Anterior Cruciate Ligament Reconstruction Upregulates Vitamin D Activating Pathways within the Quadriceps
Authors: Fry Jean L. , Brightwell Camille R. , Latham Christine M. , Munson Brooke D. , Noehren Brian , Fry Christopher S. 
Published: May 2022

Anterior cruciate ligament (ACL) tears are prevalent sport‐related injuries that result in protracted quadriceps atrophy and weakness despite surgical reconstruction and physical therapy. Vitamin D and its receptor are integral for skeletal muscle development and homeostasis. In addition, vitamin D status is positively associated with muscle size and strength. If the vitamin D receptor and related enzymes are upregulated with ACL injury and reconstruction, then overall vitamin D status may be an important effector of recovery. Concentrations of vitamin D needed to support optimal recovery may differ from typical clinical cut points used to diagnose vitamin D deficiency, which could have powerful therapeutic implications following ACL injury and reconstruction. This is especially important given recent research showing over 50% of athletes have vitamin D inadequacy. We hypothesized that subjects with circulating vitamin D sufficient to suppress parathyroid hormone secretion (≥ 40 ng/mL) would be better protected against quadriceps atrophy at the conclusion of rehabilitation. Muscle biopsies (vastus lateralis) were obtained from ACL‐injured and Healthy limbs of 16 participants (6M,10F; 19±5yr, body mass index: 25.1±3.7kg/m2) prior to surgical reconstruction, 1 week following reconstruction (Recon), and following the completion of post‐surgical rehabilitation (6 months following surgery, Post‐Rehab). RNA was isolated from all quadriceps biopsies, followed by library construction, sequencing, and bioinformatics analysis. Transcript abundance of vitamin D activating pathway regulators (vitamin D receptor [VDR], and Cytochrome P450 2R1 [CYP2R1] and Family 27 Subfamily B Member 1 [CYP27B1]) was assessed following analysis using DESeq2 and false discovery rate (FDR) adjustment of p‐values. Additionally, 25‐hydroxy vitamin D (25(OH)D) was assessed in serum samples obtained at the time of reconstruction surgery. Quadriceps fiber cross‐sectional area was measured via laminin immunohistochemistry. Recon (1 week post‐surgery) muscle showed increased transcript abundance of the vitamin D receptor, VDR(elevated 7.8 fold, FDR<0.05), and vitamin D‐activating pathway regulators CYP2R1 (elevated 1.5 fold, FDR<0.05) and CYP27B1 (elevated 2.6 fold, FDR<0.05) when compared to the Healthy limb. Following the completion of rehabilitation at the 6‐month follow‐up, participants with 25(OH)D above 40 ng/mL showed 6.4% atrophy of quadriceps fiber size compared to 22.8% atrophy in participants with 25(OH)D below 40 ng/mL (p=0.06). These findings provide evidence for surgically induced elevations in the vitamin D activating pathway within skeletal muscle. Upregulation of the vitamin D activating pathway may support quadriceps mitochondrial health and satellite cell activity and self‐renewal capacity, both of which are compromised following ACL reconstruction. In addition, elevated vitamin D concentrations may protect against protracted quadriceps atrophy. Future research should investigate if manipulation of vitamin D concentrations can enhance quadriceps functional recovery following ACL reconstruction.