Biology subjects

Karlsson, G.

Publications and source records attributed to Karlsson, G..

13 recordsLinked to original sources

bioRxiv2025

Chromatin Accessibility Shapes Developmental-Specific Lineage Plasticity in Hematopoiesis

Cell differentiation is governed by dynamic changes in chromatin accessibility, and its dysregulation underlies multiple disease states. Prior to birth, development of the hematopoietic system constitutes a period of broad differentiation potential, with certain immune cells arising exclusively during ontogeny. While age is known to affect lineage bias, the underlying molecular differences driving lineage preference in fetal and adult human hematopoietic stem and progenitor cells (HSPCs) remain unclear. Through single-cell cultures of hematopoietic stem cells (HSCs), we observed that fetal cells frequently generate mixed-lineage colonies, whereas adult HSCs are largely restricted to myeloid output. To investigate how these lineage preferences were encoded at the chromatin level, we performed single-cell ATAC-sequencing on first-trimester HSPCs. While adult HSCs showed enrichment of lineage-specific transcription factor motifs, fetal cells lacked such enrichment, consistent with their broader differentiation potential. We additionally uncovered a developmental-specific plasticity in fetal lymphoid progenitors, manifested as a hybrid lympho-myeloid chromatin program not present in adult progenitors. Additionally, the motif for PAX5, master regulator of B cell development, showed markedly reduced accessibility in fetal cells, indicating a more plastic and less committed lymphoid state. This enhanced embryonic lineage plasticity may underlie the prenatal susceptibility to mutational drivers of acute lymphoblastic leukemia.

developmental biology↗

bioRxiv2025

Multi-agent AI enables evidence-based cell annotation in single-cell transcriptomics

Cell type annotation remains a critical bottleneck, with current methods often inaccurate and requiring extensive manual validation, particularly in disease contexts. While large language models (LLMs) show promise, they can be unreliable due to hallucinations. We developed CyteType, a multi-agent framework that generates competing hypotheses grounded in full expression data and study context, validates against external databases, and iteratively self-evaluates. Comprehensive benchmarking demonstrates that CyteType substantially outperforms reference-based and LLM-based methods, with self-generated confidence scores reliably identifying trustworthy annotations. CyteType transforms cell type annotation from label assignment into evidence-grounded biological discovery. Python (AnnData compatible): https://github.com/NygenAnalytics/CyteType R (Seurat compatible): https://github.com/NygenAnalytics/CyteTypeR

bioinformatics↗

bioRxiv2024

Towards understanding of allostery in MALT1: a possible role of interdomain motions as revealed by NMR and AlphaFold

Mucosa-associated lymphoid tissue lymphoma-translocation protein 1 (MALT1) has emerged as an attractive target for the development of modulatory compounds, particularly in the treatment of lymphoma and other cancers. While the three-dimensional structure of MALT1(PCASP-Ig3)339-719 has been previously determined through X-ray analysis, its dynamic behaviour in solution has remained largely unexplored. We present here inaugural dynamic analyses of the apo MALT1(PCASP-Ig3)339-719 form along with its mutated variant, E549A. This investigation harnessed an array of NMR relaxation techniques, including longitudinal and transverse 15N auto-relaxation, heteronuclear NOE, transverse cross-correlated relaxation and NOE measurements between side-chain methyl groups. Our findings unequivocally confirm that MALT1(PCASP-Ig3)339-719 exists solely as a monomer in solution, and demonstrate that the two domains display semi-independent movements in relation to each other. Our extensive dynamic study, covering a range of time scales, along with the assessment of diverse conformational populations for MALT1(PCASP-Ig3)339-719, by Molecular Dynamic simulations, Alpha Fold modelling and PCA analysis, shed light at potential mechanisms underlying the allosteric regulation of this enzyme, and the specific importance of interdomain motions.

molecular biology↗

bioRxiv2023

CD9 marks myeloid/MegE-biased human hematopoiesis

Rare hematopoietic stem cells make up an infrequent but critical population in the bone marrow (BM), maintaining and replenishing the entire hematopoietic system. Importantly, despite sharing the unique stem cell properties of multilineage differentiation and self-renewal, individual HSCs are functionally heterogeneous, and this heterogeneity increases during aging. While HSCs in young mice are qualitatively more similar, ageing is marked by an increased size of the HSC pool and substantial functional variation of individual HSCs. CD9 is a cell surface marker that is highly expressed in HSCs in mice, while CD9 expression within the human HSC population has been reported to be low during neonatal hematopoiesis. Here, we have investigated CD9 expression levels in the human HSPC population over time and identified that early in life; CD9 is infrequent in HSCs, but marks progenitor populations with low engraftment potential and high proliferation capacity. However, during situations of myeloid/Megakaryocyte-erythoid (MegE) biased hematopoiesis, such as during ageing or in leukemia, there is a substantial increase of CD9 expression in HSPCs. Thus, CD9 represents an HSC marker for myeloid/MegE-biased hematopoiesis.

cell biology↗

bioRxiv2023

Single cell multi-omics analysis of chronic myeloid leukemia links cellular heterogeneity to therapy response

The advent of tyrosine kinase inhibitors (TKIs) as treatment of chronic myeloid leukemia (CML) is a paradigm in molecularly targeted cancer therapy. Nonetheless, TKI insensitive leukemia stem cells (LSCs) persist in most patients even after years of treatment. The sustained presence, heterogeneity and evolvability of LSCs are imperative for disease progression as well as recurrence during treatment-free remission (TFR). However, dynamic changes among LSC sub-populations upon TKI therapy impede their measurement and targeting. Here, we used cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) to generate high-resolution single cell multiomics maps from CML patients at diagnosis, retrospectively stratified by BCR::ABL1IS (%) following 12 months of TKI therapy as per European LeukemiaNet (ELN) recommendations. Simultaneous measurement of global gene expression profiles together with >40 surface markers from the same cells revealed that each patient harbored a unique composition of stem and progenitor cells at diagnosis demonstrating that cellular heterogeneity is a hallmark of CML. The patients with treatment failure after 12 months of therapy had markedly higher abundance of molecularly defined primitive cells at diagnosis compared to the optimal responders. Furthermore, deconvolution of an independent dataset of CML patient-derived bulk transcriptomes (n=59) into constituent cell populations showed that the proportion of primitive cells versus lineage primed sub-populations significantly connected with the TKI-treatment outcome. The multiomic feature landscape enabled visualization of the primitive fraction as a heterogenous mixture of molecularly distinct Lin-CD34+CD38-/low BCR::ABL1+ LSCs and BCR::ABL1- hematopoietic stem cells (HSCs) in variable ratio across patients and guided their prospective isolation by a combination of CD26 and CD35 cell surface markers. We for the first time show that BCR::ABL1+ LSCs and BCR::ABL1- HSCs can be distinctly separated as CD26+CD35- and CD26-CD35+ respectively. In addition, we found the relative proportion of CD26-CD35+ HSCs to be higher in optimal responders when compared to treatment failures, at diagnosis as well as following 3 months of TKI therapy, and that the LSC/HSC ratio was increased in patients with prospective treatment failure. Collectively, the patient-specific cellular heterogeneity multiomics maps build a framework towards understanding therapy response and adapting treatment by devising strategies that either extinguish TKI-insensitive LSCs or engage the immune effectors to suppress the residual leukemogenic cells.

cancer biology↗

bioRxiv2021

A Combined Immunophenotypic And Transcriptional Single-Cell Map Of First Trimester Human Fetal Liver Hematopoiesis

Knowledge of human fetal blood development and how it differs from adult is highly relevant for our understanding of congenital blood and immune disorders as well as childhood leukemia, the latter known to originate in utero. Blood production during development occurs in waves that overlap in time and space adding to heterogeneity, which necessitates single cell approaches. Here, a combined single cell immunophenotypic and transcriptional map of first trimester primitive blood development is presented. Using CITE-seq (Cellular Indexing of Transcriptomes and Epitopes by Sequencing) the molecular profile of established immunophenotypic gated progenitors was analyzed in the fetal liver (FL). Classical markers for hematopoietic stem cells (HSCs) such as CD90 and CD49F were largely preserved, whereas CD135 (FLT3) and CD123 (IL3R) had a ubiquitous expression pattern capturing heterogenous populations. Direct molecular comparison with an adult bone marrow (BM) data set revealed that HSC-like cells were less frequent in FL, whereas cells with a lympho-myeloid signature were more abundant. Furthermore, an erythro-myeloid primed multipotent progenitor cluster was identified, potentially representing a transient, FL-specific progenitor. Based on the projection performed, up- and downregulated genes between fetal and adult cells were analyzed. In general, cell cycle pathways, including MYC targets were shown to be upregulated in fetal cells, whereas gene sets involved in inflammation and human leukocyte antigen (HLA) complex were downregulated. Importantly, a fetal core molecular signature was identified that could discriminate certain types of infant and childhood leukemia from adult counterparts. Our detailed single cell map presented herein emphasizes molecular as well as immunophenotypic differences between fetal and adult primitive blood cells, of significance for future studies of pediatric leukemia and blood development in general.

developmental biology↗

bioRxiv2021

Scarf: A toolkit for memory efficient analysis of large-scale single-cell genomics data

The increasing capacity to perform large-scale single-cell genomic experiments continues to outpace the computational requirements to efficiently handle growing datasets. Herein we present Scarf, a modularly designed Python package that seamlessly interoperates with other single-cell toolkits and allows for memory-efficient single-cell analysis of millions of cells on a laptop or low-cost devices like single board computers. We demonstrate Scarfs memory and compute-time efficiency by applying it to the largest existing single-cell RNA-Seq and ATAC-Seq datasets. Scarf wraps memory-efficient implementations of a graph-based t-stochastic neighbour embedding and hierarchical clustering algorithm. Moreover, Scarf performs accurate reference-anchored mapping of datasets while maintaining memory efficiency. By implementing a novel data downsampling algorithm, Scarf additionally can generate representative sampling of cells from a given dataset wherein rare cell populations and lineage differentiation trajectories are conserved. Together, Scarf provides a framework wherein any researcher can perform advanced processing, downsampling, reanalysis, and integration of atlas-scale datasets on standard laptop computers.

bioinformatics↗

bioRxiv2021

Single-Cell Multiomics Reveals Distinct Cell States at the Top of Human Hematopoietic Hierarchy

The advent of single cell (Sc) genomics has challenged the dogma of haematopoiesis as a tree-like structure of stepwise lineage commitment through distinct and increasingly restricted progenitor populations. Instead, analysis of ScRNA-seq has proposed that the earliest events in human hematopoietic stem cell (HSC) differentiation are characterized by only subtle molecular changes, with hematopoietic stem and progenitor cells (HSPCs) existing as a continuum of low-primed cell-states that gradually transition into a specific lineage (CLOUD-HSPCs). Here, we combine ScRNA-seq, ScATAC-seq and cell surface proteomics to dissect the heterogeneity of CLOUD-HSPCs at different stages of human life. Within CLOUD-HSPCs, pseudotime ordering of both mRNA and chromatin data revealed a bifurcation of megakaryocyte/erythroid and lympho/myeloid trajectories immediately downstream a subpopulation with an HSC-specific enhancer signature. Importantly, both HSCs and lineage-restricted progenitor populations could be prospectively isolated based on correlation of their molecular signatures with CD35 and CD11A expression, respectively. Moreover, we describe the changes that occur in this heterogeneity as hematopoiesis develops from neonatal to aged bone marrow, including an increase of HSCs and depletion of lympho-myeloid biased MPPs. Thus, this study dissects the heterogeneity of human CLOUD-HSPCs revealing distinct HSPC-states of relevance in homeostatic settings such as ageing.

cell biology↗

bioRxiv2021

Genome-wide association study on 13,167 individuals identifies regulators of hematopoietic stem and progenitor cell levels in human blood

Understanding how hematopoietic stem and progenitor cells (HSPCs) are regulated is of central importance for the development of new therapies for blood disorders and stem cell transplantation. To date, HSPC regulation has been extensively studied in vitro and in animal models, but less is known about the mechanisms in vivo in humans. Here, in a genome-wide association study on 13,167 individuals, we identify 9 significant and 2 suggestive DNA sequence variants that influence HSPC (CD34+) levels in human blood. The identified loci associate with blood disorders, harbor known and novel HSPC genes, and affect gene expression in HSPCs. Interestingly, our strongest association maps to the PPM1H gene, encoding an evolutionarily conserved serine/threonine phosphatase never previously implicated in stem cell biology. PPM1H is expressed in HSPCs, and the allele that confers higher blood CD34+ cell levels downregulates PPM1H. By functional fine-mapping, we find that this downregulation is caused by the variant rs772557-A, which abrogates a MYB transcription factor binding site in PPM1H intron 1 that is active in specific HSPC subpopulations, including hematopoietic stem cells, and interacts with the promoter by chromatin looping. Furthermore, rs772557-A selectively increases HSPC subpopulations in which the MYB site is active, and PPM1H shRNA- knockdown increased CD34+ and CD34+90+ cell proportions in umbilical cord blood cultures. Our findings represent the first large-scale association study on a stem cell trait, illuminating HSPC regulation in vivo in humans, and identifying PPM1H as a novel inhibition target that can potentially be utilized clinically to facilitate stem cell harvesting for transplantation.

genomics↗

bioRxiv2020

Nabo - a framework to define leukemia-initiating cells and differentiation in single-cell RNA-sequencing data

Single-cell transcriptomics facilitates innovative approaches to define and identify cell types within tissues and cell populations. An emerging interest in the cancer field is to assess the heterogeneity of transformed cells, including the identification of tumor-initiating cells based on similarities to their normal counterparts. However, such cell mapping is often confounded by the large effects on total gene expression programs introduced by strong perturbations such as an oncogenic event. Here, we present Nabo, a novel computational method that allows mapping of cells from one population to the most similar cells in a reference population, independently of confounding changes to gene expression programs initiated by perturbation. We validated this method on multiple datasets from different sources and platforms and show that Nabo achieves higher rates of accuracy than conventional classification methods. Nabo is available as an integrated toolkit for preprocessing, cell mapping, differential gene expression identification, and visualization of single-cell RNA-Seq data. For exploratory studies, Nabo includes methods to help evaluate the reliability of cell mapping results. We applied Nabo on droplet-based single-cell RNA-Seq data of healthy and oncogene-induced (MLL-ENL) hematopoietic progenitor cells (GMLPs) differentiating in vitro. Despite a substantial cellular heterogeneity resulting from differentiation of GMLPs and the large transcriptional effects induced by the fusion oncogene, Nabo could pinpoint the specific cell stage where differentiation arrest occurs, which included an immunophenotypic definition of the tumor-initiating population. Thus, Nabo allows for relevant comparison between target and control cells, without being confounded by differences in population heterogeneity.

bioinformatics↗

bioRxiv2020

Concurrent stem- and lineage-affiliated chromatin programs precede hematopoietic lineage restriction

The emerging notion of hematopoietic stem- and progenitor cells (HSPCs) as a low-primed cloud without sharply demarcated gene expression programs raises the question on how cellular fate options emerge, and at which stem-like stage lineage priming is initiated. Here we investigated single-cell chromatin accessibility of Lineage-, cKit+, Sca1+ (LSK) HSPCs spanning the early differentiation landscape. Application of a signal-processing algorithm to detect transition points corresponding to massive alterations in accessibility of 571 transcription factor-motifs revealed a population of LSK FMS-like tyrosine kinase 3(Flt3)intCD9high cells that concurrently display stem-like and lineage-affiliated chromatin signatures pointing to a simultaneous gain of both Lympho-Myeloid and Megakaryocyte-Erythroid programs. Molecularly and functionally, these cells position between stem cells and committed progenitors, display multi-lineage capacity in vitro and in vivo, but lack self-renewal activity. This integrative molecular analysis resolves the heterogeneity of cells along hematopoietic differentiation and permits investigation of chromatin-mediated transition between multipotency and lineage restriction.

cell biology↗

bioRxiv2020

Signaling mechanism of phytochromes in solution

Phytochrome proteins guide the red/far-red photoresponse of plants, fungi, and bacteria. The proteins change their structure in response to light, thereby altering their biochemical activity. Crystal structures suggest that the mechanism of signal transduction from the chromophore to the output domains involves refolding of the so-called PHY tongue. It is currently not clear how the two other notable structural features of the phytochrome superfamily, the helical spine and a figure-of-eight knot, are involved in photoconversion. Here, we present solution NMR data of the complete photosensory core module from D. radiodurans (Dr BphP). Photoswitching between the resting and active states induces changes in amide chemical shifts, residual dipolar couplings, and relaxation dynamics. All observables indicate a photoinduced structural change in the knot region and lower part of the helical spine. This implies that a conformational signal is transduced from the chromophore to the helical spine through the PAS and GAF domains. The new pathway underpins functional studies of plant phytochromes and may explain photo-sensing by phytochromes under biological conditions.

biochemistry↗

bioRxiv2018

HSCs contribute actively to native multilineage hematopoiesis but with reduced differentiation capacity upon aging

A hallmark of adult hematopoiesis is the continuous replacement of blood cells with limited lifespans. It is well established that adult hematopoietic stem cells (HSCs) are active contributors to these processes after transplantation, yet their role in native hematopoiesis has recently been called into question. Here, we use inducible lineage tracing from genetically marked adult HSCs to explore their roles in the steady state. We show that adult HSCs contribute robustly to all lineages via intermediate progenitor cells, but with neglible production of hematopoietic cells with a known fetal origin. We further reveal that the timing for regeneration of distinct blood lineages varies substantially. Finally, HSC contribution to multilineage hematopoiesis in aged animals declines with increasing age. Therefore, while HSCs are active contributors to native adult hematopoiesis, it appears that the numerical increase of HSCs is a physiologically relevant compensatory mechanism to account for a reduced differentiation capacity with age.

developmental biology↗