Following are our main results:

1.      A gradient of SDF-1 or baclofen repels nerve growth cone, which can be blocked by pertussis-toxin. 

2.      As with other established guidance cues, growth cone repulsion triggered by SDF-1 or baclofen can be converted to attraction by elevating cytosolic second messenger levels.  In our study, elevation of cytosolic cGMP levels makes the conversion.

3.      The study of downstream signaling showed that PLC-PKC/IP3 pathways mediate the cytoplasmic transduction of guidance signals downstream from GPCRs.  Under normal condition, PKC pathway dominates over IP3 pathway, leading to the repulsive turning response.  In the presence of cGMP analogues, the repulsion is converted to attraction.  That the cGMP level can modulate PKC-dependent turning response is further supported by the finding that a gradient of PKC activation induced by a PMA gradient is sufficient to induce either repulsive or attractive turning, depending on the cGMP level.  The cGMP-dependent signal appears to act on PKC and/or its downstream effectors, which in turn regulate cytoskeletal arrangements required for the turning responses.

Our results give the following implication:

1.      GPCRs can mediate chemotaxis of amoeba, leukocyte, migrating neurons and axons, suggesting that all kinds of cell motility may share similar molecular and cellular mechanisms;

2.      SDF-1, a chemokine well known to attract leukocyte, may serve as guidance cues for axon guidance in vitro, although the in vivo function of SDF-1 in axon guidance in unclear;

3.      SDF-1, through its G Protein-coupled receptor CXCR4, can trigger leukocyte chemotaxis, neuronal migration, axon guidance and metastasis of malignancy cancer cell, suggesting a variety of physiological functions of this molecule in different system.  In our study, we figure out the PLC-PKC/IP3 pathways in transmitting SDF-1 guidance information, this may provide some clue for us to better understand the signaling mechanisms of SDF-1 in other systems;

4.      Most of the previous reports showed that SDF-1 only attract migrating cells, including leukocytes, neurons, and cancer cells.  However, in our study, we provide evidence that SDF-1 may attract or repel axons, depending on the level of cytosolic second messenger.  This finding raises the possibility that SDF-1 or other chemokines may repel the migrating cells if we manipulate the cytosolic levels of second messengers.  This may provide a new strategy to treat some autoimmune disease, which is caused by over-aggregation of leukocytes.  Similarly, in the treatment of some neurological diseases such as Alzheimer’s disease or Parkinson’s disease, control the migration direction of the transplanted neural stem cells may help the transplanted cells to migrate more accurately to their targets area. 

  Key borad:  G protein-coupled receptor,  growth cone guidance,  chemokine