Deficient expression of insulin receptor substrate-1 (IRS-1) fails to block insulin-like growth factor-I (IGF-I) stimulation of brain growth and myelination

Identification of the molecular mechanism of insulin-like growth factor-1 (IGF-1): a promising therapeutic target for neurodegenerative diseases associated with metabolic syndrome

Neurodegenerative disorders are accompanied by neuronal degeneration and glial dysfunction, resulting in cognitive, psychomotor, and behavioral impairment. Multiple factors including genetic, environmental, metabolic, and oxidant overload contribute to disease progression. Recent evidences suggest that metabolic syndrome is linked to various neurodegenerative diseases. Metabolic syndrome (MetS) is known to be accompanied by symptoms such as hyperglycemia, abdominal obesity, hypertriglyceridemia, and hypertension. Despite advances in knowledge about the pathogenesis of neurodegenerative disorders, effective treatments to combat neurodegenerative disorders caused by MetS have not been developed to date. Insulin growth factor-1 (IGF-1) deficiency has been associated with MetS-related pathologies both in-vivo and in-vitro. IGF-1 is essential for embryonic and adult neurogenesis, neuronal plasticity, neurotropism, angiogenesis, metabolic function, and protein clearance in the brain. Here, we review the evidence for the potential therapeutic effects of IGF-1 in the neurodegeneration related to metabolic syndrome. We elucidate how IGF-1 may be involved in molecular signaling defects that occurs in MetS-related neurodegenerative disorders and highlight the importance of IGF-1 as a potential therapeutic target in MetS-related neurological diseases.

Understanding the Role of Oxidative Stress, Neuroinflammation and Abnormal Myelination in Excessive Aggression Associated with Depression: Recent Input from Mechanistic Studies

Aggression and deficient cognitive control problems are widespread in psychiatric disorders, including major depressive disorder (MDD). These abnormalities are known to contribute significantly to the accompanying functional impairment and the global burden of disease. Progress in the development of targeted treatments of excessive aggression and accompanying symptoms has been limited, and there exists a major unmet need to develop more efficacious treatments for depressed patients. Due to the complex nature and the clinical heterogeneity of MDD and the lack of precise knowledge regarding its pathophysiology, effective management is challenging. Nonetheless, the aetiology and pathophysiology of MDD has been the subject of extensive research and there is a vast body of the latest literature that points to new mechanisms for this disorder. Here, we overview the key mechanisms, which include neuroinflammation, oxidative stress, insulin receptor signalling and abnormal myelination. We discuss the hypotheses that have been proposed to unify these processes, as many of these pathways are integrated for the neurobiology of MDD. We also describe the current translational approaches in modelling depression, including the recent advances in stress models of MDD, and emerging novel therapies, including novel approaches to management of excessive aggression, such as anti-diabetic drugs, antioxidant treatment and herbal compositions.

Intranasal insulin modulates cerebrospinal fluid markers of neuroinflammation in mild cognitive impairment and Alzheimer's disease: a randomized trial

Intranasal insulin (INI) has shown promise as a treatment for Alzheimer's disease (AD) in pilot clinical trials. In a recent phase 2 trial, participants with mild cognitive impairment (MCI) or AD who were treated with INI with one of two delivery devices showed improved cerebral spinal fluid (CSF) biomarker profiles and slower symptom progression compared with placebo. In the cohort which showed benefit, we measured changes in CSF markers of inflammation, immune function and vascular integrity and assessed their relationship with changes in cognition, brain volume, and CSF amyloid and tau concentrations. The insulin-treated group had increased CSF interferon-γ (p = 0.032) and eotaxin (p = 0.049), and reduced interleukin-6 (p = 0.048) over the 12 month trial compared to placebo. Trends were observed for increased CSF macrophage-derived chemokine for the placebo group (p = 0.083), and increased interleukin-2 in the insulin-treated group (p = 0.093). Insulin-treated and placebo groups showed strikingly different patterns of associations between changes in CSF immune/inflammatory/vascular markers and changes in cognition, brain volume, and amyloid and tau concentrations. In summary, INI treatment altered the typical progression of markers of inflammation and immune function seen in AD, suggesting that INI may promote a compensatory immune response associated with therapeutic benefit.

Motor Neuron Diseases and Neuroprotective Peptides: A Closer Look to Neurons

Motor neurons (MNs) are specialized neurons responsible for muscle contraction that specifically degenerate in motor neuron diseases (MNDs), such as amyotrophic lateral sclerosis (ALS), spinal and bulbar muscular atrophy (SBMA), and spinal muscular atrophy (SMA). Distinct classes of MNs degenerate at different rates in disease, with a particular class named fast-fatigable MNs (FF-MNs) degenerating first. The etiology behind the selective vulnerability of FF-MNs is still largely under investigation. Among the different strategies to target MNs, the administration of protective neuropeptides is one of the potential therapeutic interventions. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide with beneficial effects in many neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, and more recently SBMA. Another neuropeptide that has a neurotrophic effect on MNs is insulin-like growth factor 1 (IGF-1), also known as somatomedin C. These two peptides are implicated in the activation of neuroprotective pathways exploitable in the amelioration of pathological outcomes related to MNDs.

Differential involvement of insulin receptor substrate (IRS)-1 and IRS-2 in brain insulin signaling is associated with the effects on amyloid pathology in a mouse model of Alzheimer's disease

Insulin signaling has been implicated in the metabolism as well as aging and longevity. Type 2 diabetes mellitus and its core pathology, insulin resistance, has also been implicated in the development of Alzheimer's disease (AD) and amyloid-β deposition in humans. By contrast, genetic ablation of the insulin/IGF-1 signaling (IIS) pathway components, e.g. insulin receptor substrate (IRS)-2, has been documented to suppress amyloid-β accumulation in the brains of transgenic mice overexpressing AD mutant β-amyloid precursor protein (APP). Therefore, the brain IIS may be a key modifiable molecular target in the pathophysiology of AD. IRS-1 and IRS-2 are critical nodes in IIS as substrates for insulin receptor and IGF-1 receptor, although the functional differences between IRS-1 and IRS-2 in the adult brain are yet to be explored. To examine their relative contribution to the brain IIS activity and AD pathomechanism, we generated APP transgenic mice lacking either IRS-1 or IRS-2. IRS-1 deficiency had little effects on the brain IIS pathway associated with compensatory activation of IRS-2, whereas IRS-2 deficiency was not fully compensated by activation of IRS-1, and the downstream activation of Akt also was significantly compromised. Pathological analyses of the cortical tissues showed that the biochemical levels of soluble and insoluble amyloid-β, the amyloid-β histopathology, and tau phosphorylation were not affected by the absence of IRS-1, in contrast to the marked alteration in IRS-2 deleted mice. These results suggest the predominance of IRS-2 in the brain IIS, and support the hypothesis that reduced IIS exerts anti-amyloid effects in the brain.

One Hour-Post-load Plasma Glucose ≥155 mg/dl in Healthy Glucose Normotolerant Subjects Is Associated With Subcortical Brain MRI Alterations and Impaired Cognition: A Pilot Study

Background: Glucose alterations are associated with impaired cognition. The 1-h-post-load plasma glucose ≥155 mg/dl in non-diabetic subjects confers an increased risk of cardiovascular events and diabetes. This pilot study aimed to investigate whether the 1-h-post-load plasma glucose ≥155 mg/dl negatively affects the subcortical regions of the brain and the cognitive functions. Methods: We enrolled 32 non-diabetic subjects. Patients were divided into two groups based on 1-h- post-load plasma glucose value > or < 155 mg/dl: normal glucose tolerance (NGT) 1-h-high and NGT 1-h-low subjects. All subjects underwent 3 Tesla MRI and standard neuropsychological tests. Results: NGT 1-h-high subjects showed significantly lower values of both right (4.9 ± 0.9 vs. 5.1 ± 0.9 ml) and left (4.8 ± 1.1 vs. 5.1 ± 1.1 ml) hippocampal hemisphere volume, while right hemisphere hippocampal diffusivity was lower in the NGT 1-h-high group (10.0 ± 0.6 vs. 10.6 ± 0.5 10^-4 mm²s^-1). NGT 1-h-high subjects also showed a poorer memory performance. In particular, for both Rey Auditory Verbal Learning Task (RAVLT)-immediate-recall and Free and Cued Selective Reminding Test (FCSRT)-delayed total recall, we found lower cognitive test scores in the NGT-1 h-high group (26.5 ± 6.3 and 10.4 ± 0.3, respectively). Conclusions: One-hour-post-load hyperglycemia is associated with morpho-functional subcortical brain alterations and poor memory performance tests.

Insulin resistance and bioenergetic manifestations: Targets and approaches in Alzheimer's disease

AIM

Insulin has a well-established role in cognition, neuronal detoxification and synaptic plasticity. Insulin transduction affect neurotransmitter functions, influence bioenergetics and regulate neuronal survival through regulating glucose energy metabolism and downward pathways.

METHODS

A systematic literature review of PubMed, Medline, Bentham, Scopus and EMBASE (Elsevier) databases was carried out with the help of the keywords like "Alzheimer's disease; Hypometabolism; Oxidative stress; energy failure in AD, Insulin; Insulin resistance; Bioenergetics" till June 2020. The review was conducted using the above keywords to collect the latest articles and to understand the nature of the extensive work carried out on insulin resistance and bioenergetic manifestations in Alzheimer's disease.

KEY FINDINGS

The article sheds light on insulin resistance mediated hypometabolic state on pathological progression of AD. The disrupted insulin signaling has pathological outcome in form of disturbed glucose homeostasis, altered bioenergetic state which increases build-up of senile plaques (Aβ), neurofibrillary tangles (τ), decline in transportation of glucose and activation of inflammatory pathways. The mechanistic link of insulin resistant state with therapeutically explorable potential transduction pathways is the focus of the reviewed work.

SIGNIFICANCE

The present work opines that the mechanism by which the insulin resistance mediates dysregulation of bioenergetics and progresses to neurodegenerative state holds the tangible potential to succeed in the development of novel dementia therapies. Further, hypometabolic complications and altered insulin signaling may be explored as a mechanistic relation between bioenergetic deficits and AD.

Brain insulin resistance in type 2 diabetes and Alzheimer disease: concepts and conundrums

Considerable overlap has been identified in the risk factors, comorbidities and putative pathophysiological mechanisms of Alzheimer disease and related dementias (ADRDs) and type 2 diabetes mellitus (T2DM), two of the most pressing epidemics of our time. Much is known about the biology of each condition, but whether T2DM and ADRDs are parallel phenomena arising from coincidental roots in ageing or synergistic diseases linked by vicious pathophysiological cycles remains unclear. Insulin resistance is a core feature of T2DM and is emerging as a potentially important feature of ADRDs. Here, we review key observations and experimental data on insulin signalling in the brain, highlighting its actions in neurons and glia. In addition, we define the concept of 'brain insulin resistance' and review the growing, although still inconsistent, literature concerning cognitive impairment and neuropathological abnormalities in T2DM, obesity and insulin resistance. Lastly, we review evidence of intrinsic brain insulin resistance in ADRDs. By expanding our understanding of the overlapping mechanisms of these conditions, we hope to accelerate the rational development of preventive, disease-modifying and symptomatic treatments for cognitive dysfunction in T2DM and ADRDs alike.

Deciphering Brain Insulin Receptor and Insulin-Like Growth Factor 1 Receptor Signalling

Insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) are highly conserved receptor tyrosine kinases that share signalling proteins and are ubiquitously expressed in the brain. Central application of insulin or IGF1 exerts several similar physiological outcomes, varying in strength, whereas disruption of the corresponding receptors in the brain leads to remarkably different effects on brain size and physiology, thus highlighting the unique effects of the corresponding hormone receptors. Central insulin/IGF1 resistance impacts upon various levels of the IR/IGF1R signalling pathways and is a feature of the metabolic syndrome and neurodegenerative diseases such as Alzheimer's disease. The intricacy of brain insulin and IGF1 signalling represents a challenge for the identification of specific IR and IGF1R signalling differences in pathophysiological conditions. The present perspective sheds light on signalling differences and methodologies for specifically deciphering brain IR and IGF1R signalling.

Mammalian target of rapamycin promotes oligodendrocyte differentiation, initiation and extent of CNS myelination

Prior studies support a role for mammalian target of rapamycin (mTOR) signaling in oligodendrocyte differentiation and myelination. Here we use Cre-recombinase driven by the CNP promoter to generate a mouse line with oligodendrocyte-specific knockdown of mTOR (mTOR cKO) in the CNS. We provide evidence that mTOR is necessary for proper oligodendrocyte differentiation and myelination in the spinal cord. Specifically, the number of mature oligodendrocytes was reduced, and the initiation and extent of myelination were impaired during spinal cord development. Consistent with these data, myelin protein expression, including myelin basic protein, proteolipid protein, myelin oligodendrocyte glycoprotein, and myelin-associated glycoprotein, was delayed in the spinal cord. Hypomyelination of the spinal cord persisted into adulthood, as did the reduction in numbers of mature oligodendrocytes. In the cortex, the structure of myelin appeared normal during development and in the adult; however, myelin protein expression was delayed during development and was abnormal in the adult. Myelin basic protein was significantly reduced in all regions at postnatal day 25. These data demonstrate that mTOR promotes oligodendrocyte differentiation and CNS myelination in vivo and show that the requirement for mTOR varies by region with the spinal cord most dependent on mTOR.

Insulin, IGF-1 and GLP-1 signaling in neurodegenerative disorders: targets for disease modification?

Insulin and Insulin Growth Factor-1 (IGF-1) play a major role in body homeostasis and glucose regulation. They also have paracrine/autocrine functions in the brain. The Insulin/IGF-1 signaling pathway contributes to the control of neuronal excitability, nerve cell metabolism and cell survival. Glucagon like peptide-1 (GLP-1), known as an insulinotropic hormone has similar functions and growth like properties as insulin/IGF-1. Growing evidence suggests that dysfunction of these pathways contribute to the progressive loss of neurons in Alzheimer's disease (AD) and Parkinson's disease (PD), the two most frequent neurodegenerative disorders. These findings have led to numerous studies in preclinical models of neurodegenerative disorders targeting insulin/IGF-1 and GLP-1 signaling with currently available anti-diabetics. These studies have shown that administration of insulin, IGF-1 and GLP-1 agonists reverses signaling abnormalities and has positive effects on surrogate markers of neurodegeneration and behavioral outcomes. Several proof-of-concept studies are underway that attempt to translate the encouraging preclinical results to patients suffering from AD and PD. In the first part of this review, we discuss physiological functions of insulin/IGF-1 and GLP-1 signaling pathways including downstream targets and receptors distribution within the brain. In the second part, we undertake a comprehensive overview of preclinical studies targeting insulin/IGF-1 or GLP-1 signaling for treating AD and PD. We then detail the design of clinical trials that have used anti-diabetics for treating AD and PD patients. We close with future considerations that treat relevant issues for successful translation of these encouraging preclinical results into treatments for patients with AD and PD.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

Signaling mechanisms regulating myelination in the central nervous system

The precise and coordinated production of myelin is essential for proper development and function of the nervous system. Diseases that disrupt myelin, including multiple sclerosis, cause significant functional disability. Current treatment aims to reduce the inflammatory component of the disease, thereby preventing damage resulting from demyelination. However, therapies are not yet available to improve natural repair processes after damage has already occurred. A thorough understanding of the signaling mechanisms that regulate myelin generation will improve our ability to enhance repair. in this review, we summarize the positive and negative regulators of myelination, focusing primarily on central nervous system myelination. Axon-derived signals, extracellular signals from both diffusible factors and the extracellular matrix, and intracellular signaling pathways within myelinating oligodendrocytes are discussed. Much is known about the positive regulators that drive myelination, while less is known about the negative regulators that shift active myelination to myelin maintenance at the appropriate time. Therefore, we also provide new data on potential negative regulators of CNS myelination.

Neurodevelopmental effects of insulin-like growth factor signaling

Insulin-like growth factor (IGF) signaling greatly impacts the development and growth of the central nervous system (CNS). IGF-I and IGF-II, two ligands of the IGF system, exert a wide variety of actions both during development and in adulthood, promoting the survival and proliferation of neural cells. The IGFs also influence the growth and maturation of neural cells, augmenting dendritic growth and spine formation, axon outgrowth, synaptogenesis, and myelination. Specific IGF actions, however, likely depend on cell type, developmental stage, and local microenvironmental milieu within the brain. Emerging research also indicates that alterations in IGF signaling likely contribute to the pathogenesis of some neurological disorders. This review summarizes experimental studies and shed light on the critical roles of IGF signaling, as well as its mechanisms, during CNS development.

IRS-2 Deficiency impairs NMDA receptor-dependent long-term potentiation

The beneficial effects of insulin and insulin-like growth factor I on cognition have been documented in humans and animal models. Conversely, obesity, hyperinsulinemia, and diabetes increase the risk for neurodegenerative disorders including Alzheimer's disease (AD). However, the mechanisms by which insulin regulates synaptic plasticity are not well understood. Here, we report that complete disruption of insulin receptor substrate 2 (Irs2) in mice impairs long-term potentiation (LTP) of synaptic transmission in the hippocampus. Basal synaptic transmission and paired-pulse facilitation were similar between the 2 groups of mice. Induction of LTP by high-frequency conditioning tetanus did not activate postsynaptic N-methyl-D-aspartate (NMDA) receptors in hippocampus slices from Irs2(-/-) mice, although the expression of NR2A, NR2B, and PSD95 was equivalent to wild-type controls. Activation of Fyn, AKT, and MAPK in response to tetanus stimulation was defective in Irs2(-/-) mice. Interestingly, IRS2 was phosphorylated during induction of LTP in control mice, revealing a potential new component of the signaling machinery which modulates synaptic plasticity. Given that IRS2 expression is diminished in Type 2 diabetics as well as in AD patients, these data may reveal an explanation for the prevalence of cognitive decline in humans with metabolic disorders by providing a mechanistic link between insulin resistance and impaired synaptic transmission.

The role of insulin-like growth factor-I in the physiopathology of hearing

Insulin-like growth factor-I (IGF-I) belongs to the family of polypeptides of insulin, which play a central role in embryonic development and adult nervous system homeostasis by endocrine, autocrine, and paracrine mechanisms. IGF-I is fundamental for the regulation of cochlear development, growth, and differentiation, and its mutations are associated with hearing loss in mice and men. Low levels of IGF-I have been shown to correlate with different human syndromes showing hearing loss and with presbyacusis. Animal models are fundamental to understand the genetic, epigenetic, and environmental factors that contribute to human hearing loss. In the mouse, IGF-I serum levels decrease with aging and there is a concomitant hearing loss and retinal degeneration. In the Igf1(-/-) null mouse, hearing loss is due to neuronal loss, poor innervation of the sensory hair cells, and age-related stria vascularis alterations. In the inner ear, IGF-I actions are mediated by intracellular signaling networks, RAF, AKT, and p38 MAPK protein kinases modulate the expression and activity of transcription factors, as AP1, MEF2, FoxM1, and FoxP3, leading to the regulation of cell cycle and metabolism. Therapy with rhIGF-I has been approved in humans for the treatment of poor linear growth and certain neurodegenerative diseases. This review will discuss these findings and their implications in new IGF-I-based treatments for the protection or repair of hearing loss.

Insulin receptor substrate 2 expression and involvement in neuronal insulin resistance in diabetic neuropathy

Insulin signaling depends on tyrosine phosphorylation of insulin receptor substrates (IRSs) to mediate downstream effects; however, elevated serine phosphorylation of IRS impairs insulin signaling. Here, we investigated IRS protein expression patterns in dorsal root ganglia (DRG) of mice and whether their signaling was affected by diabetes. Both IRS1 and IRS2 are expressed in DRG; however, IRS2 appears to be the prevalent isoform and is expressed by many DRG neuronal subtypes. Phosphorylation of Ser(731)IRS2 was significantly elevated in DRG neurons from type 1 and type 2 diabetic mice. Additionally, Akt activation and neurite outgrowth in response to insulin were significantly decreased in DRG cultures from diabetic ob/ob mice. These results suggest that DRG neurons express IRS proteins that are altered by diabetes similar to other peripheral tissues, and insulin signaling downstream of the insulin receptor may be impaired in sensory neurons and contribute to the pathogenesis of diabetic neuropathy.

IGF1 molecular anomalies demonstrate its critical role in fetal, postnatal growth and brain development

The phenotype caused by human genetic insulin-like growth factor-I (IGF-I) defects is characterised by the association of intrauterine and postnatal growth retardation with sensorineural deafness and intellectual deficit. This syndrome is extremely rare and only four cases have been reported. Addition clinical features may include microcephaly and later in life adiposity and insulin resistance. Partial gonadal dysfunction and osteoporosis may also be present. A case of partial IGF-I deficiency has recently been described and was associated with pre- and postnatal growth retardation and microcephaly but the developmental delay was mild and hearing tests were normal. IGF-I deficiency is transmitted as an autosomal recessive trait and is caused by homozygous mutations in the IGF1 gene. Currently these patients can benefit from recombinant IGF-I which is now available for treatment. These observations demonstrate that the integrity of IGF-I signalling is important for normal growth and brain development.

Dystroglycan modulates the ability of insulin-like growth factor-1 to promote oligodendrocyte differentiation

The adhesion receptor dystroglycan positively regulates terminal differentiation of oligodendrocytes, but the mechanism by which this occurs remains unclear. Using primary oligodendrocyte cultures, we identified and examined a connection between dystroglycan and the ability of insulin-like growth factor-1 (IGF-1) to promote oligodendrocyte differentiation. Consistent with previous reports, treatment with exogenous IGF-1 caused an increase in MBP protein that was preceded by activation of PI3K (AKT) and MAPK (ERK) signaling pathways. The extracellular matrix protein laminin was further shown to potentiate the effect of IGF-1 on oligodendrocyte differentiation. Depletion of the laminin receptor dystroglycan using siRNA, however, blocked the ability of IGF-1 to promote oligodendrocyte differentiation of cells grown on laminin, suggesting a role for dystroglycan in IGF-1-mediated differentiation. Indeed, loss of dystroglycan led to a reduction in the ability of IGF-1 to activate MAPK, but not PI3K, signaling pathways. Pharmacological inhibition of MAPK signaling also prevented IGF-1-induced increases in myelin basic protein (MBP), indicating that MAPK signaling was necessary to drive IGF-1-mediated enhancement of oligodendrocyte differentiation. Using immunoprecipitation, we found that dystroglycan, the adaptor protein Grb2, and insulin receptor substrate-1 (IRS-1), were associated in a protein complex. Taken together, our results suggest that the positive regulatory effect of laminin on oligodendrocyte differentiation may be attributed, at least in part, to dystroglycan's ability to promote IGF-1-induced differentiation.

IGF-1-stimulated protein synthesis in oligodendrocyte progenitors requires PI3K/mTOR/Akt and MEK/ERK pathways

Insulin-like growth factor-1 (IGF-1) interacts with the Type I receptor to activate two main signaling pathways, the mitogen-activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) and the phosphatidylinositol 3-kinase (PI3K)-Akt cascades, which mediate proliferation or survival of oligodendrocyte (OL) progenitors (OLPs). In other cellular systems, mammalian target of rapamycin (mTOR) and the p70 S6 kinase are downstream effectors that phosphorylate translation initiation factors (e.g. eIF-4E), their regulators (e.g. 4E-binding protein 1, 4E-BP1) and ribosomal protein S6 (S6). The aim of this study was to determine whether these pathways are involved in IGF-1-stimulated protein synthesis, important for growth and differentiation of OLs. Rat cultured OLPs were treated with IGF-1 with or without inhibitors of PI3K (LY294002 or Wortmannin), mTOR (rapamycin), MEK (PD98059), and Akt (III or IV), as well as an adenovirus encoding a dominant negative form of Akt. Protein synthesis, as assessed by [(35)S]-methionine incorporation, was stimulated by IGF-1 and required the upstream activation of PI3K, Akt, mTOR and MEK/ERK. Concordant with the experiments using protein kinase inhibitors, western blotting revealed that IGF-1 stimulates phosphorylation of Akt, mTOR, ERK, S6 and 4E-BP1. Activation of S6 and inactivation of 4E-BP1, necessary for protein synthesis to take place, were dependent on the upstream activation of PI3K and mTOR. Finally, IGF-1 consistently stimulated protein synthesis through mTOR in differentiating OLPs but mRNA transcription was not required at day 4, indicating a differential role of IGF-1 throughout OL development.

IRS-2 branch of IGF-1 receptor signaling is essential for appropriate timing of myelination

Insulin-like growth factor (IGF)-1 increases proliferation, inhibits apoptosis and promotes differentiation of oligodendrocytes and their precursor cells, indicating an important function for IGF-1 receptor (IGF-1R) signaling in myelin development. The insulin receptor substrates (IRS), IRS-1 and -2 serve as intracellular IGF-1R adaptor proteins and are expressed in neurons, oligodendrocytes and their precursors. To address the role of IRS-2 in myelination, we analyzed myelination in IRS-2 deficient (IRS-2(-/-)) mice and age-matched controls during postnatal development. Interestingly, expression of the most abundant myelin proteins, myelin basic protein and proteolipid protein was reduced in IRS-2(-/-) brains at postnatal day 10 (P10) as compared to controls. myelin basic protein immunostaining in P10-IRS-2(-/-) mice revealed a reduced immunostaining, but an unchanged regional distribution pattern. In cerebral myelin isolates at P10 unaltered relative expression of different myelin proteins was found, indicating quantitatively reduced but not qualitatively altered myelination. Interestingly, up-regulation of IRS-1 expression and increased IGF-1R signaling were observed in IRS-2(-/-) mice at P10-14, indicating a compensatory mechanism to overcome IRS-2 deficiency. Adult IRS-2(-/-) mice showed unaltered myelination and motor function. Furthermore, in neuronal/brain-specific insulin receptor knockout mice myelination was unchanged. Thus, our experiments reveal that IGF-1R/IRS-2 mediated signals are critical for appropriate timing of myelination in vivo.

Cell-specific effects of insulin receptor substrate-1 deficiency on normal and IGF-I-mediated colon growth

Insulin-like growth factor I (IGF-I) potently stimulates intestinal growth. Insulin receptor substrate-1 (IRS-1) mediates proliferative and antiapoptotic actions of IGF-I in cell lines, but its in vivo relevance in intestine is not defined. This study tested the hypothesis that there is cell type-specific dependence on IRS-1 as a mediator of IGF-I action. Length, mass, crypt cell proliferation, and apoptosis were measured in small intestine and colon of IRS-1-null mice and wild-type (WT) littermates and in colon of IRS-1-null or WT mice expressing IGF-I transgenes. Expression of IGF-I receptor and signaling intermediates was examined in intestine of WT and IRS-1-null mice, cultured intestinal epithelial cells, and myofibroblasts. Absolute IRS-1 deficiency reduced mucosal mass in jejunum and colon, but effects were more pronounced in colon. Muscularis mass was decreased in both segments. In IGF-I transgenics, IRS-1 deficiency significantly attenuated IGF-I-stimulated growth of colonic mucosa and abolished antiapoptotic but not mitogenic effects of IGF-I transgene on crypt cells. IGF-I-induced muscularis growth was unaffected by IRS-1 deficiency. In intestinal epithelial cells, IRS-1 was expressed at higher levels than IRS-2 and was preferentially activated by IGF-I. In contrast, IGF-I activated both IRS-1 and IRS-2 in intestinal myofibroblasts and IRS-2 activation was upregulated in IRS-1-null myofibroblasts. We conclude that the intestinal epithelium but not the muscularis requires IRS-1 for normal trophic actions of IGF-I and that IRS-1 is required for antiapoptotic but not mitogenic effects of IGF-I in the intestinal crypts in vivo.

Identification and characterization of an insulin receptor substrate 4-interacting protein in rat brain: implications for longevity

The hypothalamus is organized as a collection of distinct, autonomously active nuclei that regulate discrete functions, such as feeding activity and metabolism. We used suppression subtractive hybridization (SSH) to identify genes that are enriched in the hypothalamus of the rat brain. We screened a subtractive library of 160 clones, and 4 genes that were predominantly expressed in the hypothalamus, compared to other brain regions. The mRNA for a member of the WD-repeat family of proteins, WDR6, was abundantly expressed in the hypothalamus, and we found that WDR6 interacted with insulin receptor substrate 4 (IRS-4) in the rat brain. Interestingly, WDR6 gene expression in the hypothalamic arcuate nucleus was decreased by caloric restriction, and in growth hormone (GH)-antisense transgenic rats, both of which are associated with an increased life span. Insulin-like growth factor (IGF)-I and insulin treatment increased WDR6 gene expression in mouse hypothalamus-derived GT1-7 cells. Our results might suggest that WDR6 participates in insulin/IGF-I signaling and the regulation of feeding behavior and longevity in the brain.

Insulin-like growth factor actions during development of neural stem cells and progenitors in the central nervous system

Insulin-like growth factor-I (IGF-I) plays a key role in normal development. Recent studies show that IGF-I exerts a wide variety actions in the central nervous system during development as well as in adulthood. This report reviews recent developments on IGF-I actions and its mechanisms in the central nervous system, with a focus on its actions during the development of neural stem cells and progenitors. Available data strongly indicate that IGF-I shortens the length of the cell cycle in neuron progenitors during embryonic life and has an influence on the growth of all neural cell types. The phosphatidylinositol-3 kinase/Akt and mitogen-activated protein kinase pathways seem to be the predominant mediators of IGF-I-stimulated neural cell proliferation and survival. IGF-I actions, however, likely depend on cell type, developmental stage, and microenvironmental milieu.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

Astrocyte-specific overexpression of insulin-like growth factor-I promotes brain overgrowth and glial fibrillary acidic protein expression

Insulin-like growth factor-I (IGF-I) is widely expressed in the central nervous system (CNS). Whereas during normal development IGF-I is expressed predominantly by neurons and to a much lesser degree by glial cells, its expression in astrocytes, and often in microglia, is increased during and/or after variety of CNS injuries. Recently we have generated a new line of IGF-I Tg mice, called IGF-I(Ast/Tet-Off) Tg mice, in which IGF-I transgene is expressed specifically in astrocytes and is tightly controlled by the tetracycline analog doxycycline. In this study we examined whether IGF-I derived from astrocytes is capable of promoting neural cell growth during development. When the IGF-I transgene is allowed to be expressed, IGF-I(Ast/Tet-Off) Tg mice exhibit markedly increases in 1) brain weight; 2) brain DNA and protein abundance; and 3) number of neurons, oligodendrocytes, and astrocytes, as well as myelination, findings similar to those observed in our other lines of Tg mice that express IGF-I transgene predominantly in neurons. Unlike Tg mice with neuron-specific IGF-I expression, which manifest marked increases in the concentrations of oligodendrocyte/myelin-specific proteins, however, IGF-I(Ast/Tet-Off) Tg mice exhibit an increase in the concentration of glial fibrillary acidic protein, an astrocyte-specific protein. Furthermore, when transgene expression is blunted, brain overgrowth in IGF-I(Ast/Tet-Off) Tg mice ceases. Our data indicate that astrocyte-derived IGF-I is capable of promoting neural cells growth in vivo. Our data also suggest that IGF-I's actions in CNS depend in part on the location of its expression and cellular microenvironment and that continuous presence of IGF-I expression is necessary for brain overgrowth.

Insulin receptor substrate-4 signaling in quiescent rat hepatocytes and in regenerating rat liver

This study was designed to characterize insulin receptor substrate-4 (IRS-4) in isolated rat hepatocytes and to examine its role in liver regeneration. Subcellular fractionation revealed that 85% of IRS-4 is located at isolated hepatocyte plasma membranes. The distribution of IRS-4 among intracellular compartments remained unchanged in insulin-stimulated cells. Two bands corresponding to 145 and 138 kd were observed in immunoblotting experiments. Immunoprecipitation of hepatocyte lysates with a highly specific antibody against IRS-4 led to an insulin and insulin-like growth factor 1 (IGF-1)-dependent increase in phosphotyrosine residues of the 145-kd band. IRS-4 was found to be associated with Src homology 2 (SH2) domain-containing proteins (phosphatidylinositol 3-kinase [PI 3-kinase] and Src homology phosphatase [SHP-2]) and with protein kinase C zeta (PKC zeta). Insulin and IGF-1 elicited a rapid and dose-dependent binding of these 3 proteins to IRS-4. These data suggest that IRS-4 is insulin-/IGF-1-activated by phosphorylation and not by translocation, inducing the recruitment of SH2 domain-containing proteins and PKC zeta to the membrane. To evaluate the possible role of IRS-4 in liver regeneration, we also examined this system after partial hepatectomy (PH). One day after PH, IRS-1 expression increased, consistent with a stimulatory role in the regenerative process, whereas it decreased 7 days after liver resection. This drastic IRS-1 depletion occurred at the expense of increased IRS-2 and IRS-4 expression 7 days after PH. In addition, at this period of time after surgery, the in vivo insulin stimulation of remnant rat livers showed an increase in IRS-4/PI 3-kinase association. Given that 1 and 7 days after PH isolated hepatocytes responded similarly to insulin in terms of induced cell proliferation, a compensatory role is proposed for IRS-2/4 induction. In conclusion, IRS-4 is activated by insulin and IGF-1-like IRS-1 in rat hepatocytes, and the induced expression of IRS-4 is a compensatory mechanism that plays a role in conditions of liver regeneration.

Astrocyte differentiation states and glioma formation

Gliomas are the most common primary malignancy in human central nervous system. Many similarities in cell morphology and expression of markers exist between cancerous cells and normal undifferentiated progenitor cells. At the molecular level, many important gene products are causally implicated in both the glial differentiation process and glial neoplasm formation. These observations raise the question of to what degree cell differentiation state influences glioma formation. In this review, we discuss new insights into the parallels between glial differentiation and glioma formation as well as the potential application of differentiation-inducing therapy.