The Src Homology 2 Domain Protein Shep1 Plays an Important Role in the Penetration of Olfactory Sensory Axons into the Forebrain

Abstract

Shep1 is a multidomain signaling protein that forms a complex with Cas, a key scaffolding component of integrin signaling pathways, to promote the migration of non-neuronal cells. However, the physiological function of Shep1 in the nervous system remains unknown. Interestingly, we found that Shep1 and Cas are both concentrated in the axons of developing olfactory sensory neurons (OSNs). These neurons extend their axons from the olfactory epithelium to the olfactory bulb located at the anterior tip of the forebrain. However, in developing Shep1 knock-out mice, we did not detect penetration of OSN axons across the pial basement membrane surrounding the olfactory bulb, suggesting that Shep1 function is important for the establishment of OSN connections with the olfactory bulb. Interestingly, we observed reduced levels of Cas tyrosine phosphorylation in OSN axons of Shep1 knock-out mice, suggesting compromised Cas signaling function. Indeed, when embedded in a three-dimensional gel of basement membrane proteins, explants from Shep1 knock-out olfactory epithelium extend neuronal processes less efficiently than explants from control epithelium. Furthermore, ectopic expression of Shep1 in non-neuronal cells promotes cell migration through a collagen gel. Later in development, loss of Shep1 function also causes a marked reduction in olfactory bulb size and disruption of bulb lamination, which may be primarily attributable to the defective innervation. The greatly reduced OSN connections and hypoplasia of the olfactory bulb, likely resulting in anosmia, are reminiscent of the symptoms of Kallmann syndrome, a human developmental disease that can be caused by mutations in a growing number of genes.

The SH2 domain protein Shep1 regulates the in vivo signaling function of the scaffolding protein Cas

“…However, deficiencies in Shep1-Cas function may result in defects in axon growth or dendritic spine morphology [38, 39, 44], abnormalities that may only be revealed by a more detailed analysis. Indeed, in other studies we found that the axons of developing olfactory sensory neurons fail to connect to their synaptic targets in the olfactory bulb in Shep1 knockout mice [23]. We also detected morphological abnormalities in the dendritic spines of hippocampal pyramidal neurons of adult Shep1 knockout mice (Wang and Pasquale, unpublished data).…”

“…Nevertheless, the Shep1ΔN fragments are not sufficient to promote Src activity and Cas phosphorylation, highlighting the importance of the SH2 domain in the physiological function of Shep1. Consistent with a functional deficiency of Shep1ΔN, we found that a Shep1 fragment similar to those expressed in the knockout mice does not promote cell invasion through collagen in Transwell assays when ectopically expressed in HEK 293 cells [23]. Similarly, a truncated form of BCAR3/AND-34 without the N-terminus and the SH2 domain has also been recently reported to be functionally deficient in breast cancer cells [17].…”

“…3B). This suggests suckling defects, which may be due to abnormalities in the olfactory system [23, 24]. Hence, insufficient feeding due to a defective sense of smell may be responsible for the observed perinatal lethality [25, 26].…”

“…In the nervous system, this could be important in developmental processes such as axon outgrowth and perhaps guidance [38, 39]. We indeed found that Shep1 knockout mice have severe defects in the penetration of olfactory sensory axon across the brain basal lamina [23]. …”

Monocyte Adhesion and Plaque Recruitment During Atherosclerosis Development Is Regulated by the Adapter Protein Chat-H/SHEP1

“…However, the corresponding domain on Chat-H appears to lack enzymatic activity 14 . Chat-H and Chat are identical except for a unique N-terminal 180 amino acid sequence that is present on Chat-H. Chat-H was cloned as a protein that positively regulated TCR signaling and IL-2 production in T cells 15 , whereas Chat was shown to mediate penetration of olfactory sensory axons into the forebrain 16 .…”