Impact of blunting astrocyte activity on hippocampal synaptic plasticity in a mouse model of early Alzheimer's disease based on amyloid-β peptide exposure

Experimental modeling of Alzheimer's disease: Translational lessons from cross-taxon analyses

Alzheimer's disease (AD) is a severely debilitating neurodegenerative disease with a rapidly increasing global prevalence, poorly understood causes, and no efficient treatments. Experimental models are valuable for studying AD pathogenesis, including amyloid beta and tau accumulation, synaptic dysfunction, and neuroinflammation. While no model fully reproduces the disease, we take an evolutionary biology approach to discuss available models across taxa, from mammals (rodents, primates) to zebrafish, Drosophila melanogaster, and Caenorhabditis elegans. Evaluating their strengths and limitations provides insight into disease mechanisms and may refine research strategies for improved diagnostics and therapeutic screening. Traditional models have significantly contributed to AD research, yet their translational limitations highlight the need for physiologically relevant alternatives. Integrating humanized rodent models, zebrafish, organoids, and induced pluripotent stem cell-based systems-along with advances in bioengineering and genetic editing-may offer a more comprehensive framework to bridge the gap between preclinical research and clinical application. HIGHLIGHTS: Experimental models across rodents, primates, zebrafish, fruit flies, and worms provide key insights into Alzheimer's disease (AD). Cross-taxon comparisons assess strengths and weaknesses in AD models. Evolutionary biology approaches refine experimental strategies for AD research. Diverse animal models improve understanding of AD pathogenesis. Cross-species models enhance diagnostics and therapeutic strategy development.

Astrocytic NLRP3 cKO mitigates depression-like behaviors induced by mild TBI in mice

BACKGROUND

Reports indicate that depression is a common mental health issue following traumatic brain injury (TBI). Our prior research suggests that Nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3)-related neuroinflammation, modulated by glial cells such as astrocytes, is likely to play a crucial role in the progression of anxiety and cognitive dysfunction. However, there is limited understanding of the potential of astrocytic NLRP3 in treating depression under mild TBI condition. This study aimed to determine whether astrocytic NLRP3 knockout (KO) could mitigate depressive-like behaviors following mild TBI and explore potential variations in such behaviors between genders post-mild TBI.

METHODS

Mild TBI was induced in mice using Feeney's weight-drop method. Behavioral assessments included neurological severity scores (NSS), social interaction test (SI), tail suspension test (TST), and forced swimming test (FST). Pathological changes were evaluated through immunofluorescence and local field potential (LFP) recordings at various time points post-injury.

RESULTS

Our findings indicated that astrocyte-specific NLRP3 KO decreased cleaved caspase-1 colocalized with astrocytes, decreased pathogenic astrocytes and increased Postsynaptic density protein 95 (PSD95) intensity, and significantly alleviated mild TBI-induced depression-like behaviors. It also led to the upregulation of protective astrocytes and apoptosis-associated factors, including cleaved caspase-3 post-mild TBI. Additionally, astrocyte-specific NLRP3 deletion resulting in improved θ and γ power and θ-γ phase coupling in the social interaction test (SI). Notably, under mild TBI conditions, astrocyte-specific NLRP3 exhibited greater neuroprotective effects in female knockout mice compared to males.

CONCLUSION

Astrocyte NLRP3 knockout demonstrated a protective mechanism in mice subjected to mild TBI, possibly attributed to the inhibition of pyroptosis through the NLRP3 signaling pathway in astrocytes.

Neuroprotective effect of naringin by modulation of klotho and HMGB1- TLR4 axis in PTZ-induced kindling in mice

BACKGROUND

Naringin has demonstrated various neuroprotective effects; however, its anti-inflammatory and cognitive properties, particularly through the regulation of HMGB1-TLR4 and Klotho, have not been explored in the context of epilepsy.

METHOD

Kindling was induced in Swiss albino mice by administering pentylenetetrazole (PTZ) 25 mg/kg intraperitoneally (i.p.). Naringin (40 mg/kg and 80 mg/kg) was administered orally for 6 weeks. The severity of seizures was assessed using the Racine scale. Cognitive function was evaluated by measuring step-down latency and transfer latency. The levels of GABA, glutamate, IL-1β, IL-1R1, IL-6, HMGB1, TLR4, TNF-α, Klotho, and ADAM-10 were quantified using enzyme-linked immunosorbent assay (ELISA) techniques.

RESULTS

Naringin significantly attenuated PTZ-induced seizures at both doses (p < 0.01 for 40 mg/kg; p < 0.0001 for 80 mg/kg) compared to the PTZ group. Additionally, it enhanced retention latency in both step-down latency (p < 0.01 for 40 mg/kg; p < 0.0001 for 80 mg/kg) and transfer latency (p < 0.05 for both doses) compared to the PTZ group. Furthermore, it increased Klotho and ADAM-10 levels in both the hippocampus and cortex (p < 0.01 for 40 mg/kg; p < 0.001 for 80 mg/kg, respectively). Levels of HMGB1, TLR4, and pro-inflammatory cytokines were significantly decreased in both the hippocampus and cortex compared to the PTZ group.

CONCLUSION

Naringin exhibited anti-epileptic effects by regulating neurotransmitter levels and preventing PTZ-induced kindling. Additionally, it demonstrated neuroprotective effects on cognition and attenuated neuroinflammation. These findings suggest that naringin may be a potential therapeutic agent for epilepsy-associated cognitive dysfunction, warranting further studies for clinical translation.

Modification of astrocytic Cx43 hemichannel activity in animal models of AD: modulation by adenosine A2A receptors

Increasing evidence implicates astrocytic dysfunction in Alzheimer's disease (AD), a neurodegenerative disorder characterised by progressive cognitive loss. The accumulation of amyloid-β (Aβ) plaques is a histopathological hallmark of AD and associated with increased astrocyte reactivity. In APP/PS1 mice modelling established AD (9 months), we now show an altered astrocytic morphology and enhanced activity of astrocytic hemichannels, mainly composed by connexin 43 (Cx43). Hemichannel activity in hippocampal astrocytes is also increased in two models of early AD: (1) mice with intracerebroventricular (icv) administration of Aβ1-42, and (2) hippocampal slices superfused with Aβ1-42 peptides. In hippocampal gliosomes of APP/PS1 mice, Cx43 levels were increased, whereas mice administered icv with Aβ1-42 only displayed increased Cx43 phosphorylation levels. This suggests that hemichannel activity might be differentially modulated throughout AD progression. Additionally, we tested if adenosine A2A receptor (A2AR) blockade reversed alterations of astrocytic hemichannel activity and found that the pharmacological blockade or genetic silencing (global and astrocytic) of A2AR prevented Aβ-induced hemichannel dysregulation in hippocampal slices, although A2AR genetic silencing increased the activity of astroglial hemichannels in control conditions. In primary cultures of astrocytes, A2AR-related protective effect was shown to occur through a protein kinase C (PKC) pathway. Our results indicate that the dysfunction of hemichannel activity in hippocampal astrocytes is an early event in AD, which is modulated by A2AR.

Astrocyte- and NMDA receptor-dependent slow inward currents differently contribute to synaptic plasticity in an age-dependent manner in mouse and human neocortex

Slow inward currents (SICs) are known as excitatory events of neurons elicited by astrocytic glutamate via activation of extrasynaptic NMDA receptors. By using slice electrophysiology, we tried to provide evidence that SICs can elicit synaptic plasticity. Age dependence of SICs and their impact on synaptic plasticity was also investigated in both on murine and human cortical slices. It was found that SICs can induce a moderate synaptic plasticity, with features similar to spike timing-dependent plasticity. Overall SIC activity showed a clear decline with aging in humans and completely disappeared above a cutoff age. In conclusion, while SICs contribute to a form of astrocyte-dependent synaptic plasticity both in mice and humans, this plasticity is differentially affected by aging. Thus, SICs are likely to play an important role in age-dependent physiological and pathological alterations of synaptic plasticity.

Adenosine A2A Receptor Up-Regulation Pre-Dates Deficits of Synaptic Plasticity and of Memory in Mice Exposed to Aβ1-42 to Model Early Alzheimer's Disease

The intracerebroventricular (icv) injection of amyloid peptides (Aβ) models Alzheimer's disease (AD) in mice, as typified by the onset within 15 days of deficits of memory and of hippocampal long-term potentiation (LTP) that are prevented by the blockade of adenosine A2A receptors (A2AR). Since A2AR overfunction is sufficient to trigger memory deficits, we tested if A2AR were upregulated in hippocampal synapses before the onset of memory deficits to support the hypothesis that A2AR overfunction could be a trigger of AD. Six to eight days after Aβ-icv injection, mice displayed no alterations of hippocampal dependent memory; however, they presented an increased excitability of hippocampal synapses, a slight increase in LTP magnitude in Schaffer fiber-CA1 pyramid synapses and an increased density of A2AR in hippocampal synapses. A2AR blockade with SCH58261 (50 nM) normalized excitability and LTP in hippocampal slices from mice sacrificed 7-8 days after Aβ-icv injection. Fifteen days after Aβ-icv injection, mice displayed evident deficits of hippocampal-dependent memory deterioration, with reduced hippocampal CA1 LTP but no hyperexcitability and a sustained increase in synaptic A2AR, which blockade restored LTP magnitude. This shows that the upregulation of synaptic A2AR precedes the onset of deterioration of memory and of hippocampal synaptic plasticity, supporting the hypothesis that the overfunction of synaptic A2AR could be a trigger of memory deterioration in AD.

Astrocytic A2A receptors silencing negatively impacts hippocampal synaptic plasticity and memory of adult mice

Astrocytes are wired to bidirectionally communicate with neurons namely with synapses, thus shaping synaptic plasticity, which in the hippocampus is considered to underlie learning and memory. Adenosine A_2A receptors (A_2A R) are a potential candidate to modulate this bidirectional communication, since A_2A R regulate synaptic plasticity and memory and also control key astrocytic functions. Nonetheless, little is known about the role of astrocytic A_2A R in synaptic plasticity and hippocampal-dependent memory. Here, we investigated the impact of genetic silencing astrocytic A_2A R on hippocampal synaptic plasticity and memory of adult mice. The genetic A_2A R silencing in astrocytes was accomplished by a bilateral injection into the CA1 hippocampal area of a viral construct (AAV5-GFAP-GFP-Cre) that inactivate A_2A R expression in astrocytes of male adult mice carrying "floxed" A_2A R gene, as confirmed by A_2A R binding assays. Astrocytic A_2A R silencing alters astrocytic morphology, typified by an increment of astrocytic arbor complexity, and led to deficits in spatial reference memory and compromised hippocampal synaptic plasticity, typified by a reduction of LTP magnitude and a shift of synaptic long-term depression (LTD) toward LTP. These data indicate that astrocytic A_2A R control astrocytic morphology and influence hippocampal synaptic plasticity and memory of adult mice in a manner different from neuronal A_2A R.

The role of ADAM10 in astrocytes: Implications for Alzheimer's disease

Much of the early research into AD relies on a neuron-centric view of the brain, however, evidence of multiple altered cellular interactions between glial cells and the vasculature early in AD has been demonstrated. As such, alterations in astrocyte function are widely recognized a contributing factor in the pathogenesis of AD. The processes by which astrocytes may be involved in AD make them an interesting target for therapeutic intervention, but in order for this to be most effective, there is a need for the specific mechanisms involving astrocyte dysfunction to be investigated. "α disintegrin and metalloproteinase" 10 (ADAM10) is capable of proteolytic cleavage of the amyloid precursor protein which prevents amyloid-β generation. As such ADAM10 has been identified as an interesting enzyme in AD pathology. ADAM10 is also known to play a role in a significant number of cellular processes, most notable in notch signaling and in inflammatory processes. There is a growing research base for the involvement of ADAM10 in regulating astrocytic function, primarily from an immune perspective. This review aims to bring together available evidence for ADAM10 activity in astrocytes, and how this relates to AD pathology.