Decreased ACKR3 (CXCR7) function causes oculomotor synkinesis in mice and humans.

Oculomotor synkinesis is the involuntary movement of the eyes or eyelids with a voluntary attempt at a different movement. The chemokine receptor CXCR4 and its ligand CXCL12 regulate oculomotor nerve development; mice with loss of either molecule have oculomotor synkinesis. In a consanguineous family with congenital ptosis and elevation of the ptotic eyelid with ipsilateral abduction, we identified a co-segregating homozygous missense variant (c.772G>A) in ACKR3, which encodes an atypical chemokine receptor that binds CXCL12 and functions as a scavenger receptor, regulating levels of CXCL12 available for CXCR4 signaling. The mutant protein (p.V258M) is expressed and traffics to the cell surface but has a lower binding affinity for CXCL12. Mice with loss of Ackr3 have variable phenotypes that include misrouting of the oculomotor and abducens nerves. All embryos show oculomotor nerve misrouting, ranging from complete misprojection in the midbrain, to aberrant peripheral branching, to a thin nerve, which aberrantly innervates the lateral rectus (as seen in Duane syndrome). The abducens nerve phenotype ranges from complete absence, to aberrant projections within the orbit, to a normal trajectory. Loss of ACKR3 in the midbrain leads to downregulation of CXCR4 protein, consistent with reports that excess CXCL12 causes ligand-induced degradation of CXCR4. Correspondingly, excess CXCL12 applied to ex vivo oculomotor slices causes axon misrouting, similar to inhibition of CXCR4. Thus, ACKR3, through its regulation of CXCL12 levels, is an important regulator of axon guidance in the oculomotor system; complete loss causes oculomotor synkinesis in mice, while reduced function causes oculomotor synkinesis in humans.

Ex Vivo Oculomotor Slice Culture from Embryonic GFP-Expressing Mice for Time-Lapse Imaging of Oculomotor Nerve Outgrowth.

Accurate eye movements are crucial for vision, but the development of the ocular motor system, especially the molecular pathways controlling axon guidance, has not been fully elucidated. This is partly due to technical limitations of traditional axon guidance assays. To identify additional axon guidance cues influencing the oculomotor nerve, an ex vivo slice assay to image the oculomotor nerve in real-time as it grows towards the eye was developed. E10.5 IslMN-GFP embryos are used to generate ex vivo slices by embedding them in agarose, slicing on a vibratome, then growing them in a microscope stage-top incubator with time-lapse photomicroscopy for 24-72 h. Control slices recapitulate the in vivo timing of outgrowth of axons from the nucleus to the orbit. Small molecule inhibitors or recombinant proteins can be added to the culture media to assess the role of different axon guidance pathways. This method has the advantages of maintaining more of the local microenvironment through which axons traverse, not axotomizing the growing axons, and assessing the axons at multiple points along their trajectory. It can also identify effects on specific subsets of axons. For example, inhibition of CXCR4 causes axons still within the midbrain to grow dorsally rather than ventrally, but axons that have already exited ventrally are not affected.

Loss of CXCR4/CXCL12 Signaling Causes Oculomotor Nerve Misrouting and Development of Motor Trigeminal to Oculomotor Synkinesis.

Purpose: Proper control of eye movements is critical to vision, but relatively little is known about the molecular mechanisms that regulate development and axon guidance in the ocular motor system or cause the abnormal innervation patterns (oculomotor synkinesis) seen in developmental disorders and after oculomotor nerve palsy. We developed an ex vivo slice assay that allows for live imaging and molecular manipulation of the growing oculomotor nerve, which we used to identify axon guidance cues that affect the oculomotor nerve. Methods: Ex vivo slices were generated from E10.5 IslMN-GFP embryos and grown for 24 to 72 hours. To assess for CXCR4 function, the specific inhibitor AMD3100 was added to the culture media. Cxcr4cko/cko:Isl-Cre:ISLMN-GFP and Cxcl12KO/KO:ISLMN-GFP embryos were cleared and imaged on a confocal microscope. Results: When AMD3100 was added to the slice cultures, oculomotor axons grew dorsally (away from the eye) rather than ventrally (toward the eye). Axons that had already exited the midbrain continued toward the eye. Loss of Cxcr4 or Cxcl12 in vivo caused misrouting of the oculomotor nerve dorsally and motor axons from the trigeminal motor nerve, which normally innervate the muscles of mastication, aberrantly innervated extraocular muscles in the orbit. This represents the first mouse model of trigeminal-oculomotor synkinesis. Conclusions: CXCR4/CXCL12 signaling is critical for the initial pathfinding decisions of oculomotor axons and their proper exit from the midbrain. Failure of the oculomotor nerve to innervate its extraocular muscle targets leads to aberrant innervation by other motor neurons, indicating that muscles lacking innervation may secrete cues that attract motor axons.

Ocular Motor Nerve Development in the Presence and Absence of Extraocular Muscle.

Purpose: To spatially and temporally define ocular motor nerve development in the presence and absence of extraocular muscles (EOMs). Methods: Myf5cre mice, which in the homozygous state lack EOMs, were crossed to an IslMN:GFP reporter line to fluorescently label motor neuron cell bodies and axons. Embryonic day (E) 11.5 to E15.5 wild-type and Myf5cre/cre:IslMN:GFP whole mount embryos and dissected orbits were imaged by confocal microscopy to visualize the developing oculomotor, trochlear, and abducens nerves in the presence and absence of EOMs. E11.5 and E18.5 brainstems were serially sectioned and stained for Islet1 to determine the fate of ocular motor neurons. Results: At E11.5, all three ocular motor nerves in mutant embryos approached the orbit with a trajectory similar to that of wild-type. Subsequently, while wild-type nerves send terminal branches that contact target EOMs in a stereotypical pattern, the Myf5cre/cre ocular motor nerves failed to form terminal branches, regressed, and by E18.5 two-thirds of their corresponding motor neurons died. Comparisons between mutant and wild-type embryos revealed novel aspects of trochlear and oculomotor nerve development. Conclusions: We delineated mouse ocular motor nerve spatial and temporal development in unprecedented detail. Moreover, we found that EOMs are not necessary for initial outgrowth and guidance of ocular motor axons from the brainstem to the orbit but are required for their terminal branching and survival. These data suggest that intermediate targets in the mesenchyme provide cues necessary for appropriate targeting of ocular motor axons to the orbit, while EOM cues are responsible for terminal branching and motor neuron survival.