Tissue‐Specific Abundance of NAMPT, CLOCK, and BMAL1 Proteins in Murine Tissues

Abstract

Circadian Locomotor Output Cycles Kaput (CLOCK) and Aryl hydrocarbon receptor nuclear translocator‐like (BMAL1) are nuclear transcription factors that regulate protein expression oscillations in peripheral tissues. CLOCK and BMAL1 form a heterodimer complex that initiates the transcription of circadian‐regulated genes like nicotinamide phosphoribosyl transferase (NAMPT), the rate‐limiting enzyme in NAD biosynthesis. The abundance of these molecular clock proteins likely varies in a tissue‐ specific manner. We investigated protein abundance of CLOCK, BMAL1, and NAMPT in mammalian tissues.10 male ICR mice were euthanized at ∼11:00AM. Various skeletal muscles (quadriceps (Q), gastrocnemius (G), extensor digitorum longus (EDL), soleus (S)), heart (H), liver (L), brown adipose tissue (BAT), visceral adipose tissue (VAT), and subcutaneous adipose tissue (SAT) were removed and immunoblots for NAMPT, BMAL1, and CLOCK were performed.Relative CLOCK protein abundance was high in liver tissue, while BMAL1 expression was most highly expressed in mouse subcutaneous adipose tissue, visceral adipose tissue and heart. We detected statistically significant elevations in NAMPT protein concentrations in mitochondria‐dense murine tissues (soleus muscle, heart, liver, brown adipose tissue).In conclusion, relative CLOCK and BMAL 1 protein abundances are not clearly correlated within tissues, even though these proteins form a heterodimer complex to initiate transcription of circadian‐regulated genes. These data could indicate that these proteins have additional functions within the cell that do not require their interaction. Highly oxidative tissues contained the highest amount of NAMPT, which may suggest that circadian variations are especially pronounced in mitochondria‐dense tissues.