Background: Traumatic brain injury (TBI) is a temporary or permanent impairment of the brain caused by a blow or jolt to the head. The severity of the injury may range from mild, where a person might be briefly confused or unconsciousness, to severe where a person suffers an extended period of unconsciousness or amnesia after the injury. Additional changes include problems with thinking, sensation, movement, language, and concentration. Brain injury can result in personality and emotional changes, irritability, tiredness, depression, violence, and the inability to carry out basic, everyday tasks. Of the 1.4 million who sustain a TBI each year in the United States, 50,000 die, 235,000 are hospitalized, and 1.1 million are treated and released from an emergency department. Military duties increase the risk of sustaining a TBI. Though military service does expose personnel to the risk of a penetrative brain injury, such as caused by a bullet or shrapnel, an even greater risk exists for a TBI caused by a concussive blast wave as a result of an explosive. The high occurrence of explosions due to improvised explosive devices (IEDs) in combat theaters such as Iraq and Afghanistan means American troops in the field are at a high risk of incurring a TBI. Military personnel are consistently at risk of being injured by powerful munitions, which can also cause a brain injury through concussive force. Other causes of TBI in the military are bullets and fragments, motor vehicle-traffic crashes, assaults, and falls. For most troops with post-concussion or mild TBI, recovery time will be within a few weeks or months, although a small percentage will have persistent symptoms. Patients with moderate to severe TBI may never fully recover their pre-injury function. Their lives and those of their loved ones will have been permanently changed. Many troops who go to war and have severe TBI will require a lifetime of health support services and treatment.
Calpains, a group of calcium-activated neutral proteases, play a key role in neuropathologic events following TBI. Neuronal calpain activation has been observed within minutes to hours after brain trauma in animals, suggesting that calpains are an early mediator of neuronal damage. Whereas transient calpain activation triggers numerous cell signaling and remodeling events involved in normal physiological processes, the sustained calpain activation produced by trauma is associated with neuron death and axonal degeneration leading to significant loss of neurological function in multiple models of TBI. Nonetheless, the causal relationship between calpain activation and neuronal death is not fully understood. Much remains to be learned regarding the endogenous regulatory mechanisms for controlling calpain activity and the in vivo substrates affected by calpain.
TBI-induced calpain activation is responsible for the proteolytic fragmentation of the neuroprotective and multifunctional cellular prion protein (PrPC). The loss of PrPC structural integrity contributes to neuronal death, spinal cord damage, and, finally, to functional impairment in vivo. It is reasonable to assume that maintaining the levels of functional PrPC and/or preventing PrPC degradation would result in continued neuronal survival and functionality. Further, post-traumatic inhibition of calpains, either directly or indirectly through targets related to intracellular calcium regulation, is associated with attenuation of functional and behavioral deficits, axonal pathology, and cell death in animal models of TBI.
We hypothesize that PrPC expression provides protection against neuron loss in TBI; however, the overactivation of calpain increases cellular death due to PrPC degradation and loss of its neuroprotective and neuronal survival functions. Our objective is to enhance neuroprotection and reduce/prevent neurodegeneration by simultaneously suppressing calpain activity and maintaining intact PrPC expression/functionality by examining a new pharmacological inhibitor, SNJ-1945, to prevent calpain activation, in combination with (a) excitotoxicity-challenged isolated neurons, (b) TBI-challenged mouse lines expressing various levels of intact PrPC, and (c) TBI-challenged genetically altered mice expressing a calpain-resistant modified PrPC. These studies will unequivocally prove that by increasing PrPC expression and by suppressing calpain activity we will be able to provide robust protection against the loss of countless neurons following TBI and thus maximize patient functionally recovery.
Specific Aims: To test our central hypothesis, we will generate and utilize a novel and innovative genetically modified mice. By using genetically modified mice, we will try to provide evidence that PrPC expression provides neuroprotection against spinal cord injury while calpain contributes to spinal cord injury. So if we can pharmacologically suppress calpain activity with a drug inhibitor while increasing PrPC expression, the combination will provide the most robust protection.
Impact: Our proposal has the following military-relevant high-impact deliverables: (i) definitive proof-of-principle data that calpain inhibitor drug SNJ-1945 can preserve PrPC from damage and improve functional outcome in animals, suggesting that further drug development and clinical studies of TBI would be beneficial; and (ii) synergistic proof-of-principle results that modified PrPC expression can protect against TBI, providing the basis for an adenovirus vector-based PrPC gene therapy to deliver PrPC as a neuroprotectant following TBI.