Modeling traumatic brain injury with human brain organoids

Leveraging iPSC technology to assess neuro-immune interactions in neurological and psychiatric disorders

Communication between the immune and the nervous system is essential for human brain development and homeostasis. Disruption of this intricately regulated crosstalk can lead to neurodevelopmental, psychiatric, or neurodegenerative disorders. While animal models have been essential in characterizing the role of neuroimmunity in development and disease, they come with inherent limitations due to species specific differences, particularly with regard to microglia, the major subset of brain resident immune cells. The advent of induced pluripotent stem cell (iPSC) technology now allows the development of clinically relevant models of the central nervous system that adequately reflect human genetic architecture. This article will review recent publications that have leveraged iPSC technology to assess neuro-immune interactions. First, we will discuss the role of environmental stressors such as neurotropic viruses or pro-inflammatory cytokines on neuronal and glial function. Next, we will review how iPSC models can be used to study genetic risk factors in neurological and psychiatric disorders. Lastly, we will evaluate current challenges and future potential for iPSC models in the field of neuroimmunity.

Opening the black box of traumatic brain injury: a holistic approach combining human 3D neural tissue and an in vitro traumatic brain injury induction device

Traumatic brain injury (TBI) is caused by a wide range of physical events and can induce an even larger spectrum of short- to long-term pathophysiologies. Neuroscientists have relied on animal models to understand the relationship between mechanical damages and functional alterations of neural cells. These in vivo and animal-based in vitro models represent important approaches to mimic traumas on whole brains or organized brain structures but are not fully representative of pathologies occurring after traumas on human brain parenchyma. To overcome these limitations and to establish a more accurate and comprehensive model of human TBI, we engineered an in vitro platform to induce injuries via the controlled projection of a small drop of liquid onto a 3D neural tissue engineered from human iPS cells. With this platform, biological mechanisms involved in neural cellular injury are recorded through electrophysiology measurements, quantification of biomarkers released, and two imaging methods [confocal laser scanning microscope (CLSM) and optical projection tomography (OPT)]. The results showed drastic changes in tissue electrophysiological activities and significant releases of glial and neuronal biomarkers. Tissue imaging allowed us to reconstruct the injured area spatially in 3D after staining it with specific nuclear dyes and to determine TBI resulting in cell death. In future experiments, we seek to monitor the effects of TBI-induced injuries over a prolonged time and at a higher temporal resolution to better understand the subtleties of the biomarker release kinetics and the cell recovery phases.

Functional bioengineered models of the central nervous system

The functional complexity of the central nervous system (CNS) is unparalleled in living organisms. Its nested cells, circuits and networks encode memories, move bodies and generate experiences. Neural tissues can be engineered to assemble model systems that recapitulate essential features of the CNS and to investigate neurodevelopment, delineate pathophysiology, improve regeneration and accelerate drug discovery. In this Review, we discuss essential structure-function relationships of the CNS and examine materials and design considerations, including composition, scale, complexity and maturation, of cell biology-based and engineering-based CNS models. We highlight region-specific CNS models that can emulate functions of the cerebral cortex, hippocampus, spinal cord, neural-X interfaces and other regions, and investigate a range of applications for CNS models, including fundamental and clinical research. We conclude with an outlook to future possibilities of CNS models, highlighting the engineering challenges that remain to be overcome.

Simulating traumatic brain injury in vitro: developing high throughput models to test biomaterial based therapies

Traumatic brain injuries are serious clinical incidents associated with some of the poorest outcomes in neurological practice. Coupled with the limited regenerative capacity of the brain, this has significant implications for patients, carers, and healthcare systems, and the requirement for life-long care in some cases. Clinical treatment currently focuses on limiting the initial neural damage with long-term care/support from multidisciplinary teams. Therapies targeting neuroprotection and neural regeneration are not currently available but are the focus of intensive research. Biomaterial-based interventions are gaining popularity for a range of applications including biomolecule and drug delivery, and to function as cellular scaffolds. Experimental investigations into the development of such novel therapeutics for traumatic brain injury will be critically underpinned by the availability of appropriate high throughput, facile, ethically viable, and pathomimetic biological model systems. This represents a significant challenge for researchers given the pathological complexity of traumatic brain injury. Specifically, there is a concerted post-injury response mounted by multiple neural cell types which includes microglial activation and astroglial scarring with the expression of a range of growth inhibitory molecules and cytokines in the lesion environment. Here, we review common models used for the study of traumatic brain injury (ranging from live animal models to in vitro systems), focusing on penetrating traumatic brain injury models. We discuss their relative advantages and drawbacks for the developmental testing of biomaterial-based therapies.

Modeling of Neurodegenerative Diseases: 'Step by Step' and 'Network' Organization of the Complexes of Model Systems

Neurodegenerative diseases have acquired the status of one of the leading causes of death in developed countries, which requires creating new model systems capable of accurately reproducing the mechanisms underlying these pathologies. Here we analyzed modern model systems and their contribution to the solution of unexplored manifestations of neuropathological processes. Each model has unique properties that make it the optimal tool for modeling certain aspects of neurodegenerative disorders. We concluded that to optimize research, it is necessary to combine models into complexes that include organisms and artificial systems of different organizational levels. Such complexes can be organized in two ways. The first method can be described as "step by step", where each model for studying a certain characteristic is a separate step that allows using the information obtained in the modeling process for the gradual study of increasingly complex processes in subsequent models. The second way is a 'network' approach. Studies are carried out with several types of models simultaneously, and experiments with each specific type are adjusted in conformity with the data obtained from other models. In our opinion, the 'network' approach to combining individual model systems seems more promising for fundamental biology as well as diagnostics and therapy.

A benchtop brain injury model using resected donor tissue from patients with Chiari malformation

The use of live animal models for testing new therapies for brain and spinal cord repair is a controversial area. Live animal models have associated ethical issues and scientific concerns regarding the predictability of human responses. Alternative models that replicate the 3D architecture of the central nervous system have prompted the development of organotypic neural injury models. However, the lack of reliable means to access normal human neural tissue has driven reliance on pathological or post-mortem tissue which limits their biological utility. We have established a protocol to use donor cerebellar tonsillar tissue surgically resected from patients with Chiari malformation (cerebellar herniation towards the foramen magnum, with ectopic rather than diseased tissue) to develop an in vitro organotypic model of traumatic brain injury. Viable tissue was maintained for approximately 2 weeks with all the major neural cell types detected. Traumatic injuries could be introduced into the slices with some cardinal features of post-injury pathology evident. Biomaterial placement was also feasible within the in vitro lesions. Accordingly, this 'proof-of-concept' study demonstrates that the model offers potential as an alternative to the use of animal tissue for preclinical testing in neural tissue engineering. To our knowledge, this is the first demonstration that donor tissue from patients with Chiari malformation can be used to develop a benchtop model of traumatic brain injury. However, significant challenges in relation to the clinical availability of tissue were encountered, and we discuss logistical issues that must be considered for model scale-up.

Emerging Bioelectronics for Brain Organoid Electrophysiology

Human brain organoids are generated from three-dimensional (3D) cultures of human induced pluripotent stem cells and embryonic stem cells, which partially replicate the development and complexity of the human brain. Many methods have been used to characterize the structural and molecular phenotypes of human brain organoids. Further understanding the electrophysiological phenotypes of brain organoids requires advanced electrophysiological measurement technologies to achieve long-term stable 3D recording over the time course of the organoid development with single-cell, millisecond spatiotemporal resolution. In this review, first, we briefly introduce the development, generation, and applications of human brain organoids. We then discuss the conventional methods used for characterizing the morphological, genetic, and electrical properties of brain organoids. Next, we highlight the need for characterizing electrophysiological properties of brain organoids in a minimally invasive manner. In particular, we discuss recent advances in the multi-electrode array (MEA), 3D bioelectronics, and flexible bioelectronics and their applications in brain organoid electrophysiological measurement. In addition, we introduce the recently developed cyborg organoids platform as an emerging tool for the long-term stable 3D characterization of the brain organoids electrophysiology at high spatiotemporal resolution. Finally, we discuss the perspectives of new technologies that could achieve the high-throughput, multimodal characterizations from the same brain organoids.

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 Traumatic Brain Injury in Human Cerebral Organoids

Traumatic brain injury (TBI) is a head injury that disrupts the normal brain structure and function. TBI has been extensively studied using various in vitro and in vivo models. Most of the studies have been done with rodent models, which may respond differently to TBI than human nerve cells. Taking advantage of the recent development of cerebral organoids (COs) derived from human induced pluripotent stem cells (iPSCs), which resemble the architecture of specific human brain regions, here, we adapted the controlled cortical impact (CCI) model to induce TBI in human COs as a novel in vitro platform. To adapt the CCI procedure into COs, we have developed a phantom brain matrix, matching the mechanical characteristics of the brain, altogether with an empty mouse skull as a platform to allow the use of the stereotactic CCI equipment on COs. After the CCI procedure, COs were histologically prepared to evaluate neurons and astrocyte populations using the microtubule-associated protein 2 (MAP2) and the glial fibrillary acidic protein (GFAP). Moreover, a marker of metabolic response, the neuron-specific enolase (NSE), and cellular death using cleaved caspase 3 were also analyzed. Our results show that human COs recapitulate the primary pathological changes of TBI, including metabolic alterations related to neuronal damage, neuronal loss, and astrogliosis. This novel approach using human COs to model TBI in vitro holds great potential and opens new alternatives for understanding brain abnormalities produced by TBI, and for the development and testing of new therapeutic approaches.

Cultures and cures: neurodiversity and brain organoids

BACKGROUND

Research with cerebral organoids is beginning to make significant progress in understanding the etiology of autism spectrum disorder (ASD). Brain organoid models can be grown from the cells of donors with ASD. Researchers can explore the genetic, developmental, and other factors that may give rise to the varieties of autism. Researchers could study all of these factors together with brain organoids grown from cells originating from ASD individuals. This makes brain organoids unique from other forms of ASD research. They are like a multi-tool, one with significant versatility for the scope of ASD research and clinical applications. There is hope that brain organoids could one day be used for precision medicine, like developing tailored ASD drug treatments.

MAIN BODY

Brain organoid researchers often incorporate the medical model of disability when researching the origins of ASD, especially when the research has the specific aim of potentially finding tailored clinical treatments for ASD individuals. The neurodiversity movement-a developmental disability movement and paradigm that understands autism as a form of natural human diversity-will potentially disagree with approaches or aims of cerebral organoid research on ASD. Neurodiversity advocates incorporate a social model of disability into their movement, which focuses more on the social, attitudinal, and environmental barriers rather than biophysical or psychological deficits. Therefore, a potential conflict may arise between these perspectives on how to proceed with cerebral organoid research regarding neurodevelopmental conditions, especially ASD.

CONCLUSIONS

Here, we present these perspectives and give at least three initial recommendations to achieve a more holistic and inclusive approach to cerebral organoid research on ASD. These three initial starting points can build bridges between researchers and the neurodiversity movement. First, neurodiverse individuals should be included as co-creators in both the scientific process and research communication. Second, clinicians and neurodiverse communities should have open and respectful communication. Finally, we suggest a continual reconceptualization of illness, impairment, disability, behavior, and person.