Region-Specific Interrelations between Apoptotic Proteins Expression and DNA Fragmentation in the Neonatal Rat Brain

BAX regulates dendritic spine development via mitochondrial fusion

BAX is a Bcl-2 family protein acting on apoptosis. It also promotes mitochondrial fusion by interacting with the mitochondrial fusion protein Mitofusin (Mfn1 and Mfn2). Neuronal mitochondria are important for the development and modification of dendritic spines, which are subcellular compartments accommodating excitatory synapses in postsynaptic neurons. The abundance of dendritic mitochondria influences dendritic spine development. Mitochondrial fusion is essential for mitochondrial homeostasis. Here, we show that in the hippocampal neuron of BAX knockout mice, mitochondrial fusion is impaired, leading to decreases in mitochondrial length and total mitochondrial mass in dendrites. Notably, BAX knockout mice also have fewer dendritic spines and less cellular Adenosine 5'triphosphate (ATP) in dendrites. The spine and ATP changes are abolished by restoring mitochondria fusion via overexpressing Mfn1 and Mfn2. These findings indicate that BAX-mediated mitochondrial fusion in neurons is crucial for the development of dendritic spines and the maintenance of cellular ATP levels.

Dexmedetomidine attenuates propofol-induced apoptosis of neonatal hippocampal astrocytes by inhibiting the Bcl2l1 signalling pathway

Apoptosis shapes brain structure and function during early life. However, aberrant apoptosis, including that associated with the general anaesthetic propofol, is undesirable. Dexmedetomidine (DEX) provides potential neuroprotection against apoptosis, but the underlying mechanism remains unknown. We exposed neonatal rodent hippocampal astrocytes to propofol alone and in combination with DEX and yohimbine (an α₂ -adrenergic receptor antagonist), then evaluated cell viability using the MTT assay. The underlying regulatory mechanism associated with apoptosis-related genes was detected using a combinational strategy including double immunofluorescent staining, real-time reverse transcription polymerase chain reaction (RT-PCR), western blot, and flow cytometry. Propofol reduced matrix metallopeptidase 9 (MMP9) in cultured astrocytes, and activated the rno-miR-665/Bcl2-like 1 (Bcl2l1)/cleaved caspase 9 (CC9)/cleaved caspase 3 (CC3) pathway. Combinations incorporating propofol with A-1155463 (a selective Bcl2l1 inhibitor) or MMP9 antagomir reduced Bcl2l1 and promoted apoptosis. Co-culture of propofol with Bcl2l1 or with MMP9 agomir was sufficient to decrease the pro-apoptotic effects of propofol. Interestingly, DEX alone had no significant effect on apoptosis. When combined with propofol, however, the negative effects of propofol on the MMP9 and apoptosis-related genes (Bcl2l1, CC9, and CC3) were reduced. Furthermore, yohimbine pretreatment blocked the neuroprotective effects of DEX. Rno-miR-665 was also found to reduce MMP9 expression in propofol-treated hippocampal astrocytes. Taken together, the results indicate that DEX pretreatment reduces propofol-associated pro-apoptosis in developing astrocytes via downregulation of anti-apoptotic signalling mediated by Bcl2l1.

Anoxia ameliorates the dexamethasone-induced neurobehavioral alterations in the neonatal male rat pups

Glucocorticoids and hypoxia are two essential factors affecting the brain development during labor and delivery. In addition to the neurobehavioral alterations induced separately by these factors, glucocorticoids can attenuate the deleterious consequences of severe hypoxia-ischemia on the brain development, acting as a neuroprotective agent in combination with hypoxia. The role of hypoxia in the combined action with corticosteroids is less clear. Severe hypoxia-ischemia results in the massive activation of caspase-3, masking any other effects of hypoxia on the neonatal brain exposed to glucocorticoids. As a result, the effects of mild hypoxia on the developing brain pretreated with glucocorticoids remain unclear. To analyze this problem, 2-day-old male rats were treated with dexamethasone (DEX) before the subsequent exposure to mild 10-min anoxia or normoxia. The treatment with only DEX resulted in the delay in the development of the negative geotaxis reaction and in the decrease in locomotor activity of the neonatal male pups. The mild anoxic event attenuated these DEX-induced neurobehavioral alterations. The treatment with DEX, but not the mild anoxic exposure alone, resulted in the delayed upregulation of active caspase-3 in the prefrontal cortex and in the brainstem of the male pups. This glucocorticoid-induced upregulation of active caspase-3 was prevented by the anoxic event. The present findings evidence that mild anoxia is capable of ameliorating the glucocorticoid-induced neurodevelopmental alterations in the neonatal rats if the artificial or the naturally occurring increase in the levels of glucocorticoids occurred just before the episode of hypoxia.

proBDNF Is a Major Product of bdnf Gene Expressed in the Perinatal Rat Cortex

In the developing brain, mature brain derived neurotrophic factor (mBDNF) and its precursor (proBDNF) exhibit prosurvival and proapoptotic functions, respectively. However, it is still unknown whether mBDNF or proBDNF is a major form of neurotrophin expressed in the immature brain, as well as if the level of active caspase-3 correlates with the levels of BDNF forms during normal brain development. Here we found that both proBDNF and mBDNF were expressed abundantly in the rat brainstem, hippocampus and cerebellum between embryonic day 20 and postnatal day 8. The levels of mature neurotrophin as well as mBDNF to proBDNF ratios negatively correlated with the expression of active caspase-3 across brain regions. The immature cortex was the only structure, in which proBDNF was the major product of bdnf gene, especially in the cortical layers 2-3. And only in the cortex, the expression of BDNF precursor positively correlated with the levels of active caspase-3. These findings suggest that proBDNF alone may play an important role in the regulation of naturally occurring cell death during cortical development.

Hyperactivity and depression-like traits in Bax KO mice

The Bax gene is a member of the Bcl-2 gene family and its pro-apoptotic Bcl-associated X (Bax) protein is believed to be crucial in regulating apoptosis during neuronal development as well as following injury. With the advent of mouse genomics, mice lacking the pro-apoptotic Bax gene (Bax KO) have been extensively used to study how cell death helps to determine synaptic circuitry formation during neurodevelopment and disease. Surprisingly, in spite of its wide use and the association of programmed neuronal death with motor dysfunctions and depression, the effects of Bax deletion on mice spontaneous locomotor activity and depression-like traits are unknown. Here we examine the behavioral characteristics of Bax KO male mice using classical paradigms to evaluate spontaneous locomotor activity and depressive-like responses. In the open field, Bax KO animals exhibited greater locomotor activity than their control littermates. In the forced swimming test, Bax KO mice displayed greater immobility times, a behavior despair state, when compared to controls. Collectively, our findings corroborate the notion that a fine balance between cell survival and death early during development is critical for normal brain function later in life. Furthermore, it points out the importance of considering depressive-like and hyperactivity behavioral phenotypes when conducting neurodevelopmental and other studies using the Bax KO strain.

Dexamethasone suppresses the locomotor response of neonatal rats to novel environment

Locomotion of animals in the novel environment is determined by two main factors-the intrinsic motor activity and the specific locomotor response to novelty. Glucocorticoids alter neurobehavioral development of mammals and its locomotor manifestations. However, it remains unclear whether the intrinsic and/or the novelty-induced activity are affected by glucocorticoids during early life. Here, the principal component analysis was used to determine the main factors that underlie alterations in locomotion of rat pups treated with dexamethasone. It was shown that neonatal rats exhibited an enhanced locomotion in the novel environment beginning from postnatal day (PD) 5. We found for the first time that this reaction was significantly suppressed by dexamethasone. The effect was specific to the novelty-induced component of behavior, while the intrinsic locomotor activity was not affected by glucocorticoid treatment. The suppression of the behavioral response to novelty was maximal at PD7 and vanquished at PD10-11. In parallel with the hormonal effect on the behavior, dexamethasone upregulated the main cell death executor-active caspase-3 in the prefrontal cortex of 7-day old rats. Thus, dexamethasone-induced alterations in the novelty-related behavior may be the earliest visible signs of the brain damage that could lead to forthcoming depressive state or schizophrenia, emerging as a result of neonatal stress or glucocorticoid treatment.

Acute Antiapoptotic Effects of Hydrocortisone in the Hippocampus of Neonatal Rats

Natural glucocorticoid hydrocortisone was suggested as a potent substitution for dexamethasone in the treatment of bronchopulmonary dysplasia in neonates. The aim of this study was to investigate whether hydrocortisone is able to affect the expression of apoptotic genes and the intensity of naturally occurring cell death in the developing rat hippocampus. Hormone treatment decreased procaspase-3 and active caspase-3 levels as well as DNA fragmentation intensity in the hippocampal formation of one-week-old rats in 6 h after injection. These changes were accompanied by an upregulation of antiapoptotic protein Bcl-XL, while expression of proapoptotic protein Bax remained unchanged. The action of hydrocortisone was glucocorticoid receptor-independent, as the selective glucocorticoid receptor agonist dexamethasone did not affect either apoptotic protein levels or DNA fragmentation intensity in the hippocampal region. The data are the first evidences for in vivo antiapoptotic effects of hydrocortisone in the developing hippocampus.

Increased expression of the anti-apoptotic protein Bcl-xL in the brain is associated with resilience to stress-induced depression-like behavior

Clinical observations and the results of animal studies have implicated changes in neuronal survival and plasticity in both the etiology of mood disorders, especially stress-induced depression, and anti-depressant drug action. Stress may predispose individuals toward depression through down-regulation of neurogenesis and an increase in apoptosis in the brain. Substantial individual differences in vulnerability to stress are evident in humans and were found in experimental animals. Recent studies revealed an association between the brain anti-apoptotic protein B cell lymphoma like X, long variant (Bcl-xL) expression and individual differences in behavioral vulnerability to stress. The ability to increase Bcl-xL gene expression in the hippocampus in response to stress may be an important factor for determining the resistance to the development of stress-induced depression. Treatment with anti-depressant drugs may change Bcl-xL response properties. In the rat brainstem, expression of this anti-apoptotic gene becomes sensitive to swim stress during the long-term fluoxetine treatment, an effect that appeared concomitantly with the anti-depressant-like action of the drug in the forced swim test, suggesting that Bcl-xL may be a new target for depression therapy. The processes and pathways linking stress stimuli to behavior via intracellular anti-apoptotic protein are discussed here in the context of Bcl-xL functions in the mechanisms of individual differences in behavioral resilience to stress and anti-depressant-induced effects on the behavioral despair.

Immunohistochemical analysis of active caspase-3 expression in structures of neonatal brain

The hippocampal fields of neonatal rats differ by the level of active caspase-3: dentate gyrus >CA3>CA1>CA2. In the dentate gyrus it was 70% of its maximum value in the cortex, while in CA2 it corresponded to the minimum level in the brain stem. Taking into account the role of caspase-3 in apoptosis, these differences can indicate different intensity of programmed cell death in different fields of the forming hippocampus.