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Germ cell mitogenic activity is associated with nerve growth factor‐like protein(s)

The mitogenicity of germ cell proteins released from round spermatids (RS) and pachytene spermatocytes (PS) was investigated. Germ cells were isolated by centrifugal elutriation from 90‐day‐old rat testes and incubated in a supplement enriched culture media that lacked exogenous proteins. The conditioned culture media of RS and PS were dialysed/concentrated and lyophilized to prepare RS protein (RSP) and PS protein (PSP). Mitogenic activity of RSP and PSP was determined by 3H‐thymidine incorporation into Swiss 3T3 fibroblasts. RSP and PSP stimulated 3H‐thymidine incorporation by fibroblasts in a dose‐dependent manner. At a higher concentration of RSP (300 μg/ml), fibroblast proliferation was stimulated from 6‐ to 20‐fold of control cultures, whereas PSP (300 μg/ml) stimulated fibroblast proliferation 2.5‐fold of control cultures. Since RSP exhibited substantially greater mitogenic activity than PSP we further investigated the RSP mitogenic substance(s) by immunoneutralization with antibodies against several growth factors. The mitogenic activity of RSP was significantly reduced by treatment with nerve growth factor (NGF) antibody, while neither the treatment of RSP with acidic fibroblast growth factor (aFGF) antibody, nor basic fibroblast growth factor (bFGF) antibody significantly modified the mitogenic activity of RSP. Interestingly, murine NGF‐β, recombinant human NGF‐β, and bovine serum albumin (BSA) did not exhibit mitogenic activity on 3T3 fibroblasts. Nevertheless, the presence of a NGF‐like protein in RS and PS was confirmed by indirect immunofluorescence staining with a murine NGF antibody. Subsequently, a Western blot analysis with the NGF antibody identified two immunoreactive bands of 41 ± 2 kDa and 51 ± 1 kDa in both RSP and PSP under reduced conditions. These germ cell NGF‐like proteins were apparently different from similarly prepared murine and human NGFs (13 kDa) in their molecular weight. Furthermore, neurite outgrowth from pheochromocytoma cells (PC‐12), a functional bioassay for NGF‐like activity, was stimulated by addition of RSP and PSP to the culture media of the PC‐12 cells. These results demonstrate mitogenic activity in germ cell proteins (RSP and PSP) and identify a NGF‐like protein(s) which is associated with most of this activity.