Ex-vivo structural biology

In light chain amyloidosis (AL), amyloid deposition of monoclonal immunoglobulin light chains (LCs) in vital organs is associated with their severe dysfunction. AL amyloidosis is a polymorphic disease: the variability among LCs is such that virtually every monoclonal protein is unique in its amino acid sequence. AL clinical phenotype is also variable, but heart involvement is particularly frequent (~75% of cases) and dramatically worsens patients’ prognosis.

To understand the molecular determinants of LC amyloidogenicity, we biochemically characterized a set of amyloidogenic AL-causing LCs and compared them to control non-amyloidogenic ones.

Our data indicate that fold stability tend to be lower and protein flexibility higher in AL-LCs compared to non-AL-LCs [1]. Clinical data showed clearly that soluble circulating LC molecules display strong toxicity in vivo. By using the biophysical criteria as above, we designed a more thermodynamically and kinetically stable mutant of an AL-LC. Tests on cardiac cells and on a in vivo model show that soluble toxicity strongly correlate with protein instability [2].

Then we set to study the structure of aggregated LC, amyloid fibrils were extracted from an autoptic cardiac sample from an AL patient. In collaboration with the Cryo-EM facility at the University of Milan, the structure of fibrils ex vivo was determined at 4.0 Å resolution. The fibrils consist of one protomer formed by two segments of the variable domain for a total of 76 residues. Lower order regions of the electron density account for the parts of the LC molecules unstructured in the fibril [3]. Fibrils ex vivo are always formed by full length LCs and by many different fragments but the role of proteolysis in LC aggregation is under debate.

We performed the first molecular characterization of LC fragments from ex vivo fibrils indicating that proteolysis is mainly a post aggregation process [4].

References

• [1] Concurrent structural and biophysical traits link with immunoglobulin light chains amyloid propensity. Oberti et al Sci. Rep. 2017. Doi: 10.1038/s41598-017-16953-7
• [2] Inherent Biophysical Properties Modulate the Toxicity of Soluble Amyloidogenic Light Chains. Maritan et al JMB 2019. Doi: 10.1016/j.jmb.2019.12.015
• [3] Cryo-EM structure of cardiac amyloid fibrils from an immunoglobulin light chain AL amyloidosis patient. Swuec et al Nat Comm 2019. Doi: 10.1038/s41467-019-09133-w
• [4] Mass spectrometry characterization of light chain fragmentation sites in cardiac AL amyloidosis: insights into the timing of proteolysis. Lavatelli et al JBC 2020. Doi: 10.1074/jbc.RA120.013461