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Loss of insulin-like growth factor-1 signaling in astrocytes disrupts glutamate handling.

ABSTRACT: Insulin-like Growth Factor-1 (IGF-1) has been studied extensively for its ability to promote neuronal growth and excitability. Declining levels of IGF-1 have been correlated with impaired learning and memory as well as an increased risk of neurodegenerative diseases. While neuronal regulation by IGF-1 is well understood, the role of IGF-1 in influencing astrocyte function requires further exploration. Astrocytes regulate many aspects of the brain microenvironment, including controlling glutamate-glutamine cycling, which ultimately supports neuronal metabolism, neurotransmission, and protection from over stimulation. In this study, we examined whether IGF-1 acts through its cognate receptor, IGFR, to alter astrocytic glutamate handling. We utilized both small molecule IGFR inhibitors and Cre-driven genetic approaches to reduce IGFR in vivo and in cultured rodent astrocytes. When IGFR was knocked out of primary astrocytes derived from igfrf/f mice using AAV5-CMV-Cre, significant reductions in glutamate uptake were observed. Similarly, inhibition of IGFR with picropodophyllotoxin for 2 h, as well as 24 h, reduced glutamate uptake in vitro. Mechanistically, short-term inhibition of IGFR resulted in a significant decrease in glutamate transporter availability on the cell surface, as assessed by biotinylation. Long-term inhibition of IGFR led to significant reductions in mRNA expression of glutamate transport machinery, as assessed with qPCR. Reduced glutamate transporter mRNA was also observed in the brains of astrocyte-specific IGFR-deficient mice, three to four months after knock-out was induced with tamoxifen. Interestingly, long-term IGF-1 inhibition also resulted in an increase in adenosine triphosphate-stimulated glutamate release, though no change in adenosine triphosphate-stimulated calcium flux was observed nor were any changes in purinergic receptor protein expression. Together, these data suggest that reduced IGF-1 signaling will favor an accumulation of extrasynaptic glutamate, which may contribute to neurodegeneration in disease states where IGF-1 levels are low. Cover Image for this issue: doi: 10.1111/jnc.14534.

SUBMITTER: Prabhu D

PROVIDER: S-EPMC7416791 | biostudies-literature | 2019 Dec

REPOSITORIES: biostudies-literature

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Loss of insulin-like growth factor-1 signaling in astrocytes disrupts glutamate handling.

Prabhu Disha D Khan Sariya M SM Blackburn Katherine K Marshall Jessica P JP Ashpole Nicole M NM

Journal of neurochemistry 20191020 6

Insulin-like Growth Factor-1 (IGF-1) has been studied extensively for its ability to promote neuronal growth and excitability. Declining levels of IGF-1 have been correlated with impaired learning and memory as well as an increased risk of neurodegenerative diseases. While neuronal regulation by IGF-1 is well understood, the role of IGF-1 in influencing astrocyte function requires further exploration. Astrocytes regulate many aspects of the brain microenvironment, including controlling glutamate ...[more]

PMID: 31563149

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Project description:OBJECTIVE:A decline in mitochondrial function and biogenesis as well as increased reactive oxygen species (ROS) are important determinants of aging. With advancing age, there is a concomitant reduction in circulating levels of insulin-like growth factor-1 (IGF-1) that is closely associated with neuronal aging and neurodegeneration. In this study, we investigated the effect of the decline in IGF-1 signaling with age on astrocyte mitochondrial metabolism and astrocyte function and its association with learning and memory. METHODS:Learning and memory was assessed using the radial arm water maze in young and old mice as well as tamoxifen-inducible astrocyte-specific knockout of IGFR (GFAP-CreTAM/igfrf/f). The impact of IGF-1 signaling on mitochondrial function was evaluated using primary astrocyte cultures from igfrf/f mice using AAV-Cre mediated knockdown using Oroboros respirometry and Seahorse assays. RESULTS:Our results indicate that a reduction in IGF-1 receptor (IGFR) expression with age is associated with decline in hippocampal-dependent learning and increased gliosis. Astrocyte-specific knockout of IGFR also induced impairments in working memory. Using primary astrocyte cultures, we show that reducing IGF-1 signaling via a 30-50% reduction IGFR expression, comparable to the physiological changes in IGF-1 that occur with age, significantly impaired ATP synthesis. IGFR deficient astrocytes also displayed altered mitochondrial structure and function and increased mitochondrial ROS production associated with the induction of an antioxidant response. However, IGFR deficient astrocytes were more sensitive to H2O2-induced cytotoxicity. Moreover, IGFR deficient astrocytes also showed significantly impaired glucose and A? uptake, both critical functions of astrocytes in the brain. CONCLUSIONS:Regulation of astrocytic mitochondrial function and redox status by IGF-1 is essential to maintain astrocytic function and coordinate hippocampal-dependent spatial learning. Age-related astrocytic dysfunction caused by diminished IGF-1 signaling may contribute to the pathogenesis of Alzheimer's disease and other age-associated cognitive pathologies.

Project description:PurposeInsulin and insulin-like growth factors (IGFs) are putative regulators of cell proliferation and differentiation during lens development. Transgenic mice that overexpress IGF-1 in the lens have been previously described. To further understand the ocular functions of this growth factor family, the in vivo effects of insulin expression on lens development were investigated using transgenic mice.MethodsExpression of insulin receptor (IR) and IGF-1 receptor (IGF-1R) in mouse lens was examined by reverse-transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. Transgenic mice that overexpress insulin in the lens were generated using two different promoters: a fiber-cell specific alphaA-crystallin (alphaA) promoter and a modified alphaA-promoter linked to the chicken delta1-crystallin enhancer (called the deltaenalphaA promoter). The deltaenalphaA promoter is active in both lens epithelial and fiber cells. The lens phenotypes were analyzed by histology and immunohistochemistry. Protein expression was examined by western blotting.ResultsNormal mouse lenses express both the insulin receptor (IR) and the IGF-1 receptor (IGF-1R), and their expression is highest at the lens periphery where the germinative and transitional zones are located. In transgenic mice, insulin expression in the lens induced cataract formation. The severity of the cataracts reflected the level of transgene expression, independent of the type of promoter used. In severely affected families, the spherical shape of the lens was altered and the lenses were smaller than normal. Histological analysis showed no evidence of premature differentiation of the anterior epithelial cells. In contrast to the IGF-1 mice, insulin transgenic mice exhibited an anterior shift in the location of the germinative and transitional zones, leading to a reduction of the lens epithelial compartment. Additional alterations included expansion of the lens transitional zone, variable nuclear positioning in the lens bow region, and inhibition of fiber cell denucleation and terminal differentiation.ConclusionsElevated intraocular insulin does not enhance proliferation nor induce differentiation of mouse lens epithelial cells. Since an increase in IGF-1 causes a posterior shift of the lens geminative and transitional zones, while an increase in insulin causes an anterior shift of these zones, our results suggest that these two growth factors may work together to control the location of this structural domain during normal lens development. Our data also suggest that increased insulin-signaling activity in the lens can antagonize the endogenous signals that are responsible for fiber cell maturation and terminal differentiation.

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Loss of insulin-like growth factor-1 signaling in astrocytes disrupts glutamate handling.

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Loss of insulin-like growth factor-1 signaling in astrocytes disrupts glutamate handling.

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