Extracellular Factors Affecting 
Neuron Development 
Louis F. Reichardt, Ph.D. — Investigator 
Dr. Reichardt is also Professor of Physiology and of Biochemistry and Biophysics at the University of 
California, San Francisco. He received his Ph.D. degree in biochemistry from Stanford University for work 
on control of gene expression by a bacterial virus, the bacteriophage X. Dr. Reichardt entered the field of 
neurobiology as a postdoctoral fellow in Paul Patterson 's laboratory at Harvard University, where he 
studied factors that regulate the transmitter phenotype of individual neurons. Among his honors are a 
McKnight Scholars Award, a Sloan Award, and a Guggenheim Fellowship. 
MY laboratory is investigating molecules in 
the extracellular environment of neurons 
(conducting nerve cells) that direct their devel- 
opment in vivo. These include trophic (nutri- 
tive) factors, exemplified by nerve growth factor 
(NGF), and molecules in the extracellular matrix 
or on the surface of cells that serve as substrates 
for the growth of axons (long nerve fibers) . Such 
molecules help to regulate neuronal survival, 
axon growth, and synapse (nerve junction) 
formation. 
Neurons require contact with targets to survive 
during development. Experimentally increasing 
or decreasing the volume of target tissue corre- 
spondingly increases or decreases neuronal sur- 
vival. To explain these target influences, it has 
been proposed that target organs synthesize tro- 
phic factors required for the survival of the inner- 
vating neurons. Defects in the synthesis of these 
factors or in the neuron's ability to respond to 
them may explain some neurodegenerative 
disorders. 
The key to understanding trophic factors is to 
identify and understand the actions of their re- 
ceptors. One of the proteins that functions as a 
receptor for the family of NGF-related trophic 
factors is called the low-affinity NGF receptor. 
Antibodies to this protein have been prepared 
and used to show that it is distinct from a second 
protein that constitutes a distinct high-affinity re- 
ceptor for NGF. Work in the laboratories of David 
Kaplan, Mariano Barbacid, and their colleagues 
has recently shown that this second protein is the 
trk proto-oncogene product, an NGF-tyrosine ki- 
nase. The same antibodies have been used to 
show that NGF-dependent survival and axon 
growth by response neurons do not require the 
low-affinity receptor. These antibodies are being 
used to examine the role of this class of receptor 
in mediating other responses to NGF. 
Our laboratory has devoted considerable effort 
to identifying molecules that promote the groMT:h 
of neuronal processes. We have tried to identify 
both the molecules that axons recognize in their 
environment and the receptors that neurons use 
for binding to these molecules. Our results have 
shown that laminin, an adhesive protein, is by far 
the most active of the glycoproteins that cells se- 
crete into the extracellular matrix. Other pro- 
teins that have similar activities include fibronec- 
tin, thrombospondin, and vitronectin. 
We have also shown that neurons utilize a fam- 
ily of receptors, the integrins, to bind to laminin 
and other matrix glycoproteins. Distinct recep- 
tors appear to mediate neuronal adhesion and 
growth cone motility on laminin, fibronectin, 
and collagen. In the past two years we have puri- 
fied several of these proteins and isolated clones 
encoding subunits of the receptors. Specific anti- 
bodies to these subunits have been prepared and 
are being used to study their regulation . Of partic- 
ular interest, we have identified the receptors 
used by both peripheral neurons and retinal neu- 
rons to interact with laminin, a heterotrimer as- 
sembled from three different gene products — A, 
Bl, and B2. Josh Sanes and Eva Engvall have 
shown that at least two different A-like genes and 
two different Bl -like genes exist and are differen- 
tially expressed, making it possible to assemble 
four different isoforms of laminin, which are dif- 
ferentially distributed in embryos. We have 
shown that individual integrin heterodimers dis- 
tinguish between these isoforms, making it possi- 
ble for cells to exhibit different responses to indi- 
vidual isoforms. Work in the past year has also 
identified candidate receptors for thrombospon- 
din and tenascin. 
Evidence suggests that the activity of integrin 
receptors may modulate the behavior of axons in 
vivo. In studies on regulation of a^jSi, the major 
laminin receptor in the neuroretina, we have 
shown that receptor function can be regulated on 
several levels. First, expression of the genes en- 
coding the two subunits is regulated. Retinal gan- 
glion cells lose responsiveness to laminin when 
they contact their synaptic partners in the optic 
tectum, and this reflects down-regulation of ex- 
pression of the gene. Second, other neurons in 
the retina modulate responses to laminin by ex- 
hibiting changes in the activity of integrin recep- 
tors on their surfaces. These changes occur rap- 
idly and can be modulated by external and 
internal agents. Yet a third level of regulation ap- 
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