Reparative Effects of Poloxamer P188 in Astrocytes Exposed to Controlled Microcavitation

Diblock Copolymers of Poly(ethylene oxide)-b-poly(propylene oxide) Stabilize a Blood-Brain Barrier Model under Oxidative Stress

The blood-brain barrier (BBB) is a highly restrictive barrier at the interface between the brain and the vascular system. Even under BBB dysfunction, it is extremely difficult to deliver therapies across the barrier, limiting the options for treatment of neurological injuries and disorders. To circumvent these challenges, there is interest in developing therapies that directly engage with the damaged BBB to restore its function. Previous studies revealed that poloxamer 188 (P188), a water-soluble triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), partially mitigated BBB dysfunction in vivo. In the context of stabilization of the damaged BBB, the mechanism of action of PEO-PPO block copolymers is unknown, and there has been minimal exploration of polymers beyond P188. In this study, a human-based in vitro BBB model under oxidative stress was used to investigate polymer-BBB interactions since oxidative stress is closely linked with BBB dysfunction in many neurological injuries and disorders. PEO-PPO block copolymers of varied numbers of chemically distinct blocks, PEO block length, and functionality of the end group of the PPO block were assessed for their efficacy in improving key physiological readouts associated with BBB dysfunction. While treatment with P188 did not mitigate damage in the in vitro BBB model, treatment with three diblock copolymers improved barrier integrity under oxidative stress to a similar extent. Of the considered variations in the block copolymer design, the reduction in the number of chemically distinct blocks had the strongest influence on therapeutic function. The demonstrated efficacy of three alternative PEO-PPO diblock copolymers in this work reveals the potential of these polymers as a class of therapeutics that directly treat the damaged BBB, expanding the options for treatment of neurological injuries and disorders.

Cavitation in blunt impact traumatic brain injury

Traumatic brain injury (TBI) poses a major public health challenge. No proven therapies for the condition exist so protective equipment that prevents or mitigates these injuries plays a critical role in minimizing the societal burden of this condition. Our ability to optimize protective equipment depends on our capacity to relate the mechanics of head impact events to morbidity and mortality. This capacity, in turn, depends on correctly identifying the mechanisms of injury. For several decades, a controversial theory of TBI biomechanics has attributed important classes of injury to cavitation inside the cranial vault during blunt impact. This theory explains counter-intuitive clinical observations, including the coup-contre-coup pattern of injury. However, it is also difficult to validate experimentally in living subjects. Also, blunt impact TBI is a broad term that covers a range of different head impact events, some of which may be better described by cavitation theory than others. This review surveys what has been learned about cavitation through mathematical modeling, physical modeling, and experimentation with living tissues and places it in context with competing theories of blunt injury biomechanics and recent research activity in the field in an attempt to understand what the theory has to offer the next generation of innovators in TBI biomechanics.

Effects of poloxamer 188 on traumatic brain injury

Traumatic Brain Injury (TBI) is a major cause of severe disability and death, resulting in significant health care and economic burden. Poloxamer 188, a synthetic tri-block copolymer approved by the FDA, has been studied for its potential effects on traumatic brain injury (TBI). The neuroprotective abilities of P188 have attracted significant attention. This systematic review aims to compile evidence of P188's effect on the treatment of TBI. A comprehensive literature search was conducted using PubMed, SCOPUS, and Google Scholar databases, which yielded 20 articles that satisfied the inclusion criteria. These articles have shown direct protective effects of P188 on brain tissue following TBI, including restitution of the increase cell membrane permeability, attenuation of neuronal necrosis and apoptosis, improvement of mitochondrial viability, reduction in axonal disruption, and restoration of the blood brain barrier. In animals, P188 has been shown to improve sensorimotor functions, as well as spatial learning and memory.

P188 Therapy in In Vitro Models of Traumatic Brain Injury

Traumatic brain injury (TBI) is a significant cause of morbidity and mortality worldwide. Varied mechanisms of injury contribute to the heterogeneity of this patient population as demonstrated by the multiple published grading scales and diverse required criteria leading to diagnoses from mild to severe. TBI pathophysiology is classically separated into a primary injury that is characterized by local tissue destruction as a result of the initial blow, followed by a secondary phase of injury constituted by a score of incompletely understood cellular processes including reperfusion injury, disruption to the blood-brain barrier, excitotoxicity, and metabolic dysregulation. There are currently no effective pharmacological treatments in the wide-spread use for TBI, in large part due to challenges associated with the development of clinically representative in vitro and in vivo models. Poloxamer 188 (P188), a Food and Drug Administration-approved amphiphilic triblock copolymer embeds itself into the plasma membrane of damaged cells. P188 has been shown to have neuroprotective properties on various cell types. The objective of this review is to provide a summary of the current literature on in vitro models of TBI treated with P188.

Poloxamer 188 Exerts Direct Protective Effects on Mouse Brain Microvascular Endothelial Cells in an In Vitro Traumatic Brain Injury Model

Traumatic Brain Injury (TBI), the main contributor to morbidity and mortality worldwide, can disrupt the cell membrane integrity of the vascular endothelial system, endangering blood-brain barrier function and threatening cellular subsistence. Protection of the vascular endothelial system might enhance clinical outcomes after TBI. Poloxamer 188 (P188) has been shown to improve neuronal function after ischemia/reperfusion (I/R) injury as well as after TBI. We aimed to establish an in vitro compression-type TBI model, comparing mild-to-moderate and severe injury, to observe the direct effects of P188 on Mouse Brain Microvascular Endothelial Cells (MBEC). Confluent MBEC were exposed to normoxic or hypoxic conditions for either 5 or 15 h (hours). 1 h compression was added, and P188 was administered during 2 h reoxygenation. A direct effect of P188 on MBEC was tested by assessing cell number/viability, cytotoxicity/membrane damage, metabolic activity, and total nitric oxide production (tNOp). While P188 enhanced cell number/viability, metabolic activity, and tNOp, an increase in cytotoxicity/membrane damage after mild-to-moderate injury was prevented. In severely injured MBEC, P188 improved metabolic activity only. P188, present during reoxygenation, influenced MBEC function directly in simulated I/R and compression-type TBI.

Mechanically Induced Cavitation in Biological Systems

Cavitation bubbles form in soft biological systems when subjected to a negative pressure above a critical threshold, and dynamically change their size and shape in a violent manner. The critical threshold and dynamic response of these bubbles are known to be sensitive to the mechanical characteristics of highly compliant biological systems. Several recent studies have demonstrated different biological implications of cavitation events in biological systems, from therapeutic drug delivery and microsurgery to blunt injury mechanisms. Due to the rapidly increasing relevance of cavitation in biological and biomedical communities, it is necessary to review the current state-of-the-art theoretical framework, experimental techniques, and research trends with an emphasis on cavitation behavior in biologically relevant systems (e.g., tissue simulant and organs). In this review, we first introduce several theoretical models that predict bubble response in different types of biological systems and discuss the use of each model with physical interpretations. Then, we review the experimental techniques that allow the characterization of cavitation in biologically relevant systems with in-depth discussions of their unique advantages and disadvantages. Finally, we highlight key biological studies and findings, through the direct use of live cells or organs, for each experimental approach.

Evaluation of In Vitro Neuronal Protection by Postconditioning with Poloxamer 188 Following Simulated Traumatic Brain Injury

Traumatic brain injury (TBI) leads to morbidity and mortality worldwide. Reperfusion after ischemia adds detrimental injury to cells. Ischemia/reperfusion (I/R) injures cells in a variety of ways including cell membrane disruption. Hence, methods to improve endogenous membrane resealing capacity are crucial. Poloxamer (P) 188, an amphiphilic triblock copolymer, was found to be effective against I/R and mechanical injury in various experimental settings. The aim of this study was to establish an in vitro mouse neuronal TBI model and, further, to investigate if postconditioning with P188 directly interacts with neurons after compression and simulated I/R injury, when administered at the start of reoxygenation. Cellular function was assessed by cell number/viability, mitochondrial viability, membrane damage by lactated dehydrogenase (LDH) release and FM1-43 incorporation as well as apoptosis-activation by Caspase 3. Five hours hypoxia ± compression with 2 h reoxygenation proved to be a suitable model for TBI. Compared to normoxic cells not exposed to compression, cell number and mitochondrial viability decreased, whereas membrane injury by LDH release/FM1-43 dye incorporation and Caspase 3 activity increased in cells exposed to hypoxic conditions with compression followed by reoxygenation. P188 did not protect neurons from simulated I/R and/or compression injury. Future research is indicated.

Poloxamer 188 Protects Isolated Adult Mouse Cardiomyocytes from Reoxygenation Injury

Reperfusion injury is a complex pathological event involving processes that can lead to further disruption of the cell membrane and function following an ischemic event. Return of blood flow allows for the needed reperfusion; however, for a period of time before remaining viable cells stabilize, reperfusion results in additional cellular injury. In cardiomyocytes, loss of membrane integrity allows abnormal influx of extracellular calcium, leading to hyper-contracture and cell death. Methods to improve the membrane integrity of cardiomyocytes overwhelmed by pathological disruptions, such as reperfusion injury, are needed to prevent cell death, because of the myocardium's limited ability to regenerate. Research has shown administration of the copolymer P(oloxamer) 188 before ischemia/reperfusion can protect cardiomyocytes through membrane stabilization. This study sought to determine whether the administration of P188 at the beginning of the clinically more relevant time of reperfusion after ischemia will attenuate any additional damage to cardiomyocytes by stabilizing membrane integrity to allow the cells to maintain function. Using an in-vitro cardiomyocyte model subjected to hypoxia/reoxygenation to simulate ischemia/reperfusion injury, we show that reoxygenation significantly potentiates the injury caused by hypoxia itself. P188, with its unique combination of hydrophobic and hydrophilic chemical properties, and only delivered at the beginning of reoxygenation, dose-dependently protected cardiomyocytes from injury due to reoxygenation by repairing cell membranes, decreasing calcium influx, and maintaining cellular morphology. Our study also shows the hydrophobic portion of P188 is necessary for the stabilization of cell membrane integrity in providing protection to cardiomyocytes against reoxygenation injury.

Engineering Delivery of Nonbiologics Using Poly(lactic-co-glycolic acid) Nanoparticles for Repair of Disrupted Brain Endothelium

Traumatic brain injury (TBI) is known to alter the structure and function of the blood-brain barrier (BBB). Blunt force or explosive blast impacting the brain can cause neurological sequelae through the mechanisms that remain yet to be fully elucidated. For example, shockwaves propagating through the brain have been shown to create a mechanical trauma that may disrupt the BBB. Indeed, using tissue engineering approaches, the shockwave-induced mechanical injury has been shown to modulate the organization and permeability of the endothelium tight junctions. Because an injury to the brain endothelium typically induces a high expression of E-selectin, we postulated that upregulation of this protein after an injury can be exploited for diagnosis and potential therapy through targeted nanodelivery to the injured brain endothelium. To test this hypothesis, we engineered poly(lactic-co-glycolic acid) (PLGA) nanoparticles to encapsulate therapeutic nonbiologics and decorated them with ligands to specifically target the E-selectin. A high level of the conjugated nanoparticles was found inside the injured cells. Repair of the injury site was then quantitatively measured and analyzed. To summarize, exploiting the tunable properties of PLGA, a targeted drug delivery strategy has been developed and validated, which combines the specificity of ligand/receptor interaction with therapeutic reagents. Such a strategy could be used to provide a potential theragnostic approach for the treatment of modulated brain endothelium associated with TBI.

Modulation of in vitro Brain Endothelium by Mechanical Trauma: Structural and Functional Restoration by Poloxamer 188

Brain injuries caused by an explosive blast or blunt force is typically presumed to associate with mechanical trauma to the brain tissue. Recent findings from our laboratory suggest that shockwaves produced by a blast can generate micron-sized bubbles in the tissue. The collapse of microbubbles (i.e., microcavitation) may induce a mechanical trauma and compromise the integrity of the blood-brain endothelium (BBE). To test our hypothesis, we engineered a BBE model to determine the effect of microbubbles on the structural and functional changes in the BBE. Using monolayers of mouse primary brain microvascular endothelial cells, the permeability coefficient was measured following simulated blast-induced microcavitation. This event down-regulated the expression of tight junction markers, disorganized the cell-cell junction, and increased permeability. Since poloxamers have been shown to rescue damaged cells, the cells were treated with the FDA-approved poloxamer 188 (P188). The results indicate P188 recovered the permeability, restored the tight junctions, and suppressed the expressions of matrix metalloproteinases. The biomimetic interface we developed appears to provide a systematic approach to replicate the structure and function of BBE, determine its alteration in response to traumatic brain injury, and test potential therapeutic treatments to repair the damaged brain endothelium.

Astrocyte Viability and Functionality in Spatially Confined Microcavitation Zone

Blast-induced traumatic brain injury (bTBI) can result in cell/tissue damage and lead to clinical and neuropsychiatric symptoms. Shock waves from a blast propagate through the brain and initiate cascades of mechanical and physiological events that can adversely affect the brain function. Although studies using animal models and brain slices have shown macroscale changes in the brain tissue in response to blast, systematic elucidation of coupling mechanisms is currently lacking. One mechanism that has been postulated and demonstrated repeatedly is the blast-induced generation and subsequent collapse of micron-size bubbles (i.e., microcavitation). Using a custom-designed exposure system, we have previously reported that upon collapsing of microbubbles, astrocytes exhibited changes in the cell viability, cellular biomechanics, production of reactive oxygen species, and activation of apoptotic signaling pathways. In this paper, we have applied microfabrication techniques and seeded astrocytes in a spatially controlled manner to determine the extent of cell damage from the site of the collapse of microbubbles. Such a novel experimental design is proven to facilitate our effort to examine the altered cell viability and functionality by monitoring the transient calcium spiking activity in real-time. We now report that the effect of microcavitation depends on the distance from which cells are seeded, and the cell functionality assessed by calcium dynamics is significantly diminished in the cells located within ∼800 μm of the collapsing microbubbles. Both calcium influx across the cell membrane via N-type calcium channels and intracellular calcium store are altered in response to microcavitation. Finally, the FDA-approved poloxamer 188 (P188) was used to reconstitute the compromised cell membrane and restore the cell's reparative capability. This finding may lead to a feasible treatment for partially mitigating the tissue damage associated with bTBI.

Acceleration-induced pressure gradients and cavitation in soft biomaterials

The transient, dynamic response of soft materials to mechanical impact has become increasingly relevant due to the emergence of numerous biomedical applications, e.g., accurate assessment of blunt injuries to the human body. Despite these important implications, acceleration-induced pressure gradients in soft materials during impact and the corresponding material response, from small deformations to sudden bubble bursts, are not fully understood. Both through experiments and theoretical analyses, we empirically show, using collagen and agarose model systems, that the local pressure in a soft sample is proportional to the square of the sample depth in the impact direction. The critical acceleration that corresponds to bubble bursts increases with increasing gel stiffness. Bubble bursts are also highly sensitive to the initial bubble size, e.g., bubble bursts can occur only when the initial bubble diameter is smaller than a critical size (≈10 μm). Our study gives fundamental insight into the physics of injury mechanisms, from blunt trauma to cavitation-induced brain injury.

Muscle membrane integrity in Duchenne muscular dystrophy: recent advances in copolymer-based muscle membrane stabilizers

The scientific premise, design, and structure-function analysis of chemical-based muscle membrane stabilizing block copolymers are reviewed here for applications in striated muscle membrane injury. Synthetic block copolymers have a rich history and wide array of applications from industry to biology. Potential for discovery is enabled by a large chemical space for block copolymers, including modifications in block copolymer mass, composition, and molecular architecture. Collectively, this presents an impressive chemical landscape to leverage distinct structure-function outcomes. Of particular relevance to biology and medicine, stabilization of damaged phospholipid membranes using amphiphilic block copolymers, classified as poloxamers or pluronics, has been the subject of increasing scientific inquiry. This review focuses on implementing block copolymers to protect fragile muscle membranes against mechanical stress. The review highlights interventions in Duchenne muscular dystrophy, a fatal disease of progressive muscle deterioration owing to marked instability of the striated muscle membrane. Biophysical and chemical engineering advances are presented that delineate and expand upon current understanding of copolymer-lipid membrane interactions and the mechanism of stabilization. The studies presented here serve to underscore the utility of copolymer discovery leading toward the therapeutic application of block copolymers in Duchenne muscular dystrophy and potentially other biomedical applications in which membrane integrity is compromised.