Activin A binds to perlecan through its pro-region that has heparin/heparan sulfate binding activity.

Activin A, a member of the transforming growth factor-ß family, plays important roles in hormonal homeostasis and embryogenesis. In this study, we produced recombinant human activin A and examined its abilities to bind to extracellular matrix proteins. Recombinant activin A expressed in 293-F cells was purified as complexes of mature dimeric activin A with its pro-region. Among a panel of extracellular matrix proteins tested, recombinant activin A bound to perlecan and agrin, but not to laminins, nidogens, collagens I and IV, fibronectin, and nephronectin. The binding of recombinant activin A to perlecan was inhibited by heparin and high concentrations of NaCl and abolished by heparitinase treatment of perlecan, suggesting that activin A binds to the heparan sulfate chains of perlecan. In support of this possibility, recombinant activin A was capable of directly binding to heparin and heparan sulfate chains. Site-directed mutagenesis of recombinant activin A revealed that clusters of basic amino acid residues, Lys(259)-Lys(263) and Lys(270)-Lys(272), in the pro-region were required for binding to perlecan. Interestingly, deletion of the peptide segment Lys(259)-Gly(277) containing both basic amino acid clusters from the pro-region did not impair the activity of activin A to stimulate Smad-dependent gene expressions, although it completely ablated the perlecan-binding activity. The binding of activin A to basement membrane heparan sulfate proteoglycans through the basic residues in the pro-region was further confirmed by in situ activin A overlay assays using frozen tissue sections. Taken together, the present results indicate that activin A binds to heparan sulfate proteoglycans through its pro-region and thereby regulates its localization within tissues.

Identification and characterization of nCLP2, a novel C1q family protein expressed in the central nervous system.

The C1q family is characterized by the C-terminally conserved globular C1q (gC1q) domain. Although more than 30 C1q family proteins have been identified in mammals, many of them remain ill-defined with respect to their molecular and biological properties. Here, we report on a novel C1q family protein specifically expressed in the central nervous system (CNS), which we designated neural C1q-like protein (nCLP) 2. nCLP2 was secreted as disulphide-bonded multimers comprising trimeric units. The multimers were stabilized by interchain disulphide bonds involving the cysteine residues in the N-terminal variable region and the C-terminal gC1q domain. The expression of nCLP2 was restricted to several brain regions and retina, including regions associated with memory formation (i.e. hippocampus, entorhinal cortex, anterodorsal thalamic nucleus). Immunoelectron microscopy revealed that nCLP2 was localized in the mossy fibre axons of hippocampal granule cells and their synaptic boutons and clefts, implying that nCLP2 was anterogradely transported in mossy fibres and secreted from the presynaptic termini. These results suggest that nCLP2 plays roles in synaptic function and maintenance in the CNS.

Identification and recombinant production of human laminin alpha4 subunit splice variants.

Laminins, the major basement membrane glycoproteins, are composed of three subunits. We identified a splice variant of the human laminin alpha4 subunit transcript containing 21 extra nucleotides. A heptapeptide sequence, MDCPTIS, was inserted close to the two cysteine residues possibly involved in the intersubunit disulfide bonds. Both the authentic alpha4 subunit (alpha4A) and the variant with the heptapeptide insertion (alpha4B) were readily secreted as laminin-8 trimers (alpha4Abeta1gamma1 or alpha4Bbeta1gamma1) upon cotransfection with expression vectors for the beta1 and gamma1 subunits. The purified recombinant laminin-8 containing the alpha4B subunit was more potent in promoting cell spreading than that containing alpha4A, raising the possibility that the alternative splicing of the alpha4 subunit transcript regulates the cell-adhesive activity of laminin-8. Since both alpha4A and alpha4B transcripts were detected by RT-PCR in several human cell lines, these two isoforms of laminin-8 with differing cell-adhesive activities are present in the basement membranes of human tissues.