The present invention provides methods and agents for the treatment of TWEAK-related conditions, including cardiac, liver, kidney, lung, adipose, skeletal, muscle, neuronal, bone and cartilage conditions. The invention also provides methods for identifying TWEAK agonists or antagonists for the treatment of TWEAK-related conditions. Additionally, the invention provides transgenic animals that express an exogenous DNA encoding a TWEAK polypeptide, or fragments, analogs, or muteins thereof, and methods for using such animals to identify TWEAK agonists or antagonists. The invention further provides methods for diagnosing a disease based on TWEAK expression. The invention also provides methods for affecting cellular differentiation of progenitor cells using TWEAK polypeptides, agonists, or antagonists.