Biology subjects

Hartmann, B.

Publications and source records attributed to Hartmann, B..

11 recordsLinked to original sources

bioRxiv2026

Impaired renal base excretion in secretin receptor knock-out mice during prolonged base-loading

AimSecretin was recently found to play a pivotal role in the renal adaptation to acute base excess. Here, secretin increases pendrin-dependent HCO3- secretion from the beta-intercalated cells in the cortical collecting ducts. Whether secretin and its receptor play a role during prolonged base-loading remains unknown. MethodsUrine and blood acid-base analyses were carried out in secretin receptor (SCTR) KO and WT mice at baseline and after 1 and up to 8 days of base-loading with NaHCO3-enriched drinking water. Changes in pendrin protein abundance and function were assessed by immunoblotting and isolated tubule perfusion experiments. Plasma secretin levels and renal SCTR expression were assessed after 24 hours of acid/base-loading by radioimmunoassay and qPCR, respectively. ResultsSCTR KO mice responded with diminished urine alkalization and a lesser reduction of urinary acid excretion when base-loaded for 48 hours. Concordantly, SCTR KO mice presented with increased blood base retention compared with WTs. Base-loaded SCTR WT and KO mice showed comparable total pendrin protein abundance. Despite this, pendrin function was markedly lower in SCTR KO mice. Base-loaded mice had higher plasma secretin and renal SCTR levels compared with acid-loaded mice. Higher arterial HCO - associated with higher renal SCTR mRNA expression. ConclusionLoss of the SCTR diminishes renal base excretion capacity and exacerbates systemic base accumulation during prolonged base-loading. Further, plasma secretin and renal SCTR mRNA levels are modulated by acid-base intake. These findings further support a central role of secretin and its receptor in the regulation of both acute and prolonged base excess. Key pointsO_LIIn mice, the receptor of the gut hormone secretin is required for the kidney to adequately handle a sustained base load. In mice with secretin receptor knockout, renal base excretion is impaired and metabolic alkalosis is aggravated and slower to resolve. C_LIO_LIBase-loaded knockout mice up-regulate pendrin protein abundance normally, the bicarbonate-secreting Cl-/HCO3-exchanger in collecting-duct {beta}-intercalated cells, but exhibit lower pendrin function. This suggests that the defect lies in the activation of pendrin, rather than in the regulation of its abundance. C_LIO_LIRenal secretin-receptor expression increases with higher arterial bicarbonate and plasma secretin, and renal secretin receptor expression, levels are lower in acid-loaded mice compared to base-loaded mice. This indicates that the secretin/secretin receptor system is itself affected by acid-base status/acid-base intake. C_LI

physiology↗

bioRxiv2025

Viscoelastic characterization of cells in microfluidic channels with 3D hydrodynamic focusing

The viscoelastic nature of biological cells has emerged as an increasingly important research subject due to its relevance for cellular functions under physiological and pathological conditions. Advancements in microfluidics have made this technology a promising tool to study the viscoelasticity of cells. However, significant challenges remain, including the complex distribution of stresses acting on cells depending on the channel geometry, and the difficulty of keeping cells in the focal plane for imaging. Here, we report a new approach using hyperbolic channels for measuring cell viscoelasticity. A channel height much larger than the typical cell size minimized shear stresses so that normal stresses in the hyperbolic region dominated the stress distribution. Reducing the complexity of the stress-strain relationship allowed us to use polyacrylamide microgel beads to calibrate the stress curve. Additionally, we introduced 3D hydrodynamic focusing which enabled us to focus cells and microgel beads in the center of the channel. Finally, Kelvin-Voigt and power-law rheology models were employed to extract the mechanical properties of microgel beads and human leukemia HL60 cells. The measurement technique described here will help establish the viscoelastic properties of cells as an important readout in biophysical research in health and disease.

biophysics↗

bioRxiv2024

Selective Agonism of Liver and Gut FXR Prevents Cholestasis and Intestinal Atrophy in Parenterally Fed Neonatal Pigs

BACKGROUND & AIMSWe aimed to investigate the relative efficacy of feeding different bile acids in preventing PNALD in neonatal pigs. METHODSNewborn pigs given total parenteral nutrition (TPN) combined with minimal enteral feeding of chenodeoxycholic acid (CDCA), or increasing doses of obeticholic acid (OCA) for 19 days. RESULTSEnteral OCA (5 and 15 mg/kg), but not CDCA (30 mg/kg) reduced blood cholestasis markers compared to TPN controls and increased bile acids in the gallbladder and intestine. Major bile acids in the liver and distal intestine were CDCA, HCA, HDCA and OCA, and their relative proportions were increased by the type of bile acid (CDCA or OCA) given enterally. High doses of OCA increased the total NR1H4-agonistic bile acid profile in the liver and intestine above 50% total bile acids. Both CDCA and OCA treatments suppressed hepatic cyp7a1 expression, but only OCA increased hepatobiliary transporters, ABCB11, ABCC$ and ABCB1. Plasma phytosterol levels were reduced and biliary levels were increased by CDCA and OCA and hepatic sterol transporters, abcg5/8, expression were increased by OCA. Both CDCA and OCA increased plasma FGF19 and OCA increased intestinal FGF19, FABP6, and SLC51A. Both CDCA and OCA increased intestinal mucosal growth, whereas CDCA increased the plasma GLP-2, GLP-1 and GIP. CONCLUSIONSEnteral OCA prevented cholestasis and phytosterolemia by increased hepatic bile acid and sterol transport via induction of hepatobiliary transporter FXR target genes and not by suppression of bile acid synthesis genes. We also showed an intestinal trophic action of OCA that demonstrates a dual clinical benefit of FXR agonism in the prevention of PNALD in piglets.

physiology↗

bioRxiv2024

Proinflammatory cytokine-induced alpha-cell impairment in human islet microtissues is partially restored by dual incretin receptor agonism

Aims/hypothesisIn type 1 diabetes, the counterregulatory glucagon response to low plasma glucose is impaired. The resulting increased risk of hypoglycaemia necessitates novel strategies to ameliorate alpha-cell impairment. Here, we aimed to establish an in vitro model of alpha-cell impairment in type 1 diabetes using human islet microtissues (MTs) exposed to proinflammatory cytokines. Additionally, we investigated the therapeutic potential of incretin receptor agonists in improving alpha-cell responses to low glucose. MethodsHuman islet MTs were exposed to proinflammatory cytokines (IL-1{beta}, IFN-{gamma}, and TNF-) for 1 day (short-term) and 6 days (long-term). Alpha-cell function was assessed by sequential glucose-dependent secretion assays at 2.8 and 16.7 mmol/l glucose, followed by glucagon measurements. Additional evaluations included ATP content, caspase-3/7 activity, chemokine secretion, and expression of islet transcription factors and hormones. The effects of incretin receptor agonist treatment (glucose-dependent insulinotropic polypeptide (GIP) analogue [D-Ala2]-GIP {+/-} liraglutide) alongside or after cytokine exposure were also investigated, focusing on low glucose-dependent glucagon secretion. ResultsShort-term cytokine exposure increased glucagon secretion at both 2.8 and 16.7 mmol/l glucose. In contrast, long-term cytokine exposure caused dose-dependent suppression of glucagon secretion at 2.8 mmol/l glucose, resembling a type 1 diabetes phenotype. Long-term cytokine exposure also diminished somatostatin secretion, reduced ATP content, increased caspase 3/7 activity, and decreased islet transcription factor and hormone expression. Despite cytokine-induced impairment, alpha cells partially retained secretory capacity to L-arginine stimulation. Treatment with incretin receptor agonists during long-term cytokine exposure did not prevent alpha-cell impairment. However, acute treatment with [D-Ala2]-GIP {+/-} liraglutide or the single-molecule dual agonist tirzepatide partially restored glucagon secretion at low glucose. Conclusions/interpretationLong-term cytokine exposure of human islet MTs impaired glucagon secretion to low glucose, creating a type 1 diabetes alpha-cell phenotype. This cytokine-induced alpha-cell impairment was partially restored by [D-Ala2]-GIP {+/-} liraglutide and tirzepatide, respectively. Research in contextO_ST_ABSWhat is already known about this subject?C_ST_ABSO_LIThe counterregulatory alpha-cell response to low glucose is impaired in type 1 diabetes, increasing the risk of hypoglycaemia. C_LIO_LILimited translatability of rodent islet findings highlights the need for human islet models. C_LIO_LIActions of the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) have mainly been studied in the context of type 2 diabetes and hyperglycaemia but less in type 1 diabetes and hypoglycaemia. C_LI What is the key question?O_LICan alpha-cell impairment in type 1 diabetes be modelled in vitro by exposing human islet microtissues (MTs) to proinflammatory cytokines, and could incretins protect against this? C_LI What are the new findings?O_LILong-term (6-day) exposure to proinflammatory cytokines produces a type 1 diabetes phenotype of alpha-cell impairment to low glucose in islet MTs C_LIO_LIAcute dual treatment with incretin receptor agonists partially restored glucose-dependent glucagon secretion in cytokine-exposed islet MTs -- an effect mainly carried by GIP receptor agonism and not opposed by GLP-1 receptor agonism. C_LI How might this impact on clinical practice in the foreseeable future?O_LIInvestigating incretin receptor agonists in a preclinical in vitro model of alpha-cell impairment may reveal their potential and fast-track their use as safeguards against hypoglycaemia in type 1 diabetes. C_LI

cell biology↗

bioRxiv2023

Inhibition of dipeptidyl peptidase 4 allows accurate measurement of GLP-1 secretion in mice

Dipeptidyl peptidase (DPP)-4 and neprilysin (NEP) within few minutes degrade glucagon-like peptide 1 (GLP-1) in mice, generating small, inactive fragments of the peptide. Commercially available sandwich ELISA kits may not accurately detect intact GLP-1 (that is GLP-1[7-36]NH2) and these moieties, leading to underestimation of secretion and potentially misleading results. Single-site antibody approaches may pick up some fragments, yet require large plasma volumes and the accuracy is uncertain. We aimed to find a way to stabilize GLP-1 in mice allowing reliable measurement with sensitive commercially available ELISA kits. Non-anesthetized male C57Bl/6JRj mice were subjected to an oral glucose tolerance test (OGTT; 2 g/kg glucose via oral gavage). Blood was drawn from the retrobulbar plexus before and repeatedly during the OGTT and total and intact GLP-1 were measured by commercially available sandwich ELISA kits (Mercodia and Alpco, respectively). In blood samples taken 15 minutes after the glucose load, there were no increase in plasma GLP-1 concentration. There was a small insignificant increase in total GLP-1 (1-2 pmol/L) in samples taken at t=5 and t=10 minutes following the OGTT, but no rise in intact GLP-1. We then administered control (saline), or a DPP-4 inhibitor (valine pyrrolidide, 0.1 {micro}mol/g or sitagliptin, 10 mg/kg) with or without a NEP-inhibitor (sacubitril, 0.3 mg/kg) 30 minutes before the OGTT. In the four inhibitor groups, intact GLP-1 increased during the OGTT (levels ranging between 4.04 [SD 2.87] and 15.53 [SD 6.76] pmol/L). The combination of sitagliptin with sacubitril gave the largest increase in intact GLP-1 levels. Finally, after injecting male C57Bl/6JRj mice with a known dose of GLP-1(7-36)NH2, the peak GLP-1 levels were significantly higher during sitagliptin, but not with the combination of sitagliptin/sacubitril. Both inhibitor groups, however, showed prolonged half-life of the GLP-1 plasma disappearance. We conclude that for measurements of GLP-1 secretion in mice with commercially available sandwich ELISA kits, it is necessary to consider both timing of blood sampling and in vivo inhibition of DPP-4. The described approach allows improved estimates of GLP-1 secretion for future studies. It is a limitation that DPP-4 and NEP inhibition may have metabolic effects by stabilizing the intact GLP-1 peptide (e.g. influencing levels of insulin and glucagon).

physiology↗

bioRxiv2022

GIP receptor reduces osteoclast activity and improves osteoblast survival by activating multiple signaling pathways

Bone is a dynamic tissue that is remodeled throughout life by bone resorbing osteoclasts and bone forming osteoblasts, to adapt to physiological or mechanical demands. These processes are impaired in osteoporosis, and understanding how bone remodeling is regulated could improve anti-osteoporotic treatments. Clinical investigations show that short-term treatment with glucose-dependent insulinotropic polypeptide (GIP) acutely decreases serum markers of bone resorption and may increase bone formation. However, evidence for direct effects of GIP intracellular signaling and functions in mature human osteoclasts and osteoblasts have not been investigated. We report that the GIP receptor (GIPR) is robustly expressed in mature human osteoclasts. Exposure of osteoclasts to GIP inhibits osteoclastogenesis, delays bone resorption, and increases osteoclast apoptosis by acting upon multiple signaling pathways (cAMP, Src, Akt, calcium, p38) to impair nuclear translocation of nuclear factor of activated T cells 1 (NFATc1) and nuclear factor-{kappa}B (NF{kappa}B). Human osteoblasts also express GIPR, and GIP improves osteoblast survival via cAMP and Akt-mediated pathways. GIP treatment of co-cultures of osteoclasts and osteoblasts also decreased bone resorption. Antagonizing GIPR with GIP(3-30)NH2 abolished the effects of GIP on osteoclasts and osteoblasts. This study demonstrates that GIP inhibits bone resorption and improves survival of human osteoblasts, which could increase bone mass and strength, supporting clinical investigations of the effect of GIP on bone. Moreover, this study demonstrates that GIPR agonism could be beneficial in the treatment of disorders of bone remodeling, such as osteoporosis. One-sentence SummaryGIP acts directly on bone cells to regulate bone remodeling

cell biology↗

bioRxiv2020

Nucleosome assembly and disassembly pathways.

Nucleosome assembly and disassembly play a central role in the regulation of gene expression. Here we use PhAST (Photochemical Analysis of Structural Transitions) to monitor at the base pair level, structural alterations induced all along DNA upon histone binding or release. By offering the first consistent, detailed comparison of nucleosome assembly and disassembly in vitro, we are able to reveal similarities and differences between the two processes. We identify multiple intermediate states characterised by specific PhAST signatures; revealing a complexity that goes beyond the known sequential events involving (H3-H4)2 tetramer and H2A-H2B heterodimers. Such signatures localise and quantify the extent of the asymmetry of DNA/histone interactions with respect to the nucleosome dyad. This asymmetry is therefore defined by the localisation and amplitude of the signals. The localisation of the signal is consistent between assembly and disassembly and dictated by the DNA sequence. However, the amplitude component of this asymmetry not only evolves during the assembly and disassembly but does so differently between the two processes. Understanding the regulation of gene expression requires a complete knowledge of nucleosome dynamics. Our unexpected observation of differences between assembly and disassembly opens up new avenues to define the role of the DNA sequence in these processes. Overall, we provide new insights into how the intrinsic properties of DNA are integrated into a holistic mechanism that controls chromatin structure. Statement of SignificanceThis manuscript addresses the question of nucleosome dissociation compares with association. We used PhAST which is a non-intrusive photochemical technique to follow nucleosome dynamics at base pair resolution. We observed structural asymmetry during nucleosome turnover. We also showed for the first time that the process of nucleosome dissociation is not a reversal of association. This asymmetry favours intermediate states involved in chromatin organisation suggesting novel models for the role of nucleosome turnover in the epigenetic regulation of gene expression.

molecular biology↗

bioRxiv2019

Ancestral regulatory mechanisms specify conserved midbrain circuitry in arthropods and vertebrates

Corresponding attributes of neural development and function suggest arthropod and vertebrate brains may have an evolutionarily conserved organization. However, the underlying mechanisms have remained elusive. Here we identify a gene regulatory and character identity network defining the deutocerebral-tritocerebral boundary (DTB) in Drosophila. We show this network comprises genes homologous to those directing midbrain-hindbrain boundary (MHB) formation in vertebrates and their closest chordate relatives. Genetic tracing reveals that the embryonic DTB gives rise to adult midbrain circuits that in flies control auditory and vestibular information processing and motor coordination, as do MHB-derived circuits in vertebrates. DTB-specific gene expression and function is directed by cis-regulatory elements (CREs) of developmental control genes that include homologs of mammalian Zinc finger of the cerebellum and Purkinje cell protein 4. Moreover, Drosophila DTB-specific CREs correspond to regulatory sequences of human ENGRAILED-2, PAX-2 and DACHSHUND-1 that direct MHB-specific expression in the embryonic mouse brain. Together, these findings imply ancestral regulatory mechanisms mediating the genetic specification of midbrain-cerebellar circuitry for balance and motor control that may predated the radiation of cephalic nervous systems across the animal kingdom.

neuroscience↗

bioRxiv2019

Deciphering the combinatorial interaction landscape

From cellular activation to drug combinations, the control of biological systems involves multiple stimuli that can elicit complex nonlinear interactions. To elucidate the functions and logic of stimulus interactions, we developed SAIL (Synergistic/Antagonistic Interaction Learner). SAIL uses a machine learning classifier trained to categorize interactions across a complete taxonomy of possible combinatorial effects. The strategy resolves the most informative interactions, and helps infer their functions and regulatory mechanisms. SAIL-predicted interaction mechanisms controlling key immune functions were experimentally validated. SAIL can integrate results from multiple datasets to derive general properties of how cells respond to multiple stimuli. Using public immunological datasets, we assembled a fine-grained landscape of [~]30000 interactions. Analysis of the landscape shows the context-dependent functions of individual modulators, and reveals a probabilistic algebra that links the separate and combined stimulus effects. SAIL is available through a user friendly interface to resolve the effect of stimulus and drug combinations.

bioinformatics↗

bioRxiv2019

Secretin release after Roux-en-Y Gastric Bypass reveals a population of glucose-sensitive S-cells in distal small intestine

ObjectiveGastrointestinal hormones contribute to the beneficial effects of Roux-en-Y gastric bypass surgery (RYGB) on glycemic control. Secretin is secreted from duodenal S-cells in response to low luminal pH, but it is unknown whether its secretion is altered after RYGB and if secretin contributes to the post-operative improvement in glycemic control. We hypothesized that secretin secretion increases after RYGB as a result of the diversion of nutrients to more distal parts of the small intestine, and thereby affects islet hormone release.\n\nMethodsA specific secretin radioimmunoassay was developed, evaluated biochemically, and used to quantify plasma concentrations of secretin in 13 obese individuals before, 1 week after and 3 months after RYGB. Distribution of secretin and its receptor was assessed by RNA-sequencing, mass-spectrometry and in situ hybridization in human and rat tissues. Isolated, perfused rat intestine and pancreas were used to explore the molecular mechanism underlying glucose-induced secretin secretion and to study direct effects of secretin on glucagon, insulin and somatostatin secretion. Secretin was administered alone or in combination with GLP-1 to non-sedated rats to evaluate effects on glucose regulation.\n\nResultsPlasma postprandial secretin was more than doubled in humans after RYGB (P<0.001). The distal small intestine harbored secretin expressing cells in both rats and humans. Glucose increased secretion of secretin in a sodium-glucose co-transporter dependent manner when administered to the distal part but not into the proximal part of the rat small intestine. Secretin stimulated somatostatin secretion (fold change: 1.59, P<0.05) from the perfused rat pancreas but affected neither insulin (P=0.2) nor glucagon (P=0.97) secretion. When administered to rats in vivo, insulin secretion was attenuated and glucagon secretion increased (P=0.04), while blood glucose peak time was delayed (from 15 min to 45 min) and gastric emptying time prolonged (P=0.004).\n\nConclusionGlucose-sensing secretin cells located in the distal part of the small intestine may contribute to increased plasma concentrations observed after RYGB. The metabolic role of the distal S-cells warrants further studies.

physiology↗

bioRxiv2018

Histone deposition plays an active role in recombination-dependent replication to balance genome2 stability upon replication stress

Replication stress poses a serious threat to genome and epigenome stability. Recombination-Dependent-Replication (RDR) ensures DNA synthesis resumption from arrested forks. Despite the identification of chromatin restoration pathways during DNA repair processes, crosstalk coupling RDR and chromatin assembly is largely unexplored. Here, we addressed the contribution of chromatin assembly to replication stress in fission yeast. We expressed a mutated histone (H3-H113D) to genetically impair replication-dependent chromatin assembly by destabilizing (H3-H4)2 tetramer. We established that DNA synthesis-dependent histone deposition is required for the completion of RDR. Histone deposition prevents joint-molecules from Rqh1-dependent disassembly, a crosstalk contributing to cell survival upon replication stress but channeling RDR towards deleterious events. Asf1 and CAF-1 act in RDR and CAF-1 recruitment to DNA synthesis associated to RDR requires the HR factor Rad52. Our data establish that CAF-1 counteracts Rqh1 activity at sites of replication stress by promoting repair synthesis-coupled histone deposition. These results demonstrate that histone deposition plays an active role in fine-tuning RDR, a benefit counterbalanced by stabilizing at-risk joint molecules for genome stability.

genetics↗