TABLE 5.
Pathways implicated in Rett syndrome and Fragile X syndrome.
| Source | Region/Sample | Experiment | Findings | References |
| Brain-derived neurotrophic factor signaling pathway | ||||
| Mecp2–/ Y–, Conditional BDNF-over-expression | CA2 neurons | Behavioral assessment, electrophysiology, immunohistochemistry | BDNF overexpression reverses some RTT phenotypes | Chang et al., 2006 |
| Patients with RTT | Serum and CSF | Patient studies | No changes in BDNF expression in patients with RTT vs. healthy controls | Vanhala et al., 1998 |
| Mecp2–/y, patients with RTT | Cerebrum, frontal cortex, whole brains (mice) | Chromatin immunoprecipitation, qPCR | ↓BDNF expression↑; TrkB | Abuhatzira et al., 2007; Deng et al., 2007 |
| Mecp2–/y | Hippocampal glutamatergic neurons, CA1 | Immunofluorescence | ↓PSD95; (1–3)IGF1 restores PSD95 levels | Chao et al., 2007; Tropea et al., 2009 |
| Fmr1–/y | Hippocampal neurons (P0) | Western blotting | ↓PSD95 localization; restored by inhibition of mTORC-S6K1 signaling | Yang et al., 2019 |
| Fmr1–/y | Primary cortical cultures (14–15DIV; E15), hippocampal neuronal cultures (E19) | Immunofluorescence, western blotting, mRNA stability assay | ↓PSD95; FMRP binds to and stabilize Psd95 mRNA | Todd et al., 2003; Zalfa et al., 2007 |
| Insulin-like growth factor 1 signaling pathway | ||||
| Mecp2–/y | CSF | Patient studies | No changes in IGF1 expression | Riikonen, 2003 |
| Mecp2–/y, Mecp2–/+ | Motor cortex, cortical slices | Behavioral assessment, immunocytochemistry, electrophysiology | IGF1 improves several RTT symptoms incl. cortical plasticity | Tropea et al., 2009; Castro et al., 2014; Khwaja et al., 2014; O’Leary et al., 2018 |
| Fmr1–/y | Testes | Western blot, Igf1r knockout | Correcting IGF1R levels reduces macro-orchidism | Wise, 2017 |
| Fmr1–/y, Patients with FXS | Mice (14 weeks), primary hippocampal cell cultures (17 DIV) | Clinical trials, behavioral assessment, kinase assays | NNZ-2566 administration improves patients’ symptom scoring | Deacon et al., 2015; Berry-Kravis et al., 2020 |
| Cyclic adenosine monophosphate (cAMP) response element binding protein signaling pathway | ||||
| MECP2T158M/T158M hESC, MECP2- V247fs-MT iPSC, Mecp2–/+ | hESC differentiated into forebrain neurons, iPSC | Electrophysiology, western blotting, behavioral assessment | ↓CREB; Correcting CREB levels improves some RTT phenotypes | Bu et al., 2017 |
| SWR/J mice, dfmr13/+ Drosophila | SWR/J mice (15–25 weeks, | qPCR, western blotting, immunofluorescence, behavioral assessment | Fmr1 is bound by CREB | Kanellopoulos et al., 2012; Rani and Prasad, 2015 |
| Phosphatidylinositol-3-kinases signaling pathway | ||||
| Mecp2308, Mecp2 tml.lJae | Male (Mecp2308, 5 months), cortical neurons (P1) | Behavioral assessment, qPCR, western blotting, immune-histochemistry | ↓PI3K pathway activation | Ricciardi et al., 2011; Yuan et al., 2020 |
| Fmr1–/y | Primary hippocampal neurons (E17) | Western blotting, immune-histochemistry, kinase assays | ↑PI3K pathway activation | Gross et al., 2010 |
| Fmr1–/y | Cortex, cerebellum (P11–13), hippocampus (P28–32) Hippocampal slices (4–6 weeks) | Immuno-histochemistry, bioinformatics, qPCR, electrophysiology | ↑mTOR phosphorylation | Sharma et al., 2010; Casingal et al., 2020 |
| Mitogen-activated protein kinase signaling pathway | ||||
| Mecp2–/y | Motor cortex, cortical slices | Behavioral assessment, immuno-cytochemistry, electrophysiology | ↓MAPK activation; rhIGF1 increases activation | Castro et al., 2014 |
| Fmr1–/y | Mice (14 weeks), primary hippocampal cell cultures (17 DIV) | Behavioral assessment, kinase assays | ↑MAPK activation; Corrected by NNZ-2566 | Deacon et al., 2015 |
| Bioenergetics | ||||
| Patients with RTT | 34 patients with RTT, 37 healthy controls | Metabolomic analysis | Metabolic dysfunction, oxidative stress. | Neul et al., 2020 |
| Mecp2tm1.1Bird, Patients with RTT | Isolated microglia, primary hippocampal cell cultures, fibroblasts isolated from patients with RTT | Immunofluorescence, qPCR, imaging studies, western blotting, bioenergetic assays | ↓Microglial viability; ↓Microglia numbers; ↑ROS; ↓ATP production; ↑Glutamate | Jin et al., 2015; Crivellari et al., 2021 |
| Fmr1–/y | Fibroblasts cell lines, synaptosomes, primary hippocampal (E19) and cortical cell cultures (P0–P2) | Behavioral assessment, immune-precipitation, western blotting, qPCR, bioenergetic assay | Mitochondrial proton leak | Licznerski et al., 2020 |
| Fmr1–/y | Brain slices, macrophages, total brain; (2–6 months) | Bioenergetic assay, | ↑ROS; ↑Lipid peroxidation; ↑Protein oxidation; ↑NADPH oxidase activity | El Bekay et al., 2007 |
| Fmr1–/y | Various | Review | ↑Metabolites from superoxide attack on lipids ↑ROS | Maurin et al., 2014 |
Main findings of studies reporting signaling and pathway dysfunctions in Rett syndrome and Fragile X syndrome. Mecp2, methyl-CpG binding protein 2 gene; BDNF, brain-derived neurotrophic factor; RTT, Rett syndrome; CSF, cerebrospinal fluid; TrkB, tropomycin receptor kinase B; Fmr1, fragile X mental retardation 1 gene, PSD95, postsynaptic density protein 95; DIV, days in vitro; FRMP, fragile X mental retardation protein; IGF1, insulin-like growth factor 1; Igf1r/IGF1R, insulin-like growth factor 1 receptor; NNZ-2566, (1–3)IGF1 tripeptide; CREB, cAMP response element binding protein; hESC, human embryonic stem cells; iPSC, induced pluripotent stem cells; PI3K, phosphatidylinositol-3-kinase; mTOR, mammalian target of rapamycin; MAPK, mitogen-activated protein kinase; rhIGF1, recombinant human insulin-like growth factor 1; ROS, reactive oxygen species.