Netrin
Research supports that new axons tend to follow previously traced pathways rather than being guided by netrins or related chemotropic factors.
[5] The netrin family is composed mostly of secreted proteins which serve as bifunctional signals: attracting some neurons while repelling others during the development of the brain.
Expressed in the midline of all animals possessing bilateral symmetry, they can act as long or short range signals during neurogenesis.
In order to carry out their functions, netrins interact with specific receptors: DCC or UNC-5, depending on whether they are trying to attract or repel neurons, respectively.
There is a high degree of conservation in the secondary structure of netrins, which has several domains which are homologous with laminin at the amino terminal end.
The C-terminal domain is where most of the variation is found between species and contains different amino acids which allow interaction with specific proteins in extracellular matrix or on the cell surface.
The differences in terms of structure and function have led to the identifications of several different types of netrins including netrin-1, netrin-3, and netrins-G.[6] Netrin-1 is found in the floor plate and neuroepithelial cells of the ventral region of the spinal cord, as well as other locations in the nervous system including the somatic mesoderm, pancreas and cardiac muscle.
Studies in multiple organisms including, mice, rats, chicks, the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster and the zebrafish Danio rerio have indicated that secreted netrins are bifunctional, meaning that they can act as either attractants or repellants in directing axonal extension.
[6] Studies of central nervous system (CNS) development in chick and rodent models have identified the netrin-1 protein as a particularly important vertebrate axonal guidance cue.
Most significantly, it was observed that the specialized cells of the floor plate located at the ventral midline of the embryonic brain secrete netrin-1, which resulted in a protein gradient.
Evidence suggests that this gradient is essential for the long-range function of UNC-6 in guiding the initial circumferential migration of axons to the ventral midline and that the UNC-40 receptor mediates the attractive response.
[5][10] Recently, scientists have characterized many of the cellular mechanisms by which netrin-1 binding to DCC motivates axonal attraction through at least three independent signaling pathways.
In a second possible pathway, phosphatidylinositol transfer protein α (PITP) binds to phosphorylated DCC which induces phospholipase C (PLC) to increase the ratio of cAMP to cGMP.
Evidence suggests that this increased calcium is responsible for the activation of Rho GTPases, Cdc42 Rac1 and the nuclear transcription factor NFAT which can all initiate growth cone extension.
Additional studies have also shown that netrin-induced signaling between DCC downstream targets NcK, and Wiskott–Aldrich syndrome protein WASP trigger Rac1 and Cdc42 and subsequently axonal growth.
Likewise, multiple defects were observed in C. elegans unc-40 mutants; however, errors in migration patterns were more profoundly affected by mutations in the unc-5 gene, indicating that binding of the netrin-1 homologue UNC-6 to the UNC-5 receptor alone can repel axonal growth.
[5][14] It is currently hypothesized that long range chemorepulsion involves initiation of the Arachidonic acid pathway upon netrin-1 interaction with the DCC/UNC-5 complex.
This pathway increases the intracellular levels of 12-HPETE (12-Hydroperoxy-5, 8, 10, 14-Eicosatetraenoic Acid), which induces cGMP signaling and subsequently causes a decrease in the cAMP/cGMP ratio.
A similar RhoA-mediated mechanism is proposed for short range chemorepulsion whereby netrin-1 binding to UNC-5 homodimers alone induces tyrosine phosphorylation requiring FAK and Src, which as a result activates RhoA.
These netrins surround endoderm buds in the basement membrane, preventing distal tip cells from expressing DCC and UNC5B.
This has been observed in the human colon epithelium, where higher levels of natural cell death at the upper portion of the villi correlated with a smaller gradient of netrin-1.
[24] Because netrin-1 has been found to be upregulated in tumors, recent research has attempted to identify netrin-1 as a biomarker for the onset of cancer in the human body.
[25] Netrin-3 appears to be specifically expressed in Neublastoma (a paediatric tumour) and in small cell lung cancer (SCLC) where it correlates with a bad patient prognosis.
In avian and mouse model organisms suffering from neuroblastoma, interfering with the netrin-1 autocrine loop in malignant tumors leads to cell death.
