A high-content platform for physiological profiling and unbiased classification of individual neurons

Harnessing the potential of human induced pluripotent stem cells, functional assays and machine learning for neurodevelopmental disorders

Neurodevelopmental disorders (NDDs) affect 4.7% of the global population and are associated with delays in brain development and a spectrum of impairments that can lead to lifelong disability and even mortality. Identification of biomarkers for accurate diagnosis and medications for effective treatment are lacking, in part due to the historical use of preclinical model systems that do not translate well to the clinic for neurological disorders, such as rodents and heterologous cell lines. Human-induced pluripotent stem cells (hiPSCs) are a promising in vitro system for modeling NDDs, providing opportunities to understand mechanisms driving NDDs in human neurons. Functional assays, including patch clamping, multielectrode array, and imaging-based assays, are popular tools employed with hiPSC disease models for disease investigation. Recent progress in machine learning (ML) algorithms also presents unprecedented opportunities to advance the NDD research process. In this review, we compare two-dimensional and three-dimensional hiPSC formats for disease modeling, discuss the applications of functional assays, and offer insights on incorporating ML into hiPSC-based NDD research and drug screening.

Physiological profiling of cannabidiol reveals profound inhibition of sensory neurons

ABSTRACT

Cannabidiol (CBD), the main nonpsychoactive cannabinoid of cannabis, holds promise for nonaddictive treatment of pain. Although preclinical studies have been encouraging, well-controlled human trials have been largely unsuccessful. To investigate this dichotomy and better understand the actions of CBD, we used high-content calcium imaging with automated liquid handling and observed broad inhibition of neuronal activation by a host of ionotropic and metabotropic receptors, including transient receptor potential (Trp) and purinergic receptors, as well as mediators of intracellular calcium cycling. To assess the effect of CBD on overall nociceptor electrical activity, we combined the light-activated ion channel channelrhodposin in TRPV1-positive nociceptors and a red-shifted calcium indicator and found that 1 µM CBD profoundly increased the optical threshold for calcium flux activation. Experiments using traditional whole-cell patch-clamp showed increase of nociceptor activation threshold at submicromolar concentrations, but with unusually slow kinetics, as well as block of voltage-activated currents. To address a more integrated capacity of CBD to influence nociceptor sensitization, a process implicated in multiple pain states, we found that submicromolar concentrations of CBD inhibited sensitization by the chemotherapeutic drug vincristine. Taken together, these results demonstrate that CBD can reduce neuronal activity evoked by a strikingly wide range of stimuli implicated in pain signaling. The extensive effects underscore the need for further studies at substantially lower drug concentrations, which are more likely to reflect physiologically relevant mechanisms. The slow kinetics and block raise biophysical questions regarding the lipophilic properties of CBD and its action on channels and receptors within membranes.

Interleukin-1α links peripheral CaV2.2 channel activation to rapid adaptive increases in heat sensitivity in skin

Neurons have the unique capacity to adapt output in response to changes in their environment. Within seconds, sensory nerve endings can become hypersensitive to stimuli in response to potentially damaging events. The underlying behavioral response is well studied, but several of the key signaling molecules that mediate sensory hypersensitivity remain unknown. We previously discovered that peripheral voltage-gated CaV2.2 channels in nerve endings in skin are essential for the rapid, transient increase in sensitivity to heat, but not to mechanical stimuli, that accompanies intradermal capsaicin. Here we report that the cytokine interleukin-1α (IL-1α), an alarmin, is necessary and sufficient to trigger rapid heat and mechanical hypersensitivity in skin. Of 20 cytokines screened, only IL-1α was consistently detected in hind paw interstitial fluid in response to intradermal capsaicin and, similar to behavioral sensitivity to heat, IL-1α levels were also dependent on peripheral CaV2.2 channel activity. Neutralizing IL-1α in skin significantly reduced capsaicin-induced changes in hind paw sensitivity to radiant heat and mechanical stimulation. Intradermal IL-1α enhances behavioral responses to stimuli and, in culture, IL-1α enhances the responsiveness of Trpv1-expressing sensory neurons. Together, our data suggest that IL-1α is the key cytokine that underlies rapid and reversible neuroinflammatory responses in skin.

Interleukin-1α links peripheral CaV2.2 channel activation to rapid adaptive increases in heat sensitivity in skin

Neurons have the unique capacity to adapt output in response to changes in their environment. Within seconds, sensory nerve endings can become hypersensitive to stimuli in response to potentially damaging events. The underlying behavioral response is well studied, but several of the key signaling molecules that mediate sensory hypersensitivity remain unknown. We previously discovered that peripheral voltage-gated CaV2.2 channels in nerve endings in skin are essential for the rapid, transient increase in sensitivity to heat, but not to mechanical stimuli, that accompanies intradermal capsaicin. Here we report that the cytokine interleukin-1α (IL-1α), an alarmin, is necessary and sufficient to trigger rapid heat and mechanical hypersensitivity in skin. Of 20 cytokines screened, only IL-1α was consistently detected in hind paw interstitial fluid in response to intradermal capsaicin and, similar to behavioral sensitivity to heat, IL-1α levels were also dependent on peripheral CaV2.2 channel activity. Neutralizing IL-1α in skin significantly reduced capsaicin-induced changes in hind paw sensitivity to radiant heat and mechanical stimulation. Intradermal IL-1α enhances behavioral responses to stimuli and, in culture, IL-1α enhances the responsiveness of Trpv1-expressing sensory neurons. Together, our data suggest that IL-1α is the key cytokine that underlies rapid and reversible neuroinflammatory responses in skin.

Human OPRM1 and murine Oprm1 promoter driven viral constructs for genetic access to μ-opioidergic cell types

With concurrent global epidemics of chronic pain and opioid use disorders, there is a critical need to identify, target and manipulate specific cell populations expressing the mu-opioid receptor (MOR). However, available tools and transgenic models for gaining long-term genetic access to MOR+ neural cell types and circuits involved in modulating pain, analgesia and addiction across species are limited. To address this, we developed a catalog of MOR promoter (MORp) based constructs packaged into adeno-associated viral vectors that drive transgene expression in MOR+ cells. MORp constructs designed from promoter regions upstream of the mouse Oprm1 gene (mMORp) were validated for transduction efficiency and selectivity in endogenous MOR+ neurons in the brain, spinal cord, and periphery of mice, with additional studies revealing robust expression in rats, shrews, and human induced pluripotent stem cell (iPSC)-derived nociceptors. The use of mMORp for in vivo fiber photometry, behavioral chemogenetics, and intersectional genetic strategies is also demonstrated. Lastly, a human designed MORp (hMORp) efficiently transduced macaque cortical OPRM1+ cells. Together, our MORp toolkit provides researchers cell type specific genetic access to target and functionally manipulate mu-opioidergic neurons across a range of vertebrate species and translational models for pain, addiction, and neuropsychiatric disorders.

Scalable generation of sensory neurons from human pluripotent stem cells

Development of new non-addictive analgesics requires advanced strategies to differentiate human pluripotent stem cells (hPSCs) into relevant cell types. Following principles of developmental biology and translational applicability, here we developed an efficient stepwise differentiation method for peptidergic and non-peptidergic nociceptors. By modulating specific cell signaling pathways, hPSCs were first converted into SOX10+ neural crest, followed by differentiation into sensory neurons. Detailed characterization, including ultrastructural analysis, confirmed that the hPSC-derived nociceptors displayed cellular and molecular features comparable to native dorsal root ganglion (DRG) neurons, and expressed high-threshold primary sensory neuron markers, transcription factors, neuropeptides, and over 150 ion channels and receptors relevant for pain research and axonal growth/regeneration studies (e.g., TRPV1, NAV1.7, NAV1.8, TAC1, CALCA, GAP43, DPYSL2, NMNAT2). Moreover, after confirming robust functional activities and differential response to noxious stimuli and specific drugs, a robotic cell culture system was employed to produce large quantities of human sensory neurons, which can be used to develop nociceptor-selective analgesics.

The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors

BACKGROUND AND PURPOSE

Peripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca²⁺ -sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the μ opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown.

EXPERIMENTAL APPROACH

We tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous μ receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, μ- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors.

KEY RESULTS

Sigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to μ receptors, suggesting that sigma-1R participate in TRPV1-μ receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist.

CONCLUSION AND IMPLICATIONS

Sigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing μ receptor activity.

Phenotypic screen identifies the natural product silymarin as a novel anti-inflammatory analgesic

Sensory neuron hyperexcitability is a critical driver of pathological pain and can result from axon damage, inflammation, or neuronal stress. G-protein coupled receptor signaling can induce pain amplification by modulating the activation of Trp-family ionotropic receptors and voltage-gated ion channels. Here, we sought to use calcium imaging to identify novel inhibitors of the intracellular pathways that mediate sensory neuron sensitization and lead to hyperexcitability. We identified a novel stimulus cocktail, consisting of the SSTR2 agonist L-054,264 and the S1PR3 agonist CYM5541, that elicits calcium responses in mouse primary sensory neurons in vitro as well as pain and thermal hypersensitivity in mice in vivo. We screened a library of 906 bioactive compounds and identified 24 hits that reduced calcium flux elicited by L-054,264/CYM5541. Among these hits, silymarin, a natural product derived from milk thistle, strongly reduced activation by the stimulation cocktail, as well as by a distinct inflammatory cocktail containing bradykinin and prostaglandin E2. Silymarin had no effect on sensory neuron excitability at baseline, but reduced calcium flux via Orai channels and downstream mediators of phospholipase C signaling. In vivo, silymarin pretreatment blocked development of adjuvant-mediated thermal hypersensitivity, indicating potential use as an anti-inflammatory analgesic.