Person:

Hemphill, Matthew Allen

Vasospasm of the cerebrovasculature is a common manifestation of blast-induced traumatic brain injury (bTBI) reported among combat casualties in the conflicts in Afghanistan and Iraq. Cerebral vasospasm occurs more frequently, and with earlier onset, in bTBI patients than in patients with other TBI injury modes, such as blunt force trauma. Though vasospasm is usually associated with the presence of subarachnoid hemorrhage (SAH), SAH is not required for vasospasm in bTBI, which suggests that the unique mechanics of blast injury could potentiate vasospasm onset, accounting for the increased incidence. Here, using theoretical and in vitro models, we show that a single rapid mechanical insult can induce vascular hypercontractility and remodeling, indicative of vasospasm initiation. We employed high-velocity stretching of engineered arterial lamellae to simulate the mechanical forces of a blast pulse on the vasculature. An hour after a simulated blast, injured tissues displayed altered intracellular calcium dynamics leading to hypersensitivity to contractile stimulus with endothelin-1. One day after simulated blast, tissues exhibited blast force dependent prolonged hypercontraction and vascular smooth muscle phenotype switching, indicative of remodeling. These results suggest that an acute, blast-like injury is sufficient to induce a hypercontraction-induced genetic switch that potentiates vascular remodeling, and cerebral vasospasm, in bTBI patients.

Over the past decade, investigators have attempted to establish the pathophysiological mechanisms by which non-penetrating injuries damage the brain. Several studies have implicated either membrane poration or ion channel dysfunction pursuant to neuronal cell death as the primary mechanism of injury. We hypothesized that traumatic stimulation of integrins may be an important etiological contributor to mild Traumatic Brain Injury. In order to study the effects of forces at the cellular level, we utilized two hierarchical, in vitro systems to mimic traumatic injury to rat cortical neurons: a high velocity stretcher and a magnetic tweezer system. In one system, we controlled focal adhesion formation in neurons cultured on a stretchable substrate loaded with an abrupt, one dimensional strain. With the second system, we used magnetic tweezers to directly simulate the abrupt injury forces endured by a focal adhesion on the neurite. Both systems revealed variations in the rate and nature of neuronal injury as a function of focal adhesion density and direct integrin stimulation without membrane poration. Pharmacological inhibition of calpains did not mitigate the injury yet the inhibition of Rho-kinase immediately after injury reduced axonal injury. These data suggest that integrin-mediated activation of Rho may be a contributor to the diffuse axonal injury reported in mild Traumatic Brain Injury.

Traumatic Brain Injury (TBI) is linked to a diverse range of diffuse pathological damage for which there is a severe lack of therapeutic options. A major limitation to drug development is the inability to identify causal mechanisms that link head trauma to the multitude of secondary injury cascades that underlie neuropathology. To elucidate these relationships, it is important to consider how physical forces are transmitted through the brain across multiple spatial scales ranging from the whole head to the sub-cellular level. In doing so, the mechanical behavior of the brain is typically characterized solely by its material properties and biological structure. Alternatively, forces transmitted through distinct cellular and extracellular structures have been shown to influence physiological processes in multiple cell types through the transduction of mechanical forces into cellular chemical responses. As an essential component of various biological processes, these mechanotransduction events are regulated by mechanical cues directed through extracellular matrix (ECM) and cell adhesion molecules (CAM) to mechanosensitive intra-cellular structures such as focal adhesions (FAs). Using a series of in vitro models, we have implicated FAs in the cellular mechanism of traumatic axonal injury by showing that forces directed through these structures potentiate injury levels and, moreover, that inhibition of FA-mediated signaling pathways may be neuroprotective. In addition, we show that localizing trauma forces through specific brain ECM results in differential injury rates, further implicating mechanosensitive cell-ECM linkages in the mechanism of TBI. Therefore, we show that FAs play a major role in axonal injury at low strain magnitudes indicating that cellular mechanotransduction may be an important mechanism underlying the initiation of cell and sub-cellular injuries ultimately responsible for the diffuse pathological damage and clinical symptoms observed in diffuse axonal injury. Furthermore, since these mechanisms may present the earliest events in the complex sequelae associated with TBI, they also represent potential therapeutic opportunities.