By David N. Leff
Editor's note: Science Scan is a roundup of recently published biotechnology-relevant research.
To a poet, the eyes are windows to the soul; to an ophthalmologist, windows opening on the brain. The retina of the eye is a neuronal outpost of the brain, being the origin of the optic nerve that carries the messages of vision into the cerebral cortex.
In a mature mammal, the neuronal ganglion cells of the retina can't grow back their axons after the optic nerve is injured. Instead, they die off from apoptosis - programmed cell death. But neurosurgeons at Harvard-affiliated Boston Children's Hospital reversed this morbid process by inflicting a small puncture on the lenses of rats. This ocular wound enhanced survival of the retinal ganglion cells (RGC), which then regenerated their axons. These grew past the injury into optic nerves the researchers had experimentally crushed with jewelers' tweezers, 2 millimeters behind the eye.
They describe this finding in the June 15, 2000, issue of The Journal of Neuroscience. Their paper's title: "Lens injury stimulates axon regeneration in the mature rat optic nerve." It reported, "Combining nerve injury with lens puncture leads to an eightfold increase in RGC survival and a 100-fold increase in the number of axons regenerating beyond the crush site." The article added, "The pro-regenerative effects of lens puncture appear to be mediated through activated macrophages."
In a procedure vaguely reminiscent of vein transplants in cardiac bypass surgery, they then dissected out segments of sciatic nerve from the legs of other rats, and implanted 1- millimeter lengths of these neurons into the eyeball's vitreous body behind the pierced lens.
"In the present study," the co-authors concluded, "lens puncture and nerve injury had independent and synergistic effects on growth-associated protein expression. Whereas axotomy alone induced only a weak and transient response, the effect of lens puncture persisted for at least three weeks, and the two combined resulted in an induction that was considerably more than addition."
They have tentatively identified the mechanism and factors responsible for this regeneration, for which Boston Life Sciences Inc. holds the exclusive license from Children's Hospital.
Rare, Fatal Fabry's Disease, Marked By Endless Lipid Buildup, Yields To Substrate Inhibitor
Although Fabry's disease fatally affects two or more of the body's major organs, it is named for the German dermatologist, Johannes Fabry (1860-1930), who described peculiar warts called angiokeratomas scattered over the thighs, buttocks and genitalia of his patients.
Fabry's disease is an X-linked - hence, males only - inherited deficiency of a key lysosomal enzyme, alpha-galactosidase, in the body. Its lack causes unremitting buildup of a sphingolipid, globotriaosylceramide, in lysosomes. A cellular version of garbage dumpsters, lysosomes inhabit all cells that use enzymes to dissolve specific molecular bonds and digest waste products.
Fabry's disease brings early death from heart or cerebral complications of hypertension, and kidney failure. There is no proved or approved treatment for this malady, which attacks an estimated 6,000 to 10,000 individuals in the U.S. alone.
A new experimental drug, developed at the University of Michigan, Ann Arbor, is the first therapeutic shown to correct the causes of Fabry's disease, albeit so far in mice. It's reported in the June 2000 Journal of Clinical Investigation, under the title: "Reduction of globotriaosylceramide in Fabry's disease mice by substrate deprivation."
Other researchers have tried to replace the Fabry's missing enzyme, as is successfully done in treating Gaucher's disease - another lipid storage condition - with exogenous, recombinant beta-glucosidase. Instead, the Michigan co-authors concentrated on blocking glycolipid formation by inhibiting a key substrate enzyme the cell needs to produce these excess molecules.
As they report, male mice who got twice-daily intraperitoneal injections of the inhibiting compound for eight weeks harbored glycolipid levels lower than when treatment began. Nor did they lose weight or develop toxic side effects of the drug.
"Now that we know it works in mice," observed the paper's senior author, nephrologist James Shayman, "our next step is preclinical drug development. We hope to show efficacy with oral intake, so the drug will not need to be given intravenously."
Although labeled an "orphan" because of its rarity, Fabry's disease is one of seven lysosomal storage disorders, or lipidoses, which together affect 20,000 to 30,000 Americans. Notable among these diseases: Gaucher's, Tay-Sachs, Sandhoff's. Although caused by different missing enzymes, all seven feature the accumulation of similar glycolipids in different organs of the body. The Michigan team plans to test its inhibitor in all these related disorders.
Antisense Strategy Reverses Hyperglycemia, Curbs Insulin Resistance, In Diabetic Mice
PTEN won its bones as a wannabe tumor suppressor gene by showing up mutated in endometrial cancer cells. Now it turns up playing a hand in treating diabetes.
On Monday afternoon, June 12, 2000, molecular biologist Robert McKay, of Isis Pharmaceuticals Inc., in Carlsbad, Calif., reported as much to the 60th American Diabetes Association meeting in San Antonio. His presentation bore the title: "Specific inhibition of PTEN expression with an antisense nucleotide normalizes plasma glucose in db/db [diabetic] mice."
For openers, McKay noted that "the modulation of metabolic processes by insulin is primarily controlled by the activation of the PI3 kinase signal transduction pathway. Whether inhibiting this signaling plays any role in insulin resistance - the key disorder afflicting Type II diabetics - remains unclear." The PTEN protein expressed by PTEN is a cytosolic phosphatase that dephosphorylates the lipid second messenger produced by PI3 kinase, he explained.
To determine if activating the PI3 pathway might help reverse that resistance, and so lower blood glucose levels in diabetic mice, the Isis team resorted to an antisense strategy. They injected male diabetic animals with their proprietary PTEN oligonucleotide twice a week intraperitoneally. At the end of four weeks, their average glucose level was 127 milligrams per deciliter, as compared to 418 in saline-treated controls.
"The data," McKay concluded, "demonstrate that inhibition of PTEN expression with an antisense oligonucleotide reverses hyperglycemia in diabetic mice, and suggests that PTEN may play a role in insulin resistance in vivo."