Another Chapter in Going from Blood to Brain

PLASTICITY IN MICE AND MEN

Mezey recalls that her first indication that bone marrow cells could become neurons came by accident in the early 1990s when she was trying to determine if microglia came from bone marrow. "While we were doing those studies, I accidentally noticed that there were several other cell types that seemed to derive from the bone marrow," says Mezey. In, 2000, Mezey and coworkers conducted gender mismatch studies in mice, and demonstrated that bone marrow stem cells which migrated to the brain differentiated into cells expressing a neuron-specific antigen.⁶

The two Hot Papers examined bone marrow-derived cells in different brain regions, and they characterized plasticity differently. Mezey argues that the transplanted cells actually transdifferentiated, or changed fate, adopting gene-expression patterns specific to the brain region in which they were found. Blau says this might not be surprising in regions like the hippocampus, where neurons are generated in adulthood. "You can imagine a cell coming in there and making a new neuron from something else," she says. Purkinje neurons, on the other hand, are highly elaborate cells not generated after birth. "So it's not surprising that in that case we saw fusion with the Purkinje cells," Blau says. "But in both cases it's a matter of turning on a new program of gene expression – one that a blood cell would never express."

Recent studies, cast doubt on some of these conclusions. In mid-2003, Luc Vallieres and Paul Sawchenko transplanted green fluorescent protein (GFP)-expressing bone marrow cells in irradiated mice and found that even though some donor-derived cells can be found in the brain, none have a neural phenotype.⁷ "These cells are not neurons, they are macrophages," says Vallieres. Likewise, Irving Weissman and colleagues at Stanford found that, with the exception of rare Purkinje-cell fusions, donor bone marrow cells in irradiated mice or unirradiated parabiotic mice, retain hematopoietic lineage. Coauthor Amy Wagers of Harvard Medical School notes that even in the Purkinje case, it's unlikely that the fused cells were neuron-like. "In the Weimann paper, they demonstrated activation of one transgene, but that doesn't show reprogramming of the entire gene transcription program of that hematopoeitic nucleus to act like a Purkinje neuron. It might be that it activates one or two genes, but not all of them," she says. This question will be resolved by further nuclear reprogramming studies, predicts Blau.

© 2003 National Academy of Science

A Purkinje cell from a female human has both an X (red arrow) and a Y chromosome (green arrow). The panel at right shows blue counterstain removed to highlight the probes. Though the evidence could suggest transdifferentiation of transplanted male bone marrow cells, less than half of the Purkinje nucleus is encompassed in the sample. (Scale bar, 20 μm). (From J.M. Weimann et al., Proc Natl Acad Sci, 100: 2088–93, 2003).

Vallieres says that the immunohistochemistry methods used in both the Mezey and Blau experiments may have incorrectly identified perivascular macrophages as neurons. These cells look like neurons, he says, and because they are phagocytes, they express receptors that can bind to antibodies. "The antibody used by Mezey and Blau to stain neurons... can be recognized by immunoglobulin-binding proteins expressed on perivascular macrophages, generating false positive results," Vallieres writes in an E-mail. Blau contends that this may be true for the hippocampus or cortex, but not for Purkinje cells, which are at least 100-fold larger than macrophages and have a "distinctive arborized morphology."

A ROLE IN THERAPY

Although the two Hot Papers posited a different mechanism for bone marrow-to-brain contribution, both authors came to similar conclusions about the implications of their findings. "We both speculated that it may be serving a positive function in neurodegenerative disease, repairing tissues that are damaged," says Blau, who notes that earlier studies in animals showed that tissue damage enhances bone marrow cell contribution.

Wagers contends that even if bone marrow cells do in fact contribute to brain, the contribution is too low to be therapeutically beneficial. "But if you wanted to take an approach of trying to enhance that contribution, then it would be important to know the mechanism by which it was occurring," she states.

© 2003 National Academy of Science

The neuronal marker NeuN (green) colocalized with the Y chromosome (red) in neocortex (left) and hippocampus (right) from female patients transplanted with male hematopoietic stem cells (scale bars, 10 μm). (From E. Mezey et al., Proc Natl Acad Sci, 100: 1364–9, 2003).

Blau suggests that signals from cell injury might recruit circulating bone marrow cells to fuse with a dying neuronal cell, whose nucleus eventually takes over and expresses neuron-specific genes. Others suggest that bone marrow-derived cells deliver trophic factors that prevent apoptosis and encourage endogenous neurogenesis and/or repair. "It would not be very realistic to believe that you're going to take a bone marrow cell and squirt it in there and have it magically make a brain," says neurologist David Hess of the Medical College of Georgia in Augusta.

Mezey agrees, citing her own unpublished research with stroke-induced mice, in which transplanted bone marrow cells surround injured neurons in small numbers and appear to keep them alive, possibly by secreting growth factors. "We have to figure out what these growth factors are," Mezey says.

Blau acknowledges that there is still some debate over the role of bone marrow-derived cells in the brain's response to injury. "I think that the jury's not in yet on whether bone marrow contribution to Purkinje cells is beneficial or not. It definitely happens. Why it happens we don't know," she says.

Vallieres, for example, suggests that bone marrow cells, at least in the case of fusion, may in fact be aiding the demise of injured Purkinje neurons. "This would explain why we don't see a lot of these cells in the cerebellum," he says. Hess counters that the Purkinje cells in Blau's paper do not morphologically resemble dying cells.

Although Vallieres doubts that bone marrow cell transplantation can be used to repair brain injuries, he is optimistic that they can be used as a vehicle for gene therapy instead. "I think bone marrow-derived cells could prove useful for treating a range of neurological injuries and disorders," he says.

Still, a fair number of researchers remain hopeful that a method to coax bone marrow stem cells to adopt a neural lineage may one day be developed. "It's very much a hot and open point of debate at this point, and I think it's going to stay that way until we get some more definitive experiments done," says the University of Florida's Scott.