Studies of laboratory animals with damaged spinal cords indicate that drugs able to stop cells from responding to excess glutamate could minimize destruction as well. Agents that selectively block glutamate receptors of the so-called AMPA class, a kind abundant on oligodendrocytes and neurons, seem to be particularly effective at limiting the final extent of a lesion and the related disability. Certain AMPA receptor antagonists have already been tested in early human trials as a therapy for stroke, and related compounds could enter safety studies in patients with spinal cord injury within several years.
Much of the early cell loss in the injured spinal cord occurs by necrosis, a process in which cells essentially become passive victims of murder. In the past few years, neurobiologists have also documented a more active form of cell death, somewhat akin to suicide, in the cord. Days or weeks after the initial trauma, a wave of this cell suicide, or apoptosis, frequently sweeps through oligodendrocytes as many as four segments from the trauma site. This discovery, too, has opened new doors for protective therapy. Rats given apoptosisinhibiting drugs retained more ambulatory ability after a traumatic spinal cord injury than did untreated rats.
In the past few years, biologists have identified many substances, called neurotrophic factors, that also promote neuronal and glial cell survival. A related substance, GM-1 ganglioside (Sygen), is now being evaluated for limiting cord injury in humans. Ultimately, interventions for reducing secondary damage in the spinal cord will probably enlist a variety of drugs given at different times to thwart specific mechanisms of death in distinct cell populations.
The best therapy would not only reduce the extent of an injury but also repair damage. A key component of that repair would be stimulating the regeneration of damaged axons—that is, inducing their elongation and reconnection with appropriate target cells.
Although neurons in the central nervous system of adult mammals generally fail to regenerate damaged axons, this lapse does not stem from an intrinsic property of those cells. Rather the fault lies with shortcomings in their environment. After all, neurons elsewhere in the body and in the immature spinal cord and brain regrow axons readily, and animal experiments have shown that the right environment can induce axons of the spinal cord to extend quite far.
Then, Induce Regeneration
One shortcoming of the cord environment turns out to be an overabundance of molecules that actively inhibit axonal regeneration—some of them in myelin. The scientists who discovered these myelin-related inhibitors have produced a molecule named IN-1 (inhibitorneutralizing antibody) that blocks the action of those inhibitors. They have also demonstrated that infusion of mouse-derived IN-1 into the injured rat spinal cord can lead to long-distance regrowth of some interrupted axons. And when pathways controlling front paw activity are severed, treated animals regain some paw motion, whereas untreated animals do not. The rodent antibody would be destroyed by the human immune system, but workers are developing a humanized version for testing in people.
Many other inhibitory molecules have now been found as well, including some produced by astrocytes and a number that reside in the extracellular matrix (the scaffolding between cells). Given this array, it seems likely that combination therapies will be needed to counteract or shut down the production of multiple inhibitors at once.
Beyond removing the “brakes” on axonal regrowth, a powerful tactic would supply substances that actively promote axonal extension. The search for such factors began with studies of nervous system development. Decades ago scientists isolated nerve growth factor (NGF), a neurotrophic factor that supports the survival and development of the peripheral nervous system. Subsequently, this factor turned out to be part of a family of proteins that both enhance neuronal survival and favor the outgrowth of axons. Many other families of neurotrophic factors with similar talents have been identified as well. For instance, the molecule neurotrophin- 3 (NT-3) selectively encourages the growth of axons that descend into the spinal cord from the brain.