Alzheimer’s disease, pet cats, feline neuropathology, aging, feline cognitive dysfunction syndrome

Alzheimer’s disease (AD) is the most common neurodegenerative disease in humans, accounting for nearly two thirds of dementia cases and affecting roughly 35 million people worldwide [1]. The disease is characterized by pathologic accumulations of two types of protein aggregates in specifc brain regions, which include plaques of amyloid beta (Aβ) peptide in the neuropil, and neurofbrillary tangles (NFTs) composed of hyperphosphorylated tau (pTau) protein found both within neurons and within the neuropil as ghost tangles [2-3], but generally both of these accumulations are thought to be necessary for progressive neuronal dysfunction, neuronal loss and cognitive dysfunction in AD. The focus of pharmacologic trials has shifted towards treating individuals in early stages of disease, with the goal to slow or prevent disease progression and limit disability [3-4]. Genetically engineered rodent models have been useful to identify candidate drugs for early preclinical studies, however, candidate drugs validated in these models have generally been unsuccessful in clinical trials [5]. Challenges translating therapies from mice to humans may be explained by inherent differences in neuroanatomy, lifespan, and physiology between the species, as well as the fact that most mouse models result from manipulation of one or more specific genes implicated in less common familial forms of the disease rather than sporadic AD. Understanding the mechanisms and pathogenesis in an animal model that naturally develops neuropathologic lesions similar to AD is almost certainly necessary for development of effective translational therapeutic strategies targeting early stages of the disease.
Several animal species have been shown to develop one or more age-related lesions that are comparable to AD. Ideally, animal models should possess 2 histopathologic hallmarks of AD: Aβ peptide deposition (plaques) and tau pathology (NFTs) [1]. In general, nonhuman primates (NHPs) and dogs develop spontaneous Aβ deposition with age, but do not reliably recapitulate tau pathology [6-7]. Wild-type rodents do not spontaneously form plaques or NFTs, but genetically engineered mice have been developed that have single, double or multiple mutations in genes responsible for the production of the Aβ and/or tau proteins. Plaques are formed in these models by overexpression of mutant human amyloid precursor protein (APP), resulting in overexpression of total Aβ beyond physiologic levels or increasing the proportion of Aβ^1-42, which is more prone to pathogenic aggregation and has a higher neurotoxicity profle comparted to Aβ^1-40. NFTs occur in some mouse models expressing mutations of microtubule associated protein tau (MAPT), which causefrontotemporal dementia, not AD, in people. Although helpful in preclinical trials, candidate drugs tested in rodent transgenic models have very poor performance in clinical trials [5]. By contrast, a limited number of studies have shown that domestic cats can spontaneously develop both histopathologic hallmarks of AD (Aβ deposition and NFTs), as well as associated neuronal loss, in a pattern of distribution similar to humans that progresses with age [5- 6] (Table 1). Additional studies involving large cohorts are needed to further characterize neuropathologic similarities between pet cats and humans and to correlate neuronal lesions with cognitive dysfunction.

Aβ plaques are derived from amyloid precursor protein (APP), which is an integral membrane protein made by neurons and other brain cells, coded by the APP gene [3]. When APP is processed, it is cleaved at an extracellular domain followed by an intracellular domain, and follows one of two pathways. The non-amyloidogenic pathway involves cleavage of the extracellular domain of APP by an α-secretase, followed by intramembrous -secretase cleavage, which prohibits the production of Aβ peptide. Conversely, the amyloidogenic pathway involves cleavage of the extracellular domain of APP followed by intramembranous γ-secretase, which produces neurotoxic Aβ peptides. The C terminus of A peptide is variable, ranging from 36-43 amino acids in length, based on alterations in the location of cleavage by γ-secretase [2]. In general, the two dominant forms of Aβ in AD are Aβ^1-40 and Aβ^1-42. Aβ^1-42 is less soluble, more aggregation prone, and more pathogenic than Aβ^1-40 [1]. Once Aβ is produced, it accumulates in the extracellular spaces of the brain (parenchymal deposits) and may also be seen in vascular walls (cerebral amyloid angiopathy, CAA). CAA can be seen with, but is not exclusive to AD.
Parenchymal Aβ deposits may be classifed into subtypes based on morphology, fbrils (B-sheet conformation), and the surrounding elements (degenerative neurites [i.e. dystrophic neurites] and reactive astrocytes and microglia) [8]. In general, there are two main subtypes: diffuse and focal deposits [1-2]. The focal deposits are composed of a focal spherical dense core of fbrillar Aβ (Aβ40 and Aβ1- 42) that stains positive for Congo red and thioflavin S. Focal deposits that contain few reactive glia and dystrophic neurites (and thus lack a layered structure) are referred to as primitive plaques. At a later stage, when they are centrally surrounded by more extensive dystrophic neurons and reactive glia, they are referred to as mature or neuritic plaques (NPs). The diffuse form of A deposits (DP) make up the diffuse or amorphous plaque (DP), which are predominantly Aβ^1-42. Morphologically they are large (>50 um), poorly limited, stain negative for Congo red and Thioflavin S, are not surrounded by dystrophic neurites, and typically do not have a surrounding glial reaction. While both DPs and NPs can be found in normal aging, the degree of cognitive impairment in AD patients is correlated with the severity of NPs, but not DPs. In addition to routine H&E and special stains, Aβ deposits can also be detected by IHC using antibodies such as Aβ^1-16 (6E10), Aβ^8-17 (6F3D), and Aβ^17-24 (4G8).
Topographically, parenchymal Aβ deposits are located almost exclusively in the gray matter. The brain regions involved depend on stage of disease, which can be classifed into fve successive phases (Thal Phases). The main regions involved by phase include: neocortex in phase I; hippocampus and entorhinal cortex in phase II; neostriatum and diencephalic nuclei in phase III; brainstem nuclei in phase IV; cerebellum and additional brainstem nuclei (pontine nuclei and locus coeruleus) in phase V [2, 5]. Deposition of plaques typically occurs frst as DPs, followed by primitive plaques which progress to NPs.
In cats, Aβ deposition has been widely documented to increase with age, beginning by 7.5 years, and typically by 10 years of age [9-10]. Morphologically these are present predominantly as DPs (Aβ^1-42), and are negative for Congo Red and thioflavin. They are most commonly found in the cerebral cortex, with extension to the hippocampus and basal ganglia. This is comparable to the distribution of DPs in humans, which are most commonly found in the cerebral cortex, basal ganglia, and cerebellar cortex [3]. Although typical NPs have not been identifed in cats, another form of Aβ deposition (intracellular Aβ oligomers) has been found to accumulate within hippocampal pyramidal cells of cats 14 years and older [6]. The intracellular Aβ oligomers were composed of hexamers and dodecomers, and found in the same brain regions as NFTs with associated neuronal loss, similar to AD patients. Collectively, Aβ deposition in cats is most similar to early forms present during normal aging and AD in humans.
Several other species have been shown to accumulate A deposition with age including pet dogs and nonhuman primates (NHPs), in comparison to cats and humans (Figure 1). Similar to cats, both dogs and NHPs demonstrate agerelated deposition of Aβ the brain reminiscent of some but not all features of AD. In dogs, the Aβ peptide is identical in sequence to humans, with deposits mainly as DPs beginning at 8 years of age, composed primarily of Aβ1- 42 with minimal Aβ^1-40, in similar topographic locations to cats and humans [11-12]. Neuritic plaques of Aβ^1-40 and Aβ^1-42 may occur less commonly, and mature NPs are very rare in dogs [1]. In NHPs, age-related Aβ deposition has been documented in great apes, old world monkeys, new world monkeys, and prosimians, although there is a high degree of variability between groups in the age of onset, distribution, amount, and appearance of lesions. In general, DPs are most common across the groups, although old world monkeys display more NP plaques compared to other animals, beginning around 30 years of age [4, 7].

Tau is a microtubule-binding phosphoprotein in axons that mediates axonal transport, encoded by the MAPT gene [2-3]. Alternative splicing of MAPT yields a total of 6 isoforms in the human adult brain. Specifically, the isoforms differ by the number of amino acids inserted at the N terminal aspect of the protein (0, 1 or 2 producing 0N, 1N or 2N) and by the presence of three or four amino acid repeats (3R and 4R) in the C terminal part of the protein, which contain the microtubule binding domains [13]. In AD (a taupathy), tau is shifted from primarily axonal locations to a somatic-dendritic distribution, becomes hyperphosphorylated, ubiquinated, and loses its ability to bind to microtubules, resulting in a misfolded, insoluble tau protein. The specifc tau isoforms that accumulate in humans vary across different taupathies. pTau aggregates in AD consist of hyperphosporylated 3R and 4R isoforms [2]. In addition, the active form glycogen synthase kinase-3 (GSK3), a tau protein kinase, has been found in association with tau lesions and at increased expression levels in AD brains [13]. Tau aggregates may be present in the neuron cell body, dendrites, and axons. In the cell body, tau aggregates progress from pretangle lesions (diffuse or granular phospho-tau immunoreactivity in the absence of fbrillary structure) then become fbrillary, forming neurofbrillary tangles (NFTs). NFTs that form in the neuronal cytoplasm result in neuronal degeneration and death, after which the NFTs persist in the neuropil as ghost tangles. Tau aggregates may also accumulate in dendrites, where they are known as neuropil threads. Lastly, tau aggregates can be present in axons, where they are visualized as fne tau-positive processes that make up the neuritic component (crown) of the focal plaque.
Tau aggregates are usually detected by IHC using immunostaining with anti-tau antibodies such as AT8, PHF1, and anti-3R and anti-4R. Special stains such as silver stains and thioflavin S can also be used [1]. Ultrastructurally, tau aggregates in AD are made of two filaments paired in a helical structure (PHFs). Straight filaments have also been seen, but usually these are more suggestive of other taupathies such as progressive supranuclear palsy (PSP) and Pick’s disease (PiD) [13]. In general, the development of Aβ plaques is considered to precede and accentuate the develop of NFTs in human AD [14]. The burden of tau pathology progresses from medial temporal lobe to neocortex describred through Braak stages, as detected by AT8 immunohistochemistry. The stages sequentially involve the medial temporal cortex (stage I), hippocampus CA1 (stage II), subiculum and other regions of hippocampus and medial temporal cortex (stage III and IV) and other areas of neocortex (stage V and VI) [2].
Aging cats have been shown to develop tauopathies that share many features to human AD. For example, adult cats express 6 total tau isoforms, including the hyperphosphorylated 3R and 4R isoforms, which can form aggregates in the presence of Aβ as in human AD [6, 13]. Cats also show increased expression of GSK3 (tau protein kinase) in association with tauopathies and regions of neuronal loss at 15 years of age [15]. Morphologically, neurons with intracellular phosphorylated tau in aging cats have been noted to display a sprouting response similar to human AD [15]. Several forms of tau aggregates reported in cats include: pretangles, threads, dystrophic neurites, NFTs and ghost tangles [16]. Ultrastructurally, cat NFTs are similar to those in humans, consisting of some straight flaments but mostly paired twisted patterns of flaments. These features of feline NFTs accompanied by neuronal loss and occurring in intracellular oligomers in the same brain region (hippocampus), have also been described in transgenic mouse models and human AD patients, and thus represent a potentially high impact naturally occurring model of the disease [5] compared to pet dogs and NHP’s (Figure 2).
Although most other aging nonhuman animals do not spontaneously develop NFTs and neuronal loss, abnormal tau phosphorylation has been described in a number of species including sheep and goats, bison, bears, degu, and wolverines [1, 13]. Dogs have not been shown to develop NFTs, but do show increased phorphorylation of tau that may represent pretangle pathology. In NHPs, NFTs are relatively rare, and when present are usually focal, mild, and not associated with neuronal loss [4, 7].

Authors’ contributions

Jenna Klug wrote the first draft. All co-authors contributed to editing and additions. Warren Ladiges did the final editing.

Conflicts of interest

The authors declare no conflicts of interest.

Consent for publication

All authors consent to the publication of this manuscript.

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