Understanding the biomechanics of brain edema
Brain swelling is a major complication of TBI. Current clinical practice is to systemically inject hypertonic saline to draw water out of the brain, which often fails to control swelling. My laboratory has applied the biomechanical framework of triphasic mixture theory to explain, from a thermodynamic viewpoint, why brain swells. We first reported that triphasic theory predicted changes in volume of dead brain tissue in different osmotic environments (Elkin et al., 2010b). Our findings suggested that if the fixed charge could be reduced, swelling could be reduced. In a follow-up study, we demonstrated that treatment with chondroitinase ABC to remove the glycosaminglycan (GAG) chondroitin sulfate significantly reduced the FCD and significantly reduced swelling by 50% (Elkin et al., 2011b).
Cell penetrating peptides as therapeutic delivery vehicles
The delivery of therapeutics, especially proteins, into cells is hampered by the presence of the plasma membrane. However, a class of peptides, called cell penetrating peptides (CPP), has the ability to deliver cargoes including proteins, oligonucleotides, and quantum dots intracellularly. In collaboration with Dr. Scott Banta from Chemical Engineering at Columbia University, my laboratory is studying the mechanism of internalization of the archetypical CPP, TAT, to determine if it may be useful for treating TBI. We have found that the initial steps of TAT internalization require an electrostatic interaction between the positively charged TAT and cell surface GAG. Experimental modification of GAG content correlated positively with TAT transduction efficiency (Simon et al., 2009). These results are significant because GAG expression is increased after TBI, suggesting that TAT may be a useful delivery vehicle in this setting. To test this, we have shown that mechanical injury of astrocytes increases GAG content and also improves TAT transduction. Furthermore, the TAT-mediated delivery of a peptide inhibitor to c-Jun N-terminal kinase attenuated astrocyte activation, a detrimental cellular response to mechanical stimuli (Kang et al., 2011). These studies are also significant because they resolve two controversies in the CPP literature. The first was whether TAT was capable of transducing primary cells or only cell lines as typically reported. Our studies showed that delivery was dependent on GAG content which could vary even for the same cell type grown under different culture conditions (Simon et al., 2009, Kang et al., 2011). The second was whether TAT could deliver cargoes transcellularly across a cellular barrier like the blood brain barrier (BBB) and into cells on the other side. Through the use of an in vitro BBB model, we showed that TAT cannot cross an intact BBB, but injuries such as ischemia compromise the BBB integrity and allow delivery to cells on the other side (Simon et al., 2010). This work continues as we search for novel CPPs which have the ability to cross the BBB.