Immune Cell Protein Sparks Retinal Cell Regrowth

The sight of a macrophage closing in on a wounded cell is one of the most vivid in biology. Yet these steely scavengers appear to have a kinder, gentler side. Researchers at Children’s Hospital Boston report that macrophages secrete a protein that helps injured retinal nerve cells regrow their axons.

Image courtesy of Larry Benowitz

Long and short of it. Optic nerve fibers of rats injected with the macrophage protein oncomodulin along with mannose and forskolin (bottom) display much greater growth past the site of injury than controls (top).

Yuqin Yin, Larry Benowitz, and colleagues injected the protein, oncomodulin, along with two cofactors, into the vitreous fluid of rats whose optic nerves had been injured. The nerve fibers, which originate in the retinal ganglion cells, exhibited a five- to seven-fold increase in regrowth past the site of injury. What is more, the fibers regrew even when oncomodulin was given three days after injury. The findings appear in the May 16 online Nature Neuroscience and the June print issue of the journal.

It has been known for some time that neurons injured in the periphery can regrow, but damage in the central nervous system was thought to be irreparable. Several years ago, Benowitz, HMS associate professor of neurosurgery, and his colleagues discovered that injured retinal ganglion cells can regenerate axons when macrophages are activated in the eye. They set out to discover which macrophage-secreted molecules might be responsible.

Using biochemical methods, Yin, HMS instructor in surgery, Benowitz, and colleagues isolated the small protein oncomodulin. To test its regenerative powers, they added the protein, along with two known axon-simulating molecules, mannose and forskolin, to cultured retinal cells. Retinal ganglion fibers exhibited greater growth than controls and also than cells treated only with growth factors or mannose and forskolin.

The most stunning results came when they moved into living rats. Their in vivo finding—that maimed retinal ganglion cells exhibit a five- to seven-fold increase in fiber regrowth—raises a host of questions. To begin, how exactly is the macrophage’s message triggering regeneration? Yin and colleagues believe that oncomodulin may work as a signaling molecule. Indeed, they found that it binds to a receptor on the surface of the retinal ganglion cells and appears to trigger a transcriptional cascade.

Immune cells are rarely found in the central nervous system, so why would retinal ganglion cells be receptive to oncomodulin? The researchers suggest that oncomodulin may be released from some other source, possibly in the developing brain. It would not be the first time an immune system molecule was found to work as a signaling molecule during development (see Focus, June 23, 2000, DMS Symposium).

In fact, Benowitz believes that oncomodulin may play a broader role inside and outside the nervous system. “To me the main story is that oncomodulin is a new signaling molecule which, in the nervous system, has these very strong effects on regeneration,” he said.

For many, the real story might be its potential for triggering regeneration in a host of nerve-maiming diseases, including certain forms of blindness, spinal cord injuries, and neurodegenerative diseases. Benowitz believes such treatments are many years away. “Macrophages make a lot of molecules, some of which are going to be beneficial and some of which are going to be cytotoxic,” he said. “What we would really like to do is to just isolate all the good guys and use those by themselves.”

—Misia Landau

Fighting Fat with Vitamin D

New research identifies the vitamin D receptor and its ligand calcitriol as critical early players in the cascade that causes certain cells to differentiate into fat cells. The study, by Anthony Hollenberg, HMS associate professor of medicine at Beth Israel Deaconess Medical Center, and colleagues, appears in the April 21 Journal of Biological Chemistry.

The scientists were focusing on the molecular underpinnings of adipocyte differentiation as a means to better understand metabolic diseases. “Fat cells are not just storing the extra calories we eat,” said Hollenberg. “The adipocyte is an important endocrine organ. Like the pancreas or pituitary, the fat cell makes key hormones that regulate metabolism.”

Hollenberg and colleagues studied the eight-day process of adipocyte differentiation in a model cell line in vitro, triggering differentiation with a cocktail of hormones. Previous research had identified the protein C/EBP-beta as the precursor of two other proteins, C/EBP-alpha and PPAR-gamma, which in turn activate the required genes for differentiation. Iphigenia Tzameli, HMS instructor in medicine at BID, screened the cell culture prior to C/EBP-beta activation and identified the vitamin D receptor as a precursor to this cascade.

Since nuclear receptors such as the one for vitamin D have different actions in the presence and absence of their ligand, HMS research associate Jeffrey Blumberg and research fellow Inna Astapova treated some cell cultures with calcitriol, the vitamin D receptor ligand, and left others untreated. In the absence of calcitriol, cells differentiated. But when the researchers added the ligand at the beginning of the differentiation program, liganded receptor curtailed the expression of C/EBP-beta, C/EBP-alpha, and PPAR-gamma, interrupting the entire transcription cascade and preventing fat-cell formation.

The discovery suggests that calcitriol may play a role in obesity. Recent nutrition research provides some evidence that vitamin D may combat obesity, said Hollenberg, but the molecular mechanism identified in this study requires getting vitamin D into the fat cell, not just into the bloodstream, where calcitriol normally flows after ingestion or sun exposure.

Before considering calcitriol as an obesity treatment, these findings must be confirmed in mouse models, something Hollenberg and collaborator Jeffrey Flier, the George C. Reisman professor of medicine at HMS and BID, are undertaking as part of a larger metabolic disease project. “Intracellular concentrations of active vitamin D seem to be an important toggle in regulating fat cell differentiation and lipid genesis in vitro,” said Hollenberg. “We now want to know if that has relevance to what’s going on in vivo.”

—Elizabeth Dougherty

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