Impaired synaptic plasticity and cAMP response element-binding protein activation in Ca2+/calmodulin-dependent protein kinase type IV/Gr-deficient mice.

The Ca(2+)/calmodulin-dependent protein kinase type IV/Gr (CaMKIV/Gr) is a key effector of neuronal Ca(2+) signaling; its function was analyzed by targeted gene disruption in mice. CaMKIV/Gr-deficient mice exhibited impaired neuronal cAMP-responsive element binding protein (CREB) phosphorylation and Ca(2+)/CREB-dependent gene expression. They were also deficient in two forms of synaptic plasticity: long-term potentiation (LTP) in hippocampal CA1 neurons and a late phase of long-term depression in cerebellar Purkinje neurons. However, despite impaired LTP and CREB activation, CaMKIV/Gr-deficient mice exhibited no obvious deficits in spatial learning and memory. These results support an important role for CaMKIV/Gr in Ca(2+)-regulated neuronal gene transcription and synaptic plasticity and suggest that the contribution of other signaling pathways may spare spatial memory of CaMKIV/Gr-deficient mice.

Targeted disruption of the Oct-2 locus in a B cell provides genetic evidence for two distinct cell type-specific pathways of octamer element-mediated gene activation.

The Oct-2 protein is a tissue-specific POU-homeodomain transcription factor. It has been considered to represent a developmental regulator of immunoglobulin gene expression by virtue of its interaction with a functionally essential octamer element found in immunoglobulin gene promoters. This proposal has been most strongly challenged by several in vitro transcription analyses which have shown that the related ubiquitous factor Oct-1 can activate transcription from immunoglobulin gene promoters as efficiently as Oct-2. We have genetically analyzed Oct-2 function by using gene targeting to disrupt both alleles of the locus in the murine B cell line WEHI-231. This cell line expresses productively rearranged immunoglobulin genes as well as the Oct-2 gene at high levels which are comparable to those observed in activated murine splenic B cells. In spite of a drastic reduction in Oct-2 levels (20-fold), no effect was observed on the expression of endogenous immunoglobulin genes or on the activity of a transfected immunoglobulin promoter or a heterologous promoter with a single octamer element. In contrast, expression of a reporter construct containing multiple octamer motifs upstream of a heterologous promoter was severely reduced in the double-disruptant cells. The differential responses of the single- and multiple-octamer motif reporter constructs in the mutant B cells are unlikely to be a consequence of differing concentration requirements for activation by Oct-2. The two constructs are activated equivalently over the same range of Oct-2 concentration in a non-B cell. These results provide genetic support for the existence of an Oct-2-independent, but octamer element-dependent, B cell-specific pathway for immunoglobulin gene transcription. They also genetically reveal a distinct Oct-2-dependent pathway of octamer-mediated gene activation. This study demonstrates the feasibility of targeting a diploid locus in a somatic mammalian cell line. Extension of this approach to genes encoding other transcription factors will allow a genetic dissection of their functions within the context of cell lines representing various differentiation states.

BLyF, a novel cell-type- and stage-specific regulator of the B-lymphocyte gene mb-1.

The mb-1 gene encodes an integral membrane protein that appears to be required for the surface expression and signalling function(s) of the immunoglobulin receptor on B lymphocytes. The gene is expressed in a lineage-restricted manner. It is activated early in B-cell ontogeny, continues to be expressed in mature B cells, but is turned off in terminally differentiated plasma cells. We have identified the mb-1 promoter and functionally tested its activity by transient transfections. A 737-bp promoter fragment preferentially stimulates accurately initiated transcription in mb-1-expressing B cells. Deletion analysis of the promoter suggests the presence of two functional domains, proximal and distal. Both domains independently activate transcription from a heterologous promoter. The distal domain functions in a cell-type- and stage-specific manner, activating transcription in B cells but not in T cells or plasma cells. A 25-bp element within this domain is necessary and sufficient for activity. This element is recognized by a novel cell-type- and stage-specific transcription factor termed BLyF. The binding of BLyF completely correlates with the ability of the regulatory element to stimulate transcription. Thus, BLyF appears to positively regulate transcription of the mb-1 gene. Our results also suggest that the inactivity of the mb-1 locus in plasma cells is not simply due to the loss of BLyF activity.