Human mini brains and spinal cords in a dish: Modeling strategies, current challenges, and prospective advances.

Engineered three-dimensional (3D) in vitro and ex vivo neural tissues, also known as "mini brains and spinal cords in a dish," can be derived from different types of human stem cells via several differentiation protocols. In general, human mini brains are micro-scale physiological systems consisting of mixed populations of neural progenitor cells, glial cells, and neurons that may represent key features of human brain anatomy and function. To date, these specialized 3D tissue structures can be characterized into spheroids, organoids, assembloids, organ-on-a-chip and their various combinations based on generation procedures and cellular components. These 3D CNS models incorporate complex cell-cell interactions and play an essential role in bridging the gap between two-dimensional human neuroglial cultures and animal models. Indeed, they provide an innovative platform for disease modeling and therapeutic cell replacement, especially shedding light on the potential to realize personalized medicine for neurological disorders when combined with the revolutionary human induced pluripotent stem cell technology. In this review, we highlight human 3D CNS models developed from a variety of experimental strategies, emphasize their advances and remaining challenges, evaluate their state-of-the-art applications in recapitulating crucial phenotypic aspects of many CNS diseases, and discuss the role of contemporary technologies in the prospective improvement of their composition, consistency, complexity, and maturation.

Modeling gain-of-function and loss-of-function components of SPAST-based hereditary spastic paraplegia using transgenic mice.

Hereditary spastic paraplegia (HSP) is a disease in which dieback degeneration of corticospinal tracts, accompanied by axonal swellings, leads to gait deficiencies. SPG4-HSP, the most common form of the disease, results from mutations of human spastin gene (SPAST), which is the gene that encodes spastin, a microtubule-severing protein. The lack of a vertebrate model that recapitulates both the etiology and symptoms of SPG4-HSP has stymied the development of effective therapies for the disease. hSPAST-C448Y mice, which express human mutant spastin at the ROSA26 locus, display corticospinal dieback and gait deficiencies but not axonal swellings. On the other hand, mouse spastin gene (Spast)-knockout (KO) mice display axonal swellings but not corticospinal dieback or gait deficiencies. One possibility is that reduced spastin function, resulting in axonal swellings, is not the cause of the disease but exacerbates the toxic effects of the mutant protein. To explore this idea, Spast-KO and hSPAST-C448Y mice were crossbred, and the offspring were compared with the parental lines via histological and behavioral analyses. The crossbred animals displayed axonal swellings as well as earlier onset, worsened gait deficiencies and corticospinal dieback compared with the hSPAST-C448Y mouse. These results, together with observations on changes in histone deacetylases 6 and tubulin modifications in the axon, indicate that each of these three transgenic mouse lines is valuable for investigating a different component of the disease pathology. Moreover, the crossbred mice are the best vertebrate model to date for testing potential therapies for SPG4-HSP.

A cellular approach to understanding and treating Gulf War Illness.

Gulf War Illness (GWI), a disorder suffered by approximately 200,000 veterans of the first Gulf War, was caused by exposure to low-level organophosphate pesticides and nerve agents in combination with battlefield stress. To elucidate the mechanistic basis of the brain-related symptoms of GWI, human-induced pluripotent stem cells (hiPSCs) derived from veterans with or without GWI were differentiated into forebrain glutamatergic neurons and then exposed to a Gulf War (GW) relevant toxicant regimen consisting of a sarin analog and cortisol, a human stress hormone. Elevated levels of total and phosphorylated tau, reduced microtubule acetylation, altered mitochondrial dynamics/transport, and decreased neuronal activity were observed in neurons exposed to the toxicant regimen. Some of the data are consistent with the possibility that some veterans may have been predisposed to acquire GWI. Wistar rats exposed to a similar toxicant regimen showed a mild learning and memory deficit, as well as cell loss and tau pathology selectively in the CA3 region of the hippocampus. These cellular responses offer a mechanistic explanation for the memory loss suffered by veterans with GWI and provide a cell-based model for screening drugs and developing personalized therapies for these veterans.

New hypothesis for the etiology of SPAST-based hereditary spastic paraplegia.

Mutations of the SPAST gene are the chief cause of hereditary spastic paraplegia. Controversy exists in the medical community as to whether the etiology of the disease is haploinsufficiency or toxic gain-of-function properties of the mutant spastin proteins. In recognition of strong reasons that support each possible mechanism, here we present a novel perspective, based in part on new studies with mouse models and in part on the largest study to date on patients with the disease. We posit that haploinsufficiency does not cause the disease but makes the corticospinal tracts vulnerable to a second hit, which is usually the mutant spastin proteins but could also be proteins generated by mutations of other genes that may or may not cause the disease on their own.

Hereditary spastic paraplegia: gain-of-function mechanisms revealed by new transgenic mouse.

Mutations of the SPAST gene, which encodes the microtubule-severing protein spastin, are the most common cause of hereditary spastic paraplegia (HSP). Haploinsufficiency is the prevalent opinion as to the mechanism of the disease, but gain-of-function toxicity of the mutant proteins is another possibility. Here, we report a new transgenic mouse (termed SPASTC448Y mouse) that is not haploinsufficient but expresses human spastin bearing the HSP pathogenic C448Y mutation. Expression of the mutant spastin was documented from fetus to adult, but gait defects reminiscent of HSP (not observed in spastin knockout mice) were adult onset, as is typical of human patients. Results of histological and tracer studies on the mouse are consistent with progressive dying back of corticospinal axons, which is characteristic of the disease. The C448Y-mutated spastin alters microtubule stability in a manner that is opposite to the expectations of haploinsufficiency. Neurons cultured from the mouse display deficits in organelle transport typical of axonal degenerative diseases, and these deficits were worsened by depletion of endogenous mouse spastin. These results on the SPASTC448Y mouse are consistent with a gain-of-function mechanism underlying HSP, with spastin haploinsufficiency exacerbating the toxicity of the mutant spastin proteins. These findings reveal the need for a different therapeutic approach than indicated by haploinsufficiency alone.

Nrf2-Inducers Counteract Neurodegeneration in Frataxin-Silenced Motor Neurons: Disclosing New Therapeutic Targets for Friedreich's Ataxia.

Oxidative stress is actively involved in Friedreich's Ataxia (FA), thus pharmacological targeting of the antioxidant machinery may have therapeutic value. Here, we analyzed the relevance of the antioxidant phase II response mediated by the transcription factor Nrf2 on frataxin-deficient cultured motor neurons and on fibroblasts of patients. The in vitro treatment of the potent Nrf2 activator sulforaphane increased Nrf2 protein levels and led to the upregulation of phase II antioxidant enzymes. The neuroprotective effects were accompanied by an increase in neurites' number and extension. Sulforaphane (SFN) is a natural compound of many diets and is now being used in clinical trials for other pathologies. Our results provide morphological and biochemical evidence to endorse a neuroprotective strategy that may have therapeutic relevance for FA. The findings of this work reinforce the crucial importance of Nrf2 in FA and provide a rationale for using Nrf2-inducers as pharmacological agents.

Cytoskeletal dynamics during in vitro neurogenesis of induced pluripotent stem cells (iPSCs).

Patient-derived induced pluripotent stem cells (iPSCs) provide a novel tool to investigate the pathophysiology of poorly known diseases, in particular those affecting the nervous system, which has been difficult to study for its lack of accessibility. In this emerging and promising field, recent iPSCs studies are mostly used as "proof-of-principle" experiments that are confirmatory of previous findings obtained from animal models and postmortem human studies; its promise as a discovery tool is just beginning to be realized. A recent number of studies point to the functional similarities between in vitro neurogenesis and in vivo neuronal development, suggesting that similar morphogenetic and patterning events direct neuronal differentiation. In this context, neuronal adhesion, cytoskeletal organization and cell metabolism emerge as an integrated and unexplored processes of human neurogenesis, mediated by the lack of data due to the difficult accessibility of the human neural tissue. These observations raise the necessity to understand which are the players controlling cytoskeletal reorganization and remodeling. In particular, we investigated human in vitro neurogenesis using iPSCs of healthy subjects to unveil the underpinnings of the cytoskeletal dynamics with the aim to shed light on the physiologic events controlling the development and the functionality of neuronal cells. We validate the iPSCs system to better understand the development of the human nervous system in order to set the bases for the future understanding of pathologies including developmental disorders (i.e. intellectual disability), epilepsy but also neurodegenerative disorders (i.e. Friedreich's Ataxia). We investigate the changes of the cytoskeletal components during the 30days of neuronal differentiation and we demonstrate that human neuronal differentiation requires a (time-dependent) reorganization of actin filaments, intermediate filaments and microtubules; and that immature neurons present a finely regulated localization of Glu-, Tyr- and Acet-TUBULINS. This study advances our understanding on cytoskeletal dynamics with the hope to pave the way for future therapies that could be potentially able to target cytoskeletal based neurodevelopmental and neurodegenerative diseases.

TBCE Mutations Cause Early-Onset Progressive Encephalopathy with Distal Spinal Muscular Atrophy.

Tubulinopathies constitute a family of neurodevelopmental/neurodegenerative disorders caused by mutations in several genes encoding tubulin isoforms. Loss-of-function mutations in TBCE, encoding one of the five tubulin-specific chaperones involved in tubulin folding and polymerization, cause two rare neurodevelopmental syndromes, hypoparathyroidism-retardation-dysmorphism and Kenny-Caffey syndrome. Although a missense mutation in Tbce has been associated with progressive distal motor neuronopathy in the pmn/pmn mice, no similar degenerative phenotype has been recognized in humans. We report on the identification of an early-onset and progressive neurodegenerative encephalopathy with distal spinal muscular atrophy resembling the phenotype of pmn/pmn mice and caused by biallelic TBCE mutations, with the c.464T>A (p.Ile155Asn) change occurring at the heterozygous/homozygous state in six affected subjects from four unrelated families originated from the same geographical area in Southern Italy. Western blot analysis of patient fibroblasts documented a reduced amount of TBCE, suggestive of rapid degradation of the mutant protein, similarly to what was observed in pmn/pmn fibroblasts. The impact of TBCE mutations on microtubule polymerization was determined using biochemical fractionation and analyzing the nucleation and growth of microtubules at the centrosome and extracentrosomal sites after treatment with nocodazole. Primary fibroblasts obtained from affected subjects displayed a reduced level of polymerized α-tubulin, similarly to tail fibroblasts of pmn/pmn mice. Moreover, markedly delayed microtubule re-polymerization and abnormal mitotic spindles with disorganized microtubule arrangement were also documented. Although loss of function of TBCE has been documented to impact multiple developmental processes, the present findings provide evidence that hypomorphic TBCE mutations primarily drive neurodegeneration.

Biallelic Mutations in TBCD, Encoding the Tubulin Folding Cofactor D, Perturb Microtubule Dynamics and Cause Early-Onset Encephalopathy.

Microtubules are dynamic cytoskeletal elements coordinating and supporting a variety of neuronal processes, including cell division, migration, polarity, intracellular trafficking, and signal transduction. Mutations in genes encoding tubulins and microtubule-associated proteins are known to cause neurodevelopmental and neurodegenerative disorders. Growing evidence suggests that altered microtubule dynamics may also underlie or contribute to neurodevelopmental disorders and neurodegeneration. We report that biallelic mutations in TBCD, encoding one of the five co-chaperones required for assembly and disassembly of the αß-tubulin heterodimer, the structural unit of microtubules, cause a disease with neurodevelopmental and neurodegenerative features characterized by early-onset cortical atrophy, secondary hypomyelination, microcephaly, thin corpus callosum, developmental delay, intellectual disability, seizures, optic atrophy, and spastic quadriplegia. Molecular dynamics simulations predicted long-range and/or local structural perturbations associated with the disease-causing mutations. Biochemical analyses documented variably reduced levels of TBCD, indicating relative instability of mutant proteins, and defective ß-tubulin binding in a subset of the tested mutants. Reduced or defective TBCD function resulted in decreased soluble α/ß-tubulin levels and accelerated microtubule polymerization in fibroblasts from affected subjects, demonstrating an overall shift toward a more rapidly growing and stable microtubule population. These cells displayed an aberrant mitotic spindle with disorganized, tangle-shaped microtubules and reduced aster formation, which however did not alter appreciably the rate of cell proliferation. Our findings establish that defective TBCD function underlies a recognizable encephalopathy and drives accelerated microtubule polymerization and enhanced microtubule stability, underscoring an additional cause of altered microtubule dynamics with impact on neuronal function and survival in the developing brain.

Frataxin silencing alters microtubule stability in motor neurons: implications for Friedreich's ataxia.

To elucidate the pathogenesis of axonopathy in Friedreich's Ataxia (FRDA), a neurodegenerative disease characterized by axonal retraction, we analyzed the microtubule (MT) dynamics in an in vitro frataxin-silenced neuronal model (shFxn). A typical feature of MTs is their "dynamic instability", in which they undergo phases of growth (polymerization) and shrinkage (depolymerization). MTs play a fundamental role in the physiology of neurons and every perturbation of their dynamicity is highly detrimental for neuronal functions. The aim of this study is to determine whether MTs are S-glutathionylated in shFxn and if the glutathionylation triggers MT dysfunction. We hypothesize that oxidative stress, determined by high GSSG levels, induces axonal retraction by interfering with MT dynamics. We propose a mechanism of the axonopathy in FRDA where GSSG overload and MT de-polymerization are strictly interconnected. Indeed, using a frataxin-silenced neuronal model we show a significant reduction of neurites extension, a shift of tubulin toward the unpolymerized fraction and a consistent increase of glutathione bound to the cytoskeleton. The live cell imaging approach further reveals a significant decrease in MT growth lifetime due to frataxin silencing, which is consistent with the MT destabilization. The in vitro antioxidant treatments trigger the axonal re-growth and the increase in stable MTs in shFxn, thus contributing to identify new neuronal targets of oxidation in this disease and providing a novel approach for antioxidant therapies.

High concentrations of H2O2 trigger hypertrophic cascade and phosphatase and tensin homologue (PTEN) glutathionylation in H9c2 cardiomyocytes.

Cardiac hypertrophy occurs in response to different stimuli and is mainly characterized by an enlargement of cardiomyocyte size. During hypertrophy, cardiomyocytes undergo not only radical changes of the cellular architecture but also activation of signaling cascades that counteract the atrophy genes. Experimental studies highlighted that chronic low concentrations of H2O2, induce a hypertrophic phenotype, while higher levels of H2O2 promote apoptosis. In this study, we explored the early and long-term hypertrophic effects of high concentrations of H2O2 on H9c2 rat cardiomyocytes. We found that 2-h stimulation with 200µM H2O2 caused an early dramatic reduction of cell viability, accompanied, 5-days later, by increased cell size and up-regulation of atrial natriuretic peptide transcription. This hypertrophic phenotype is associated to increased Akt phosphorylation and a consequent reduction of the FOXO3a and atrogin-1 gene expression. Moreover, we observed that H2O2 caused the overexpression of miR-212/miR-132 cluster concomitantly to a down-regulation of PTEN transcript without changes in its protein expression. Noteworthy, we found that the treatment of cardiomyocytes with H2O2 further led to an increase of oxidized glutathione and glutathionylation of proteins, including PTEN. In conclusion, our results permit to reconstruct the molecular cascade triggering the cardiomyocyte hypertrophy upon high concentrations of H2O2.

The cytoskeletal arrangements necessary to neurogenesis.

During the process of neurogenesis, the stem cell committed to the neuronal cell fate starts a series of molecular and morphological changes. The understanding of the physio-pathology of mechanisms controlling the molecular and morphological changes occurring during neuronal differentiation is fundamental to the development of effective therapies for many neurologic diseases. Unfortunately, our knowledge of the biological events occurring in the cell during neuronal differentiation is still poor. In this study, we focus preliminarily on the relevance of the cytoskeletal rearrangements, which earlier drive the morphology of the neuronal precursors, and later the migrating/mature neurons. In fact, neuritogenesis, neurite branching, outgrowth and retraction are seminal to the development of a fully functional nervous system. With this in mind, we highlight the importance of iPSC technology to study the processes of cytoskeletal-driven morphological changes during neuronal differentiation.

Systemic Redox Biomarkers in Neurodegenerative Diseases.

Neurodegenerative diseases are characterized by a gradual and selective loss of neurons. ROS overload has been proved to occur early in this heterogeneous group of disorders, indicating oxidative stress as a primer factor underlying their pathogenesis. Given the importance of a better knowledge of the cause/effect of oxidative stress in the pathogenesis and evolution of neurodegeneration, recent efforts have been focused on the identification and determination of stable markers that may reflect systemic oxidative stress. This review provides an overview of these systemic redox biomarkers and their responsiveness to antioxidant therapies. Redox biomarkers can be classified as molecules that are modified by interactions with ROS in the microenvironment and antioxidant molecules that change in response to increased oxidative stress. DNA, lipids (including phospholipids), proteins and carbohydrates are examples of molecules that can be modified by excessive ROS in vivo. Some modifications have direct effects on molecule functions (e.g. to inhibit enzyme function), but others merely reflect the degree of oxidative stress in the local environment. Testing of redox biomarkers in neurodegenerative diseases has 3 important goals: 1) to confirm the presence or absence of systemic oxidative stress; 2) to identify possible underlying (and potentially reversible) causes of neurodegeneration; and 3) to estimate the severity of the disease and the risk of progression. Reflecting pathological processes occurring in the whole body, redox biomarkers may pinpoint novel therapeutic targets and lead to diagnose diseases before they are clinically evident.

Frataxin silencing inactivates mitochondrial Complex I in NSC34 motoneuronal cells and alters glutathione homeostasis.

Friedreich's ataxia (FRDA) is a hereditary neurodegenerative disease characterized by a reduced synthesis of the mitochondrial iron chaperon protein frataxin as a result of a large GAA triplet-repeat expansion within the first intron of the frataxin gene. Despite neurodegeneration being the prominent feature of this pathology involving both the central and the peripheral nervous system, information on the impact of frataxin deficiency in neurons is scant. Here, we describe a neuronal model displaying some major biochemical and morphological features of FRDA. By silencing the mouse NSC34 motor neurons for the frataxin gene with shRNA lentiviral vectors, we generated two cell lines with 40% and 70% residual amounts of frataxin, respectively. Frataxin-deficient cells showed a specific inhibition of mitochondrial Complex I (CI) activity already at 70% residual frataxin levels, whereas the glutathione imbalance progressively increased after silencing. These biochemical defects were associated with the inhibition of cell proliferation and morphological changes at the axonal compartment, both depending on the frataxin amount. Interestingly, at 70% residual frataxin levels, the in vivo treatment with the reduced glutathione revealed a partial rescue of cell proliferation. Thus, NSC34 frataxin silenced cells could be a suitable model to study the effect of frataxin deficiency in neurons and highlight glutathione as a potential beneficial therapeutic target for FRDA.

Frataxin deficiency leads to reduced expression and impaired translocation of NF-E2-related factor (Nrf2) in cultured motor neurons.

Oxidative stress has been implicated in the pathogenesis of Friedreich's Ataxia (FRDA), a neurodegenerative disease caused by the decreased expression of frataxin, a mitochondrial protein responsible of iron homeostasis. Under conditions of oxidative stress, the activation of the transcription factor NF-E2-related factor (Nrf2) triggers the antioxidant cellular response by inducing antioxidant response element (ARE) driven genes. Increasing evidence supports a role for the Nrf2-ARE pathway in neurodegenerative diseases. In this study, we analyzed the expression and the distribution of Nrf2 in silenced neurons for frataxin gene. Decreased Nrf2 mRNA content and a defective activation after treatment with pro-oxidants have been evidenced in frataxin-silenced neurons by RT-PCR and confocal microscopy. The loss of Nrf2 in FRDA may greatly enhance the cellular susceptibility to oxidative stress and make FRDA neurons more vulnerable to injury. Our findings may help to focus on this promising target, especially in its emerging role in the neuroprotective response.

Transglutaminase 2 transamidation activity during first-phase insulin secretion: natural substrates in INS-1E.

Transglutaminase 2 (TG2) is a multifunctional protein with Ca(2+)-dependent transamidating and G protein activity. Previously, we reported that tgm2 -/- mice have an impaired insulin secretion and that naturally occurring TG2 mutations associated with familial, early-onset type 2 diabetes, show a defective transamidating activity. Aim of this study was to get a better insight into the role of TG2 in insulin secretion by identifying substrates of TG2 transamidating activity in the pancreatic beta cell line INS-1E. To this end, we labeled INS-1E that are capable of secreting insulin upon glucose stimulation in the physiologic range, with an artificial acyl acceptor (biotinamido-pentylamine) or donor (biotinylated peptide), in basal condition and after stimulus with glucose for 2, 5, and 8 min. Biotinylated proteins were analyzed by two-dimensional electrophoresis and mass spectrometry. In addition, subcellular localization of TG2 in human endocrine pancreas was studied by electron microscopy. Among several TG2's transamidating substrates in INS-1E, mass spectrometry identified cytoplasmic actin (a result confirmed in human pancreatic islet), tropomyosin, and molecules that participate in insulin granule structure (e.g., GAPDH), glucose metabolism, or [Ca(2+)] sensing (e.g., calreticulin). Physical interaction between TG2 and cytoplasmic actin during glucose-stimulated first-phase insulin secretion was confirmed by co-immunoprecipitation. Electron microscopy revealed that TG2 is localized close to insulin and glucagon granules in human pancreatic islet. We propose that TG2's role in insulin secretion may involve cytoplasmic actin remodeling and may have a regulative action on other proteins during granule movement. A similar role of TG2 in glucagon secretion is also suggested.