Calpain I Null Mice Display Lymphoid Hyperplasia.

Calpains are ubiquitous calcium-regulated cysteine proteases that have been implicated in cytoskeletal organization, homeostasis, cell proliferation, cell motility, and apoptosis. The conventional calpains include mu-calpain (calpain I) that requires micromolar calcium for half maximal activation and m-calpain (calpain II) that functions at millimolar calcium concentration in vitro. To evaluate the physiological function(s) of mu-calpain, we utilized genetically altered mu-calpain null mice as the model system. Spleen biopsy of mu-calpain null adult mice showed consistent expansion of white pulp (lymphoid hyperplasia) due to an increase in number and size of follicles and PALS (periarteriolar lymphatic sheets), relative to matched wild type mice. Also, cervical lymph nodes of mu-calpain null mice showed lymphoid hyperplasia and marked plasmacytosis. The majority of mu-calpain null mice displayed multi-organ lymphoid infiltration. Using immunofluorescence labeling and multiparameter flow cytometry, the cellular composition of the hematopoietic compartment was quantified in both bone marrow and spleen of mu-calpain null mice. The mu-calpain null mice exhibited an overall increase in the number of B-cell lineage; showing ~40% increase of B220⁺ cells in the bone marrow and ~19% increase of B220⁺ cells in the spleen. Interestingly, T-cell compartment (CD3⁺, CD4, CD8⁺) and NK lineage (NK1.1⁺) cells were significantly reduced. Myeloid specific GR1⁺, CD11b⁺ granulocytic cell lineage, and megakaryocytic CD41⁺ cells were also noticeably increased in the bone marrow of mu-calpain null mice. Together, these results reveal a central role of mu-calpain in the maintenance of lymphoid homeostasis, contributing to the regulation of B-cell lineage. Ongoing experiments involve investigating quantitative effects of mu-calpain deletion on the survival and apoptotic cell death of B220⁺ B-cell lineage in response to various insults such as Fas ligand, cycloheximide, irradiation, 9-AC, and staurosporine. The outcome of these studies will elucidate the precise biochemical function(s) of mu-calpain by identifying key substrates that regulate apoptosis in a physiologically relevant environment.