No abstract

Skeletal muscle aging is a major cause of disability and frailty in the elderly. The progressive impairment of skeletal muscle function with aging was recently linked to a disequilibrium between damage and repair. Macrophages participate in muscle tissue repair, first as pro‐inflammatory M1 subtype and then as anti‐inflammatory M2 subtype. However, information on the presence of macrophages in skeletal muscle is still sporadic and the effect of aging on macrophage phenotype remains unknown. In this study, we sought to characterize the polarization status of macrophages in skeletal muscle of persons across a wide range of ages. We found that most macrophages in human skeletal muscle are M2, and that this number increased with advancing age. On the contrary, M1 macrophages declined with aging, making the total number of macrophages invariant with older age. Notably, M2 macrophages colocalized with increasing intermuscular adipose tissue (IMAT) in aging skeletal muscle. Similarly, aged BALB/c mice showed increased IMAT and M2 macrophages in skeletal muscle, accompanied by slightly increased collagen protein production. Collectively, we report that polarization of macrophages to the major M2 subtype is associated with IMAT and propose that increased M2 in aged skeletal muscle may impact upon muscle metabolism associated with aging.

Mammalian circRNAs can influence different cellular processes by interacting with proteins and other nucleic acids. Here, we used ribonucleoprotein immunoprecipitation (RIP) analysis to identify systematically the circRNAs associated with the cancer-related protein AUF1. Among the circRNAs interacting with AUF1 in HeLa (human cervical carcinoma) cells, we focused on hsa_circ_0032434 (circPCNX), an abundant target of AUF1. Overexpression of circPCNX specifically interfered with the binding of AUF1 to p21 (CDKN1A) mRNA, thereby promoting p21 mRNA stability and elevating the production of p21, a major inhibitor of cell proliferation. Conversely, silencing circPCNX increased AUF1 binding to p21 mRNA, reducing p21 production and promoting cell division. Importantly, eliminating the AUF1-binding region of circPCNX abrogated the rise in p21 levels and rescued proliferation. Therefore, we propose that the interaction of circPCNX with AUF1 selectively prevents AUF1 binding to p21 mRNA, leading to enhanced p21 mRNA stability and p21 protein production, thereby suppressing cell growth.

Skeletal muscle aging is a major cause of disability and frailty in the elderly. The progressive impairment of skeletal muscle with aging was recently linked to a disequilibrium between damage and repair. Macrophages participate in muscle tissue repair first as pro-inflammatory M1 subtype and then as anti-inflammatory M2 subtype. However, information on the presence of macrophages in skeletal muscle is still sporadic and the effect of aging on macrophage phenotype remains unknown. In this study, we sought to characterize the polarization status of macrophages in human skeletal muscle at different ages. We found that most macrophages in human skeletal muscle are M2, and that this number increased with advancing age. On the contrary, M1 macrophages declined with aging, making the total number of macrophages invariant with older age. Notably, M2 macrophages co-localized with increasing intermuscular adipose tissue (IMAT) in aging skeletal muscle. Old BALB/c mice showed increased IMAT and regenerating myofibers in skeletal muscle, accompanied by elevated expression of adipocyte markers and M2 cytokines. Collectively, we report that polarization of macrophages to the major M2 subtype is associated with IMAT, and propose that increased M2 in aged skeletal muscle may reflect active repair of aging-associated muscle damage.

GRSF1 is a mitochondrial RNA-binding protein important for maintaining mitochondrial function. We found that GRSF1 is highly expressed in cultured skeletal myoblasts differentiating into myotubes. To understand the physiological function of GRSF1 in vivo, we generated mice in which GRSF1 was specifically ablated in skeletal muscle. The conditional knockout mice (Grsf1cKO) appeared normal until 7-9 months of age. Importantly, however, a reduction of muscle endurance compared to wild-type controls was observed in 16-to 18-month old Grsf1cKO mice. Transcriptomic analysis revealed more than 200 mRNAs differentially expressed in Grsf1cKO muscle at this age. Notably, mRNAs encoding proteins involved in mitochondrial function, inflammation, and ion transport, including Mgarp, Cxcl10, Nfkb2, and Sln mRNAs, were significantly elevated in aged Grsf1cKO muscle. Our findings suggest that GRSF1 deficiency exacerbates the functional decline of aged skeletal muscle, likely through multiple downstream effector proteins.

Preclinical studies indicate that up-regulation of carbonic anhydrase (CA) by temozolomide (TMZ), via a mechanism requiring the proto-oncogene BCL-3, promotes resistance to therapy in glioblastoma (GBM) cells. Moreover, the CA inhibitor, acetazolamide (ACZ), sensitizes patient-derived GBM cells and xenografts to TMZ. These findings led to the current Phase I study investigating the safety and efficacy of adding ACZ to adjuvant TMZ in patients with newly diagnosed, MGMT-methylated malignant glioma. 24 patients were enrolled (23 GBM and one Grade III IDH-mutant astrocytoma), median age was 53.5 and mean KPS 91. ACZ was given on days 1-21 of each adjuvant TMZ cycle (250 mg BID days 1-7, increased to 500 mg BID days 8-21). No patient experienced the primary outcome of regimen limiting toxicity (RLT) and there were only three grade III adverse events deemed likely unrelated to ACZ. For the secondary endpoints of overall and progression free survival (OS and PFS, respectively), only the 23 GBM patients were included (22 IDH-wildtype and 1 IDH-mutant). From diagnosis, median PFS was 18.8 months (95% CI: 10.4-23.0) and median OS was 25.0 months (95% CI: 19.9-28.4). Median time from diagnosis to consent was 2.9 months. As of April 2021, only 7 of 23 deaths had occurred. 2-year OS% was 68.2%. Further analysis of secondary endpoints including 6-month objective response rate (ORR) and biomarker analysis of BCL-3 by IHC will be available in the coming months. In sum, the data indicate that addition of ACZ to TMZ is safe and does not lead to reduced TMZ dosing. Also, compared to historical data, interim outcomes suggest that addition of ACZ may substantially improve PFS and 2-year overall survival. These findings support the hypothesis that ACZ acts as a chemosensitizer of alkylating chemotherapy in GBM and support examination of this regimen in a randomized, placebo-controlled study.