🦠Regenerative Medicine Engineering Unit 14 – Neural Regeneration in Neuroengineering

Basics of Neural Anatomy

• Neurons are the fundamental units of the nervous system consisting of a cell body, dendrites, and an axon
  • Cell body contains the nucleus and other organelles essential for cellular function
  • Dendrites are branched extensions that receive signals from other neurons
  • Axon is a long, thin fiber that transmits electrical signals to other neurons or target cells
• Glial cells provide support, protection, and maintenance for neurons
  • Astrocytes regulate neurotransmitter levels, maintain the blood-brain barrier, and provide metabolic support
  • Oligodendrocytes form myelin sheaths around axons in the central nervous system (CNS) to facilitate signal transmission
  • Microglia are the immune cells of the CNS, responding to injury and infection
• The CNS includes the brain and spinal cord, while the peripheral nervous system (PNS) consists of nerves outside the brain and spinal cord
• Synapses are specialized junctions between neurons where neurotransmitters are released to transmit signals
• The blood-brain barrier is a selective barrier formed by endothelial cells, astrocytes, and pericytes that regulates the passage of substances between the bloodstream and the CNS

Mechanisms of Neural Injury

• Traumatic brain injury (TBI) occurs due to external forces causing damage to brain tissue
  • Primary injury results from the initial mechanical insult, leading to cell death and disruption of the blood-brain barrier
  • Secondary injury develops over time and involves inflammation, oxidative stress, and excitotoxicity
• Spinal cord injury (SCI) can result in partial or complete loss of motor and sensory function below the level of injury
  • Compression, contusion, or transection of the spinal cord can cause immediate damage to neurons and glial cells
  • Secondary injury mechanisms, such as inflammation and glial scar formation, can hinder regeneration
• Neurodegenerative diseases, such as Alzheimer's and Parkinson's, involve progressive loss of specific neuronal populations
  • Accumulation of misfolded proteins (amyloid-beta in Alzheimer's, alpha-synuclein in Parkinson's) contributes to neuronal dysfunction and death
• Ischemic stroke occurs when blood flow to the brain is disrupted, leading to oxygen and nutrient deprivation
  • Excitotoxicity, oxidative stress, and inflammation contribute to neuronal death in the affected area
• Axonal injury can result from mechanical forces, leading to disruption of axonal transport and potential axonal degeneration
  • Wallerian degeneration occurs distal to the site of injury, involving axonal fragmentation and myelin breakdown

Principles of Neural Regeneration

• Neurogenesis is the process of generating new neurons from neural stem or progenitor cells
  • Adult neurogenesis occurs in specific regions of the brain, such as the subventricular zone and the dentate gyrus of the hippocampus
  • Enhancing neurogenesis could potentially replace lost neurons in neurodegenerative diseases or following injury
• Axonal regeneration involves the regrowth of damaged axons to restore connectivity
  • In the CNS, axonal regeneration is limited due to the presence of inhibitory factors and the formation of a glial scar
  • Strategies to promote axonal regeneration include neutralizing inhibitory molecules, providing growth-promoting substrates, and modulating the immune response
• Remyelination is the process of forming new myelin sheaths around demyelinated axons
  • Oligodendrocyte precursor cells (OPCs) can differentiate into mature oligodendrocytes and remyelinate axons
  • Promoting remyelination can restore efficient signal transmission and protect axons from further damage
• Synaptic plasticity refers to the ability of synapses to strengthen or weaken in response to activity
  • Long-term potentiation (LTP) and long-term depression (LTD) are forms of synaptic plasticity that underlie learning and memory
  • Harnessing synaptic plasticity mechanisms could help restore functional connectivity following injury or disease
• Neuromodulation involves the use of electrical, magnetic, or pharmacological interventions to modulate neural activity
  • Deep brain stimulation (DBS) is used to treat movement disorders, such as Parkinson's disease, by delivering electrical stimulation to specific brain regions
  • Transcranial magnetic stimulation (TMS) is a non-invasive technique that uses magnetic fields to stimulate or inhibit cortical activity

Biomaterials for Neural Repair

• Hydrogels are three-dimensional networks of hydrophilic polymers that can mimic the extracellular matrix of neural tissue
  • Injectable hydrogels can be delivered minimally invasively and conform to the shape of the injury site
  • Hydrogels can be functionalized with bioactive molecules, such as growth factors or cell adhesion peptides, to promote regeneration
• Nanofibers can provide topographical cues and directional guidance for axonal growth
  • Electrospinning is a common technique for fabricating aligned nanofiber scaffolds
  • Nanofibers can be made from natural (collagen, silk fibroin) or synthetic (polycaprolactone, poly(lactic-co-glycolic acid)) polymers
• Conductive polymers, such as polypyrrole and polyaniline, can facilitate electrical stimulation and recording of neural activity
  • Conductive polymers can be used to coat electrodes or fabricate scaffolds for neural tissue engineering
  • Electrical stimulation through conductive polymers can promote neurite outgrowth and direct axonal regeneration
• Microfluidic devices can be used to create controlled microenvironments for studying neural regeneration
  • Compartmentalized microfluidic devices can separate neuronal cell bodies from axons, allowing for targeted interventions
  • Microfluidic platforms can be used for high-throughput screening of drugs or biomolecules that promote regeneration
• Decellularized extracellular matrix (dECM) can provide a natural scaffold for neural tissue engineering
  • dECM can be derived from neural tissue sources, such as brain or spinal cord, to preserve tissue-specific biochemical and structural cues
  • dECM scaffolds can support neuronal survival, differentiation, and axonal growth

Stem Cell Approaches in Neuroengineering

• Neural stem cells (NSCs) are self-renewing, multipotent cells that can differentiate into neurons, astrocytes, and oligodendrocytes
  • NSCs can be derived from embryonic, fetal, or adult sources, as well as induced pluripotent stem cells (iPSCs)
  • Transplantation of NSCs has shown promise in promoting regeneration and functional recovery in animal models of neural injury and disease
• Mesenchymal stem cells (MSCs) are multipotent cells that can be isolated from various tissues, such as bone marrow, adipose tissue, and umbilical cord
  • MSCs have immunomodulatory and neuroprotective properties, making them attractive candidates for neural repair
  • MSCs can secrete neurotrophic factors and exosomes that promote neuronal survival and regeneration
• iPSCs are derived from adult somatic cells that have been reprogrammed to a pluripotent state
  • iPSCs can be differentiated into patient-specific neural cell types, enabling personalized cell replacement therapies
  • Gene editing techniques, such as CRISPR-Cas9, can be used to correct genetic mutations in iPSCs before differentiation and transplantation
• Organoids are three-dimensional, self-organizing structures that recapitulate aspects of organ development and function
  • Cerebral organoids can be generated from human pluripotent stem cells and used to model brain development and disease
  • Spinal cord organoids can be used to study spinal cord injury and test regenerative strategies in vitro
• Cell encapsulation involves enclosing cells within a semipermeable membrane or matrix to protect them from the immune system and provide a controlled microenvironment
  • Encapsulated stem cells can be implanted into the brain or spinal cord to deliver neurotrophic factors and promote regeneration
  • Encapsulation materials can be designed to allow for the exchange of nutrients and waste while preventing immune cell infiltration

Neurotrophic Factors and Growth Cues

• Neurotrophic factors are proteins that support the survival, growth, and differentiation of neurons
  • Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrophic factor (GDNF) are examples of well-studied neurotrophic factors
  • Neurotrophic factors can be delivered locally or systemically to promote neuronal survival and axonal regeneration following injury
• Growth cues are molecules that guide axonal growth and direct neurite outgrowth during development and regeneration
  • Netrins, semaphorins, and ephrins are families of guidance molecules that can attract or repel axons
  • Incorporating growth cues into biomaterial scaffolds can provide directional guidance for regenerating axons
• Chondroitinase ABC (ChABC) is a bacterial enzyme that degrades chondroitin sulfate proteoglycans (CSPGs), which are inhibitory components of the glial scar
  • Delivery of ChABC can reduce CSPG-mediated inhibition and promote axonal regeneration through the glial scar
  • ChABC can be combined with other regenerative strategies, such as cell transplantation or biomaterial scaffolds, to enhance regeneration
• Rho-associated protein kinase (ROCK) inhibitors can promote axonal regeneration by modulating the actin cytoskeleton
  • ROCK inhibitors, such as Y-27632 and fasudil, can reduce myelin-associated inhibition and promote neurite outgrowth
  • Local delivery of ROCK inhibitors can be achieved through biomaterial scaffolds or nanoparticles
• Neurotrophic factor gene therapy involves delivering genes encoding neurotrophic factors to the injured or diseased nervous system
  • Viral vectors, such as adeno-associated virus (AAV) or lentivirus, can be used to deliver neurotrophic factor genes to specific cell types or regions
  • Gene therapy can provide sustained, local expression of neurotrophic factors to promote regeneration and neuroprotection

Electrical Stimulation Techniques

• Functional electrical stimulation (FES) involves applying electrical currents to paralyzed muscles to restore movement
  • FES can be used to assist with walking, grasping, and other functional tasks in individuals with SCI or stroke
  • Implantable FES systems can provide long-term stimulation and improve quality of life
• Spinal cord stimulation (SCS) involves delivering electrical stimulation to the spinal cord to modulate sensory and motor function
  • SCS can be used to treat chronic pain, spasticity, and other neurological conditions
  • Epidural and transcutaneous SCS systems can be used to deliver stimulation at different spinal levels
• Deep brain stimulation (DBS) involves implanting electrodes in specific brain regions to modulate neural activity
  • DBS is used to treat movement disorders, such as Parkinson's disease, essential tremor, and dystonia
  • DBS can also be investigated as a potential treatment for psychiatric disorders, such as depression and obsessive-compulsive disorder
• Optogenetics is a technique that uses light to control the activity of genetically modified neurons
  • Light-sensitive ion channels, such as channelrhodopsin-2 (ChR2), can be expressed in specific neuronal populations
  • Optogenetic stimulation can be used to investigate neural circuits and modulate activity with high spatial and temporal precision
• Transcranial direct current stimulation (tDCS) is a non-invasive technique that delivers weak electrical currents to the brain through scalp electrodes
  • tDCS can modulate cortical excitability and has been investigated for the treatment of neurological and psychiatric disorders
  • tDCS can also be used to enhance cognitive function and motor learning in healthy individuals

Emerging Technologies and Future Directions

• 3D bioprinting involves using additive manufacturing techniques to create complex, three-dimensional structures containing living cells
  • 3D bioprinting can be used to fabricate patient-specific neural tissue constructs with precise control over cell placement and scaffold architecture
  • Bioinks containing neural cells, hydrogels, and growth factors can be used to create functional neural tissue for regenerative applications
• Brain-computer interfaces (BCIs) enable direct communication between the brain and external devices
  • Invasive BCIs involve implanting electrodes in the brain to record neural activity and control prosthetic devices or computer cursors
  • Non-invasive BCIs, such as those based on electroencephalography (EEG) or functional near-infrared spectroscopy (fNIRS), can be used for communication and rehabilitation
• Exosomes are small, extracellular vesicles that contain proteins, lipids, and nucleic acids
  • Exosomes secreted by stem cells have been shown to promote neural regeneration and functional recovery in animal models of neural injury
  • Engineered exosomes can be used as targeted delivery vehicles for therapeutic molecules, such as siRNAs or growth factors
• Gene editing technologies, such as CRISPR-Cas9, can be used to modify the genome of cells for regenerative applications
  • CRISPR-Cas9 can be used to correct genetic mutations associated with neurodegenerative diseases in patient-derived iPSCs
  • Gene editing can also be used to enhance the regenerative potential of transplanted cells or to modulate the host immune response
• Artificial intelligence (AI) and machine learning (ML) can be applied to analyze large datasets and guide the design of regenerative strategies
  • AI and ML can be used to identify novel therapeutic targets, predict the efficacy of treatments, and optimize biomaterial properties
  • Deep learning algorithms can be used to analyze imaging data and assess the outcomes of regenerative interventions