retinal ganglion cells appear to contact the somata 
and axons of these chiasmic neurons. Thus these 
neurons are strong candidates to provide growth- 
promoting and directional cues for retinal ganglion 
cell growth cones in the optic chiasm and in the 
optic tract pathway. 
In collaboration with Dr. Ellen Pure of the 
Rockefeller University, Dr. Reichardt and his col- 
leagues have shown that the neurons in the optic 
chiasm express a hyaluronic acid-binding recep- 
tor named H-CAM or CD44. This has made it possi- 
ble to remove these neurons by injection of 
CD44-specific antibodies and complement. In the 
absence of these neurons, growth cones of retinal 
ganglion cells fail to enter the optic chiasm and 
can be visualized as long as four days later at the 
junction between the optic stalk and chiasm. In 
collaboration with Mark Siegel of Genentech, it 
has been possible to show that the morphology of 
the neuroepithelium is normal after ablation of 
the chiasmic neurons. Thus the results argue that 
these neurons are necessary for normal develop- 
ment of the visual pathway. Although there is no 
evidence at present supporting a direct role for 
CD44 in visual pathway development, the possi- 
bility is being actively investigated. 
Work has continued on characterizing receptors 
for extracellular matrix (ECM) proteins in the retina 
and visual projection. Dr. Blaise Bossy previously 
isolated cDNAs encoding a novel integrin a subunit 
named that associates with the j8, subunit and is 
strongly expressed on axons in the primary visual 
projection and many other axon tracts. He has ex- 
pressed the ctgiSi receptor but has not yet identified 
an ECM protein as a ligand. Michael DeFreitas has 
obtained strong evidence that the integrin a^^^ 
functions as a receptor for thrombospondin. Both 
ttjiSj and thrombospondin II are expressed in the 
embryonic retina. Dr. Barbara Varnum-Finney has ev- 
idence that a jSj integrin also functions as a receptor 
for tenascin, another ECM protein that is strongly 
expressed in the embryonic central nervous system. 
Regulation of Integrin Receptor 
Function in Neurons 
The functions of some integrins are down- 
regulated on neurons at times correlating with tar- 
get innervation and are up-regulated by manipula- 
tions that prevent or disrupt target innervation. Dr. 
Ivan de Curtis has found evidence in the neuroretina 
for both transcriptional and post-translational regu- 
lation of the functions of laminin-binding integrins. 
In the past year, function-blocking antibodies and 
cytochemistry have been used to extend this work. 
Two major laminin receptors — olS\ ^'"'d a^^^ — 
are expressed in developing retina. Antibody inhibi- 
tion experiments show that the a^jS, receptor ac- 
counts for most but not all interactions of early 
retinal neurons with laminin. At later developmen- 
tal stages, this receptor is expressed on neurons that 
bind laminin poorly. Binding to laminin is enhanced 
by the antibody TASC, which activates integrins by 
binding to the jSj subunit. Binding to laminin is abol- 
ished by inhibitory antibodies to /3i and is reduced 
by inhibitory antibodies to a^,. 
The results imply that integrins can exist in more 
than one activity state on neuronal cell surfaces. In 
the early developing retina, a^i^i present on neu- 
rons and neuroepithelial cells in a comparatively 
active conformation. At later times the same integrin 
is present in a comparatively inactive state. Thus 
integrin activity is developmentally regulated. Tran- 
scription of o;^ is also regulated during development 
and also contributes to changes in neuronal respon- 
siveness to laminin. In collaboration with this labo- 
ratory. Dr. Clayton Buck (University of Pennsyl- 
vania) has shown that the TASC monoclonal 
antibody binds an epitope in the "neck" of the inte- 
grin spatially distant from the ligand-binding 
pocket. Work in progress seeks to understand the 
mechanisms by which the this antibody activates the 
functions of these integrins. Since regulators of ac- 
tivity can in principle cause rapid changes in inte- 
grin function that quickly affect growth cone guid- 
ance, the laboratory is also seeking to identify 
endogenous mediators of integrin activity. 
Integrins in Caenorhabditis elegans 
In collaboration with Dr. Cynthia Kenyon's labora- 
tory (University of California, San Francisco), Sonya 
Gettner has identified an integrin /3 subunit that is 
widely expressed in C. elegans. The sequence, geno- 
mic organization, and genomic position of this pro- 
tein's gene are now known. The protein associates 
with at least three putative integrin a subunits. Cyto- 
chemistry indicates it is widely expressed through- 
out the C. elegans life cycle. 
In collaboration with Dr. Ed Hedgecock (Johns 
Hopkins University), it has been possible to show 
that a gene, pat-3, encodes this integrin /3 subunit. 
Ms. Gettner has sequenced several mutants in this 
gene and has shown that each introduces a nonsense 
or missense mutation into the |8 subunit. The muta- 
tions have distinct phenotypes but affect cell attach- 
ment, cell morphogenesis, cell migration, and pro- 
cess outgrowth by many different cell types. The 
mutations include a null, a partial deficiency, and a 
probable neomorph. Thus this system may be valu- 
NEUROSCIENCE 425 
