We have completed several studies whose focus was to analyze the regulatory elements in the promoter regions of the B cell lineage specific genes CD19, CD20, and CD22. These studies identified several known and novel regulatory elements in their promoters. In the CD19 promoter are two cis-elements, which bind the transcription factor BSAP. BSAP has emerged as key regulator of B cell development and B cell function. To further understand the role of BSAP in B cell physiology we have developed several mouse lines in which BSAP has been expressed transgenically. These mice have several interesting phenotypic abnormalities, including an increased number of B cells and an expansion of two compartments of immature B cells called fraction C and C? in the bone marrow; the C? expansion is most striking and suggests an impairment in the normal B cell apoptosis that occurs during B cell development. The spleens of the transgenic mice are larger than the normal controls. The transgenic B cells have normal proliferative rates to mitogenic signals, but have decreased rates of apoptosis following B cell activation. The transgenic mice also have a marked expansion in their splenic marginal zones; marginal zone B cells are important in generating antibody responses to polysaccharide antigens, which are present in capsules of gram-positive bacteria. When challenged with thymus-dependent antigens, the transgenic mice develop a normal antibody response, but when challenged with a thymus-independent antigen (polysaccharide) they have a markedly increased IgG response compared to litter mate controls, a result consistent with their expanded marginal zone. Analysis of BSAP expression in normal mice by immunohistochemistry revealed very strong expression in the marginal zone B cells. We have also observed that germinal center B cells, particularly centroblasts, have very low levels of BSAP, while mantle zone and memory B cells have much higher levels. In addition, expression of BSAP in a T cell line partially protected it from an apoptotic signal. In contrast to the normal low levels of BSAP expression in normal germinal center B cells, a variety of follicular B cell lymphomas express very high BSAP levels. Overall these results are consistent with a role for BSAP in protecting B cells from apoptosis and in the genesis of marginal zone B cells. The very low expression of BSAP in centroblasts suggests that survival of B cells in the germinal center is associated with an upregulation of BSAP expression and BSAP target gene expression We have also continued our analysis of the HB9 -/- mice; HB9 is a transcription factor that was found to be expressed in activated B lymphocytes, hematopoietic progenitors, pancreas, and developing motor neurons. HB9 deficient mice were found to die at the time of birth. We have determined that the reason that these mice die at this time is because they are unable to breathe, which results from the lack of a phrenic nerve. We also found that these mice have a reduced number of motor neurons and a compensatory increase in interneurons. Using an HB9 specific antibody we have been able to perform both immunohistochemistry and Western blotting. We verified that the HB9-/- mice lack HB9 and analyzed the expression of HB9 in normal developing motor neurons, pancreas, and hematopoietic tissues. Surprisingly, we have also found expression of HB9 in the developing tongue. While HB9 is expressed in fetal liver, spleen, and bone marrow no abnormalities in hematopoiesis or lymphoid development has been observed in HB9 -/- mice. The HB9 -/- mice lack a population of cells normally present in the endocrine portion of the pancreas. The nature of these cells needs to be determined. The HB9 -/- mice will prove to be an extremely valuable resource to investigators interested in motor neuron function and may provide some interesting insights into pancreatic development.
| Thaler, J; Harrison, K; Sharma, K et al. (1999) Active suppression of interneuron programs within developing motor neurons revealed by analysis of homeodomain factor HB9. Neuron 23:675-87 |
| Harrison, K A; Thaler, J; Pfaff, S L et al. (1999) Pancreas dorsal lobe agenesis and abnormal islets of Langerhans in Hlxb9-deficient mice. Nat Genet 23:71-5 |
