Expression of rat insulin-like growth factor binding protein-6 in the brain, spinal cord, and sensory ganglia

Insulin-Like Growth Factor Signaling in Alzheimer's Disease: Pathophysiology and Therapeutic Strategies

Alzheimer's disease (AD) is the leading cause of dementia among the elderly population, posing a significant public health challenge due to limited therapeutic options that merely delay cognitive decline. AD is associated with impaired energy metabolism and reduced neurotrophic signaling. The insulin-like growth factor (IGF) signaling pathway, crucial for central nervous system (CNS) development, metabolism, repair, cognition, and emotion regulation, includes IGF-1, IGF-2, IGF-1R, IGF-2R, insulin receptor (IR), and six insulin-like growth factor binding proteins (IGFBPs). Research has identified abnormalities in IGF signaling in individuals with AD and AD models. Dysregulated expression of IGFs, receptors, IGFBPs, and disruptions in downstream phosphoinositide 3-kinase-protein kinase B (PI3K/AKT) and mitogen-activated protein kinase (MAPK) pathways collectively increase AD susceptibility. Studies suggest modulating the IGF pathway may ameliorate AD pathology and cognitive decline. This review explores the CNS pathophysiology of IGF signaling in AD progression and assesses the potential of targeting the IGF system as a novel therapeutic strategy. Further research is essential to elucidate how aberrant IGF signaling contributes to AD development, understand underlying molecular mechanisms, and evaluate the safety and efficacy of IGF-based treatments.

The neurobiology of insulin-like growth factor I: From neuroprotection to modulation of brain states

After decades of research in the neurobiology of IGF-I, its role as a prototypical neurotrophic factor is undisputed. However, many of its actions in the adult brain indicate that this growth factor is not only involved in brain development or in the response to injury. Following a three-layer assessment of its role in the central nervous system, we consider that at the cellular level, IGF-I is indeed a bona fide neurotrophic factor, modulating along ontogeny the generation and function of all the major types of brain cells, contributing to sculpt brain architecture and adaptive responses to damage. At the circuit level, IGF-I modulates neuronal excitability and synaptic plasticity at multiple sites, whereas at the system level, IGF-I intervenes in energy allocation, proteostasis, circadian cycles, mood, and cognition. Local and peripheral sources of brain IGF-I input contribute to a spatially restricted, compartmentalized, and timed modulation of brain activity. To better define these variety of actions, we consider IGF-I a modulator of brain states. This definition aims to reconcile all aspects of IGF-I neurobiology, and may provide a new conceptual framework in the design of future research on the actions of this multitasking neuromodulator in the brain.

Identification of secreted factors in dental pulp cell-conditioned medium optimized for neuronal growth

With their potent regenerative and protective capacities, stem cell-derived conditioned media emerged as an effective alternative to cell therapy, and have a prospect to be manufactured as pharmaceutical products for tissue regeneration applications. Our study investigates the neuroregenerative potential of human dental pulp cells (DPCs) conditioned medium (CM) and defines an optimization strategy of DPC-CM for enhanced neuronal outgrowth. Primary sensory neurons from mouse dorsal root ganglia were cultured with or without DPC-CM, and the lengths of βIII-tubulin positive neurites were measured. The impacts of several manufacturing features as the duration of cell conditioning, CM storage, and preconditioning of DPCs with some factors on CM functional activity were assessed on neurite length. We observed that DPC-CM significantly enhanced neurites outgrowth of sensory neurons in a concentration-dependent manner. The frozen storage of DPC-CM had no impact on experimental outcomes and 48 h of DPC conditioning is optimal for an effective activity of CM. To further understand the regenerative feature of DPC-CM, we studied DPC secretome by human growth factor antibody array analysis and revealed the presence of several factors involved in either neurogenesis, neuroprotection, angiogenesis, and osteogenesis. The conditioning of DPCs with the B-27 supplement enhanced significantly the neuroregenerative effect of their secretome by changing its composition in growth factors. Here, we show that DPC-CM significantly stimulate neurite outgrowth in primary sensory neurons. Moreover, we identified secreted protein candidates that can potentially promote this promising regenerative feature of DPC-CM.

The insulin-like growth factor-1 system in the adult mammalian brain and its implications in central maternal adaptation

Our knowledge on the bioavailability and actions of insulin-like growth factor-1 (IGF-1) has markedly expanded in recent years as novel mechanisms were discovered on IGF binding proteins (IGFBPs) and their ability to release IGF-1. The new discoveries allowed a better understanding of the endogenous physiological actions of IGF-1 and also its applicability in therapeutics. The focus of the present review is to summarize novel findings on the neuronal, neuroendocrine and neuroplastic actions of IGF-1 in the adult brain. As most of the new regulatory mechanisms were described in the periphery, their implications on brain IGF system will also be covered. In addition, novel findings on the effects of IGF-1 on lactation and maternal behavior are described. Based on the enormous neuroplastic changes related to the peripartum period, IGF-1 has great but largely unexplored potential in maternal adaptation of the brain, which is highlighted in the present review.

Insulin-like growth factor binding protein-6 delays replicative senescence of human fibroblasts

Cellular senescence can be induced by a variety of mechanisms, and recent data suggest a key role for cytokine networks to maintain the senescent state. Here, we have used a proteomic LC-MS/MS approach to identify new extracellular regulators of senescence in human fibroblasts. We identified 26 extracellular proteins with significantly different abundance in conditioned media from young and senescent fibroblasts. Among these was insulin-like growth factor binding protein-6 (IGFBP-6), which was chosen for further analysis. When IGFBP-6 gene expression was downregulated, cell proliferation was inhibited and apoptotic cell death was increased. Furthermore, downregulation of IGFBP-6 led to premature entry into cellular senescence. Since IGFBP-6 overexpression increased cellular lifespan, the data suggest that IGFBP-6, in contrast to other IGF binding proteins, is a negative regulator of cellular senescence in human fibroblasts.

Toward a comprehensive neurobiology of IGF-I

Insulin-like growth factor I (IGF-I) belongs to an ancient family of hormones already present in early invertebrates. The insulin family is well characterized in mammals, although new members have been described recently. Since its characterization over 50 years ago, IGF-I has been considered a peptide mostly involved in the control of body growth and tissue remodeling. Currently, its most prominent recognized role is as a quasi-universal cytoprotectant. This role connects IGF-I with regulation of lifespan and with cancer, two areas of very active research in relation to this peptide. In the brain, IGF-I was formerly considered a neurotrophic factor involved in brain growth, as many other neurotrophic factors. Other aspects of the neurobiology of IGF-I are gradually emerging and suggest that this growth factor has a prominent role in brain function as a whole. During development IGF-I is abundantly expressed in many areas, whereas once the brain is formed its expression is restricted to a few regions and in very low quantities. However, the adult brain appears to have an external input from serum IGF-I, where this anabolic peptide is abundant. Thus, serum IGF-I has been proven to be an important modulator of brain activity, including higher functions such as cognition. Many of these functions can be ascribed to its tissue-remodeling activity as IGF-I modulates adult neurogenesis and angiogenesis. Other activities are cytoprotective; indeed, IGF-I can be considered a key neuroprotective peptide. Still others pertain to the functional characteristics of brain cells, such as cell excitability. Through modulation of membrane channels and neurotransmission, IGF-I impinges directly on neuronal plasticity, the cellular substrate of cognition. However, to fully understand the role of IGF-I in the brain, we have to sum the actions of locally produced IGF-I to those of serum IGF-I, and this is still pending. Thus, an integrated view of the role played by IGF-I in the brain is not yet possible. An operational approach to overcome this limitation would be to consider IGF-I as a signal coupling environmental influences on body metabolism with brain function. Or in a more colloquial way, we may say that IGF-I links body "fitness" with brain fitness, providing a mechanism to the roman saying "mens sana in corpore sano."

Differential modulation of Sonic-hedgehog-induced cerebellar granule cell precursor proliferation by the IGF signaling network

The molecular mechanisms regulating organ growth and size remain unclear. Sonic hedgehog (SHH) signaling is a major player in the regulation of cerebellar development: SHH is secreted by Purkinje neurons and acts on the proliferation of granule cell precursors (GCPs) in the external germinal layer. These then become postmitotic and form the internal granular layer but do so in the presence of SHH ligand, begging the question of how the proliferative response to SHH signaling is downregulated in differentiating GCPs. Here, we have determined the precise cellular localization of the expression of insulin-like growth factor (IGF) network components in the developing mouse cerebellum and show that this network modulates the proliferative effects of SHH signaling on GCPs. IGF1 and IGF2 are potent mitogens for GCPs and both synergize with SHH in inducing GCP proliferation. Whereas the proliferative activity of IGF1 or IGF2 on GCPs does not require intact SHH signaling, aspects of SHH activity on GCP proliferation require signaling through the IGF receptor 1. Moreover, we find that 3 of the IGF-binding proteins, IGFBP2, IGFBP3 and IGFBP5, inhibit IGF1/2-induced cell proliferation, whereas IGFBP5 also inhibits SHH-induced GCPs proliferation. This novel function of IGFBP5 that we have uncovered demonstrates the exquisite regulation of SHH signaling by different components of the IGF network.

Expression profiles of Insulin-like growth factor binding protein-like 1 in the developing mouse forebrain

Insulin-like growth factor binding protein-like 1 (IGFBPL-1) has a domain structure that resembles one in the insulin-like growth factor binding proteins (IGFBPs), which is a domain known to be bound by insulin-like growth factors (IGFs) with high affinity and to thereby affect cell growth and metabolism. However, the distribution and functions of IGFBPL-1 in the brain are not known. Thus, to clarify IGFBPL-1 expression profiles during development, we have investigated the expression patterns of IGFBPL-1 mRNA in developing mouse brains using in situ hybridization, focusing especially on the forebrain. On embryonic day (E)12, IGFBPL-1mRNA expressions were localized in the preplate of the neopallium, dorsal thalamus, the differentiating field of the epithalamus, and the hippocampus. On E14, strong expression was recognized in the subventricular zone (SVZ) and intermediate zone (IMZ) in the neopallium, hippocampus, the differentiating field of the epithalamus, and the dorsal thalamus. E14 expressions were generally the strongest among all the stages examined and signals decreased thereafter. At E16 and E18, IGFBPL-1mRNA was strongly detected in the hippocampus and the SVZ of the neopallium. In postnatal stages, mRNA was found only in the granule cell layer of the dentate gyrus (DG). Notably, at P20, expression was restricted to the deep fraction of the granule cell layer, the location of late-born DG neurons.

Expression of IGFBPs in the developing mouse submandibular and von Ebner's glands

The expression of insulin-like growth factor binding proteins (IGFBPs) in the developing mouse submandibular and von Ebner's glands was determined by in situ hybridization and by an immunohistochemical method. In the submandibular glands, IGFBP-2 and IGFBP-4 mRNAs were expressed in the terminal end-buds (TEB) at E13-E17, concomitant with epithelial branching. IGFBP-3 mRNA was expressed in the mesenchyme surrounding the TEB; and IGFBP-5 mRNA, in the ducts. At E17, IGFBP-5 mRNA expression was observed not only in the ducts but also in the TEB. Similarly, IGFBP-4 mRNA expression was observed not only in the TEB but also in the mesenchyme. After birth, IGFBP-4 expression was observed only in the connective tissue and disappeared by P14. That of IGFBP-7 appeared at P1 and was observed in the connective tissue until P21. The IGFBP-5 mRNA expression pattern after birth was the same as that seen at E17, but at P21 IGFBP-5 was immunohistochemically expressed only in the duct. The mRNA level of IGFBP-2 expression at postnatal days was weak, but its protein was detected in the ducts and acini at P14-P21. In von Ebner's glands, which appeared at the base of the circumvallate papillae at E17, only IGFBP-2 and IGFBP-4 mRNAs were expressed in the ducts and acini. Postnatally, IGFBP-4 was substituted by IGFBP-5 in the same region. Immunohistochemically, IGFBP-5 and IGFBP-2 were expressed in the ducts and acini at P14-P21. Throughout the study, IGFBP-6 was not detected by in situ hybridization, the immunoreactivity for it was observed in the nerve fibers of submandibular and von Ebner's glands. These data support a role for these molecules as local mediators of salivary growth and differentiation.

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.

Modulation of the cytokine network in human adult astrocytes by human herpesvirus-6A

Human herpesvirus-6A (HHV-6A) is a common pathogen whose role in CNS disorders including multiple sclerosis remains controversial. To understand how HHV-6A could influence inflammatory pathways in the CNS, we infected cultured human adult astrocytes and examined the expression of 268 cytokines, chemokines, growth factors and their receptors by gene profiling. HHV-6 infection alone had little effect on the astrocyte gene profile but strongly altered the astrocyte response to proinflammatory cytokines. Under those conditions astrocytes express higher levels of anti-inflammatory mediators including IL-10 and IL-11, chemotactic factors, growth factors and factors controlling type I interferon production. Our data suggest that HHV-6 itself does not evoke a pro-inflammatory response in astrocytes but rather triggers immune modulatory factors in the face of inflammation.

Distinct expression pattern of insulin-like growth factor family in rodent taste buds

The insulin-like growth factor (IGF) system is an important regulator of growth and differentiation in a variety of tissues. In the present study, the expression of IGF family members in the taste buds of mice and rats was examined. By reverse transcriptase polymerase chain reaction (RT-PCR) analysis, mRNA of IGF-I and -II, IGF-I receptor (IGF-IR), insulin receptor (insulin R), and IGF-binding protein (IGFBP)-2, -3, -4, -5, and -6 was detected in the taste bud-containing epithelium of the circumvallate papillae of mice. As suggested by the study using degenerate PCR (McLaughlin [2000] J. Neurosci. 20:5679-5688), IGF-IR was expressed in most of the taste bud cells of adult mice, as found by immunohistochemistry, and in those of postnatal day (P) 6 mice by in situ hybridization. Insulin R, which has strong homology to IGF-IR, was also detected in most of the taste bud cells of mice by immunohistochemistry and in situ hybridization. IGF-I immunoreactivity was detected in a few taste bud cells and in the epithelium surrounding taste buds. Northern blot analysis revealed that the amount of IGF-I mRNA in taste bud-containing epithelium was very low compared with that in liver. IGF-II immunoreactivity was weakly detected in mouse taste buds and the surrounding epithelium. In the rat tissue, a subset of the taste bud cells was positive for IGF-II. Among the six IGFBPs, IGFBP-2, -5, and -6 were detected in the mouse taste buds: IGFBP-2 and -5 immunoreactivity was seen in the majority of the taste bud cells, whereas IGFBP-6 immunoreactivity was found in the nerve fibers innervating the taste buds. In situ hybridization study also revealed that IGFBP-2 and -5 mRNA was synthesized in the taste buds of P6 mice and that the expression of these mRNAs overlapped in von Ebner's glands. These data reveal that IGF-I and -II might be produced in taste bud cells and (or) surrounding lingual epithelium and act through IGF-IR and insulin R locally in a paracrine and autocrine manner. The activity of these IGFs may be modulated through their interaction with IGFBP-2, -5, and 6.

Cell death in the nervous system: lessons from insulin and insulin-like growth factors

Programmed cell death is an essential process for proper neural development. Cell death, with its similar regulatory and executory mechanisms, also contributes to the origin or progression of many or even all neurodegenerative diseases. An understanding of the mechanisms that regulate cell death during neural development may provide new targets and tools to prevent neurodegeneration. Many studies that have focused mainly on insulin-like growth factor-I (IGF-I), have shown that insulin-related growth factors are widely expressed in the developing and adult nervous system, and positively modulate a number of processes during neural development, as well as in adult neuronal and glial physiology. These factors also show neuroprotective effects following neural damage. Although some specific actions have been demonstrated to be anti-apoptotic, we propose that a broad neuroprotective role is the foundation for many of the observed functions of the insulin-related growth factors, whose therapeutical potential for nervous system disorders may be greater than currently accepted.

Neuroprotective effects of insulin-like growth factor-binding protein ligand inhibitors in vitro and in vivo

The role of brain insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs) in neuroprotection was further investigated using in vitro and in vivo models of cerebral ischemia by assessing the effects of IGF-I, IGF-II, and high affinity IGFBP ligand inhibitors (the peptide [Leu24, 59, 60, Ala31]hIGF-I (IGFBP-LI) and the small molecule NBI-31772 (1-(3,4-dihydroxybenzoyl)-3-hydroxycarbonyl-6, 7-dihydroxyisoquinoline), which pharmacologically displace and elevate endogenous, bioactive IGFs from IGFBPs. Treatment with IGF-I, IGF-II, or IGFBP-LI (2 microg/mL) significantly (P < 0.05) reduced CA1 damage in organotypic hippocampal cultures resulting from 35 minutes of oxygen and glucose deprivation by 71%, 60%, and 40%, respectively. In the subtemporal middle cerebral artery occlusion (MCAO) model of focal ischemia, intracerebroventricular (icv) administration of IGF-I and IGF-II at the time of artery occlusion reduced ischemic brain damage in a dose-dependent manner, with maximum reductions in total infarct size of 37% (P < 0.01) and 38% (P < 0.01), respectively. In this model of MCAO, i.c.v. administration of NBI-31772 at the time of ischemia onset also dose-dependently reduced infarct size, and the highest dose (100 microg) significantly reduced both total (by 40%, P < 0.01) and cortical (by 43%, P < 0.05) infarct volume. In the intraluminal suture MCAO model, administration of NBI-31772 (50 microg i.c.v.) at the time of artery occlusion reduced both cortical infarct volume (by 40%, P < 0.01) and brain swelling (by 24%, P < 0.05), and it was still effective when treatment was delayed up to 3 hours after the induction of ischemia. These results further define the neuroprotective properties of IGFs and IGFBP ligand inhibitors in experimental models of cerebral ischemia.

Expression of IGF-II, IGFBP-2, -5, and -6 in meningiomas with different brain invasiveness

Meningiomas show clinical characteristics that vary from very benign to clearly malignant with rapid invasive growth and metastases. This study was undertaken to analyze the expression of members in the insulin-like growth factor (IGF) system in meningiomas showing different degrees of brain invasion. Tissue samples from 16 meningiomas were analyzed for members in the IGF family by mRNA in situ hybridization. The meningiomas comprised three groups: I. Benign meningiomas that did not interfere with the arachnoid plane and showed no edema. II. Benign meningiomas that did not respect the arachnoid plane and tumors that caused edema. III. Aggressive and malignant meningiomas that caused edema and showed brain invasion. IGF-II mRNA was identified in all tumors analyzed, and with a clear increase in expression observed in group III tumors. IGFBP-2 mRNA was detected in equal levels in all tumors. IGFBP-5 mRNA levels were highest in the benign group without edema (I) of meningiomas whereas IGFBP-6 mRNA levels were highest in the group with brain invasion (III). Brain invasiveness in degrees from respect of the arachnoid membrane progressing to frank brain invasion correlated with increases in IGF-II and IGFBP-6 expression. High levels of IGFBP-6 may not inhibit IGF-II actions, which is known to be growth promoting in tumors. Instead, IGFBP-6 appears to have an importance for the characteristics of edema and brain invasion in meningiomas.

Effects of early undernutrition on the brain insulin-like growth factor-I system

Undernutrition reduces circulating concentrations of insulin-like growth factor (IGF)-I, but how it affects the brain IGF system, especially during development, is largely unknown. We have studied IGF-I, IGF-II, IGF receptor and IGF binding protein (BP)-2 mRNA expression in the hypothalamus, cerebellum and cerebral cortex of neonatal rats that were food restricted beginning on gestational day 16. One group was refed starting on postnatal day 14. Rats were killed on postnatal day 8 or 22. Undernutrition did not produce an overall reduction in brain weight at either age but, at 22 days, both the cerebellum and hypothalamus weighed significantly less. At 8 days, no change was detected in the central IGF axis in response to undernutrition. However, in 22-day-old undernourished rats, IGF-I and IGF receptor mRNA expression were increased in both the hypothalamus and cerebellum, while IGFBP-2 was decreased, but only in the hypothalamus. Refeeding had no effect on any of these parameters. These results suggest that the hypothalamus and cerebellum respond to malnutrition and the decrease in circulating IGF-I, a peptide fundamental for growth and development, by increasing the local production of both the growth factor and its receptor in attempt to maintain normal development.