Protease-Activatable Porphyrin Molecular Beacon for Osteoarthritis Management

Despite the substantial burden posed by osteoarthritis (OA) globally, difficult challenges remain in achieving early OA diagnosis and adopting effective disease-modifying treatments. In this study, we use a biomolecular approach to address these limitations by creating an inherently theranostic molecular beacon whose imaging and therapeutic capabilities are activated by early pathological changes in OA. This platform comprised (1) a peptide linker substrate for metalloproteinase-13 (MMP-13), a pathological protease upregulated in OA, which was conjugated to (2) a porphyrin moiety with inherent multimodal imaging, photodynamic therapy, and drug delivery capabilities, and (3) a quencher that silences the porphyrin's endogenous fluorescence and photoreactivity when the beacon is intact. In diseased OA tissue with upregulated MMP-13 expression, this porphyrin molecular beacon (PP_MMP13B) was expected to undergo sequence-specific cleavage, yielding porphyrin fragments with restored fluorescence and photoreactivity that could, respectively, be used as a readout of MMP-13 activity within the joint for early OA imaging and disease-targeted photodynamic therapy. This study focused on the synthesis and characterization of PP_MMP13B, followed by a proof-of-concept evaluation of its OA imaging and drug delivery potential. In solution, PP_MMP13B demonstrated 90% photoactivity quenching in its intact form and robust MMP-13 activation, yielding a 13-fold increase in fluorescence post-cleavage. In vitro, PP_MMP13B was readily uptaken and activated in an MMP-13 cell expression-dependent manner in primary OA synoviocytes without exuding significant cytotoxicity. This translated into effective intra-articular cartilage (to a 50 μm depth) and synovial uptake and activation of PP_MMP13B in a destabilization of the medial meniscus OA mouse model, yielding strong fluorescence contrast (7-fold higher signal than background) at the diseased joint site. These results provide the foundation for further exploration of porphyrin molecular beacons for image-guided OA disease stratification, effective articular delivery of disease-modify agents, and OA photodynamic therapy.

MicroRNA In Situ Hybridization in Paraffin-Embedded Human Articular Cartilage and Mouse Knee Joints

MicroRNA (miRNA) in situ hybridization (ISH) is a highly sensitive method that allows for the detection of expression and distribution of miRNAs in fixed paraffin-embedded tissues. MiRNA ISH requires time-consuming optimization based on the tissue type analyzed, method of tissue fixation, and miRNA detection probe. Here, we provide the optimized miRNA ISH protocol for human cartilage and mouse whole knee joints that also entails the necessary steps for sample collection, processing, and preparation for high-quality ISH staining.

MicroRNA-34a-5p Promotes Joint Destruction During Osteoarthritis

Objective

MicroRNA‐34a‐5p (miR‐34a‐5p) expression is elevated in the synovial fluid of patients with late‐stage knee osteoarthritis (OA); however, its exact role and therapeutic potential in OA remain to be fully elucidated. This study was undertaken to examine the role of miR‐34a‐5p in OA pathogenesis.

Methods

Expression of miR‐34a‐5p was determined in joint tissues and human plasma (n = 71). Experiments using miR‐34a‐5p mimic or antisense oligonucleotide (ASO) treatment were performed in human OA chondrocytes, fibroblast‐like synoviocytes (FLS) (n = 7–9), and mouse OA models, including destabilization of the medial meniscus (DMM; n = 22) and the accelerated, more severe model of mice fed a high‐fat diet and subjected to DMM (n = 11). Wild‐type (WT) mice (n = 9) and miR‐34a–knockout (KO) mice (n = 11) were subjected to DMM. Results were expressed as the mean ± SEM and analyzed by t‐test or analysis of variance, with appropriate post hoc tests. P values less than 0.05 were considered significant. RNA sequencing was performed on WT and KO mouse chondrocytes.

Results

Expression of miR‐34a‐5p was significantly increased in the plasma, cartilage, and synovium of patients with late‐stage OA and in the cartilage and synovium of mice subjected to DMM. Plasma miR‐34a‐5p expression was significantly increased in obese patients with late‐stage OA, and in the plasma and knee joints of mice fed a high‐fat diet. In human OA chondrocytes and FLS, miR‐34a‐5p mimic increased key OA pathology markers, while miR‐34a‐5p ASO improved cellular gene expression. Intraarticular miR‐34a‐5p mimic injection induced an OA‐like phenotype. Conversely, miR‐34a‐5p ASO injection imparted cartilage‐protective effects in the DMM and high‐fat diet/DMM models. The miR‐34a–KO mice exhibited protection against DMM‐induced cartilage damage. RNA sequencing of WT and KO chondrocytes revealed a putative miR‐34a‐5p signaling network.

Conclusion

Our findings provide comprehensive evidence of the role and therapeutic potential of miR‐34a‐5p in OA.

The complex landscape of microRNAs in articular cartilage: biology, pathology, and therapeutic targets

The disabling degenerative disease osteoarthritis (OA) is prevalent among the global population. Articular cartilage degeneration is a central feature of OA; therefore, a better understanding of the mechanisms that maintain cartilage homeostasis is vital for developing effective therapeutic interventions. MicroRNAs (miRs) modulate cell signaling pathways and various processes in articular cartilage via posttranscriptional repression of target genes. As dysregulated miRs frequently alter the homeostasis of articular cartilage, modulating select miRs presents a potential therapeutic opportunity for OA. Here, we review key miRs that have been shown to modulate cartilage-protective or -destructive mechanisms and signaling pathways. Additionally, we use an integrative computational biology approach to provide insight into predicted miR gene targets that may contribute to OA pathogenesis, and highlight the complexity of miR signaling in OA by generating both unique and overlapping gene targets of miRs that mediate protective or destructive effects. Early OA detection would enable effective prevention; thus, miRs are being explored as diagnostic biomarkers. We discuss these ongoing efforts and the applicability of miR mimics and antisense inhibitors as potential OA therapeutics.

High-fat diet-induced acceleration of osteoarthritis is associated with a distinct and sustained plasma metabolite signature

Metabolic changes induced by high fat diet (HFD) that contribute to osteoarthritis (OA) are poorly understood. We investigated longitudinal changes to metabolites and their contribution to OA pathogenesis in response to HFD. HFD-fed mice exhibited acceleration of spontaneous age-related and surgically-induced OA compared to lean diet (LD)-fed mice. Using metabolomics, we identified that HFD-fed mice exhibited a distinct and sustained plasma metabolite signature rich in phosphatidylcholines (PC) and lysophosphatidylcholines (lysoPCs), even after resumption of normal chow diet. Using receiver operator curve analysis and prediction modelling, we showed that the concentration of these identified metabolites could efficiently predict the type of diet and OA risk with an accuracy of 93%. Further, longitudinal evaluation of knee joints of HFD- compared to LD- fed mice showed a greater percentage of leptin-positive chondrocytes. Mechanistic data showed that leptin-treated human OA chondrocytes exhibited enhanced production of lysoPCs and expression of autotaxin and catabolic MMP-13. Leptin-induced increased MMP13 expression was reversed by autotaxin inhibition. Together, this study is the first to describe a distinct and sustained HFD-induced metabolite signature. This study suggests that in addition to increased weight, identified metabolites and local leptin-signaling may also contribute in part, towards the accelerated OA-phenotype observed in HFD mice.

Identification of microRNA-181a-5p and microRNA-4454 as mediators of facet cartilage degeneration

Osteoarthritis (OA) of spine (facet joints [FJs]) is one of the major causes of severe low back pain and disability worldwide. The degeneration of facet cartilage is a hallmark of FJ OA. However, endogenous mechanisms that initiate degeneration of facet cartilage are unknown, and there are no disease-modifying therapies to stop FJ OA. In this study, we have identified microRNAs (small noncoding RNAs) as mediators of FJ cartilage degeneration. We first established a cohort of patients with varying degrees of facet cartilage degeneration (control group: normal or mild facet cartilage degeneration; FJ OA group: moderate to severe facet cartilage degeneration) and then screened 2,100 miRNAs and identified 2 miRNAs (miR-181a-5p and miR-4454) that were significantly elevated in FJ OA cartilage compared with control facet cartilage. We further explored their role, function, and signaling mechanisms using computational, in vitro functional, and in vivo studies. We specifically indicate that miR-181a-5p and miR-4454 are involved in promoting inflammatory, catabolic, and cell death activity in FJ chondrocytes. This is the first report to our knowledge that identifies miR-181a-5p and miR-4454 as mediators of cartilage degeneration in FJs and potential therapeutic targets for stopping cartilage degeneration.

PPARγ deficiency results in severe, accelerated osteoarthritis associated with aberrant mTOR signalling in the articular cartilage

OBJECTIVES

We have previously shown that peroxisome proliferator-activated receptor gamma (PPARγ), a transcription factor, is essential for the normal growth and development of cartilage. In the present study, we created inducible cartilage-specific PPARγ knockout (KO) mice and subjected these mice to the destabilisation of medial meniscus (DMM) model of osteoarthritis (OA) to elucidate the specific in vivo role of PPARγ in OA pathophysiology. We further investigated the downstream PPARγ signalling pathway responsible for maintaining cartilage homeostasis.

METHODS

Inducible cartilage-specific PPARγ KO mice were generated and subjected to DMM model of OA. We also created inducible cartilage-specific PPARγ/mammalian target for rapamycin (mTOR) double KO mice to dissect the PPARγ signalling pathway in OA.

RESULTS

Compared with control mice, PPARγ KO mice exhibit accelerated OA phenotype with increased cartilage degradation, chondrocyte apoptosis, and the overproduction of OA inflammatory/catabolic factors associated with the increased expression of mTOR and the suppression of key autophagy markers. In vitro rescue experiments using PPARγ expression vector reduced mTOR expression, increased expression of autophagy markers and reduced the expression of OA inflammatory/catabolic factors, thus reversing the phenotype of PPARγ KO mice chondrocytes. To dissect the in vivo role of mTOR pathway in PPARγ signalling, we created and subjected PPARγ-mTOR double KO mice to the OA model to see if the genetic deletion of mTOR in PPARγ KO mice (double KO) can rescue the accelerated OA phenotype observed in PPARγ KO mice. Indeed, PPARγ-mTOR double KO mice exhibit significant protection/reversal from OA phenotype.

SIGNIFICANCE

PPARγ maintains articular cartilage homeostasis, in part, by regulating mTOR pathway.

mTOR: a potential therapeutic target in osteoarthritis?

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by the progressive loss of articular cartilage, remodeling of the subchondral bone, and synovial inflammation. Mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that controls critical cellular processes such as growth, proliferation, and protein synthesis. Recent studies suggest that mTOR plays a vital role in cartilage growth and development and in altering the articular cartilage homeostasis as well as contributing to the process of cartilage degeneration associated with OA. Both pharmacological inhibition and genetic deletion of mTOR have been shown to reduce the severity of OA in preclinical mouse models. In this review article, we discuss the roles of mTOR in cartilage development, in maintaining articular cartilage homeostasis, and its potential as an OA therapeutic target.

Restoring SIRT6 Expression in Hutchinson-Gilford Progeria Syndrome Cells Impedes Premature Senescence and Formation of Dysmorphic Nuclei

OBJECTIVES

Mice overexpressing SIRT6 live longer than wild-type mice while SIRT6 knockout mice exhibit similar degenerative phenotypes as individuals with Hutchinson-Gilford progeria syndrome (HGPS). Thus, we sought to test whether levels of SIRT6 are reduced in cells from individuals with HGPS and whether restored SIRT6 expression may impede premature aging phenotypes.

METHODS

Levels of endogenous SIRT6 and progerin in HGPS and normal fibroblasts were assessed by Western blotting and immunofluorescence. A tetracycline-inducible system was utilized to test whether progerin causes a rapid reduction in SIRT6 protein. SIRT6 was overexpressed in HGPS cells via lentiviral infection with biological endpoints including senescence-associated β-galactosidase (SA-β-gal) positivity, frequency of nuclear atypia, the number of 53BP1-positive DNA damage foci and growth rates.

RESULTS

Typical HGPS fibroblasts express lower levels of SIRT6 than fibroblasts from normal and atypical HGPS donors. Experimental induction of progerin did not cause a detectable reduction of SIRT6 protein. However, overexpression of SIRT6 in HGPS cells was associated with a reduced frequency of SA-β-gal positivity, fewer misshapen nuclei, fewer DNA damage foci, and increased growth rates.

CONCLUSIONS

Typical HGPS fibroblasts exhibit reduced levels of SIRT6 protein via a mechanism that remains to be elucidated. Our findings suggest that restoring SIRT6 expression in HGPS cells may partially impede senescence and the formation of dysmorphic nuclei. © 2015 S. Karger AG, Basel.