Neuroprotective Role of Mitochondrial Uncoupling Protein 2 in Cerebral Stroke

Abstract

The uncoupling proteins (UCPs) are mitochondrial transporter proteins involved in proton conductance across inner mitochondrial membrane (IMM). UCP2, which is one of the members of this class of proteins, has a wide but restricted tissue distribution including brain. Its physiologic role according to emerging evidences, although still not clear, indicate that distribution of UCP2 may be related to regulation of mitochondria membrane potential (ΔΨm), production of reactive oxygen species (ROS), preservation of calcium homeostasis, modulation of neuronal activity, and eventually inhibition of cellular damage. These factors are very important in determining the fate of neurons and damage progression in the brain during various neurodegenerative diseases including cerebral stroke. Recent evidence indicates that an increased expression and activity of UCP2 are well correlated with neuronal survival after stroke and trauma. This review briefly covers the present understanding of UCP2, which eventually may be beneficial to understand the precise role of UCP2 to develop strategy to identify its potential therapeutic application.

Introduction

The uncoupling proteins (UCPs) are inner mitochondrial membrane (IMM) anion-carrier protein that transport protons (H⁺) to the mitochondrial matrix and in turn dissipates the proton motive force (Δp) as heat. Recent investigations have showed that the tissue distribution of UCPs regulates various important biologic functions (Echtay, 2007). In addition, the fact that oxidative phosphorylation is not completely coupled in mitochondria in vivo or in vitro, highlights the functional importance of these proteins (Andrews et al, 2005). Among various UCPs (UCPl, −3, −4, and −5), UCP2 is widely expressed in various tissues and implicated in diverse pathologic conditions such as obesity and diabetes (Hidaka et al, 2000; Zhang et al, 2001; Wang et al, 2007), neurodegenerative diseases (Mattiasson et al, 2003; Sullivan et al, 2003; Diano et al, 2003; Deierborg et al, 2008), atherosclerosis (Blanc et al, 2003), cancer (Derdak et al, 2008), and sepsis (Roshon et al, 2003).

Emerging evidence suggests that UCP2 may play an important role during cerebral stroke by regulating mitochondrial potential (ΔΨm) and energy balance, neuroendocrine and autonomic functions, reactive oxygen species (ROS) production and fatty acid anion transport, cell death and inflammation (Richard et al, 2001; Mattiasson et al, 2003; Mattiasson and Sullivan, 2006; Deierborg et al, 2008). In the brain, UCP2 is predominantly expressed in fast-firing neurons, microglia, neutrophils, residing and invading monocytes, cells of the choroids plexus as well as endothelial cells in several parts of the fore-, mid-, and hindbrain (Richard et al, 1998, 2001). Interestingly, UCP2 has varied mRNA expression and protein disproportion in various species (Richard et al, 2001). This may be because of different transcription and translation activities of UCP2, and which is influenced by metabolic rate, free fatty acid (FFA), nucleotides, coenzyme Q, superoxide anion (O₂⁻), developmental stage, etc. The distinct role of UCPs, in particular UCP2, in pathophysiology at present is a matter of debate; this review article will look into the present understanding of UCP2 and emphasis will be made to highlight the potential neuroprotective role of UCP2 in stroke and diabetes.

Structure and action of uncoupling protein-2

The functional structure of UCP2 is a homodimer composed of 309 amino-acid residues. It is present on chromosomes 11 and 7 in the human and mouse, respectively. Uncoupling protein-2 exhibits 59%, 71%, 33%, and 38% structural similarities with UCPl, −3, −4, and −5, respectively (Fleury and Sanchis, 1999; Vidal-Puig et al, 1997). The rat UCP2 shares approximately 99% and 95% sequence homology with the UCP2 of the mouse and human, respectively, which emphasizes the remarkable conservation of UCP2 (Hidaka et al, 1998).

The primary function of UCPs in the mitochondria is translocation of protons, which are pumped out against concentration gradient during the flow of electrons through respiratory chain complexes I—IV (Figure 1, refer legend for details). The proton electrochemical gradient, the Δp, established by these protons is used to synthesize adinosine triphosphate (ATP) by F0Fl-ATPase (complex V). However, the consumption of oxygen during electron transport chain (ETC) may not be completely used to concomitant ATP generation because of proton leak through UCPs, and the resultant energy of the Δp is dissipated as heat. The protons may be directly transported by UCPs or indirectly through a process called fatty acid cycling across the IMM (Figure 2; Klingenberg and Echtay, 2001; Jezek, 1999). The exact mechanism of proton transport although is not clear, however, both processes suggested may uncouple the oxidation from ATP synthesis.

Figure 1

Schematic diagram of mitochondria, showing mitochondrial electron transport chain (ETC) complexes (I–V), UCP2, ANT, and VDAC. During the respiration, the free energy generated by oxidation of substrates is conserved in reduced molecules of nicotinamide adenine dinucleotide (NADH) and FADH₂. These reducing equivalents donate the electron to protein complexes and the conserved energy in these molecules is used to transport protons to the intermembrane space. This leads to the establishment of a proton motive force (Δp), and transmembrane potential, which is negative inside (around −180 mV). The electrons from the complexes I and II are accepted by mobile electron carrier coenzyme Q (CoQ), which donates these electrons to the complex III, whereas cytochrome C shuttle these electron between complexes III and IV. The complex IV then reduces molecular oxygen to water. The energy that is conserved as Δp during electron shuttle is used by complex V to drive synthesis of adinosine triphosphate (ATP) from adinosine diphosphate (ADP) and inorganic phosphate (Pi), and as a result, the protons are transported back to the matrix through complex V Alternatively, protons are also transported to matrix because of mild uncoupling or proton leak by uncoupling proteins (UCPs). This process decreases the Δp and which in turn affect the synthesis of ATP The conserved energy during this uncoupling is dissipated in the form of heat. The expression of UCPs is increased by superoxide anions (O₂⁻), which are formed during leakage of electron from complexes I and III. These electrons are taken up by molecular oxygen to form O₂⁻. Under normal physiologic conditions, O₂⁻ is simultaneously dismutated by SOD2 (MnSOD) to hydrogen peroxide (H₂O₂) and H₂O₂ is further reduced to water by glutathione peroxidase (GPX) to avoid possible build up of oxidative stress. However, under mitochondrial stress, O₂⁻ may react with nitric oxide (NO) to form highly reactive peroxynitrite (ONOO⁻) and H₂O₂ in the presence of Fe²⁺ form hydroxyl radicals (⁻OH). These radicals have potential to damage proteins, lipids and DNA. Moreover, mitochondrial dysfunction may allow release of certain proteins involved in apoptotic cell-death activation. The mild uncoupling because of UCPs can attenuate the production of O₂⁻ and ultimately prevent mitochondrial dysfunction and cell-death activation. ANT, adenine nucleotide translocase; CYP, D-cyclophilin-D; Cyt C, cytochrome C; IMM, inner mitochondrial membrane; IMS, intermembrane space; OMM, outer mitochondrial membrane; UCP2, uncoupling protein 2; TTFA, thenoyl trifluoracetone; VDAC, voltage-dependent anion channel.

Figure 2

Drawing shows proposed mechanism of uncoupling protein-2 (UCP2)-mediated uncoupling. UCP2 transport protons directly or with the help of free fatty acid (FFA) to the matrix from intermembrane space (IMS). FFA carries protons through UCPs with or without coenzyme Q (CoQ) and then FFA returns back. However, electroneutral, protonated fatty acids flip across the inner mitochondrial membrane (IMM) and release the protons in the matrix. The fatty acid anion is transported back by UCP2 and above cycle is repeated to sustain the mitochondria function.

Neuroprotective role of uncoupling protein-2 in cerebral stroke

Cerebral Stroke and Mitochondria

Cerebral stroke has been increasingly recognized as a medical emergency because of sudden onset, and activates several intricate and overlapping cell-survival/damaging mechanisms. The damaging mechanisms may proceed through necrotic or apoptotic cell death and dependent on the dynamic interplay among the various cells in affected territory (Mehta et al, 2007). The mitochondrion has emerged as a crucial organelle in regulating cell death during cerebral ischemia, besides playing a major role in the regulation of energy metabolism, ROS production, and Ca²⁺ homeostasis. Numerous reports suggest that mitochondria is the major source of ROS (Kudin et al, 2008) and produce O₂⁻ and hydrogen peroxide (H₂O₂) radicals (please refer to Figure 1 and its legend for more details about ROS production in the mitochondria) during transient/ permanent focal and global cerebral ischemia (Peters et al, 1998; Murakami et al, 1998; Kim et al, 2002; Dirnagl et al, 1995). This can be supported by the evidence that suggest superoxide dismutase, SOD2^−/+ (MnSOD) mice have increased mitochondrial dysfunction, infarct size, neural apoptosis, and neurologic deficit, whereas, SOD2 overexpression have increased mitochondrial tolerance (Silva et al, 2005).

In addition, mitochondria can functions as calcium-buffering organelle. It can efficiently take up Ca²⁺ to avoid Ca²⁺ fluctuations and maintain cellular bioenergetics (White and Reynolds, 1997). However, after glutamate excitotoxicity and cerebral ischemia, the intracellular Ca²⁺ is increased (Castilho et al, 1998; Kristian and Siesjo, 1998). The Ca²⁺ influx through glutamate-dependent N-methyl D-aspartate (NMDA) receptor is particularly toxic to neurons (Tymianski et al, 1993). This can cause major mitochondrial ultrastructural alterations because of its spatial proximity to NMDA receptor and preferential exposure to high levels of Ca²⁺ (Peng and Greenamyre, 1998; Pivovarova et al, 1999). These events can induce calcium-dependent change of ΔΨm and opening of mitochondrial membrane transition pore (MPTP; Isaev et al, 1996). The ΔΨm may in addition be impaired because of excessive excretion of protons or inhibition of proton re-entry into the matrix. This condition considerably increases the ΔΨm, called the hyperpolarization because of the build up of strong proton gradient. The consequence of altered ΔΨm has been proposed as the point of no return in apoptotic signaling, which may be critical during cerebral ischemia (Iijima et al, 2003a, b ; Iijima, 2006). This opens the MPTP in the IMM and allows the release of soluble proteins such as cytochrome C, apoptosis-inducing factor, and Smac/Diablo, etc. and accumulated Ca²⁺ (Bernardi and Petronilli, 1996). The release of these proteins then activates apoptotic cell-death pathways in caspase-3-dependent or -independent manner. Therefore, the change in the ΔΨm, mitochondrial membrane permeability, and activation of cell-death pathways are perhaps interconnected and can be exploited to modulate cell death.

Possible Role of Uncoupling Protein-2 in Regulation of Reactive Oxygen Species

The brain, owing to its high lipid content and possibly lesser antioxidants, is highly prone to ROS induced damage during cerebral ischemia and reperfusion. The ROS is generated when ETC slows down or stalls during the inadequate supply of adinosine diphosphate (ADP) or dysfunction of ATP synthase. In such conditions, the electron carrying intermediates have an increased chance to transfer single electron to molecular oxygen (O₂) and the protons pumped out of the matrix can no longer return back (Mattiasson and Sullivan, 2006). This build up the ΔΨm and the increased ΔΨm reported during reoxygenation is linked to the ROS generation (Skulachev, 1998; Gergely et al, 2002). Interestingly, it has been suggested that uncoupled or noncoupled respiration decreases the intracellular oxygen, and thereby, prevent nonenzymatic one-electron reduction of O₂ to O₂⁻ (Skulachev, 1996). This hypothesis received attention when it was found that macrophages from UCP2^−/− mouse generated more ROS than wild-type mouse (80% increase; Arsenijevic et al, 2000). The mitochondrial proton conductance through UCPs is increased by matrix O₂⁻ in the presence of FFA such as 4-hydroxy-2-nonenal and is inhibited by purine nucleotides (Echtay et al, 2002, 2003).

Recently, Mattiasson et al (2003) has proposed that UCP2 could be one of the targets to provide neuroprotection in stroke by alleviating the mitochondrial ROS production and shifting the ROS from mitochondrial to the extramitochondrial space (Andrews et al, 2008). This facilitates the change in cellular redox state, and thus, UCP2 may contribute to redox signaling by direct interaction with the ETC. This can be further supported by the evidence suggesting a significant and specific increase in the ΔΨm and intracellular ROS level in endothelial cells pretreated with antisense oligonucleotides directed against UCP2 mRNA (Duval et al, 2002). Accumulative evidence also suggests that UCP2^−/− mice has a higher ROS level that delays liver regeneration, increased atherosclerotic lesion and pancreatic islet cell dysfunction (Horimoto et al, 2004; Blanc et al, 2003; Krauss et al, 2003). The elevated ROS level can be reversed by overexpression of UCP2 (Park et al, 2005). Thus, ROS-suppressing property of UCP2 can inhibit apoptosis, whereas inhibiting UCP2 expression increases tumor necrosis factor-α-induced expression of markers of ROS accumulation and apoptosis (Derdak et al, 2008; Degasperi et al, 2008). Likewise, UCP2/3 overexpression negatively regulated the levels of lipid peroxidation and free radicals together with a positive regulation of neuronal mitochondrial number (Diano et al, 2003; Coppola et al, 2007). Thus, the elevated expression of UCP2 and UCP5 in the ischemic lesions of embolic stroke cases, compared with the intact area, may be linked to neuroprotection by modulating mitochondrial ROS production (Nakase et al, 2007; Kim et al, 2007). Contradictory to the above reports, the concept that UCP2 is linked with ROS production is challenged by two types of reports. First, treatment with p-(trifluoromethoxyl)-phenyl-hydrazone (FCCP) or mitochondrial depolarization induced by veratridine is not paralleled with significant alteration in the ROS generation (Johnson-Cadwell et al, 2007; Tretter and Adam-Vizi, 2007). Secondly, UCP2^−/− mice showed resistance to cerebral ischemic injury (de Bilbao et al, 2004). It is noteworthy to mention that permanent middle cerebral artery occlusion (MCAO) model was used in this study. In contrast to transient MCAO, the ROS production after permanent MCAO does not seem to be a major contributor in ischemic damage progressions. It has been reported that reperfusion produced a burst in ROS formation after 1 h of MCAO whereas increase of ROS production was much less prominent after permanent MCAO (Peters et al, 1998). Studies from SOD1 (CuZnSOD) knockout animals further supported the notion that ROS production is significantly different between transient and permanent MCAO. Thus, although overexpression of SOD1 failed to protect neurons after a permanent MCAO (Chan et al, 1993), SOD1 overexpression significantly reduced cell death after a transient MCAO (Saito et al, 2003). These further strengthen the argument that the negative effect of UCP2 observed by de Bilbao et al (2004) is probably ascribed to the permanent MCAO model being used.

The UCP2-dependent modulation of ROS production is directly linked to ΔΨm, suggesting that small reduction in the ΔΨm because of mild uncoupling has significant effect on ROS generation (Teshima et al, 2003). Thus, UCP2 appears to decrease the redox pressure on the mitochondria and offer intriguing target to develop potential strategy to provide neuroprotection.

Uncoupling Protein-2 and Mitochondrial Potential

The proton gradient established on transfer of protons during the ETC generates the ΔΨm and provides the driving force to synthesize ATP along with the back transport of protons to the matrix. However, under limiting supply of ADP required for synthesis of ATP through FOF1-ATPase, increased mitochondrial respiration because of supply of nicotinamide adenine dinucleotide from glycolytic pathway, inability of FOF1-ATPase to cope up with the energy demand, and intracellular alkalization as during postischemic reoxygenation may lead to mitochondrial membrane hyperpolarization. (Di Lisa and Bernardi, 1998; Gergely et al, 2002; Back et al, 2000; Iijima et al, 2003b; McLeod et al, 2004; Iijima, 2006). The hyperpolarization during ischemic and long-term hypoxic condition is associated with increased ROS production (Nicholls and Ward, 2000) and mitochondrial Ca²⁺ accumulation (Smith et al, 2003). Interestingly, the mild uncoupling because of FCCP reduced the ROS generation besides decreases the ΔΨm (Yu et al, 2006). In addition, the pharmacologic induction of mitochondrial uncoupling by 2,4-dinitrophenol 1 h after reperfusion reduced infarct volume by 40% after 2 h of focal cerebral ischemia (Maragos and Korde, 2004). Emerging evidence suggest that UCP2 decreases the ΔΨm by transporting the protons back to the matrix. This has been linked to reduction in ROS generation (Arsenijevic et al, 2000; Mattiasson et al, 2003). Besides decreasing ΔΨm, UCP2 maintained the ΔΨm after oxygen glucose deprivation in primary cortical neurons overexpressing UCP2 in comparison to depolarization in control cells (Mattiasson et al, 2003). In addition, the induction of UCP2 in subpopulations of neuron and microglia after brain lesion is inversely correlated with caspase-3 activation (Bechmann et al, 2002), which suggests the role of UCP2 in regulation of cell death by stabilizing ΔΨm (Nicholls and Ward, 2000; Sullivan et al, 2003; Diano et al, 2003). These evidence, therefore, provide justification to target UCP2 for neuroprotection in excitotoxicity, stroke, and related conditions.

Uncoupling Protein-2, Adinosine Triphosphate, and Mitochondrial Proliferation

The UCP2-dependent transport of protons dissociate oxidation from phosphorylation (ATP production) and dissipates energy as heat. This makes the condition a little more complicated after cerebral ischemia where ATP levels are reported to decrease (26%), although, likely recover to normal levels (80%) during the initial 4 h of reperfusion (Sun et al, 1995). The depletion of ATP during cerebral ischemia depends on the duration and severity of ischemic stress. Thus, profound ischemia lasting for a certain time, impair the ionic homeostasis and consecutively affect the cellular activity. Interestingly, it has been reported that despite the large increases in UCP expression, ATP production was enhanced in oxidative tissues (Short et al, 2001) and decreased intracellular ATP levels were accompanied by decreased UCP2 mRNA expression (Cheng et al, 2003). Similarly, in the brain and the muscle, the induction of uncoupling by UCP2 and UCP3 increased the ATP/ADP ratio (Garcia-Martinez et al, 2001; Horvath et al, 2003), suggesting that ATP production is maintained alternatively by mitochondrial proliferation (Wu et al, 1999; Blumcke et al, 1999). Recently, Andrews et al (2008) have reported that gut-derived hormone ghrelin increased mitochondrial number in neuropeptide Y neuronal perikarya in wild-type but not UCP2^−/− mice. Uncoupling protein-2 is linked to transcription factors, nuclear respiratory factor 1 (Nrf1) and peroxisome proliferator-activated receptor-γ coactivator-1 (PGC-1) that was found to stimulate the mitochondrial biogenesis (Wu et al, 1999; Andrews et al, 2008), suggesting that mitochondrial proliferation is controlled by UCP2 (Coppola et al, 2007). Thus, it seems apparent that greater number of mitochondria can produce higher amounts of ATP without a corresponding increase in free radical production as compared with the normal number of tightly coupled mitochondria (Diano et al, 2003). In addition, it is tempting to speculate that the delicate balance between mitochondrial uncoupling and ATP production is maintained by signaling mechanisms that activate mitochondrial proliferation (Diano et al, 2003; Coppola et al, 2007; Andrews et al, 2008). Moreover, occurrence of such mechanisms may be of vital importance during cerebral ischemia.

Uncoupling Protein-2 and Calcium

The excitotoxic elevation of intraneuronal Ca²⁺ is one of the major events during glutamate excitotoxicity and cerebral ischemia in mediating brain injury. The elevated intracellular Ca²⁺ is readily taken up by mitochondria through the electrophoretic channel called uniporter, when electrochemical gradient is established across IMM. The mitochondrial Ca²⁺ is effluxed by Ca^2+/2H⁺ and/or Ca²⁺ /3Na⁺ exchanger depending on the ΔΨm as decrease of ΔΨm promotes Ca²⁺ release, whereas elevation of ΔΨm favors Ca²⁺ influx. However, vast mitochondria Ca²⁺ sequestration contributes to the formation of MPTP under cerebral ischemic condition. Moreover, mitochondrial Ca²⁺ elevation also stimulate the ROS production and FFA, which also promote the opening of MPTP. The MPTP allows free movement of apoptogenic molecules and as a result can activate cell-death cascades. Therefore, Ca²⁺ signals that are converged in mitochondria seem to regulate mitochondrial-dependent apoptotic cell death.

It is proposed that UCP2-mediated decrease of ΔΨm may be involved in the reduction of mitochondrial Ca²⁺ and production of ROS (Horvath et al, 2003), which subsequently can reduce the probability of MPTP and activation of cell-death cascades (Richard et al, 2001; Mattiasson et al, 2003). The UCP2-mediated potential drop, although may not be significant enough to affect the Ca²⁺ sequestration but may be advantageous to reduce the rate of Ca²⁺ uptake (Nicholls, 2002; Mattiasson and Sullivan, 2006). Therefore, it appears reasonable that UCP2-induced decrease of ΔΨm could limit the overloading of mitochondria with Ca²⁺, and hence decrease the potential for apoptotic events (Horvath et al, 2003). However, contradictory results existed (Trenker et al, 2007). Obviously, additional studies are necessary to further approve the importance of UCP2 in cellular signaling. Nevertheless, modulation of UCP2 may seem imperative in limiting the mitochondrial dysfunction and subsequent cell death in various neurodegenerative diseases including cerebral ischemia (Mattiasson et al, 2003).

Uncoupling Protein-2 and Neuronal Activity

In addition to the regulation of various cellular aspects of autonomic, endocrine, and metabolic processes, it has been proposed that the uncoupling property of UCP2 may be important in temperature regulation in the microenvironment of presynaptic terminals (Fleury et al, 1997; Horvath et al, 1999). This appears notable as UCP2 was reported to be localized at axons and axon terminals that are actively involved in synaptic vesicle formation, trafficking, and finally neurotransmitter release (Horvath et al, 1999). Moreover, the significant increase in the number of neuronal axons in the ischemic periphery of multiple infarction cases together with increase in UCP2 expression highlight the importance of UCP2 in such mechanisms (Nakase et al, 2007). The thermogenic capability of UCP2 although is not clear, however, the cells expressing c-fos after cold exposure are innervated by UCP2 containing axons (Horvath et al, 1999). Uncoupling protein-2 expression might be under the control of peroxisome proliferator-activated receptor-β (PPARβ; Aubert et al, 1997; Wang et al, 2003) as the expression of UCP2 was not upregulated in PPARβ knockout mice after cerebral ischemia (Arsenijevic et al, 2006). PPARs are ligand-activated transcription factors that regulate the expression of genes involved in fatty acid uptake and oxidation, lipid metabolism, and inflammation by binding to specific response elements within promoters (Kersten et al, 2000). Among the different types (α, β, and γ) of PPARs, PPARβ is the only one widely expressed in neurons and oligodendrocytes (Cullingford et al, 1998; Vanden Heuvel, 2007). The deletion of PPARβ gene in mice dramatically exacerbated the lesion volume in mice after cerebral ischemia (Arsenijevic et al, 2006; Pialat et al, 2007). The gene expression initiated by PPARs is regulated by coactivators and PGC-1, a cold-inducible coactivator of PPARs that is highly expressed in tissues such as brain with high metabolic rates (Puigserver et al, 1998; Rosenfeld and Glass, 2001). It is involved in activation of many aspects of the adaptive thermogenic program (Wang et al, 2003). Proliferator-activated receptor-γ coactivator-1 was found to stimulate the mitochondrial biogenesis and respiration in muscle cells through an induction of UCP2 (Wu et al, 1999). Thus, it seems logical that the physical interaction of PGC-1 with PPARβ (Wang et al, 2003) stimulates the induction of UCP2 and regulate the temperature important for synaptic transmission through UCP2. Additionally, many of the hypothalamic and thalamic nuclei that are known to participate in the regulation of body temperature, expresses UCP2 (Horvath et al, 1999; Deierborg et al, 2008). These reports suggest that the location of UCP2 at or nearaxon and axon terminals may be important in thermogenic buffering required to accelerate synaptic transmission and interneuronal communication for central homeostasis (Horvath et al, 1999).

Cerebral ischemia and diabetes

The consequence of stroke is often enhanced by risk factors such as diabetes, which increases the incidence of stroke in particular the ischemic stroke by 4 to 12 folds (Bemeur et al, 2007). In addition, between 20% and 50% of all patients presenting with stroke may be hyperglycemic at the time of admission (Scott et al, 1999). The neuronal glucose level in diabetic hyperglycemia is suggested to increase up by fourfold (Tomlinson and Gardiner, 2008), and increased glucose level or preischemic hyperglycemia is negatively related to stroke outcome (Li et al, 1994, 2000; Parsons et al, 2002). Studies have suggested that acute hyperglycemia and chronic diabetes can aggravate brain damage because of transient focal or forebrain ischemia (Nedergaard, 1987; Siesjo et al, 1996; Li and Siesjo, 1997; Li et al, 1998; Gisselsson et al, 1999) by glutamate excitotoxicity (Gaitonde et al, 1987; Li et al, 2000) and increased ROS generation in metabolically challenged tissue (Nelson et al, 1992, Nishikawa et al, 2000; Muranyi et al, 2006). The ROS generation is suggested to be a connecting link between high glucose, mitochondrial dysfunction, and apoptosis (Russell et al, 2002). Similarly, the excessive glucose supply is suggested to upregulate the glucose transporters and ΔΨm (hyperpolarization), which might be detrimental to the brain cells (Mastrocola et al, 2005). In addition, an evolution of infarction is accompanied by repeated waves of spreading depression. The consequence of these effects is the extension of core region or conversion of penumbra into the core in a short time.

Uncoupling Protein-2 and Diabetic Cerebral Ischemia

The brain, owing to high energy demands, is exclusively dependent on glucose for energy production. However, the persistent or regular episodic increase in glucose level, as in the case of diabetic hyperglycemic, may be damaging to neurons (Tomlinson and Gardiner, 2008). Additionally, the restoration of blood circulation after cerebral ischemia under hyperglycemic condition, leads to pathophysiologic manifestation of increased glucose such as increased ROS generation and mitochondrial dysfunction in the neurons. It has been suggested that cellular energy and mitochondrial membrane function in response to glucose are regulated by UCPs (Marti et al, 2001; Brown et al, 2002). The property of UCP2 to leak protons across the IMM may reduce ROS and mitochondrial potential. These mechanisms (ROS and potential alteration) appear to be involved in damage progression in cerebral ischemia under diabetic condition (Bemeur et al, 2007). The overexpression of UCP2 negatively regulates glucose-stimulated insulin secretion in β-cell of pancreas and the amount of ATP generated from glucose (Zhang et al, 2001). The expression of UCP2 is stimulated in vitro and in vivo by hyperglycemia, FFA, and O₂⁻ (Zhang et al, 2001; Joseph et al, 2002; Krauss et al, 2003), which are increased after acute brain injury. Thus it seems reasonable to target UCP2 to reduce or restrict the progression of ischemic damage during high glucose. Furthermore, evidence suggests that UCP2 in the presence of hyperglycemia remarkably prevents the catastrophic loss of IMM potential, Ca²⁺ overload, decreases ROS production, and glucose-induced neuronal programmed cell death (Diano et al, 2003; Teshima et al, 2003; Vincent et al, 2004).

Although UCP2 may offer neuroprotective effect after stroke with or without preexisting hyperglycemic condition, glucose sensing negatively controlled by UCP2, reduces release of insulin from β-cell and may increase the blood glucose level. Recently, it has been reported that an increase in UCP2 in the brain makes the glucose-excited neurons less sensitive to glucose (Parton et al, 2007). The study conducted on wild-type and UCP2^−/− mice fed on a high fat diet revealed that the abolished glucose-sensing in the wild type is reversed in UCP2^−/− animals (Parton et al, 2007). Such impairment in glucose sensing by UCP2 may be one of the reasons to prevent the high glucose induced brain damage under the condition of high-fat diet, obesity, and diabetes. Moreover, Chen et al (2006) have reported that rosiglitazone, a PPAR-γ agonist and currently used as antidiabetic drug, can induce UCP2 expression and thereby provide the correlative evidence of PPAR-γ-UCP2-dependent neuroprotection in ischemic brain injury. These conclusions strengthen the proposal demonstrating UCP2 as a neuromodulator and neuroprotector (Horvath et al, 2003).

Conclusions

The emerging evidence suggests that UCPs play a very important role in mitochondrial functioning by acting as proton transporters. Their expression, activity, and distribution in various types of tissue appear to be a critical determinant in number of diseases including cerebral stroke. Recent studies have proposed the role for UCP2 in regulation of ΔΨm, ROS, calcium, and neuronal activity, which seems to have significant impact on cellular homeostasis and neuroprotection. These factors support the fact that exploring the potential role of UCP2 might be beneficial to develop the appropriate strategy to reduce progression of damage after stroke and other neurodegenerative diseases. In addition, continuous research is imperative to translate the promising role of UCP2 into suitable therapeutic strategy.