Synthesis of cell-penetrating peptides and their application in neurobiology

Transient apoptosis inhibition in donor stem cells improves hematopoietic stem cell transplantation

During hematopoietic stem cell transplantation, a substantial number of donor cells are lost because of apoptotic cell death. Transplantation-associated apoptosis is mediated mainly by the proapoptotic BCL-2 family proteins BIM and BMF, and their proapoptotic function is conserved between mouse and human stem and progenitor cells. Permanent inhibition of apoptosis in donor cells caused by the loss of these BH3-only proteins improves transplantation outcome, but recipients might be exposed to increased risk of lymphomagenesis or autoimmunity. Here, we address whether transient inhibition of apoptosis can serve as a safe but efficient alternative to improve the outcome of stem cell transplantation. We show that transient apoptosis inhibition by short-term overexpression of prosurvival BCL-XL, known to block BIM and BMF, is not only sufficient to increase the viability of hematopoietic stem and progenitor cells during engraftment but also improves transplantation outcome without signs of adverse pathologies. Hence, this strategy represents a promising and novel therapeutic approach, particularly under conditions of limited donor stem cell availability.

CPP-Assisted Intracellular Drug Delivery, What Is Next?

For the past 20 years, we have witnessed an unprecedented and, indeed, rather miraculous event of how cell-penetrating peptides (CPPs), the naturally originated penetrating enhancers, help overcome the membrane barrier that has hindered the access of bio-macromolecular compounds such as genes and proteins into cells, thereby denying their clinical potential to become potent anti-cancer drugs. By taking the advantage of the unique cell-translocation property of these short peptides, various payloads of proteins, nucleic acids, or even nanoparticle-based carriers were delivered into all cell types with unparalleled efficiency. However, non-specific CPP-mediated cell penetration into normal tissues can lead to widespread organ distribution of the payloads, thereby reducing the therapeutic efficacy of the drug and at the same time increasing the drug-induced toxic effects. In view of these challenges, we present herein a review of the new designs of CPP-linked vehicles and strategies to achieve highly effective yet less toxic chemotherapy in combating tumor oncology.

Anti-angiogenesis through noninvasive to minimally invasive intraocular delivery of the peptide CC12 identified by in vivo-directed evolution

Anti-vascular endothelial growth factor (VEGF) therapies are widely used for the treatment of neovascular fundus diseases such as diabetic retinopathy. However, these agents need to be injected intravitreally, because their strong hydrophilicity and high molecular weight prevent them from penetrating cell membranes and complex tissue barriers. Moreover, the repeated injections that are required can cause infection and tissue injury. In this study, we used in vivo-directed evolution phage display technology to identify a novel dodecapeptide, named CC12, with the ability to penetrate the ocular barrier in a noninvasive (via conjunctival sac instillation) or minimally invasive (via retrobulbar injection) manner. KV11, an antiangiogenesis peptide previously demonstrated to inhibit pathological neovascularization in the retina, was then used as a model antiangiogenesis cargo for CC12. We found that conjugation of KV11 peptide with CC12 peptide facilitated the delivery of KV11 to the retina, resulting in significant inhibition of retinal neovascularization development via topical application without tissue toxicity. Collectively, our data of multilevel evaluations demonstrate that CC12 may enable the noninvasive to minimally invasive intraocular delivery of antiangiogenic therapeutics.

Sambucus williamsii induced embryonic stem cells differentiated into neurons

The pluripotent stem cells, including embryonic stem cells (ESCs), are capable of self-renewal and differentiation into any cell type, thus making them the focus of many clinical application studies. However, the efficiency of ESCs differentiated into neurons needs to improve. In this study, we tried to increase efficiently to a neural fate in the presence of various transitional Chinese medicines through a three-step differentiation strategy. From extracts of 10 transitional Chinese medicine candidates, we determined that Sambucus williamsii (SW) extract triggers the up-regulation of Nestin and Tuj1 (neuron cells markers) gene expression levels. After determining the different concentrations of SW extract, the number of neurons in the 200 μg/ml SW extract group was higher than the control, 50, 100, and 400 μg/ml SW extract groups. In addition, the number of neurons in the 200 μg/ml SW extract group was higher and higher after each time passage (three times). We also detected the Oct4, Sox2 (stem cells markers), Tuj1, and Nestin genes expression levels by RT-PCR. In the differentiated process, Oct4 and Sox2 genes decreased while the Tuj1 and Nestin genes expression levels increased. In summary, we demonstrated that SW could induce pluripotent stem cells differentiated into neurons. Thus, SW might become a powerful material for neurons–differentiating strategies.

Internalization routes of cell-penetrating melanoma antigen peptides into human dendritic cells

Optimized delivery of antigens combined with sustainable maturation of dendritic cells (DCs) is crucial for generation of effective antitumoral immune responses. Multiple approaches for ex vivo antigen loading and improvement in immunogenicity have been described. We have recently established a single-step protocol consisting of a fusion peptide (a sequence of the melanoma antigen Melan-A and a cationic cell-penetrating HIV TAT domain) bound in complexes with a toll-like receptor agonist. As the exact cellular uptake mechanisms of TAT-coupled antigens have been a matter of considerable debate and significantly depend on cell type, cargo and concentrations, we evaluated internalization routes into human immature DCs in comparison with non-phagocytic cell lines. We found that Melan-A-TAT fusion peptide uptake by DCs is mainly energy dependent, superior compared with polylysine-coupled Melan-A and significantly higher in DCs as compared with Jurkat cells or HUVECs. Furthermore, we could track the uptake of the fusion peptide exclusively through early endosomes to lysosome compartments after 90 min by fluorescence microscopy and immunoelectron microscopy. Specific endocytosis inhibitors revealed major internalization of the fusion peptide by DCs via clathrin-mediated endocytosis, whereas uptake by non-phagocytic HUVECs differed significantly, involving macropinocytosis as well as clathrin-mediated endocytosis. As our understanding of the processes involved in internalization of TAT-coupled cargos by human DCs broadens, and DC activation becomes available by single-step procedures as described, further development of simultaneous DC maturation and intra-cellular peptide targeting is warranted.

TAT-Hsp70 induces neuroprotection against stroke via anti-inflammatory actions providing appropriate cellular microenvironment for transplantation of neural precursor cells

Heat-shock protein 70 (Hsp70) protects against cerebral ischemia, which is attributed to its chaperone activity. However, recent reports also describe pro-inflammatory actions of Hsp70 via activation of Toll-like receptors (TLR). Using membrane-permeable transactivator of transcription (TAT)-Hsp70, we analyzed TAT-Hsp70-induced neuroprotection and its underlying mechanism after cerebral ischemia in mice. Infusion of TAT-Hsp70 reduced infarct volume and enhanced blood-brain barrier integrity on day 3 poststroke, when given no later than 12 hours. The latter was associated with reduction of microglial activation, although upregulation of pro-inflammatory TLR-2/4 was observed both in verum and in control animals. Nevertheless, protein abundance and nuclear translocation of downstream nuclear factor kappa B (NF-κB) as well as proteasomal degradation of the NF-κB regulator Ikappa B alpha (IκB-α) were significantly reduced by TAT-Hsp70. TAT-Hsp70-induced neuroprotection and functional recovery were restricted to 4 weeks only. However, TAT-Hsp70 provided an appropriate extracellular milieu for delayed intravenous transplantation of adult neural precursor cells (NPCs). Thus, NPCs that were grafted 28 days poststroke induced long-term neuroprotection for at least 3 months, which was not due to integration of grafted cells but rather due to paracrine effects of transplanted NPCs. Conclusively, TAT-Hsp70 ameliorates postischemic inflammation via proteasome inhibition, thus providing an appropriate extracellular milieu for delayed NPC transplantation and culminating in long-term neuroprotection.

Curb challenges of the "Trojan Horse" approach: smart strategies in achieving effective yet safe cell-penetrating peptide-based drug delivery

Cell-penetrating peptide (CPP)-mediated intracellular drug delivery system, often specifically termed as "the Trojan horse approach", has become the "holy grail" in achieving effective delivery of macromolecular compounds such as proteins, DNA, siRNAs, and drug carriers. It is characterized by the unique cell- (or receptor-), temperature-, and payload-independent mechanisms, therefore offering potent means to improve poor cellular uptake of a variety of macromolecular drugs. Nevertheless, this "Trojan horse" approach also acts like a double-edged sword, causing serious safety and toxicity concerns to normal tissues or organs for in vivo application, due to lack of target selectivity of the powerful cell penetrating activity. To overcome this problem of potent yet non-selective penetration vs. targeting delivery, a number of "smart" strategies have been developed in recent years, including controllable CPP-based drug delivery systems based on various stimuli-responsive mechanisms. This review article provides a fundamental understanding of these smart systems, as well as a discussion of their real-time in vivo applicability.

Physiology and pathophysiology of the blood-brain barrier: P-glycoprotein and occludin trafficking as therapeutic targets to optimize central nervous system drug delivery

The blood-brain barrier (BBB) is a physical and metabolic barrier that separates the central nervous system from the peripheral circulation. Central nervous system drug delivery across the BBB is challenging, primarily because of the physical restriction of paracellular diffusion between the endothelial cells that comprise the microvessels of the BBB and the activity of efflux transporters that quickly expel back into the capillary lumen a wide variety of xenobiotics. Therapeutic manipulation of protein trafficking is emerging as a novel means of modulating protein function, and in this minireview, the targeting of the trafficking of 2 key BBB proteins, P-glycoprotein and occludin, is presented as a novel, reversible means of optimizing central nervous system drug delivery.

Transduction of neural precursor cells with TAT-heat shock protein 70 chaperone: therapeutic potential against ischemic stroke after intrastriatal and systemic transplantation

Novel therapeutic concepts against cerebral ischemia focus on cell-based therapies in order to overcome some of the side effects of thrombolytic therapy. However, cell-based therapies are hampered because of restricted understanding regarding optimal cell transplantation routes and due to low survival rates of grafted cells. We therefore transplanted adult green fluorescence protein positive neural precursor cells (NPCs) either intravenously (systemic) or intrastriatally (intracerebrally) 6 hours after stroke in mice. To enhance survival of NPCs, cells were in vitro protein-transduced with TAT-heat shock protein 70 (Hsp70) before transplantation followed by a systematic analysis of brain injury and underlying mechanisms depending on cell delivery routes. Transduction of NPCs with TAT-Hsp70 resulted in increased intracerebral numbers of grafted NPCs after intracerebral but not after systemic transplantation. Whereas systemic delivery of either native or transduced NPCs yielded sustained neuroprotection and induced neurological recovery, only TAT-Hsp70-transduced NPCs prevented secondary neuronal degeneration after intracerebral delivery that was associated with enhanced functional outcome. Furthermore, intracerebral transplantation of TAT-Hsp70-transduced NPCs enhanced postischemic neurogenesis and induced sustained high levels of brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, and vascular endothelial growth factor in vivo. Neuroprotection after intracerebral cell delivery correlated with the amount of surviving NPCs. On the contrary, systemic delivery of NPCs mediated acute neuroprotection via stabilization of the blood-brain-barrier, concomitant with reduced activation of matrix metalloprotease 9 and decreased formation of reactive oxygen species. Our findings imply two different mechanisms of action of intracerebrally and systemically transplanted NPCs, indicating that systemic NPC delivery might be more feasible for translational stroke concepts, lacking a need of in vitro manipulation of NPCs to induce long-term neuroprotection.

Blocking p55PIK signaling inhibits proliferation and induces differentiation of leukemia cells

p55PIK, a regulatory subunit of phosphatidylinositol 3-kinases, promotes cell cycle progression by interacting with cell cycle modulators such as retinoblastoma protein (Rb) via its unique amino-terminal 24 amino-acid residue (N24). Overexpression of N24 specifically inhibits these interactions and leads to cell cycle arrest. Herein, we describe the generation of a fusion protein (Tat transactivator protein (TAT)-N24) that contains the protein transduction domain and N24, and examined its effects on the proliferation and differentiation of leukemia cells. TAT-N24 not only blocks cell proliferation but remarkably induces differentiation of leukemia cells in vitro and in vivo. Systemically administered TAT-N24 also significantly decreases growth of leukemia cell tumors in animal models. Furthermore, overexpression of p55PIK in leukemia cells leads to increased proliferation; however, TAT-N24 blocks this effect and concomitantly induces differentiation. There is significant upregulation of p55PIK mRNA and protein expression in leukemia cells from patients. TAT-N24 inhibits cell cycle progression and induces differentiation of bone marrow cells derived from patients with several different types of leukemia. These results show that cell-permeable N24 peptide induces leukemia cell differentiation and suggest that p55PIK may be a novel drug target for the treatment of hematopoetic malignancies.

Peptides based on the presenilin-APP binding domain inhibit APP processing and Aβ production through interfering with the APP transmembrane domain

Presenilins (PSENs) form the catalytic component of the γ-secretase complex, responsible for intramembrane proteolysis of amyloid precursor protein (APP) and Notch, among many other membrane proteins. Previously, we identified a PSEN1-binding domain in APP, encompassing half of the transmembrane domain following the amyloid β (Aβ) sequence. Based on this, we designed peptides mimicking this interaction domain with the aim to selectively block APP processing and Aβ generation through interfering with enzyme-substrate binding. We identified a peptide sequence that, when fused to a virally derived translocation peptide, significantly lowered Aβ production (IC(50): 317 nM) in cell-free and cell-based assays using APP-carboxy terminal fragment as a direct γ-secretase substrate. Being derived from the APP sequence, this inhibitory peptide did not affect NotchΔE γ-cleavage, illustrating specificity and potential therapeutic value. In cell-based assays, the peptide strongly suppressed APP shedding, demonstrating that it exerts the inhibitory effect already upstream of γ-secretase, most likely through steric hindrance.

Engineered biological entities for drug delivery and gene therapy protein nanoparticles

The development of genetic engineering techniques has speeded up the growth of the biotechnological industry, resulting in a significant increase in the number of recombinant protein products on the market. The deep knowledge of protein function, structure, biological interactions, and the possibility to design new polypeptides with desired biological activities have been the main factors involved in the increase of intensive research and preclinical and clinical approaches. Consequently, new biological entities with added value for innovative medicines such as increased stability, improved targeting, and reduced toxicity, among others have been obtained. Proteins are complex nanoparticles with sizes ranging from a few nanometers to a few hundred nanometers when complex supramolecular interactions occur, as for example, in viral capsids. However, even though protein production is a delicate process that imposes the use of sophisticated analytical methods and negative secondary effects have been detected in some cases as immune and inflammatory reactions, the great potential of biodegradable and tunable protein nanoparticles indicates that protein-based biotechnological products are expected to increase in the years to come.

Classes and prediction of cell-penetrating peptides

The classical view on how peptides enter cells has been changed due to the development in the research field of cell-penetrating peptides (CPPs). During the last 15 years, more than 100 peptide sequences have been published to enter cells and also to bring different biological cargoes with them. Here, we present an overview of CPPs, mainly trying to analyze their common properties yielding the prediction of their cell-penetrating properties. Furthermore, examples of recent research, ideas on classification and uptake mechanisms, as well as a summary of the therapeutic potential of CPPs are presented.

Transcription factor-based modulation of neural stem cell differentiation using direct protein transduction

In contrast to conventional gene transfer strategies, the direct introduction of recombinant proteins into cells bypasses the risk of insertional mutagenesis and offers an alternative to genetic intervention. Here, we explore whether protein transduction of the gliogenic transcription factor Nkx2.2 can be used to promote oligodendroglial differentiation of mouse embryonic stem cell (ESC)-derived neural stem cells (NSC). To that end, a recombinant cell-permeant form of Nkx2.2 protein was generated. Exposure of ESC-derived NSC to the recombinant protein and initiation of differentiation resulted in a two-fold increase in the number of oligodendrocytes. Furthermore, Nkx2.2-transduced cells exhibited a more mature oligodendroglial phenotype. Comparative viral gene transfer studies showed that the biological effect of Nkx2.2 protein transduction is comparable to that obtained by lentiviral transduction. The results of this proof-of-concept study depict direct intracellular delivery of transcription factors as alternative modality to control lineage differentiation in NSC cultures without genetic modification.

Cell-Penetrating Fragments of the Cdk5 Regulatory Subunit Are Protective in Models of Neurodegeneration

Cdk5 is essential for neuronal differentiation processes in the brain. Activation of Cdk5 requires the association with the mostly neuron-specific p35 or p39. Overactivation of CDK5 by cleavage of p35 into p25 is thought to be involved in neurodegenerative processes. Here, we have tested an approach to inhibit pathological Cdk5 activation with a Tat-linked dominant-negative fragment of p25. It reduced cell death induced by staurosporine and showed a tendency to alleviate manganese-induced cell death, while it did not protect against 6-OHDA toxicity. Our results suggest that the Tat technique is a suitable tool to inhibit dysregulated CDK5.

Dendritic Guanidines as Efficient Analogues of Cell Penetrating Peptides

The widespread application of cell penetrating agents to clinical therapeutics and imaging agents relies on the ability to prepare them on a large scale and to readily conjugate them to their cargos. Dendritic analogues of cell penetrating peptides, with multiple guanidine groups on their peripheries offer advantages as their high symmetry allows them to be efficiently synthesized, while orthogonal functionalities at their focal points allow them to be conjugated to cargo using simple synthetic methods. Their chemical structures and properties are also highly tunable as their flexibility and the number of guanidine groups can be tuned by altering the dendritic backbone or the linkages to the guanidine groups. This review describes the development of cell-penetrating dendrimers based on several different backbones, their structure-property relationships, and comparisons of their efficacies with those of known cell penetrating peptides. The toxicities of these dendritic guanidines are also reported as well as their application towards the intracellular delivery of biologically significant cargos including proteins and nanoparticles.

Protection of hippocampal neurogenesis by TAT-Bcl-x(L) after cerebral ischemia in mice

Endogenous neurogenesis persists in the subgranular zone (SGZ) of the adult rodent brain. Cerebral ischemia stimulates endogenous neurogenesis involving proliferation, migration and differentiation of SGZ-derived neural precursor cells (NPC). However, the biological meaning of this phenomenon is limited by poor survival of NPC. In order to study the effects of an acute neuroprotective treatment on hippocampal endogenous neurogenesis after transient cerebral ischemia in mice, we applied a fusion protein consisting of the TAT domain of the HI virus with the anti-apoptotic Bcl-x(L). Intravenous injection of TAT-Bcl-x(L) resulted in reduced hippocampal cell injury for up to 4weeks after stroke as assessed by TUNEL and NeuN staining. This was in line with a TAT-Bcl-x(L)-mediated reduced postischemic microglia activation. Analysis of endogenous hippocampal cell proliferation revealed an increased number of BrdU(+) cells in the TAT-Bcl-x(L) group 4weeks after stroke compared to animals treated with saline and TAT-HA (negative control). Cell proliferation in non-ischemic sham operated animals was not affected by TAT-Bcl-x(L). Twenty-eight days after stroke co-expression of BrdU(+) cells with the immature neuronal marker doublecortin was significantly increased in TAT-Bcl-x(L) animals. Although TAT-Bcl-x(L) treatment also resulted in an increased number of BrdU(+) cells expressing the mature neuronal marker NeuN, the total amount of these cells was low. These data show that TAT-Bcl-x(L) treatment yields both postischemic sustained hippocampal neuroprotection and increased survival of NPC rather than an induction of endogenous neurogenesis itself.

Cell entry of arginine-rich peptides is independent of endocytosis

Arginine-rich peptides are a subclass of cell-penetrating peptides that are taken up by living cells and can be detected freely diffusing inside the cytoplasm and nucleoplasm. This phenomenon has been attributed to either an endocytic mode of uptake and a subsequent release from vesicles or to direct membrane penetration (transduction). To distinguish between both possibilities, we have blocked endocytic pathways suggested to be involved in uptake of cell-penetrating peptides. We have then monitored by confocal microscopy the uptake and distribution of the cell-penetrating transactivator of transcription (TAT) peptide into living mammalian cells over time. To prevent side effects of chemical inhibitors, we used genetically engineered cells as well as different temperature. We found that a knockdown of clathrin-mediated endocytosis and a knock-out of caveolin-mediated endocytosis did not affect the ability of TAT to enter cells. In addition, the TAT peptide showed the same intracellular distribution throughout the cytoplasm and nucleus as in control cells. Even incubation of cells at 4 °C did not abrogate TAT uptake nor change its intracellular distribution. We therefore conclude that this distribution results from TAT peptide that directly penetrated (transduced) the plasma membrane. The formation of nonselective pores is unlikely, because simultaneously added fluorophores were not taken up together with the TAT peptide. In summary, although the frequency and kinetics of TAT transduction varied between cell types, it was independent of endocytosis.

Membrane-permeable Bcl-xL prevents MPTP-induced dopaminergic neuronal loss in the substantia nigra

The anti-apoptotic Bcl-xL is a promising agent to prevent neurodegeneration in Parkinson's disease, which is characterized by a demise of dopaminergic neurons. We linked Bcl-xL to a peptide that allows its delivery across biological membranes and the blood-brain barrier. We tested the fusion protein in two models of Parkinson's Disease. Cell-permeable Bcl-xL protected neuroblastoma cells from the selective neurotoxin 1-methyl-4-phenylpyridinium. Furthermore, its systemic application in aged mice protected dopaminergic neurons following administration of MPTP as revealed by counting of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra pars compacta. Hence, we present that a cell-permeable form of an anti-apoptotic protein can be delivered to CNS neurons through its systemic application, and we provide the proof that the delivery of this protein to the CNS neurons effectively prevents neuronal cell death in models of chronic neurodegenerative diseases.