these patients, likely as behavioral manifestations of damage to these brain regions. GABAergic dysfunction from HIV infection and viral proteins such as transactivator of transcription (Tat) have been well documented. We recently demonstrated that the neuron specific Cl- extruder, K+ Cl- cotransporter 2 (KCC2), is diminished after exposure to HIV proteins, including Tat, resulting in disrupted GABAAR-mediated hyperpolarization and inhibition. Here, we utilized doxycycline (DOX)-inducible, GFAP-driven HIV-1 Tat transgenic mice to further explore this phenomenon. After two weeks of Tat expression, we found no changes in hippocampal KCC2 levels, but a significant decrease in the striatum that was associated with hyperlocomotion in the open field assay. We were able to restore KCC2 activity and baseline locomotion with the KCC2 enhancer, CLP290. Additionally, we found that CLP290, whose mechanism of action has yet to be described, acts to restore phosphorylation of serine 940 resulting in increased KCC2 membrane localization. We also examined neuronal subpopulation contributions to the noted effects and found significant differences. Dopamine D2 receptor-expressing medium spiny neurons (MSNs) were selectively vulnerable to Tat-induced KCC2 loss, with no changes observed in dopamine D1 receptor-expressing MSNs. These results suggest that disinhibition/diminished hyperpolarization of dopamine D2 receptor-expressing MSNs can manifest as increased locomotion in this context. They further suggest that KCC2 activity might be a therapeutic target to alleviate motor disturbances related to HIV.

HIV and opiates dysregulate K+- Cl- cotransporter 2 (KCC2) to cause GABAergic dysfunction in primary human neurons and Tat-transgenic mice

Approximately half of people infected with HIV (PWH) exhibit HIV-associated neuropathology (neuroHIV), even when receiving combined antiretroviral therapy. Opiate use is widespread in PWH and exacerbates neuroHIV. While neurons themselves are not infected, they incur sublethal damage and GABAergic disruption is selectively vulnerable to viral and inflammatory factors released by infected/affected glia. Here, we demonstrate diminished K+-Cl- cotransporter 2 (KCC2) levels in primary human neurons after exposure to HIV-1 or HIV-1 proteins ± morphine. Resulting disruption of GABAAR-mediated hyperpolarization/inhibition was shown using genetically-encoded voltage (Archon1) and calcium (GCaMP6f) indicators. The HIV proteins Tat (acting through NMDA receptors) and R5-gp120 (acting via CCR5) but not X4-tropic gp120 (acting via CXCR4), and morphine (acting through μ-opioid receptors) all induced KCC2 loss. We demonstrate that modifying KCC2 levels or function, or antagonizing NMDAR, CCR5 or MOR rescues KCC2 and GABAAR-mediated hyperpolarization/inhibition in HIV, Tat, or gp120 ± morphine-exposed neurons. Using an inducible, Tat-transgenic mouse neuroHIV model, we found that chronic exposure to Tat also reduces KCC2. Our results identify KCC2 as a novel therapeutic target for ameliorating the pathobiology of neuroHIV, including PWH exposed to opiates.