HONG KONG – Treatment with low levels of carbon monoxide (CO) may offer a novel dual approach to treating traumatic brain injury (TBI), by promoting neurogenesis and preventing the death of pericytes, cells that play multiple roles in blood flow in the brain and the blood-brain barrier.

A U.S./Korean collaborative study reported those findings in the Sept. 26, 2016, of Nature Medicine.

Multiple mechanisms of apoptotic cell death are triggered by TBI, followed by adaptive responses as the injured brain tissue attempts to recover from injury.

At low levels, CO acts both as a second messenger and as a neuromodulator. That prompted researchers from Harvard University in Boston and Hanyang University College of Medicine and Seoul National University, Seoul, to assess whether CO might also have dual therapeutic efficacy in a mouse model of TBI.

They treated the mice with CORM-3, a CO-releasing molecule that has been shown to have anti-inflammatory properties and to reduce brain damage in a rat model of hemorrhagic stroke.

"We used a standard model of TBI known as the controlled cortical impact model, whereby a computer-controlled rod is applied to the brain surface to cause a contusion," said study leader Eng H. Lo, professor of radiology and neurology at Massachusetts General Hospital, Harvard Medical School.

In that model, treatment with CORM-3 was shown to reduce pericyte death and slowed progression of neurological deficits. In contrast, although treatment with the antioxidant free radical scavenger N-tert-butyl-a-phenylnitrone (PBN) also reduced pericyte death, neurological deficits were unimproved.

Contractile endothelial cells of the capillaries that are also involved in paracrine signaling in the general circulation, pericytes play important roles in the brain, where they help to sustain the blood-brain barrier and have other homeostatic and hemostatic functions.

"In this mouse model, treatment with CORM-3 was shown to reduce pericyte death and improve neurological recovery, with improved motor skills and memory being seen in various tests," Lo told BioWorld Today.

"Because both CORM-3 and PBN seemed to prevent pericyte cell death to a similar extent, but only CORM-3 rescued behavioral recovery . . . this suggests that beyond preventing cell death, something else must be happening to improve recovery."

Further testing revealed that compared to vehicle-treated controls and PBN-treated mice, animals treated with CORM-3 showed higher levels of phosphorylated neural nitric oxide (NO) synthase within multipotent neural stem cells (NSCs), from which neurons and other nervous system cells are derived.

That is a significant finding, as "it suggests that CORM-3, or perhaps some other means of providing CO to the brain, may improve neural stem cell function," Lo said.

Moreover, inhibition of NO synthase lessened the CORM-3-mediated increase in the number of cells that stained positive for both the neuronal biomarker NeuN and 5-bromo-2- deoxyuridine, a marker for proliferating cells, in vivo, consequently interfering with neurological recovery after TBI.

Therefore "providing CO seemed to up-regulate NO and improve the neural stem cell response, while blocking NO appeared to prevent this," noted Lo. "So taken together, this suggests that the ability of CORM-3 to improve NSC function may require the ability of CO to promote NO signaling."

Because under microscopic examination NSCs seemed to be located in close proximity to pericytes, the researchers questioned whether intracellular cross-talk between pericytes and NSCs might also be induced by CORM-3, thereby promoting neurogenesis.

They demonstrated that in oxygen- and glucose-deprived pericyte cultures, conditioned cell culture medium collected after CORM-3 treatment enhanced in vitro differentiation of NSCs into mature neurons under hypoxic conditions. "This supports the findings of the in vivo experiments," noted Lo.

Taken together, those findings therefore suggest that CO treatment may provide a therapeutic approach for TBI by preventing pericyte death, rescuing crosstalk with NSCs and promoting neurogenesis.

However, while Lo was optimistic regarding the possibility of those latest research findings leading to the development of effective new treatment approaches for TBI, he also expressed caution in that respect.

"It is important to remember that we are still far away from such treatments, as a lot more basic research work is needed to define carefully the mechanisms involved, before jumping into humans," he said. "Far too often, we proclaim success and rush into clinical trials before we sort out all the details. It is easy to be overoptimistic, but perhaps it might be better to be conservative and cautious."

Nevertheless, "our ultimate [research] objective is to find a clinically viable way to manipulate this CO-NO signaling between pericytes and NSCs, so we can improve brain protection and repair in TBI. Importantly, this research area may also be useful for the future treatment of stroke and other central nervous system disorders."