*4.1.1 Ischemic stroke*

*Connectivity and Functional Specialization in the Brain*

regulating intracellular endothelial pH levels [62]. The Na+

prevent excitotoxicity, similar to endothelial transporters.

CNS, contributing to neurological deficits [68].

play an active role in regulating CBF and permeability of the BBB.

exchanger cotransporter mediates Ca2+ efflux from endothelium into brain ISF, which maintains low intracellular Ca2+ levels in the microvascular endothelium [34]. Abluminal transient receptor potential (TRP) channels, also known as non- selective Ca2+ conducting cation channels, are expressed in both arterial endothelium and brain microvascular endothelial cell lines. TRP channels regulate Ca2 influx into brain endothelium, which in turn promotes the release of soluble factors such as NO, prostaglandins, and endothelial-derived hyperpolarizing factor initiating endothelium-dependent vasodilation [63]. BBB dysfunction also generates a leak of molecules across it, enabling considerable vascular fluid movement across the microvascular endothelium and the development of vasogenic edema [64]. Increased expression and activity of Na-K-Cl cotransporter (NKCC), sodiumhydrogen antiporter 1 and 2 (NHE1 and NHE2), and TRP channels promote the

**Pericyte transporters.** Recent studies suggest that pericytes also express several transporters, receptors, and ion channels (**Figure 2**), such as carbohydrate transporters like the insulin-regulated glucose transporter GLUT4 and GLUT10 [65] and AA transporters, including the high-affinity excitatory AA transporter EAAT2, sodium-dependent neutral AA transporter SLC6A17, sodium and chloride dependent transporter SLC6A20 for small AA including glycine and proline, GABA transporter-1 and 2 (GAT1; GAT2), and the cationic AA transporter CAT2 [34, 65]. These transporters contribute to the removal of excitatory AA from the brain to

As endothelial cells, pericytes express lipoprotein receptor LRP1, mediating cellular uptake followed by its intracellular degradation and clearance [66] Pericytes regulate cerebrovascular integrity in an APOE-dependent way, inhibiting the proinflammatory CypA-MMP-9 pathway which prevents the degradation of BBB's TJ and basement membrane proteins [67]. These findings support that pericytes

BBB's integrity is essential for the normal functioning of the nervous system. It comes as no surprise then that its disruption initiates and perpetuates several neurological pathophysiological events. Although the nature and extent of such changes vary from every condition, one key commonality is the breakdown of BBB and the detrimental functioning of the NVU [4]. The BBB prevents neurotoxic plasma components, blood cells, and pathogens from entering the brain (integrity of BBB). At the same time, the BBB regulates transport of molecules into and out of the central nervous system (CNS) (permeability of BBB). In cerebrovascular diseases, BBB breakdown and dysfunction leads to leakages of components into the

The cells of the NVU are extremely sensitive to a number of different substances, including pro-inflammatory cytokines (e.g. IL-1, IL-6, TNF-α, interferon-γ), lipid mediators, oxidative compounds (free radical), vasogenic agents (e.g. glutamate, serotonin, histamine) and other endogenous stimuli (e.g.

pathophysiological conditions and changes of their levels in BBB is a critical event in the development and progression of CNS dysfunction [69]. In some cases, increased BBB permeability is a consequence of the pathology, such as with ischemic stroke

and intracellular Ca2+). Many of these substances are released under

and the extracellular release of HCO3

, generating a subsequent gradient osmotical that force the

− ,


mediates the entry of intracellular Cl<sup>−</sup>

, and Cl−

water movement across the BBB.

**4. BBB dysfunction**

influx of Na<sup>+</sup>

**32**

extracellular K+

In ischemic stroke, there is a sudden cessation of blood supply to the brain tissue, which translates into reduced oxygen and glucose delivery, both essential for ATP production. Depletion of ATP levels can lead to impaired functioning of Na/K-ATPase and Ca+2ATPase activity, generating ion- gradient failure and abnormal intracellular ion accumulation. By contrast, endothelial transporters' activity, such as Na/H ion-exchanger and Na-K-Cl cotransporter are stimulated. This secondarily induces increased Na+ , Cl− , and water across to the barrier and into the brain parenchyma, which results in characteristic cytotoxic edema secondary to ischemia [71]. The stimulation of this transporter's activity also triggers endothelial cell Na+ accumulation, generating swelling that contributes to BBB breakdown [72]. The Na+ cellular uptake depolarizes the cell's membrane, opening voltage-gated ion channels and promoting Ca2+ further cell uptake. These changes, in turn, prompt the release of excitatory neurotransmitters, which can be toxic [71]. BBB's breakdown in stroke occurs in a biphasic subacute fashion [70]. In the initial hit, activated metalloproteinases MMP-2 attack tight junction proteins. This activation is mediated by membrane-type MMP (MMP-14) and the fur gene expression, regulated by hypoxia-induced factor 1a (HIF-1a) [73]. Decreased expression and disorganization of tight junction constituent proteins, claudins, are the first signs of BBB damage, with further dysfunction of influx and efflux BBB transporters' expression. These changes are limit the hypoxic area and revert after the acute insult [70].

After 24 and 48 hours post-reperfusion, a non-reversible second phase takes place. Proinflammatory local cytokines activate inducible and freely available metalloproteinases MMP-3 and MMP-9, whose destructive activity characterizes this phase [74]. The most abundant cytokines present in focal cerebral ischemic areas are TNF-alfa and IL-1b [75] and have also been observed to decrease the expression of occludin and ZO-1 [76]. Cyclooxygenase-2 also plays a role in this second and more harmful opening of the BBB. Although this inflammation is local and mainly initiated by the activation of glia and pericytes, the BBB's damage and opening allow monocytes and neutrophils' entrance, perpetuating and amplifying the local inflammatory response [74]. The breakage of BBB in ischemic stroke is also the precursor of further complications such as the hemorrhagic transformation of the infarcted parenchyma [77]. A schematic view of ischemic stroke and intracerebral hemorrhage mechanisms are shown in **Figure 3**.

BBB can also be disrupted by the action of reactive oxygen species (ROS) and ensuing oxidative stress. Superoxide anion (O2−) is a known mediator of cellular damage after ischemic stroke. Under oxidative stress conditions such as stroke, superoxide dismutase's (SOD) metabolic capacity of controlling the biological activity of O2− gets surpassed. When combined with nitric oxide (NO), O2− forms peroxynitrite, a cytotoxic and proinflammatory molecule that can initiate and amplify BBB's injury by its ability to nitrosylate tyrosine and inducing endothelial damage [78]. Oxidative stress plays a critical role in ischemia/reperfusion (I/R)

#### **Figure 3.**

*A schematic view of ischemic stroke and intracerebral hemorrhage mechanisms. Activation of glutamate receptors following ischemic stroke leads to excitotoxicity and calcium influx, this impairs the neuronal homeostasis leading to activation of several calcium dependent pathways that include proteases and nucleases. Reperfusion aggravates the cerebral parenchyma damage by forming free radicals that damage the membranes and proteins. Further, the opening of mitochondrial permeability transition pore releases various proapoptotic molecules that activate apoptotic cell death in cerebral parenchyma. In intracerebral hemorrhage the initial bleed causes physical disruption of the cellular architecture of the brain and the mass of the haematoma may increase intracranial pressure which can compress brain regions, increasing neuronal death result of necrotic and apoptotic mechanisms depending on the severity of insult. Created with BioRender.com.*

induced brain injury by stroke, various mechanisms in the neurovascular bed can trigger oxidative stress, including mitochondrial dysfunction, increase vasogenic mediators, glutamate release, and depletion of antioxidant defense system. Mitochondria are both important intracellular organelles for energy metabolism organelles, the main intracellular source of ROS, and important targets for I/R brain injury [79]. During stroke, inflammatory cytokines, oxidative stress, and Ca+2 overload stimulate the mitochondria, inducing the production of higher ROS levels, thereby triggering the mitochondrial necrosis pathway and leading to cell death [80]. In addition, endothelial cells, and immune cells produce large amounts of ROS during the cerebral ischemia phase, which in turn induce the activation of nuclear factor-κappa B (NF-κB), inducible endothelial nitric oxide (iNOS), and proinflammatory factors, triggering the upregulation of vascular endothelial cell adhesion molecules and causing BBB permeability [81].

#### *4.1.2 Intracerebral hemorraghe*

Between 10 and 15% of all strokes in the USA are intracerebral hemorrhages, which has higher morbidity and mortality when compared to ischemic strokes. Multiple etiologies are associated with ICH, hypertension being the more common, followed by amyloid pathology, especially in older populations, vascular malformations, and coagulopathies [72]. After the initial bleed, there can be a continuous bleed for the next 24 hours, the so-called hematoma expansion. A delayed vascular disruption occurs after the first 24 hours; this includes BBB dysfunction, which can associate with edema formation and an influx of leukocytes into the brain parenchyma [82].

The role of ischemia in ICH-induced brain injury is controversial, as a reduction in blood flow may be a result rather than the cause of brain damage. This suggests that BBB's increased permeability is due to the direct effect of certain blood components (thrombin, fibrin, and hemoglobin, iron) or to the inflammatory response to these components [72]. This phenomenon may include further peripheral cell

**35**

BBB disruption [94].

*4.2.2 Parkinson's disease*

*Blood-Brain Barrier Dysfunction in the Detrimental Brain Function*

infiltration and microglia activation, which may promote the higher secretion of proinflammatory cytokines and the activation of MMPs, as previously described in

Alzheimer's disease (AD) pathological hallmark is the accumulation of amyloid beta plaque deposits, which suggests the imbalance between its production and clearance rates may be due to a leaky BBB. The BBB dysfunction itself can also promote and accelerate the process of further AB production [84]. Diminished expression and dysfunction of ABC transporters at the BBB have been found in AD mice models [85], and two crucial BBB transporters in Abeta BBB's flow dynamics, p-glycoprotein LRP1, and RAGE have been identified as functionally impaired in AD. Expression of LRP1, which is in charge of the efflux of brain-derived Ab into blood across the BBB, is remarkably low at the BBB in AD patients' and AD models' brains [86]. Verapamil-PET studies in patients with mild AD, an exam that clinically assesses p-glycoprotein function, have found reduced activity of this transport in frontal, posterior cingulate, and the parietooccipital cortices, as well as in the hippocampus [87]. RAGE is a vital transporter that regulates the influx of circulating soluble ab intro the brain, which may promote neuroinflammation. Patients with AD develop increased levels of this transporter receptor both in brain endothelium

There is enough evidence that associates AD with vascular disease at a pathological level [89]. Cerebral vessel pathology is not only a significant risk factor for AD but can also cause BBB disruption, as is the case with cerebral amyloid angiopathy [90]. Furthermore, changes in vascular biomarkers have been observed in preclinical AD before the development of cognitive impairment, and even before increases in routine AD biomarkers [91]. These findings support the two-hit vascular hypothesis of AD suggests that BBB dysfunction and brain hypoperfusion secondary to blood vessel damage may be the first hit that leads to ab accumulation and neuronal injury [92]. There is also evidence that at least two out of three BBB's main three cell lines are significantly compromised in AD. Accelerated pericyte degeneration and BBB breakdown is a distinguishing characteristic of AD-ApoE4 carriers mouse models [93]. On the other hand, astrocytic dysfunction, which has also been seen in AD models [84], may explain the hyperactivity of RAGE and hypoactivity of LRP1 in these patients' BBB. The pericyte degeneration initiates multiple pathways of neurodegeneration owing to the entry of several neurotoxic blood-derived proteins, including plasminogen, thrombin and fibrinogen which enter different areas of the CNS [93]. Plasmin, which is generated from circulating plasminogen, degrades the neuronal matrix protein laminin, thereby promoting neuronal injury. High concentrations of thrombin mediate neurotoxicity and memory impairment and accelerate

PD is one of the most prevalent neurodegenerative diseases after AD. It is characterized by filamentous and oligomeric α-synuclein (α-syn) accumulation, and degeneration of dopaminergic neurons in the substantia nigra leading to motor impairments [34]. Ever since the publication of Braak et al. studies, there is consensus that Parkinson's disease starts in the peripheral system and reaches the central nervous system in a retrograde (axon terminal to soma) spread of Lewy pathology.

*DOI: http://dx.doi.org/10.5772/intechopen.94572*

**4.2 Alteration of BBB by neurological disorders**

ischemic stroke [83].

*4.2.1 Alzheimer's disease*

and mural cells of the BBB [88].

infiltration and microglia activation, which may promote the higher secretion of proinflammatory cytokines and the activation of MMPs, as previously described in ischemic stroke [83].
