Human recombinant palmitoyl-protein thioesterase-1 (PPT1) for preclinical evaluation of enzyme replacement therapy for infantile neuronal ceroid lipofuscinosis

Identification of New Modulators and Inhibitors of Palmitoyl-Protein Thioesterase 1 for CLN1 Batten Disease and Cancer

Palmitoyl-protein thioesterase 1 (PPT1) is an understudied enzyme that is gaining attention due to its role in the depalmitoylation of several proteins involved in neurodegenerative diseases and cancer. PPT1 is overexpressed in several cancers, specifically cholangiocarcinoma and esophageal cancers. Inhibitors of PPT1 lead to cell death and have been shown to enhance the killing of tumor cells alongside known chemotherapeutics. PPT1 is hence a viable target for anticancer drug development. Furthermore, mutations in PPT1 cause a lysosomal storage disorder called infantile neuronal ceroid lipofuscinosis (CLN1 disease). Molecules that can inhibit, stabilize, or modulate the activity of this target are needed to address these diseases. We used PPT1 enzymatic assays to identify molecules that were subsequently tested by using differential scanning fluorimetry and microscale thermophoresis. Selected compounds were also tested in neuroblastoma cell lines. The resulting PPT1 screening data was used for building machine learning models to help select additional compounds for testing. We discovered two of the most potent PPT1 inhibitors reported to date, orlistat (IC50 178.8 nM) and palmostatin B (IC50 11.8 nM). When tested in HepG2 cells, it was found that these molecules had decreased activity, indicating that they were likely not penetrating the cells. The combination of in vitro enzymatic and biophysical assays enabled the identification of several molecules that can bind or inhibit PPT1 and may aid in the discovery of modulators or chaperones. The molecules identified could be used as a starting point for further optimization as treatments for other potential therapeutic applications outside CLN1 disease, such as cancer and neurological diseases.

Treatment of CLN1 disease with a blood-brain barrier penetrating lysosomal enzyme

Neuronal ceroid lipofuscinosis type 1(CLN1 disease) is a rare autosomal recessive lysosomal storage disease caused by genetic defects of palmitoyl protein thioesterase-1(PPT1), leading to accumulation of lipofuscin granules in brain and progressive neurodegeneration. Psychomotor regression, seizures, loss of vision, and movement disorder begin in infancy and result in early death. Currently, no disease-modifying therapy is available.

We report a 68-month-old boy with CLN1 treated on a compassionate use basis weekly for 26 months with a PPT1 enzyme fused to an anti-insulin receptor antibody (AGT-194), thereby enabling penetration of the blood-brain barrier (BBB). During treatment, no side effects were observed, while seizure frequency decreased, life quality improved, and the boy's general condition remained stable.

This case documents for the first time that treatment of CLN1 is principally feasible by an intravenous BBB penetrating enzyme replacement therapy using PPT1 fused with the human insulin receptor. Monitoring of side effects raised no unacceptable or unexpected safety concerns.Observed improvement of life quality related to ameliorated epilepsy control raises hope that further robust clinical trials including patients in earlier stages of disease will show positive results.

Cross-species efficacy of enzyme replacement therapy for CLN1 disease in mice and sheep

CLN1 disease, also called infantile neuronal ceroid lipofuscinosis (NCL) or infantile Batten disease, is a fatal neurodegenerative lysosomal storage disorder resulting from mutations in the CLN1 gene encoding the soluble lysosomal enzyme palmitoyl-protein thioesterase 1 (PPT1). Therapies for CLN1 disease have proven challenging because of the aggressive disease course and the need to treat widespread areas of the brain and spinal cord. Indeed, gene therapy has proven less effective for CLN1 disease than for other similar lysosomal enzyme deficiencies. We therefore tested the efficacy of enzyme replacement therapy (ERT) by administering monthly infusions of recombinant human PPT1 (rhPPT1) to PPT1-deficient mice (Cln1-/-) and CLN1R151X sheep to assess how to potentially scale up for translation. In Cln1-/- mice, intracerebrovascular (i.c.v.) rhPPT1 delivery was the most effective route of administration, resulting in therapeutically relevant CNS levels of PPT1 activity. rhPPT1-treated mice had improved motor function, reduced disease-associated pathology, and diminished neuronal loss. In CLN1R151X sheep, i.c.v. infusions resulted in widespread rhPPT1 distribution and positive treatment effects measured by quantitative structural MRI and neuropathology. This study demonstrates the feasibility and therapeutic efficacy of i.c.v. rhPPT1 ERT. These findings represent a key step toward clinical testing of ERT in children with CLN1 disease and highlight the importance of a cross-species approach to developing a successful treatment strategy.

Blood-brain barrier delivery for lysosomal storage disorders with IgG-lysosomal enzyme fusion proteins

The majority of lysosomal storage diseases affect the brain. Treatment of the brain with intravenous enzyme replacement therapy is not successful, because the recombinant lysosomal enzymes do not cross the blood-brain barrier (BBB). Biologic drugs, including lysosomal enzymes, can be re-engineered for BBB delivery as IgG-enzyme fusion proteins. The IgG domain of the fusion protein is a monoclonal antibody directed against an endogenous receptor-mediated transporter at the BBB, such as the insulin receptor or the transferrin receptor. This receptor transports the IgG across the BBB, in parallel with the endogenous receptor ligand, and the IgG acts as a molecular Trojan horse to ferry into brain the lysosomal enzyme genetically fused to the IgG. The IgG-enzyme fusion protein is bi-functional and retains both high affinity binding for the BBB receptor, and high lysosomal enzyme activity. IgG-lysosomal enzymes are presently in clinical trials for treatment of the brain in Mucopolysaccharidosis.

Ocular Manifestations of Neuronal Ceroid Lipofuscinoses

Neuronal ceroid lipofuscinoses (NCLs) are a group of rare neurodegenerative storage disorders associated with devastating visual prognosis, with an incidence of 1/1,000,000 in the United States and comparatively higher incidence in European countries. The pathophysiological mechanisms causing NCLs occur due to enzymatic or transmembrane defects in various sub-cellular organelles including lysosomes, endoplasmic reticulum, and cytoplasmic vesicles. NCLs are categorized into different types depending upon the underlying cause i.e., soluble lysosomal enzyme deficiencies or non-enzymatic deficiencies (functions of identified proteins), which are sub-divided based on an axial classification system. In this review, we have evaluated the current evidence in the literature and reported the incidence rates, underlying mechanisms and currently available management protocols for these rare set of neuroophthalmological disorders. Additionally, we also highlighted the potential therapies under development that can expand the treatment of these rare disorders beyond symptomatic relief.

GFAP hyperpalmitoylation exacerbates astrogliosis and neurodegenerative pathology in PPT1-deficient mice

This study reports that the intermediate filament protein GFAP is modified with protein palmitoylation. Increased GFAP expression and palmitoylation is involved in astrocyte proliferation and astrogliosis. We demonstrate that GFAP palmitoylation is regulated by PPT1, a palmitoylprotein thioesterase linked to a childhood neurodegenerative disorder, infantile neuronal ceroid lipofuscinosis. A palmitoylation-defective mutant of GFAP attenuates astrogliosis and the concurrent pathology in a loss-of-function PPT1 mouse. We conclude that accumulation of palmitoylated GFAP contributes to the pathogenesis of astrogliosis and neurodegeneration, suggesting that targeting the modified cysteine in GFAP may be a potential therapeutic strategy for the treatment of infantile neuronal ceroid lipofuscinosis and other neurodegenerative disorders.

The homeostasis of protein palmitoylation and depalmitoylation is essential for proper physiological functions in various tissues, in particular the central nervous system (CNS). The dysfunction of PPT1 (PPT1-KI, infantile neuronal ceroid lipofuscinosis [INCL] mouse model), which catalyze the depalmitoylation process, results in serious neurodegeneration accompanied by severe astrogliosis in the brain. Endeavoring to determine critical factors that might account for the pathogenesis in CNS by palm-proteomics, glial fibrillary acidic protein (GFAP) was spotted, indicating that GFAP is probably palmitoylated. Questions concerning if GFAP is indeed palmitoylated in vivo and how palmitoylation of GFAP might participate in neural pathology remain unexplored and are waiting to be investigated. Here we show that GFAP is readily palmitoylated in vitro and in vivo; specifically, cysteine-291 is the unique palmitoylated residue in GFAP. Interestingly, it was found that palmitoylated GFAP promotes astrocyte proliferation in vitro. Furthermore, we showed that PPT1 depalmitoylates GFAP, and the level of palmitoylated GFAP is overwhelmingly up-regulated in PPT1-knockin mice, which lead us to speculate that the elevated level of palmitoylated GFAP might accelerate astrocyte proliferation in vivo and ultimately led to astrogliosis in INCL. Indeed, blocking palmitoylation by mutating cysteine-291 into alanine in GFAP attenuate astrogliosis, and remarkably, the concurrent neurodegenerative pathology in PPT1-knockin mice. Together, these findings demonstrate that hyperpalmitoylated GFAP plays critical roles in regulating the pathogenesis of astrogliosis and neurodegeneration in the CNS, and most importantly, pinpointing that cysteine-291 in GFAP might be a valuable pharmaceutical target for treating INCL and other potential neurodegenerative diseases.

Protein Delivery by Peptide-Based Stealth Liposomes: A Biomolecular Insight into Enzyme Replacement Therapy

Infantile neural ceroid lipofuscinosis (INCL) is a lysosomal storage disorder characterized by mutations in the CLN1 gene that leads to lack of the lysosomal enzyme palmitoyl-protein thioesterase-1 (PPT1), which causes the progressive death of cortical neurons. Enzyme replacement therapy (ERT) is one of the most promising treatments, but its translation toward a clinical use is hampered by the need to deliver the enzyme to the central nervous system and a more detailed understanding of its capability to restore physiologic conditions at the biochemical and protein level, beyond the simple regulation of enzymatic activity. Targeted nanoparticles can promote protein delivery to the central nervous system and affect biological pathways inside cells. Here, we describe an innovative peptide-based stealth nanoparticle that inhibits serum protein adsorption exploiting transferrin-driven internalization to convey the PPT1 enzyme to transferrin receptor-mediated pathways (endocytosis in this work, or transcytosis, in perspective, in vivo). These enzyme-loaded nanoparticles were able to restore stable levels of enzymatic activity in CLN1 patient's fibroblasts, comparable with the free enzyme, demonstrating that delivery after encapsulation in the nanocarrier does not alter uptake or intracellular trafficking. We also investigate, for the first time, dysregulated pathways of proteome and palmitoylome and their alteration upon enzyme delivery. Our nanoparticles were able of halving palmitoylated protein levels restoring conditions similar to the normal cells. From proteomic analysis, we also highlighted the reduction of the different groups of proteins after treatments with the free or encapsulated enzyme. In conclusion, our system is able to deliver the enzyme to a model of CLN1 disease restoring normal conditions in cells. Investigation of molecular details of pathologic state and enzyme-based correction reveals dysregulated pathways with unprecedented details for CLN1. Finally, we unveil for the first time the dysregulation landscape of palmitoylome and proteome in primary patient-derived fibroblasts and their modifications in response to enzyme administration. These findings will provide a guideline for the validation of future therapeutic strategies based on enzyme replacement therapy or acting at different metabolic levels.

Allosteric regulation of lysosomal enzyme recognition by the cation-independent mannose 6-phosphate receptor

The cation-independent mannose 6-phosphate receptor (CI-MPR, IGF2 receptor or CD222), is a multifunctional glycoprotein required for normal development. Through the receptor's ability to bind unrelated extracellular and intracellular ligands, it participates in numerous functions including protein trafficking, lysosomal biogenesis, and regulation of cell growth. Clinically, endogenous CI-MPR delivers infused recombinant enzymes to lysosomes in the treatment of lysosomal storage diseases. Although four of the 15 domains comprising CI-MPR's extracellular region bind phosphorylated glycans on lysosomal enzymes, knowledge of how CI-MPR interacts with ~60 different lysosomal enzymes is limited. Here, we show by electron microscopy and hydroxyl radical protein footprinting that the N-terminal region of CI-MPR undergoes dynamic conformational changes as a consequence of ligand binding and different pH conditions. These data, coupled with X-ray crystallography, surface plasmon resonance and molecular modeling, allow us to propose a model explaining how high-affinity carbohydrate binding is achieved through allosteric domain cooperativity.

Bi-functional IgG-lysosomal enzyme fusion proteins for brain drug delivery

Most lysosomal storage disorders affect the central nervous system. However, lysosomal enzymes do not cross the blood-brain barrier (BBB), and intravenous enzyme infusion is not effective for the brain. Lysosomal enzymes can be re-engineered for BBB transport as IgG-enzyme fusion proteins, where the IgG domain is a monoclonal antibody (MAb) against an endogenous BBB receptor/transporter, and which acts as a molecular Trojan horse to deliver the enzyme to brain. However, the problem is retention of high enzyme activity following enzyme fusion to the IgG. The present investigation shows this is possible with a versatile approach that employs fusion of the enzyme to either the IgG heavy chain or light chain using a long flexible linker. The model IgG is a chimeric monoclonal antibody (MAb) against the human insulin receptor (HIR). The enzyme activity of the HIRMAb-enzyme fusion protein is preserved for hexosaminidase A, which is mutated in Tay Sachs disease, for protein palmitoylthioesterase-1, which is mutated in Batten disease type 1, acid sphingomyelinase, which is mutated in Niemann Pick disease type A, and beta galactosidase-1, which is mutated in GM1 gangliosidosis.

Therapeutic landscape for Batten disease: current treatments and future prospects

Batten disease (also known as neuronal ceroid lipofuscinoses) constitutes a family of devastating lysosomal storage disorders that collectively represent the most common inherited paediatric neurodegenerative disorders worldwide. Batten disease can result from mutations in 1 of 13 genes. These mutations lead to a group of diseases with loosely overlapping symptoms and pathology. Phenotypically, patients with Batten disease have visual impairment and blindness, cognitive and motor decline, seizures and premature death. Pathologically, Batten disease is characterized by lysosomal accumulation of autofluorescent storage material, glial reactivity and neuronal loss. Substantial progress has been made towards the development of effective therapies and treatments for the multiple forms of Batten disease. In 2017, cerliponase alfa (Brineura), a tripeptidyl peptidase enzyme replacement therapy, became the first globally approved treatment for CLN2 Batten disease. Here, we provide an overview of the promising therapeutic avenues for Batten disease, highlighting current FDA-approved clinical trials and prospective future treatments.

Emerging new roles of the lysosome and neuronal ceroid lipofuscinoses

Neuronal Ceroid Lipofuscinoses (NCLs), commonly known as Batten disease, constitute a group of the most prevalent neurodegenerative lysosomal storage disorders (LSDs). Mutations in at least 13 different genes (called CLNs) cause various forms of NCLs. Clinically, the NCLs manifest early impairment of vision, progressive decline in cognitive and motor functions, seizures and a shortened lifespan. At the cellular level, all NCLs show intracellular accumulation of autofluorescent material (called ceroid) and progressive neuron loss. Despite intense studies the normal physiological functions of each of the CLN genes remain poorly understood. Consequently, the development of mechanism-based therapeutic strategies remains challenging. Endolysosomal dysfunction contributes to pathogenesis of virtually all LSDs. Studies within the past decade have drastically changed the notion that the lysosomes are merely the terminal degradative organelles. The emerging new roles of the lysosome include its central role in nutrient-dependent signal transduction regulating metabolism and cellular proliferation or quiescence. In this review, we first provide a brief overview of the endolysosomal and autophagic pathways, lysosomal acidification and endosome-lysosome and autophagosome-lysosome fusions. We emphasize the importance of these processes as their dysregulation leads to pathogenesis of many LSDs including the NCLs. We also describe what is currently known about each of the 13 CLN genes and their products and how understanding the emerging new roles of the lysosome may clarify the underlying pathogenic mechanisms of the NCLs. Finally, we discuss the current and emerging therapeutic strategies for various NCLs.

Neural stem cells for disease modeling and evaluation of therapeutics for infantile (CLN1/PPT1) and late infantile (CLN2/TPP1) neuronal ceroid lipofuscinoses

BACKGROUND

Infantile and late infantile neuronal ceroid lipofuscinoses (NCLs) are lysosomal storage diseases affecting the central nervous system (CNS). The infantile NCL (INCL) is caused by mutations in the PPT1 gene and late-infantile NCL (LINCL) is due to mutations in the TPP1 gene. Deficiency in PPT1 or TPP1 enzyme function results in lysosomal accumulation of pathological lipofuscin-like material in the patient cells. There is currently no small-molecular drug treatment for NCLs.

RESULTS

We have generated induced pluripotent stem cells (iPSC) from three patient dermal fibroblast lines and further differentiated them into neural stem cells (NSCs). Using these new disease models, we evaluated the effect of δ-tocopherol (DT) and hydroxypropyl-β-cyclodextrin (HPBCD) with the enzyme replacement therapy as the control. Treatment with the relevant recombinant enzyme or DT significantly ameliorated the lipid accumulation and lysosomal enlargement in the disease cells. A combination therapy of δ-tocopherol and HPBCD further improved the effect compared to that of either drug used as a single therapy.

CONCLUSION

The results demonstrate that these patient iPSC derived NCL NSCs are valid cell- based disease models with characteristic disease phenotypes that can be used for study of disease pathophysiology and drug development.

Gene Therapy Approaches to Treat the Neurodegeneration and Visual Failure in Neuronal Ceroid Lipofuscinoses

Neuronal ceroid lipofuscinoses (NCLs) are a group of fatal, inherited lysosomal storage disorders mostly affecting the central nervous system of children. Symptoms include vision loss, seizures, motor deterioration and cognitive decline ultimately resulting in premature death. Studies in animal models showed that the diseases are amenable to gene supplementation therapies, and over the last decade, major advances have been made in the (pre)clinical development of these therapies. This mini-review summarises and discusses current gene therapy approaches for NCL targeting the brain and the eye.

Synergistic effects of treating the spinal cord and brain in CLN1 disease

Infantile neuronal ceroid lipofuscinosis (INCL, or CLN1 disease) is an inherited neurodegenerative storage disorder caused by a deficiency of the lysosomal enzyme palmitoyl protein thioesterase 1 (PPT1). It was widely believed that the pathology associated with INCL was limited to the brain, but we have now found unexpectedly profound pathology in the human INCL spinal cord. Similar pathological changes also occur at every level of the spinal cord of PPT1-deficient (Ppt1^-/- ) mice before the onset of neuropathology in the brain. Various forebrain-directed gene therapy approaches have only had limited success in Ppt1^-/- mice. Targeting the spinal cord via intrathecal administration of an adeno-associated virus (AAV) gene transfer vector significantly prevented pathology and produced significant improvements in life span and motor function in Ppt1^-/- mice. Surprisingly, forebrain-directed gene therapy resulted in essentially no PPT1 activity in the spinal cord, and vice versa. This leads to a reciprocal pattern of histological correction in the respective tissues when comparing intracranial with intrathecal injections. However, the characteristic pathological features of INCL were almost completely absent in both the brain and spinal cord when intracranial and intrathecal injections of the same AAV vector were combined. Targeting both the brain and spinal cord also produced dramatic and synergistic improvements in motor function with an unprecedented increase in life span. These data show that spinal cord pathology significantly contributes to the clinical progression of INCL and can be effectively targeted therapeutically. This has important implications for the delivery of therapies in INCL, and potentially in other similar disorders.

Safety and potential efficacy of gemfibrozil as a supportive treatment for children with late infantile neuronal ceroid lipofuscinosis and other lipid storage disorders

Neuronal Ceroid Lipofuscinosis (NCL), also known as Batten disease, is a group of genetically distinct lysosomal disorders that mainly affect the central nervous system, resulting in progressive motor and cognitive decline primarily in children. Multiple distinct genes involved in the metabolism of lipids have been identified to date with various mutations in this family of diseases. There is no cure for these diseases but some new therapeutic approaches have been tested that offer more hope than the standard palliative care. Many of the therapeutic advances require invasive procedures but some progress in slowing the disease has been found and more options can be expected in the future. We also review the literature on children with disease/conditions other than NCL for the non-invasive use, safety, and tolerability of a lipid-lowering drug, gemfibrozil, as a potential treatment for NCLs. Gemfibrozil has shown efficacy in an animal model of NCL known as CLN2 (late infantile classic juvenile) and has been shown to be safe for lowering lipids in children. Among the 200 non-NCL children found in the published literature who were treated with gemfibrozil for NCL-related problems, only 3 experienced adverse events, including 2 with muscle pain and 1 with localized linear IgA bullous dermatitis. We conclude that gemfibrozil is safe for long-term use in children, causes minimal adverse events, is well tolerated, and may delay the progression of NCLs. Gemfibrozil may potentially be an alternative to more invasive therapeutic approaches currently under investigation and has the potential to be used in combination with other therapeutic approaches.

Using Patient-Specific Induced Pluripotent Stem Cells and Wild-Type Mice to Develop a Gene Augmentation-Based Strategy to Treat CLN3-Associated Retinal Degeneration

Juvenile neuronal ceroid lipofuscinosis (JNCL) is a childhood neurodegenerative disease with early-onset, severe central vision loss. Affected children develop seizures and CNS degeneration accompanied by severe motor and cognitive deficits. There is no cure for JNCL, and patients usually die during the second or third decade of life. In this study, independent lines of induced pluripotent stem cells (iPSCs) were generated from two patients with molecularly confirmed mutations in CLN3, the gene mutated in JNCL. Clinical-grade adeno-associated adenovirus serotype 2 (AAV2) carrying the full-length coding sequence of human CLN3 was generated in a U.S. Food and Drug Administration-registered cGMP facility. AAV2-CLN3 was efficacious in restoring full-length CLN3 transcript and protein in patient-specific fibroblasts and iPSC-derived retinal neurons. When injected into the subretinal space of wild-type mice, purified AAV2-CLN3 did not show any evidence of retinal toxicity. This study provides proof-of-principle for initiation of a clinical trial using AAV-mediated gene augmentation for the treatment of children with CLN3-associated retinal degeneration.

Moving towards effective therapeutic strategies for Neuronal Ceroid Lipofuscinosis

The Neuronal Ceroid Lipofuscinoses (NCLs) are a family of autosomal recessive neurodegenerative disorders that annually affect 1:100,000 live births worldwide. This family of diseases results from mutations in one of 14 different genes that share common clinical and pathological etiologies. Clinically, the diseases are subcategorized into infantile, late-infantile, juvenile and adult forms based on their age of onset. Though the disease phenotypes may vary in their age and order of presentation, all typically include progressive visual deterioration and blindness, cognitive impairment, motor deficits and seizures. Pathological hallmarks of NCLs include the accumulation of storage material or ceroid in the lysosome, progressive neuronal degeneration and massive glial activation. Advances have been made in genetic diagnosis and counseling for families. However, comprehensive treatment programs that delay or halt disease progression have been elusive. Current disease management is primarily targeted at controlling the symptoms rather than "curing" the disease. Recognizing the growing need for transparency and synergistic efforts to move the field forward, this review will provide an overview of the therapeutic approaches currently being pursued in preclinical and clinical trials to treat different forms of NCL as well as provide insight to novel therapeutic approaches in development for the NCLs.

Histochemical localization of palmitoyl protein thioesterase-1 activity

Infantile neuronal ceroid lipofuscinosis (INCL, Infantile Batten disease) is an invariably fatal neurodegenerative pediatric disorder caused by an inherited mutation in the PPT1 gene. Patients with INCL lack the lysosomal enzyme palmitoyl protein thioesterase-1 (PPT1, EC 3.1.2.22), resulting in intracellular accumulation of autofluorescent storage material and subsequent neuropathology. The Ppt1(-/-) mouse is deficient in PPT1 activity and represents a useful animal model of INCL that recapitulates most of the clinical and pathological aspects of the disease. Preclinical therapeutic experiments performed in the INCL mouse include CNS-directed gene therapy and recombinant enzyme replacement therapy; both seek to re-establish therapeutic levels of the deficient enzyme. We present a novel method for the histochemical localization of PPT1 activity in the Ppt1(-/-) mouse. By utilizing the substrate CUS-9235, tissues known to be positive for PPT1 activity turn varying intensities of blue. Presented here are histochemistry data showing the staining pattern in Ppt1(-/-), wild type, and Ppt1(-/-) mice treated with enzyme replacement therapy or AAV2/9-PPT1-mediated gene therapy. Results are paired with quantitative biochemistry data that confirm the ability of CUS-9235 to detect and localize PPT1 activity. This new method complements the current tools for the study of INCL and evaluation of effective therapies.

Reversible Cysteine Acylation Regulates the Activity of Human Palmitoyl-Protein Thioesterase 1 (PPT1)

Mutations in the depalmitoylating enzyme gene, PPT1, cause the infantile form of Neuronal Ceroid Lipofuscinosis (NCL), an early onset neurodegenerative disease. During recent years there have been different therapeutic attempts including enzyme replacement. Here we show that PPT1 is palmitoylated in vivo and is a substrate for two palmitoylating enzymes, DHHC3 and DHHC7. The palmitoylated protein is detected in both cell lysates and medium. The presence of PPT1 with palmitoylated signal peptide in the cell medium suggests that a subset of the protein is secreted by a nonconventional mechanism. Using a mutant form of PPT1, C6S, which was not palmitoylated, we further demonstrate that palmitoylation does not affect intracellular localization but rather that the unpalmitoylated form enhanced the depalmitoylation activity of the protein. The calculated Vmax of the enzyme was significantly affected by the palmitoylation, suggesting that the addition of a palmitate group is reminiscent of adding a noncompetitive inhibitor. Thus, we reveal the existence of a positive feedback loop, where palmitoylation of PPT1 results in decreased activity and subsequent elevation in the amount of palmitoylated proteins. This positive feedback loop is likely to initiate a vicious cycle, which will enhance disease progression. The understanding of this process may facilitate enzyme replacement strategies.

Intrathecal enzyme replacement therapy improves motor function and survival in a preclinical mouse model of infantile neuronal ceroid lipofuscinosis

The neuronal ceroid lipofuscinoses (NCLs) are a group of related hereditary lysosomal storage disorders characterized by progressive loss of neurons in the central nervous system resulting in dementia, loss of motor skills, seizures and blindness. A characteristic intralysosomal accumulation of autofluorescent storage material occurs in the brain and other tissues. Three major forms and nearly a dozen minor forms of NCL are recognized. Infantile-onset NCL (CLN1 disease) is caused by severe deficiency in a soluble lysosomal enzyme, palmitoyl-protein thioesterase-1 (PPT1) and no therapy beyond supportive care is available. Homozygous Ppt1 knockout mice reproduce the known features of the disease, developing signs of motor dysfunction at 5 months of age and death around 8 months. Direct delivery of lysosomal enzymes to the cerebrospinal fluid is an approach that has gained traction in small and large animal models of several other neuropathic lysosomal storage diseases, and has advanced to clinical trials. In the current study, Ppt1 knockout mice were treated with purified recombinant human PPT1 enzyme delivered to the lumbar intrathecal space on each of three consecutive days at 6 weeks of age. Untreated PPT1 knockout mice and wild-type mice served as additional controls. Four enzyme concentration levels (0, 2.6, 5.3 and 10.6 mg/ml of specific activity 20 U/mg) were administered in a volume of 80 μl infused over 8 min. Each group consisted of 16-20 mice. The treatment was well tolerated. Disease-specific survival was 233, 267, 272, and 284days for each of the four treatment groups, respectively, and the effect of treatment was highly significant (p<0.0001). The timing of motor deterioration was also delayed. Neuropathology was improved as evidenced by decreased autofluorescent storage material in the spinal cord and a decrease in CD68 staining in the cortex and spinal cord. The improvements in motor function and survival are similar to results reported for preclinical studies involving other lysosomal storage disorders, such as CLN2/TPP1 deficiency, for which intraventricular ERT is being offered in clinical trials. If ERT delivery to the CSF proves to be efficacious in these disorders, PPT1 deficiency may also be amenable to this approach.

Towards a new understanding of NCL pathogenesis

The Neuronal Ceroid Lipofuscinoses (NCLs, Batten disease) are a group of inherited neurodegenerative disorders that have been traditionally grouped together on the basis of certain shared clinical and pathological features. However, as the number of genes that appear to cause new forms of NCL continues to grow, it is timely to reassess our understanding of the pathogenesis of these disorders and what groups them together. The various NCL subtypes do indeed share features of a build-up of autofluorescent storage material, progressive neuron loss and activation of the innate immune system. The characterisation of animal models has highlighted the selective nature of neuron loss and its intimate relationship with glial activation, rather than the generalised build-up of storage material. More recent data provide evidence for the pathway-dependent nature of pathology, the contribution of glial dysfunction, and the involvement of new brain regions previously thought to be unaffected, and it is becoming apparent that pathology extends beyond the brain. These data have important implications, not just for therapy, but also for our understanding of these disorders. However, looking beneath these broadly similar pathological themes evidence emerges for marked differences in the nature and extent of these events in different forms of NCL. Indeed, given the widely different nature of the mutated gene products it is perhaps more surprising that these disorders resemble each other as much as they do. Such data raise the question whether we should rethink the collective grouping of these gene deficiencies together, or whether it would be better to consider them as separate entities. This article is part of a Special Issue entitled: Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease).

Experimental therapies in the neuronal ceroid lipofuscinoses

The neuronal ceroid lipofuscinoses represent a group of severe childhood lysosomal storage diseases. With at least 13 identified variants they are the most common cause of inherited neurodegeneration in children. These diseases share common pathological characteristics including motor problems, vision loss, seizures, and cognitive decline, culminating in premature death. Currently, no form of the disease can be treated or cured, with only palliative care to minimise discomfort. This review focuses on current and potentially ground-breaking clinical trials, including small molecule, enzyme replacement, stem cell, and gene therapies, in the development of effective treatments for the various disease subtypes. This article is part of a Special Issue entitled: "Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease)".

Identification of palmitoyl protein thioesterase 1 in human THP1 monocytes and macrophages and characterization of unique biochemical activities for this enzyme

The profiles of serine hydrolases in human and mouse macrophages are similar yet different. For instance, human macrophages express high levels of carboxylesterase 1 (CES1), whereas mouse macrophages have minimal amounts of the orthologous murine CES1. On the other hand, macrophages from both species exhibit limited expression of the canonical 2-arachidonoylglycerol (2-AG) hydrolytic enzyme, MAGL. Our previous study showed CES1 was partly responsible for the hydrolysis of 2-AG (50%) and prostaglandin glyceryl esters (PG-Gs) (80-95%) in human THP1 monocytes and macrophages. However, MAGL and other endocannabinoid hydrolases, FAAH, ABHD6, and ABHD12, did not have a role because of limited expression or no expression. Thus, another enzyme was hypothesized to be responsible for the remaining 2-AG hydrolysis activity following chemical inhibition and immunodepletion of CES1 (previous study) or CES1 gene knockdown (this study). Here we identified two candidate serine hydrolases in THP1 cell lysates by activity-based protein profiling (ABPP)-MUDPIT and Western blotting: cathepsin G and palmitoyl protein thioesterase 1 (PPT1). Both proteins exhibited electrophoretic properties similar to those of a serine hydrolase in THP1 cells detected by gel-based ABPP at 31-32 kDa; however, only PPT1 exhibited lipolytic activity and hydrolyzed 2-AG in vitro. Interestingly, PPT1 was strongly expressed in THP1 cells but was significantly less reactive than cathepsin G toward the activity-based probe, fluorophosphonate-biotin. KIAA1363, another serine hydrolase, was also identified in THP1 cells but did not have significant lipolytic activity. On the basis of chemoproteomic profiling, immunodepletion studies, and chemical inhibitor profiles, we estimated that PPT1 contributed 32-40% of 2-AG hydrolysis activity in the THP1 cell line. In addition, pure recombinant PPT1 catalyzed the hydrolysis of 2-AG, PGE2-G, and PGF2α-G, although the catalytic efficiency of hydrolysis of 2-AG by PPT1 was ~10-fold lower than that of CES1. PPT1 was also insensitive to several chemical inhibitors that potently inhibit CES1, such as organophosphate poisons and JZL184. This is the first report to document the expression of PPT1 in a human monocyte and macrophage cell line and to show PPT1 can hydrolyze the natural substrates 2-AG and PG-Gs. These findings suggest that PPT1 may participate in endocannabinoid metabolism within specific cellular contexts and highlights the functional redundancy often exhibited by enzymes involved in lipid metabolism.

Neuroprotection and lifespan extension in Ppt1(-/-) mice by NtBuHA: therapeutic implications for INCL

Infantile neuronal ceroid lipofuscinosis (INCL) is a devastating childhood neurodegenerative lysosomal storage disease (LSD) that has no effective treatment. It is caused by inactivating mutations in the palmitoyl-protein thioesterase-1 (PPT1) gene. PPT1-deficiency impairs the cleavage of thioester linkage in palmitoylated proteins (constituents of ceroid), preventing degradation by lysosomal hydrolases. Consequently, accumulation of lysosomal ceroid leads to INCL. Thioester linkage is cleaved by nucleophilic attack. Hydroxylamine, a potent nucleophilic cellular metabolite, may have therapeutic potential for INCL but its toxicity precludes clinical application. Here we report that a hydroxylamine-derivative, N-(tert-Butyl) hydroxylamine (NtBuHA), is non-toxic, cleaves thioester linkage in palmitoylated proteins and mediates lysosomal ceroid depletion in cultured cells from INCL patients. Importantly, in Ppt1^−/− mice, which mimic INCL, NtBuHA crossed the blood-brain-barrier, depleted lysosomal ceroid, suppressed neuronal apoptosis, slowed neurological deterioration and extended lifespan. Our findings provide the proof of concept that thioesterase-mimetic and antioxidant small molecules like NtBuHA are potential drug-targets for thioesterase deficiency diseases like INCL.

Considerations for the treatment of infantile neuronal ceroid lipofuscinosis (infantile Batten disease)

The infantile form of neuronal ceroid lipofuscinosis (ie, infantile Batten disease) is the most rapidly progressing type and is caused by an inherited deficiency in the lysosomal enzyme palmitoyl protein thioesterase 1. The absence of enzyme activity leads to progressive accumulation of autofluorescent material in many cell types, particularly neurons of the central nervous system. Clinical signs of infantile neuronal ceroid lipofuscinosis appear between 6 months and 1 year of age and include vision loss, cognitive decline, motor deficits, seizures, and premature death, typically by 3 to 5 years of age. There is currently no effective treatment. However, preclinical experiments in the murine model of infantile neuronal ceroid lipofuscinosis have shown that gene therapy, enzyme replacement, stem cell transplantation, and small-molecule drugs, alone or in combination, can significantly slow disease progression. A more thorough understanding of the underlying pathogenesis of infantile neuronal ceroid lipofuscinosis will identify new therapeutic targets.

Pathogenesis and therapies for infantile neuronal ceroid lipofuscinosis (infantile CLN1 disease)

The neuronal ceroid lipofuscinoses (NCL, Batten disease) are a group of inherited neurodegenerative diseases. Infantile neuronal ceroid lipofuscinosis (INCL, infantile Batten disease, or infantile CLN1 disease) is caused by a deficiency in the soluble lysosomal enzyme palmitoyl protein thioesterase-1 (PPT1) and has the earliest onset and fastest progression of all the NCLs. Several therapeutic strategies including enzyme replacement, gene therapy, stem cell-mediated therapy, and small molecule drugs have resulted in minimal to modest improvements in the murine model of PPT1-deficiency. However, more recent studies using various combinations of these approaches have shown more promising results; in some instances more than doubling the lifespan of PPT1-deficient mice. These combination therapies that target different pathogenic mechanisms may offer the hope of treating this profoundly neurodegenerative disorder. Similar approaches may be useful when treating other forms of NCL caused by deficiencies in soluble lysosomal proteins. Different therapeutic targets will need to be identified and novel strategies developed in order to effectively treat forms of NCL caused by deficiencies in integral membrane proteins such as juvenile neuronal ceroid lipofuscinosis. Finally, the challenge with all of the NCLs will lie in early diagnosis, improving the efficacy of the treatments, and effectively translating them into the clinic. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

Use of model organisms for the study of neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses are a group of fatal progressive neurodegenerative diseases predominantly affecting children. Identification of mutations that cause neuronal ceroid lipofuscinosis, and subsequent functional and pathological studies of the affected genes, underpins efforts to investigate disease mechanisms and identify and test potential therapeutic strategies. These functional studies and pre-clinical trials necessitate the use of model organisms in addition to cell and tissue culture models as they enable the study of protein function within a complex organ such as the brain and the testing of therapies on a whole organism. To this end, a large number of disease models and genetic tools have been identified or created in a variety of model organisms. In this review, we will discuss the ethical issues associated with experiments using model organisms, the factors underlying the choice of model organism, the disease models and genetic tools available, and the contributions of those disease models and tools to neuronal ceroid lipofuscinosis research. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

Intravenous high-dose enzyme replacement therapy with recombinant palmitoyl-protein thioesterase reduces visceral lysosomal storage and modestly prolongs survival in a preclinical mouse model of infantile neuronal ceroid lipofuscinosis

PPT1-related neuronal ceroid lipofuscinosis (NCL) is a lysosomal storage disorder caused by deficiency in a soluble lysosomal enzyme, palmitoyl-protein thioesterase-1 (PPT1). Enzyme replacement therapy (ERT) has not been previously examined in a preclinical animal model. Homozygous PPT1 knockout mice reproduce the known features of the disease, developing signs of motor dysfunction at 5 months of age and death by around 8 months. In the current study, PPT1 knockout mice were treated with purified recombinant PPT1 (0.3 mg, corresponding to 12 mg/kg or 180 U/kg for a 25 g mouse) administered intravenously weekly either 1) from birth; or 2) beginning at 8 weeks of age. The treatment was surprisingly well tolerated and neither anaphylaxis nor antibody formation was observed. In mice treated from birth, survival increased from 236 to 271 days (p<0.001) and the onset of motor deterioration was similarly delayed. In mice treated beginning at 8 weeks, no increases in survival or motor performance were seen. An improvement in neuropathology in the thalamus was seen at 3 months in mice treated from birth, and although this improvement persisted it was attenuated by 7 months. Outside the central nervous system, substantial clearance of autofluorescent storage material in many tissues was observed. Macrophages in spleen, liver and intestine were especially markedly improved, as were acinar cells of the pancreas and tubular cells of the kidney. These findings suggest that ERT may be an option for addressing visceral storage as part of a comprehensive approach to PPT1-related NCL, but more effective delivery methods to target the brain are needed.

Mouse models of neuronal ceroid lipofuscinoses: useful pre-clinical tools to delineate disease pathophysiology and validate therapeutics

The neuronal ceroid lipofuscinoses (NCL, also known as Batten disease) is a devastating neurodegenerative diseases caused by mutations in either soluble enzymes or membrane-associated structural proteins that result in lysosome dysfunction. Different forms of NCL were defined initially by age of onset, affected population and/or type of storage material but collectively represent the most prevalent pediatric hereditary neurovisceral storage disorder. Specific gene mutations are now known for each subclass of NCL in humans that now largely define the disease: cathepsin D (CTSD) for congenital (CLN10 form); palmitoyl protein thioesterase 1 (PPT1) for infantile (CLN1 form); tripeptidyl peptidase 1 (TPP1) for classic late infantile (CLN2 form); variant late infantile-CLN5, CLN6 or CLN8 for variant late infantile forms; and CLN3 for juvenile (CLN3 form). Several mouse models of NCL have been developed, or in some cases exist sporadically, that exhibit mutations producing a progressive neurodegenerative phenotype similar to that observed in human NCL. The study of these mouse models of NCL has dramatically advanced our knowledge of NCL pathophysiology and in some cases has helped delineate the function of proteins mutated in human NCL. In addition, NCL mutant mice have been tested for several different therapeutic approaches and as such they have become important pre-clinical models for validating treatment options. In this review we will assess the current state of mouse models of NCL with regards to their unique pathophysiology and how these mice have helped investigators achieve a better understanding of human NCL disease and therapy.

Therapeutic approaches to the challenge of neuronal ceroid lipofuscinoses

The Neuronal Ceroid Lipofuscinoses (NCLs) are lysosomal storage diseases (LSDs) affecting the central nervous system (CNS), with generally recessive inheritance. They are characterized by pathological lipofuscin-like material accumulating in cells. The clinical phenotypes at all onset ages show progressive loss of vision, decreasing cognitive and motor skills, epileptic seizures and premature death, with dementia without visual loss prominent in the rarer adult forms. Eight causal genes, CLN10/CTSD, CLN1/PPT1, CLN2/TPP1, CLN3, CLN5, CLN6, CLN7/MFSD8, CLN8, with more than 265 mutations and 38 polymorphisms (http://www.ucl.ac.uk/ncl) have been described. Other NCL genes are hypothesized, including CLN4 and CLN9; CLCN6, CLCN7 and possibly SGSH are under study. Some therapeutic strategies applied to other LSDs with significant systemic involvement would not be effective in NCLs due to the necessity of passing the blood brain barrier to prevent the neurodegeneration, repair or restore the CNS functionality. There are therapies for the NCLs currently at preclinical stages and under phase 1 trials to establish safety in affected children. These approaches involve enzyme replacement, gene therapy, neural stem cell replacement, immune therapy and other pharmacological approaches. In the next decade, progress in the understanding of the natural history and the biochemical and molecular cascade of events relevant to the pathogenesis of these diseases in humans and animal models will be required to achieve significant therapeutic advances.

Interactions of the proteins of neuronal ceroid lipofuscinosis: clues to function

Neuronal ceroid lipofuscinoses (NCL) are caused by mutations in eight different genes, are characterized by lysosomal accumulation of autofluorescent storage material, and result in a disease that causes degeneration of the central nervous system (CNS). Although functions are defined for some of the soluble proteins that are defective in NCL (cathepsin D, PPT1, and TPP1), the primary function of the other proteins defective in NCLs (CLN3, CLN5, CLN6, CLN7, and CLN8) remain poorly defined. Understanding the localization and network of interactions for these proteins can offer clues as to the function of the NCL proteins and also the pathways that will be disrupted in their absence. Here, we present a review of the current understanding of the localization, interactions, and function of the proteins associated with NCL.