Alzheimer’s Disease (AD) is one of the main progressive neurodegenerative disorders and the most common cause of dementia affecting principally the elderly (Bateman et al., 2012). The histopathology is characterized by brain atrophy, deposits of amyloid β (Aβ) peptides in the extracellular matrix, neurofibrillary tangles (mainly tau protein), loss of neurons and synapses and dystrophic neurites (Hansen et al., 2018).

The increased number of Aβ amyloid plaques in the extracellular matrix, due to the imbalance in the production vs. clearance, is believed to be the principal pathogenic mechanism (Selkoe and Hardy, 2016). Familial AD is characterized by excessive production of Aβ, caused by a mutation in amyloid precursor protein (APP) or in the APP processing enzyme. However, Familial AD is extremely rare, while the majority of the AD cases are “sporadic” and occur late in life. Late AD is thought to be a result of genetic and environmental factors, and mainly aging that reduce the brain’s ability to clear Aβ (Mawuenyega et al., 2010; Wildsmith et al., 2013).

Microglial activation was initially thought to be incidental and triggered by the accumulation of amyloid deposits. Interestingly, it has been established recently that many genes found in or near AD risk loci are genes mainly expressed in microglia (Hemonnot et al., 2019). Among these genes, Apoliprotein E (APOE), SP1l, TREM2, or CD33 code for proteins that are expressed principally or exclusively in microglia (Verheijen and Sleegers, 2018).

Microglia, the major inflammatory cells of the brain, play a dual role in AD. On the one hand, they play a protective role by clearing the deposits of Aβ peptides increasing the phagocytic activity (Miners et al., 2011) and on the other hand, they play a cytotoxic role by releasing cytotoxic substances and pro-inflammatory cytokines in response to reactive gliosis activated by the Aβ aggregates (Glass et al., 2010).

Neuroinflammation is driven mostly by glial cells such as microglia and astrocytes (Forloni and Balducci, 2018). Microglia, which represent around 10–15% of human brain cells, are immune cells that first respond to nervous system changes (Kwon and Koh, 2020; Liu et al., 2021). Microglia are categorized mainly in two opposite phenotypes depending on their specific markers that define cell type and state; the pro-inflammatory M1 (classical activation) and M2 (alternative activation) phenotypes (Mills et al., 2000; Bi et al., 2021). Depending on the activated phenotype, microglia can produce either cytotoxic or neuroprotective effects. The classical activation is related to pro-inflammatory cytokine production, such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and reactive oxygen species (ROS) or nitric oxide (NO) production. Meanwhile, the alternative activation promotes anti-inflammatory response with increased IL-4, IL-10, CD36 and phagocytic response expressing CD163, insulin-like growth factor 1 (IGF-1) and brain derived neurotrophic factor (BDNF) (Le et al., 2016; Chen et al., 2021; Figure 1). The microglial polarization has not been supported by single-cell RNA-seq, while this transcriptomic analysis have been used to distinguish between diseased associated microglia (DAM) or activated response microglia (ARM) (Keren-Shaul et al., 2017; Sala Frigerio et al., 2019). Transcriptome data show that during neurodegenerative diseases both phenotypes, the neurotoxic and the neuroprotective, are expressed (Sarlus and Heneka, 2017; Figure 1).

During AD, microglial activation is associated with Aβ deposits in human and mice brains (Kamphuis et al., 2012; Olmos-Alonso et al., 2016). Indeed, activated microglia in AD mouse models express increased inflammatory markers CD36, CD14, CD11c, major histocompatibility complex-II (MHC-II), and inducible nitric oxide synthase (iNOS), as well as M1 phenotype markers.

The inflammatory response is typically composed of three main stages. First, toll-like receptor (TLR)-mediated NF-κB formation. This leads to an increase of mainly members of the nod-like receptor (NLR) that assemble the inflammasome that third, activates caspase-1, which cleaves the precursor protein and release IL-1β (Yin et al., 2016).

While anti-inflammatory cytokines may have a deleterious role in AD (Guillot-Sestier et al., 2015), TLR activation and IL-1β secretion may also have protective effects (Shaftel et al., 2007; Richard et al., 2008).

In healthy brains, microglia regulate the correct development and function of synapses and synaptic plasticity. Microglia-synapse disruption may contribute to synapse loss, dysfunction and, consequently, disease (Hong et al., 2016). There are many studies concerning the effect of changes in cytokines, chemokines or ROS production in microglial activation. However, little is known about the effects of changes in the intracellular ionic homeostasis and microglial activation (Izquierdo et al., 2019). Ion channels are involved in many microglial functions, such as cytokine production, migration, production or proliferation, among others.

Microglia membrane express different ion channels, such as Ca²⁺-, K⁺-, Na⁺-, H⁺- and Cl^–-channels, in order to face all physiological functions. For instance, Ca²⁺-channels are important for intracellular Ca²⁺ homeostasis in microglia. Store-operated Ca²⁺-channels, voltage-gated Ca²⁺-channels and transient receptor potential channels control Ca²⁺ signaling for microglial activation (Luo et al., 2021). Inward rectifier channels and voltage-gated K⁺-channels as well as Cl^–-channels (volume regulated Cl^–-channels and chloride intracellular channels), are not only necessary for cell hyperpolarization, but for cell activation and proliferation. This is because they supply the driving force that allow an increase intracellular Ca²⁺ concentration via Ca²⁺-channels (Nguyen et al., 2017).

Voltage-gated and acid-sensing Na⁺-channels are also relevant in these non-excitable cells to regulate cell migration, phagocytosis, and secretion of cytokines (Pappalardo et al., 2016). Finally, voltage-gated H⁺-channels are also important to regulate cells’ pH.

Glial cells express the voltage-gated K⁺-channels K_V1.3 and K_V1.5. These channels activity changes in microglial activation by modifying their relative expression. In fact, one characteristic of activated inflammatory cells is an increased expression and function of the K_V1.3 channels (Pérez-Verdaguer et al., 2016). In quiescent cells K_V1.5 regulates the proliferation rate (Pannasch et al., 2006; Gubiè et al., 2021) while K_V1.3 is dominant in activated microglial cells (Gubiè et al., 2021). Besides, K_V1.5 seems to be essential for NO production (Pannasch et al., 2006), but causes cell cycle arrest. On the other hand, K_V1.3 participates in microglial proliferation and migration, as well as in the cytokine release (Charolidi et al., 2015; Stebbing et al., 2015). LPS-mediated microglial activation induces cytokine release, but decreases proliferation (Pannasch et al., 2006). In response to Aβ accumulation K_V1.3, K_V1.5 and calcium-activated K⁺-channels (KCa3.1, KCa2.3, or BK channels) increase voltage-dependent Ca²⁺ entry provoking a disruption in Ca²⁺ homeostasis and consequently neurodegeneration (Dolga et al., 2012; Kumar et al., 2016; Huang et al., 2021). Among all these channels, the most studied is K_V1.3.

There is a controversy about human neurogenesis. Some authors conclude that hippocampal neurogenesis is extremely rare in the adult brain, as they did not detect new neurons in the dental gyrus (Sorrells et al., 2018). However, other authors observed immature neurons, neuroblast and neural progenitor cells in aged human hippocampus (Boldrini et al., 2018; Tobin et al., 2019).

In the adult mammalian brain, neural stem cells (NSCs) are localized in two major neurological niches, the subgranular zone of the hippocampus (SGZ) and the subventricular zone of the lateral ventricle (SVZ). These cells retain the ability to proliferate and differentiate into neurons and glial cells (Moreno-Cugnon et al., 2019).

Some studies report that microglia in the hippocampus are more active than in other brain regions, playing an important role in refining neuronal circuits (Rao et al., 2022). Furthermore, microglia release several cytokines that promote microglial migration, neuroblast generation and neurogenesis and is considered a crucial component for determine NSC fate (Shigemoto-Mogami et al., 2014; Geribaldi-Doldán et al., 2021).

The hippocampus is one of the most affected brain regions in AD with altered dentate granule cells. There have been several studies using NSC/induced pluripotent stem cells (iPSCs) derived from Alzheimer patients with the objective of promoting neurogenesis and ameliorating the progression of the disease (Wu et al., 2021). In one of these studies, for example, they demonstrate that the release of pro-inflammatory cytokines, such as TNF-α, IL-1β, and IGF-1 by microglia enhance the dopaminergic differentiation of neural stem cells and promote neurogenesis (Boyd et al., 2021; Schmidt et al., 2021).

As mentioned, impaired CX3CR1 has been described in AD (Guedes et al., 2018). In the hippocampus, this impairment has been linked to adult hippocampal neurogenesis disruption, with spatial and fear-memory and motor learning loss due mainly to the increase of IL-1β by microglial activation (Parkitny and Maletic-Savatic, 2021). Remarkably, the importance of IL-1β in adult hippocampal neurogenesis was recognized over a decade ago when it was associated with anti-proliferative and anti-neurogenic effects (Crampton et al., 2012). Moreover, PRR seem to be implicated in the modulation of adult neurogenesis as they are expressed also in neural progenitor cells (NPCs), providing communication pathways from apoptotic or injured cells (Parkitny and Maletic-Savatic, 2021).

To date, there is no treatment to cure or prevent AD. Current treatments are only useful in slowing down the progression of the disease and in managing some behavioral and cognitive symptoms of AD patients. Because of the complex pathophysiology, sometimes the treatment needs to be a combination of therapies. Currently, only six treatments are approved in the US. These include three cholinesterase inhibitors (donepezil, galantamine, and rivastigmine), one N-methyl-D-aspartate receptor antagonist (memantine) (Cummings et al., 2019), a fixed-dose combination with donepezil and memantine and finally, the recently approved aducanumab, a human monoclonal antibody that targets, and reduces Aβ accumulations in the brain.

As mentioned, during AD there is a release of cytotoxic substances and pro-inflammatory cytokines by the M1 activation provoking neuronal damage and aggravating AD pathology (Heneka et al., 2015). In this M1 activated state, K_V1.3 channels are upregulated. Little is known about the mechanism in which these channels are activated and the consequences in AD. Recent studies have analyzed the effect of K_V1.3 blockers on microglial profiles in AD models and confirm that pro-inflammatory and neurotoxic microglia functions are reduced with different K_V1.3 inhibitors (Figure 2B). K_V1.3 blockade decreases cerebral amyloid load, enhances hippocampal neuronal plasticity, and improves behavioral deficits by a reduction of microglia activation and inflammatory cytokines levels in transgenic AD mouse models (Chen et al., 2018; Maezawa et al., 2018; Ma et al., 2020). Accordingly, K_V1.3 blockers inhibit microglia-mediated neurotoxicity in culture (Fordyce et al., 2005) and protect mice from microglia-mediated radiation-induced brain injury in vivo (Peng et al., 2014).

Classification of K_V1.3 channel blockers depends on their selectivity and blockade potency. The most effective inhibitors are the natural peptides such as the sea anemone Stichodactyla helianthus toxin ShK and scorpion toxins HsTx, OSK1 and Vm24. These molecules present high affinity for different channels. An analog of Shk, the ShK-223, diminished the activity of the pro-inflammatory microglia and elevated Aβ clearance in AD models (Rangaraju et al., 2018; Ramesha et al., 2021). HsTX1 [R14A] mutant, is a potent, selective and highly stable peptide inhibitor. It has been shown that this inhibitor reduces the release of TNF-α and IL-6 by LPS-mediated BV-2 microglia activation improving neuroinflammation (Nicolazzo et al., 2022).

Beside these peptides, there are different synthetic organic small-molecules inhibitors such as, PAP−1 [5-(4-phenoxybutoxy) psoralen], Psora−4, dihydroquinoline, benzamides, clofazimine, furoquinoline, acridinone, furochromene−7−thione, diphenoxylate, and several analogs are used. All these inhibitors differ in the potency to block K_V1.3 channel and the selectivity for the target (Gubiè et al., 2021).

In rodents, PAP-1 and ShK-223 reduce the expression and production of several cytokines (IL-1β, IL-4, IL-5, IL-10, IL-12, IFNγ and TNF-α) (Nguyen et al., 2017; Zayas-Arrabal et al., 2021) and decreased Aβ plaque burden in the 5xFAD mice brain. Furthermore, these molecules increased Aβ phagocytosis by microglia and not blood derived monocytes due to the K_V1.3 channel blockade (Maezawa et al., 2018; Ramesha et al., 2021). The pharmacological blockade of the channel in AD mice model promotes synaptogenesis and polarizes microglial phenotype toward M2 (Ramesha et al., 2021).

However, the use of pharmacological K_V1.3 blockers may have several adverse effects. ShK and HsTX1 blockers, for instance, have difficulties to penetrating the intestinal mucosa so they can’t be taken orally and they neither can cross the BBB. Therefore, there is a need for cell penetrating peptides to assist the passage of the drug across the cell membrane (Wang et al., 2020). Other problem relates to the selective of K_V1.3 blockade. It is important to mention that K_V1.3 is also expressed in mitochondria. This channel controls cell-proliferation and has an important role in cellular respiration (Styles et al., 2021). Thus, the use of some small-molecule K_V1.3 blockers as therapy could also block mitochondrial channels, inducing apoptosis in cancer cells (Teisseyre et al., 2019). According to recent studies, the most potent and selective small-molecule K_V1.3 inhibitor available is PAP-1 (Peruzzo et al., 2020).

However, PAP-1, PSORA-4 and some derivatives produce apoptosis in cancer cell lines. These blockers inhibit K_V1.3 channels both at the plasma and mitochondrial membranes, causing an increased ROS production and, finally, apoptosis. The activation of the apoptotic pathway by these inhibitors is mainly due to cancer cells’ massive ROS release (Checchetto et al., 2019). Therefore, the therapeutic use of these inhibitors in AD may induce microglial apoptosis by further increasing the cell’s basal level of ROS. Additional studies need to be performed in order to determine the cytotoxicity of this K_V1.3 channel blocker in AD.

In summary, K_V1.3 plays important roles in regulating membrane potential, preventing depolarization and controlling Ca²⁺ signaling events reducing microglia activation. However, further investigation is needed to achieve a deeper understanding of the role of K_V1.3 in the microglial immune response and to identify specific pathways for enhancement of Aβ plaque formation or NPC differentiation. Moreover, the potential pharmacological use of drugs targeting K_V1.3 channels requires further characterization.

MR and JU wrote the article. AV and OC reviewed. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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