Long-term retention of ECM hydrogel after implantation into a sub-acute stroke cavity reduces lesion volume.

Salvaging or functional replacement of damaged tissue caused by stroke in the brain remains a major therapeutic challenge. In situ gelation and retention of a hydrogel bioscaffold composed of 8mg/mL extracellular matrix (ECM) can induce a robust invasion of cells within 24h and potentially promote a structural remodeling to replace lost tissue. Herein, we demonstrate a long-term retention of ECM hydrogel within the lesion cavity. A decrease of approximately 32% of ECM volume is observed over 12weeks. Lesion volume, as measured by magnetic resonance imaging and histology, was reduced by 28%, but a battery of behavioral tests (bilateral asymmetry test; footfault; rotameter) did not reveal a therapeutic or detrimental effect of the hydrogel. Glial scarring and peri-infarct astrocytosis were equivalent between untreated and treated animals, potentially indicating that permeation into host tissue is required to exert therapeutic effects. These results reveal a marked difference of biodegradation of ECM hydrogel in the stroke-damaged brain compared to peripheral soft tissue repair. Further exploration of these structure-function relationships is required to achieve a structural remodeling of the implanted hydrogel, as seen in peripheral tissues, to replace lost tissue and promote behavioral recovery. STATEMENT OF SIGNIFICANCE: In situ gelation of ECM is essential for its retention within a tissue cavity. The brain is a unique environment with restricted access that necessitates image-guided delivery through a thin needle to access tissue cavities caused by stroke, as well as other conditions, such as traumatic brain injury or glioma resection. Knowledge about a brain tissue response to implanted hydrogels remains limited, especially in terms of long-term effects and potential impact on behavioral function. We here address the long-term retention of hydrogel within the brain environment, its impact on behavioral function, as well as its ability to reduce further tissue deformation caused by stroke. This study highlights considerable differences in the brain's long-term response to an ECM hydrogel compared to peripheral soft tissue. It underlines the importance of understanding the effect of the structural presence of a hydrogel within a cavity upon host brain tissue and behavioral function. As demonstrated herein, ECM hydrogel can fill a cavity long-term to reduce further progression of the cavity, while potentially serving as a reservoir for local drug or cell delivery.

ECM hydrogel for the treatment of stroke: Characterization of the host cell infiltrate.

Brain tissue loss following stroke is irreversible with current treatment modalities. The use of an acellular extracellular matrix (ECM), formulated to produce a hydrogel in situ within the cavity formed by a stroke, was investigated as a method to replace necrotic debris and promote the infiltration of host brain cells. Based on magnetic resonance imaging measurements of lesion location and volume, different concentrations of ECM (0, 1, 2, 3, 4, 8 mg/mL) were injected at a volume equal to that of the cavity (14 days post-stroke). Retention of ECM within the cavity occurred at concentrations >3 mg/mL. A significant cell infiltration into the ECM material in the lesion cavity occurred with an average of ∼36,000 cells in the 8 mg/mL concentration within 24 h. An infiltration of cells with distances of >1500 µm into the ECM hydrogel was observed, but the majority of cells were at the tissue/hydrogel boundary. Cells were typically of a microglia, macrophage, or neural and oligodendrocyte progenitor phenotype. At the 8 mg/mL concentration, ∼60% of infiltrating cells were brain-derived phenotypes and 30% being infiltrating peripheral macrophages, polarizing toward an M2-like anti-inflammatory phenotype. These results suggest that an 8 mg/mL ECM concentration promotes a significant acute endogenous repair response that could potentially be exploited to treat stroke.