Fnip1 regulates skeletal muscle fibre type specification, fatigue resistance, and suspectibility to muscular dystrophy

Folliculin (FLCN) and the associated folliculin-interacting proteins FNIP1 and FNIP2 have been shown to play a role in cell metabolism through regulation of the AMPK-mTOR pathways. Previously Hasumi et al. (2012) reported that selective deletion of Flcn in mouse skeletal muscle resulted in an increase in mitochondrial biogenesis and muscle fibre specification (discussed in this blog post).

Reyes et al. (2014) have now contributed more to the understanding of muscle fibre specification using a Fnip1-/- mouse (described in Park et al. 2012) which also shows altered muscle fibre specification from type IIb (fast twitch) to type I (slow twitch). Usually Fnip1 protein is expressed in type IIb muscle fibres but not in type I fibres. However in theFnip1 null mice there was an increased proportion of type I fibres indicated by an increase in mitochondrial gene transcripts, an increase in oxygen consumption indicative of an increase of functional mitochondria, and type I fibre specific physiology.

FLCN and FNIP1/2 interact with each other but also directly interact with the metabolicAMPK – a key regulator in energy homeostasis and mitochondrial biogenesis but also skeletal muscle fibre specification in response to exercise (Hardie, 2011). AMPK activation results in activation of the transcriptional regulators PGC1α and PGC1β and subsequent downstream programmes for mitochondrial biogenesis and oxidative metabolism. Within skeletal muscle activation of AMPK and PGC1 α, often associated with endurance exercise, also increases type I fibre specification (Hambrecht et al., 1997).

Fnip1 interaction inhibits AMPK activity and subsequently, as seen in this paper, a loss of Fnip1 results in increased phosphorylation of AMPK and increased levels of downstream pathway components, such as PGC1α, as detected by qPCR and western blot. This suggests that perturbed Fnip1 activity, as seen with FLCN loss, results in an increase in basal AMPK activity which in turn leads to changes in PGC1α expression and activity.

Fnip1-/- PGC1a-/- double null mice show a reduction in aberrant muscle specification, suggesting that the muscle fibre specification changes seen in the Fnip1-/- mice are dependent on PGC1α induction and as such the authors conclude that PGC1α is an essential mediator of fibre specification. Although the exact connection between Fnip1 loss and an increase in PGC1α activity is not fully understood it is likely to be mediated by AMPK activity. The authors suggest that under normal conditions Flcn and Fnip1 work as a complex to inhibit AMPK thereby reducing PGC1α expression and oxidative metabolism.

Although AMPK activation has been reported, via activation of TSC, which inhibits mTOR activity, thereby minimising ATP consumption and cell growth (Gowans et al. 2014), the Fnip1-/- muscle showed an increase in mTOR activity. The changes in mTOR activity were determined not to play a role in muscle fibre specification but may indicate a novel role for Fnip1 in coupling AMPK to mTOR. It is also possible that the increase in intracellular ATP has, through a negative feedback loop, resulted in AMPK inactivation thereby disrupting mTOR regulation as suggested previously in cardiac hypertrophy associated with a loss of FLCN (Hasumi et al 2014).

The loss of FLCN resulting in hyperactivity of AMPK and PGC1α, increased mitochondrial biogenesis and changes to energy levels has been previously reported (Yan et al. 2014). In BHD an increase in mitochondrial respiration has been linked to increased activation of the HIF pathway known to increase tumourigenic potential with increased expression of PGC1α and HIF1 having been identified in BHD renal carcinoma samples (Klomp et al. 2010, Preston et al. 2011). Whilst disruptions to the AMPK-mTOR regulatory pathways are of great importance in BHD research, the exact role played FNIP1 is not entirely understood. What Reyes et al. indicate however, based on the loss of Fnip1 being sufficient to induce an increase in AMPK activity and mitochondrial biogenesis, is that FNIP1 does play an important role that requires further investigation.

Interestingly the group also report a reduction in muscle damage in the Dmdmdx-4CV mouse model of Duchenne Muscular Dystrophy (DMD) when Fnip1 is absent. As the specific overexpression of PGC1α in skeletal muscle has a similarly protective effect (Chan et al. 2014) it suggests that the reduction in muscle loss seen with the inhibition of Fnip1 may, in part, be through induction of PGC1α.

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