Proteolipid protein (PLP) is the most abundant integral membrane protein in compact central nervous system myelin, and null mutations of the PLP1 gene cause spastic paraplegia type 2 (SPG2). SPG2 patients and PLP-deficient mice exhibit only moderate abnormalities of myelin but progressive degeneration of long axons. Since Plp1 gene products are detected in a subset of neurons it has been suggested that the loss of neuronal Plp1 expression could be the cause of the axonal pathology. To test this hypothesis, we created mice with a floxed Plp1 allele for selective Cre-mediated recombination in neurons. We find that recombination of Plp1 in excitatory projection neurons does not cause neuropathology, whereas oligodendroglial targeting of Plp1 is sufficient to cause the entire neurodegenerative spectrum of SPG2 including axonopathy and secondary neuroinflammation. We conclude that PLP-dependent loss of oligodendroglial support is the primary cause of axonal degeneration in SPG2.

The velocity of nerve conduction is moderately enhanced by larger axonal diameters and potently sped up by myelination of axons. Myelination thus allows rapid impulse propagation with reduced axonal diameters; however, no myelin-dependent mechanism has been reported that restricts radial growth of axons. By label-free proteomics, STED-microscopy and cryo-immuno electron-microscopy we here identify CMTM6 (chemokine-like factor-like MARVEL-transmembrane domain-containing family member-6) as a myelin protein specifically localized to the Schwann cell membrane exposed to the axon. We find that disruption of Cmtm6-expression in Schwann cells causes a substantial increase of axonal diameters but does not impair myelin biogenesis, radial sorting or integrity of axons. Increased axonal diameters correlate with accelerated sensory nerve conduction and sensory responses and perturbed motor performance. These data show that Schwann cells utilize CMTM6 to restrict the radial growth of axons, which optimizes nerve function.

Proteolipid protein (PLP) is the most abundant integral membrane protein in central nervous system (CNS) myelin. Expression of the Plp‐gene in oligodendrocytes is not essential for the biosynthesis of myelin membranes but required to prevent axonal pathology. This raises the question whether the exceptionally high level of PLP in myelin is required later in life, or whether high‐level PLP expression becomes dispensable once myelin has been assembled. Both models require a better understanding of the turnover of PLP in myelin in vivo. Thus, we generated and characterized a novel line of tamoxifen‐inducible Plp‐mutant mice that allowed us to determine the rate of PLP turnover after developmental myelination has been completed, and to assess the possible impact of gradually decreasing amounts of PLP for myelin and axonal integrity. We found that 6 months after targeting the Plp‐gene the abundance of PLP in CNS myelin was about halved, probably reflecting that myelin is slowly turned over in the adult brain. Importantly, this reduction by 50% was sufficient to cause the entire spectrum of neuropathological changes previously associated with the developmental lack of PLP, including myelin outfoldings, lamellae splittings, and axonal spheroids. In comparison to axonopathy and gliosis, the infiltration of cytotoxic T‐cells was temporally delayed, suggesting a corresponding chronology also in the genetic disorders of PLP‐deficiency. High‐level abundance of PLP in myelin throughout adult life emerges as a requirement for the preservation of white matter integrity.

Protein zero (P0) is the major structural component of peripheral myelin. Lack of this adhesion protein from Schwann cells causes a severe dysmyelinating neuropathy with secondary axonal degeneration in humans with the neuropathy Dejerine-Sottas syndrome (DSS) and in the corresponding mouse model (P0(null)-mice). In the mammalian CNS, the tetraspan-membrane protein PLP is the major structural myelin constituent and required for the long-term preservation of myelinated axons, which fails in hereditary spastic paraplegia (SPG type-2) and the relevant mouse model (Plp(null)-mice). The Plp-gene is also expressed in Schwann cells but PLP is of very low abundance in normal peripheral myelin; its function has thus remained enigmatic. Here we show that the abundance of PLP but not of other tetraspan myelin proteins is strongly increased in compact peripheral myelin of P0(null)-mice. To determine the functional relevance of PLP expression in the absence of P0, we generated P0(null)*Plp(null)-double-mutant mice. Compared with either single-mutant, P0(null)*Plp(null)-mice display impaired nerve conduction, reduced motor functions, and premature death. At the morphological level, axonal segments were frequently non-myelinated but in a one-to-one relationship with a hypertrophic Schwann cell. Importantly, axonal numbers were reduced in the vital phrenic nerve of P0(null)*Plp(null)-mice. In the absence of P0, thus, PLP also contributes to myelination by Schwann cells and to the preservation of peripheral axons. These data provide a link between the Schwann cell-dependent support of peripheral axons and the oligodendrocyte-dependent support of central axons.

Abstract Spinal Muscular Atrophy (SMA) is a fatal neurodegenerative disease of newborns and children caused by mutations or deletions of the Survival of motoneuron gene 1 (SMN1) resulting in low levels of the SMN protein. While neuromuscular degeneration is the cardinal symptom of the disease, the reduction of the ubiquitously expressed SMN additionally elicits non-motoneuron symptoms. Impaired bone development is a key feature of SMA but it is yet unknown whether this is an indirect functional consequence of muscle weakness or caused by bone-intrinsic mechanisms. Therefore, we radiologically examined SMA patients in a prospective, non-randomized cohort study characterizing bone size and bone mineral density (BMD) and performed equivalent measurements in pre-symptomatic SMA mice. BMD as well as lumbar vertebral body size were significantly reduced in SMA patients. This growth defect but not BMD reduction was confirmed in SMA mice by μCT before the onset of neuromuscular symptoms indicating that it is at least partially independent of neuromuscular degeneration. Interestingly, the number of chondroblasts in the hypertrophic zone of the growth plate was significantly reduced. This was underlined by RNAseq and expression data from developing SMA mice vertebral bodies which revealed molecular changes related to cell division and cartilage remodeling. Together, these findings suggest a bone intrinsic defect in SMA. This phenotype may not be rescued by novel drugs which enhance SMN levels in the central nervous system only.