Longitudinal Lower Airway Microbial Signatures of Acute Cellular Rejection in Lung Transplantation.

Rationale: Acute cellular rejection (ACR) after lung transplant is a leading risk factor for chronic lung allograft dysfunction. Prior studies have demonstrated dynamic microbial changes occurring within the allograft and gut that influence local adaptive and innate immune responses. However, the lung microbiome's overall impact on ACR risk remains poorly understood. Objectives: To evaluate whether temporal changes in microbial signatures were associated with the development of ACR. Methods: We performed cross-sectional and longitudinal analyses (joint modeling of longitudinal and time-to-event data and trajectory comparisons) of 16S rRNA gene sequencing results derived from lung transplant recipient lower airway samples collected at multiple time points. Measurements and Main Results: Among 103 lung transplant recipients, 25 (24.3%) developed ACR. In comparing samples acquired 1 month after transplant, subjects who never developed ACR demonstrated lower airway enrichment with several oral commensals (e.g., Prevotella and Veillonella spp.) than those with current or future (beyond 1 mo) ACR. However, a subgroup analysis of those who developed ACR beyond 1 month revealed delayed enrichment with oral commensals occurring at the time of ACR diagnosis compared with baseline, when enrichment with more traditionally pathogenic taxa was present. In longitudinal models, dynamic changes in α-diversity (characterized by an initial decrease and a subsequent increase) and in the taxonomic trajectories of numerous oral commensals were more commonly observed in subjects with ACR. Conclusions: Dynamic changes in the lower airway microbiota are associated with the development of ACR, supporting its potential role as a useful biomarker or in ACR pathogenesis.

Structural basis of chromatin regulation by histone variant H2A.Z.

The importance of histone variant H2A.Z in transcription regulation has been well established, yet its mechanism-of-action remains enigmatic. Conflicting evidence exists in support of both an activating and a repressive role of H2A.Z in transcription. Here we report cryo-electron microscopy (cryo-EM) structures of nucleosomes and chromatin fibers containing H2A.Z and those containing canonical H2A. The structures show that H2A.Z incorporation results in substantial structural changes in both nucleosome and chromatin fiber. While H2A.Z increases the mobility of DNA terminus in nucleosomes, it simultaneously enables nucleosome arrays to form a more regular and condensed chromatin fiber. We also demonstrated that H2A.Z's ability to enhance nucleosomal DNA mobility is largely attributed to its characteristic shorter C-terminus. Our study provides the structural basis for H2A.Z-mediated chromatin regulation, showing that the increase flexibility of the DNA termini in H2A.Z nucleosomes is central to its dual-functions in chromatin regulation and in transcription.

Multimodal opioid-sparing pain management after lung transplantation and the impact of liposomal bupivacaine intercostal nerve block.

Opioid analgesics are commonly used post-lung transplant, but have many side effects and are associated with worse outcomes. We conducted a retrospective review of all lung transplant recipients who were treated with a multimodal opioid-sparing pain protocol. The use of liposomal bupivacaine intercostal nerve block was variable due to hospital restrictions. The primary objective was to describe opioid requirements and patient-reported pain scores early post-lung transplant and to assess the impact of intraoperative liposomal bupivacaine intercostal nerve block. We treated 64 lung transplant recipients with our protocol. Opioid utilization decreased to a mean of 43 milligram oral morphine equivalents by postoperative day 4. Median pain scores peaked at 4 on postoperative day 1 and decreased thereafter. Only three patients were discharged home with opioids, all of whom were taking opioid agonist therapy pre-transplant for opioid use disorder. Patients who received liposomal bupivacaine intercostal nerve block in the operating room had a significant reduction in opioid consumption over postoperative day 1 through 4 (228 mg vs. 517 mg, P= .032). A multimodal opioid-sparing pain management protocol is feasible and resulted in weaning of opioids prior to hospital discharge.

One-year immunologic outcomes of lung transplantation utilizing hepatitis C-viremic donors.

Little is known about the effects of hepatitis C viremia on immunologic outcomes in the era of direct-acting antivirals. We conducted a prospective, single-arm trial of lung transplantation from hepatitis C-infected donors into hepatitis C-naïve recipients (n = 21). Recipients were initiated on glecaprevir-pibrentasvir immediately post-transplant and were continued on therapy for a total of 8 weeks. A control group of recipients of hepatitis C-negative lungs were matched 1:1 on baseline variables (n = 21). The primary outcome was the frequency of acute cellular rejection over 1-year post-transplant. Treatment with glecaprevir-pibrentasvir was well tolerated and resulted in viremia clearance after a median of 16 days of therapy (IQR 10-24 days). At one year, there was no difference in incidence of acute cellular rejection (71.4% vs. 85.7%, P = .17) or rejection requiring treatment (33.3% vs. 57.1%, P = .12). Mean cumulative acute rejection scores were similar between groups (.46 [SD ± .53] vs. .52 [SD ± .37], P = .67). Receipt of HCV+ organs was not associated with acute rejection on unadjusted Cox regression analysis (HR .55, 95% CI .28-1.11, P = .09), or when adjusted for risk factors known to be associated with acute rejection (HR .57, 95% CI .27-1.21, P = .14). Utilization of hepatitis C infected lungs with immediate treatment leads to equivalent immunologic outcomes at 1 year.

A Comparison of Prothrombin Complex Concentrate and Recombinant Activated Factor VII for the Management of Bleeding With Cardiac Surgery.

Bleeding following cardiac surgery that warrants transfusion of blood products is associated with significant complications, including increased mortality at 1 year following surgery. Factor concentrates, such as prothrombin complex concentrate (PCC), or recombinant activated factor VII (rFVIIa) have been used off-label for bleeding in cardiac surgery that is refractory to conventional therapy. The objective of this retrospective study is to assess the hemostatic effectiveness of 4-factor PCC or rFVIIa for bleeding after a broad range of cardiac surgeries. Patients were included if they were at least 18 years of age and had undergone cardiac surgery with bleeding requiring intervention with 4-factor PCC or rFVIIa. There were no differences observed in the number of packed red blood cells (4-factor PCC: 2 units vs. rFVIIa: 2 units), fresh frozen plasma (0 units vs. 1 unit) or platelet (2 units vs. 2 units) transfusions following the administration of 4-factor PCC or rFVIIa. The patients in the rFVIIa group, required more cryoprecipitate than those in the 4-factor PCC group (4-factor PCC: 2 units (range 0-6) vs. rFVIIa: 2 units (range 0-8), p = 0.03). There were no differences in secondary outcomes of chest tube output at 2, 6, 12 and 24 hours, nor was there a difference in reexploration rates or the median length of stay in the intensive care unit. Thromboembolic complications at 30 days were similar between the two groups (4-factor PCC: 13% vs. rFVIIa 26%, p = 0.08). The total median dose requirement for 4-factor PCC was 1000 units (15 units/kg) and 2 mg (20 mcg/kg) for rFVIIa. The results demonstrate feasibility of utilizing the minimum amount of drug in order to achieve a desired effect. Both 4-factor PCC and rFVIIa appear to be safe and effective options for the management of bleeding associated with cardiac surgery.

Clinical Use of Cangrelor After Percutaneous Coronary Intervention in Patients Requiring Mechanical Circulatory Support.

BACKGROUND: Patients with cardiogenic shock after percutaneous coronary intervention (PCI) may require mechanical circulatory support (MCS). The combination of dual antiplatelet therapy with cangrelor and continuous anticoagulation required for MCS may increase the risk of bleeding. OBJECTIVE: The objective of the study is to describe the complications and outcomes of patients who received cangrelor during MCS following PCI. METHODS: This is a single-center, retrospective, observational case series of 17 patients who received cangrelor while on MCS from June 2017 to September 2019. RESULTS: In a case series of 17 patients, 8 patients (47%) were supported with an Impella device and 4 patients (24%) with venoarterial (VA) extracorporeal membrane oxygenation (ECMO); 5 required (29%) concomitant VA ECMO and Impella support in the setting of cardiogenic shock. All patients received triple antithrombotic therapy with aspirin, heparin, and cangrelor. Cangrelor was commonly initiated at a median dose of 0.75 (range 0.5-4) µg/kg/min. Cangrelor dose adjustments included changes in increments up to 0.25 µg/kg/min with review of P2Y12 levels. A total of 10 patients (59%) experienced a bleeding event, most commonly located at the peripheral cannulation site (40%) and in the gastrointestinal tract (30%). Seven (70%) and 3 (30%) of the bleeding complications were classified as major and minor, respectively. No patient developed in-stent thrombosis during the hospitalization; 14 (82%) patients survived their MCS course. CONCLUSION AND RELEVANCE: This case series suggests that cangrelor doses less than 0.75 µg/kg/min may be beneficial. Larger studies should evaluate alternative dosing regimens.

Initial tacrolimus weight-based dosing strategy in allogeneic hematopoietic stem-cell transplantation.

Tacrolimus is a mainstay medication for graft-versus-host disease (GVHD) prophylaxis in combination with other immunosuppressive agents. Achieving therapeutic tacrolimus levels is vital in preventing acute GVHD (aGVHD), while supratherapeutic levels may increase risk of toxicity and relapse. We performed a single center retrospective chart review including all adult patients post-allogeneic hematopoietic stem-cell transplantation who received initial tacrolimus continuous intravenous infusion for GVHD prophylaxis between June 1, 2017 and December 31, 2019. The primary outcome was the percent of patients with an initial therapeutic tacrolimus level, defined as 5-12 ng/mL, after empiric weight-based dosing at 0.02 mg/kg/day. Secondary outcomes included evidence of tacrolimus toxicity within seven days of initiation, incidence of aGVHD by day 100, and relapse after six months. An initial therapeutic level was achieved in 47% of patients with a median initial level of 12.4 ng/mL. Fifty-two percent of patients had supratherapeutic levels. No significant nephrotoxicity, hepatotoxicity, or neurotoxicity occurred within a week of starting tacrolimus or at neutrophil engraftment. Grade II-IV aGVHD by day 100 was observed in 22% of patients, and relapse after six months was found in 16% of patients. These results have led to consideration of an empiric 20% dose reduction to 0.016 mg/kg/day or an expanded initial tacrolimus target of 5-15 ng/mL as there was low aGVHD incidence and no increased risk of toxicity.

Therapeutic blockade of inflammation in severe COVID-19 infection with intravenous N-acetylcysteine.

Glucose 6-phosphate dehydrogenase (G6PD) deficiency facilitates human coronavirus infection due to glutathione depletion. G6PD deficiency may especially predispose to hemolysis upon coronavirus disease-2019 (COVID-19) infection when employing pro-oxidant therapy. However, glutathione depletion is reversible by N-acetylcysteine (NAC) administration. We describe a severe case of COVID-19 infection in a G6PD-deficient patient treated with hydroxychloroquine who benefited from intravenous (IV) NAC beyond reversal of hemolysis. NAC blocked hemolysis and elevation of liver enzymes, C-reactive protein (CRP), and ferritin and allowed removal from respirator and veno-venous extracorporeal membrane oxygenator and full recovery of the G6PD-deficient patient. NAC was also administered to 9 additional respirator-dependent COVID-19-infected patients without G6PD deficiency. NAC elicited clinical improvement and markedly reduced CRP in all patients and ferritin in 9/10 patients. NAC mechanism of action may involve the blockade of viral infection and the ensuing cytokine storm that warrant follow-up confirmatory studies in the setting of controlled clinical trials.

Management and tolerability of glecaprevir-pibrentasvir pharmacotherapy in hepatitis C viremic heart and lung transplant recipients.

We conducted a retrospective review of thoracic transplant recipients (22 heart and 16 lung transplant recipients) prospectively enrolled in a single-center observational study of HCV NAT+ organ transplantation in HCV NAT- recipients. All recipients were treated with 8 weeks of glecaprevir-pibrentasvir (GP) for HCV viremia in addition to standard triple immunosuppression post-transplant. Thoracic transplant recipients of HCV NAT- organs were used as a control (24 heart and 22 lung transplant recipients). Our primary outcome was to assess the effect of GP on tacrolimus dose requirements. Secondary objectives included assessing drug interactions with common post-transplant medications, adverse effects, and the need to hold or discontinue GP therapy. The median tacrolimus concentration-to-dose ratio (CDR) in the cohort was 184 (99-260) during GP therapy and 154 (78-304) over the first month after GP (P = .79). Trends in median tacrolimus CDR were similar on a per-week basis and per-patient basis. In three instances, concomitant posaconazole and GP led to hyperbilirubinemia and interruption of posaconazole. GP therapy was held in one heart transplant recipient and discontinued in another due to unresolving hyperbilirubinemia. Utilization of GP to treat HCV viremia post-thoracic transplant is feasible and safe, but requires modifications to post-transplant pharmacotherapy and careful monitoring for adverse effects.

Outcomes of the Treatment with Glecaprevir/Pibrentasvir following heart transplantation utilizing hepatitis C viremic donors.

BACKGROUND: The use of direct-acting antivirals (DAA) has expanded transplantation from hepatitis C viremic donors (HCV-VIR). Our team has conducted an open-label, prospective trial to assess outcomes transplanting HCV viremic hearts. Glecaprevir/pibrentasvir (GLE/PIB) was our sole DAA. METHODS: Serial quantitative hepatitis C virus (HCV) RNA PCR was obtained to assess HCV viral titers. Between January 2018 and June 2019, a total of 50 recipients were transplanted. Of these, 22/50 (44%) were from HCV-VIR, the remaining 28 from non-viremic (HCV NON-VIR) donors. An 8-week course of GLE/PIB was initiated at 1 week post-transplant. RESULTS: There was no difference in demographic or clinical parameters between groups. All 22 recipients of HCV-VIR transplants became viremic. GLE/PIB was effective in decreasing viremia to undetectable levels by 6 weeks post-transplant in all patients. The median time to first undetectable HCV quantitative PCR was (4.3 weeks, IQR: 4-5.7 weeks). All patients demonstrated sustained undetectable viral load through 1-year follow-up. There was no difference in survival at one year between HCV NON-VIR 28/28: (100%) vs HCV-VIR 21/22 (95%) recipients. CONCLUSIONS: Our center reports excellent outcomes in transplanting utilizing hearts from HCV-VIR donors. No effect on survival or co-morbidity was found. An 8-week GLE/PIB course was safe and effective when initiated approximately 1 week post-transplant.

Sedation Requirements in Patients on Venovenous or Venoarterial Extracorporeal Membrane Oxygenation.

Background:There is a paucity of data evaluating optimal dosing strategies of commonly utilized opioids and sedatives for patients on extracorporeal membrane oxygenation (ECMO) support where pharmacokinetic and pharmacodynamic variables of these administered agents are altered. Objective: To assess the daily dosing requirement of sedatives and analgesics for patients on venovenous (VV) and venoarterial (VA) ECMO after the initial ECMO cannulation period. Methods: We performed a retrospective, observational study of adult patients receiving sedation and analgesia while receiving ECMO support for at least 24 hours. Patients cannulated at an outside hospital more than 24 hours before transfer, those with a history of intravenous drug use or acute alcohol withdrawal, or those who died within 48 hours of ECMO initiation were excluded. Results: We evaluated 26 patients on ECMO, including 13 on VV and 13 on VA ECMO. The median dose of fentanyl was 140 µg/h, with the VV group requiring a higher dose compared with the VA group (167 vs 106 µg/h, P < 0.001). The median doses of dexmedetomidine and propofol were 0.7 µg/kg/h and 26 µg/kg/min, respectively, with no significant differences between groups (P = 0.38 and P = 0.24, respectively). The median daily doses of fentanyl, dexmedetomidine, and propofol did not significantly increase throughout the time on ECMO support. Conclusions and Relevance: We found that the overall opioid daily dosing requirements were lower than previously reported in the literature. Additionally, light sedation strategies with a target RASS of -1 to 0 are feasible in this patient population.

Safety of the Peripheral Administration of Vasopressor Agents.

Vasopressors are an integral component of the management of septic shock and are traditionally given via a central venous catheter (CVC) due to the risk of tissue injury and necrosis if extravasated. However, the need for a CVC for the management of septic shock has been questioned, and the risk of extravasation and incidence of severe injury when vasopressors are given via a peripheral venous line (PVL) remains poorly defined. We performed a retrospective chart review of 202 patients who received vasopressors through a PVL. The objective was to describe the vasopressors administered peripherally, PVL size and location, the incidence of extravasation events, and the management of extravasation events. The primary vasopressors used were norepinephrine and phenylephrine. The most common PVL sites used were the forearm and antecubital fossa. The incidence of extravasation was 4%. All of the events were managed conservatively; none required an antidote or surgical management. Vasopressors were restarted at another peripheral site in 88% of the events. The incidence of extravasation was similar to prior studies. The use of a PVL for administration of vasopressors can be considered in patients with a contraindication to a CVC.

Venous Thromboembolism Prophylaxis: A Narrative Review With a Focus on the High-Risk Critically Ill Patient.

Venous thromboembolism (VTE) is a major health concern associated with significant morbidity and mortality. Critically ill patients are at an increased risk of VTE compared to general medical patients due to unique risk factors: prolonged immobilization, invasive lines and devices, certain medications, and acquired thrombophilia. Furthermore, VTE in the critically ill is associated with increased duration of mechanical ventilation, increased length of intensive care unit and hospital stay, and a trend toward increased mortality. Clinical practice guidelines therefore recommend VTE prophylaxis with either subcutaneous heparin or low-molecular-weight heparin for all critically ill patients without contraindication. Yet, many patients will develop VTE despite appropriate pharmacologic prophylaxis, which has led to interest in risk-stratifying critically ill patients for more aggressive prophylaxis strategies. Recent research identified patients at highest risk of failure of thromboprophylaxis and provided insight into the pathophysiologic mechanisms. Obesity and the receipt of vasopressors are 2 risk factors consistently identified in observational studies; further clinical data support decreased absorption of anticoagulant administered via the subcutaneous route as the likely mechanism behind thromboprophylaxis failure in these patient populations. Several studies have investigated novel thromboprophylaxis strategies to circumvent pharmacokinetic limitations in patients who are obese or on vasopressors: increased fixed-dose, weight-based subcutaneous, or continuous intravenous infusion of a prophylactic dose of anticoagulant has shown promise in limited studies; however, the results have yet to demonstrate superiority compared to current standard-of-care. This review discusses observational studies identifying patients at risk of thromboprophylaxis failure and critiques clinical studies evaluating novel thromboprophylaxis strategies in high-risk, critically ill patients with a focus on their limitations. Future studies are currently being conducted that will provide further guidance into the appropriate use of individualized thromboprophylaxis.

A single-center intervention to discontinue postoperative antibiotics after spinal fusion.

INTRODUCTION: Postoperative antibiotics (PA) are often administered to patients after instrumented spinal surgery until all drains are removed to prevent surgical site infections (SSI). This practice is discouraged by numerous medical society guidelines, so our institutional Neurosurgery Quality Improvement Committee decided to discontinue use of PA for this population. METHODS: We retrospectively reviewed data for patients who had instrumented spinal surgery at our institution for seven months before and after this policy change and compared the frequency of SSI and development of antibiotic related complications in patients who received PA to those who did not (non-PA). RESULTS: We identified 188 PA patients and 158 non-PA patients. Discontinuation of PA did not result in an increase in frequency of SSI (2% of PA patients vs. 0.6% of non-PA patients, p = .4). Growth of resistant bacteria was not significantly reduced in the non-PA period in comparison to the PA period (2% in the PA period and 1% in the non-PA period). The cost of antibiotics for PA patients was $5,499.62, whereas the cost of antibiotics for the non-PA patients was $0. On a per patient basis, the cost associated with antibiotics and resistant infections was significantly greater for patients who received PA than for those who did not (median of $26.32 with IQR $9.87-$46.06 vs. median of $0 with IQR $0-$0; p < .0001). CONCLUSION: After discontinuing PA for patients who had instrumented spinal procedures, we did not observe an increase in the frequency of SSI. We did, however, note that there was a non-significant decrease in the frequency of growth of resistant organisms. These findings suggest that patients in this population do not need PA, and complications can be reduced if PA are withheld.

Multilayer Model of Pharmacy Participation in the Antimicrobial Stewardship Program at a Large Academic Medical Center.

Purpose: Leveraging pharmacy personnel resources for the purpose of antimicrobial stewardship program (ASP) operations presents a challenging task. We describe our experience integrating all pharmacists into an ASP, and evaluate the impact on ASP interventions, antimicrobial utilization, rate of selected hospital-onset infections and readmission. Summary: During a study period (January 1 to December 31, 2015), a total of 14 552 ASP-related pharmacy interventions were performed (ASP clinical pharmacotherapy specialists [CPS] n = 4025; non-ASP CPS n = 4888; hospital pharmacists n = 5639). Sixty percent of interventions by ASP CPS were initiated utilizing the dedicated ASP phone, and 40% through prospective audit and feedback. Non-ASP CPS performed interventions during bedside rounds (dose adjustment 23%, initiate new or alternative anti-infective 21%, discontinue antibiotic(s) 12%, therapeutic drug monitoring 11%, de-escalation 4%), whereas hospital pharmacists participated at the point of verification (dose adjustment 75%, restricted antibiotic verification 15%, and reporting major drug-drug interactions 4%). The acceptance rate of interventions by providers and clinicians was >90% for all groups. Annual aggregate antimicrobial use decreased by 6.4 days of therapy/1000 patient-days (DOT/1000 PD; P = 1.0). Ceftriaxone use increased by 8.4 DOT/1000 PD (P = .029) without a significant compensatory increase in the use of antipseudomonal agents. Sustained low rates of hospital-onset Clostridium difficile (CDI) and carbapenem-resistant Enterobacteriaceae (CRE) infections were observed in 2015 compared with the prior year (1.1 and 1.2 cases/1000 PD, 0.2 and 0.1 cases/1000 PD, respectively). Thirty-day readmission rate decreased by 0.6% (P = .019). Conclusions: Integration of all pharmacists into ASP activities based on the level of patient care and responsibilities is an effective strategy to expand clinical services provided by ASP.

Trophic dynamics of shrinking Subarctic lakes: naturally eutrophic waters impart resilience to rising nutrient and major ion concentrations.

Shrinking lakes were recently observed for several Arctic and Subarctic regions due to increased evaporation and permafrost degradation. Along with lake drawdown, these processes often boost aquatic chemical concentrations, potentially impacting trophic dynamics. In particular, elevated chemical levels may impact primary productivity, which may in turn influence populations of primary and secondary consumers. We examined trophic dynamics of 18 shrinking lakes of the Yukon Flats, Alaska, that had experienced pronounced increases in nutrient (>200 % total nitrogen, >100 % total phosphorus) and ion concentrations (>100 % for four major ions combined) from 1985-1989 to 2010-2012, versus 37 stable lakes with relatively little chemical change over the same period. We found that phytoplankton stocks, as indexed by chlorophyll concentrations, remained unchanged in both shrinking and stable lakes from the 1980s to 2010s. Moving up the trophic ladder, we found significant changes in invertebrate abundance across decades, including decreased abundance of five of six groups examined. However, these decadal losses in invertebrate abundance were not limited to shrinking lakes, occurring in lakes with stable surface areas as well. At the top of the food web, we observed that probabilities of lake occupancy for ten waterbird species, including adults and chicks, remained unchanged from the period 1985-1989 to 2010-2012. Overall, our study lakes displayed a high degree of resilience to multi-trophic cascades caused by rising chemical concentrations. This resilience was likely due to their naturally high fertility, such that further nutrient inputs had little impact on waters already near peak production.

Pronounced chemical response of Subarctic lakes to climate-driven losses in surface area.

Losses in lake area have been observed for several Arctic and Subarctic regions in recent decades, with unknown consequences for lake ecosystems. These reductions are primarily attributed to two climate-sensitive mechanisms, both of which may also cause changes in water chemistry: (i) increased imbalance of evaporation relative to inflow, whereby increased evaporation and decreased inflow act to concentrate solutes into smaller volumes; and (ii) accelerated permafrost degradation, which enhances sublacustrine drainage while simultaneously leaching previously frozen solutes into lakes. We documented changes in nutrients [total nitrogen (TN), total phosphorus (TP)] and ions (calcium, chloride, magnesium, sodium) over a 25 year interval in shrinking, stable, and expanding Subarctic lakes of the Yukon Flats, Alaska. Concentrations of all six solutes increased in shrinking lakes from 1985-1989 to 2010-2012, while simultaneously undergoing little change in stable or expanding lakes. This created a present-day pattern, much weaker or absent in the 1980s, in which shrinking lakes had higher solute concentrations than their stable or expanding counterparts. An imbalanced evaporation-to-inflow ratio (E/I) was the most likely mechanism behind such changes; all four ions, which behave semiconservatively and are prone to evapoconcentration, increased in shrinking lakes and, along with TN and TP, were positively related to isotopically derived E/I estimates. Moreover, the most conservative ion, chloride, increased >500% in shrinking lakes. Conversely, only TP concentration was related to probability of permafrost presence, being highest at intermediate probabilities. Overall, the substantial increases of nutrients (TN >200%, TP >100%) and ions (>100%) may shift shrinking lakes towards overly eutrophic or saline states, with potentially severe consequences for ecosystems of northern lakes.

Multi-trophic resilience of boreal lake ecosystems to forest fires.

Fires are the major natural disturbance in the boreal forest, and their frequency and intensity will likely increase as the climate warms. Terrestrial nutrients released by fires may be transported to boreal lakes, stimulating increased primary productivity, which may radiate through multiple trophic levels. Using a before-after-control-impact (BACI) design, with pre- and postfire data from burned and unburned areas, we examined effects of a natural fire across several trophic levels of boreal lakes, from nutrient and chlorophyll levels, to macroinvertebrates, to waterbirds. Concentrations of total nitrogen and phosphorus were not affected by the fire. Chlorophyll a levels were also unaffected, likely reflecting the stable nutrient concentrations. For aquatic invertebrates, we found that densities of three functional feeding groups did not respond to the fire (filterers, gatherers, scrapers), while two groups increased (shredders, predators). Amphipods accounted for 98% of shredder numbers, and we hypothesize that fire-mediated habitat changes may have favored their generalist feeding and habitat ecology. This increase in amphipods may, in turn, have driven increased predator densities, as amphipods were the most numerous invertebrate in our lakes and are commonly taken as prey. Finally, abundance of waterbird young, which feed primarily on aquatic invertebrates, was not affected by the fire. Overall, ecosystems of our study lakes were largely resilient to forest fires, likely due to their high initial nutrient concentrations and small catchment sizes. Moreover, this resilience spanned multiple trophic levels, a significant result for ecologically similar boreal regions, especially given the high potential for increased fires with future climate change.

Structural mechanism of HP1α-dependent transcriptional repression and chromatin compaction.

Heterochromatin protein 1 (HP1) plays a central role in establishing and maintaining constitutive heterochromatin. However, the mechanisms underlying HP1-nucleosome interactions and their contributions to heterochromatin functions remain elusive. In this study, we employed a multidisciplinary approach to unravel the interactions between human HP1α and nucleosomes. We have elucidated the cryo-EM structure of an HP1α dimer bound to an H2A.Z nucleosome, revealing that the HP1α dimer interfaces with nucleosomes at two distinct sites. The primary binding site is located at the N-terminus of histone H3, specifically at the trimethylated K9 (K9me3) region, while a novel secondary binding site is situated near histone H2B, close to nucleosome superhelical location 4 (SHL4). Our biochemical data further demonstrates that HP1α binding influences the dynamics of DNA on the nucleosome. It promotes DNA unwrapping near the nucleosome entry and exit sites while concurrently restricting DNA accessibility in the vicinity of SHL4. This study offers a model that explains how HP1α functions in heterochromatin maintenance and gene silencing, particularly in the context of H3K9me-dependent mechanisms. Additionally, it sheds light on the H3K9me-independent role of HP1 in responding to DNA damage.