Improved Posterolateral Lumbar Spinal Fusion Using a Biomimetic, Nanocomposite Scaffold Augmented by Autologous Platelet-Rich Plasma

Remodeling of the human bony skeleton is constantly occurring with up to 10% annual bone volume turnover from osteoclastic and osteoblastic activity. A shift toward resorption can result in osteoporosis and pathologic fractures, while a shift toward deposition is required after traumatic, or surgical injury. Spinal fusion represents one such state, requiring a substantial regenerative response to immobilize adjacent vertebrae through bony union. Autologous bone grafts were used extensively prior to the advent of advanced therapeutics incorporating exogenous growth factors and biomaterials. Besides cost constraints, these applications have demonstrated patient safety concerns. This study evaluated the regenerative ability of a nanostructured, magnesium-doped, hydroxyapatite/type I collagen scaffold (MHA/Coll) augmented by autologous platelet-rich plasma (PRP) in an orthotopic model of posterolateral lumbar spinal fusion. After bilateral decortication, rabbits received either the scaffold alone (Group 1) or scaffold with PRP (Group 2) to the anatomic right side. Bone regeneration and fusion success compared to internal control were assessed by DynaCT with 3-D reconstruction at 2, 4, and 6 weeks postoperatively followed by comparative osteogenic gene expression and representative histopathology. Both groups formed significantly more new bone volume than control, and Group 2 subjects produced significantly more trabecular and cortical bone than Group 1 subjects. Successful fusion was seen in one Group 1 animal (12.5%) and 6/8 Group 2 animals (75%). This enhanced effect by autologous PRP treatment appears to occur via astounding upregulation of key osteogenic genes. Both groups demonstrated significant gene upregulation compared to vertebral bone controls for all genes. Group 1 averaged 2.21-fold upregulation of RUNX2 gene, 3.20-fold upregulation of SPARC gene, and 3.67-fold upregulation of SPP1 gene. Depending on anatomical subgroup (cranial, mid, caudal scaffold portions), Group 2 had significantly higher average expression of all genes than both control and Group 1–RUNX2 (8.23–19.74 fold), SPARC (18.67–55.44 fold), and SPP1 (46.09–90.65 fold). Our data collectively demonstrate the osteoinductive nature of a nanostructured MHA/Coll scaffold, a beneficial effect of augmentation with autologous PRP, and an ability to achieve clinical fusion when applied together in an orthotopic model. This has implications both for future study and biomedical innovation of bone-forming therapeutics.

Link protein N-terminal peptide and fullerol promote matrix production and decrease degradation enzymes in rabbit annulus cells

PURPOSE

Intervertebral disc degeneration is a major cause of back pain. Novel therapies for prevention or reversal of disc degeneration are needed. It is desirable for potential therapies to target both inflammation and matrix degeneration.

MATERIALS AND METHODS

The combined regenerative potential of link protein N-terminal peptide (LN) and fullerol on annulus fibrosus (AF) cells was evaluated in a 3D culture model.

RESULTS

Interleukin-1α (IL-1α)-induced AF cell degeneration was counteracted by fullerol, LN, and fullerol + LN, with the latter having the greatest effect on matrix production as evaluated by real-time polymerase chain reaction and glycosaminoglycan assay. IL-1α-induced increases in pro-inflammatory mediators (interleukin-6 and cyclooxygenase-2) and matrix metalloproteinases (MMP-1, -2, -9, and -13) were also counteracted by fullerol and LN.

CONCLUSION

Our data demonstrate that LN and fullerol individually, and in combination, promote matrix production and have anti-inflammatory and anti-catabolic effects on AF cells.

Nanoparticle fullerol alleviates radiculopathy via NLRP3 inflammasome and neuropeptides

The present study aimed to evaluate the analgesic effect of the antioxidant nanoparticle fullerol in a mouse radiculopathy and a dorsal root ganglion (DRG) culture models. Intervertebral disk degeneration causes significant hyperalgesia and nerve inflammation. Pain sensitization and inflammatory reaction were counteracted by fullerol when disk material was bathed in 10 or 100μM of fullerol prior to implantation. Immunohistochemistry showed similar massive IBA1 positive macrophage infiltration surrounding implanted disk material among groups, but IL-1β and IL-6 expression was decreased in the fullerol treated group. In the DRG explant culture, after treatment with TNF-α, the expression of IL-1β, NLRP3, and caspase 1 was significantly increased but this was reversed by the addition of fullerol. In addition, fullerol also decreased the expression of substance P and CGRP in the cultured DRGs. Nanoparticle fullerol effectively counteracts pain sensitization and the inflammatory cascade caused by disk degeneration.

Growth-Factor Nanocapsules That Enable Tunable Controlled Release for Bone Regeneration

Growth factors are of great potential in regenerative medicine. However, their clinical applications are largely limited by the short in vivo half-lives and the narrow therapeutic window. Thus, a robust controlled release system remains an unmet medical need for growth-factor-based therapies. In this research, a nanoscale controlled release system (degradable protein nanocapsule) is established via in situ polymerization on growth factor. The release rate can be finely tuned by engineering the surface polymer composition. Improved therapeutic outcomes can be achieved with growth factor nanocapsules, as illustrated in spinal cord fusion mediated by bone morphogenetic protein-2 nanocapsules.

The Evaluation and Observation of "Hidden" Hypertrophy of Cervical Ligamentum Flavum, Cervical Canal, and Related Factors Using Kinetic Magnetic Resonance Imaging

Study Design Retrospective cohort study. Objective The objective was to measure the change of flavum ligament diameter during positional changes of the cervical spine using kinetic magnetic resonance imaging (MRI) and to examine the correlational diameter changes of the flavum ligament, disk bulging, and the spinal canal from extension to flexion positions. Methods One hundred eight-nine patients underwent kinetic MRI in neutral, extension, and flexion positions. The diameters of cervical ligamentum flavum, disk bulging, and cervical spinal canal and the disk degeneration grade and Cobb angles were measured from C2-C3 to C7-T1. Results In all, 1,134 cervical spinal segments from 189 patients were included. There was a 0.26 ± 0.85-mm average increase in the diameter of the ligamentum flavum from flexion to extension, and 62.70% of the segments had increased ligamentum flavum diameter from flexion to extension. For all segments of the 189 patients, the cervical spinal canal diameters had an average decrease at the disk level of 0.56 ± 1.21 mm from flexion to extension. For all segments with cervical spinal canal narrowing ≥1 mm from flexion to extension view, the ligamentum flavum diameters at C3-C4 to C5-C6 had significant increases compared with patients with spinal canal narrowing < 1 mm (p < 0.05). For patients with ligamentum flavum hypertrophy of ≥1 mm from the flexion to extension view, the cervical spinal canal diameters at C2-C3, C4-C5, and C5-C6 had significant decreases compared with patients with ligamentum flavum hypertrophy of <1 mm (p < 0.05). Conclusion The "hidden" hypertrophy of ligamentum flavum was significant at C4-C5 and C5-C6 and significantly contributes to the stenosis of cervical spinal canal in the extension position.

The Occupancy of the Components in the Cervical Spine and Their Changes with Extension and Flexion

Study Design Retrospective case series. Objectives The kinematics of the cervical spine has been investigated by many researchers. However, the occupancy of the disk bulges, spinal cord, ligamentum flavum, and the rest of the canal as well as the changes of these structures with motion have not yet been investigated. The goal of this study is to investigate these dynamic changes. Methods The kinetic magnetic resonance images of 248 patients (124 men and 124 women) were evaluated, and the occupancy of each structure for each cervical level at neutral, flexion, and extension were calculated. Results Whole canal anteroposterior (AP) diameters showed significant differences between neutral-extension and flexion-extension at the C4-C5 and C5-C6 levels (p < 0.05). The mean disk bulges showed significant differences between neutral-flexion and flexion-extension at the C4-C5, C5-C6, C6-C7, and C7-T1 levels (p < 0.01). The mean spinal canal AP diameter showed significant differences between flexion-extension and neutral-extension at the C3-C4, C4-C5, C5-C6, and C6-C7 levels (p < 0.05). There were significant differences between neutral-flexion at the C4-C5, C5-C6, and C6-C7 levels (p < 0.05). The mean thickness of the ligamentum flavum showed significant differences between flexion-extension at the C3-C4, C4-C5, C5-C6, and C6-C7 levels (p < 0.001). There were significant differences between neutral-extension at the C3-C4 and C5-C6 levels (p < 0.05). There were significant differences between neutral-flexion at the C5-C6 and C6-C7 levels (p < 0.05). The mean thickness of the spinal cord showed significant differences between neutral-flexion at the C2-C3 and C3-C4 levels (p < 0.05). There were significant differences between flexion-extension at the C3-C4 and C4-C5 levels (p < 0.01). The rest of the canal showed significant differences between neutral-extension and flexion-extension at the C3-C4, C4-C5, C5-C6, and C6-C7 levels (p < 0.005). There were significant differences between neutral-flexion at the C5-C6 and C6-C7 levels (p < 0.01). Conclusions The occupancy of each structure in the cervical spine for each level was revealed by this study. In addition, the dynamic changes in the cervical spine with flexion and extension were seen to have different characteristics for each level.

The effect of bone morphogenetic protein-2 injection at different time points on intervertebral disk degeneration in a rat tail model

STUDY DESIGN

Prospective in vivo rat tail model of disk degeneration comparing the effects of recombinant human bone morphogenetic protein-2 (rhBMP-2) injection over various time points and grades of degeneration.

OBJECTIVE

To evaluate the effect of timing and disk grade on rhBMP-2 injection in a rat tail model of disk degeneration.

SUMMARY OF BACKGROUND DATA

rhBMP-2 stimulates the proliferation of intervertebral disk cells and the secretion of extracellular matrix. However, few in vivo studies have demonstrated whether rhBMP-2 also improves disk degeneration and the severity of disk degeneration beyond which disks cannot be recovered by rhBMP-2 treatment.

METHODS

Two coccygeal disks of each rodent subject were punctured percutaneously using an 18 G needle. At 4 weeks after the puncture, disks demonstrating induced degeneration were divided into 3 groups. Groups 1, 2, and 3 were treated with 7.5 μg rhBMP-2 or phosphate buffered saline by injection into the disk at 4, 6, and 8 weeks postpuncture, respectively. Plain radiographs and magnetic resonance images (MRIs) were obtained on the day of puncture and every 2 weeks thereafter until sacrifice. At 6 weeks after injection, each group was killed and examined with histologic and immunohistochemical analysis.

RESULTS

According to MRI disk grade evaluation of the degenerative disk, rhBMP-2 significantly improved degeneration grade in group 1 at 2 weeks after injection. According to radiographic disk height index, groups 1 and 2 showed a trend toward improvement at 2 weeks after rhBMP-2 injection. Chondrogenic differentiation was noted on immunohistochemical staining of many disks treated with rhBMP-2.

CONCLUSIONS

rhBMP-2 injection of degenerated disks at 4 weeks postpuncture induced a transient improvement in disk grade on MRI and stimulated chondrogenic differentiation. These data suggest rhBMP-2 as a potential therapy for degenerative disk disease.

A novel osteogenic oxysterol compound for therapeutic development to promote bone growth: activation of hedgehog signaling and osteogenesis through smoothened binding

Osteogenic factors are often used in orthopedics to promote bone growth, improve fracture healing, and induce spine fusion. Osteogenic oxysterols are naturally occurring molecules that were shown to induce osteogenic differentiation in vitro and promote spine fusion in vivo. The purpose of this study was to identify an osteogenic oxysterol more suitable for clinical development than those previously reported, and evaluate its ability to promote osteogenesis in vitro and spine fusion in rats in vivo. Among more than 100 oxysterol analogues synthesized, Oxy133 induced significant expression of osteogenic markers Runx2, osterix (OSX), alkaline phosphatase (ALP), bone sialoprotein (BSP), and osteocalcin (OCN) in C3H10T1/2 mouse embryonic fibroblasts and in M2-10B4 mouse marrow stromal cells. Oxy133-induced activation of an 8X-Gli luciferase reporter, its direct binding to Smoothened, and the inhibition of Oxy133-induced osteogenic effects by the Hedgehog (Hh) pathway inhibitor, cyclopamine, demonstrated the role of Hh pathway in mediating osteogenic responses to Oxy133. Oxy133 did not stimulate osteogenesis via BMP or Wnt signaling. Oxy133 induced the expression of OSX, BSP, and OCN, and stimulated robust mineralization in primary human mesenchymal stem cells. In vivo, bilateral spine fusion occurred through endochondral ossification and was observed in animals treated with Oxy133 at the fusion site on X-ray after 4 weeks and confirmed with manual assessment, micro-CT (µCT), and histology after 8 weeks, with equal efficiency to recombinant human bone morphogenetic protein-2 (rhBMP-2). Unlike rhBMP-2, Oxy133 did not induce adipogenesis in the fusion mass and resulted in denser bone evidenced by greater bone volume/tissue volume (BV/TV) ratio and smaller trabecular separation. Findings here suggest that Oxy133 has significant potential as an osteogenic molecule with greater ease of synthesis and improved time to fusion compared to previously studied oxysterols. Small molecule osteogenic oxysterols may serve as the next generation of bone anabolic agents for therapeutic development.

Effect of head and neck positioning on cerebral perfusion during shoulder arthroscopy in beach chair position

The aim of this prospective cohort study was to investigate the effect of head and neck positioning on cerebral perfusion during shoulder arthroscopy in the beach chair position. Regional cerebral tissue oxygen saturation (rSO2) was monitored intraoperatively using near-infrared spectroscopy on 51 consecutive patients undergoing arthroscopic shoulder surgery in the beach chair position. The head of each subject was manipulated by the examiner and sequentially positioned for 45 seconds in terminal flexion, extension, bilateral rotation, and bilateral lateral bending. Decreases in rSO2 of 20% or greater from baseline were defined as a cerebral desaturation event (CDE). The association between head and neck position and cerebral perfusion was assessed. Eight percent of patients (4/51) experienced CDE during head and neck positioning. Body mass index was found to be a risk factor for CDE (p = .05). When comparing preoperative baseline rSO2 to intraoperative supine and intraoperative upright rSO2, there was no significant decrease in saturation levels for any of the six tested positions. Frequent intraoperative evaluations of the head and neck position as well as careful preoperative positioning may reduce the risk of position-related complications in patients undergoing elective shoulder arthroscopy in the beach chair position. In this study's patient population, however, head and neck position was not found to cause significant cerebral desaturation for the time period tested compared to preoperative baselines.

Cerebral desaturation events during shoulder arthroscopy in the beach chair position: patient risk factors and neurocognitive effects

BACKGROUND

Patients undergoing shoulder surgery in the beach chair position may be at increased risk for serious neurocognitive complications due to cerebral ischemia. We sought to define the incidence, patient risk factors, and clinical sequelae of intraoperative cerebral desaturation events.

METHODS

Regional cerebral tissue oxygen saturation (rSO2) was monitored intra-operatively using near-infrared spectroscopy (NIRS) on 50 consecutive patients. The Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) was administered to each patient pre- and postoperatively. Intra-operative decreases in rSO2 of 20% or greater were defined as cerebral desaturation events (CDE). The association between intraoperative CDE and postoperative cognitive decline was assessed.

RESULTS

The incidence of intraoperative CDE in our series was 18% (9/50). Increased body mass index (BMI) was found to have a statistically significant association with intraoperative CDE (mean BMI 37.32 vs 28.59, P < .0001). There was no statistical significance in pre- vs postoperative RBANS either in composite scores or any of the sub-indices in either group.

CONCLUSION

The degree and duration of cerebral ischemia required to produce neurocognitive dysfunction in this patient population remains undefined; however, cerebral oximetry with NIRS allows prompt identification and treatment of decreased cerebral perfusion decreasing the risk of this event. Increased BMI was found to be a statistically significant patient risk factor for the development of intra-operative CDE. The transient intra-operative CDEs were not associated with postoperative cognitive dysfunction in our patient series. We believe protocols aimed at detecting and reversing CDE minimize the risk of neurocognitive dysfunction and improve patient safety.

Factors affecting dynamic foraminal stenosis in the lumbar spine

BACKGROUND CONTEXT

Lumbar foraminal stenosis is a common clinical problem and a significant cause of lower extremity radiculopathy. Minimal in vivo data exists quantifying changes in foraminal area (FA) as the spine moves from flexion to extension in the lumbar spine or on the relationship between FA and lumbar segmental angular motion, translational motion (TM), or disc bulge migration.

PURPOSE

To use kinetic magnetic resonance imaging (kMRI) to evaluate changes in dimensions of lumbar neural foramina during weight bearing in neutral, flexion, and extension positions. To evaluate the relationship between foraminal stenosis and lumbar segmental angular motion, TM, and disc bulge migration.

STUDY DESIGN

A retrospective radiographic study.

PATIENT SAMPLE

Forty-five patients with a mean age of 44 years undergoing kMRI for symptoms of low back pain or radiculopathy.

OUTCOME MEASURES

Magnetic resonance imaging measurements of FA, angular motion, TM, and disc bulge migration.

METHODS

Kinetic magnetic resonance imaging of the lumbar spine was reviewed in 45 patients with low back pain or radiculopathy, and parasagittal images were evaluated for changes in neural foraminal dimensions in various degrees of motion with weight bearing. The changes in foraminal dimension were correlated to the amount of segmental angular motion, TM, and disc bulge migration at each level. Neural foramina were also assessed qualitatively by Wildermuth criteria. Only those foramina that were clearly visualized with well-defined anatomic boundaries in all three positions were taken into consideration. Patients with previous surgery, tumor, and scoliosis were excluded from the study.

RESULTS

There was a significant decrease in the FA from flexion to neutral (p<.05) at all levels except L5-S1 and from neutral to extension at all levels (p<.05). The average percent decrease in FA was 30.0% with the greatest decrease from flexion to extension occurring at L2-L3 (167-107 mm(2)) and the smallest change occurring at L5-S1 (135-106 mm(2)) (p<.05). The magnitude of change in FA increased as angular motion at a segment increased. The mean change in FA was 32.3 mm(2) when angular motion was less than 5° and was 75.16 mm(2) when angular motion exceeded 15°. The extent of disc bulging posteriorly in the neural foramen was also correlated with the reduction in the FA from flexion to extension, but TM had no effect.

CONCLUSIONS

Foraminal area decreased significantly in extension compared with flexion and neutral on MRI. Lumbar disc bulge migration and angular motion at each level contributed independently to the decrease in FA in extension, whereas TM had no effect on FA.

Secreted phosphoprotein 24 kD (Spp24) and Spp14 affect TGF-β induced bone formation differently

Transforming growth factor-β (TGF-β) and bone morphogenetic proteins (BMPs) have opposing but complementary functions in directing bone growth, repair, and turnover. Both are found in the bone matrix. Proteins that bind to and affect the activity of these growth factors will determine the relative abundance of the growth factors and, therefore, regulate bone formation. Secreted phosphoprotein 24 kD (Spp24) is a bone matrix protein that has been demonstrated to bind to and affect the activity of BMPs. The arginine-rich carboxy terminus of Spp24 is proteolytically processed to produce three other predictable truncation products (Spp18.1, Spp16.0, and Spp14.5). In this work, we report that kinetic data obtained by surface plasmon resonance demonstrate that Spp24 and the three C-terminal truncation products all bind to TGF-β1 and TGF-β2 with a similar but somewhat less affinity than they bind BMP-2; that, as in the case of BMP-2, the full-length (FL) form of Spp24 binds TGF-β with greater affinity than do the truncation products; that FL-Spp24 inhibits TGF-β2 induced bone formation in vivo, but Spp14.5 does not; and that co-administration of FL-Spp24 or Spp14.5 with TGF-β2 in vivo is associated with a reduction in the amount of cartilage, relative to new bone, present at the site of injection. This finding is consistent with the observation that low-dose TGF-β administration in vivo is associated with greater bone formation than high-dose TGF-β administration, and suggests that one function of Spp24 and its truncation products is to down-regulate local TGF-β activity or availability during bone growth and development. The similarities and differences of the interactions between Spp24 proteins and TGF-β compared to the interaction of the Spp24 proteins and BMPs have significant implications with respect to the regulation of bone metabolism and with respect to engineering therapeutic proteins for skeletal disorders.

Dynamic changes of the ligamentum flavum in the cervical spine assessed with kinetic magnetic resonance imaging

The purpose of this article is to quantify changes in thickness of the ligamentum flavum (LF) associated with motion of the cervical spine and to compare the thickness of the LF at each cervical level using kinetic magnetic resonance imaging (kMRI). Two hundred fifty-seven symptomatic patients (129 men; 128 women) underwent kMRI in neutral, flexion, and extension positions. Midsagittal images were digitally marked and electronically analyzed by spine surgeons. Thickness of LF in the cervical region from C2-3 to C7-T1 was measured in all three positions. LF at C7-T1 was significantly thicker than C2-3 to C6-7 in neutral, flexion, and extension positions (p < 0.05). LF was significantly thicker in extension than in flexion at C3-4 to C6-7. LF thickness increases with extension and decreases with flexion. LF is uniquely thick at C6-7 and at C7-T1 in the extension position, which may predispose these levels to cord compression syndromes and associated neuropathies.

A review of demineralized bone matrices for spinal fusion: the evidence for efficacy

BACKGROUND AND PURPOSE

Autologous Iliac Crest Bone Grafting (ICBG) is considered the gold-standard graft choice for spinal arthrodesis; however, it is associated with donor site morbidity and a limited graft supply. Bone graft alternatives to replace autograft and augment arthrodesis are a topic of ongoing research. This article will review properties of Demineralized Bone Matrix (DBM) and review the evidence for its use, including animal models and human clinical trials.

METHODS

A systematic and critical review of the English-language literature was conducted on Pubmed, Cochrane, CINAHL, and Google Scholar using search key terms such as 'Demineralized Bone Matrix', 'Spine' and 'Fusion'. Papers that were included were original research articles in peer-reviewed journals that investigated fusion outcomes. Scientific validity of articles was appraised using the PRISMA methodology. Articles were critically examined and compared according to study design, DBM type, outcomes, and results. Primary outcome of interest was fusion rate. Secondary outcomes included Oswestry Disability Index; Short Form-36 survey; Odom's criteria; Visual Analog Scale neurologic pain score; Japanese Orthopedic Association myelopathy score; Neck Disability and Ishihara Curvature Indices; and pseudarthrosis and surgical failure rates.

RESULTS

Demineralized Bone Matrix has been evaluated in animal models and human clinical trials of spine fusion. Results of animal studies indicate variation in performance within and among DBM products. The majority of human clinical trials report high fusion rates when DBM is employed as a graft extender or a graft enhancer. Few prospective randomized controlled trials have been performed comparing DBM to autologous iliac crest bone graft in spine fusion.

CONCLUSIONS

Although many animal and human studies demonstrate comparable efficacy of DBM when combined with autograft or compared to autograft alone, additional high level of evidence studies are required to clearly define the indications for its use in spine fusion surgeries and the appropriate patient population that will benefit from DBM.

Analysis of Relationship between Paraspinal Muscle Fatty Degeneration and Cervical Spine Motion Using Kinetic Magnetic Resonance Imaging

The alignment and mobility of the cervical spine is influenced by factors related to the vertebral bodies, intervertebral discs, ligaments, facet joints, and muscles. Few reports have described the role played by the paraspinal muscles in cervical spine mobility. In this study, we investigate the relationship between fatty degeneration of the paraspinal muscles and cervical motion as assessed with kinetic magnetic resonance imaging (kMRI). One hundred eighty-eight symptomatic patients underwent cervical kMRI in neutral, flexion, and extension positions. We quantified cervical paraspinal muscle fatty infiltration and measured angular variation and translational motion at each cervical level, and the global Cobb angle. Cervical paraspinal muscle fatty degeneration demonstrated a pattern in which C3 and C7 had significantly more fatty infiltration than C4, C5, and C6. Additionally, when the normal group was compared with the fatty degeneration group with respect to angular variation, translational motion, and Cobb angle, no significant differences were found except in angular variation at the C3-C4 level. In conclusion, we found a significantly larger quantity of fatty degeneration in the paraspinal muscles at C3 and C7 than the middle cervical levels. Also, we demonstrate that fatty degeneration does not significantly affect cervical lordotic alignment or mobility characteristics.

FKBP12, the 12-kDa FK506-binding protein, is a physiologic regulator of the cell cycle

FKBP12, the 12-kDa FK506-binding protein, is a ubiquitous abundant protein that acts as a receptor for the immunosuppressant drug FK506, binds tightly to intracellular calcium release channels and to the transforming growth factor beta (TGF-beta) type I receptor. We now demonstrate that cells from FKBP12-deficient (FKBP12(-/-)) mice manifest cell cycle arrest in G(1) phase and that these cells can be rescued by FKBP12 transfection. This arrest is mediated by marked augmentation of p21(WAF1/CIP1) levels, which cannot be further augmented by TGF-beta1. The p21 up-regulation and cell cycle arrest derive from the overactivity of TGF-beta receptor signaling, which is normally inhibited by FKBP12. Cell cycle arrest is prevented by transfection with a dominant-negative TGF-beta receptor construct. TGF-beta receptor signaling to gene expression can be mediated by SMAD, p38, and ERK/MAP kinase (extracellular signal-regulated kinase/mitogen-activated protein kinase) pathways. SMAD signaling is down-regulated in FKBP12(-/-) cells. Inhibition of ERK/MAP kinase fails to affect p21 up-regulation. By contrast, activated phosphorylated p38 is markedly augmented in FKBP12(-/-) cells and the p21 up-regulation is prevented by an inhibitor of p38. Thus, FKBP12 is a physiologic regulator of cell cycle acting by normally down-regulating TGF-beta receptor signaling.

Cardiac defects and altered ryanodine receptor function in mice lacking FKBP12

FKBP12, a cis-trans prolyl isomerase that binds the immunosuppressants FK506 and rapamycin, is ubiquitously expressed and interacts with proteins in several intracellular signal transduction systems. Although FKBP12 interacts with the cytoplasmic domains of type I receptors of the transforming growth factor-beta (TGF-beta) superfamily in vitro, the function of FKBP12 in TGF-beta superfamily signalling is controversial. FKBP12 also physically interacts stoichiometrically with multiple intracellular calcium release channels including the tetrameric skeletal muscle ryanodine receptor (RyR1). In contrast, the cardiac ryanodine receptor, RyR2, appears to bind selectively the FKBP12 homologue, FKBP12.6. To define the functions of FKBP12 in vivo, we generated mutant mice deficient in FKBP12 using embryonic stem (ES) cell technology. FKBP12-deficient mice have normal skeletal muscle but have severe dilated cardiomyopathy and ventricular septal defects that mimic a human congenital heart disorder, noncompaction of left ventricular myocardium. About 9% of the mutants exhibit exencephaly secondary to a defect in neural tube closure. Physiological studies demonstrate that FKBP12 is dispensable for TGF-beta-mediated signalling, but modulates the calcium release activity of both skeletal and cardiac ryanodine receptors.

Nitric oxide protects the skeletal muscle Ca2+ release channel from oxidation induced activation

Reactive oxygen intermediates and nitric oxide modulate the contractile function of skeletal muscle fibers, possibly via direct interaction with the Ca2+ release channel. Oxidants produce disulfide bonds between subunits of the Ca2+ release channel tetramer, and this is accompanied by an increase in channel activity. The sulfhydryl alkylating agent N-ethylmaleimide has three distinct effects on Ca2+ release channel activity: first, channel activity is decreased (phase 1); then with continued exposure the activity is dramatically increased (phase 2); and finally, the channel is again inhibited (phase 3) (Aghdasi, B., Zhang, J. Z., Wu, Y., Reid, M. B., and Hamilton, S. L., (1997) J. Biol. Chem. 272, 3739-3749). Both H2O2 and nitric oxide (NO) block the phase 1 inhibitory effect of N-ethylmaleimide. NO donors, at concentrations that have no detectable effect on channel activity, block intersubunit cross-linking and prevent activation of the channel by the disulfide inducing agent, diamide. These findings support a model in which NO modulates the activity of the Ca2+ release channel by preventing oxidation of regulatory sulfhydryls. However, higher concentrations of NO donors activate the channel and produce intersubunit cross-links, supporting a bifunctional effect of NO on channel activity. Low NO concentrations prevent oxidation of the Ca2+ release channel whereas higher concentrations oxidize it.

Functional interactions between cytoplasmic domains of the skeletal muscle Ca2+ release channel

The skeletal muscle Ca2+ release channel (RYR1), which plays a critical role in excitation-contraction coupling, is a homotetramer with a subunit molecular mass of 565 kDa. Oxidation of the channel increases its activity and produces intersubunit cross-links within the RYR1 tetramer (Aghdasi, B., Zhang, J., Wu, Y., Reid, M. B., and Hamilton, S. L. (1997) J. Biol. Chem. 272, 3739-3748). Alkylation of hyperreactive sulfhydryls on RYR1 with N-ethylmaleimide (NEM) inhibits channel function and blocks the intersubunit cross-linking. We used calpain and tryptic cleavage, two-dimensional SDS-polyacrylamide gel electrophoresis, N-terminal sequencing, sequence-specific antibody Western blotting, and [14C]NEM labeling to identify the domains involved in these effects. Our data are consistent with a model in which 1) diamide, an oxidizing agent, simultaneously produces an intermolecular cross-link between adjacent subunits within the RYR1 tetramer and an intramolecular cross-link within a single subunit; 2) all of the cysteines involved in both cross-links are in either the region between amino acids approximately 2100 and 2843 or the region between amino acids 2844 and 4685; 3) oxidation exposes a new calpain cleavage site in the central domain of the RYR1 (in the region around amino acid 2100); 4) sulfhydryls that react most rapidly with NEM are located in the N-terminal domain (between amino acids 426 and 1396); 5) alkylation of the N-terminal cysteines completely inhibits the formation of both inter- and intrasubunit cross-links. In summary, we present evidence for interactions between the N-terminal region and the putatively cytoplasmic central domains of RYR1 that appear to influence subunit-subunit interactions and channel activity.

Modulation of skeletal muscle Ca2(+)-release channel activity by sphingosine

The effect of D-erythro-C18-sphingosine (sphingosine) and related compounds on the Ca(2+)-release channel (ryanodine binding protein) was examined on rabbit skeletal muscle membranes, on the purified ryanodine binding protein, and on the channel reconstituted into planar lipid bilayers. Sphingosine inhibited [3H]ryanodine binding to sarcoplasmic reticulum (SR) membranes in a dose-dependent manner similar to published results (R. A. Sabbadini, R. Betto, A. Teresi, G. Fachechi-Cassano, and G. Salviati. J. Biol. Chem. 267: 15475-15484, 1992). The sphingolipid also inhibited [3H]ryanodine binding to the purified ryanodine binding protein. Our results demonstrate that the inhibition of [3H]ryanodine binding by sphingosine is due to an increased rate of dissociation of bound [3H]ryanodine from SR membranes and a decreased rate of association of [3H]ryanodine to the high-affinity site. Unlike other modulators of the Ca(2+)-release channel, sphingosine can remove bound [3H]ryanodine from the high-affinity site within minutes. Sphingosine increased the rate of dissociation of [3H]ryanodine bound to a solubilized proteolytic fragment derived from the carboxy terminus of the ryanodine binding protein (cleavage at Arg4475). Sphingosine also inhibited the activity of the Ca(2+)-release channel incorporated into planar lipid bilayers. Taken together, the data provide evidence for a direct effect of sphingosine on the Ca(2+)-release channel. Sphingosine is a noncompetitive inhibitor at the high-affinity ryanodine binding site, and it interacts with a site between Arg4475 and the carboxy terminus of the Ca(2+)-release channel.

A carboxy-terminal peptide of the alpha 1-subunit of the dihydropyridine receptor inhibits Ca(2+)-release channels

Excitation-contraction coupling in skeletal muscle is thought to involve a physical interaction between the alpha 1-subunit of the dihydropyridine receptor (DHPR) and the sarcoplasmic reticulum (SR) Ca(2+)-release channel (also known as the ryanodine receptor). Considerable evidence has accumulated to suggest that the cytoplasmic loop between domains II and III of the DHPR alpha 1-subunit is at least partially responsible for this interaction. Other parts of this subunit or other subunits may, however, contribute to the functional and/or structural coupling between these two proteins. A synthetic peptide corresponding to a conserved sequence located between amino acids 1487 and 1506 in the carboxy terminus of the alpha 1-subunit inhibits both [3H]ryanodine binding to skeletal and cardiac SR membranes and the activity of skeletal SR Ca(2+)-release channels reconstituted into planar lipid bilayers. A second, multiantigenic peptide synthesized to correspond to the same sequence inhibits both binding and channel activity at lower concentrations than the linear peptide. These peptides slow the rate at which [3H]ryanodine binds to its high-affinity binding site and decrease the rate at which [3H]ryanodine dissociates from this site. A third polypeptide synthesized in Escherichia coli and corresponding to amino acids 1381-1627 and encompassing the above sequence has similar effects. This portion of the alpha 1-subunit of the transverse tubule DHPR is therefore a candidate for contributing to the interaction of this protein with the Ca(2+)-release channel.

Multiple classes of sulfhydryls modulate the skeletal muscle Ca2+ release channel

Two sulfhydryl reagents, N-ethylmaleimide (NEM), an alkylating agent, and diamide, an oxidizing agent, were examined for effects on the skeletal muscle Ca2+ release channel. NEM incubated with the channel for increasing periods of time displays three distinct phases in its functional effects on the channel reconstituted into planar lipid bilayers; first it inhibits, then it activates, and finally it again inhibits channel activity. NEM also shows a three-phase effect on the binding of [3H]ryanodine by first decreasing binding (phase 1), followed by a recovery of the binding (phase 2), and then a final phase of inhibition (phase 3). In contrast, diamide 1) activates the channel, 2) enhances [3H]ryanodine binding, 3) cross-links subunits within the Ca2+ release channel tetramer, and 4) protects against phase 1 inhibition by NEM. All diamide effects can be reversed by the reducing agent, dithiothreitol. Diamide induces intersubunit dimer formation of both the full-length 565-kDa subunit of the channel and the 400-kDa generated by endogenous calpain digestion, suggesting that the cross-link does not involve sulfhydryls within the N-terminal 170-kDa fragment of the protein. NEM under phase 1 conditions blocks the formation of the intersubunit cross-links by diamide. In addition, single channels activated by diamide are further activated by the addition of NEM. Diamide either cross-links phase 1 sulfhydryls or causes a conformational change in the Ca2+ release channel which leads to inaccessibility of phase 1 sulfhydryls to NEM alkylation. The data presented here lay the groundwork for mapping the location of one of the sites of subunit-subunit contact in the Ca2+ release channel tetramer and for identifying the functionally important sulfhydryls of this protein.

Interaction between ryanodine and neomycin binding sites on Ca2+ release channel from skeletal muscle sarcoplasmic reticulum

Neomycin is a potent inhibitor of skeletal muscle sarcoplasmic reticulum (SR) calcium release. To elucidate the mechanism of inhibition, the effects of neomycin on the binding of [3H]ryanodine to the Ca2+ release channel and on its channel activity when reconstituted into planar lipid bilayer were examined. Equilibrium binding of [3H]ryanodine was partially inhibited by neomycin. Inhibition was incomplete at high neomycin concentrations, indicating noncompetitive inhibition rather than direct competitive inhibition. Neomycin and [3H]ryanodine can bind to the channel simultaneously and, if [3H]ryanodine is bound first, the addition of neomycin will slow the dissociation of [3H]ryanodine from the high affinity site. Neomycin also slows the association of [3H]ryanodine with the high affinity binding site. The neomycin binding site, therefore, appears to be distinct from the ryanodine binding site. Dissociation of [3H]ryanodine from trypsin-treated membranes or from a solubilized 14 S complex is also slowed by neomycin. This complex is composed of polypeptides derived from the carboxyl terminus of the Ca2+ release channel after Arg-4475 (Callaway, C., Seryshev, A., Wang, J. P., Slavik, K., Needleman, D. H., Cantu, C., Wu, Y., Jayaraman, T., Marks, A. R., and Hamilton, S. L. (1994) J. Biol. Chem. 269, 15876-15884). The proteolytic 14 S complex isolated with ryanodine bound produces a channel upon reconstitution into planar lipid bilayers, and its activity is inhibited by neomycin. Our data are consistent with a model in which the ryanodine binding sites, the neomycin binding sites, and the channel-forming portion of the Ca2+ release channel are located between Arg-4475 and the carboxyl terminus.