Regional Immunoreactivity of Pax6 in the Neurogenic Zone After Chronic Prenatal Hypoxia

Differential Expression of Pax6 Following Bilateral Common Carotid Artery Occlusion

BACKGROUND/AIM

Chronic cerebral hypoperfusion causes neuronal damage involving cognitive impairment and development of dementia. Permanent bilateral common carotid artery occlusion (BCCAO) in rat models is used to study chronic cerebral hypoperfusion. Pax6 is used as an early neurogenesis marker which affects the maturation of neuronal cells. However, the expression of PAX 6 after BCCAO is not well understood. In this study, we investigated the expression of PAX6 in the neurogenic zones after BCCAO to evaluate the effects of Pax6 on chronic hypoperfusion.

MATERIALS AND METHODS

Chronic hypoperfusion was induced by BCCAO. Common carotid artery was laid parallel to the vagus nerve and separated from it. Both arteries were occluded using 4-0 silk sutures. Rats who underwent bi-common carotid artery occlusion formed in the BCCAO group, while unoperated rats served as the control group. Brain samples were obtained on days 3 and 14 after BCCAO and subjected to immunohisto-chemistry with NeuN and western blotting for Pax6 and HIF1α.

RESULTS

Compared to the control, the expression of Pax6 increased three days after surgery but did not differ on day 14, while that of NeuN showed the opposite trend. The expression of HIF1α increased three days after surgery.

CONCLUSION

Bilateral common carotid artery occlusion induced early neurogenesis at three days after BCCAO but this result was not maintained at fourteen days after BCCAO.

Ghrelin Regulates Expression of the Transcription Factor Pax6 in Hypoxic Brain Progenitor Cells and Neurons

The nature of brain impairment after hypoxia is complex and recovery harnesses different mechanisms, including neuroprotection and neurogenesis. Experimental evidence suggests that hypoxia may trigger neurogenesis postnatally by influencing the expression of a variety of transcription factors. However, the existing data are controversial. As a proof-of-principle, we subjected cultured cerebral cortex neurons, cerebellar granule neurons and organotypic cerebral cortex slices from rat brains to hypoxia and treated these cultures with the hormone ghrelin, which is well-known for its neuroprotective functions. We found that hypoxia elevated the expression levels and stimulated nuclear translocation of ghrelin’s receptor GHSR1 in the cultured neurons and the acute organotypic slices, whereas ghrelin treatment reduced the receptor expression to normoxic levels. GHSR1 expression was also increased in cerebral cortex neurons of mice with induced experimental stroke. Additional quantitative analyses of immunostainings for neuronal proliferation and differentiation markers revealed that hypoxia stimulated the proliferation of neuronal progenitors, whereas ghrelin application during the phase of recovery from hypoxia counteracted these effects. At the mechanistic level, we provide a link between the described post-ischemic phenomena and the expression of the transcription factor Pax6, an important regulator of neural progenitor cell fate. In contrast to the neurogenic niches in the brain where hypoxia is known to increase Pax6 expression, the levels of the transcription factor in cultured hypoxic cerebral cortex cells were downregulated. Moreover, the application of ghrelin to hypoxic neurons normalised the expression levels of these factors. Our findings suggest that ghrelin stimulates neurogenic factors for the protection of neurons in a GHSR1-dependent manner in non-neurogenic brain areas such as the cerebral cortex after exposure to hypoxia.

Effects of Prenatal Hypoxia on Nervous System Development and Related Diseases

The fetal origins of adult disease (FOAD) hypothesis, which was proposed by David Barker in the United Kingdom in the late 1980s, posited that adult chronic diseases originated from various adverse stimuli in early fetal development. FOAD is associated with a wide range of adult chronic diseases, including cardiovascular disease, cancer, type 2 diabetes and neurological disorders such as schizophrenia, depression, anxiety, and autism. Intrauterine hypoxia/prenatal hypoxia is one of the most common complications of obstetrics and could lead to alterations in brain structure and function; therefore, it is strongly associated with neurological disorders such as cognitive impairment and anxiety. However, how fetal hypoxia results in neurological disorders remains unclear. According to the existing literature, we have summarized the causes of prenatal hypoxia, the effects of prenatal hypoxia on brain development and behavioral phenotypes, and the possible molecular mechanisms.

IGF-1 Protects Neurons in the Cortex and Subventricular Zone in a Periventricular Leucomalacia Model

BACKGROUND/AIM

Chronic cerebral hypoperfusion affects early and mature neurons in the subventricular zone (SVZ) and cerebral cortex. Herein, we investigated the effects of insulin-like growth factor-1 (IGF-1), a neurogenesis-promoting agent, on neurons in these regions in periventricular leucomalacia (PVL) model rats.

MATERIALS AND METHODS

Following right carotid artery ligation, the rats were placed in a hypoxia chamber and injected with recombinant IGF-1 (0.1 and 1 μg/μl). Their brain sections were immunohistochemically analysed using anti-nestin and anti-NeuN antibodies.

RESULTS

The numbers of early-neuronal cells in the SVZ and mature neurons in the cerebral cortex were higher and lower, respectively, in the PVL group than in the control group. The number of NeuN-positive cells was significantly higher in the IGF-treated group than in the PVL group.

CONCLUSION

PVL increased the number of early neuronal cells in the SVZ, reducing the survival of mature neurons in the cerebral cortex; IGF-1 reversed these effects.

Immunoreactivity of MAPK Signaling in a Rat Model of Intrauterine Growth Retardation Induced by Uterine Artery Ligation

BACKGROUND/AIM

Intrauterine growth retardation (IUGR) causes very low birth weight and is related to the morbidity and mortality of the newborn. In our previous study, expression of brain-derived neurotrophic factor (BDNF) was found reduced in the cerebral cortex and dentate gyrus of fetuses with IUGR. BDNF protected cortical neurons against hypoxic injury via activation of the extracellular signal-related kinase (ERK) pathway. The aim of the current study was to observe the immunoreactivity of ERK in mature neurons and proliferating cells.

MATERIALS AND METHODS

Uterine artery ligation was performed at 17 days of gestation (dg). Rat fetuses were obtained at 21 dg using cesarean section. Fetuses were designated either to the growth retardation (GR) group when removed from the horn with uterine artery ligation, or to the control group when removed from the other horn with the untied artery. Immunohistochemistry was performed with primary antibodies on paraffin-embedded forebrain sections.

RESULTS

The density and proportion of cells expressing PCNA, ERK, and phosphate ERK in the subventricular zone (SVZ) was not different between the control and GR group. The density and proportion of NeuN- and phosphate ERK-positive cells in the cerebral parietal cortex was lower in the GR group, compared to the control group.

CONCLUSION

Although IUGR had no effect on the proliferation of cells in the SVZ, it reduced neuronal survival in the cerebral parietal cortex, which was associated with the decrease of pERK-positive cell density and proportion in the cerebral cortex.

Differential Expression of Vascular Endothelial Growth Factor in the Cortex and Hippocampus upon Cerebral Hypoperfusion

BACKGROUND/AIM

Vascular endothelial growth factor (VEGF) provides tolerance against ischemic brain injury, yet, the pattern of VEGF expression in the neurogenic zones following chronic cerebral hypoperfusion has not been studied. Here we evaluated the immunoreactivity of VEGF in a rat model of chronic cerebral hypoperfusion.

MATERIALS AND METHODS

Chronic hypoperfusion was induced by bilateral common carotid artery ligation in rats. Immunohistochemistry was performed against hypoxia-inducible factor-1α (HIF-1α) and VEGF on brain sections.

RESULTS

The density of HIF1α-positive cells in the hypoxia group was increased in the cerebral cortex and hippocampus. Further, the density of VEGF-positive cells was significantly higher in the hypoxia group compared to the control group in the cerebral cortex whereas it was similar in the subventricular zone, and in the dentate gyrus in the hippocampus between the two groups.

CONCLUSION

The pattern of VEGF expression varies in different brain regions following chronic cerebral hypoperfusion.

Hypoxia of Rats Subjected to Carotid Artery Ligation Results in Impaired Neurogenesis and Reduced Number of Cortical Neurons

BACKGROUND/AIM

Cerebral ischemia is a major cause of abnormal brain development. In a cerebral ischemia model, periventricular leukomalacia (PVL), white matter lesion and a decrease in the number of subcortical neurons were observed. The aim of this study was to investigate the effect of hypoxia on neurogenesis and cell survival.

MATERIALS AND METHODS

In seven-day postnatal rats, the right carotid artery was ligated. The rats were incubated either in a regular normoxic chamber (control group) or in a hypoxic chamber (PVL group, 8% 02 and 92% N2 at 37°C) for 2 h. Nestin- and NeuN-positive neurons were detected by immunohistochemistry.

RESULTS

The densities of nestin-immunoreactivity (IR) cells in the cerebral parietal cortex and subventricular zone were increased with hypoxia. NeuN-IR cells in the cerebral cortex were significantly decreased in the PVL group.

CONCLUSION

Perinatal white matter injury induced neurogenesis, while the survival of neurons was decreased in the cerebral cortex.

Role of Prenatal Hypoxia in Brain Development, Cognitive Functions, and Neurodegeneration

This review focuses on the role of prenatal hypoxia in the development of brain functions in the postnatal period and subsequent increased risk of neurodegenerative disorders in later life. Accumulating evidence suggests that prenatal hypoxia in critical periods of brain formation results in significant changes in development of cognitive functions at various stages of postnatal life which correlate with morphological changes in brain structures involved in learning and memory. Prenatal hypoxia also leads to a decrease in brain adaptive potential and plasticity due to the disturbance in the process of formation of new contacts between cells and propagation of neuronal stimuli, especially in the cortex and hippocampus. On the other hand, prenatal hypoxia has a significant impact on expression and processing of a variety of genes involved in normal brain function and their epigenetic regulation. This results in changes in the patterns of mRNA and protein expression and their post-translational modifications, including protein misfolding and clearance. Among proteins affected by prenatal hypoxia are a key enzyme of the cholinergic system-acetylcholinesterase, and the amyloid precursor protein (APP), both of which have important roles in brain function. Disruption of their expression and metabolism caused by prenatal hypoxia can also result, apart from early cognitive dysfunctions, in development of neurodegeneration in later life. Another group of enzymes affected by prenatal hypoxia are peptidases involved in catabolism of neuropeptides, including amyloid-β peptide (Aβ). The decrease in the activity of neprilysin and other amyloid-degrading enzymes observed after prenatal hypoxia could result over the years in an Aβ clearance deficit and accumulation of its toxic species which cause neuronal cell death and development of neurodegeneration. Applying various approaches to restore expression of neuronal genes disrupted by prenatal hypoxia during postnatal development opens an avenue for therapeutic compensation of cognitive dysfunctions and prevention of Aβ accumulation in the aging brain and the model of prenatal hypoxia in rodents can be used as a reliable tool for assessment of their efficacy.