The actin-binding protein caldesmon is in spleen and lymph nodes predominately expressed by smooth-muscle cells, reticular cells, and follicular dendritic cells.

Reticular cells and follicular dendritic cells (FDCs) build up a framework that underlies the compartmentalization of spleens and lymph nodes. Subpopulations of reticular cells express the smooth-muscle isoform of actin, indicative of a specialized contractile apparatus. We have investigated the distribution of the actin-binding protein caldesmon in spleen and lymph nodes of mice and rats. Caldesmon modulates contraction and regulates cell motility. Alternative splicing of transcripts from a single gene results in high-molecular-mass isoforms (h-caldesmon) that are predominately expressed by smooth-muscle cells (SMCs), and low-molecular-mass isoforms (l-caldesmon) that are thought to be widely distributed in non-muscle tissues, but the distribution of caldesmon in spleen and lymph nodes has not been reported. We have performed Western blot analysis and immunohistochemistry using four different antibodies against caldesmon, among these a newly developed polyclonal antibody directed against recombinant mouse caldesmon. Western blot analysis showed the preponderance of l-caldesmon in spleen and lymph nodes. Our results from immunohistochemistry demonstrate caldesmon in SMCs, as expected, but also in reticular cells and FDCs, and suggest that the isoform highly expressed by reticular cells is l-caldesmon. In spleen of SCID mice, caldesmon was expressed by reticular cells in the absence of lymphocytes.

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