Disentangling the signaling complexity of nerve growth factor receptors by CRISPR /Cas9

A p75 neurotrophin receptor-sparing nerve growth factor protects retinal ganglion cells from neurodegeneration by targeting microglia

BACKGROUND AND PURPOSE

Retinal ganglion cells (RGCs) are the output stage of retinal information processing, via their axons forming the optic nerve (ON). ON damage leads to axonal degeneration and death of RGCs, and results in vision impairment. Nerve growth factor (NGF) signalling is crucial for RGC operations and visual functions. Here, we investigate a new neuroprotective mechanism of a novel therapeutic candidate, a p75-less, TrkA-biased NGF agonist (hNGFp) in rat RGC degeneration, in comparison with wild type human NGF (hNGFwt).

EXPERIMENTAL APPROACH

Both neonate and adult rats, whether subjected or not to ON lesion, were treated with intravitreal injections or eye drops containing either hNGFp or hNGFwt. Different doses of the drugs were administered at days 1, 4 or 7 after injury for a maximum of 10 days, when immunofluorescence, electrophysiology, cellular morphology, cytokine array and behaviour studies were carried out. Pharmacokinetic evaluation was performed on rabbits treated with hNGFp ocular drops.

RESULTS

hNGFp exerted a potent RGC neuroprotection by acting on microglia cells, and outperformed hNGFwt in rescuing RGC degeneration and reducing inflammatory molecules. Delayed use of hNGFp after ON lesion resulted in better outcomes compared with treatment with hNGFwt. Moreover, hNGFp-based ocular drops were less algogenic than hNGFwt. Pharmacokinetic measurements revealed that biologically relevant quantities of hNGFp were found in the rabbit retina.

CONCLUSIONS AND IMPLICATIONS

Our data point to microglia as a new cell target through which NGF-induced TrkA signalling exerts neuroprotection of the RGC, emphasizing hNGFp as a powerful treatment to tackle retinal degeneration.

Structural basis for the transmembrane signaling and antidepressant-induced activation of the receptor tyrosine kinase TrkB

Neurotrophin receptors of the Trk family are involved in the regulation of brain development and neuroplasticity, and therefore can serve as targets for anti-cancer and stroke-recovery drugs, antidepressants, and many others. The structures of Trk protein domains in various states upon activation need to be elucidated to allow rational drug design. However, little is known about the conformations of the transmembrane and juxtamembrane domains of Trk receptors. In the present study, we employ NMR spectroscopy to solve the structure of the TrkB dimeric transmembrane domain in the lipid environment. We verify the structure using mutagenesis and confirm that the conformation corresponds to the active state of the receptor. Subsequent study of TrkB interaction with the antidepressant drug fluoxetine, and the antipsychotic drug chlorpromazine, provides a clear self-consistent model, describing the mechanism by which fluoxetine activates the receptor by binding to its transmembrane domain.

Cholinergic neurodegeneration and cholesterol metabolism dysregulation by constitutive p75NTR signaling in the p75exonIII-KO mice

Degeneration of basal forebrain cholinergic neurons (BFCNs) is a hallmark of Alzheimer's disease (AD). However, few mouse models of AD recapitulate the neurodegeneration of the cholinergic system. The p75 neurotrophin receptor, p75NTR, has been associated with the degeneration of BFCNs in AD. The senescence-accelerated mouse prone number 8 (SAMP8) is a well-accepted model of accelerated and pathological aging. To gain a better understanding of the role of p75NTR in the basal forebrain during aging, we generated a new mouse line, the SAMP8-p75exonIII-/-. Deletion of p75NTR in the SAMP8 background induces an increase in the number of BFCNs at birth, followed by a rapid decline during aging compared to the C57/BL6 background. This decrease in the number of BFCNs correlates with a worsening in the Y-maze memory test at 6 months in the SAMP8-p75exonIII-/-. We found that SAMP8-p75exonIII-/- and C57/BL6-p75exonIII-/- mice expressed constitutively a short isoform of p75NTR that correlates with an upregulation of the protein levels of SREBP2 and its targets, HMGCR and LDLR, in the BF of both SAMP8-p75exonIII-/- and C57/BL6-p75exonIII-/- mice. As the neurodegeneration of the cholinergic system and the dysregulation of cholesterol metabolism are implicated in AD, we postulate that the generated SAMP8-p75exonIII-/- mouse strain might constitute a good model to study long-term cholinergic neurodegeneration in the CNS. In addition, our results support the role of p75NTR signaling in cholesterol biosynthesis regulation.

Low Forces Push the Maturation of Neural Precursors into Neurons

Mechanical stimulation modulates neural development and neuronal activity. In a previous study, magnetic "nano-pulling" is proposed as a tool to generate active forces. By loading neural cells with magnetic nanoparticles (MNPs), a precise force vector is remotely generated through static magnetic fields. In the present study, human neural stem cells (NSCs) are subjected to a standard differentiation protocol, in the presence or absence of nano-pulling. Under mechanical stimulation, an increase in the length of the neural processes which showed an enrichment in microtubules, endoplasmic reticulum, and mitochondria is found. A stimulation lasting up to 82 days induces a strong remodeling at the level of synapse density and a re-organization of the neuronal network, halving the time required for the maturation of neural precursors into neurons. The MNP-loaded NSCs are then transplanted into mouse spinal cord organotypic slices, demonstrating that nano-pulling stimulates the elongation of the NSC processes and modulates their orientation even in an ex vivo model. Thus, it is shown that active mechanical stimuli can guide the outgrowth of NSCs transplanted into the spinal cord tissue. The findings suggest that mechanical forces play an important role in neuronal maturation which could be applied in regenerative medicine.

Human TrkAR649W mutation impairs nociception, sweating and cognitive abilities: a mouse model of HSAN IV

A functional nerve growth factor NGF-Tropomyosin Receptor kinase A (TrkA) system is an essential requisite for the generation and maintenance of long-lasting thermal and mechanical hyperalgesia in adult mammals. Indeed, mutations in the gene encoding for TrkA are responsible for a rare condition, named Hereditary Sensory and Autonomic Neuropathy type IV (HSAN IV), characterized by the loss of response to noxious stimuli, anhidrosis and cognitive impairment. However, to date, there is no available mouse model to properly understand how the NGF-TrkA system can lead to pathological phenotypes that are distinctive of HSAN IV. Here, we report the generation of a knock-in mouse line carrying the HSAN IV TrkAR649W mutation. First, by in vitro biochemical and biophysical analyses, we show that the pathological R649W mutation leads to kinase-inactive TrkA also affecting its membrane dynamics and trafficking. In agreement with the HSAN IV human phenotype, TrkAR649W/m mice display a lower response to thermal and chemical noxious stimuli, correlating with reduced skin innervation, in addition to decreased sweating in comparison to TrkAh/m controls. Moreover, the R649W mutation decreases anxiety-like behavior and compromises cognitive abilities, by impairing spatial-working and social memory. Our results further uncover unexplored roles of TrkA in thermoregulation and sociability. In addition to accurately recapitulating the clinical manifestations of HSAN IV patients, our findings contribute to clarifying the involvement of the NGF-TrkA system in pain sensation.