Surrogate Light Chain Expression Is Required to Establish Immunoglobulin Heavy Chain Allelic Exclusion during Early B Cell Development

AbstractAllelic exclusion at the IgH locus was examined in B lineage cells of wild-type mice and mice unable to express the surrogate light chain molecule λ5 using a single-cell PCR approach. By analyzing B precursor cells containing two VHDHJH rearrangements, we found that in wild-type animals, cells are allelically excluded as soon as μ chains are expressed. Furthermore, we provide evidence that in cells expressing Dμ proteins VH→DHJH rearrangement is inhibited. In contrast, in the absence of λ5 protein, B precursor cells were allelically “included,” indicating that allelic exclusion at the IgH locus requires expression of the pre-B cell receptor either containing a μ chain or a Dμ chain. However, although μ chain double-producing B precursor cells are generated in λ5-deficient mice, such cells were not detected among surface immunoglobulin positive B cells

Rearrangement and Expression of Immunoglobulin Light Chain Genes Can Precede Heavy Chain Expression during Normal B Cell Development in Mice

In mouse mutants incapable of expressing μ chains, VκJκ joints are detected in the CD43+ B cell progenitors. In agreement with these earlier results, we show by a molecular single cell analysis that 4–7% of CD43+ B cell progenitors in wild-type mice rearrange immunoglobulin (Ig)κ genes before the assembly of a productive VHDHJH joint. Thus, μ chain expression is not a prerequisite to Igκ light chain gene rearrangements in normal development. Overall, ∼15% of the total CD43+ B cell progenitor population carry Igκ gene rearrangements in wild-type mice. Together with the results obtained in the mouse mutants, these data fit a model in which CD43+ progenitors rearrange IgH and Igκ loci independently, with a seven times higher frequency in the former. In addition, we show that in B cell progenitors VκJκ joining rapidly initiates κ chain expression, irrespective of the presence of a μ chain

Differential subunit composition of the G protein–activated inward-rectifier potassium channel during cardiac development

Parasympathetic slowing of the heart rate is predominantly mediated by acetylcholine-dependent activation of the G protein–gated potassium (K(+)) channel (I(K,ACh)). This channel is composed of 2 inward-rectifier K(+) (Kir) channel subunits, Kir3.1 and Kir3.4, that display distinct functional properties. Here we show that subunit composition of I(K,ACh) changes during embryonic development. At early stages, I(K,ACh) is primarily formed by Kir3.1, while in late embryonic and adult cells, Kir3.4 is the predominant subunit. This change in subunit composition results in reduced rectification of I(K,ACh), allowing for marked K(+) currents over the whole physiological voltage range. As a consequence, I(K,ACh) is able to generate the membrane hyperpolarization that underlies the strong negative chronotropy occurring in late- but not early-stage atrial cardiomyocytes upon application of muscarinic agonists. Both strong negative chronotropy and membrane hyperpolarization can be induced in early-stage cardiomyocytes by viral overexpression of the mildly rectifying Kir3.4 subunit. Thus, a switch in subunit composition is used to adopt I(K,ACh) to its functional role in adult cardiomyocytes