1. What are Glucocorticoids?
Glucocorticoids (from Greek “γλυκός” meaning ”sweet” and cortex) are a class of steroid hormones that are produced in the adrenal cortex. In human the main glucocorticoid (GC) is cortisol, whereas in rodents, the main glucocorticoid is corticosterone. GC hormones are vital for the homeostatic regulation of the bodily functions, and their actions span a hierarchy of control levels. Accordingly, GCs act on the basic molecular level to control critical functions such the cell cycle, cellular metabolism, viability, synaptic plasticity and the immune response. At the top of the hierarchical pyramid, GCs fine-tune the stress behavior and cognitive function. It is well-known and as every human has experienced, high levels of GCs caused by stressful situations impair memory. Also, GCs are very important for fetal and brain development.
2. The Glucocorticoid Receptor
GCs bind onto their receptor, the Glucocorticoid Receptor (GR), which is located in the cytoplasm of the cell. GR belongs to the superfamily of the ligand-activated nuclear transcription factors. Hence, in the absence of ligand, GR is located in the cellular cytoplasm in a complex with chaperone proteins such as HSP90, HSP60 and FKBP51 (1). Upon hormone binding, the receptor is shuttled in the nucleus (Fig.2). There it recognizes and associates with partially pallidromic regulatory DNA areas in target genes, called Glucocorticoid Responsive Elements (i.e GREs) (2,3) . GR can either act as a transcriptional activator, by directly binding onto the promoter of the target gene. Alternatively, GR interacts with other co-factors and causes trans-repression of target genes (1, 4) . Structure of the GR: GR shares the modular structure common among the steroid receptors. It consists of a variable N-terminal domain (NTD), a highly conserved DNA binding domain with two zinc finger motifs (DBD), a hinge region, and a C-terminal hormone binding domain (5). The DBD is conserved throughout the members of the nuclear receptorsuperfamily and almostall vertebrate species (6). The hinge region participates in receptor–ligand binding.
3. GCs and Adverse Effects on Fetal and Infant Brain Development
GC hormones are potent stimulators of organ maturation. Due to this property, synthetic GC analogs are vastly used in prenatal medicine. Specifically, in pregnancies under the risk of pre-term delivery (10% of pregnancies in North America) . GC administration is the preferred route followed, to accelerate fetal lung maturation (12,13). Moreover, other clinical conditions of the mother or the fetus make use of synthetic GC administration such as allergies, asthma, and when the fetus is suspected to suffer from Congenital Adrenal Hyperplasia (CAH) (14). Synthetic GCs are marginally different from their endogenous equivalents and more potent agonists of the GR. Specifically, dexamethasone (DEX) one of the most widely used synthetic GC is a very potent GR agonist. DEX is less sensitive to degradation by 11β-hydroxysteroid dehydrogenase-2 (11β-HSD2) (8, 14); a key enzyme that transforms active GCs to inactive 11 keto-analogs in the fetoplacental area. Therefore, 11β-HSD2 reduces the exposure of the fetus to GCs.
Despite the benefits of DEX, accumulating data suggest a dark side of DEX. Specifically, clinical follow-up studies have indicated that DEX can have detrimental effects on brain development. Specifically, in infants and school aged children having exposed to DEX around birth, cognitive and behavioral deficits such as reduced IQ, poor social interaction and difficulty in coping with stress have been documented. Moreover, imaging data have pointed to reduced cortical convolution and reduced head circumference of DEX exposed infants in respect to untreated controls. Even more significantly , animal studies on prenatal DEX treatment, have documented that brain development can be permanently impaired by DEX. For example, Uno et al.(1990 and 1994) demonstrated that the size of the hippocampal structure is reduced by DEX treatment in utero in rhesus macaques (15, 16).
Importantly, the observed impairments in the brain architecture could be the result of changes in different processes. Hence, the formation of the brain can be disrupted by altered migration of neuronal cells,or by impairments in the proliferation of Neural Progenitor Cells from which neurons originate. Accordingly, a series of in vitro studies demonstrated that exposure to GCs inhibits neural progenitor and stem cell proliferation and changes the balance between proliferation and differentiation (17, 18). The anti- proliferative effects of GCs are not limited in embryonic NPCs as increased GCs due to stress, or DEX exposure cause the same effects in adult NPCs of the hippocampus. Moreover, GCs are potent inhibitors of the proliferation of tumor cell lines such as medulloblastoma, neuroblastoma and osteosarcoma cells (17, 18).
In the light of the clinical and experimental observations on GC’s dynamics in the developing brain and NPCs, it is very important to understand the mechanisms by which they can impair brain development. Results from this line of research could be applied for improving the current GC-based treatments and the mode of DEX use in perinatal medicine.
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