Zhang H, Wu L, Pchitskaya E, Zakharova O, Saito T, Saido T, Bezprozvanny I. Neuronal Store-Operated Calcium Entry and Mushroom Spine Loss in Amyloid Precursor Protein Knock-In Mouse Model of Alzheimer's Disease. J Neurosci. 2015 Sep 30;35(39):13275-86. PubMed.
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The APP-KI mouse model is valuable in that it avoids the impact of chronic high overexpression of APP that signals through sAPPα and a multitude of APP binding proteins, most obviously those that interact with the C terminal cytoplasmic domain when it is intact as well as when it is released as a γ-secretase fragment. This multitude of confounding signal transduction events generated by APP overexpression was deliberately circumvented in the APP-KI model, which uses multiple mutations rather than APP overexpression to achieve pathogenic levels of Aβ42. Because the knock-in retains the natural regulation, it can elevate APP expression in feedback loops that may upregulate APP focally in response to inflammation or other events, but it avoids the consequences of tonic abnormal APP overexpression. PS1 mutations, the other commonly used tool to increase pathological Aβ42, also has consequences for many substrates that will not be seen in other forms of FAD or sporadic AD. Since PS1 mutations are sufficient to cause changes in calcium signaling, the use of PS1 mutants to study events in spines downstream from aberrant calcium regulation has been problematic. Hence, Zhang and colleagues have very reasonably employed the APP KI model to confirm the presence of their STIM2 neuronal calcium entry-mediated, mushroom spine-loss pathway in a model with Aβ42 elevation in the absence of mutant PS1 or APP overexpression.
This provides an elegant in vivo system to confirm in vitro evidence from electrophysiological studies that simply added Aβ42 to neurons and implicated mGluR5 as an Aβ oligomer target relevant to synaptic deficits. Since Aβ42 does many things to glia and other targets and precipitates tauopathy, the clinical impact of simply targeting mGluR5 is open to question. That said, elevated calcium in vivo in other AD models has been beautifully demonstrated by Backsai and colleagues and it is hard to believe that it wouldn't be a problem for normal neuronal function. Since the elevated calcium in neurons is manifested in a kind of subclinical excitotoxicity and there is some evidence for a benefit from Levetiracetam/Keppra, this paper points to one reasonable approach to try to go upstream and prevent the chronic aberrant elevation of neuronal calcium influx.
Seeing the advantages of the APP-KI model, we got permission to use Saido's APP-KI mice several years ago. However, due to difficulty of NIA funding in recent years, we have not had resources to develop a colony.View all comments by Gregory Cole
Rosalind Franklin University/The Chicago Medical School
In recent years, the AD field has been undergoing a bit of a paradigm shift in attempts to understand mechanisms of memory loss, with an increased focus on synaptic structure and function. This study by Zhang et al., and their earlier Neuron paper (Sun et al., 2014), provide a much-needed, step-by-step walk through the mechanisms by which the memory-supporting mushroom spines are lost in AD brains. Importantly, the proposed pathway incorporates many of the key suspects of AD pathology (e.g., Aβ42, ER calcium dyshomeostasis, and glutamate signaling) and thus provides a means to accommodate seemingly disparate theories of synaptic loss into a linear sequence of events culminating in impaired memory function. I look forward to the next study!View all comments by Grace Stutzmann
MRC Laboratory of Molecular Biology
Postsynaptic dendritic spines are believed to play an important role in learning and memory. They are classified into mushroom, thin, and stubby spines, based on morphology. Of these, mushroom spines are believed to be stable “memory spines.” Persistent activation of Ca2+/calmodulin-dependent protein kinase II is needed for the long-term stability of mushroom spines. It requires neuronal store-operated Ca2+ entry (nSOC), which is gated by stromal interaction molecule 2 (STIM2). This study shows that hippocampal mushroom spines were reduced in an amyloid precursor protein knock-in model of Aβ deposition (the APP-NL-F line). This line has the advantage that APP is not overexpressed, yet Aβ42 production is increased and Aβ deposits form. The loss of mushroom spines was caused by dysregulated Ca2+ homoeostasis, consequent to overstimulation of mGluR5 receptors by Aβ42. Similar findings had previously been obtained in other mouse models.
However, the behavioral changes were always subtle. Do these animals develop Alzheimer’s disease? Would a human being with this pathology and these symptoms be diagnosed with Alzheimer’s disease? Probably not. What these mice lack are tau inclusions and extensive neurodegeneration. The association between cognitive impairment and tau deposits is well established; it has been shown that tau aggregation is a key mediator of neurodegeneration. It will be interesting to see if the APP NL-F line will help us to understand the largely enigmatic connection between the assemblies of Aβ and tau.View all comments by Michel Goedert
The idea of upregulating STIM2 to reduce calcium and thereby treat AD is interesting, but problematical with regard to how to do it practically. Targeting the downstream protease activation may achieve similar outcomes in a more translatable way. Several years ago, Arancio and colleagues showed that E64, which inhibits cysteine proteases including calpains, improved memory and long-term potentiation deficits in APPswe-overexpressing mice. In hippocampal cells, Halpain and colleagues showed that CA074Me, which inhibits the cysteine proteases cathepsin B and L, prevented NMDA induced dendritic loss. It would be interesting to see if E64 and CA-074Me treatment also improves mushroom budding in the APP KI mice.
Trinchese F, Fa' M, Liu S, Zhang H, Hidalgo A, Schmidt SD, Yamaguchi H, Yoshii N, Mathews PM, Nixon RA, Arancio O. Inhibition of calpains improves memory and synaptic transmission in a mouse model of Alzheimer disease. J Clin Invest. 2008 Aug;118(8):2796-807. PubMed.
Graber S, Maiti S, Halpain S. Cathepsin B-like proteolysis and MARCKS degradation in sub-lethal NMDA-induced collapse of dendritic spines. Neuropharmacology. 2004 Oct;47(5):706-13. PubMed.View all comments by Greg Hook
While the APP-KI model offers an invaluable lens for elucidating insights on how amyloid influences neurons and disease cascades downstream of amyloid, an important objective is the identification of candidate therapeutics that modify disease progression during preclinical stages, possibly before significant amyloid deposits are observable—an endeavor that would be facilitated by characterizing changes to neurons, most likely associated with aging, that influence malformed protein quantities in aging brains.
A challenge that continues to complicate drug development for targeting the preclinical progression of disease is integrating epidemiological evidence into AD-modeling paradigms (to model the possible influence of aberrations in metabolism and decreased mitochondrial capacity associated with aging, for example); incorporating the cellular processes associated with aging that may influence malformed proteins into animal models (to better observe how an aging cell influences amyloid/tau, rather than how amyloid/tau influences a cell); and then evaluating candidate therapeutics that abrogate processes that influence levels of malformed proteins utilizing animal models that capture the influences of aging on disease.View all comments by James Wallace
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