Recombinant Human Neuroplastin 65 Protein, CF

Neuroplastin 65 (Np65; also SDR1) is a 62‑75 kDa glycoprotein member of the CD147/Bsg family, Ig superfamily of molecules (1‑3). Although originally called either gp65 or stromal cell‑derived factor‑1, it was renamed neuroplastin to reflect its role in synapse remodeling (4). Np65 is a type I transmembrane (TM) protein that is expressed on the cell surface. The human Np65 precursor is 398 amino acids (aa) in length, and contains a 28 aa signal sequence, a 311 aa extracellular region, a 21 aa TM segment (aa 340‑360), and a 38 aa cytoplasmic domain (5, 6). Like other CD147 family members, it contains multiple Ig‑like domains in its extracellular region (three, in the case of Np65), and a positively charged Glu residue in its TM segment. Presumably, the Glu facilitates Np65 interaction with other TM proteins (1). With respect to the three Ig‑like domains, the N‑terminal domain is a V‑type, while the remaining two qualify as I‑type domains (2, 7). Through the V‑type domain, Np65 achieves homodimerization in trans, while the I‑type domain mediates protein‑protein interactions in cis (1, 2, 7). There are multiple splice forms associated the the Np65 gene. One is well characterized and termed Np55. This shows a Glu substitution for aa‑31‑147, resulting in the loss of the homodimerizing V‑type Ig‑like domain (6, 7). Although the designation “55” suggests a fixed native MW of 55 kDa, it is reported to range between 44 and 65 kDa in SDS‑Page (5). A second isoform impacts the cytoplasmic domain and shows a deletion of aa 372‑375. Since the Np65 cytoplasmic domain is not believed to participate in any downstream signaling events, its significance is unclear (2). Finally, an 18 aa substitution for aa 320‑398 has also been reported. Mature human Np65 shares 95% aa sequence identity with mouse Np65 in the extracellular domain.

Unlike Np55, which is widely, if not ubiquitously, expressed, Np65 expression is believed to be restricted to synapses of neurons and neuronal cell types of the CNS (2, 7). In human, Np65 is reported to be expressed by retinal photoreceptors (8), Purkinje and granule cells of the cerebellum (3), and pyramidal neurons throughout the various CA regions of the hippocampus (3). In all cases, it would appear that Np65 is both a pre‑ and postsynaptic protein that has at least two functions. First, it serves as a cell adhesion molecule, forming homodimers in trans via its V‑type Ig‑like domain and, second, it forms a complex with FGFR1 in cis via its second I-type Ig‑like domain, initiating downstream signaling through the FGF receptor (2, 7, 9). Although the particulars are unclear, at least two signaling pathways are involved. One involves p38 MAP kinase that, when activated, promotes GluR1cycling away from the cell membrane. This has a net effect of blocking LTP induction in the hippocampus (2, 9, 10). ERK1/2 and CaMKII can also be activated, resulting in the initiation of neurite outgrowth (2, 9). The diversity of outcomes associated with a common ligand‑receptor pair suggests the potential for a highly nuanced response program that may involve additional receptor elements.