Biogel Promotes Brain Tissue Growth and Limb Recovery after Stroke in Mice

U.S. scientists have developed a bioengineered gel that can promote the growth of new blood vessels and neurons in stroke-damaged mice, resulting in what researchers call “dramatic” improvements in limb function.

Scientists at the University of California, Los Angeles (UCLA), and Duke University developed an immune-modulating angiogenic gel containing vascular endothelial growth factor (VEGF) and heparin, which could be injected directly into the brain cavity that is caused by stroke. Tests in stroke-damaged mice confirmed that the gel promoted blood vessel formation and the growth of neural networks into the cavity, which promoted functional recovery.

“We tested this in laboratory mice to determine if it would repair the brain in a model of stroke, and lead to recovery,” said S. Thomas Carmichael, Ph.D., professor and chair of neurology at UCLA. “This study indicated that new brain tissue can be regenerated in what was previously just an inactive brain scar after stroke.”

The researchers report on their developments in Nature Materials, in a paper entitled “Dual-Function Injectable Angiogenic Biomaterial for the Repair of Brain Tissue following Stroke.”

A stroke causes tissue damage and generates an essentially dead cavity in the brain that is devoid of normal brain tissue, neurons, and vasculature, and which over time fills with fibrotic scar tissue, the authors explain. This fibrotic cavity is associated with the functional disability that stroke patients suffer. Stroke also stimulates a local inflammatory response that can further hamper recovery. And while angiogenesis after stroke is associated with better outcomes in stroke patients but, as the authors write, “therapeutic manipulation of angiogenesis in the brain is problematic.”

The stroke cavity does, however, represent an ideal site as a transplant location because it abuts the edges of the infarct area, out from where new neurons and blood vessels could feasibly grow. “A successful strategy for brain repair after stroke would deliver a molecule that stimulates angiogenesis and neural regeneration, reduces local inflammation, removes the barrier to cellular infiltration in the stroke site, and introduces a scaffold that can serve as a physical support onto which a neuronal network can grow.”

To this end, the researchers developed a nonfibrous hydrogel composed of hyaluronic acid (HA) – which has previously been shown to promote neural differentiation – as a scaffold, into which they added heparin nanoparticles (nH) and incorporated high-density clusters of extracellular matrix-bound VEGF (hcV).

Experimental mice received injections of the engineered gel directly into the cavities caused by stroke, within five days of injury. Compared with control animals, the brains of animals treated using the gel demonstrated much lower inflammatory responses and developed a bed of mature blood vessels within the stroke cavity, alongside which new neurons grew and generated neural networks. This was something that had not seen before. “This is unexpected in a traditional delivery of VEGF in the brain and indicates that the clustered nanoparticle presentation of VEGF in the hydrogel promotes elements of normal vascular development,” the team writes. Results of further analyses also demonstrated that the axonal growth was due to revascularization of the stroke cavity, rather than a direct effect of VEGF on sprouting neurons.

Encouragingly a number of tests indicated that the treated animals recovered a degree of limb control, with one assessment demonstrating that stroke-damaged mice receiving the gel injections recovered limb dexterity to the point that they became faster at manipulating and eating a piece of pasta than control animals.

Additional biochemical tests confirmed that the functional recovery was linked to angiogenesis, which supported axonal network formation in the stroke cavity. “…the delivery of an engineered immune-modulating angiogenic biomaterial directly to the stroke cavity promotes tissue formation de novo, and results in axonal networks along these generated blood vessels,” the authors state. “This regenerated tissue produces functional recovery through the established axonal networks.”

What isn’t clear is the mechanism behind these functional improvements. “The new axons could actually be working,” comments Tatiana Segura, Ph.D., a former professor of chemical and biomeolecular engineering at UCLA, who is now a professor in the department of biomedical engineering at Duke University. “Or the new tissue could be improving the performance of the surrounding, unharmed brain tissue.”

The researchers conclude that their studies demonstrate that injections of the engineered VEGF-containing hydrogel directly into the stroke cavity can induce the formation of a vascular and neuronal structure that leads to behavioral improvement.  “HA gel + hcV induced the formation of a robust, mature and highly developed vascular bed within the stroke cavity and patterned axonal ingrowth along these vessels,” they write. “These findings support a process of coordinated vascular and axonal growth in a developing neural tissue inside a normally fibrotic brain scar.” And whereas previous studies have demonstrated that administering VEGF directly can cause inflammation, the heparin acts to counterbalance this inflammatory effect of VEGF, creating an environment that promotes repair and leads to the formation of new brain tissue.

The reported studies were designed to explore recovery in acute stroke, which is five days after stroke induction in mice, and within two months of stroke in humans. The researchers next aim to study if brain tissue can be regenerated in mice much longer after the initial stroke injury. Chronic stroke affects more than 6 million people in the U.S.

The team claims their development may also have more widespread applications, and “lays the groundwork for the use of angiogenic materials to repair other neurologically diseased tissues.”