Protein Aggregation in Hematopoietic Stem Cells

About This Grant

ABSTRACT Hematopoietic stem cells (HSCs) regenerate blood and immune cells throughout life. Unfortunately, HSC function declines with age. Age-related defects in HSCs lead to anemia, impaired immunity, bone marrow failure, and cancer. Thus, understanding mechanisms that contribute to HSC aging is critical for developing strategies to enhance regeneration and tissue function in older adults. Proteostasis dysfunction contributes to several age- associated pathologies, but has not been examined as a mechanism of HSC aging. We recently discovered that HSCs are particularly dependent on proteostasis to preserve their self-renewal. However, misfolded proteins arise in HSCs and must be eliminated to preserve HSC fitness. Canonically, the proteasome serves as the primary pathway for degradation of misfolded proteins, but we recently discovered that HSCs preferentially traffic misfolded proteins to aggresomes. Aggresomes are cytosolic inclusion bodies containing misfolded and aggregated proteins that are typically substrates for a selective form of autophagy. Although aggresomes were thought to specifically form in response to stress, we made the surprising discovery that most young HSCs contain aggresomes at steady state in vivo, and that Bag3 deficiency impairs aggresome formation, HSC fate determination, and self-renewal. We also found that HSC aging is associated with a severe loss of aggresomes. This discovery revealed a new and unexplored aspect of HSC physiology and aging, but why HSCs preferentially form aggresomes and the impact of their loss on aging is unknown. Based on preliminary data, we hypothesize that HSCs preferentially traffic misfolded proteins to aggresomes to promote fitness and longevity by preventing accumulation of protein aggregates and amassing proteomic resources that support regenerative activation and confer resistance to nutrient deprivation. Furthermore, we propose that diminished aggresome formation in aging HSCs results from defects in cellular polarity and subsequent accumulation of protein aggregates contributes to declines in HSC function during aging. In Aim 1, we will use Bag3 knockout mice to test if aggresome formation is required to protect HSCs against accumulation of pathologic protein aggregates under steady state and stress conditions. We will also examine if HSCs preferentially store misfolded proteins in aggresomes to sequester resources to fuel activation and buffer against nutrient deprivation. In Aim 2, we will test if reduced aggresome formation induces Hsf1 activation, and test if enhancing Hsf1 activity rescues Bag3-deficient HSCs. In Aim 3, we will examine why aggresomes are lost in aging HSCs. Aggresomes are polarized, but cell polarity is lost in aging HSCs. We will test if disrupting polarity impairs aggresome formation in HSCs, and determine if rescuing polarity by inhibiting Cdc42 restores aggresomes in old HSCs. Research outcomes will uncover why HSCs form aggresomes and reveal how protein aggregation contributes to HSC aging. These studies will identify strategies to manipulate proteostasis to enhance HSC fitness, prevent blood diseases and extend human healthspan.