Epidermal growth factor treatment of female mice that express APOE4 at an age of advanced pathology mitigates behavioral and cerebrovascular dysfunction

Abstract

APOE4 is a major genetic risk factor for Alzheimer's disease and high amyloid-β (Aβ) levels in the brain are a pathological hallmark of the disease. However, the contribution of specific APOE-modulated Aβ-dependent and Aβ-independent functions to cognitive decline remain unclear. Increasing evidence supports a role of APOE in modulating cerebrovascular function, however whether ameliorating this dysfunction can improve behavioral function is still under debate. We have previously demonstrated that systemic epidermal growth factor (EGF) treatment, which is important for vascular function, at early stages of pathology (treatment from 6 to 8 months) is beneficial for recognition and spatial memory and cerebrovascular function in female mice that express APOE4. These data raise the important question of whether EGF can improve APOE4-associated cerebrovascular and behavioral dysfunction when treatment is initiated at an age of advanced pathology. Positive findings would support the development of therapies that target cerebrovascular dysfunction associated with APOE4 in aging and AD in individuals with advanced cognitive impairment. Therefore, in this study female mice that express APOE4 in the absence (E4FAD-mice) or presence (E4FADþ mice) of Aβ overproduction were treated from 8 to 10 months of age systemically with EGF. EGF treatment mitigated behavioral dysfunction in recognition memory and spatial learning and improved hippocampal neuronal function in both E4FADþ and E4FAD-mice, suggesting that EGF treatment improves Aβ-independent APOE4-associated deficits. The beneficial effects of EGF treatment on behavior occurred in tandem with improved markers of cerebrovascular function, including lower levels of fibrinogen, lower permeability when assessed by MRI and higher percent area coverage of laminin and CD31 in the hippocampus. These data suggest a mechanistic link among EGF signaling, cerebrovascular function and APOE4-associated behavioral deficits in mice with advanced AD-relevant pathology.

Systemic Candesartan Treatment Modulates Behavior, Synaptic Protein Levels, and Neuroinflammation in Female Mice That Express Human APOE4

“…Therefore, E4FAD− and E4FAD+ mice were utilized to test whether candesartan modulates functions specific to the interaction of Aβ and APOE4 or were generally applicable to APOE4 . Previous research in E4FAD− and E4FAD+ mice has demonstrated that alterations in behavior and neuronal protein levels occurs earlier in female mice (∼6 months) than in male mice (∼8 months) ( Wolf et al, 2013 ; Thomas et al, 2016 , 2017 ; Tai et al, 2017 ; Zaldua et al, 2020 ). Therefore, candesartan treatments were initiated at different ages for female and male E4FAD mice to try and broadly match AD-relevant pathology at the start of treatment.…”

“…Further, although the AT1 receptor is expressed on brain endothelial cells and excessive AT1 receptor signaling has been shown to directly impair endothelial cell function ( Fleegal-DeMotta et al, 2009 ), in our study candesartan treatment did not result in changes to vascular permeability (fibrinogen) in female E4FAD− and E4FAD+ mice. Therefore, blood-brain barrier dysfunction may be too far advanced in E4FAD mice at the ages evaluated in this study ( Thomas et al, 2016 , 2017 ; Zaldua et al, 2020 ) for candesartan to have exerted a beneficial effect, or the contribution of the AT1 receptor to cerebrovascular dysfunction is minimal in E4FAD mice. For either event, the result may be that a disrupted blood-brain barrier in E4FAD mice enabled candesartan to cross into the brain.…”

“…E4FAD− mice express the human APOE4 gene under the endogenous mouse promoter, and E4FAD+ mice express the human APOE4 gene and overproduce human Aβ42 through the expression of 5xFAD autosomal dominant mutations. APOE4 is associated with age-dependent changes in behavior and neuronal function in the absence of high levels of human Aβ in vivo , including in E4FAD− mice ( Wolf et al, 2013 ; Thomas et al, 2016 , 2017 ; Zaldua et al, 2020 ). Therefore, E4FAD− and E4FAD+ mice were utilized to test whether candesartan modulates functions specific to the interaction of Aβ and APOE4 or were generally applicable to APOE4 .…”

“…Impaired memory-relevant behavior with APOE4 has been reported both in the presence and absence of Aβ in vivo ( Thomas et al, 2016 , 2017 ; Zaldua et al, 2020 ), and data are conflicting on whether ARB treatment considerably modulates memory-relevant behavior in AD-relevant mouse models ( Wang et al, 2007 ; Mogi et al, 2008 ; Takeda et al, 2009 ; Tsukuda et al, 2009 ; Danielyan et al, 2010 ; Ferrington et al, 2011 , 2012 ; Ongali et al, 2014 , 2016 ; Torika et al, 2016 , 2017 , 2018 ; Royea et al, 2017 ; Trigiani et al, 2018 ). Therefore, one of our goals was to evaluate whether candesartan treatment could modulate memory-relevant behavior in E4FAD− and E4FAD+ mice.…”

“…APOE can modulate neuronal function in the brain through a number of direct or indirect pathways both in the absence ( Wolf et al, 2013 ) and presence ( Carter, 2005 ; Ye et al, 2005 ; Tai et al, 2015 ) of Aβ. In vivo models have identified that APOE4 is associated with memory-relevant behavioral deficits ( Bour et al, 2008 ; Liu et al, 2015 ), greater neuroinflammation ( Guo et al, 2004 ), higher Aβ levels ( LaDu et al, 1994 ; Ye et al, 2005 ; Youmans et al, 2012 ; Tai et al, 2013 , 2014b ; Lewandowski et al, 2020 ), and more severe cerebrovascular dysfunction ( Thomas et al, 2016 , 2017 ; Zaldua et al, 2020 ), compared to APOE3 . As described above, these are the same functions and pathologies purported to be altered by ARB treatment in mouse models expressing familial AD (FAD) mutations.…”