Table Of ContentNeurobiology of Spinal Cord Injury
Contemporary Neuroscience
Neurobiology ofS pinal Cord Injury, Molecular Mechanisms ofD ementia,
edited by Robert G. Kalb and edited by Wilma Wasco and
Stephen M. Strittmatter, 2000 Rudolph E. Tanzi, 1997
Cerebral Signal Transduction: From Neurotransmitter Transporters: Struc
First to Fourth Messengers, edited ture, Function, and Regulation,
by Maarten E. A. Reith, 2000 edited by Maarten E. A. Reith,
Central Nervous System Diseases: 1997
Innovative Animal Models from Lab Motor Activity and Movement Disorders:
to Clinic, edited by Dwaine F. Research Issues and Applications,
Emerich, Reginald L. Dean, III, edited by Paul R. Sanberg, Klaus
and Paul R. Sanberg, 2000 Peter Ossenkopp, and Martin
Mitochondrial Inhibitors and Neurode Kavaliers, 1996
generative Disorders, edited by Paul Neurotherapeutics: Emerging Strate
R. Sanberg, Hitoo Nishino, and gies, edited by Linda M. Pullan and
Cesario V. Borlongan, 2000 Jitendra Patel, 1996
Cerebral Ischemia: Molecular and Neuron-Glia Interrelations During
Cellular Pathophysiology, edited by Phylogeny: II. Plasticity
Wolfgang Walz, 1999 and Regeneration. edited by Antonia
Cell Transplantation for Neurological Vernadakis and Betty I. Roots, 1995
Disorders, edited by Thomas B.
Neuron-Glia Interrelations During
Freeman and Hakan Widner, 1998
Phylogeny: I. Phylogeny
Gene Therapy for Neurological
and Ontogeny of Glial Cells, edited
Disorders and Brain Tumors, edited
by Antonia Vernadakis
by E. Antonio Chiocca and Xandra
and Betty I. Roots, 1995
0. Breakefield, 1998
The Biology ofN europeptide Y and
Highly Selective Neurotoxins: Basic
Related Peptides, edited
and Clinical Applications, edited by
by William F. Colmers and Claes
Richard M. Kostrzewa, 1998
Wahlestedt, 1993
Neuroinjlammation: Mechanisms and
Psychoactive Drugs: Tolerance and
Management, edited by Paul L.
Sensitization, edited
Wood, 1998
by A. J. Goudie and M. W.
Neuroprotective Signal Transduction,
Emmett-Oglesby, 1989
edited by Mark P. Mattson, 1998
Clinical Pharmacology of Cerebral Experimental Psychopharmacology,
Ischemia, edited by Gert J. Ter edited by Andrew J. Greenshaw
Horst and Jakob Korf, 1997 and Colin T. Dourish, 1987
Neurobiology of
Spinal Cord Injury
Edited by
Robert G. Kalb,
MD
Stephen M. Strittmatter,
MD, PHD
School of Medicine, Yale University
New Haven, CT
Springer Science+B usiness Media, LLC
© 2000 Springer Science+Business Media New York
Originally published by Humana Press Inc in 2000
Softcover reprint of the hardcover 1st edition 2000
All rights reserved.
No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any
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permission from the Publisher.
All authored papers, comments, opinions, conclusions, or recommendations are those of the author(s),
and do not necessarily reflect the views of the publisher.
This publication is printed on acid-free paper. @
ANSI 239.48-1984 (American Standards Institute) Permanence of Paper for Printed Library Materials.
Cover design by Patricia F. Cleary.
Cover illustration: A motor neuron and its synaptic inputs. Immunohistology for synaptophysin (green
puncta), a marker for presynaptic terminals, is combined with Dii labeling of a motor neuron axon,
proximal dendrites, and cell body. Optimizing motor function after spinal cord injury will depend on
improving the ability of excitatory synapses to drive motor neuron activation. Photograph by Drs. Laising
Yen and Robert Kalb.
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Authorization to photocopy items for internal or personal use, or the internal or personal use of specific
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10 9 8 7 6 5 4 3 2 I
Library of Congress Cataloging-in-Publication Data
Neurobiology of spinal cord injury I edited by Robert G. Kalb, Stephen M. Strittmatter.
p. em. -- (Contemporary neuroscience)
Includes bibliographical references and index.
ISBN 978-1-61737-126-4 ISBN 978-1-59259-200-5 (eBook)
DOI 10.1007/978-1-59259-200-5
I. Spinal cord--Wounds and injuries--Pathophysiology. I. Kalb, Robert G. II. Strittmatter,
Stephen M. III. Series.
[DNLM: I. Spinal Cord Injuries--physiopathology. 2. Neurobiology.
3. Spinal Cord Injuries--therapy. WL 400 N4938 2000]
RD594.3.N4686 2000
61 7.4' 82044--dc21
DNLM/DLC
for Library of Congress 99-23563
CIP
Preface
Neurobiological Research in SCI
Suggests a Multimodality Approach to Therapy
Neuroscientists representing a wide variety of disciplines are drawn
to the problem of spinal cord injury (SCI). One of the many reasons for this
is humanistic: the severity of neurologic deficits can have a devastating
impact on most aspects of an individual's functioning. As compassionate
individuals we cannot help but believe that a small lesion, tragically local
ized, can be amenable to a therapeutic intervention. Interest in spinal cord
injury research also stems from the view that the overall problem can be
broken down into a number of understandable and experimentally tractable
smaller problems (see Table 1) . The clearness and discreteness of the issues
to be addressed help focus the research effort. The hope is that if one could
amalgamate the progress made in these smaller arenas, a significant overall
benefit would be available for patients. This book highlights the major areas
of basic science research in which progress is being made today in our battle
against the problem of spinal cord injury.
Important advances in developing effective intervention to promote
functional recovery after spinal cord injury depends on animal models. The
utility of complete or partial spinal cord transection models is complemented
by studies in which the spinal cord is injured by dropping a weight of known
mass from a fixed distance onto the cord. This has led to reproducible
lesions and functional deficits. There is a remarkable concordance between
this experimentally induced lesion and that seen in postmortem specimens
from injured human spinal cords. In their chapter, Drs. Beattie and
Bresnahan describe a number of important insights gleaned from this model
system. First, there is a significant amount of delayed apoptotic death of
neurons and glia both at the lesion site and remotely. It stands to reason that
prevention of this cell death will have important consequences for recovery
of function. Second, anatomical studies of the cystic lesion induced by the
trauma reveal a complex mix of astrocytes, Schwann cells, inflammatory
cells, as well as axons of dorsal root ganglion cells and spared descending
axons in various states of demyelination. Some of the cellular components
of the contusion lesion matrix appear to arise from ependymal progenitor
cells. If new neurons or glial cells were incorporated to the contusion site,
v
Preface
Vl
they might integrate into functional circuitry and/or help form tissue bridges
for regrowing axons.
Though the glial component of the injury is important (as we will see),
one of the major reasons for functional impairment after spinal cord injury
is the death of neurons. Effective neuroprotective strategies after spinal cord
insult will therefore depend on understanding the mechanisms of neuronal
death. A major advance in this field has come from the focus on intracellular
Ca2+. Intracellular Ca2+ is a fundamental regulator of cellular physiology
and, as such, its concentration and subcellular distribution is highly regu
lated. Derangement of this regulation plays a central role in neuronal death.
In their chapter, Chu et al. discuss the controversies surrounding the mecha
nism by which rises in intracellular Ca2+ lead to cell death. In particular they
focus on the extent to which pathogenic deregulation of Ca2+ homeostasis is
a function of the ion's intracellular concentration, its spatial distribution, or
the route of entry into cells.
After injury to the cervical or thoracic spinal cord, the neural elements in
the lumbar enlargement are deprived of inputs from higher command
centers (brainstem and cortex), but are in a certain sense otherwise intact.
Can the isolated lumbar enlargement generate the patterned muscle activa
tion required for locomotion or is this capacity dependent on information
encoded by the descending inputs? The functional capacity of the deaf
ferented lumbar cord has been investigated in lamprey, rodents, and cats and
under certain experimental conditions exhibits a remarkable ability to
generate the neural activity subserving locomotion. Thus, there is great in
terest in understanding the functional capacity of the isolated lumbar cord.
Rossignol et al. describe a set of experiments using cats to define the intrin
sic capacity of the isolated lumbar cord to generate locomotion. Through a
combination oflesion studies and pharmacological manipulations, this work
is beginning to outline the cellular and biochemical mechanisms involved.
A particularly exciting set of experiments provides compelling evidence for
spinal learning-the ability of the isolated cord to make adaptations to new
environmental demands (such as stepping over an obstruction or walking on
a tilted surface). The circuitry inherent within the isolated lumbar spinal
cord and its ability to adapt to changing demands may form an important
substrate for therapeutic intervention.
A more detailed anatomical and electrophysiological analysis of the neu
ral substrate underlying the locomotor generating ability of the isolated
lumbar spinal cord requires a more experimentally accessible system.
Dr. Cazalets has pioneered the use of an ex vivo neonatal rat spinal cord
Preface
Vll
preparation and details work on the location and properties of the major
central pattern generator for fictive locomotion. Though many regions within
the spinal cord can be experimentally manipulated to generate oscillatory
firing of neurons, the neural circuitry of the lower thoracic and upper lumbar
spinal cord are likely to be the dominant central pattern generator for
hindlimb locomotor activity. The region around the central canal, particu
larly ventrally, appears to be key. If this region is undamaged after a spinal
cord insult, then stimulation by regrowing axons or pharmacological means
might form the basis for functional restitution.
Assuming sufficient numbers of neurons and glia can be coaxed to
survive the primary and secondary wave of cell death and that the pattern
generators for locomotion are intact, a central concern focuses on restoring
the continuity between the brain and the distal spinal cord. Steeves and
Tetzlaff provide an overview of molecules known to promote axonal regen
eration in SCI and those which inhibit regeneration. Growth-promoting
molecules include proteins expressed within regenerating neurons, such as
GAP-43 and Tal-tubulin. In addition, a number ofneurotrophins can in
duce axonal sprouting, and the effects of these molecules in SCI are re
viewed. The ability of certain axonal guidance molecules, such as CAMs,
semaphorins, netrins, and ephrins, to promote axonal regeneration is also
discussed. The major inhibitory factors are those derived from CNS myelin
and from astrocytes. In SCI, the growth inhibitors exert a predominant
effect over the growth-promoting factors. To increase functional recovery
after SCI, intervention to both decrease growth-inhibiting effects and
increase growth-promoting effects must be considered.
The extension of axons is primarily regulated from their distal end, a special
ization termed the growth cone. A number of endogenous macro-molecules,
including some present in injured spinal cords, can repel axons and prevent
axonal regeneration. We review the molecular events that transduce the
presence of extracellular signals into a cessation of axonal extension. Par
ticular emphasis is placed on the mechanism of action of the semaphorins, since
they are well studied, and on components of CNS myelin, because they are
likely to underlie the failure of axonal regeneration after SCI. As the normal
functioning of the growth cone becomes understood, pharmacological methods
to overcome the failure of axonal growth cone advance in SCI might be
come obvious.
If the milieu that a growing axon encounters is permissive or even
promotes growth, ultimately the axon elaboration machinery must be en
gaged. Peter Baas reviews the role of microtubules in forming axon struc-
viii Preface
ture and determining axonal elongation rates. The biochemistry and cell
biology of microtubules are reviewed with special focus on their contribu
tion to axonal extension. Evidence suggests that microtubules are nucleated
from a site near the centrosome and then transported into axons. The protein
dynein is the motor responsible for transporting microtubules into and down
the axon. A number of microtubule associated proteins (MAPs) are likely to
regulate microtubule formation and transport. Although the functional orga
nization of these proteins in developing axons is becoming clear, their role
in facilitating or preventing axonal regeneration remains less so. Further
investigation of such mechanisms in SCI might provide novel opportunities
to promote axon regeneration and recovery of function.
From the perspective of problems incurred by spinal cord injury that may
be amenable to therapeutic intervention in the near future, three have
received the most attention: (1) How do we keep the maximal number of
neurons and glial cells alive after the injury? (2) How do we promote the
extension of axons past the lesion site? (3) How can we promote myelina
tion of surviving or newly formed axons so that they can efficiently transmit
action potentials distally? Remarkably, research from a number of different
labs has indicated that cell transplants and neurotrophic factors may have
utility for each of these problems. Work from Barbara Bregman's lab indi
cates that transplanting fetal spinal cord tissue and similtaneously providing
neurotrophic factors can have a beneficial effect on both neuronal survival
and axon growth. Though it has long been clear that trophic factors are
important for the survival and differentiation of immature neurons, it has
only been recently that evidence has been accrued that they also play a role
in mature neuronal survival. One important advance is the recognition that
different neurons (i.e., brainstem, cortical, etc.) have distinct trophic factor
dependence for survival and axon elaboration. When combined with a suitable
substrate for growth (such as fetal spinal cord transplants), trophic
factors remarkably enhance neuronal survival and promote axonal growth
and sprouting. These interventions lead to functionally significant improve
ment in animal behavior, particularly when applied to immature animals.
The challenge will be to adapt this strategy to maximize the benefits for
mature animals.
Since cell transplantation strategies hold great hope for functional
recovery after SCI, what other choices exist beyond the use of fetal spinal
cord tissue? Bartolomei and Greer consider the various cell transplantation
strategies now being developed. Transplanted tissue has included fetal
nervous system, peripheral nervous system Schwann cells, and more recently
olfactory ensheathing cells. The olfactory ensheathing cell appears to hold
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