[Clinical aspects and diagnosis of polyneuropathies]

Activation of the RAGE pathway: a general mechanism in the pathogenesis of polyneuropathies?

OBJECTIVES

Binding of ligands to the receptor for advanced glycation end products (RAGE) results in activation of the transcription factor NF-kappaB and subsequent expression of NF-kappaB regulated cytokines and is a possible pathomechanism in diabetic and in vasculitic polyneuropathies (PNP). We wanted to investigate whether the newly discovered RAGE pathway also contributes to the pathogenesis of various other PNP.

METHODS

The presence of the RAGE ligand Nepsilon-Carboxymethyllysine (CML), the receptor itself and NF-kappaBp65 was studied in sural nerve biopsies of patients with alcohol-associated PNP (n=5), PNP owing to vitamin B12 deficiency (n=5), chronic inflammatory demyelinating PNP (CIDP, n=10), Charcot-Marie-Tooth disease (CMT) I or II (n= 10), PNP caused by monoclonal gammopathy of unknown significance (MGUS) (n=5), idiopathic PNP (n=10) and five normal controls by immunohistochemistry. Biopsies of either ten patients with diabetic and vasculitic PNP served as positive controls.

RESULTS

CML, RAGE and NF-kappaBp65 were found in co-localization in epineurial vessels in PNP owing to vitamin B12 deficiency, diabetes and vasculitis and in the perineurium in diabetic PNP, vasculitic PNP and in some cases in CIDP and vitamin B12 deficiency. Only diabetic subjects demonstrated co-expression of the three antigens in endoneurial vessels. Increased CML, RAGE and NF-kappaBp65 expression was detected in endoneurial and epineurial mononuclear cells in CIDP and in vasculitic PNP. Additionally, RAGE expression in Schwann cells was significantly increased in diabetic PNP.

DISCUSSION

These data suggest that activation of the RAGE pathway might contribute to the pathogenesis of CIDP, PNP owing to vitamin B12 deficiency, diabetes and vasculitis, whereas it does not seem to be involved in the pathogenesis of PNP owing to alcohol, MGUS, CMT I or II and idiopathic PNP.

Loss of pain perception in diabetes is dependent on a receptor of the immunoglobulin superfamily

Molecular events that result in loss of pain perception are poorly understood in diabetic neuropathy. Our results show that the receptor for advanced glycation end products (RAGE), a receptor associated with sustained NF-kappaB activation in the diabetic microenvironment, has a central role in sensory neuronal dysfunction. In sural nerve biopsies, ligands of RAGE, the receptor itself, activated NF-kappaBp65, and IL-6 colocalized in the microvasculature of patients with diabetic neuropathy. Activation of NF-kappaB and NF-kappaB-dependent gene expression was upregulated in peripheral nerves of diabetic mice, induced by advanced glycation end products, and prevented by RAGE blockade. NF-kappaB activation was blunted in RAGE-null (RAGE(-/-)) mice compared with robust enhancement in strain-matched controls, even 6 months after diabetes induction. Loss of pain perception, indicative of long-standing diabetic neuropathy, was reversed in WT mice treated with soluble RAGE. Most importantly, loss of pain perception was largely prevented in RAGE(-/-) mice, although they were not protected from diabetes-induced loss of PGP9.5-positive plantar nerve fibers. These data demonstrate, for the first time to our knowledge, that the RAGE-NF-kappaB axis operates in diabetic neuropathy, by mediating functional sensory deficits, and that its inhibition may provide new therapeutic approaches.