YAP and TAZ regulate Schwann cell proliferation and differentiation during peripheral nerve regeneration

YAP and TAZ are effectors of the Hippo pathway that controls multicellular development by integrating chemical and mechanical signals. Peripheral nervous system development depends on the Hippo pathway. We previously showed that loss of YAP and TAZ impairs the development of peripheral nerve as well as Schwann cell myelination. The role of the Hippo pathway in peripheral nerve regeneration has just started to be explored. After injury, Schwann cells adopt new identities to promote regeneration by converting to a repair‐promoting phenotype. While the reprogramming of Schwann cells to repair cells has been well characterized, the maintenance of such repair phenotype cannot be sustained for a very long period, which limits nerve repair in human. First, we show that short or long‐term myelin maintenance is not affected by defect in YAP and TAZ expression. Using crush nerve injury and conditional mutagenesis in mice, we also show that YAP and TAZ are regulators of repair Schwann cell proliferation and differentiation. We found that YAP and TAZ are required in repair Schwann cells for their redifferentiation into myelinating Schwann cell following crush injury. In this present study, we describe how the Hippo pathway and YAP and TAZ regulate remyelination over time during peripheral nerve regeneration.

Prohibitin 1 is essential to preserve mitochondria and myelin integrity in Schwann cells

In peripheral nerves, Schwann cells form myelin and provide trophic support to axons. We previously showed that the mitochondrial protein prohibitin 2 can localize to the axon-Schwann-cell interface and is required for developmental myelination. Whether the homologous protein prohibitin 1 has a similar role, and whether prohibitins also play important roles in Schwann cell mitochondria is unknown. Here, we show that deletion of prohibitin 1 in Schwann cells minimally perturbs development, but later triggers a severe demyelinating peripheral neuropathy. Moreover, mitochondria are heavily affected by ablation of prohibitin 1 and demyelination occurs preferentially in cells with apparent mitochondrial loss. Furthermore, in response to mitochondrial damage, Schwann cells trigger the integrated stress response, but, contrary to what was previously suggested, this response is not detrimental in this context. These results identify a role for prohibitin 1 in myelin integrity and advance our understanding about the Schwann cell response to mitochondrial damage.

Transferrin and thyroid hormone converge in the control of myelinogenesis

Myelination is a concerted mechanism tightly regulated in the brain. Although several factors are known to participate during this process, the complete sequence of events is far from being fully elucidated. Separate effects of apotransferrin (aTf) and thyroid hormone (TH) are well documented on rat myelin formation. TH promotes the maturation of oligodendrocyte progenitors (OPCs) into myelinating oligodendrocytes (OLGs), while aTf is able to induce the commitment of neural stem cells (NSCs) toward the oligodendroglial linage and favors OLG maturation. We have also demonstrated that Tf mRNA exhibited a seven-fold increase in hyperthyroid animals. These observations have led us to hypothesize that both factors may interplay during oligodendrogenesis. To assess the combined effects of aTf and TH on proper myelination in the rat brain, Tf expression and oligodendroglial maturation were evaluated at postnatal days 10 (P10) and 20 (P20) in several experimental groups. At P10, an up-regulation of both Tf mRNA and protein, as well as myelination, was found in hyperthyroid animals, while a decrease in Tf mRNA levels and myelin formation was detected in the hypothyroid group. At P20, no differences were found either in Tf mRNA or protein levels between hyperthyroid and control (Ctrol) rats, although differences in OLG differentiation remained. Also at P20, hypothyroid animals showed decreased Tf mRNA and protein levels accompanied with a less mature myelinating phenotype. Moreover, TH and aTf differentially regulate the expression of KLF9 transcription factor as well as TRα and TRβ at P10 and P20. Our results suggest that TH is necessary early in OLG development for aTf action, as exogenous aTf administration was unable to counteract the effect of low TH levels in the hypothyroid state in all the time points analyzed. Furthermore, the fact that hyperthyroidism induced an increase in Tf expression and aTf-dependent regulation of TRα strongly suggests that Tf could be involved in some of TH later effects on OLG maturation. Here we describe the possible relationship between TH and aTf and its implication in oligodendrogenesis.

Laminin α2 controls mouse and human stem cell behaviour during midbrain dopaminergic neuron development

Development of the central nervous system requires coordination of the proliferation and differentiation of neural stem cells. Here, we show that laminin alpha 2 (lm-α2) is a component of the midbrain dopaminergic neuron (mDA) progenitor niche in the ventral midbrain (VM) and identify a concentration-dependent role for laminin α2β1γ1 (lm211) in regulating mDA progenitor proliferation and survival via a distinct set of receptors. At high concentrations, lm211-rich environments maintain mDA progenitors in a proliferative state via integrins α6β1 and α7β1, whereas low concentrations of lm211 support mDA lineage survival via dystroglycan receptors. We confirmed our findings in vivo, demonstrating that the VM was smaller in the absence of lm-α2, with increased apoptosis; furthermore, the progenitor pool was depleted through premature differentiation, resulting in fewer mDA neurons. Examination of mDA neuron subtype composition showed a reduction in later-born mDA neurons of the ventral tegmental area, which control a range of cognitive behaviours. Our results identify a novel role for laminin in neural development and provide a possible mechanism for autism-like behaviours and the brainstem hypoplasia seen in some individuals with mutations of LAMA2.

Combined effects of transferrin and thyroid hormone during oligodendrogenesis In vitro

Thyroid hormones (THs) and transferrin (Tf) are factors capable of favoring myelination due to their positive effects on oligodendroglial cell (OLG) differentiation. The first notion of a combined effect of apotransferrin (aTf) and TH emerged from experiments conducted in young hyperthyroid animals, which showed a seven-fold increase in the expression of Tf mRNA and precocious myelination when compared with control animals. The mechanism underlying this phenomenon in young hyperthyroid rats could consist of an increase in Tf synthesis, which in the CNS is almost exclusively produced by OLG. Overall, our results show that, during the initial stages of OLG differentiation, Tf synthesis triggers thyroid hormone receptor alpha 1 (TRα1) expression in the subventricular zone (SVZ) and promotes proliferating cells to become responsive to this trophic factor. Exposure to TH could then regulate Tf expression through TRα1 and promote the induction of thyroid hormone receptor beta (TRβ) expression, which mediates TH effects on myelination through the activation of final OLG differentiation. This regulation of the combined effects of Tf and THs implies that both factors are fundamental actors during oligodendrogenesis. GLIA 2016;64:1879-1891.

Age-related neurodegeneration and cognitive impairments of NRMT1 knockout mice are preceded by misregulation of RB and abnormal neural stem cell development

N-terminal methylation is an important posttranslational modification that regulates protein/DNA interactions and plays a role in many cellular processes, including DNA damage repair, mitosis, and transcriptional regulation. Our generation of a constitutive knockout mouse for the N-terminal methyltransferase NRMT1 demonstrated its loss results in severe developmental abnormalities and premature aging phenotypes. As premature aging is often accompanied by neurodegeneration, we more specifically examined how NRMT1 loss affects neural pathology and cognitive behaviors. Here we find that Nrmt1-/- mice exhibit postnatal enlargement of the lateral ventricles, age-dependent striatal and hippocampal neurodegeneration, memory impairments, and hyperactivity. These morphological and behavior abnormalities are preceded by alterations in neural stem cell (NSC) development. Early expansion and differentiation of the quiescent NSC pool in Nrmt1-/- mice is followed by its subsequent depletion and many of the resulting neurons remain in the cell cycle and ultimately undergo apoptosis. These cell cycle phenotypes are reminiscent to those seen with loss of the NRMT1 target retinoblastoma protein (RB). Accordingly, we find misregulation of RB phosphorylation and degradation in Nrmt1-/- mice, and significant de-repression of RB target genes involved in cell cycle. We also identify novel de-repression of Noxa, an RB target gene that promotes apoptosis. These data identify Nα-methylation as a novel regulatory modification of RB transcriptional repression during neurogenesis and indicate that NRMT1 and RB work together to promote NSC quiescence and prevent neuronal apoptosis.

YAP and TAZ regulate remyelination in the central nervous system

Myelinating cells are sensitive to mechanical stimuli from their extracellular matrix. Ablation of YAP and TAZ mechanotransducers in Schwann cells abolishes the axon-Schwann cell recognition, myelination, and remyelination in the peripheral nervous system. It was unknown if YAP and TAZ are also required for myelination and remyelination in the central nervous system. Here we define the importance of oligodendrocyte (OL) YAP and TAZ in vivo, by specific deletion in oligodendroglial cells in adult OLs during myelin repair. Blocking YAP and TAZ expression in OL lineage cells did not affect animal viability or any major defects on OL maturation and myelination. However, using a mouse model of demyelination/remyelination, we demonstrate that YAP and TAZ modulate the capacity of OLs to remyelinate axons, particularly during the early stage of the repair process, when OL proliferation is most important. These results indicate that YAP and TAZ signaling is necessary for effective remyelination of the mouse brain.

Human iPSC-derived myelinating organoids and globoid cells to study Krabbe disease

Krabbe disease (Kd) is a lysosomal storage disorder (LSD) caused by the deficiency of the lysosomal galactosylceramidase (GALC) which cleaves the myelin enriched lipid galactosylceramide (GalCer). Accumulated GalCer is catabolized into the cytotoxic lipid psychosine that causes myelinating cells death and demyelination which recruits microglia/macrophages that fail to digest myelin debris and become globoid cells. Here, to understand the pathological mechanisms of Kd, we used induced pluripotent stem cells (iPSCs) from Kd patients to produce myelinating organoids and microglia. We show that Kd organoids have no obvious defects in neurogenesis, astrogenesis, and oligodendrogenesis but manifest early myelination defects. Specifically, Kd organoids showed shorter but a similar number of myelin internodes than Controls at the peak of myelination and a reduced number and shorter internodes at a later time point. Interestingly, myelin is affected in the absence of autophagy and mTOR pathway dysregulation, suggesting lack of lysosomal dysfunction which makes this organoid model a very valuable tool to study the early events that drive demyelination in Kd. Kd iPSC-derived microglia show a marginal rate of globoid cell formation under normal culture conditions that is drastically increased upon GalCer feeding. Under normal culture conditions, Kd microglia show a minor LAMP1 content decrease and a slight increase in the autophagy protein LC3B. Upon GalCer feeding, Kd cells show accumulation of autophagy proteins and strong LAMP1 reduction that at a later time point are reverted showing the compensatory capabilities of globoid cells. Altogether, this supports the value of our cultures as tools to study the mechanisms that drive globoid cell formation and the compensatory mechanism in play to overcome GalCer accumulation in Kd.

p38γ MAPK delays myelination and remyelination and is abundant in multiple sclerosis lesions

Multiple sclerosis is a chronic inflammatory disease in which disability results from the disruption of myelin and axons. During the initial stages of the disease, injured myelin is replaced by mature myelinating oligodendrocytes that differentiate from oligodendrocyte precursor cells. However, myelin repair fails in secondary and chronic progressive stages of the disease and with ageing, as the environment becomes progressively more hostile. This may be attributable to inhibitory molecules in the multiple sclerosis environment including activation of the p38MAPK family of kinases. We explored oligodendrocyte precursor cell differentiation and myelin repair using animals with conditional ablation of p38MAPKγ from oligodendrocyte precursors. We found that p38γMAPK ablation accelerated oligodendrocyte precursor cell differentiation and myelination. This resulted in an increase in both the total number of oligodendrocytes and the migration of progenitors ex vivo and faster remyelination in the cuprizone model of demyelination/remyelination. Consistent with its role as an inhibitor of myelination, p38γMAPK was significantly downregulated as oligodendrocyte precursor cells matured into oligodendrocytes. Notably, p38γMAPK was enriched in multiple sclerosis lesions from patients. Oligodendrocyte progenitors expressed high levels of p38γMAPK in areas of failed remyelination but did not express detectable levels of p38γMAPK in areas where remyelination was apparent. Our data suggest that p38γ could be targeted to improve myelin repair in multiple sclerosis.

A new mouse model of Charcot-Marie-Tooth 2J neuropathy replicates human axonopathy and suggest alteration in axo-glia communication

Myelin is essential for rapid nerve impulse propagation and axon protection. Accordingly, defects in myelination or myelin maintenance lead to secondary axonal damage and subsequent degeneration. Studies utilizing genetic (CNPase-, MAG-, and PLP-null mice) and naturally occurring neuropathy models suggest that myelinating glia also support axons independently from myelin. Myelin protein zero (MPZ or P0), which is expressed only by Schwann cells, is critical for myelin formation and maintenance in the peripheral nervous system. Many mutations in MPZ are associated with demyelinating neuropathies (Charcot-Marie-Tooth disease type 1B [CMT1B]). Surprisingly, the substitution of threonine by methionine at position 124 of P0 (P0T124M) causes axonal neuropathy (CMT2J) with little to no myelin damage. This disease provides an excellent paradigm to understand how myelinating glia support axons independently from myelin. To study this, we generated targeted knock-in MpzT124M mutant mice, a genetically authentic model of T124M-CMT2J neuropathy. Similar to patients, these mice develop axonopathy between 2 and 12 months of age, characterized by impaired motor performance, normal nerve conduction velocities but reduced compound motor action potential amplitudes, and axonal damage with only minor compact myelin modifications. Mechanistically, we detected metabolic changes that could lead to axonal degeneration, and prominent alterations in non-compact myelin domains such as paranodes, Schmidt-Lanterman incisures, and gap junctions, implicated in Schwann cell-axon communication and axonal metabolic support. Finally, we document perturbed mitochondrial size and distribution along MpzT124M axons suggesting altered axonal transport. Our data suggest that Schwann cells in P0T124M mutant mice cannot provide axons with sufficient trophic support, leading to reduced ATP biosynthesis and axonopathy. In conclusion, the MpzT124M mouse model faithfully reproduces the human neuropathy and represents a unique tool for identifying the molecular basis for glial support of axons.

Human iPSC-derived myelinating organoids and globoid cells to study Krabbe Disease

Krabbe disease (Kd) is a lysosomal storage disorder (LSD) caused by the deficiency of the lysosomal galactosylceramidase (GALC) which cleaves the myelin enriched lipid galactosylceramide (GalCer). Accumulated GalCer is catabolized into the cytotoxic lipid psychosine that causes myelinating cells death and demyelination which recruits microglia/macrophages that fail to digest myelin debris and become globoid cells. Here, to understand the pathological mechanisms of Kd, we used induced pluripotent stem cells (iPSCs) from Kd patients to produce myelinating organoids and microglia. We show that Kd organoids have no obvious defects in neurogenesis, astrogenesis, and oligodendrogenesis but manifest early myelination defects. Specifically, Kd organoids showed shorter but a similar number of myelin internodes than Controls at the peak of myelination and a reduced number and shorter internodes at a later time point. Interestingly, myelin is affected in the absence of autophagy and mTOR pathway dysregulation, suggesting lack of lysosomal dysfunction which makes this organoid model a very valuable tool to study the early events that drive demyelination in Kd. Kd iPSC-derived microglia show a marginal rate of globoid cell formation under normal culture conditions that is drastically increased upon GalCer feeding. Under normal culture conditions, Kd microglia show a minor LAMP1 content decrease and a slight increase in the autophagy protein LC3B. Upon GalCer feeding, Kd cells show accumulation of autophagy proteins and strong LAMP1 reduction that at a later time point are reverted showing the compensatory capabilities of globoid cells. Altogether, this supports the value of our cultures as tools to study the mechanisms that drive globoid cell formation and the compensatory mechanism in play to overcome GalCer accumulation in Kd.

Pharmacologically increasing cGMP improves proteostasis and reduces neuropathy in mouse models of CMT1

Increasing cyclic GMP activates 26S proteasomes via phosphorylation by Protein Kinase G and stimulates the intracellular degradation of misfolded proteins. Therefore, agents that raise cGMP may be useful therapeutics against neurodegenerative diseases and other diseases in which protein degradation is reduced and misfolded proteins accumulate, including Charcot Marie Tooth 1A and 1B peripheral neuropathies, for which there are no treatments. Here we increased cGMP in the S63del mouse model of CMT1B by treating for three weeks with either the phosphodiesterase 5 inhibitor tadalafil, or the brain-penetrant soluble guanylyl cyclase stimulator CYR119. Both molecules activated proteasomes in the affected peripheral nerves, reduced polyubiquitinated proteins, and improved myelin thickness and nerve conduction. CYR119 increased cGMP more than tadalafil in the peripheral nerves of S63del mice and elicited greater biochemical and functional improvements. To determine whether raising cGMP could be beneficial in other neuropathies, we first showed that polyubiquitinated proteins and the disease-causing protein accumulate in the sciatic nerves of the C3 mouse model of CMT1A. Treatment of these mice with CYR119 reduced the levels of polyubiquitinated proteins and the disease-causing protein, presumably by increasing their degradation, and improved myelination, nerve conduction, and motor coordination. Thus, pharmacological agents that increase cGMP are promising treatments for CMT1 neuropathies and may be useful against other proteotoxic and neurodegenerative diseases.