7S-NERVE GROWTH FACTOR, MOUSE

NGF is the first member of neurotrophins. In the first half of the 20th century, it had been experimentally demonstrated in embryos that the nonneuronal “periphery” is essential for the differentiation of the sensory and motor centers of the central nervous system and the sensory ganglia. By 1951, Rita Levi-Montalcini and Viktor Hamburger discovered that certain mouse tumors release a “substance” that induces nerve fibers to grow in a chick embryo. With Stanley Cohen, they isolated “nerve growth-stimulating factor,” a preparation containing both protein and nucleic acid. Subsequently, “nerve growth factor” was purified from snake venom and mouse salivary glands, and identified as a protein. Its complete amino acid sequence was determined in 1971.

NGF consists of three types of subunits, α, β, and γ, which specifically interact to form a 7S, 130-kDa complex. The 7S complex contains two identical β-subunits. Among the three types of subunits, β-subunit only has bioactivity and therefore is referred to as NGF. The human NGF gene (β-subunit, β-NGF) encodes a preproprotein of 241 aa residues (preproNGF) . After cleavage by the protease activity of the γ-subunit, a 120-aa peptide of NGF (β-subunit), which has biological activity, is generated. The β-subunit dimerizes through noncovalent interaction, and presents as a 2.5S, 26-kDa protein. NGF has been identified in mammals, birds, reptiles, amphibians, and teleosts. Six cysteines involved in the intramolecular disulfide bond are conserved among all vertebrate NGFs.

Mr. 26,518 (mouse preproNGF); 13,259 (mouse mature NGF). pI 9.3. NGF is soluble in water. Disulfide bridges are formed between Cys15 and Cys80, between Cys58 and Cys108, and between Cys68 and Cys110 (in mammalian mature NGFs).

The human preproNGF (β-NGF) gene, NGF, location 1p13.2, consists of three exons. Human NGF mRNA has 1061 bases that encode a preproNGF of 241 aa residues. ProNGF shows broad expression in the ovary (RPKM 1.3), the heart (RPKM 0.5), and 18 other tissues, including the brain.

The expression of the NGF gene is regulated by the developmental stage, neuronal innervation, and neuronal activity. NGF mRNA levels are increased during postnatal development in the rat hippocampus and basal ganglia. Both continuous release and neuronal activity-dependent release are reported. The release from cultured hippocampal neurons is enhanced more than three-fold within several minutes by a depolarizing stimulus.

NGF binds to at least two classes of receptors: tropomyosin receptor kinase A (TrkA), the high-affinity receptor, and p75NTR, the low affinity receptor. Both TrkA and p75NTR are single-transmembrane receptors. The human NTRK1 gene encodes TrkA (Trk1) of 790 aa residues. The human NGFR gene encodes p75NTR of 427 aa residues. TrkA has a tyrosine-kinase domain in the cytoplasmic region. p75NTR has no tyrosine-kinase domain. The binding activity of mature NGF: Kd=10 pM (TrkA) and 1 nM (p75NTR). ProNGF also binds to p75NTR (Kd: 100 pM) and TrkA (Kd: 1 nM).

Both TrkA and p75NTR are expressed in sensory neurons, sympathetic neurons, and cholinergic neurons. The activation of TrkA induces the survival of these neurons, promotes outgrowth of the neurite, and stimulates the synthesis of neurotransmitters. The involvement of p75NTR in apoptosis is observed in the avian retina, mouse basal forebrain, and rat cerebral cortex. p75NTR-mediated apoptosis of the rat oligodendrocytes is induced specifically by NGF but not by other neurotrophins.

NGF is the first member of the neurotrophin-family trophic factors. NGF regulates the growth, maintenance, proliferation, and survival of certain target neurons through activation of the high affinity receptor, tropomyosin receptor kinase A (TrkA). NGF is also involved in apoptosis through binding to the low affinity receptor, p75NTR. NGF consists of three types of subunits, α, β, and γ, and the β-subunit is solely responsible for the nerve growth stimulating activity of NGF.

NGF is now known to have therapeutic potential on peripheral and central nervous system diseases but also on the visual system, cutaneous wound healing, and the regulation of immune system functions. NGF shows protective effects on cholinergic neurons in the brains of Alzheimer’s disease model animals. NGF also shows promoting effects on neurite outgrowth in the aged human and rat.