Alexander Schulz

Merlin isoform 2 in neurofibromatosis type 2-associated polyneuropathy.

The autosomal dominant disorder neurofibromatosis type 2 (NF2) is a hereditary tumor syndrome caused by inactivation of the NF2 tumor suppressor gene, encoding merlin. Apart from tumors affecting the peripheral and central nervous systems, most NF2 patients develop peripheral neuropathies. This peripheral nerve disease can occur in the absence of nerve-damaging tumors, suggesting an etiology that is independent of gross tumor burden. We discovered that merlin isoform 2 (merlin-iso2) has a specific function in maintaining axonal integrity and propose that reduced axonal NF2 gene dosage leads to NF2-associated polyneuropathy. We identified a merlin-iso2–dependent complex that promotes activation of the GTPase RhoA, enabling downstream Rho-associated kinase to promote neurofilament heavy chain phosphorylation. Merlin-iso2–deficient mice exhibited impaired locomotor capacities, delayed sensory reactions and electrophysiological signs of axonal neuropathy. Sciatic nerves from these mice and sural nerve biopsies from NF2 patients revealed reduced phosphorylation of the neurofilament H subunit, decreased interfilament spacings and irregularly shaped axons.

Merlin Inhibits Neurite Outgrowth in the CNS

The neurofibromatosis type 2 gene product merlin is known to provoke gliogenic tumors as a result of its mutagenic loss. Merlins physiological anti-mitogenic function makes it unique among its ezrin–radixin–moesin (ERM) family members. Although ERM proteins and merlin are known to be expressed in glial cells of the peripheral nervous system and CNS, the neuronal expression pattern and function of merlin have been less well investigated. We report here expression of merlin in developing and mature neurons of the murine CNS. Within cerebellar Purkinje cells (PCs), merlin was localized in the soma, sprouting dendrites and axons. Merlin expression in PCs was high during the period of initial dendrite regression and declined during later phases of dendrite elongation. Consistently, merlin expression in vivo was increased in Engrailed-2-overexpressing PCs, which are characterized by a reduced dendritic extension. Furthermore, overexpression of merlin in dissociated cerebellar cultures and in neurogenic P19 cells caused a significant decline in neurite outgrowth, while, conversely, inhibition of merlin expression increased process formation. This effect was dependent on phosphorylation of serine 518 and involved the inactivation of the growth-promoting GTPase Rac. We thus provide evidence that merlin plays a pivotal role in controlling the neuronal wiring in the developing CNS.

Merlin controls the repair capacity of Schwann cells after injury by regulating Hippo/YAP activity

Loss of the Merlin tumor suppressor and activation of the Hippo signaling pathway play major roles in the control of cell proliferation and tumorigenesis. We have identified completely novel roles for Merlin and the Hippo pathway effector Yes-associated protein (YAP) in the control of Schwann cell (SC) plasticity and peripheral nerve repair after injury. Injury to the peripheral nervous system (PNS) causes a dramatic shift in SC molecular phenotype and the generation of repair-competent SCs, which direct functional repair. We find that loss of Merlin in these cells causes a catastrophic failure of axonal regeneration and remyelination in the PNS. This effect is mediated by activation of YAP expression in Merlin-null SCs, and loss of YAP restores axonal regrowth and functional repair. This work identifies new mechanisms that control the regenerative potential of SCs and gives new insight into understanding the correct control of functional nerve repair in the PNS.

Neuronal merlin influences ERBB2 receptor expression on Schwann cells through neuregulin 1 type III signalling

Merlin mutations in Neurofibromatosis type 2 cause tumorigenic transformation of Schwann cells, leading to schwannoma development. Schulz et al. show that loss of neuronally expressed merlin alone increases the susceptibility of adjacent Schwann cells to mitogenic signals through the Neuregulin1-ErbB2/3 pathway.

The importance of nerve microenvironment for schwannoma development

Schwannomas are predominantly benign nerve sheath neoplasms caused by Nf2 gene inactivation. Presently, treatment options are mainly limited to surgical tumor resection due to the lack of effective pharmacological drugs. Although the mechanistic understanding of Nf2 gene function has advanced, it has so far been primarily restricted to Schwann cell-intrinsic events. Extracellular cues determining Schwann cell behavior with regard to schwannoma development remain unknown. Here we show pro-tumourigenic microenvironmental effects on Schwann cells where an altered axonal microenvironment in cooperation with injury signals contribute to a persistent regenerative Schwann cell response promoting schwannoma development. Specifically in genetically engineered mice following crush injuries on sciatic nerves, we found macroscopic nerve swellings in mice with homozygous nf2 gene deletion in Schwann cells and in animals with heterozygous nf2 knockout in both Schwann cells and axons. However, patient-mimicking schwannomas could only be provoked in animals with combined heterozygous nf2 knockout in Schwann cells and axons. We identified a severe re-myelination defect and sustained macrophage presence in the tumor tissue as major abnormalities. Strikingly, treatment of tumor-developing mice after nerve crush injury with medium-dose aspirin significantly decreased schwannoma progression in this disease model. Our results suggest a multifactorial concept for schwannoma formation—emphasizing axonal factors and mechanical nerve irritation as predilection site for schwannoma development. Furthermore, we provide evidence supporting the potential efficacy of anti-inflammatory drugs in the treatment of schwannomas.

Multifocal Nerve Lesions and LZTR1 Germline Mutations in Segmental Schwannomatosis

Schwannomatosis is a genetic disorder characterized by the occurrence of multiple peripheral schwannomas. Segmental schwannomatosis is diagnosed when schwannomas are restricted to 1 extremity and is thought to be caused by genetic mosaicism. We studied 5 patients with segmental schwannomatosis through microstructural magnetic resonance neurography and mutation analysis of NF2, SMARCB1, and LZTR1. In 4 of 5 patients, subtle fascicular nerve lesions were detected in clinically unaffected extremities. Two patients exhibited LZTR1 germline mutations. This appears contrary to a simple concept of genetic mosaicism and suggests more complex and heterogeneous mechanisms underlying the phenotype of segmental schwannomatosis than previously thought. Ann Neurol 2016;80:625–628

Inhibition of RAS activation due to a homozygous ezrin variant in patients with profound intellectual disability.

Gain‐of‐function alterations in several components and modulators of the Ras‐MAPK pathway lead to dysregulation of the pathway and cause a broad spectrum of autosomal dominant developmental disorders, collectively known as RASopathies. These findings demonstrate the importance of tight multilevel Ras regulation to safeguard signaling output and prevent aberrant activity. We have recently identified ezrin as a novel regulatory element required for Ras activation. Homozygosity mapping and exome sequencing have now revealed the first presumably disease‐causing variant in the coding gene EZR in two siblings with a profound intellectual disability. Localization and membrane targeting of the altered ezrin protein appeared normal but molecular modeling suggested protein interaction surfaces to be disturbed. Functional analysis revealed that the altered ezrin protein is no longer able to bind Ras and facilitate its activation. Furthermore, expression of the altered ezrin protein in different cell lines resulted in abnormal cellular processes, including reduced proliferation and neuritogenesis, thus revealing a possible mechanism for its phenotype in humans.

Neuropathies in the setting of Neurofibromatosis tumor syndromes: Complexities and opportunities

ABSTRACT The term ‘Neurofibromatosis’ (NF) comprises a group of rare diseases with related clinical presentations but distinct genetic conditions. All currently known types – NF1, NF2 and Schwannomatosis – predispose afflicted individuals to the development of glial cell‐derived (gliogenic) tumors. Furthermore, the occurrence of neuropathic symptoms, which add to the overall neurologic disability of patients, has been described in all disease entities. We show that neuropathic symptoms are a common and clinically important, yet infrequently studied feature in the NF spectrum. However, the clinical relevance and respective underlying pathogenesis, varies greatly among the different NF types. In this review, we summarize and interpret the latest basic research findings, as well as clinical observations, in respect of Neurofibromatosis‐associated neuropathies. HIGHLIGHTSNeuropathic symptoms are common and clinically relevant, yet infrequently studied features of Neurofibromatosis tumor syndromes.Tumor‐independent neuropathy in NF1 is rare and its pathophysiological basis poorly understood.NF2‐related neuropathy is often progressive and most probably caused by a complex and multifactorial pathogenesis.In Schwannomatosis, unclear but chronic neuropathic pain affects 80% of patients.Since neuropathic symptoms can have severe impact on the quality of life of patients a better understanding of NF‐related neuropathies is urgently needed.

Neuron-Specific Deletion of the Nf2 Tumor Suppressor Impairs Functional Nerve Regeneration

In contrast to axons of the central nervous system (CNS), axons of the peripheral nervous system (PNS) show better, but still incomplete and often slow regeneration following injury. The tumor suppressor protein merlin, mutated in the hereditary tumor syndrome Neurofibromatosis type 2 (NF2), has recently been shown to have RhoA regulatory functions in PNS neurons—in addition to its well-characterized, growth-inhibitory activity in Schwann cells. Here we report that the conditional knockout of merlin in PNS neurons leads to impaired functional recovery of mice following sciatic nerve crush injury, in a gene-dosage dependent manner. Gross anatomical or electrophysiological alterations of sciatic nerves could not be detected. However, correlating with attenuated RhoA activation due to merlin deletion, ultrastructural analysis of nerve samples indicated enhanced sprouting of axons with reduced caliber size and increased myelination compared to wildtype animals. We conclude that deletion of the tumor suppressor merlin in the neuronal compartment of peripheral nerves results in compromised functional regeneration after injury. This mechanism could explain the clinical observation that NF2 patients suffer from higher incidences of slowly recovering facial nerve paralysis after vestibular schwannoma surgery.

Merlin Isoforms 1 and 2 Both Act as Tumour Suppressors and Are Required for Optimal Sperm Maturation

The tumour suppressor Merlin, encoded by the gene NF2, is frequently mutated in the autosomal dominant disorder neurofibromatosis type II, characterised primarily by the development of schwannoma and other glial cell tumours. However, NF2 is expressed in virtually all analysed human and rodent organs, and its deletion in mice causes early embryonic lethality. Additionally, NF2 encodes for two major isoforms of Merlin of unknown functionality. Specifically, the tumour suppressor potential of isoform 2 remains controversial. In this study, we used Nf2 isoform-specific knockout mouse models to analyse the function of each isoform during development and organ homeostasis. We found that both isoforms carry full tumour suppressor functionality and can completely compensate the loss of the other isoform during development and in most adult organs. Surprisingly, we discovered that spermatogenesis is strictly dependent on the presence of both isoforms. While the testis primarily expresses isoform 1, we noticed an enrichment of isoform 2 in spermatogonial stem cells. Deletion of either isoform was found to cause decreased sperm quality as observed by maturation defects and head/midpiece abnormalities. These defects led to impaired sperm functionality as assessed by decreased sperm capacitation. Thus, we describe spermatogenesis as a new Nf2-dependent process. Additionally, we provide for the first time in vivo evidence for equal tumour suppressor potentials of Merlin isoform 1 and isoform 2.