Is part of the molecular basis of the perineurial barrier function the lack of endogenous carbohydrate-binding proteins?

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Abstract
The sugar part of cellular glycoconjugates and specific endogenous sugar receptors, i.e., lectins, can establish a system of biological recognition based on proteincarbohydrate interactions. An assortment of labelled (neo)glycoproteins, carrying different types of sugar moieties, is synthesized to localize respective sugar receptors. With these tools, the histochemical patterns of endogenous carbohydrate‐binding receptors of the epi‐, peri‐, and endoneurium were analyzed in human sural and accessory nerves and in swine sciatic nerve. This approach is complementary to the application of plant lectins, focusing on endogenous carbohydrate‐binding proteins (lectins). In contrast to the epi‐ and endoneurium, which bound certain types of carbohydrates, such endogenous sugar receptors were histochemically not detectable in the perineurial cells. Moreover, no histochemical reaction was present in the “connective tissue septa” localized in the endoneurium in which the endoneurial vessels were embedded. This common property supplies evidence that these septa are composed of perineurial cells. They may represent a barrier in addition to the capillary endothelium. Our observations suggest histogenetical differences between the cell populations of epi‐and endoneurium vs. perineurium. This significant difference in the ability to bind carbohydrate residues, conjugated to a carrier protein, is contradictory to the assumption that perineurial cells and fibroblasts are functional variants of the same cell type. The histochemical patterns of endogenous carbohydrate‐binding receptors found in human and swine nerves were similar but not identical, with exception of the perineurium, reflecting phylogenetic differences in the expression of sugar‐binding proteins. The absence of specific sugar receptors in perineurial cells, however, seems to be a more general phenomenon. It can lead to a profoundly limited recognitive capacity of these cells within a sugar‐code system of biological information, possibly contributing to the molecular basis of the barrier function.