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Insulin receptor and its substrates. The Insulin signaling cascade is initiated by the binding of insulin to the insulin receptor, a receptor tyrosine kinase which consists of two Î± subunits and two Î² subunits that are disulfide linked into an Î±2Î²2 heterotetrameric complex (Saltiel and Kahn 2001) (Lizcano and Alessi 2002). The receptor then undergoes a series of transphosphorylation reactions during which it phosphorylates specific tyrosine residues. Activation of the insulin receptor leads to tyrosine phosphorylation of different isoforms of insulin receptor substrates (IRS), the Shc adapter protein isoforms, Grb2 and as a result triggers two separate branches of signaling one involving the Shc, Grb2 mitogen-activated protein (MAP) kinase (MAPK) cascade and the other involving activation of Phosphatidylinositol 3-kinases (PI3K) from the IRS. (Pessin, J. E., and Saltiel, 2000) (Saltiel and Kahn 2001). Activation of PI3K is followed by the phosphorylation of phosphatidylinositol (4,5) bisphosphate (PtdIns(4,5)P2) leading to the formation of Ptd(3,4,5)P3 by the catalytic subunit of PI 3-kinase, p110 as shown in Figure 1. AKT or protein kinase B (PKB), an important downstream effector of Ptd (3,4,5) P3, is recruited to the plasma membrane. Akt is activated by the master kinase 3-phosphoinositide dependent protein kinase-1 (PDPK1), also known as PDK1, which is indirectly regulated by PI3K by phosphorylating phosphatidylinositols. The phosphorylation of Akt leads to the phosphorylation of the Rab GTPase activating protein AS160 that facilitates the translocation of glucose transporter protein (GLUT4) transporters to the plasma membrane. Activation of AKT leads to the phosphorylation and inactivation of glycogen synthase kinase 3 (GSK3) (Bouché, C, 2004, Endocr. Rev). Glycogen synthase is a substrate of GSK3 and catalyses the final step in glycogen synthesis. Glycogen synthesis is inhibited by GSK3 via phosphorylation of glycogen synthase (Saltiel and Kahn, 2001).
Figure1. Insulin binds to its receptors leading to the auto-phosphorylation of the beta subunits and tyrosine phosphorylation of the insulin receptor substrate and other signaling molecules like SHC, Grb2 which result in a diverse range of signaling pathways involving MAPK cascade and PI3K activation. The catalytic subunit of PI 3-kinase, p110 catalyzes the conversion of PtdIns (4,5) P2 to Ptd (3,4,5) P3 that in turn activates and phosphorylates Akt through PDK1. Akt phosphorylation of the Rab GTPase activating protein allows the translocation of the GLUT4 transporters to the plasma membrane. Activation of Akt also inactivates and phosphorylates GSK3 that in turn promotes glycogen synthesis.
Inhibition of insulin receptor signaling
Impaired insulin signaling contributes to a wide range of metabolic abnormalities including obesity, insulin resistance and diabetes (R. A. DeFronzo 2010). Inflammatory cytokines like TNF¡ induces serine phosphorylation of IRS-1 and thereby prevent its interaction with PI3K leading to an inhibition of insulin signaling (Sykiotis GP, et al 2001) (Daniela Dietze et al 2002) (Aguirre V et al, 2002, JBC) (Hotamisligil, G. S. et al 1996). Alterations in the activity of Akt are involved in various models of diabetes and insulin resistance (N. Hay, Trends Endocrinol. Metab, 2011). Decreased Akt phosphorylation has been reported in skeletal muscle and adipocytes of patients with T2D (N. Hay, Trends Endocrinol. Metab. 2011). Defects in insulin signaling are the main cause of impaired glucose metabolism ultimately leading to insulin resistance (Sesti, G. 2006).
Although, there have been several studies showing defective insulin signaling and insulin resistance in skeletal muscle and liver, very little is known about insulin resistance in neurons. In 2011, Kim et al reported that hyperinsulinemia causes insulin resistance in dorsal root ganglion neurons by disrupting the Akt-mediated pathway. The same study also showed that chronic insulin treatment lead to a high basal Akt phosphorylation in dorsal root ganglion neurons resulting in a blunted acute phosphorylation of Akt and glycogen synthase kinase-3Î² (GSK 3Î²) (Kim B et al, 2011). Another study reported increased IRS2 expression in dorsal root ganglion neurons of type 2 diabetic mice leading to suppression of insulin signaling (C. W. Grote. Et al 2011). Impaired IRS1 signaling was noted in the hippocampi of Tg mice that resemble an Alzheimer's disease (AD) brain model indicating defective insulin signaling in these mice brains (Bomfim TR, et al 2012). However, the mechanism behind impaired insulin signaling in neurons is still not completely known and needs further investigation.
Type II diabetes & insulin resistance
Insulin resistance is central to the pathogenesis of type II diabetes mellitus (T2DM) and is a major metabolic disorder associated with obesity. Insulin resistance may be defined as the inability of some of the cells of the body to efficiently respond to insulin. Insulin resistance precedes the onset of T2DM (Prikoszovich, T et al, 2011). In the early stage of T2DM, Î² cells secrete sufficient insulin to compensate for insulin resistance and hence maintain euglycemia (Savage, D et al, 2007). However at a later stage, insulin deficiency supersedes precipitating overt diabetes and hyperglycemia (Savage, D et al 2007).
Factors inducing insulin resistance
A number of factors have been reported to induce insulin resistance in the periphery including high levels of free fatty acids, hyperinsulinemia, increased oxidative stress, inflammatory cytokines and high amino acids (Goldstein BJ, 2002) (Kraegen, E, et al 1991) (Konrad Talbot et al 2012) (Griffin ME et al, 1999).
The role of free fatty acids (FFA) in causing insulin resistance is widely known in patients with type 2 diabetes having defects in FFA metabolism (Arner P, 2002). Plasma free fatty acids (FFA) are increased in obese subjects that in turn inhibit insulin's anti-lipolytic action thereby augmenting the FFA release into the circulation (Guenther Boden, 2011). High levels of FFA are known to cause a defect in the hepatocyte insulin function (Arner P, 2002).
Hyperinsulinemia may be described, as a condition during which there is elevated levels of insulin in the blood circulation than expected relative to the level of glucose. Hyperinsulinemia is common in people with T2DM and has been considered as a major risk factor for developing insulin resistance (Shanik MH, et al 2008). Basal plasma insulin levels were increased by two to four fold in mice transfected with extra copies of the human insulin gene (Shanik MH, et al 2008). These mice also exhibited an augmented postprandial glucose associated with an amplified insulin response to glucose suggesting that hyperinsulinemia in the basal state leads to insulin resistance associated with alterations in glucose metabolism and insulin secretion (Shanik MH, et al 2008).
Increased oxidative stress has been previously reported to cause defects in insulin signaling and induce insulin resistance in skeletal muscle through the Angiotensin II induced reactive oxygen species (ROS) generation by increasing the NADPH oxidase activity (Wei Y, et al 2006). In 2008, Morris J et al reported that oxidative stress is one of the important factors for causing insulin resistance in a 6-hydroxydopamine model of Parkinson's disease where the expression of heat shock protein 25 (hsp25), a protective agent for oxidative damage was found to be reduced in striated dopamine depleted lesioned rats. The same group also observed activation of GSK3 isoforms in response to DA depletion indicating a possible defect in insulin signaling (Wei Y, et al 2006).
Tumor necrosis factor alpha (TNF ¡), an important inflammatory cytokine has been associated with insulin resistance through increased free fatty acid secretion and defective insulin signaling at the level of PI3K (L.S. Liu, Diabetes, 47 1998) (Goldstein BJ, Am J Cardiol. 2002). Studies on human adipocytes have shown that preincubation with TNF-Î± resulted in a 60-70% reduction of insulin action (L.S. Liu, Diabetes, 47 1998).
Amino acids like leucine have also been implicated in causing insulin resistance in the skeletal muscle by downregulating 5' adenosine monophosphate-activated protein kinase (AMPK) activity and increasing mTOR/p70S6K signaling (Saha, Diabetes, 2010). Activation of mTOR/p70S6K signaling has been widely known to cause peripheral insulin resistance (Niu YM, 2010) (Saha, Diabetes, 2010) (Lee DF, 2008) (Balage M, 2011). Incubation of rat extensor digitorum longus (EDL) muscles with leucine (100 and 200 Î¼M) was found to increase protein synthesis and phosphorylation of mTOR/p70S6K (Saha, Diabetes, 2010). Also high levels of leucine were found in the plasma of 3 weeks old diabetic rat brain that was restored by exogenous insulin therapy (Crandall EA, 1983). Although, there is enough evidence about the FFA, hyperinsulinemia, inflammatory cytokines and amino acids in inducing insulin resistance in the periphery, very little is known about its role in neurons.
Insulin signaling in the brain
Insulin levels in the brain
Over three decades ago, Havrankova et al. (Havrankova, J.et al, 1978) reported that the insulin receptor is widely distributed in the central nervous system with its highest expression in the olfactory bulb, followed by the cerebral cortex, hippocampus and the pre optic area, hypothalamus and amygdala. The same study also reported that the insulin concentration in the whole brain was 25 fold greater than in plasma determined by radioimmunoassay. The brain insulin levels in rats after 48 hr of streptozotocin treatment was found to be l6 ±5 ng/ml whereas the untreated control rats had an insulin concentration of 8 ±2 ng/ml in the brain (Havrankova, J.et al, 1979). However, other studies have reported different concentrations of insulin which suggest that there is no general agreement about the concentration of insulin in the brain and needs further investigation. Expression levels of the insulin receptors are developmentally regulated in a rat brain (S Kar, Comp Neurol 1993). Brain regions like thalamus, caudate-putamen and some mesencephalic and brainstem nuclei had higher concentrations of insulin receptors during neurogenesis compared to the adult rat brain (S Kar, Comp Neurol 1993). Also, glia cells were found to have lower insulin receptor levels compared to neurons and these levels decreased with age (Unger et al., 1989). It was not until 1991, that Schwartz et al to demonstrate that insulin crosses the blood brain barrier (BBB) by a saturable mechanism. Following studies showed that insulin could cross the BBB by a receptor-mediated transcytosis (William A. Banks et al, 2012). The brain endothelial cells (BEC) play a very important role in the insulin transport across the blood brain barrier as the brain endothelial cells that comprise BBB and the blood Cerebrospinal fluid (CSF) barrier contain binding sites for insulin (Frank and Pardridge, 1981), (Frank et al., 1985). The insulin binding sites at the BBB aid in the transport of insulin across the BBB and also function as receptor sites that help activating the intracellular machinery of the barrier cell (William A. Banks, et al, 2012).
Effects of insulin in the brain
Insulin has widespread effects on the brain including its role in control of food-intake and in cognition (A.F. Debons, Am J Physiol, 219 1970). The effects of peripheral insulin is very different from the central nervous system (CNS) insulin as the latter is found to have opposite effects to peripheral insulin like decreased feeding, reduced body weight and increased blood glucose levels (J.S. Hatfield, Pharmacol Biochem Behav, 1974) (J.C. Bruning, Science Washington DC, 2000) (A.F. Debons, Am J Physiol, 1970). Chronic administration of insulin into the ventricular system or the hypothalamus is reported to decrease the food intake and body weight in a dose-dependent manner wile maintaining peripheral euglycemia (Brief DJ, 1984). However, administration of insulin autoantibodies has been shown to increase the food intake and body weight in the ventromedial areas (Stockhorst U, et al 2004) (Gerozissis .K, 2003).
The role of insulin in memory and cognition has been well evidenced by some typically used studies in animals including Morris water maze or passive avoidance task that have shown to improve memory performance in rats upon administration of insulin (C.R. Park, 2001) (C.R. Park, et al 2000). An increase in insulin receptor expression was observed in the hippocampal dentate gyrus and CA1 field when rodents were trained on a spatial memory task, suggesting that synthesis of insulin receptor may be augmented in these areas as a result of learning (Zhao et al, 1999). Insulin pre-treatment improved the memory deficits in rats with hippocampal lesions, which were previously suffering from severe loss of learning and memory ability [Huppert and Piercy, 1979) (De Castro and Balagura, 1976) (Zhao et al, 2004).
Role of insulin in Alzheimer's disease
Over the past decade, the link between impaired insulin sensitivity and Alzheimer's disease (AD) has been one of the major topics of discussion (Hoyer S, 1998) (Messier.C, 1996). This is evident with observed impaired insulin signaling and abnormal levels of insulin in the CSF and plasma of patients with AD (Neumann KF, et al 2008) (Craft S, et al 1998). A defect in systemic insulin sensitivity is observed in patients with AD (Park C.R, 2001). Defective insulin receptor signaling was noted in patients with dementia of the Alzheimer type (DAT) accompanied with higher levels of non-metabolized glucose in the cerebral blood suggesting that DAT could be a kind of T2DM in the brain (Hoyer S, 1998) (Stockhorst U, et al 2004). Further, in patients with AD the levels of insulin in the CSF are lower compared to the peripheral plasma insulin levels (Craft S, et al 1998).
T2DM is associated with higher risk for cognitive decline and AD (Konrad Talbot et al J Clin Invest. 2012) (Ohara T, et al Neurology. 2011). One of the most common features in both AD and T2DM is insulin resistance which is very well described by Goldstein (Goldstein BJ, Am J Cardiol, 2002) as the inability of the cells to respond to stimulation by insulin. Role of insulin in cognition is very well documented by studies relating to AD and insulin resistance (Ohara T, et al Neurology. 2011) (L.J Neural Transm, 1998). Insulin and insulin receptor levels in the brain are decreased in patients with sporadic AD (L. Frolich, J Neural Transm, 1998). Memory performance is reported to ameliorate in patients with AD upon systemic administration of insulin under the conditions of euglycemia or hyperglycemia (Craft et al., 1999) (Craft et al., 2000) (W.Q Zhao et al, 2004). However, the same effect was not true under conditions of hyperglycemia alone (Craft et al., 1999) (Craft et al., 2000) (W.Q Zhao et al, 2004). Aggregation of Î²-amyloid (AÎ²) peptide is one of the main hallmarks of Alzheimer's disease. Studies have shown links between AÎ² and insulin signaling. Infact, AÎ² have been shown to inhibit the brain insulin-signaling cascade, which is mediated by the binding of soluble AÎ² to insulin receptor causing a downregulation of the plasma membrane insulin receptors leading to synaptic spine loss in hippocampal neuron cultures. (B. Cholerton, et al 2011) (Townsend M, et al, 2007). However, this effect was reversed upon pretreatment with insulin (De Felice F.G, et al 2009).
The Synaptic vesicle cycle
Neurotransmitter release is triggered by an action potential from a pre-synaptic nerve terminal that initiates the synaptic transmission. The neurotransmitters are released into the synaptic cleft between two neurons by a process known as exocytosis. This happens after an action potential is generated from a pre-synaptic nerve terminal. Once, the neurotransmitters are released, they are rapidly reformed to maintain sustained synaptic transmission. A process known as endocytosis to restore the synaptic vesicles achieves this. An overview of the synaptic vesicle cycle is shown in Figure 2.
Exocytosis involves two major steps namely: docking and priming. Docking is the process during which the loaded synaptic vesicles dock near the release sites. Docking is mediated by a group of proteins that form the active zone complex: rab-3 interacting molecules, Munc-13 proteins, Bassoon and Piccolo, ELKS proteins and Liprin -¡ proteins by forming a scaffold at the active zone and act as the pre-synaptic spatial organizer, thereby integrating the synaptic vesicle cycle (Schoch et al, 2006) (Abraham, C, Neuroscience 2005).
The next step is priming where the synaptic vesicles are ready to be able to fuse rapidly in response to a calcium influx. Formation of SNARE complex of proteins is one of the major events during priming where the R-SNARE protein synaptobrevin from the synaptic vesicle forms a complex with Q-SNAREs SNAP-25 and syntaxin from the PM thereby making the primed synaptic vesicle ready to be exocytosed (Sutton, 1998) (Becherer, 2006).
The synaptic vesicles now get ready to fuse quickly in response to the membrane depolarization of an action potential as shown in Figure 2. The action potential generated activates the voltage gated Ca2+ channels, causing an influx of Ca2+ that is detected by the synaptotagmin family of proteins (Thomas C. Südhof et al, 2004). Ca2+binding to synaptotagmin causes the fusion pore to open and secretion of neurotransmitters hence leading to exocytosis of the primed synaptic vesicle (Nishiki et al, 2004).
Immediately following exocytosis, synaptic vesicles are reformed by endocytosis. This involves the recovery of the synaptic vesicle proteins that are exocytosed to the PM followed by the reformation of the vesicles by endocytosis. The main pathway of endocytosis is the clathrin-mediated endocytosis (CME).
CME is initiated by a Ca2+ influx with the help of calmodulin as a sensor that provides powerful coupling of endocytosis to exocytosis thereby facilitating synaptic vesicle cycling (Wu, et al, 2009). Following dispersion of the transmembrane synaptic proteins to the PM by exocytosis, the proteins are re-assembled through interactions with the AP-2 adapter proteins complex which facilitates the clathrin and other accessory proteins (amphiphysin and endophilin) to associate with AP-2 and hence form an inward curvature of membrane in the form of a coated pit that grows to a spherical invagination (Ryan, 2006) (Jung, 2007). The guanoside triphosphate hydrolase (GTPase) then mediates pinching off the coated pit to form a single clathrin-coated vesicle (Royle, 2003). The exact mechanism behind this process is not very well understood. However, a study suggests that dynamin is recruited onto amphiphysin in its guanoside diphosphate (GDP)-bound form that facilitates the membrane cleavage and hence pinches off the coated pit (Sever S, 2002). Following detachment from the plasma membrane, the vesicles are uncoated by the chaperone hsc70 and the DNA-J domain co-chaperone auxillin. Uncoated endocytic vesicles are then re-acidified and reform the synaptic vesicles by either direct recycling or by reaching an early endosome where the synaptic vesicles are restored from the recycling endosome and reloaded with neurotransmitters thus facilitating the synaptic vesicle cycle (Südhof, 2004). However, there is still some debate about the process of reforming synaptic vesicles from the early endosome and needs further investigation.
There are different pools of synaptic vesicles during the process of synaptic transmission when they are not associated with the plasma membrane (PM) namely: reserve pool or recycling pool of vesicles. These pools are generally defined based on their position and function (Cheung G, 2011). The recycling pool is the group of vesicles that are located close to the synaptic plasma membrane and are released under moderate stimulation. On the other hand, reserve pool group of vesicles are mobilized by intense stimulation and are located farther away from the synaptic PM (Cheung G, 2011).
Figure 2. 1) Synaptic vesicles are loaded with neurotransmitters at the presynaptic nerve terminal. 2) Loaded vesicles are then docked near the plasma membrane followed by 3) priming which makes them competent for Ca2+ triggered fusion-pore opening (4) thereby triggering exocytosis of the synaptic vesicle. 5) Clathrin-mediated endocytosis retrieves the synaptic vesicles. 6) The vesicles are uncoated and re-acidified. 7) Re-acidified vesicles are either directly recycled from step 7 to step 1 or 8) reach an early endosome where the synaptic vesicles are reformed and 9) reloaded with neurotransmitters.
Role of insulin / insulin signaling in synaptic function
Insulin and insulin-like growth factor 1 (IGF-1) support neuronal growth and survival (Katarina G. Brywe, 2005). Insulin regulates synaptic plasticity by modulating activities of excitatory and inhibitory receptors such as glutamate and GABA receptors (Zhao WQ, 2004). The effects of insulin on the postsynaptic side are best known. A study by Liu et al, 1995 and (Liao and Leonard, 1999) showed that a brief exposure to insulin initiated a robust potentiation of responses to N-Methyl-D-aspartic acid (NMDA) mediated by NMDA receptor in Xenopus oocytes expressed with NMDA receptors. An increase in the tyrosine phosphorylation of the NR2A and 2B subunits of NMDA receptors was observed when rat hippocampal slices were incubated with insulin (J.M. Christie, 1999). Insulin facilitates postsynaptic receptor trafficking in gamma-amino butyric acid (GABA) receptors by mediating synaptic inhibition that plays an important role in neuronal functions associated with learning (O. Paulsen, 1988). Insulin receptors recruit functional GABA receptor to the postsynaptic membrane thereby modulating the GABA transmission (Wei-Qin Zhao, 2004). The GABAergic neurons in turn send feedback inhibitory inputs to the principal neurons as they sense the excitatory transmission and hence regulate synaptic strength (Wei-Qin Zhao, 2004). There is enough evidence about the effects of insulin/ insulin receptors in the postsynaptic side, but very little is known about the presynaptic side of the neurons.
Insulin and mitochondrial function
Insulin resistance and mitochondrial dysfunction
Insulin resistance and/or T2DM is linked with decreased mitochondrial mass and function (Morino K, Diabetes. 2006). Kelley et al found reduced mitochondrial size in obese subjects with insulin resistance and/or T2DM (Kelley et al. 2002). Magnetic resonance spectroscopy studies showed impaired insulin stimulated mitochondrial function in elderly subjects with insulin resistance (Petersen, K, et al, 2003). A couple of years later, Sparks et al. (2005) reported reduced mRNA levels of PGC1Î± and PGC-1 Î² in the muscle of healthy male subjects after three days of high fat feeding. Thiazolidinediones like pioglitazone and rosiglitazone have been reported to activate mitochondrial biogenesis in human adipose tissue (Bogacka I, Diabetes.2005). Despite these findings, it is not unambiguously clear whether the observed mitochondrial dysfunction was a primary cause of insulin resistance or a consequence of insulin resistance. Numerous studies have investigated the link between impaired mitochondrial function and insulin resistance in the skeletal muscle and liver, but very little is known about the link between insulin/insulin resistance and mitochondrial function in the brain.
Mitochondrial biogenesis in the brain
The peroxisome proliferator-activated receptor gamma (PPARÎ³) coactivator (PGC-1) family of inducible transcriptional co-activators is one of the key elements in mitochondrial biogenesis and promotes gene transcription involving mitochondrial metabolism and function (Turner, N. 2008). Most of the studies relating mitochondrial biogenesis are focused on skeletal muscle and liver. Little is known about mitochondrial biogenesis in the brain. In 2011, Hathorn T, et al reported that in a mouse model of Huntington's disease, motor deficits were improved by nicotinamide and lead to an increase in PGC-1Î± and brain-derived neurotropic factor (BDNF) gene expression. A study published in the same year (Steiner, J et al, 2011) showed that mitochondrial biogenesis in the brain was increased by exercise training in mice. Mitochondrial biogenesis related genes, PGC-1Î±, Silent mating type information regulation 2 homolog (SIRT1) and Citrate synthase were increased after 8 weeks of treadmill running in mice (Steiner, J et al, 2011). Impaired mitochondrial biogenesis, synaptic degeneration and defective axonal transport of mitochondria were observed in a mouse model (AÎ²PP) of Alzheimer's disease which was restored to normal by a mitochondria-targeted antioxidant SS31 (Calkins MJ, 2011). Although, there is a correlation between decreased mitochondrial biogenesis and synaptic degeneration, the exact mechanistic links are yet to be elucidated.
Mitochondria in the synapse
Neuronal mitochondria are known to be present mostly in the cell bodies and to a larger extent in the synaptic terminals (Nguyen et al, 1997). Mitochondria are reported to regulate synaptic calcium levels and act as calcium buffers (Cindy et al 2006). Stagnant release of mitochondrial calcium was shown to be essential for synaptic strengthening by using TPP+ to block the Na+ dependent and independent mitochondrial calcium efflux indicating that mitochondrial calcium buffering has an effect on synaptic transmission (Tang and Zucker 1997). Mitochondrial ATP is known to be very important for maintaining neurotransmission as it is involved in providing energy during many steps of the synaptic vesicle cycle (Patrik Verstreken, et al 2005). This is evident by studies on Drosophila drp1 mutants that are unable to mobilize the reserve pool vesicles due to lack of synaptic ATP (Patrik Verstreken, et al 2005). Further, supplementing the drp1 synapses with ATP facilitated the mobilization of reserve pool vesicles (Patrik Verstreken, et al 2005). In 2003, Kann et al showed that ATP production in mitochondria is regulated by synaptic activity indicating the role played by mitochondria in regulating synaptic strength.
Niemann -Pick Type C disease
Niemann- Pick Type C (NPC) disease is a rare neurodegenerative genetic disorder in humans affecting nearly 1 in 150 000 live births. It is associated with rapid neurological decline and shortens the life span to less than 20 years (Garver et al, 2007). One of the main hallmarks of this disease is cholesterol accumulation in the endosomes (L. Liscum, 1987) (Karten B et al, 2009) (Vance, 2006).
NPC disease causes progressive neurodegeneration in humans. Some of the commonly seen symptoms in NPC disease include ataxia, dystonia, dysphagia and seizures (Vance, 2006). These symptoms become apparent at a young age and can ultimately lead to death with a decade. NPC infants also suffer from neonatal jaundice and splenomegaly and a few of these infants ultimately die from acute liver failure/liver dysfunction (Vance, 2006). Another major characteristic symptom of NPC disease is the loss of Purkinje cells in the cerebellum and appearance of swollen neuritis Y.Higashi et al, Acta Neuropathol. (Berl.), 1993) (Vance, 2006). Cholesterol accumulation in the lysosomes, gangliosides (GM2 and GM3) and other lipids characterizes the biochemical phenotype of NPC deficient cells or tissues (J. Sokol et al, 1988) (Vance, 2006) (Y. Watanabe, 1998)(D. te Vruchte, 2004).
NPC disease is caused by mutations in the NPC1 or NPC2 genes (Millat et al, 1999) (Vance, 2006). NPC1 is an integral membrane protein in the membrane of the late endosomes (Higgins et al, 1999) (Karten B, et al, 2009). NPC2 is a soluble protein that was originally identified as a secretory protein in the human epididymis and was therefore originally known as HE1 (kirchhoff et al, 1996). Genetic mutations in the NPC1 gene are more common compared to mutations in the NPC2 gene (Garver, et al, 2007).
Insulin signaling in NPC disease
Altered cholesterol trafficking is the major defect observed in NPC disease (T.Y. Chang, et al, 2005) (F.D. Porter, 2010). There is enough evidence about the link between cholesterol metabolism and insulin signaling (R. Suzuki et al, 2010). Few studies have investigated insulin signaling in NPC disease. Insulin receptors have been shown to function via lipid rafts that are cholesterol-rich membrane microdomains involved in regulating various signal transduction events and membrane trafficking (Bickel PE, 2002). High plasma membrane cholesterol in the rafts has been reported to cause impaired insulin signaling (Vainio, et al 2005). In 2005, Vainio et al showed that the IR levels were increased and the receptor activation was decreased in NPC hepatocytes. They also observed an augmented association of the insulin receptor with the detergent resistant membrane (DRM) in NPC hepatocytes and plasma membrane fractions suggesting that there could be lipid imbalance in NPC hepatocytes and may contribute to insulin resistance (Saara Vainio et al, 2005). Later in 2012, Qi-Rui Ong et al, studied the expression of various proteins in the insulin signaling pathway in 9 week old brain of homozygous mutant BALB/c NPCnih mice. Phosphorylation of Akt was greatly reduced at residue T308 than S473 accompanied with a lower GSK3Î² phosphorylation and also a reduction in the insulin receptor substrate (IRS-2) in 9 week old NPCnih mouse brain (Qi-Rui Ong et al, 2012). Although there are a few studies focusing on the insulin signaling in NPC disease, till date there is no evidence about insulin resistance in the NPC diseased brain.