AtaGenix Co-Developed Tau368 Antibody Empowers Discovery: Calbindin-D28k Loss Mediates Tau-Driven Hippocampal Hyperexcitability and Cognitive Impairment

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder worldwide, and no effective preventive or therapeutic strategy currently exists. Clinical data indicate that 42%–64% of AD patients experience overt seizures or subclinical epileptic activity, with early-onset AD patients (aged 50–60) facing an 87-fold higher seizure risk compared to age-matched healthy individuals. Intracranial recordings have shown that silent hippocampal seizures and interictal spikes emerge during the preclinical stage of AD, preceding the onset of cognitive symptoms.

Among the two hallmark pathologies of AD, amyloid-β (Aβ) plaques preferentially accumulate in the medial parietal and frontal cortices, whereas Tau pathology first targets the entorhinal–hippocampal network during the prodromal stage. PET imaging data further reveal that in early-AD patients who go on to develop temporal lobe epilepsy (TLE), Tau deposition is highly asymmetric across hemispheres centered on the seizure focus, while Aβ asymmetry is comparatively modest. Nevertheless, how Tau pathology disrupts the hippocampal excitation–inhibition balance to promote TLE remains poorly understood. A research team at Zhongnan Hospital of Wuhan University, publishing in Translational Neurodegeneration, investigated the pivotal role of the calcium-binding protein Calbindin-D28k (CB) in this pathological cascade, systematically delineating:

Selective Accumulation of Tau in Hippocampal Excitatory Neurons

The research team employed a self-constructed Tg hTau368 transgenic mouse model, which uses a tetracycline-inducible (Tet-on) system driven by the neuron-specific promoter Eno2. Administration of doxycycline (Dox) induces expression of a truncated human Tau fragment (hTau368). Following two months of Dox treatment, mice exhibited phosphorylated Tau (pTau) aggregation predominantly in the hippocampus. Immunofluorescence co-staining revealed that pTau (labeled by AT8 or pTau205) co-localized with CaMKIIα-positive excitatory neurons in hippocampal CA1 and DG regions, with minimal overlap with PV- or GAD67-positive inhibitory neurons.

The Tau368 antibody used for pTau detection was co-developed by AtaGenix, providing a critical tool for precise pTau characterization throughout the experimental series. A similar pattern of selective Tau accumulation was observed in PR5 mice (overexpressing P301L mutant human Tau), further validating this distribution profile.

Figure 1. pTau accumulates predominantly in excitatory neurons of the hippocampus in TgTau-driven mice

Tau Aggregation Impairs Calcium Buffering Capacity in Hippocampal CA1 Neurons

To assess the impact of Tau aggregation on calcium dynamics, researchers injected AAV-CaMKIIα-GCaMP6f (a genetically encoded calcium indicator) into the CA1 region of Tg hTau368 mice and recorded intracellular calcium signals following KCl-induced depolarization using ex vivo calcium imaging. CA1 neurons in the Dox-treated group (with Tau pathology) showed significantly greater calcium signal changes after KCl stimulation compared to controls, while baseline fluorescence did not differ between groups. This indicates that pTau aggregation does not affect resting calcium levels, but instead impairs the neurons' ability to buffer calcium transients — a finding consistent with prior studies using Fluo-3 AM to record full-length hTau-expressing neurons.

Figure 2. Tau protein aggregation in hippocampal CA1 excitatory neurons disrupts intracellular calcium signaling

Hyperactivation of CA1/DG Neurons Triggers Acute Seizures, and Tau Pathology Heightens Seizure Susceptibility

To clarify the roles of CA1 and DG in TLE, researchers first performed local injections of kainic acid (KA) into the CA1 or DG of wild-type mice, successfully inducing characteristic seizure waveforms and confirming that hyperactivation of CA1/DG neurons is a critical driver of acute seizure generation. Using an optogenetic approach, they then delivered AAV-CaMKIIα-ChR2-mCherry into the CA1 and DG of Tg hTau368 mice and selectively activated excitatory neurons with blue light. Both dorsal and ventral hippocampal stimulation triggered seizures of varying severity, with in vivo electrophysiological recordings capturing typical ictal waveforms.

Crucially, Dox-treated Tg hTau368 mice exhibited shorter latency to generalized seizures (GS) and greater seizure severity compared to vehicle-treated controls, indicating that Tau pathology significantly lowers the seizure threshold of the hippocampal DG–CA1–EC–DG circuit.

Figure 3. Tau protein aggregation promotes susceptibility to acute seizures driven by hippocampal microcircuit hyperexcitability

Tau Pathology Drives Cerebral Hypermetabolism, Hyperexcitability Behavioral Phenotypes, and Cognitive Deficits

Given the age-dependent nature of AD, researchers induced hTau expression in 14–15-month-old Tg hTau368 mice and performed ¹⁸F-FDG PET/CT scanning, energy metabolism monitoring, and cognitive behavioral testing. PET/CT results revealed that after 1–2 months of Dox treatment, glucose metabolism in the hippocampus and olfactory bulb was significantly elevated, mirroring the cerebral hypermetabolism seen in early-stage AD. This hypermetabolic state was accompanied by increased oxygen consumption (VO₂) and higher overall energy expenditure, particularly during daytime rest periods.

Behaviorally, Dox-treated mice showed significantly reduced discrimination indices in the novel location recognition test and markedly prolonged escape latencies during Morris water maze training, reflecting clear spatial memory impairment. Hippocampal neuronal loss and glial activation were also confirmed in Dox-treated animals.

Figure 4. Aged TgTau-driven mice exhibit cerebral hypermetabolism, elevated energy expenditure, and cognitive deficits

Tau Pathology Reversibly Suppresses CB Expression and Impairs Synaptic Function

Leveraging the temporal control afforded by the Tet-on system, researchers established three experimental groups: continuous Dox treatment (Dox-on), Dox treatment for two months followed by three months of withdrawal (Dox-on-off), and vehicle control (Veh). Immunohistochemistry and Western blot analyses showed that pTau (AT8) levels in CA1 and DG were markedly elevated in the Dox-on group, with a concurrent downregulation of CB; upon Dox withdrawal (Dox-on-off group), Tau pathology largely resolved and CB expression rebounded accordingly, with synaptic proteins including PSD95 and synapsin-1 (SYN-1) recovering in parallel.

Immunofluorescence co-staining further confirmed that CB downregulation occurred specifically within AT8-positive neurons, while CB expression was relatively preserved in AT8-negative cells — pointing to a direct intracellular link between CB loss and Tau aggregation.

Figure 5. Tau pathology downregulates CB expression in the hippocampus and exacerbates Iba1+ microglial proliferation

CB Overexpression Reverses Tau-Induced Neuronal Hyperexcitability, Neuroinflammation, and Cognitive Impairment

To directly test whether CB loss mediates Tau-related neuronal hyperexcitability, researchers delivered either AAV-CaMKIIα-eGFP (control) or AAV-CaMKIIα-Calb1-eGFP (CB overexpression) into the CA1 and DG of Tg hTau368 mice via AAV vectors, establishing three groups: Veh+eGFP, Dox+eGFP, and Dox+CB. Whole-cell patch-clamp recordings revealed that CA1 pyramidal neurons in the Dox+eGFP group displayed significantly elevated amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs) compared to Veh+eGFP controls; CB overexpression (Dox+CB group) effectively reversed these changes. Furthermore, Dox+eGFP neurons exhibited elevated resting membrane potential, reduced rheobase, and increased evoked action potential frequency — all of which were restored to near-normal levels by CB overexpression. CB overexpression also significantly attenuated Tau pathology-induced Iba1+ microglial proliferation, suggesting an additional anti-neuroinflammatory effect.

Figure 6. CB overexpression in hippocampal excitatory neurons alleviates neuronal hyperexcitability caused by Tau aggregation

At the behavioral level, Dox+CB mice crossed the target quadrant significantly more frequently than Dox+eGFP mice in the Morris water maze, and showed markedly improved discrimination performance in the novel location recognition test. Spontaneous locomotor activity in the open field test did not differ across groups, ruling out confounding motor effects.

Figure 7. CB upregulation alleviates cognitive deficits caused by hippocampal Tau aggregation

CB Downregulation in AD Patient Brains Correlates with Cognitive Decline and Disease Stage

Using publicly available AD databases, researchers analyzed CB (CALB1 gene) expression at both the transcript and protein levels. Compared to healthy controls, CALB1 mRNA levels were significantly reduced in AD patient brains. Stratified analyses by cognitive function (CDR scale) and neuropathological severity (Braak staging) revealed a progressive decline in CB transcription with worsening CDR scores and advancing Braak stages. At the protein level, CB was similarly reduced in AD brains, with the degree of loss correlating with the severity of cognitive decline and clinical symptom burden. Immunohistochemical staining further confirmed a marked reduction in CB protein in hippocampal tissue from a 65-year-old AD patient, in stark contrast to an age-matched control.

Figure 8. CB deficiency is associated with cognitive decline in AD patients

This study systematically defines a Tau-driven pathological cascade underlying neuronal hyperexcitability: Tau aggregation occurs selectively in excitatory neurons of the hippocampal CA1 and DG regions, driving CB downregulation and impairing intracellular calcium buffering. The resulting disruption of calcium homeostasis leads to synaptic protein loss, elevated neuronal excitability, and heightened seizure susceptibility, ultimately culminating in cognitive impairment. CB overexpression, by restoring calcium homeostasis, effectively ameliorates this entire spectrum of pathological changes — positioning CB-mediated calcium regulation as a promising therapeutic target in AD. Corroborating evidence from AD patient databases further establishes the clinical relevance of CB loss in relation to disease progression and cognitive deterioration.