Spinal cord injury (SCI) is a devastating condition with a complex pathophysiology and there are no therapies available currently to a restore the lost neurological functions. Following a SCI, astrocytes at the site of injury become reactive and exhibit a neurotoxic (A1) phenotype, which exacerbate the neuronal death caused by the injury. In addition, the glial scar formed after SCI, which is composed primarily of reactive astrocytes, acts as a chemical and physical barrier to subsequent axonal regeneration. Biomaterials, specifically electrospun fibers, induce a migratory phenotype of astrocytes and promote regeneration of axons following acute SCI in preclinical models. However, till recently, no one has examined the potential of electrospun fibers or biomaterials in general to modulate the neurotoxic (A1) or neuroprotective (A2) reactive phenotypes of astrocytes.
Neural tissue engineering is a newly emerging field that combines biomaterials, stem cells, and chemical and physical cues to produce engineered structures with the ultimate goal of replacing/repairing in vivo brain and spinal cord tissues. There has been extensive advancement in the development of various biomaterial scaffolds into nerve guidance channels or hydrogels that more effectively stimulate spinal cord tissue regeneration. For instance, Professor Ryan Gilbert and colleagues from Rensselaer Polytechnic Institute previously showed that aligned Poly-L-lactic acid (PLLA) fiber conduits support the ingrowth and migration of astrocytes when implanted into an acute rat model of complete transection SCI. Nonetheless, there is still need for further research in order to ascertain the potential of biomaterials, including electrospun fibers, for neural tissue engineering applications such as understanding their potential in modulating the phenotype of astrocytes, the homeostatic regulators of the central nervous system.. With this in mind, a new study by Rensselaer Polytechnic Institute researchers: Dr. Manoj Gottipati, Dr. Anthony D’Amato, Dr. Alexis Ziemba and led by Professor Ryan Gilbert, in collaboration with Professor Phillip Popovich at the Ohio State University examined the ability of electrospun PLLA microfibers to influence the expression of A1/A2-specific astrocyte markers in culture. Their work is currently published in the research journal, Acta Biomaterialia.
In their approach, naïve rat spinal cord astrocytes or astrocytes primed towards the neurotoxic (A1) phenotype were cultured on fibrous scaffolds for the purposes of investigating the changes in astrocytic reactivity in response to aligned PLLA microfibers. To that end, gene expression analysis of the pan-reactive astrocyte makers (GFAP, Lcn2, SerpinA3), A1-specific markers (H2-D1, SerpinG1), and A2-specific makers (Emp1, S100a10) was done using quantitative polymerase chain reaction to elucidate the fiber-induced changes in astrocytic phenotype.
The authors found that the electrospun fibers increased the expression of the pan-reactive and A1-specific markers, demonstrating the ability of fibrous materials to induce a mild neurotoxic phenotype in astrocytes. However, when naïve or activated astrocytes were cultured on fibers in the presence of the anti-inflammatory cytokine transforming growth factor β3 (TGFβ3), the expression of A1-specific markers was greatly reduced, which in turn improved neuronal survival in culture.
In summary, the study demonstrated PLLA fibers to be highly biocompatible, and that the spinal cord astrocytes respond to their surface topography in culture. However, the PLLA fibers were seen to cause a mild increase in the neurotoxic response of the astrocytes; a drawback that could be alleviated by the inclusion of the anti-inflammatory cytokine, TGFβ3. To date there are no effective treatments that can regenerate the spinal cord after injury. Although there have been significant preclinical advances in bioengineering and regenerative medicine over the last decade, these have not translated into effective clinical therapies for spinal cord injury. In a statement to Advances in Engineering, Professor Ryan Gilbert explained that their findings clearly showed the potential of a combination of PLLA fibers and TGFβ3 as a translatable therapy to modulate the reactive astrocytic response and promote tissue repair following a spinal cord injury.
Manoj K. Gottipati, Anthony R. D’Amato, Alexis M. Ziemba, Phillip G. Popovich, Ryan J. Gilbert. TGFβ3 is neuroprotective and alleviates the neurotoxic response induced by aligned poly-l-lactic acid fibers on naïve and activated primary astrocytes. Acta Biomaterialia; Volume117 (2020) page 273–282.