Store depletion-induced h-channel plasticity rescues a channelopathy linked to Alzheimer's disease

Tau-mediated synaptic dysfunction is coupled with HCN channelopathy

INTRODUCTION

In tauopathies, altered tau processing correlates with impairments in synaptic density and function. Changes in hyperpolarization-activated cyclic nucleotide-gated (HCN) channels contribute to disease-associated abnormalities in multiple neurodegenerative diseases.

METHODS

To investigate the link between tau and HCN channels, we performed histological, biochemical, ultrastructural, and functional analyses of hippocampal tissues from Alzheimer's disease (AD), age-matched controls, Tau35 mice, and/or Tau35 primary hippocampal neurons.

RESULTS

Expression of specific HCN channels is elevated in post mortem AD hippocampus. Tau35 mice develop progressive abnormalities including increased phosphorylated tau, enhanced HCN channel expression, decreased dendritic branching, reduced synapse density, and vesicle clustering defects. Tau35 primary neurons show increased HCN channel expression enhanced hyperpolarization-induced membrane voltage "sag" and changes in the frequency and kinetics of spontaneous excitatory postsynaptic currents.

DISCUSSION

Our findings are consistent with a model in which pathological changes in tauopathies impact HCN channels to drive network-wide structural and functional synaptic deficits.

HIGHLIGHTS

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are functionally linked to the development of tauopathy. Expression of specific HCN channels is elevated in the hippocampus in Alzheimer's disease and the Tau35 mouse model of tauopathy. Increased expression of HCN channels in Tau35 mice is accompanied by hyperpolarization-induced membrane voltage "sag" demonstrating a detrimental effect of tau abnormalities on HCN channel function. Tau35 expression alters synaptic organization, causing a loosened vesicle clustering phenotype in Tau35 mice.

Alzheimer's-linked axonal changes accompany elevated antidromic action potential failure rate in aged mice

Alzheimer's disease (AD) affects both grey and white matter (WM), but considerably more is known about the former. Interestingly, WM disruption has been consistently observed and thoroughly described using imaging modalities, particularly MRI which has shown WM functional disconnections between the hippocampus and other brain regions during AD pathogenesis when early neurodegeneration and synapse loss are also evident. Nonetheless, high-resolution structural and functional analyses of WM during AD pathogenesis remain scarce. Given the importance of the myelinated axons in the WM for conveying information across brain regions, such studies will provide valuable information on the cellular drivers and consequences of WM disruption that contribute to the characteristic cognitive decline of AD. Here, we employed a multi-scale approach to investigate hippocampal WM disruption during AD pathogenesis and determine whether hippocampal WM changes accompany the well-documented grey matter losses. Our data indicate that ultrastructural myelin disruption is elevated in the alveus in human AD cases and increases with age in 5xFAD mice. Unreliable action potential propagation and changes to sodium channel expression at the node of Ranvier co-emerged with this deterioration. These findings provide important insight to the neurobiological substrates and functional consequences of decreased WM integrity and are consistent with the notion that hippocampal disconnection contributes to cognitive changes in AD.

VTA dopamine neurons are hyperexcitable in 3xTg-AD mice due to casein kinase 2-dependent SK channel dysfunction

Alzheimer's disease (AD) patients exhibit neuropsychiatric symptoms that extend beyond classical cognitive deficits, suggesting involvement of subcortical areas. Here, we investigated the role of midbrain dopamine (DA) neurons in AD using the amyloid + tau-driven 3xTg-AD mouse model. We found deficits in reward-based operant learning in AD mice, suggesting possible VTA DA neuron dysregulation. Physiological assessment revealed hyperexcitability and disrupted firing in DA neurons caused by reduced activity of small-conductance calcium-activated potassium (SK) channels. RNA sequencing from contents of single patch-clamped DA neurons (Patch-seq) identified up-regulation of the SK channel modulator casein kinase 2 (CK2). Pharmacological inhibition of CK2 restored SK channel activity and normal firing patterns in 3xTg-AD mice. These findings shed light on a complex interplay between neuropsychiatric symptoms and subcortical circuits in AD, paving the way for novel treatment strategies.

Lamotrigine rescues neuronal alterations and prevents seizure-induced memory decline in an Alzheimer's disease mouse model

Epilepsy is a comorbidity associated with Alzheimer's disease (AD), often starting many years earlier than memory decline. Investigating this association in the early pre-symptomatic stages of AD can unveil new mechanisms of the pathology as well as guide the use of antiepileptic drugs to prevent or delay hyperexcitability-related pathological effects of AD. We investigated the impact of repeated seizures on hippocampal memory and amyloid-β (Aβ) load in pre-symptomatic Tg2576 mice, a transgenic model of AD. Seizure induction caused memory deficits and an increase in oligomeric Aβ₄₂ and fibrillary species selectively in pre-symptomatic transgenic mice, and not in their wildtype littermates. Electrophysiological patch-clamp recordings in ex vivo CA1 pyramidal neurons and immunoblots were carried out to investigate the neuronal alterations associated with the behavioral outcomes of Tg2576 mice. CA1 pyramidal neurons exhibited increased intrinsic excitability and lower hyperpolarization-activated I_h current. CA1 also displayed lower expression of the hyperpolarization-activated cyclic nucleotide-gated HCN1 subunit, a protein already identified as downregulated in the AD human proteome. The antiepileptic drug lamotrigine restored electrophysiological alterations and prevented both memory deficits and the increase in extracellular Aβ induced by seizures. Thus our study provides evidence of pre-symptomatic hippocampal neuronal alterations leading to hyperexcitability and associated with both higher susceptibility to seizures and to AD-specific seizure-induced memory impairment. Our findings also provide a basis for the use of the antiepileptic drug lamotrigine as a way to counteract acceleration of AD induced by seizures in the early phases of the pathology.

Altered membrane properties but unchanged intrinsic excitability and spontaneous postsynaptic currents in an aged APPswe/PS1dE9 model of Alzheimer’s disease

Neuronal hyperexcitability in Alzheimer’s disease (AD) models is thought to either contribute to the formation of amyloid beta plaques or result from their formation. Neuronal hyperexcitability has been shown in the cerebral cortex of the widely used young APPswe/PS1dE9 mice, which have accelerated plaque formation. However, it is currently unclear if hyperexcitability also occurs in CA1 hippocampal neurons of aged animals in this model. In the present work, we have compared intrinsic excitability and spontaneous synaptic inputs from CA1 pyramidal cells of 8-month-old APPswe/PS1dE9 and wildtype control mice. We find no change in intrinsic excitability or spontaneous postsynaptic currents (PSCs) between groups. We did, however, find a reduced input resistance and an increase in hyperpolarization-activated sag current. These results are consistent with findings from other aged AD model mice, including the widely used 5xFAD and 3xTg. Together these results suggest that neuronal hyperexcitability is not a consistent feature of all AD mouse models, particularly at advanced ages.

Variability in sub-threshold signaling linked to Alzheimer's disease emerges with age and amyloid plaque deposition in mouse ventral CA1 pyramidal neurons

The hippocampus is vulnerable to deterioration in Alzheimer's disease (AD). It is, however, a heterogeneous structure, which may contribute to the differential volumetric changes along its septotemporal axis during AD progression. Here, we investigated amyloid plaque deposition along the dorsoventral axis in two strains of transgenic AD (ADTg) mouse models. We also used patch-clamp physiology in these mice to probe for functional consequences of AD pathogenesis in ventral hippocampus, which we found bears significantly higher plaque burden in the aged ADTg group compared to corresponding dorsal regions. Despite dorsoventral differences in amyloid load, ventral CA1 pyramidal neurons of aged ADTg mice exhibited subthreshold physiological changes similar to those previously reported in dorsal neurons, indicative of an HCN channelopathy, but lacked exacerbated suprathreshold accommodation. Additionally, HCN channel function could be rescued by pharmacological manipulation of the endoplasmic reticulum. These observations suggest that an AD-linked HCN channelopathy emerges in both dorsal and ventral CA1 pyramidal neurons, but that the former encounter an additional integrative obstacle in the form of reduced intrinsic excitability.

Physiology and Therapeutic Potential of SK, H, and M Medium AfterHyperPolarization Ion Channels

SK, HCN, and M channels are medium afterhyperpolarization (mAHP)-mediating ion channels. The three channels co-express in various brain regions, and their collective action strongly influences cellular excitability. However, significant diversity exists in the expression of channel isoforms in distinct brain regions and various subcellular compartments, which contributes to an equally diverse set of specific neuronal functions. The current review emphasizes the collective behavior of the three classes of mAHP channels and discusses how these channels function together although they play specialized roles. We discuss the biophysical properties of these channels, signaling pathways that influence the activity of the three mAHP channels, various chemical modulators that alter channel activity and their therapeutic potential in treating various neurological anomalies. Additionally, we discuss the role of mAHP channels in the pathophysiology of various neurological diseases and how their modulation can alleviate some of the symptoms.

Cognitive aging is associated with redistribution of synaptic weights in the hippocampus

Behaviors that rely on the hippocampus are particularly susceptible to chronological aging, with many aged animals (including humans) maintaining cognition at a young adult-like level, but many others the same age showing marked impairments. It is unclear whether the ability to maintain cognition over time is attributable to brain maintenance, sufficient cognitive reserve, compensatory changes in network function, or some combination thereof. While network dysfunction within the hippocampal circuit of aged, learning-impaired animals is well-documented, its neurobiological substrates remain elusive. Here we show that the synaptic architecture of hippocampal regions CA1 and CA3 is maintained in a young adult-like state in aged rats that performed comparably to their young adult counterparts in both trace eyeblink conditioning and Morris water maze learning. In contrast, among learning-impaired, but equally aged rats, we found that a redistribution of synaptic weights amplifies the influence of autoassociational connections among CA3 pyramidal neurons, yet reduces the synaptic input onto these same neurons from the dentate gyrus. Notably, synapses within hippocampal region CA1 showed no group differences regardless of cognitive ability. Taking the data together, we find the imbalanced synaptic weights within hippocampal CA3 provide a substrate that can explain the abnormal firing characteristics of both CA3 and CA1 pyramidal neurons in aged, learning-impaired rats. Furthermore, our work provides some clarity with regard to how some animals cognitively age successfully, while others' lifespans outlast their "mindspans."

The structure and function of TRIP8b, an auxiliary subunit of hyperpolarization-activated cyclic-nucleotide gated channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed throughout the mammalian central nervous system (CNS). These channels have been implicated in a wide range of diseases, including Major Depressive Disorder and multiple subtypes of epilepsy. The diversity of functions that HCN channels perform is in part attributable to differences in their subcellular localization. To facilitate a broad range of subcellular distributions, HCN channels are bound by auxiliary subunits that regulate surface trafficking and channel function. One of the best studied auxiliary subunits is tetratricopeptide-repeat containing, Rab8b-interacting protein (TRIP8b). TRIP8b is an extensively alternatively spliced protein whose only known function is to regulate HCN channels. TRIP8b binds to HCN pore-forming subunits at multiple interaction sites that differentially regulate HCN channel function and subcellular distribution. In this review, we summarize what is currently known about the structure and function of TRIP8b isoforms with an emphasis on the role of this auxiliary subunit in health and disease.

Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels as Drug Targets for Neurological Disorders

The hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are voltage-gated ion channels that critically modulate neuronal activity. Four HCN subunits (HCN1-4) have been cloned, each having a unique expression profile and distinctive effects on neuronal excitability within the brain. Consistent with this, the expression and function of these subunits are altered in diverse ways in neurological disorders. Here, we review current knowledge on the structure and distribution of the individual HCN channel isoforms, their effects on neuronal activity under physiological conditions, and how their expression and function are altered in neurological disorders, particularly epilepsy, neuropathic pain, and affective disorders. We discuss the suitability of HCN channels as therapeutic targets and how drugs might be strategically designed to specifically act on particular isoforms. We conclude that medicines that target individual HCN isoforms and/or their auxiliary subunit, TRIP8b, may provide valuable means of treating distinct neurological conditions.

Cardiac and neuronal HCN channelopathies

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed as four different isoforms (HCN1-4) in the heart and in the central and peripheral nervous systems. In the voltage range of activation, HCN channels carry an inward current mediated by Na⁺ and K⁺, termed I_f in the heart and I_h in neurons. Altered function of HCN channels, mainly HCN4, is associated with sinus node dysfunction and other arrhythmias such as atrial fibrillation, ventricular tachycardia, and atrioventricular block. In recent years, several data have also shown that dysfunctional HCN channels, in particular HCN1, but also HCN2 and HCN4, can play a pathogenic role in epilepsy; these include experimental data from animal models, and data collected over genetic mutations of the channels identified and characterized in epileptic patients. In the central nervous system, alteration of the I_h current could predispose to the development of neurodegenerative diseases such as Parkinson's disease; since HCN channels are widely expressed in the peripheral nervous system, their dysfunctional behavior could also be associated with the pathogenesis of neuropathic pain. Given the fundamental role played by the HCN channels in the regulation of the discharge activity of cardiac and neuronal cells, the modulation of their function for therapeutic purposes is under study since it could be useful in various pathological conditions. Here we review the present knowledge of the HCN-related channelopathies in cardiac and neurological diseases, including clinical, genetic, therapeutic, and physiopathological aspects.

Battling Neurodegenerative Diseases with Adeno-Associated Virus-Based Approaches

Neurodegenerative diseases (NDDs) are most commonly found in adults and remain essentially incurable. Gene therapy using AAV vectors is a rapidly-growing field of experimental medicine that holds promise for the treatment of NDDs. To date, effective delivery of a therapeutic gene into target cells via AAV has been a major obstacle in the field. Ideally, transgenes should be delivered into the target cells specifically and efficiently, while promiscuous or off-target gene delivery should be minimized to avoid toxicity. In the pursuit of an ideal vehicle for NDD gene therapy, a broad variety of vector systems have been explored. Here we specifically outline the advantages of adeno-associated virus (AAV)-based vector systems for NDD therapy application. In contrast to many reviews on NDDs that can be found in the literature, this review is rather focused on AAV vector selection and their testing in experimental and preclinical NDD models. Preclinical and in vitro data reveal the strong potential of AAV for NDD-related diagnostics and therapeutic strategies.

Regulation of intrinsic excitability: Roles for learning and memory, aging and Alzheimer's disease, and genetic diversity

Plasticity of intrinsic neuronal excitability facilitates learning and memory across multiple species, with aberrant modulation of this process being linked to the development of neurological symptoms in models of cognitive aging and Alzheimer's disease. Learning-related increases in intrinsic excitability of neurons occurs in a variety of brain regions, and is generally thought to promote information processing and storage through enhancement of synaptic throughput and induction of synaptic plasticity. Experience-dependent changes in intrinsic neuronal excitability rely on activity-dependent gene expression patterns, which can be influenced by genetic and environmental factors, aging, and disease. Reductions in baseline intrinsic excitability, as well as aberrant plasticity of intrinsic neuronal excitability and in some cases pathological hyperexcitability, have been associated with cognitive deficits in animal models of both normal cognitive aging and Alzheimer's disease. Genetic factors that modulate plasticity of intrinsic excitability likely underlie individual differences in cognitive function and susceptibility to cognitive decline. Thus, targeting molecular mediators that either control baseline intrinsic neuronal excitability, subserve learning-related intrinsic neuronal plasticity, and/or promote resilience may be a promising therapeutic strategy for maintaining cognitive function in aging and disease. In this review, we discuss the complementary relationship between intrinsic excitability and learning, with a particular focus on how this relationship varies as a function of age, disease state, and genetic make-up, and how targeting these factors may help to further elucidate our understanding of the role of intrinsic excitability in cognitive function and cognitive decline.

Harnessing ionic mechanisms to achieve disease modification in neurodegenerative disorders

Progressive neuronal death is the key pathogenic event leading to clinical symptoms in neurodegenerative disorders (NDDs). Neuroprotective treatments are virtually unavailable, partly because of the marked internal heterogeneity of the mechanisms underlying pathology. Targeted neuroprotection would require deep mechanistic knowledge across the entire aetiological spectrum of each NDD and the development of tailored treatments. Although ideal, this strategy appears challenging, as it would require a degree of characterization of both the disease and the patient that is currently unavailable. The alternate strategy is to search for commonalities across molecularly distinct NDD forms and exploit these for the development of drugs with broad-spectrum efficacy. In this view, mounting evidence points to ionic mechanisms (IMs) as targets with potential therapeutic efficacy across distinct NDD subtypes. The scope of this review is to present clinical and preclinical evidence supporting the link between disruption of IMs and neuronal death in specific NDDs and to critically revise past and ongoing attempts of harnessing IMs for the development of neuroprotective treatments.

Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: An Emerging Role in Neurodegenerative Diseases

Neurodegenerative diseases such as Parkinson’s disease (PD), Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), and spinal muscular atrophy (SMA) are chronic, progressive, and age-associated neurological disorders characterized by neuronal deterioration in specific brain regions. Although the specific pathological mechanisms underlying these disorders have remained elusive, ion channel dysfunction has become increasingly accepted as a potential mechanism for neurodegenerative diseases. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are encoded by the HCN1-4 gene family and conduct the hyperpolarization-activated current (I_h). These channels play important roles in modulating cellular excitability, rhythmic activity, dendritic integration, and synaptic transmission. In the present review, we first provide a comprehensive picture of the role of HCN channels in PD by summarizing their role in the regulation of neuronal activity in PD-related brain regions. Dysfunction of I_h may participate in 1-methyl-4-phenylpyridinium (MPP⁺)-induced toxicity and represent a pathogenic mechanism in PD. Given current reports of the critical role of HCN channels in neuroinflammation and depression, we also discussed the putative contribution of HCN channels in inflammatory processes and non-motor symptoms in PD. In the second section, we summarize how HCN channels regulate the formation of β-amyloid peptide in AD and the role of these channels in learning and memory. Finally, we briefly discuss the effects of HCN channels in ALS and SMA based on existing discoveries.