When tissue tears, the body fills in the gap with a scar. In the case of the spinal cord, that so-called “glial scar” is made mainly of astrocytes—or so neuroscientists believed. According to a paper in today’s Science, pericyte-enwrapping blood vessels near a spinal lesion will differentiate and move toward the injury, forming the core of an astrocyte-coated scar. Without pericytes, the wound is likely to remain a gaping hole.
“The pericyte response is dramatic,” wrote study author Christian Göritz of the Karolinska Institute in Stockholm, Sweden, in an e-mail to ARF. “Many cell types respond in one or another way to injury, but the pericyte response is by far the strongest I have ever seen.” Göritz led the study with senior author Jonas Frisén.
Scars are good, in that they fill in a wound with connective tissue. But they also put a damper on regeneration; in the spinal cord, scars block axons attempting to grow across the lesion. “It will be interesting to further investigate the role of these pericytes…to see the extent to which they contribute to axon regenerative failure in the central nervous system,” commented Ben Barres of Stanford University in Palo Alto, California, in an e-mail to ARF.
Pericytes surround blood vessels and help stabilize them. They form an integral part of the blood-brain barrier. In the central nervous system (CNS), these cells are multipotent, with the ability to differentiate into neural and glial cell types as well as connective tissue such as fibroblasts (Dore-Duffy et al., 2006; Dore-Duffy, 2008; Dore-Duffy et al., 2011). They are found throughout the body, and participate in kidney and liver fibrosis (Pinzani et al., 1992; Lin et al., 2008; Humphreys et al., 2010), as well as skin scarring (Sundberg et al., 1996). The role of pericytes in the spinal cord, however, has been “mysterious,” Barres wrote.
Göritz and Frisén have been studying injury response in several cell types (Meletis et al., 2008; Barnabé-Heider et al., 2010), and in the current study, turned their fate-mapping attention to pericytes. Göritz used Cre recombinase to selectively label the pericytes of mice—specifically, a subclass called perivascular cells—with yellow fluorescent protein (YFP). These pericytes, as well as any of their progeny, would glow yellow, allowing the researchers to track them and estimate their role in scarring. However, the markers used to identify pericytes were insufficient to convince Paula Dore-Duffy of Wayne State University School of Medicine in Detroit, Michigan; she suggested that other cell types might have been labeled, too. If so, then cell types besides pericytes could be contributing to the scar’s core.
After spinal cord lesion, the pericytes proliferated. YFP-labeled cells expanded to more than 25 times their normal numbers within nine days of injury. The descendants of the YFP-tagged cells migrated to the injury site and built an extracellular matrix there.
The core of the scar is traditionally considered to be connective tissue such as fibroblasts, Göritz noted, but the identity of the pericyte-descendant, scar-forming cells is uncertain. More work must be done to determine if the migrating pericytes stay pericyte-like, morph into fibroblasts, or become another cell type, he wrote.
The pericyte-derived cells were the first cells to reach the injury site. Astrocytes also proliferated, but their doubling pales in comparison to the explosion of the pericyte lineage. While astrocytes outnumber pericytes 10 to one in a healthy spinal cord, there were twice as many YFP-tagged cells as astrocytes in the damaged area. Together, the cell types formed a scar with a pericyte-derived center and astrocyte shell.
To confirm the importance of pericytes in scar formation, the researchers modified their mice further. In the new line, the activation of Cre not only turned on YFP in the pericytes, but it also disabled cell division by removing all ras genes. Göritz performed spinal cord hemisections and examined the injury sites 18 weeks later. Compared to control mice with normal pericyte cell division, the modified mice had fewer connective tissue cells in the core of the scar. In one-third of the ras-free mice, the injury did not even seal.
Pericytes also dissociate from blood vessels after traumatic brain injury and stroke, as well as in cases of hypoxia and in a mouse model of multiple sclerosis, Dore-Duffy said, although the mechanism of the migration is unclear (Dore-Duffy et al., 1999; Takahashi et al., 1997; Dore-Duffy et al., 2000; Dore-Duffy and Lamanna, 2007). The study authors suggest that pericyte scar formation may be a general mechanism of wound repair throughout the central nervous system, and perhaps in other organs as well. The next step, Göritz wrote, will be to find ways to modulate pericyte activity to minimize the downsides of scarring.
Pericytes might also play a role in neurodegeneration, Dore-Duffy suggested in an e-mail to ARF. A slowing of vascular activity comes with age, she noted, and pericytes are compromised in conjunction with vascular damage in Alzheimer’s disease. Pericytes contribute to the structure of the vascular system, and a poorly functioning vasculature in amyotrophic lateral sclerosis or multiple sclerosis might prevent neurons from getting the energy supply they need, she speculated.—Amber Dance
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