Nerve growth factor


Nerve growth factor is a neurotrophic factor and neuropeptide primarily involved in the regulation of growth, maintenance, proliferation, and survival of certain target neurons. It is perhaps the prototypical growth factor, in that it was one of the first to be described. Since it was first isolated by Nobel Laureates Rita Levi-Montalcini and Stanley Cohen in 1956, numerous biological processes involving NGF have been identified, two of them being the survival of pancreatic beta cells and the regulation of the immune system.

Structure

NGF is initially in a 7S, 130-kDa complex of 3 proteins - Alpha-NGF, Beta-NGF, and Gamma-NGF when expressed. This form of NGF is also referred to as proNGF. The gamma subunit of this complex acts as a serine protease, and cleaves the N-terminal of the beta subunit, thereby activating the protein into functional NGF.
The term nerve growth factor usually refers to the 2.5S, 26-kDa beta subunit of the protein, the only component of the 7S NGF complex that is biologically active.

Function

As its name suggests, NGF is involved primarily in the growth, as well as the maintenance, proliferation, and survival of nerve cells. In fact, NGF is critical for the survival and maintenance of sympathetic and sensory neurons, as they undergo apoptosis in its absence. However, several recent studies suggest that NGF is also involved in pathways besides those regulating the life cycle of neurons.

Neuronal proliferation

NGF can drive the expression of genes such as bcl-2 by binding to the TrkA receptor, which stimulates the proliferation and survival of the target neuron.
High affinity binding between proNGF, sortilin, and p75NTR can result in either survival or programmed cell death. Study results indicate that superior cervical ganglia neurons that express both p75NTR and TrkA die when treated with proNGF, while NGF treatment of these same neurons results in survival and axonal growth. Survival and PCD mechanisms are mediated through adaptor protein binding to the death domain of the p75NTR cytoplasmic tail. Survival occurs when recruited cytoplasmic adaptor proteins facilitate signal transduction through tumor necrosis factor receptor members such as TRAF6, which results in the release of nuclear factor κB transcription activator. NF-κB regulates nuclear gene transcription to promote cell survival. Alternatively, programmed cell death occurs when TRAF6 and neurotrophin receptor interacting factor are both recruited to activate c-Jun N-terminal kinase ; which phosphorylates c-Jun. The activated transcription factor c-Jun regulates nuclear transcription via AP-1 to increase pro-apoptotic gene transcription.

Proliferation of pancreatic beta cells

There is evidence that pancreatic beta cells express both the TrkA and p75NTR receptors of NGF. It has been shown that the withdrawal of NGF induces apoptosis in pancreatic beta cells, signifying that NGF may play a critical role in the maintenance and survival of pancreatic beta cells.

Regulation of the immune system

NGF plays a critical role in the regulation of both innate and acquired immunity. In the process of inflammation, NGF is released in high concentrations by mast cells, and induces axonal outgrowth in nearby nociceptive neurons. This leads to increased pain perception in areas under inflammation. In acquired immunity, NGF is produced by the Thymus as well as CD4+ T cell clones, inducing a cascade of maturation of T cells under infection.

Ovulation

NGF is abundant in seminal plasma. Recent studies have found that it induces ovulation in some mammals e.g. “induced” ovulators, such as llamas. Surprisingly, research showed that these induced animals will also ovulate when semen from on-schedule or “spontaneous” ovulators, such as cattle is used. Its significance in humans is unknown. It was previously dubbed ovulation-inducing factor in semen before it was identified as beta-NGF in 2012.

Romantic love

Recent studies found that the concentration of NGF in the blood plasma is significantly higher in individuals who have been in a romantic relationship with another person for less than 12 months , than those who are either not in a romantic relationship or have been in one for more than 12 months .
NGF can indirectly stimulate the expression of adrenocorticotrophic hormone in the hypothalamic-pituitary-adrenal axis by increasing vasopressin secretion. ACTH binds to the MC2 receptor in the zona fasciculata of the adrenal cortex, and stimulates secretion of the stress hormone cortisol. This rapid increase of cortisol in the blood plasma can induce feelings of euphoria, which may explain the initial "rush" of falling in love. Studies show that ACTH can in turn stimulate NGF secretion in both the cerebral cortex and the hypothalamus.

Mechanism of action

NGF binds with at least two classes of receptors: the tropomyosin receptor kinase A and low-affinity NGF receptor. Both are associated with neurodegenerative disorders.
When NGF binds to the TrkA receptor, it drives the homodimerization of the receptor, which in turn causes the autophosphorylation of the tyrosine kinase segment. This leads to the activation of PI 3-kinase, ras, and PLC signaling pathways. Alternatively, the p75NTR receptor can form a heterodimer with TrkA, which has higher affinity and specificity for NGF.
Studies suggest that NGF circulates throughout the entire body via the blood plasma, and is important for the overall maintenance of homeostasis.

Neuron survival

Binding interaction between NGF and the TrkA receptor facilitates receptor dimerization and tyrosine residue phosphorylation of the cytoplasmic tail by adjacent Trk receptors. Trk receptor phosphorylation sites operate as Shc adaptor protein docking sites, which undergo phosphorylation by the TrkA receptor Once the cytoplasmic adaptor protein is phosphorylated by the receptor cytoplasmic tail, cell survival is initiated through several intracellular pathways.
One major pathway leads to the activation of the serine/threonine kinase, Akt. This pathway begins with the Trk receptor complex-recruitment of a second adaptor protein called growth factor-receptor bound protein-2 along with a docking protein called Grb2-associated Binder-1. Subsequently, phosphatidylinositol-3 kinase is activated, resulting in Akt kinase activation. Study results have shown that blocking PI3K or Akt activity results in death of sympathetic neurons in culture, regardless of NGF presence. However, if either kinase is constitutively active, neurons survive even without NGF.
A second pathway contributing to cell survival occurs through activation of the mitogen-activated protein kinase kinase. In this pathway, recruitment of a guanine nucleotide exchange factor by the adaptor and docking proteins leads to activation of a membrane-associated G-protein known as Ras. The guanine nucleotide exchange factor mediates Ras activation through the GDP-GTP exchange process. The active Ras protein phosphorylates several proteins, along with the serine/threonine kinase, Raf. Raf in turn activates the MAPK cascade to facilitate ribosomal s6 kinase activation and transcriptional regulation.
Both Akt and RSK, components of the PI3K-Akt and MAPK pathways respectively, act to phosphorylate the cyclic AMP response element binding protein transcription factor. Phosphorylated CREB translocates into the nucleus and mediates increased expression of anti-apoptotic proteins, thus promoting NGF-mediated cell survival. However, in the absence of NGF, the expression of pro-apoptotic proteins is increased when the activation of cell death-promoting transcription factors such as c-Jun are not suppressed by the aforementioned NGF-mediated cell survival pathways.

History

and Stanley Cohen discovered NGF in the 1950s while faculty members at Washington University in St Louis. However, its discovery, along with the discovery of other neurotrophins, was not widely recognized until 1986, when it won the Nobel Prize in Physiology or Medicine.
Studies in 1971 determined the primary structure of NGF. This eventually led to the discovery of the NGF gene.
NGF is abundant in seminal plasma. Recent studies have found that it induces ovulation in some mammals.
Nerve Growth Factors were initially discovered due to their actions during development, but NGF are not known to be involved in the function throughout the life of the animal.

Clinical significance

Nerve growth factor prevents or reduces neuronal degeneration in animal models of neurodegenerative diseases and these encouraging results in animals have led to several clinical trials in humans. NGF promotes peripheral nerve regeneration in rats. The expression of NGF is increased in inflammatory diseases where it suppresses inflammation. NGF appears to promote myelin repair. Hence NGF may be useful for the treatment of multiple sclerosis. NGF could also be involved in various psychiatric disorders, such as dementia, depression, schizophrenia, autism, Rett syndrome, anorexia nervosa, and bulimia nervosa.
Dysregulation of NGF signaling has also been linked to Alzheimer's disease. Connective tissue cells genetically engineered to synthesize and secrete NGF and implanted in patients' basal forebrains reliably pumped out NGF, which enhanced the cells’ size and their ability to sprout new neural fibers. The treatment also rescued vulnerable cells, even if they already showed the trademark signs of Alzheimer's pathology. In some patients, these beneficial effects lasted almost 10 years after the treatment. Even patients who died responded positively to the therapy. Even pathological cells with protein clumps in their cell bodies and surroundings extended their fibers toward the NGF source, maintained a healthy size and activated pro-survival signals that boosted their stress resilience. Two other patients received direct injections of modified viruses containing the NGF gene directly to their basal forebrains. This allowed the gene to express longer in the brain.
Neurotrophins, including NGF, have been shown to affect many areas of the brain, including areas that are related to Rett syndrome, bipolar disorder, and Alzheimer's disease. Stress and/or anxiety are usually a precipitating factor in these disorders and affects levels of NGF, leading to impaired cognitive functioning.
This impaired cognitive functioning can be seen in patients with Schizophrenia. In treatment of schizophrenia, NGF levels are increased in patients using atypical antipsychotic medication, but not in patients using typical antipsychotic medications. Patients using atypical medications usually report improved cognitive performance compared to those using typical antipsychotics. Higher NGF levels from the atypical antipsychotic medications may underlie the reduction in negative symptoms of Schizophrenia relative to typical antipsychotics.
NGF has been shown to restore learning ability in rats recovering from induced alcoholism
Rett syndrome and autism often show similar signs early in life, such as slowing development and intellectual disability. One distinguishing factor is that low levels of NGF have been found in the cerebral spinal fluid of children with Rett syndrome compared to children with Autism who have relatively normal to high levels Pharmaceutical therapies with NGF-like activity can be effective in treating Rett syndrome, including better motor and cortical functioning as well as increased social communication.
Impairment of neuroplasticity and altered levels of neuro-trophins are involved in bipolar disorder. NGF has been found to be decreased overall in bipolar disorder patients. More specifically, while in a manic state NGF is especially low. This leads to elevated or irritable mood with increased energy and decreased need for sleep while in a manic state. This decreased NGF may serve as a biological marker when assessing the present state of a bipolar disorder patient. When bipolar disorder patients were treated with lithium, their NGF concentrations increased in the frontal cortex, limbic forebrain, hippocampus, and amygdala.
An increase in cortical and subcortical NGF has been found in patients with Alzheimer's disease. Alzheimer's is a neurodegenerative disease with which dysregulation of NGF signaling has also been linked, causing impaired retrograde transport of NGF to certain areas of the brain. This impairment may be caused by an atypical production or use of receptors in the brain. Stimulating NGF receptors via NGF infusion has been shown to increase blood flow and verbal episodic memory. These improvements have been longer lasting than other treatments for Alzheimer's.
Also, NGF has been shown to play a role in a number cardiovascular diseases, such as coronary atherosclerosis, obesity, type 2 diabetes, and metabolic syndrome. Reduced plasma levels of NGF and BDNF have been associated with acute coronary syndromes and metabolic syndromes. NGF is known to have insulinotropic, angiogenic, and antioxidant properties. NGF suppresses food intake.
NGF has also been shown to accelerate wound healing. There is evidence that it could be useful in the treatment of skin ulcers and cornea ulcers.
In some gynecological diseases, an elevated prostaglandin E2 is thought to stimulate production of NGF which contributes to the perception of pain and increased inflammation in endometriosis.
Monoclonal antibodies against NGF have been used in clinical trials to modulate pain. One of these is tanezumab, another is fulranumab.

Miscellaneous

Nerve growth factor may contribute to increased longevity and mental capacity. Centenarian Rita Levi-Montalcini took a daily solution in the form of eye drops, and has stated that her brain is more active now than it was four decades ago. In 2014, researchers at the Medical University of South Carolina showed that NGF level is elevated in people who performed a single 20-minute yoga session involving om-chanting and thirumoolar pranayama, when compared to a control group.

Interactions

Nerve growth factor has been shown to interact with TrkA and p75NTR.
It has recently been suggested that NGF expression may be stimulated by dehydroepiandrosterone. DHEA may also act as an agonist of both TrkA and p75NTR and activate the pathways of NGF, demonstrating neurotrophic activities similar to that of NGF.
Adrenocorticotrophic hormone can also upregulate NGF expression in the brain.