Kremen1 restricts Dkk activity during posterior lateral line development in zebrafish.

Canonical Wnt signaling plays crucial roles during development and disease. How Wnt signaling is modulated in different in vivo contexts is currently not well understood. Here, we investigate the modulation of Wnt signaling in the posterior lateral line primordium (pLLP), a cohort of ~100 cells that collectively migrate along the trunk of the zebrafish embryo. The pLLP comprises proliferative progenitor cells and organized epithelial cells that will form the mechanosensory organs of the posterior lateral line. Wnt signaling is active in the leading progenitor zone of the pLLP and restricted from the trailing zone through expression of the secreted Wnt inhibitors dkk1b and dkk2. We have identified a zebrafish strain, krm1(nl10), which carries a mutation in the kremen1 gene, a non-obligate co-receptor for the Dkk family of proteins. Previous studies have shown that Kremen1 inhibits Wnt signaling by facilitating internalization of the Kremen1-Dkk-Lrp5/6 complex. Surprisingly, we found that disruption of Kremen1 in the pLLP exhibited molecular and cellular phenotypes associated with a decrease rather than overactivation of Wnt signaling. Transplantation of wild-type cells into the mutant primordia failed to rescue the krm1(nl10) phenotype, thus revealing that the effects of Kremen1 loss are non-cell-autonomous. Finally, ectopic expression of Dkk1b-mTangerine protein revealed larger spread of the fusion protein in the mutant primordia compared with the wild type. Based on our data, we propose a novel mechanism in which Kremen1 modulates Wnt activity by restricting the range of secreted Dkk proteins during collective cell migration in the pLLP.

Lef1 is required for progenitor cell identity in the zebrafish lateral line primordium.

The zebrafish posterior lateral line (pLL) is a sensory system that comprises clusters of mechanosensory organs called neuromasts (NMs) that are stereotypically positioned along the surface of the trunk. The NMs are deposited by a migrating pLL primordium, which is organized into polarized rosettes (proto-NMs). During migration, mature proto-NMs are deposited from the trailing part of the primordium, while progenitor cells in the leading part give rise to new proto-NMs. Wnt signaling is active in the leading zone of the primordium and global Wnt inactivation leads to dramatic disorganization of the primordium and a loss of proto-NM formation. However, the exact cellular events that are regulated by the Wnt pathway are not known. We identified a mutant strain, lef1(nl2), that contains a lesion in the Wnt effector gene lef1. lef1(nl2) mutants lack posterior NMs and live imaging reveals that rosette renewal fails during later stages of migration. Surprisingly, the overall primordium patterning, as assayed by the expression of various markers, appears unaltered in lef1(nl2) mutants. Lineage tracing and mosaic analyses revealed that the leading cells (presumptive progenitors) move out of the primordium and are incorporated into NMs; this results in a decrease in the number of proliferating progenitor cells and eventual primordium disorganization. We concluded that Lef1 function is not required for initial primordium organization or migration, but is necessary for proto-NM renewal during later stages of pLL formation. These findings revealed a novel role for the Wnt signaling pathway during mechanosensory organ formation in zebrafish.

Naproxen treatment inhibits articular cartilage loss in a rat model of osteoarthritis.

The effects of naproxen, a nonsteroidal anti-inflammatory drug (NSAID), on articular cartilage degeneration in female Sprague-Dawley rats was examined. Osteoarthritis (OA) was induced by destabilization of the medial meniscus (DMM) in each knee. Rats were treated with acetaminophen (60 mg/kg), naproxen (8 mg/kg), or 1% carboxymethylcellulose (placebo) by oral gavage twice daily for 3 weeks, beginning 2 weeks after surgery. OA severity was assessed by histological Osteoarthritis Research Society International (OARSI) scoring and by measuring proximal tibia cartilage depth using contrast enhanced µCT (n = 6 per group) in specimens collected at 2, 5, and 7 weeks after surgery as well as on pristine knees. Medial cartilage OARSI scores from the DMM knees of naproxen-treated rats were statistically lower (i.e., better) than the medial cartilage OARSI scores from the DMM knees of placebo-treated rats at 5-weeks (8.7 ± 3.6 vs. 13.2 ± 2.4, p = 0.025) and 7-weeks (9.5 ± 1.2 vs. 12.5 ± 2.5, p = 0.024) after surgery. At 5 weeks after DMM surgery, medial articular cartilage depth in the proximal tibia specimens was significantly greater in the naproxen (1.78 ± 0.26 mm, p = 0.005) and acetaminophen (1.94 ± 0.12 mm, p < 0.001) treated rats as compared with placebo-treated rats (1.34 ± 0.24 mm). However, at 7 weeks (2 weeks after drug withdrawal), medial articular cartilage depth for acetaminophen-treated rats (1.36 ± 0.29 mm) was significantly reduced compared with specimens from the naproxen-treated rats (1.88 ± 0.14 mm; p = 0.004). The results indicate that naproxen treatment reduced articular cartilage degradation in the rat DMM model during and after naproxen treatment.

Chondrogenic and gliogenic subpopulations of neural crest play distinct roles during the assembly of epibranchial ganglia.

In vertebrates, the sensory neurons of the epibranchial (EB) ganglia transmit somatosensory signals from the periphery to the CNS. These ganglia are formed during embryogenesis by the convergence and condensation of two distinct populations of precursors: placode-derived neuroblasts and neural crest- (NC) derived glial precursors. In addition to the gliogenic crest, chondrogenic NC migrates into the pharyngeal arches, which lie in close proximity to the EB placodes and ganglia. Here, we examine the respective roles of these two distinct NC-derived populations during development of the EB ganglia using zebrafish morphant and mutants that lack one or both of these NC populations. Our analyses of mutant and morphant zebrafish that exhibit deficiencies in chondrogenic NC at early stages reveal a distinct requirement for this NC subpopulation during early EB ganglion assembly and segmentation. Furthermore, restoration of wildtype chondrogenic NC in one of these mutants, prdm1a, is sufficient to restore ganglion formation, indicating a specific requirement of the chondrogenic NC for EB ganglia assembly. By contrast, analysis of the sox10 mutant, which lacks gliogenic NC, reveals that the initial assembly of ganglia is not affected. However, during later stages of development, EB ganglia are dispersed in the sox10 mutant, suggesting that glia are required to maintain normal EB ganglion morphology. These results highlight novel roles for two subpopulations of NC cells in the formation and maintenance of EB ganglia: chondrogenic NC promotes the early-stage formation of the developing EB ganglia while glial NC is required for the late-stage maintenance of ganglion morphology.

Extensive spinal decussation and bilateral termination of cervical corticospinal projections in rhesus monkeys.

To examine neuroanatomical mechanisms underlying fine motor control of the primate hand, adult rhesus monkeys underwent injections of biotinylated dextran amine (BDA) into the right motor cortex. Spinal axonal anatomy was examined using detailed serial-section reconstruction and modified stereological quantification. Eighty-seven percent of corticospinal tract (CST) axons decussated in the medullary pyramids and descended through the contralateral dorsolateral tract of the spinal cord. Eleven percent of CST axons projected through the dorsolateral CST ipsilateral to the hemisphere of origin, and 2% of axons projected through the ipsilateral ventromedial CST. Notably, corticospinal axons decussated extensively across the spinal cord midline. Remarkably, nearly 2-fold more CST axons decussated across the cervical spinal cord midline (approximately 12,000 axons) than were labeled in all descending components of the CST (approximately 6,700 axons). These findings suggest that CST axons extend multiple segmental collaterals. Furthermore, serial-section reconstructions revealed that individual axons descending in either the ipsilateral or contralateral dorsolateral CST can: 1) terminate in the gray matter ipsilateral to the hemisphere of origin; 2) terminate in the gray matter contralateral to the hemisphere of origin; or 3) branch in the spinal cord and terminate on both sides of the spinal cord. These results reveal a previously unappreciated degree of bilaterality and complexity of corticospinal projections in the primate spinal cord. This bilaterality is more extensive than that of the rat CST, and may resemble human CST organization. Thus, augmentation of sprouting of these extensive bilateral CST projections may provide a novel target for enhancing recovery after spinal cord injury.