Loss of the parkinsonism-associated protein FBXO7 in glutamatergic forebrain neurons in mice leads to abnormal motor behavior and synaptic defects.

Mutations in PARK15, which encodes for the F-box protein FBXO7 have been associated with Parkinsonian Pyramidal syndrome, a rare and complex movement disorder with Parkinsonian symptoms, pyramidal tract signs and juvenile onset. Our previous study showed that systemic loss of Fbxo7 in mice causes motor defects and premature death. We have also demonstrated that FBXO7 has a crucial role in neurons as the specific deletion in tyrosine hydroxylase-positive or glutamatergic forebrain neurons leads to late-onset or early-onset motor dysfunction, respectively. In this study, we examined NEX-Cre;Fbxo7fl/fl mice, in which Fbxo7 was specifically deleted in glutamatergic projection neurons. The effects of FBXO7 deficiency on striatal integrity were investigated with HPLC and histological analyses. NEX-Cre;Fbxo7fl/fl mice revealed an increase in striatal dopamine concentrations, changes in the glutamatergic, GABAergic and dopaminergic pathways, astrogliosis and microgliosis and little or no neuronal loss in the striatum. To determine the effects on the integrity of the synapse, we purified synaptic membranes, subjected them to quantitative mass spectrometry analysis and found alterations in the complement system, endocytosis and exocytosis pathways. These neuropathological changes coincide with alterations in spontaneous home cage behavior. Taken together, our findings suggest that FBXO7 is crucial for corticostriatal projections and the synaptic integrity of the striatum, and consequently for proper motor control.

Myelinating Glia-Specific Deletion of Fbxo7 in Mice Triggers Axonal Degeneration in the Central Nervous System Together with Peripheral Neuropathy.

Myelination of axons facilitates the rapid propagation of electrical signals and the long-term integrity of axons. The ubiquitin-proteasome system is essential for proper protein homeostasis, which is particularly crucial for interactions of postmitotic cells. In our study, we examined how the E3 ubiquitin ligase FBXO7-SCF (SKP1, Cul1, F-box protein) expressed in myelinating cells affects the axon-myelin unit. Deletion of Fbxo7 in oligodendrocytes and Schwann cells in mice using the Cnp1-Cre driver line led to motor impairment due to hindlimb paresis. It did not result in apoptosis of myelinating cells, nor did it affect the proper myelination of axons or lead to demyelination. It however triggered axonal degeneration in the CNS and resulted in the severe degeneration of axons in the PNS, inducing a full-blown neuropathy. Both the CNS and PNS displayed inflammation, while the PNS was also characterized by fibrosis, massive infiltration of macrophages, and edema. Tamoxifen-induced deletion of Fbxo7, after myelination using the Plp1-CreERT2 line, led to a small number of degenerated axons and hence a very mild peripheral neuropathy. Interestingly, loss of Fbxo7 also resulted in reduced proteasome activity in Schwann cells but not in cerebellar granule neurons, indicating a specific sensitivity of the former cell type. Together, our results demonstrate an essential role for FBXO7 in myelinating cells to support associated axons, which is fundamental to the proper developmental establishment and the long-term integrity of the axon-myelin unit.SIGNIFICANCE STATEMENT The myelination of axons facilitates the fast propagation of electrical signals and the trophic support of the myelin-axon unit. Here, we report that deletion of Fbxo7 in myelinating cells in mice triggered motor impairment but had no effect on myelin biogenesis. Loss of Fbxo7 in myelinating glia, however, led to axonal degeneration in the CNS and peripheral neuropathy of the axonal type. In addition, we found that Schwann cells were particularly sensitive to Fbxo7 deficiency reflected by reduced proteasome activity. Based on these findings, we conclude that Fbxo7 is essential for the support of the axon-myelin unit and long-term axonal health.

Loss of FBXO7 (PARK15) results in reduced proteasome activity and models a parkinsonism-like phenotype in mice.

Mutations in the FBXO7 (PARK15) gene have been implicated in a juvenile form of parkinsonism termed parkinsonian pyramidal syndrome (PPS), characterized by Parkinsonian symptoms and pyramidal tract signs. FBXO7 (F-box protein only 7) is a subunit of the SCF (SKP1/cullin-1/F-box protein) E3 ubiquitin ligase complex, but its relevance and function in neurons remain to be elucidated. Here, we report that the E3 ligase FBXO7-SCF binds to and ubiquitinates the proteasomal subunit PSMA2. In addition, we show that FBXO7 is a proteasome-associated protein involved in proteasome assembly. In FBXO7 knockout mice, we find reduced proteasome activity and early-onset motor deficits together with premature death. In addition, we demonstrate that NEX (neuronal helix-loop-helix protein-1)-Cre-induced deletion of the FBXO7 gene in forebrain neurons or the loss of FBXO7 in tyrosine hydroxylase (TH)-positive neurons results in motor defects, reminiscent of the phenotype in PARK15 patients. Taken together, our study establishes a vital role for FBXO7 in neurons, which is required for proper motor control and accentuates the importance of FBXO7 in proteasome function.

Mechanistic contributions of FBXO7 to Parkinson disease.

Parkinson disease (PD) is, without doubt, a burden on modern society as the prevalence increases significantly with age. Owing to this growing number of PD cases, it is more critical than ever to understand the pathogenic mechanisms underlying PD to identify therapeutic targets. The discovery of genetic mutations associated with PD and parkinsonism paves the way toward this goal. Even though, familial forms of the disease represent the minority of PD cases and some forms are so rare that there are only a few affected families, the research on the associated genes is invaluable. Recent additions to PARK mutations are those in PARK15 that encodes the F-box protein O-type 7 (FBXO7). In this review, we highlight the recent research on FBXO7, which advances our knowledge of the etiopathological pathways and fills unexpected gaps therein, justifying the dedicated study of rare variants of PD.

Gallyas Silver Impregnation of Myelinated Nerve Fibers.

In the nervous system of vertebrates, nerve impulse propagation is accelerated by the ensheathment of neuronal axons with myelin. Myelin sheaths are molecularly specialized, lipid-rich plasma membrane extensions of Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system (CNS). To visualize myelinated nerve fibers and to allow for the morphological analyses of myelin in the brain and the spinal cord, an efficient method for silver impregnation of myelin has originally been developed by Ferenc Gallyas in 1979, referred to as Gallyas silver impregnation. Gallyas' method is based on the agyrophilic characteristic of myelin to form and bind silver particles, while this process is suppressed in tissues other than myelin. The silver particles are finally enhanced in a developing step ("physical developer"). The main advantage of this method is that it efficiently visualizes both large myelinated fiber tracts and individual myelinated axons. Here we provide our laboratory protocol that is suitable for paraffin embedded sections and the use of light microscopy based on Gallyas' original protocol and subsequent modifications by Pistorio and colleagues.