Iron homeostasis and post-hemorrhagic hydrocephalus: a review

Iron physiology is regulated by a complex interplay of extracellular transport systems, coordinated transcriptional responses, and iron efflux mechanisms. Dysregulation of iron metabolism can result in defects in myelination, neurotransmitter synthesis, and neuronal maturation. In neonates, germinal matrix-intraventricular hemorrhage (GMH-IVH) causes iron overload as a result of blood breakdown in the ventricles and brain parenchyma which can lead to post-hemorrhagic hydrocephalus (PHH). However, the precise mechanisms by which GMH-IVH results in PHH remain elusive. Understanding the molecular determinants of iron homeostasis in the developing brain may lead to improved therapies. This manuscript reviews the various roles iron has in brain development, characterizes our understanding of iron transport in the developing brain, and describes potential mechanisms by which iron overload may cause PHH and brain injury. We also review novel preclinical treatments for IVH that specifically target iron. Understanding iron handling within the brain and central nervous system may provide a basis for preventative, targeted treatments for iron-mediated pathogenesis of GMH-IVH and PHH.

Neurotensin orchestrates valence assignment in the amygdala

The ability to associate temporally segregated information and assign positive or negative valence to environmental cues is paramount for survival. Studies have shown that different projections from the basolateral amygdala (BLA) are potentiated following reward or punishment learning1-7. However, we do not yet understand how valence-specific information is routed to the BLA neurons with the appropriate downstream projections, nor do we understand how to reconcile the sub-second timescales of synaptic plasticity8-11 with the longer timescales separating the predictive cues from their outcomes. Here we demonstrate that neurotensin (NT)-expressing neurons in the paraventricular nucleus of the thalamus (PVT) projecting to the BLA (PVT-BLA:NT) mediate valence assignment by exerting NT concentration-dependent modulation in BLA during associative learning. We found that optogenetic activation of the PVT-BLA:NT projection promotes reward learning, whereas PVT-BLA projection-specific knockout of the NT gene (Nts) augments punishment learning. Using genetically encoded calcium and NT sensors, we further revealed that both calcium dynamics within the PVT-BLA:NT projection and NT concentrations in the BLA are enhanced after reward learning and reduced after punishment learning. Finally, we showed that CRISPR-mediated knockout of the Nts gene in the PVT-BLA pathway blunts BLA neural dynamics and attenuates the preference for active behavioural strategies to reward and punishment predictive cues. In sum, we have identified NT as a neuropeptide that signals valence in the BLA, and showed that NT is a critical neuromodulator that orchestrates positive and negative valence assignment in amygdala neurons by extending valence-specific plasticity to behaviourally relevant timescales.

The Sirt1 deacetylase modulates the insulin-like growth factor signaling pathway in mammals

The lifespan of the nematode, Caenorhabditis elegans, can be extended by mutations affecting components of the insulin-like growth factor (IGF) signaling cascade or by overexpression of SIR2, an NAD+-dependent protein deacetylase. The mammalian homologue of SIR2, Sirt1, has been shown to modulate the activity of FoxO, a transcription factor that is downstream of the IGF signaling system. These results suggest that Sirt1 ought to affect the IGF pathway. We report here evidence that this is the case in mice. The loss of Sirt1 protein in mice results in increased expression of the IGF binding protein IGFBP1, a secreted modulator of IGF function. A number of the anatomical characteristics of Sirt1-null mice closely resemble those of transgenic mice overexpressing IGFBP1. Our data suggest that Sirt1 is part of a regulatory loop that limits the production of IGFBP1 thereby modulating IGF signaling.

Developmental effects of ectopic expression of the glucocorticoid receptor DNA binding domain are alleviated by an amino acid substitution that interferes with homeodomain binding

Steroid hormone receptors are distinguished from other members of the nuclear hormone receptor family through their association with heat shock proteins and immunophilins in the absence of ligands. Heat shock protein association represses steroid receptor DNA binding and protein-protein interactions with other transcription factors and facilitates hormone binding. In this study, we investigated the hormone-dependent interaction between the DNA binding domain (DBD) of the glucocorticoid receptor (GR) and the POU domains of octamer transcription factors 1 and 2 (Oct-1 and Oct-2, respectively). Our results indicate that the GR DBD binds directly, not only to the homeodomains of Oct-1 and Oct-2 but also to the homeodomains of several other homeodomain proteins. As these results suggest that the determinants for binding to the GR DBD are conserved within the homeodomain, we examined whether the ectopic expression of GR DBD peptides affected early embryonic development. The expression of GR DBD peptides in one-cell-stage zebra fish embryos severely affected their development, beginning with a delay in the epibolic movement during the blastula stage and followed by defects in convergence-extension movements during gastrulation, as revealed by the abnormal patterns of expression of several dorsal gene markers. In contrast, embryos injected with mRNA encoding a GR peptide with a point mutation that disrupted homeodomain binding or with mRNA encoding the DBD of the closely related mineralocorticoid receptor, which does not bind octamer factors, developed normally. Moreover, coinjection of mRNA encoding the homeodomain of Oct-2 completely rescued embryos from the effects of the GR DBD. These results highlight the potential of DNA-independent effects of GR in a whole-animal model and suggest that at least some of these effects may result from direct interactions with homeodomain proteins.

Selective binding of steroid hormone receptors to octamer transcription factors determines transcriptional synergism at the mouse mammary tumor virus promoter

Transcriptional synergism between glucocorticoid receptor (GR) and octamer transcription factors 1 and 2 (Oct-1 and Oct-2) in the induction of mouse mammary tumor virus (MMTV) transcription has been proposed to be mediated through directed recruitment of the octamer factors to their binding sites in the viral long terminal repeat. This recruitment correlates with direct binding between the GR DNA binding domain and the POU domain of the octamer factors. In present study, in vitro experiments identified several nuclear hormone receptors to have the potential to bind to the POU domains of Oct-1 and Oct-2 through their DNA binding domains, suggesting that POU domain binding may be a property shared by many nuclear hormone receptors. However, physiologically relevant binding to the POU domain appeared to be a property restricted to only a few nuclear receptors as only GR, progesterone receptor (PR), and androgen receptor (AR), were found to interact physically and functionally with Oct-1 and Oct-2 in transfected cells. Thus GR, PR, and AR efficiently promoted the recruitment of Oct-2 to adjacent octamer motifs in the cell, whereas mineralocorticoid receptor (MR), estrogen receptor alpha, and retinoid X receptor failed to facilitate octamer factor DNA binding. For MMTV, although GR and MR both induced transcription efficiently, mutation of the promoter proximal octamer motifs strongly decreased GR-induced transcription without affecting the total level of reporter gene activity in response to MR. These results suggest that the configuration of the hormone response element within the MMTV long terminal repeat may promote a dependence for the glucocorticoid response upon the recruitment of octamer transcription factors to their response elements within the viral promoter.

Recruitment of octamer transcription factors to DNA by glucocorticoid receptor

Glucocorticoid receptor (GR) and octamer transcription factors 1 and 2 (Oct-1/2) interact synergistically to activate the transcription of mouse mammary tumor virus and many cellular genes. Synergism correlates with cooperative DNA binding of the two factors in vitro. To examine the molecular basis for these cooperative interactions, we have studied the consequences of protein-protein binding between GR and Oct-1/2. We have determined that GR binds in solution to the octamer factor POU domain. Binding is mediated through an interface in the GR DNA binding domain that includes amino acids C500 and L501. In transfected mammalian cells, a transcriptionally inert wild-type but not an L501P GR peptide potentiated transcriptional activation by Oct-2 100-fold above the level that could be attained in the cell by expressing Oct-2 alone. Transcriptional activation correlated closely with a striking increase in the occupancy of octamer motifs adjacent to glucocorticoid response elements (GREs) on transiently transfected DNAs. Intriguingly, GR-Oct-1/2 binding was interrupted by the binding of GR to a GRE. We propose a model for transcriptional cooperativity in which GR-Oct-1/2 binding promotes an increase in the local concentration of octamer factors over glucocorticoid-responsive regulatory regions. These results reveal transcriptional cooperativity through a direct protein interaction between two sequence-specific transcription factors that is mediated in a way that is expected to restrict transcriptional effects to regulatory regions with DNA binding sites for both factors.

Differential ability of flt3-ligand, interleukin-11, and Steel factor to support the generation of B cell progenitors and myeloid cells from primitive murine fetal liver cells

A variety of factors produced by stromal fibroblasts, including Flt3-ligand (FL), interleukin-11 (IL-11), Steel factor (SF), and IL-7, have been implicated in stimulating the production of pre-B cells and myeloid cells from primitive hematopoietic precursors. To investigate their relative roles in this process, either as single-acting or synergistic agents, we compared the yield and types of cells produced after 2 weeks from small numbers of Sca-1+ Lin- (i.e., B220-, Ly-1-, Gr-1-, and Ter-119-) day 14.5 murine fetal liver cells placed in stromal cell-free cultures containing all possible combinations of FL, SF, IL-7, and IL-11. None of these factors alone supported the production (or survival) of any cells beyond 1 week: only pairs of factors consisting of either FL or SF plus either IL-11 or IL-7 were effective in this regard, with FL plus IL-11 being the most potent pair (approximately 7 x 10(4) cells obtained per 100 Sca-1+ Lin- input cells). The maximum numbers of cells were produced in the presence of FL, IL-11, and IL-7: these included both B220+ and Mac-1+/Gr-1+ cells (approximately 10(6) and approximately 2 x 10(5), respectively, per 100 Sca-1+ Lin- input cells). Both of these lineages were also obtained with each of the other possible three-factor combinations, albeit with variable effectiveness. Omission of either FL or IL-7 caused the greatest reduction in the yield of B220+ cells (approximately 130-fold and approximately 80-fold, respectively). Omission of IL-11 and, to a lesser extent, FL caused the greatest reduction in the yield of Mac-1+/Gr-1+ cells (approximately 90-fold and approximately 3-fold, respectively). When fetal calf serum was replaced with a defined serum substitute, the out put of B220+ cells remained the same but myelopoiesis was consistently enhanced (approximately 5- to 20-fold). These findings support a model involving factor redundancy in the extracellular signals required to stimulate the production and amplification of both lymphoid and myeloid cells from early Sca-1+ Lin- cells. They also reveal quantitative differences in the abilities of different competent factor combinations to promote this process, which may be further modulated by the presence of undefined serum components.

Identification of properties that can distinguish primitive populations of stromal-cell-responsive lympho-myeloid cells from cells that are stromal-cell-responsive but lymphoid-restricted and cells that have lympho-myeloid potential but are also capable of competitively repopulating myeloablated recipients

We have recently described a modification of the long-term culture-initiating cell (LTC-IC) assay that allows the identification and quantitation of a subset of murine LTC-IC that are able to generate both myeloid and lymphoid progeny in vitro. These "LTC-ICML" are inseparable from either standard LTC-IC or cells with long-term in vivo lympho-myeloid repopulating potential (competitive repopulating units, or CRU) present in normal adult mouse marrow but are detected at 15- to 30-fold lower frequencies. To determine whether at least part of this discrepancy is due to a reduced sensitivity of the LTC-ICML assay, a number of strategies to try to improve the original protocol were tested. Prolongation of the second phase of the assay (when the cells are maintained under lymphoid LTC conditions) from 7 to 10 days and inclusion of Flt3 ligand (FL) in the assay used to detect the production of B-lineage cells (CFU-pre-B) increased the average yield of CFU-pre-B per LTC-ICML by a factor of 3- to 20-fold; however, neither of these manipulations increased the frequency of LTC-ICML detected. Similarly, the use of S17 fibroblasts as feeders also did not improve the sensitivity of the LTC-ICML assay. On the other hand, approximately threefold more LTC-ICML could be detected when the initial phase of the assay (when the cells are maintained under myeloid LTC conditions) was shortened from 4 weeks to 1 week. In addition, this latter modification showed the existence in adult mouse marrow of a previously unrecognized, very early B220- stage of lymphoid development characterized by a relative resistance to both 5-fluorouracil and hydrocortisone. The discovery of such cells is of significant interest; however, their presence in adult mouse marrow in numbers comparable to those of both LTC-ICML and myeloid-restricted LTC-IC imposes severe restrictions on the use of the shorter LTC-ICML assay which offset its increased sensitivity. Interestingly, the present studies also show that both LTC-IC and LTC-ICML numbers are significantly better maintained (approximately twofold to sixfold) in myeloid LTC than are CRU. This differential maintenance of LTC-IC and CRU in vitro may be related to the fact that the detection of each of these two functionally defined progenitors involves the use of different ligand receptor systems. Alternatively, it is possible that at least some LTC-IC and CRU may represent developmentally distinct cell types. Taken together, these findings suggest a model of hematopoietic stem cell regulation in which retention of totipotentiality and maintenance of responsiveness to specific regulators may not be tightly linked.

Studies of W mutant mice provide evidence for alternate mechanisms capable of activating hematopoietic stem cells

Previous studies have suggested that Steel factor (SF) can influence the behavior of many types of hematopoietic progenitor cells both in vivo and in vitro, although whether these may include the most primitive populations of totipotent repopulating cells remains controversial. To approach this question, we measured the number of Sca1+Lin-WGA+ cells, the number of cells with demonstrable myeloid (long-term culture-initiating cell [LTC-IC]) or both myeloid and lymphoid (LTC-IC(ML)) potential in 4- to 5-week-old long-term cultures containing irradiated primary marrow feeder layers, and the number of multilineage long-term in vivo repopulating cells (competitive repopulating unit [CRU]) present in the marrow of W42/+ or W41/W41 mice compared to +/+ controls. There was no significant effect of either of these W mutations on the number of Sca1+Lin-WGA+ cells and, in W41/W41 mice, neither LTC-IC nor LTC-IC(ML) populations appeared to be affected. On the other hand, although W41/W41 and W42/+ cells could both be detected in the in vivo CRU assay, their numbers were markedly reduced (17- and seven-fold, respectively) in spite of the fact that both of these W mutant genotypes contained near normal numbers of day-9 and -12 colony-forming units-spleen (CFU-S). In vitro quantitation of erythroid (burst-forming units-erythroid [BFU-E]), granulopoietic (CFU-granulocyte/macrophage [CFU-GM]), multilineage (CFU-granulocyte/erythrocyte/monocyte/macrophage [CFU-GEMM]), and pre-B clonogenic progenitors (CFU-pre-B) also revealed no differences in the numbers (or proliferative potential) of any of these cells when W41/W41 or W42/+ and normal mice were compared, although day 3 BFU-E from both types of W mutant mice showed no response to the typical enhancing effect exerted by SF on their +/+ counterparts. Taken together, these findings are consistent with the view that SF activation of c-kit receptor-induced signaling events is not a rate-limiting mechanism controlling red blood cell production during normal development until hematopoietic cells differentiate beyond the day-3 BFU-E stage. Nevertheless, normal hematopoietic stem cells do appear to be responsive to SF, since their W mutant counterparts display a disadvantage in the in vivo setting which is exaggerated under conditions of hematopoietic regeneration. On the other hand, alternative mechanisms also appear to contribute to the regulation of hematopoietic stem cell numbers in vivo and to their detection as LTC-IC in vitro.

Characterization and purification of a primitive hematopoietic cell type in adult mouse marrow capable of lymphomyeloid differentiation in long-term marrow "switch" cultures

In this report, we describe a modification of the assay for long-term culture-initiating cells (LTC-IC) that allows a subset of murine LTC-IC (designated as LTC-ICML) to express both their myeloid (M) and lymphoid (L) differentiative potentials in vitro. The modified assay involves culturing test cells at limiting dilutions on irradiated mouse marrow feeder layers for an initial 4 weeks under conditions that support myelopoiesis and then for an additional week under conditions permissive for B-lymphopoiesis. All of the clonogenic pre-B progenitors (colony-forming unit [CFU] pre-B) detected in such postswitch LTC appear to be the progeny of uncommitted cells present in the original cell suspension because exposure of lymphoid-restricted progenitors to myeloid LTC conditions for > or = 7 days was found to irreversibly terminate CFU-pre-B production and, in cultures initiated with limiting numbers of input cells (no progenitors of any type detected in > 70% of cultures 1 week after the switch), the presence of CFU-pre-B was tightly associated with the presence of myeloid clonogenic cells, regardless of the purity of the input population. Limiting dilution analysis of the proportion of negative cultures measured for different numbers of input cells showed the frequency of LTC-ICML in normal adult mouse marrow to be 1 per 5 x 10(5) cells with an enrichment of approximately 500-fold in the Sca-1+ Lin-WGA+ fraction, as was also found for competitive in vivo repopulating units (CRU) and conventionally defined LTC-IC. LTC-ICML also exhibited the same resistance to treatment in vivo with 5-fluorouracil (5-FU) as CRU and LTC-IC, thereby distinguishing these three populations from the great majority of both in vitro clonogenic cells and day 12 CFU-S. The ability to quantitate cells with dual lymphoid and myeloid differentiation potentials in vitro, without the need for their prior purification, should facilitate studies of totipotent hematopoietic stem cell regulation.