# ER Morphology in the Pathogenesis of Hereditary
## 3 ER Shaping Proteins
### Reticulon-2 (SPG12)
However, their collective role also remains vague. First, because the loss of multiple RTNs in cells and yeast has consequences that are not always consistent with their putative function [69; 79], and second because the differential expression of the four RTNs in different tissues and cell types (i.e., RTN1 and RTN3 expression is essentially limited to the nervous system [72; 77; 78] and RTN4 is expressed ubiquitously [66; 80]) complicates our understanding of their various functional combinations. These experimental observations make it difficult to envisage why so many different RTN isoforms with overlapping functions should have evolved, cooperating in different combinations to eventually achieve the identical result of generating the membrane curvature typical of ER tubules and sheet edges. Despite these difficulties, recourse to simpler model systems that lack the redundancy of vertebrates has partially helped to unravel the function of RTNs. Loss of _Drosophila_ RTN1 was shown to cause most of the tubular ER to be converted to unbranched sheets, even though, as in mice, it didn't impact the survival of flies [33; 71]. RTN1 role in shaping the ER was further confirmed by the demonstration that it engages in a functionally antagonistic interaction with the fusion protein atlastin by mediating membrane fission [71]. However, these findings raise an interesting issue: if ER morphology is linked to function, and RTNs play an important role in controlling ER shape, then why do flies with an ER obviously altered by the absence of their single RTN1 survive unscathed? One possibility is that ER shape is not so crucial under normal circumstances and becomes critical only under specific conditions that have not yet been identified. The enrichment in ER sheets observed in flies and, to some extent, in cell lines following loss of RTNs [71; 79] poses another problem as RTNs are thought to stabilize not only the high curvature of tubules but also that of sheet edges which are abundant in cells and tissues lacking RTNs due to the conversion of tubules to sheets. Other families of sequence-unrelated RHD-containing proteins, such as REEPs/DP1/Yop1p, reside in the ER and have been grouped together with RTNs as being responsible for the generation of ER curvature [70; 81; 82] and thus could be implicated in compensating for the missing RTNs. Nevertheless, in vivo data are essentially absent and in vitro evidence is available only for sparse members of these families and thus does not permit to prove the widely accepted paradigm that they are functionally equivalent in shaping the ER network. Furthermore, it seems highly unlikely that proteins belonging to different families were independently selected during the course of evolution to interchangeably perform an identical cellular function. Finally, we note that a possible compensatory effect of REEP/DP1/Yop1p would have to be specifically targeted to sheet edges, which are abundant upon loss of RTNs, without causing a similar compensation in the membrane of the tubules. However, this is mechanistically inexplicable based on the mainstream hypothesis that both families of curvature-stabilizing proteins are indiscriminately responsible for creating the curvature of sheet edges and tubules [70; 82; 83]. It is clear that our comprehension of the function/s of RTNs is rather imperfect, and the inconsistencies presented here clearly demonstrate the need to critically revise current, sometimes contradictory, ideas about ER curvature-inducing proteins and devise experimental strategies to illuminate their individual roles while refraining from extending functional findings for any of these proteins, even for evolutionarily very distant homologs, to members of other families based on assumptions rather than experimental evidence.