ereas calponin-3-expression appears to be restricted to thymic T cells, B cells display an intermediate GFP fluorescence already in the PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19712481 bone marrow pro-B/pre-B compartment, but significantly higher levels in immature and mature B cells. This implies that calponin-3 expression is turned on early and peaks in later stages, suggesting a 
possible role in both B cell development and immune function. However, conditional deletion of calponin-3 in pro-B using mb1-cre mice did not severely affect B cell development, nor did we observe any defects in overall signaling and calcium flux. Based on its expression pattern and its structural properties, with its ability to bind cytoskeletal elements on the one hand, and molecules like Erk1/2, Smad and PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19710468 PKCs on the other hand, we hypothesized that calponin-3 might be a factor downstream of pre-BCR signaling. Thus, we can only speculate about the lack of an obvious phenotype in the B cell-specific Cnn3-/- mice. First, it may be that cells lacking calponin-3 are functionally impaired with respect to signaling, but that this does not manifest in an alteration of the developmental stages or calcium mobilization. However, this is unlikely, as aberrant signaling from the pre-BCR has profound effects in the bone marrow and in the periphery. Defective activation of Erk1/2, for example, is associated with reduced cell expansion and a block at the pro-B to pre-B cell transition. Likewise, loss of PKC-mediated signaling strongly affects MedChemExpress Chrysontemin normal B cell development. Second, it is possible that other family members, or other proteins with similar structural properties, may be able to compensate for the loss of calponin-3. Given that we did not find any calponin-1 expression in B cells, a likely candidate was calponin-2. However, B cell development in Cnn2/Cnn3-double-deficient mice was also not significantly altered, arguing against a compensatory effect at least within the calponin family. A third possibility is that calponin 3 plays an inhibitory role, rather than being an activator that links receptor stimulation to downstream signaling events. As such, sequestration of molecules such as Erk1/2 or PKCs to calponin 3, and phosphorylated calponin 3 in particular, may counteract their normal function and thus may establish a negative feedback loop that limits cellular signaling. So far, we have not observed any signs of such an inhibitory function of calponin 3, and mice monitored for up to one year did not develop any signs of autoimmunity as a consequence of a putative cellular hyperactivation. However, it is possible that loss of an inhibitory calponin 3 may not lead to a severe phenotype under steady-state conditions, warranting a detailed analysis in a challenging context, e.g. in an autoimmune-prone mouse strain. Moreover, it might be interesting to define the interactome of calponin 3 in lymphocytes, both in its non-phosphorylated and in its phosphorylated state, to undercover such a possible decoy function. Fourth, it is possible that the phosphorylation of calponin-3 induced by pervanadate stimulation, although widely used in the signaling field to mimic strong receptor engagement, does not reflect a physiological condition. Despite intense efforts, we have not been able to detect calponin-3 phosphorylation upon stimulation with an anti- antibody, which may be considered a “cleaner” way to trigger preBCR signaling. On the other hand, the S2 Schneider cell data clearly demonstrate that kinases involved in pre-
