Inhibition of an Alzheimer's disease-associated form of necroptosis rescues neuronal death in mouse models

Theaflavin suppresses necroptosis by attenuating RIPK1-RIPK3-MLKL signaling and mitigates cisplatin-induced kidney injury in mice

Necroptosis is a lytic form of regulated cell death (RCD) that is dependent on receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain like pseudokinase (MLKL). This form of RCD has been implicated in various inflammatory diseases and organ injuries including cisplatin-induced acute kidney injury (AKI), thus representing a therapeutic target for such diseases. Theaflavin is an ingredient of black tea that exhibits beneficial effects on human health and has been shown to regulate pyroptosis, but its effects on necroptosis and cisplatin-induced AKI remain unclear. In this study, we found that theaflavin suppressed necroptosis in murine macrophages, MPC-5 podocytes and human HT-29 cells treated with TNF-α, Smac mimetic and IDN-6556 or LPS plus IDN-6556. The RIPK1/RIPK3/MLKL signaling axis in these cells treated with necroptosis inducers was effectively inhibited by theaflavin. The inhibition of necroptotic signaling was associated with attenuated mitochondrial dysfunction (as evidenced by decreased mitochondrial membrane potential and increased mitochondrial ROS production), reduced ubiquitination of RIPK1 and RIPK3, and blockade of necrosome. Furthermore, oral administration of theaflavin mitigated renal and hepatic injury in a mouse model of cisplatin-induced AKI. In agreement with in vitro cellular data, theaflavin decreased the levels of phosphorylated MLKL, an in vivo biomarker for necroptosis, in macrophages and other cells in the kidney and the liver of mice with cisplatin-induced AKI. Collectively, these results indicate that theaflavin can suppress necroptosis by attenuating RIPK1/RIPK3/MLKL signaling and thereby conferring protection against cisplatin-induced AKI, uncovering a previously unappreciated action of black tea components against necroptosis-related disorders.

Alzheimer's disease neuropathology and its estimation with fluid and imaging biomarkers

Alzheimer's disease (AD) is neuropathologically characterized by the extracellular deposition of the amyloid-β peptide (Aβ) and the intraneuronal accumulation of abnormal phosphorylated tau (τ)-protein (p-τ). Most frequently, these hallmark lesions are accompanied by other co-pathologies in the brain that may contribute to cognitive impairment, such as vascular lesions, intraneuronal accumulation of phosphorylated transactive-response DNA-binding protein 43 (TDP-43), and/or α-synuclein (αSyn) aggregates. To estimate the extent of these AD and co-pathologies in patients, several biomarkers have been developed. Specific tracers target and visualize Aβ plaques, p-τ and αSyn pathology or inflammation by positron emission tomography. In addition to these imaging biomarkers, cerebrospinal fluid, and blood-based biomarker assays reflecting AD-specific or non-specific processes are either already in clinical use or in development. In this review, we will introduce the pathological lesions of the AD brain, the related biomarkers, and discuss to what extent the respective biomarkers estimate the pathology determined at post-mortem histopathological analysis. It became evident that initial stages of Aβ plaque and p-τ pathology are not detected with the currently available biomarkers. Interestingly, p-τ pathology precedes Aβ deposition, especially in the beginning of the disease when biomarkers are unable to detect it. Later, Aβ takes the lead and accelerates p-τ pathology, fitting well with the known evolution of biomarker measures over time. Some co-pathologies still lack clinically established biomarkers today, such as TDP-43 pathology or cortical microinfarcts. In summary, specific biomarkers for AD-related pathologies allow accurate clinical diagnosis of AD based on pathobiological parameters. Although current biomarkers are excellent measures for the respective pathologies, they fail to detect initial stages of the disease for which post-mortem analysis of the brain is still required. Accordingly, neuropathological studies remain essential to understand disease development especially in early stages. Moreover, there is an urgent need for biomarkers reflecting co-pathologies, such as limbic predominant, age-related TDP-43 encephalopathy-related pathology, which is known to modify the disease by interacting with p-τ. Novel biomarker approaches such as extracellular vesicle-based assays and cryptic RNA/peptides may help to better detect these co-pathologies in the future.

Retinal ganglion cell vulnerability to pathogenic tau in Alzheimer's disease

Pathological tau isoforms, including hyperphosphorylated tau at serine 396 (pS396-tau) and tau oligomers (Oligo-tau), are elevated in the retinas of patients with mild cognitive impairment (MCI) due to Alzheimer's disease (AD) and AD dementia. These patients exhibit significant retinal ganglion cell (RGC) loss, however the presence of tau isoforms in RGCs and their impact on RGC integrity, particularly in early AD, have not been studied. Here, we analyzed retinal superior temporal cross-sections from 25 MCI or AD patients and 16 age- and sex-matched cognitively normal controls. Using the RGC marker ribonucleic acid binding protein with multiple splicing (RBPMS) and Nissl staining, we found a 46-56% reduction in RBPMS+ RGCs and Nissl+ neurons in the ganglion cell layer (GCL) of MCI and AD retinas (P < 0.05-0.001). RGC loss was accompanied by soma hypertrophy (10-50% enlargement, P < 0.05-0.0001), nuclear displacement, apoptosis (30-50% increase, P < 0.05-0.01), and prominent expression of granulovacuolar degeneration (GVD) bodies and GVD-necroptotic markers. Both pS396-tau and Oligo-tau were identified in RGCs, including in hypertrophic cells. PS396-tau+ and Oligo-tau+ RGC counts were significantly increased by 2.1-3.5-fold in MCI and AD retinas versus control retinas (P < 0.05-0.0001). Tauopathy-laden RGCs strongly inter-correlated (rP=0.85, P < 0.0001) and retinal tauopathy associated with RGC reduction (rP=-0.40-(-0.64), P < 0.05-0.01). Their abundance correlated with brain pathology and cognitive deficits, with higher tauopathy-laden RGCs in patients with Braak stages (V-VI), clinical dementia ratings (CDR = 3), and mini-mental state examination (MMSE ≤   26) scores. PS396-tau+ RGCs in the central and mid-periphery showed the closest associations with disease status, while Oligo-tau+ RGCs in the mid-periphery exhibited the strongest correlations with brain pathology (NFTs, Braak stages, ABC scores; rS=0.78-0.81, P < 0.001-0.0001) and cognitive decline (MMSE; rS=-0.79, P = 0.0019). Overall, these findings identify a link between pathogenic tau in RGCs and RGC degeneration in AD, involving apoptotic and GVD-necroptotic cell death pathways. Future research should validate these results in larger and more diverse cohorts and develop RGC tauopathy as a potential noninvasive biomarker for early detection and monitoring of AD progression.

Therapeutic implications of necroptosis activation in Alzheimer´s disease

In recent years, a growing body of research has unveiled the involvement of the necroptosis pathway in the pathogenesis of Alzheimer's disease (AD). This evidence has shed light on the mechanisms underlying neuronal death in AD, positioning necroptosis at the forefront as a potential target for therapeutic intervention. This review provides an update on the current knowledge on this emerging, yet rapidly advancing topic, encompassing all published studies that present supporting proof of the role of the necroptosis pathway in the neurodegenerative processes of AD. The implication of misfolded tau and amyloid-β (Aβ) aggregates is highlighted, with evidence suggesting their direct or indirect involvement in necroptosis activation. In summary, the review underscores the significance of understanding the complex interplay between necroptosis, protein aggregates, and neurodegeneration in AD. The findings advocate for a comprehensive approach, combining therapeutic and early diagnostic strategies, to intervene in the disease process before irreversible damage occurs.