Bill Greenough’s research career is exemplary of how the pursuit of a basic research
question can lead to clinically applicable knowledge and ultimately to the solution of
clinical questions.
Greenough’s research has pursued two main themes sequentially and in parallel, the
cellular mechanisms of nervous system plasticity and the involvement of these
mechanisms in clinical pathologies, particularly (and more recently) the fragile X mental
retardation syndrome.
Greenough’s early work focused on structural plasticity in the nervous system in the
period following early postnatal development. Prior work had shown that early postnatal
sensory deprivation altered the structure of the nervous system during “critical or
sensitive periods” in early postnatal development. Greenough’s work addressed plasticity
of brain information storage in later development and adulthood. Combining the
“environmental enrichment” procedure pioneered by Donald Hebb and Mark Rosenzweig
with quantitative optical and electron microscopic morphological methods drawn from
Scheibel, Sholl and others, Greenough’s early studies emphasized two kinds of synaptic
plasticity, one of synapse structure (involving several components) and the other of
synapse number. Structural synapse plasticity included changes in synaptic size, vesicle
aggregate number and position, protein synthetic apparatus, association with glial
processes and other changes in what appeared to be pre-existing synapses. Plasticity of
synapse number included both the gain and the loss of synapses as a function of
experience.
The loss of synapses with experience in later development, which Greenough first
observed in monkeys in collaboration with Jennifer Lund, Ronald Boothe and
Greenough’s student Kathy Wrege (now Nordeen) was surprising but was confirmed by
contemporary work by Cragg and Huttenlocher and now stands as a tenet of the field.
Greenough subsequently showed (with Chang) that this loss was not random but
patterned and that the detailed circuitry of adulthood emerged through both the loss and
the addition of synapses, with both processes sometimes occurring simultaneously, as in
the development of the whisker barrels of somatosensory cortex.
This synapse loss phenomenon and the continuing synapse plasticity seen through later
development and adulthood led Greenough to posit a revision of the traditional view of
critical and sensitive periods for the effects of experience on behavioral and brain
development. The traditional view had arisen through the observations of behavioral
development of classical ethologists such as Lorenz and experimentalists such as
Gottlieb. Clearly the plasticity of early development was often very time- and agedependent and had very specific and outcome-constrained implications for the
organization of the developing nervous system. The visual system either worked well or
it worked poorly in a limited number of highly predictable ways, depending upon
experience. By contrast, in later development, experience painted with a broad brush,
with subtle differences in experience having different, parallel effects upon neural
organization, and, as others showed, upon cognitive organization as well.
To account for these seemingly disparate effects of experience during development,
Greenough proposed the replacement of the critical and sensitive period
conceptualizations with those of Experience-Expectant and Experience-Dependent
information storage arising from contact with the environment. The ExperienceExpectant process was one in which the nervous system was set up to receive specific
guiding or formative information at specific developmental points, such as patterned
visual and acoustic experience, or somesthetic experience as studied in the whisker barrel
system. Heree, as Hubel and Wiesel and many others were showing, synapse loss – the
elimination of functionally inappropriate connections – was an important part of the
emergence of patterned circuitry. By contrast, other effects of experience, such as those
affecting cognitive function, were less time- and age-dependent and varied much more
widely in their details across individuals. These Greenough described as ExperienceDependent, where the outcomes of developmental experiences were reflected in
functional brain organization and behavioral competence but where the specific endpoints
of development were defined by the multifaceted nature of experiences of the individual
rather than by the need for specific experiences common to all members of the species.
This conceptualization has had a broad impact in the field of child development and
developmental psychobiology and a growing impact in neuroscience as scientists have
increasingly begun to recognize the limitations of the simple critical-sensitive period
view.
The knowledge arising from this work also extended beyond their applications to brain
and behavioral development. In a series of studies, Greenough demonstrated that many of
the synaptic changes that occurred as a consequence of experience manipulations during
development also occurred in specific functional systems with learning. Dendritic
growth and synapse formation occurred in motor cortex with motor learning, for
example, and were specific to the hemisphere involved when learning was restricted to
one forelimb. Parallel specificity was seen in cerebellar paramedian lobule, and both
cerebellum and motor cortex exhibited different responses to motor skill acquisition (e.g.,
synapse formation and modification) and to activation without learning, where, for
example, capillaries were altered independently of synapses. Other changes in nonneuronal cellular compartments of the brain were less easily separated: glial envelopment
of synapses seemed a necessary component of learning-induced synaptic plasticity or
complex environment exposure, and such a basic phenomenon as myelination of axons
was strongly affected by experiences such as exposure to an enriched environment, even
in mature adult animals. These results reinforced a growing view that there was not a
single cellular change, such as formation of new synapses or alteration of existing ones,
that underlay adult long-term memories. That is, just as others had shown that different
brain regions were associated with different kinds of memories, so too, multiple cellular
changes were associated with the processes of long-term memory. Particularly important,
the interactions of non-neuronal and neuronal elements appeared to be important parts of
the process—memory did not seem to be an exclusive property of synapses.
Greenough’s involvement in fragile X research arose as a consequence of these basic
research approaches to developmental and adult information storage mechanisms. His
group had discovered that protein synthesis at and near synapses, first examined by
Steward, was enhanced by experience in animals housed in an enriched environment. It
was at this point that Greenough and Eberwine first met—Jim gave a talk on the
identification of dendrite specific mRNAs while visiting the University of Illinois, and,
immediately following the talk, they devised a procedure to combine their approaches to
identify which mRNAs were being translated in response to synaptic activation in a
synaptoneurosome preparation that Greenough and then new colleague Ivan Jeanne
Weiler had developed. When this procedure was implemented, the first protein identified
as synthesized at or near synapses in this preparation was FMRP, the fragile X mental
retardation protein. Synthesis of FMRP was increased at activated synapses.
Subsequent to this Greenough and the Greenough-Eberwine-Weiler team have made very
substantive contributions to the understanding of the roles of FMRP and the clinical
syndrome associated with its absence, and the basic research techniques Greenough
implemented for studying neural plasticity have been enormously valuable. Using
quantitative neuroanatomy, Greenough and his group have described the principal
neuropathologies associated with the disorder and have made a convincing case that they
arise from a failure of the synaptic pruning mechanisms studied in his earlier work. Both
the immature appearance of synapses and the greater numbers of synapses in the adult
human and mouse model phenotypes appear to arise from the absence of a patterned
synapse loss process, which is most evident in work on the somatosensory whisker barrel
cortex of the mouse. Thus the syndrome seems to involve a relatively more
nonselectively organized (or disorganized) nervous system, [suggesting the possible
efficacy of treatments involving the building of ancillary circuitry, such as the extensive,
repetitive training that does seem to have positive effects].
In molecular biological work, Greenough and colleagues have shown that protein
synthesis at synapses is regulated by and very largely dependent upon the presence of
FMRP.
References:
Psychology-Neuroscience:
Richard F Thompson, PHD
Univ Southern California
Neurosci Program
HNB 122 University Park
Los Angeles CA 90089-2520 USA
213-740-7350
thompson@usc.edu
Larry R Squire, PhD
VA Med Ctr
V116A 3350 La Jolla Village Dr
San Diego CA 92161 USA
858-552-8585 ext.3628
Fax: 858-552-7457
LSquire@UCSD.edu
James L McGaugh, PhD
Univ California, Irvine
Ctr Neurobiol Learning & Memory
Irvine CA 92697 USA
949-824-5401
Fax: 949-824-2952
jlmcgaug@uci.edu
Specific references regarding the impact of the work in the
area of Child Development Research
Charles A. Nelson, Ph.D.
Director, Center for Neurobehavioral Development
MMC#507
420 Delaware St. SE
University of Minnesota
Minneapolis, MN 55455
canelson@umn.edu
(612) 624-3878 (office)
FAX: 612-625-1530
Bruce Pennington
Also, you might contact Suzanne Wandersman at the American Psychological
Association. She would have, I believe, handled my materials with regard to the APA
Distinguished Scientific Contribution Award several years ago. The letters would be way
out of date (probably gone except I have the feeling that APA never throws anything
away), but the names might include ones I haven’t thought of.
Suzanne Wandersman
Director for Governance Affairs
Science Directorate
American Psychological Association
750 First Street, NE
Washington, DC 20002-4242
(202) 336-6000
(202) 336-5953 FAX
swandersman@apa.org
Cell Biology-Fragile X
Question marks indicate that the individual never has written a letter for me so far as I
know, and, while I assume they are familiar with the work, I cannot be sure of their
opinion of it.
Oswald Steward, PhD
Col Med Univ California Irvine
Dir, Reeve-Irvine Res Ctr
1105 Gillespie Neurosci Res Facility
Irvine CA 92697-4292 USA
949-824-8908
Fax: 949-824-2625
osteward@uci.edu
Edouard W. Khandjian (Hasn’t written but knows work)
Unite de Recherche en Genetique Humaine et Moleculaire
Centre de Recherche Hopital Saint-Francois d'Assise
Centre Hospitalier Universitaire de Quebec
Quebec, QC, Canada G1L 3L5
direct: 418 525 4444 (ext. 3484)
Fax: 418 525 4195
edward.khandjian@crsfa.ulaval.ca
?Rusty (Fred H.) Gage , PhD –
Salk Inst
Lab Genetics
10010 N Torrey Pines Rd
La Jolla CA 92037 USA
858-453-4100 ext.1012
Fax: 858-597-0824
gage@salk.edu
?Andre Hoogeveen
Department of Clinical Genetics
Erasmus University
Rotterdam, The Netherlands
hoogeveen@ikg.fgg.eur.nl
?Ben A. Oostra
Department of Clinical Genetics
Erasmus University
Rotterdam, The Netherlands
b.oostra@erasmusmc.nl