**3. Experimental findings supporting protein misfolding as a neurodegenerative mechanism in CCH**

The causative role of CCH in cognitive impairment and AD has been reported in several studies using the BCCAO model, an experimental paradigm of easy application [28]. Differential mechanisms have been proposed as a potential link between CCH and neurodegeneration, including synaptic dysfunction, oxidative stress, neuronal loss, white matter lesion, neuroinflammation, and protein misfolding [28]. The latter appears as a novel target for neuroprotection, according to cumulative evidence [11]. **Figure 1** illustrates how CCH alters proteostasis network, leading to protein misfolding and neurodegeneration.

Degeneration of hippocampal neurons was attributed to proteostasis network destruction and protein aggregation induced by BCCAO [33]. This murine model has also led to a sustained increase in autophagy related-proteins Beclin-1, light chain 3B (LC3-B), and P62, suggesting a defensive reaction against protein misfolding. In this study, although CBF returned to baseline, cognitive failure was irreversible and attributed to Aβ aggregation in the hippocampus [34]. This brain region is characterized by its vulnerability to CCH [35] and its association with memory and learning dysfunction in AD [36]. Hippocampal degeneration has also been related to BCCAO-induced macroautophagy and endoplasmic reticulum (ER) stress, as it was inferred from the expression of light chain 3 II (LC3-II), Beclin 1, CCAAT/ enhancer-binding protein, and C/EBP homologous protein [37].

Besides BCCAO, oxygen-glucose deprivation (OGD) provokes autophagy upregulation and apoptosis [35]. Recent evidence extended these findings, reporting CCH-induced high levels of LC3-II and Beclin-1 along with ultrastructural markers of apoptosis in CA1 neurons, including nuclear pycnosis, autophagosomes, and autolysosomes. ER fragmentation and spatial working memory impairment appeared as subcellular and functional correlates [38]. In addition to autophagy [34, 35, 38] and macroautophagy [37], ubiquitin-proteasome system (UPS) appears as another proteostatic pathway altered as a consequence of experimental CCH, which leads to CA1 degeneration. Long-term decrease peptidase activity and accumulation of ubiquitinated protein aggregates were observed after ligating the left vein and draining the transverse sinus and the bilateral external carotid arteries [39]. Previous studies from this laboratory had suggested the removal of misfolded proteins was impaired by UPS downregulation [40], and cognitive decline might be associated with long-term potentiation inhibition [41].

Along with aggregation of extracellular Aβ, intracellular phosphorylated tau protein deposition constitutes a hallmark of AD [42]. Tau hyperphosphorylation was observed as a result of unilateral common carotid artery occlusion (UCCAO),

#### **Figure 1.**

*Protein misfolding as a neurodegenerative mechanism and novel neuroprotective target in CCH. Chronic cerebral hypoperfusion impairs proteostasis network, inducing protein misfolding. Under cell stress, proteostasis network surveillance systems degrade proteins through different mechanisms. Depending on the nature of misfolded proteins, different systems are activated such as ubiquitin-proteasome system or macroautophagy. Protein aggregates, are recognized by molecular chaperones, ubiquitinated and delivered to the autophagosome via Beclin-1 complex. Neuroprotective agents, which target proteostasis network, emerge as promising treatments for cognitive impairment following CCH.*

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In Vivo *Studies of Protein Misfolding and Neurodegeneration Induced by Metabolic Syndrome…*

acetylglucosamine, dysregulation of synaptic proteins, and memory deficits [43]. According to previous findings, brain glucose metabolic dysfunction might downregulate tau O-GlcNAcylation mediated by tau hyperphosphorylation [44–46]. Recent studies have confirmed and extended this finding, suggesting CCH might exacerbate tau hyperphosphorylation in AD-rodents, either after UCCAO in mice [47] or BCCAO in rats [48]. Similarly, previous evidence prompted CCH might precipitate AD neuropathology since BCAS seemed to accelerate Aβ aggregation, the same process found in amyloid protein precursor (APP)-transgenic (APP-Tg) mice [49]. In fact, aberrant processing of APP has been reported after BCCAO [50].

Since MetS is associated with an increased risk of cerebral ischemia, recent investigations developed murine models of MetS combined with experimental paradigms of ischemia-reperfusion injury to study the impact of ischemia associated with MetS. Wistar rats fed a high-fat diet for 20 weeks were more susceptible to BCCAO-reperfusion than normal diet (ND)-fed animals, showing worsening in microvascular dysfunction and oxidative stress. These results show that MetS increases the vulnerability of the ischemic brain to damage, whereby BCCAO exacerbates cerebrovascular disease previously induced by HFD [51]. Similarly, BCCAO, followed by reperfusion, aggravated microvascular alterations in obese Zucker rats compared with lean Wistar controls. Lesions in the cortex and striatum were largely more pronounced in obese Zucker rats, suggesting MetS could increase the risk of adverse outcomes following a brain hypoperfusion-

Novel findings support the hypothesis that the brain under obesity's conditions is more vulnerable to ischemic injury. A brief episode of transient ischemia (TI) was provoked in obese gerbils, commonly known as desert rats. After 12 weeks of HFD, the rodents underwent a 2-min experimental TI. Hyperglycemia, cholesterolemia, and triglyceridemia observed in gerbils fed with a HFD were associated with a massive loss of pyramidal neurons in the hippocampal CA1 region 5 days after TI, indicating that a short-lived episode of TI could evoke neuronal damage along with pre-existing MetS. Increased levels of dihydroethidium, 4-hydroxynonenal, tumor necrosis factor-α, and interleukin-1β indicated brain injury was mediated by oxidative stress and neuroinflammation. On the contrary, ND gerbils did not exhibit neuronal death as a consequence of acute TI. In addition, these control animals could develop cerebral ischemic tolerance against a subsequent severer episode of TI [53]. Previous studies from the same laboratory have deepened the role of mammalian target of rapamycin (mTOR) in the pathogenesis of metabolic and neurological diseases and demonstrated that obesity and its related metabolic dysfunction might exacerbate the impact of TI in certain brain areas, including cerebral cortex,

Conversely, cerebral ischemia itself might lead to glucose deregulation, a pathognomonic feature of MetS. Experimental evidence combining rodent models of ischemia and MetS shows that ischemic hippocampal neuronal death hampers glucose homeostasis, decreasing insulin secretion, which is later exacerbated by a HFD. In this study, gerbils were subjected to an 8-min BCCAO or a sham operation followed by either 11 or 40% fat diet for 7, 14, or 28 days. Although the initial occlusion provoked a 70% decrease in CA1 neurons, only HFD and longer ischemic periods resulted in larger hippocampal cell death. Similarly, glucose intolerance measured through the oral glucose tolerance test (OGTT) was overrated in gerbils

together with decreased post-translational tau O-GlcNAcylation by β-N-

*DOI: http://dx.doi.org/10.5772/intechopen.92603*

**4. Combining experimental models**

striatum, and hippocampus (CA1-3 regions) [54–56].

reperfusion event [52].

In Vivo *Studies of Protein Misfolding and Neurodegeneration Induced by Metabolic Syndrome… DOI: http://dx.doi.org/10.5772/intechopen.92603*

together with decreased post-translational tau O-GlcNAcylation by β-Nacetylglucosamine, dysregulation of synaptic proteins, and memory deficits [43]. According to previous findings, brain glucose metabolic dysfunction might downregulate tau O-GlcNAcylation mediated by tau hyperphosphorylation [44–46]. Recent studies have confirmed and extended this finding, suggesting CCH might exacerbate tau hyperphosphorylation in AD-rodents, either after UCCAO in mice [47] or BCCAO in rats [48]. Similarly, previous evidence prompted CCH might precipitate AD neuropathology since BCAS seemed to accelerate Aβ aggregation, the same process found in amyloid protein precursor (APP)-transgenic (APP-Tg) mice [49]. In fact, aberrant processing of APP has been reported after BCCAO [50].
