The TAT protein transduction domain enhances the neuroprotective effect of glial-cell-line-derived neurotrophic factor after optic nerve transection

Encapsulated cell technology: Delivering cytokines to treat posterior ocular diseases

Encapsulated cell technology (ECT) is a targeted delivery method that uses the genetically engineered cells in semipermeable polymer capsules to deliver cytokines. Thus far, ECT has been extensively utilized in pharmacologic research, and shows enormous potentials in the treatment of posterior segment diseases. Due to the biological barriers within the eyeball, it is difficult to attain effective therapeutic concentration in the posterior segment through topical administration of drug molecules. Encouragingly, therapeutic cytokines provided by ECT can cross these biological barriers and achieve sustained release at the desired location. The encapsulation system uses permeable materials that allow growth factors and cytokines to diffuse efficiently into retinal tissue. Moreover, the ECT based treatment can be terminated timely when we need to retrieve the implant, which makes the therapy reversible and provides a safer alternative for intraocular gene therapy. Meanwhile, we also place special emphasis on optimizing encapsulation materials and enhancing preservation techniques to achieve the stable release of growth factors and cytokines in the eyeball. This technology holds great promise for the treatment of patients with dry AMD, RP, glaucoma and MacTel. These findings would enrich our understandings of ECT and promote its future applications in treatment of degenerative retinopathy. This review comprises articles evaluating the exactness of artificial intelligence-based formulas published from 2000 to March 2024. The papers were identified by a literature search of various databases (PubMed/MEDLINE, Google Scholar, Cochrane Library and Web of Science).

Roles of growth factors in eye development and ophthalmic diseases

Growth factors are active substances secreted by a variety of cells, which act as messengers to regulate cell migration, proliferation and differentiation. Many growth factors are involved in the eye development or the pathophysiological processes of eye diseases. Growth factors such as vascular endothelial growth factor and basic fibroblast growth factor mediate the occurrence and development of diabetic retinopathy, choroidal neovascularization, cataract, diabetic macular edema, and other retinal diseases. On the other hand, growth factors like nerve growth factor, ciliary neurotrophic factor, glial cell line-derived neurotrophic factor, pigment epithelial-derived factor and granulocyte colony-stimulating factor are known to promote optic nerve injury repair. Growth factors are also related to the pathogenesis of myopia. Fibroblast growth factor, transforming growth factor-β, and insulin-like growth factor regulate scleral thickness and influence the occurrence and development of myopia. This article reviews growth factors involved in ocular development and ocular pathophysiology, discusses the relationship between growth factors and ocular diseases, to provide reference for the application of growth factors in ophthalmology.

Brain Pathogenesis and Potential Therapeutic Strategies in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy that affects multiple systems including the muscle and heart. The mutant CTG expansion at the 3'-UTR of the DMPK gene causes the expression of toxic RNA that aggregate as nuclear foci. The foci then interfere with RNA-binding proteins, affecting hundreds of mis-spliced effector genes, leading to aberrant alternative splicing and loss of effector gene product functions, ultimately resulting in systemic disorders. In recent years, increasing clinical, imaging, and pathological evidence have indicated that DM1, though to a lesser extent, could also be recognized as true brain diseases, with more and more researchers dedicating to develop novel therapeutic tools dealing with it. In this review, we summarize the current advances in the pathogenesis and pathology of central nervous system (CNS) deficits in DM1, intervention measures currently being investigated are also highlighted, aiming to promote novel and cutting-edge therapeutic investigations.

PTD4 Peptide Increases Neural Viability in an In Vitro Model of Acute Ischemic Stroke

Ischemic stroke is a disturbance in cerebral blood flow caused by brain tissue ischemia and hypoxia. We optimized a multifactorial in vitro model of acute ischemic stroke using rat primary neural cultures. This model was exploited to investigate the pro-viable activity of cell-penetrating peptides: arginine-rich Tat(49–57)-NH₂ (R⁴⁹KKRRQRRR⁵⁷-amide) and its less basic analogue, PTD4 (Y⁴⁷ARAAARQARA⁵⁷-amide). Our model included glucose deprivation, oxidative stress, lactic acidosis, and excitotoxicity. Neurotoxicity of these peptides was excluded below a concentration of 50 μm, and PTD4-induced pro-survival was more pronounced. Circular dichroism spectroscopy and molecular dynamics (MD) calculations proved potential contribution of the peptide conformational properties to neuroprotection: in MD, Tat(49–57)-NH₂ adopted a random coil and polyproline type II helical structure, whereas PTD4 adopted a helical structure. In an aqueous environment, the peptides mostly adopted a random coil conformation (PTD4) or a polyproline type II helical (Tat(49–57)-NH₂) structure. In 30% TFE, PTD4 showed a tendency to adopt a helical structure. Overall, the pro-viable activity of PTD4 was not correlated with the arginine content but rather with the peptide’s ability to adopt a helical structure in the membrane-mimicking environment, which enhances its cell membrane permeability. PTD4 may act as a leader sequence in novel drugs for the treatment of acute ischemic stroke.

The delicate balance between neurotoxicity and neuroprotection in the context of HIV-1 infection

Human immunodeficiency virus type-1 (HIV-1) causes a spectrum of neurological impairments, termed HIV-associated neurocognitive disorder (HAND), following the infiltration of infected cells into the brain. Even though the implementation of antiretroviral therapy reduced the systemic viral load, the prevalence of HAND remains unchanged and infected patients develop persisting neurological disturbances affecting their quality of life. As a result, HAND have gained importance in basic and clinical researches, warranting the need of developing new adjunctive treatments. Nonetheless, a better understanding of the molecular and cellular mechanisms remains necessary. Several studies consolidated their efforts into elucidating the neurotoxic signaling leading to HAND including the deleterious actions of HIV-1 viral proteins and inflammatory mediators. However, the scope of these studies is not sufficient to address all the complexity related to HAND development. Fewer studies focused on an altered neuroprotective capacity of the brain to respond to HIV-1 infection. Neurotrophic factors are endogenous polyproteins involved in neuronal survival, synaptic plasticity, and neurogenesis. Any defects in the processing or production of these crucial factors might compose a risk factor rendering the brain more vulnerable to neuronal damages. Due to their essential roles, they have been investigated for their diverse interplays with HIV-1 infection. In this review, we present a complete description of the neurotrophic factors involved in HAND. We discuss emerging concepts for their therapeutic applications and summarize the complex mechanisms that down-regulate their production in favor of a neurotoxic environment. For certain factors, we finally address opposing roles that rather lead to increased inflammation.

Lentivirally administered glial cell line-derived neurotrophic factor promotes post-ischemic neurological recovery, brain remodeling and contralesional pyramidal tract plasticity by regulating axonal growth inhibitors and guidance proteins

Owing to its potent longterm neuroprotective and neurorestorative properties, glial cell line-derived neurotrophic factor (GDNF) is currently studied in neurodegenerative disease clinical trials. However, little is known about the longterm effect of GDNF on neurological recovery, brain remodeling and neuroplasticity in the post-acute phase of ischemic stroke. In a comprehensive set of experiments, we examined the effects of lentiviral GDNF administration after ischemic stroke. GDNF reduced neurological deficits, neuronal injury, blood-brain barrier permeability in the acute phase in mice. As compared with control, enhanced motor-coordination and spontaneous locomotor activity were noted in GDNF-treated mice, which were associated with increased microvascular remodeling, increased neurogenesis and reduced glial scar formation in the peri-infarct tissue. We observed reduced brain atrophy and increased plasticity of contralesional pyramidal tract axons that crossed the midline in order to innervate denervated neurons in the ipsilesional red and facial nuclei. Contralesional axonal plasticity by GDNF was associated with decreased abundance of the axonal growth inhibitors brevican and versican in contralesional and ipsilesional brain tissue, reduced abundance of the growth repulsive guidance molecule ephrin b1 in contralesional brain tissue, increased abundance of the midline growth repulsive protein Slit1 in contralesional brain tissue and reduced abundance of Slit1's receptor Robo2 in ipsilesional brain tissue. These data indicate that GDNF potently induces longterm neurological recovery, peri-infarct brain remodeling and contralesional neuroplasticity, which are associated with the fine-tuned regulation of axonal growth inhibitors and guidance molecules that facilitate the growth of contralesional corticofugal axons in the direction to the ipsilesional hemisphere.

Gene Therapy for Inherited Retinal Degeneration

Inherited retinal degeneration (IRD), a group of rare retinal diseases that primarily lead to the progressive loss of retinal photoreceptor cells, can be inherited in all modes of inheritance: autosomal dominant (AD), autosomal recessive (AR), X-linked (XL), and mitochondrial. Based on the pattern of inheritance of the dystrophy, retinal gene therapy has 2 main strategies. AR, XL, and AD IRDs with haploinsufficiency can be treated by inserting a functional copy of the gene using either viral or nonviral vectors (gene augmentation). Different types of viral vectors and nonviral vectors are used to transfer plasmid DNA both in vitro and in vivo. AD IRDs with gain-of-function mutations or dominant-negative mutations can be treated by disrupting the mutant allele with (and occasionally without) gene augmentation. This review article aims to provide an overview of ocular gene therapy for treating IRDs using gene augmentation with viral or nonviral vectors or gene disruption through different gene-editing tools, especially with the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system.

Hypoxia-Inducible Factor-1α Target Genes Contribute to Retinal Neuroprotection

Hypoxia-inducible factor (HIF) is a transcription factor that facilitates cellular adaptation to hypoxia and ischemia. Long-standing evidence suggests that one isotype of HIF, HIF-1α, is involved in the pathogenesis of various solid tumors and cardiac diseases. However, the role of HIF-1α in retina remains poorly understood. HIF-1α has been recognized as neuroprotective in cerebral ischemia in the past two decades. Additionally, an increasing number of studies has shown that HIF-1α and its target genes contribute to retinal neuroprotection. This review will focus on recent advances in the studies of HIF-1α and its target genes that contribute to retinal neuroprotection. A thorough understanding of the function of HIF-1α and its target genes may lead to identification of novel therapeutic targets for treating degenerative retinal diseases including glaucoma, age-related macular degeneration, diabetic retinopathy, and retinal vein occlusions.

G-quartet oligonucleotide mediated delivery of proteins into photoreceptors and retinal pigment epithelium via intravitreal injection

There is currently no available method to efficiently deliver proteins across the plasma membrane of photoreceptor or retinal pigment epithelium (RPE) cells in vivo. Thus, current clinical application of recombinant proteins in ophthalmology is limited to the use of proteins that perform their biological function extracellularly. The ability to traverse biological membranes would enable the mobilization of a significantly larger number of proteins with previously well characterized properties. Nucleolin is abundantly present on the surface of rapidly dividing cells including cancer cells. Surprisingly, nucleolin is also present on the surface of photoreceptor cell bodies. Here we investigated whether nucleolin can be utilized as a gateway for the delivery of proteins into retinal cells following intravitreal injection. AS1411 is a G-quartet aptamer capable of targeting nucleolin. Subsequent to intravitreal injection, fluorescently labeled AS1411 localized to various retinal cell types including the photoreceptors and RPE. AS1411 linked to streptavidin (a ∼50 kDa protein) via a biotin bridge enabled the uptake of Streptavidin into photoreceptors and RPE. AS1411-Streptavidin conjugate applied topically to the cornea allowed for uptake of the conjugate into the nucleus and cytoplasm of corneal endothelial cells. Clinical relevance of AS1411 as a delivery vehicle was strongly indicated by demonstration of the presence of cell surface nucleolin on the photoreceptors, inner neurons and ganglion cells of human retina. These data support exploration of AS1411 as a means of delivering therapeutic proteins to diseased retina.

Penetrating the cell membrane, thermal targeting and novel anticancer drugs: the development of thermally targeted, elastin-like polypeptide cancer therapeutics

Therapeutic peptides offer important cancer treatment approaches. Designed to inhibit oncogenes and other oncoproteins, early therapeutic peptides applications were hampered by pharmacokinetic properties now addressed through tumor targeting strategies. Active targeting with environmentally responsive biopolymers or macromolecules enhances therapeutics accumulation at tumor sites; passive targeting with macromolecules, or liposomes, exploits angiogenesis and poor lymphatic drainage to preferentially accumulate therapeutics within tumors. Genetically engineered, thermally-responsive, elastin-like polypeptides use both strategies and cell-penetrating peptides to further intratumoral cell uptake. This review describes the development and application of cell-penetrating peptide-elastin-like polypeptide therapeutics for the thermally targeted delivery of therapeutic peptides.

Role of neurotrophic factor alterations in the neurodegenerative process in HIV associated neurocognitive disorders

Migration of HIV infected cells into the CNS is associated with a spectrum of neurological disorders, ranging from milder forms of HIV-associated neurocognitive disorders (HAND) to HIV-associated dementia (HAD). These neuro-psychiatric syndromes are related to the neurodegenerative pathology triggered by the release of HIV proteins and cytokine/chemokines from monocytes/macrophages into the CNS -a condition known as HIV encephalitis (HIVE). As a result of more effective combined anti-retroviral therapy patients with HIV are living longer and thus the frequency of HAND has increased considerably, resulting in an overlap between the neurodegenerative pathology associated with HIV and that related to aging. In fact, HIV infection is believed to hasten the aging process. The mechanisms through which HIV and aging lead to neurodegeneration include: abnormal calcium flux, excitotoxicity, signaling abnormalities, oxidative stress and autophagy defects. Moreover, recent studies have shown that defects in the processing and transport of neurotrophic factors such as fibroblast growth factors (FGFs), neural growth factor (NGF) and brain-derived growth factor (BDNF) might also play a role. Recent evidence implicates alterations in neurotrophins in the pathogenesis of neurodegeneration associated with HAND in the context of aging. Here, we report FGF overexpression curtails gp120-induced neurotoxicity in a double transgenic mouse model. Furthermore, our data show disparities in brain neurotrophic factor levels may be exacerbated in HIV patients over 50 years of age. In this review, we discuss the most recent findings on neurotrophins and HAND in the context of developing new therapies to combat HIV infection in the aging population.

Motor neuron trophic factors: therapeutic use in ALS?

The modest effects of neurotrophic factor (NTF) treatment on lifespan in both animal models and clinical studies of Amyotropic Lateral Sclerosis (ALS) may result from any one or combination of the four following explanations: 1.) NTFs block cell death in some physiological contexts but not in ALS; 2.) NTFs do not rescue motoneurons (MNs) from death in any physiological context; 3.) NTFs block cell death in ALS but to no avail; and 4.) NTFs are physiologically effective but limited by pharmacokinetic constraints. The object of this review is to critically evaluate the role of both NTFs and the intracellular cell death pathway itself in regulating the survival of spinal and cranial (lower) MNs during development, after injury and in response to disease. Because the role of molecules mediating MN survival has been most clearly resolved by the in vivo analysis of genetically engineered mice, this review will focus on studies of such mice expressing reporter, null or other mutant alleles of NTFs, NTF receptors, cell death or ALS-associated genes.

Novel RasGRF1-derived Tat-fused peptides inhibiting Ras-dependent proliferation and migration in mouse and human cancer cells

Mutations of RAS genes are critical events in the pathogenesis of different human tumors and Ras proteins represent a major clinical target for the development of specific inhibitors to use as anticancer agents. Here we present RasGRF1-derived peptides displaying both in vitro and in vivo Ras inhibitory properties. These peptides were designed on the basis of the down-sizing of dominant negative full-length RasGRF1 mutants. The over-expression of these peptides can revert the phenotype of K-RAS transformed mouse fibroblasts to wild type, as monitored by several independent biological readouts, including Ras-GTP intracellular levels, ERK activity, morphology, proliferative potential and anchorage independent growth. Fusion of the RasGRF1-derived peptides with the Tat protein transduction domain allows their uptake into mammalian cells. Chemically synthesized Tat-fused peptides, reduced to as small as 30 residues on the basis of structural constraints, retain Ras inhibitory activity. These small peptides interfere in vitro with the GEF catalyzed nucleotide dissociation and exchange on Ras, reduce cell proliferation of K-RAS transformed mouse fibroblasts, and strongly reduce Ras-dependent IGF-I-induced migration and invasion of human bladder cancer cells. These results support the use of RasGRF1-derived peptides as model compounds for the development of Ras inhibitory anticancer agents.

Nuclear targeted delivery of macromolecules to retina and cornea

BACKGROUND

Cell-penetrating peptides (CPPs) can deliver molecules into cells by binding and penetrating the plasma membrane. However, the majority of CPPs get trapped in endosomes, resulting in degradation of the cargo molecule and inefficient delivery to the nucleus. The present study investigates the potential use of a nucleolin binding peptide (NBP) for the delivery of macromolecules including fluorophores, recombinant protein and DNA to the nuclei of ocular tissues in vivo.

METHODS

Fluorescent dyes covalently linked to NBP or NBP-green fluorescent protein fusion protein were injected intravitreally or subretinally or topically applied to the cornea. Frozen sections were prepared for quantification of transduction. Delivery of plasmid DNA was studied using luciferase and LacZ DNA compacted with pegylated NBP. Levels of luciferase were quantified, and LacZ expression was localized in ocular tissues.

RESULTS

We found that NBP-directed fluorophores exhibited retinal and corneal transduction. Subretinal injection transduced cell types throughout the retina, including photoreceptors, retinal pigment epithelium and neuronal cells. Intravitreal injection transduced neuronal cells in the retina, as well as cells in the cornea. Topically applied NBP lead to transduction of the superficial epithelial layer of the cornea. NBP localized to the nucleus upon exogenous application in vivo. Pegylated NBP nanoparticles significantly improved delivery and expression of transgenes over DNA alone without any measureable toxicity.

CONCLUSIONS

The results obtained in the present study demonstrate that NBP can deliver small and large molecules into retinal and corneal cells and plasmid DNA into retinal cells and hence may be useful for the delivery of therapeutics to the eye.

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.

Cell Permeable Peptides: A Promising Tool to Deliver Neuroprotective Agents in the Brain

The inability of most drugs to cross the blood-brain barrier and/or plasma membrane limits their use for biomedical applications in the brain. Cell Permeable Peptides (CPPs) overcome this problem and are effective in vivo, crossing the plasma membrane and the blood-brain barrier. CPPs deliver a wide variety of compounds intracellularly in an active form. In fact, many bioactive cargoes have neuroprotective properties, and due to their ability to block protein-protein interactions, offer exciting perspectives in the clinical setting. In this review we give an overview of the Cell Permeable Peptides strategy to deliver neuroprotectants against neurodegeneration in the CNS.

Cell penetrating peptide POD mediates delivery of recombinant proteins to retina, cornea and skin

Recently we described a novel cell penetrating peptide, peptide for ocular delivery (POD) that could deliver small molecules including fluorescent dyes into retinal cells. The objective of the current study was to examine whether biologically relevant macromolecules such as proteins, genetically fused with POD could also be delivered into retinal tissues in vivo. We generated a POD-GFP fusion protein and examined its cell and tissue penetrating properties. We found that endogenously expressed POD-GFP fusion protein localized to the nucleus, suggesting that POD acts as a nuclear localization signal. Adenovirus (Ad) vectors expressing POD-GFP fusion protein were constructed and the recombinant protein was purified from Ad-infected human embryonic retinoblasts (HER). Exogenously supplied POD-GFP fusion protein rapidly transduced A549 and HER cells and colocalized in part with markers of late endosomes, from which it could escape. Following subretinal delivery, POD-GFP localized to the retinal pigment epithelium and the photoreceptor cell bodies. When injected into the vitreous, POD-GFP localized to the ganglion cells and the inner nuclear layer of the retina as well as the lens capsule. Topical application of POD-GFP to ocular surfaces resulted in uptake by the corneal epithelium. POD-GFP also transduced non-ocular tissues, including the epidermis of the skin following topical application.

Ciliary neurotrophic factor infused intracerebroventricularly shows reduced catabolic effects when linked to the TAT protein transduction domain

Ciliary neurotrophic factor (CNTF) regulates the differentiation and survival of a wide spectrum of developing and adult neurons, including motor neuron loss after injury. We recently described a cell-penetrant recombinant human CNTF (rhCNTF) molecule, formed by fusion with the human immunodeficiency virus-1 transactivator of transcription (TAT) protein transduction domain (TAT-CNTF) that, upon subcutaneous administration, retains full neurotrophic activity without cytokine-like side-effects. Although the CNTF receptor is present in hypothalamic nuclei, which are involved in the control of energy, rhCNTF but not TAT-CNTF stimulates signal transducers and activators of transcription 3 phosphorylation in the rat hypothalamus after subcutaneous administration. This could be due limited TAT-CNTF distribution in the hypothalamus and/or altered intracellular signaling by the fusion protein. To explore these possibilities, we examined the effect of intracerebroventricular administration of TAT-CNTF in male adult rats. TAT-CNTF-induced weight loss, although the effect was smaller than that seen with either rhCNTF or leptin (which exerts CNTF-like effects via its receptor). In contrast to rhCNTF and leptin, TAT-CNTF neither induced morphological changes in adipose tissues nor increased uncoupling protein 1 expression in brown adipose tissue, a characteristic feature of rhCNTF and leptin. Acute intracerebroventricular administration of TAT-CNTF induced a less robust phosphorylation of signal transducers and activators of transcription 3 in the hypothalamus, compared with rhCNTF. The data show that fusion of a protein transduction domain may change rhCNTF CNS distribution, while further strengthening the utility of cell-penetrating peptide technology to neurotrophic factor biology beyond the neuroscience field.

1,25-Dihydroxyvitamin D3 administration to 6-hydroxydopamine-lesioned rats increases glial cell line-derived neurotrophic factor and partially restores tyrosine hydroxylase expression in substantia nigra and striatum

It has previously been demonstrated that 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] administration, whether in cell cultures or in vivo to rats, increases glial cell line-derived neurotrophic factor (GDNF) expression levels, suggesting that this hormone may have beneficial effects in neurodegenerative disorders. This study was carried out to explore the effects of 1,25(OH)(2)D(3) administration in a 6-OHDA-lesioned rat model of Parkinson's disease on GDNF and tyrosine hydroxylase (TH) expression in substantia nigra (SN) and striatum. Two groups of animals received 1,25(OH)(2)D(3) intraperitoneally, the first group 7 days before the unilateral injection of 6-OHDA into the medial forebrain bundle (MFB) and the second group 21 days (days 21-28) after the unilateral injection of 6-OHDA. Animals of both groups were sacrificed on day 28. In addition, two other groups received a unilateral injection of either saline or 6-OHDA into the MFB. Rats were killed, and the SN and striatum were then removed for GDNF and TH determination. Striatal GDNF protein expression was increased on the ipsilateral with respect to the contralateral side after 6-OHDA injection alone as well as in 1,25(OH)(2)D(3)-treated rats before or after 6-OHDA administration. As expected, 6-OHDA injection induced an ipsilateral decrease in TH-immunopositive neuronal cell bodies and axonal terminals in the SN and striatum. However, treatment with 1,25(OH)(2)D(3) before and after 6-OHDA injection partially restored TH expression in SN. These data suggest that 1,25(OH)(2)D(3) may help to prevent dopaminergic neuron damage.

PTD-mediated delivery of anti-cell death proteins/peptides and therapeutic enzymes

Millions of unnecessary cells are removed from our body everyday by apoptosis to ensure our survivals. Apoptosis is a highly coordinated process. Failure in apoptotic regulation results in disease. A large number of studies have demonstrated that accelerated apoptosis is involved in degenerative diseases, ischemic injuries, immunodeficiency and infertility. These studies have also revealed the molecular mechanisms of apoptosis signal transduction to provide therapeutic targets. On the other hand, protein transduction technology has been developed to deliver full-length proteins to various tissues including the brain. So far, many studies have shown that in vivo delivery of therapeutic proteins/peptides, including anti-apoptotic proteins, an anti-oxidant enzyme, a neuroprotectant, enzymes involved in purine or tyrosine metabolism, caspase inhibitors, c-Jun N-terminal kinase inhibitors and an NF-kappaB inhibitor, by protein transduction technology mitigates various diseases in animal models.

Synthesis of cell-penetrating peptides and their application in neurobiology

Short basic amino acid sequences, often called cell-penetrating peptides (CPPs), allow the delivery of proteins and other molecules into cells and across the blood-brain barrier (BBB). Although the ability of basic proteins to facilitate such trafficking is known for a long time, only the application of genetic methods and overexpression of fusion proteins in Escherichia coli has lead to a wide application of CPP in many research areas, including signal transduction, cancer, angiogenesis, apoptosis, bone development, cardioprotection, cell cycle, neurobiology, and many others. For the neuroscientist, CPPs are particularly attractive, as a number of articles in the last 5 years have reported their use for neuronal rescue in a number of models for neurodegenerative diseases in vitro and in vivo in rats, mice, or gerbils. Here, we give a detailed description of the protein purification methodology and applications in neuroscience.

Application of a blood-brain-barrier-penetrating form of GDNF in a mouse model for Parkinson's disease

Glial-cell-line-derived neurotrophic factor (GDNF) promotes mesencephalic dopaminergic neuronal survival in several in vitro and in vivo models. As the demise of dopaminergic neurons is the cause for Parkinson's disease (PD) symptoms, GDNF is a promising agent for its treatment. However, this neurotrophin is unable to cross the blood-brain barrier, which has complicated its clinical use. Therefore, ways to deliver GDNF into the central nervous system in an effective manner are needed. The HIV-1-Tat-derived cell-penetrating peptide (CPP) provides a means to deliver fusion proteins into the brain. We generated a fusion protein between the 11 amino acid CPP of Tat and the rat GDNF mature protein to deliver GDNF across the blood-brain barrier. We showed previously that Tat-GDNF enhances the neuroprotective effect of GDNF in in vivo models for nerve trauma and ischemia. Here, we tested its effect in a subchronic scheme of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) application into the mouse as a model for PD to evaluate the effect of Tat-GDNF fusion protein in dopaminergic neuron survival. We showed that the fusion protein did indeed reach the dopaminergic neurons. However, the in vivo application of Tat-GDNF did not provide neuroprotection of dopaminergic neurons, as revealed by immunohistochemistry and counting of the number of tyrosine-hydroxylase-immunoreactive neurons in the substantia nigra pars compacta. Possibly, GDNF does protect nigro-striatal projections of those neurons that survive MPTP treatment but does not increase the number of surviving dopaminergic neurons. A concomitant treatment of Tat-GDNF with an anti-apoptotic Tat-fusion protein might be beneficial.

TAT-GDNF in neurodegeneration and ischemic stroke

The delivery of proteins across the blood-brain barrier is severely limited by their size and biochemical properties. Numerous peptides have been characterized in recent years that prevent neuronal death in vitro, but cannot be used therapeutically, since they do not cross cell membrane barriers. It has been shown in the 1990s that the HIV TAT protein is able to cross cell membranes even when coupled with larger peptides. It appears, therefore, that TAT fusion proteins may enter the brain, even when used systemically. Indeed, the systemic delivery of a TAT protein linked with glial-derived neurotrophic factor (GDNF) successfully transduced central nervous system (CNS) neurons in mice. When administered after optic nerve transection and focal cerebral ischemia, TAT-GDNF protected retinal ganglion cells and brain neurons from cell death, elevated tissue Bcl-XL levels and attenuated the activity of the executioner caspase-3. These findings demonstrate the in vivo efficacy of fusion proteins in clinically relevant disease models, raising hopes that neuroprotection may become eventually feasible in human patients.

Bcl-xL increases axonal numbers but not axonal elongation from rat retinal explants

The Bcl-2 family of proteins has been characterized as a key regulator of cell death programs. In addition, these proteins also play important roles in cellular differentiation, such as axonal growth. The role of Bcl-2 family members on axonal regeneration and neurite extension has been controversial so far. Here, we examine the influence of Bcl-x(L) on axonal regeneration from adult retina explants in vitro. We delivered recombinant Bcl-x(L) into retinal tissue, mediated by the Tat-protein transduction domain, and observed its effect on retinal axon extension. We found that Bcl-x(L) increased the number of regenerating neurites, but did not increase their length. Our results indicate that Bcl-x(L) stimulates axonal initiation but not axonal elongation after crush injury to retinal explants, without altering the number of surviving neurons.

PTD-XIAP protects against cerebral ischemia by anti-apoptotic and transcriptional regulatory mechanisms

Caspases play a major role in the infarction process that follows occlusion of cerebral arteries and are important targets for stroke therapy. We have generated three fusion proteins that link various domains of the X chromosome-linked inhibitor of apoptosis (XIAP), a potent caspase inhibitor, to the protein transduction domain (PTD) of HIV-1/Tat, and have tested their efficacy after distal occlusion of the middle cerebral artery (dMCAO) in mice. PTD-XIAP failed to accumulate in brain structures after intravenous (iv) delivery, but properly transduced cortical cells when applied topically. Shorter constructs efficiently targeted the lesion after iv delivery. All proteins retained their caspase inhibitory activity and significantly reduced infarct volumes. PTD-XIAP reversed long-term impairments in the water maze test. Sequential activation of transcription factors was observed, suggesting that the effects of XIAP are mediated by both direct inhibition of apoptotic mechanisms and secondary regulation of transcription factors involved in neuronal survival.

Peptide-enhanced cellular internalization of proteins in neuroscience

Over the last 15 years, many publications described the use of peptide sequences that have been dubbed cell penetrating peptides (CPP), Trojan Horse peptides, protein transduction domains, or membrane-translocating sequences. These mostly positively charged domains bring attached cargo across biological membranes. One of the reasons for the interest in CPP is their potential as delivery tools to enhance the pharmacodynamics of drugs otherwise poorly bioavailable. In particular, the neuroscientist aiming to deliver a protein or other compound into the brain for analytical or therapeutic reasons is faced with the challenge that few drugs cross the blood-brain barrier. CPP are valuable tools to overcome the plasma membrane or the blood-brain barrier in basic research, and in relevant models of neural disease, and will hopefully help to increase the precious few treatments or even cures for people with diseases of the brain and nervous system. Here, we review applications in neuroscience and recent insights into the mechanism of CPP-mediated trafficking.

Neuroprotection by hypoxic preconditioning: HIF-1 and erythropoietin protect from retinal degeneration

Hypoxic exposure of cells or organisms induces expression of a number of hypoxia responsive genes through the activation of the hypoxia-inducible factor-1 (HIF-1). One of the most prominent HIF-1 targets is erythropoietin that has beneficial effects on ischemia-related injury in the brain. Exposure to low environmental oxygen concentrations can be used as a preconditioning paradigm to protect cells or tissues against a variety of harmful conditions. Here, we summarize recent work on neuroprotection of retinal photoreceptors and ganglion cells induced by hypoxic preconditioning or by systemically elevated levels of Epo in mouse plasma.

FK506 blocks activation of the intrinsic caspase cascade after optic nerve crush

Retinal ganglion cells die by apoptosis after optic nerve crush. FK506 has been shown to be neuroprotective in this model but the mechanism(s) by which it exerts these actions remains unknown. We and others have shown that caspase 9 is cleaved in the retina in other injury models and we hypothesized that the neuroprotection observed with FK506 was mediated by interference with caspase 9 activation. The present study examined the cellular localization of caspase 9 cleavage after intraorbital optic nerve crush in rats, the time course of caspase 9 cleavage after optic nerve crush and the ability of orally administered FK506 to block caspase 9 cleavage after optic nerve crush. We show by immunohistochemistry that cleaved caspase 9 is present in retinal ganglion cells (identified by prior backlabelling) after optic nerve crush. Immunoblot analysis showed that caspase 9 cleavage is significantly elevated 5 and 8 days after optic nerve crush. We show that orally administered FK506 reaches the retina and is pharmacologically active in retinal tissue. Furthermore, the oral administration of FK506 5 mg kg(-1) day(-1) blocks the cleavage of caspase 9 at both time points. These data suggest that caspase 9 activation may play an important role in retinal ganglion cell death following optic nerve crush and that the neuroprotection seen with FK506 may be mediated by interfering with the activation of caspase 9.

Expression of the protein inhibitor of nitric oxide synthase in the adult rat retina before and after optic nerve lesion

The molecular messenger nitric oxide (NO) not only serves a number of physiologic functions, but is also involved in the pathophysiology of neurodegeneration. It is produced by the nitric oxide synthase (NOS) isoenzymes. One of the many players regulating NOS activity is the Protein Inhibitor of NOS, PIN. To gain further insight into the mechanisms of NOS regulation and NO-mediated cell death after nerve trauma, we examined PIN expression in a standard model of lesion-induced neurodegeneration, the rat optic nerve transsection model. In both the axotomized retinae and the control retinae PIN expression was predominantly observed in the retinal ganglion cell layer. Optic nerve lesion did neither change the amount of PIN mRNA, as determined by in situ hybridization and real-time RT-PCR, nor did it change the amount of PIN as determined by immunohistochemistry and Western blot analysis. These results suggest that in our model, NOS activity is not regulated by altered PIN levels, which contributes to our understanding of apoptotic mechanisms in injured neurons.

Delivery of bioactive molecules into the cell: the Trojan horse approach

In recent years, vast amounts of data on the mechanisms of neural de- and regeneration have accumulated. However, only in disproportionally few cases has this led to efficient therapies for human patients. Part of the problem is to deliver cell death-averting genes or gene products across the blood-brain barrier (BBB) and cellular membranes. The discovery of Antennapedia (Antp)-mediated transduction of heterologous proteins into cells in 1992 and other "Trojan horse peptides" raised hopes that often-frustrating attempts to deliver proteins would now be history. The demonstration that proteins fused to the Tat protein transduction domain (PTD) are capable of crossing the BBB may revolutionize molecular research and neurobiological therapy. However, it was only recently that PTD-mediated delivery of proteins with therapeutic potential has been achieved in models of neural degeneration in nerve trauma and ischemia. Several groups have published the first positive results using protein transduction domains for the delivery of therapeutic proteins in relevant animal models of human neurological disorders. Here, we give an extensive review of peptide-mediated protein transduction from its early beginnings to new advances, discuss their application, with particular focus on a critical evaluation of the limitations of the method, as well as alternative approaches. Besides applications in neurobiology, a large number of reports using PTD in other systems are included as well. Because each protein requires an individual purification scheme that yields sufficient quantities of soluble, transducible material, the neurobiologist will benefit from the experiences of other researchers in the growing field of protein transduction.

Erythropoietin protects from axotomy-induced degeneration of retinal ganglion cells by activating ERK-1/-2

Apart from its hematopoietic function, erythropoietin (Epo) exerts neuroprotective activity upon reduced oxygenation or ischemia of brain, retina, and spinal cord. To examine whether Epo has an impact on the retrograde degeneration of retinal ganglion cells (RGCs) following optic nerve transection in vivo, we made use of our transgenic mouse line tg21 that constitutively expresses human Epo preferentially in neuronal cells without inducing polycythemia. We show that the tg21 retina expresses human Epo and that RGCs in this mouse line carry the Epo receptor. Upon axotomy, the RGCs of Epo transgenic tg21 mice were protected against degeneration, as compared with wild-type control animals. Western blot analysis revealed decreased phosphorylation levels of STAT-5 and reduced expression of Bcl-XL in RGCs of axotomized tg21 animals, suggesting that the corresponding pathways are not crucial for Epo's neuroprotective activity. Increased phosphorylation levels of ERK-1/-2 and Akt, as well as decreased caspase-3 activity, however, were observed in injured tg21 retinae. Injection of selective inhibitors of ERK-1/-2 (PD98059) or Akt (Wortmannin) pathways into the vitreous space revealed that transgenic Epo protected the RGCs by a pathway involving ERK-1/-2 but not Akt. In view that axotomy-induced degeneration of RGC occurs slowly, and considering the earlier data on the safety and efficacy of Epo in human stroke patients, we predict the clinical implementation of recombinant human Epo not only in patients with acute ischemic stroke, but also with more delayed degenerative neurological diseases.