Fetal human cortical neurons grown in culture: morphological differentiation, biochemical correlates and development of electrical activity

Properties of human central nervous system neurons in a glia-depleted (isolated) culture system

BACKGROUND

Current methods for studying human neurons depend on a feeder layer of astroglia supplemented with animal serum to support the growing neurons. These requirements undermine many of the advantages provided by in vitro cell culture approaches when compared with more complex in vivo techniques.

NEW METHOD

Here, we identified a reliable marker (MHCI) that allows for direct isolation of primary neurons from fetal human brain. We utilized a magnetic labeling and isolation technique to separate neurons from other neural cells. We established a defined condition, omitting the astroglial supports that could maintain the human neurons for varying amounts of time.

RESULTS

We showed that the new method significantly improved the purity of human neurons in culture without the need for further chemical/mechanical enrichment. We demonstrated the suitability of these neurons for functional studies including Rho-kinase dependent regulation of neurite outgrowth and ensheathment in co-cultures with oligodendrocyte progenitor cells derived from fetal human brain.

COMPARISON WITH EXISTING METHODS

The accountability for neuron-only seeding and the controllable density allows for better neuronal maturation and better visualization of the different neuronal compartments. The higher purity culture constitutes an effective system to study and screen for compounds that impact neuron biology without potential confounding effects from glial crowding.

CONCLUSIONS

High purity human neurons generated using the improved method will enable enhanced reliability in the discovery and development of drugs with neuroregenerative and neuroprotective activity.

Reprogramming of HUVECs into induced pluripotent stem cells (HiPSCs), generation and characterization of HiPSC-derived neurons and astrocytes

Neurodegenerative diseases are characterized by chronic and progressive structural or functional loss of neurons. Limitations related to the animal models of these human diseases have impeded the development of effective drugs. This emphasizes the need to establish disease models using human-derived cells. The discovery of induced pluripotent stem cell (iPSC) technology has provided novel opportunities in disease modeling, drug development, screening, and the potential for “patient-matched” cellular therapies in neurodegenerative diseases. In this study, with the objective of establishing reliable tools to study neurodegenerative diseases, we reprogrammed human umbilical vein endothelial cells (HUVECs) into iPSCs (HiPSCs). Using a novel and direct approach, HiPSCs were differentiated into cells of central nervous system (CNS) lineage, including neuronal, astrocyte and glial cells, with high efficiency. HiPSCs expressed embryonic genes such as nanog, sox2 and Oct-3/4, and formed embryoid bodies that expressed markers of the 3 germ layers. Expression of endothelial-specific genes was not detected in HiPSCs at RNA or protein levels. HiPSC-derived neurons possess similar morphology but significantly longer neurites compared to primary human fetal neurons. These stem cell-derived neurons are susceptible to inflammatory cell-mediated neuronal injury. HiPSC-derived neurons express various amino acids that are important for normal function in the CNS. They have functional receptors for a variety of neurotransmitters such as glutamate and acetylcholine. HiPSC-derived astrocytes respond to ATP and acetylcholine by elevating cytosolic Ca²⁺ concentrations. In summary, this study presents a novel technique to generate differentiated and functional HiPSC-derived neurons and astrocytes. These cells are appropriate tools for studying the development of the nervous system, the pathophysiology of various neurodegenerative diseases and the development of potential drugs for their treatments.

Nerve growth factor metabolic dysfunction in Down's syndrome brains

Basal forebrain cholinergic neurons play a key role in cognition. This neuronal system is highly dependent on NGF for its synaptic integrity and the phenotypic maintenance of its cell bodies. Basal forebrain cholinergic neurons progressively degenerate in Alzheimer's disease and Down's syndrome, and their atrophy contributes to the manifestation of dementia. Paradoxically, in Alzheimer's disease brains, the synthesis of NGF is not affected and there is abundance of the NGF precursor, proNGF. We have shown that this phenomenon is the result of a deficit in NGF's extracellular metabolism that compromises proNGF maturation and exacerbates its subsequent degradation. We hypothesized that a similar imbalance should be present in Down's syndrome. Using a combination of quantitative reverse transcription-polymerase chain reaction, enzyme-linked immunosorbent assay, western blotting and zymography, we investigated signs of NGF metabolic dysfunction in post-mortem brains from the temporal (n = 14), frontal (n = 34) and parietal (n = 20) cortex obtained from subjects with Down's syndrome and age-matched controls (age range 31-68 years). We further examined primary cultures of human foetal Down's syndrome cortex (17-21 gestational age weeks) and brains from Ts65Dn mice (12-22 months), a widely used animal model of Down's syndrome. We report a significant increase in proNGF levels in human and mouse Down's syndrome brains, with a concomitant reduction in the levels of plasminogen and tissue plasminogen activator messenger RNA as well as an increment in neuroserpin expression; enzymes that partake in proNGF maturation. Human Down's syndrome brains also exhibited elevated zymogenic activity of MMP9, the major NGF-degrading protease. Our results indicate a failure in NGF precursor maturation in Down's syndrome brains and a likely enhanced proteolytic degradation of NGF, changes which can compromise the trophic support of basal forebrain cholinergic neurons. The alterations in proNGF and MMP9 were also present in cultures of Down's syndrome foetal cortex; suggesting that this trophic compromise may be amenable to rescue, before frank dementia onset. Our study thus provides a novel paradigm for cholinergic neuroprotection in Alzheimer's disease and Down's syndrome.

Granule-derived granzyme B mediates the vulnerability of human neurons to T cell-induced neurotoxicity

Multiple sclerosis (MS) is considered an autoimmune disease of the CNS and is characterized by inflammatory cells infiltrating the CNS and inducing demyelination, axonal loss, and neuronal death. Recent evidence strongly suggests that axonal and neuronal degeneration underlie the progression of permanent disability in MS. In this study, we report that human neurons are selectively susceptible to the serine-protease granzyme B (GrB) isolated from cytotoxic T cell granules. In vitro, purified human GrB induced neuronal death to the same extent as the whole activated T cell population. On the contrary, activated T cells isolated from GrB knockout mice failed to induce neuronal injury. We found that following internalization through various parts of neurons, GrB accumulated in the neuronal soma. Within the cell body, GrB diffused out of endosomes possibly through a perforin-independent mechanism and induced subsequent activation of caspases and cleavage of α-tubulin. Inhibition of caspase-3, a well-known substrate for GrB, significantly reduced GrB-mediated neurotoxicity. We demonstrated that treatment of neurons with mannose-6-phosphate prevented GrB entry and inhibited GrB-mediated neuronal death, suggesting mannose-6-phosphate receptor-dependent endocytosis. Together, our data unveil a novel mechanism by which GrB induces selective neuronal injury and suggest potential new targets for the treatment of inflammatory-mediated neurodegeneration in diseases such as MS.

CD4+CD25+CD127dimFoxp3+ T cells are cytotoxic for human neurons

MS lesions are characterized by destruction of myelin and significant neuronal and axonal loss. Preliminary studies with the use of T(regs) in the mouse model of MS have been extremely encouraging. However, recent studies with human cells have shown the presence of different subpopulations of T cells within the CD4(+)CD25(+)Foxp3(+) T cell phenotype, some of which do not have regulatory functions. These findings suggest a potential difference between mouse and human in the regulatory phenotype. Here, we show that human activated CD4(+)CD25(+)Foxp3(+) T cells are neurotoxic in vitro. These cells expressed high levels of the cytotoxic molecule GrB and had no suppressive effect. On the contrary, they produced IFN-γ and low IL-17, suggesting a shift toward a T(H)1 phenotype. Thus, our data confirm the presence of a nonregulatory cytotoxic subpopulation within the human CD4(+)CD25(+)Foxp3(+) T cells and suggest further studies on the human regulatory phenotype prior to any potential therapeutic application.

Electrical excitability of early neurons in the human cerebral cortex during the second trimester of gestation

Information about development of the human cerebral cortex (proliferation, migration, and differentiation of neurons) is largely based on postmortem histology. Physiological properties of developing human cortical neurons are difficult to access experimentally and therefore remain largely unexplored. Animal studies have shown that information about the arousal of electrical activity in individual cells within fundamental cortical zones (subventricular zone [SVZ], intermediate zone, subplate [SP], and cortical plate [CP]) is necessary for understanding normal brain development. Here we ask where, in what cortical zone, and when, in what gestational week (gw), human neurons acquire the ability to generate nerve impulses (action potentials [APs]). We performed electrical recordings from individual cells in acute brain slices harvested postmortem from the human fetal cerebral cortex (16-22 gw). Tetrodotoxin-sensitive Na(+) current occurs more frequently among CP cells and with significantly greater peak amplitudes than in SVZ. As early as 16 gw, a relatively small population of CP neurons (27%) was able to generate sodium APs upon direct current injection. Neurons located in the SP exhibited the highest level of cellular differentiation, as judged by their ability to fire repetitive APs. At 19 gw, a fraction of human CP and SP neurons possess beta IV spectrin-positive axon initial segments populated with voltage-gated sodium channels (PanNav). These results yield the first physiological characterization of developing human fetal cortical neurons with preserved morphologies in intact surrounding brain tissue.

Radial glia cells in the developing human brain

Human radial glia (RG) share many of the features described in rodents, but also have a number of characteristics unique to the human brain. Results obtained from different mammalian species including human and non-human primates reveal differences in the involvement of RG in neurogenesis and oligodendrogenesis and in the timing of the initial expression of typical RG immunomarkers. A common problem in studying the human brain is that experimental procedures using modern molecular and genetic methods, such as in vivo transduction with retroviruses or creation of knockout or transgenic mutants, are not possible. Nevertheless, abundant and valuable information about the development of the human brain has been revealed using postmortem human material. Additionally, a combination and spectrum of in vitro techniques are used to gain knowledge about normal developmental processes in the human brain, including better understanding of RG as progenitor cells. Molecular and functional characterization of multipotent progenitors, such as RG, is important for future cell replacement therapies in neurological and psychiatric disorders, which are often resistant to conventional treatments. The protracted time of development and larger size of the human brain could provide insight into processes that may go unnoticed in the much smaller rodent cortex, which develops over a much shorter period. With that in mind, we summarize results on the role of RG in the human fetal brain.

Neuroprotective role of minocycline in co-cultures of human fetal neurons and microglia

Bacterial infections during pregnancy often result in premature birth and neonatal white matter damage. During these infections, microglia, the resident immune cells of the CNS, undergo activation and contribute to further neuronal damage of the CNS. Minocycline, a second-generation tetracycline antibiotic, inhibits microglial activation and protects neurons in rodents but data about its effects on human cells are limited. We studied the mechanism of the neuroprotective effect of minocycline in either purified cell cultures or co-cultures of microglia and neurons from human fetal brain during inflammation induced by lipopolysaccharide (LPS). In neuron/microglial co-cultures, minocycline treatment prevented activation and proliferation of microglia and protected neurons as demonstrated by decreased neuronal cell death and a shift of Bcl-2 family proteins toward anti-apoptotic ratio. Notably, neither minocycline nor LPS had an effect on neurons in purified neuronal cultures. The ability of minocycline to regulate activation of human fetal microglia might be relevant in therapies used towards treating neonatal CNS infections.

Functional neural development from human embryonic stem cells: accelerated synaptic activity via astrocyte coculture

How a naive human neuroepithelial cell becomes an electrophysiologically active neuron remains unknown. Here, we describe the early physiological development of neurons differentiating from naive human embryonic stem (hES) cells. We found that differentiating neuronal cells progressively decrease their resting membrane potential, gain characteristic Na+ and K+ currents, and fire mature action potentials by 7 weeks of differentiation. This is similar to the maturation pattern observed in animals, albeit on a greatly expanded time scale. An additional 3 weeks of differentiation resulted in neurons that could fire repetitive trains of action potentials in response to depolarizing current pulses. The onset of spontaneous synaptic activity also occurred after 7 weeks of differentiation, in association with the differentiation of astrocytes within the culture. Cocultures of hES cell-derived neuroepithelial cells with exogenous astrocytes significantly accelerated the onset of synaptic currents but did not alter action potential generation. These findings suggest that the development of membrane characteristics and action potentials depend on the intrinsic maturation of Na+ and K+ currents, whereas synaptic transmission is enhanced by astrocytes, which may be achieved independently of the maturation of action potentials. Furthermore, we found that although astrocyte-conditioned medium accelerated synaptic protein localization, it did not increase synaptic activity, suggesting a contact-dependent mechanism by which astrocytes augment synaptic activity. These results lay the foundation for future studies examining the functional development of human neurons and provide support for the potential application of human cells in restorative neuronal therapies.

Different conformations of amyloid beta induce neurotoxicity by distinct mechanisms in human cortical neurons

Characterization of soluble oligomeric amyloid beta (Abeta) species in the brains of Alzheimer's disease (AD) patients and transgenic models has raised the possibility that different conformations of Abeta may contribute to AD pathology via different mechanisms. To characterize the toxic effect of different Abeta conformations, we tested side by side the effect of well characterized Abeta oligomers (AbetaOs), Abeta-derived diffusible ligands (ADDLs), and fibrillar Abeta (Abetaf) preparations in human cortical neurons (HCNs). Both AbetaOs and ADDLs bind rapidly and with high affinity to synaptic contacts and cellular membranes. AbetaOs (5 microm) induced rapid and massive neuronal death. Calcium influx accelerated, but was not required for, AbetaO toxicity. AbetaOs elicited a stereotyped succession of cellular changes consistent with the activation of a mitochondrial death apoptotic pathway. At low concentrations AbetaOs caused chronic and subtler mitochondrial alterations but minimal cell death. ADDLs induced similar toxic changes as AbetaOs but on a fivefold longer time scale. Higher concentrations of Abetaf and longer incubation times were required to produce widespread neuritic dystrophy but modest HCN cell death. Thus various Abeta species may play relevant roles in AD, causing neurotoxicity by distinct non-overlapping mechanisms affecting neuronal function and viability over multiple time courses.

Human central nervous system tissue culture: a historical review and examination of recent advances

Tissue culture has been and continues to be widely used in medical research. Since the beginning of central nervous system (CNS) tissue culture nearly 100 years ago, the scientific community has contributed innumerable protocols and materials leading to the current wide variety of culture systems. While nonhuman cultures have traditionally been more widely used, interest in human CNS tissue culture techniques has accelerated since the middle of the last century. This has been fueled largely by the desire to model human physiology and disease in vitro with human cells. We review the history of human CNS tissue culture summarizing advances that have led to the current breadth of options available. The review addresses tissue sources, culture initiation, formats, culture ware, media, supplements and substrates, and maintenance. All of these variables have been influential in the development of culturing options and the optimization of culture survival and propagation.

Vulnerability of human neurons to T cell-mediated cytotoxicity

Axonal and neuronal loss occurs in inflammatory diseases of the CNS such as multiple sclerosis. The cause of the loss remains unclear. We report that polyclonally activated T cells align along axons and soma of cultured human neurons leading to substantial neuronal death. This occurs in an allogeneic and syngeneic manner in the absence of added Ag, requires T cells to be activated, and is mediated through cell contact-dependent mechanisms involving FasL, LFA-1, and CD40 but not MHC class I. Activated CD4(+) and CD8(+) T cell subsets are equally neuronal cytotoxic. In contrast to neurons, other CNS cell types (oligodendrocytes and astrocytes) are not killed by T cells. These results demonstrate for the first time the high and selective vulnerability of human neurons to T cells, and suggest that when enough activated T cells accumulate in the CNS, neuronal cytotoxicity can result through Ag-independent non-MHC class I mechanisms.

An efficient method for the culturing and generation of neurons and astrocytes from second trimester human central nervous system tissue

The isolation, culturing and expansion of human neural progenitors cells has important potential clinical applications in cellular transplantation strategies as well as in developmental studies involving the central nervous system (CNS). This study describes an efficient method to culture neurons and astrocytes as primary cultures, as well as from proliferative progenitor cells derived from second trimester fetal CNS tissue. Second trimester fetal human tissue was mechanically dissociated and subjected to trypsin-dissociation and trituration. The resulting suspension was passed over a Percoll density gradient. The middle (second) fraction of cells was centrifuged to yield a homogenous population of cells with 80%-90% viability. These cells were either cultured directly on laminin coated dishes with defined medium supplemented with fetal bovine serum or in defined medium supplemented with growth factors including epidermal growth factor, basic fibroblast growth factor and leukemia inhibitory factor. The primary cell cultures yielded neurons and astrocytes after 3-5 days in vitro verified by immunostaining with MAP2ab and GFAP. Cells exposed to growth factor supplemented medium formed free-floating spheres within one week. Upon growth factor removal and plating on laminin-coated dishes, brain derived spheres gave rise to neurons, astrocytes and oligodendrocytes; spinal cord derived spheres generated only astrocytes. This protocol describes an efficient method to generate and culture neurons and astrocytes from second trimester human CNS tissue that may be useful in transplantation and developmental studies.