Effects of neuraminidase on DNA synthesis, proliferation and endocytosis of cultivated arterial smooth muscle cells

Heterodimerization of the sialidase NEU1 with the chaperone protective protein/cathepsin A prevents its premature oligomerization

Lysosomal neuraminidase-1 (NEU1) forms a multienzyme complex with beta-galactosidase and protective protein/cathepsin A (PPCA). Because of its association with PPCA, which acts as a molecular chaperone, NEU1 is transported to the lysosomal compartment, catalytically activated, and stabilized. However, the mode(s) of association between these two proteins both en route to the lysosome and in the multienzyme complex has remained elusive. Here, we have analyzed the hydrodynamic properties of PPCA, NEU1, and a complex of the two proteins and identified multiple binding sites on both proteins. One of these sites on NEU1 that is involved in binding to PPCA can also bind to other NEU1 molecules, albeit with lower affinity. Therefore, in the absence of PPCA, as in the lysosomal storage disease galactosialidosis, NEU1 self-associates into chain-like oligomers. Binding of PPCA can reverse self-association of NEU1 by causing the disassembly of NEU1-oligomers and the formation of a PPCA-NEU1 heterodimeric complex. The identification of binding sites between the two proteins allowed us to create innovative structural models of the NEU1 oligomer and the PPCA-NEU1 heterodimeric complex. The proposed mechanism of interaction between NEU1 and its accessory protein PPCA provides a rationale for the secondary deficiency of NEU1 in galactosialidosis.

Neuraminidase-1, a subunit of the cell surface elastin receptor, desialylates and functionally inactivates adjacent receptors interacting with the mitogenic growth factors PDGF-BB and IGF-2

We recently established that the elastin-binding protein, which is identical to the spliced variant of beta-galactosidase, forms a cell surface-targeted complex with two proteins considered "classic lysosomal enzymes": protective protein/cathepsin A and neuraminidase-1 (Neu1). We also found that cell surface-residing Neu1 can desialylate neighboring microfibrillar glycoproteins and facilitate the deposition of insoluble elastin, which contributes to the maintenance of cellular quiescence. Here we provide evidence that cell surface-residing Neu1 contributes to a novel mechanism that limits cellular proliferation by desialylating cell membrane-residing sialoglycoproteins that directly propagate mitogenic signals. We demonstrated that treatment of cultured human aortic smooth muscle cells (SMCs) with either a sialidase inhibitor or an antibody that blocks Neu1 activity induced significant up-regulation in SMC proliferation in response to fetal bovine serum. Conversely, treatment with Clostridium perfringens neuraminidase (which is highly homologous to Neu1) decreased SMC proliferation, even in cultures that did not deposit elastin. Further, we found that pretreatment of aortic SMCs with exogenous neuraminidase abolished their mitogenic responses to recombinant platelet-derived growth factor (PDGF)-BB and insulin-like growth factor (IGF)-2 and that sialidosis fibroblasts (which are exclusively deficient in Neu1) were more responsive to PDGF-BB and IGF-2 compared with normal fibroblasts. Furthermore, we provide direct evidence that neuraminidase caused the desialylation of both PDGF and IGF-1 receptors and diminished the intracellular signals induced by the mitogenic ligands PDGF-BB and IGF-2.

Plasma membrane desialylation and DNA synthesis are induced via adrenergic receptors of the beta 1-subtype in mouse parotid glands

Induction of DNA synthesis and plasma membrane desialylation in the early prereplicative period in isoproterenol-stimulated mouse parotid glands seem to occur in a close relationship. beta 1, beta 2 or both subtypes of adrenergic receptors could be involved in those responses. To discriminate between those alternatives, dose-dependency studies were addressed to assess the abilities of the selective beta-adrenergic agonists dobutamine (beta 1) and salbutamol (beta 2) to induce plasma membrane desialylation and DNA synthesis. Likewise, the abilities of the selective beta-adrenoceptor antagonists atenolol (beta 1) and ICI 118,551 (beta 2) to interfere with the induction of both responses were also studied. Dobutamine was found to be at least 10-fold more potent than salbutamol in inducing DNA synthesis whereas ICI 118,551 was much weaker than atenolol, as inhibitor of the agonist-induced DNA synthesis. On the other hand, dobutamine was also more potent than salbutamol in provoking plasma membrane desialylation. However, whenever an increase in the rate of DNA synthesis was induced by either agonist, a constant 50% reduction in the levels of sialic acid in the plasma membranes was observed. In the same direction, atenolol, at variance to ICI 118,551, was able to produce a full suppression of the plasma membrane desialylation induced by either selective agonist. Taken together, these results suggest that in mouse parotid glands, both DNA synthesis and plasma membrane desialylation during the early prereplicative period are induced preferentially via adrenergic receptors of the beta 1-subtype.