Chronic hypoxia impairs murine hippocampal development and depletes the postnatal progenitor pool by attenuating mammalian target of rapamycin signaling

Brainstem auditory-evoked responses among children afflicted by severely hypoxic CHD

MAIN AIM

To electrophysiologically determine the impact of moderate to severe chronic hypoxia (H) resulting from a wide array of CHD (HCHD) conditions on the integrity of brainstem function.

MATERIALS AND METHODS

Applying brainstem auditory-evoked response methodology, 30 chronically afflicted HCHD patients, who already had undergone heart surgery, were compared to 28 healthy control children (1-15 yo) matched by age, gender and socioeconomic condition. Blood oxygen saturation was clinically determined and again immediately before brainstem auditory-evoked response testing.

RESULTS

Among HCHD children, auditory wave latencies (I, III and V) were significantly longer (medians: I, 2.02 ms; III, 4.12 ms, and; V, 6.30 ms) compared to control (medians: I, 1.67ms; III, 3.72 ms, and; V, 5.65 ms), as well as interpeak intervals (HCHD medians: I-V, 4.25 ms, and; III-V, 2.25ms; control medians: I-V, 3.90 ms and, III-V, 1.80 ms) without significant differences in wave amplitudes between groups. A statistically significant and inverse correlation between average blood oxygen saturation of each group (control, 94%; HCHD, 78%) and their respective wave latencies and interpeak intervals was found.

CONCLUSIONS

As determined by brainstem auditory-evoked responses, young HCHD patients manifestly show severely altered neuronal conductivity in the auditory pathway strongly correlated with their hypoxic condition. These observations are strongly supported by different brainstem neurological and image studies showing that alterations, either in microstructure or function, result from the condition of chronic hypoxia in CHD. The non-altered wave amplitudes are indicative of relatively well-preserved neuronal relay nuclei.

Alterations in Amygdala/Hippocampal Volume Ratios in Children with Obstructive Sleep Apnea Syndrome Caused by Adenotonsillar Hypertrophy

BACKGROUND Intermittent hypoxemia can cause changes in certain brain structures. However, in pediatric patients with obstructive sleep apnea (OSA) caused by adenotonsillar hypertrophy (ATH), there is only limited information on the effect of ATH-induced OSA on brain structures. This study sought to investigate alterations in amygdala and hippocampal volumes in children with OSA by ATH. MATERIAL AND METHODS Magnetic resonance imaging scans were applied in children who had ATH-induced OSA (ATH/OSA) and in healthy children. Amygdala and hippocampus volumes and adenoid sizes were measured on MRI volumetric images. The ratio of adenoid size/nasopharyngeal depth was used to describe the severity of adenoid hypertrophy. The clinical variables of the involved subjects were investigated. RESULTS One hundred ATH/OSA children and 100 healthy children without ATH/OSA participated in the study. The ATH/OSA children had higher amygdala volumes and amygdala/hippocampus volume ratios but lower hippocampus volumes than healthy controls, and the amygdala/hippocampus volume ratios were correlated with disease duration and hypoxemia conditions. However, our data showed that amygdala/hippocampus volume ratios were not correlated with the ratios of adenoid size/nasopharyngeal depth in the ATH/OSA children. In addition, the ratio of adenoid size/nasopharyngeal depths in ATH/OSA children was higher than that in healthy children in each subgroup based on the age of participants. CONCLUSIONS Compared to healthy controls, amygdala/hippocampus volume ratios are increased in children with ATH/OSA.

Acute Prenatal Hypoxia in Rats Affects Physiology and Brain Metabolism in the Offspring, Dependent on Sex and Gestational Age

Hypoxia is damaging to the fetus, but the developmental impact may vary, with underlying molecular mechanisms unclear. We demonstrate the dependence of physiological and biochemical effects of acute prenatal hypoxia (APH) on sex and gestational age. Compared to control rats, APH on the 10th day of pregnancy (APH-10) increases locomotion in both the male and female offspring, additionally increasing exploratory activity and decreasing anxiety in the males. Compared to APH-10, APH on the 20th day of pregnancy (APH-20) induces less behavioral perturbations. ECG is changed similarly in all offspring only by APH-10. Sexual dimorphism in the APH outcome on behavior is also observed in the brain acetylation system and 2-oxoglutarate dehydrogenase reaction, essential for neurotransmitter metabolism. In view of the perturbed behavior, more biochemical parameters in the brains are assessed after APH-20. Of the six enzymes, APH-20 significantly decreases the malic enzyme activity in both sexes. Among 24 amino acids and dipeptides, APH-20 increases the levels of only three amino acids (Phe, Thr, and Trp) in male offspring, and of seven amino acids (Glu, Gly, Phe, Trp, Ser, Thr, Asn) and carnosine in the female offspring. Thus, a higher reactivity of the brain metabolism to APH stabilizes the behavior. The behavior and brain biochemistry demonstrate sexually dimorphic responses to APH at both gestational stages, whereas the APH effects on ECG depend on gestational age rather than sex.

Cognitive impairment caused by hypoxia: from clinical evidences to molecular mechanisms

Hypoxia is a state of reduced oxygen supply and excessive oxygen consumption. According to the duration of hypoxic period, it can be classified as acute and chronic hypoxia. Both acute and chronic hypoxia could induce abundant neurological deficits. Although there have been significant advances in the pathophysiological injuries, few studies have focused on the cognitive dysfunction. In this review, we focused on the clinical evidences and molecular mechanisms of cognitive impairment under acute and chronic hypoxia. Hypoxia can impair several cognitive domains such as attention, learning and memory, procession speed and executive function, which are similar in acute and chronic hypoxia. The severity of cognitive deficit correlates with the duration and degree of hypoxia. Recovery can be achieved after acute hypoxia, while sequelae or even dementia can be observed after chronic hypoxia, perhaps due to the different molecular mechanisms. Cardiopulmonary compensatory response, glycolysis, oxidative stress, calcium overload, adenosine, mitochondrial disruption, inflammation and excitotoxicity contribute to the molecular mechanisms of cognitive deficit after acute hypoxia. During the chronic stage of hypoxia, different adaptive responses, impaired neurovascular coupling, apoptosis, transcription factors-mediated inflammation, as well as Aβ accumulation and tau phosphorylation account for the neurocognitive deficit. Moreover, brain structural changes with hippocampus and cortex atrophy, ventricle enlargement, senile plaque and neurofibrillary tangle deposition can be observed under chronic hypoxia rather than acute hypoxia.

Morphology of the Dentate Gyrus in a Large Cohort of Sudden Infant Deaths-Interrelation Between Features but Not Diagnosis

Morphological differences in the dentate gyrus (DG) have been reported in sudden unexpected deaths in infancy (SUDI), with the feature of focal granule cell (GC) bilamination (FGCB) reported as increased in unexplained SUDI, including sudden infant death syndrome (SIDS), compared with explained SUDI (eSUDI). However, it remains to be determined how these morphologies relate to each other and their extent along the anteroposterior length. This retrospective study evaluated the prevalence of FGCB, single or clustered ectopic GCs, granule cell dispersion (GCD), heterotopia, hyperconvolution, gaps, thinning, blood vessel dissection (BVD), and cuffing (BV cuffing), in an Australian SUDI cohort, and compared the prevalence of these features in eSUDI and unexplained SUDI. We analyzed 850 formalin-fixed paraffin-embedded serial and subserial sections of the hippocampus at the level of the lateral geniculate nucleus from 90 infants, and identified GCD in 97% of infants, single ectopic cells, hyperconvolution, thinning, and BVD in 60%-80%, heterotopia in 36%, gaps, clusters of ectopic cells and BV cuffing in 9%-15%, and FGCB in 18%. These features are clustered within 3-5 serial sections. The presence of FGCB correlated with single ectopic GCs and hyperconvolution. There were no differences in the prevalence of these features between unexplained SUDI (n = 74) and eSUDI (n = 16). Our findings highlight that DG morphological features are highly localized, extending 14-35 µm at their focal location(s) along the anteroposterior length. Consequently, multiple sections along the longitudinal extent are required to identify them. No feature differentiated SUDI from eSUDI in our cohort, thus we cannot conclude that any of these features are abnormal and it remains to be determined their functional significance.

Experience-Dependent Regulation of Cajal-Retzius Cell Networks in the Developing and Adult Mouse Hippocampus

In contrast to their near-disappearance in the adult neocortex, Cajal-Retzius cells have been suggested to persist longer in the hippocampus. A distinctive feature of the mature hippocampus, not maintained by other cortical areas, is its ability to sustain adult neurogenesis. Here, we have investigated whether environmental manipulations affecting hippocampal postnatal neurogenesis have a parallel impact on Cajal-Retzius cells. We used multiple mouse reporter lines to unequivocally identify Cajal-Retzius cells and quantify their densities during postnatal development. We found that exposure to an enriched environment increased the persistence of Cajal-Retzius cells in the hippocampus, but not in adjacent cortical regions. We did not observe a similar effect for parvalbumin-expressing interneurons, which suggested the occurrence of a cell type-specific process. In addition, we did not detect obvious changes either in Cajal-Retzius cell electrophysiological or morphological features, when compared with what previously reported in animals not exposed to enriched conditions. However, optogenetically triggered synaptic output of Cajal-Retzius cells onto local interneurons was enhanced, consistent with our observation of higher Cajal-Retzius cell densities. In conclusion, our data reveal a novel form of hippocampal, cell type-specific, experience-dependent network plasticity. We propose that this phenomenon may be involved in the regulation of enrichment-dependent enhanced hippocampal postnatal neurogenesis.

Reduction of brain volumes after neonatal cardiopulmonary bypass surgery in single-ventricle congenital heart disease before Fontan completion

BackgroundLittle is known about the relationship between brain volumes and neurodevelopmental outcome at 2 years of age in children with single-ventricle congenital heart disease (CHD). We hypothesized that reduced brain volumes may be associated with adverse neurodevelopmental outcome.MethodsVolumetric segmentation of cerebral magnetic resonance imaging (MRI) scans was carried out in 44 patients without genetic comorbidities and in 8 controls. Neurodevelopmental outcome was assessed with the Bayley-III scales.ResultsGray matter (GM), deep GM, white matter (WM), and cerebrospinal fluid (CSF) volumes were 611±59, 43±4.5, 277±30, and 16.4 ml, respectively (interquartile range (IQR) 13.1, 23.3 ml). Children undergoing neonatal cardiopulmonary bypass surgery showed smaller deep GM (P=0.005) and WM (P=0.021) volumes. Brain volumes were smaller in patients compared with controls (GM: P=0.017, deep GM: P=0.012, and WM: P=0.015), whereas CSF volumes were greater (P=0.014). Of all intracranial volumes, only CSF volume was associated with neurodevelopmental outcome, accounting for 21% (P=0.011) of variability in the cognitive composite score when combined with common risk factors in a multivariable analysis.ConclusionIncreased CSF volume represents a significant risk factor for neurodevelopmental impairment in children with single-ventricle CHD. Later assessments are warranted to determine the prognostic role of intracranial volumes for long-term outcome.

Everolimus improves memory and learning while worsening depressive- and anxiety-like behavior in an animal model of depression

Everolimus (EVR) is an orally-administered rapamycin analog that selectively inhibits the mammalian target of rapamycin (mTOR) kinase (mainly mTORC1 and likely mTORC2) and the related signaling pathway. mTOR is a serine/threonine protein kinase regulating multiple important cellular functions; dysfunction of mTOR signaling has also been implicated in the pathophysiology of several neurological, neurodegenerative, developmental and cognitive disorders. EVR is widely used as an anti-neoplastic therapy and more recently in children with tuberous sclerosis complex (TSC). However, no clear correlation exists between EVR use and development of central side effects e.g. depression, anxiety or cognitive impairment. We studied the effects of a 3 weeks administration of EVR in mice chronically treated with betamethasone 21-phosphate disodium (BTM) as a model of depression and cognitive decline. EVR treatment had detrimental effects on depressive- and anxiety-like behavior while improving cognitive performance in both control (untreated) and BTM-treated mice. Such effects were accompanied by an increased hippocampal neurogenesis and synaptogenesis. Our results therefore might support the proposed pathological role of mTOR dysregulation in depressive disorders and confirm some previous data on the positive effects of mTOR inhibition in cognitive decline. We also show that EVR, possibly through mTOR inhibition, may be linked to the development of anxiety. The increased hippocampal neurogenesis by EVR might explain its ability to improve cognitive function or protect from cognitive decline. Our findings suggest some caution in the use of EVR, particularly in the developing brain; patients should be carefully monitored for their psychiatric/neurological profiles in any clinical situation where an mTOR inhibitor and in particular EVR is used e.g. cancer treatment, TSC or immunosuppression.

Perinatal chronic hypoxia induces cortical inflammation, hypomyelination, and peripheral myelin-specific T cell autoreactivity

pCH is an important risk factor for brain injury and long-term morbidity in children, occurring during the developmental stages of neurogenesis, neuronal migration, and myelination. We show that a rodent model of pCH results in an early decrease in mature myelin. Although pCH does increase progenitor oligodendrocytes in the developing brain, BrdU labeling revealed a loss in dividing progenitor oligodendrocytes, indicating a defect in mature cell replacement and myelinogenesis. Mice continued to exhibited hypomyelination, concomitant with long-term impairment of motor function, weeks after cessation of pCH. The implication of a novel neuroimmunologic interplay, pCH also induced a significant egress of infiltrating CD4 T cells into the developing brain. This pCH-mediated neuroinflammation included oligodendrocyte-directed autoimmunity, with an increase in peripheral myelin-specific CD4 T cells. Thus, both the loss of available, mature, myelin-producing glial cells and an active increase in autoreactive, myelin-specific CD4 T cell infiltration into pCH brains may contribute to early pCH-induced hypomyelination in the developing CNS. The elucidation of potential mechanisms of hypoxia-driven autoimmunity will expand our understanding of the neuroimmune axis during perinatal CNS disease states that may contribute to long-term functional disability.

Evaluation of the impact of the cancer therapy everolimus on the central nervous system in mice

Cancer and treatments may induce cognitive impairments in cancer patients, and the causal link between chemotherapy and cognitive dysfunctions was recently validated in animal models. New cancer targeted therapies have become widely used, and their impact on brain functions and quality of life needs to be explored. We evaluated the impact of everolimus, an anticancer agent targeting the mTOR pathway, on cognitive functions, cerebral metabolism, and hippocampal cell proliferation/vascular density in mice. Adult mice received everolimus daily for 2 weeks, and behavioral tests were performed from 1 week after the last treatment. Everolimus-treated mice displayed a marked reduction in weight gain from the last day of the treatment period. Ex vivo analysis showed altered cytochrome oxidase activity in selective cerebral regions involved in energy balance, food intake, reward, learning and memory modulation, sleep/wake cycle regulation, and arousal. Like chemotherapy, everolimus did not alter emotional reactivity, learning and memory performances, but in contrast to chemotherapy, did not affect behavioral flexibility or reactivity to novelty. In vivo hippocampal neural cell proliferation and vascular density were also unchanged after everolimus treatments. In conclusion, two weeks daily everolimus treatment at the clinical dose did not evoke alteration of cognitive performances evaluated in hippocampal- and prefrontal cortex-dependent tasks that would persist at one to four weeks after the end of the treatment completion. However, acute everolimus treatment caused selective CO modifications without altering the mTOR effector P70S6 kinase in cerebral regions involved in feeding behavior and/or the sleep/wake cycle, at least in part under control of the solitary nucleus and the parasubthalamic region of the hypothalamus. Thus, this area may represent a key target for everolimus-mediating peripheral modifications, which has been previously associated with symptoms such as weight loss and fatigue.

Developmental and adult GAP-43 deficiency in mice dynamically alters hippocampal neurogenesis and mossy fiber volume

Growth-associated protein-43 (GAP-43) is a presynaptic protein that plays key roles in axonal growth and guidance and in modulating synapse formation. Previous work has demonstrated that mice lacking one allele of this gene (GAP-43+/- mice) exhibit hippocampal structural abnormalities, impaired spatial learning and stress-induced behavioral withdrawal and anxiety, behaviors that are dependent on proper hippocampal circuitry and function. Given the correlation between hippocampal function, synaptic connectivity and neurogenesis, we tested if behaviorally naïve GAP-43+/- mice had alterations in either neurogenesis or synaptic connectivity in the hippocampus during early postnatal development and young adulthood, and following behavior testing in older adults. To test our hypothesis, we examined hippocampal cell proliferation (Ki67), number of immature neuroblasts (doublecortin, DCX) and mossy fiber volume (synaptoporin) in behaviorally naïve postnatal day 9 (P9) and P26, and behaviorally experienced 5- to 7-month-old GAP-43+/- and +/+ littermate mice. P9 GAP-43+/- mice had fewer Ki67+ and DCX+ cells compared to +/+ mice, particularly in the posterior dentate gyrus, and smaller mossy fiber volume in the same region. In young adulthood, however, male GAP-43+/- mice had more Ki67+ and DCX+ cells and greater mossy fiber volume in the posterior dentate gyrus relative to male +/+ mice. These increases were not seen in females. In 5- to 7-month-old GAP-43+/- mice (whose behaviors were the focus of our prior publication), there was no global change in the number of proliferating or immature neurons relative to +/+ mice. However, more detailed analysis revealed fewer proliferative DCX+ cells in the anterior dentate gyrus of male GAP-43+/- mice compared to male +/+ mice. This reduction was not observed in females. These results suggest that young GAP-43+/- mice have decreased hippocampal neurogenesis and synaptic connectivity, but slightly older mice have greater hippocampal neurogenesis and synaptic connectivity. In conjunction with our previous study, these findings suggest that GAP-43 is dynamically involved in early postnatal and adult hippocampal neurogenesis and synaptic connectivity, possibly contributing to the GAP-43+/- behavioral phenotype.

Contrasting effects of chronic, systemic treatment with mTOR inhibitors rapamycin and metformin on adult neural progenitors in mice

The chronic and systemic administration of rapamycin extends life span in mammals. Rapamycin is a pharmacological inhibitor of mTOR. Metformin also inhibits mTOR signaling but by activating the upstream kinase AMPK. Here we report the effects of chronic and systemic administration of the two mTOR inhibitors, rapamycin and metformin, on adult neural stem cells of the subventricular region and the dendate gyrus of the mouse hippocampus. While rapamycin decreased the number of neural progenitors, metformin-mediated inhibition of mTOR had no such effect. Adult-born neurons are considered important for cognitive and behavioral health, and may contribute to improved health span. Our results demonstrate that distinct approaches of inhibiting mTOR signaling can have significantly different effects on organ function. These results underscore the importance of screening individual mTOR inhibitors on different organs and physiological processes for potential adverse effects that may compromise health span.

Pharmacological inhibition of the mTOR pathway impairs hippocampal development in mice

Brain injury is an important cause of morbidity in infants at risk for exposure to chronic hypoxia. Using a transgenic mouse that expresses green fluorescent protein (GFP) within this progenitor population we have previously shown that exposure to chronic hypoxia significantly decreases the progenitor stem cell pool in the dentate gyrus of the hippocampus that is in part mediated by inhibition of the mammalian target of rapamycin (mTOR) pathway. Hence we hypothesized that pharmacological inhibition of the mTOR pathway using rapamycin will alter the progenitor stem cell pool and impair the development of the dentate gyrus. We find that prolonged inhibition of the mTOR pathway causes a decrease in the early progenitor stem cell pool, demonstrated by decreased GFP-expressing progenitors, which persists long term. However there is a significant increase in proliferating progenitor cell pool, as seen by increased BrdU that is coupled with increased apoptosis thereby leading to fewer Neu N-expressing mature neurons. Further inhibition of the mTOR pathway leads to depletion of the astrocyte and microglial pool in the dentate gyrus as well. Overall our findings demonstrate that pharmacological inhibition of the mTOR pathway leads to impaired development of the DG, raising the concern that in young children could impair cognitive development.

Mammalian target of rapamycin signaling is a key regulator of the transit-amplifying progenitor pool in the adult and aging forebrain

Adult forebrain neurogenesis is dynamically regulated. Multiple families of niche-derived cues have been implicated in this regulation, but the precise roles of key intracellular signaling pathways remain vaguely defined. Here, we show that mammalian target of rapamycin (mTOR) signaling is pivotal in determining proliferation versus quiescence in the adult forebrain neural stem cell (NSC) niche. Within this niche, mTOR complex-1 (mTORC1) activation displays stage specificity, occurring in transiently amplifying (TA) progenitor cells but not in GFAP+ stem cells. Inhibiting mTORC1 depletes the TA progenitor pool in vivo and suppresses epidermal growth factor (EGF)-induced proliferation within neurosphere cultures. Interestingly, mTORC1 inhibition induces a quiescence-like phenotype that is reversible. Likewise, mTORC1 activity and progenitor proliferation decline within the quiescent NSC niche of the aging brain, while EGF administration reactivates the quiescent niche in an mTORC1-dependent manner. These findings establish fundamental links between mTOR signaling, proliferation, and aging-associated quiescence in the adult forebrain NSC niche.

Survival of exfoliated epithelial cells: a delicate balance between anoikis and apoptosis

The recovery of exfoliated cells from biological fluids is a noninvasive technology which is in high demand in the field of translational research. Exfoliated epithelial cells can be isolated from several body fluids (i.e., breast milk, urines, and digestives fluids) as a cellular mixture (senescent, apoptotic, proliferative, or quiescent cells). The most intriguing are quiescent cells which can be used to derive primary cultures indicating that some phenotypes retain clonogenic potentials. Such exfoliated cells are believed to enter rapidly in anoikis after exfoliation. Anoikis can be considered as an autophagic state promoting epithelial cell survival after a timely loss of contact with extracellular matrix and cell neighbors. This paper presents current understanding of exfoliation along with the influence of methodology on the type of gastrointestinal epithelial cells isolated and, finally, speculates on the balance between anoikis and apoptosis to explain the survival of gastrointestinal epithelial cells in the environment.