Cell death in the rat hippocampus in a model of prenatal brain injury: time course and expression of death-related proteins

Region- and age-specific effects of perinatal phthalate exposure on developmental cell death and adult anatomy of dorsal and ventral hippocampus and associated cognitive behaviors

Humans are exposed to phthalates, a class of endocrine-disrupting chemicals used in food packaging/processing, PVC plastics, and personal care products. Gestational exposure may lead to adverse neurodevelopmental outcomes. In a rat model, perinatal exposure to an environmentally relevant mixture and dose of phthalates leads to increased developmental apoptosis in the medial prefrontal cortex (mPFC) and a subsequent reduction in neurons and in cognitive flexibility measured in adults of both sexes (Sellinger et al., 2021b; Kougias et al., 2018b). However, whether these effects generalize to other cognitive regions, like the hippocampus, is less well understood as existing studies used single phthalates at large doses, unrepresentative of human exposure. In the current study, patterns of naturally occurring cell death were first established in the dorsal and ventral hippocampal subfields (CA3 and CA1). Both dorsal and ventral CA3 reached high levels of cell death on P2 while levels in dorsal and ventral CA1 peaked on P5 in both sexes. Exposure to a phthalate mixture (0.2 and 1 mg/kg/day) throughout gestation through postnatal day 10 resulted in subtle age- and region-specific decreases in developmental cell death, however there were no significant changes in adult neuron number or associated behaviors: the Morris water maze and social recognition. Therefore, perinatal exposure to a low dose mixture of phthalates does not result in the dramatic structural and behavioral changes seen with high doses of single phthalates. This study also adds to our understanding of the distinct neurodevelopmental effects of phthalates on different brain regions.

Synthetic female gonadal hormones alter neurodevelopmental programming and behavior in F1 offspring

The increased prevalence of neurodevelopmental disorders during the last half-century led us to investigate the potential for intergenerational detrimental neurodevelopmental effects of synthetic female gonadal hormones, typically used in contraceptive pills. We examined 3 separate cohorts of mice over the span of 2 years, a total of 150 female F₀ mice and over 300 male and female rodents from their F₁ progeny. We demonstrate that F₁ male offsprings of female mice previously exposed to the synthetic estrogen 17α-ethinylestradiol (EE2) in combination with the synthetic progestin Norethindrone, exhibit neurodevelopmental and behavioral differences compared to control mice. Because the EE2 + Norethindrone administration resulted in gene expression changes in the exposed F₀ mice ovaries persisting after the end of treatment, it is likely that the synthetic hormone treatment caused changes in the germline cells and that led to altered neurodevelopment in the offsprings. An altered gene expression pattern was discovered in the frontal cortex of male mice from the first offspring (F_1.1) at infancy and an ADHD-like hyperactive locomotor behavior was exhibited in young male mice from the second offspring (F_1.2) of female mice treated with contraceptive pill doses of EE2 + Norethindrone prior to pregnancy. The intergenerational neurodevelopmental effects of EE2 + Norethindrone treatment were sex specific, predominantly affecting males. Our observations in mice support the hypothesis that the use of synthetic contraceptive hormones is a potential environmental factor impacting the prevalence of human neurodevelopmental disorders. Additionally, our results indicate that contraceptive hormone drug safety assessments may need to be extended to F₁ offspring.

Gonadal hormone modulation of intracellular calcium as a mechanism of neuroprotection

Hormones have wide-ranging effects throughout the nervous system, including the ability interact with and modulate many aspects of intracellular calcium regulation and calcium signaling. Indeed, these interactions specifically may help to explain the often opposing or paradoxical effects of hormones, such as their ability to both promote and prevent neuronal cell death during development, as well as reduce or exacerbate damage following an insult or injury in adulthood. Here, we review the basic mechanisms underlying intracellular calcium regulation-perhaps the most dynamic and flexible of all signaling molecules-and discuss how gonadal hormones might manipulate these mechanisms to coordinate diverse cellular responses and achieve disparate outcomes. Additional future research that specifically addresses questions of sex and hormone effects on calcium signaling at different ages will be critical to understanding hormone-mediated neuroprotection.

Sex and steroid hormones in early brain injury

Brain injury during development can have severe, long-term consequences. Using an array of animal models, we have an understanding of the etiology of perinatal brain injury. However, we have only recently begun to address the consequences of endogenous factors such as genetic sex and developmental steroid hormone milieu. Our limited understanding has sometimes led researchers to make over-generalizing and potentially dangerous statements regarding treatment for brain injury. Therefore this review acts as a cautionary tale, speaking to our need to understand the effects of sex and steroid hormone environment on the response to brain trauma in the neonate.

What can development teach us about menopause?

Development and aging are often mirror image processes and this may be equally true in the effects of estradiol, a potent endogenous steroid regulating brain development as well as a therapeutic used to relieve the negative components of perimenopause. Both the developing and perimenopausal brain are characterized by a sensitive period of hormone responsiveness, and in both cases, the neurotransmitters GABA and glutamate, as well as synaptogenesis and cell proliferation are major hormone targets. This review compares and contrasts the effects of estradiol on the developing and aging brain and highlights new avenues of exploration and therapeutic development.

Effects of 17beta-estradiol replacement on the apoptotic effects caused by ovariectomy in the rat hippocampus

AIMS

The aim of the present study was to investigate the effects of different periods of ovariectomy and 17beta-estradiol replacement on apoptotic cell death and expression of members of the Bcl-2 family in the rat hippocampus.

MAIN METHODS

Hippocampi were obtained from rats in proestrus, ovariectomized (15 days, 21 days and 36 days), ovariectomized for 15 days and then treated with 17beta-estradiol for 7 or 21 days, and rats ovariectomized and immediately treated with 17beta-estradiol for 21 days. The expression of Bcl-2 and Bax and the number of apoptotic cells were determined.

KEY FINDINGS

Ovariectomy decreased Bcl-2 expression and increased Bax expression and the number of apoptotic cells. Replacement with 17beta-estradiol (21 days) throughout the post-ovariectomy period reduced the number of apoptotic cells to the control levels, and prevented the effects of ovariectomy on Bax expression, but only partially restored the Bcl-2 expression. After 15 days of ovariectomy, the replacement with 17beta-estradiol for 21 days, but not for 7 days, restored the Bcl-2 and Bax expression and the percentage of apoptotic cells to the levels found in the proestrus control.

SIGNIFICANCE

The present results show that a physiological concentration of 17beta-estradiol may help maintain long-term neuronal viability by regulating the expression of members of the Bcl-2 family. Even after a period of hormonal deprivation, treatment with 17beta-estradiol is able to restore the expression of Bax and Bcl-2 to control levels, but the duration of the treatment is a key factor to obtain the desired effect. These data provide new understanding into the mechanisms contributing to the neuroprotective action of estrogen.

Structural remodeling of gray matter astrocytes in the neonatal pig brain after hypoxia/ischemia

Astrocytes play a vital role in the brain; their structural integrity and sustained function are essential for neuronal viability, especially after injury or insult. In this study, we have examined the response of astrocytes to hypoxia/ischemia (H/I), employing multiple methods (immunohistochemistry, iontophoretic cell injection, Golgi-Kopsch staining, and D-aspartate uptake) in a neonatal pig model of H/I. We have identified morphological changes in cortical gray matter astrocytes in response to H/I. Initial astrocytic changes were evident as early as 8 h post-insult, before histological evidence for neuronal damage. By 72 h post-insult, astrocytes exhibited significantly fewer processes that were shorter, thicker, and had abnormal terminal swellings, compared with astrocytes from control brains that exhibited a complex structure with multiple fine branching processes. Quantification and image analysis of astrocytes at 72 h post-insult revealed significant decreases in the average astrocyte size, from 686 microm(2) in controls to 401 microm(2) in H/I brains. Sholl analysis revealed a significant decrease (>60%) in the complexity of astrocyte branching between 5 and 20 microm from the cell body. D-Aspartate uptake studies revealed that the H/I insult resulted in impaired astrocyte function, with significantly reduced clearance of the glutamate analog, D-aspartate. These results suggest that astrocytes may be involved in the pathophysiological events of H/I brain damage at a far earlier time point than first thought. Developing therapies that prevent or reverse these astrocytic changes may potentially improve neuronal survival and thus might be a useful strategy to minimize brain damage after an H/I insult.

Impact of sex and hormones on new cells in the developing rat hippocampus: a novel source of sex dimorphism?

The hippocampus is a key brain region regulating complex cognitive and emotional responses, and is implicated in the etiology of depressive and anxiety disorders, many of which exhibit some degree of sex difference. The male rat hippocampus is consistently reported to be slightly but significantly larger than the female. The majority of studies on the development of volumetric sex differences have focused on the effects of estradiol (E2), with relatively few focusing on androgens. We examined the impact of both E2 and androgens on newly born cells in the developing rat hippocampus, and report that neonatal males have significantly more 5-bromo-2'-deoxyuridine-5'-monophosphate (BrdU)+ cells than females. Both testosterone (T) and dihydrotestosterone treatment of females significantly increased the number of BrdU+ cells, an effect blocked by the androgen receptor antagonist, flutamide. However, only T significantly increased the number of neuronal nuclear antigen+ neurons in the female rat hippocampus. Interestingly, E2 treatment also increased BrdU+ cells in females, but had no effect on neuron number. Instead, E2 and T significantly increased the number of newly born glial fibrillary acidic protein or glutamine synthetase+ glial cells in females, indicating that androgens and E2 may act independently to achieve distinct endpoints. Quantification of pyknotic cells at two different developmental time points indicates no sex difference in the number of cells dying, suggesting, but not proving, that gonadal steroids are promoting cell genesis.

Estradiol and the developing brain

Estradiol is the most potent and ubiquitous member of a class of steroid hormones called estrogens. Fetuses and newborns are exposed to estradiol derived from their mother, their own gonads, and synthesized locally in their brains. Receptors for estradiol are nuclear transcription factors that regulate gene expression but also have actions at the membrane, including activation of signal transduction pathways. The developing brain expresses high levels of receptors for estradiol. The actions of estradiol on developing brain are generally permanent and range from establishment of sex differences to pervasive trophic and neuroprotective effects. Cellular end points mediated by estradiol include the following: 1) apoptosis, with estradiol preventing it in some regions but promoting it in others; 2) synaptogenesis, again estradiol promotes in some regions and inhibits in others; and 3) morphometry of neurons and astrocytes. Estradiol also impacts cellular physiology by modulating calcium handling, immediate-early-gene expression, and kinase activity. The specific mechanisms of estradiol action permanently impacting the brain are regionally specific and often involve neuronal/glial cross-talk. The introduction of endocrine disrupting compounds into the environment that mimic or alter the actions of estradiol has generated considerable concern, and the developing brain is a particularly sensitive target. Prostaglandins, glutamate, GABA, granulin, and focal adhesion kinase are among the signaling molecules co-opted by estradiol to differentiate male from female brains, but much remains to be learned. Only by understanding completely the mechanisms and impact of estradiol action on the developing brain can we also understand when these processes go awry.

Androgens predispose males to GABAA-mediated excitotoxicity in the developing hippocampus

Clinical evidence and animal models indicate greater brain damage in newborn males following injury. In adults, glutamate is the primary source of excitotoxic cell death and the steroid, estradiol, is neuroprotective. In neonatal brain, membrane depolarization following activation of GABAA receptors is the major source of excitation. Consequent influx of calcium via L-type channels is normally trophic, but becomes excitotoxic during periods of excessive activation of GABAA receptors, such as hypoxia-ischemia, alcohol exposure and seizures. The use of sex-specific hippocampal cultures revealed greater cell death induced by the GABAA agonist, muscimol, in male- versus female-derived cultures. Pretreatment with the androgen, dihydrotestosterone (DHT) increased muscimol-induced death in both sexes. Exploration of calcium dynamics indicated that, counter to expectation, female neurons achieved higher [Ca2+]i than male, but the calcium transient duration was shorter due to faster rise and decay. However, a second exposure to muscimol within minutes of the first, caused significant attenuation of [Ca2+]i in female neurons. In contrast, while male neurons exposed to muscimol for the first time exhibited lower maximal [Ca2+]i, when exposed to muscimol again there was no attenuation in [Ca2+]i. The latter effect was induced in females by DHT, and inversely correlated with the amount of gamma2 subunit of the GABAA receptor. This novel effect of androgen on GABA-mediated excitotoxicty suggests a unique opportunity for a sex-specific therapeutic approach involving antagonism of the androgen receptor in neonatal males at risk for brain injury.

Granulocyte-colony stimulating factor inhibits apoptotic neuron loss after neonatal hypoxia-ischemia in rats

Neonatal hypoxia-ischemia (HI) is an important clinical problem with few effective treatments. Granulocyte-colony stimulating factor (G-CSF) is an endogenous peptide hormone of the hematopoietic system that has been shown to be neuroprotective in focal ischemia in vivo and is currently in phase I/II clinical trials for ischemic stroke in humans. We tested G-CSF in a rat model of neonatal hypoxia-ischemia in postnatal day 7 unsexed rat pups. Three groups of animals were used: hypoxia-ischemia (HI, n=67), hypoxia-ischemia with G-CSF treatment (HI+G, n=65), and healthy control (C, n=53). G-CSF (50 microg/kg, subcutaneous) was administered 1 h after HI and given on four subsequent days (five total injections). Animals were euthanized 24 h, 1, 2, and 3 weeks after HI. Assessment included brain weight, histology, immunohistochemistry, and Western blotting. G-CSF treatment was associated with improved quantitative brain weight and qualitative Nissl histology after hypoxia-ischemia. TUNEL demonstrated reduced apoptosis in group HI+G. Western blot demonstrated decreased expression of Bax and cleaved caspase-3 in group HI+G. G-CSF treatment was also associated with increased expression of STAT3, Bcl-2, and Pim-1, all of which may have participated in the anti-apoptotic effect of the drug. We conclude that G-CSF ameliorates hypoxic-ischemic brain injury and that this may occur in part by an inhibition of apoptotic cell death.

Metabolic analysis of mouse brains that have compromised iron storage

Iron is a critical component of the CNS that must be tightly regulated; too little iron can result in energy insufficiency and too much iron can result in oxidative stress. The intracellular iron storage protein ferritin is central to the regulation of iron. In this study, we determined the neurochemical profile of brains of animals deficient in heavy-chain ferritin (H-ferritin) using high-resolution magic angle spin proton magnetic resonance spectroscopy (HR-MAS (1)H MRS). Spectra of 2 mm-thick coronal tissue punches ( approximately 4 mg) were obtained using a CPMG pulse sequence on Bruker Avance 500 and quantified (nmol/mg tissue) using customized LCModel software (16 metabolites). In H-ferritin deficient mice, we found significant increases in striatal glutamate, hippocampal choline, and N-acetyl-aspartyl-glutamate in both the cortex and the hippocampus (t-test, p < 0.05). Neurochemical profiling with principal component analysis (PCA) revealed increased glutamate in the hippocampus, striatum, and ventral tegmental area (VTA) in H-ferritin deficient animals as compared to wild-type. While lactate was increased in the VTA of deficient animals, it was decreased in the striatum. Also, GABA was increased in both cortical and striatal regions of deficient mice. These changes reveal the importance of proper iron management for maintaining neurochemical balance and provide new evidence for region specific differences in neurochemical profiles as a result of compromised ability of neurons to store iron while overall iron status is normal. Because H-ferritin is predominantly expressed in neurons, the neurochemical profile is suggestive of neuronal iron deficiency and may have relevance to the functional consequences associated with brain iron deficiency.

Intrauterine growth restriction due to uteroplacental vascular insufficiency leads to increased hypoxia-induced cerebral apoptosis in newborn piglets

Uteroplacental vascular insufficiency in humans is a common cause of intrauterine growth restriction (IUGR) and is associated with an increased incidence of perinatal asphyxia and neurodevelopmental disorders compared to normal weight newborns. Experimental models that provide an opportunity to analyze the pathogenesis of these relationships are limited. Here, we used neonatal pigs from large litters in which there were piglets of normal birth weight (for controls) and of low birth weight (for uteroplacental vascular insufficiency). Hypoxia was induced in paired littermates by reducing the fraction of inspired oxygen to 4% for 25 min. Brain tissue was collected 4 h post-hypoxia. Cerebral levels of apoptosis were quantified morphologically and verified with caspase-3 activity and TUNEL. Expression of Bcl-2, Bcl-XL and Bax proteins was investigated using immunohistochemistry. Cellular positivity for Bcl-2 was consistently higher in the non-apoptotic white matter of the hypoxic IUGR animals compared with their littermates and reached significance at P < 0.05 in several pairs of littermates. Alterations in Bax showed a trend towards higher expression in the hypoxic IUGR littermates but rarely reached significance. The IUGR piglets showed a significantly greater amount of apoptosis in response to the hypoxia than the normal weight piglets, suggesting an increased vulnerability to apoptosis in the IUGR piglets.

Evidence that intracellular Ca2+ mediates the effect of α-ketoisocaproic acid on the phosphorylating system of cytoskeletal proteins from cerebral cortex of immature rats

In this study we investigated the involvement of Ca2+ on the effects of alpha-ketoisocaproic acid (KIC), the main metabolite accumulating in maple syrup urine disease (MSUD), on the phosphorylating system associated with the intermediate filament (IF) proteins in slices from cerebral cortex of 9-day-old rats. We first observed that KIC significantly decreased the in vitro phosphorylation of IF proteins in brain slices. KIC-induced dephosphorylation was mediated especially by the protein phosphatase PP2B, a Ca2+-dependent protein phosphatase, but also by PP2A. We also demonstrated the involvement of Ca2+-dependent mechanisms in the KIC effects using the specific L-voltage-dependent Ca2+ channels (L-VDCC) inhibitor nifedipine, the NMDA antagonist DL-AP5 and the intracellular Ca2+ chelator BAPTA-AM. Blockage of Ca2+ channels or chelating intracellular Ca2+ completely prevented the effects of KIC on the phosphorylating system associated to IF proteins. In addition, we verified that KIC increased 45Ca2+ uptake in brain slices after 3 and 30 min incubation. Taken together, our present data indicate that KIC increase intracellular Ca2+ levels, probably promoting the activation of calcineurin. These results might be associated with the increased dephosphorylation of the IF proteins in slices of cerebral cortex of immature rats exposed to KIC at similar concentrations from those found in blood and tissues of patients with MSUD.

Apoptosis in perinatal hypoxic-ischemic brain injury: how important is it and should it be inhibited?

The discovery of safe and effective therapies for perinatal hypoxia-ischemia (HI) and stroke remains an unmet goal of perinatal medicine. Hypothermia and antioxidants such as allopurinol are currently under investigation as treatments for neonatal HI. Drugs targeting apoptotic mechanisms are currently being studied in adult diseases such as cancer, stroke, and trauma and have been proposed as potential therapies for perinatal HI and stroke. Before developing antiapoptosis therapies for perinatal brain injury, we must determine whether this form of cell death plays an important role in these injuries and if the inhibition of these pathways promotes more benefit than harm. This review summarizes current evidence for apoptotic mechanisms in perinatal brain injury and addresses issues pertinent to the development of antiapoptosis therapies for perinatal HI and stroke.