Lecture 9

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Cell and Systems Biology
Melanie Woodin

CSB332H1S L9; Feb. 8, 2012 assembly of actin filaments (and other prots needed) on 1 side Axon Guidance of growth cone. Read: Pg. 558-562  The cascade of local clustering & consequent depletion of signaling molecs @ rest of cell o  Provide local activation & global inhibition  Required for amplification of gradient signal.   respond more effectively on upper-left side, also decreases response in regions furthest from gradient  (global inhibition on right side)  Prots get localized  growth cone turning Gradient of Ca is important  Hign Ca grad  attraction  Low Ca grad  repulsion  GCs of cultured Xenopus spinal neurons exhibit adaptation  For both attraction & during chemotactic migration, undergoing consecutive phases repulsion, Ca level above of desensitization & resensitization in presence of guidance baseline, just more so for factor netrin-1 attraction  With more time (90min) resensitizes to gradient  now   Activate cascades responsive to the chemoattractant  Adapt to envt o Ex. Phosphorylation or prot kinases – need certain levels of Ca to activate  Large grad of Ca coming from neuron but not important for act pot – important for many other things  Images of Ca-binding growth cone – been bathed in fluorescent Ca-sensitive indicator  Calcium signals induced by a netrin-1 gradient correlate w adaptation.  Fluorescence images of a GC loaded w a Ca2+-sensitive fluorescent indicator (Oregon green BAPTA-1-dextran).  Neuron was incubated w netrin-1 (5 ng ml-1) in the bath 30 min (a) or 90 min (b) before exposure to a netrin-1 gradient.  A: apply grad @ t=0 o Desensitized to netrin so growth cone not turned towards gradient, overall no global change in amt of Ca  B: resensitzation since more time passed  increase in yellow signal – indicates Ca levels increased in growth cone  so  Retina projections ganglia to optic tectum (analygous to LGN in growth cone grows towards gradient vertebrates?) in midbrain  Ca = important in signaling other molecs (ex. prots) for gene  Retina divided in nasal & temporal regions expression  Optic tectum divided btwn anterior & posterior  Nasal  innervates posterior  Temporal innverates anterior o Due to conc grads  Eph = rec  So growth growth growing from temporal region has much higher conc of Eph recs  Nasal region has lower expression of Eph recs o Ligands that activate Eph recs = Ephrins, found in optic  Black ovals = rec for chemoattractant tectum  In encountering a shallow gradient of guidance molecs (yellow  Critically important: when ephrin binds to rec  repulsive, cloud)  shallow gradient of rec activation (red oval)  inhibitory reaction  growth cone moves away from that clustering of activated recs. region o More red ovals close to gradient  Ganglion cell in temporal retina  Initial clustering  triggers recruitment & further clustering of o If grow into tectum, will choose anterior region since less cytoplasmic adaptors & effectors  eventually orientated
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