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Updated: May 21, 2025
Author Spotlight: Investigating Early Events and Long-Term Effects of ACL Injuries for Osteoarthritis Progression
Published on: September 29, 2023
Molecular mapping of articular cartilage CXCR4 expression after ACL injury via a novel small molecule-based probe
Michael D Newton1, Mackenzie M Fleischer2, Howard W T Matthew3
1Department of Orthopaedic Surgery, University of Michigan, 109 Zina Pitcher Place #2278, Ann Arbor, MI 48109, United States of America; Department of Orthopaedic Surgery, Beaumont Hospital, 3811 W 13 Mile Rd, Royal Oak, MI 48073, United States of America; Department of Chemical Engineering, Wayne State University, 5050 Anthony Wayne Dr, Detroit, MI 48202, United States of America.
Purpose:
Molecular imaging is a powerful modality to spatially resolve molecular changes across tissues, but application to articular cartilage remains limited. CXCR4 is an established marker of chondrocyte hypertrophy and potential therapeutic target for osteoarthritis. The purpose of this study was to develop and apply a CXCR4-targeted, near-infrared fluorescent (NIR) probe to a rat model of post-traumatic osteoarthritis (PTOA).
Methods:
A CXCR4-targeted, small molecule-based NIR probe ("Cy7-AMD") was synthesized. Sensitivity and specificity of Cy7-AMD to CXCR4 was validated in vitro (HUVECs), and ex vivo (rat osteochondral explants). To induce PTOA, female Lewis rats underwent noninvasive anterior cruciate ligament (ACL) rupture. At 7- and 28-days post-injury, injured/contralateral femora and tibiae were dissected, incubated in Cy7-AMD vs a non-targeting control, and imaged via NIR imaging, as well as conventional and contrast-enhanced micro-computed tomography. Imaging datasets were co-registered, cartilage tissue volumes were segmented, and paired cartilage thickness and NIR signal maps were generated and analyzed for PTOA-relevant changes.
Results:
Compared to a non-targeting control probe, in vitro and ex vivo assays confirm sensitivity and specificity of Cy7-AMD to CXCR4. Flow cytometry confirmed high correspondence between Cy7-AMD- and antibody-based measurement of CXCR4 expression. Cy7-AMD rapidly equilibrated within cartilage, and fluorescent histology confirmed full-thickness penetration. Injured femoral cartilage exhibited heterogeneous CXCR4 expression, with increased signal deviation compared to contralateral femora. Spatial CXCR4 expression patterns correlated to cartilage thickness patterns; high CXCR4 expression at boundaries of low-thickness lesions suggests an association between CXCR4 expression and cartilage loss.
Conclusions:
Small molecule-based probes are advantageous for mapping spatial patterns of molecular expression in rodent articular cartilage, deepening our understanding of PTOA progression.
Insights
A novel near-infrared fluorescent probe targeting CXCR4 was developed and validated in a rat osteoarthritis model. This probe effectively maps molecular changes in cartilage, aiding understanding of post-traumatic osteoarthritis progression.
Area of Science:
- Biomedical imaging
- Molecular imaging
- Osteoarthritis research
Background:
- Articular cartilage molecular changes are crucial in osteoarthritis (OA).
- CXCR4 is a key marker for chondrocyte hypertrophy and a potential OA therapeutic target.
- Current molecular imaging in cartilage is limited.
Purpose of the Study:
- Develop and apply a CXCR4-targeted, near-infrared fluorescent (NIR) probe.
- Utilize the probe in a rat model of post-traumatic osteoarthritis (PTOA).
Main Methods:
- Synthesized a small molecule-based NIR probe (Cy7-AMD) targeting CXCR4.
- Validated probe sensitivity and specificity in vitro and ex vivo.
- Induced PTOA in rats via ACL rupture and analyzed injured/contralateral tissues using NIR imaging and micro-CT.
Main Results:
- Cy7-AMD demonstrated high sensitivity and specificity for CXCR4, correlating with antibody-based measurements.
- The probe achieved full-thickness cartilage penetration.
- Heterogeneous CXCR4 expression in injured cartilage correlated with cartilage loss, suggesting a link to PTOA progression.
Conclusions:
- Small molecule-based probes offer advantages for mapping molecular expression in cartilage.
- This approach enhances understanding of PTOA progression at a molecular level.
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