Group living in highland tuco-tucos (Ctenomys opimus) persists despite a catastrophic decline in population density.

Identifying the factors that favor group living is central to studies of animal social behavior. One demographic parameter that is expected to substantially shape spatial and social relationships is population density. Specifically, high population densities may favor group living by constraining opportunities to live alone. In contrast, low densities may allow individuals to spread out within the habitat, leading to a reduction in the prevalence or size of social groups. Abrupt changes in density following natural catastrophic events provide important opportunities to evaluate the effects of population density on patterns of spatial and social organization. As part of long-term studies of the behavioral ecology of a population of highland tuco-tucos (Ctenomys opimus) at Monumento Natural Laguna de los Pozuelos, Jujuy Province, Argentina, we monitored the demographic and behavioral consequences of a flood that inundated our study site during December 2012. Unlike most species of Ctenomys studied to date, highland tuco-tucos are group living, meaning that multiple adults share burrow systems and nest sites. Despite a post-flood reduction in population density of ~75%, animals present on the study site during the 2013 breeding season continued to live in multi-adult social units (groups). No differences between pre- and post-flood home range sizes were detected and although between-unit spatial overlap was reduced in 2013, overlap within social units did not differ from that in pre-flood years. Animals assigned to the same social unit in 2013 had not lived together during 2012, indicating that post-flood groups were not simply the remnants of those present prior to the flood. Collectively, these findings indicate that group living in highland tuco-tucos is not driven by the density of conspecifics in the habitat. In addition to enhancing understanding of the adaptive bases for group living in Ctenomys, our analyses underscore the power of catastrophic events to generate insights into fundamental aspects of social behavior.

Robustness to leg loss in Opiliones: A review and framework considerations for future research.

Animals have evolved behavioral and morphological traits that allow them to respond to environmental challenges. However, these traits may have long-term consequences that could impact an animal's performance, fitness, and welfare. Several species in a group of the arachnid order of Opiliones release their legs voluntarily to escape predators. These animals use their legs for locomotion, sensation, and reproduction. Here, we first compile data across species in the suborder Eupnoi, showing that more than half of individuals are found missing legs. Then, we review recent work on the ultimate and proximate implications of leg loss in Opiliones. Field and laboratory experiments showed that leg loss (a) did not affect their survival or mating success and (b) compromised the kinematics and energetics of locomotion, but individuals recovered velocity and acceleration quickly. These findings demonstrate that these animals display robustness, i.e., the ability to withstand and overcome the potential consequences of bodily damage. This may explain why leg loss is so common and prevalent in Opiliones. Additionally, we encourage researchers to consider expanding their hypotheses beyond traditional adaptationist and ableist lenses and incorporate a comprehensive examination of animal welfare when studying animals' responses to bodily damage. Finally, we highlight avenues for future research in Opiliones, namely assessing how individuals move in three-dimensional environments, the neural plasticity aiding recovery post-leg loss, applications for bio-inspired design, and evidence-based animal welfare measures.

SARS-CoV-2 Disease Severity and Cycle Threshold Values in Children Infected during Pre-Delta, Delta, and Omicron Periods, Colorado, USA, 2021-2022.

In adults, viral load and disease severity can differ by SARS-CoV-2 variant, patterns less understood in children. We evaluated symptomatology, cycle threshold (Ct) values, and SARS-CoV-2 variants among 2,299 pediatric SARS-CoV-2 patients (0-21 years of age) in Colorado, USA, to determine whether children infected with Delta or Omicron had different symptom severity or Ct values than during earlier variants. Children infected during the Delta and Omicron periods had lower Ct values than those infected during pre-Delta, and children <1 year of age had lower Ct values than older children. Hospitalized symptomatic children had lower Ct values than asymptomatic patients. Compared with pre-Delta, more children infected during Delta and Omicron were symptomatic (75.4% pre-Delta, 95.3% Delta, 99.5% Omicron), admitted to intensive care (18.8% pre-Delta, 39.5% Delta, 22.9% Omicron), or received oxygen support (42.0% pre-Delta, 66.3% Delta, 62.3% Omicron). Our data reinforce the need to include children, especially younger children, in pathogen surveillance efforts.

Lessons Learned from Cross-Systems Approach to COVID-19 Pandemic Response in Juvenile Justice System, Colorado, USA.

The global COVID-19 pandemic illustrates the importance of a close partnership between public health and juvenile justice systems when responding to communicable diseases. Many setting-specific obstacles must be navigated to respond effectively to limit disease transmission and negative health outcomes while maintaining necessary services for youth in confinement facilities. The response requires multidisciplinary expertise and collaboration to address unique considerations. Public health mitigation strategies must balance the risk for disease against the negative effects of restrictions. Key aspects of the COVID-19 response in the juvenile justice system of Colorado, USA, involved establishing robust communication and data reporting infrastructures, building a multidisciplinary response team, adapting existing infection prevention guidelines, and focusing on a whole-person health approach to infection prevention. We examine lessons learned and offer recommendations on pandemic emergency response planning and managing a statewide public health emergency in youth confinement settings that ensure ongoing readiness.

A randomized controlled trial of graded exposure treatment (GET living) for adolescents with chronic pain.

ABSTRACT: Graded exposure treatment (GET) is a theory-driven pain treatment that aims to improve functioning by exposing patients to activities previously feared and avoided. Combining key elements of GET with acceptance-based exposure, GET Living (GL) was developed for adolescents with chronic pain (GL). Based on robust treatment effects observed in our single-case experimental design pilot trial of GL (NCT01974791), we conducted a 2-arm randomized clinical trial comparing GL with multidisciplinary pain management (MPM) comprised of cognitive behavioral therapy and physical therapy for pain management (NCT03699007). A cohort of 68 youth with chronic musculoskeletal pain (M age 14.2 years; 81% female) were randomized to GL or MPM. Owing to COVID-19 restrictions, 54% of participants received zoom video delivered care. Assessments were collected at baseline, discharge, as well as at 3-month and 6-month follow-up. Primary outcomes were self-reported pain-related fear and avoidance. Secondary outcomes were child functional disability and parent protective responses to child pain. As hypothesized, GL improved in primary and secondary outcomes at 3-month follow-up. Contrary to our superiority hypothesis, there was no significant difference between GL and MPM. Patients reported both GL and MPM (in person and video) as credible and were highly satisfied with the treatment experience. Next steps will involve examining the single-case experimental design data embedded in this trial to facilitate an understanding of individual differences in treatment responses (eg, when effects occurred, what processes changed during treatment within the treatment arm). The current findings support GET Living and MPM for youth with chronic pain.

Cardiac human bitter taste receptors contain naturally occurring variants that alter function.

Bitter taste receptors (T2R) are a subfamily of G protein-coupled receptors that enable humans to detect aversive and toxic substances. The ability to discern bitter compounds varies between individuals and is attributed mainly to naturally occurring T2R polymorphisms. T2Rs are also expressed in numerous non-gustatory tissues, including the heart, indicating potential contributions to cardiovascular physiology. In this study. T2Rs that have previously been identified in human cardiac tissues (T2Rs - 10, 14, 30, 31, 46 and 50) and their naturally occurring polymorphisms were functionally characterised. The ligand-dependent signaling responses of some T2R variants were completely abolished (T2R30 Leu252 and T2R46 Met228), whereas other receptor variants had moderate changes in their maximal response, but not potency, relative to wild type. Using a cAMP fluorescent biosensor, we reveal the productive coupling of T2R14, but not the T2R14 Phe201 variant, to endogenous Gαi. Modeling revealed that these variants resulted in altered interactions that generally affected ligand binding (T2R30 Leu252) or Gα protein interactions (T2R46 Met228 and T2R14 Phe201), rather than receptor structural stability. Interestingly, this study is the first to show a difference in signaling for T2R50 Tyr203 (rs1376251) which has been associated with cardiovascular disease. The observation of naturally occurring functional variation in the T2Rs with the greatest expression in the heart is important, as their discovery should prove useful in deciphering the role of T2Rs within the cardiovascular system.

Plasma membrane preassociation drives ß-arrestin coupling to receptors and activation.

ß-arrestin plays a key role in G protein-coupled receptor (GPCR) signaling and desensitization. Despite recent structural advances, the mechanisms that govern receptor-ß-arrestin interactions at the plasma membrane of living cells remain elusive. Here, we combine single-molecule microscopy with molecular dynamics simulations to dissect the complex sequence of events involved in ß-arrestin interactions with both receptors and the lipid bilayer. Unexpectedly, our results reveal that ß-arrestin spontaneously inserts into the lipid bilayer and transiently interacts with receptors via lateral diffusion on the plasma membrane. Moreover, they indicate that, following receptor interaction, the plasma membrane stabilizes ß-arrestin in a longer-lived, membrane-bound state, allowing it to diffuse to clathrin-coated pits separately from the activating receptor. These results expand our current understanding of ß-arrestin function at the plasma membrane, revealing a critical role for ß-arrestin preassociation with the lipid bilayer in facilitating its interactions with receptors and subsequent activation.

SARS-CoV-2 Seroprevalence Compared with Confirmed COVID-19 Cases among Children, Colorado, USA, May-July 2021.

To compare SARS-CoV-2 antibody seroprevalence among children with seropositive confirmed COVID-19 case counts (case ascertainment by molecular amplification) in Colorado, USA, we conducted a cross-sectional serosurvey during May-July 2021. For a convenience sample of 829 Colorado children, SARS-CoV-2 seroprevalence was 36.7%, compared with prevalence of 6.5% according to individually matched COVID-19 test results reported to public health. Compared with non-Hispanic White children, seroprevalence was higher among Hispanic, non-Hispanic Black, and non-Hispanic other race children, and case ascertainment was significantly lower among Hispanic and non-Hispanic Black children. This serosurvey accurately estimated SARS-CoV-2 prevalence among children compared with confirmed COVID-19 case counts and revealed substantial racial/ethnic disparities in infections and case ascertainment. Continued efforts to address racial and ethnic differences in disease burden and to overcome potential barriers to case ascertainment, including access to testing, may help mitigate these ongoing disparities.

Activity-based anorexia in adolescent female rats causes changes in brain mitochondrial dynamics.

Anorexia Nervosa (AN) is associated with a high rate of morbidity and mortality as well as a high rate of relapse. The molecular mechanisms underlying the progression of the disorder or the relapses are largely unknown. Patients with AN have been shown to have increased oxidative stress, but its involvement in the development in the disease is unknown. We have previously shown that adolescent female rats undergoing the activity-based anorexia (ABA) paradigm also show signs of oxidative stress. Due to their role in the release of reactive oxygen species (ROS), mitochondria are of high interest in diseases exhibiting oxidative stress. In this study, the impact of ABA on brain mitochondrial dynamics was examined. We found transient changes in the medial prefrontal cortex, hypothalamus, and hippocampus following 25% weight loss and changes in the amygdala at a 10-day weight recovery timepoint. These changes point towards damage in the mitochondria contributing to the oxidative stress.

Heat shock factor HSF1 regulates BDNF gene promoters upon acute stress in the hippocampus, together with pCREB.

Heat shock factor 1 (HSF1) is a master stress-responsive transcriptional factor, protecting cells from death. However, its gene regulation in vivo in the brain in response to neuronal stimuli remains elusive. Here, we investigated its direct regulation of the brain-derived neurotrophic factor (BDNF) gene (Bdnf) in response to acute neuronal stress stimuli in the brain. The results of immunohistochemistry and chromatin immunoprecipitation quantitative PCR (ChIP-qPCR) showed that administration of kainic acid (a glutamate receptor agonist inducing excitotoxity) to young adult mice induced HSF1 nuclear translocation and its binding to multiple Bdnf promoters in the hippocampus. Footshock, a physical stressor used for learning, also induced HSF1 binding to selected Bdnf promoters I and IV. This is, to our knowledge, the first demonstration of HSF1 gene regulation in response to neuronal stimuli in the hippocampus in vivo. HSF1 binding sites (HSEs) in Bdnf promoters I and IV were also detected when immunoprecipitated by an antibody of phosphorylated (p)CREB (cAMP-responsive element-binding protein), suggesting their possible interplay in acute stress-induced Bdnf transcription. Interestingly, their promoter binding patterns differed by KA and footshock, suggesting that HSF1 and pCREB orchestrate to render fine-tuned promoter control depending on the types of stress. Further, HSF1 overexpression increased Bdnf promoter activity in a luciferase assay, while virus infection of constitutively active-form HSF1 increased levels of BDNF mRNA and protein in vitro in primary cultured neurons. These results indicated that HSF1 activation of Bdnf promoter was sufficient to induce BDNF expression. Taken together, these results suggest that HSF1 promoter-specific control of Bdnf gene regulation plays an important role in neuronal protection and plasticity in the hippocampus in response to acute stress, possibly interplaying with pCREB.

ADRA1A-Gαq signalling potentiates adipocyte thermogenesis through CKB and TNAP.

Noradrenaline (NA) regulates cold-stimulated adipocyte thermogenesis1. Aside from cAMP signalling downstream of ß-adrenergic receptor activation, how NA promotes thermogenic output is still not fully understood. Here, we show that coordinated α1-adrenergic receptor (AR) and ß3-AR signalling induces the expression of thermogenic genes of the futile creatine cycle2,3, and that early B cell factors, oestrogen-related receptors and PGC1α are required for this response in vivo. NA triggers physical and functional coupling between the α1-AR subtype (ADRA1A) and Gαq to promote adipocyte thermogenesis in a manner that is dependent on the effector proteins of the futile creatine cycle, creatine kinase B and tissue-non-specific alkaline phosphatase. Combined Gαq and Gαs signalling selectively in adipocytes promotes a continual rise in whole-body energy expenditure, and creatine kinase B is required for this effect. Thus, the ADRA1A-Gαq-futile creatine cycle axis is a key regulator of facultative and adaptive thermogenesis.

Knowledge of genetic test results among caregivers and individuals with spinal muscular atrophy.

Spinal muscular atrophy (SMA) is a progressive recessive genetic disease. Early identification is critical for achieving maximal treatment benefit. Survival motor neuron (SMN) 2 copy number may be a needed descriptor of disease severity than SMA type. Therefore, we assessed knowledge of SMN2 copy number among those with SMA and their caregivers via a phone survey. Only patients with SMA (or their caregivers) registered in the Cure SMA database with no SMN2 copy number on file were eligible. Descriptive results are reported. Backward stepwise multinomial logistic regressions determined if specific factors predicted knowledge of SMN2 copy number. Engagement with the SMA community (odds ratio [OR] 1.82; p<0.0001), ability to walk (OR 1.74; p = 0.006), and current age at time of survey (OR = 0.98; p<0.0001) each positively predicted knowledge of SMN2 copy number. Of 806 completed surveys, the majority (n = 452; 56.3%) did not know SMN2 copy numbers for themselves (n = 190; 62.5%) or their loved ones (n = 261; 52.4%). Of these, 66 respondents (8.2%) said genetic testing had not been done. Motor function increased linearly with increasing SMN2 copy number. SMN2 copy number is emerging as a critical descriptor of severity for SMA as type becomes more obsolete with early drug treatment. Communication of SMN2 copy numbers is recommended as a standard part of the treatment plan.

How Carvedilol activates ß2-adrenoceptors.

Carvedilol is among the most effective ß-blockers for improving survival after myocardial infarction. Yet the mechanisms by which carvedilol achieves this superior clinical profile are still unclear. Beyond blockade of ß1-adrenoceptors, arrestin-biased signalling via ß2-adrenoceptors is a molecular mechanism proposed to explain the survival benefits. Here, we offer an alternative mechanism to rationalize carvedilol's cellular signalling. Using primary and immortalized cells genome-edited by CRISPR/Cas9 to lack either G proteins or arrestins; and combining biological, biochemical, and signalling assays with molecular dynamics simulations, we demonstrate that G proteins drive all detectable carvedilol signalling through ß2ARs. Because a clear understanding of how drugs act is imperative to data interpretation in basic and clinical research, to the stratification of clinical trials or to the monitoring of drug effects on the target pathway, the mechanistic insight gained here provides a foundation for the rational development of signalling prototypes that target the ß-adrenoceptor system.

Two Cases of Monkeypox-Associated Encephalomyelitis - Colorado and the District of Columbia, July-August 2022.

Monkeypox virus (MPXV) is an orthopoxvirus in the Poxviridae family. The current multinational monkeypox outbreak has now spread to 96 countries that have not historically reported monkeypox, with most cases occurring among gay, bisexual, and other men who have sex with men (1,2). The first monkeypox case in the United States associated with this outbreak was identified in May 2022 in Massachusetts (1); monkeypox has now been reported in all 50 states, the District of Columbia (DC), and one U.S. territory. MPXV is transmitted by close contact with infected persons or animals; infection results in a febrile illness followed by a diffuse vesiculopustular rash and lymphadenopathy. However, illness in the MPXV current Clade II outbreak has differed: the febrile prodrome is frequently absent or mild, and the rash often involves genital, anal, or oral regions (3,4). Although neuroinvasive disease has been previously reported with MPXV infection (5,6), it appears to be rare. This report describes two cases of encephalomyelitis in patients with monkeypox disease that occurred during the current U.S. outbreak. Although neurologic complications of acute MPXV infections are rare, suspected cases should be reported to state, tribal, local, or territorial health departments to improve understanding of the range of clinical manifestations of and treatment options for MPXV infections during the current outbreak.

Rapid Transition to Virtual Assessment and Treatment in an Interdisciplinary Randomized Clinical Trial for Youth With Chronic Pain: Adaptations and Implications for Future Trials.

OBJECTIVES: COVID-19 abruptly halted in-person clinical care and research requiring a shift to virtual assessment and treatment. This unexpected transition of a 2-arm randomized controlled trial (RCT) examining interdisciplinary graded exposure treatment (GET Living) compared with multidisciplinary pain management for youth with chronic pain provided an opportunity to implement the first remotely delivered exposure treatment and remotely delivered biomechanical assessment for pediatric chronic pain. Here we describe these new approaches and provide lessons learned to inform future efforts in digital health care. METHODS: A total of 68 youth (M=14.2 y; 80.9% female) were enrolled in the RCT (n=31 in-person, n=5 hybrid, n=32 virtual, n=9 withdrew). Of those withdrawn, n=3 withdrew due to COVID-19 related reasons. Some RCT elements required slight modification (eg, e-consent, actigraphy deployment, recruitment, and screening), while others were significantly altered (eg, session format and lab-based biomechanical assessment). Data from exit interviews were also examined to assess perspectives on the virtual format transition. RESULTS: Results showed an increased enrollment rate when virtual care was an option (70.7%) compared with in-person (44.3%). Equivalent rates of completion for daily assessment (in-person, 72.8%; virtual, 73.3) were also observed, and participants described enhanced experience when able to complete exercises and exposures in their home environment during session (vs. a rehabilitation gym) allowing for genuine in vivo exposures (eg, household chores, riding bicycles). DISCUSSION: Overall, our data demonstrate acceptability, feasibility, and equivalent patient engagement to virtual treatment. Novel methods implemented in this RCT can inform trial design and measures of clinical endpoints for future digital health interventions.

Adolescent female rats recovered from the activity-based anorexia display blunted hedonic responding.

OBJECTIVE: As patients with anorexia nervosa tend to "like" palatable tastants less than controls, we set out to model this preclinically by using the taste reactivity test (TRT) to assess hedonic state in rats following weight restoration from a bout of activity-based anorexia (ABA). METHOD: Female rats (n = 31) were surgically implanted with an intraoral catheter, which allowed experimenters to assess baseline TRT to six tastants. Following baseline TRT, animals were either exposed to the activity-based anorexia condition (ABA; 1.5HR chow/ad lib wheel until 25% weight loss), kept sedentary (SED; ad lib chow/locked wheel), given access to running wheels with ad lib chow access (RW; ad lib chow/wheel), or were body weight matched to the ABA group (BWM; restricted chow/locked wheel). Following 25% weight loss, wheels were locked and food returned to ABA rats. Paired RW groups had their wheels locked and paired BWM rats were given ad lib access to food. Animals were given 10 days to recover prior to a second TRT. Videos were analyzed for liking (tongue protrusions) and disliking (gape) behaviors. RESULTS: The ABA group displayed a significant within-subject reduction in cumulative lick responses to water and 1 M sucrose. Additionally, we found the SED and ABA group displayed a significant within-subject reduction in cumulative lick responses to .1 M sucrose. Positive hedonic responses did not decline in either the BWM or the RW groups. DISCUSSION: The data show a novel phenomenon that a history of ABA results in an anhedonia phenotype that mirrors aspects of AN. SIGNIFICANCE STATEMENT: Patients recovered from anorexia nervosa report anhedonia, or the lack of pleasure in consuming palatable foods. Unfortunately, the biological mechanism underpinning anhedonia in anorexia nervosa is not well understood. The current study assessed hedonic state in adolescent female rats prior to and 10 days recovered following the activity-based anorexia paradigm. Age-matched, running wheel-matched and body weight-matched control groups were also tested at the same time points.

Sex, not social behavior, predicts fecal glucocorticoid metabolite concentrations in a facultatively social rodent, the highland tuco-tuco (Ctenomys opimus).

Social relationships may influence circulating glucocorticoid levels, particularly in group-living species in which individuals regularly engage in interactions with conspecifics. The effects of such interactions appear to vary, with greater social contact being associated with increased glucocorticoid concentrations in some species but decreased concentrations in others. These distinct responses raise intriguing questions regarding relationships among social behavior, individual phenotypes, and glucocorticoid physiology. To explore such relationships in a free-living mammal with a dynamic social organization, we quantified variation in baseline glucocorticoids in a population of highland tuco-tucos (Ctenomys opimus) from Jujuy Province, Argentina. These subterranean rodents are facultatively social, with lone and group-living individuals regularly occurring within the same population. To assess potential endocrine correlates of this behavioral variability, we examined differences in baseline fecal glucocorticoid metabolite (fGCm) concentrations as a function of social group size and composition as well as several metrics of social behavior derived from social network analyses. Despite marked variability in social relationships among the 37 (12 male, 25 female) free-living tuco-tucos sampled, none of the measures of social behavior examined were significant predictors of variation in fGCm concentrations. In contrast, individual variation in glucocorticoid metabolites was best explained by sex, with males having higher fGCm concentrations than females. These analyses provide the first characterization of the glucocorticoid physiology of highland tuco-tucos and underscore the potential importance of intrinsic phenotypic factors (e.g., sex) in shaping glucocorticoid variation in free-living mammals.

Role of miR-181c in Diet-induced obesity through regulation of lipid synthesis in liver.

We recently identified a nuclear-encoded miRNA (miR-181c) in cardiomyocytes that can translocate into mitochondria to regulate mitochondrial gene mt-COX1 and influence obesity-induced cardiac dysfunction through the mitochondrial pathway. Because liver plays a pivotal role during obesity, we hypothesized that miR-181c might contribute to the pathophysiological complications associated with obesity. Therefore, we used miR-181c/d-/- mice to study the role of miR-181c in hepatocyte lipogenesis during diet-induced obesity. The mice were fed a high-fat (HF) diet for 26 weeks, during which indirect calorimetric measurements were made. Quantitative PCR (qPCR) was used to examine the expression of genes involved in lipid synthesis. We found that miR-181c/d-/- mice were not protected against all metabolic consequences of HF exposure. After 26 weeks, the miR-181c/d-/- mice had a significantly higher body fat percentage than did wild-type (WT) mice. Glucose tolerance tests showed hyperinsulinemia and hyperglycemia, indicative of insulin insensitivity in the miR-181c/d-/- mice. miR-181c/d-/- mice fed the HF diet had higher serum and liver triglyceride levels than did WT mice fed the same diet. qPCR data showed that several genes regulated by isocitrate dehydrogenase 1 (IDH1) were more upregulated in miR-181c/d-/- liver than in WT liver. Furthermore, miR-181c delivered in vivo via adeno-associated virus attenuated the lipogenesis by downregulating these same lipid synthesis genes in the liver. In hepatocytes, miR-181c regulates lipid biosynthesis by targeting IDH1. Taken together, the data indicate that overexpression of miR-181c can be beneficial for various lipid metabolism disorders.

Stimulation of the four isoforms of receptor tyrosine kinase ErbB4, but not ErbB1, confers cardiomyocyte hypertrophy.

Epidermal growth factor (EGF) receptors (ErbB1-ErbB4) promote cardiac development and growth, although the specific EGF ligands and receptor isoforms involved in growth/repair versus pathology remain undefined. We challenged ventricular cardiomyocytes with EGF-like ligands and observed that selective activation of ErbB4 (the receptor for neuregulin 1 [NRG1]), but not ErbB1 (the receptor for EGF, EGFR), stimulated hypertrophy. This lack of direct ErbB1-mediated hypertrophy occurred despite robust activation of extracellular-regulated kinase 1/2 (ERK) and protein kinase B. Hypertrophic responses to NRG1 were unaffected by the tyrosine kinase inhibitor (AG1478) at concentrations that are selective for ErbB1 over ErbB4. NRG1-induced cardiomyocyte enlargement was suppressed by small interfering RNA (siRNA) knockdown of ErbB4 and ErbB2, whereas ERK phosphorylation was only suppressed by ErbB4 siRNA. Four ErbB4 isoforms exist (JM-a/JM-b and CYT-1/CYT-2), generated by alternative splicing, and their expression declines postnatally and following cardiac hypertrophy. Silencing of all four isoforms in cardiomyocytes, using an ErbB4 siRNA, abrogated NRG1-induced hypertrophic promoter/reporter activity, which was rescued by coexpression of knockdown-resistant versions of the ErbB4 isoforms. Thus, ErbB4 confers cardiomyocyte hypertrophy to NRG1, and all four ErbB4 isoforms possess the capacity to mediate this effect.