Zinc plays a fundamental role in various brain functions, including the regulation of neurotransmission, endocrine pathways, and neurogenesis (Baltaci et al., 2022; Choi et al., 2020; Gumus et al., 2023; Wang et al., 2023). Dysregulation of zinc homeostasis has been increasingly linked to aging and neurodegenerative conditions such as Alzheimer's disease (AD) (Adlard and Bush, 2018; Brewer, 2012; Danscher et al., 1997; Friedlich et al., 2004; Stoltenberg et al., 2005; Sun et al., 2022).

Zinc plays a critical role in retinal physiology, with the retina containing one of the highest zinc concentrations in the body. Zn is essential for photoreceptor function, visual cycle enzymes, and antioxidant defense systems that protect against light-induced oxidative damage (Ugarte and Osborne, 2001). Dysregulation of zinc metabolism has been implicated in retinal pathologies, including age-related macular degeneration and diabetic retinopathy, making it an important therapeutic target (Gilbert et al., 2019). ZnT3 has been reported to be localized throughout rodent retinal layers, which may suggest its role in zinc homeostasis regulation in the retina (Redenti and Chappell, 2004).

Within the brain, zinc homeostasis is tightly regulated by transporter proteins, notably ZnT3, which mediates zinc efflux into synaptic vesicles (McAllister and Dyck, 2017), and ZIP3, which facilitates zinc influx into cells (Eide, 2004; Palmiter and Huang, 2004; Zhang et al., 2010). ZnT3 is primarily expressed in the brain, particularly in the hippocampus, amygdala and cerebral cortex, where it localised to synaptic vesicle membranes (Beyer et al., 2009). This transporter is particularly abundant in mossy fiber terminals of hippocampal CA3 pyramidal neurons (Wenzel et al., 1997). Within the central nervous system, ZIP3 is abundantly expressed in the hippocampus pyramidal neurons and is found to the plasma membrane and lysosomes (Dufner-Beattie et al., 2005, Qian et al., 2011). Disruption of these transport mechanisms can lead to synaptic dysfunction, contributing to the cognitive decline observed in AD (Bjorklund et al., 2012; Lee et al., 2012; Zhang et al., 2010). Studies have shown that ZnT3 expression declines with age, particularly in the cerebral cortex, and this reduction is exacerbated in AD, correlating with impaired learning and memory (Adlard et al., 2010; Whitfield et al., 2014).

ZnT3 knockout (ZnT3-KO) mice, which lack detectable synaptic vesicle zinc, exhibit significant cognitive deficits, highlighting ZnT3’s critical role in maintaining cognitive function (Adlard et al., 2010; Cole et al., 1999; Martel et al., 2010; Sindreu et al., 2011). Similarly, ZIP3 knockdown models have revealed a reduction in Zn2 + uptake, highlighting the importance of both transporters in maintaining zinc homeostasis(Kelleher and Lönnerdal, 2005; Wang et al., 2004; Zhao and Eide, 1996a, Zhao and Eide, 1996b). Examining ZIP3 (Baltaci and Yuce, 2018) alongside ZnT3 facilitates the determination of whether alterations in zinc influx mechanisms complement ZnT3-related changes in synaptic zinc storage. This provides a more comprehensive perspective on zinc dysregulation in AD, consistent with current understanding of zinc transport proteins. Given the central role of zinc dysregulation in AD pathology, there is growing interest in exploring zinc-related biomarkers for early disease detection.

There are no curative treatments for AD, and early diagnosis is essential for improving patient outcomes. However, the diagnosis of AD relies on invasive, resource intensive tests that require a specific clinical phenotype (amyloid-positive and tau-positive biomarkers), often resulting in underdiagnosis and recognition only at late stages (Dubois et al., 2021). This diagnostic challenge has driven interest in exploring non-invasive methods, particularly the retina, as a potential site for early detection.

The retina, an extension of the central nervous system, shares developmental, anatomical, and functional similarities with the brain, making it an attractive target for AD biomarker research. Notably, amyloid β-protein (Aβ) and neurofibrillary tangles (NFTs), hallmark features of AD, have been identified in retinal tissue (He et al., 2014; Wang and Mao, 2021). Emerging evidence, including studies from our group, has demonstrated parallel alterations in the distribution of transition metals such as copper, iron, and zinc in the retina and brain of AD models, supporting the potential of the retina as a window into cerebral pathology (Hosseinpour Mashkani et al., 2023; Mashkani et al., 2024).

This study aimed to determine whether the expression patterns of zinc transporter proteins ZnT3 and ZIP3 in the retina mirror those observed in the brain during AD progression. Using APP/PS1 and wild-type mouse models at different ages, as well as postmortem human retinal and hippocampal tissues, we investigated age- and disease-related changes in these transporters. Additionally, to explore the functional role of ZnT3 in regulating tissue zinc levels, we quantified zinc concentrations in ZnT3 knockout and wild-type mice.