Increasing evidence supports the hypothesis that polyphenols protect neurons against changes associated with cerebral ischemia [23
]. Ischemic stroke is caused by an interruption of cerebral blood flow, which can lead to failure of energy production, activation of proteases, impairment of metabolism, free radical production, excitotoxicity, and altered calcium homeostasis [24
]. Cerebral ischemia is associated with a severe deterioration in the ability of neuronal mitochondria to function effectively; this seriously compromises oxidative phosphorylation, a key mechanism by which ATP is produced. Apart from ATP depletion, dysfunctional mitochondria may increase ROS production and are unable to maintain a normal [Ca2+
. This causes depolarization of the cytoplasmic membrane potential. Such changes can contribute to both Ca2+
-induced membrane damage and increase levels of Ca2+
-induced proteases, the extent of ROS-mediated cell damage (to membranes and DNA) and lipid peroxidation [28
]. Thus, potential neuroprotectants seek to reduce oxidative stress and mitochondrial damage. It is thus reasonable to suggest that the therapeutic use of polyphenols (which possess strong antioxidant activities) may reduce cell damage, thus improving cell viability, following cerebral ischemia [29
Recently, knowledge of the beneficial roles played by polyphenols derived from terrestrial plants in protection from neurodegenerative diseases such as stroke has greatly increased, but only very limited information is available on the polyphenol components of marine plants. Brown algae are popular health foods in Korea and Japan. Ecklonia cava
, a representative perennial brown alga of the Ecklonia
group, is a member of the Laminariaceae
of the order Laminariales [13
]. As mentioned above, purified polyphenols from Ecklonia cava
have been reported to exert multiple biological activities, although their neuroprotective effect of ECP on stroke has received only limited attention. In our previous study, ECP protected hippocampal slices of the gerbil from global ischemia induced by bilateral common carotid artery occlusion, consistent with in vitro
data showing that ECP protected against oxygen-glucose deprivation-induced cytotoxic death of the SH-SY5Y cell line. Although these results were the first experimental demonstration of the neuroprotective efficacy of ECP in the context of cerebral ischemia, some fundamental limitations of the work are evident.
First, global ischemia is not the only form of cerebral ischemia. It is generally accepted that two types of ischemia exist: these are global ischemia, affecting wide areas of brain tissue, and focal ischemia, confined to a specific region of the brain. The former condition occurs when blood flow to the brain is halted, or drastically reduced, as when cardiac arrest occurs. The latter condition eventuates when a blood clot occludes a cerebral vessel; indeed, the condition is clinically less common than focal ischemia. To overcome this limitation, in the present study we adapted the focal ischemia model to permit in vivo screening of the therapeutic efficacy of ECP in the context of cerebral ischemia.
Second, although the human neuroblastoma cell line, SH-SY5Y, is commonly employed for in vitro
neurotoxicological testing, because the cells were originally derived from neurons, are very sensitive to neuronal inhibitors, and are morphologically similar to neurons, doubts have been expressed concerning functional mimicry of neurons by such cells. To overcome this limitation, in the present study we used RA-induced differentiated SH-SY5Y cells to conduct supportive in vitro
experiments. Indeed, SH-SY5Y cells can differentiate, both morphologically and functionally, into neuron-like cells when cultured in the presence of low levels of RA [32
]. Such differentiation, accompanied by neurite extension, is also associated with increased electrical excitability of the cytoplasmic membrane [35
] and alterations in the expression levels of certain G-protein coupled receptor subunits [36
]. Given that a neuron-like phenotype is shown by differentiated SH-SY5Y cells, the present study was undertaken to examine changes in cell viability and [Ca2+
after pretreatment with different amounts of ECP.
We also considered the effects of ECP on post-ischemic brain edema. Such edema is an essential component of ischemic injury [37
], being characterized by abnormal accumulation of exudate in the parenchyma, resulting in volumetric enlargement of the tissue. Acute brain edema usually radically affects clinical prognosis after occurrence of ischemia, because edema can lead to brain herniation, irreversible brain damage, and, eventually, death. Brain edema is of two distinct types, distinguished by the origin and location of fluid, and is classified as either cytotoxic or vasogenic [38
]. The former is a neuronal form of swelling; fluid accumulates in the cytosol. Vasogenic edema is caused by breakdown of the blood-brain barrier, resulting in fluid accumulation in the extracellular space. Reductions in the levels of either type of edema may improve neurobehavioral morbidity [39
] and recirculation in the cerebral microvasculature [40
]. As we have shown in the present study, ECP treatment can effectively reduce the extent of edema; this limits the extent of ischemia-induced secondary damage and may have a role to play in neuroprotection against ischemic injury [42
In summary, we suggest that ECP plays a beneficial role in the treatment of cerebral ischemia and neurological deficits caused by this condition, at least partly by reducing the extent of brain edema, inhibiting neuronal apoptosis, and blocking the rise in cytotoxic [Ca2+]i. It remains to be established whether ECP is also helpful in the treatment of other neurodegenerative conditions including parkinsonism and Alzheimer's disease.