Role of Tissue Type Plasminogen Activator in TBI
To Determine the Role of Tissue Type Plasminogen Activator in Models of
Traumatic Brain Injury
Chief Investigator: Dr Robert Medcalf
Associate Investigators: Associate Professor Cristina Morganti-Kossmann, Professor
Harald Schmidt
Lead Organisation: Monash University
TAC Neurotrauma Funding: $578,064
Project Dates: 5 April 2007 – 4 October 2010
Background:
Traumatic brain injury (TBI) is the leading cause of death and disability in individuals
under the age of 45. Following trauma, a complex series of neurochemical and
signalling changes are initiated that cause severe damage to the blood-brain barrier.
The blood-brain barrier (BBB) is a structure that protects the brain from the entrance
of potentially harmful substances present in the blood and maintains the homeostasis
of the central nervous system (CNS). Breakdown of the BBB causes bleeding as well
as a build-up of the fluid in the brain. Tissue plasminogen activator (t-PA) is a
protease that facilitates the breakdown of blood clots in the circulation via its
conversion of plasminogen into its active form, plasmin. t-PA has a completely
different function in the CNS, where it contributes to learning and memory. The
activity of t-PA in the brain is regulated by specific inhibitors, namely plasminogen
activator inhibitor-1 (PAI-1), neuroserpin and protease-nexin 1. Studies involving
models of acute ischemic stroke have indicated that t-PA can exacerbate BBB damage
in a process that involves it binding to a cell surface receptor know as low-density
lipoprotein receptor related protein (LRP). t-PA can also promote the activation of the
matrix metalloproteinase (MMP) family of enzymes. This is important because
MMPs have the capacity to directly break down the structural support of the BBB.
Earlier studies suggested that the absence of t-PA reduced the severity of TBI, yet the
mechanisms underlying this are unknown.
Aims:
To use genetic and biochemical approaches to investigate the role of t-PA in
compromising BBB integrity following TBI, and to characterise the mechanisms
underlying this process.
Methods:
Experimental TBI was induced in mice that have normal levels of t-PA and
genetically modified mice that lack t-PA or have extremely high levels of t-PA in the
brain. The extent of TBI and damage to the BBB as a result of trauma was
investigated. Various compounds targeted at t-PA were injected into the brain after
trauma and their effects on BBB permeability examined.
Results:
t-PA levels were shown to correlate with injury severity as well as the extent of BBB
permeability post-trauma. It was also shown that t-PA activity is increased by ~30%
in the damaged brain following trauma and that an increase in t-PA activity in the
brain is directly linked to activation of the MMP system in trauma. In addition, the
deleterious effects of t-PA at promoting TBI severity occurs via its association with
its inhibitors, notably PAI-1, which in turn triggers deleterious signalling processes
leading to BBB damage.
Conclusions: These results suggest that t-PA promotes TBI severity increasing BBB
permeability via the formation of t-PA-inhibitor complexes and signalling via LDL
receptors that may in turn lead to an increase in MMP recruitment. Inhibition of t-PAcomplex formation or subsequent downstream signalling via LDL receptors would
offer a new approach to minimise the severity of TBI.
Publications:
SASHINDRANATH M, SAMSON AL, DOWNES CE, CRACK PJ, LAWRENCE AJ, LI Q-X, PING
NG AQ, JONES NC, FARRUGIA J, ABDELLA E, VASSALLI J-D, MADANI R, MEDCALF RL.
Compartment- and context-specific changes in tissue-type plasminogen activator (tPA) activity
following brain injury and pharmacological stimulation. Laboratory Investigation. 2011;91:1079-1091.
Presentations:
MEDCALF R. Role of tissue type plasminogen activator in TBI. Trauma Melbourne; 2009 November
20-21, Melbourne, Australia.
SASHINDRANATH M, KARADIMOS D, BECKHAM S, GALLE A, FARRUGIA J, MADANI R,
VASSALLI JD, MEDCALF RL. Increased permeability of the blood-brain barrier and more severe
motor impairment following traumatic brain injury in mice over-expressing tissue type plasminogen
activator (t-PA). The Second Joint Symposium of the International and National Neurotrauma
Societies; 2009 September 7-11, Santa Barbara, USA.
SASHINDRANATH M, KARADIMOS D, BECKHAM S, GALLE A, FARRUGIA J, MADANI R,
VASSALLI JD, MEDCALF RL. Neuronal Over-expression of t-PA accelerates MMP-9 activation
following traumatic brain injury. XIIth International Workshop on Molecular & Cellular Biology of
Plasminogen Activation; 2009 March 31-April 4, Cold Spring Harbour, USA.
SASHINDRANATH M, KARADIMOS D, BECKHAM S, GALLE A, MEDCALF RL. Neuronal
Over-expression of Tissue-type Plasminogen Activator correlates with Increased Matrix
Metalloproteinase-9 Activation following Traumatic Brain Injury. 29th Annual Meeting of the
Australian Neuroscience Society; 2009 January 27-30, Canberra, Australia.
SASHINDRANATH M, SALES E, FARRUGIA J, KARADIMOS D, BECKHAM S, GALLE A,
MADANI R, VASSALLI J-D, MEDCALF R. Increased permeability of the blood-brain barrier and
more severe motor impairment following traumatic brain injury in mice over-expressing tissue type
plasminogen activator (t-PA). National and International Neurotrauma Societies; 2009 September 7-11,
Santa Barbara, USA.
SASHINDRANATH M, KARADIMOS D, MEDCALF RL. Increased endogenous tissue typeplasminogen activator (T-PA) correlates with increased matrix metalloproteinase-9 activation
following traumatic brain injury. 29th Annual Meeting of the Australian Neuroscience Society; 2009
January 27-30, Canberra, Australia.
SASHINDRANATH M, SALES E AND MEDCALF RL. Tissue-type plasminogen activation, the
blood brain barrier and traumatic brain injury. 20th International Congress on Fibrinolysis and
Proteolysis; 2010 August 24-28, Amsterdam, Netherlands.
SAMSON AL, BORG RJ, SASHINDRANATH M, AU AE-L, NIEGO B, CODY S AND MEDCALF
RL. Identification of proteins that selectively misfold within the acutely injured brain and accelerate tPA-mediated plasmin formation: implications for ischemic stroke. 20th International Congress on
Fibrinolysis and Proteolysis; 2010 August 24-28, Amsterdam, Netherlands.
SASHINDRANATH M, SAMSON AL, ABDELLA E, LAWRENCE AJ, TARLAC V, DOWNES CE,
CRACK PJ, AND MEDCALF RL. Quantitation of net tissue-type plasminogen activator activity
following CNS stimulation and injury using a rapid and reliable amidolytic assay. 20th International
Congress on Fibrinolysis and Proteolysis; 2010 August 24-28, Amsterdam, Netherlands.
MEDCALF RL. t-PA, the blood brain barrier and Traumatic brain injury. Gordon Research
Conference; 2010 February 14-19, Ventura, California, USA.
SASHINDRANATH M, KARADIMOS D, BECKHAM S, GALLE A, FARRUGIA J, MADANI R,
VASSALLI JD, MEDCALF RL. t-PA, the Blood Brain Barrier and Traumatic Brain Injury. Gordon
Research Conference; 2010 February 14-19, Ventura, California, USA.
Other Grants:
NHMRC Project Grant ID: 606660
Chief investigators: A/Prof Robert Medcalf
To understand the role of the plasminogen activating and matrix metalloproteinase
systems in traumatic brain injury ($481,500)
NHMRC equipment grant fund ($28,000) to contribute towards the cost of a state of
the art device to determine changes in motor function in mice and rats following TBI
and in other neurological conditions. This device is called the Mouse DigiGait system.