Cell-penetrating peptides (CPPs) are small
peptides (typically 5-25
amino acids), which are used to facilitate the delivery of normally non-permeable cargos such as other
peptides,
proteins,
nucleic acids, or drugs into cells. However, several recent studies have demonstrated that the TAT
CPP has neuroprotective properties. Therefore, in this study, we assessed the TAT and three other CPPs (penetratin, Arg-9, Pep-1) for their neuroprotective properties in cortical neuronal cultures following exposure to
glutamic acid,
kainic acid, or in vitro
ischemia (
oxygen-
glucose deprivation). Arg-9,
penetratin, and TAT-D displayed consistent and high level neuroprotective activity in both the
glutamic acid (IC50: 0.78, 3.4, 13.9 μM) and
kainic acid (IC50: 0.81, 2.0, 6.2 μM) injury models, while Pep-1 was ineffective. The TAT-D
isoform displayed similar efficacy to the TAT-L
isoform in the
glutamic acid model. Interestingly, Arg-9 was the only
CPP that displayed efficacy when washed-out prior to
glutamic acid exposure. Neuroprotection following in vitro
ischemia was more variable with all
peptides providing some level of neuroprotection (IC50; Arg-9: 6.0 μM, TAT-D: 7.1 μM,
penetratin/Pep-1: >10 μM). The positive control
peptides JNKI-1D-TAT (JNK inhibitory
peptide) and/or PYC36L-TAT (AP-1 inhibitory
peptide) were neuroprotective in all models. Finally, in a post-
glutamic acid treatment experiment, Arg-9 was highly effective when added immediately after, and mildly effective when added 15 min post-insult, while the JNKI-1D-TAT control
peptide was ineffective when added post-insult. These findings demonstrate that different CPPs have the ability to inhibit neurodamaging events/pathways associated with excitotoxic and ischemic
injuries. More importantly, they highlight the need to interpret neuroprotection studies when using CPPs as delivery agents with caution. On a positive note, the cytoprotective properties of CPPs suggests they are ideal carrier molecules to deliver
neuroprotective drugs to the CNS following injury and/or potential
neuroprotectants in their own right.