The cell biology of systemic insulin function

“Insulin regulates appetite by reducing expression of neuropeptide Y and Agouti-related peptide (orexigenic) and, conversely, elevating expression of pro-opiomelanocortin (anorexigenic; Schwartz et al., 2000).”

“In turn, Akt activates endothelial NO synthase to catalyze the conversion of l-arginine to NO (Palmer et al., 1988; Zeng et al., 2000).”

“…of the hormone’s journey in the body have been correlated and often causally related to insulin resistance, hypertension, and type 2 diabetes (Taniguchi et al., 2006; Hoehn et al., 2008; Odegaard and Chawla, 2013; Boucher et al., 2014; DeFronzo et al., 2015; Samuel and Shulman, 2016;…”

“…2018) and CEA CAM1 expression (Lee, 2011) in obesity • Impaired vasoactive effects of insulin during insulin resistance, including capillary recruitment (de Jongh et al., 2004; Clerk et al., 2006; Keske et al., 2009); reduced insulin delivery to muscle in obesity and diabetes (Broussard et al.,…”

“, 2013) as well as a number of proteins of the GLUT4 vesicle fusion machinery (Aslamy and Thurmond, 2017); the underlying defects include alterations in the maintenance of the storage compartment (Foley et al., 2011; Samuel and Shulman, 2012) and in the insulin-derived signals that trigger GLUT4 vesicle release from s…”

“…often causally related to insulin resistance, hypertension, and type 2 diabetes (Taniguchi et al., 2006; Hoehn et al., 2008; Odegaard and Chawla, 2013; Boucher et al., 2014; DeFronzo et al., 2015; Samuel and Shulman, 2016; Haeusler et al., 2018; also see other important highlights in the text box).”

“…often causally related to insulin resistance, hypertension, and type 2 diabetes (Taniguchi et al., 2006; Hoehn et al., 2008; Odegaard and Chawla, 2013; Boucher et al., 2014; DeFronzo et al., 2015; Samuel and Shulman, 2016; Haeusler et al., 2018; also see other important highlights in the text box).”

“NO is a potent vasodilator that rapidly diffuses to the vessels’ outer layer of smooth muscle cells, where it activates intracellular guanylate cyclase to increase cyclic guanosine monophosphate production (Arnold et al., 1977).”

“…the role of IDE in hepatic insulin clearance in vivo is controversial, as some studies report that loss of IDE results in hyperinsulinemia (Farris et al., 2003; Abdul-Hay et al., 2011), whereas others observed no changes in systemic circulating insulin in the absence of the enzyme…”

“Consistent with earlier studies, liver-specific IR knockout mice provided direct evidence that receptor-mediated degradation regulates systemic insulin levels and that impairments in this process lead to severe hyperinsulinemia that, in turn, contributes to whole-body insulin resistance (Michael et al., 2000).”

“Notably, alterations in insulin renal clearance prolong the permanence of insulin in the blood (Dahl et al., 1989), evincing the importance of this process to insulin’s half-life in the circulation.”

“As a consequence of endothelial NO production, insulin stimulates dilation of arteries and arterioles (Steinberg et al., 1994; Vincent et al., 2002).”

“Hence, glucose not only controls the Ca2+ signals that trigger insulin secretion but also generates signals that improve the efficacy of Ca2+ on the secretory process, likely by acting on the Ca2+ sensitivity of diverse components in the pathway (Henquin, 2000).”

“…autophosphorylation by inducing structural rearrangement of the transmembrane domains to bring them into close proximity with each other (Gutmann et al., 2018), and the consequent activation of the IR tyrosine kinase toward phosphorylation of its major substrates IRS1,2 (Copps and White, 2012).”

“The TBC domain of AS160/TBC1D4 defines its GAP activity toward Rab family small GTPases.”

“Insulin granule pools and their intracellular traffic Most insulin granules (perhaps 75–95% of an estimated 10,000) are stored within the β-cell cytoplasm at some distance away from the cell membrane (Rorsman and Renström, 2003).”

“In turn, Akt activates endothelial NO synthase to catalyze the conversion of l-arginine to NO (Palmer et al., 1988; Zeng et al., 2000).”

“This ensures that a portion of dietary glucose is effectively stored in the liver and is only released to the rest of the body upon cessation of insulin action (between meals) or upon metabolic demand enacted by other “counterregulatory” hormones (Samuel and Shulman, 2018).”

“…often causally related to insulin resistance, hypertension, and type 2 diabetes (Taniguchi et al., 2006; Hoehn et al., 2008; Odegaard and Chawla, 2013; Boucher et al., 2014; DeFronzo et al., 2015; Samuel and Shulman, 2016; Haeusler et al., 2018; also see other important highlights in the text box).”

“Conversely, central defects in insulin action are emerging as a potential contributor to the development of Alzheimer’s disease (Griffith et al., 2018), possibly as a result of abnormal phosphorylation of tau protein (Kleinridders et al., 2014).”

“The portal circulation delivers insulin into the capillaries of the sinusoids, which are not supported by a basement membrane and their endothelial cells contain fenestrations (Wisse, 1970; Braet et al., 1995), together permitting the exchange of contents between the blood and the surrounding liver cells.”

“Two major consequences of the PIP3 burst relevant for GLUT4 translocation are activation of the kinases Akt1, 2 (Brozinick and Birnbaum, 1998; Wang et al., 1999) and of the Rho-family GTPase Rac1 (Chiu et al., 2011).”

“The journey of insulin in the body Journal of Cell Biology https://doi.org/10.1083/jcb.201802095 11 PTP-1B (Yamada et al., 2005; Fukuda et al., 2009; Bakke et al., 2013; Garrido-Sanchez et al., 2013; Fig.”

“Phosphorylation sites on IRS1,2 constitute entropic information to attract class I PI3K, which rapidly generates membrane domains enriched in PI(3,4,5)P3 (PIP3) within minutes (Ruderman et al., 1990).”

“…MacDonald, 2017; Gandasi et al., 2017) and impaired pulsatile secretion of insulin in diabetic individuals (Lang et al., 1981; Hollingdal et al., 2000; Laedtke et al., 2000) • Reduced hepatic insulin clearance (Jung et al., 2018) and CEA CAM1 expression (Lee, 2011) in obesity • Impaired vasoactive…”

“GLUT4 vesicle retention near the membrane also involves the exocyst subunit Exo70 (Lizunov et al., 2012).”

“HID-1 is required for homotypic fusion of immature secretory granules during maturation. eLife.”

“Insulin acting centrally also evokes efferent inputs into peripheral tissue metabolism (Ferris and Kahn, 2016), contributing to the suppression of gluconeogenesis in the liver and the counterregulatory response to hypoglycemia (Diggs-Andrews et al., 2010).”

“…in obesity and diabetes (Broussard et al., 2016) • Diminished GLUT4 translocation to the muscle membrane in diabetic rodents and humans (Klip et al., 1990; Zierath et al., 1996; Garvey et al., 1998; Hoehn et al., 2008; Czech, 2017) and lowered expression of Rac1 (Sylow et al., 2013) as well as a…”

“…phase resulting from fusion and secretion by already “docked and primed” secretory granules that in human lasts up to 10 min, and a subsequent second-phase secretion that is associated with actin reorganization thought to allow granule recruitment to the plasma membrane (Wang and Thurmond, 2009).”

“In adipocytes, Rab10 promotes GLUT4 mobilization from the perinuclear region toward the plasma membrane (Sano et al., 2007; Bruno et al., 2016), specifically by interacting with Sec16A (Sano et al., 2007; Bruno et al., 2016).”

“…often causally related to insulin resistance, hypertension, and type 2 diabetes (Taniguchi et al., 2006; Hoehn et al., 2008; Odegaard and Chawla, 2013; Boucher et al., 2014; DeFronzo et al., 2015; Samuel and Shulman, 2016; Haeusler et al., 2018; also see other important highlights in the text box).”

“Gap junction coupling between β-cells via connexin36 plays a critical role, the loss of which results in dysregulation of insulin secretion (Ravier et al., 2005).”

“Thought to be insulin unresponsive in the past, the central nervous system is well recognized to be exquisitely responsive to the incoming hormone (Porte et al., 2005).”

“…the initial gain in surface GLUT4 emanates from the storage compartment, whereas maintenance of the steady state involves the endosomal route (Bryant and Gould, 2011; Kandror and Pilch, 2011; Stöckli et al., 2011; Bogan, 2012; Leto and Saltiel, 2012; Klip et al., 2014; Jaldin-Fincati et al., 2017).”

“Insulin action in target tissues is temporally linked to these vascular effects (Barrett et al., 2009); in particular, the full stimulation of skeletal muscle glucose uptake in vivo is contingent on prior NO-mediated vasodilation (Vincent et al., 2003; Bradley et al., 2013).”

“…journey in the body have been correlated and often causally related to insulin resistance, hypertension, and type 2 diabetes (Taniguchi et al., 2006; Hoehn et al., 2008; Odegaard and Chawla, 2013; Boucher et al., 2014; DeFronzo et al., 2015; Samuel and Shulman, 2016; Haeusler et al., 2018; also…”

“As outlined above, insulin crosses the blood–brain barrier through a receptor-mediated process (Woods et al., 2003).”

“For example, feedback from oscillatory Ca2+ signals controls mitochondrial ATP generation (Tarasov et al., 2012), and recent work shows that Ca2+ oscillations and the ER protein TMEM24 interact at ER–plasma membrane contact sites to maintain phosphatidylinositol levels required for β-cell signaling…”

“The majority of insulin that binds to hepatic IRs is degraded (Duckworth, 1988).”

“, 2018) and CEA CAM1 expression (Lee, 2011) in obesity • Impaired vasoactive effects of insulin during insulin resistance, including capillary recruitment (de Jongh et al., 2004; Clerk et al., 2006; Keske et al., 2009); reduced insulin delivery to muscle in obesity and diabetes (Broussard et al.”

“In addition, these IR are important sites sensing the hormone to stimulate important functions such as reabsorption of sodium, phosphate, and glucose (Rabkin et al., 1984; Hale and Coward, 2013).”

“As endosomes acidify, any remaining insulin or partially degraded insulin that escaped complete degradation by IDE dissociates from the IR (Murphy et al., 1984; Fig.”

“…the initial gain in surface GLUT4 emanates from the storage compartment, whereas maintenance of the steady state involves the endosomal route (Bryant and Gould, 2011; Kandror and Pilch, 2011; Stöckli et al., 2011; Bogan, 2012; Leto and Saltiel, 2012; Klip et al., 2014; Jaldin-Fincati et al., 2017).”

“Activated Akt1,2 migrates to the cytosol and intracellular membranes (Zheng and Cartee, 2016), where it phosphorylates AS160, a substrate of 160 kD more appropriately named TBC1D4 (Sano et al., 2003; Lansey et al., 2012).”

“Although rich information is being gathered through electrophysiological approaches (van der Heide et al., 2005; Könner et al., 2007; Korol et al., 2018), there is a rich opportunity to explore additional mechanisms through the advent of real-time intravital imaging of the central nervous system…”

“…the initial gain in surface GLUT4 emanates from the storage compartment, whereas maintenance of the steady state involves the endosomal route (Bryant and Gould, 2011; Kandror and Pilch, 2011; Stöckli et al., 2011; Bogan, 2012; Leto and Saltiel, 2012; Klip et al., 2014; Jaldin-Fincati et al., 2017).”

“…the initial gain in surface GLUT4 emanates from the storage compartment, whereas maintenance of the steady state involves the endosomal route (Bryant and Gould, 2011; Kandror and Pilch, 2011; Stöckli et al., 2011; Bogan, 2012; Leto and Saltiel, 2012; Klip et al., 2014; Jaldin-Fincati et al., 2017).”

“Hence, the level of CEA CAM1 at the plasma membrane and its phosphorylation may impart physiological fine-tuning regulation to the process of insulin clearance.”

“In addition to the Rab13–MIC AL-L2–α-actinin4 connection, GLUT4 vesicles tether to actin filaments via Myosin 1c (Bose et al., 2002; Boguslavsky et al., 2012).”

“, 2000) prevent phosphorylation of myosin light chain required for cytoskeletal cross-bridge formation and contraction (Lee et al., 1997; Mizuno et al., 2008), thereby resulting in vessel relaxation (Fig.”

“This degradation is coupled to pulsatile delivery, such that the liver preferentially clears insulin that arrives in pulses (Meier et al., 2005).”

“Importantly, this translates into oscillations of insulin secretion from isolated islets, again with a periodicity of 1–5 min (Bergsten et al., 1994).”

“Thus, communication between cells within an islet likely contributes to the well-described phenomenon of insulin secretory oscillations, which occur in healthy humans with a periodicity of 5–10 min (Satin et al., 2015).”

“When the membrane potential depolarizes sufficiently (approximately −50 mV), the activation of voltage-dependent Na+ and Ca2+ channels cause repetitive action potential spiking and a rise in intracellular Ca2+ (Rorsman et al., 2012).”

“This restricts GLUT4 mobility beneath the membrane, a phenomenon nicely documented through total internal reflection fluorescence microscopy of muscle and adipose cells (Bai et al., 2007; Xiong et al., 2010; Boguslavsky et al., 2012; Lizunov et al., 2012).”

“As a consequence of endothelial NO production, insulin stimulates dilation of arteries and arterioles (Steinberg et al., 1994; Vincent et al., 2002).”

“This includes the glucose- and Cdc42-dependent activation of Rac1, which, when released from an inhibitory RhoGDI and in its GTP-bound form, promotes cortical actin remodeling, perhaps via an interaction with gelsolin (Kalwat and Thurmond, 2013).”

“The journey of insulin in the body Journal of Cell Biology https://doi.org/10.1083/jcb.201802095 2 required for insulin gene expression and contribute to the regulation of INS transcription in response to glucose and autocrine insulin signaling (Andrali et al., 2008).”

“…the role of IDE in hepatic insulin clearance in vivo is controversial, as some studies report that loss of IDE results in hyperinsulinemia (Farris et al., 2003; Abdul-Hay et al., 2011), whereas others observed no changes in systemic circulating insulin in the absence of the enzyme…”

“This is brought about through formation of a SNA RE complex between VAMP2 on the vesicles and synatxin4 and SNAP23 on the plasma membrane (Cheatham et al., 1996; Foster and Klip, 2000; Thurmond and Pessin, 2001).”

“5–1 nmol/liter in the fasted state and rises to ∼5 nmol/liter after a meal (Pørksen et al., 1995; Song et al., 2000).”

“…classical G-protein–coupled receptors via Gαs-activation of adenylate cyclase to increase cAMP, which causes PKA-dependent phosphorylation of the exocytic machinery and PKA-independent effects mediated by Epac2A to promote the release of Ca2+ from intracellular stores (Kolic and MacDonald, 2015).”

“…is being gathered through electrophysiological approaches (van der Heide et al., 2005; Könner et al., 2007; Korol et al., 2018), there is a rich opportunity to explore additional mechanisms through the advent of real-time intravital imaging of the central nervous system (Forli et al., 2018).”

“Mutations that alter the glucose-dependent activity of GK effectively adjust the set point for whole-body glucose homeostasis (Gloyn et al., 2003).”

“…be not the IR (Meezan et al., 1988; Sato et al., 1991; Nielsen, 1993, 1994) but possibly scavenger receptors such as megalin (member of the low-density lipoprotein receptor family; Christensen et al., 1998; Kolman et al., 2009) and cubilin, proteins that recover a number of proteins by endocytosis.”

“Pulsatile insulin delivery to the liver is an important physiological signal that regulates both hepatic insulin action (Matveyenko et al., 2012) and insulin clearance (Meier et al.”

“…insulin-derived signals that trigger GLUT4 vesicle release from storage and interaction with the plasma membrane • Compromised glomerular function in obesity (Kanasaki et al., 2013) that may alter insulin bioavailability; sodium retention and down-regulation of the natriuretic peptide system in…”

“…of the GLUT4 vesicle fusion machinery (Aslamy and Thurmond, 2017); the underlying defects include alterations in the maintenance of the storage compartment (Foley et al., 2011; Samuel and Shulman, 2012) and in the insulin-derived signals that trigger GLUT4 vesicle release from storage and…”

“Loss of key SNA RE proteins results in impaired insulin secretion (Liang et al., 2017).”

“Two major consequences of the PIP3 burst relevant for GLUT4 translocation are activation of the kinases Akt1, 2 (Brozinick and Birnbaum, 1998; Wang et al., 1999) and of the Rho-family GTPase Rac1 (Chiu et al., 2011).”

“, 2016) • Diminished GLUT4 translocation to the muscle membrane in diabetic rodents and humans (Klip et al., 1990; Zierath et al., 1996; Garvey et al., 1998; Hoehn et al., 2008; Czech, 2017) and lowered expression of Rac1 (Sylow et al.”

“…insulin egress from the circulation toward tissues (Kubota et al., 2011; Majumdar et al., 2012; Meijer et al., 2016; Konishi et al., 2017), opposing studies challenge a receptor-mediated (Vicent et al., 2003; Duncan et al., 2008; Williams et al., 2018) or saturable process (Steil et al., 1996).”

“Conversely, central defects in insulin action are emerging as a potential contributor to the development of Alzheimer’s disease (Griffith et al., 2018), possibly as a result of abnormal phosphorylation of tau protein (Kleinridders et al., 2014).”

“Intriguingly, there are studies of additional participation of Ca2+-regulated proteins such as Doc2b, Tctex1d2, and E-Syt1 (Lalioti et al., 2009; Friedrich et al., 2010; Shimoda et al., 2015), and insulin-dependent Ca2+-spikes have been recorded in muscle cells (Contreras-Ferrat et al., 2014),…”

“Subsequent increases in the cytosolic ATP/ADP ratio control cell membrane potential by inhibiting ATP-sensitive K+ (KATP) channels, eliciting a membrane depolarization that is modulated by a number of additional ion channels (Fig.”

“The formation and fidelity of the SNA RE complex mediating granule docking is regulated by a number of proteins such as Munc18 and syntaxin isoforms (Gandasi and Barg, 2014; Zhu et al., 2015).”

“…insulin in the circulation (Kolka and Bergman, 2012), and several studies have documented that the capillary endothelium is a barrier to insulin delivery to muscle in vivo, maintaining a disequilibrium between circulating and interstitial insulin levels (Jansson et al., 1993; Herkner et al., 2003).”

“These emanate directly from the storage compartment; however, insulin also appears to redirect vesicles from this compartment toward the general recycling endosomes, from whence they reach the cell periphery in the form of somewhat larger vesicles (Xu et al., 2011).”

“The portal circulation delivers insulin into the capillaries of the sinusoids, which are not supported by a basement membrane and their endothelial cells contain fenestrations (Wisse, 1970; Braet et al., 1995), together permitting the exchange of contents between the blood and the surrounding liver cells.”

“…clearance Although hepatocytes are not exclusive in their ability to internalize insulin, they highly express the transmembrane glycoprotein CEA CAM1 (carcinoembryonic antigen–related cell adhesion molecule 1), which mediates rapid and effective IR-mediated insulin endocytosis (Najjar, 2002).”

“Intriguingly, there are studies of additional participation of Ca2+-regulated proteins such as Doc2b, Tctex1d2, and E-Syt1 (Lalioti et al., 2009; Friedrich et al., 2010; Shimoda et al., 2015), and insulin-dependent Ca2+-spikes have been recorded in muscle cells (Contreras-Ferrat et al., 2014),…”

“In adipocytes, Rab10 is the preferred GTPase in GLUT4 translocation (Sano et al., 2008), whereas in muscle cells, it is Rab8A and Rab13 (Ishikura et al., 2007; Sun et al., 2010, 2014, 2016).”

“However, another prevailing model proposes that the majority of vesicles furnishing the cell membrane with GLUT4 contain the fusogenic protein VAMP2, which segregates away with the storage compartment and is largely absent from recycling endosomes (Randhawa et al., 2000, 2004; Török et al., 2004).”

“The portal vein delivers insulin from the pancreas to the liver in discrete pulses that occur every ∼5 min (Song et al., 2000), where the amplitude of these insulin pulses is 0.5–1 nmol/liter in the fasted state and rises to ∼5 nmol/liter after a meal (Pørksen et al., 1995; Song et al., 2000).”

“…of IDE in hepatic insulin clearance in vivo is controversial, as some studies report that loss of IDE results in hyperinsulinemia (Farris et al., 2003; Abdul-Hay et al., 2011), whereas others observed no changes in systemic circulating insulin in the absence of the enzyme (Steneberg et al., 2013).”

“…monophosphate–dependent reductions in intracellular Ca2+ concentration (Carvajal et al., 2000) prevent phosphorylation of myosin light chain required for cytoskeletal cross-bridge formation and contraction (Lee et al., 1997; Mizuno et al., 2008), thereby resulting in vessel relaxation (Fig.”

“Nonetheless, renal insulin clearance may explain the curious fact that type 1 diabetic patients with onset renal failure can end up reducing their requirement for injected insulin (Rubenstein and Spitz, 1968; Rabkin et al., 1984).”

“Endosomal signaling may have a differential impact from that emanating exclusively from the cell surface, akin to the differential location-based signaling of the EGF receptor (Bergeron et al., 2016).”

“The TBC domain of AS160/TBC1D4 defines its GAP activity toward Rab family small GTPases.”

“Subsequently, carboxypeptidase E removes C-terminal basic amino acids from the resulting peptide chains, yielding mature insulin consisting of A- and B-peptide chains linked by disulfide bonds (Hutton, 1994).”

“, 2011), whereas others observed no changes in systemic circulating insulin in the absence of the enzyme (Steneberg et al., 2013).”

“After binding, 125I-insulin–IR complexes move to the base of the microvilli, where they associate with clathrin-coated pits (Pilch et al., 1983) and internalize by clathrin-mediated endocytosis (Fehlmann et al., 1982).”

“…scrutiny in vitro, as cell culture studies have rendered inconsistent results regarding the precise role of the endothelial IR in the uptake of fluorescently conjugated insulin, potentially dependent on their niche origin: microvascular (Azizi et al., 2015) or macrovascular (Wang et al., 2008).”

“The resulting Rac1 activation leads to a dynamic remodeling of cortical actin filaments via cycles of actin filament branching enacted by Arp2/3 and actin severing enacted by cofilin, which is best mapped in muscle cells (Chiu et al., 2010) and tissue (Sylow et al., 2013; Fig.”

“After binding, 125I-insulin–IR complexes move to the base of the microvilli, where they associate with clathrin-coated pits (Pilch et al., 1983) and internalize by clathrin-mediated endocytosis (Fehlmann et al.”

“…1 and 2 (IRS1,2; adaptor proteins that can bind to the IR to initiate signal transduction) have less severe hyperinsulinemia (Dong et al., 2008) than mice lacking the IR (Michael et al., 2000; Cohen et al., 2007), suggesting that canonical insulin signaling via IRS1,2 may…”

“Pyruvate generated from glycolysis enters the mitochondria via mitochondrial pyruvate carriers (Patterson et al., 2014), where the TCA cycle–dependent generation of NADH promotes the export of H+ from the mitochondrial matrix by the electron transport chain, and then generation of ATP from ADP by ATP synthase, which i…”

“With continued insulin circulation (∼30 min), the hormone induces relaxation of larger, upstream resistance vessels to further promote limb blood flow (Baron et al., 1996).”

“This includes the transfer of insulin across cells of the blood– brain barrier (Jialal et al., 1984; Gray et al., 2017), an important conduit for the now-recognized neuronal actions of the hormone evinced by the neuron-specific IR gene depletion (Brüning et al., 2000).”

“The journey of insulin in the body Journal of Cell Biology https://doi.org/10.1083/jcb.201802095 11 PTP-1B (Yamada et al., 2005; Fukuda et al., 2009; Bakke et al., 2013; Garrido-Sanchez et al., 2013; Fig.”

“Mechanistically, CEA CAM1 is phosphorylated by the IR, enhancing the formation of an insulin– IR–CEA CAM1 complex (Najjar et al., 1995; Fig.”

“• Defective insulin exocytosis from diabetic β-cells (Ferdaoussi and MacDonald, 2017; Gandasi et al., 2017) and impaired pulsatile secretion of insulin in diabetic individuals (Lang et al.”

“Controversy about the mechanism of insulin transit across the microvasculature also arises upon scrutiny in vitro, as cell culture studies have rendered inconsistent results regarding the precise role of the endothelial IR in the uptake of fluorescently conjugated insulin, potentially dependent on their niche origin:…”

“…expression of the IR or vascular insulin signaling is required for overall insulin egress from the circulation toward tissues (Kubota et al., 2011; Majumdar et al., 2012; Meijer et al., 2016; Konishi et al., 2017), opposing studies challenge a receptor-mediated (Vicent et al., 2003; Duncan et…”

“Moreover, although insulin internalization is not required for many of the metabolic actions of insulin in the liver, the internalized insulin receptor (IR) continues to signal at least within early endosomes (Bevan et al., 1995).”

“The story of its discovery is legendary and was awarded the Nobel Prize in Physiology or Medicine in 1923 (Karamitsos, 2011), but the journey of this hormone in the body has not been “romanced” as much.”

“This “neuroinsular network” was most recently demonstrated by elegant 3D imaging techniques in rodent (Tang et al., 2018a) and human (Tang et al., 2018b) pancreata.”

“, 1988) before acidification occurs (Hamel et al., 1991).”

“2 B and in recent modeling that integrates metabolic, electrical, and Ca2+ feedback to produce these oscillations (Bertram et al., 2018).”

“In adipocytes, Rab10 is the preferred GTPase in GLUT4 translocation (Sano et al., 2008), whereas in muscle cells, it is Rab8A and Rab13 (Ishikura et al., 2007; Sun et al., 2010, 2014, 2016).”

“In adipocytes, Rab10 is the preferred GTPase in GLUT4 translocation (Sano et al., 2008), whereas in muscle cells, it is Rab8A and Rab13 (Ishikura et al., 2007; Sun et al., 2010, 2014, 2016).”

“…et al., 2014), where the TCA cycle–dependent generation of NADH promotes the export of H+ from the mitochondrial matrix by the electron transport chain, and then generation of ATP from ADP by ATP synthase, which itself appears dependent on mitochondrial Ca2+ uptake (Tarasov et al., 2013).”

“…insulin egress from the circulation toward tissues (Kubota et al., 2011; Majumdar et al., 2012; Meijer et al., 2016; Konishi et al., 2017), opposing studies challenge a receptor-mediated (Vicent et al., 2003; Duncan et al., 2008; Williams et al., 2018) or saturable process (Steil et al., 1996).”

“Insulin granules in apposition to the plasma membrane dock with the membrane through the coordinated interaction and recruitment of exocytic SNA RE proteins that include SNAP-25, VAMP-8, and syntaxins 1A and 3 (Gaisano, 2017).”

“, ATP and pyruvate) within a range of glucose concentrations that matches the normal physiological range for plasma glucose homeostasis (Meglasson et al., 1983).”

“In addition to the Rab13–MIC AL-L2–α-actinin4 connection, GLUT4 vesicles tether to actin filaments via Myosin 1c (Bose et al., 2002; Boguslavsky et al., 2012).”

“Although still unknown for hepatocytes, IR autophosphorylation is required for insulin uptake by CHO cells (Carpentier et al., 1992).”

“…β-cells (Ferdaoussi and MacDonald, 2017; Gandasi et al., 2017) and impaired pulsatile secretion of insulin in diabetic individuals (Lang et al., 1981; Hollingdal et al., 2000; Laedtke et al., 2000) • Reduced hepatic insulin clearance (Jung et al., 2018) and CEA CAM1 expression (Lee, 2011) in…”

“In addition, active Rab10 binds to Exoc6/6b (Sano et al., 2015), and the Rab10 effector RalA and its GEF, Rlf, interact with exocyst components (Karunanithi et al., 2014).”

“RAB IF/MSS4 is a Rab-stabilizing holdase chaperone required for GLUT4 exocytosis.”

“, 2000) • Reduced hepatic insulin clearance (Jung et al., 2018) and CEA CAM1 expression (Lee, 2011) in obesity • Impaired vasoactive effects of insulin during insulin resistance, including capillary recruitment (de Jongh et al.”

“…be not the IR (Meezan et al., 1988; Sato et al., 1991; Nielsen, 1993, 1994) but possibly scavenger receptors such as megalin (member of the low-density lipoprotein receptor family; Christensen et al., 1998; Kolman et al., 2009) and cubilin, proteins that recover a number of proteins by endocytosis.”

“This is brought about through formation of a SNA RE complex between VAMP2 on the vesicles and synatxin4 and SNAP23 on the plasma membrane (Cheatham et al., 1996; Foster and Klip, 2000; Thurmond and Pessin, 2001).”

“The degradation process begins on the membrane immediately after insulin binding, where some insulin is reported to be partially degraded by extracellular IDE before internalization (Yokono et al., 1982).”

“…adaptor proteins that can bind to the IR to initiate signal transduction) have less severe hyperinsulinemia (Dong et al., 2008) than mice lacking the IR (Michael et al., 2000; Cohen et al., 2007), suggesting that canonical insulin signaling via IRS1,2 may not participate in insulin clearance.”

“, 2016), specifically by interacting with Sec16A (Sano et al., 2007; Bruno et al., 2016).”

“This restricts GLUT4 mobility beneath the membrane, a phenomenon nicely documented through total internal reflection fluorescence microscopy of muscle and adipose cells (Bai et al., 2007; Xiong et al., 2010; Boguslavsky et al., 2012; Lizunov et al., 2012).”

“However, another prevailing model proposes that the majority of vesicles furnishing the cell membrane with GLUT4 contain the fusogenic protein VAMP2, which segregates away with the storage compartment and is largely absent from recycling endosomes (Randhawa et al., 2000, 2004; Török et al., 2004).”

“It is understood that insulin promotes GLUT4 exit from retention in the storage compartment (Xu and Kandror, 2002; Coster et al., 2004; Martin et al., 2006; Bogan et al., 2012) and also regulates GLUT4 vesicle tethering, docking, and fusion with the plasma membrane through mechanisms that borrow…”

“Several elements contribute to the genesis and maintenance of this storage compartment, including sortilin (Huang et al., 2013), the Rab GTPase-activating protein (GAP) AS160/TBC1D4, syntaxin 6/16 (Bryant and Gould, 2011; Klip et al., 2014), the cleavable tether protein TUG (Belman et al., 2015),…”

“In adipocytes, Rab10 is the preferred GTPase in GLUT4 translocation (Sano et al., 2008), whereas in muscle cells, it is Rab8A and Rab13 (Ishikura et al., 2007; Sun et al., 2010, 2014, 2016).”

“Intriguingly, there are studies of additional participation of Ca2+-regulated proteins such as Doc2b, Tctex1d2, and E-Syt1 (Lalioti et al., 2009; Friedrich et al., 2010; Shimoda et al., 2015), and insulin-dependent Ca2+-spikes have been recorded in muscle cells (Contreras-Ferrat et al., 2014),…”

“…(Lang et al., 1981; Hollingdal et al., 2000; Laedtke et al., 2000) • Reduced hepatic insulin clearance (Jung et al., 2018) and CEA CAM1 expression (Lee, 2011) in obesity • Impaired vasoactive effects of insulin during insulin resistance, including capillary recruitment (de Jongh et al.,…”

“…that insulin promotes GLUT4 exit from retention in the storage compartment (Xu and Kandror, 2002; Coster et al., 2004; Martin et al., 2006; Bogan et al., 2012) and also regulates GLUT4 vesicle tethering, docking, and fusion with the plasma membrane through mechanisms that borrow…”

“SOR CS1 is necessary for normal insulin secretory granule biogenesis in metabolically stressed β cells.”

“…compartment, including sortilin (Huang et al., 2013), the Rab GTPase-activating protein (GAP) AS160/TBC1D4, syntaxin 6/16 (Bryant and Gould, 2011; Klip et al., 2014), the cleavable tether protein TUG (Belman et al., 2015), and, in human muscle, clathrin heavy chain 22 (Vassilopoulos et al., 2009).”

“first reported a delay in muscle insulin action relative to the rise in insulin in the circulation (Kolka and Bergman, 2012), and several studies have documented that the capillary endothelium is a barrier to insulin delivery to muscle in vivo, maintaining a disequilibrium between circulating and interstitial insulin…”

“…of this storage compartment, including sortilin (Huang et al., 2013), the Rab GTPase-activating protein (GAP) AS160/TBC1D4, syntaxin 6/16 (Bryant and Gould, 2011; Klip et al., 2014), the cleavable tether protein TUG (Belman et al., 2015), and, in human muscle, clathrin heavy chain 22…”

“The journey of insulin in the body Journal of Cell Biology https://doi.org/10.1083/jcb.201802095 11 PTP-1B (Yamada et al., 2005; Fukuda et al., 2009; Bakke et al., 2013; Garrido-Sanchez et al., 2013; Fig.”

“Notably, alterations in insulin renal clearance prolong the permanence of insulin in the blood (Dahl et al., 1989), evincing the importance of this process to insulin’s half-life in the circulation.”

“Lastly, and importantly, the fusion event requires insulin-induced actin polymerization, evincing the contribution of the actin cytoskeleton at different steps in the process of GLUT4 translocation (Lopez et al., 2009).”

“The majority of insulin that binds to hepatic IRs is degraded (Duckworth, 1988).”

“…imaging of the muscle distribution of somewhat high doses of fluorescent insulin injected into the circulation was best fitted to a model of distribution that does not obey saturation kinetics, suggesting that the IR may not be a major conduit under these conditions (Williams et al., 2018).”

“, 2013) as well as a number of proteins of the GLUT4 vesicle fusion machinery (Aslamy and Thurmond, 2017); the underlying defects include alterations in the maintenance of the storage compartment (Foley et al.”

“In addition, these IR are important sites sensing the hormone to stimulate important functions such as reabsorption of sodium, phosphate, and glucose (Rabkin et al., 1984; Hale and Coward, 2013).”

“…of the GLUT4 vesicle fusion machinery (Aslamy and Thurmond, 2017); the underlying defects include alterations in the maintenance of the storage compartment (Foley et al., 2011; Samuel and Shulman, 2012) and in the insulin-derived signals that trigger GLUT4 vesicle release from storage and…”

“In addition, active Rab10 binds to Exoc6/6b (Sano et al., 2015), and the Rab10 effector RalA and its GEF, Rlf, interact with exocyst components (Karunanithi et al., 2014).”

“Early electron microscopy studies revealed that IRs bind 125I-insulin on microvilli (interdigitations) of the hepatocyte membrane (Carpentier et al., 1985).”

“The storage compartment is in dynamic communication with recycling endosomes (Coster et al., 2004; Karylowski et al., 2004; Kandror and Pilch, 2011).”

“Beyond first- and second-pass insulin clearance, the hepatocytes are essential metabolic responders to insulin, where one of the major actions of the hormone is to suppress gluconeogenesis and glycogenolysis (Lin and Accili, 2011).”

“The journey of insulin in the body Journal of Cell Biology https://doi.org/10.1083/jcb.201802095 11 PTP-1B (Yamada et al., 2005; Fukuda et al., 2009; Bakke et al., 2013; Garrido-Sanchez et al., 2013; Fig.”

“, 2009); in particular, the full stimulation of skeletal muscle glucose uptake in vivo is contingent on prior NO-mediated vasodilation (Vincent et al., 2003; Bradley et al., 2013).”

“paracellular) and supporting mechanisms remain a matter of debate (Lee and Klip, 2016).”

“Recently, Epac2A was also shown to regulate insulin granule priming (Alenkvist et al., 2017).”

“In addition, active Rab10 binds to Exoc6/6b (Sano et al., 2015), and the Rab10 effector RalA and its GEF, Rlf, interact with exocyst components (Karunanithi et al.”

“Tethering may be regulated by insulin, as stimulation leads to phosphorylation of Exo84 (Uhm et al., 2017).”

“…the crucial role of CEA CAM1 in hepatic insulin clearance, impairments in insulin-stimulated hepatic CEA CAM1 phosphorylation or whole-body depletion of hepatic CEA CAM1 cause severe hyperinsulinemia and, consequently, insulin resistance and hyperglycemia (Poy et al., 2002; Russo et al., 2017).”

“The formation and fidelity of the SNA RE complex mediating granule docking is regulated by a number of proteins such as Munc18 and syntaxin isoforms (Gandasi and Barg, 2014; Zhu et al., 2015).”

“, 2009); reduced insulin delivery to muscle in obesity and diabetes (Broussard et al., 2016) • Diminished GLUT4 translocation to the muscle membrane in diabetic rodents and humans (Klip et al.”

“Here, IRs on the epithelial cells bind insulin and transport it intracellularly for degradation (Nielsen et al., 1987).”

“, 2017), where it begins to degrade receptor-bound insulin in endosomes (Yonezawa et al., 1988) before acidification occurs (Hamel et al.”

“Insulin acting centrally also evokes efferent inputs into peripheral tissue metabolism (Ferris and Kahn, 2016), contributing to the suppression of gluconeogenesis in the liver and the counterregulatory response to hypoglycemia (Diggs-Andrews et al., 2010).”

“The normal process of glucose uptake via GLUT-4 is closely dependent on the physiological functioning of healthy mitochondria [134] that produce the energy needed for glucose uptake in peripheral tissues [134].”

“Other acetate mediated signals may indirectly contribute to insulin secretion, such as via gut-derived GLP-1 and vagal activation of the parasympathetic nervous system [176].”

“Insulin is another anabolic hormone acting on myocytes (Tokarz et al., 2018).”

“Hyperinsulinemia can manifest as an elevation in basal/fasting circulating insulin and/or as a potentiation of post-prandial insulin secretion [71,72], and can result from insulin hypersecretion or reduced systemic insulin clearance, or both [73-76].”

“Receptor-mediated insulin uptake followed by insulin degradation by hepatocytes is the basic mechanism of hepatic insulin clearance [53].”

“This occurs when the membrane potential reaches approximately −50 mV [20].”

“Receptor-mediated insulin uptake followed by insulin degradation in hepatocytes constitutes the basic mechanism of hepatic insulin clearance (204).”

“Glucose sensing leads to a b -cell response that ultimately leads to insulin secretion [24].”

“Insulin is a hormone produced by pancreatic β cells that increases glucose uptake by the cells, thereby reducing blood glucose levels [60].”

“Under normal circumstances, Cdc42 and Rac1 activation is needed for this process [58,63,67].”

“This occurs through regulation of a range of physiological processes involved in metabolism, growth, differentiation, and cell survival (Boucher et al., 2014; Tokarz et al., 2018).”

“…diameter and account for approximately 0.2% of the total surface area of the capillary endothelium and, to a lesser extent, large paracellular gaps, which are estimated to be 25 to 30 nm in diameter and account for 0.002% to 0.02% of the total surface area of the capillary endothelium (19, 29, 45).”

“Thus, defects of GLUT4 may lead to insulin resistance and glucose intolerance [53,54].”

“By maintaining blood sugar levels and interacting with hormones like glucagon, insulin ensures a balanced regulation of energy storage and utilization across the body [56].”

“Second-phase insulin secretion requires the continual mobilization of insulin granules from synthetic parts to the plasma membrane, and this process mainly relies on filamentous actin (F-actin) remodelling regulated by Cdc42 and its downstream factors [34].”

“In this process, thermogenic adipocytes serve as a cellular ‘rheostat’ that senses the glucose status and contributes significantly to whole-body energy homeostasis.(65,66) Given that brown and beige adipocytes can take up glucose positively, (18)F-fluorodeoxyglucose positron emission tomography combined with computed…”

“Insulin action begins after the hormone binds to INSR, which activates IRS-1 [2,37].”

“Insulin is a peptide hormone that is expressed by the β-pancreatic cells in response to blood glucose [335].”

“…(PK) profile must be balanced with the receptor engagement; thus, the strategy requires attenuating IR binding to decrease clearance by receptor mediated endocytosis (RME) (Tokarz et al., 2018), as well as mitigating renal filtration (Ferrannini et al., 1983; Henriksen et al., 1987).”

“This shift closes ATP-sensitive potassium channels, depolarizing the cell membrane and opening voltage-dependent calcium channels where calcium influx is a trigger of insulin granule exocytosis (12).”

“Indeed, the actions of insulin mainly contribute to glucose uptake by skeletal muscle and adipose tissue and reduce liver gluconeogenesis and glycogenolysis (Titchenell et al., 2017; Tokarz et al., 2018).”

“Insulin resistance arises when peripheral tissues, such as skeletal muscle, liver, and adipose tissue exhibit decreased sensitivity to insulin, resulting in impaired uptake and utilization of glucose [10].”

“The hormones involved in stress regulation include INS, GH, GC, thyroid hormones, etc. INS has the effect of enhancing anabolism and promoting energy deposition in the body [41].”

“This phosphorylates other substrates and delivers insulin to early/late endosomes for degradation before the receptor recycles back to the surface membrane [5].”

“It is caused by decreased uptake of glucose by insulin sensitive cells (hepatocytes, myocytes, adipocytes) [20].”

“Insulin signaling also plays an important role in nitric oxide production, which is considered a strong vasodilator and a substance with anti-atherosclerosis properties [7].”

“Unprocessed insulin passes the liver and is delivered into the systemic (post-hepatic) circulation to exert its actions in peripheral tissues (such as muscles and fat cells) [89].”

“Rab10 then interacts with RAS-like oncoprotein A and membrane-to-cortex linker Myosin-Ic to stimulate the translocation of GLUT4 from cytoplasm to membrane, enhancing glucose uptake [181].”

“Both have been demonstrated to participate in insulin receptor-dependent endocytosis [143,144] and insulin-degrading processes [145,146], which ultimately mediate circulating insulin levels.”

“The first family contains glucose transporter (GLUT) proteins, which are stereospecific facilitative glucose transporters, with net transport driven by the concentration gradient of glucose across the membrane [1].”

“Abnormal blood glucose levels, called hypo- or hyperglycemia, stimulate pancreatic insulin secretion and glucagon [50].”

“Approximately 50 e 80% of insulin may be depleted from the blood by hepatic uptake and degradation during fi rst-pass insulin clearance [9,10].”

“These effects are achieved by insulin binding to the heterotetrameric insulin receptor, which has intrinsic tyrosine kinase activity [29].”

“As insulin and C-peptide undergo different elimination mechanisms with a high hepatic first-pass clearance of insulin (31), we next investigated possible links between LDL cholesterol and insulin clearance that could potentially explain these contrary results.”

“Insulin resistance as a result of decreased sensitivity to the ligand and decreased ligand availability presents as a significant problem in many pathological conditions [118].”

“The liver plays an important role in regulating homeostatic insulin levels and removing it from the bloodstream (Tokarz et al. 2018).”

“Insulin plays a vital role in the fate of macronutrients, mainly carbohydrates, to achieve optimal energy usage while regulating glucose levels within the bloodstream to a set threshold of approximately 70 mg/dL (3.9 mmol/L) [1,2].”

“To decrease the blood glucose concentration back to normal, insulin promotes cellular uptake of glucose and synthesis of glycogen, fat, and protein.(36) The following sections review insulin as a hepatic mitogen.”

“Elements like insulin promote glucose uptake after binding to cellular receptors [5].”

“52 To investigate this dysfunction further, we investigated changes in both RNA and protein expression involved in different facets of beta cell glucose metabolism from fixed pancreatic samples.”

“Adipose tissue receives insulin signals and is responsible for plasma glucose clearance by stimulating its uptake and utilization (1).”

“Adipocytes are well acknowledged for actively responding to insulin signal for regulating systemic glucose and lipid homeostasis with the involvement of classic Akt phosphorylation and Glut4 translocation [3].”

“improving two key components: β -cell functional integrity and hepatic insulin sensitivity [5].”

“Furthermore, they run in parallel with the Ras-MAPK pathway (mitogen-activated proteins kinases such as MEK, MAPK/KRE), with some crossover (7, 12).”

“The maintenance of normal glucose tolerance depends on a delicate balance between insulin action in peripheral tissues and insulin secretion [30].”

“Furthermore, under physiological conditions, insulin secretion is meticulously regulated, dynamically adjusting in response to various factors such as blood glucose levels (134, 135).”