Depending on the inflammatory milieu, injury can result either in a tissue's complete regeneration or in its degeneration and fibrosis, the latter of which could potentially lead to permanent organ failure. Yet how inflammatory cells regulate matrix-producing cells involved in the reparative process is unknown. Here we show that in acutely damaged skeletal muscle, sequential interactions between multipotent mesenchymal progenitors and infiltrating inflammatory cells determine the outcome of the reparative process. We found that infiltrating inflammatory macrophages, through their expression of tumor necrosis factor (TNF), directly induce apoptosis of fibro/adipogenic progenitors (FAPs). In states of chronic damage, however, such as those in mdx mice, macrophages express high levels of transforming growth factor β1 (TGF-β1), which prevents the apoptosis of FAPs and induces their differentiation into matrix-producing cells. Treatment with nilotinib, a kinase inhibitor with proposed anti-fibrotic activity, can block the effect of TGF-β1 and reduce muscle fibrosis in mdx mice. Our findings reveal an unexpected anti-fibrotic role of TNF and suggest that disruption of the precisely timed progression from a TNF-rich to a TGF-β-rich environment favors fibrotic degeneration of the muscle during chronic injury.

Acute skeletal muscle injury triggers an expansion of fibro/adipogenic progenitors (FAPs) and a transient stage of fibrogenesis characterized by extracellular matrix deposition. While the perpetuation of such phase can lead to permanent tissue scarring, the consequences of its suppression remain to be studied. Using a model of acute muscle damage we were able to determine that pharmacological inhibition of FAP expansion by Nilotinib, a tyrosine kinase inhibitor with potent antifibrotic activity, exerts a detrimental effect on myogenesis during regeneration. We found that Nilotinib inhibits the damage-induced expansion of satellite cells in vivo, but it does not affect in vitro proliferation, suggesting a non cell-autonomous effect. Nilotinib impairs regenerative fibrogenesis by preventing the injury-triggered expansion and differentiation of resident CD45(-):CD31(-):α7integrin(-):Sca1(+) mesenchymal FAPs. Our data support the notion that the expansion of FAPs and transient fibrogenesis observed during regeneration play an important trophic role toward tissue-specific stem cells.

Kidney injury is characterized by persisting inflammation and fibrosis, yet mechanisms by which inflammatory signals drive fibrogenesis remain poorly defined. RNA sequencing of fibrotic kidneys from patients with CKD identified a metabolic gene signature comprising loss of mitochondrial and oxidative phosphorylation gene expression with a concomitant increase in regulators and enzymes of glycolysis under the control of PGC1 and MYC transcription factors, respectively. We modeled this metabolic switch , in experimental murine models of kidney injury, and in human kidney stromal cells (SCs) and human kidney organoids. In mice, MYC and the target genes thereof became activated in resident SCs early after kidney injury, suggesting that acute innate immune signals regulate this transcriptional switch. , stimulation of purified human kidney SCs and human kidney organoids with IL-1 recapitulated the molecular events observed , inducing functional metabolic derangement characterized by increased MYC-dependent glycolysis, the latter proving necessary to drive proliferation and matrix production. MYC interacted directly with sequestosome 1/p62, which is involved in proteasomal degradation, and modulation of p62 expression caused inverse effects on MYC expression. IL-1 stimulated autophagy flux, causing degradation of p62 and accumulation of MYC. Inhibition of the IL-1R signal transducer kinase IRAK4 or inhibition of MYC as well as in human kidney organoids abrogated fibrosis and reduced tubular injury. Our findings define a connection between IL-1 and metabolic switch in fibrosis initiation and progression and highlight IL-1 and MYC as potential therapeutic targets in tubulointerstitial diseases.

Pathologies characterized by lipomatous infiltration of craniofacial structures as well as certain forms of lipodystrophies suggest the existence of a distinct adipogenic program in the cephalic region of mammals. Using lineage tracing, we studied the origin of craniofacial adipocytes that accumulate both in cranial fat depots and during ectopic lipomatous infiltration of craniofacial muscles. We found that unlike their counterparts in limb muscle, a significant percentage of cranial adipocytes is derived from the neural crest (NC). In addition, we identified a population of NC-derived Lin 2 /a7 2 /CD34 1 /Sca-1 1 fibro/adipogenic progenitors (NC-FAPs) that resides exclusively in the mesenchyme of cephalic fat and muscle. Comparative analysis of the adipogenic potential, impact on metabolism, and contribution to the regenerative response of NC-FAPs and mesoderm-derived FAPs (M-FAPs) suggests that these cells are functionally indistinguishable. While both NC-and M-FAPs express mesenchymal markers and promyogenic cytokines upon damage-induced activation, NC-FAPs additionally express components of the NC developmental program. Furthermore, we show that craniofacial FAP composition changes with age, with young mice containing FAPs that are almost exclusively of NC origin, while NC-FAPs are progressively replaced by M-FAPs as mice age. Based on these results, we propose that in the adult, ontogenetically distinct FAPs form a diffused system reminiscent of the endothelium, which can originate from multiple developmental intermediates to seed all anatomical locations.

The suprachiasmatic nucleus plays a key role in the circadian secretion of adrenocortical hormones. However, there is evidence from mouse studies that components of the circadian clock are also expressed within the adrenal gland itself. In the present study we performed genome-wide expression profiling to determine whether the adrenal gland of rhesus monkeys shows temporal gene expression across a 24-h period. We identified 322 transcripts with rhythmic patterns of expression and found that the phase distribution of cycling transcripts varied across the day, with more genes showing activation during the night. We classified the transcripts by their function and clustered them according to their participation in common biochemical pathways: 1) catecholamine synthesis and reuptake; 2) cholesterol cleavage and dehydroepiandrosterone sulfate synthesis; 3) protein synthesis and turnover; and 4) the circadian clock mechanism. In an additional experiment, we assessed the expression of various clock genes at two time points, 12 h apart. We found that expression of Bmal1 and Cry1 was higher at 1300 h, or zeitgeber time 6, whereas expression of Per1 was higher at 0100 h (zeitgeber time 18). Expression levels of Rev-erbalpha were higher at 0100 h than at 1300 h (P<0.05), and immunohistochemistry revealed a strong expression of this transcription factor specifically in chromaffin cells of the adrenal medulla. Taken together, the data indicate that the primate adrenal gland shows rhythmic expression of genes associated with cell biology and synthesis of steroids and catecholamines. Moreover, they strongly imply the existence of an intrinsic circadian clock.

The cardiac stroma contains multipotent mesenchymal progenitors. However, lineage relationships within cardiac stromal cells are still poorly understood. Here, we identify heartresident PDGFRa + Sca-1 + cells as cardiac Fibro/Adipogenic Progenitors (cFAPs) and show that they respond to ischemic damage by generating fibrogenic cells. Pharmacological blockade of this differentiation step with an anti-fibrotic tyrosine kinase inhibitor decreases post-myocardial infarction (MI) remodeling and leads to improvements in heart function. In the undamaged heart, activation of cFAPs through lineage-specific deletion of the quiescence factor Hic1 reveals additional pathogenic potential, causing fibro-fatty infiltration of the myocardium and driving major pathological features of Arrhythmogenic Cardiomyopathy (AC). Highlights• A subpopulation of PDGFRa + , Sca-1 + cells, previously considered to be a sub-type of cardiac fibroblasts, are multipotent mesenchymal progenitors,• Cardiac damage triggers the differentiation of PDGFRa + Sca-1 + cells into Sca-1cells expressing a fibrogenic transcriptional programme,• Blockade of the cFAP-to-fibroblast transition by Nilotinib ameliorated cardiac dysfunction post-MI and modulated cardiac remodelling.• Studies performed on a model of experimentally-induced AC confirmed that cFAPs are a source of both cardiac fibroblasts and adipocytes in vivo.• Conversely, in the undamaged heart, activation of cFAPs by means of lineage-specific deletion of transcription factor Hic1, resulted in fibro/fatty cardiac degeneration and pathological alterations reminiscent of AC. Collectively, our findings show that a proportion of what are commonly termed "fibroblasts" are actually multipotent .

No abstract

Pericytes are tissue-resident mesenchymal progenitor cells anatomically associated with the vasculature that have been shown to participate in tissue regeneration. Here, we tested the hypothesis that kidney pericytes, derived from FoxD1 mesodermal progenitors during embryogenesis, are necessary for postnatal kidney homeostasis. Diphtheria toxin delivery to FoxD1Cre::RsDTR transgenic mice resulted in selective ablation of >90% of kidney pericytes but not other cell lineages. Abrupt increases in plasma creatinine, blood urea nitrogen, and albuminuria within 96 h indicated acute kidney injury in pericyte-ablated mice. Loss of pericytes led to a rapid accumulation of neutral lipid vacuoles, swollen mitochondria, and apoptosis in tubular epithelial cells. Pericyte ablation led to endothelial cell swelling, reduced expression of vascular homeostasis markers, and peritubular capillary loss. Despite the observed injury, no signs of the acute inflammatory response were observed. Pathway enrichment analysis of genes expressed in kidney pericytes in vivo identified basement membrane proteins, angiogenic factors, and factors regulating vascular tone as major regulators of vascular function. Using novel microphysiological devices, we recapitulated human kidney peritubular capillaries coated with pericytes and showed that pericytes regulate permeability, basement membrane deposition, and microvascular tone. These findings suggest that through the active support of the microvasculature, pericytes are essential to adult kidney homeostasis.