Steady-state and regenerative hematopoiesis occurs normally in mice in the absence of GDF11.

Tightly regulated production of mature blood cells is essential for health and survival in vertebrates and dependent on discrete populations of blood-forming (hematopoietic) stem and progenitor cells. Prior studies suggested that inhibition of growth differentiation factor 11 (GDF11) through soluble activin receptor type II (ActRII) ligand traps or neutralizing antibodies promotes erythroid precursor cell maturation and red blood cell formation in contexts of homeostasis and anemia. As Gdf11 is expressed by mature hematopoietic cells, and erythroid precursor cell expression of Gdf11 has been implicated in regulating erythropoiesis, we hypothesized that genetic disruption of Gdf11 in blood cells might perturb normal hematopoiesis or recovery from hematopoietic insult. Contrary to these predictions, we found that deletion of Gdf11 in the hematopoietic lineage in mice does not alter erythropoiesis or erythroid precursor cell frequency under normal conditions or during hematopoietic recovery after irradiation and transplantation. In addition, although hematopoietic cell-derived Gdf11 may contribute to the pool of circulating GDF11 protein during adult homeostasis, loss of Gdf11 specifically in the blood system does not impair hematopoietic stem cell function or induce overt pathological consequences. Taken together, these results reveal that hematopoietic cell-derived Gdf11 is largely dispensable for native and transplant-induced blood formation.

Dysregulation of IL-33/ST2 signaling and myocardial periarteriolar fibrosis.

Microvascular dysfunction in the heart and its association with periarteriolar fibrosis may contribute to the diastolic dysfunction seen in heart failure with preserved ejection fraction. Interleukin-33 (IL-33) prevents global myocardial fibrosis in a pressure overloaded left ventricle by acting via its receptor, ST2 (encoded by the gene, Il1rl1); however, whether this cytokine can also modulate periarteriolar fibrosis remains unclear. We utilized two approaches to explore the role of IL-33/ST2 in periarteriolar fibrosis. First, we studied young and old wild type mice to test the hypothesis that IL-33 and ST2 expression change with age. Second, we produced pressure overload in mice deficient in IL-33 or ST2 by transverse aortic constriction (TAC). With age, IL-33 expression increased and ST2 expression decreased. These alterations accompanied increased periarteriolar fibrosis in aged mice. Mice deficient in ST2 but not IL-33 had a significant increase in periarteriolar fibrosis following TAC compared to wild type mice. Thus, loss of ST2 signaling rather than changes in IL-33 expression may contribute to periarteriolar fibrosis during aging or pressure overload, but manipulating this pathway alone may not prevent or reverse fibrosis.

Analysis of Cre-mediated genetic deletion of Gdf11 in cardiomyocytes of young mice.

Administration of active growth differentiation factor 11 (GDF11) to aged mice can reduce cardiac hypertrophy, and low serum levels of GDF11 measured together with the related protein, myostatin (also known as GDF8), predict future morbidity and mortality in coronary heart patients. Using mice with a loxP-flanked ("floxed") allele of Gdf11 and Myh6-driven expression of Cre recombinase to delete Gdf11 in cardiomyocytes, we tested the hypothesis that cardiac-specific Gdf11 deficiency might lead to cardiac hypertrophy in young adulthood. We observed that targeted deletion of Gdf11 in cardiomyocytes does not cause cardiac hypertrophy but rather leads to left ventricular dilation when compared with control mice carrying only the Myh6-cre or Gdf11-floxed alleles, suggesting a possible etiology for dilated cardiomyopathy. However, the mechanism underlying this finding remains unclear because of multiple confounding effects associated with the selected model. First, whole heart Gdf11 expression did not decrease in Myh6-cre; Gdf11-floxed mice, possibly because of upregulation of Gdf11 in noncardiomyocytes in the heart. Second, we observed Cre-associated toxicity, with lower body weights and increased global fibrosis, in Cre-only control male mice compared with flox-only controls, making it challenging to infer which changes in Myh6-cre;Gdf11-floxed mice were the result of Cre toxicity versus deletion of Gdf11. Third, we observed differential expression of cre mRNA in Cre-only controls compared with the cardiomyocyte-specific knockout mice, also making comparison between these two groups difficult. Thus, targeted Gdf11 deletion in cardiomyocytes may lead to left ventricular dilation without hypertrophy, but alternative animal models are necessary to understand the mechanism for these findings. NEW & NOTEWORTHY We observed that targeted deletion of growth differentiation factor 11 in cardiomyocytes does not cause cardiac hypertrophy but rather leads to left ventricular dilation compared with control mice carrying only the Myh6-cre or growth differentiation factor 11-floxed alleles. However, the mechanism underlying this finding remains unclear because of multiple confounding effects associated with the selected mouse model.

Selective direct motor cortical influence during naturalistic climbing.

It remains poorly resolved when and how motor cortical output directly influences limb muscle activity through descending projections, which impedes mechanistic understanding of cortical movement control. Here we addressed this in mice performing an ethologically inspired all-limb climbing behavior. We quantified the direct influence of forelimb primary motor cortex (caudal forelimb area, CFA) on muscle activity comprehensively across the muscle activity states that occur during climbing. We found that CFA informs muscle activity pattern, mainly by selectively activating certain muscles while exerting much smaller, bidirectional effects on their antagonists. From Neuropixel recordings, we identified linear combinations (components) of motor cortical activity that covary with these effects, finding that these components differ from those that covary with muscle activity or kinematics. Collectively, our results reveal an instructive direct motor cortical influence on limb muscles that is selective within a motor behavior and reliant on a new type of neural activity subspace.

Distinct recruitment of feedforward and recurrent pathways across higher-order areas of mouse visual cortex.

Cortical visual processing transforms features of the external world into increasingly complex and specialized neuronal representations. These transformations arise in part through target-specific routing of information; however, within-area computations may also contribute to area-specific function. Here, we sought to determine whether higher order visual cortical areas lateromedial (LM), anterolateral (AL), posteromedial (PM), and anteromedial (AM) have specialized anatomical and physiological properties by using a combination of whole-cell recordings and optogenetic stimulation of primary visual cortex (V1) axons in vitro. We discovered area-specific differences in the strength of recruitment of interneurons through feedforward and recurrent pathways, as well as differences in cell-intrinsic properties and interneuron densities. These differences were most striking when comparing across medial and lateral areas, suggesting that these areas have distinct profiles for net excitability and integration of V1 inputs. Thus, cortical areas are not defined simply by the information they receive but also by area-specific circuit properties that enable specialized filtering of these inputs.