Search

Hematopoietic stem cells (HSCs) and multipotent progenitor cells (MPPs) arise in successive waves during ontogeny, and their properties change significantly throughout life. Ontological changes in HSCs/MPPs underlie corresponding changes in mechanisms of pediatric leukemia initiation. As HSCs and MPPs progress from fetal to neonatal, juvenile and adult stages of life, they undergo transcriptional and epigenetic reprogramming that modifies immune output to meet age-specific pathogenic challenges. Some immune cells arise exclusively from fetal HSCs/MPPs. We propose that this layered immunity instructs cell fates that underlie a parallel layered leukemogenicity. Indeed, some pediatric leukemias, such as juvenile myelomonocytic leukemia, myeloid leukemia of Down syndrome, and infant pre-B-cell acute lymphoblastic leukemia, are age-restricted. They only present during infancy or early childhood. These leukemias likely arise from fetal progenitors that lose competence for transformation as they age. Other childhood leukemias, such as non-infant pre-B-cell acute lymphoblastic leukemia and acute myeloid leukemia, have mutation profiles that are common in childhood but rare in morphologically similar adult leukemias. These differences could reflect temporal changes in mechanisms of mutagenesis or changes in how progenitors respond to a given mutation at different ages. Interactions between leukemogenic mutations and normal developmental switches offer potential targets for therapy.

Behavioral timescale synaptic plasticity (BTSP) is a type of non-Hebbian synaptic plasticity reported to underlie place field formation in the hippocampal CA1 neurons. Despite this important function, the molecular mechanisms underlying BTSP are poorly understood. The α-Calcium-calmodulin-dependent protein kinase II (αCaMKII) is activated by synaptic transmission-mediated calcium influx and its subsequent phosphorylation is central to synaptic plasticity. Because the activity of αCaMKII is known to outlast the event triggering phosphorylation, we hypothesized it could be involved in the extended timescale of the BTSP process. To examine the role of αCaMKII in BTSP, we performed whole-cell in-vivo and in-vitro recordings in CA1 pyramidal neurons from mice engineered to have a point mutation at the autophosphorylation site (T286A) causing accelerated signaling kinetics. Here we demonstrate a profound deficit in synaptic plasticity, strongly suggesting that αCaMKII signaling is required for BTSP. This study elucidates part of the molecular mechanism of BTSP and provides insight into the function of αCaMKII in place cell formation and ultimately learning and memory.TeaserThe molecular mechanisms of BTSP have been revealed to require the autophosphorylation of CaMKII.

VAF concordance

Supplementary Data file including supplementary methods and clinical case vignettes.

Reasons for not using recommended targeted therapy.

Patient sequencing data, by patient number and diagnosis.

How episodic memories are stored within brains is poorly understood. While certain memory-retaining neurons have been potentially identified1–3, it is unclear if they retain learned information4,5. Further, there is considerable evidence that neuronal activity is unstable and may require additional mechanisms to support robust memory6–11. To examine these issues, we recorded the activity of a hippocampal CA1 neuronal population for 7 days as mice learned cued reward locations. These data and modelling results suggest that two place cell (PC) pools, distinguished by place field (PF) stability, are formed each day (transient: ~1.5 days; sustained: ~2 weeks)8. Notably, the proportions of these pools changed across the week as unstable transient PCs were progressively replaced by sustained PCs, markedly enhancing the stability of the total representation. This growing stable representation contained behaviorally relevant information and sustained PCs became active immediately at the start of each session. Finally, the initial formation of sustained PCs was associated with a higher rate and efficacy of behavioral timescale synaptic plasticity (BTSP) and these PCs showed elevated and more reliable activity. It, therefore, appears that BTSP stabilizes particularly informative PCs, incorporating them into an expanding and readily retrievable representation that displays hallmarks of a long-lasting memory.

Hematopoietic stem cells (HSCs) and lineage committed hematopoietic progenitor cells (HPCs) undergo profound shifts in gene expression during neonatal and juvenile stages of life. Temporal changes in HSC/HPC gene expression underlie concomitant changes in self-renewal capacity, lineage biases and hematopoietic output. Moreover, they can modify disease phenotypes. For example, childhood leukemias have distinct driver mutation profiles relative to adult leukemias, and they may arise from distinct cells of origin. The putative relationship between neonatal HSC/HPC ontogeny and childhood blood disorders highlights the importance of understanding how, at a mechanistic level, HSCs transition from fetal to adult transcriptional states. In this perspective piece, we summarize recent work showing that the transition is uncoordinated and imprecisely timed. We discuss implications of these findings, including mechanisms that might enable neonatal HSCs and HPCs to acquire adult-like properties over a drawn-out period of time, in lieu of precise gene regulatory networks. The transition from fetal to adult transcriptional programs coincides with a pulse of type I interferon signaling that activates many genes associated with the adult-like state. This pulse may sensitize HSCs/HPCs to mutations that drive leukemogenesis shortly after birth. If we can understand how developmental switches modulate HSC and HPC fate after birth – both under normal circumstances and in the setting of disease-causing mutations – we can potentially reprogram these switches to treat or prevent childhood leukemias.

Type I interferon (IFN-1) regulates gene expression and hematopoiesis both during development and in response to inflammatory stress. We previously showed that during development, hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs) induce IFN-1 target genes shortly before birth in mice. This coincides with the onset of a transition to adult hematopoiesis, and it drives expression of genes associated with antigen presentation. However, it is not clear whether perinatal IFN-1 modulates hematopoietic output, as has been observed in contexts of inflammation. We have characterized hematopoiesis at several different stages of blood formation, from HSCs to mature blood cells, and found that loss of the IFN-1 receptor (IFNAR1) leads to depletion of several phenotypic HSC and MPP subpopulations in neonatal and juvenile mice. Committed lymphoid and myeloid progenitor populations simultaneously expand. These changes had surprisingly little effect on production of more differentiated blood cells. Cellular Indexing of Transcriptomes and Epitopes by sequencing (CITE-seq) resolved the discrepancy between the extensive changes in progenitor numbers and modest changes in hematopoiesis, revealing stability in most MPP populations in Ifnar1-deficient neonates when the populations were identified based on gene expression rather than surface marker phenotype. Thus, basal IFN-1 signaling has only modest effects on hematopoiesis. Discordance between transcriptionally- and phenotypically-defined MPP populations may impact interpretations of how IFN-1 shapes hematopoiesis in other contexts, such as aging or inflammation.KEY POINTSLoss of type I Interferon signaling in neonatal mice depletes immature blood progenitors without compromising postnatal hematopoiesisProgenitor populations remain intact when measured by single cell transcriptomes rather than surface marker phenotypes

The leukocyte NADPH oxidase 2 (NOX2) plays a key role in pathogen killing and immunoregulation. Genetic defects in NOX2 result in chronic granulomatous disease (CGD), associated with microbial infections and inflammatory disorders, often involving the lung. Alveolar macrophages (AMs) are the predominant immune cell in the airways at steady state, and limiting their activation is important, given the constant exposure to inhaled materials, yet the importance of NOX2 in this process is not well understood. In this study, we showed a previously undescribed role for NOX2 in maintaining lung homeostasis by suppressing AM activation, in CGD mice or mice with selective loss of NOX2 preferentially in macrophages. AMs lacking NOX2 had increased cytokine responses to Toll-like receptor-2 (TLR2) and TLR4 stimulation ex vivo. Moreover, between 4 and 12 week of age, mice with global NOX2 deletion developed an activated CD11bhigh subset of AMs with epigenetic and transcriptional profiles reflecting immune activation compared with WT AMs. The presence of CD11bhigh AMs in CGD mice correlated with an increased number of alveolar neutrophils and proinflammatory cytokines at steady state and increased lung inflammation after insults. Moreover, deletion of NOX2 preferentially in macrophages was sufficient for mice to develop an activated CD11bhigh AM subset and accompanying proinflammatory sequelae. In addition, we showed that the altered resident macrophage transcriptional profile in the absence of NOX2 is tissue specific, as those changes were not seen in resident peritoneal macrophages. Thus, these data demonstrate that the absence of NOX2 in alveolar macrophages leads to their proinflammatory remodeling and dysregulates alveolar homeostasis.

Acute myeloid leukemia (AML) arises when leukemia-initiating cells, defined by a primary genetic lesion, acquire subsequent molecular changes whose cumulative effects bypass tumor suppression. The changes that underlie AML pathogenesis not only provide insights into the biology of transformation but also reveal novel therapeutic opportunities. However, backtracking these events in transformed human AML samples is challenging, if at all possible. Here, we approached this question using a murine in vivo model with an MLL-ENL fusion protein as a primary molecular event. Upon clonal transformation, we identified and extensively verified a recurrent codon-changing mutation (Arg

Synaptic plasticity, the activity-dependent change in neuronal connection strength, has long been considered an important component of learning and memory. Computational and engineering work corroborate the power of learning through the directed adjustment of connection weights. Here we review the fundamental elements of four broadly categorized forms of synaptic plasticity and discuss their functional capabilities and limitations. Although standard, correlation-based, Hebbian synaptic plasticity has been the primary focus of neuroscientists for decades, it is inherently limited. Three-factor plasticity rules supplement Hebbian forms with neuromodulation and eligibility traces, while true supervised types go even further by adding objectives and instructive signals. Finally, a recently discovered hippocampal form of synaptic plasticity combines the above elements, while leaving behind the primary Hebbian requirement. We suggest that the effort to determine the neural basis of adaptive behavior could benefit from renewed experimental and theoretical investigation of more powerful directed types of synaptic plasticity.

Learning-related changes in brain activity are thought to underlie adaptive behaviours1,2. For instance, the learning of a reward site by rodents requires the development of an over-representation of that location in the hippocampus3–6. How this learning-related change occurs remains unknown. Here we recorded hippocampal CA1 population activity as mice learned a reward location on a linear treadmill. Physiological and pharmacological evidence suggests that the adaptive over-representation required behavioural timescale synaptic plasticity (BTSP)7. BTSP is known to be driven by dendritic voltage signals that we proposed were initiated by input from entorhinal cortex layer 3 (EC3). Accordingly, the CA1 over-representation was largely removed by optogenetic inhibition of EC3 activity. Recordings from EC3 neurons revealed an activity pattern that could provide an instructive signal directing BTSP to generate the over-representation. Consistent with this function, our observations show that exposure to a second environment possessing a prominent reward-predictive cue resulted in both EC3 activity and CA1 place field density that were more elevated at the cue than at the reward. These data indicate that learning-related changes in the hippocampus are produced by synaptic plasticity directed by an instructive signal from the EC3 that seems to be specifically adapted to the behaviourally relevant features of the environment.

Learning-related changes in brain activity are thought to underlie adaptive behaviours

Hodgkin lymphoma (HL) is a highly curable form of cancer, and current treatment regimens are focused on improving treatment efficacy while decreasing the risk of late effects of treatment. The NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines) for pediatric HL provide recommendations on the workup, diagnostic evaluation, and treatment of classic HL, including principles of pathology, imaging, staging, systemic therapy, and radiation therapy. This portion of the NCCN Guidelines focuses on the management of pediatric classic HL in the upfront and relapsed/refractory settings.

Despite a remarkable increase in the genomic profiling of cancer, integration of genomic discoveries into clinical care has lagged behind. We report the feasibility of rapid identification of targetable mutations in 153 pediatric patients with relapsed/refractory or high-risk leukemias enrolled on a prospective clinical trial conducted by the LEAP Consortium. Eighteen percent of patients had a high confidence Tier 1 or 2 recommendation. We describe clinical responses in the 14% of patients with relapsed/refractory leukemia who received the matched targeted therapy. Further, in order to inform future targeted therapy for patients, we validated variants of uncertain significance, performed ex vivo drug-sensitivity testing in patient leukemia samples, and identified new combinations of targeted therapies in cell lines and patient-derived xenograft models. These data and our collaborative approach should inform the design of future precision medicine trials. Significance: Patients with relapsed/refractory leukemias face limited treatment options. Systematic integration of precision medicine efforts can inform therapy. We report the feasibility of identifying targetable mutations in children with leukemia and describe correlative biology studies validating therapeutic hypotheses and novel mutations. See related commentary by Bornhauser and Bourquin, p. 1322. This article is highlighted in the In This Issue feature, p. 1307

MLL rearrangements are translocation mutations that cause both acute lymphoblastic leukemia and acute myeloid leukemia (AML). These translocations can occur as sole clonal driver mutations in infant leukemias, suggesting that fetal or neonatal hematopoietic progenitors may be exquisitely sensitive to transformation by MLL fusion proteins. To test this possibility, we used transgenic mice to induce one translocation product, MLL-ENL, during fetal, neonatal, juvenile and adult stages of life. When MLL-ENL was induced in fetal or neonatal mice, almost all died of AML. In contrast, when MLL-ENL was induced in adult mice, most survived for >1 year despite sustained transgene expression. AML initiation was most efficient when MLL-ENL was induced in neonates, and even transient suppression of MLL-ENL in neonates could prevent AML in most mice. MLL-ENL target genes were induced more efficiently in neonatal progenitors than in adult progenitors, consistent with the distinct AML initiation efficiencies. Interestingly, transplantation stress mitigated the developmental barrier to leukemogenesis. Since fetal/neonatal progenitors were highly competent to initiate MLL-ENL–driven AML, we tested whether Lin28b, a fetal master regulator, could accelerate leukemogenesis. Surprisingly, Lin28b suppressed AML initiation rather than accelerating it. This may explain why MLL rearrangements often occur before birth in human infant leukemia patients, but transformation usually does not occur until after birth, when Lin28b levels decline. Our findings show that the efficiency of MLL-ENL–driven AML initiation changes through the course of pre- and postnatal development, and developmental programs can be manipulated to impede transformation.

Hematopoietic stem cells (HSC) balance differentiation and self-renewal to promote maintenance of mature blood cell lineages. The mechanisms that regulate HSC expansion during fetal hematopoietic development and adult HSC maintenance remain significant knowledge gaps. BCLAF1 (Bcl2-associated factor 1), a transcriptional regulator, is highly expressed in hematopoietic cells. We recently defined its function as a modulator of ETS-family transcription factor activity in developing B cells. Our objective in this study is to determine the molecular mechanism of BCLAF1 in regulation of fetal and adult HSCs. We used mouse models with selective deletion of Bclaf1 in hematopoietic cells to test whether BCLAF1 regulates HSC development and function. We find that loss of Bclaf1 results in significantly reduced numbers of fetal HSCs. The reduced HSC numbers persist in adult mice; but there is no additional decline or development of bone marrow failure with aging. In competitive transplant experiments, Bclaf1-deficient fetal and adult HSCs have defective repopulation and self-renewal capacity resulting in reduced reconstitution of all hematopoietic populations. Bclaf1-deficient HSCs do not have abnormalities in emergence, proliferation, cell death, or homing activity. Using single cell RNA-sequencing of fetal HSCs, we find that loss of BCLAF1 results in reduction of a subpopulation of HSCs and upregulation of the early response genes Jun, Fos, and Egr1. Collectively, these findings suggest BCLAF1 restrains activity of immediate early genes to preserve HSC self-renewal capacity and promote HSC expansion during both fetal development and repopulation of hematopoiesis after HSC transplant. Supported by grants from Children's Discovery Institute, St. Louis Children's Hospital Foundation, Siteman Cancer Center, and NIH (T32 AI007163).

SUMMARY The myeloid tumor suppressor KMT2C is recurrently deleted in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML), particularly therapy-related MDS/AML (t-MDS/t-AML), as part of larger chromosome 7 deletions. Here, we show that KMT2C deletions convey a selective advantage to hematopoietic stem cells (HSCs) after chemotherapy treatment that may precipitate t-MDS/t-AML. Kmt2c deletions markedly enhance murine HSC self-renewal capacity without altering proliferation rates. Haploid Kmt2c deletions convey a selective advantage only when HSCs are driven into cycle by a strong proliferative stimulus, such as chemotherapy. Cycling Kmt2c-deficient HSCs fail to differentiate appropriately, particularly in response to interleukin-1. Kmt2c deletions mitigate histone methylation/acetylation changes that accrue as HSCs cycle after chemotherapy, and they impair enhancer recruitment during HSC differentiation. These findings help explain why Kmt2c deletions are more common in t-MDS/t-AML than in de novo AML or clonal hematopoiesis: they selectively protect cycling HSCs from differentiation without inducing HSC proliferation themselves., Graphical Abstract, In Brief Chen et al. use loss-of-function mice to study the role Kmt2c/MLL3 in hematopoietic stem cell (HSC) self-renewal. They show that haploid Kmt2c deletions impede differentiation when HSCs are driven into cycle by chemotherapy, especially in conjunction with interleukin-1 exposure. This conveys a selective advantage that may promote therapy-related leukemias.

Summary Adult hematopoietic stem cell (HSC) self-renewal requires precise control of protein synthesis, but fetal and adult HSCs have distinct self-renewal mechanisms and lineage outputs. This raises the question of whether protein synthesis rates change with age. Here, we show that protein synthesis rates decline during HSC ontogeny, yet erythroid protein synthesis rates increase. A ribosomal mutation that impairs ribosome biogenesis (Rpl24Bst/+) disrupts both fetal and adult HSC self-renewal. However, the Rpl24Bst/+ mutation selectively impairs fetal erythropoiesis at differentiation stages that exhibit fetal-specific attenuation of protein synthesis. Developmental changes in protein synthesis thus differentially sensitize hematopoietic stem and progenitor cells to impaired ribosome biogenesis., Graphical Abstract, Highlights • Fetal HSCs synthesize much more protein per hour than young adult HSCs in vivo • Fetal erythroid progenitors synthesize less protein than adult erythroid progenitors • Differences in protein synthesis distinguish fetal and adult erythroid differentiation • Rpl24Bst/+ impairs fetal and adult HSCs, but only impairs fetal erythroid progenitors, In this report, Magee and Signer show that protein synthesis rates decline during HSC ontogeny, but erythroid protein synthesis rates increase. A mutation that impairs ribosome biogenesis (Rpl24Bst/+) disrupts both fetal and adult HSC self-renewal. However, the Rpl24Bst/+ mutation selectively impairs fetal erythropoiesis. Developmental changes in protein synthesis differentially sensitize hematopoietic stem and progenitor cells to impaired ribosome biogenesis.

Toll-like receptor 2 (TLR2) expression is increased on hematopoietic stem and progenitor cells (HSPCs) of patients with myelodysplastic syndromes (MDS), and enhanced TLR2 signaling is thought to contribute to MDS pathogenesis. Notably, TLR2 heterodimerizes with TLR1 or TLR6, and while high TLR2 is associated with lower-risk disease, high TLR6, but not TLR1, correlates with higher-risk disease. This raises the possibility of heterodimer-specific effects of TLR2 signaling in MDS, and in the work described here, we tested the effects of specific modulation of TLR1/2 versus TLR2/6 signaling on premalignant HSPCs. Indeed, chronic stimulation of TLR2/6, but not TLR1/2, accelerates leukemic transformation in the NHD13 mouse model of MDS, and conversely, loss of TLR6, but not TLR1, slows this process. TLR2/6 stimulation expands premalignant HSPCs, and chimeric mouse studies revealed that cell-autonomous signaling contributes to this expansion. Finally, TLR2/6 stimulation is associated with an enrichment of Myc and mTORC1 activities. While Myc inhibition partially suppressed the TLR2/6 agonist-mediated expansion of premalignant HSPCs, inhibition of mTORC1 exacerbated it, suggesting that these pathways play opposite roles in regulating the effects of TLR2/6 ligation on HSPCs. Together, these data reveal heterodimer-specific effects of TLR2 signaling on premalignant HSPCs, with TLR2/6 signaling promoting their expansion and leukemic transformation.

SummaryLearning requires neural adaptations thought to be mediated by activity-dependent synaptic plasticity. A relatively non-standard form of synaptic plasticity driven by dendritic plateau potentials has been reported to underlie place field formation in hippocampal CA1 neurons. Here we found that this behavioral timescale synaptic plasticity (BTSP) can also reshape existing place fields via bidirectional synaptic weight changes that depend on the temporal proximity of plateau potentials to pre-existing place fields. When evoked near an existing place field, plateau potentials induced less synaptic potentiation and more depression, suggesting BTSP might depend inversely on postsynaptic activation. However, manipulations of place cell membrane potential and computational modeling indicated that this anti-correlation actually results from a dependence on current synaptic weight such that weak inputs potentiate and strong inputs depress. A network model implementing this bidirectional synaptic learning rule suggested that BTSP enables population activity, rather than pairwise neuronal correlations, to drive neural adaptations to experience.

Animals strategically scan the environment to form an accurate perception of their surroundings. Here we investigated the neuronal representations that mediate this behavior. Ca2+ imaging and selective optogenetic manipulation during an active sensing task reveals that layer 5 pyramidal neurons in the vibrissae cortex produce a diverse and distributed representation that is required for mice to adapt their whisking motor strategy to changing sensory cues. The optogenetic perturbation degraded single-neuron selectivity and network population encoding through a selective inhibition of active dendritic integration. Together the data indicate that active dendritic integration in pyramidal neurons produces a nonlinearly mixed network representation of joint sensorimotor parameters that is used to transform sensory information into motor commands during adaptive behavior. The prevalence of the layer 5 cortical circuit motif suggests that this is a general circuit computation.