Influenza Virus Z-RNAs Induce ZBP1-Mediated Necroptosis.

Influenza A virus (IAV) is a lytic RNA virus that triggers receptor-interacting serine/threonine-protein kinase 3 (RIPK3)-mediated pathways of apoptosis and mixed lineage kinase domain-like pseudokinase (MLKL)-dependent necroptosis in infected cells. ZBP1 initiates RIPK3-driven cell death by sensing IAV RNA and activating RIPK3. Here, we show that replicating IAV generates Z-RNAs, which activate ZBP1 in the nucleus of infected cells. ZBP1 then initiates RIPK3-mediated MLKL activation in the nucleus, resulting in nuclear envelope disruption, leakage of DNA into the cytosol, and eventual necroptosis. Cell death induced by nuclear MLKL was a potent activator of neutrophils, a cell type known to drive inflammatory pathology in virulent IAV disease. Consequently, MLKL-deficient mice manifest reduced nuclear disruption of lung epithelia, decreased neutrophil recruitment into infected lungs, and increased survival following a lethal dose of IAV. These results implicate Z-RNA as a new pathogen-associated molecular pattern and describe a ZBP1-initiated nucleus-to-plasma membrane "inside-out" death pathway with potentially pathogenic consequences in severe cases of influenza.

Necroptosis blockade prevents lung injury in severe influenza.

Severe influenza A virus (IAV) infections can result in hyper-inflammation, lung injury and acute respiratory distress syndrome1-5 (ARDS), for which there are no effective pharmacological therapies. Necroptosis is an attractive entry point for therapeutic intervention in ARDS and related inflammatory conditions because it drives pathogenic lung inflammation and lethality during severe IAV infection6-8 and can potentially be targeted by receptor interacting protein kinase 3 (RIPK3) inhibitors. Here we show that a newly developed RIPK3 inhibitor, UH15-38, potently and selectively blocked IAV-triggered necroptosis in alveolar epithelial cells in vivo. UH15-38 ameliorated lung inflammation and prevented mortality following infection with laboratory-adapted and pandemic strains of IAV, without compromising antiviral adaptive immune responses or impeding viral clearance. UH15-38 displayed robust therapeutic efficacy even when administered late in the course of infection, suggesting that RIPK3 blockade may provide clinical benefit in patients with IAV-driven ARDS and other hyper-inflammatory pathologies.

Molecular Coevolution of Nuclear and Nucleolar Localization Signals inside the Basic Domain of HIV-1 Tat.

During evolution, viruses had to adapt to an increasingly complex environment of eukaryotic cells. Viral proteins that need to enter the cell nucleus or associate with nucleoli possess nuclear localization signals (NLSs) and nucleolar localization signals (NoLSs) for nuclear and nucleolar accumulation, respectively. As viral proteins are relatively small, acquisition of novel sequences seems to be a more complicated task for viruses than for eukaryotes. Here, we carried out a comprehensive analysis of the basic domain (BD) of HIV-1 Tat to show how viral proteins might evolve with NLSs and NoLSs without an increase in protein size. The HIV-1 Tat BD is involved in several functions, the most important being the transactivation of viral transcription. The BD also functions as an NLS, although it is substantially longer than a typical NLS. It seems that different regions in the BD could function as NLSs due to its enrichment with positively charged amino acids. Additionally, the high positive net charge inevitably causes the BD to function as an NoLS through a charge-specific mechanism. The integration of NLSs and NoLSs into functional domains enriched with positively charged amino acids might be a mechanism that allows the condensation of different functional sequences in small protein regions and, as a result, reduces protein size, influencing the origin and evolution of NLSs and NoLSs in viruses. IMPORTANCE Here, we investigated the molecular mechanism of nuclear localization signal (NLS) and nucleolar localization signal (NoLS) integration into the basic domain of HIV-1 Tat (49RKKRRQRRR57) and found that these two supplementary functions (i.e., function of NLS and function of NoLS) are embedded in the basic domain amino acid sequence. The integration of NLSs and NoLSs into functional domains of viral proteins enriched with positively charged amino acids is a mechanism that allows the concentration of different functions within small protein regions. Integration of NLS and NoLS into functional protein domains might have influenced the viral evolution, as this could prevent an increase in the protein size.

ZBP1/DAI-Dependent Cell Death Pathways in Influenza A Virus Immunity and Pathogenesis.

Influenza A viruses (IAV) are members of the Orthomyxoviridae family of negative-sense RNA viruses. The greatest diversity of IAV strains is found in aquatic birds, but a subset of strains infects other avian as well as mammalian species, including humans. In aquatic birds, infection is largely restricted to the gastrointestinal tract and spread is through feces, while in humans and other mammals, respiratory epithelial cells are the primary sites supporting productive replication and transmission. IAV triggers the death of most cell types in which it replicates, both in culture and in vivo. When well controlled, such cell death is considered an effective host defense mechanism that eliminates infected cells and limits virus spread. Unchecked or inopportune cell death also results in immunopathology. In this review, we discuss the impact of cell death in restricting virus spread, supporting the adaptive immune response and driving pathogenesis in the mammalian respiratory tract. Recent studies have begun to shed light on the signaling pathways underlying IAV-activated cell death. These pathways, initiated by the pathogen sensor protein ZBP1 (also called DAI and DLM1), cause infected cells to undergo apoptosis, necroptosis, and pyroptosis. We outline mechanisms of ZBP1-mediated cell death signaling following IAV infection.

RIPK3-driven cell death during virus infections.

The programmed self-destruction of infected cells is a powerful antimicrobial strategy in metazoans. For decades, apoptosis represented the dominant mechanism by which the virus-infected cell was thought to undergo programmed cell death. More recently, however, new mechanisms of cell death have been described that are also key to host defense. One such mechanism in vertebrates is programmed necrosis, or "necroptosis", driven by receptor-interacting protein kinase 3 (RIPK3). Once activated by innate immune stimuli, including virus infections, RIPK3 phosphorylates the mixed lineage kinase domain-like protein (MLKL), which then disrupts cellular membranes to effect necroptosis. Emerging evidence demonstrates that RIPK3 can also mediate apoptosis and regulate inflammasomes. Here, we review studies on the mechanisms by which viruses activate RIPK3 and the pathways engaged by RIPK3 that drive cell death.

Proliferation, cancer, and aging-novel functions of the nucleolar methyltransferase fibrillarin?

Fibrillarin is an essential nucleolar protein that catalyzes the 2'-O-methylation of ribosomal RNAs. Recently, experimental data have begun to accumulate that suggest that fibrillarin can influence various cellular processes, development of pathological processes, and even aging. The exact mechanism by which fibrillarin can influence these processes has not been found, but some experimental data indicate that up- or downregulation of fibrillarin can modify the ribosome structure and, thus, causе an alteration in relative efficiency with which various mRNAs are translated. Here, we discuss recent studies on the potential roles of fibrillarin in the regulation of cell proliferation, cancer progression, and aging.

Continued Statin Prescriptions After Adverse Reactions and Patient Outcomes: A Cohort Study.

BACKGROUND: Many patients discontinue statin treatment, often after having a possible adverse reaction. The risks and benefits of continued statin therapy after an adverse reaction are not known. OBJECTIVE: To examine the relationship between continuation of statin therapy (any prescription within 12 months after an adverse reaction) and clinical outcomes. DESIGN: Retrospective cohort study. SETTING: Primary care practices affiliated with 2 academic medical centers. PARTICIPANTS: Patients with a presumed adverse reaction to a statin between 2000 and 2011. MEASUREMENTS: Information on adverse reactions to statins was obtained from structured electronic medical record data or natural-language processing of narrative provider notes. The primary composite outcome was time to a cardiovascular event (myocardial infarction or stroke) or death. RESULTS: Most (81%) of the adverse reactions to statins were identified from the text of electronic provider notes. Among 28 266 study patients, 19 989 (70.7%) continued receiving statin prescriptions after the adverse reaction. Four years after the presumed adverse event, the cumulative incidence of the composite primary outcome was 12.2% for patients with continued statin prescriptions, compared with 13.9% for those without them (difference, 1.7% [95% CI, 0.8% to 2.7%]; P < 0.001). In a secondary analysis of 7604 patients for whom a different statin was prescribed after the adverse reaction, 2014 (26.5%) had a documented adverse reaction to the second statin, but 1696 (84.2%) of those patients continued receiving statin prescriptions. LIMITATIONS: The risk for recurrent adverse reactions to statins could not be established for the entire sample. It was also not possible to determine whether patients actually took the statins. CONCLUSION: Continued statin prescriptions after an adverse reaction were associated with a lower incidence of death and cardiovascular events. PRIMARY FUNDING SOURCE: Chinese National Key Program of Clinical Science, National Natural Science Foundation of China, and Young Scientific Research Fund of Peking Union Medical College Hospital.

Discontinuation of statins in routine care settings: a cohort study.

BACKGROUND: Systematic data on discontinuation of statins in routine practice of medicine are limited. OBJECTIVE: To investigate the reasons for statin discontinuation and the role of statin-related events (clinical events or symptoms believed to have been caused by statins) in routine care settings. DESIGN: A retrospective cohort study. SETTING: Practices affiliated with Brigham and Women's Hospital and Massachusetts General Hospital in Boston. PATIENTS: Adults who received a statin prescription between 1 January 2000 and 31 December 2008. MEASUREMENTS: Information on reasons for statin discontinuations was obtained from a combination of structured electronic medical record entries and analysis of electronic provider notes by validated software. RESULTS: Statins were discontinued at least temporarily for 57 292 of 107 835 patients. Statin-related events were documented for 18 778 (17.4%) patients. Of these, 11 124 had statins discontinued at least temporarily; 6579 were rechallenged with a statin over the subsequent 12 months. Most patients who were rechallenged (92.2%) were still taking a statin 12 months after the statin-related event. Among the 2721 patients who were rechallenged with the same statin to which they had a statin-related event, 1295 were receiving the same statin 12 months later, and 996 of them were receiving the same or a higher dose. LIMITATIONS: Statin discontinuations and statin-related events were assessed in practices affiliated with 2 academic medical centers. Utilization of secondary data could have led to missing or misinterpreted data. Natural-language-processing tools used to compensate for the low (30%) proportion of reasons for statin discontinuation documented in structured electronic medical record fields are not perfectly accurate. CONCLUSION: Statin-related events are commonly reported and often lead to statin discontinuation. However, most patients who are rechallenged can tolerate statins long-term. This suggests that many of the statin-related events may have other causes, are tolerable, or may be specific to individual statins rather than the entire drug class. PRIMARY FUNDING SOURCE: National Library of Medicine, Diabetes Action Research and Education Foundation, and Chinese National Key Program of Clinical Science.

EXIST: EXamining rIsk of excesS adiposiTy-Machine learning to predict obesity-related complications.

Background: Obesity is associated with an increased risk of multiple conditions, ranging from heart disease to cancer. However, there are few predictive models for these outcomes that have been developed specifically for people with overweight/obesity. Objective: To develop predictive models for obesity-related complications in patients with overweight and obesity. Methods: Electronic health record data of adults with body mass index 25-80 kg/m2 treated in primary care practices between 2000 and 2019 were utilized to develop and evaluate predictive models for nine long-term clinical outcomes using a) Lasso-Cox models and b) a machine-learning method random survival forests (RSF). Models were trained on a training dataset and evaluated on a test dataset over 100 replicates. Parsimonious models of <10 variables were also developed using Lasso-Cox. Results: Over a median follow-up of 5.6 years, study outcome incidence in the cohort of 433,272 patients ranged from 1.8% for knee replacement to 11.7% for atherosclerotic cardiovascular disease. Harrell C-index averaged over replicates ranged from 0.702 for liver outcomes to 0.896 for death for RSF, and from 0.694 for liver outcomes to 0.891 for death for Lasso-Cox. The Harrell C-index for parsimonious models ranged from 0.675 for liver outcomes to 0.850 for knee replacement. Conclusions: Predictive modeling can identify patients at high risk of obesity-related complications. Interpretable Cox models achieve results close to those of machine learning methods and could be helpful for population health management and clinical treatment decisions.

Following the money: copy-paste of lifestyle counseling documentation and provider billing.

BACKGROUND: Evidence suggests that copy-pasted components of electronic notes may not reliably reflect the care delivered. Federal agencies have raised concerns that such components may be used to justify inappropriately inflated claims for reimbursement. It is not known whether copied information is used to justify higher evaluation and management (E&M) charges. METHODS: This retrospective cohort study aimed to assess the relationship between the level of evaluation and management (E&M) charges and the method of documentation (none, distinct or copied) of lifestyle counseling (diet, exercise and weight loss) for patients with diabetes mellitus. To determine the association, an ordered multinomial logistic regression model that corrected for clustering within individual providers and patients and adjusted for patient and encounter characteristics was utilized. E&M charge level served as the primary outcome variable. Patients were included if they were followed by primary care physicians affiliated with two academic hospitals for a minimum of two years between 01/01/2000 and 12/13/2009. RESULTS: Lifestyle counseling was documented in 65.4% of 155,168 primary care encounters of 16,164 patients. Copied counseling was identified in 12,527 encounters. In multivariable analysis higher E&M charges were associated with older patient age, longer notes, treatment with insulin, medication changes and acute complaints. However, copied lifestyle counseling was associated with a decrease of 70.5% in the odds of higher E&M charge levels when time spent on counseling (required to justify higher charges based on counseling) was recorded (p<0.0001). This finding is opposite to what would have been expected if the impetus for copied documentation of lifestyle counseling was an increase in submitted E&M charges. CONCLUSION: There is no evidence that copied documentation of lifestyle counseling is used to justify higher evaluation and management charges. Higher charges were generally associated with indicators of complexity of care.

Assessment of Sex Disparities in Nonacceptance of Statin Therapy and Low-Density Lipoprotein Cholesterol Levels Among Patients at High Cardiovascular Risk.

Importance: Many patients at high cardiovascular risk-women more commonly than men-are not receiving statins. Anecdotally, it is common for patients to not accept statin therapy recommendations by their clinicians. However, population-based data on nonacceptance of statin therapy by patients are lacking. Objectives: To evaluate sex disparities in nonacceptance of statin therapy and assess their association with low-density lipoprotein (LDL) cholesterol control. Design, Setting, and Participants: A retrospective cohort study was conducted from January 1, 2019, to December 31, 2022, of statin-naive patients with atherosclerotic cardiovascular disease, diabetes, or LDL cholesterol levels of 190 mg/dL (to convert to millimoles per liter, multiply by 0.0259) or more who were treated at Mass General Brigham between January 1, 2000, and December 31, 2018. Exposure: Recommendation of statin therapy by the patient's clinician, ascertained from the combination of electronic health record prescription data and natural language processing of electronic clinician notes. Main Outcomes and Measures: Time to achieve an LDL cholesterol level of less than 100 mg/dL. Results: Of 24 212 study patients (mean [SD] age, 58.8 [13.0] years; 12 294 women [50.8%]), 5308 (21.9%) did not accept the initial recommendation of statin therapy. Nonacceptance of statin therapy was more common among women than men (24.1% [2957 of 12 294] vs 19.7% [2351 of 11 918]; P < .001) and was similarly higher in every subgroup in the analysis stratified by comorbidities. In multivariable analysis, female sex was associated with lower odds of statin therapy acceptance (0.82 [95% CI, 0.78-0.88]). Patients who did vs did not accept a statin therapy recommendation achieved an LDL cholesterol level of less than 100 mg/dL over a median of 1.5 years (IQR, 0.4-5.5 years) vs 4.4 years (IQR, 1.3-11.1 years) (P < .001). In a multivariable analysis adjusted for demographic characteristics and comorbidities, nonacceptance of statin therapy was associated with a longer time to achieve an LDL cholesterol level of less than 100 mg/dL (hazard ratio, 0.57 [95% CI, 0.55-0.60]). Conclusions and Relevance: This cohort study suggests that nonacceptance of a statin therapy recommendation was common among patients at high cardiovascular risk and was particularly common among women. It was associated with significantly higher LDL cholesterol levels, potentially increasing the risk for cardiovascular events. Further research is needed to understand the reasons for nonacceptance of statin therapy by patients and to develop methods to ensure that all patients receive optimal therapy in accordance with their preferences and priorities.

Effectiveness of interruptive alerts in increasing application functionality utilization: a controlled trial.

Medical applications frequently contain a wide range of functionalities. Users are often unaware of all of the functionalities available. More effective ways of delivering information about available functionalities to the users are needed. We conducted a pseudo-randomized controlled trial to determine whether interruptive alerts will increase utilization of several functionalities by the users of the Pre-Admission Medication List (PAML) Builder application at two academic medical centers. In a log-linear model, alerts increased total utilization of the promoted functionalities per PAML built by 70% compared to the controls at the site level (p<0.0001). At the user level, frequency of utilization of the PAML Builder functionalities by individual users increased by 0.03 for every extra alert shown to the user (p<0.0001). Alerts led to a nearly 2-fold increase in utilization of the promoted functionalities. Interruptive alerts are an effective method of delivering information about application functionalities to users.

Reasons for discontinuing insulin and factors associated with insulin discontinuation in patients with type 2 diabetes mellitus: a real-world evidence study.

BACKGROUND: Evidence suggests that insulin therapy of patients with type 2 diabetes mellitus (T2DM) is frequently discontinued. However, the reasons for discontinuing insulin and factors associated with insulin discontinuation in this patient population are not well understood. METHODS: We conducted a retrospective cohort study of adults with T2DM prescribed insulin between 2010 and 2017 at Partners HealthCare. Reasons for discontinuing insulin and factors associated with insulin discontinuation were studied using electronic medical records (EMR) data. Natural language processing (NLP) was applied to identify reasons from unstructured clinical notes. Factors associated with insulin discontinuation were extracted from structured EMR data and evaluated using multivariable logistic regression. RESULTS: Among 7009 study patients, 2957 (42.2%) discontinued insulin within 12 months after study entry. Most patients who discontinued insulin (2121 / 71.7%) had reasons for discontinuation documented. The most common reasons were improving blood glucose control (33.2%), achieved weight loss (18.5%) and initiation of non-insulin diabetes medications (16.7%). In multivariable analysis adjusted for demographics and comorbidities, patients were more likely to discontinue either basal or bolus insulin if they were on a basal-bolus regimen (OR 1.6, 95% CI 1.3 to 1.8; p <  0.001) or were being seen by an endocrinologist (OR 2.6; 95% CI 2.2 to 3.0; p <  0.001). CONCLUSIONS: In this large real-world evidence study conducted in an area with a high penetration of health insurance, insulin discontinuation countenanced by healthcare providers was common. In most cases it was linked to achievement of glycemic control, achieved weight loss and initiation of other diabetes medications. Factors associated with and stated reasons for insulin discontinuation were different from those previously described for non-adherence to insulin therapy, identifying it as a distinct clinical phenomenon.

Patient-provider discussions of bariatric surgery and subsequent weight changes and receipt of bariatric surgery.

OBJECTIVE: The purpose of this study was to determine whether patients who discuss bariatric surgery with their providers are more likely to undergo the procedure and to lose weight. METHODS: A retrospective cohort study of adults with BMI ≥ 35 kg/m2 treated between 2000 and 2015 was conducted to analyze the relationship between a discussion of bariatric surgery in the first year after study entry and weight changes (primary outcome) and receipt of bariatric surgery (secondary outcome) over 2 years after study entry. Natural language processing was used to identify the documentation of bariatric surgery discussion in electronic provider notes. RESULTS: Out of 30,560 study patients, a total of 2,659 (8.7%) discussed bariatric surgery with their providers. The BMI of patients who discussed bariatric surgery decreased by 2.18 versus 0.21 for patients who did not (p < 0.001). In a multivariable analysis, patients who discussed bariatric surgery with their providers lost more weight (by 1.43 [change in BMI]; 95% CI: 1.29-1.57) and had greater odds (10.2; 95% CI: 9.0-11.6; p < 0.001) of undergoing bariatric surgery. CONCLUSIONS: Clinicians rarely discussed bariatric surgery with their patients. Patients who did have this discussion were more likely to lose weight and to undergo bariatric surgery.

"Summary Page": a novel tool that reduces omitted data in research databases.

BACKGROUND: Data entry errors are common in clinical research databases. Omitted data are of particular concern because they are more common than erroneously inserted data and therefore could potentially affect research findings. However, few affordable strategies for their prevention are available. METHODS: We have conducted a prospective observational study of the effect of a novel tool called "Summary Page" on the frequency of correction of omitted data errors in a radiation oncology research database between July 2008 and March 2009. "Summary Page" was implemented as an optionally accessed screen in the database that visually integrates key fields in the record. We assessed the frequency of omitted data on the example of the Date of Relapse field. We considered the data in this field to be omitted for all records that had empty Date of Relapse field and evidence of relapse elsewhere in the record. RESULTS: A total of 1,156 records were updated and 200 new records were entered in the database over the study period. "Summary Page" was accessed for 44% of all updated records and for 69% of newly entered records. Frequency of correction of the omitted date of cancer relapse was six-fold higher in records for which "Summary Page" was accessed (p = 0.0003). CONCLUSIONS: "Summary Page" was strongly associated with an increased frequency of correction of omitted data errors. Further, controlled, studies are needed to confirm this finding and elucidate its mechanism of action.

Prevalence and factors affecting home blood pressure documentation in routine clinical care: a retrospective study.

BACKGROUND: Home blood pressure (BP) is closely linked to patient outcomes. However, the prevalence of its documentation has not been examined. The objective of this study was to analyze the prevalence and factors affecting documentation of home BP in routine clinical care. METHODS: A retrospective study of 142,973 encounters of 9,840 hypertensive patients with diabetes from 2000 to 2005 was performed. The prevalence of recorded home BP and the factors associated with its documentation were analyzed. We assessed validity of home BP information by comparing the difference between home and office BP to previously published prospective studies. RESULTS: Home BP was documented in narrative notes for 2.08% of encounters where any blood pressure was recorded and negligibly in structured data (EMR flowsheets). Systolic and diastolic home BP in narrative notes were lower than office BP readings by 9.6 and 2.5 mm Hg, respectively (p < 0.0001 for both), consistent with prospective data. Probability of home BP documentation increased by 23.0% for each 10 mm Hg of office systolic BP (p < 0.0001), by 6.2% for each $10,000 in median income of zip code (p = 0.0055), and by 17.7% for each decade in the patient's age (p < 0.0001). CONCLUSIONS: Home BP readings provide a valid representation of the patient's condition, yet are seldom documented despite their potential utility in both patient care and research. Strong association between higher patient income and home BP documentation suggests that the cost of the monitors may be a limiting factor; reimbursement of home BP monitoring expenses should be pursued.

Origin of the nuclear proteome on the basis of pre-existing nuclear localization signals in prokaryotic proteins.

BACKGROUND: The origin of the selective nuclear protein import machinery, which consists of nuclear pore complexes and adaptor molecules interacting with the nuclear localization signals (NLSs) of cargo molecules, is one of the most important events in the evolution of eukaryotic cells. How proteins were selected for import into the forming nucleus remains an open question. RESULTS: Here, we demonstrate that functional NLSs may be integrated in the nucleotide-binding domains of both eukaryotic and prokaryotic proteins and may coevolve with these domains. CONCLUSION: The presence of sequences similar to NLSs in the DNA-binding domains of prokaryotic proteins might have created an advantage for nuclear accumulation of these proteins during evolution of the nuclear-cytoplasmic barrier, influencing which proteins accumulated and became compartmentalized inside the forming nucleus (i.e., the content of the nuclear proteome). REVIEWERS: This article was reviewed by Sergey Melnikov and Igor Rogozin. OPEN PEER REVIEW: Reviewed by Sergey Melnikov and Igor Rogozin. For the full reviews, please go to the Reviewers' comments section.

Necroptosis restricts influenza A virus as a stand-alone cell death mechanism.

Influenza A virus (IAV) activates ZBP1-initiated RIPK3-dependent parallel pathways of necroptosis and apoptosis in infected cells. Although mice deficient in both pathways fail to control IAV and succumb to lethal respiratory infection, RIPK3-mediated apoptosis by itself can limit IAV, without need for necroptosis. However, whether necroptosis, conventionally considered a fail-safe cell death mechanism to apoptosis, can restrict IAV-or indeed any virus-in the absence of apoptosis is not known. Here, we use mice selectively deficient in IAV-activated apoptosis to show that necroptosis drives robust antiviral immune responses and promotes effective virus clearance from infected lungs when apoptosis is absent. We also demonstrate that apoptosis and necroptosis are mutually exclusive fates in IAV-infected cells. Thus, necroptosis is an independent, "stand-alone" cell death mechanism that fully compensates for the absence of apoptosis in antiviral host defense.

The GAR domain integrates functions that are necessary for the proper localization of fibrillarin (FBL) inside eukaryotic cells.

Fibrillarin (FBL) is an essential nucleolar protein that participates in pre-rRNA methylation and processing. The methyltransferase domain of FBL is an example of an extremely well-conserved protein domain in which the amino acid sequence was not substantially modified during the evolution from Archaea to Eukaryota. An additional N-terminal glycine-arginine-rich (GAR) domain is present in the FBL of eukaryotes. Here, we demonstrate that the GAR domain is involved in FBL functioning and integrates the functions of the nuclear localization signal and the nucleolar localization signal (NoLS). The methylation of the arginine residues in the GAR domain is necessary for nuclear import but decreases the efficiency of nucleolar retention via the NoLS. The presented data indicate that the GAR domain can be considered an evolutionary innovation that integrates several functional activities and thereby adapts FBL to the highly compartmentalized content of the eukaryotic cell.

Switching of cardiac troponin I between nuclear and cytoplasmic localization during muscle differentiation.

The nuclear accumulation of proteins may depend on the presence of short targeting sequences, which are known as nuclear localization signals (NLSs). Here, we found that NLSs are predicted in some cytosolic proteins and examined the hypothesis that these NLSs may be functional under certain conditions. As a model, human cardiac troponin I (hcTnI) was used. After expression in cultured non-muscle or undifferentiated muscle cells, hcTnI accumulated inside nuclei. Several NLSs were predicted and confirmed by site-directed mutagenesis in hcTnI. Nuclear import occurred via the classical karyopherin-α/ß nuclear import pathway. However, hcTnI expressed in cultured myoblasts redistributed from the nucleus to the cytoplasm, where it was integrated into forming myofibrils after the induction of muscle differentiation. It appears that the dynamic retention of proteins inside cytoplasmic structures can lead to switching between nuclear and cytoplasmic localization.