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Neurotrophin-3, or NT-3 is a neurotrophic factor, in the NGF (Nerve Growth Factor)-family of neurotrophins. It is a protein growth factor which has activity on certain neurons of the peripheral and central nervous system; it helps to support the survival and differentiation of existing neurons, and encourages the growth and differentiation of new neurons and synapses. NT-3 was the third neurotrophic factor to be characterized, after nerve growth factor (NGF) and BDNF (Brain Derived Neurotrohic Factor).
Although the vast majority of neurons in the mammalian brain are formed prenatally, parts of the adult brain retain the ability to grow new neurons from neural stem cells; a process known as neurogenesis. Neurotrophins are chemicals that help to stimulate and control neurogenesis. NT-3 is unique in the number of neurons it can potentially stimulate, given its ability to activate two of the receptor tyrosine kinase neurotrophin receptors (TrkC and TrkB - see below). Mice born without the ability to make NT-3 have loss of proprioceptive and subsets of mechanoreceptive sensory neurons.
Mechanism of action for NT-3Edit
NT-3 binds three receptors on the surface of cells which are capable of responding to this growth factor, TrkC (pronounced "Track C"), is apparently the "physiologic" receptor, in that it binds with greatest affinity to NT-3. However, NT-3 is capable of binding and signaling through a TrkC-related receptors called TrkB. Finally, NT-3 also binds a second-receptor type besides Trk receptors, called the LNGFR (for "low affinity nerve growth factor receptor).
TrkC is a receptor tyrosine kinase (meaning it mediates its actions by causing the addition of phosphate molecules on certain tyrosines in the cell, activating cellular signaling). As mentioned above, there are other related Trk receptors, TrkA and TrkB. Also as mentioned, there are other neurotrophic factors structurally related to NT-3: NGF (for Nerve Growth Factor), BDNF (for Brain Derived Neurotrophic Factor) and NT-4 (for Neurotrophin-4). While TrkB mediates the effects of BDNF, NT-4, and NT-3, TrkA binds and is activated by NGF, and TrkC binds and is activated only by NT-3.
The other NT-3 receptor, the LNGFR, plays a somewhat less clear role. Some researchers have shown the LNGFR binds and serves as a "sink" for neurotrophins. Cells which express both the LNGFR and the Trk receptors might therefore have a greater activity - since they have a higher "microconcentration" of the neurotrophin. It has also been shown, however, that the LNGFR may signal a cell to die via apoptosis - so therefore cells expressing the LNGFR in the absence of Trk receptors may die rather than live in the presence of a neurotrophin.
 Neurotrophin-3: a neurotrophic factor related to NGF and BDNF. Science. 1990 Mar 23;247(4949 Pt 1):1446-51.
 trkC, a new member of the trk family of tyrosine protein kinases, is a receptor for neurotrophin-3. Cell. 1991 Sep 6;66(5):967-79
 trkC, a receptor for neurotrophin-3, is widely expressed in the developing nervous system and in non-neuronal tissues. Development. 1993 Jun;118(2):463-75.
 Similarities and differences in the way neurotrophins interact with the Trk receptors in neuronal and nonneuronal cells. Neuron. 1993 Feb;10(2):137-49.
 Targeted Mutation in the Neurotrophin-3 Gene Results in loss of Muscle Sensory Neurons. 1994. Proc Natl Acad Sci USA 91:11844-11848
 Disruption of the neurotrophin-3 receptor gene trkC eliminates la muscle afferents and results in abnormal movements. 1994. Nature 368:249-251
|Hormones and endocrine glands - edit|
Hypothalamus: - TRH - CRH - GnRH - GHRH - somatostatin - dopamine | Posterior pituitary: vasopressin - oxytocin - lipotropin | Anterior pituitary: GH - ACTH - TSH - LH - FSH - prolactin - MSH - endorphins - lipotropin
Thyroid: T3 and T4 - calcitonin | Parathyroid: PTH | Adrenal medulla: epinephrine - norepinephrine | Adrenal cortex: aldosterone - cortisol - DHEA | Pancreas: glucagon- insulin - somatostatin | Ovary: estradiol - progesterone - inhibin - activin | Testis: testosterone - AMH - inhibin | Pineal gland: melatonin | Kidney: renin - EPO - calcitriol - prostaglandin | Heart atrium: ANP
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