Lysophosphatidic acid (LPA) is a bioactive lipid that signals through G protein-coupled receptors (LPA1-6) and regulates multiple cellular processes, including fibrosis. Although LPA signaling has bee Show more
Lysophosphatidic acid (LPA) is a bioactive lipid that signals through G protein-coupled receptors (LPA1-6) and regulates multiple cellular processes, including fibrosis. Although LPA signaling has been implicated in fibrotic diseases in several organs, its role in skeletal muscle remains unclear. Here, we show that LPA/LPA1 signaling promotes fibrogenesis after sciatic nerve transection. Denervation induces differential expression of LPA signaling axis components and a transient early increase in intramuscular LPA levels. Pharmacological inhibition of LPA1/3 with Ki16425, or genetic deletion of LPA1, reduces extracellular matrix accumulation and expansion of fibro/adipogenic progenitors (FAPs) in denervated muscle. Although LPA blockade suppresses atrophy-related gene expression, it does not fully preserve myofiber size. Mechanistically, denervation increases YAP/TAZ expression, nuclear localization in FAPs, and transcriptional activity, effects that are attenuated by LPA axis inhibition. Furthermore, pharmacological inhibition of YAP/TAZ with verteporfin reduces fibrosis after denervation, supporting their role as critical downstream mediators. Finally, transient denervation activates the LPA axis, promotes muscle fibrosis, reduces axonal density in the sciatic nerve, and increases neuromuscular junction instability, effects reversed by Ki16425. Together, these findings identify the LPA/LPA1/YAP/TAZ pathway as a key driver of denervation-induced muscle fibrosis and a potential therapeutic target in neuromuscular disorders. Show less
Post-transcriptional regulatory processes may change transcript levels and affect cell-to-cell variability or noise. We study small-RNA downregulation to elucidate its effects on noise in the iron hom Show more
Post-transcriptional regulatory processes may change transcript levels and affect cell-to-cell variability or noise. We study small-RNA downregulation to elucidate its effects on noise in the iron homeostasis network of Escherichia coli In this network, the small-RNA RyhB undergoes stoichiometric degradation with the transcripts of target genes in response to iron stress. Using single-molecule fluorescence in situ hybridization, we measured transcript numbers of the RyhB-regulated genes sodB and fumA in individual cells as a function of iron deprivation. We observed a monotonic increase of noise with iron stress but no evidence of theoretically predicted, enhanced stoichiometric fluctuations in transcript numbers, nor of bistable behavior in transcript distributions. Direct detection of RyhB in individual cells shows that its noise is much smaller than that of these two targets, when RyhB production is significant. A generalized two-state model of bursty transcription that neglects RyhB fluctuations describes quantitatively the dependence of noise and transcript distributions on iron deprivation, enabling extraction of in vivo RyhB-mediated transcript degradation rates. The transcripts' threshold-linear behavior indicates that the effective in vivo interaction strength between RyhB and its two target transcripts is comparable. Strikingly, the bacterial cell response exhibits Fur-dependent, switch-like activation instead of a graded response to iron deprivation. Show less