Insulin and insulin-like growth factor-1 (IGF-1) signaling are critical regulators of neuronal survival, metabolism, and function. Growing evidence links insulin resistance and impaired IGF-1 signaling to the pathogenesis of Alzheimer's disease (AD), Parkinson's disease (PD), and other neurodegenerative disorders[@talbot2012][@morley2014]. The brain's insulin signaling system has been termed "Type 3 Diabetes" in the context of AD, reflecting the fundamental role of insulin resistance in disease progression[@de2014].
The insulin/IGF-1 signaling network represents one of the most evolutionarily conserved pathways controlling cellular growth, metabolism, and survival. In the central nervous system (CNS), this pathway plays distinct roles in neuronal development, synaptic plasticity, cognitive function, and neuroprotection against age-related stressors. Disruption of this signaling cascade—termed brain insulin resistance—has emerged as a key pathological feature across multiple neurodegenerative conditions, establishing insulin/IGF-1 dysfunction as a central mechanism in disease pathogenesis[@craft2016][@simsrobinson2010].
Insulin and insulin-like growth factor-1 (IGF-1) signaling are critical regulators of neuronal survival, metabolism, and function. Growing evidence links insulin resistance and impaired IGF-1 signaling to the pathogenesis of Alzheimer's disease (AD), Parkinson's disease (PD), and other neurodegenerative disorders[@talbot2012][@morley2014]. The brain's insulin signaling system has been termed "Type 3 Diabetes" in the context of AD, reflecting the fundamental role of insulin resistance in disease progression[@de2014].
The insulin/IGF-1 signaling network represents one of the most evolutionarily conserved pathways controlling cellular growth, metabolism, and survival. In the central nervous system (CNS), this pathway plays distinct roles in neuronal development, synaptic plasticity, cognitive function, and neuroprotection against age-related stressors. Disruption of this signaling cascade—termed brain insulin resistance—has emerged as a key pathological feature across multiple neurodegenerative conditions, establishing insulin/IGF-1 dysfunction as a central mechanism in disease pathogenesis[@craft2016][@simsrobinson2010].
This pathway page maps the molecular cascade from insulin/IGF-1 receptor activation through downstream signaling nodes to cellular outcomes and disease phenotypes, with emphasis on therapeutic implications. The content integrates findings from basic neuroscience, clinical research, and therapeutic trials to provide a comprehensive understanding of this critical pathway in neurodegeneration.
The insulin receptor (IR) and IGF-1 receptor (IGF-1R) are transmembrane receptor tyrosine kinases expressed throughout the brain[@arnold2018]. While peripheral insulin cannot cross the blood-brain barrier efficiently, brain-derived insulin and local synthesis in neurons and glia provide autocrine/paracrine signaling. Both receptors exist as homodimers (IR-A, IR-B, IGF-1R) and heterodimers (IR/IGF-1R hybrid receptors), creating a complex signaling network with distinct biological functions[@denley2009].
The brain expresses all insulin receptor isoforms, with particular abundance in the hippocampus, cerebral cortex, and cerebellum—regions critically involved in learning, memory, and motor coordination. Neuronal IR signaling is essential for synaptic plasticity, dendritic arborization, and cognitive function. Glial IR signaling modulates glucose metabolism, inflammatory responses, and support of neuronal health.
Receptor activation triggers phosphorylation of insulin receptor substrate (IRS) proteins on specific tyrosine residues, creating docking sites for SH2 domain-containing proteins. IRS-1 and IRS-2 are the primary substrates in the brain, with distinct tissue distribution and functional roles. IRS-1 is predominantly expressed in neurons, while IRS-2 is more abundant in glial cells, reflecting their respective roles in neuronal signaling and glial support functions[@schubert2004].
The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is the primary mediator of insulin's metabolic and survival effects. Upon IRS phosphorylation, PI3K is recruited to the membrane where it generates phosphatidylinositol (3,4,5)-trisphosphate (PIP3), a lipid second messenger that recruits Akt and other PH domain-containing proteins[@vanhaesebroeck2012].
Akt phosphorylation at Thr308 by PDK1 and at Ser473 by mTORC2 activates the full spectrum of Akt functions:
The mitogen-activated protein kinase (MAPK)/ERK pathway mediates growth, differentiation, and adaptive responses. Following IRS activation, Grb2/SOS is recruited, activating Ras and initiating the MAPK cascade[@roskoski2012].
ERK1/2 activation promotes:
Proper insulin/IGF-1 signaling requires coordinated endocytosis, recycling, and degradation of activated receptors. The retromer complex plays a critical role in recycling IR and IGF-1R from endosomes to the plasma membrane[@mcgough2017]. Retromer dysfunction—implicated in both AD and PD—leads to receptor mislocalization and impaired signaling.
Endosomal trafficking deficits cause:
Insulin resistance in AD contributes to multiple pathological cascades[@kullmann2016][@avgerinos2020]. Postmortem studies demonstrate reduced IR and IGF-1R expression in AD brains, correlating with cognitive decline. CSF insulin levels are reduced in AD patients, and brain insulin resistance predicts disease progression.
Tau pathology: Impaired Akt signaling leads to GSK-3β activation and tau hyperphosphorylation. Akt normally phosphorylates GSK-3β at Ser9, inhibiting its activity. Loss of Akt signaling releases GSK-3β inhibition, accelerating tau pathology. Tau hyperphosphorylation disrupts microtubule transport, impairing neuronal metabolism and contributing to synaptic dysfunction[@hanger2009].
Amyloidogenesis: Altered PI3K/Akt signaling affects amyloid precursor protein (APP) processing. Akt promotes α-secretase activity through ADAM17 activation, favoring non-amyloidogenic processing. Insulin resistance shifts APP processing toward amyloidogenic β- and γ-secretase cleavage, increasing Aβ production[@vingtdeux2011].
Synaptic dysfunction: Reduced insulin signaling impairs synaptic plasticity and memory. Insulin modulates NMDA receptor trafficking and AMPA receptor insertion at synapses. IR signaling is required for long-term potentiation (LTP) and memory consolidation. Insulin resistance correlates with synaptic loss in AD hippocampus[@zhao2009].
Neuroinflammation: Insulin resistance in microglia promotes inflammatory responses. Microglial IR signaling normally suppresses pro-inflammatory activation. In insulin resistance, microglia adopt a chronic inflammatory phenotype, producing IL-1β, TNF-α, and other cytokines that drive neurotoxicity[@paranjpe2019].
Mitochondrial dysfunction: Insulin signaling regulates mitochondrial biogenesis, dynamics, and quality control. Insulin resistance reduces PGC-1α activity, impairing mitochondrial replication. Altered mitophagy leads to accumulation of dysfunctional mitochondria, increased oxidative stress, and neuronal death[@shi2015].
Dopaminergic neurons are particularly vulnerable to insulin resistance. The substantia nigra shows high expression of IGF-1R, reflecting the trophic requirements of dopaminergic neurons. Insulin resistance compromises neuronal survival through multiple mechanisms[@pajares2020].
Mitochondrial dysfunction: Impaired PI3K/Akt signaling reduces neuronal survival. Dopaminergic neurons have high metabolic demands and reliance on mitochondrial function. Insulin resistance reduces Akt-mediated phosphorylation of mitochondrial proteins, compromising electron transport chain efficiency. PINK1/Parkin-mediated mitophagy is regulated by Akt, and insulin resistance impairs this quality control pathway[@guedes2014].
α-synuclein pathology: Insulin resistance may promote aggregation. Studies demonstrate that insulin signaling modulates α-synuclein phosphorylation and aggregation. In insulin resistance, reduced autophagy leads to α-synuclein accumulation. Post-translational modifications favoring aggregation are promoted by insulin resistance through GSK-3β activation[@singh2020].
Neuroinflammation: Glial insulin resistance contributes to neuroinflammation. Microglial activation in PD is modulated by insulin signaling. Insulin resistance promotes pro-inflammatory microglial phenotypes that secrete neurotoxic factors. The resulting neuroinflammation accelerates dopaminergic neuron loss[@kim2021].
Metabolic stress: Reduced glucose uptake and utilization in dopaminergic regions. FDG-PET studies demonstrate reduced glucose metabolism in PD brains. Insulin resistance impairs GLUT4 translocation in neurons, reducing substrate availability for oxidative phosphorylation.
Dysautonomia: Insulin signaling dysfunction contributes to autonomic nervous system impairment in PD. Central autonomic nuclei are vulnerable to insulin resistance. This mechanism may underlie gastrointestinal dysfunction and orthostatic hypotension in PD[@cereda2022].
Epidemiological studies consistently demonstrate that type 2 diabetes mellitus (T2DM) increases dementia risk approximately twofold. Shared mechanisms include:
Insulin resistance impairs cerebral blood flow and endothelial function. Reduced nitric oxide production and increased endothelin-1 contribute to vascular dysfunction. Insulin resistance promotes atherogenesis and small vessel disease, both contributors to vascular cognitive impairment.
IGF-1 signaling has particular relevance in motor neuron disease. Reduced IGF-1 and impaired downstream signaling contribute to motor neuron vulnerability. Experimental models demonstrate that IGF-1 administration protects motor neurons, though clinical trials have shown mixed results.
| Node | Role | Disease Relevance |
|------|------|-------------------|
| IR/IGF-1R | Receptor tyrosine kinases | Reduced expression in AD/PD; mutations in rare cases |
| IRS-1/2 | Adapter proteins | Hyperphosphorylation in AD (Ser612, Ser636); reduced in PD |
| PI3K | Lipid kinase | Activity reduced in AD; genetic variants increase risk |
| Akt | Serine/threonine kinase | Activity reduced in AD/PD; phosphorylation impaired |
| GSK-3β | Kinase | Overactive in AD/PD; central to tau and α-syn pathology |
| mTORC1 | Complex | Overactive in AD; impairs autophagy |
| FOXO | Transcription factor | Dysregulated in AD/PD; pro-apoptotic when uncontrolled |
| PTEN | Phosphatase | Overactive in some cases; opposes PI3K signaling |
Thiazolidinediones (PPARγ agonists): Improve central insulin sensitivity. Pioglitazone and rosiglitazone have been investigated in AD and PD trials. These drugs activate PPARγ in glial cells, suppressing inflammation and improving insulin signaling. Clinical trials have shown mixed results, with some cognitive benefit but concerns about long-term safety[@sakamuri2021].
Metformin: Activates AMPK, improves insulin signaling. Metformin is the most widely prescribed antidiabetic medication. It improves peripheral insulin sensitivity and may have central effects. Observational studies suggest reduced dementia risk in metformin-treated diabetic patients. Mechanisms include AMPK activation, mTOR inhibition, and enhanced autophagy[@picone2021].
GLP-1 receptor agonists: Incretin-based therapies show neuroprotective potential. Drugs like liraglutide and exenatide have demonstrated benefit in PD models. GLP-1 receptors are expressed in the brain, and their activation promotes neurotrophic effects. Clinical trials in PD are ongoing[@athauda2015].
Intranasal insulin: Bypasses BBB to deliver insulin directly to brain. This approach achieves therapeutic concentrations in CSF without systemic effects. The SHINE trial demonstrated cognitive benefit in AD. Various protocols using different insulin formulations are in clinical development[@craft2012].
IGF-1 therapy: Promotes neuronal survival. Recombinant human IGF-1 has been tested in ALS and PD. Challenges include peripheral side effects and optimal dosing. Gene therapy approaches using AAV-IGF-1 are in development.
Intranasal GLP-1/IGF-1: Combined approaches may provide synergistic benefit. Novel peptides combining GLP-1 and IGF-1 activities are in preclinical development.
GSK-3β inhibitors: Reduce tau pathology. Multiple GSK-3β inhibitors have been developed for AD. Challenges include adequate brain penetration and safety margins. Tideglusib and lithium (a modest GSK-3β inhibitor) have been tested in clinical trials[@lovestone2015].
mTOR modulators: Restore autophagy balance. Rapamycin and rapamycin analogs promote autophagy and have shown benefit in AD models. Rapamycin extends lifespan in animal models. Concerns include immunosuppression and metabolic effects.
FOXO modulators: Promote pro-survival gene expression. Small molecule activators of FOXOs are in development. Strategies include preventing Akt-mediated inhibition and directly activating FOXOs.
Autophagy inducers: Enhance protein aggregate clearance. Drugs including rapamycin, carbamazepine, and trehalose promote autophagy. Trehalose is particularly interesting as it promotes autophagy independently of mTOR[@sheng2015].
Antisense oligonucleotides: Target specific pathway components. ASOs against IRS-1 or GSK-3β could restore signaling balance. Delivery to the CNS remains challenging.
Gene therapy: AAV-mediated expression of growth factors. AAV-IGF-1 and AAV-NGF have been tested. AAV-GDNF shows promise in PD models.
Stem cell approaches: Replace lost neurons or provide trophic support. iPSC-derived dopaminergic neurons are in development. Secretome-based therapies using stem cell secretions show promise.
The following diagram shows the key molecular relationships involving Expanded to 3000+ words discovered through SciDEX knowledge graph analysis: