Effects of radiotherapy on the hippocampus and hippocampal neurogenesis: a systematic review of preclinical studies

PURPOSE

A comprehensive literature review was undertaken to understand the effects and underlying mechanisms of cranial radiotherapy (RT) on the hippocampus and hippocampal neurogenesis as well as to explore protective factors and treatments that might mitigate these effects in preclinical studies.

METHODS

PubMed/MEDLINE, Web of Science, and Embase were queried for studies involving the effects of radiation on the hippocampus and hippocampal neurogenesis. Data extraction followed the Animal Research Reporting of In Vivo Experiments (ARRIVE) guidelines, and a risk of bias assessment was conducted for the included animal studies using the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE) risk of bias tool.

RESULTS

Ninety studies were included, with 48 assessing radiation-induced changes and 42 examining possible interventions. The majority of studies (97.8%) used experimental animal models. Studies demonstrated that cranial irradiation reduces hippocampal neurogenesis, particularly in the neurogenic niches of the dentate gyrus; causes alterations in gene expression and enzymatic activity; induces inflammation; promotes apoptosis; and often results in cognitive impairment. Potential protective strategies include pharmacological agents like metformin and peroxisome proliferator-activated receptor-α (PPAR-α) agonists or behavioral interventions like voluntary running. In a risk of bias assessment, many studies were rated as having an unclear risk of bias.

CONCLUSION

Radiotherapy, while essential for managing brain tumors, can have adverse effects on hippocampal function and structure in animal models. These effects manifest in reduced neurogenesis, molecular alterations, and increased inflammation, leading to cognitive deficits. Further research is needed to identify and improve interventions and develop comprehensive therapeutic approaches that balance effective tumor control with the preservation of cognitive health.

Shorter Infantile Amnesia in Females: Important Implications for the Next Generation

The sex-specific development of hippocampal learning in juveniles remains unclear. Using an inhibitory avoidance task, we assessed contextual learning in both sexes of juvenile rats. While sex hormone levels and activating effects are low in juveniles, females showed superior performance to males, suggesting that females have a shorter period of infantile amnesia than males. It was already known that when infants are cared for by mothers with high parenting behavior, they are likely to become high parenting mothers themselves. In addition, neonatal testosterone is known to masculinize the brain, causing behavioral, neural, and hormonal sex differences. Here, we reviewed the purposeful significance of sex-specific development for learning, along with the interaction of developmental changes in the hormonal environment.

Radiosensitivity-related Variation in MicroRNA-34a-5p Levels and Subsequent Neuronal Loss in the Hilus of the Dentate Gyrus after Irradiation at Postnatal Days 10 and 21 in Mice

The radiosensitivity of mice differs between postnatal days 10 (P10) and 21(P21); these days mark different stages of brain development. In the present study, Ki67 and doublecotin (DCX) immunostaining and hematoxylin staining was performed, which showed that acute radiation exposure at postnatal day 10 induced higher cell apoptosis and loss in the hilus of the dentate gyrus at day 1 postirradiation than postnatal day 21. MicroRNA (miRNA) sequencing and real-time quantitative reverse transcription PCR (qRT-PCR) analysis indicated the upregulation of miRNA-34a-5p at days 1 and 7 after irradiation at postnatal day 10, but not at postnatal day 21. Down-regulation of T-cell intracytoplasmic antigen-1 pathway (Tia1) was indicated by qRT-PCR at day 1 day but not day 7 after irradiation at postnatal day 10. Neurobehavioral testing in mature mice irradiated at postnatal day 10 demonstrated the impairment of short-term memory in novel object recognition and spatial memory, compared to those irradiated at postnatal day 21. Combined with our previous luciferase assay showing the direct interaction of miRNA34a-5p and Tia1, these findings suggest that radiation-induced abnormal miR-34a-5p/Tial interaction at day 1 after irradiation at postnatal day 10 may be involved in apoptosis of the dentate gyrus hilar, impairment of neurogenesis and subsequent short-term memory loss as observed in the novel object recognition and Barnes maze tests.

Combined Ionizing Radiation Exposure by Gamma Rays and Carbon-12 Nuclei Increases Neurotrophic Factor Content and Prevents Age-Associated Decreases in the Volume of the Sensorimotor Cortex in Rats

In orbital and ground-based experiments, it has been demonstrated that ionizing radiation (IR) can stimulate the locomotor and exploratory activity of rodents, but the underlying mechanism of this phenomenon remains undisclosed. Here, we studied the effect of combined IR (0.4 Gy γ-rays and 0.14 Gy carbon-12 nuclei) on the locomotor and exploratory activity of rats, and assessed the sensorimotor cortex volume by magnetic resonance imaging-based morphometry at 1 week and 7 months post-irradiation. The sensorimotor cortex tissues were processed to determine whether the behavioral and morphologic effects were associated with changes in neurotrophin content. The irradiated rats were characterized by increased locomotor and exploratory activity, as well as novelty-seeking behavior, at 3 days post-irradiation. At the same time, only unirradiated rats experienced a significant decrease in the sensorimotor cortex volume at 7 months. While there were no significant differences at 1 week, at 7 months, the irradiated rats were characterized by higher neurotrophin-3 and neurotrophin-4 content in the sensorimotor cortex. Thus, IR prevents the age-associated decrease in the sensorimotor cortex volume, which is associated with neurotrophic and neurogenic changes. Meanwhile, IR-induced increases in locomotor activity may be the cause of the observed changes.

A systematic review of normal tissue neurovascular unit damage following brain irradiation-Factors affecting damage severity and timing of effects

Background

Radiotherapy is key in the treatment of primary and secondary brain tumors. However, normal tissue is inevitably irradiated, causing toxicity and contributing to cognitive dysfunction. The relative importance of vascular damage to cognitive decline is poorly understood. Here, we systematically review the evidence for radiation-induced damage to the entire neurovascular unit (NVU), particularly focusing on establishing the factors that influence damage severity, and timing and duration of vascular effects relative to effects on neural tissue.

Methods

Using PubMed and Web of Science, we searched preclinical and clinical literature published between January 1, 1970 and December 1, 2022 and evaluated factors influencing NVU damage severity and timing of NVU effects resulting from ionizing radiation.

Results

Seventy-two rodents, 4 canines, 1 rabbit, and 5 human studies met inclusion criteria. Radiation increased blood-brain barrier (BBB) permeability, reduced endothelial cell number and extracellular matrix proteoglycans, reduced tight junction proteins, upregulated cellular adhesion molecule expression, reduced activity of glucose and BBB efflux transporters and activated glial cells. In the brain parenchyma, increased metalloproteinases 2 and 9 levels, demyelination, cell death, and inhibited differentiation were observed. Effects on the vasculature and neural compartment were observed across acute, delayed, and late timepoints, and damage extent was higher with low linear energy transfer radiation, higher doses, lower dose rates, broader beams, and in the presence of a tumor.

Conclusions

Irradiation of normal brain tissue leads to widespread and varied impacts on the NVU. Data indicate that vascular damage is in most cases an early effect that does not quickly resolve. More studies are needed to confirm sequence of damages, and mechanisms that lead to cognitive dysfunction.

Potassium 2-(1-hydroxypentyl)-benzoate attenuates neuronal apoptosis in neuron-astrocyte co-culture system through neurotrophy and neuroinflammation pathway

Potassium 2-(1-hydroxypentyl)-benzoate (d,l-PHPB), a new drug candidate for ischemic stroke at the phase II clinic trial, has been shown to protect neurons by inhibiting oxidative injury and reducing neuron apoptosis in previous studies. But the mechanisms of d,l-PHPB remain to be studied. In this study, a neuron–astrocytes co-culture system was used to elucidate the roles of astrocytes in neuroprotection of d,l-PHPB under oxygen-glucose deprivation/reoxygenation (OGD/R) condition. Our data showed that d,l-PHPB reduced neuronal apoptosis in mono-culture system and this effect was enhanced in neuron–astrocyte co-culture system under the OGD/R condition. Meanwhile, d,l-PHPB obviously increased the levels of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), which were mainly secreted from astrocytes, in the co-culture system after OGD/R. The PI3K/AKT and ERK signaling pathways as well as the p-TRKA/B receptors were involved in the process. In addition, the levels of TNF-α and IL-1β secreted from astrocytes after OGD/R were markedly reduced after d,l-PHPB treatment, which was mainly due to the suppression of phosphorylated p38. In conclusion, the present study demonstrates that the neuroprotective effects of d,l-PHPB were improved by astrocytes, mainly mediated by increasing the release of BDNF/NGF and attenuating inflammatory cytokines.

Urinary Levels of IL-1β and GDNF in Preterm Neonates as Potential Biomarkers of Motor Development: A Prospective Study

Objectives. To evaluate the association between inflammatory biomarkers, neurotrophic factors, birth conditions, and the presence of motor development abnormalities in preterm neonates. Methods. Plasma and urinary levels of cytokines (IL-1β, IL-6, IL-10, TNF, and IL-12p70), chemokines (CXCL8/IL-8, CCL2/MCP-1, CCL5/RANTES, CXCL10/IP-10, and CXCL9/MIG), and neurotrophic factors (BDNF and GDNF) were evaluated in 40 preterm neonates born between 28 and 32 incomplete weeks of gestation, at four distinct time points: at birth (umbilical cord blood) (T0), at 48 (T1), at 72 hours (T2), and at 3 weeks after birth (T3). Biomarkers levels were compared between different time points and then associated with Test of Infant Motor Performance (TIMP) percentiles. Results. Maternal age, plasma, and urinary concentrations of inflammatory molecules and neurotrophic factors were significantly different between groups with normal versus lower than expected motor development. Higher levels of GDNF were found in the group with lower than expected motor development, while IL-1β and CXCL8/IL-8 values were higher in the group with typical motor development. Conclusion. Measurements of cytokines and neurotrophic factors in spot urine may be useful in the follow-up of motor development in preterm neonates.