Expression of semaphorin 3A and its receptors in the human intervertebral disc: potential role in regulating neural ingrowth in the degenerate intervertebral disc

Abstract

IntroductionIntervertebral disc (IVD) degeneration is considered a major underlying factor in the pathogenesis of chronic low back pain. Although the healthy IVD is both avascular and aneural, during degeneration there is ingrowth of nociceptive nerve fibres and blood vessels into proximal regions of the IVD, which may contribute to the pain. The mechanisms underlying neural ingrowth are, however, not fully understood. Semaphorin 3A (sema3A) is an axonal guidance molecule with the ability to repel nerves seeking their synaptic target. This study aimed to identify whether members of the Class 3 semaphorins were expressed by chondrocyte-like cells of the IVD addressing the hypothesis that they may play a role in repelling axons surrounding the healthy disc, thus maintaining its aneural condition.MethodsHuman IVD samples were investigated using reverse transcription polymerase chain reaction (RT-PCR) to identify gene expression of sema3A, 3F and their receptors: neuropilins (1 and 2) and plexins (A1-4). Sema3A protein was also localised within sections of normal and degenerate human IVD and immunopositivity quantified. Serial sections were stained using PGP9.5 and CD31 to correlate semaphorin 3A expression with nerve and blood vessel ingrowth, respectively.ResultsSema3A protein was expressed highly in the healthy disc, primarily localised to the outer annulus fibrosus. In degenerate samples, sema3A expression decreased significantly in this region, although cell clusters within the degenerate nucleus pulposus exhibited strong immunopositivity. mRNA for sema3A receptors was also identified in healthy and degenerate tissues. CD31 and PGP9.5 were expressed most highly in degenerate tissues correlating with low expression of sema3A.ConclusionsThis study is the first to establish the expression of semaphorins and their receptors in the human IVD with a decrease seen in the degenerate painful IVD. Sema3A may therefore, amongst other roles, act as a barrier to neuronal ingrowth within the healthy disc.

Aging and degeneration of the intervertebral disc: review of basic science

“…8 It is assumed that the IVD cells lose the anti-angiogenic and anti-axonogenic factors in the degenerative process. 25,60,61 Thus, two neurotrophins have revealed a close relationship with this process: NGF-β (nerve growth factor) ( Figure 2D) and BDNF (brain-derived neurotrophic factor) ( Table 1). However, BDNF is the neurotrophic most closely related to nociceptive sensitivity in the IVD.…”

“…64 The results of research on the interaction between the axonogenesis process, angiogenesis and remodeling of the disc extracellular matrix [65][66][67][68] make it clear that the constituents of the IVD ECM act as preventive agents of this process, as do the molecules of the collapsins group (semaphorin). 60,69 Other evidence has been found, such as the modulation of the expression of these neurotrophins through IL-1β, 49,70,71 the proportionality between the severity of disc degeneration and the amount of cytokines detected, 49 and the association between the reduction of semaphorins and greater expression of PECAM-1 in the tissue. 60 One of the most interesting data currently verified in the literature was the ability of the NP cells to produce mediators capable of inhibiting or stimulating ( Figure 2D) nerve growth to this region of the IVD, proving the theory that there are anti-angiogenic and antiaxonogenic agents in the disc tissue, and that these cells have the ability to modulate nerve growth into the tissue.…”

“…60,69 Other evidence has been found, such as the modulation of the expression of these neurotrophins through IL-1β, 49,70,71 the proportionality between the severity of disc degeneration and the amount of cytokines detected, 49 and the association between the reduction of semaphorins and greater expression of PECAM-1 in the tissue. 60 One of the most interesting data currently verified in the literature was the ability of the NP cells to produce mediators capable of inhibiting or stimulating ( Figure 2D) nerve growth to this region of the IVD, proving the theory that there are anti-angiogenic and antiaxonogenic agents in the disc tissue, and that these cells have the ability to modulate nerve growth into the tissue. 61,68 Genetics and disc degeneration Few genetic associations have been made in the field of disc degeneration.…”

Effects of proinflammatory cytokines on axonal outgrowth from adult rat lumbar dorsal root ganglia using a novel three-dimensional culture system

“…Although the precise mechanisms are not well understood, various factors have been posited to regulate nerve ingrowth into the lumbar IVD [9][10][11][12][13][14]. In terms of biochemical stimulation, it is widely held that causal links between proinflammatory cytokines and neurotrophic factors, particularly nerve growth factor (NGF), influence the growth potential of DRG neurons into the IVDs [15,16].…”

Análise do envelhecimento e degeneração de discos intervertebrais humanos cervicais e lombares

“…A maior parte da literatura pertinente utiliza espécimes doentes, portanto sintomáticos, obtidos em cirurgias de hérnia discal (protrusão, extrusão e sequestro) 23,37,124,133,156 ; espondilólise/espondilolistese 28,39 ; escoliose 161 e cervicalgia/lombalgia (dissectomia) 22,123,153,164 . Trabalhos que relacionam indivíduos sintomáticos x assintomáticos admitindo esses últimos como controle normal, exibem um número menor desses espécimes, por serem de origem cadavérica 20,22,31,36 .…”