Publisher Correction: Dynamic changes to lipid mediators support transitions among macrophage subtypes during muscle regeneration.

In the version of this article initially published, two arrows in the far right plot of Fig. 3c were aimed incorrectly, and the error bars were missing in Fig. 6e,f. In Fig. 3c, the arrow labeled '5-LOX' should be aimed at the plot measuring LXB4, and the arrow labeled 'LTA4H' should be aimed at the plot measuring LTB4. The errors have been corrected in the HTML and PDF versions of the article.

Dynamic changes to lipid mediators support transitions among macrophage subtypes during muscle regeneration.

Muscle damage elicits a sterile immune response that facilitates complete regeneration. Here, we used mass spectrometry-based lipidomics to map the mediator lipidome during the transition from inflammation to resolution and regeneration in skeletal muscle injury. We observed temporal regulation of glycerophospholipids and production of pro-inflammatory lipid mediators (for example, leukotrienes and prostaglandins) and specialized pro-resolving lipid mediators (for example, resolvins and lipoxins) that were modulated by ibuprofen. These time-dependent profiles were recapitulated in sorted neutrophils and Ly6Chi and Ly6Clo muscle-infiltrating macrophages, with a distinct pro-resolving signature observed in Ly6Clo macrophages. RNA sequencing of macrophages stimulated with resolvin D2 showed similarities to transcriptional changes found during the temporal transition from Ly6Chi macrophage to Ly6Clo macrophage. In vivo, resolvin D2 increased Ly6Clo macrophages and functional improvement of the regenerating muscle. These results reveal dynamic lipid mediator signatures of innate immune cells and provide a proof of concept for their exploitable effector roles in muscle regeneration.

PCTR1 Enhances Repair and Bacterial Clearance in Skin Wounds.

Tissue injury elicits an inflammatory response that facilitates host defense. Resolution of inflammation promotes the transition to tissue repair and is governed, in part, by specialized pro-resolving mediators (SPM). The complete structures of a novel series of cysteinyl-SPM (cys-SPM) were recently elucidated, and proved to stimulate tissue regeneration in planaria and resolve acute inflammation in mice. Their functions in mammalian tissue repair are of interest. Here, nine structurally distinct cys-SPM were screened and PCTR1 uniquely enhanced human keratinocyte migration with efficacy similar to epidermal growth factor. In skin wounds of mice, PCTR1 accelerated closure. Wound infection increased PCTR1 that coincided with decreased bacterial burden. Addition of PCTR1 reduced wound bacteria levels and decreased inflammatory monocytes/macrophages, which was coupled with increased expression of genes involved in host defense and tissue repair. These results suggest that PCTR1 is a novel regulator of host defense and tissue repair, which could inform new approaches for therapeutic management of delayed tissue repair and infection.

Resolvin D1 promotes efferocytosis in aging by limiting senescent cell-induced MerTK cleavage.

Inflammation-resolution is mediated by the balance between specialized pro-resolving mediators (SPMs) like resolvin D1 (RvD1) and pro-inflammatory factors, like leukotriene B4 (LTB4). A key cellular process of inflammation-resolution is efferocytosis. Aging is associated with defective inflammation-resolution and the accumulation of pro-inflammatory senescent cells (SCs). Therefore, understanding mechanism(s) that underpin this impairment is a critical gap. Here, using a model of hind limb ischemia-reperfusion (I/R) remote lung injury, we present evidence that aging is associated with heightened inflammation, impaired SPM:LT ratio, defective efferocytosis, and a decrease in MerTK levels in injured lungs. Treatment with RvD1 mitigated I/R lung injury in aging, promoted efferocytosis, and prevented the decrease of MerTK in injured lungs from old mice. Old MerTK cleavage-resistant mice (MerTKCR) exhibited less neutrophils or polymorpho nuclear cells infiltration and had improved efferocytosis compared with old WT controls. Mechanistically, macrophages that were treated with conditioned media (CM) from senescent cells had increased MerTK cleavage, impaired efferocytosis, and a defective RvD1:LTB4 ratio. Macrophages from MerTKCR mice were resistant to CM-induced efferocytosis defects and had an improved RvD1:LTB4 ratio. RvD1-stimulated macrophages prevented CM-induced MerTK cleavage and promoted efferocytosis. Together, these data suggest a new mechanism and a potential therapy to promote inflammation-resolution and efferocytosis in aging.

Engineering subarachnoid trabeculae with electrospun poly(caprolactone) (PCL) scaffolds to study leptomeningeal metastasis in medulloblastoma.

Leptomeningeal metastasis (LM) occurs when cancer cells infiltrate the subarachnoid space (SAS) and metastasize to the fibrous structures that surround the brain and spinal cord. These structures include the leptomeninges (i.e., the pia mater and arachnoid mater), as well as subarachnoid trabeculae, which are collagen-rich fibers that provide mechanical structure for the SAS, support resident cells, and mediate flow of cerebrospinal fluid (CSF). Although there is a strong expectation that the presence of fibers within the SAS influences LM to be a major driver of tumor progression and lethality, exactly how trabecular architecture relates to the process of metastasis in cancer is poorly understood. This lack of understanding is likely due in part to the difficulty of accessing and manipulating this tissue compartment in vivo. Here, we utilized electrospun polycaprolactone (PCL) to produce structures bearing remarkable morphological similarity to native SAS fiber architecture. First, we profiled the native architecture of leptomeningeal and trabecular fibers collected from rhesus macaque monkeys, evaluating both qualitative and quantitative differences in fiber ultrastructure for various regions of the CNS. We then varied electrospinning parameters to produce a small library of PCL scaffolds possessing distinct architectures mimicking the range of fiber properties observed in vivo. For proof of concept, we studied the metastasis-related behaviors of human pediatric medulloblastoma cells cultured in different fiber microenvironments. These studies demonstrated that a more open, porous fiber structure facilitates DAOY cell spread across and infiltration into the meningeal mimic. Our results present a new tissue engineered model of the subarachnoid space and affirm the expectation that fiber architecture plays an important role in mediating metastasis-related behaviors in an in vitro model of pediatric medulloblastoma.

Resolvin D1 promotes the targeting and clearance of necroptotic cells.

Inflammation-resolution is a protective response that is mediated by specialized pro-resolving mediators (SPMs). The clearance of dead cells or efferocytosis is a critical cellular program of inflammation-resolution. Impaired efferocytosis can lead to tissue damage in prevalent human diseases, like atherosclerosis. Therefore understanding mechanisms associated with swift clearance of dead cells is of utmost clinical importance. Recently, the accumulation of necroptotic cells (NCs) was observed in human plaques and we postulated that this is due to defective clearance programs. Here we present evidence that NCs are inefficiently taken up by macrophages because they have increased surface expression of a well-known "don't eat me" signal called CD47. High levels of CD47 on NCs stimulated RhoA-pMLC signaling in macrophages that promoted "nibbling", rather than whole-cell engulfment of NCs. Anti-CD47 blocking antibodies limited RhoA-p-MLC signaling and promoted whole-cell NC engulfment. Treatment with anti-CD47 blocking antibodies to Ldlr-/- mice with established atherosclerosis decreased necrotic cores, limited the accumulation of plaque NCs and increased lesional SPMs, including Resolvin D1 (RvD1) compared with IgG controls. Mechanistically, RvD1 promoted whole-cell engulfment of NCs by decreasing RhoA signaling and activating CDC42. RvD1 specifically targeted NCs for engulfment by facilitating the release of the well-known "eat me signal" called calreticulin from macrophages in a CDC42 dependent manner. Lastly, RvD1 enhanced the clearance of NCs in advanced murine plaques. Together, these results suggest new molecules and signaling associated with the clearance of NCs, provide a new paradigm for the regulation of inflammation-resolution, and offer a potential treatment strategy for diseases where NCs underpin the pathology.