Growth hormone and prolactin regulate human neural stem cell regenerative activity.

We have previously shown that the growth hormone (GH)/prolactin (PRL) axis has a significant role in regulating neuroprotective and/or neurorestorative mechanisms in the brain and that these effects are mediated, at least partly, via actions on neural stem cells (NSCs). Here, using NSCs with properties of neurogenic radial glia derived from fetal human forebrains, we show that exogenously applied GH and PRL promote the proliferation of NSCs in the absence of epidermal growth factor or basic fibroblast growth factor. When applied to differentiating NSCs, they both induce neuronal progenitor proliferation, but only PRL has proliferative effects on glial progenitors. Both GH and PRL also promote NSC migration, particularly at higher concentrations. Since human GH activates both GH and PRL receptors, we hypothesized that at least some of these effects may be mediated via the latter. Migration studies using receptor-specific antagonists confirmed that GH signals via the PRL receptor promote migration. Mechanisms of receptor signaling in NSC proliferation, however, remain to be elucidated. In summary, GH and PRL have complex stimulatory and modulatory effects on NSC activity and as such may have a role in injury-related recovery processes in the brain.

Proteolytic activity during cortical development is distinct from that involved in hypoxic ischemic injury.

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases involved in brain development and the etiology of adult cerebral injuries. In this study, we determined the MMP-2 and 9 responses following hypoxic ischemia (HI) injury in the developing brain. First, we characterized the developmental changes of MMP activity in the rat brain from embryonic day 18 (E18) to postnatal day 120 (P120). MMP-2 activity was high from E18 to P3 and decreased with age (P< or =0.001), while MMP-9 activity was not detectable. MMP-2 immunoreactivity was closely associated with differentiating cortical plate and subplate neurons. Next, we characterized the proteolytic changes after unilateral HI brain injury in 3- (P3) and 21- (P21) day-old rats. Zymography revealed that in the P21 rat brain, MMP-9 activity (150 and 92 kDa forms) was increased at 6 h and remained elevated 24 h post-injury in the ipsilateral injured hemisphere (P< or =0.001), whereas there was a gradual increase in MMP-2 (65 kDa) activity, reaching a peak at 5 days (P< or =0.001). Similarly, quantitative real time polymerase chain reaction (qRT-PCR) indicated significant elevations in MMP-9 and MMP-2 mRNA expression in the injured cortex (P< or =0.05) and hippocampus (P< or =0.05) at 1 and 5 days post-injury, respectively in the P21 rat brain. In the P3 rat brain, zymography results revealed that both pro (92 kDa) and cleaved (87 kDa) MMP-9 activities were upregulated in the ipsilateral injured hemisphere from 6 h to 1 day after injury (P< or =0.001). In contrast, cleaved MMP-2 (60 kDa) was only moderately upregulated at 6 h (P< or =0.01), while pro MMP-2 (65 kDa) levels were unaffected. MMP-9 mRNA expression was also increased at 6 h (P< or =0.05) following injury at P3, whereas MMP-2 expression remained unchanged compared with the uninjured contralateral hemisphere. Immunohistochemistry indicated that MMP-9 protein expression was localized predominantly to neurons and peri-vascular astrocytes in the affected regions at early time points, whereas MMP-2 was present on reactive astrocytes surrounding the infarct at later time points. Together, these results indicate that MMP-2 may be primarily associated with the development and differentiation of cortical plate neurons and wound recovery processes. Conversely, MMP-9 appeared to be associated with more acute processes during the period of lesion development.

Prolactin is involved in glial responses following a focal injury to the juvenile rat brain.

A cerebral growth hormone axis is activated following brain injury in the rat and treatment with growth hormone is neuroprotective. We have now investigated whether the closely related prolactin axis has similar properties following injury to the developing rat brain. From one day following a unilateral hypoxic ischemic injury, prolactin immunoreactivity was increased in the affected cortex parallel to the development of the injury (P<0.001). Initial prolactin and prolactin receptor staining on penumbral neurons progressively decreased whereas astrocytes remained strongly immunopositive. Reactive microglia also became strongly prolactin immunoreactive. Unlike growth hormone, central treatment with prolactin failed to rescue neurons in this paradigm. This was confirmed in vitro; rat prolactin failed to protect neurons under conditions for which growth hormone was neuroprotective. However, prolactin had trophic and pro-proliferative effects on glia (P<0.001). We confirmed the expression of the prolactin receptor in vitro by reverse transcriptase polymerase chain reaction, and show its strong association with astrocytes as compared with neurons by immunocytochemistry. In summary, we show for the first time that hypoxia ischemia induces a robust activation of the prolactin axis in regions of the cerebral cortex affected by injury. The lack of neuroprotective properties in vivo and in vitro indicates that, unlike growth hormone, prolactin is not directly involved in neuronal rescue in the injured brain. Its strong relation to glial reactions and its gliatrophic effects suggest that the prolactin axis is primarily involved in a gliogenic response during recovery from cerebral injury.

Prenatal stress in the rat alters 5-HT1A receptor binding in the ventral hippocampus.

Exposure of a pregnant woman to physical and/or psychological stress might affect her offspring by promoting the development of various learning, behavioral and/or mood disorders in later life. The 5-HT1A and 5-HT2A receptors are prominently implicated in the modulation of anxiety and mood-related behaviors. Using a semi-quantitative radiolabel immunocytochemical analysis (immunobinding), we studied the effect of prenatal stress on binding of these two receptor subtypes in the hippocampus of 4-week-old male and female Fischer 344 rats. Levels of 5-HT1A immunobinding in the ventral hippocampus, which is primarily implicated in emotional processing, were significantly decreased in male offspring after prenatal stress. A trend towards a decrease was observed in the ventral hippocampus of females. In contrast, 5-HT1A immunobinding within the dorsal hippocampus, which is mainly related to learning and memory, was not affected by prenatal stress in offspring of either gender. Likewise, no significant differences between control and prenatally stressed rats were observed for levels of 5-HT2A immunobinding in either part of the hippocampus or gender. The observed reduction in hippocampal 5-HT1A receptor binding in male offspring after prenatal stress may have important consequences for adult anxiety- and depressive-like behavior.

Prenatal stress and neonatal rat brain development.

Chronic or repeated stress during human fetal brain development has been associated with various learning, behavioral, and/or mood disorders, including depression in later life. The mechanisms accounting for these effects of prenatal stress are not fully understood. The aim of this study was to investigate the effects of prenatal stress on early postnatal brain development, a disturbance of which may contribute to this increased vulnerability to psychopathology. We studied the effects of prenatal stress on fetal growth, stress-induced corticosterone secretion, brain cell proliferation, caspase-3-like activity and brain-derived neurotrophic factor protein content in newborn Fischer 344 rats. In addition to a slight reduction in birth weight, prenatal stress was associated with elevated corticosterone levels (33.8%) after 1 h of maternal deprivation on postnatal day 1, whereas by postnatal day 8 this pattern was reversed (-46.5%). Further, prenatal stress resulted in an approximately 50% decrease in brain cell proliferation just after birth in both genders with a concomitant increase in caspase-3-like activity within the hippocampus at postnatal day 1 (36.1%) and at postnatal day 5 (females only; 20.1%). Finally, brain-derived neurotrophic factor protein content was reduced in both the olfactory bulbs (-24.6%) and hippocampus (-28.2%) of prenatally stressed male offspring at postnatal days 1 and 5, respectively. These detrimental central changes observed may partly explain the increased susceptibility of prenatally stressed subjects to mood disorders including depression in later life.

Growth hormone as a neuronal rescue factor during recovery from CNS injury.

There is growing evidence to suggest that growth hormone plays a role in the growth and development of the CNS. Specifically, growth hormone has been implicated in promoting brain growth, myelination, neuronal arborisation, glial differentiation and cognitive function. Here we investigate if growth hormone has a role in the recovery from an unilateral hypoxic-ischaemic brain injury. Using moderate (15 min hypoxia) and severe (60 min hypoxia) models of hypoxic-ischaemia in juvenile rats and standard immunohistochemical techniques, we found intense growth hormone-like immunoreactivity present within regions of cell loss by 3 days (P<0.05). Growth hormone-like immunoreactivity was observed on injured neurones, myelinated axons, glial cells within and surrounding infarcted tissue and on the choroid plexus plus ependymal cells within the injured hemisphere. The pattern of immunoreactivity suggests that (a) growth hormone (or a growth hormone-like substance) is transported via the cerebrospinal fluid and (b) that growth hormone (or a growth hormone-like substance) is acting in a neurotrophic manner specifically targeted to injured neurones and glia. To test this hypothesis we treated a moderate hypoxic-ischaemic brain injury with 20 microg of rat growth hormone by intracerebroventricular infusion starting 2 h after injury (n=12/group). After 3 days the animals were killed and the extent of neuronal loss quantified. Growth hormone treatment reduced neuronal loss in the frontoparietal cortex (P<0.001), hippocampus (P<0.01) and dorsolateral thalamus (P<0.01) but not in the striatum. This spatial distribution of the neuroprotection conveyed by growth hormone correlates with the spatial distribution of the constitutive neural growth hormone receptor, but not with the neuroprotection offered by insulin-like growth factor-I treatment in this model. These results suggest that some of the neuroprotective effects of growth hormone are mediated directly through the growth hormone receptor and do not involve insulin-like growth factor-I induction.In summary, we have found that a growth hormone-like factor increased in the brain in the days after injury. In addition, treatment with growth hormone soon after an hypoxic-ischaemic injury reduced the extent of neuronal loss. These results further suggest that a neural growth hormone axis is activated during recovery from injury and that this may act to restrict the extent of neuronal death.

A role for the somatotropic axis in neural development, injury and disease.

This review article discusses the roles of the somatotropic axis in the growth and development of the normal central nervous system (CNS) and during recovery from brain injury. Classically, the actions of pituitary-derived growth hormone (GH) have been reported to be primarily mediated via the induction of hepatic insulin-like growth factor-I (IGF-I). GH receptors (GHRs), however, have now been identified in many body tissues and shown to have both endocrine and local actions, some of which are IGF-I independent. Within the brain, GHRs are widely located across a range of cellular phenotypes, yet little is known regarding their function or endogenous ligand. It is now becoming accepted that GH, like IGF-I, is integrally involved in the growth and development of the normal CNS. Following brain injury, IGF-I mRNA is induced, primarily within reactive microglia. The resultant IGF-I protein appears to have a dual role, first as an endogenous neurotropic and anti-apoptotic agent acting directly on stressed cells, and second as a prohormone for generation of the N-terminal tripeptide of IGF-I, glycine-proline-glutamate (GPE), and the resulting des-N-(1-3)-IGF-I, both of which have specific neuroprotective properties. Our work on deciphering the upstream regulators of injury-induced IGF-I has revealed that a GH-like substance is strongly upregulated after brain injury and specifically associated with stressed neurons and glia. Subsequent to this finding, GH administered centrally 2 hours after a hypoxic-ischemic brain injury in juvenile rats was found to provide significant neuroprotection, interestingly, in a spatiotemporal pattern distinct from the neuroprotection offered by IGF-I. The implications of these findings in regard to the growth, development and injury response of the CNS are discussed.

Alterations in the neural growth hormone axis following hypoxic-ischemic brain injury.

Recently, there has been considerable interest in determining the role of the growth hormone receptor (GHR) in the central nervous system (CNS). The aim of this study was to investigate the role of circulating growth hormone (GH) and the neural GHR after hypoxic-ischemic (HI) brain injury in the 21-day old rat. We observed growth hormone receptor/binding protein (GHR/BP) immunoreactivity to be rapidly upregulated following a severe unilateral HI injury. There was a biphasic increase with an initial rise occurring in blood vessels within a few hours after injury followed by a secondary rise evident by 3 days post-hypoxia in microglia/macrophages, some of which are destined to express insulin-like growth factor-I (IGF-I). There was also an increased immunoreactivity in reactive astrocytes, some of which were in the process of dividing. Subsequently, we attempted to activate the endothelial GHR/BP which was found to be increased after injury by treating with 15 microgram g-1 day-1 s.c. bGH for 7 days. Circulating concentrations of IGF-I fell after injury and were restored with GH treatment (P=0.001), whereas treatment of normal animals had no effect on serum IGF-I. Peripheral GH treatment increased the cerebrospinal fluid (CSF) concentration of immunoreactive IGF-I in the injured rats (P=0.017). GH treatment also reversed the systemic catabolism caused by the injury but had no significant neuroprotective effects. These results indicate that GH therapy can be used to reverse the systemic catabolism that occurs after CNS injury. In addition, these data suggest a role for the neural GHR during the recovery from brain injury, both in terms of the induction of IGF-I and in terms of glial proliferation.

Co-ordinated and cellular specific induction of the components of the IGF/IGFBP axis in the rat brain following hypoxic-ischemic injury.

Insulin-like growth factor 1 (IGF-1) is induced after hypoxic-ischemic (HI) brain injury, and therapeutic studies suggest that IGF-1 may restrict delayed neuronal and glial cell loss. We have used a well-characterised rat model of HI injury to extend our understanding of the modes of action of the IGF system after injury. The induction of the IGF system by injury was examined by in situ hybridization, immunohistochemistry, Northern blot analysis, RNase protection assay and reverse transcriptase-polymerase chain reaction (RT-PCR). IGF-1 accumulated in blood vessels of the damaged hemisphere within 5 h after a severe injury. By 3 days, IGF-1 mRNA was expressed by reactive microglia in regions of delayed neuronal death, and immunoreactive IGF-1 was associated with these microglia and reactive astrocytes juxtaposed to surviving neurones surrounding the infarct. Total IGF-1 receptor mRNA was unchanged by the injury. IGFBP-2 mRNA was strongly induced in reactive astrocytes throughout the injured hemisphere, and IGFBP-3 and IGFBP-5 mRNA were moderately induced in reactive microglia and neurones of the injured hippocampus, respectively. IGFBP-6 mRNA was induced in the damaged hemisphere by 3 days and increased protein was seen on the choroid plexus, ependyma and reactive glia. In contrast, insulin II was not induced. These results indicate cell type-specific expression for IGF-1, IGFBP-2,3,5 and 6 after injury. Our findings suggest that the IGF-1 produced by microglia after injury is transferred to perineuronal reactive astrocytes expressing IGFBP-2. Thus, modulation of IGF-1 action by IGFBP-2 might represent a key mechanism that restricts neuronal cell loss following HI brain injury.

Asphyxial brain injury--the role of the IGF system.

Transient neural injuries, such as asphyxia, can trigger considerable delayed neuronal death. Inappropriate induction of apoptosis is thought to play an important role in this process. Our studies have shown marked changes in the IGF system in the brain in response to these injuries with an induction of insulin growth factor (IGF)-1 and insulin growth factor binding protein (IGFBP)-2 and IGFBP-3 in glial cells in the region of injury. This suggests that the IGF-1 system may be an endogenous neuroprotective system. Earlier administration of IGF-1 - 2 h after injury reduced the phase of secondary neuronal loss suggesting that IGF-1 may well have therapeutic potential as a neuronal rescue agent. The action of IGF-1 appears to involve binding proteins, transport to the site of injury and the IGF-1 receptor and inhibition of apoptosis, but might also involve generation of GPE which itself appears to be neuroprotective. Together these results indicate considerable potential of these agents to treat stroke, perinatal asphyxia and other forms of acute brain injury.

Some effects of post-translational N-terminal acetylation of the human embryonic zeta globin protein.

Using site-directed mutagenesis we have produced the first mutant form of a human embryonic haemoglobin. We have mutated the N-terminal Ser residue of the zeta-chain of haemoglobin Portland, zeta 2 gamma 2, (which is normally acetylated) to a Val (which possesses a free amine terminus). The protein spontaneously assembles into a fully functional tetramer which shows cooperative oxygen binding. Determination of the reactivity of the mutant protein with 2,3-diphosphoglycerate indicates that the mutation process does not lead to any major disruption of the protein structure. A comparison of the properties of the mutant and wild-type proteins identifies a significant role for the normal N-terminal acetylation of the zeta-chain with regard to the alkaline Bohr effect and the sensitivity of the oxygen affinity of the protein towards chloride ions. The possible physiological significance of this modification is discussed.