P5. Monocarboxylate transporter expression directly affects degeneration of the intervertebral disc in mice

Abstract

<h3>BACKGROUND CONTEXT</h3> Disc degeneration is the central component involved in numerous common spinal pathologies. Low back pain remains a leading cause of disability in the world, yet treatment remains nonspecific and of poor efficacy. Degenerative disc disease (DDD) affects millions of people every year, yet the mechanisms driving degeneration remain poorly understood. Recent work has shown that high levels of intracellular lactate are involved in driving disc degeneration. Whether this loss of ability to export lactate is specific to a few models or common across modes of degeneration is yet to be determined. <h3>PURPOSE</h3> We utilized different mouse models of intervertebral disc (IVD) degeneration in order to better understand the role of lactate and lactate transport in DDD. <h3>STUDY DESIGN/SETTING</h3> Randomized, controlled animal trial. <h3>PATIENT SAMPLE</h3> Female mice (C57BL/6J, n=30). <h3>OUTCOME MEASURES</h3> Mice were euthanized at 2, 4 and 8 weeks. The IVD was evaluated by histological and immunofluorescence (IF) analysis. RNA extraction from disc tissue was analyzed with QPCR. Data are presented as means +/-SD. <h3>METHODS</h3> Female mice (C57BL/6J, n=30) were randomly assigned to one of four models of disc degeneration: 1) lumbar disc poke, 2) tail disc poke, 3) spinal instability and 4) sham. In group 1, a retroperitoneal approach exposed the IVD of the lumbar spine, and a 27G needle was used to injure the disc. In group 2, the needle was inserted in the tail IVD. In group 3, lumbar instability is induced by resection of bilateral facet joints and supra/interspinous ligaments. A sham group was used for each. <h3>RESULTS</h3> In group 1 and 2, the degenerative process at 2, 4 and 8 weeks was characterized by loss of nucleus pulposus (NP) cells and the gradual increase in matrix components. The distinction between NP and annulus fibrosus (AF) or endplate cartilage (ECP) was lost. Sham mice had no significant disc degeneration. There was increased expression of COL10 and MMP13 in the NP and decreased expression of GLUT1 (NP marker) in the disc-poke groups. The expression of a central lactate transporter (MCT4) was also reduced in injured mice, and there was an increase in catabolic chondrocyte markers. In group 2 there was also decreasedhexokinase-2 (HK2) expression and an increase in MCT3 expression. In group 3, disordered AF was seen 2 weeks postoperatively, with decreased disc space and fewer NP cells. The proteoglycan inner layer of AF and periphery of NP were decreased at 8 weeks. If confirmed an increase in COL10 and MMP13 in the inner AF and NP. Both NP and CEP of sham mice maintained strong expression of MCT4 while the NP and CEP in injured mice had reduced expression of MCT4. <h3>CONCLUSIONS</h3> Our results demonstrate a common molecular pathway whereby discs degenerate after direct injury or after becoming unstable. In our model there is rapid degeneration of the IVD in mice who exhibit up/down-regulation of several key markers. Importantly, MCT4 is down-regulated, while MCT3 is upregulated. Decreased MCT4 expression is a key and common step in the degeneration of the IVD. While MCT4 is associated with lactate exportation, and its loss results in elevated intracellular lactate and disc degradation, MCT3 is rarely expressed and may be acting as a "rescue" lactate transporter here. Decreased HK2 gene expression led to reduced glycolysis and mitochondrial dysfunction, leading to increased formation of ROS and oxidative stress; HK2 may be important to disc degeneration. We demonstrated a regulatory role for lactate transport in DDD, with decreased lactate exportation common to different degenerative mechanisms. Promoting MCT4 or MCT3 gene expression may represent a novel therapeutic option in DDD. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.