An iPSC model of hereditary sensory neuropathy-1 reveals L-serine-responsive deficits in neuronal ganglioside composition and axoglial interactions

Advances and challenges in modeling inherited peripheral neuropathies using iPSCs

Inherited peripheral neuropathies (IPNs) are a group of diseases associated with mutations in various genes with fundamental roles in the development and function of peripheral nerves. Over the past 10 years, significant advances in identifying molecular disease mechanisms underlying axonal and myelin degeneration, acquired from cellular biology studies and transgenic fly and rodent models, have facilitated the development of promising treatment strategies. However, no clinical treatment has emerged to date. This lack of treatment highlights the urgent need for more biologically and clinically relevant models recapitulating IPNs. For both neurodevelopmental and neurodegenerative diseases, patient-specific induced pluripotent stem cells (iPSCs) are a particularly powerful platform for disease modeling and preclinical studies. In this review, we provide an update on different in vitro human cellular IPN models, including traditional two-dimensional monoculture iPSC derivatives, and recent advances in more complex human iPSC-based systems using microfluidic chips, organoids, and assembloids.

Serine Palmitoyltransferase (SPT)-related Neurodegenerative and Neurodevelopmental Disorders

Motor neuron diseases and peripheral neuropathies are heterogeneous groups of neurodegenerative disorders that manifest with distinct symptoms due to progressive dysfunction or loss of specific neuronal subpopulations during different stages of development. A few monogenic, neurodegenerative diseases associated with primary metabolic disruptions of sphingolipid biosynthesis have been recently discovered. Sphingolipids are a subclass of lipids that form critical building blocks of all cellular and subcellular organelle membranes including the membrane components of the nervous system cells. They are especially abundant within the lipid portion of myelin. In this review, we will focus on our current understanding of disease phenotypes in three monogenic, neuromuscular diseases associated with pathogenic variants in components of serine palmitoyltransferase, the first step in sphingolipid biosynthesis. These include hereditary sensory and autonomic neuropathy type 1 (HSAN1), a sensory predominant peripheral neuropathy, and two neurodegenerative disorders: juvenile amyotrophic lateral sclerosis affecting the upper and lower motor neurons with sparing of sensory neurons, and a complicated form of hereditary spastic paraplegia with selective involvement of the upper motor neurons and more broad CNS neurodegeneration. We will also review our current understanding of disease pathomechanisms, therapeutic approaches, and the unanswered questions to explore in future studies.

Human genetic defects of sphingolipid synthesis

Sphingolipids are ubiquitous lipids, present in the membranes of all cell types, the stratum corneum and the circulating lipoproteins. Autosomal recessive as well as dominant diseases due to disturbed sphingolipid biosynthesis have been identified, including defects in the synthesis of ceramides, sphingomyelins and glycosphingolipids. In many instances, these gene variants result in the loss of catalytic function of the mutated enzymes. Additional gene defects implicate the subcellular localization of the sphingolipid-synthesizing enzyme, the regulation of its activity, or even the function of a sphingolipid-transporter protein. The resulting metabolic alterations lead to two major, non-exclusive types of clinical manifestations: a neurological disease, more or less rapidly progressive, associated or not with intellectual disability, and an ichthyotic-type skin disorder. These phenotypes highlight the critical importance of sphingolipids in brain and skin development and homeostasis. The present article reviews the clinical symptoms, genetic and biochemical alterations, pathophysiological mechanisms and therapeutic options of this relatively novel group of metabolic diseases.

A humanized chemogenetic system inhibits murine pain-related behavior and hyperactivity in human sensory neurons

Hyperexcitability in sensory neurons is known to underlie many of the maladaptive changes associated with persistent pain. Chemogenetics has shown promise as a means to suppress such excitability, yet chemogenetic approaches suitable for human applications are needed. PSAM4-GlyR is a modular system based on the human α7 nicotinic acetylcholine and glycine receptors, which responds to inert chemical ligands and the clinically approved drug varenicline. Here, we demonstrated the efficacy of this channel in silencing both mouse and human sensory neurons by the activation of large shunting conductances after agonist administration. Virally mediated expression of PSAM4-GlyR in mouse sensory neurons produced behavioral hyposensitivity upon agonist administration, which was recovered upon agonist washout. Stable expression of the channel led to similar reversible suppression of pain-related behavior even after 10 months of viral delivery. Mechanical and spontaneous pain readouts were also ameliorated by PSAM4-GlyR activation in acute and joint pain inflammation mouse models. Furthermore, suppression of mechanical hypersensitivity generated by a spared nerve injury model of neuropathic pain was also observed upon activation of the channel. Effective silencing of behavioral hypersensitivity was reproduced in a human model of hyperexcitability and clinical pain: PSAM4-GlyR activation decreased the excitability of human-induced pluripotent stem cell-derived sensory neurons and spontaneous activity due to a gain-of-function NaV1.7 mutation causing inherited erythromelalgia. Our results demonstrate the contribution of sensory neuron hyperexcitability to neuropathic pain and the translational potential of an effective, stable, and reversible humanized chemogenetic system for the treatment of pain.

Divergent amino acid and sphingolipid metabolism in patients with inherited neuro-retinal disease

OBJECTIVES

The non-essential amino acids serine, glycine, and alanine, as well as diverse sphingolipid species, are implicated in inherited neuro-retinal disorders and are metabolically linked by serine palmitoyltransferase (SPT), a key enzyme in membrane lipid biogenesis. To gain insight into the pathophysiological mechanisms linking these pathways to neuro-retinal diseases we compared patients diagnosed with two metabolically intertwined diseases: macular telangiectasia type II (MacTel), hereditary sensory autonomic neuropathy type 1 (HSAN1), or both.

METHODS

We performed targeted metabolomic analyses of amino acids and broad sphingolipids in sera from a cohort of MacTel (205), HSAN1 (25) and Control (151) participants.

RESULTS

MacTel patients exhibited broad alterations of amino acids, including changes in serine, glycine, alanine, glutamate, and branched-chain amino acids reminiscent of diabetes. MacTel patients had elevated 1-deoxysphingolipids but reduced levels of complex sphingolipids in circulation. A mouse model of retinopathy indicates dietary serine and glycine restriction can drive this depletion in complex sphingolipids. HSAN1 patients exhibited elevated serine, lower alanine, and a reduction in canonical ceramides and sphingomyelins compared to controls. Those patients diagnosed with both HSAN1 and MacTel showed the most significant decrease in circulating sphingomyelins.

CONCLUSIONS

These results highlight metabolic distinctions between MacTel and HSAN1, emphasize the importance of membrane lipids in the progression of MacTel, and suggest distinct therapeutic approaches for these two neurodegenerative diseases.

Specific Deoxyceramide Species Correlate with Expression of Macular Telangiectasia Type 2 (MacTel2) in a SPTLC2 Carrier HSAN1 Family

Hereditary sensory and autonomic neuropathy type 1 (HSAN1/HSN1) is a peripheral neuropathy most commonly associated with pathogenic variants in the serine palmitoyltransferase complex (SPTLC1, SPTLC2) genes, which are responsible for sphingolipid biosynthesis. Recent reports have shown that some HSAN1 patients also develop macular telangiectasia type 2 (MacTel2), a retinal neurodegeneration with an enigmatic pathogenesis and complex heritability. Here, we report a novel association of a SPTLC2 c.529A>G p.(Asn177Asp) variant with MacTel2 in a single member of a family that otherwise has multiple members afflicted with HSAN1. We provide correlative data to suggest that the variable penetrance of the HSAN1/MacTel2-overlap phenotype in the proband may be explained by levels of certain deoxyceramide species, which are aberrant intermediates of sphingolipid metabolism. We provide detailed retinal imaging of the proband and his HSAN1+/MacTel2- brothers and suggest mechanisms by which deoxyceramide levels may induce retinal degeneration. This is the first report of HSAN1 vs. HSAN1/MacTel2 overlap patients to comprehensively profile sphingolipid intermediates. The biochemical data here may help shed light on the pathoetiology and molecular mechanisms of MacTel2.

Insulin-regulated serine and lipid metabolism drive peripheral neuropathy

Diabetes represents a spectrum of disease in which metabolic dysfunction damages multiple organ systems including liver, kidneys and peripheral nerves1,2. Although the onset and progression of these co-morbidities are linked with insulin resistance, hyperglycaemia and dyslipidaemia3-7, aberrant non-essential amino acid (NEAA) metabolism also contributes to the pathogenesis of diabetes8-10. Serine and glycine are closely related NEAAs whose levels are consistently reduced in patients with metabolic syndrome10-14, but the mechanistic drivers and downstream consequences of this metabotype remain unclear. Low systemic serine and glycine are also emerging as a hallmark of macular and peripheral nerve disorders, correlating with impaired visual acuity and peripheral neuropathy15,16. Here we demonstrate that aberrant serine homeostasis drives serine and glycine deficiencies in diabetic mice, which can be diagnosed with a serine tolerance test that quantifies serine uptake and disposal. Mimicking these metabolic alterations in young mice by dietary serine or glycine restriction together with high fat intake markedly accelerates the onset of small fibre neuropathy while reducing adiposity. Normalization of serine by dietary supplementation and mitigation of dyslipidaemia with myriocin both alleviate neuropathy in diabetic mice, linking serine-associated peripheral neuropathy to sphingolipid metabolism. These findings identify systemic serine deficiency and dyslipidaemia as novel risk factors for peripheral neuropathy that may be exploited therapeutically.

Emergence of nociceptive functionality and opioid signaling in human induced pluripotent stem cell-derived sensory neurons

ABSTRACT

Induced pluripotent stem cells (iPSCs) have enabled the generation of various difficult-to-access cell types such as human nociceptors. A key challenge associated with human iPSC-derived nociceptors (hiPSCdNs) is their prolonged functional maturation. While numerous studies have addressed the expression of classic neuronal markers and ion channels in hiPSCdNs, the temporal development of key signaling cascades regulating nociceptor activity has remained largely unexplored. In this study, we used an immunocytochemical high-content imaging approach alongside electrophysiological staging to assess metabotropic and ionotropic signaling of large scale-generated hiPSCdNs across 70 days of in vitro differentiation. During this period, the resting membrane potential became more hyperpolarized, while rheobase, action potential peak amplitude, and membrane capacitance increased. After 70 days, hiPSCdNs exhibited robust physiological responses induced by GABA, pH shift, ATP, and capsaicin. Direct activation of protein kinase A type II (PKA-II) through adenylyl cyclase stimulation with forskolin resulted in PKA-II activation at all time points. Depolarization-induced activation of PKA-II emerged after 35 days of differentiation. However, effective inhibition of forskolin-induced PKA-II activation by opioid receptor agonists required 70 days of in vitro differentiation. Our results identify a pronounced time difference between early expression of functionally important ion channels and emergence of regulatory metabotropic sensitizing and desensitizing signaling only at advanced stages of in vitro cultivation, suggesting an independent regulation of ionotropic and metabotropic signaling. These data are relevant for devising future studies into the development and regulation of human nociceptor function and for defining time windows suitable for hiPSCdN-based drug discovery.

In vitro models for investigating itch

Itch (pruritus) is a sensation that drives a desire to scratch, a behavior observed in many animals. Although generally short-lasting and not causing harm, there are several pathological conditions where chronic itch is a hallmark symptom and in which prolonged scratching can induce damage. Finding medications to counteract the sensation of chronic itch has proven difficult due to the molecular complexity that involves a multitude of triggers, receptors and signaling pathways between skin, immune and nerve cells. While much has been learned about pruritus from in vivo animal models, they have limitations that corroborate the necessity for a transition to more human disease-like models. Also, reducing animal use should be encouraged in research. However, conducting human in vivo experiments can also be ethically challenging. Thus, there is a clear need for surrogate models to be used in pre-clinical investigation of the mechanisms of itch. Most in vitro models used for itch research focus on the use of known pruritogens. For this, sensory neurons and different types of skin and/or immune cells are stimulated in 2D or 3D co-culture, and factors such as neurotransmitter or cytokine release can be measured. There are however limitations of such simplistic in vitro models. For example, not all naturally occurring cell types are present and there is also no connection to the itch-sensing organ, the central nervous system (CNS). Nevertheless, in vitro models offer a chance to investigate otherwise inaccessible specific cell–cell interactions and molecular pathways. In recent years, stem cell-based approaches and human primary cells have emerged as viable alternatives to standard cell lines or animal tissue. As in vitro models have increased in their complexity, further opportunities for more elaborated means of investigating itch have been developed. In this review, we introduce the latest concepts of itch and discuss the advantages and limitations of current in vitro models, which provide valuable contributions to pruritus research and might help to meet the unmet clinical need for more refined anti-pruritic substances.

Genetic pain loss disorders

Genetic pain loss includes congenital insensitivity to pain (CIP), hereditary sensory neuropathies and, if autonomic nerves are involved, hereditary sensory and autonomic neuropathy (HSAN). This heterogeneous group of disorders highlights the essential role of nociception in protecting against tissue damage. Patients with genetic pain loss have recurrent injuries, burns and poorly healing wounds as disease hallmarks. CIP and HSAN are caused by pathogenic genetic variants in >20 genes that lead to developmental defects, neurodegeneration or altered neuronal excitability of peripheral damage-sensing neurons. These genetic variants lead to hyperactivity of sodium channels, disturbed haem metabolism, altered clathrin-mediated transport and impaired gene regulatory mechanisms affecting epigenetic marks, long non-coding RNAs and repetitive elements. Therapies for pain loss disorders are mainly symptomatic but the first targeted therapies are being tested. Conversely, chronic pain remains one of the greatest unresolved medical challenges, and the genes and mechanisms associated with pain loss offer new targets for analgesics. Given the progress that has been made, the coming years are promising both in terms of targeted treatments for pain loss disorders and the development of innovative pain medicines based on knowledge of these genetic diseases.

Towards bridging the translational gap by improved modeling of human nociception in health and disease

Despite numerous studies which have explored the pathogenesis of pain disorders in preclinical models, there is a pronounced translational gap, which is at least partially caused by differences between the human and rodent nociceptive system. An elegant way to bridge this divide is the exploitation of human-induced pluripotent stem cell (iPSC) reprogramming into human iPSC-derived nociceptors (iDNs). Several protocols were developed and optimized to model nociceptive processes in health and disease. Here we provide an overview of the different approaches and summarize the knowledge obtained from such models on pain pathologies associated with monogenetic sensory disorders so far. In addition, novel perspectives offered by increasing the complexity of the model systems further to better reflect the natural environment of nociceptive neurons by involving other cell types in 3D model systems are described.

Neurological update: hereditary neuropathies

In this update, we review the recent discovery of autosomal recessive variants in sorbitol dehydrogenase as one of the commonest and potentially treatable causes of hereditary motor neuropathy and CMT2. We also report on recent therapeutic advances in hereditary neuropathy including the use of lipid nanoparticle sequestered antisense oligonucleotides in CMT1A and lipid nanoparticle delivered CRISPR-Cas9 gene editing in ATTR amyloidosis.