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Semaphorin6A acts as a gate keeper between the central and the peripheral nervous system.

Mauti O, Domanitskaya E, Andermatt I, Sadhu R, Stoeckli ET - Neural Dev (2007)

Bottom Line: Ablation of the boundary cap resulted in emigration of motoneurons from the ventral spinal cord along the ventral roots.Loss of either PlexinA4 or Sema6D function had an effect only at the dorsal root entry site but not at the ventral motor axon exit point.At the dorsal root entry site it organizes the segregation of dorsal roots.

View Article: PubMed Central - HTML - PubMed

Affiliation: Developmental Neuroscience, Institute of Zoology, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland. Olivier.Mauti@zool.uzh.ch

ABSTRACT

Background: During spinal cord development, expression of chicken SEMAPHORIN6A (SEMA6A) is almost exclusively found in the boundary caps at the ventral motor axon exit point and at the dorsal root entry site. The boundary cap cells are derived from a population of late migrating neural crest cells. They form a transient structure at the transition zone between the peripheral nervous system (PNS) and the central nervous system (CNS). Ablation of the boundary cap resulted in emigration of motoneurons from the ventral spinal cord along the ventral roots. Based on its very restricted expression in boundary cap cells, we tested for a role of Sema6A as a gate keeper between the CNS and the PNS.

Results: Downregulation of Sema6A in boundary cap cells by in ovo RNA interference resulted in motoneurons streaming out of the spinal cord along the ventral roots, and in the failure of dorsal roots to form and segregate properly. PlexinAs interact with class 6 semaphorins and are expressed by both motoneurons and sensory neurons. Knockdown of PlexinA1 reproduced the phenotype seen after loss of Sema6A function both at the ventral motor exit point and at the dorsal root entry site of the lumbosacral spinal cord. Loss of either PlexinA4 or Sema6D function had an effect only at the dorsal root entry site but not at the ventral motor axon exit point.

Conclusion: Sema6A acts as a gate keeper between the PNS and the CNS both ventrally and dorsally. It is required for the clustering of boundary cap cells at the PNS/CNS interface and, thus, prevents motoneurons from streaming out of the ventral spinal cord. At the dorsal root entry site it organizes the segregation of dorsal roots.

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Lack of Sema6A and Sema6D in dorsal BCCs results in aberrant segregation of dorsal roots. (a) In control embryos axon bundles from each dorsal root ganglion extend to the DREZ in a well organized manner. Roots from adjacent DRGs are segregated and they are all of the same length (dashed bars). (b) In contrast, in embryos lacking Sema6A, roots from adjacent DRGs are no longer segregated (arrowheads). The arrangement of roots arising from individual DRGs is strongly disorganized and roots are often formed by fibers from two adjacent DRGs (arrowheads in (b)). (c) Similarly, roots are disorganized in embryos lacking Sema6D (arrowheads). In addition the length of the roots varied more in the absence of Sema6D (compare dashed bars in (c)). (d) Strong phenotypes were seen in 71% of the embryos lacking Sema6A and in 68% of the embryos lacking Sema6D. Only 13% of the embryos injected with an EGFP plasmid had a comparable phenotype. Downregulation of Sema6B resulted in aberrant DRG shapes and root arrangement in 30% of the embryos. (e) The shapes of DRGs were classified as arc-like when the distance between the most anterior and the most posterior fiber emanating from the DRG was the same as the anteroposterior diameter of the DRG; as bell-shaped when the fibers spread an anteroposterior length that was bigger than the diameter of the DRG; and as mushroom-like when the fibers entered the dorsal spinal cord in a segment that was shorter than the diameter of the DRG. Note that the diameter of the mushroom-like DRGs was smaller than the diameter of arc-like or bell-shaped DRGs. Bar: 200 μm.
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Figure 3: Lack of Sema6A and Sema6D in dorsal BCCs results in aberrant segregation of dorsal roots. (a) In control embryos axon bundles from each dorsal root ganglion extend to the DREZ in a well organized manner. Roots from adjacent DRGs are segregated and they are all of the same length (dashed bars). (b) In contrast, in embryos lacking Sema6A, roots from adjacent DRGs are no longer segregated (arrowheads). The arrangement of roots arising from individual DRGs is strongly disorganized and roots are often formed by fibers from two adjacent DRGs (arrowheads in (b)). (c) Similarly, roots are disorganized in embryos lacking Sema6D (arrowheads). In addition the length of the roots varied more in the absence of Sema6D (compare dashed bars in (c)). (d) Strong phenotypes were seen in 71% of the embryos lacking Sema6A and in 68% of the embryos lacking Sema6D. Only 13% of the embryos injected with an EGFP plasmid had a comparable phenotype. Downregulation of Sema6B resulted in aberrant DRG shapes and root arrangement in 30% of the embryos. (e) The shapes of DRGs were classified as arc-like when the distance between the most anterior and the most posterior fiber emanating from the DRG was the same as the anteroposterior diameter of the DRG; as bell-shaped when the fibers spread an anteroposterior length that was bigger than the diameter of the DRG; and as mushroom-like when the fibers entered the dorsal spinal cord in a segment that was shorter than the diameter of the DRG. Note that the diameter of the mushroom-like DRGs was smaller than the diameter of arc-like or bell-shaped DRGs. Bar: 200 μm.

Mentions: The strong effect on motoneurons and the fact that SEMA6A was expressed also in BCCs at the DREZ prompted us to analyze the effect of Sema6A downregulation on sensory afferents. Loss of Sema6A in dorsal BCCs had a severe effect on the arrangement of dorsal roots (Figure 3). In control embryos analyzed at HH25/26, fibers emanating from a single DRG formed, on average, four to five well separated fiber bundles or roots that entered the dorsal spinal cord via the DREZ. Roots derived from neighboring DRGs were clearly segregated (Figure 3a). This was not the case after downregulation of Sema6A in dorsal BCCs (Figure 3b). In 71% of these embryos the arrangement of dorsal roots and their number were severely perturbed (Figure 3d). Furthermore, the shape of the DRGs was more variable than in control embryos, including many DRGs with a bell shape; that is, with a distance between the most anterior and the most posterior fiber entering the spinal cord that was larger than the anteroposterior size of the DRG (Figure 3e). In control embryos these two lengths were identical, resulting in an arc-like shape of the DRG. In addition to the embryos exhibiting a strong phenotype, we found 18% with a weak phenotype (Figure 3d). In these embryos no bell-shaped DRGs were found despite the fact that the number and arrangement of roots varied. In more than 70% of the embryos lacking Sema6A in BCCs, we found no segregation between adjacent DRGs; that is, roots were formed by fibers emanating from two adjacent DRGs. Only 12% of the embryos treated with double-stranded RNA (dsRNA) derived from SEMA6A were normal. In 58% of the control-treated embryos, DRGs and their roots were normal (Figure 3a,d). Only 13% of them exhibited a strong phenotype.


Semaphorin6A acts as a gate keeper between the central and the peripheral nervous system.

Mauti O, Domanitskaya E, Andermatt I, Sadhu R, Stoeckli ET - Neural Dev (2007)

Lack of Sema6A and Sema6D in dorsal BCCs results in aberrant segregation of dorsal roots. (a) In control embryos axon bundles from each dorsal root ganglion extend to the DREZ in a well organized manner. Roots from adjacent DRGs are segregated and they are all of the same length (dashed bars). (b) In contrast, in embryos lacking Sema6A, roots from adjacent DRGs are no longer segregated (arrowheads). The arrangement of roots arising from individual DRGs is strongly disorganized and roots are often formed by fibers from two adjacent DRGs (arrowheads in (b)). (c) Similarly, roots are disorganized in embryos lacking Sema6D (arrowheads). In addition the length of the roots varied more in the absence of Sema6D (compare dashed bars in (c)). (d) Strong phenotypes were seen in 71% of the embryos lacking Sema6A and in 68% of the embryos lacking Sema6D. Only 13% of the embryos injected with an EGFP plasmid had a comparable phenotype. Downregulation of Sema6B resulted in aberrant DRG shapes and root arrangement in 30% of the embryos. (e) The shapes of DRGs were classified as arc-like when the distance between the most anterior and the most posterior fiber emanating from the DRG was the same as the anteroposterior diameter of the DRG; as bell-shaped when the fibers spread an anteroposterior length that was bigger than the diameter of the DRG; and as mushroom-like when the fibers entered the dorsal spinal cord in a segment that was shorter than the diameter of the DRG. Note that the diameter of the mushroom-like DRGs was smaller than the diameter of arc-like or bell-shaped DRGs. Bar: 200 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2238753&req=5

Figure 3: Lack of Sema6A and Sema6D in dorsal BCCs results in aberrant segregation of dorsal roots. (a) In control embryos axon bundles from each dorsal root ganglion extend to the DREZ in a well organized manner. Roots from adjacent DRGs are segregated and they are all of the same length (dashed bars). (b) In contrast, in embryos lacking Sema6A, roots from adjacent DRGs are no longer segregated (arrowheads). The arrangement of roots arising from individual DRGs is strongly disorganized and roots are often formed by fibers from two adjacent DRGs (arrowheads in (b)). (c) Similarly, roots are disorganized in embryos lacking Sema6D (arrowheads). In addition the length of the roots varied more in the absence of Sema6D (compare dashed bars in (c)). (d) Strong phenotypes were seen in 71% of the embryos lacking Sema6A and in 68% of the embryos lacking Sema6D. Only 13% of the embryos injected with an EGFP plasmid had a comparable phenotype. Downregulation of Sema6B resulted in aberrant DRG shapes and root arrangement in 30% of the embryos. (e) The shapes of DRGs were classified as arc-like when the distance between the most anterior and the most posterior fiber emanating from the DRG was the same as the anteroposterior diameter of the DRG; as bell-shaped when the fibers spread an anteroposterior length that was bigger than the diameter of the DRG; and as mushroom-like when the fibers entered the dorsal spinal cord in a segment that was shorter than the diameter of the DRG. Note that the diameter of the mushroom-like DRGs was smaller than the diameter of arc-like or bell-shaped DRGs. Bar: 200 μm.
Mentions: The strong effect on motoneurons and the fact that SEMA6A was expressed also in BCCs at the DREZ prompted us to analyze the effect of Sema6A downregulation on sensory afferents. Loss of Sema6A in dorsal BCCs had a severe effect on the arrangement of dorsal roots (Figure 3). In control embryos analyzed at HH25/26, fibers emanating from a single DRG formed, on average, four to five well separated fiber bundles or roots that entered the dorsal spinal cord via the DREZ. Roots derived from neighboring DRGs were clearly segregated (Figure 3a). This was not the case after downregulation of Sema6A in dorsal BCCs (Figure 3b). In 71% of these embryos the arrangement of dorsal roots and their number were severely perturbed (Figure 3d). Furthermore, the shape of the DRGs was more variable than in control embryos, including many DRGs with a bell shape; that is, with a distance between the most anterior and the most posterior fiber entering the spinal cord that was larger than the anteroposterior size of the DRG (Figure 3e). In control embryos these two lengths were identical, resulting in an arc-like shape of the DRG. In addition to the embryos exhibiting a strong phenotype, we found 18% with a weak phenotype (Figure 3d). In these embryos no bell-shaped DRGs were found despite the fact that the number and arrangement of roots varied. In more than 70% of the embryos lacking Sema6A in BCCs, we found no segregation between adjacent DRGs; that is, roots were formed by fibers emanating from two adjacent DRGs. Only 12% of the embryos treated with double-stranded RNA (dsRNA) derived from SEMA6A were normal. In 58% of the control-treated embryos, DRGs and their roots were normal (Figure 3a,d). Only 13% of them exhibited a strong phenotype.

Bottom Line: Ablation of the boundary cap resulted in emigration of motoneurons from the ventral spinal cord along the ventral roots.Loss of either PlexinA4 or Sema6D function had an effect only at the dorsal root entry site but not at the ventral motor axon exit point.At the dorsal root entry site it organizes the segregation of dorsal roots.

View Article: PubMed Central - HTML - PubMed

Affiliation: Developmental Neuroscience, Institute of Zoology, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland. Olivier.Mauti@zool.uzh.ch

ABSTRACT

Background: During spinal cord development, expression of chicken SEMAPHORIN6A (SEMA6A) is almost exclusively found in the boundary caps at the ventral motor axon exit point and at the dorsal root entry site. The boundary cap cells are derived from a population of late migrating neural crest cells. They form a transient structure at the transition zone between the peripheral nervous system (PNS) and the central nervous system (CNS). Ablation of the boundary cap resulted in emigration of motoneurons from the ventral spinal cord along the ventral roots. Based on its very restricted expression in boundary cap cells, we tested for a role of Sema6A as a gate keeper between the CNS and the PNS.

Results: Downregulation of Sema6A in boundary cap cells by in ovo RNA interference resulted in motoneurons streaming out of the spinal cord along the ventral roots, and in the failure of dorsal roots to form and segregate properly. PlexinAs interact with class 6 semaphorins and are expressed by both motoneurons and sensory neurons. Knockdown of PlexinA1 reproduced the phenotype seen after loss of Sema6A function both at the ventral motor exit point and at the dorsal root entry site of the lumbosacral spinal cord. Loss of either PlexinA4 or Sema6D function had an effect only at the dorsal root entry site but not at the ventral motor axon exit point.

Conclusion: Sema6A acts as a gate keeper between the PNS and the CNS both ventrally and dorsally. It is required for the clustering of boundary cap cells at the PNS/CNS interface and, thus, prevents motoneurons from streaming out of the ventral spinal cord. At the dorsal root entry site it organizes the segregation of dorsal roots.

Show MeSH
Related in: MedlinePlus