Phlpp1 is associated with human intervertebral disc degeneration and its deficiency promotes healing after needle puncture injury in mice

Abstract

Back pain is a leading cause of global disability and is strongly associated with intervertebral disc (IVD) degeneration (IDD). Hallmarks of IDD include progressive cell loss and matrix degradation. The Akt signaling pathway regulates cellularity and matrix production in IVDs and its inactivation is known to contribute to a catabolic shift and increased cell loss via apoptosis. The PH domain leucine-rich repeat protein phosphatase (Phlpp1) directly regulates Akt signaling and therefore may play a role in regulating IDD, yet this has not been investigated. The aim of this study was to investigate if Phlpp1 has a role in Akt dysregulation during IDD. In human IVDs, Phlpp1 expression was positively correlated with IDD and the apoptosis marker cleaved Caspase-3, suggesting a key role of Phlpp1 in the progression of IDD. In mice, 3 days after IVD needle puncture injury, Phlpp1 knockout (KO) promoted Akt phosphorylation and cell proliferation, with less apoptosis. At 2 and 8 months after injury, Phlpp1 deficiency also had protective effects on IVD cellularity, matrix production, and collagen structure as measured with histological and immunohistochemical analyses. Specifically, Phlpp1-deletion resulted in enhanced nucleus pulposus matrix production and more chondrocytic cells at 2 months, and increased IVD height, nucleus pulposus cellularity, and extracellular matrix deposition 8 months after injury. In conclusion, Phlpp1 has a role in limiting cell survival and matrix degradation in IDD and research targeting its suppression could identify a potential therapeutic target for IDD.

Pleckstrin homology (PH) domain and Leucine Rich Repeat Phosphatase 1 (Phlpp1) Suppresses Parathyroid Hormone Receptor 1 (Pth1r) Expression and Signaling During Bone Growth

“…Further enhancement of cell proliferation as a result of Phlpp1 inhibition has potential implications beyond chondrocytes. Phlpp1 deletion promotes cell proliferation in a model of intervertebral disc degeneration, (19 ) and regulation of cell apoptosis by Phlpp1 is implicated in ischemic brain and spinal cord injury, ( 20,22 ) colitis, ( 21 ) cardiac dysfunction, ( 66 ) pancreatic beta‐cell survival, ( 67 ) and tumor activity. ( 8 ) As such, enhancement of signals associated with Phlpp1 inhibition could have a profound impact on various physiological and disease processes.…”

“…( 12 ) Phlpp1/2 have been implicated as tumor suppressors, ( 9,13,14 ) but also regulate metabolic processes, ( 15 ) inflammation, ( 16 ) and tissue regeneration following injury. ( 17–24 ) Phlpp1 is overexpressed in human osteoarthritic tissue and Phlpp1 inactivation improves murine posttraumatic osteoarthritis. ( 17 ) Phlpp1 deletion accelerates chondrocyte maturation in vitro, preserves articular cartilage in vivo, and increases mobility in mice with posttraumatic osteoarthritis.…”

Single Impact Injury of Vertebral Endplates Without Structural Disruption, Initiates Disc Degeneration Through Piezo1 Mediated Inflammation and Metabolism Dysfunction

“…These changes were consistent with the previous studies showing loss of proteoglycans and less organized collagens during the initial phase of disc degeneration. 29 – 31 …”

Inhibition of PHLPP1/2 phosphatases rescues pancreatic β-cells in diabetes

“…PHLPP1-KO mice develop normally with no anatomical defects, consistent with previously reported studies showing that PHLPP1-KO mice are viable and show no overt changes in growth, anatomy, or development ( Chen et al, 2013 ; Masubuchi et al, 2010 ). In addition, mice with systemic deletion/inhibition of PHLPP1 show promising neuro-, cardio-, and intestine-protection as well as tissue regeneration in several pathological settings ( Chen et al, 2013 ; Hwang et al, 2018 ; Jackson et al, 2018 ; Moc et al, 2015 ; Wen et al, 2015 ; Zhang et al, 2019 , 2020 ). In our long-term mouse studies, no tumorigenic features were observed in PHLPP1-KO mice; they live to a relatively old age without development of tumors.…”