Peroxisomes are essential metabolic organelles which must communicate and interact extensively with their environment to exchange metabolites and coordinate cellular responses. This communication is often established through membrane contact sites (MCSs), where membranes of two organelles are physically tethered to enable rapid transfer of small molecules, which are crucial for coordinating cellular functions and hence human health. Identifying, understanding and characterising these MCSs and their regulation has been a challenging research topic in the current cell biology field.
In this thesis, I characterised two isoforms of the Acyl-CoA Binding Domain Protein 4 (ACBD4) in order to understand the physiological role of ACBD4 in the cell. Our group previously identified ACBD4 isoform 2 (ACBD4.2) as a tail-anchored peroxisomal
membrane protein. This protein belongs to the ACBD family, which binds acyl-CoA fatty acids via their acyl-CoA binding (ACB) domain. This isoform was described to be involved in the formation of a MCS by tethering with the ER protein, VAPB (Vesicle-Associated membrane Protein B). Looking at the two other uncharacterised isoforms of ACBD4, I found that ACBD4 isoform 1 (ACBD4.1) is localised only in the nucleus, and ACBD4 isoform 3 (ACBD4.3) is predominantly nuclear but also located at
peroxisomes. I explored their physiological role by analysing a BioID, which revealed candidate proximal interaction partners in the nucleus. Notably, the peroxisomal protein ACBD5 (ACBD Protein 5) was identified as a binding partner of ACBD4.3 but
not ACBD4.1. Similar to ACBD4.2, ACBD5 is also described as a tail-anchored peroxisomal protein involved in the interaction with VAPB to enable the exchange of lipids and metabolites between peroxisomes and the ER. We have confirmed that
ACBD5 interacts with ACBD4.3, supporting the idea that there is communication between peroxisomes and the nucleus, although the function of this, and the role of ACBD4.3 in this process, are not yet understood. Notably, the peroxisomal localisation
of ACBD4.3 seems to be dependent on its acyl-CoA fatty acid binding state, as ACB domain mutants decreased localisation at peroxisomes. Additionally, more prolonged expression of ACBD4.3 led to a rearrangement of the ER around peroxisomes,
suggesting a role of this protein in the peroxisome-ER hub. Further studies intend to investigate the impact of ACBD4.3 on peroxisome and nuclear function as well as on peroxisome-ER communication.
In parallel, I investigated the regulation of ACBD5 as an important mediator of peroxisome-ER contacts, which have a huge impact on maintaining cellular well-being. Therefore, I generated an endogenously tagged ACBD5 reporter cell line by CRIS PITCh to study the response of ACBD5 protein levels to different compounds. A small compound screen using a luminescence read-out of endogenous ACBD5 levels revealed that ACBD5 may be modulated by different compounds which activate PPARα signalling, although nothing significant was revealed. Further studies will explore other compounds that could impact lipid metabolism and extend to an
extensive compound screen in cooperation with Novartis. The identified compounds may prove useful to modulate organelle contacts and consequently improve cell performance in pathophysiological conditions.