Sensory axon guidance with semaphorin 6A and nerve growth factor in a biomimetic choice point model

The Lamellar Cells of Vertebrate Meissner and Pacinian Corpuscles: Development, Characterization, and Functions

Sensory corpuscles, or cutaneous end-organ complexes, are complex structures localized at the periphery of Aβ-axon terminals from primary sensory neurons that primarily work as low-threshold mechanoreceptors. Structurally, they consist, in addition to the axons, of non-myelinating Schwann-like cells (terminal glial cells) and endoneurial- and perineurial-related cells. The terminal glial cells are the so-called lamellar cells in Meissner and Pacinian corpuscles. Lamellar cells are variably arranged in sensory corpuscles as a “coin stack” in the Meissner corpuscles or as an “onion bulb” in the Pacinian ones. Nevertheless, the origin and protein profile of the lamellar cells in both morphotypes of sensory corpuscles is quite similar, although it differs in the expression of mechano-gated ion channels as well as in the composition of the extracellular matrix between the cells. The lamellar cells have been regarded as supportive cells playing a passive role in the process of genesis of the action potential, i.e., the mechanotransduction process. However, they express ion channels related to the mechano–electric transduction and show a synapse-like mechanism that suggest neurotransmission at the genesis of the electrical action potential. This review updates the current knowledge about the embryonic origin, development modifications, spatial arrangement, ultrastructural characteristics, and protein profile of the lamellar cells of cutaneous end-organ complexes focusing on Meissner and Pacinian morphotypes.

Biodegradable Microcapsules Loaded with Nerve Growth Factor Enable Neurite Guidance and Synapse Formation

Neurological disorders and traumas often involve loss of specific neuronal connections, which would require intervention with high spatial precision. We have previously demonstrated the biocompatibility and therapeutic potential of the layer-by-layer (LbL)-fabricated microcapsules aimed at the localized delivery of specific channel blockers to peripheral nerves. Here, we explore the potential of LbL-microcapsules to enable site-specific, directional action of neurotrophins to stimulate neuronal morphogenesis and synaptic circuit formation. We find that nanoengineered biodegradable microcapsules loaded with nerve growth factor (NGF) can guide the morphological development of hippocampal neurons in vitro. The presence of NGF-loaded microcapsules or their clusters increases the neurite outgrowth rate while boosting neurite branching. Microcapsule clusters appear to guide the trajectory of developing individual axons leading to the formation of functional synapses. Our observations highlight the potential of NGF-loaded, biodegradable LbL-microcapsules to help guide axonal development and possibly circuit regeneration in neuropathology.

3D biofabrication for soft tissue and cartilage engineering

Soft tissue injuries (STIs) affect patients of all age groups and represent a common worldwide clinical problem, resulting from conditions including trauma, infection, cancer and burns. Within the spectrum of STIs a mixture of tissues can be injured, ranging from skin to underlying nerves, blood vessels, tendons and cartilaginous tissues. However, significant limitations affect current treatment options and clinical demand for soft tissue and cartilage regenerative therapies continues to rise. Improving the regeneration of soft tissues has therefore become a key area of focus within tissue engineering. As an emerging technology, 3D bioprinting can be used to build complex soft tissue constructs "from the bottom up," by depositing cells, growth factors, extracellular matrices and other biomaterials in a layer-by-layer fashion. In this way, regeneration of cartilage, skin, vasculature, nerves, tendons and other bodily tissues can be performed in a patient specific manner. This review will focus on recent use of 3D bioprinting and other biofabrication strategies in soft tissue repair and regeneration. Biofabrication of a variety of soft tissue types will be reviewed following an overview of available cell sources, bioinks and bioprinting techniques.

Semaphorins in Angiogenesis and Autoimmune Diseases: Therapeutic Targets?

The axonal guidance molecules, semaphorins, have been described to function both physiologically and pathologically outside of the nervous system. In this review, we focus on the vertebrate semaphorins found in classes 3 through 7 and their roles in vascular development and autoimmune diseases. Recent studies indicate that while some of these vertebrate semaphorins promote angiogenesis, others have an angiostatic function. Since some semaphorins are also expressed by different immune cells and are known to modulate immune responses, they have been implicated in autoimmune disorders such as multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus and systemic sclerosis. We conclude this review by addressing strategies targeting semaphorins as potential therapeutic agents for angiogenesis and autoimmune diseases.

Biofabrication of neural microphysiological systems using magnetic spheroid bioprinting

The high attrition rate of neuro-pharmaceuticals as they proceed to market necessitates the development of clinically-relevant in vitro neural microphysiological systems that can be utilized during the preclinical screening phase to assess the safety and efficacy of potential compounds. Historically, proposed models have adhered to two distinct approaches; those that are biologically relevant (e.g.-organoids, spheroids) or those that provide engineering control (e.g.-bioprinting, microfluidics). Separately, these approaches fail to fully recapitulate the complex hierarchical structure of the nervous system, limiting their clinical applications. Furthermore, the reliance on manual implementation present in many models fails to effectively scale up or satisfy the consistency standards required for widespread industry adoption. This work serves as a proof-of-concept for merging the two approaches to create a neural microphysiological system that overcomes their individual limitations. Spinal cord spheroids, fabricated using magnetic nanoparticles, are positioned in a three-dimensional hydrogel construct using magnetic bioprinting. Resulting constructs demonstrate both localized cell-cell interactions and long-distance projections that mimic in vivo structure. The use of magnetic nanoparticles for spheroid formation provides batch-to-batch consistency in size and shape and reduces the reliance on trained experimenters for accurate placing for culture. Taken together, this combination approach provides the first steps towards developing a simple approach for integrating spheroid, hydrogel culture, and bioprinting as an alternative to more specialized and expensive processes.

Effect of surgically guided axonal regrowth into a 3-way-conduit (isogeneic trifurcated aorta) on functional recovery after facial-nerve reconstruction: Experimental study in rats

BACKGROUND

The "post-paralytic syndrome" after facial nerve reconstruction has been attributed to (i) malfunctioning axonal guidance at the fascicular (branches) level, (ii) collateral branching of the transected axons at the lesion site, and (iii) intensive intramuscular terminal sprouting of regenerating axons which causes poly-innervation of the neuromuscular junctions (NMJ).

OBJECTIVE

The first two reasons were approached by an innovative technique which should provide the re-growing axons optimal conditions to elongate and selectively re-innervate their original muscle groups.

METHODS

The transected facial nerve trunk was inserted into a 3-way-conduit (from isogeneic rat abdominal aorta) which should "guide" the re-growing facial axons to the three main branches of the facial nerve (zygomatic, buccal and marginal mandibular). The effect of this method was tested also on hypoglossal axons after hypoglossal-facial anastomosis (HFA). Coaptational (classic) FFA (facial-facial anastomosis) and HFA served as controls.

RESULTS

When compared to their coaptation (classic) alternatives, both types of 3-way-conduit operations (FFA and HFA) promoted a trend for reduction in the collateral axonal branching (the proportion of double- or triple-labelled perikarya after retrograde tracing was slightly reduced). In contrast, poly-innervation of NMJ in the levator labii superioris muscle was increased and vibrissal (whisking) function was worsened.

CONCLUSIONS

The use of 3-way-conduit provides no advantages to classic coaptation. Should the latter be impossible (too large interstump defects requiring too long interpositional nerve grafts), this type of reconstruction may be applied. (230 words).

Methods for fabrication and evaluation of a 3D microengineered model of myelinated peripheral nerve

OBJECTIVE

The cost and low success rates of the neurological drug development pipeline have diverted the pharmaceutical industry to 'nerve-on-a-chip' systems as preclinical models to streamline drug development. We present a novel micro-engineered 3D hydrogel platform for the culture of myelinated embryonic peripheral neural tissue to serve as an effective in vitro model for electrophysiological and histological analysis that could be adopted for preclinical testing.

APPROACH

Dorsal root ganglions (DRG) from 15 d old embryonic rats were cultured in 3D hydrogel platforms. The interaction between Schwann cells (SC) and neurons during axonal development and regeneration affects the direction of growth and the synthesis of myelin sheaths. Induction of myelination was performed with two approaches: the addition of exogenous SC and promoting migration of endogenous SC.

MAIN RESULTS

Histological analysis of the preparation utilizing exogenous SC showed aligned, highly fasciculated axonal growth with noticeable myelin sheaths around axons. Separately, electrophysiological testing of the preparation utilizing endogenous SC showed increased amplitude of the compound action potential and nerve conduction velocity in the presence of ascorbic acid (AA).

SIGNIFICANCE

This platform has immense potential to be a useful and translatable in vitro testing tool for drug discovery and myelination studies.

Comparison of visible and UVA phototoxicity in neural culture systems micropatterned with digital projection photolithography

Photopolymerization provides a favorable method for hydrogel formation due to its simplicity, convenience, and versatility. However, the light exposure required to initiate photopolymerization is known to have a cytotoxic effect on encapsulated cells. Here, a 3D in vitro model of the nervous system microenvironment, micropatterned through the use of digital projection photolithography using a single hydrogel formulation that cross-links similarly under ultraviolet A (UVA, 315-400 nm) and visible light (400-700 nm) exposure, is presented. This setup allowed for the investigation of neuronal responses to different light wavelengths and exposure times during photoencapsulation, while ruling out effects due to the hydrogel formulation or photoinitiators used. Cellular studies-including neurite viability, DNA fragmentation, and neurite outgrowth for both UVA and visible light irradiation, the most common spectra used in biological photomicropatterning applications-were performed to assess the effect of light source on neuronal cultures. These studies indicated that while cell death occurs after exposure to either spectrum, visible light was less phototoxic than UVA, when using comparable levels of irradiation, and interestingly, glial cells were more susceptible to phototoxicity than neuronal cells. Thus, while utilizing visible light for micropatterning and cell encapsulation for nervous system applications is beneficial, it is helpful to keep the light exposure low to ensure optimal neuronal survival and growth. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 134-144, 2019.

The development of human digital Meissner's and Pacinian corpuscles

Meissner's and Pacinian corpuscles are cutaneous mechanoreceptors responsible for different modalities of touch. The development of these sensory formations in humans is poorly known, especially regarding the acquisition of the typical immunohistochemical profile related to their full functional maturity. Here we used a panel of antibodies (to specifically label the main corpuscular components: axon, Schwann-related cells and endoneurial-perineurial-related cells) to investigate the development of digital Meissner's and Pacinian corpuscles in a representative sample covering from 11 weeks of estimated gestational age (wega) to adulthood. Development of Pacinian corpuscles starts at 13 wega, and it is completed at 4 months of life, although their basic structure and immunohistochemical characteristics are reached at 36 wega. During development, around the axon, a complex network of S100 positive Schwann-related processes is progressively compacted to form the inner core, while the surrounding mesenchyme is organized and forms the outer core and the capsule. Meissner's corpuscles start to develop at 22 wega and complete their typical morphology and immunohistochemical profile at 8 months of life. In developing Meissner's corpuscles, the axons establish complex relationships with the epidermis and are progressively covered by Schwann-like cells until they complete the mature arrangement late in postnatal life. The present results demonstrate an asynchronous development of the Meissner's and Pacini's corpuscles and show that there is not a total correlation between morphological and immunohistochemical maturation. The correlation of the present results with touch-induced cortical activity in developing humans is discussed.

3D Neural Culture in Dual Hydrogel Systems

3D in vitro culture systems may yield physiological outcomes that more closely approximate in vivo behavior. A number of fabrication techniques and hydrogel scaffold materials are available to researchers, but often their implementation is complex and seemingly prohibitive. Herein, we describe a simplistic and adaptable dual hydrogel photolithography method utilized to engineer advanced in vitro systems for studies of neuronal development and characterization.

Regulatory role of NGFs in neurocognitive functions

Nerve growth factors (NGFs), especially the prototype NGF and brain-derived neurotrophic factor (BDNF), have a diverse array of functions in the central nervous system through their peculiar set of receptors and intricate signaling. They are implicated not only in the development of the nervous system but also in regulation of neurocognitive functions like learning, memory, synaptic transmission, and plasticity. Evidence even suggests their role in continued neurogenesis and experience-dependent neural network remodeling in adult brain. They have also been associated extensively with brain disorders characterized by neurocognitive dysfunction. In the present article, we aimed to make an exhaustive review of literature to get a comprehensive view on the role of NGFs in neurocognitive functions in health and disease. Starting with historical perspective, distribution in adult brain, implied molecular mechanisms, and developmental basis, this article further provides a detailed account of NGFs’ role in specified neurocognitive functions. Furthermore, it discusses plausible NGF-based homeostatic and adaptation mechanisms operating in the pathogenesis of neurocognitive disorders and has presents a survey of such disorders. Finally, it elaborates on current evidence and future possibilities in therapeutic applications of NGFs with an emphasis on recent research updates in drug delivery mechanisms. Conclusive remarks of the article make a strong case for plausible role of NGFs in comprehensive regulation of the neurocognitive functions and pathogenesis of related disorders and advocate that future research should be directed to explore use of NGF-based mechanisms in the prevention of implicated diseases as well as to target these molecules pharmacologically.

Organ-on-a-chip devices advance to market

To curb the high cost of drug development, there is an urgent need to develop more predictive tissue models using human cells to determine drug efficacy and safety in advance of clinical testing. Recent insights gained through fundamental biological studies have validated the importance of dynamic cell environments and cellular communication to the expression of high fidelity organ function. Building on this knowledge, emerging organ-on-a-chip technology is poised to fill the gaps in drug screening by offering predictive human tissue models with methods of sophisticated tissue assembly. Organ-on-a-chip start-ups have begun to spawn from academic research to fill this commercial space and are attracting investment to transform the drug discovery industry. This review traces the history, examines the scientific foundation and envisages the prospect of these renowned organ-on-a-chip technologies. It serves as a guide for new members of this dynamic field to navigate the existing scientific and market space.

Microphysiological Human Brain and Neural Systems-on-a-Chip: Potential Alternatives to Small Animal Models and Emerging Platforms for Drug Discovery and Personalized Medicine

Translational challenges associated with reductionist modeling approaches, as well as ethical concerns and economic implications of small animal testing, drive the need for developing microphysiological neural systems for modeling human neurological diseases, disorders, and injuries. Here, we provide a comprehensive review of microphysiological brain and neural systems-on-a-chip (NSCs) for modeling higher order trajectories in the human nervous system. Societal, economic, and national security impacts of neurological diseases, disorders, and injuries are highlighted to identify critical NSC application spaces. Hierarchical design and manufacturing of NSCs are discussed with distinction for surface- and bulk-based systems. Three broad NSC classes are identified and reviewed: microfluidic NSCs, compartmentalized NSCs, and hydrogel NSCs. Emerging areas and future directions are highlighted, including the application of 3D printing to design and manufacturing of next-generation NSCs, the use of stem cells for constructing patient-specific NSCs, and the application of human NSCs to 'personalized neurology'. Technical hurdles and remaining challenges are discussed. This review identifies the state-of-the-art design methodologies, manufacturing approaches, and performance capabilities of NSCs. This work suggests NSCs appear poised to revolutionize the modeling of human neurological diseases, disorders, and injuries.

The Amyloid Precursor Protein Is a Conserved Receptor for Slit to Mediate Axon Guidance

The amyloid precursor protein (APP) is a receptor-like membrane protein. Although APP processing and β-amyloid production play a central role in Alzheimer's disease (AD) pathogenesis, the physiological function of APP remains elusive. Here, we identify APP as a novel receptor for Slit that mediates axon guidance and neural circuit formation. APP deficiency abolishes the Slit repulsive effect in a 3D olfactory explant culture, consistent with its callosal projection deficit in vivo and reminiscent of Slit loss. Inactivation of APP ortholog APL-1 in Caenorhabditis elegans results in pioneer axon mistargeting and genetic analysis places APL-1 in the SLT-1 (Slit)/SAX-3 (Robo) repulsive pathway. Slit binds to APP through the E1 domain, which triggers APP ectodomain shedding and recruitment of the intracellular FE65 and Pak1 complex and associated Rac1 GTPase activation. Our study establishes APP as a novel receptor for Slit ligand mediating axon guidance and neural circuit formation.

Semaphorin 3C Released from a Biocompatible Hydrogel Guides and Promotes Axonal Growth of Rodent and Human Dopaminergic Neurons

Cell therapy in experimental models of Parkinson's disease replaces the lost dopamine neurons (DAN), but we still need improved methods to guide dopaminergic axons (DAx) of grafted neurons to make proper connections. The protein Semaphorin 3C (Sema3C) attracts DAN axons and enhances their growth. In this work, we show that the hydrogel PuraMatrix, a self-assembling peptide-based matrix, incorporates Sema3C and releases it steadily during 4 weeks. We also tested if hydrogel-delivered Sema3C attracts DAx using a system of rat midbrain explants embedded in collagen gels. We show that Sema3C released by this hydrogel attracts DAx, in a similar way to pretectum, which is known to attract growing DAN axons. We assessed the effect of Sema3C on the growth of DAx using microfluidic devices. DAN from rat midbrain or those differentiated from human embryonic stem cells showed enhanced axonal extension when exposed to hydrogel-released Sema3C, similar to soluble Sema3C. Notably, DAN of human origin express the cognate Sema3C receptors, Neuropilin1 and Neuropilin2. These results show that PuraMatrix is able to incorporate and release Sema3C, and such delivery guides and promotes the axonal growth of DAN. This biocompatible hydrogel might be useful as a Sema3C carrier for in vivo studies in parkinsonian animal models.

Co-immobilization of semaphorin3A and nerve growth factor to guide and pattern axons

Immobilization of axon guidance cues offers a powerful tissue regenerative strategy to control the presentation and spatial location of these biomolecules. We use our previously developed immobilization strategy to specifically tether recombinant biotinylated nerve growth factor (bNGF) and biotinylated semaphorin3A (bSema3A) to chitosan films as an outgrowth and guidance platform. DRG neurite length and number for a range of single cues of immobilized bNGF or bSema3A were examined to determine a concentration response. Next single and dual cues of bNGF and bSema3A were immobilized and DRG guidance was assessed in response to a step concentration change from zero. Overall, immobilized groups caused axon extension, retraction and turning depending on the ratio of bNGF and bSema3A immobilized in the encountered region. This response indicated the exquisite sensitivity of DRG axons to both attractive and repulsive tethered cues. bSema3A concentrations of 0.10 and 0.49 ng/mm(2), when co-immobilized with bNGF (at 0.86 and 0.43 ng/mm(2) respectively), caused axons to turn away from the co-immobilized region. Immunocytochemical analysis showed that at these bSema3A concentrations, axons inside the co-immobilized region display microtubule degradation and breakdown of actin filaments. At the lowest bSema3A concentration (0.01 ng/mm(2)) co-immobilized with a higher bNGF concentration (2.16 ng/mm(2)), neurite lengths are shorter in the immobilized area, but bNGF dominates the guidance mechanism as neurites are directed toward the immobilized region. Future applications can pattern these cues in various geometries and gradients in order to better modulate axon guidance in terms of polarity, extension and branching.

STATEMENT OF SIGNIFICANCE

Nervous system formation and regeneration requires key molecules for guiding the growth cone and nervous system patterning. In vivo these molecules work in conjunction with one another to modulate axon guidance, and often they are tethered to limit spatial distribution. The novelty of this research is that we provide a specific attachment method to immobilize an attractive signal, nerve growth factor, along with an inhibitory cue, semaphorin3A, to a substrate in order to analyze the interplay of these proteins on axon guidance responses. The scientific impact of this manuscript is that we show that dual-cued platforms are necessary in order to finetune and tailor specific axon responses for varying neuronal regenerative purposes.

Microengineered peripheral nerve-on-a-chip for preclinical physiological testing

The use of advanced in vitro testing is a powerful tool to develop predictive cellular assays suitable for improving the high attrition rates of novel pharmaceutical compounds. A microscale, organotypic model of nerve tissue with physiological measures that mimic clinical nerve compound action potential (CAP) and nerve fiber density (NFD) tests may be more predictive of clinical outcomes, enabling a more cost-effective approach for selecting promising lead compounds with higher chances of late-stage success. However, the neurological architecture, physiology, and surrounding extracellular matrix are hard to mimic in vitro. Using a dual hydrogel construct and explants from rat embryonic dorsal root ganglia, the present study describes an in vitro method for electrophysiological recording of intra- and extra-cellular recordings using a spatially-controlled, microengineered sensory neural fiber tract. Specifically, these 3D neural cultures exhibit both structural and functional characteristics that closely mimic those of afferent sensory peripheral fibers found in vivo. Our dual hydrogel system spatially confines growth to geometries resembling nerve fiber tracts, allowing for a high density of parallel, fasciculated neural growth. Perhaps more importantly, outputs resembling clinically relevant test criteria, including the measurement of CAP and NFD are possible through our advanced model. Moreover, the 3D hydrogel constructs allow flexibility in incorporated cell type, geometric fabrication, and electrical manipulation, providing a viable assay for systematic culture, perturbation, and testing of biomimetic neural growth for mechanistic studies necessitating physiologically-relevant readouts.